A human antibody epitope map of Pfs230D1 derived from analysis of individuals vaccinated with a malaria transmission-blocking vaccine

Targeting Bottlenecks in Malaria Transmission: Antibody-Epitope Descriptions Guide the Design of Next-Generation Biomedical Interventions

Malaria continues to pose a significant burden to global health. Thus, a strong need exists for the development of a diverse panel of intervention strategies and modalities to combat malaria and achieve elimination and eradication goals. Deploying interventions that target bottlenecks in the transmission life cycle of the causative agent of malaria, Plasmodium parasites, is an attractive strategy. The development of highly potent antibody-based biologics, including vaccines, can be greatly facilitated by an in-depth molecular understanding of antibody-epitope interactions. Here, we provide an overview of structurally characterized antibodies targeting lead vaccine candidates expressed during the bottlenecks of the Plasmodium life cycle which include the pre-erythrocytic and sexual stages. The repeat region of the circumsporozoite protein (CSP), domain 1 of Pfs230 and domains 1 and 3 of Pfs48/45 are critical Plasmodium regions targeted by the most potent antibodies at the two bottlenecks of transmission, with other promising targets emerging and requiring further characterization.

Structure of endogenous Pfs230:Pfs48/45 in complex with potent malaria transmission-blocking antibodies

The Pfs230:Pfs48/45 complex is essential for malaria parasites to infect mosquitoes and forms the basis for current leading transmission-blocking vaccine candidates, yet little is known about its molecular assembly. Here, we used cryogenic electron microscopy to elucidate the structure of the endogenous Pfs230:Pfs48/45 complex bound to six potent transmission-blocking antibodies. Pfs230 consists of multiple domain clusters rigidified by interactions mediated through insertion domains. Membrane-anchored Pfs48/45 forms a disc-like structure and interacts with a short C-terminal peptide on Pfs230 that is critical for Pfs230 membrane-retention in vivo. Analyses of Pfs48/45- and Pfs230-targeted antibodies identify conserved epitopes on the Pfs230:Pfs48/45 complex and provides a structural paradigm for complement-dependent activity of Pfs230-targeting antibodies. Altogether, the Pfs230:Pfs48/45 antibody-complex structure presented improves our understanding of malaria transmission biology and the mechanisms of action of transmission-blocking antibodies, informing the development of next-generation transmission-blocking interventions.

Target-agnostic identification of human antibodies to Plasmodium falciparum sexual forms reveals cross-stage recognition of glutamate-rich repeats

Circulating sexual stages of Plasmodium falciparum (Pf) can be transmitted from humans to mosquitoes, thereby furthering the spread of malaria in the population. It is well established that antibodies can efficiently block parasite transmission. In search for naturally acquired antibodies targets on sexual stages, we established an efficient method for target-agnostic single B cell activation followed by high-throughput selection of human monoclonal antibodies (mAbs) reactive to sexual stages of Pf in the form of gametes and gametocyte extracts. We isolated mAbs reactive against a range of Pf proteins including well-established targets Pfs48/45 and Pfs230. One mAb, B1E11K, was cross-reactive to various proteins containing glutamate-rich repetitive elements expressed at different stages of the parasite life cycle. A crystal structure of two B1E11K Fab domains in complex with its main antigen, RESA, expressed on asexual blood stages, showed binding of B1E11K to a repeating epitope motif in a head-to-head conformation engaging in affinity-matured homotypic interactions. Thus, this mode of recognition of Pf proteins, previously described only for Pf circumsporozoite protein (PfCSP), extends to other repeats expressed across various stages. The findings augment our understanding of immune-pathogen interactions to repeating elements of the Plasmodium parasite proteome and underscore the potential of the novel mAb identification method used to provide new insights into the natural humoral immune response against Pf.

Malaria vaccines: a new era of prevention and control

Malaria killed over 600,000 people in 2022, a death toll that has not improved since 2015. Additionally, parasites and mosquitoes resistant to existing interventions are spreading across Africa and other regions. Vaccines offer hope to reduce the mortality burden: the first licensed malaria vaccines, RTS,S and R21, will be widely deployed in 2024 and should substantially reduce childhood deaths. In this Review, we provide an overview of the malaria problem and the Plasmodium parasite, then describe the RTS,S and R21 vaccines (the first vaccines for any human parasitic disease), summarizing their benefits and limitations. We explore next-generation vaccines designed using new knowledge of malaria pathogenesis and protective immunity, which incorporate antigens and platforms to elicit effective immune responses against different parasite stages in human or mosquito hosts. We describe a decision-making process that prioritizes malaria vaccine candidates for development in a resource-constrained environment. Future vaccines might improve upon the protective efficacy of RTS,S or R21 for children, or address the wider malaria scourge by preventing pregnancy malaria, reducing the burden of Plasmodium vivax or accelerating malaria elimination.

T-Cell Epitope-Based Vaccines: A Promising Strategy for Prevention of Infectious Diseases

With the development of novel vaccine strategies, T-cell epitope-based vaccines have become promising prophylactic and therapeutic tools against infectious diseases that cannot be controlled via traditional vaccines. T-cell epitope-based vaccines leverage specific immunogenic peptides to elicit protective T-cell responses against infectious pathogens. Compared to traditional vaccines, they provide superior efficacy and safety, minimizing the risk of adverse side effects. In this review, we summarized and compared the prediction and identification methods of T-cell epitopes. By integrating bioinformatic prediction and experimental validation, efficient and precise screening of T-cell epitopes can be achieved. Importantly, we delved into the development approaches to diverse T-cell epitope-based vaccines, comparing their merits and demerits, as well as discussing the prevalent challenges and perspectives in their applications. This review offers fresh perspectives for the formulation of safe and efficacious epitope-based vaccines for the devastating diseases against which no vaccines are currently available.

Multistage protective anti-CelTOS monoclonal antibodies with cross-species sterile protection against malaria

CelTOS is a malaria vaccine antigen that is conserved in Plasmodium and other apicomplexan parasites and plays a role in cell-traversal. The structural basis and mechanisms of CelTOS-induced protective immunity to parasites are unknown. Here, CelTOS-specific monoclonal antibodies (mAbs) 7g7 and 4h12 demonstrated multistage activity, protecting against liver infection and preventing parasite transmission to mosquitoes. Both mAbs demonstrated cross-species activity with sterile protection against in vivo challenge with transgenic parasites containing either P. falciparum or P. vivax CelTOS, and with transmission reducing activity against P. falciparum. The mAbs prevented CelTOS-mediated pore formation providing insight into the protective mechanisms. X-ray crystallography and mutant-library epitope mapping revealed two distinct broadly conserved neutralizing epitopes. 7g7 bound to a parallel dimer of CelTOS, while 4h12 bound to a novel antiparallel dimer architecture. These findings inform the design of antibody therapies and vaccines and raise the prospect of a single intervention to simultaneously combat P. falciparum and P. vivax malaria.

Antibody gene features associated with binding and functional activity in malaria vaccine-derived human mAbs

The impact of adjuvants on malaria vaccine-induced antibody repertoire is poorly understood. Here, we characterize the impact of two adjuvants, Alhydrogel® and AS01, on antibody clonotype diversity, binding and function, post malaria vaccination. We expressed 132 recombinant anti-Pfs230D1 human monoclonal antibodies (mAbs) from participants immunized with malaria transmission-blocking vaccine Pfs230D1, formulated with either Alhydrogel® or AS01. Anti-Pfs230D1 mAbs generated by Alhydrogel® formulation showed higher binding frequency to Pfs230D1 compared to AS01 formulation, although the frequency of functional mAbs was similar between adjuvant groups. Overall, the AS01 formulation induced anti-Pfs230D1 functional antibodies from a broader array of germline sequences versus the Alhydrogel® formulation. All mAbs using IGHV1-69 gene from the Alhydrogel® cohort bound to recombinant Pfs230D1, but did not block parasite transmission to mosquitoes, similar to the IGHV1-69 mAbs isolated from the AS01 cohort. These findings may help inform vaccine design and adjuvant selection for immunization with Plasmodium antigens.

Structure-based design of a Plasmodium vivax Duffy-binding protein immunogen focuses the antibody response to functional epitopes

The Duffy-binding protein (DBP) is a promising antigen for a malaria vaccine that would protect against clinical symptoms caused by Plasmodium vivax infection. Region II of DBP (DBP-II) contains the receptor-binding domain that engages host red blood cells, but DBP-II vaccines elicit many non-neutralizing antibodies that bind distal to the receptor-binding surface. Here, we engineered a truncated DBP-II immunogen that focuses the immune response to the receptor-binding surface. This immunogen contains the receptor-binding subdomain S1S2 and lacks the immunodominant subdomain S3. Structure-based computational design of S1S2 identified combinatorial amino acid changes that stabilized the isolated S1S2 without perturbing neutralizing epitopes. This immunogen elicited DBP-II-specific antibodies in immunized mice that were significantly enriched for blocking activity compared to the native DBP-II antigen. This generalizable design process successfully stabilized an integral core fragment of a protein and focused the immune response to desired epitopes to create a promising new antigen for malaria vaccine development.

Diversity and selection analyses identify transmission-blocking antigens as the optimal vaccine candidates in Plasmodium falciparum

BACKGROUND

A highly effective vaccine for malaria remains an elusive target, at least in part due to the under-appreciated natural parasite variation. This study aimed to investigate genetic and structural variation, and immune selection of leading malaria vaccine candidates across the Plasmodium falciparum's life cycle.

METHODS

We analysed 325 P. falciparum whole genome sequences from Zambia, in addition to 791 genomes from five other African countries available in the MalariaGEN Pf3k Database. Ten vaccine antigens spanning three life-history stages were examined for genetic and structural variations, using population genetics measures, haplotype network analysis, and 3D structure selection analysis.

FINDINGS

Among the ten antigens analysed, only three in the transmission-blocking vaccine category display P. falciparum 3D7 as the dominant haplotype. The antigens AMA1, CSP, MSP119 and CelTOS, are much more diverse than the other antigens, and their epitope regions are under moderate to strong balancing selection. In contrast, Rh5, a blood stage antigen, displays low diversity yet slightly stronger immune selection in the merozoite-blocking epitope region. Except for CelTOS, the transmission-blocking antigens Pfs25, Pfs48/45, Pfs230, Pfs47, and Pfs28 exhibit minimal diversity and no immune selection in epitopes that induce strain-transcending antibodies, suggesting potential effectiveness of 3D7-based vaccines in blocking transmission.

INTERPRETATION

These findings offer valuable insights into the selection of optimal vaccine candidates against P. falciparum. Based on our results, we recommend prioritising conserved merozoite antigens and transmission-blocking antigens. Combining these antigens in multi-stage approaches may be particularly promising for malaria vaccine development initiatives.

FUNDING

Purdue Department of Biological Sciences; Puskas Memorial Fellowship; National Institute of Allergy and Infectious Diseases (U19AI089680).

RBD design increases the functional antibody titers elicited by SARS-CoV-2 spike vaccination

Most COVID-19 vaccines contain the SARS-CoV-2 spike protein as an antigen, but they lose efficacy as neutralizing antibody titers wane and escape variants emerge. Modifying the spike antigen to increase neutralizing antibody titers would help counteract this decrease in titer. We previously used a structure-based computational design method to identify nine amino acid changes in the receptor-binding domain (RBD) of spike that stabilize the RBD and increase the neutralizing antibody titers elicited by vaccination. Here, we introduce those enhancing amino acid changes into a full-length spike (FL-S-2P) ectodomain representative of most approved vaccine antigens. These amino acid changes can be incorporated into the FL-S-2P protein without negatively effecting expression or stability. Furthermore, the amino acid changes improved functional antibody titers in both mice and monkeys following vaccination. These amino acid changes could increase the duration of protection conferred by most COVID-19 vaccines.

Target-agnostic identification of human antibodies to Plasmodium falciparum sexual forms reveals cross stage recognition of glutamate-rich repeats

Circulating sexual stages of Plasmodium falciparum (Pf) can be transmitted from humans to mosquitoes, thereby furthering the spread of malaria in the population. It is well established that antibodies (Abs) can efficiently block parasite transmission. In search for naturally acquired Ab targets on sexual stages, we established an efficient method for target-agnostic single B cell activation followed by high-throughput selection of human monoclonal antibodies (mAbs) reactive to sexual stages of Pf in the form of gamete and gametocyte extract. We isolated mAbs reactive against a range of Pf proteins including well-established targets Pfs48/45 and Pfs230. One mAb, B1E11K, was cross-reactive to various proteins containing glutamate-rich repetitive elements expressed at different stages of the parasite life cycle. A crystal structure of two B1E11K Fab domains in complex with its main antigen, RESA, expressed on asexual blood stages, showed binding of B1E11K to a repeating epitope motif in a head-to-head conformation engaging in affinity-matured homotypic interactions. Thus, this mode of recognition of Pf proteins, previously described only for PfCSP, extends to other repeats expressed across various stages. The findings augment our understanding of immune-pathogen interactions to repeating elements of the Plasmodium parasite proteome and underscore the potential of the novel mAb identification method used to provide new insights into the natural humoral immune response against Pf .

Impact Statement

A naturally acquired human monoclonal antibody recognizes proteins expressed at different stages of the Plasmodium falciparum lifecycle through affinity-matured homotypic interactions with glutamate-rich repeats.

Vaccines and monoclonal antibodies: new tools for malaria control

SUMMARYMalaria remains one of the biggest health problems in the world. While significant reductions in malaria morbidity and mortality had been achieved from 2000 to 2015, the favorable trend has stalled, rather significant increases in malaria cases are seen in multiple areas. In 2022, there were 249 million estimated cases, and 608,000 malaria-related deaths, mostly in infants and children aged under 5 years, globally. Therefore, in addition to the expansion of existing anti-malarial control measures, it is critical to develop new tools, such as vaccines and monoclonal antibodies (mAbs), to fight malaria. In the last 2 years, the first and second malaria vaccines, both targeting Plasmodium falciparum circumsporozoite proteins (PfCSP), have been recommended by the World Health Organization to prevent P. falciparum malaria in children living in moderate to high transmission areas. While the approval of the two malaria vaccines is a considerable milestone in vaccine development, they have much room for improvement in efficacy and durability. In addition to the two approved vaccines, recent clinical trials with mAbs against PfCSP, blood-stage vaccines against P. falciparum or P. vivax, and transmission-blocking vaccine or mAb against P. falciparum have shown promising results. This review summarizes the development of the anti-PfCSP vaccines and mAbs, and recent topics in the blood- and transmission-blocking-stage vaccine candidates and mAbs. We further discuss issues of the current vaccines and the directions for the development of next-generation vaccines.

A Self-Assembling Pfs230D1-Ferritin Nanoparticle Vaccine Has Potent and Durable Malaria Transmission-Reducing Activity

Malaria is caused by eukaryotic protozoan parasites of the genus Plasmodium. There are 249 million new cases and 608,000 deaths annually, and new interventions are desperately needed. Malaria vaccines can be divided into three categories: liver stage, blood stage, or transmission-blocking vaccines. Transmission-blocking vaccines prevent the transmission of disease by the mosquito vector from one human to another. Pfs230 is one of the leading transmission-blocking vaccine antigens for malaria. Here, we describe the development of a 24-copy self-assembling nanoparticle vaccine comprising domain 1 of Pfs230 genetically fused to H. pylori ferritin. The single-component Pfs230D1-ferritin construct forms a stable and homogenous 24-copy nanoparticle with good production yields. The nanoparticle is highly immunogenic, as two low-dose vaccinations of New Zealand White rabbits elicited a potent and durable antibody response with high transmission-reducing activity when formulated in two distinct adjuvants suitable for translation to human use. This single-component 24-copy Pfs230D1-ferritin nanoparticle vaccine has the potential to improve production pipelines and the cost of manufacturing a potent and durable transmission-blocking vaccine for malaria control.

Diversity and selection analyses identify transmission-blocking antigens as the optimal vaccine candidates in Plasmodium falciparum

Background

A highly effective vaccine for malaria remains an elusive target, at least in part due to the under-appreciated natural parasite variation. This study aimed to investigate genetic and structural variation, and immune selection of leading malaria vaccine candidates across the Plasmodium falciparum's life cycle.

Methods

We analyzed 325 P. falciparum whole genome sequences from Zambia, in addition to 791 genomes from five other African countries available in the MalariaGEN Pf3k Rdatabase. Ten vaccine antigens spanning three life-history stages were examined for genetic and structural variations, using population genetics measures, haplotype network analysis, and 3D structure selection analysis.

Findings

Among the ten antigens analyzed, only three in the transmission-blocking vaccine category display P. falciparum 3D7 as the dominant haplotype. The antigens AMA1, CSP, MSP119 and CelTOS, are much more diverse than the other antigens, and their epitope regions are under moderate to strong balancing selection. In contrast, Rh5, a blood stage antigen, displays low diversity yet slightly stronger immune selection in the merozoite-blocking epitope region. Except for CelTOS, the transmission-blocking antigens Pfs25, Pfs48/45, Pfs230, Pfs47, and Pfs28 exhibit minimal diversity and no immune selection in epitopes that induce strain-transcending antibodies, suggesting potential effectiveness of 3D7-based vaccines in blocking transmission.

Interpretations

These findings offer valuable insights into the selection of optimal vaccine candidates against P. falciparum. Based on our results, we recommend prioritizing conserved merozoite antigens and transmission-blocking antigens. Combining these antigens in multi-stage approaches may be particularly promising for malaria vaccine development initiatives.

Funding

Purdue Department of Biological Sciences; Puskas Memorial Fellowship; National Institute of Allergy and Infectious Diseases (U19AI089680).

Structure-guided design of VAR2CSA-based immunogens and a cocktail strategy for a placental malaria vaccine

Placental accumulation of Plasmodium falciparum infected erythrocytes results in maternal anemia, low birth weight, and pregnancy loss. The parasite protein VAR2CSA facilitates the accumulation of infected erythrocytes in the placenta through interaction with the host receptor chondroitin sulfate A (CSA). Antibodies that prevent the VAR2CSA-CSA interaction correlate with protection from placental malaria, and VAR2CSA is a high-priority placental malaria vaccine antigen. Here, structure-guided design leveraging the full-length structures of VAR2CSA produced a stable immunogen that retains the critical conserved functional elements of VAR2CSA. The design expressed with a six-fold greater yield than the full-length protein and elicited antibodies that prevent adhesion of infected erythrocytes to CSA. The reduced size and adaptability of the designed immunogen enable efficient production of multiple variants of VAR2CSA for use in a cocktail vaccination strategy to increase the breadth of protection. These designs form strong foundations for the development of potent broadly protective placental malaria vaccines.

mRNA vaccines expressing malaria transmission-blocking antigens Pfs25 and Pfs230D1 induce a functional immune response

Malaria transmission-blocking vaccines (TBV) are designed to inhibit the sexual stage development of the parasite in the mosquito host and can play a significant role in achieving the goal of malaria elimination. Preclinical and clinical studies using protein-protein conjugates of leading TBV antigens Pfs25 and Pfs230 domain 1 (Pfs230D1) have demonstrated the feasibility of TBV. Nevertheless, other promising vaccine platforms for TBV remain underexplored. The recent success of mRNA vaccines revealed the potential of this technology for infectious diseases. We explored the mRNA platform for TBV development. mRNA constructs of Pfs25 and Pfs230D1 variously incorporating signal peptides (SP), GPI anchor, and Trans Membrane (TM) domain were assessed in vitro for antigen expression, and selected constructs were evaluated in mice. Only mRNA constructs with GPI anchor or TM domain that resulted in high cell surface expression of the antigens yielded strong immune responses in mice. These mRNA constructs generated higher transmission-reducing functional activity versus the corresponding alum-adjuvanted protein-protein conjugates used as comparators. Pfs25 mRNA with GPI anchor or TM maintained >99% transmission reducing activity through 126 days, the duration of the study, demonstrating the potential of mRNA platform for TBV.

Pfs230 Domain 7 is targeted by a potent malaria transmission-blocking monoclonal antibody

Malaria transmission-blocking vaccines (TBVs) aim to induce antibodies that block Plasmodium parasite development in the mosquito midgut, thus preventing mosquitoes from becoming infectious. While the Pro-domain and first of fourteen 6-Cysteine domains (Pro-D1) of the Plasmodium gamete surface protein Pfs230 are known targets of transmission-blocking antibodies, no studies to date have discovered other Pfs230 domains that are functional targets. Here, we show that a murine monoclonal antibody (mAb), 18F25.1, targets Pfs230 Domain 7. We generated a subclass-switched complement-fixing variant, mAb 18F25.2a, using a CRISPR/Cas9-based hybridoma engineering method. This subclass-switched mAb 18F25.2a induced lysis of female gametes in vitro. Importantly, mAb 18F25.2a potently reduced P. falciparum infection of Anopheles stephensi mosquitoes in a complement-dependent manner, as assessed by standard membrane feeding assays. Together, our data identify Pfs230 Domain 7 as target for transmission-blocking antibodies and provide a strong incentive to study domains outside Pfs230Pro-D1 as TBV candidates.

Designing an effective malaria vaccine targeting Plasmodium vivax Duffy-binding protein

Malaria caused by the Plasmodium vivax parasite is a major global health burden. Immunity against blood-stage infection reduces parasitemia and disease severity. Duffy-binding protein (DBP) is the primary parasite protein responsible for the invasion of red blood cells and it is a leading subunit vaccine candidate. An effective vaccine, however, is still lacking despite decades of interest in DBP as a vaccine candidate. This review discusses the reasons for targeting DBP, the challenges associated with developing a vaccine, and modern structural vaccinology methods that could be used to create an effective DBP vaccine. Next-generation DBP vaccines have the potential to elicit a broadly protective immune response and provide durable and potent protection from P. vivax malaria.

A potent and durable malaria transmission-blocking vaccine designed from a single-component 60-copy Pfs230D1 nanoparticle

Malaria transmission-blocking vaccines (TBVs) reduce disease transmission by breaking the continuous cycle of infection between the human host and the mosquito vector. Domain 1 (D1) of Pfs230 is a leading TBV candidate and comprises the majority of transmission-reducing activity (TRA) elicited by Pfs230. Here we show that the fusion of Pfs230D1 to a 60-copy multimer of the catalytic domain of dihydrolipoyl acetyltransferase protein (E2p) results in a single-component nanoparticle composed of 60 copies of the fusion protein with high stability, homogeneity, and production yields. The nanoparticle presents a potent human transmission-blocking epitope within Pfs230D1, indicating the antigen is correctly oriented on the surface of the nanoparticle. Two vaccinations of New Zealand White rabbits with the Pfs230D1 nanoparticle elicited a potent and durable antibody response with high TRA when formulated in two distinct adjuvants suitable for translation to human use. This single-component nanoparticle vaccine may play a key role in malaria control and has the potential to improve production pipelines and the cost of manufacturing of a potent and durable TBV.

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.

Antibody gene features associated with binding and functional activity in vaccine-derived human mAbs targeting malaria parasites

Adjuvants have been essential to malaria vaccine development, but their impact on the vaccine-induced antibody repertoire is poorly understood. Here, we used cDNA sequences from antigen-specific single memory B cells to express 132 recombinant human anti-Pfs230 monoclonal antibodies (mAbs). Alhydrogel®-induced mAbs demonstrated higher binding to Pfs230D1, although functional activity was similar between adjuvants. All Alhydrogel® mAbs using IGHV1-69 gene bound to recombinant Pfs230D1, but none blocked parasite transmission to mosquitoes; similarly, no AS01 mAb using IGHV1-69 blocked transmission. Functional mAbs from both Alhydrogel® and AS01 vaccines used IGHV3-21 and IGHV3-30 genes. Antibodies with the longest CDR3 sequences were associated with binding but not functional activity. This study assesses adjuvant effects on antibody clonotype diversity during malaria vaccination.

Extending the range of Plasmodium falciparum transmission blocking antibodies

Recent work demonstrating that asymptomatic carriers of P. falciparum parasites make up a large part of the infectious reservoir highlights the need for an effective malaria vaccine. Given the historical challenges of vaccine development, multiple parasite stages have been targeted, including the sexual stages required for transmission. Using flow cytometry to efficiently screen for P. falciparum gamete/zygote surface reactivity, we identified 82 antibodies that bound live P. falciparum gametes/zygotes. Ten antibodies had significant transmission-reducing activity (TRA) in a standard membrane feeding assay and were subcloned along with 9 nonTRA antibodies as comparators. After subcloning, only eight of the monoclonals obtained have significant TRA. These eight TRA mAbs do not recognize epitopes present in any of the current recombinant transmission-blocking vaccine candidates, Pfs230D1M, Pfs48/45.6C, Pf47 D2 and rPfs25. One TRA mAb immunoprecipitates two surface antigens, Pfs47 and Pfs230, that are expressed by both gametocytes and gametes/zygotes. These two proteins have not previously been reported to associate and the recognition of both by a single TRA mAb suggests the Pfs47/Pfs230 complex is a new vaccine target. In total, Pfs230 was the dominant target antigen, with five of the eight TRA mAbs and 8 of 11 nonTRA gamete/zygote surface reactive mAbs interacting with Pfs230. Of the three remaining TRA mAbs, two recognized non-reduced, parasite-produced Pfs25 and one bound non-reduced, parasite-produced Pfs48/45. None of the TRA mAbs bound protein on an immunoblot of reduced gamete/zygote extract and two TRA mAbs were immunoblot negative, indicating none of the new TRA epitopes are linear. The identification of eight new TRA mAbs that bind epitopes not included in any of the constructs currently under advancement as transmission-blocking vaccine candidates may provide new targets worthy of further study.