Restricted or absent immune responses in human populations to Plasmodium falciparum gamete antigens that are targets of malaria transmission-blocking antibodies

Malaria transmission-blocking vaccines Pfs230D1-EPA and Pfs25-EPA in Alhydrogel in healthy Malian adults; a phase 1, randomised, controlled trial

BACKGROUND

Malaria transmission-blocking vaccines target mosquito-stage parasites and will support elimination programmes. Gamete vaccine Pfs230D1-EPA/Alhydrogel induced superior activity to zygote vaccine Pfs25-EPA/Alhydrogel in malaria-naive US adults. Here, we compared these vaccines in malaria-experienced Malians.

METHODS

We did a pilot safety study then double-blind, block-randomised, comparator-controlled main-phase trial in malaria-intense Bancoumana, Mali. 18-50-year-old healthy non-pregnant, non-breastfeeding consenting adult residents were randomly assigned (1:1:1:1) to receive four doses at months 0, 1, 4·5, and 16·5 of either 47 μg Pfs25, 40 μg Pfs230D1 or comparator (Twinrix or Menactra)-all co-administered with normal saline for blinding-or 47 μg Pfs25 plus 40 μg Pfs230D1 co-administered. We documented safety and tolerability (primary endpoint in the as-treated populations) and immunogenicity (secondary endpoint in the as-treated populations: ELISA, standard-membrane-feeding assay, and mosquito direct skin feed assay). This trial is registered at ClinicalTrials.gov, NCT02334462.

FINDINGS

Between March 19, and June 2, 2015, we screened 471 individuals. Of 225 enrolled for the pilot and main cohorts, we randomly assigned 25 participants to pilot safety cohort groups of five (20%) to receive a two-dose series of Pfs25-EPA/Alhydrogel (16 μg), Pfs230D1-EPA/Alhydrogel (15 μg) or comparator, followed by Pfs25-EPA/Alhydrogel (16 μg) plus Pfs230D1-EPA/Alhydrogel (15 μg) or comparator plus saline. For the main cohort, we enrolled 200 participants between May 11 and June 2, 2015, to receive a four-dose series of 47 μg Pfs25-EPA/Alhydrogel plus saline (n=50 [25%]; Pfs25), 40 μg Pfs230D1-EPA/Alhydrogel plus saline (n=49 [25%]; Pfs230D1), 47 μg Pfs25-EPA/Alhydrogel plus 40 μg Pfs230D1-EPA/Alhydrogel (n=50 [25%]; Pfs25 plus Pfs230D1), or comparator (Twinrix or Menactra) plus saline (n=51 [25%]). Vaccinations were well tolerated in the pilot safety and main phases. Most vaccinees became seropositive after two Pfs230D1 or three Pfs25 doses; peak titres increased with each dose thereafter (Pfs230D1 geometric mean: 77·8 [95% CI 56·9-106·3], 146·4 [108·3-198·0], and 410·2 [301·6-558·0]; Pfs25 geometric mean 177·7 [130·3-242·4] and 315·7 [209·9-474·6]). Functional activity (mean peak transmission-reducing activity) appeared for Pfs230D1 (74·5% [66·6-82·5]) and Pfs25 plus Pfs230D1 (68·6% [57·3-79·8]), after the third dose and after the fourth dose (88·9% [81·7-96·2] for Pfs230D1 and 85·0% [78·4-91·5] Pfs25 plus Pfs230D1) but not for Pfs25 (58·2% [49·1-67·3] after the third dose and 58·2% [48·5-67·9] after the fourth dose). Pfs230D1 transmission-reducing activity (73·7% [64·1-83·3]) persisted 10 weeks after the fourth dose. Transmission-reducing activity of 80% was estimated at 1659 ELISA units for Pfs25, 218 for Pfs230D1, and 223 for Pfs230D1 plus Pfs25. After 3850 direct skin feed assays, 35 participants (12 Pfs25, eight Pfs230D1, five Pfs25 plus Pfs230D1, and ten comparator) had transmitted parasites at least once. The proportion of positive assays in vaccine groups (Pfs25 33 [3%] of 982 [-0·013 to 0·014], Pfs230D1 22 [2%] of 954 [-0·005 to 0·027], and combination 11 [1%] of 940 [-0·024 to 0·002]) did not differ from that of the comparator (22 [2%] of 974), nor did Pfs230D1 and combination groups differ (-0·024 to 0·001).

INTERPRETATION

Pfs230D1 but not Pfs25 vaccine induces durable serum functional activity in Malian adults. Direct skin feed assays detect parasite transmission to mosquitoes but increased event rates are needed to assess vaccine effectiveness.

FUNDING

Intramural Research Program of the National Institute of Allergy and Infectious Diseases and US National Institutes of Health.

A human antibody epitope map of the malaria vaccine antigen Pfs25

Pfs25 is a leading antigen for a malaria transmission-blocking vaccine and shows moderate transmission-blocking activity and induction of rapidly decreasing antibody titers in clinical trials. A comprehensive definition of all transmission-reducing epitopes of Pfs25 will inform structure-guided design to enhance Pfs25-based vaccines, leading to potent transmission-blocking activity. Here, we compiled a detailed human antibody epitope map comprising epitope binning data and structures of multiple human monoclonal antibodies, including three new crystal structures of Pfs25 in complex with transmission-reducing antibodies from Malian volunteers immunized with Pfs25 conjugated to EPA and adjuvanted with AS01. These structures revealed additional epitopes in Pfs25 capable of reducing transmission and expanded this characterization to malaria-exposed humans. This work informs immunogen design to focus the antibody response to transmission-reducing epitopes of Pfs25, enabling development of more potent transmission-blocking vaccines for malaria.

The Virtues and Vices of Pfs230: From Vaccine Concept to Vaccine Candidate

Among the Plasmodium falciparum surface antigens reported by Richard Carter and his colleagues decades ago, Pfs230 is currently the target of the most advanced candidate for a malaria transmission-blocking vaccine. First identified by its orthologue in the avian malaria parasite Plasmodium gallinaceum, the large cysteine-rich 14-domain Pfs230 antigen is displayed on the surface of gametes that emerge in the mosquito midgut. Gametes lacking Pfs230 cannot bind to red blood cells nor develop further into oocysts. Human antibodies against Pfs230 lyse gametes in the presence of complement, which largely explains serum transmission-blocking activity in Pfs230 antisera. A protein-protein conjugate vaccine that incorporates the first domain of the Pfs230 antigen induced greater serum transmission-reducing activity versus a similarly manufactured Pfs25 vaccine in U.S. trials, and is currently in phase II field trials in Mali.

Efforts to Develop Pfs25 Vaccines

Acknowledging the fallibilities of recalling events from more than three decades ago, the recollection of Richard Carter's impact on the identification and development of Pfs25, a major surface protein of Plasmodium falciparum zygotes and ookinetes, and target of malaria transmission-blocking vaccines, remains unassailable. In fondest memories of Richard Carter's many contributions, herein retells some memorable events along the tortuous journey toward the development of Pfs25 vaccines.

Vector microbiota manipulation by host antibodies: the forgotten strategy to develop transmission-blocking vaccines

Human and animal pathogens that are transmitted by arthropods are a global concern, particularly those vectored by ticks (e.g. Borrelia burgdorferi and tick-borne encephalitis virus) and mosquitoes (e.g. malaria and dengue virus). Breaking the circulation of pathogens in permanent foci by controlling vectors using acaricide-based approaches is threatened by the selection of acaricide resistance in vector populations, poor management practices and relaxing of control measures. Alternative strategies that can reduce vector populations and/or vector-mediated transmission are encouraged worldwide. In recent years, it has become clear that arthropod-associated microbiota are involved in many aspects of host physiology and vector competence, prompting research into vector microbiota manipulation. Here, we review how increased knowledge of microbial ecology and vector-host interactions is driving the emergence of new concepts and tools for vector and pathogen control. We focus on the immune functions of host antibodies taken in the blood meal as they can target pathogens and microbiota bacteria within hematophagous arthropods. Anti-microbiota vaccines are presented as a tool to manipulate the vector microbiota and interfere with the development of pathogens within their vectors. Since the importance of some bacterial taxa for colonization of vector-borne pathogens is well known, the disruption of the vector microbiota by host antibodies opens the possibility to develop novel transmission-blocking vaccines.

Professor Richard Carter (1945-2021)

The malaria research community lost a pioneer when Professor Richard Carter passed away at the age of 76 on 4 September 2021. Richard was an exceptionally brilliant malariologist, always inquisitive and gifted with an unorthodox way of thinking.

Transmission-Blocking Vaccines: Old Friends and New Prospects

In the progression of the life cycle of Plasmodium falciparum, a small proportion of asexual parasites differentiate into male or female sexual forms called gametocytes. Just like their asexual counterparts, gametocytes are contained within the infected host's erythrocytes (RBCs). However, unlike their asexual partners, they do not exit the RBC until they are taken up in a blood meal by a mosquito. In the mosquito midgut, they are stimulated to emerge from the RBC, undergo fertilization, and ultimately produce tens of thousands of sporozoites that are infectious to humans. This transmission cycle can be blocked by antibodies targeting proteins exposed on the parasite surface in the mosquito midgut, a process that has led to the development of candidate transmission-blocking vaccines (TBV), including some that are in clinical trials. Here we review the leading TBV antigens and highlight the ongoing search for additional gametocyte/gamete surface antigens, as well as antigens on the surfaces of gametocyte-infected erythrocytes, which can potentially become a new group of TBV candidates.

Plasmodium falciparum genotype and gametocyte prevalence in children with uncomplicated malaria in coastal Ghana

BACKGROUND

Plasmodium falciparum gametocytes are vital to sustaining malaria transmission. Parasite densities, multiplicity of infection as well as asexual genotype are features that have been found to influence gametocyte production. Measurements of the prevalence of Plasmodium sp. gametocytes may serve as a tool to monitor the success of malaria eradication efforts.

METHODS

Whole blood was collected from 112 children aged between 6 months and 13 years with uncomplicated P. falciparum malaria attending three health facilities in southern Ghana from June to August, 2014 before (day 0) and 4 days after completion of anti-malaria drug treatment (day 7). Malaria parasites were observed by microscopy and polymerase chain reaction (PCR); submicroscopic gametocyte carriage was measured by a Pfs25 (PF3D7_1031000) mRNA real time reverse transcriptase polymerase chain reaction (RT-PCR). Parasite genotyping was performed on gDNA extracted from dried filter paper blood blots by amplification of the polymorphic regions of msp1 (PF3D7_0930300) and msp2 (PF3D7_0206800) using PCR.

RESULTS

Microscopy estimated 3.1% (3/96) of the total population to carry gametocytes on day 0, which decreased to 2.1% (2/96) on day 7. In contrast, reverse transcriptase-real time PCR (RT-PCR) analysis of a subset of 35 samples estimated submicroscopic gametocyte carriage to be as high as 77% (27/35) using primers specific for Pfs25 (CT < 35) on day 0 and by day 7 this only declined to 60% (21/35). Genotyping the msp2 gene identified higher levels of MOI than the msp1 gene.

CONCLUSIONS

Although below detection by microscopy, gametocyte prevalence at submicroscopic levels are high in this region and emphasize the need for more effective elimination approaches like the development of transmission-blocking vaccines and safer gametocytocidal drugs.

Associations Between Helminth Infections, Plasmodium falciparum Parasite Carriage and Antibody Responses to Sexual and Asexual Stage Malarial Antigens

Infections with helminths and Plasmodium spp. overlap in their geographical distribution. It has been postulated that helminth infections may influence malarial transmission by altering Plasmodium falciparum gametocytogenesis. This cross-sectional study assessed the effect of helminth infections on P. falciparum gametocyte carriage and on humoral immune responses to sexual stage antigens in Gabon. Schistosoma haematobium and filarial infections as well as P. falciparum asexual forms and gametocyte carriage were determined. The antibody responses measured were to sexual (Pfs230, Pfs48/45) and asexual P. falciparum antigens (AMA1, MSP1, and GLURP). A total of 287 subjects were included. The prevalence of microscopically detectable P. falciparum asexual parasites was higher in S. haematobium-infected subjects in comparison to their uninfected counterparts (47% versus 26%, P = 0.003), but this was not different when filarial infections were considered. Plasmodium falciparum gametocyte carriage was similar between Schistosoma- or filaria-infected and uninfected subjects. We observed a significant decrease of Pfs48/45 immunoglobulin G titer in S. haematobium-infected subjects (P = 0.037), whereas no difference was seen for Pfs230 antibody titer, nor for antibodies to AMA1, MSP1, or GLURP. Our findings suggest an effect of S. haematobium on antibody responses to some P. falciparum gametocyte antigens that may have consequences for transmission-blocking immunity.

Plasmodium falciparum Gametocyte-Specific Antibody Profiling Reveals Boosting through Natural Infection and Identifies Potential Markers of Gametocyte Exposure

Malaria elimination efforts would benefit from vaccines that block transmission of Plasmodium falciparum gametocytes from humans to mosquitoes. A clear understanding of gametocyte-specific antibody responses in exposed populations could help determine whether transmission-blocking vaccines (TBV) would be boosted by natural gametocyte exposure, and also inform the development of serologic tools to monitor gametocyte exposure in populations targeted for malaria elimination. To this end, plasma was collected from Malian children and adults before and after the 6-month malaria season and probed against a microarray containing 1,204 P. falciparum proteins. Using publicly available proteomic data, we classified 91 proteins as gametocyte specific and 69 as proteins not expressed by gametocytes. The overall breadth and magnitude of gametocyte-specific IgG responses increased during the malaria season, although they were consistently lower than IgG responses to nongametocyte antigens. Notably, IgG specific for the TBV candidates Pfs48/45 and Pfs230 increased during the malaria season. In addition, IgGs specific for the gametocyte proteins Pfmdv1, Pfs16, PF3D7_1346400, and PF3D7_1024800 were detected in nearly all subjects, suggesting that seroconversion to these proteins may be a sensitive indicator of gametocyte exposure, although further studies are needed to determine the specificity and kinetics of these potential serologic markers. These findings suggest that TBV-induced immunity would be boosted through natural gametocyte exposure, and that antibody responses to particular antigens may reliably indicate gametocyte exposure.

Heat-precipitation allows the efficient purification of a functional plant-derived malaria transmission-blocking vaccine candidate fusion protein

Malaria is a vector-borne disease affecting more than two million people and accounting for more than 600,000 deaths each year, especially in developing countries. The most serious form of malaria is caused by Plasmodium falciparum. The complex life cycle of this parasite, involving pre-erythrocytic, asexual and sexual stages, makes vaccine development cumbersome but also offers a broad spectrum of vaccine candidates targeting exactly those stages. Vaccines targeting the sexual stage of P. falciparum are called transmission-blocking vaccines (TBVs). They do not confer protection for the vaccinated individual but aim to reduce or prevent the transmission of the parasite within a population and are therefore regarded as an essential tool in the fight against the disease. Malaria predominantly affects large populations in developing countries, so TBVs need to be produced in large quantities at low cost. Combining the advantages of eukaryotic expression with a virtually unlimited upscaling potential and a good product safety profile, plant-based expression systems represent a suitable alternative for the production of TBVs. We report here the high level (300 μg/g fresh leaf weight (FLW)) transient expression in Nicotiana benthamiana leaves of an effective TBV candidate based on a fusion protein F0 comprising Pfs25 and the C0-domain of Pfs230, and the implementation of a simple and cost-effective heat treatment step for purification that yields intact recombinant protein at >90% purity with a recovery rate of >70%. The immunization of mice clearly showed that antibodies raised against plant-derived F0 completely blocked the formation of oocysts in a malaria transmission-blocking assay (TBA) making F0 an interesting TBV candidate or a component of a multi-stage malaria vaccine cocktail.

Recent developments in malaria vaccinology

The development of a highly effective malaria vaccine remains a key goal to aid in the control and eventual eradication of this devastating parasitic disease. The field has made huge strides in recent years, with the first-generation vaccine RTS,S showing modest efficacy in a Phase III clinical trial. The updated 2030 Malaria Vaccine Technology Roadmap calls for a second generation vaccine to achieve 75% efficacy over two years for both Plasmodium falciparum and Plasmodium vivax, and for a vaccine that can prevent malaria transmission. Whole-parasite immunisation approaches and combinations of pre-erythrocytic subunit vaccines are now reporting high-level efficacy, whilst exciting new approaches to the development of blood-stage and transmission-blocking vaccine subunit components are entering clinical development. The development of a highly effective multi-component multi-stage subunit vaccine now appears to be a realistic ambition. This review will cover these recent developments in malaria vaccinology.

Anti-Pfs25 human plasma reduces transmission of Plasmodium falciparum isolates that have diverse genetic backgrounds

Pfs25 is a leading candidate for a malaria transmission-blocking vaccine whose potential has been demonstrated in a phase 1 trial with recombinant Pfs25 formulated with Montanide ISA51. Because of limited sequence polymorphism, the anti-Pfs25 antibodies induced by this vaccine are likely to have transmission-blocking or -reducing activity against most, if not all, field isolates. To test this hypothesis, we evaluated transmission-blocking activities by membrane feeding assay of anti-Pfs25 plasma from the Pfs25/ISA51 phase 1 trial against Plasmodium falciparum parasites from patients in two different geographical regions of the world, Thailand and Burkina Faso. In parallel, parasite isolates from these patients were sequenced for the Pfs25 gene and genotyped for seven microsatellites. The results indicate that despite different genetic backgrounds among parasite isolates, the Pfs25 sequences are highly conserved, with a single nonsynonymous nucleotide polymorphism detected in 1 of 41 patients in Thailand and Burkina Faso. The anti-Pfs25 immune plasma had significantly higher transmission-reducing activity against parasite isolates from the two geographical regions than the nonimmune controls (P < 0.0001).

Naturally acquired immune responses to Plasmodium falciparum sexual stage antigens Pfs48/45 and Pfs230 in an area of seasonal transmission

Acquisition of immunity to Plasmodium falciparum sexual stages is a key determinant for reducing human-mosquito transmission by preventing the fertilization and the development of the parasite in the mosquito midgut. Naturally acquired immunity against sexual stages may therefore form the basis for the development of transmission-blocking vaccines, but studies conducted to date offer little in the way of consistent findings. Here, we describe the acquisition of antigametocyte immune responses in malaria-exposed individuals in Burkina Faso. A total of 719 blood samples were collected in a series of three cross-sectional surveys at the start, peak, and end of the wet season. The seroprevalence of antibodies with specificity for the sexual stage antigens Pfs48/45 and Pfs230 was 2-fold lower (22 to 28%) than that for an asexual blood stage antigen glutamate-rich protein (GLURP) (65%) or for the preerythrocytic stage antigen circumsporozoite protein (CSP) (54%). The youngest children responded at frequencies similar to those for all four antigens but, in contrast with the immune responses to GLURP and CSP that increased with age independently of season and area of residence, there was no evidence for a clear age dependence of responses to Pfs48/45 and Pfs230. Anti-Pfs230 antibodies were most prevalent at the peak of the wet season (P < 0.001). Our findings suggest that naturally acquired immunity against Pfs48/45 and Pfs230 is a function of recent exposure rather than of cumulative exposure to gametocytes.

The novel putative transporter NPT1 plays a critical role in early stages of Plasmodium berghei sexual development

Transmission of Plasmodium species from a mammalian host to the mosquito vector requires the uptake, during an infected blood meal, of gametocytes, the precursor cells of the gametes. Relatively little is known about the molecular mechanisms involved in the developmental switch from asexual development to sexual differentiation or the maturation and survival of gametocytes. Here, we show that a gene coding for a novel putative transporter, NPT1, plays a crucial role in the development of Plasmodium berghei gametocytes. Parasites lacking NPT1 are severely compromised in the production of gametocytes and the rare gametocytes produced are unable to differentiate into fertile gametes. This is the earliest block in gametocytogenesis obtained by reverse genetics and the first to demonstrate the role of a protein with a putative transport function in sexual development. These results and the high degree of conservation of NPT1 in Plasmodium species suggest that this protein could be an attractive target for the development of novel drugs to block the spread of malaria.

N-terminal prodomain of Pfs230 synthesized using a cell-free system is sufficient to induce complement-dependent malaria transmission-blocking activity

The aim of a malaria transmission-blocking vaccine is to block the development of malaria parasites in the mosquito and thus prevent subsequent infection of the human host. Previous studies have demonstrated that the gametocyte/gamete surface protein Pfs230 can induce transmission-blocking immunity and have evaluated Escherichia coli-produced Pfs230 as a transmission-blocking vaccine candidate. In this study, we used the wheat germ cell-free expression system to produce N-terminal fragments of Pfs230 and evaluated the transmission-blocking activity of antisera raised against the recombinant Pfs230 protein. The rabbit antisera reacted to the surface of cultured gametocytes and gametes of the Plasmodium falciparum NF54 line, recognized the 360-kDa form of parasite-produced Pfs230 by Western blot assay, and reduced the infectivity of NF54 parasites to Anopheles stefensi mosquitoes in the presence of complement in a standard membrane feeding assay. Thus, our data demonstrate that the N-terminal pro domain of Pfs230 is sufficient to induce complement-dependent transmission-blocking activity against P. falciparum.

Epidemiology and infectivity of Plasmodium falciparum and Plasmodium vivax gametocytes in relation to malaria control and elimination

Malaria remains a major cause of morbidity and mortality in the tropics, with Plasmodium falciparum responsible for the majority of the disease burden and P. vivax being the geographically most widely distributed cause of malaria. Gametocytes are the sexual-stage parasites that infect Anopheles mosquitoes and mediate the onward transmission of the disease. Gametocytes are poorly studied despite this crucial role, but with a recent resurgence of interest in malaria elimination, the study of gametocytes is in vogue. This review highlights the current state of knowledge with regard to the development and longevity of P. falciparum and P. vivax gametocytes in the human host and the factors influencing their distribution within endemic populations. The evidence for immune responses, antimalarial drugs, and drug resistance influencing infectiousness to mosquitoes is reviewed. We discuss how the application of molecular techniques has led to the identification of submicroscopic gametocyte carriage and to a reassessment of the human infectious reservoir. These components are drawn together to show how control measures that aim to reduce malaria transmission, such as mass drug administration and a transmission-blocking vaccine, might better be deployed.

The IC(50) of anti-Pfs25 antibody in membrane-feeding assay varies among species

Plasmodium falciparum surface protein 25 (Pfs25) is a candidate for transmission-blocking vaccines (TBVs). Anti-Pfs25 antibodies block the development of oocysts in membrane-feeding assays and we have shown the activity correlates with antibody titer. In this study, we purified Pfs25-specific IgGs to convert antibody titer to microg/mL and determined the amount of antibody required to inhibit 50% of oocyst development (IC(50)). The IC(50) were, 15.9, 4.2, 41.2, and 85.6microg/mL for mouse, rabbit, monkey and human, respectively, and the differences among species were significant. Anti-Pfs25 sera from rabbit, monkey and human showed different patterns of competition against 6 mouse monoclonal antibodies, and the avidity of antibodies among four species were also different. These data suggests that information obtained from animal studies which assess efficacy of TBV candidates may be difficult to translate to human immunization.

The role of MHC- and non-MHC-associated genes in determining the human immune response to malaria antigens

Individual susceptibility to malaria infection, disease and death is influenced by host genotype, parasite virulence and a number of environmental factors including malaria-specific immunity. Immune responses are themselves determined by a combination of host genes and environmental effects. The extent to which host genotype limits the spectrum of possible immune responses may influence the outcome of infection and has consequences for vaccine design. Associations have been observed between human major histocompatibility complex (MHC) genotype and susceptibility to severe malaria, but no similar associations have been observed for mild malarial disease or for specific antibody responses to defined malaria antigens. Epidemiological studies have shown that, in practice, neither T helper cell nor antibody responses to malaria parasites are limited by host MHC genotype, but have revealed that genes lying outside the MHC may influence T cell proliferative responses. These genes have yet to be identified, but possible candidates include T cell receptor (TcR) genes, and genes involved in TcR gene rearrangements. More importantly, perhaps, longitudinal epidemiological studies have shown that the anti-malarial antibody repertoire is selective and becomes fixed in malaria-immune individuals, but is independent of host genotype. These findings suggest that the antibody repertoire may be determined, at least in part, by stochastic events. The first of these is the generation of the T and B cell repertoire, which results from random gene recombinations and somatic mutation and is thus partially independent of germline genes. Secondly, of the profusion of immunogenic peptides which are processed and presented by antigen presenting cells, a few will, by chance, interact with T and B cell surface antigen receptors of particularly high affinity. These T and B cell clones will be selected, will expand and may come to dominate the immune response, preventing the recognition of variant epitopes presented by subsequent infections - a process known as original antigenic sin or clonal imprinting. The immune response of an individual thus reflects the balance between genetic and stochastic effects. This may have important consequences for subunit vaccine development.

A very large C-loop in EGF domain IV is characteristic of the P28 family of ookinete surface proteins

The P28 family of proteins are 28 kDa proteins expressed on the surface of sexual stages--zygote, ookinete and young oocyst stages--of Plasmodium species when the parasite resides inside the mosquito midgut. Together with P25 proteins, P28 proteins protect the parasite from the harsh proteolytic environment prevailing inside the mosquito midgut. Vaccines against these proteins induce antibodies in vertebrate hosts that are capable of inhibiting parasite development in the mosquito midgut, thus preventing transmission of the parasite from the mosquito to another human host. These transmission-blocking vaccines are helpful in reducing the burden caused by malaria, which affects 300-600 million, and kills 1-3 million, people annually. The purpose of this study was to structurally characterise six members of the P28 family of ookinete surface proteins with the help of homology modelling, to compare these proteins in terms of transmission blocking and host parasite interactions, and to analyse phylogenetic relationships within the P28 family and with the P25 family. Our results indicate that all the members of the P28 family studied have four EGF domains arranged in triangular fashion with a very big C loop present in EGF domain IV, which could serve as a diagnostic feature of the P28 family as this loop is absent in the P25 family of ookinete surface proteins. The models of the P28 family of ookinete surface proteins obtained may help in understanding the biology of the parasite inside the mosquito midgut, and in designing transmission-blocking vaccines against malaria in the absence of experimentally determined structures of these important surface proteins.

Vaccines against malaria - an update

Malaria vaccine discovery and development follow two principal strategies. Most subunit vaccines are designed to mimic naturally acquired immunity that develops over years upon continuous exposure to Plasmodium transmission. Experimental model vaccines, such as attenuated live parasites and transmission-blocking antigens, induce immune responses superior to naturally acquired immunity. The promises and hurdles of the different tracks towards an effective and affordable vaccine against malaria are discussed.

Long-lasting and transmission-blocking activity of antibodies to Plasmodium falciparum elicited in mice by protein conjugates of Pfs25

Malaria is a leading cause of morbidity and mortality, estimated to cause >1 million childhood deaths annually. Plasmodium falciparum causes the most severe form of the disease. There is as yet no licensed vaccine for this disease, despite over a half century of research. In this study, we investigated a transmission-blocking vaccine candidate, the ookinete surface protein Pfs25. Antibodies against Pfs25, drawn in during a bite, can block parasite development in the mosquito midgut, preventing transmission to other individuals. Pfs25 is a low-molecular-weight protein, by itself not immunogenic. To increase its immunogenicity, we investigated several methods of conjugating Pfs25 to itself and to other proteins: recombinant Pseudomonas aeruginosa exotoxin A, and ovalbumin, using amide, hydrazone, or thioether linkages. All conjugates were immunogenic and induced booster responses in mice. The scheme to form amide bonds between proteins by using adipic acid dihydrizide as a linker produced the most immunogenic conjugates. Adsorption of the conjugates onto aluminum hydroxide further increased the antibody response. Remarkably, the antibody levels 3 or 7 months after the last injection were significantly higher than those 1 wk after that injection. The observed transmission-blocking activity of immune sera correlated with antibody levels measured by ELISA.

Plasmodium sexual stage antigens

The sexual stages of the malaria parasite are highly specialized cells adapted to withstand major environmental changes during their development. They also induce immune responses that may affect the outcome of the infection in the mosquito. In this review Pietro Alano considers the nature and the role of the antigens expressed by Plasmodium sexual stages.

Does malaria suffer from lack of memory?

It is widely perceived that immunity to malaria is, to an extent, defective and that one component of this defective immune response is the inability to induce or maintain long-term memory responses. If true, this is likely to pose problems for development of an effective vaccine against malaria. In this article, we critically review and challenge this interpretation of the epidemiological and experimental evidence. While evasion and modulation of host immune responses clearly occurs and naturally acquired immunity is far from optimal, mechanisms to control blood-stage parasites are acquired and maintained by individuals living in endemic areas, allowing parasite density to be kept below the threshold for induction of acute disease. Furthermore, protective immunity to severe pathology is achieved relatively rapidly and is maintained in the absence of boosting by re-infection. Nevertheless, there are significant challenges to overcome. The need for multiple infections to acquire immunity means that young children remain at risk of infection for far too long. Persistent or frequent exposure to antigen seems to be required to maintain anti-parasite immunity (premunition). Lastly, pre-erythrocytic and sexual stages of the life cycle are poorly immunogenic, and there is little evidence of effective pre-erythrocytic or transmission-blocking immunity at the population level. While these problems might theoretically be due to defective immunological memory, we suggest alternative explanations. Moreover, we question the extent to which these problems are malaria-specific rather than generic (i.e. result from inherent limitations of the vertebrate immune system).

Parasite infectivity and immunity to Plasmodium falciparum gametocytes in Gambian children

Immunity to the sexual stages of Plasmodium falciparum can be induced during natural infections. Characterization of this immunity may facilitate the design of a transmission-blocking vaccine (TBV). This study aimed to assess the prevalence and serological correlates of functional transmission-blocking immunity in Gambian children (aged 1-4 years old) who were P. falciparum gametocyte carriers. Serological assays showed 100% response to fixed, whole parasites but only 42% to live gametes. Responses to the antigens Pfs230 and Pfs48/45 were 54.1% and 37.3%, respectively, in an IgG1 ELISA. 14/55 sera were capable of reducing the infectivity of laboratory isolate NF54 in a standard membrane-feeding assay (SMFA). This activity was strongly correlated with IgG1 responses to Pfs48/45 (r = 0.49, P < 0.001) and to a serological reaction with epitopes of the same molecule (r = 0.38, P = 0.003). A weaker correlation was observed with IgG1 to Pfs230 (r = 0.29, P = 0.03). In direct membrane feeding assays (DMFA) with autologous isolates, sera from 4/29 children showed transmission-blocking activity. There was no correlation with serological assays and the DMFA or between the SMFA and DMFA. This may be caused by variation in sexual stage antigens and/or alternative modes of transmission-blocking immunity, both of which have implications for vaccine implementation.

Effect of CpG oligodeoxynucleotides on the immunogenicity of Pfs25, a Plasmodium falciparum transmission-blocking vaccine antigen

Antibodies directed against Pfs25, a protein present on the surface of zygotes and ookinetes of Plasmodium falciparum, completely block pathogen transmission. We evaluated the immunomodulatory effect of CpG oligodeoxynucleotides (ODN) on the immunogenicity of recombinant Pfs25 (rPfs25) formulated in alum (Al). Immunization of mice with rPfs25 plus CpG ODN improved both the antibody titer (a 30-fold-higher antibody response than that with rPfs25-Al alone) and avidity. Coadministration of CpG ODN dramatically enhanced the titer of immunoglobulin G2A (IgG2a) compared to the titer of the IgG1-dominant response caused by rPfs25-Al alone, and the sera from the CpG ODN-coadministered group completely blocked the transmission of P. falciparum parasites to mosquitoes, as determined by membrane feeding assays. However, transmission-blocking experiments revealed that blocking efficacy was dependent on high-titer antibody levels, independent of isotypes. These results suggest that CpG ODN can be used as an adjuvant to enhance the immunogenicity of rPfs25 as a malaria transmission-blocking vaccine.

Induction of Plasmodium falciparum transmission-blocking antibodies in nonhuman primates by a combination of DNA and protein immunizations

Malaria transmission-blocking vaccination can effectively reduce and/or eliminate transmission of parasites from the human host to the mosquito vector. The immunity achieved by inducing an antibody response to surface antigens of male and female gametes and parasite stages in the mosquito. Our laboratory has developed DNA vaccine constructs, based on Pfs25 (a Plasmodium falciparum surface protein of 25 kDa), that induce a transmission-blocking immune response in mice (C. A. Lobo, R. Dhar, and N. Kumar, Infect. Immun. 67:1688-1693, 1999). To evaluate the safety, immunogenicity, and efficacy of the Pfs25 DNA vaccine in nonhuman primates, we immunized rhesus macaques (Macaca mulatta) with a DNA vaccine plasmid encoding Pfs25 or a Pfg27-Pfs25 hybrid or with the plasmid (empty plasmid) alone. Immunization with four doses of these DNA vaccine constructs elicited antibody titers that were high but nonetheless unable to reduce the parasite's infectivity in membrane feeding assays. Further boosting of the antibody response with recombinant Pfs25 formulated in Montanide ISA-720 increased antibody titers (30-fold) and significantly blocked transmission of P. falciparum gametocytes to Anopheles mosquitoes (approximately 90% reduction in oocyst numbers in the midgut). Our data show that a DNA prime-protein boost regimen holds promise for achieving transmission-blocking immunity in areas where malaria is endemic and could be effective in eradicating malaria in isolated areas where the level of malaria endemicity is low.

Transmission-blocking vaccine of vivax malaria

Malaria remains one of the leading causes of both morbidity and mortality of humans residing in tropical countries. For many malarious regions outside of Africa, development of effective transmission-blocking vaccines will require coverage against both Plasmodium falciparum and Plasmodium vivax. The genes coding for two potential P. vivax transmission-blocking antigens, Pvs25 and Pvs28, have been cloned. Mice vaccinated with yeast-produced recombinant proteins Pvs25 and Pvs28 adsorbed to aluminum hydroxide developed strong antibody responses against the immunogens. The development of oocysts in mosquitoes was completely inhibited when these antisera were ingested with the P. vivax Salvador (Sal) I strain-infected chimpanzee blood. In a large collection of P. vivax field isolates, we found only 5 nucleotide changes that would result in amino acid substitutions in Pvs25. In contrast, the Pvs28 gene had 22 nucleotide changes that would result in conservative amino acid substitutions. How the antigenic polymorphism of Pvs25 and Pvs28 would affect the efficacy of Sal I based vaccine remains to be elucidated. Clinical trials with Pvs25 and the P. falciparum ortholog Pfs25 are in preparation.

Plasmodium falciparum and P. vivax gametocyte-specific exoantigens stimulate proliferation of TCR gammadelta+ lymphocytes

Immune modulation of Plasmodium vivax and P. falciparum gametocytes occurs over the course of erythrocytic infection. The response is linked to proliferative and inflammatory responses, which may be stimulated by stage-specific gametocyte proteins. Stage-specific exoantigens were purified from supernatants of P. falciparum and P. vivax gametocyte cultures, and either primary or secondary postinfection lymphocytes were stimulated for proliferation. Five of 25 exoantigens purified from P. falciparum gametocyte cultures and 6 of 28 exoantigens isolated from P. vivax were gametocyte stage specific. Metabolic labeling of soluble P. falciparum gametocyte proteins confirmed synthesis and secretion of 5 stage-specific exoantigens, with molecular masses of 118, 62, 52, 37, and 33 kDa. Purified gametocyte exoantigens within the range of 50 to 100 kDa stage-specifically stimulated proliferation of lymphocytes from postprimary P. falciparum infections, and from postprimary and secondary P. vivax infection patients with homologous purified exoantigens. T-cell receptor (TCR)gammadelta+, and CD3+ CD8+ and CD3+ CD4- CD8- T cells were specifically upregulated from P. falciparum primary- and P. vivax secondary-infection lymphocytes, respectively, using gametocyte stage-specific exoantigens. CD25+ was the major activation marker expressed by CD3+ and gammadelta T cells when stimulated with gametocyte exoantigens. None of the T cell markers was significantly upregulated using gametocyte stage-specific exoantigens with primary-infection P. vivax lymphocytes.

Plasmodium sex determination and transmission to mosquitoes

In order to be transmitted by their mosquito vector, malaria parasites undergo sexual reproduction, which occurs between specialized male and female parasites (gametes) within the blood meal in the mosquito. Nothing was known about how Plasmodium determines the sex of its gametocytes (gamete precursors), which are produced in the vertebrate host. Recently, erythropoietin, the vertebrate hormone controlling erythropoiesis in response to anaemia, was implicated in Plasmodium sex determination in animal models of malaria. This review examines the available information and addresses the relevance of such a sex determining mechanism for Plasmodium falciparum transmission to mosquitoes, with special reference to low gametocytaemias.

Current developments in malaria transmission-blocking vaccines

Malaria is still a leading cause of morbidity and mortality in human populations. Problems, including drug-resistant parasites and insecticide resistant mosquitoes, ensure the continued hold of malaria in the tropics and sub-tropics. Each year around 100 million cases of malaria result in at least 50,000 deaths outside of sub-Saharan Africa; within sub-Saharan Africa itself, malaria causes around one million child deaths per year. New approaches for malaria control are badly needed and much effort has gone to develop malaria vaccines. In addition to giving personal protection, most such vaccines would also tend to reduce the transmission of malaria. One class of vaccine is being developed specifically for this purpose--the malaria transmission-blocking vaccines (TBV). TBVs are based upon antigens expressed on the surface of the sexual and mosquito mid-gut stages of malaria parasites. These antigens are the targets of antibodies induced by vaccination of the host and ingested with the parasites in a mosquito blood meal. The antibodies act by inhibiting the parasite's development within the mosquito itself and they thereby prevent the onward transmission of the parasites. TBVs could contribute to the total interruption of malaria transmission in many locations with relatively low transmission rates, mostly outside sub-Saharan Africa. Under almost all transmission rates, however, TBVs would help reduce malaria incidence and malaria-related morbidity and mortality. Promising recombinant TBV candidate antigens for the two main human malaria parasite species, Plasmodium falciparum and Plasmodium vivax, have been produced and tested in the laboratory; one has undergone early clinical trials.

Differential ability of specific regions of Plasmodium falciparum sexual-stage antigen, Pfs230, to induce malaria transmission-blocking immunity

Antibodies raised against an Escherichia coli-produced recombinant protein encoding a 76-kDa section (region C) of malaria transmission-blocking vaccine candidate, Pfs230, have previously been shown to significantly reduce the ability of Plasmodium falciparum parasites to infect mosquitoes (71.2-89.8%). To further define the region of the Pfs230 required for transmission-blocking activity, four recombinant proteins each encoding a section of region C (Pfs230 amino acids 443-1132) were produced using the same E. coli expression system and tested for immunogenicity in mice: (i) r230/MBP.C5' encodes the first half of region C (amino acids 443-791, six cysteines); (ii) r230/MBP.CM1 encodes only cysteine motif (CM) 1 (amino acids 583-913, eight cysteines); (iii) r230/MBP.C1.6 (amino acids 453-913, eight cysteines) also includes all of CM1; and (iv) r230/MBP.C2 encodes only CM2 (amino acids 914-1268, 11 cysteines). All the recombinant proteins induced antibodies that recognized parasite-produced Pfs230, but the titre of the Pfs230 specific-antibodies generated varied, C = C1.6 = C5' > CM1 > CM2. Two recombinants, r230/MBP.C5' and r230/MBP.C1.6, induced antibody titres that were equivalent to or greater than the titre generated by r230/MBP.C. However, in contrast to r230/MBP.C, none of the recombinants induced antibodies that effectively blocked parasite infectivity to mosquitoes. This suggests that the inclusion of amino acids 914-1132 is important for the production of the transmission-blocking epitope present in region C.

A single-chain antibody fragment specific for the Plasmodium berghei ookinete protein Pbs21 confers transmission blockade in the mosquito midgut

Mouse monoclonal antibody 13.1 (mAb 13.1) directed against Pbs21, a 21-kDa sexual-stage surface protein of Plasmodium berghei, is known to inhibit oocyst development from gametocytes and ookinetes in the mosquito midgut. To examine the properties and potential uses of a single-chain antibody fragment (scFv) for blocking transmission of malaria parasites to mosquitoes, we have cloned and sequenced the genes encoding variable regions of the immunoglobulin heavy and light chains (V(H) and V(L)) of mAb 13.1. The V(H) and V(L) genes were assembled as an scFv gene, and expressed in a baculovirus expression system. Following purification of 13.1 scFv, Western blotting and inhibition ELISA assays confirmed that 13.1 scFv retained the binding specificity of the parent mAb 13.1 for Pbs21. Furthermore, 13.1 scFv bound to the surface of P. berghei ookinetes, and blocked oocyst development in the mosquito midgut by at least 93%, as assessed by oocyst counts in mosquitoes. We suggest that the 13.1 scFv gene could be useful not only in studying the mechanism of transmission blockade, but also in generating, by mosquito germline transformation, a model system to evaluate the production of mosquitoes refractory to malaria.

Immunogenicity of malaria transmission-blocking vaccine candidate, y230.CA14 following crosslinking in the presence of tetanus toxoid

Proteolytically processed 310 kDa form of Plasmodium falciparum gamete surface antigen, Pfs230, is the target of malaria transmission-blocking monoclonal antibodies. To design a recombinant malaria transmission-blocking subunit vaccine, the amino terminus of the 310 kDa surface-exposed form of Pfs230 was mapped to amino acids (aa) 522 and 584 using a series of peptides and recombinant proteins encoding distinct regions of Pfs230. Antiserum generated against an Escherichia coli-produced recombinant protein, spanning the Pfs230 processing site and extending into the cysteine domains, r230/MBP.C (aa 443-1132), reduced parasite infectivity by 71.2-89.8%. To determine if the region spanning the cleavage site blocked malaria transmission when produced as a secreted protein by Saccharomyces cerevisiae, y230.CA14 (aa 467-584) was generated, purified, emulsified in adjuvant and used to vaccinate mice. In contrast to E. coli-produced r230/MBP.C, the immune response generated against y230. CA14 was very weak. To enhance the response, y230.CA14 was mixed with tetanus toxoid, chemically crosslinked, repurifed, and its immunogenicty compared with unconjugated y230.CA14. Conjugated-y230. CA14/TT required fewer booster injections to induce an immune response against Pfs230 and the antibodies generated reacted with the surface of intact gametes and immunoprecipitated radiolabelled Pfs230 extracted from 125I surface-labelled gametes to a greater extent. After seven injections, all y230.CA14 vaccinated mice developed anti-Pfs230 antibodies and the isotype profile was the same. In addition to enhancing the initial immune response generated against y230.CA14, conjugation focuses the immune response toward epitopes within the region of Pfs230 present on the surface of the gamete.

Metabolic changes of the malaria parasite during the transition from the human to the mosquito host

Plasmodium falciparum is an obligate human parasite that is the causative agent of the most lethal form of human malaria. Transmission of P. falciparum to a new human host requires a mosquito vector within which sexual replication occurs. P. falciparum replicates as an intracellular parasite in man and as an extracellular parasite in the mosquito, and it undergoes multiple developmental changes in both hosts. Changes in the environment and the activities of parasites in these various life-cycle stages are likely to be reflected in changes in the metabolic needs and capabilities of the parasite. Most of our knowledge of the metabolic capabilities of P. falciparum is derived from studies of the asexual erythrocytic cycle of the parasite, the portion of the parasite life cycle found in infected humans that is responsible for malarial symptoms. Efforts to control transmission and to understand the sometimes unique biology of this parasite have led to information about the metabolic capabilities of sexual and/or sporogonic stages of these parasites. This review focuses on comparing and contrasting the carbohydrate, nucleic acid, and protein synthetic capabilities of asexual erythrocytic stages and sexual stages of P. falciparum.

The biology of Plasmodium falciparum transmission stages

The most important function of any parasite is to secure transmission to new hosts. The gametocyte, the stage which has become developmentally committed to the sexual cycle, provides a critical link in the transmission of Plasmodium falciparum from the human host to the anopheline mosquito vector. It is therefore imperative that our determination to understand the biology of the gametocyte is greater than the technical obstacles which have resulted in the gametocyte being left very much out of the limelight by the intensive investigation of the asexual bloodstream parasite. Here we explore the areas of gametocyte biology which by nature of their relevance to control and pathology as well as basic biology, are the subjects of investigation in our laboratory. We also point out areas in need of particular attention.

Malaria parasite development in mosquitoes

Mosquitoes of the genus Anopheles transmit malaria parasites to humans. Anopheles mosquito species vary in their vector potential because of environmental conditions and factors affecting their abundance, blood-feeding behavior, survival, and ability to support malaria parasite development. In the complex life cycle of the parasite in female mosquitoes, a process termed sporogony, mosquitoes acquire gametocyte-stage parasites from blood-feeding on an infected host. The parasites carry out fertilization in the midgut, transform to ookinetes, then oocysts, which produce sporozoites. Sporozoites invade the salivary glands and are transmitted when the mosquito feeds on another host. Most individual mosquitoes that ingest gametocytes do not support development to the sporozoite stage. Bottle-necks occur at every stage of the cycle in the mosquito. Powerful new techniques and approaches exist for evaluating malaria parasite development and for identifying mechanisms regulating malaria parasite-vector interactions. This review focuses on those interactions that are important for the development of new approaches for evaluating and blocking transmission in nature.

Leukocytes in a Plasmodium falciparum-infected blood meal reduce transmission of malaria to Anopheles mosquitoes

Mosquitoes are infected with Plasmodium falciparum by taking a blood meal from a gametocyte carrier. Since a mosquito takes a volume of 1 to 2 microl, a blood meal may contain 1 x 10(4) to 3 x 10(4) leukocytes (WBC). The majority of WBC are composed of neutrophils which may phagocytose and kill developing gametes inside the mosquito midgut. Phagocytosis was measured in vitro by a luminol-dependent chemiluminescence (CL) assay. In the presence of P. falciparum gametes, sera from areas of endemicity had an increased CL response compared to controls. In mosquito membrane feeding experiments some such sera showed a transmission reduction which was related to the presence of viable WBC. The results of this study suggest that phagocytosis of opsonized gametes inside the mosquito midgut occurs and can contribute to a reduction in the transmission of P. falciparum parasites.

Complement-mediated lysis of Plasmodium falciparum gametes by malaria-immune human sera is associated with antibodies to the gamete surface antigen Pfs230

Antibodies to the sexual-stage surface antigens of Plasmodium falciparum, Pfs230 and Pfs48/45, can abolish the infectivity of gametes to mosquitoes; these antigens have been proposed as candidates for inclusion in a malaria transmission-blocking vaccine. One possible mechanism of antibody-mediated transmission blocking is complement-mediated gamete lysis. We have used a panel of human sera from geographically distinct regions where malaria is endemic to investigate whether this may be a mechanism of naturally acquired transmission-blocking immunity to P. falciparum. By immunoprecipitation, we have shown that antibody recognition of Pfs230 and Pfs48/45 is limited, despite universal exposure to P. falciparum gametocytes. In vitro complement-mediated lysis of P. falciparum gametes was positively associated with the presence of antibodies to Pfs230 but not with antibodies to the N-terminal region of the precursor molecule (Pfs260), which is shed from the gametocyte surface at the time of gametogenesis. Similarly, antibodies to two other gametocyte-specific proteins, Pfs48/45 and Pfg27/25, were not associated with gamete lysis. All sera which mediate gamete lysis contain immunoglobulin G1 (IgG1) and/or IgG3 antibodies to gamete surface proteins as determined by an enzyme-linked immunosorbent assay. These data suggest that Pfs230 is a major target of complement-fixing antibodies which may be important for antibody-mediated transmission-blocking immunity.

Transmission-blocking vaccines: uses and current status of development

Malaria continues to cause incomprehensible human suffering throughout most of the tropics and subtropics: in sub-Saharan Africa it is estimated that 2 million children die each year as a direct cause of infection with Plasmodium. Vector control and malaria chemotherapy that were previously effective in controlling and treating malaria are now largely ineffective due to insecticide-resistant mosquitoes and drug-resistant parasites. As alternatives to these mainstays of control, an intensive effort to develop subunit vaccines targeted at various stages of the life has been undertaken. One such vaccine, directed against the sexual and sporogonic stages and referred to as a transmission-blocking vaccine, offers the hope of controlling malaria in geographically isolated areas, preventing re-introduction of the parasite in malaria-free zones, blocking the spread of drug-resistant or vaccine escape mutants, and reducing exposure to "virulent" strains of parasites. A series of potential transmission-blocking vaccine candidates have identified and the genes encoding these surface proteins have now been isolated and sequenced. One such vaccine candidate, Pfs25, is now being tested in human Phase I safety and immunogenicity studies. Here the use and status of transmission-blocking vaccines are reviewed.

The biology of malarial parasite in the mosquito--a review

The purpose of this review is to summarize the biology of Plasmodium in the mosquito including recent data to contribute to better understanding of the developmental interaction between mosquito and malarial parasite. The entire sporogonic cycle is discussed taking into consideration different parasite/vector interactions and factors affecting parasite development to the mosquito.

Stage-specific processing of Pfs230, a Plasmodium falciparum transmission-blocking vaccine candidate

During Plasmodium falciparum gametocytogenesis, Pfs230, a malaria transmission-blocking vaccine candidate, is expressed as a 360 kDa protein localized to the parasitophorous vacuole/parasite plasma membrane. When gametocytes emerge from red blood cells, as they do when taken up in a blood meal by a mosquito, Pfs230 is processed from a 360 kDa form to a 310 kDa form the latter of which is exposed on the exterior surface of gametes. The 50 kDa portion of Pfs230, removed from the amino-terminus of the 360 kDa form, contains 25 contiguous glutamates and an EEVG16 repeat. Analogous to other P. falciparum proteins, the repeat region appears to be immunodominant. As the gamete emerges from the red blood cell and is exposed to the antibodies in the blood meal, cleavage of the immunodominant region of Pfs230 may contribute to an immune evasion strategy by the parasite.

Recombinant Pfs230, a Plasmodium falciparum gametocyte protein, induces antisera that reduce the infectivity of Plasmodium falciparum to mosquitoes

Six regions of malaria transmission-blocking target antigen, Pfs230, encoding 80% of the 363-kDa protein, were expressed as recombinant proteins in E. coli as fusions with maltose-binding protein (MBP). Antisera generated against amylose-purified recombinant Pfs230/MBP fusion proteins (r230/MBP.A-r230/MBP.F) all recognized the 360-kDa form of parasite-produced Pfs230 by immunoblot. However, only antisera against the four carboxy regions (C-F) of Pfs230 and not the two amino regions (A and B) recognized the 310-kDa form of Pfs230, the form expressed on the surface of gametes. The data suggest that the 310-kDa form of Pfs230 arises from the cleavage of 50 kDa from the amino terminus of the 360-kDa form. Furthermore, antisera against r230/MBP.C bound to the surface of intact gametes and significantly reduced (by 71.2-89.8% (rank sum analysis, P < 0.01)) the infectivity of P. falciparum parasites to mosquitoes. This is the first report of a recombinant form of a P. falciparum gametocyte protein capable of inducing antisera that reduce malaria parasite infectivity to mosquitoes.