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.

Comparison of carrier proteins to conjugate malaria transmission blocking vaccine antigens, Pfs25 and Pfs230.

Malaria transmission blocking vaccines (TBV) target the sexual stage of the parasite and have been pursued as a stand-alone vaccine or for combination with pre-erythrocytic or blood stage vaccines. Our efforts to develop TBV focus primarily on two antigens, Pfs25 and Pfs230. Chemical conjugation of these poorly immunogenic antigens to carrier proteins enhances their immunogenicity, and conjugates of these antigens to Exoprotein A (EPA) are currently under evaluation in clinical trials. Nonetheless, more potent carriers may augment the immunogenicity of these antigens for a more efficacious vaccine; here, we evaluate a series of proteins to identify such a carrier. Pfs25 and Pfs230 were chemically conjugated to 4 different carriers [tetanus toxoid (TT), a recombinant fragment of tetanus toxin heavy chain (rTThc), recombinant CRM197 produced in Pseudomonas fluorescens (CRM197) or in E. coli (EcoCRM®)] and compared to EPA conjugates in mouse immunogenicity studies. Conjugates of each antigen formulated in Alhydrogel® elicited similar antibody titers but showed differences in functional activity. At a 0.5 µg dose, Pfs230 conjugated to TT, CRM197 and EcoCRM® showed significantly higher functional activity compared to EPA. When formulated with the more potent adjuvant GLA-LSQ, all 4 alternate conjugates induced higher antibody titers as well as increased functional activity compared to the EPA conjugate. IgG subclass analysis of Pfs230 conjugates showed no carrier-dependent differences in the IgG profile. While Alhydrogel® formulations induced a Th2 dominant immune response, GLA-LSQ formulations induced a mixed Th1/Th2 response.

Outer membrane protein complex as a carrier for malaria transmission blocking antigen Pfs230.

Malaria transmission blocking vaccines (TBV) target the mosquito stage of parasite development by passive immunization of mosquitoes feeding on a vaccinated human. Through uptake of vaccine-induced antibodies in a blood meal, mosquito infection is halted and hence transmission to another human host is blocked. Pfs230 is a gametocyte and gamete surface antigen currently under clinical evaluation as a TBV candidate. We have previously shown that chemical conjugation of poorly immunogenic TBV antigens to Exoprotein A (EPA) can enhance their immunogenicity. Here, we assessed Outer Membrane Protein Complex (OMPC), a membrane vesicle derived from Neisseria meningitidis, as a carrier for Pfs230. We prepared Pfs230-OMPC conjugates with varying levels of antigen load and examined immunogenicity in mice. Chemical conjugation of Pfs230 to OMPC enhanced immunogenicity and functional activity of the Pfs230 antigen, and OMPC conjugates achieved 2-fold to 20-fold higher antibody titers than Pfs230-EPA/AdjuPhos® at different doses. OMPC conjugates were highly immunogenic even at low doses, indicating a dose-sparing effect. EPA conjugates induced an IgG subclass profile biased towards a Th2 response, whereas OMPC conjugates induced a strong Th1-biased immune response with high levels of IgG2, which can benefit Pfs230 antibody functional activity, which depends on complement activation. OMPC is a promising carrier for Pfs230 vaccines.