In vitro production of infectious Plasmodium falciparum sporozoites.

An effective vaccine is needed for the prevention and elimination of malaria. The only immunogens that have been shown to have a protective efficacy of more than 90% against human malaria are Plasmodium falciparum (Pf) sporozoites (PfSPZ) manufactured in mosquitoes (mPfSPZ)1-7. The ability to produce PfSPZ in vitro (iPfSPZ) without mosquitoes would substantially enhance the production of PfSPZ vaccines and mosquito-stage malaria research, but this ability is lacking. Here we report the production of hundreds of millions of iPfSPZ. iPfSPZ invaded human hepatocytes in culture and developed to mature liver-stage schizonts expressing P. falciparum merozoite surface protein 1 (PfMSP1) in numbers comparable to mPfSPZ. When injected into FRGhuHep mice containing humanized livers, iPfSPZ invaded the human hepatocytes and developed to PfMSP1-expressing late liver stage parasites at 45% the quantity of cryopreserved mPfSPZ. Human blood from FRGhuHep mice infected with iPfSPZ produced asexual and sexual erythrocytic-stage parasites in culture, and gametocytes developed to PfSPZ when fed to mosquitoes, completing the P. falciparum life cycle from infectious gametocyte to infectious gametocyte without mosquitoes or primates.

Mosquito bite immunization with radiation-attenuated Plasmodium falciparum sporozoites: safety, tolerability, protective efficacy and humoral immunogenicity.

BACKGROUND: In this phase 1 clinical trial, healthy adult, malaria-naïve subjects were immunized with radiation-attenuated Plasmodium falciparum sporozoites (PfRAS) by mosquito bite and then underwent controlled human malaria infection (CHMI). The PfRAS model for immunization against malaria had previously induced >90 % sterile protection against homologous CHMI. This study was to further explore the safety, tolerability and protective efficacy of the PfRAS model and to provide biological specimens to characterize protective immune responses and identify protective antigens in support of malaria vaccine development. METHODS: Fifty-seven subjects were screened, 41 enrolled and 30 received at least one immunization. The true-immunized subjects received PfRAS via mosquito bite and the mock-immunized subjects received mosquito bites from irradiated uninfected mosquitoes. Sera and peripheral blood mononuclear cells (PBMCs) were collected before and after PfRAS immunizations. RESULTS: Immunization with PfRAS was generally safe and well tolerated, and repeated immunization via mosquito bite did not appear to increase the risk or severity of AEs. Local adverse events (AEs) of true-immunized and mock-immunized groups consisted of erythaema, papules, swelling, and induration and were consistent with reactions from mosquito bites seen in nature. Two subjects, one true- and one mock-immunized, developed large local reactions that completely resolved, were likely a result of mosquito salivary antigens, and were withdrawn from further participation as a safety precaution. Systemic AEs were generally rare and mild, consisting of headache, myalgia, nausea, and low-grade fevers. Two true-immunized subjects experienced fever, malaise, myalgia, nausea, and rigours approximately 16 h after immunization. These symptoms likely resulted from pre-formed antibodies interacting with mosquito salivary antigens. Ten subjects immunized with PfRAS underwent CHMI and five subjects (50 %) were sterilely protected and there was a significant delay to parasitaemia in the other five subjects. All ten subjects developed humoral immune responses to whole sporozoites and to the circumsporozoite protein prior to CHMI, although the differences between protected and non-protected subjects were not statistically significant for this small sample size. CONCLUSIONS: The protective efficacy of this clinical trial (50 %) was notably less than previously reported (>90 %). This may be related to differences in host genetics or the inherent variability in mosquito biting behavior and numbers of sporozoites injected. Differences in trial procedures, such as the use of leukapheresis prior to CHMI and of a longer interval between the final immunization and CHMI in these subjects compared to earlier trials, may also have reduced protective efficacy. This trial has been retrospectively registered at ISRCTN ID 17372582, May 31, 2016.

TAP-mediated processing of exoerythrocytic antigens is essential for protection induced with radiation-attenuated Plasmodium sporozoites.

MHC class I dependent CD8(+) T cells are essential for protection induced by radiation-attenuated Plasmodium sporozoites (RAS) in murine malaria models. Apart from the mechanism of activation of CD8(+) T cells specific for the circumsporozoite protein, the major sporozoite antigen (Ag), CD8(+) T cells specific for other exoerythrocytic Ags that have been shown to mediate protection have not been thoroughly investigated. Specifically, mechanisms of processing and presentation of exoerythrocytic Ags, which includes liver stage (LS) Ags, remain poorly understood. We hypothesize that as exogenous proteins, LS Ags are processed by mechanisms involving either the TAP-dependent phagosomal-to-cytosol or TAP-independent vacuolar pathway of cross-presentation. We used TAP-deficient mice to investigate whether LS Ag mediated induction of naïve CD8(+) T cells and their recall during sporozoite challenge occur by the TAP-dependent or TAP-independent pathways. On the basis of functional attributes, CD8(+) T cells were activated via the TAP-independent pathway during immunizations with Plasmodium berghei RAS; however, IFN-γ(+) CD8(+) T cells previously induced by P. berghei RAS in TAP-deficient mice failed to be recalled against sporozoite challenge and the mice became parasitemic. On the basis of these observations, we propose that TAP-associated Ag processing is indispensable for sterile protection induced with P. berghei RAS.

First-in-human evaluation of genetically attenuated Plasmodium falciparum sporozoites administered by bite of Anopheles mosquitoes to adult volunteers.

BACKGROUND: Immunization with genetically engineered, attenuated malaria parasites (GAP) that arrest during liver infection confers sterile protection in mouse malaria models. A first generation Plasmodium falciparum GAP (Pf p52(-)/p36(-) GAP) was previously generated by deletion of two pre-erythrocytic stage-expressed genes (P52 and P36) in the NF54 strain. METHODS: A first-in-human, proof-of-concept, safety and immunogenicity clinical trial in six human volunteers was conducted. Exposure consisted of delivery of Pf p52(-)/p36(-) GAP sporozoites via infected Anopheles mosquito bite with a five-bite/volunteer exposure followed by an approximately 200-bite exposure/volunteer one month later. RESULTS: The exposures were well tolerated with mild to moderate local and systemic reactions. All volunteers remained blood stage negative after low dose exposure. Five volunteers remained blood stage negative after high dose exposure. One volunteer developed peripheral parasitemia twelve days after high dose exposure. Together the findings indicate that Pf p52(-)/p36(-) GAP was severely but not completely attenuated. All six volunteers developed antibodies to CSP. Furthermore, IFN-γ responses to whole sporozoites and multiple antigens were elicited in 5 of 6 volunteers, with both CD4 and CD8 cell cytokine production detected. CONCLUSION: Severe attenuation and favorable immune responses following administration of a first generation Pf p52(-)/p36(-) GAP suggests that further development of live-attenuated strains using genetic engineering should be pursued.

The effect of antiretrovirals on Plasmodium falciparum liver stages.

HIV and malaria overlap geographically, but the full impact of different antiretrovirals on malaria remains poorly understood. We examined the antimalarial activity of the HIV protease inhibitors lopinavir and saquinavir and the non-nucleoside reverse transcriptase inhibitor nevirapine on Plasmodium falciparum liver stages. Our results demonstrate that the HIV PI lopinavir inhibits liver stage parasites at clinically relevant concentrations, that is, at drug levels achieved in HIV-infected patients on standard dosing regimens. Because drugs that inhibit liver stages target parasites when they are present in lower numbers, these results might have implications for eradication efforts.

Transgenic parasites stably expressing full-length Plasmodium falciparum circumsporozoite protein as a model for vaccine down-selection in mice using sterile protection as an endpoint.

Circumsporozoite protein (CSP) of Plasmodium falciparum is a protective human malaria vaccine candidate. There is an urgent need for models that can rapidly down-select novel CSP-based vaccine candidates. In the present study, the mouse-mosquito transmission cycle of a transgenic Plasmodium berghei malaria parasite stably expressing a functional full-length P. falciparum CSP was optimized to consistently produce infective sporozoites for protection studies. A minimal sporozoite challenge dose was established, and protection was defined as the absence of blood-stage parasites 14 days after intravenous challenge. The specificity of protection was confirmed by vaccinating mice with multiple CSP constructs of differing lengths and compositions. Constructs that induced high NANP repeat-specific antibody titers in enzyme-linked immunosorbent assays were protective, and the degree of protection was dependent on the antigen dose. There was a positive correlation between antibody avidity and protection. The antibodies in the protected mice recognized the native CSP on the parasites and showed sporozoite invasion inhibitory activity. Passive transfer of anti-CSP antibodies into naive mice also induced protection. Thus, we have demonstrated the utility of a mouse efficacy model to down-select human CSP-based vaccine formulations.

Quantification of sporozoite invasion, migration, and development by microscopy and flow cytometry.

There is an important role for in vitro assays to better understand the initial steps of malaria infection. In this section, we describe both microscopy-based and flow cytometry-based sporozoite invasion, migration and development assays with the rodent malaria parasites, Plasmodium berghei and Plasmodium yoelii, and the human malaria parasite, Plasmodium falciparum.

Clinical trial in healthy malaria-naïve adults to evaluate the safety, tolerability, immunogenicity and efficacy of MuStDO5, a five-gene, sporozoite/hepatic stage Plasmodium falciparum DNA vaccine combined with escalating dose human GM-CSF DNA.

When introduced in the 1990s, immunization with DNA plasmids was considered potentially revolutionary for vaccine development, particularly for vaccines intended to induce protective CD8 T cell responses against multiple antigens. We conducted, in 1997-1998, the first clinical trial in healthy humans of a DNA vaccine, a single plasmid encoding Plasmodium falciparum circumsporozoite protein (PfCSP), as an initial step toward developing a multi-antigen malaria vaccine targeting the liver stages of the parasite. As the next step, we conducted in 2000-2001 a clinical trial of a five-plasmid mixture called MuStDO5 encoding pre-erythrocytic antigens PfCSP, PfSSP2/TRAP, PfEXP1, PfLSA1 and PfLSA3. Thirty-two, malaria-naïve, adult volunteers were enrolled sequentially into four cohorts receiving a mixture of 500 µg of each plasmid plus escalating doses (0, 20, 100 or 500 µg) of a sixth plasmid encoding human granulocyte macrophage-colony stimulating factor (hGM-CSF). Three doses of each formulation were administered intramuscularly by needle-less jet injection at 0, 4 and 8 weeks, and each cohort had controlled human malaria infection administered by five mosquito bites 18 d later. The vaccine was safe and well-tolerated, inducing moderate antigen-specific, MHC-restricted T cell interferon-γ responses but no antibodies. Although no volunteers were protected, T cell responses were boosted post malaria challenge. This trial demonstrated the MuStDO5 DNA and hGM-CSF plasmids to be safe and modestly immunogenic for T cell responses. It also laid the foundation for priming with DNA plasmids and boosting with recombinant viruses, an approach known for nearly 15 y to enhance the immunogenicity and protective efficacy of DNA vaccines.

HIV nonnucleoside reverse transcriptase inhibitors and trimethoprim-sulfamethoxazole inhibit plasmodium liver stages.

BACKGROUND: Although nonnucleoside reverse transcriptase inhibitors (NNRTIs) are usually part of first-line treatment regimens for human immunodeficiency virus (HIV), their activity on Plasmodium liver stages remains unexplored. Additionally, trimethoprim-sulfamethoxazole (TMP-SMX), used for opportunistic infection prophylaxis in HIV-exposed infants and HIV-infected patients, reduces clinical episodes of malaria; however, TMP-SMX effect on Plasmodium liver stages requires further study. METHODS: We characterized NNRTI and TMP-SMX effects on Plasmodium liver stages in vivo using Plasmodium yoelii. On the basis of these results, we conducted in vitro studies assessing TMP-SMX effects on the rodent parasites P. yoelii and Plasmodium berghei and on the human malaria parasite Plasmodium falciparum. RESULTS: Our data showed NNRTI treatment modestly reduced P. yoelii liver stage parasite burden and minimally extended prepatent period. TMP-SMX administration significantly reduced liver stage parasite burden, preventing development of patent parasitemia in vivo. TMP-SMX inhibited development of rodent and P. falciparum liver stage parasites in vitro. CONCLUSIONS: NNRTIs modestly affect liver stage Plasmodium parasites, whereas TMP-SMX prevents patent parasitemia. Because drugs that inhibit liver stages target parasites when they are present in lower numbers, these results may have implications for eradication efforts. Understanding HIV drug effects on Plasmodium liver stages will aid in optimizing treatment regimens for HIV-exposed and HIV-infected infected patients in malaria-endemic areas.

Early transcriptional responses of HepG2-A16 liver cells to infection by Plasmodium falciparum sporozoites.

Invasion of hepatocytes by Plasmodium sporozoites deposited by Anopheles mosquitoes, and their subsequent transformation into infective merozoites is an obligatory step in the initiation of malaria. Interactions between the sporozoites and hepatocytes lead to a distinct, complex and coordinated cellular and systemic host response. Little is known about host liver cell response to sporozoite invasion, or whether it is primarily adaptive for the parasite, for the host, or for both. Our present study used gene expression profiling of human HepG2-A16 liver cells infected with Plasmodium falciparum sporozoites to understand the host early cellular events and factors influencing parasite infectivity and sporozoite development. Our results show that as early as 30 min following wild-type, non-irradiated sporozoite exposure, the expressions of at least 742 genes was selectively altered. These genes regulate diverse biological functions, such as immune processes, cell adhesion and communications, metabolism pathways, cell cycle regulation, and signal transduction. These functions reflect cellular events consistent with initial host cell defense responses, as well as alterations in host cells to sustain sporozoites growth and survival. Irradiated sporozoites gave very similar gene expression pattern changes, but direct comparative analysis between liver gene expression profiles caused by irradiated and non-irradiated sporozoites identified 29 genes, including glypican-3, that were specifically up-regulated only in irradiated sporozoites. Elucidating the role of this subset of genes may help identify the molecular basis for the irradiated sporozoites inability to develop intrahepatically, and their usefulness as an immunogen for developing protective immunity against pre-erythrocytic stage malaria.

Disruption of the Plasmodium falciparum liver-stage antigen-1 locus causes a differentiation defect in late liver-stage parasites.

The malaria parasite Plasmodium falciparum infects humans and first targets the liver where liver-stage parasites undergo pre-erythrocytic replication. Liver-stage antigen-1 (LSA-1) is currently the only identified P. falciparum protein for which expression is restricted to liver stages. Yet, the importance of LSA-1 for liver-stage parasite development remains unknown. Here we deleted LSA-1 in the NF54 strain of P. falciparum and analysed the lsa-1(-) parasites throughout their life cycle. lsa-1(-) sporozoites had normal gliding motility and invasion into hepatocytes. Six days after infection of a hepatocytic cell line, lsa-1(-) parasites exhibited a moderate phenotype with an ~50% reduction of late liver-stage forms when compared with wild type. Strikingly, lsa-1(-) parasites growing in SCID/Alb-uPA mice with humanized livers showed a severe defect in late liver-stage differentiation and exo-erythrocytic merozoite formation 7 days after infection, a time point when wild-type parasites develop into mature merozoites. The lsa-1(-) parasites also showed aberrant liver-stage expression of key parasite proteins apical membrane antigen-1 and circumsporozoite protein. Our data show that LSA-1 plays a critical role during late liver-stage schizogony and is thus important in the parasite transition from the liver to blood. LSA-1 is the first P. falciparum protein identified to be required for this transitional stage of the parasite life cycle.

Multiple antigen peptide vaccines against Plasmodium falciparum malaria.

The multiple antigen peptide (MAP) approach is an effective method to chemically synthesize and deliver multiple T-cell and B-cell epitopes as the constituents of a single immunogen. Here we report on the design, chemical synthesis, and immunogenicity of three Plasmodium falciparum MAP vaccines that incorporated antigenic epitopes from the sporozoite, liver, and blood stages of the life cycle. Antibody and cellular responses were determined in three inbred (C57BL/6, BALB/c, and A/J) strains, one congenic (HLA-A2 on the C57BL/6 background) strain, and one outbred strain (CD1) of mice. All three MAPs were immunogenic and induced both antibody and cellular responses, albeit in a somewhat genetically restricted manner. Antibodies against MAP-1, MAP-2, and MAP-3 had an antiparasite effect that was also dependent on the mouse major histocompatibility complex background. Anti-MAP-1 (CSP-based) antibodies blocked the invasion of HepG2 liver cells by P. falciparum sporozoites (highest, 95.16% in HLA-A2 C57BL/6; lowest, 11.21% in BALB/c). Furthermore, antibodies generated following immunizations with the MAP-2 (PfCSP, PfLSA-1, PfMSP-1(42), and PfMSP-3b) and MAP-3 (PfRAP-1, PfRAP-2, PfSERA, and PfMSP-1(42)) vaccines were able to reduce the growth of blood stage parasites in erythrocyte cultures to various degrees. Thus, MAP-based vaccines remain a viable option to induce effective antibody and cellular responses. These results warrant further development and preclinical and clinical testing of the next generation of candidate MAP vaccines that are based on the conserved protective epitopes from Plasmodium antigens that are widely recognized by populations of divergent HLA types from around the world.

Immunization with pre-erythrocytic antigen CelTOS from Plasmodium falciparum elicits cross-species protection against heterologous challenge with Plasmodium berghei.

BACKGROUND: The Plasmodium protein Cell-traversal protein for ookinetes and sporozoites (CelTOS) plays an important role in cell traversal of host cells in both, mosquito and vertebrates, and is required for successful malaria infections. CelTOS is highly conserved among the Plasmodium species, suggesting an important functional role across all species. Therefore, targeting the immune response to this highly conserved protein and thus potentially interfering with its biological function may result in protection against infection even by heterologous species of Plasmodium. METHODOLOGY/PRINCIPAL FINDINGS: To test this hypothesis, we developed a recombinant codon-harmonized P. falciparum CelTOS protein that can be produced to high yields in the E. coli expression system. Inbred Balb/c and outbred CD-1 mice were immunized with various doses of the recombinant protein adjuvanted with Montanide ISA 720 and characterized using in vitro and in vivo analyses. CONCLUSIONS/SIGNIFICANCE: Immunization with PfCelTOS resulted in potent humoral and cellular immune responses and most importantly induced sterile protection against a heterologous challenge with P. berghei sporozoites in a proportion of both inbred and outbred mice. The biological activity of CelTOS-specific antibodies against the malaria parasite is likely linked to the impairment of sporozoite motility and hepatocyte infectivity. The results underscore the potential of this antigen as a pre-erythrocytic vaccine candidate and demonstrate for the first time a malaria vaccine that is cross-protective between species.

Characterization of immunoglobulin G antibodies to Plasmodium falciparum sporozoite surface antigen MB2 in malaria exposed individuals.

BACKGROUND: MB2 protein is a sporozoite surface antigen on the human malaria parasite Plasmodium falciparum. MB2 was identified by screening a P. falciparum sporozoite cDNA expression library using immune sera from a protected donor immunized via the bites of P. falciparum-infected irradiated mosquitoes. It is not known whether natural exposure to P. falciparum also induces the anti-MB2 response and if this response differs from that in protected individuals immunized via the bites of P. falciparum infected irradiated mosquitoes. The anti-MB2 antibody response may be part of a robust protective response against the sporozoite. METHODS: Fragments of polypeptide regions of MB2 were constructed as recombinant fusions sandwiched between glutathione S-transferase and a hexa histidine tag for bacterial expression. The hexa histidine tag affinity purified proteins were used to immunize rabbits and the polyclonal sera evaluated in an in vitro inhibition of sporozoite invasion assay. The proteins were also used in immunoblots with sera from a limited number of donors immunized via the bites of P. falciparum infected irradiated mosquitoes and plasma and serum obtained from naturally exposed individuals in Kenya. RESULTS: Rabbit polyclonal antibodies targeting the non-repeat region of the basic domain of MB2 inhibited sporozoites entry into HepG2-A16 cells in vitro. Analysis of serum from five human volunteers that were immunized via the bites of P. falciparum infected irradiated mosquitoes that developed immunity and were completely protected against subsequent challenge with non-irradiated parasite also had detectable levels of antibody against MB2 basic domain. In contrast, in three volunteers not protected, anti-MB2 antibodies were below the level of detection. Sera from protected volunteers preferentially recognized a non-repeat region of the basic domain of MB2, whereas plasma from naturally-infected individuals also had antibodies that recognize regions of MB2 that contain a repeat motif in immunoblots. Sequence analysis of eleven field isolates and four laboratory strains showed that these antigenic regions of the basic domain of the MB2 gene are highly conserved in parasites obtained from different parts of the world. Moreover, anti-MB2 antibodies also were detected in the plasma of 83% of the individuals living in a malaria endemic area of Kenya (n = 41). CONCLUSION: A preliminary analysis of the human humoral response against MB2 indicates that it may be an additional highly conserved target for immune intervention at the pre-erythrocytic stage of P. falciparum life cycle.

Preerythrocytic, live-attenuated Plasmodium falciparum vaccine candidates by design.

Falciparum malaria is initiated when Anopheles mosquitoes transmit the Plasmodium sporozoite stage during a blood meal. Irradiated sporozoites confer sterile protection against subsequent malaria infection in animal models and humans. This level of protection is unmatched by current recombinant malaria vaccines. However, the live-attenuated vaccine approach faces formidable obstacles, including development of accurate, reproducible attenuation techniques. We tested whether Plasmodium falciparum could be attenuated at the early liver stage by genetic engineering. The P. falciparum genetically attenuated parasites (GAPs) harbor individual deletions or simultaneous deletions of the sporozoite-expressed genes P52 and P36. Gene deletions were done by double-cross-over recombination to avoid genetic reversion of the knockout parasites. The gene deletions did not affect parasite replication throughout the erythrocytic cycle, gametocyte production, mosquito infections, and sporozoite production rates. However, the deletions caused parasite developmental arrest during hepatocyte infection. The double-gene deletion line exhibited a more severe intrahepatocytic growth defect compared with the single-gene deletion lines, and it did not persist. This defect was assessed in an in vitro liver-stage growth assay and in a chimeric mouse model harboring human hepatocytes. The strong phenotype of the double knockout GAP justifies its human testing as a whole-organism vaccine candidate using the established sporozoite challenge model. GAPs might provide a safe and reproducible platform to develop an efficacious whole-cell malaria vaccine that prevents infection at the preerythrocytic stage.

Distinct malaria parasite sporozoites reveal transcriptional changes that cause differential tissue infection competence in the mosquito vector and mammalian host.

The malaria parasite sporozoite transmission stage develops and differentiates within parasite oocysts on the Anopheles mosquito midgut. Successful inoculation of the parasite into a mammalian host is critically dependent on the sporozoite's ability to first infect the mosquito salivary glands. Remarkable changes in tissue infection competence are observed as the sporozoites transit from the midgut oocysts to the salivary glands. Our microarray analysis shows that compared to oocyst sporozoites, salivary gland sporozoites upregulate expression of at least 124 unique genes. Conversely, oocyst sporozoites show upregulation of at least 47 genes (upregulated in oocyst sporozoites [UOS genes]) before they infect the salivary glands. Targeted gene deletion of UOS3, encoding a putative transmembrane protein with a thrombospondin repeat that localizes to the sporozoite secretory organelles, rendered oocyst sporozoites unable to infect the mosquito salivary glands but maintained the parasites' liver infection competence. This phenotype demonstrates the significance of differential UOS expression. Thus, the UIS-UOS gene classification provides a framework to elucidate the infectivity and transmission success of Plasmodium sporozoites on a whole-genome scale. Genes identified herein might represent targets for vector-based transmission blocking strategies (UOS genes), as well as strategies that prevent mammalian host infection (UIS genes).

Safety and clinical outcome of experimental challenge of human volunteers with Plasmodium falciparum-infected mosquitoes: an update.

BACKGROUND: Challenge of volunteers by the bites of membrane-fed anopheline mosquitoes infected with Plasmodium falciparum was reported in 1986. In 1997, an analysis of experience with 118 volunteers indicated that mosquito inoculation of P. falciparum could be a safe, well-tolerated, reproducible, and efficient method of challenge. METHODS: We reviewed the records of 47 volunteers challenged at our institution with the NF54 isolate of P. falciparum between 1998 and 2002. We also reviewed data from 17 published studies of experimental challenge conducted since 1996. RESULTS: At our institution, the time to onset of first symptoms (incubation period) was 8.9 days, and the time to first detectable parasitemia on blood smear (prepatent period) was 10.5 days. All volunteers became symptomatic. Most symptoms were mild to moderate, although 21% of volunteers had at least 1 severe symptom. None developed complicated or severe malaria, and all were cured. Laboratory assessments demonstrated modest, short-term abnormalities typical of malaria. Review of 17 published studies demonstrated that an additional 367 volunteers received experimental challenge safely with similar outcomes. CONCLUSIONS: In total, data from 532 volunteers demonstrate that experimental challenge is safe and results in predictable incubation and prepatent periods. Our findings support the continued use of this method for testing efficacy of vaccines and drugs against P. falciparum.

Development, characterization and immunogenicity of a multi-stage, multi-valent Plasmodium falciparum vaccine antigen (FALVAC-1A) expressed in Escherichia coli.

A synthetic multistage, multi-epitope Plasmodium falciparum malaria antigen (FALVAC-1A) was designed and evaluated in silico, and then the gene was constructed and expressed in Escherichia coli. The FALVAC-1A protein was purified by inclusion body isolation, followed by affinity and ion exchange chromatography. Although FALVAC-1A was a synthetic antigen, it folded to a specific, but as yet incompletely defined, molecular conformation that was stable and comparable from lot to lot. When formulated with four different adjuvants, FALVAC-1A was highly immunogenic in rabbits, inducing not only ELISA reactivity to the cognate antigen and most of its component epitopes, but also in vitro activity against P. falciparum parasites as demonstrated by inhibition of sporozoite invasion, antibody dependent cellular inhibition and the immunofluorescence assay.

The Plasmodium falciparum sexual development transcriptome: a microarray analysis using ontology-based pattern identification.

The sexual stages of malarial parasites are essential for the mosquito transmission of the disease and therefore are the focus of transmission-blocking drug and vaccine development. In order to better understand genes important to the sexual development process, the transcriptomes of high-purity stage I-V Plasmodium falciparum gametocytes were comprehensively profiled using a full-genome high-density oligonucleotide microarray. The interpretation of this transcriptional data was aided by applying a novel knowledge-based data-mining algorithm termed ontology-based pattern identification (OPI) using current information regarding known sexual stage genes as a guide. This analysis resulted in the identification of a sexual development cluster containing 246 genes, of which approximately 75% were hypothetical, exhibiting highly-correlated, gametocyte-specific expression patterns. Inspection of the upstream promoter regions of these 246 genes revealed putative cis-regulatory elements for sexual development transcriptional control mechanisms. Furthermore, OPI analysis was extended using current annotations provided by the Gene Ontology Consortium to identify 380 statistically significant clusters containing genes with expression patterns characteristic of various biological processes, cellular components, and molecular functions. Collectively, these results, available as part of a web-accessible OPI database (http://carrier.gnf.org/publications/Gametocyte), shed light on the components of molecular mechanisms underlying parasite sexual development and other areas of malarial parasite biology.

An immunologically cryptic epitope of Plasmodium falciparum circumsporozoite protein facilitates liver cell recognition and induces protective antibodies that block liver cell invasion.

Circumsporozoite, a predominant surface protein, is involved in invasion of liver cells by Plasmodium sporozoites, which leads to malaria. We have previously reported that the amino terminus region (amino acids 27-117) of P. falciparum circumsporozoite protein plays a critical role in the invasion of liver cells by the parasite. Here we show that invasion-blocking antibodies are induced by a polypeptide encoding these 91 amino acids, only when it is presented in the absence of the rest of the protein. This suggests that when present in the whole protein, the amino terminus remains immunologically cryptic. A single reactive epitope was identified and mapped to a stretch of 21 amino acids from position 93 to 113. The epitope is configurational in nature, since its recognition was affected by deleting as little as 3 amino acids from either end of the 21-residue peptide. Lysine 104, the only known polymorphic position in the epitope, affected its recognition by the antibodies, and its conversion to leucine in the protein led to a substantial loss of binding activity of the protein to the hepatocytes. This indicated that in the protein, the epitope serves as a binding ligand and facilitates the interaction between sporozoite and hepatic cells. When considered along with the observation that in its native state this motif is immunologically unresponsive, we suggest that hiding functional moieties of the protein from the immune system is an evasion strategy to preserve liver cell binding function and may be of importance in designing anti-sporozoite vaccines.