Vaccine Containing the Three Allelic Variants of the Plasmodium vivax Circumsporozoite Antigen Induces Protection in Mice after Challenge with a Transgenic Rodent Malaria Parasite

Plasmodium vivax is the most common species that cause malaria outside of the African continent. The development of an efficacious vaccine would contribute greatly to control malaria. Recently, using bacterial and adenoviral recombinant proteins based on the P. vivax circumsporozoite protein (CSP), we demonstrated the possibility of eliciting strong antibody-mediated immune responses to each of the three allelic forms of P. vivax CSP (PvCSP). In the present study, recombinant proteins representing the PvCSP alleles (VK210, VK247, and P. vivax-like), as well as a hybrid polypeptide, named PvCSP-All epitopes, were generated. This hybrid containing the conserved C-terminal of the PvCSP and the three variant repeat domains in tandem were successfully produced in the yeast Pichia pastoris. After purification and biochemical characterization, they were used for the experimental immunization of C57BL/6 mice in a vaccine formulation containing the adjuvant Poly(I:C). Immunization with a recombinant protein expressing all three different allelic forms in fusion elicited high IgG antibody titers reacting with all three different allelic variants of PvCSP. The antibodies targeted both the C-terminal and repeat domains of PvCSP and recognized the native protein on the surface of P. vivax sporozoites. More importantly, mice that received the vaccine formulation were protected after challenge with chimeric Plasmodium berghei sporozoites expressing CSP repeats of P. vivax sporozoites (Pb/PvVK210). Our results suggest that it is possible to elicit protective immunity against one of the most common PvCSP alleles using soluble recombinant proteins expressed by P. pastoris. These recombinant proteins are promising candidates for clinical trials aiming to develop a multiallele vaccine against P. vivax malaria.

UIS2: A Unique Phosphatase Required for the Development of Plasmodium Liver Stages

Plasmodium salivary sporozoites are the infectious form of the malaria parasite and are dormant inside salivary glands of Anopheles mosquitoes. During dormancy, protein translation is inhibited by the kinase UIS1 that phosphorylates serine 59 in the eukaryotic initiation factor 2α (eIF2α). De-phosphorylation of eIF2α-P is required for the transformation of sporozoites into the liver stage. In mammalian cells, the de-phosphorylation of eIF2α-P is mediated by the protein phosphatase 1 (PP1). Using a series of genetically knockout parasites we showed that in malaria sporozoites, contrary to mammalian cells, the eIF2α-P phosphatase is a member of the PP2C/PPM phosphatase family termed UIS2. We found that eIF2α was highly phosphorylated in uis2 conditional knockout sporozoites. These mutant sporozoites maintained the crescent shape after delivery into mammalian host and lost their infectivity. Both uis1 and uis2 were highly transcribed in the salivary gland sporozoites but uis2 expression was inhibited by the Pumilio protein Puf2. The repression of uis2 expression was alleviated when sporozoites developed into liver stage. While most eukaryotic phosphatases interact transiently with their substrates, UIS2 stably bound to phosphorylated eIF2α, raising the possibility that high-throughput searches may identify chemicals that disrupt this interaction and prevent malaria infection.

Malaria is transmitted to humans by female mosquitoes as they take a blood meal. Plasmodium sporozoites are the infectious and quiescent forms of malaria parasites, which reside in the salivary glands of mosquitoes. Global protein synthesis is inhibited in sporozoites through phosphorylation of the translational factor eIF2α. However, the development of the parasites in the host liver requires de-phosphorylation of eIF2α-P. We find that a unique Plasmodium phosphatase termed UIS2 de-phosphorylates eIF2α-P in malaria. The eIF2α is highly phosphorylated in the uis2 mutant sporozoites. The uis2 mutant parasites did not change their morphology after delivery into the host and could not properly infect the host. We also showed that UIS2 expression was inhibited by the Pumilio protein Puf2. However, this repression was relieved when sporozoites developed into liver stage. In sum, our findings revealed a new mechanism that evolved to control eIF2α dephosphorylation and suggest that identification of UIS2 inhibitors may be useful in anti-malaria therapy.

Human immune system mice immunized with Plasmodium falciparum circumsporozoite protein induce protective human humoral immunity against malaria

In this study, we developed human immune system (HIS) mice that possess functional human CD4+ T cells and B cells, named HIS-CD4/B mice. HIS-CD4/B mice were generated by first introducing HLA class II genes, including DR1 and DR4, along with genes encoding various human cytokines and human B cell activation factor (BAFF) to NSG mice by adeno-associated virus serotype 9 (AAV9) vectors, followed by engrafting human hematopoietic stem cells (HSCs). HIS-CD4/B mice, in which the reconstitution of human CD4+ T and B cells resembles to that of humans, produced a significant level of human IgG against Plasmodium falciparum circumsporozoite (PfCS) protein upon immunization. CD4+ T cells in HIS-CD4/B mice, which possess central and effector memory phenotypes like those in humans, are functional, since PfCS protein-specific human CD4+ T cells secreting IFN-γ and IL-2 were detected in immunized HIS-CD4/B mice. Lastly, PfCS protein-immunized HIS-CD4/B mice were protected from in vivo challenge with transgenic P. berghei sporozoites expressing the PfCS protein. The immune sera collected from protected HIS-CD4/B mice reacted against transgenic P. berghei sporozoites expressing the PfCS protein and also inhibited the parasite invasion into hepatocytes in vitro. Taken together, these studies show that our HIS-CD4/B mice could mount protective human anti-malaria immunity, consisting of human IgG and human CD4+ T cell responses both specific for a human malaria antigen.

Inhibition by stabilization: targeting the Plasmodium falciparum aldolase-TRAP complex

BACKGROUND

Emerging resistance of the malaria parasite Plasmodium to current therapies underscores the critical importance of exploring novel strategies for disease eradication. Plasmodium species are obligate intracellular protozoan parasites. They rely on an unusual form of substrate-dependent motility for their migration on and across host-cell membranes and for host cell invasion. This peculiar motility mechanism is driven by the 'glideosome', an actin-myosin associated, macromolecular complex anchored to the inner membrane complex of the parasite. Myosin A, actin, aldolase, and thrombospondin-related anonymous protein (TRAP) constitute the molecular core of the glideosome in the sporozoite, the mosquito stage that brings the infection into mammals.

METHODS

Virtual library screening of a large compound library against the PfAldolase-TRAP complex was used to identify candidate compounds that stabilize and prevent the disassembly of the glideosome. The mechanism of these compounds was confirmed by biochemical, biophysical and parasitological methods.

RESULTS

A novel inhibitory effect on the parasite was achieved by stabilizing a protein-protein interaction within the glideosome components. Compound 24 disrupts the gliding and invasive capabilities of Plasmodium parasites in in vitro parasite assays. A high-resolution, ternary X-ray crystal structure of PfAldolase-TRAP in complex with compound 24 confirms the mode of interaction and serves as a platform for future ligand optimization.

CONCLUSION

This proof-of-concept study presents a novel approach to anti-malarial drug discovery and design. By strengthening a protein-protein interaction within the parasite, an avenue towards inhibiting a previously "undruggable" target is revealed and the motility motor responsible for successful invasion of host cells is rendered inactive. This study provides new insights into the malaria parasite cell invasion machinery and convincingly demonstrates that liver cell invasion is dramatically reduced by 95 % in the presence of the small molecule stabilizer compound 24.

Self-assembled peptide nanofibers raising durable antibody responses against a malaria epitope

Biomaterials that modulate innate and adaptive immune responses are receiving increasing interest as adjuvants for eliciting protective immunity against a variety of diseases. Previous results have indicated that self-assembling β-sheet peptides, when fused with short peptide epitopes, can act as effective adjuvants and elicit robust and long-lived antibody responses. Here we investigated the mechanism of immunogenicity and the quality of antibody responses raised by a peptide epitope from Plasmodium falciparum circumsporozoite (CS) protein, (NANP)(3),conjugated to the self-assembling peptide domain Q11. The mechanism of adjuvant action was investigated in knockout mice with impaired MyD88, NALP3, TLR-2, or TLR-5 function, and the quality of antibodies raised against (NANP)(3)-Q11 was assessed using a transgenic sporozoite neutralizing (TSN) assay for malaria infection. (NANP)(3)-Q11 self-assembled into nanofibers, and antibody responses lasted up to 40 weeks in C57BL/6 mice. The antibody responses were T cell- and MyD88-dependent. Sera from mice primed with either irradiated sporozoites or a synthetic peptide, (T1BT*)(4)-P3C, and boosted with (NANP)(3)-Q11 showed significant increases in antibody titers and significant inhibition of sporozoite infection in TSN assays. In addition, two different epitopes could be self-assembled together without compromising the strength or duration of the antibody responses raised against either of them, making these materials promising platforms for self-adjuvanting multi-antigenic immunotherapies.

Antibodies to Plasmodium circumsporozoite protein (CSP) inhibit sporozoite's cell traversal activity

Plasmodium sporozoites are deposited in the skin of the mammalian host by Anopheles mosquitoes. To continue the life cycle, the sporozoites have to invade the host's hepatocytes, where they transform into exoerythrocytic forms (EEFs) inside a parasitophorous vacuole. During their route from the skin to the liver, the parasites traverse the capillary epithelium in the dermis to enter the blood circulation, and cross the endothelium of liver sinusoids to enter the parenchyma. Cell traversal by sporozoites is usually measured by quantifying dyes that enter or are released from cells during incubation with salivary gland sporozoites. These methods do not distinguish cell traversal from cell wounding. Here we validate an assay that quantifies cell traversal of sporozoites through monolayers of MDCK cells that form tight junctions. We compared cell traversal of wt sporozoites and of parasites lacking the Type I membrane protein TLP (TRAP-like protein) previously implicated in cell traversal. We provide direct evidence that TLP ko sporozoites are defective in cell traversal and that they are retained inside the MDCK cytoplasm. We then used the MDCK assay to study the effect of a monoclonal antibody (3D11) to the circumsporozoite protein (CSP) on the parasite's cell traversal. We show that 3D11 inhibits cell traversal at nanomolar concentrations. We conclude that antibodies elicited by CSP-based vaccines are likely to inhibit the migration of sporozoites from the skin to the liver.

Identification of non-CSP antigens bearing CD8 epitopes in mice immunized with irradiated sporozoites

Immunization of BALB/c mice with irradiated sporozoites (IrSp) of Plasmodium yoelii can lead to sterile immunity. The circumsporozoite protein (CSP) plays a dominant role in protection. Nevertheless after hyper-immunization with IrSp, complete protection is obtained in CSP-transgenic BALB/c mice that are T-cell tolerant to the CSP and cannot produce antibodies [CSP-Tg/JhT(-/-)]. This protection is mediated exclusively by CD8(+) T cells [1]. To identify the non-CSP protective T cell antigens, we studied the properties of 34 P. yoelii sporozoite antigens that are predicted to be secreted and to contain strong Kd-restricted CD8(+) T cell epitopes. The synthetic peptides corresponding to the epitopes were used to screen for the presence of peptide-specific CD8(+) T cells secreting interferon-γ (IFN-γ) in splenocytes from CSP-Tg/JhT(-/-) BALB/c mice hyper immunized with IrSp. However, the numbers of IFN-γ-secreting splenocytes specific for the non-CSP antigen-derived peptides were 20-100 times lower than those specific for the CSP-specific peptide. When mice were immunized with recombinant adenoviruses expressing selected non-CSP antigens, the animals were not protected against challenge with P. yoelii sporozoites although large numbers of CD8(+) specific T cells were generated.

The Plasmodium eukaryotic initiation factor-2alpha kinase IK2 controls the latency of sporozoites in the mosquito salivary glands

Sporozoites, the invasive form of malaria parasites transmitted by mosquitoes, are quiescent while in the insect salivary glands. Sporozoites only differentiate inside of the hepatocytes of the mammalian host. We show that sporozoite latency is an active process controlled by a eukaryotic initiation factor-2alpha (eIF2alpha) kinase (IK2) and a phosphatase. IK2 activity is dominant in salivary gland sporozoites, leading to an inhibition of translation and accumulation of stalled mRNAs into granules. When sporozoites are injected into the mammalian host, an eIF2alpha phosphatase removes the PO4 from eIF2alpha-P, and the repression of translation is alleviated to permit their transformation into liver stages. In IK2 knockout sporozoites, eIF2alpha is not phosphorylated and the parasites transform prematurely into liver stages and lose their infectivity. Thus, to complete their life cycle, Plasmodium sporozoites exploit the mechanism that regulates stress responses in eukaryotic cells.

From the circumsporozoite protein to the RTS, S/AS candidate vaccine

The RTS,S/AS01(E) malaria vaccine candidate has recently entered Phase 3 testing. Reaching this important milestone is the culmination of more than 20 years of research and development by GlaxoSmithKline and partners and collaborators. The vaccine has been developed to protect young children and infants living in Sub-Saharan Africa against clinical and severe disease caused by Plasmodium falciparum infection. Over the past 9 years, RTS,S/AS has been evaluated in multiple Phase 2 studies. The vaccine was shown to have a favorable safety profile and to be well tolerated in all age groups in which it was tested, including the intended target population of infants and young children in Sub-Saharan Africa. Data obtained so far suggest that RTS,S/AS can be co-administered with other vaccines included in the routine Expanded Program of Immunization (EPI). In Phase 2 testing, the vaccine candidate was shown to confer significant protection against P. falciparum infection and clinical disease, including severe malaria. Furthermore, a trend towards an indirect beneficial effect of the vaccine on non-malarial morbidities has been observed in several trials. In this paper, we will describe the genesis of the RTS,S/AS concept, including the rationale for selecting the circumsporozoite protein (CSP) as the target antigen. Early development history of the vaccine will be briefly described. We will present the most salient results from recent Phase 2 studies conducted in the target pediatric population, which have led to the decision to progress RTS,S/AS to Phase 3 testing. If the Phase 3 results confirm the observations made during Phase 2 testing, the RTS,S/AS vaccine, when broadly implemented and judiciously integrated with other malaria-prevention measures, would have a major public-health impact in Sub-Saharan Africa.

Conserved protective mechanisms in radiation and genetically attenuated uis3(-) and uis4(-) Plasmodium sporozoites

Immunization with radiation attenuated Plasmodium sporozoites (RAS) elicits sterile protective immunity against sporozoite challenge in murine models and in humans. Similarly to RAS, the genetically attenuated sporozoites (GAPs) named uis3(-), uis4(-) and P36p(-) have arrested growth during the liver stage development, and generate a powerful protective immune response in mice. We compared the protective mechanisms in P. yoelii RAS, uis3(-) and uis4(-) in BALB/c mice. In RAS and GAPs, sterile immunity is only achieved after one or more booster injections. There were no differences in the immune responses to the circumsporozoite protein (CSP) generated by RAS and GAPs. To evaluate the role of non-CSP T-cell antigens we immunized antibody deficient, CSP-transgenic BALB/c mice, that are T cell tolerant to CSP, with P. yoelii RAS or with uis3(-) or uis4(-) GAPs, and challenged them with wild type sporozoites. In every instance the parasite liver stage burden was approximately 3 logs higher in antibody deficient CSP transgenic mice as compared to antibody deficient mice alone. We conclude that CSP is a powerful protective antigen in both RAS and GAPs viz., uis3(-) and uis4(-) and that the protective mechanisms are similar independently of the method of sporozoite attenuation.

Plasmodium pre-erythrocytic stages: what's new?

The pre-erythrocytic (PE) phase of malaria infection, which extends from injection of sporozoites into the skin to the release of the first generation of merozoites, has traditionally been the 'black box' of the Plasmodium life cycle. However, since the advent of parasite transfection technology 13 years ago, our understanding of the PE phase in cellular and molecular terms has dramatically improved. Here, we review and comment on the major developments in the field in the past five years. Progress has been made in many diverse areas, including identifying and characterizing new proteins of interest, imaging parasites in vivo, understanding better the cell biology of hepatocyte infection and developing new vaccines against PE stages of the parasite.

Plasmodium circumsporozoite protein promotes the development of the liver stages of the parasite

The liver stages of malaria are clinically silent but have a central role in the Plasmodium life cycle. Liver stages of the parasite containing thousands of merozoites grow inside hepatocytes for several days without triggering an inflammatory response. We show here that Plasmodium uses a PEXEL/VTS motif to introduce the circumsporozoite (CS) protein into the hepatocyte cytoplasm and a nuclear localization signal (NLS) to enter its nucleus. CS outcompetes NFkappaB nuclear import, thus downregulating the expression of many genes controlled by NFkappaB, including those involved in inflammation. CS also influences the expression of over one thousand host genes involved in diverse metabolic processes to create a favorable niche for the parasite growth. The presence of CS in the hepatocyte enhances parasite growth of the liver stages in vitro and in vivo. These findings have far reaching implications for drug and vaccine development against the liver stages of the malaria parasite.

Experimental basis for the development of a synthetic vaccine against Plasmodium falciparum malaria sporozoites

Malaria continues to cause extensive morbidity and mortality in man. The exact number of individuals affected is not known. Estimates vary from 200 to 400 million, and more than one million die each year. Protective immunity against malaria can be obtained by vaccination with irradiated sporozoites. The protective antigens are polypeptides (circumsporozoite [CS] proteins) which cover the surface membrane of the parasite. CS proteins contain species-specific immunodominant epitopes, formed by tandem repeated sequences of amino acids. The dominant epitope of Plasmodium falciparum is represented in the synthetic peptide asparagine-alanine-asparagine-proline repeated in tandem three times; that is, (NANP)3. Monoclonal antibodies and most or all polyclonal human antibodies to P. falciparum sporozoites react with (NANP)3. Polyclonal antibodies raised against the synthetic peptide (NANP)3 react with the surface of the parasite and neutralize its infectivity. Since (NANP)3 repeats are present worldwide in CS proteins from P. falciparum, this epitope is a logical target for vaccine development.

Exposure of Plasmodium sporozoites to the intracellular concentration of potassium enhances infectivity and reduces cell passage activity

Malaria sporozoites migrate through several cells prior to a productive invasion that involves the formation of a parasitophorous vacuole (PV) where sporozoites undergo transformation into Exo-erythorcytic forms (EEFs). The precise mechanism leading to sporozoite activation for invasion is unknown, but prior traversal of host cells is required. During cell migration sporozoites are exposed to large shifts in K(+) concentration. We report here that incubation of sporozoites to the intracellular K(+) concentration enhances 8-10 times the infectivity of Plasmodium berghei and 4-5 times the infectivity of Plasmodium yoelli sporozoites for a hepatocyte cell line, while simultaneously decreasing cell passage activity. The K(+) enhancing effect was time and concentration dependent, and was significantly decreased by K(+) channel inhibitors. Potassium-treated P. berghei sporozoites also showed enhanced numbers of EEFs in non-permissive cell lines. Treated sporozoites had reduced infectivity for mice, but infectivity was enhanced upon Kupffer cell depletion. Transcriptional analysis of K(+) treated and control sporozoites revealed a high degree of correlation in their levels of gene expression, indicating that the observed phenotypic changes are not due to radical changes in gene transcription. Only seven genes were upregulated by more than two-fold in K(+) treated sporozoites. The highest level was noted in PP2C, a phosphatase known to dephosphorylate the AKT potassium channel in plants.

Aldolase provides an unusual binding site for thrombospondin-related anonymous protein in the invasion machinery of the malaria parasite

An actomyosin motor located underneath the plasma membrane drives motility and host-cell invasion of apicomplexan parasites such as Plasmodium falciparum and Plasmodium vivax, the causative agents of malaria. Aldolase connects the motor actin filaments to transmembrane adhesive proteins of the thrombospondin-related anonymous protein (TRAP) family and transduces the motor force across the parasite surface. The TRAP-aldolase interaction is a distinctive and critical trait of host hepatocyte invasion by Plasmodium sporozoites, with a likely similar interaction crucial for erythrocyte invasion by merozoites. Here, we describe 2.4-A and 2.7-A structures of P. falciparum aldolase (PfAldo) obtained from crystals grown in the presence of the C-terminal hexapeptide of TRAP from Plasmodium berghei. The indole ring of the critical penultimate Trp-residue of TRAP fits snugly into a newly formed hydrophobic pocket, which is exclusively delimited by hydrophilic residues: two arginines, one glutamate, and one glutamine. Comparison with the unliganded PfAldo structure shows that the two arginines adopt new side-chain rotamers, whereas a 25-residue subdomain, forming a helix-loop-helix unit, shifts upon binding the TRAP-tail. The structural data are in agreement with decreased TRAP binding after mutagenesis of PfAldo residues in and near the induced TRAP-binding pocket. Remarkably, the TRAP- and actin-binding sites of PfAldo seem to overlap, suggesting that both the plasticity of the aldolase active-site region and the multimeric nature of the enzyme are crucial for its intriguing nonenzymatic function in the invasion machinery of the malaria parasite.

Modeling the interaction between aldolase and the thrombospondin-related anonymous protein, a key connection of the malaria parasite invasion machinery

A complex molecular motor empowers substrate-dependent motility and host cell invasion in malaria parasites. The interaction between aldolase and the transmembrane adhesin thrombospondin-related anonymous protein (TRAP) transduces the motor force across the parasite surface. Here, we analyzed this interaction by using state-of-the-art flexible docking. Besides algorithms to account for induced fit in the side-chains of the Plasmodium falciparum aldolase (PfAldo) structure, we used additional in silico receptors modeled upon crystallographic structures of evolutionarily related aldolases to incorporate enzyme backbone flexibility, and to overcome structure inaccuracies due to the relatively low resolution (3.0 A) of the genuine PfAldo structure. Our results indicate that, in spite of multiple intermolecular contacts, only the six C-terminal residues of the TRAP cytoplasmic tail bind in an ordered manner to PfAldo. This portion of TRAP targets the PfAldo active site, with its n-1 Trp residue, which is essential for this interaction, buried within the PfAldo catalytic pocket. Docking of a TRAP peptide bearing a Trp to Ala mutation rendered the lower energy configurations either bound weakly outside the active site or not bound to PfAldo at all. The position of the bound TRAP peptide, and particularly the close proximity between the carbonyl of its n-2 Asp residue and the experimentally determined position of the phosphate-6 group of fructose 1,6-phosphate bound to mammalian aldolases, predicts an inhibitory effect of TRAP on catalysis. Enzymatic and TRAP-binding assays using mutant PfAldo molecules strongly support the overall structural model. These results might provide the initial framework for the identification of novel antiparasitic compounds.

The circumsporozoite protein is an immunodominant protective antigen in irradiated sporozoites

Malaria infection starts when mosquitoes inject sporozoites into the skin. The parasites enter the blood stream and make their way to the liver where they develop into the exo-erythrocytic forms (EEFs). Immunization with irradiated sporozoites (IrSp) leads to robust protection against malaria infection in rodents, monkeys and humans by eliciting antibodies to circumsporozoite protein (CS) that inhibit sporozoite infectivity, and T cells that destroy the EEFs. To study the role of non-CS antigens in protection, we produced CS transgenic mice that were tolerant to CS T-cell epitopes. Here we show that in the absence of T-cell-dependent immune responses to CS, protection induced by immunization with two doses of IrSp was greatly reduced. Thus, although hundreds of other Plasmodium genes are expressed in sporozoites and EEFs, CS is a dominant protective antigen. Nevertheless, sterile immunity could be obtained by immunization of CS transgenics with three doses of IrSp.

Characterization of an Aldolase-binding Site in the Wiskott-Aldrich Syndrome Protein

The thrombospondin-related anonymous protein (TRAP) is an essential transmembrane molecule in Plasmodium sporozoites. TRAP displays adhesive motifs on the extracellular portion, whereas its cytoplasmic tail connects to actin via aldolase, thus driving parasite motility and host cell invasion. The minimal requirements for the TRAP binding to aldolase were scanned here and found to be shared by different human proteins, including the Wiskott-Aldrich syndrome protein (WASp) family members. In vitro and in vivo binding of WASp members to aldolase was characterized by biochemical, deletion mapping, mutagenesis, and co-immunoprecipitation studies. As in the case of TRAP, the binding of WASp to aldolase is competitively inhibited by the enzyme substrate/products. Furthermore, TRAP and WASp, but not other unrelated aldolase binders, compete for the binding to the enzyme in vitro. Together, our results define a conserved aldolase binding motif in the WASp family members and suggest that aldolase modulates the motility and actin dynamics of mammalian cells. These findings along with the presence of similar aldolase binding motifs in additional human proteins, some of which indeed interact with aldolase in pull-down assays, suggest supplementary, non-glycolytic roles for this enzyme.

Mutational analysis of the GPI-anchor addition sequence from the circumsporozoite protein of Plasmodium

The plasma membrane of Plasmodium sporozoites is uniformly covered by the glycosylphosphatidylinositol (GPI)-anchored circumsporozoite (CS) protein. Sporozoites form in the mosquito midgut through a budding process that occurs within a multinucleate oocyst underneath the basal lamina of the gut. Earlier genetic studies established that normal sporozoite development requires CS. Mutant parasites lacking CS [CS (-)] do not form sporozoites. Ultrastructural analysis of the oocysts from these parasites revealed that there is an early block in the cytokinesis that occurs within the multinucleate oocysts to generate individual sporozoites. Parasites that are hypomorphic for CS expression gave rise to sporozoites with abnormal morphology. These results proved that CS plays a direct role in the maturation of oocysts and in the normal budding of sporozoites. In this article, we examined if the membrane localization of CS via a GPI-anchor, is crucial for its function during sporozoite formation. We generated three mutants in Plasmodium berghei CS, CS-DeltaGPI, CS-TM1 and CS-TM2. In CS-DeltaGPI, we deleted the signal sequence required for the addition of a GPI-anchor to CS. The resulting protein was found only in the cytoplasm of the oocyst. In CS-TM1 and CS-TM2, the GPI-anchor addition sequence of CS was substituted by the transmembrane domain and truncated (to different degrees) cytoplasmic tail of Plasmodium thrombospondin-related anonymous protein (TRAP). The resulting CS protein was detected on the plasma membrane of the oocysts. The amount of CS in the mutants was similar to that of wild type. The sporozoite budding and development were abrogated in both CS-DeltaGPI and CS-TM mutants. The ultrastructure of the mutant oocysts was indistinguishable from that of the CS (-) parasites. Our results suggest that the GPI-anchor of the CS protein is required for sporogenesis.

Exit of Plasmodium sporozoites from oocysts is an active process that involves the circumsporozoite protein

Plasmodium sporozoites develop within oocysts residing in the mosquito midgut. Mature sporozoites exit the oocysts, enter the hemolymph, and invade the salivary glands. The circumsporozoite (CS) protein is the major surface protein of salivary gland and oocyst sporozoites. It is also found on the oocyst plasma membrane and on the inner surface of the oocyst capsule. CS protein contains a conserved motif of positively charged amino acids: region II-plus, which has been implicated in the initial stages of sporozoite invasion of hepatocytes. We investigated the function of region II-plus by generating mutant parasites in which the region had been substituted with alanines. Mutant parasites produced normal numbers of sporozoites in the oocysts, but the sporozoites were unable to exit the oocysts. In in vitro as well, there was a profound delay, upon trypsin treatment, in the release of mutant sporozoites from oocysts. We conclude that the exit of sporozoites from oocysts is an active process that involves the region II-plus of CS protein. In addition, the mutant sporozoites were not infective to young rats. These findings provide a new target for developing reagents that interfere with the transmission of malaria.

Malaria affects hundreds of millions of people, and kills at least 1 million children per year. The infective stages of the malaria parasites, named “sporozoites,” are found in the salivary gland of Anopheles mosquitoes, and are injected along with the saliva during blood feeding. From the skin, sporozoites enter the blood circulation and invade liver cells where the parasites multiply. When they exit the liver, these parasites infect blood cells and can cause severe symptoms. If ingested by mosquitoes, the blood-stage parasites continue their lifecycle in the insect stomach. Thousands of sporozoites are formed within a cyst-like structure (oocyst). The sporozoites come out of the oocyst and infect the salivary gland, where they remain until injected back into humans. Malaria parasites are increasingly resistant to drugs, mosquitoes are difficult to eliminate, and effective vaccines are not yet available. New tools to combat malaria are urgently needed. One exciting approach, although the application is in the distant future, is to release in endemic areas genetically modified mosquitoes that are resistant to parasite growth. This paper provides a new target for generating these “transmission-block” mosquitoes and shows that the exit of sporozoites from the oocysts is an active process that requires the enzymatic digestion of components of the oocyst wall. If these enzymes are inhibited in transgenic mosquitoes, sporozoites will never reach the salivary gland.

Yellow fever 17D as a vaccine vector for microbial CTL epitopes: protection in a rodent malaria model

The yellow fever vaccine 17D (17D) is safe, and after a single immunizing dose, elicits long-lasting, perhaps lifelong protective immunity. One of the major challenges facing delivery of human vaccines in underdeveloped countries is the need for multiple injections to achieve full efficacy. To examine 17D as a vector for microbial T cell epitopes, we inserted the H-2K^d–restricted CTL epitope of the circumsporozoite protein (CS) of Plasmodium yoelii between 17D nonstructural proteins NS2B and NS3. The recombinant virus, 17D-Py, was replication competent and stable in vitro and in vivo. A single subcutaneous injection of 10⁵ PFU diminished the parasite burden in the liver by ∼70%. The high level of protection lasted between 4 and 8 wk after immunization, but a significant effect was documented even 24 wk afterwards. Thus, the immunogenicity of a foreign T cell epitope inserted into 17D mimics some of the remarkable properties of the human vaccine. Priming with 17D-Py followed by boosting with irradiated sporozoites conferred sterile immunity to 90% of the mice. This finding indicates that the immune response of vaccine-primed individuals living in endemic areas could be sustained and magnified by the bite of infected mosquitoes.

Transcriptome of axenic liver stages of Plasmodium yoelii

Plasmodium liver stages or early exo-eythrocytic forms (EEFs) contain antigens that are essential for achieving sterile, protective immunity against malaria. Yet, attempts at identifying these antigens have been hampered by the challenge of obtaining large numbers of purified EEFs, uncontaminated with hepatocyte material. Using a recently described system for producing axenically cultured EEFs from Plasmodium yoelii, we have constructed a cDNA library and generated 1453 expressed sequence tags (ESTs) resulting in 652 unique transcripts. Analysis of the library provides insight into processes required for the initiation and development of Plasmodium liver stages, such as protein degradation, cell cycle progression and nutrient transport. Analysis of the gene expression profile of liver stages, as revealed by this library, suggests that liver stages represent a shift from "sporozoite-like" to "blood-stage-like". This is the first study of the transcriptional repertoire of Plasmodium liver stages.

Plasmodium sporozoite molecular cell biology

Plasmodium sporozoites display complex phenotypes including gliding motility and invasion of and transmigration through cells in the mosquito vector and the vertebrate host. Sporozoite studies have been difficult to perform because of technical concerns. Nevertheless, they have already provided insights into several aspects of sporozoite biology, shared in part with other apicomplexan invasive stages. Structure/function analysis of the thrombospondin-related anonymous protein paved the way to the understanding of the molecular mechanisms of apicomplexan gliding motility and host cell invasion. Functional studies of circumsporozoite protein revealed its role in Plasmodium sporozoite morphogenesis in addition to its well-known function in host cell invasion. Transcriptional surveys, which facilitate the investigation of gene expression programs that control sporozoite phenotypes, have revealed a high degree of previously unappreciated complexity and novel proteins that mediate sporozoite host cell infection.

A surface phospholipase is involved in the migration of plasmodium sporozoites through cells

Plasmodium sporozoites, injected by mosquitoes into the skin of the host, traverse cells during their migration to hepatocytes where they continue their life cycle. The mechanisms used by the parasite to rupture the plasma membrane of the host cells are not known. Here we report the presence of a phospholipase on the surface of Plasmodium berghei sporozoites (P. berghei phospholipase; Pb PL) and demonstrate that it is involved in the establishment of a malaria infection in vivo. Pb PL is highly conserved among the Plasmodium species. The protein is about 750 amino acids, with a predicted signal sequence and a carboxyl terminus that is 32% identical to the vertebrate lecithin:cholesterol acyltransferase, a secreted phospholipase. Pb PL contains a motif characteristic of lipases and a catalytic triad of a serine, aspartate, and histidine that is found in several phospholipases. We have verified its lipase and membrane lytic activity in vitro, using recombinant baculovirus-expressed protein. To study its role in vivo, we have disrupted the P. berghei PL open reading frame and generated mutants in its active site. During an infection through mosquito bite, the infectivity of the knock-out parasites in the liver is decreased by approximately 90%. The prepatent period of the resulting blood infection is 1 day longer as compared with wild type. Further, the mutant sporozoites are impaired in their ability to cross epithelial cell layers. Thus, the Pb PL functions as a lipase to damage cell membranes and facilitates sporozoite passage through cells during their migration from the skin to the bloodstream.

Quantitative Plasmodium sporozoite neutralization assay (TSNA)

The circumsporozoite (CS) protein is the major surface protein of Plasmodium sporozoites. Antibodies to the immunodominant repeat domain of CS immobilize sporozoites and prevent infection of hepatocytes. Plasmodium falciparum vaccines containing CS repeats are undergoing human trials in endemic areas, and proof of efficacy has been obtained. The correlates of protection are under investigation. Levels of anti-repeat antibodies in the serum of the human volunteers have been measured mostly by enzyme-linked immunosorbent assay (ELISA) and IFA. Assays that measure the effect of the serum antibodies on parasite infectivity (serum neutralization assays SNAs) are not usually performed because they require a susceptible host and P. falciparum sporozoites are highly infectious only to humans. To overcome this limitation, we developed a new assay named transgenic sporozoite neutralization assay (TSNA) that uses as neutralization target, a transgenic rodent malaria parasite Plasmodium berghei that bears the P. falciparum CS repeats [CS(Pf)]. Following incubation with human serum, CS(Pf) infectivity of HepG2 cells is evaluated by real-time PCR. We have compared ELISA titers and TSNAs in a limited number of sera from humans immunized with (T1B)4 MAP, a peptide vaccine containing P. falciparum CS repeats. A comparison between the two assays did not reach significance (p=0.175) when analyzed by non-parametric Spearman correlation method. Ongoing human trials of CS-based vaccines should provide an opportunity to determine whether TSNAs will provide better correlates of protective immunity than ELISA assays.

Sites of interaction between aldolase and thrombospondin-related anonymous protein in plasmodium

Gliding motility and host cell invasion by apicomplexan parasites are empowered by an acto-myosin motor located underneath the parasite plasma membrane. The motor is connected to host cell receptors through trans-membrane invasins belonging to the thrombospondin-related anonymous protein (TRAP) family. A recent study indicates that aldolase bridges the cytoplasmic tail of MIC2, the homologous TRAP protein in Toxoplasma, and actin. Here, we confirm these unexpected findings in Plasmodium sporozoites and identify conserved features of the TRAP family cytoplasmic tail required to bind aldolase: a subterminal tryptophan residue and two noncontiguous stretches of negatively charged amino acids. The aldolase substrate and other compounds that bind to the active site inhibit its interaction with TRAP and with F-actin, suggesting that the function of the motor is metabolically regulated. Ultrastructural studies in salivary gland sporozoites localize aldolase to the periphery of the secretory micronemes containing TRAP. Thus, the interaction between aldolase and the TRAP tail takes place during or preceding the biogenesis of the micronemes. The release of their contents in the anterior pole of the parasite upon contact with the target cells should bring simultaneously aldolase, TRAP and perhaps F-actin to the proper subcellular location where the motor is engaged.

Defective sorting of the thrombospondin-related anonymous protein (TRAP) inhibits Plasmodium infectivity

Thrombospondin-related anonymous protein (TRAP) is a type 1 transmembrane protein that plays an essential role in gliding motility and cell invasion by Plasmodium sporozoites. It is stored in micronemes-secretory organelles located primarily in the apical end of the parasites and is also found on the parasite surface. The mechanisms that target TRAP and other sporozoite proteins to micronemes and subsequently to the parasite surface are not known. Here we report that the micronemal and surface localization of TRAP requires a tyrosine-based motif located in its cytoplasmic tail. This motif is analogous to the YXXphi motif (Y: tyrosine, X: any amino acid; phi: hydrophobic amino acid) that targets eukaryotic proteins to certain sub-cellular compartments and to the plasma membrane. Abrogating the Y motif substantially reduces micronemal and cell surface localization of TRAP. The infectivity of mutant parasites is substantially inhibited. However, there is no significant difference in the amounts of TRAP secreted into the culture medium by wild type and mutant parasites, suggesting that TRAP destined for secretion bypasses micronemal localization.

Myosin A tail domain interacting protein (MTIP) localizes to the inner membrane complex of Plasmodium sporozoites

Apicomplexan host cell invasion and gliding motility depend on the parasite's actomyosin system located beneath the plasma membrane of invasive stages. Myosin A (MyoA), a class XIV unconventional myosin, is the motor protein. A model has been proposed to explain how the actomyosin motor operates but little is known about the components, topology and connectivity of the motor complex. Using the MyoA neck and tail domain as bait in a yeast two-hybrid screen we identified MTIP, a novel 24 kDa protein that interacts with MyoA. Deletion analysis shows that the 15 amino-acid C-terminal tail domain of MyoA, rather than the neck domain, specifically interacts with MTIP. In Plasmodium sporozoites MTIP localizes to the inner membrane complex (IMC), where it is found clustered with MyoA. The data support a model for apicomplexan motility and invasion in which the MyoA motor protein is associated via its tail domain with MTIP, immobilizing it at the outer IMC membrane. The head domain of the immobilized MyoA moves actin filaments that, directly or via a bridging protein, connect to the cytoplasmic domain of a transmembrane protein of the TRAP family. The actin/TRAP complex is then redistributed by the stationary MyoA from the anterior to the posterior end of the zoite, leading to its forward movement on a substrate or to penetration of a host cell.

Cutting edge: a new tool to evaluate human pre-erythrocytic malaria vaccines: rodent parasites bearing a hybrid Plasmodium falciparum circumsporozoite protein

Malaria vaccines containing the Plasmodium falciparum Circumsporozoite protein repeat domain are undergoing human trials. There is no simple method to evaluate the effect of vaccine-induced responses on P. falciparum sporozoite infectivity. Unlike the rodent malaria Plasmodium berghei, P. falciparum sporozoites do not infect common laboratory animals and only develop in vitro in human hepatocyte cultures. We generated a recombinant P. berghei parasite bearing P. falciparum Circumsporozoite protein repeats. These hybrid sporozoites are fully infective in vivo and in vitro. Monoclonal and polyclonal Abs to P. falciparum repeats neutralize hybrid parasite infectivity, and mice immunized with a P. falciparum vaccine are protected against challenge with hybrid sporozoites.

Infectivity-associated changes in the transcriptional repertoire of the malaria parasite sporozoite stage

Injection of Plasmodium salivary gland sporozoites into the vertebrate host by Anopheles mosquitoes initiates malaria infection. Sporozoites develop within oocysts in the mosquito midgut and then enter and mature in the salivary glands. Although morphologically similar, oocyst sporozoites and salivary gland sporozoites differ strikingly in their infectivity to the mammalian host, ability to elicit protective immune responses, and cell motility. Here, we show that differential gene expression coincides with these dramatic phenotypic differences. Using suppression subtractive cDNA hybridization we identified highly up-regulated mRNAs transcribed from 30 distinct genes in salivary gland sporozoites. Of those genes, 29 are not significantly expressed in the parasite's blood stages. The most frequently recovered transcript encodes a protein kinase. Developmental up-regulation of specific mRNAs in the infectious transmission stage of Plasmodium indicates that their translation products may have unique roles in hepatocyte infection and/or development of liver stages.

Plasmodium sporozoite invasion into insect and mammalian cells is directed by the same dual binding system

Plasmodium sporozoites, the transmission form of the malaria parasite, successively invade salivary glands in the mosquito vector and the liver in the mammalian host. Sporozoite capacity to invade host cells is mechanistically related to their ability to glide on solid substrates, both activities depending on the transmembrane protein TRAP. Here, we show that loss-of- function mutations in two adhesive modules of the TRAP ectodomain, an integrin-like A-domain and a thrombospondin type I repeat, specifically decrease sporozoite invasion of host cells but do not affect sporozoite gliding and adhesion to cells. Irrespective of the target cell, i.e. in mosquitoes, rodents and cultured human or hamster cells, sporozoites bearing mutations in one module are less invasive, while those bearing mutations in both modules are non-invasive. In Chinese hamster ovary cells, the TRAP modules interact with distinct cell receptors during sporozoite invasion, and thus act as independently active pass keys. As these modules are also present in other members of the TRAP family of proteins in Apicomplexa, they may account for the capacity of these parasites to enter many cell types of phylogenetically distant origins.

Levels of circumsporozoite protein in the Plasmodium oocyst determine sporozoite morphology

The sporozoite stage of the Plasmodium parasite is formed by budding from a multinucleate oocyst in the mosquito midgut. During their life, sporozoites must infect the salivary glands of the mosquito vector and the liver of the mammalian host; both events depend on the major sporozoite surface protein, the circumsporozoite protein (CS). We previously reported that Plasmodium berghei oocysts in which the CS gene is inactivated do not form sporozoites. Here, we analyzed the ultrastructure of P.berghei oocyst differentiation in the wild type, recombinants that do not produce or produce reduced amounts of CS, and corresponding complemented clones. The results indicate that CS is essential for establishing polarity in the oocyst. The amounts of CS protein correlate with the extent of development of the inner membranes and associated microtubules underneath the oocyst outer membrane, which normally demarcate focal budding sites. This is a first example of a protein controlling both morphogenesis and infectivity of a parasite stage.

Exploring the transcriptome of the malaria sporozoite stage

Most studies of gene expression in Plasmodium have been concerned with asexual and/or sexual erythrocytic stages. Identification and cloning of genes expressed in the preerythrocytic stages lag far behind. We have constructed a high quality cDNA library of the Plasmodium sporozoite stage by using the rodent malaria parasite P. yoelii, an important model for malaria vaccine development. The technical obstacles associated with limited amounts of RNA material were overcome by PCR-amplifying the transcriptome before cloning. Contamination with mosquito RNA was negligible. Generation of 1,972 expressed sequence tags (EST) resulted in a total of 1,547 unique sequences, allowing insight into sporozoite gene expression. The circumsporozoite protein (CS) and the sporozoite surface protein 2 (SSP2) are well represented in the data set. A BLASTX search with all tags of the nonredundant protein database gave only 161 unique significant matches (P(N) < or = 10(-4)), whereas 1,386 of the unique sequences represented novel sporozoite-expressed genes. We identified ESTs for three proteins that may be involved in host cell invasion and documented their expression in sporozoites. These data should facilitate our understanding of the preerythrocytic Plasmodium life cycle stages and the development of preerythrocytic vaccines.

Gene targeting in the rodent malaria parasite Plasmodium yoelii

It is anticipated that the sequencing of Plasmodium falciparum genome will soon be completed. Rodent models of malaria infection and stable transformation systems provide powerful means of using this information to study gene function in vivo. To date, gene targeting has only been developed for one rodent malaria species, Plasmodium berghei. Another rodent species, Plasmodium yoelii, however, is favored to study the mechanisms of protective immunity to the pre-erythrocytic stages of infection and vaccine development. In addition, it offers the opportunity to investigate unique aspects of pathogenesis of blood stage infection. Here, we report on the stable transfection and gene targeting of P. yoelii. Purified late blood stage schizonts were used as targets for electroporation with a plasmid that contains a pyrimethamine-resistant form of the P. berghei dihydrofolate reductase-thymidylate synthase (Pbdhfr-ts) fused to green fluorescent protein (gfp) gene. After drug selection, fluorescent parasites contained intact, non-rearranged plasmids that remain stable under drug-pressure. In addition, we used another dhfr-ts/gfp based plasmid to disrupt the P. yoelii trap (thrombospondin-related anonymous protein) locus by site-specific integration. The phenotype of P. yoelii TRAP knockout was identical to that previously reported for the P. berghei TRAP knockout. In the absence of TRAP, the erythrocytic cycle, gametocyte and oocyst development of the mutant parasites were indistinguishable from wild type (WT). Although the sporozoites appeared morphologically normal, they failed to glide and to invade the salivary glands of mosquitoes.

Complementation of Plasmodium berghei TRAP knockout parasites using human dihydrofolate reductase gene as a selectable marker

Previously we have used the Plasmodium dihydrofolate reductase thymidylate synthase (DHFR-TS) selectable marker to generate Plasmodium berghei TRAP null mutant parasites. These TRAP null mutants do not glide and they showed a great reduction in their ability to infect mosquito salivary glands and the hepatocytes of the vertebrate host. Thus far, complementation of these knockout parasites was not possible due to the lack of additional selectable markers. Recently, a new selectable marker, based on the human dihydrofolate reductase (hDHFR) gene, has been developed which confers resistance to the antifolate drug WR99210. This drug has been found to be highly active against pyrimethamine-sensitive and -resistant strains of P. berghei. In this study, we have used the hDHFR gene as a second selectable marker for the complementation of P. berghei TRAP null mutant parasites. Restoration of the TRAP null mutant parasites to the wild-type phenotype was achieved in this study via autonomously replicating episomes bearing a wild-type copy of the TRAP gene. This is the first report of complementation of a mutant phenotype in malaria parasites.

The ubiquitin-proteasome pathway plays an essential role in proteolysis during Trypanosoma cruzi remodeling

Here, we document for the first time the presence of the 26S proteasome and the ubiquitin pathway in a protozoan parasite that is in an early branch in the eukaryotic lineage. The 26S proteasome of Trypanosoma cruzi epimastigotes was identified as a high molecular weight complex (1400 kDa) with an ATP-dependent chymotrypsin-like activity against the substrate Suc-LLVY-Amc. This activity was inhibited by proteasome inhibitors and showed same electrophorectic migration pattern as yeast 26S proteasome in nondenaturating gels. About 30 proteins in a range of 25-110 kDa were detected in the purified T. cruzi 26S proteasome. Antibodies raised against the AAA family of ATPases from eukaryotic 26S proteasome and the T. cruzi 20S core specifically recognized components of T. cruzi 26S. To confirm the biological role of 26S in this primitive eukaryotic parasite, we analyzed the participation of the ubiquitin (Ub)-proteasome system in protein degradation during the time of parasite remodeling. Protein turnover in trypomastigotes was proteasome and ATP-dependent and was enhanced during the transformation of the parasites into amastigotes. If 20S proteasome activity is inhibited, ubiquitinated proteins accumulate in the parasites. As expected from the profound morphological changes that occur during transformation, cytoskeletal proteins associated with the flagellum are targets of the ubiquitin-proteasome pathway.

Migration of Plasmodium sporozoites through cells before infection

Intracellular bacteria and parasites typically invade host cells through the formation of an internalization vacuole around the invading pathogen. Plasmodium sporozoites, the infective stage of the malaria parasite transmitted by mosquitoes, have an alternative mechanism to enter cells. We observed breaching of the plasma membrane of the host cell followed by rapid repair. This mode of entry did not result in the formation of a vacuole around the sporozoite, and was followed by exit of the parasite from the host cell. Sporozoites traversed the cytosol of several cells before invading a hepatocyte by formation of a parasitophorous vacuole, in which they developed into the next infective stage. Sporozoite migration through several cells in the mammalian host appears to be essential for the completion of the life cycle.

Phosphatidylinositol-specific phospholipase C (PI-PLC) cleavage of GPI-anchored surface molecules of Trypanosoma cruzi triggers in vitro morphological reorganization of trypomastigotes

Trypanosoma cruzi trypomastigotes treated with phosphatidylinositol-specific phospholipase C (PI-PLC) in vitro are rapidly induced to differentiate into round forms. Using confocal microscopy, we were able to show that trypomastigotes treated with PI-PLC initiate the process of flagellum remodeling by 30 sec after contact with the enzyme and amastigote-like forms are detected as early as 10 min after PI-PLC treatment. Scanning and transmission electron microscopy indicate that trypomastigotes undergo a previously undescribed process of flagellum circularization and internalization. Analysis of the flagellar complex with monoclonal antibody 4D9 shows heterogeneous labeling among the parasites, suggesting a remodeling of these molecules. After PI-PLC treatment, parasites rapidly lose the surface marker Ssp-3 and 24 h post-treatment they begin to exhibit a circular nucleus and a rod-shaped kinetoplast. By flow cytometry analysis and confocal microscopy, the Ssp-4 amastigote-specific epitope can be detected on the parasite surface. This indicates that the release of trypomastigote GPI-anchored molecules by exogenous PI-PLC in vitro can trigger morphological changes.

Antibodies against thrombospondin-related anonymous protein do not inhibit Plasmodium sporozoite infectivity in vivo

Thrombospondin-related anonymous protein (TRAP), a candidate malaria vaccine antigen, is required for Plasmodium sporozoite gliding motility and cell invasion. For the first time, the ability of antibodies against TRAP to inhibit sporozoite infectivity in vivo is evaluated in detail. TRAP contains an A-domain, a well-characterized adhesive motif found in integrins. We modeled here a three-dimensional structure of the TRAP A-domain of Plasmodium yoelii and located regions surrounding the MIDAS (metal ion-dependent adhesion site), the presumed business end of the domain. Mice were immunized with constructs containing these A-domain regions but were not protected from sporozoite challenge. Furthermore, monoclonal and rabbit polyclonal antibodies against the A-domain, the conserved N terminus, and the repeat region of TRAP had no effect on the gliding motility or sporozoite infectivity to mice. TRAP is located in micronemes, secretory organelles of apicomplexan parasites. Accordingly, the antibodies tested here stained cytoplasmic TRAP brightly by immunofluorescence. However, very little TRAP could be detected on the surface of sporozoites. In contrast, a dramatic relocalization of TRAP onto the parasite surface occurred when sporozoites were treated with calcium ionophore. This likely mimics the release of TRAP from micronemes when a sporozoite contacts its target cell in vivo. Contact with hepatoma cells in culture also appeared to induce the release of TRAP onto the surface of sporozoites. If large amounts of TRAP are released in close proximity to its cellular receptor(s), effective competitive inhibition by antibodies may be difficult to achieve.