Apical membrane antigen 1, a major malaria vaccine candidate, mediates the close attachment of invasive merozoites to host red blood cells

Apical membrane antigen 1 (AMA-1) of Plasmodium merozoites is established as a candidate molecule for inclusion in a human malaria vaccine and is strongly conserved in the genus. We have investigated its function in merozoite invasion by incubating Plasmodium knowlesi merozoites with red cells in the presence of a previously described rat monoclonal antibody (MAb R31C2) raised against an invasion-inhibitory epitope of P. knowlesi AMA-1 and then fixing the material for ultrastructural analysis. We have found that the random, initial, long-range (12 nm) contact between merozoites and red cells occurs normally in the presence of the antibody, showing that AMA-1 plays no part in this stage of attachment. Instead, inhibited merozoites fail to reorientate, so they do not bring their apices to bear on the red cell surface and do not make close junctional apical contact. We conclude that AMA-1 may be directly responsible for reorientation or that the molecule may initiate the junctional contact, which is then presumably dependent on Duffy binding proteins for its completion.

The domain on the Duffy blood group antigen for binding Plasmodium vivax and P. knowlesi malarial parasites to erythrocytes

Plasmodium vivax and the related simian malarial parasite P. knowlesi use the Duffy blood group antigen as a receptor to invade human erythrocytes and region II of the parasite ligands for binding to this erythrocyte receptor. Here, we identify the peptide within the Duffy blood group antigen of human and rhesus erythrocytes to which the P. vivax and P. knowlesi ligands bind. Peptides from the NH2-terminal extracellular region of the Duffy antigen were tested for their ability to block the binding of erythrocytes to transfected Cos cells expressing on their surface region II of the Duffy-binding ligands. The binding site on the human Duffy antigen used by both the P. vivax and P. knowlesi ligands maps to a 35-amino acid region. A 34-amino acid peptide from the equivalent region of the rhesus Duffy antigen blocked the binding of P. vivax to human erythrocytes, although the P. vivax ligand expressed on Cos cells does not bind rhesus erythrocytes. The binding of the rhesus peptide, but not the rhesus erythrocyte, to the P. vivax ligand was explained by interference of carbohydrate with the binding process. Rhesus erythrocytes, treated with N-glycanase, bound specifically to P. vivax region II. Thus, the interaction of P. vivax ligand with human and rhesus erythrocytes appears to be mediated by a peptide-peptide interaction. Glycosylation of the rhesus Duffy antigen appears to block binding of the P. vivax ligand to rhesus erythrocytes.

Protective CD8+ T lymphocytes in primates immunized with malaria sporozoites

Live attenuated malaria vaccines are more potent than the recombinant protein, bacterial or viral platform vaccines that have been tested, and an attenuated sporozoite vaccine against falciparum malaria is being developed for humans. In mice, attenuated malaria sporozoite vaccines induce CD8⁺ T cells that kill parasites developing in the liver. We were curious to know if CD8⁺ T cells were also important in protecting primates against malaria. We immunized 9 rhesus monkeys with radiation attenuated Plasmodium knowlesi sporozoites, and found that 5 did not develop blood stage infections after challenge with live sporozoites. We then injected 4 of these protected monkeys with cM-T807, a monoclonal antibody to the CD8 molecule which depletes T cells. The fifth monkey received equivalent doses of normal IgG. In 3 of the 4 monkeys receiving cM-T807 circulating CD8⁺ T cells were profoundly depleted. When re-challenged with live sporozoites all 3 of these depleted animals developed blood stage malaria. The fourth monkey receiving cM-T807 retained many circulating CD8⁺ T cells. This monkey, and the vaccinated monkey receiving normal IgG, did not develop blood stage malaria at re-challenge with live sporozoites. Animals were treated with antimalarial drugs and rested for 4 months. During this interval CD8⁺ T cells re-appeared in the circulation of the depleted monkeys. When all vaccinated animals received a third challenge with live sporozoites, all 5 monkeys were once again protected and did not develop blood stage malaria infections. These data indicate that CD8⁺ T cells are important effector cells protecting monkeys against malaria sporozoite infection. We believe that malaria vaccines which induce effector CD8+ T cells in humans will have the best chance of protecting against malaria.

Antigenic variation in malaria: a 3' genomic alteration associated with the expression of a P. knowlesi variant antigen

Antigenic variation of malaria parasites was discovered in P. knowlesi, using a schizont-infected cell agglutination (SICA) assay to detect variant antigens expressed at the surface of infected erythrocytes. Later studies utilizing stable clones, Pk1(A+) and its direct derivative, Pk1(B+)1+, showed that SICA[+] clones express distinct parasite-encoded antigens of approximately 200 kDa. Here we identify a P. knowlesi variant antigen gene and cDNA and demonstrate that it encodes the 205 kDa variant antigen expressed by B+ parasites. This gene belongs to a multigene family, which we term SICAvar. Its ten-exon structure with seven cysteine-rich coding modules is unique compared to P. falciparum var genes. Further, we highlight a 3' genomic alteration that we predict is related to SICAvar gene switching.

Environmental risk factors and exposure to the zoonotic malaria parasite Plasmodium knowlesi across northern Sabah, Malaysia: a population-based cross-sectional survey

BACKGROUND

Land use changes disrupt ecosystems, altering the transmission of vector-borne diseases. These changes have been associated with increasing incidence of zoonotic malaria caused by Plasmodium knowlesi; however, the population-level distributions of infection and exposure remain unknown. We aimed to measure prevalence of serological exposure to P knowlesi and assess associated risk factors.

METHODS

We did an environmentally stratified, population-based, cross-sectional survey across households in the Kudat, Kota Marudu, Pitas, and Ranau districts in northern Sabah, Malaysia, encompassing a range of ecologies. Using blood samples, the transmission intensity of P knowlesi and other malaria species was measured by specific antibody prevalence and infection detected using molecular methods. Proportions and configurations of land types were extracted from maps derived from satellite images; a data-mining approach was used to select variables. A Bayesian hierarchical model for P knowlesi seropositivity was developed, incorporating questionnaire data about individual and household-level risk factors with selected landscape factors.

FINDINGS

Between Sept 17, 2015, and Dec 12, 2015, 10 100 individuals with a median age of 25 years (range 3 months to 105 years) were sampled from 2849 households in 180 villages. 5·1% (95% CI 4·8-5·4) were seropositive for P knowlesi, and marked historical decreases were observed in the transmission of Plasmodium falciparum and Plasmodium vivax. Nine Plasmodium spp infections were detected. Age, male sex, contact with macaques, forest use, and raised house construction were positively associated with P knowlesi exposure, whereas residing at higher geographical elevations and use of insecticide were protective. Agricultural and forest variables, such as proportions and fragmentation of land cover types, predicted exposure at different spatial scales from households.

INTERPRETATION

Although few infections were detected, P knowlesi exposure was observed in all demographic groups and was associated with occupational factors. Results suggest that agricultural expansion and forest fragmentation affect P knowlesi exposure, supporting linkages between land use change and P knowlesi transmission.

FUNDING

UK Medical Research Council, Natural Environment Research Council, Economic and Social Research Council, and Biotechnology and Biosciences Research Council.

Exposure and infection to Plasmodium knowlesi in case study communities in Northern Sabah, Malaysia and Palawan, The Philippines

BACKGROUND

Primarily impacting poor, rural populations, the zoonotic malaria Plasmodium knowlesi is now the main cause of human malaria within Malaysian Borneo. While data is increasingly available on symptomatic cases, little is known about community-level patterns of exposure and infection. Understanding the true burden of disease and associated risk factors within endemic communities is critical for informing evidence-based control measures.

METHODOLOGY/PRINCIPAL FINDINGS

We conducted comprehensive surveys in three areas where P. knowlesi transmission is reported: Limbuak, Pulau Banggi and Matunggung, Kudat, Sabah, Malaysia and Bacungan, Palawan, the Philippines. Infection prevalence was low with parasites detected by PCR in only 0.2% (4/2503) of the population. P. knowlesi PkSERA3 ag1 antibody responses were detected in 7.1% (95% CI: 6.2-8.2%) of the population, compared with 16.1% (14.6-17.7%) and 12.6% (11.2-14.1%) for P. falciparum and P. vivax. Sero-prevalence was low in individuals <10 years old for P. falciparum and P. vivax consistent with decreased transmission of non-zoonotic malaria species. Results indicated marked heterogeneity in transmission intensity between sites and P. knowlesi exposure was associated with agricultural work (OR 1.63; 95% CI 1.07-2.48) and higher levels of forest cover (OR 2.40; 95% CI 1.29-4.46) and clearing (OR 2.14; 95% CI 1.35-3.40) around houses. Spatial patterns of P. knowlesi exposure differed from exposure to non-zoonotic malaria and P. knowlesi exposed individuals were younger on average than individuals exposed to non-zoonotic malaria.

CONCLUSIONS/SIGNIFICANCE

This is the first study to describe serological exposure to P. knowlesi and associated risk factors within endemic communities. Results indicate community-level patterns of infection and exposure differ markedly from demographics of reported cases, with higher levels of exposure among women and children. Further work is needed to understand these variations in risk across a wider population and spatial scale.

Multistage multiantigen heterologous prime boost vaccine for Plasmodium knowlesi malaria provides partial protection in rhesus macaques

A nonhuman primate model for malaria vaccine development allowing reliable, stringent sporozoite challenge and evaluation of both cellular and antibody responses is needed. We therefore constructed a multicomponent, multistage DNA vaccine for the simian malaria species Plasmodium knowlesi including two preerythrocytic-stage antigens, the circumsporozoite protein (PkCSP) and sporozoite surface protein 2 (PkSSP2), and two blood stage antigens, apical merozoite antigen 1 (PkAMA1) and merozoite surface protein 1 (PkMSP1p42), as well as recombinant canarypox viruses encoding the four antigens (ALVAC-4). The DNA vaccine plasmids expressed the corresponding antigens in vitro and induced antiparasite antibodies in mice. Groups of four rhesus monkeys received three doses of a mixture of the four DNA vaccine plasmids and a plasmid encoding rhesus granulocyte-monocyte colony-stimulating factor, followed by boosting with a single dose of ALVAC-4. Three groups received the priming DNA doses by different routes, either by intramuscular needle injection, by intramuscular injection with a needleless injection device, the Biojector, or by a combination of intramuscular and intradermal routes by Biojector. Animals immunized by any route developed antibody responses against sporozoites and infected erythrocytes and against a recombinant PkCSP protein, as well as gamma interferon-secreting T-cell responses against peptides from PkCSP. Following challenge with 100 P. knowlesi sporozoites, 1 of 12 experimental monkeys was completely protected and the mean parasitemia in the remaining monkeys was significantly lower than that in 4 control monkeys. This model will be important in preclinical vaccine development.

Protection of rhesus macaques against lethal Plasmodium knowlesi malaria by a heterologous DNA priming and poxvirus boosting immunization regimen

We tested a cytokine-enhanced, multiantigen, DNA priming and poxvirus boosting vaccine regimen for prevention of malaria in the Plasmodium knowlesi-rhesus macaque model system. Animals were primed with a mixture of DNA plasmids encoding two preerythrocytic-stage proteins and two erythrocytic-stage proteins from P. knowlesi and combinations of the cytokines granulocyte-macrophage colony-stimulating factor, interleukin-4, and tumor necrosis factor alpha and were boosted with a mixture of four recombinant, attenuated vaccinia virus strains encoding the four P. knowlesi antigens. Two weeks after boosting, the geometric mean immunofluorescence titers in the immunized groups against sporozoites and infected erythrocytes ranged from 160 to 8,096 and from 1,810 to 5,120, respectively. The geometric mean anti-P. knowlesi circumsporozoite protein (PkCSP) titers ranged from 1,761 to 24,242. Peripheral blood mononuclear cells (PBMC) from the immunized monkeys produced gamma interferon (IFN-gamma) in response to incubation with pooled peptides from the PkCSP at frequencies of 10 to 571 spot-forming cells/10(6) PBMC. Following challenge with 100 infectious P. knowlesi sporozoites, 2 of 11 immunized monkeys were sterilely protected, and 7 of the 9 infected monkeys resolved their parasitemias spontaneously. In contrast, all four controls became infected and required treatment for overwhelming parasitemia. Early protection was strongly associated with IFN-gamma responses against a pool of peptides from the preerythrocytic-stage antigen, PkCSP. These findings demonstrate that a multistage, multiantigen, DNA priming and poxvirus boosting vaccine regimen can protect nonhuman primates from an otherwise lethal malaria sporozoite challenge.

Vaccination with Plasmodium knowlesi AMA1 formulated in the novel adjuvant co-vaccine HT™ protects against blood-stage challenge in rhesus macaques

Plasmodium falciparum apical membrane antigen 1 (PfAMA1) is a leading blood stage vaccine candidate. Plasmodium knowlesi AMA1 (PkAMA1) was produced and purified using similar methodology as for clinical grade PfAMA1 yielding a pure, conformational intact protein. Combined with the adjuvant CoVaccine HT™, PkAMA1 was found to be highly immunogenic in rabbits and the efficacy of the PkAMA1 was subsequently tested in a rhesus macaque blood-stage challenge model. Six rhesus monkeys were vaccinated with PkAMA1 and a control group of 6 were vaccinated with PfAMA1. A total of 50 µg AMA1 was administered intramuscularly three times at 4 week intervals. One of six rhesus monkeys vaccinated with PkAMA1 was able to control parasitaemia, upon blood stage challenge with P. knowlesi H-strain. Four out of the remaining five showed a delay in parasite onset that correlated with functional antibody titres. In the PfAMA1 vaccinated control group, five out of six animals had to be treated with antimalarials 8 days after challenge; one animal did not become patent during the challenge period. Following a rest period, animals were boosted and challenged again. Four of the six rhesus monkeys vaccinated with PkAMA1 were able to control the parasitaemia, one had a delayed onset of parasitaemia and one animal was not protected, while all control animals required treatment. To confirm that the control of parasitaemia was AMA1-related, animals were allowed to recover, boosted and re-challenged with P. knowlesi Nuri strain. All control animals had to be treated with antimalarials by day 8, while five out of six PkAMA1 vaccinated animals were able to control parasitaemia. This study shows that: i) Yeast-expressed PkAMA1 can protect against blood stage challenge; ii) Functional antibody levels as measured by GIA correlated inversely with the day of onset and iii) GIA IC₅₀ values correlated with estimated in vivo growth rates.

Expression and serologic activity of a soluble recombinant Plasmodium vivax Duffy binding protein

Plasmodium vivax Duffy binding protein (DBP) is a conserved functionally important protein. P. vivax DBP is an asexual blood-stage malaria vaccine candidate because adhesion of P. vivax DBP to its erythrocyte receptor is essential for the parasite to continue development in human blood. We developed a soluble recombinant protein of P. vivax DBP (rDBP) and examined serologic activity to it in residents of a region of high endemicity. This soluble rDBP product contained the cysteine-rich ligand domain and most of the contiguous proline-rich hydrophilic region. rDBP was expressed as a glutathione S-transferase (GST) fusion protein and was isolated from GST by thrombin treatment of the purified fusion protein bound on glutathione agarose beads. P. vivax rDBP was immunogenic in rabbits and induced antibodies that reacted with P. vivax and Plasmodium knowlesi merozoites. Human sera from adult residents of a region of Papua New Guinea where malaria is highly endemic or P. vivax-infected North American residents reacted with rDBP in an immunoblot and an enzyme-linked immunosorbent assay. The reactivity to reduced, denatured P. vivax rDBP and the cross-reactivity with P. knowlesi indicated the presence of immunogenic conserved linear B-cell epitopes. A more extensive serologic survey of Papua New Guinea residents showed that antibody response to P. vivax DBP is common and increases with age, suggesting a possible boosting of the antibody response in some by repeated exposure to P. vivax. A positive humoral response to P. vivax DBP correlated with a significantly higher response to P. vivax MSP-1(19). The natural immunogenicity of this DBP should strengthen its usefulness as a vaccine.

Malaria Elimination: Time to Target All Species

Important strides have been made within the past decade toward malaria elimination in many regions, and with this progress, the feasibility of eradication is once again under discussion. If the ambitious goal of eradication is to be achieved by 2040, all species of Plasmodium infecting humans will need to be targeted with evidence-based and concerted interventions. In this perspective, the potential barriers to achieving global malaria elimination are discussed with respect to the related diversities in host, parasite, and vector populations. We argue that control strategies need to be reorientated from a sequential attack on each species, dominated by Plasmodium falciparum to one that targets all species in parallel. A set of research themes is proposed to mitigate the potential setbacks on the pathway to a malaria-free world.

Plasmodium vivax, P. cynomolgi, and P. knowlesi: identification of homologue proteins associated with the surface of merozoites

We have identified a Plasmodium vivax merozoite surface protein (MSP) that migrates on SDS-polyacrylamide gels at a Mr of about 185 kDa. This protein was recognized by a P. vivax monoclonal antibody (mAb) that localizes the protein by immunofluorescence to the surface of merozoites and also immunoprecipitates this protein from NP-40 detergent extracts of [35S]methionine metabolically radiolabeled P. vivax schizonts. The P. vivax MSP does not become biosynthetically radiolabeled with [3H]glucoamine, [3H]myristate, [3H]palmitate, or [3H]mannose, indicating that this P. vivax MSP is not posttranslationally modified and bound to the merozoite membrane by a glycosylphosphatidylinositol (GPI) lipid anchor. Thus, in this respect, this protein is different from members of the MSP-1 protein family and from MSP-2 and MSP-4 of P. falciparum. The mAb cross-reacts with and outlines the surface of P. cynomolgi merozoites and immunoprecipitates a 150-kDa P. cynomolgi homologue. The mAb was used as an affinity reagent to purify the native homologous MSP from NP-40 extracts of P. cynomolgi mature schizonts in order to develop a specific polyclonal antiserum. The resulting anti-PcyMSP rabbit antiserum cross-reacts strongly with the P. vivax 185-kDa MSP and also recognizes an analogous 110-kDa protein from P. knowlesi. We have determined via an immunodepletion experiment that the 110-kDa P. knowlesi MSP corresponds to the PK 110 protein partially characterized earlier (Perler et al. 1987). The potential of P. vivax MSP as a vaccine candidate was addressed by conducting in vitro inhibition of erythrocyte invasion assays, and the IgG fraction of both the P. vivax MSP mAb and the P. cynomolgi MSP rabbit antiserum significantly inhibited entry of P. vivax merozoites. We denote, on a preliminary basis, these antigenically related merozite surface proteins PvMSP-185, PcyMSP-150, and PkMSP-110.

Sterile protection against Plasmodium knowlesi in rhesus monkeys from a malaria vaccine: comparison of heterologous prime boost strategies

Using newer vaccine platforms which have been effective against malaria in rodent models, we tested five immunization regimens against Plasmodium knowlesi in rhesus monkeys. All vaccines included the same four P. knowlesi antigens: the pre-erythrocytic antigens CSP, SSP2, and erythrocytic antigens AMA1, MSP1. We used four vaccine platforms for prime or boost vaccinations: plasmids (DNA), alphavirus replicons (VRP), attenuated adenovirus serotype 5 (Ad), or attenuated poxvirus (Pox). These four platforms combined to produce five different prime/boost vaccine regimens: Pox alone, VRP/Pox, VRP/Ad, Ad/Pox, and DNA/Pox. Five rhesus monkeys were immunized with each regimen, and five Control monkeys received a mock vaccination. The time to complete vaccinations was 420 days. All monkeys were challenged twice with 100 P. knowlesi sporozoites given IV. The first challenge was given 12 days after the last vaccination, and the monkeys receiving the DNA/Pox vaccine were the best protected, with 3/5 monkeys sterilely protected and 1/5 monkeys that self-cured its parasitemia. There was no protection in monkeys that received Pox malaria vaccine alone without previous priming. The second sporozoite challenge was given 4 months after the first. All 4 monkeys that were protected in the first challenge developed malaria in the second challenge. DNA, VRP and Ad5 vaccines all primed monkeys for strong immune responses after the Pox boost. We discuss the high level but short duration of protection in this experiment and the possible benefits of the long interval between prime and boost.

Antibodies raised against receptor-binding domain of Plasmodium knowlesi Duffy binding protein inhibit erythrocyte invasion

Erythrocyte invasion by malaria parasites requires specific receptor-ligand interactions. Plasmodium vivax and Plasmodium knowlesi are completely dependent on binding the Duffy blood group antigen to invade human erythrocytes. P. knowlesi invades rhesus erythrocytes by multiple pathways using the Duffy antigen as well as alternative receptors. Plasmodium falciparum binds sialic acid residues on glycophorin A as well as other sialic acid-independent receptors to invade human erythrocytes. Parasite proteins that mediate these interactions belong to a family of erythrocyte binding proteins, which includes the P. vivax Duffy binding protein, 175 kDa P. falciparum erythrocyte binding antigen (EBA-175), P. knowlesi alpha protein, which binds human and rhesus Duffy antigens, and P. knowlesi beta and gamma proteins, which bind Duffy-independent receptors on rhesus erythrocytes. The receptor-binding domains of these proteins lie in conserved, N-terminal, cysteine-rich regions that are referred to as region II. Here, we have examined the feasibility of inhibiting erythrocyte invasion with antibodies directed against receptor-binding domains of erythrocyte binding proteins. Region II of P. knowelsi alpha protein (Pk(alpha)RII), which binds the Duffy antigen, was expressed as a secreted protein in insect cells and purified from culture supernatants. Rabbit antibodies raised against recombinant Pk(alpha)RII were tested for inhibition of erythrocyte binding and invasion. Antibodies raised against Pk(alpha)RII inhibit P. knowlesi invasion of both human and rhesus erythrocytes. These data provide support for the development of recombinant vaccines based on the homologous binding domains of P. vivax Duffy binding protein and P. falciparum EBA-175.

ELISPOT assay for detection of peptide specific interferon-gamma secreting cells in rhesus macaques

A reliable procedure to measure antigen specific T cell responses in rhesus macaques is required to determine the efficacy of vaccines and immunotherapies. The currently available T cell assays are poorly quantifiable or technically difficult to perform. Classical 51Cr-release cytotoxic T cell (CTL) assays are cumbersome and difficult to quantitate reproducibly. Detection of specific T-cell using MHC-peptide tetrameric complexes is highly sensitive, but requires knowledge of MHC type and prior identification of T cell epitopes. We therefore developed a rhesus interferon-gamma (IFN-gamma) ELISPOT assay capable of detecting IFN-gamma secretion in response to stimulation with pooled 20-mer peptides. Peripheral blood mononuclear cells (PBMCs) from rhesus monkeys immunized with a DNA vaccine and recombinant canary pox encoding the Plasmodium knowlesi circumsporozoite protein (PkCSP) were incubated with pools of peptides from PkCSP. Positive responses to peptide pools and individual peptides ranging from 100 to 450 spot forming cells (SFC)/10(6) PBMC were detected in four of four immunized monkeys and in zero of two control monkeys. In two monkeys studied in detail, the IFN-gamma response was focussed on a single 20-mer peptide, QGDGANAGQPQAQGDGANAG, and was dependent on CD4(+), but not CD8(+), T cells. Background responses in control monkeys and preimmunization PBMCs ranged from 10 to 50 SFC/10(6) PBMC. The average within assay and between assay coefficients of variation (CV) for this peptide ELISPOT were 21.9 and 24.7%, respectively. This peptide IFN-gamma assay will be a useful tool for evaluation of T cell responses in rhesus macaques.

Proteomic studies of Plasmodium knowlesi SICA variant antigens demonstrate their relationship with P. falciparum EMP1

Malaria variant antigens are encoded by large multigene families and expressed on the surface of infected erythrocytes. The Plasmodium knowlesi Schizont-infected cell agglutination (SICA) antigens are encoded by the SICAvar multigene family, and the P. falciparum erythrocyte membrane protein-1 (PfEMP1) antigens are encoded by the var gene family. Although these variant antigens share many fundamental features, P. knowlesi and P. falciparum are phylogenetically distantly related, and so far a significant level of sequence identity has not been observed in alignments of either the SICAvar and var gene families or their encoded proteins. In support of their orthologous relationship, however, here we demonstrate through proteomic analysis that the P. knowlesi SICA variant antigens share significant common sequences with P. falciparum EMP1 molecules. As many as forty P. knowlesi SICA peptides show identity with a particular P. falciparum EMP1, mapping throughout all characterized domains, including the externally exposed cysteine-rich domains that are characteristic of both proteins. These findings provide further validation of the classical in vivo P. knowlesi-rhesus monkey model system for advancing our understanding of the immunobiology of antigenic variation and variant antigen gene expression in Plasmodium.

PacBio assembly of a Plasmodium knowlesi genome sequence with Hi-C correction and manual annotation of the SICAvar gene family

Plasmodium knowlesi has risen in importance as a zoonotic parasite that has been causing regular episodes of malaria throughout South East Asia. The P. knowlesi genome sequence generated in 2008 highlighted and confirmed many similarities and differences in Plasmodium species, including a global view of several multigene families, such as the large SICAvar multigene family encoding the variant antigens known as the schizont-infected cell agglutination proteins. However, repetitive DNA sequences are the bane of any genome project, and this and other Plasmodium genome projects have not been immune to the gaps, rearrangements and other pitfalls created by these genomic features. Today, long-read PacBio and chromatin conformation technologies are overcoming such obstacles. Here, based on the use of these technologies, we present a highly refined de novo P. knowlesi genome sequence of the Pk1(A+) clone. This sequence and annotation, referred to as the 'MaHPIC Pk genome sequence', includes manual annotation of the SICAvar gene family with 136 full-length members categorized as type I or II. This sequence provides a framework that will permit a better understanding of the SICAvar repertoire, selective pressures acting on this gene family and mechanisms of antigenic variation in this species and other pathogens.

Cytomegalovirus vectors expressing Plasmodium knowlesi antigens induce immune responses that delay parasitemia upon sporozoite challenge

The development of a sterilizing vaccine against malaria remains one of the highest priorities for global health research. While sporozoite vaccines targeting the pre-erythrocytic stage show great promise, it has not been possible to maintain efficacy long-term, likely due to an inability of these vaccines to maintain effector memory T cell responses in the liver. Vaccines based on human cytomegalovirus (HCMV) might overcome this limitation since vectors based on rhesus CMV (RhCMV), the homologous virus in rhesus macaques (RM), elicit and indefinitely maintain high frequency, non-exhausted effector memory T cells in extralymphoid tissues, including the liver. Moreover, RhCMV strain 68-1 elicits CD8+ T cells broadly recognizing unconventional epitopes exclusively restricted by MHC-II and MHC-E. To evaluate the potential of these unique immune responses to protect against malaria, we expressed four Plasmodium knowlesi (Pk) antigens (CSP, AMA1, SSP2/TRAP, MSP1c) in RhCMV 68-1 or in Rh189-deleted 68-1, which additionally elicits canonical MHC-Ia-restricted CD8+ T cells. Upon inoculation of RM with either of these Pk Ag expressing RhCMV vaccines, we obtained T cell responses to each of the four Pk antigens. Upon challenge with Pk sporozoites we observed a delayed appearance of blood stage parasites in vaccinated RM consistent with a 75-80% reduction of parasite release from the liver. Moreover, the Rh189-deleted RhCMV/Pk vectors elicited sterile protection in one RM. Once in the blood, parasite growth was not affected. In contrast to T cell responses induced by Pk infection, RhCMV vectors maintained sustained T cell responses to all four malaria antigens in the liver post-challenge. The delayed appearance of blood stage parasites is thus likely due to a T cell-mediated inhibition of liver stage parasite development. As such, this vaccine approach can be used to efficiently test new T cell antigens, improve current vaccines targeting the liver stage and complement vaccines targeting erythrocytic antigens.

Cross-species reactivity of antibodies against Plasmodium vivax blood-stage antigens to Plasmodium knowlesi

Malaria is caused by multiple different species of protozoan parasites, and interventions in the pre-elimination phase can lead to drastic changes in the proportion of each species causing malaria. In endemic areas, cross-reactivity may play an important role in the protection and blocking transmission. Thus, successful control of one species could lead to an increase in other parasite species. A few studies have reported cross-reactivity producing cross-immunity, but the extent of cross-reactive, particularly between closely related species, is poorly understood. P. vivax and P. knowlesi are particularly closely related species causing malaria infections in SE Asia, and whilst P. vivax cases are in decline, zoonotic P. knowlesi infections are rising in some areas. In this study, the cross-species reactivity and growth inhibition activity of P. vivax blood-stage antigen-specific antibodies against P. knowlesi parasites were investigated. Bioinformatics analysis, immunofluorescence assay, western blotting, protein microarray, and growth inhibition assay were performed to investigate the cross-reactivity. P. vivax blood-stage antigen-specific antibodies recognized the molecules located on the surface or released from apical organelles of P. knowlesi merozoites. Recombinant P. vivax and P. knowlesi proteins were also recognized by P. knowlesi- and P. vivax-infected patient antibodies, respectively. Immunoglobulin G against P. vivax antigens from both immune animals and human malaria patients inhibited the erythrocyte invasion by P. knowlesi. This study demonstrates that there is extensive cross-reactivity between antibodies against P. vivax to P. knowlesi in the blood stage, and these antibodies can potently inhibit in vitro invasion, highlighting the potential cross-protective immunity in endemic areas.

In recent years, malaria initiatives have increasingly shifted focus from achieving malaria control to achieving malaria elimination. However, the interventions used are leading to drastic changes in the proportions of different Plasmodium species causing clinical infection, particularly within Southeast Asia. Little is known about how these different parasite species interact/compete in nature or whether exposure to one species could cause some level of protection against another. We examined cross-reactive antibody responses to key parasite proteins with roles in red blood cell invasion and identified novel cross-species reactivity among the closest of malaria affecting the human population (P. vivax and P. knowlesi). This comprehensive analysis provides evidence that cross-reactive immunity could play an important role in areas where species distributions are perturbed by malaria control measures, and future efforts to identify the specific cross-reactive epitopes involved would be invaluable both to our understanding of malaria immunity and vaccine development.

Cytokine responses during acute simian Plasmodium cynomolgi and Plasmodium knowlesi infections

Experimental infection of non-human primates with simian malaria parasites offers a controlled system to study malarial immunity. Plasmodium cynomolgi (P. vivax-like) and P. knowlesi (P. falciparum-like) infections in the rhesus monkey were used as a model to test the hypothesis that initial acute infection stimulates type 1/pro-inflammatory cytokine expression followed by a gradual type 2/anti-inflammatory response upon re-infection. This study analyzed cytokine gene expression (interleukin-12, interferon-gamma, tumor necrosis factor-alpha = type 1; interleukin-4, interleukin-10 = type 2) using a semi-quantitative reverse transcriptase-polymerase chain reaction in monkeys infected with each of the parasites (three per group). Clinicoparasitologic and serologic parameters were also monitored. Monkeys were re-infected to assess whether enhanced immunity could increase parasite clearance. The immune response to P. cynomolgi infection in rhesus monkeys seemed to be mediated by anti-parasite, pro-inflammatory responses during primary infection with a transition to protective type 2 responses after repeat infection. The immune responses to P. knowlesi infection were more varied. Anti-inflammatory responses were more prevalent during primary infection. Repeat infection stimulated a wide variety of responses; most included expression of tumor necrosis factor-alpha, a cytokine that has been associated with inflammatory and host-destructive effects (weight loss, fever, anemia). These observations further confirmed that the simian malaria/rhesus monkey model is well suited for studies on the regulation of immunity to acute Plasmodium infection.

Using Plasmodium knowlesi as a model for screening Plasmodium vivax blood-stage malaria vaccine targets reveals new candidates

Plasmodium vivax is responsible for the majority of malaria cases outside Africa. Unlike P. falciparum, the P. vivax life-cycle includes a dormant liver stage, the hypnozoite, which can cause infection in the absence of mosquito transmission. An effective vaccine against P. vivax blood stages would limit symptoms and pathology from such recurrent infections, and therefore could play a critical role in the control of this species. Vaccine development in P. vivax, however, lags considerably behind P. falciparum, which has many identified targets with several having transitioned to Phase II testing. By contrast only one P. vivax blood-stage vaccine candidate based on the Duffy Binding Protein (PvDBP), has reached Phase Ia, in large part because the lack of a continuous in vitro culture system for P. vivax limits systematic screening of new candidates. We used the close phylogenetic relationship between P. vivax and P. knowlesi, for which an in vitro culture system in human erythrocytes exists, to test the scalability of systematic reverse vaccinology to identify and prioritise P. vivax blood-stage targets. A panel of P. vivax proteins predicted to function in erythrocyte invasion were expressed as full-length recombinant ectodomains in a mammalian expression system. Eight of these antigens were used to generate polyclonal antibodies, which were screened for their ability to recognize orthologous proteins in P. knowlesi. These antibodies were then tested for inhibition of growth and invasion of both wild type P. knowlesi and chimeric P. knowlesi lines modified using CRISPR/Cas9 to exchange P. knowlesi genes with their P. vivax orthologues. Candidates that induced antibodies that inhibited invasion to a similar level as PvDBP were identified, confirming the utility of P. knowlesi as a model for P. vivax vaccine development and prioritizing antigens for further follow up.

Transformed Toxoplasma gondii tachyzoites expressing the circumsporozoite protein of Plasmodium knowlesi elicit a specific immune response in rhesus monkeys

Toxoplasma gondii tachyzoites were transformed with the coding sequence of the circumsporozoite (CS) protein of the primate malaria parasite Plasmodium knowlesi. A single inoculation of live transformed tachyzoites elicited an antibody response directed against the immunodominant repeat epitope (EQPAAGAGG)2 of the P. knowlesi CS protein in rhesus monkeys. Notably, these animals failed to show a positive serum conversion against T. gondii. Antibodies against Toxoplasma antigens were detected only after a second inoculation with a higher number of transformed tachyzoites. This boost induced an increased antibody response against the P. knowlesi CS protein associated with immunoglobulin class switching, thus demonstrating the establishment of immunological memory. These results indicate that the Toxoplasma-derived CS protein is efficiently recognized by the monkey immune system and represents an immunodominant antigen in transformed parasites.

Identification and validation of a novel panel of Plasmodium knowlesi biomarkers of serological exposure

BACKGROUND

Plasmodium knowlesi is the most common cause of malaria in Malaysian Borneo, with reporting limited to clinical cases presenting to health facilities and scarce data on the true extent of transmission. Serological estimations of transmission have been used with other malaria species to garner information about epidemiological patterns. However, there are a distinct lack of suitable serosurveillance tools for this neglected disease.

METHODOLOGY/PRINCIPAL FINDINGS

Using in silico tools, we designed and expressed four novel P. knowlesi protein products to address the distinct lack of suitable serosurveillance tools: PkSERA3 antigens 1 and 2, PkSSP2/TRAP and PkTSERA2 antigen 1. Antibody prevalence to these antigens was determined by ELISA for three time-points post-treatment from a hospital-based clinical treatment trial in Sabah, East Malaysia (n = 97 individuals; 241 total samples for all time points). Higher responses were observed for the PkSERA3 antigen 2 (67%, 65/97) across all time-points (day 0: 36.9% 34/92; day 7: 63.8% 46/72; day 28: 58.4% 45/77) with significant differences between the clinical cases and controls (n = 55, mean plus 3 SD) (day 0 p<0.0001; day 7 p<0.0001; day 28 p<0.0001). Using boosted regression trees, we developed models to classify P. knowlesi exposure (cross-validated AUC 88.9%; IQR 86.1-91.3%) and identified the most predictive antibody responses.

CONCLUSIONS/SIGNIFICANCE

The PkSERA3 antigen 2 had the highest relative variable importance in all models. Further validation of these antigens is underway to determine the specificity of these tools in the context of multi-species infections at the population level.

A SICAvar switching event in Plasmodium knowlesi is associated with the DNA rearrangement of conserved 3′ non-coding sequences

Plasmodium knowlesi variant antigens are expressed at the surface of infected erythrocytes and are encoded by the Schizont Infected Cell Agglutination variant antigen (SICAvar) multigene family. The 3' region of the SICAvar gene locus encoding the 205 kDa variant antigen expressed in the Pk1(B+)1+ parasites was found to be altered compared to the Pk1(A+) parental clone. Here we report that this alteration is the result of a DNA rearrangement and that the original and altered 205 SICAvar alleles appear to encode bona fide variant antigens. Importantly, 205A and 205B SICAvar RNA sequences are detectable in similar apparent quantities as determined by quantitative real-time reverse-transcription polymerase chain reaction (qRT-PCR) amplification experiments. However, expression of the 205 kDa SICA protein at the surface of the infected erythrocyte is not characteristic of the Pk1(A+) parasites and the 205 SICAvar transcript has not been detected in Pk1(A+) parasites by northern blot analysis. Furthermore, we report that many distinct SICAvar transcripts were detected in P. knowlesi Pk1(B+)1+ cDNA library hybridization screens. Of special interest, in light of these data, distinctive differences at the 3' end of the 205A and 205B alleles are observed, which may be of functional importance. An analysis of the 3' untranslated region (UTR) of SICAvar genes in more than 100 sequences revealed a surprising common sequence pattern characterized by blocks of imperfect, GT-rich, heptad repeated motifs (Block I), followed by A and T rich homopolymers (Block II) and in a large number of genes, GC-rich segments (Block III). We show that this region undergoes extensive recombination and that the preferential stability of the 205 SICAvar transcript in Pk1(B+)1+ parasites may be associated with the presence of its specific Block III sequences. We speculate that the conserved yet polymorphic SICAvar 3'UTR sequences, and comparable regions in P. falciparum var genes, function in the stage-specific and developmentally regulated post-transcriptional gene silencing (PTGS) of variant antigen transcripts.