BACKGROUND. Vaccines that block human-to-mosquito Plasmodium transmission are needed for malaria eradication, and clinical trials have targeted zygote antigen Pfs25 for decades. We reported that a Pfs25 protein-protein conjugate vaccine formulated in alum adjuvant induced serum functional activity in both US and Malian adults. However, antibody levels declined rapidly, and transmission-reducing activity required 4 vaccine doses. Functional immunogenicity and durability must be improved before advancing transmission-blocking vaccines further in clinical development. We hypothesized that the prefertilization protein Pfs230 alone or in combination with Pfs25 would improve functional activity. METHODS. Transmission-blocking vaccine candidates based on gamete antigen Pfs230 or Pfs25 were conjugated with Exoprotein A, formulated in Alhydrogel, and administered to mice, rhesus macaques, and humans. Antibody levels were measured by ELISA and transmission-reducing activity was assessed by the standard membrane feeding assay. RESULTS. Pfs25-EPA/Alhydrogel and Pfs230D1-EPA/Alhydrogel induced similar serum functional activity in mice, but Pfs230D1-EPA induced significantly greater activity in rhesus monkeys that was enhanced by complement. In US adults, 2 vaccine doses induced complement-dependent activity in 4 of 5 Pfs230D1-EPA/Alhydrogel recipients but no significant activity in 5 Pfs25-EPA recipients, and combination with Pfs25-EPA did not increase activity over Pfs230D1-EPA alone. CONCLUSION. The complement-dependent functional immunogenicity of Pfs230D1-EPA represents a significant improvement over Pfs25-EPA in this comparative study. The rhesus model is more predictive of the functional human immune response to Pfs230D1 than is the mouse model. TRIAL REGISTRATION. ClinicalTrials.gov NCT02334462. FUNDING. Intramural Research Program of the National Institute of Allergy and Infectious Diseases, National Institutes of Health.

Background/Objectives: Rapid development of vaccines against SARS-CoV-2 was pivotal to controlling the COVID-19 pandemic. The emergency also provided a rare opportunity to test novel vaccine platforms such as mRNA in large clinical trials. Most of the early vaccines used SARS-CoV-2 Spike protein as the target antigen. Nevertheless, subsequent studies have shown that Receptor Binding Domain (RBD) of Spike also can yield efficacious vaccines, and we previously demonstrated that chemical conjugation of RBD to a carrier protein, EcoCRM^®, enhanced antibody responses and induced strong virus neutralization activity in mice. Methods: Here, we compared the immunogenicity of this conjugate to that of an approved mRNA vaccine from Pfizer/BioNTech in rhesus macaques over a period of nine months. Results: AS01-adjuvanted RBD conjugate induced a similar or better antibody response, receptor binding inhibition, and virus neutralization activity against different variants of SARS-CoV-2, compared to mRNA. IgG subclass profiles induced by conjugate and mRNA vaccines were initially dominated by IgG1 and IgG3 then switched to IgG2 and IgG4 dominant profiles during the subsequent six-month period. Polyclonal immune sera from the conjugate and mRNA had similar antibody avidity at multiple time points. Conclusions: In summary, antibody responses in rhesus macaques induced by the RBD-EcoCRM conjugate and the Spike mRNA vaccine are very similar. These results demonstrate the potential for the RBD-EcoCRM conjugate as a vaccine against SARS-CoV-2.

Functional antibody responses to malaria transmission-blocking vaccines (TBVs) are assessed using the standard membrane feeding assay (SMFA). This assay quantifies the percentage reduction of oocyst levels in mosquitoes fed gametocytes mixed with antisera/antibodies, referred to as transmission-reducing activity (TRA). As TBVs advance to large clinical trials, new scalable assays are needed to characterize vaccine responses. Here, we developed an epitope-specific competitive ELISA platform (P230Compete) for TBV candidate Pfs230D1, based on single-chain variable fragments against epitopes recognized by human monoclonal antibodies with high TRA. We quantified functional epitope-specific antibody responses (F) in phase I Pfs230D1-EPA/AS01 vaccine trial participants, using 171 serum samples collected at 2 postvaccination time points. Five antibody features were examined by P230Compete, including total IgG (reported as ELISA units, EU _F ), IgG subclasses (IgG1 _F , IgG3 _F , IgG4 _F ), and bound complement factor C1q (C1q _F ). EU _F and IgG1 _F demonstrated strong correlation and excellent prediction of TRA (≥80%) in logistic regression analysis (AUC of 0.81 for both assays after dose 3, and 0.80 and 0.76 after dose 4). Furthermore, combining EU _F and IgG1 _F showed even better predictive performance at each time point. P230Compete offers a promising proxy assay to replace SMFA in late-stage Pfs230D1 trials.