Plasmodium pre-erythrocytic stages: what's new?

Application of machine and deep learning algorithms in optical microscopic detection of Plasmodium: A malaria diagnostic tool for the future

Machine and deep learning techniques are prevalent in the medical discipline due to their high level of accuracy in disease diagnosis. One such disease is malaria caused by Plasmodium falciparum and transmitted by the female anopheles mosquito. According to the World Health Organisation (WHO), millions of people are infected annually, leading to inevitable deaths in the infected population. Statistical records show that early detection of malaria parasites could prevent deaths and machine learning (ML) has proved helpful in the early detection of malarial parasites. Human error is identified to be a major cause of inaccurate diagnostics in the traditional microscopy malaria diagnosis method. Therefore, the method would be more reliable if human expert dependency is restricted or entirely removed, and thus, the motivation of this paper. This study presents a systematic review to understand the prevalent machine learning algorithms applied to a low-cost, portable optical microscope in the automation of blood film interpretation for malaria parasite detection. Peer-reviewed papers were downloaded from selected reputable databases eg. Elsevier, IEEExplore, Pubmed, Scopus, Web of Science, etc. The extant literature suggests that convolutional neural network (CNN) and its variants (deep learning) account for 41.9% of the microscopy malaria diagnosis using machine learning with a prediction accuracy of 99.23%. Thus, the findings suggest that early detection of the malaria parasite has improved through the application of CNN and other ML algorithms on microscopic malaria parasite detection.

Conditional Gene Deletion in Mammalian and Mosquito Stages of Plasmodium berghei Using Dimerizable Cre Recombinase

Genome editing in the malaria parasite Plasmodium relies on homologous recombination and requires parasite transfection in asexual blood stages. Therefore, conditional genetic approaches are needed to delete genes that are essential during blood stage replication. Among these, the dimerizable Cre (DiCre) recombinase system has emerged as a powerful approach for conditional gene knockout in Plasmodium parasites. In this system, the Cre recombinase is expressed in the form of two separate, enzymatically inactive polypeptides. Rapamycin-induced heterodimerization of the two components restores recombinase activity, leading to site-specific excision of floxed DNA sequences. Here, we describe methods to generate genetically modified DiCre-expressing Plasmodium berghei mutants by introducing Lox sites upstream and downstream of a gene of interest and to induce conditional excision of the floxed gene in different stages of the parasite life cycle. Administration of rapamycin to P. berghei-infected mice allows conditional gene deletion in the asexual erythrocytic stages. Rapamycin-induced gene excision can also be achieved in P. berghei sexual blood stages prior to transmission to mosquitoes, or during sporogony by treating P. berghei-infected mosquitoes, both methods allowing functional studies in P. berghei mosquito stages. Finally, rapamycin can be administered to in vitro cell cultures in order to induce gene excision in P. berghei liver stages. Subsequent phenotyping allows for the analysis of essential gene function across the parasite life cycle stages.

CXCR4 regulates Plasmodium development in mouse and human hepatocytes

In the livers of Plasmodium-infected mammalian hosts, the rod-shaped mosquito-stage parasites develop into spherical exoerythrocytic forms, subsequently forming the erythrocyte-stage parasites and eventually causing malaria. Here, Bando et al. identify CXCR4 as a host factor for Plasmodium liver-stage development.

The liver stage of the etiological agent of malaria, Plasmodium, is obligatory for successful infection of its various mammalian hosts. Differentiation of the rod-shaped sporozoites of Plasmodium into spherical exoerythrocytic forms (EEFs) via bulbous expansion is essential for parasite development in the liver. However, little is known about the host factors regulating the morphological transformation of Plasmodium sporozoites in this organ. Here, we show that sporozoite differentiation into EEFs in the liver involves protein kinase C ζ–mediated NF-κB activation, which robustly induces the expression of C-X-C chemokine receptor type 4 (CXCR4) in hepatocytes and subsequently elevates intracellular Ca²⁺ levels, thereby triggering sporozoite transformation into EEFs. Blocking CXCR4 expression by genetic or pharmacological intervention profoundly inhibited the liver-stage development of the Plasmodium berghei rodent malaria parasite and the human Plasmodium falciparum parasite. Collectively, our experiments show that CXCR4 is a key host factor for Plasmodium development in the liver, and CXCR4 warrants further investigation for malaria prophylaxis.

Design and Synthesis of Novel Hybrid Molecules against Malaria

The effective treatment of malaria can be very complex: Plasmodium parasites develop in multiple stages within a complex life cycle between mosquitoes as vectors and vertebrates as hosts. For the full and effective elimination of parasites, an effective drug should be active against the earliest stages of the Plasmodium infection: liver stages (reduce the progress of the infection), blood stages (cure the clinical symptoms), and gametocytes (inhibit the transmission cycle). Towards this goal, here we report the design, the synthetic methodology, and the characterization of novel hybrid agents with combined activity against Plasmodium liver stages and blood stages and gametocytes. The divergent synthetic approach allows the access to differently linked primaquine-chloroquine hybrid templates in up to eight steps.

Malaria vaccine development and how external forces shape it: an overview

The aim of this paper is to analyse the current status and scientific value of malaria vaccine approaches and to provide a realistic prognosis for future developments. We systematically review previous approaches to malaria vaccination, address how vaccine efforts have developed, how this issue may be fixed, and how external forces shape vaccine development. Our analysis provides significant information on the various aspects and on the external factors that shape malaria vaccine development and reveal the importance of vaccine development in our society.

Towards an optimized inhibition of liver stage development assay (ILSDA) for Plasmodium falciparum

BACKGROUND

Experimental vaccines targeting Plasmodium falciparum have had some success in recent years. These vaccines use attenuated parasites, recombinant sporozoite proteins, or DNA and virus combinations to induce cell-mediated immune responses and/or antibodies targeting sporozoite surface proteins. To capitalize on the success of these vaccines and understand the mechanisms by which these vaccines function, it is important to develop assays that measure correlates of protection in volunteers. The inhibition of liver stage development assay (ILSDA) tests antibodies for the ability to block sporozoite development in hepatocytes. As such the ILSDA is an excellent candidate assay to identify correlates of humoral protection, particularly against the liver stage of malaria infection. In addition, the ILSDA can be used as a tool to evaluate novel sporozoite antigens for future vaccine development. Historically the ILSDA has suffered from low sporozoite infection rates, absence of standardized reagents, and the subjectivity associated with the traditional primary outcome measures, which depend on microscopy of stained hepatocyte cultures. This study worked to significantly improve sporozoite infection rates in hepatocytes, modify key steps in the assay protocol to reduce experimental variability, and demonstrate the utility of the ILSDA in testing antibodies targeting the circumsporozoite protein.

METHODS

Cryopreserved primary human hepatocytes, Plasmodium falciparum sporozoites, and circumsporozoite antibodies were used to optimize the ILSDA.

RESULTS

Inoculation of cryopreserved primary human hepatocytes with Plasmodium falciparum sporozoites improved liver stage development in the ILSDA compared to HCO4 cells. In the ILSDA, circumsporozoite antibodies suppressed liver stage development in cryopreserved primary human hepatocytes in a concentration-dependent manner. Antibody-mediated suppression of parasite development in the ILSDA at a 96-hour endpoint was more robust than the 24-hour endpoint.

CONCLUSIONS

ILSDA performance is improved by the use of cryopreserved primary human hepatocytes, expediting interactions between sporozoites and hepatocytes, and extending the assay endpoint.

The high immunogenicity induced by modified sporozoites' malarial peptides depends on their phi (ϕ) and psi (ψ) angles

The importance of CSP- and STARP-derived ϕ and ψ dihedral angles in mHABP structure was analysed by (1)H NMR in the search for molecules which can be included as components of a first-line-of-defence Plasmodium falciparum sporozoite multi-epitope vaccine against the most lethal form of human malaria. Most of the aforementioned dihedral angles were left-hand-like polyproline type II (PPII(L)) structures whilst others had right-hand-like α-helix (α(R)), thus allowing mHABPS to fit better into MHCII molecules and thereby form an appropriate pMHCII complex and also establish the H-bonds which stabilise such complex and by this means induce an appropriate immune response. This information has great implications for vaccine development, malaria being one of them.

A nonintegrative lentiviral vector-based vaccine provides long-term sterile protection against malaria

Trials testing the RTS,S candidate malaria vaccine and radiation-attenuated sporozoites (RAS) have shown that protective immunity against malaria can be induced and that an effective vaccine is not out of reach. However, longer-term protection and higher protection rates are required to eradicate malaria from the endemic regions. It implies that there is still a need to explore new vaccine strategies. Lentiviral vectors are very potent at inducing strong immunological memory. However their integrative status challenges their safety profile. Eliminating the integration step obviates the risk of insertional oncogenesis. Providing they confer sterile immunity, nonintegrative lentiviral vectors (NILV) hold promise as mass pediatric vaccine by meeting high safety standards. In this study, we have assessed the protective efficacy of NILV against malaria in a robust pre-clinical model. Mice were immunized with NILV encoding Plasmodium yoelii Circumsporozoite Protein (Py CSP) and challenged with sporozoites one month later. In two independent protective efficacy studies, 50% (37.5-62.5) of the animals were fully protected (p = 0.0072 and p = 0.0008 respectively when compared to naive mice). The remaining mice with detectable parasitized red blood cells exhibited a prolonged patency and reduced parasitemia. Moreover, protection was long-lasting with 42.8% sterile protection six months after the last immunization (p = 0.0042). Post-challenge CD8+ T cells to CSP, in contrast to anti-CSP antibodies, were associated with protection (r = -0.6615 and p = 0.0004 between the frequency of IFN-g secreting specific T cells in spleen and parasitemia). However, while NILV and RAS immunizations elicited comparable immunity to CSP, only RAS conferred 100% of sterile protection. Given that a better protection can be anticipated from a multi-antigen vaccine and an optimized vector design, NILV appear as a promising malaria vaccine.

3D structure and immunogenicity of Plasmodium falciparum sporozoite induced associated protein peptides as components of fully-protective anti-malarial vaccine

SIAP-1 and SIAP-2 are proteins which are implicated in early events involving Plasmodium falciparum infection of the Anopheles mosquito vector and the human host. High affinity HeLa and HepG2 cell binding conserved peptides have been previously identified in these proteins, i.e. SIAP-1 34893 ((421)KVQGLSYLLRRKNGTKHPVY(440)) and SIAP-1 34899 ((541)YVLNSKLLNSRSFDKFKWIQ(560)) and SIAP-2 36879 ((181)LLLYSTNSEDNLDISFGELQ(200)). When amino acid sequences have been properly modified (replacements shown in bold) they have induced high antibody titres against sporozoites in Aotus monkeys (assessed by IFA) and in the corresponding recombinant proteins (determined by ELISA and Western blot). (1)H NMR studies of these conserved native and modified high activity binding peptides (HABPs) revealed that all had α-helical structures in different locations and lengths. Conserved and corresponding modified HABPs displayed different lengths between the residues fitting into MHCII molecule pockets 1-9 and different amino acid orientation based on their different HLA-DRβ1(∗) binding motifs and binding registers, suggesting that such modifications were associated with making them immunogenic. The results suggested that these modified HAPBs could be potential targets for inclusion as components of a fully-effective, minimal sub-unit based, multi-epitope, and multistage anti-malarial vaccine.

Superior antimalarial immunity after vaccination with late liver stage-arresting genetically attenuated parasites

While subunit vaccines have shown partial efficacy in clinical trials, radiation-attenuated sporozoites (RAS) remain the "gold standard" for sterilizing protection against Plasmodium infection in human vaccinees. The variability in immunogenicity and replication introduced by the extensive, random DNA damage necessary to generate RAS could be overcome by genetically attenuated parasites (GAP) designed via gene deletion to arrest at defined points during liver-stage development. Here, we demonstrate the principle that late liver stage-arresting GAP induce larger and broader CD8 T cell responses that provide superior protection in inbred and outbred mice compared to RAS or early-arresting GAP immunizations. Late liver stage-arresting GAP also engender high levels of cross-stage and cross-species protection and complete protection when administered by translationally relevant intradermal or subcutaneous routes. Collectively, our results underscore the potential utility of late liver stage-arresting GAP as broadly protective next-generation live-attenuated malaria vaccines and support their potential as a powerful model for identifying antigens to generate cross-stage protection.

Cutting edge: attrition of Plasmodium-specific memory CD8 T cells results in decreased protection that is rescued by booster immunization

Sterile protection against infection with Plasmodium sporozoites requires high numbers of memory CD8 T cells. However, infections with unrelated pathogens, as may occur in areas endemic to malaria, can dramatically decrease pre-existing memory CD8 T cells. It remains unknown whether unrelated infections will compromise numbers of Plasmodium-specific memory CD8 T cells and thus limit the duration of antimalarial immunity generated by subunit vaccination. We show that P. berghei circumsporozoite-specific memory CD8 T cells underwent significant attrition in numbers in mice subjected to unrelated infections. Attrition was associated with preferential loss of effector memory CD8 T cells and reduced immunity to P. berghei sporozoite challenge. However, and of relevance to deployment of Plasmodium vaccines in areas endemic to malaria, attrition of memory CD8 T cells was reversed by booster immunization, which restored protection. These data suggest that regular booster immunizations may be required to sustain protective vaccine-induced Plasmodium-specific memory CD8 T cells in the face of attrition caused by unrelated infections.

Conservation and divergence of known apicomplexan transcriptional regulons

BACKGROUND

The apicomplexans are a diverse phylum of parasites causing an assortment of diseases including malaria in a wide variety of animals and lymphoproliferation in cattle. Little is known about how these varied parasites regulate their transcriptional regulons. Even less is known about how regulon systems, consisting of transcription factors and target genes together with their associated biological process, evolve in these diverse parasites.

RESULTS

In order to obtain insights into the differences in transcriptional regulation between these parasites we compared the orthology profiles of putative malaria transcription factors across species and examined the enrichment patterns of four binding sites across eleven apicomplexans. About three-fifths of the factors are broadly conserved in several phylogenetic orders of sequenced apicomplexans. This observation suggests the existence of regulons whose regulation is conserved across this ancient phylum. Transcription factors not broadly conserved across the phylum are possibly involved in regulon systems that have diverged between species. Examining binding site enrichment patterns in light of transcription factor conservation patterns suggests a second mode via which regulon systems may diverge - rewiring of existing transcription factors and their associated binding sites in specific ways. Integrating binding sites with transcription factor conservation patterns also facilitated prediction of putative regulators for one of the binding sites.

CONCLUSIONS

Even though transcription factors are underrepresented in apicomplexans, the distribution of these factors and their associated regulons reflect common and family-specific transcriptional regulatory processes.

Malaria: targeting parasite and host cell kinomes

Malaria still remains one of the deadliest infectious diseases, and has a tremendous morbidity and mortality impact in the developing world. The propensity of the parasites to develop drug resistance, and the relative reluctance of the pharmaceutical industry to invest massively in the developments of drugs that would offer only limited marketing prospects, are major issues in antimalarial drug discovery. Protein kinases (PKs) have become a major family of targets for drug discovery research in a number of disease contexts, which has generated considerable resources such as kinase-directed libraries and high throughput kinase inhibition assays. The phylogenetic distance between malaria parasites and their human host translates into important divergences in their respective kinomes, and most Plasmodium kinases display atypical properties (as compared to mammalian PKs) that can be exploited towards selective inhibition. Here, we discuss the taxon-specific kinases possessed by malaria parasites, and give an overview of target PKs that have been validated by reverse genetics, either in the human malaria parasite Plasmodium falciparum or in the rodent model Plasmodium berghei. We also briefly allude to the possibility of attacking Plasmodium through the inhibition of human PKs that are required for survival of this obligatory intracellular parasite, and which are targets for other human diseases.

Functional immunoassays using an in-vitro malaria liver-stage infection model: where do we go from here?

For more than 25 years, the ISI assay and ILSDA have been used to study the development of the malaria parasite in the liver, to discover and characterize sporozoite and liver-stage antigens, to support the development of malaria vaccine candidates, and to search for immunological correlates of protection in animals and in humans. Although both assays have been limited by low sporozoite invasion rates, significant biological variability, and the subjective nature of manually counting hepatocytes containing parasites as the read-out, they have nevertheless been useful tools for exploring parasite biology. This review describes the origin, application and current status of these assays, critically discusses the need for improvements, and explores the roles of these assays in supporting the development of an effective vaccine against Plasmodium falciparum malaria.

Calcium-dependent signaling and kinases in apicomplexan parasites

Calcium controls many critical events in the complex life cycles of apicomplexan parasites including protein secretion, motility, and development. Calcium levels are normally tightly regulated and rapid release of calcium into the cytosol activates a family of calcium-dependent protein kinases (CDPKs), which are normally characteristic of plants. CDPKs present in apicomplexans have acquired a number of unique domain structures likely reflecting their diverse functions. Calcium regulation in parasites is closely linked to signaling by cyclic nucleotides and their associated kinases. This Review summarizes the pivotal roles that calcium- and cyclic nucleotide-dependent kinases play in unique aspects of parasite biology.