Genetic polymorphism and evidence of signatures of selection in the Plasmodium falciparum circumsporozoite protein gene in Tanzanian regions with different malaria endemicity

Background

In 2021 and 2023, the World Health Organization approved RTS, S/AS01 and R21/Matrix M malaria vaccines, respectively, for routine immunization of children in African countries with moderate to high transmission. These vaccines are made of Plasmodium falciparum circumsporozoite protein (Pfcsp) but polymorphisms in this gene raises concerns regarding strain-specific responses and the long-term efficacy of these vaccines. This study assessed the Pfcsp genetic diversity, population structure and signatures of selection among parasites from areas of different malaria transmission in mainland Tanzania, to generate baseline data before the introduction of the malaria vaccines in the country.

Methods

The analysis involved 589 whole genome sequences generated by and as part of the MalariaGEN Community Project. The samples were collected between 2013 and January 2015 from five regions of mainland Tanzania: Morogoro and Tanga (Muheza) (moderate transmission areas), and Kagera (Muleba), Lindi (Nachingwea), and Kigoma (Ujiji) (high transmission areas). Wright's inbreeding coefficient (Fws), Wright's fixation index (FST), principal component analysis, nucleotide diversity, and Tajima's D were used to assess within-host parasite diversity, population structure and natural selection.

Results

Based on Fws (< 0.95), there was high polyclonality (ranged from 69.23% in Nachingwea to 56.9% in Muheza). No population structure was detected in the Pfcsp gene in the five regions (mean FST= 0.0068). The average nucleotide diversity (π), nucleotide differentiation (K) and haplotype diversity (Hd) in the five regions were 4.19, 0.973 and 0.0035, respectively. The C-terminal region of Pfcsp showed high nucleotide diversity at Th2R and Th3R regions. Positive values for the Tajima's D were observed in the Th2R and Th3R regions consistent with balancing selection. The Pfcsp C-terminal sequences had 50 different haplotypes (H_1 to H_50) and only 2% of sequences matched the 3D7 strain haplotype (H_50).

Conclusions

The findings demonstrate high diversity of the Pfcsp gene with limited population differentiation. The Pfcsp gene showed positive Tajima's D values for parasite populations, consistent with balancing selection for variants within Th2R and Th3R regions. This data is consistent with other studies conducted across Africa and worldwide, which demonstrate low 3D7 haplotypes and little population structure. Therefore, additional research is warranted, incorporating other regions and more recent data to comprehensively assess trends in genetic diversity within this important gene. Such insights will inform the choice of alleles to be included in the future vaccines.

Comparative analyses of functional antibody-mediated inhibition with anti-circumsporozoite monoclonal antibodies against transgenic Plasmodium berghei

BACKGROUND

Acquired functional inhibitory antibodies are one of several humoral immune mechanisms used to neutralize foreign pathogens. In vitro bioassays are useful tools for quantifying antibody-mediated inhibition and evaluating anti-parasite immune antibodies. However, a gap remains in understanding of how antibody-mediated inhibition in vitro translates to inhibition in vivo. In this study, two well-characterized transgenic Plasmodium berghei parasite lines, PbmCh-luc and Pb-PfCSP(r), and murine monoclonal antibodies (mAbs) specific to P. berghei and Plasmodium falciparum circumsporozoite protein (CSP), 3D11 and 2A10, respectively, were used to evaluate antibody-mediated inhibition of parasite development in both in vitro and in vivo functional assays.

METHODS

IC50 values of mAbs were determined using an established inhibition of liver-stage development assay (ILSDA). For the in vivo inhibition assay, mice were passively immunized by transfer of the mAbs and subsequently challenged with 5.0 × 103 sporozoites via tail vein injection. The infection burden in both assays was quantified by luminescence and qRT-PCR of P. berghei 18S rRNA normalized to host GAPDH.

RESULTS

The IC50 values quantified by relative luminescence of mAbs 3D11 and 2A10 were 0.396 µg/ml and 0.093 µg/ml, respectively, against transgenic lines in vitro. Using the highest (> 90%) inhibitory antibody concentrations in a passive transfer, an IC50 of 233.8 µg/ml and 181.5 µg/ml for mAbs 3D11 and 2A10, respectively, was observed in vivo. At 25 µg (250 µg/ml), the 2A10 antibody significantly inhibited liver burden in mice compared to control. Additionally, qRT-PCR of P. berghei 18S rRNA served as a secondary validation of liver burden quantification.

CONCLUSIONS

Results from both experimental models, ILSDA and in vivo challenge, demonstrated that increased concentrations of the homologous anti-CSP repeat mAbs increased parasite inhibition. However, differences in antibody IC50 values between parasite lines did not allow a direct correlation between the inhibition of sporozoite invasion in vitro by ILSDA and the inhibition of mouse liver stage burden. Further studies are needed to establish the conditions for confident predictions for the in vitro ILSDA to be a predictor of in vivo outcomes using this model system.

Antibodies elicited by Plasmodium falciparum circumsporozoite proteins lacking sequentially deleted C-terminal amino acids reveal mouse strain and epitopes specific differences

Malaria affects ∼ ¼ billion people globally and requires the development of additional tools to aid in elimination efforts. The recently approved RTS,S/AS01 vaccine represents a positive step, however, the moderate efficacy necessitates the development of more efficacious vaccines. PfCSP is a key target antigen for pre-erythrocytic vaccines aimed at preventing Plasmodium falciparum malaria infections. Epitopes within the central repeat region and at the junction of the repeat and N-terminal domain are well documented as major protective B cell epitopes. On the other hand, a majority of antibodies against the epitopes in the C-terminal domain, have been shown to be non-protective against sporozoite challenge. The C-terminal domain, however, contains CD4+ and CD8+ T cell epitopes previously shown to be important for regulating immune responses. The present study was designed to further explore the immunomodulatory potential of the C-terminal domain using DNA vaccines encoding PfCSP with sequential C-terminal truncations following known T cell epitopes. Five DNA vaccines encoding different truncations of PfCSP within the C-terminal domain were administered via intramuscular route and in vivo electroporation for effective immunogenicity. Protection in mice was evaluated by challenge with transgenic P. berghei expressing PfCSP. In Balb/c mice, antibody responses and protective efficacy were both affected progressively with sequential deletion of C-terminal amino acid residues. Similar studies in C57Bl/6 mice revealed that immunizations with plasmids encoding truncated PfCSP showed partial protection from sporozoite challenge with no significant differences in antibody titers observed compared to full-length PfCSP DNA immunized mice. Further analysis revealed murine strain-specific differences in the recognition of specific epitopes.

Genetic variations of Plasmodium falciparum circumsporozoite protein and the impact on interactions with human immunoproteins and malaria vaccine efficacy

In October 2021, the world's first malaria vaccine RTS,S was endorsed by WHO for broad use in children, despite its low efficacy. This study examined polyclonal infections and the associations of parasite genetic variations with binding affinity to human leukocyte antigen (HLA). Multiplicity of infection was determined by amplicon deep sequencing of PfMSP1. Genetic variations in PfCSP were examined across 88 samples from Ghana and analyzed together with 1655 PfCSP sequences from other African and non-African isolates. Binding interactions of PfCSP peptide variants and HLA were predicted using NetChop and HADDOCK. High polyclonality was detected among infections, with each infection harboring multiple non-3D7 PfCSP variants. Twenty-seven PfCSP haplotypes were detected in the Ghanaian samples, and they broadly represented PfCSP diversity across Africa. The number of genetic differences between 3D7 and non-3D7 PfCSP variants does not influence binding to HLA. However, CSP peptide length after proteolytic degradation significantly affects its molecular weight and binding affinity to HLA. Despite the high diversity of HLA, the majority of the HLAI and II alleles interacted/bound with all Ghana CSP peptides. Multiple non-3D7 strains among P. falciparum infections could impact the effectiveness of RTS,S. Longer peptides of the Th2R/Th3R CSP regions should be considered in future versions of RTS,S.

Humoral Immune Responses to P. falciparum Circumsporozoite Protein (Pfcsp) Induced by the RTS, S Vaccine - Current Update

Malaria vaccines targeting the circumsporozoite protein (CSP) of the P. falciparum parasite have been overall relatively promising. RTS, S is a pre-erythrocytic recombinant protein-based malaria vaccine that targets CSP. RTS, S effectiveness shows some limited success regardless of its 58% efficacy for severe disease. P. falciparum circumsporozoite protein (Pfcsp) has stood to be the main candidate protein for most pre-erythrocytic stage vaccines. Studies on the structural and biophysical characteristics of antibodies specific to CSP (anti-CSP) are underway to achieve fine specificity with the CSP polymorphic regions. More recent studies have proposed the use of different kinds of monoclonal antibodies, the use of appropriate adjuvants, ideal vaccination dose and frequency, and improved targeting of particular epitopes for the robust production of functional antibodies and high complement-fixing activity as other potential methods for achieving long-lasting RTS, S. This review highlights recent findings regarding humoral immune responses to CSP elicited by RTS, S vaccine.

Genetic polymorphism of circumsporozoite protein (CSP) in Plasmodium malariae isolates from Malaysia

Information about Plasmodium malariae is scanty worldwide due to its "benign" nature and low infection rates. Consequently, studies on the genetic polymorphisms of P. malariae are lacking. Here, we report genetic polymorphisms of 28 P. malariae circumsporozoite protein (Pmcsp) isolates from Malaysia which were compared with those in other regions in Asia as well as those from Africa. Phylogenetic analysis revealed that most Malaysian P. malariae isolates clustered together but independently from other Asian isolates. Low nucleotide diversity was observed in Pmcsp non-repeat regions in contrast to high nucleotide diversity observed in non-repeat regions of Plasmodium knowlesi CSP gene, the current major cause of malaria in Malaysia. This study contributes to the characterisation of naturally occurring polymorphisms in the P. malariae CSP gene.

Seven-year kinetics of RTS, S/AS01-induced anti-CSP antibodies in young Kenyan children

BACKGROUND

RTS,S/AS01, the leading malaria vaccine has been recommended by the WHO for widespread immunization of children at risk. RTS,S/AS01-induced anti-CSP IgG antibodies are associated with the vaccine efficacy. Here, the long-term kinetics of RTS,S/AS01-induced antibodies was investigated.

METHODS

150 participants were randomly selected from the 447 children who participated in the RTS,S/AS01 phase IIb clinical trial in 2007 from Kilifi-Kenya. Cumulatively, the retrospective follow-up period was 93 months with annual plasma samples collection. The levels of anti-CSP IgM, total IgG, IgG1, IgG2, IgG3, and IgG4 antibodies were then determined using an enzyme-linked immunosorbent assay.

RESULTS

RTS,S/AS01 induced high levels of anti-CSP IgG antibodies which exhibited a rapid waning over 6.5 months post-vaccination, followed by a slower decay over the subsequent years. RTS,S/AS01-induced anti-CSP IgG antibodies remained elevated above the control group levels throughout the 7 years follow-up period. The anti-CSP IgG antibodies were mostly IgG1, IgG3, IgG2, and to a lesser extent IgG4. IgG2 predominated in later timepoints. RTS,S/AS01 also induced high levels of anti-CSP IgM antibodies which increased above the control group levels by month 3. The controls exhibited increasing levels of the anti-CSP IgM antibodies which caught up with the RTS,S/AS01 vaccinees levels by month 21. In contrast, there were no measurable anti-CSP IgG antibodies among the controls.

CONCLUSION

RTS,S/AS01-induced anti-CSP IgG antibodies kinetics are consistent with long-lived but waning vaccine efficacy. Natural exposure induces anti-CSP IgM antibodies in children, which increases with age, but does not induce substantial levels of anti-CSP IgG antibodies.

Live attenuated rubella vectors expressing Plasmodium falciparum circumsporozoite protein (Pf-CSP) provide a novel malaria vaccine platform in the rhesus macaque

There is an urgent need for a malaria vaccine that can prevent severe disease in young children and adults. Despite earlier work showing an immunological mechanism for preventing infection and reducing disease severity, there is currently no reliable vaccine that can provide durable protection. In part, this may reflect a limited number of ways that the host can respond to the NANP repeat sequences of circumsporozoite protein (CSP) in the parasite. In addition, it may reflect antigenic escape by the parasite from protective antibodies. To be successful, a vaccine must protect against repeated exposure to infected mosquitoes in endemic areas. We have created a series of live viral vectors based on the rubella vaccine strain that express multiple tandem repeats of NANP, and we demonstrate immunogenicity in a rhesus macaque model. We tested the vectors in a sequential immunization strategy. In the first step, the animals were primed with CSP-DNA vaccine and boosted with rubella/CSP vectors. In the second step, we gave rubella/CSP vectors again, followed by recombinant CSP protein. Following the second step, antibody titers were comparable to adult exposure to malaria in an endemic area. The antibodies were specific for native CSP protein on sporozoites, and they persisted for at least 1½ years in two out of three macaques. Given the safety profile of rubella vaccine in children, these vectors could be most useful in protecting young children, who are at greatest risk of severe malarial disease.

Identification of immunodominant epitopes in allelic variants VK210 and VK247 of Plasmodium Vivax Circumsporozoite immunogen

Plasmodium vivax-induced malaria is among the leading causes of morbidity and mortality in sub-tropical and tropical regions and infect 2.85 billion people globally. The continual rise and propagation of resistance against anti-malarial drugs is a prerequisite to develop a potent vaccine candidate for Plasmodium vivax (P. vivax). Circumsporozoite protein (CSP) is an important immunogen of malaria parasite that has the conserved CSP structure as an immune dominant B-cell epitope. In current study, we focused on designing multi-epitope vaccines (MEVs) using various immunoinformatics tools against Pakistani based allelic variants VK210 and VK247 of P. vivax CSP (PvCSP) gene. Antigenicity, allergic potential and physicochemical parameters of both PvCSP variants were assessed for the designed MEVs and they were within acceptable range suitable for post experimental investigations. The three-dimensional structures of both MEVs have been predicted ab initio, optimized, and validated by using different online servers. The both MEVs candidates were stable and free from aggregation-prone regions. The stability of both MEVs had been improved by a disulfide engineering approach. To estimate the binding energy and stability of the MEVs, molecular docking simulation and binding free energy calculations with TLR-4 immune receptor have been conducted. The docking score of PvCSP210 and PvCSP247 for TLR-4 was -6.34 kJ/mol and - 2.3 kJ/mol, respectively. For PvCSP210-TLR4 system, mean RMSD was 4.96 Å while PvCSP247-TLR4 system, average RMSD was 4.49 Å. The binding free energy of PvCSP210-TLR4 complex and PvCSP247-TLR4 complex was -50.49/-117.15 kcal/mol (MMGBSA/MMPSA) and -52.94/-96.26 kcal/mol (MMGBSA/MMPSA), respectively. The expression of both MEVs produced in Escherichia coli K12 expression system by in silico cloning was significant. Immune simulation revealed that the proposed MEVs induce strong humoral and cellular immunological responses, in addition to significant production of interleukins and cytokines. In conclusions, we believed that the MEVs proposed in current research, using combine approach of immunoinformatics, structural biology and biophysical approaches, could induce protective and effective immune responses against P. vivax and the experimental validation of our findings could contribute to the development of potential malaria vaccine.

Molecular and morphological identification of suspected Plasmodium vivax vectors in Central and Eastern Sudan

Background

In spite of the global effort to eliminate malaria, it remains the most significant vector-borne disease of humans. Plasmodium falciparum is the dominant malaria parasite in sub-Saharan Africa. However, Plasmodium vivax is becoming widely spread throughout Africa. The overuse of vector control methods has resulted in a remarkable change in the behaviour of mosquito that feeds on human as well as on vector composition. The aim of this study was to identify Anopheles mosquito species in vivax malaria endemic regions and to investigate their role in P. vivax circumsporozoite protein (Pvcsp) allele diversity.

Methods

Mosquito samples were collected from Central Sudan (Rural Khartoum and Sennar) and Eastern Sudan (New Halfa, Kassala state) using pyrethrum spray catch (PSC) and CDC light traps. Mosquitoes were identified using appropriate morphological identification keys and Anopheles gambiae complex were confirmed to species level using molecular analysis. A subset of blood-fed anopheline mosquitoes were dissected to determine the presence of natural infection of malaria parasites. In addition, the rest of the samples were investigated for the presence of Pvcsp gene using nested-PCR.

Results

A total of 1037 adult anopheline mosquitoes were collected from New Halfa (N = 467), Rural Khartoum (N = 132), and Sennar (N = 438). Morphological and molecular identification of the collected mosquitoes revealed the presence of Anopheles arabiensis (94.2%), Anopheles funestus (0.5%), and Anopheles pharoensis (5.4%). None of the dissected mosquitoes (N = 108) showed to be infected with malaria parasite. Overall P. vivax infectivity rate was 6.1% (63/1037) by Pvcsp nested PCR. Co-dominance of An. arabiensis and An. pharoensis is reported in Sennar state both being infected with P. vivax.

Conclusion

This study reported P. vivax infection among wild-caught anopheline mosquitoes in Central and Eastern Sudan. While An. arabiensis is the most abundant vector observed in all study areas, An. funestus was recorded for the first time in New Halfa, Eastern Sudan. The documented Anopheles species are implicated in Pvcsp allele diversity. Large-scale surveys are needed to identify the incriminated vectors of P. vivax malaria and determine their contribution in disease transmission dynamics.

Low dose recombinant full-length circumsporozoite protein-based Plasmodium falciparum vaccine is well-tolerated and highly immunogenic in phase 1 first-in-human clinical testing

Plasmodium falciparum circumsporozoite protein (CSP) is a major sporozoite surface protein and a key target of pre-erythrocytic malaria subunit vaccines. A full-length recombinant CSP (rCSP) based strategy could be advantageous, as this antigen includes a region critical to sporozoite cell attachment and hepatocyte invasion. The adjuvant Glucopyranosyl Lipid A-liposome Quillaja saponaria 21 (GLA-LSQ) functions as a TLR4 agonist, promotes antigen-specific T_H1 responses and stimulates cytotoxic T cell production. To date, one study has reported the clinical acceptability of GLA-LSQ. We present interim results of a phase 1 first-in-human dose-escalation clinical trial of full-length rCSP vaccine given with or without GLA-LSQ adjuvant. Participants experienced only mild to moderate related solicited adverse events. The lowest adjuvanted vaccine dose achieved >90-fold rise in geometric mean anti-CSP IgG antibody titer. These favorable safety and immunogenicity results confirm the immunostimulatory capacity of this relatively new adjuvant and support next steps in clinical product development. Trial registration: ClinicalTrials.gov Identifier NCT03589794 (registered 18 July 2018).

Breadth of humoral immune responses to the C-terminus of the circumsporozoite protein is associated with protective efficacy induced by the RTS,S malaria vaccine

The circumsporozoite protein (CSP) is the main surface antigen of malaria sporozoites, a prime vaccine target, and is known to have polymorphisms in the C-terminal region. Vaccines using a single allele may have lower efficacy against genotypic variants. Recent studies have found evidence suggesting the efficacy of the CSP-based RTS,S malaria vaccine may be limited against P. falciparum CSP alleles that diverge from the 3D7 vaccine allele, particularly in this polymorphic C-terminal region. In order to assess the breadth of the RTS,S-induced antibody responses against CSP C-terminal antigenic variants, we used a novel multiplex assay to measure reactivity of serum samples from a recent RTS,S study against C-terminal peptides from 3D7 and seven additional CSP alleles that broadly represent the genetic diversity found in circulating P. falciparum field isolates. We found that responses to the variants showed, on average, a ~ 30-fold reduction in reactivity relative to the vaccine-matched 3D7 allele. The extent of this reduction, ranging from 21 to 69-fold, correlated with the number of polymorphisms between the variants and 3D7. We calculated antibody breadth of each sample as the median relative reactivity to the seven CSP variants compared to 3D7. Surprisingly, protection from 3D7 challenge in the RTS,S study was associated with higher C-terminal antibody breadth. These findings suggest CSP C-terminal-specific avidity or fine-specificity may play a role in RTS,S-mediated protection and that breadth of C-terminal CSP-specific antibody responses may be a marker of protection.

Structural ordering of the Plasmodium berghei circumsporozoite protein repeats by inhibitory antibody 3D11

Plasmodium sporozoites express circumsporozoite protein (CSP) on their surface, an essential protein that contains central repeating motifs. Antibodies targeting this region can neutralize infection, and the partial efficacy of RTS,S/AS01 - the leading malaria vaccine against P. falciparum (Pf) - has been associated with the humoral response against the repeats. Although structural details of antibody recognition of PfCSP have recently emerged, the molecular basis of antibody-mediated inhibition of other Plasmodium species via CSP binding remains unclear. Here, we analyze the structure and molecular interactions of potent monoclonal antibody (mAb) 3D11 binding to P. berghei CSP (PbCSP) using molecular dynamics simulations, X-ray crystallography, and cryoEM. We reveal that mAb 3D11 can accommodate all subtle variances of the PbCSP repeating motifs, and, upon binding, induces structural ordering of PbCSP through homotypic interactions. Together, our findings uncover common mechanisms of antibody evolution in mammals against the CSP repeats of Plasmodium sporozoites.

Genetic diversity of circumsporozoite protein in Plasmodium knowlesi isolates from Malaysian Borneo and Peninsular Malaysia

Background

Understanding the genetic diversity of candidate genes for malaria vaccines such as circumsporozoite protein (csp) may enhance the development of vaccines for treating Plasmodium knowlesi. Hence, the aim of this study is to investigate the genetic diversity of non-repeat regions of csp in P. knowlesi from Malaysian Borneo and Peninsular Malaysia.

Methods

A total of 46 csp genes were subjected to polymerase chain reaction amplification. The genes were obtained from P. knowlesi isolates collected from different divisions of Sabah, Malaysian Borneo, and Peninsular Malaysia. The targeted gene fragments were cloned into a commercial vector and sequenced, and a phylogenetic tree was constructed while incorporating 168 csp sequences retrieved from the GenBank database. The genetic diversity and natural evolution of the csp sequences were analysed using MEGA6 and DnaSP ver. 5.10.01. A genealogical network of the csp haplotypes was generated using NETWORK ver. 4.6.1.3.

Results

The phylogenetic analysis revealed indistinguishable clusters of P. knowlesi isolates across different geographic regions, including Malaysian Borneo and Peninsular Malaysia. Nucleotide analysis showed that the csp non-repeat regions of zoonotic P. knowlesi isolates obtained in this study underwent purifying selection with population expansion, which was supported by extensive haplotype sharing observed between humans and macaques. Novel variations were observed in the C-terminal non-repeat region of csp.

Conclusions

The csp non-repeat regions are relatively conserved and there is no distinct cluster of P. knowlesi isolates from Malaysian Borneo and Peninsular Malaysia. Distinctive variation data obtained in the C-terminal non-repeat region of csp could be beneficial for the design and development of vaccines to treat P. knowlesi.

Genetic polymorphism and natural selection of circumsporozoite protein in Myanmar Plasmodium vivax

Background

Circumsporozoite surface protein (CSP) of malaria parasites has been recognized as one of the leading vaccine candidates. Clinical trials of vaccines for vivax malaria incorporating Plasmodium vivax CSP (PvCSP) have demonstrated their effectiveness in preventing malaria, at least in part. However, genetic diversity of pvcsp in the natural population remains a major concern.

Methods

A total of 171 blood samples collected from patients infected with Plasmodium vivax in Myanmar were analysed in this study. The pvcsp was amplified by polymerase chain reaction, followed by cloning and sequencing. Polymorphic characteristics and natural selection of pvcsp population in Myanmar were analysed using DNASTAR, MEGA6 and DnaSP programs. The polymorphic pattern and natural selection of publicly accessible global pvcsp sequences were also comparatively analysed.

Results

Myanmar pvcsp sequences were divided into two subtypes VK210 and VK247 comprising 143 and 28 sequences, respectively. The VK210 subtypes showed higher levels of genetic diversity and polymorphism than the VK247 subtypes. The N-terminal non-repeat region of pvcsp displayed limited genetic variations in the global population. Different patterns of octapeptide insertion (ANKKAEDA in VK210 and ANKKAGDA in VK247) and tetrapeptide repeat motif (GGNA) were identified in the C-terminal region of global pvcsp population. Meanwhile, the central repeat region (CRR) of Myanmar and global pvcsp, both in VK210 and VK247 variants, was highly polymorphic. The high level of genetic diversity in the CRR has been attributed to the different numbers, types and combinations of peptide repeat motifs (PRMs). Interestingly, 27 and 5 novel PRMs were found in Myanmar VK210 and VK247 variants, respectively.

Conclusion

Comparative analysis of the global pvcsp population suggests a complex genetic profile of pvcsp in the global population. These results widen understanding of the genetic make-up of pvcsp in the global P. vivax population and provide valuable information for the development of a vaccine based on PvCSP.

Genetic polymorphism of Plasmodium falciparum circumsporozoite protein on Bioko Island, Equatorial Guinea and global comparative analysis

BACKGROUND

Plasmodium falciparum circumsporozoite protein (PfCSP) is a potential malaria vaccine candidate, but various polymorphisms of the pfcsp gene among global P. falciparum population become the major barrier to the effectiveness of vaccines. This study aimed to investigate the genetic polymorphisms and natural selection of pfcsp in Bioko and the comparison among global P. falciparum population.

METHODS

From January 2011 to December 2018, 148 blood samples were collected from P. falciparum infected Bioko patients and 96 monoclonal sequences of them were successfully acquired and analysed with 2200 global pfcsp sequences mined from MalariaGEN Pf3k Database and NCBI.

RESULTS

In Bioko, the N-terminus of pfcsp showed limited genetic variations and the numbers of repetitive sequences (NANP/NVDP) were mainly found as 40 (35%) and 41 (34%) in central region. Most polymorphic characters were found in Th2R/Th3R region, where natural selection (p > 0.05) and recombination occurred. The overall pattern of Bioko pfcsp gene had no obvious deviation from African mainland pfcsp (Fst = 0.00878, p < 0.05). The comparative analysis of Bioko and global pfcsp displayed the various mutation patterns and obvious geographic differentiation among populations from four continents (p < 0.05). The global pfcsp C-terminal sequences were clustered into 138 different haplotypes (H_1 to H_138). Only 3.35% of sequences matched 3D7 strain haplotype (H_1).

CONCLUSIONS

The genetic polymorphism phenomena of pfcsp were found universal in Bioko and global isolates and the majority mutations located at T cell epitopes. Global genetic polymorphism and geographical characteristics were recommended to be considered for future improvement of malaria vaccine design.

Evaluation of Chimpanzee Adenovirus and MVA Expressing TRAP and CSP from Plasmodium cynomolgi to Prevent Malaria Relapse in Nonhuman Primates

Plasmodium vivax is the world’s most widely distributed human malaria parasite, with over 2.8 billion people at risk in Asia, the Americas, and Africa. The 80–90% new P. vivax malaria infections are due to relapses which suggest that a vaccine with high efficacy against relapses by prevention of hypnozoite formation could lead to a significant reduction in the prevalence of P. vivax infections. Here, we describe the development of new recombinant ChAdOx1 and MVA vectors expressing P. cynomolgi Thrombospondin Related Adhesive Protein (PcTRAP) and the circumsporozoite protein (PcCSP). Both were shown to be immunogenic in mice prior to their assessment in rhesus macaques. We confirmed good vaccine-induced humoral and cellular responses after prime-boost vaccination in rhesus macaques prior to sporozoite challenge. Results indicate that there were no significant differences between mock-control and vaccinated animals after challenge, in terms of protective efficacy measured as the time taken to 1st patency, or as number of relapses. This suggests that under the conditions tested, the vaccination with PcTRAP and PcCSP using ChAdOx1 or MVA vaccine platforms do not protect against pre-erythrocytic malaria or relapses despite good immunogenicity induced by the viral-vectored vaccines.

Putative circumsporozoite protein (CSP) of Plasmodium vivax is considerably distinct from the well-known CSP and plays a role in the protein ubiquitination pathway

Amidst technical challenges which limit successful culture and genetic manipulation of P. vivax parasites, we used a computational approach to identify a critical target with evolutionary significance. The putative circumsporozoite protein on chromosome 13 of P. vivax (PvpuCSP)is distinct from the well-known vaccine candidate PfCSP. The aim of this study was to understand the role of PvpuCSP and its relatedness to the well-known CSP. The study revealed PvpuCSP as a membrane bound E3 ubiquitin ligase involved in ubiquitination. It has a species-specific tetra-peptide unit which is differentially repeated in various P. vivax strains. The PvpuCSP is different from CSP in terms of stage-specific expression and function. Since E3 ubiquitin ligases are known antimalarial drug targets targeting the proteasome pathway, PvpuCSP, with evolutionary connotation and a key role in orchestrating protein degradation in P. vivax, can be explored as a potential drug target.

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PvpuCSP is predicted as E3 ubiquitin ligase, a part of ubiquitination pathway.

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Tetra-peptide tandem repeat at C terminal of PvpuCSP is exclusive to P. vivax.

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Moderately expressed during all parasitic stages in host and vector

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Partially disordered protein with both structured domains and two distinct IDRs

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A transmembrane protein with highly conserved functional domain across Apicomplexa

Protective efficacy of peptides from Plasmodium vivax circumsporozoite protein

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Short repeat-region peptides from PvCSP on a VLP protect against malaria.

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The AGDR tetramer from PvCSP VK210 can, on a VLP, also protect against malaria.

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Full-length PvCSP is much less protective as a vaccine than truncated PvCSP.

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Region I and II peptides confer no protection against malaria presented on a VLP.

Vivax malaria is a major cause of morbidity and mortality worldwide, with several million clinical cases per year and 2.5 billion at risk of infection. A vaccine is urgently needed but the most advanced malaria vaccine, VMP001, confers only very low levels of protection against vivax malaria challenge in humans. VMP001 is based on the circumsporozoite protein (CSP) of Plasmodium vivax. Here a virus-like particle, Qβ, is used as a platform to generate very high levels of antibody against peptides from PvCSP in mice, in order to answer questions important to further development of P. vivax CSP (PvCSP) vaccines. Minimal peptides from the VK210 and VK247 allelic variants of PvCSP are found to be highly protective as Qβ-peptide vaccines, using transgenic P. berghei parasites expressing the homologous PvCSP allelic variant. A target of neutralising antibodies within the nonamer unit repeat of VK210, AGDR, is found, as a Qβ-peptide vaccine, to provide partial protection against malaria challenge, and enhances protective efficacy when combined with full-length PvCSP vaccination. A truncated form of PvCSP, missing the N-terminal domain, is found to confer much higher levels of protective efficacy than full-length PvCSP. Peptides derived from highly conserved areas of PvCSP, RI and RII, are found not to confer protective efficacy as Qβ-peptide vaccines.

Genotyping and Phylogenetic Analysis of Plasmodium vivax Circumsporozoite Protein (PvCSP) Gene of Clinical Isolates in South-Eastern Iran

BACKGROUND

Circumsporozoite protein (CSP) is one of the most important surface sporozoite antigens in malaria, recently considered as a candidate for vaccination. Considering the importance of CSP, this study was conducted to investigate the polymorphism and genetic diversity of Plasmodium vivax Circumsporozoite Protein (Pvcsp) in the southeastern region of Iran during 2015-2016.

METHODS

To investigate polymorphism and genetic diversity, 20 blood samples were collected from patients with P. vivax, then DNA was extracted and amplified using partial sequence of CSP gene. Polymerase chain reaction (PCR) products were sequenced and compared to sequences from genomic databases using BLAST. Genetic evaluation and phylogenic analysis were performed using MEGA7 and DnaSP5 software's on 38 sequences include 20 sequences of our study and 18 sequences of Gene Bank.

RESULTS

Eleven isolates were VK210 genotype and 9 isolates contained VK247. The result of variable segregation nucleotide site indicated that the differentiation of sequences in CSP were 25.67% in our 20 samples which are less than the 38 samples with a value of 26.67%. Comparing the ratio of dN/dS regions in the CSP gene indicates that the CSP varies more synonymously and amino acid has lower variation. Out of 38 samples, 35 unique haplotypes were identified based on 1042 nucleotide sequences in CSP, showing a variation percentage of 99.4%.

CONCLUSION

The Tajima D analyses showed that CSP gene in P. vivax had a positive number in the total analyzed sequences, which means that the P. vivax mutations are in order to select positive evolution.

Multi-functional antibodies are induced by the RTS,S malaria vaccine and associated with protection in a phase I/IIa trial

Background

RTS,S is the leading malaria vaccine candidate but only confers partial efficacy against malaria in children. RTS,S is based on the major Plasmodium falciparum sporozoite surface antigen, circumsporozoite protein (CSP). The induction of anti-CSP antibodies is important for protection; however, it is unclear how these protective antibodies function.

Methods

We quantified the induction of functional anti-CSP antibody responses in healthy malaria-naive adults (N = 45) vaccinated with RTS,S/AS01. This included the ability to mediate effector functions via the fragment crystallizable (Fc) region, such as interacting with human complement proteins and Fcγ-receptors (FcγRs) that are expressed on immune cells, which promote various immunological functions.

Results

Our major findings were (1) RTS,S-induced antibodies mediated Fc-dependent effector functions, (2) functional antibodies were generally highest after the second vaccine dose, (3) functional antibodies targeted multiple regions of CSP, (4) participants with higher levels of functional antibodies had a reduced probability of developing parasitemia following homologous challenge (P < .05), and (5) nonprotected subjects had higher levels of anti-CSP IgM.

Conclusions

Our data suggest a role for Fc-dependent antibody effector functions in RTS,S-induced immunity. Enhancing the induction of these functional activities may be a strategy to improve the protective efficacy of RTS,S or other malaria vaccines.

Clinical Trials Registration

NCT00075049

Comparison of immunogenicity and safety outcomes of a malaria vaccine FMP013/ALFQ in rhesus macaques (Macaca mulatta) of Indian and Chinese origin

Background

Indian-origin rhesus (InR) are preferred for research, but strict export restrictions continue to limit their use. Chinese-origin rhesus (ChR), although easier to procure, are genetically distinct from InR and differ in their immune response to infectious agents, such as the Simian Immunodeficiency Virus. The most advanced malaria vaccine, RTS,S (GlaxoSmithKline), is based on the circumsporozoite protein (CSP) of Plasmodium falciparum. The efficacy of RTS,S vaccine in the field remains low and short-lived; efforts are underway to improve CSP-based vaccines. Rhesus models can accelerate preclinical down-selection of the next generation of malaria vaccines. This study was used to determine if the safety and immunogenicity outcomes following vaccination with a CSP vaccine would differ in the InR and ChR models, given the genetic differences between the two sub-populations of rhesus.

Methods

The FMP013 vaccine, was composed of nearly full-length soluble P. falciparum CSP produced in Escherichia coli and was adjuvanted with the Army liposomal formulation (ALFQ). Three doses of the vaccine were administered in InR and ChR (n = 6) at 1-month intervals and the antibody and T cell responses were assessed.

Results

Local and systemic toxicity profile of FMP013 vaccine in InR and ChR were similar and they revealed that the FMP013 vaccine was safe and caused only mild and transient inflammatory adverse reactions. Following the first 2 vaccines, there was a slower acquisition of antibodies to the CSP repeat region in ChR. However after the 3rd vaccination the titers in the two models were comparable. The ChR group repeat-specific antibodies had higher avidity and ChR group showed higher inhibition of liver stage development activity compared to InR. There was no difference in T-cell responses to the FMP013 vaccine between the two models.

Conclusions

A difference in the quality of serological responses was detected between the two sub-populations of rhesus. However, both models confirmed that FMP013/ALFQ vaccine was safe, highly immunogenic, elicited functional antibodies and T-cell responses. Overall, the data suggests that rhesus of Indian and Chinese origins can be interchangeably used to compare the safety and immunogenicity of next-generation of malaria vaccines and adjuvants.

Genetic polymorphism of the N-terminal region in circumsporozoite surface protein of Plasmodium falciparum field isolates from Sudan

Background

Malaria caused by Plasmodium falciparum parasite is still known to be one of the most significant public health problems in sub-Saharan Africa. Genetic diversity of the Sudanese P. falciparum based on the diversity in the circumsporozoite surface protein (PfCSP) has not been previously studied. Therefore, this study aimed to investigate the genetic diversity of the N-terminal region of the pfcsp gene.

Methods

A cross-sectional molecular study was conducted; 50 blood samples have been analysed from different regions in Sudan. Patients were recruited from the health facilities of Khartoum, New Halfa, Red Sea, White Nile, Al Qadarif, Gezira, River Nile, and Ad Damazin during malaria transmission seasons between June to October and December to February 2017–2018. Microscopic and nested PCR was performed for detection of P. falciparum. Merozoite surface protein-1 was performed to differentiate single and multiple clonal infections. The N-terminal of the pfcsp gene has been sequenced using PCR-Sanger dideoxy method and analysed to sequences polymorphism including the numbers of haplotypes (H), segregating sites (S), haplotypes diversity (Hd) and the average number of nucleotide differences between two sequences (Pi) were obtained using the software DnaSP v5.10. As well as neutrality testing, Tajima’s D test, Fu and Li’s D and F statistics.

Results

PCR amplification resulted in 1200 bp of the pfcsp gene. Only 21 PCR products were successfully sequenced while 29 were presenting multiple clonal P. falciparum parasite were not sequenced. The analysis of the N-terminal region of the PfCSP amino acids sequence compared to the reference strains showed five different haplotypes. H1 consisted of 3D7, NF54, HB3 and 13 isolates of the Sudanese pfcsp. H2 comprised of 7G8, Dd2, MAD20, RO33, Wellcome strain, and 5 isolates of the Sudanese pfcsp. H3, H4, and H5 were found in 3 distinct isolates. Hd was 0.594 ± 0.065, and S was 12. The most common polymorphic site was A98G; other sites were D82Y, N83H, N83M, K85L, L86F, R87L, R87F, and A98S. Fu and Li’s D* test value was − 2.70818, Fu and Li’s F* test value was − 2.83907, indicating a role of negative balancing selection in the pfcsp N-terminal region. Analysis with the global pfcsp N-terminal regions showed the presence of 13 haplotypes. Haplotypes frequencies were 79.4%, 17.0%, 1.6% and 1.0% for H1, H2, H3 and H4, respectively. Remaining haplotypes frequency was 0.1% for each. Hd was 0.340 ± 0.017 with a Pi of 0.00485, S was 18 sites, and Pi was 0.00030. Amino acid polymorphisms identified in the N-terminal region of global pfcsp were present at eight positions (D82Y, N83H/M, K85L/T/N, L86F, R87L/F, A98G/V/S, D99G, and G100D).

Conclusions

Sudanese pfcsp N-terminal region was well-conserved with only a few polymorphic sites. Geographical distribution of genetic diversity showed high similarity to the African isolates, and this will help and contribute in the deployment of RTS,S, a PfCSP-based vaccine, in Sudan.

Irradiated sporozoite vaccination induces sex-specific immune responses and protection against malaria in mice

In both preclinical animal studies and human clinical trials, adult females tend to develop greater adaptive immune responses than males following receipt of either viral or bacterial vaccines. While there is currently no approved malaria vaccine, several anti-sporozoite vaccines, including RTS,S/AS01 and attenuated sporozoite vaccines, are in development, but the impact of sex and age on their efficacy remains undefined. To examine sex differences in the efficacy of anti-sporozoite stage malaria vaccination, adult (10 weeks of age) or juvenile (11 days of age) male and female C3H mice were twice vaccinated with irradiated transgenic Plasmodium berghei sporozoites expressing the P. falciparum circumsporozoite (CSP) protein and 45 days post boost vaccination, mice were challenged with transgenic P. berghei via mosquito bite or intradermal challenge. Immunization with irradiated sporozoites resulted in greater protection against challenge in adult females, which was associated with greater anti-CSP antibody production and avidity, as well as greater hepatic, but not splenic, CD8+ T cell IFNƴ production in adult females than adult males. No sex differences in adaptive immune responses or protection were observed in mice vaccinated prior to puberty, suggesting a role for sex steroid hormones. Depletion of testosterone in males increased, whereas rescue of testosterone decreased, anti-CSP antibody production, the number of antigen-specific CD8+ T cells isolated from the liver, and protection following parasite challenge. Conversely, depletion of sex steroids in female mice did not alter vaccine-induced responses or protection following challenge. These data suggest that elevated testosterone concentrations in males reduce adaptive immunity and contribute to sex differences in malaria vaccine efficacy.

Safety, toxicity and immunogenicity of a malaria vaccine based on the circumsporozoite protein (FMP013) with the adjuvant army liposome formulation containing QS21 (ALFQ)

Antibodies to Circumsporozoite protein (CSP) confer protection against controlled human malaria infection (CHMI) caused by the parasite Plasmodium falciparum. Although CSP is highly immunogenic, it does not induce long lasting protection and efforts to improve CSP-specific immunological memory and duration of protection are underway. We have previously reported that the clinical grade CSP vaccine FMP013 was immunogenic and protective against malaria challenge in mice when combined with the Army Liposomal Formulation adjuvant containing immune modulators 3D-PHAD™ and QS21 (ALFQ). To move forward with clinical evaluation, we now report the safety, toxicity and immunogenicity of clinical grade FMP013 and ALFQ in Rhesus macaques. Three groups of Rhesus (n = 6) received half or full human dose of FMP013 + ALFQ on a 0-1-2 month schedule, which showed mild local site reactions with no hematologic derangements in red blood cell homeostasis, liver function or kidney function. Immunization induced a transient systemic inflammatory response, including elevated white blood cell counts, mild fever, and a few incidences of elevated creatine kinase, receding to normal range by day 7 post vaccination. Optimal immunogenicity in Rhesus was observed using a 1 mL ALFQ + 20 µg FMP013 dose. Doubling the FMP013 antigen dose to 40 µg had no effect while halving the ALFQ adjuvant dose to 0.5 mL lowered immunogenicity. Similar to data generated in mice, FMP013 + ALFQ induced serum antibodies that reacted to all regions of the CSP molecule and a Th1-biased cytokine response in Rhesus. Rhesus antibody response to FMP013 + ALFQ was found to be non-inferior to historical benchmarks including that of RTS,S + AS01 in humans. A four-dose GLP toxicity study in rabbits confirmed no local site reactions and transient systemic inflammation associated with ALFQ adjuvant administration. These safety and immunogenicity data support the clinical progression and testing of FMP013 + ALFQ in a CHMI trial in the near future.

Induction and decay of functional complement-fixing antibodies by the RTS,S malaria vaccine in children, and a negative impact of malaria exposure

BACKGROUND

Leading malaria vaccine, RTS,S, is based on the circumsporozoite protein (CSP) of sporozoites. RTS,S confers partial protection against malaria in children, but efficacy wanes relatively quickly after primary immunization. Vaccine efficacy has some association with anti-CSP IgG; however, it is unclear how these antibodies function, and how functional antibodies are induced and maintained over time. Recent studies identified antibody-complement interactions as a potentially important immune mechanism against sporozoites. Here, we investigated whether RTS,S vaccine-induced antibodies could function by interacting with complement.

METHODS

Serum samples were selected from children in a phase IIb trial of RTS,S/AS02_A conducted at two study sites of high and low malaria transmission intensity in Manhiça, Mozambique. Samples following primary immunization and 5-year post-immunization follow-up time points were included. Vaccine-induced antibodies were characterized by isotype, subclass, and epitope specificity, and tested for the ability to fix and activate complement. We additionally developed statistical methods to model the decay and determinants of functional antibodies after vaccination.

RESULTS

RTS,S vaccination induced anti-CSP antibodies that were mostly IgG1, with some IgG3, IgG2, and IgM. Complement-fixing antibodies were effectively induced by vaccination, and targeted the central repeat and C-terminal regions of CSP. Higher levels of complement-fixing antibodies were associated with IgG that equally recognized both the central repeat and C-terminal regions of CSP. Older age and higher malaria exposure were significantly associated with a poorer induction of functional antibodies. There was a marked decay in functional complement-fixing antibodies within months after vaccination, as well as decays in IgG subclasses and IgM. Statistical modeling suggested the decay in complement-fixing antibodies was mostly attributed to the waning of anti-CSP IgG1, and to a lesser extent IgG3.

CONCLUSIONS

We demonstrate for the first time that RTS,S can induce complement-fixing antibodies in young malaria-exposed children. The short-lived nature of functional responses mirrors the declining vaccine efficacy of RTS,S over time. The negative influence of age and malaria exposure on functional antibodies has implications for understanding vaccine efficacy in different settings. These findings provide insights into the mechanisms and longevity of vaccine-induced immunity that will help inform the future development of highly efficacious and long-lasting malaria vaccines.

Plasmodium ovale curtisi and Plasmodium ovale wallikeri in Chinese travelers: Prevalence of novel genotypes of circumsporozoite protein in the African continent

Imported malaria due to Plasmodium ovale curtisi and P. ovale wallikeri infections from African countries has increased recently (2011-2014) in Chinese travelers. We report novel genotypes, their prevalence and the predominant haplotypes of P. ovale curtisi and P. ovale wallikeri circumsporozoite protein (CSP) from 20 African countries in Chinese travelers. These genotypes should be considered while designing a CSP-based vaccine against P. ovale malaria.

Insights into the early liver stage biology of Plasmodium

Even though malaria is preventable and curable, it has become a serious threat to mankind. In 2016, there were an estimated 216 million cases of malaria across the world. The biology of its causative agent, i.e. Plasmodium parasite is full of complex mechanisms. There are five Plasmodium species responsible for malaria in humans, viz. Plasmodium falciparum, P. vivax, P. malariae, P. ovale and recently identified P. knowlesi that normally infect apes. In humans, malaria is spread by the injection of Plasmodium sporozoites through the bite of infectious Anopheles' female mosquito during their blood meal. From the time of entry into human skin till the development into the asexual forms, the parasite undergoes several transformations. This review attempts to understand the science behind the pre-erythrocytic liver stage of Plasmodium. Research articles explaining parasite biology, cell-traversal, transformation stages, cell-egress process, etc. were retrieved from PubMed and google scholar database. Various known and unknown mechanisms and strategies used by the malaria parasite P. berghei in rodent models have been discussed in this review. Limited or no information was available for humans, due to technical feasibility and complexity of parasite's life cycle. Hence, it was concluded that there is an urgent need to investigate the hepatic invasion, traversal and egress mechanism of P. falciparum and P. vivax for developing novel therapeutics to fight against malaria.

Genetic polymorphism and natural selection of circumsporozoite surface protein in Plasmodium falciparum field isolates from Myanmar

BACKGROUND

Plasmodium falciparum circumsporozoite protein (PfCSP) is one of the most extensively studied malaria vaccine candidates, but the genetic polymorphism of PfCSP within and among the global P. falciparum population raises concerns regarding the efficacy of a PfCSP-based vaccine efficacy. In this study, genetic diversity and natural selection of PfCSP in Myanmar as well as global P. falciparum were comprehensively analysed.

METHODS

Blood samples were collected from 51 P. falciparum infected Myanmar patients. Fifty-one full-length PfCSP genes were amplified from the blood samples through a nested polymerase chain reaction, cloned into a TA cloning vector, and then sequenced. Polymorphic characteristics and natural selection of Myanmar PfCSP were analysed using the DNASTAR, MEGA6, and DnaSP programs. Polymorphic diversity and natural selection in publicly available global PfCSP were also analysed.

RESULTS

The N-terminal and C-terminal non-repeat regions of Myanmar PfCSP showed limited genetic variations. A comparative analysis of the two regions in global PfCSP displayed similar patterns of low genetic diversity in global population, but substantial geographic differentiation was also observed. The most notable polymorphisms identified in the N-terminal region of global PfCSP were A98G and 19-amino acid length insertion in global population with different frequencies. Major polymorphic characters in the C-terminal region of Myanmar and global PfCSP were found in the Th2R and Th3R regions, where natural selection and recombination occurred. The central repeat region of Myanmar PfCSP was highly polymorphic, with differing numbers of repetitive repeat sequences NANP and NVDP. The numbers of the NANP repeats varied among global PfCSP, with the highest number of repeats seen in Asian and Oceanian PfCSP. Haplotype network analysis of global PfCSP revealed that global PfCSP clustered into 103 different haplotypes with geographically-separated populations.

CONCLUSION

Myanmar and global PfCSP displayed genetic diversity. N-terminal and C-terminal non-repeat regions were relatively conserved, but the central repeat region displayed high levels of genetic polymorphism in Myanmar and global PfCSP. The observed geographic pattern of genetic differentiation and the points of evidence for natural selection and recombination suggest that the functional consequences of the polymorphism should be considered for developing a vaccine based on PfCSP.

Chimeric Plasmodium falciparum parasites expressing Plasmodium vivax circumsporozoite protein fail to produce salivary gland sporozoites

BACKGROUND

Rodent malaria parasites where the gene encoding circumsporozoite protein (CSP) has been replaced with csp genes from the human malaria parasites, Plasmodium falciparum or Plasmodium vivax, are used as pre-clinical tools to evaluate CSP vaccines in vivo. These chimeric rodent parasites produce sporozoites in Anopheles stephensi mosquitoes that are capable of infecting rodent and human hepatocytes. The availability of chimeric P. falciparum parasites where the pfcsp gene has been replaced by the pvcsp would open up possibilities to test P. vivax CSP vaccines in small scale clinical trials using controlled human malaria infection studies.

METHODS

Using CRISPR/Cas9 gene editing two chimeric P. falciparum parasites, were generated, where the pfcsp gene has been replaced by either one of the two major pvcsp alleles, VK210 or VK247. In addition, a P. falciparum parasite line that lacks CSP expression was also generated. These parasite lines have been analysed for sporozoite production in An. stephensi mosquitoes.

RESULTS

The two chimeric Pf-PvCSP lines exhibit normal asexual and sexual blood stage development in vitro and produce sporozoite-containing oocysts in An. stephensi mosquitoes. Expression of the corresponding PvCSP was confirmed in oocyst-derived Pf-PvCSP sporozoites. However, most oocysts degenerate before sporozoite formation and sporozoites were not found in either the mosquito haemocoel or salivary glands. Unlike the chimeric Pf-PvCSP parasites, oocysts of P. falciparum parasites lacking CSP expression do not produce sporozoites.

CONCLUSIONS

Chimeric P. falciparum parasites expressing P. vivax circumsporozoite protein fail to produce salivary gland sporozoites. Combined, these studies show that while PvCSP can partially complement the function of PfCSP, species-specific features of CSP govern full sporozoite maturation and development in the two human malaria parasites.

Genetic polymorphisms in the circumsporozoite protein of Plasmodium malariae show a geographical bias

BACKGROUND

Plasmodium malariae is characterized by its long asymptomatic persistence in the human host. The epidemiology of P. malariae is incompletely understood and is hampered by the limited knowledge of genetic polymorphisms. Previous reports from Africa have shown heterogeneity within the P. malariae circumsporozoite protein (pmcsp) gene. However, comparative studies from Asian countries are lacking. Here, the genetic polymorphisms in pmcsp of Asian isolates have been characterized.

METHODS

Blood samples from 89 symptomatic P. malariae-infected patients were collected, from Thailand (n = 43), Myanmar (n = 40), Lao PDR (n = 5), and Bangladesh (n = 1). pmcsp was amplified using semi-nested PCR before sequencing. The resulting 89 pmcsp sequences were analysed together with 58 previously published pmcsp sequences representing African countries using BioEdit, MEGA6, and DnaSP.

RESULTS

Polymorphisms identified in pmcsp were grouped into 3 populations: Thailand, Myanmar, and Kenya. The nucleotide diversity and the ratio of nonsynonymous to synonymous substitutions (dN/dS) in Thailand and Myanmar were higher compared with that in Kenya. Phylogenetic analysis showed clustering of pmcsp sequences according to the origin of isolates (Asia vs. Africa). High genetic differentiation (Fst = 0.404) was observed between P. malariae isolates from Asian and African countries. Sequence analysis of pmcsp showed the presence of tetrapeptide repeat units of NAAG, NDAG, and NAPG in the central repeat region of the gene. Plasmodium malariae isolates from Asian countries carried fewer copies of NAAG compared with that from African countries. The NAPG repeat was only observed in Asian isolates. Additional analysis of 2 T-cell epitopes, Th2R and Th3R, showed limited heterogeneity in P. malariae populations.

CONCLUSIONS

This study provides valuable information on the genetic polymorphisms in pmcsp isolates from Asia and advances our understanding of P. malariae population in Asia and Africa. Polymorphisms in the central repeat region of pmcsp showed association with the geographical origin of P. malariae isolates and can be potentially used as a marker for genetic epidemiology of P. malariae population.

Evaluating seroprevalence to circumsporozoite protein to estimate exposure to three species of Plasmodium in the Brazilian Amazon

Background

Brazil has seen a great decline in malaria and the country is moving towards elimination. However, for eventual elimination, the control program needs efficient tools in order to monitor malaria exposure and transmission. In this study, we aimed to evaluate whether seroprevalence to the circumsporozoite protein (CSP) is a good tool for monitoring the exposure to and/or evaluating the burden and distribution of Plasmodium species in the Brazilian Amazon.

Methods

Cross-sectional surveys were conducted in a rural area of Porto Velho, Rondônia state. Parasite infection was detected by microscopy and polymerase chain reaction. Antibodies to the sporozoite CSP repeats of Plasmodium vivax, P. falciparum, and P. malariae (PvCS, PfCS, and PmCS) were detected using the enzyme-linked immunosorbent assay technique. Human leukocyte antigen (HLA)-DRB1 and DQB1 genes were typed using Luminex® xMAP® technology.

Results

The prevalence of immunoglobulin G against P. vivax CSP peptide (62%) was higher than P. falciparum (49%) and P. malariae (46%) CSP peptide. Most of the studied individuals had antibodies to at least one of the three peptides (72%), 34% had antibodies to all three peptides and 28% were non-responders. Although the majority of the population was not infected at the time of the survey, 74.3% of parasite-negative individuals had antibodies to at least one of the CSPs. Importantly, among individuals carrying the haplotypes DRB1*04~DQB1*03, there was a significantly higher frequency of PfCS responders, and DRB1*16~DQB1*03 haplotype for PvCS and PfCS responders. In contrast, HLA-DRB1*01 and HLA-DQB1*05 allelic groups were associated with a lack of antibodies to P. vivax and P. falciparum CSP repeats, and the haplotype DRB1*01~DQB1*05 was also associated with non-responders, including non-responders to P. malariae.

Conclusions

Our results show that in low transmission settings, naturally acquired antibody responses against the CSP repeats of P. vivax, P. falciparum, and P. malariae in a single cross-sectional study may not represent a valuable marker for monitoring recent malaria exposure, especially in an area with a high prevalence of P. vivax. Furthermore, HLA class II molecules play an important role in antibody response and require further study with a larger sample size. It will be of interest to consider HLA analysis when using serosurveillance to monitor malaria exposure among genetically diverse populations.

Electronic supplementary material

The online version of this article (10.1186/s40249-018-0428-1) contains supplementary material, which is available to authorized users.

Human antibodies activate complement against Plasmodium falciparum sporozoites, and are associated with protection against malaria in children

BACKGROUND

Antibodies targeting Plasmodium falciparum sporozoites play a key role in human immunity to malaria. However, antibody mechanisms that neutralize sporozoites are poorly understood. This has been a major constraint in developing highly efficacious vaccines, as we lack strong correlates of protective immunity.

METHODS

We quantified the ability of human antibodies from malaria-exposed populations to interact with human complement, examined the functional effects of complement activity against P. falciparum sporozoites in vitro, and identified targets of functional antibodies. In children and adults from malaria-endemic regions, we determined the acquisition of complement-fixing antibodies to sporozoites and their relationship with antibody isotypes and subclasses. We also investigated associations with protective immunity in a longitudinal cohort of children (n = 206) residing in a malaria-endemic region.

RESULTS

We found that antibodies to the major sporozoite surface antigen, circumsporozoite protein (CSP), were predominately IgG1, IgG3, and IgM, and could interact with complement through recruitment of C1q and activation of the classical pathway. The central repeat region of CSP, included in leading vaccines, was a key target of complement-fixing antibodies. We show that antibodies activate human complement on P. falciparum sporozoites, which consequently inhibited hepatocyte cell traversal that is essential for establishing liver-stage infection, and led to sporozoite death in vitro. The natural acquisition of complement-fixing antibodies in malaria-exposed populations was age-dependent, and was acquired more slowly to sporozoite antigens than to merozoite antigens. In a longitudinal cohort of children, high levels of complement-fixing antibodies were significantly associated with protection against clinical malaria.

CONCLUSIONS

These novel findings point to complement activation by antibodies as an important mechanism of anti-sporozoite human immunity, thereby enabling new strategies for developing highly efficacious malaria vaccines. We also present evidence that complement-fixing antibodies may be a valuable correlate of protective immunity in humans.

Natural Plasmodium vivax infections in Anopheles mosquitoes in a malaria endemic area of northeastern Thailand

There was recently an outbreak of malaria in Ubon Ratchathani Province, northeastern Thailand. In the absence of information on malaria vector transmission dynamics, this study aimed to identify the anopheline vectors and their role in malaria transmission. Adult female Anopheles mosquitoes were collected monthly by human-landing catch in Na Chaluai District of Ubon Ratchathani Province during January 2014-December 2015. Field-captured mosquitoes were identified to species using morphology-based keys and molecular assays (allele-specific polymerase chain reaction, AS-PCR), and analysed for the presence of Plasmodium falciparum and Plasmodium vivax using an enzyme-linked immunosorbent assay (ELISA) for the detection of circumsporozoite proteins (CSP). A total of 1,229 Anopheles females belonging to 13 species were collected. Four anopheline taxa were most abundant: Members of the Anopheles barbirostris complex, comprising 38% of the specimens, species of the Anopheles hyrcanus group (18%), Anopheles nivipes (17%) and Anopheles philippinensis (12%). The other nine species comprised 15% of the collections. Plasmodium infections were detected in two of 668 pooled samples of heads/thoraces, Anopheles dirus (1/29) and An. philippinensis (1/97). The An. dirus pool had a mixed infection of P. vivax-210 and P. vivax-247, whereas the An. philippinensis pool was positive only for the latter protein variant. Both positive ELISA samples were confirmed by nested PCR. This study is the first to incriminate An. dirus and An. philippinensis as natural malaria vectors in the area where the outbreak occurred. This information can assist in designing and implementing a more effective malaria control programme in the province.

A potent malaria vaccine based on adenovirus with dual modifications at Hexon and pVII

Adenovirus (Ad) is thought to be one of the most promising platforms for a malaria vaccine targeted against its liver stages, because of its ability to induce a strong T-cell response against a transgene. However, a further improvement of this platform is needed in order to elicit another arm of the immunity, i.e. humoral response, against malaria. In order to augment immunogenicity and protective efficacy of Ad-based malaria vaccine, we inserted B-cell, as well as CD4+ T-cell, epitopes of Plasmodium falciparum circumsporozoite protein (PfCSP) into the capsid protein, Hexon, and the core protein, VII (pVII), of Ad, respectively, in addition to the PfCSP transgene. Insertion of PfCSP-derived B cell epitope to Hexon significantly enhanced the epitope-specific antibody response compared to AdPfCSP, an Ad vaccine expressing only PfCSP transgene. PfCSP-derived CD4+ T-cell epitope insertion into pVII augmented not only PfCSP-specific CD4+ T-cell response but also anti-PfCSP antibody response. Finally, mice immunized with AdPfCSP having both Hexon and pVII modifications were more protected than AdPfCSP or Hexon-modified AdPfCSP against challenge with transgenic rodent malaria parasites expressing the PfCSP. Overall, this study has demonstrated that Hexon and pVII-modified AdPfCSP vaccine is a promising malaria vaccine which induces strong PfCSP-specific humoral, CD4+ T-cell, and CD8+ T-cell responses and protects against infection with transgenic malaria parasites expressing the PfCSP.

Biological, immunological and functional properties of two novel multi-variant chimeric recombinant proteins of CSP antigens for vaccine development against Plasmodium vivax infection

The circumsporozoite protein (CSP) of the malaria parasite Plasmodium vivax is a major pre-erythrocyte vaccine candidate. The protein has a central repeat region that belongs to one of repeat families (VK210, VK247, and the P. vivax-like). In the present study, computer modelling was employed to select chimeric proteins, comprising the conserved regions and different arrangements of the repeat elements (VK210 and VK247), whose structure is similar to that of the native counterparts. DNA encoding the selected chimeras (named CS127 and CS712) were synthetically constructed based on E. coli codons, then cloned and expressed. Mouse monoclonal antibodies (mAbs; anti-Pv-210-CDC and -Pv-247-CDC), recognized the chimeric antigens in ELISA, indicating correct conformation and accessibility of the B-cell epitopes. ELISA using IgG from plasma samples collected from 221 Iranian patients with acute P. vivax showed that only 49.32% of the samples reacted to both CS127 and CS712 proteins. The dominant subclass for the two chimeras was IgG1 (48% of the positive responders, OD₄₉₂=0.777±0.420 for CS127; 48.41% of the positive responders, OD₄₉₂=0.862±0.423 for CS712, with no statistically significant difference P>0.05; Wilcoxon signed ranks test). Binding assays showed that both chimeric proteins bound to immobilized heparan sulphate and HepG2 hepatocyte cells in a concentration-dependent manner, saturable at 80μg/mL. Additionally, anti-CS127 and -CS712 antibodies raised in mice recognized the native protein on the surface of P. vivax sporozoite with high intensity, confirming the presence of common epitopes between the recombinant forms and the native proteins. In summary, despite structural differences at the molecular level, the expression levels of both chimeras were satisfactory, and their conformational structure retained biological function, thus supporting their potential for use in the development of vivax-based vaccine.

A VAR2CSA:CSP conjugate capable of inducing dual specificity antibody responses

BACKGROUND

Vaccine antigens targeting specific P. falciparum parasite stages are under pre-clinical and clinical development. It seems plausible that vaccine with multiple specificities will offer higher protection. With this hypothesis, we exploited the Spy-Tag/SpyCatcher conjugation system to make a, post expression, dual antigen conjugate vaccine, comprising two clinically tested antigen candidates (CSP and VAR2CSA).

METHODS

The DBL1x-DBL2x-ID2a region of VAR2CSA was genetically fused with SpyTag at N-terminus. The full-length CSP antigen was genetically fused to C-terminal SpyCatcher peptide. The covalent interaction between SpyTag/SpyCatcher enables the formation of DBL1x-DBL2x-ID2a:CSP conjugate vaccine. Immunogenicity and quality of antibody responses induced by the conjugate vaccine, as well as a control CSP-SpyCatcher vaccine, was tested in BALB/c mice.

RESULTS

Serum samples obtained from mice immunized with the conjugate vaccine were able to recognize both untagged DBL1x-DBL2x-ID2a as well as CSP antigen. Moreover, the geometric mean anti-CSP antibody titer was 1.9-fold higher in serum (at day 35 and 55 post-first immunization) from mice immunized with the conjugate vaccine, as compared to mice receiving the control vaccine.

CONCLUSION

The data obtained in this study serves as proof-of-concept for the simultaneous induction of antibodies directed against individual antigen components in a dual stage anti-malaria vaccine.

Rhesus macaque and mouse models for down-selecting circumsporozoite protein based malaria vaccines differ significantly in immunogenicity and functional outcomes

BACKGROUND

Non-human primates, such as the rhesus macaques, are the preferred model for down-selecting human malaria vaccine formulations, but the rhesus model is expensive and does not allow for direct efficacy testing of human malaria vaccines. Transgenic rodent parasites expressing genes of human Plasmodium are now routinely used for efficacy studies of human malaria vaccines. Mice have however rarely predicted success in human malaria trials and there is scepticism whether mouse studies alone are sufficient to move a vaccine candidate into the clinic.

METHODS

A comparison of immunogenicity, fine-specificity and functional activity of two Alum-adjuvanted Plasmodium falciparum circumsporozoite protein (CSP)-based vaccines was conducted in mouse and rhesus models. One vaccine was a soluble recombinant protein (CSP) and the other was the same CSP covalently conjugated to the Qβ phage particle (Qβ-CSP).

RESULTS

Mice showed different kinetics of antibody responses and different sensitivity to the NANP-repeat and N-terminal epitopes as compared to rhesus. While mice failed to discern differences between the protective efficacy of CSP versus Qβ-CSP vaccine following direct challenge with transgenic Plasmodium berghei parasites, rhesus serum from the Qβ-CSP-vaccinated animals induced higher in vivo sporozoite neutralization activity.

CONCLUSIONS

Despite some immunologic parallels between models, these data demonstrate that differences between the immune responses induced in the two models risk conflicting decisions regarding potential vaccine utility in humans. In combination with historical observations, the data presented here suggest that although murine models may be useful for some purposes, non-human primate models may be more likely to predict the human response to investigational vaccines.

Bacterial superglue generates a full-length circumsporozoite protein virus-like particle vaccine capable of inducing high and durable antibody responses

BACKGROUND

Malaria, caused by Plasmodium falciparum, continues to have a devastating impact on global health, emphasizing the great need for a malaria vaccine. The circumsporozoite protein (CSP) is an attractive target for a malaria vaccine, and forms a major component of RTS,S, the most clinically advanced malaria vaccine. The clinical efficacy of RTS,S has been moderate, yet has demonstrated the viability of a CSP-based malaria vaccine. In this study, a vaccine comprised of the full-length CSP antigen presented on a virus-like particle (VLP) is produced using a split-intein conjugation system (SpyTag/SpyCatcher) and the immunogenicity is tested in mice.

METHODS

Full-length 3d7 CSP protein was genetically fused at the C-terminus to SpyCatcher. The CSP-SpyCatcher antigen was then covalently attached (via the SpyTag/SpyCatcher interaction) to Acinetobacter phage AP205 VLPs which were modified to display one SpyTag per VLP subunit. To evaluate the VLP-display effect, the immunogenicity of the VLP vaccine was tested in mice and compared to a control vaccine containing AP205 VLPs plus unconjugated CSP.

RESULTS

Full-length CSP was conjugated at high density (an average of 112 CSP molecules per VLP) to AP205 SpyTag-VLPs. Vaccination of mice with the CSP Spy-VLP vaccine resulted in significantly increased antibody titres over a course of 7 months as compared to the control group (2.6-fold higher at 7 months after immunization). Furthermore, the CSP Spy-VLP vaccine appears to stimulate production of IgG2a antibodies, which has been linked with a more efficient clearing of intracellular parasite infection.

CONCLUSION

This study demonstrates that the high-density display of CSP on SpyTag-VLPs, significantly increases the level and quality of the vaccine-induced humoral response, compared to a control vaccine consisting of soluble CSP plus AP205 VLPs. The SpyTag-VLP platform utilized in this study constitutes a versatile and rapid method to develop highly immunogenic vaccines. It might serve as a generic tool for the cost-effective development of effective VLP-vaccines, e.g., against malaria.

A monoclonal antibody-based immunoassay to measure the antibody response against the repeat region of the circumsporozoite protein of Plasmodium falciparum

Background

The malaria vaccine candidate RTS,S/AS01 (GSK Vaccines) induces high IgG concentration against the circumsporozoite protein (CSP) of Plasmodium falciparum. In human vaccine recipients circulating anti-CSP antibody concentrations are associated with protection against infection but appear not to be the correlate of protection. However, in a humanized mouse model of malaria infection prophylactic administration of a human monoclonal antibody (MAL1C), derived from a RTS,S/AS01-immunized volunteer, directed against the CSP repeat region, conveyed full protection in a dose-dependent manner suggesting that antibodies alone are able to prevent P. falciparum infection when present in sufficiently high concentrations. A competition ELISA was developed to measure the presence of MAL1C-like antibodies in polyclonal sera from RTS,S/AS01 vaccine recipients and study their possible contribution to protection against infection.

Results

MAL1C-like antibodies present in polyclonal vaccine-induced sera were evaluated for their ability to compete with biotinylated monoclonal antibody MAL1C for binding sites on the capture antigen consisting of the recombinant protein encompassing 32 NANP repeats of CSP (R32LR). Serum samples were taken at different time points from participants in two RTS,S/AS01 vaccine studies (NCT01366534 and NCT01857869). Vaccine-induced protection status of the study participants was determined based on the outcome of experimental challenge with infected mosquito bites after vaccination. Optimal conditions were established to reliably detect MAL1C-like antibodies in polyclonal sera. Polyclonal anti-CSP antibodies and MAL1C-like antibody content were measured in 276 serum samples from RTS,S/AS01 vaccine recipients using the standard ELISA and MAL-1C competition ELISA, respectively. A strong correlation was observed between the results from these assays. However, no correlation was found between the results of either assay and protection against infection.

Conclusions

The competition ELISA to measure MAL1C-like antibodies in polyclonal sera from RTS,S/AS01 vaccine recipients was robust and reliable but did not reveal the elusive correlate of protection.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-016-1596-8) contains supplementary material, which is available to authorized users.

Development of a Plasmodium berghei transgenic parasite expressing the full-length Plasmodium vivax circumsporozoite VK247 protein for testing vaccine efficacy in a murine model

Background

The approach of using transgenic rodent malaria parasites to assess the immune system’s response to antigenic targets from a human malaria parasite has been shown to be useful for preclinical evaluation of new vaccine formulations. The transgenic Plasmodium berghei parasite line [PvCSP(VK210)/Pb] generated previously expresses the full-length circumsporozoite protein (CSP) VK210 from Plasmodium vivax. The transgenic parasite expresses one of the two most common alleles of CSP, defined by nine amino acids at the central repeat region of this protein. In the present study, a transgenic P. berghei parasite line [PvCSP(VK247)/Pb] expressing the full-length PvCSP(VK247), which is the alternative common allele, was generated and characterized.

Methods

The P. berghei expressing full-length PvCSP(VK247) was generated and examined its applicability to CSP-based vaccine research by examining its biological characteristics in mosquitoes and mice.

Results

Similar to PvCSP(VK210)/Pb, PvCSP(VK247)/Pb developed normally in mosquitoes and produced infectious sporozoites equipped to generate patent infections in mice. Invasion of HepG2 cells by PvCSP(VK247)/Pb sporozoites was inhibited by an anti-PvCSP(VK247) repeat monoclonal antibody (mAb), but not by an anti-PvCSP(VK210) repeat mAb.

Conclusions

These two transgenic parasites thus far can be used to evaluate the potential efficacy of PvCSP-based vaccine candidates encompassing the two major genetic variants in preclinical trials.

Co-administration of α-GalCer analog and TLR4 agonist induces robust CD8(+) T-cell responses to PyCS protein and WT-1 antigen and activates memory-like effector NKT cells

In the present study, the combined adjuvant effect of 7DW8-5, a potent α-GalCer-analog, and monophosphoryl lipid A (MPLA), a TLR4 agonist, on the induction of vaccine-induced CD8(+) T-cell responses and protective immunity was evaluated. Mice were immunized with peptides corresponding to the CD8(+) T-cell epitopes of a malaria antigen, a circumsporozoite protein of Plasmodium yoelii, and a tumor antigen, a Wilms Tumor antigen-1 (WT-1), together with 7DW8-5 and MPLA, as an adjuvant. These immunization regimens were able to induce higher levels of CD8(+) T-cell responses and, ultimately, enhanced levels of protection against malaria and tumor challenges compared to the levels induced by immunization with peptides mixed with 7DW8-5 or MPLA alone. Co-administration of 7DW8-5 and MPLA induces activation of memory-like effector natural killer T (NKT) cells, i.e. CD44(+)CD62L(-)NKT cells. Our study indicates that 7DW8-5 greatly enhances important synergistic pathways associated to memory immune responses when co-administered with MPLA, thus rendering this combination of adjuvants a novel vaccine adjuvant formulation.

A highly infectious Plasmodium yoelii parasite, bearing Plasmodium falciparum circumsporozoite protein

Background

Plasmodium circumsporozoite protein (CSP) is a major surface antigen present in the sporozoite (Spz) stage of a malaria parasite. RTS, S vaccine, the most clinically advanced malaria vaccine, consists of a large portion of Plasmodium falciparum CSP (PfCSP). A highly infectious, recombinant rodent malaria, Plasmodium yoelii parasite bearing a full-length PfCSP, PfCSP/Py Spz, was needed as a tool to evaluate the role of PfCSP in mediating, protective, anti-malaria immunity in a mouse model.

Methods

A transgenic parasite, PfCSP/Py Spz, was generated by inserting a construct expressing the PfCSP at the locus of the P. yoelii CSP gene by double cross-over homologous recombination. Then the biological and protective properties of PfCSP/Py Spz were determined.

Results

This PfCSP/Py parasite produced up to 30,000 Spz in mosquito salivary glands, which is equal or even higher than the number of Spz produced by wild-type P. yoelii parasites. Five bites of PfCSP/Py-infected mosquitoes could induce blood infection in BALB/c mice.

Conclusions

The current study has demonstrated a successful establishment of a transgenic P. yoelii parasite clone that is able to express a full-length PfCSP, PfCSP/Py parasite. Importantly, this PfCSP/Py parasite can be as infectious as the wild-type P. yoelii parasite both in mosquito vector and in mouse, a mammalian host. A new transgenic parasite that expresses a full-length PfCSP may become a useful tool for researchers to investigate immunity against PfCSP in a mouse model.

Endogenously-expressed NH2-terminus of circumsporozoite protein interferes with sporozoite invasion of mosquito salivary glands

Background

The circumsporozoite protein is the most abundant polypeptide expressed by sporozoites, the malaria parasite stage capable of infecting humans. Sporozoite invasion of mosquito salivary glands prior to transmission is likely mediated by a receptor/ligand-like interaction of the parasites with the target tissues, and the amino (NH₂)-terminal portion of CSP is involved in this interaction but not the TSR region on the carboxyl (C)-terminus. Peptides based on the NH₂-terminal domain could compete with the parasites for the salivary gland receptors and thus inhibit penetration.

Methods

Peptides based on the NH₂-terminus and TSR domains of the CSP from avian or human malaria parasites, Plasmodium gallinaceum and Plasmodium falciparum, respectively, were expressed endogenously in mosquito haemolymph using a transient (Sindbis virus-mediated) or stable (piggyBac-mediated transgenesis) system.

Results

Transient endogenous expression of partial NH₂-terminus peptide from P. falciparum CSP in P. gallinaceum-infected Aedes aegypti resulted in a reduced number of sporozoites in the salivary glands. When a transgenic approach was used to express a partial CSP NH₂-terminal domain from P. gallinaceum the number of sporozoites in the salivary glands did not show a difference when compared to controls. However, a significant difference could be observed when mosquitoes with a lower infection were analysed. The same result could not be observed with mosquitoes endogenously expressing peptides based on the TSR domain from either P. gallinaceum or P. falciparum.

Conclusion

These results support the conclusion that CSP partial NH₂-terminal domain can be endogenously expressed to promote a competition for the receptor used by sporozoites to invade salivary glands, and they could be used to block this interaction and reduce parasite transmission. The same effect cannot be obtained with peptides based on the TSR domain.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-016-1207-8) contains supplementary material, which is available to authorized users.

RTS,S: Toward a first landmark on the Malaria Vaccine Technology Roadmap

The Malaria Vaccine Technology Roadmap calls for a 2015 landmark goal of a first-generation malaria vaccine that has protective efficacy against severe disease and death, lasting longer than one year. This review focuses on product development efforts over the last five years of RTS,S, a pre-erythrocytic, recombinant subunit, adjuvanted, candidate malaria vaccine designed with this goal of a first-generation malaria vaccine in mind. RTS,S recently completed a successful pivotal Phase III safety, efficacy and immunogenicity study. Although vaccine efficacy was found to be modest, a substantial number of cases of clinical malaria were averted over a 3-4 years period, particularly in settings of significant disease burden. European regulators have subsequently adopted a positive opinion under the Article 58 procedure for an indication of active immunization of children aged 6 weeks up to 17 months against malaria caused by Plasmodium falciparum and against hepatitis B. Further evaluations of the benefit, risk, feasibility and cost-effectiveness of RTS,S are now anticipated through policy and financing reviews at the global and national levels.

Natural infection of Plasmodium brasilianum in humans: Man and monkey share quartan malaria parasites in the Venezuelan Amazon

Background

The quartan malaria parasite Plasmodium malariae is the widest spread and best adapted human malaria parasite. The simian Plasmodium brasilianum causes quartan fever in New World monkeys and resembles P. malariae morphologically. Since the genetics of the two parasites are nearly identical, differing only in a range of mutations expected within a species, it has long been speculated that the two are the same. However, no naturally acquired infection with parasites termed as P. brasilianum has been found in humans until now.

Methods

We investigated malaria cases from remote Yanomami indigenous communities of the Venezuelan Amazon and analyzed the genes coding for the circumsporozoite protein (CSP) and the small subunit of ribosomes (18S) by species-specific PCR and capillary based-DNA sequencing.

Findings

Based on 18S rRNA gene sequencing, we identified 12 patients harboring malaria parasites which were 100% identical with P. brasilianum isolated from the monkey, Alouatta seniculus. Translated amino acid sequences of the CS protein gene showed identical immunodominant repeat units between quartan malaria parasites isolated from both humans and monkeys.

Interpretation

This study reports, for the first time, naturally acquired infections in humans with parasites termed as P. brasilianum. We conclude that quartan malaria parasites are easily exchanged between humans and monkeys in Latin America. We hypothesize a lack of host specificity in mammalian hosts and consider quartan malaria to be a true anthropozoonosis. Since the name P. brasilianum suggests a malaria species distinct from P. malariae, we propose that P. brasilianum should have a nomenclatorial revision in case further research confirms our findings. The expansive reservoir of mammalian hosts discriminates quartan malaria from other Plasmodium spp. and requires particular research efforts.

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We found human infections with ‘Plasmodium brasilianum’, a quartan malaria parasite of New World monkeys in South America

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We show that in areas of close contact humans and non-human primates are concurrently infected with quartan malaria parasites

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We conclude that quartan malaria parasites can transcend host species boundaries with impunity

We found naturally acquired infections in humans with Plasmodium brasilianum parasites, a quartan malaria parasite which usually infects more than 35 monkey species in South America. This confirms that malaria parasites, which cause the quartan type of fever (two days without fever between fever peaks), are easily exchanged between humans and monkeys in Latin America. The wide host reservoir of quartan malaria parasites requires particular malaria research efforts.

Long-lasting humoral and cellular immune responses elicited by immunization with recombinant chimeras of the Plasmodium vivax circumsporozoite protein

The circumsporozoite protein (CSP), the most abundant surface antigen of sporozoites, has been extensively studied in different expression platforms as a vaccine candidate. Clinical trials have shown the necessity of broad and highly avid humoral immune responses together with high numbers of CSP-specific TCD4+ and TCD8+ cells, especially those producing IFN-γ, to induce protection. To this aim, we designed two distinct recombinant immunogens based on previously-described antigenic fragments of Plasmodium vivax CSP (PvCSP) to be used as vaccine candidates. The first one is a virus-like particle (VLP) comprising the repeat region of PvCSP (B and TCD4+ epitopes) within the loop of the hepatitis B virus core antigen (HBcAgPvCSP). The second one is a PvCSP multi-epitope polypeptide, rPvCSP-ME, designed based on antigenic regions of PvCSP recognized by lymphocytes of individuals from endemic areas. Mice immunized with 2 doses of these proteins, administered individually or combined and formulated in Montanide ISA 720 adjuvant, were able to induce strong effector and memory humoral responses with IgG titers ranging from 10(4) to 10(5) and avidity indexes toward full-length PvCSP reaching up to 66%, even 3 months after the last immunization. Furthermore, balanced Th1/Th2 responses were generated, as determined by titers of IgG subclasses and further confirmed by ELISPOT analyses, which detected that these vaccination protocols were able to elicit long-term IFN-γ and IL-2-secreting memory T-cells. Overall, these results show that our vaccine candidates generate, in mice, immune responses against regions within PvCSP that have been associated with protection against malaria in humans.

Evolutionary analysis of circumsporozoite surface protein and merozoite surface protein-1 (CSP and MSP-1) sequences of malaria parasites

Malaria, one of the world's most common diseases, is caused by the intracellular protozoan parasite known as Plasmodium. In this study, we have determined the evolutionary relationship of two single-copy proteins, circumsporozoite protein (CSP) and merozoite surface protein-1 (MSP-1), among Plasmodium species using various bioinformatics tools and softwares. These two proteins are major blood stage antigens of Plasmodium species. This study demonstrates that the circumsporozoite protein of Plasmodium falciparum shows similarity with Plasmodium cynomolgi and Plasmodium knowlesi. The merozoite surface protein-1 of Plasmodium coatneyi forms a monophyletic group with Plasmodium knowlesi, demonstrating their close relationship and these two species also reveal similarity between the human malaria Plasmodium vivax. This Plasmodium phylogenetic arrangement is evidently crucial to identify shared derived characters as well as particular adaptation of plasmodium species from inside and between monophyletic groups.