Many vaccine candidates induce a strong, but transient antibody response, which makes the vaccine impractical for protection over years when the timing of exposure to the relevant agent cannot be predicted (i.e., other than with travelers going to an endemic area or troops arriving on station). Therefore, the primary goal of this project is to identify those aspects of adaptive T-cell dependent antibody responses that control the persistence of high and functional antibody levels after vaccination, with a specific focus on the choice of adjuvant in the vaccine formation. Using a combination of rodent and NHP studies, we seek to identify whether and to what extent the quantity and quality of T cell help (Tfh) for B cell responses, the specific conditions involved in B cell activation by antigen, adjuvant and platform, and the state and regulation of the bone marrow plasma cell niche individually and in aggregate contribute to robust, long-lived antibody responses. These studies will employ both candidate malarial vaccine antigens in combination with a wide range of adjuvants. Adjuvants that may not be directly useful in humans will be included in the analysis, because discovery of the mechanisms(s) by which they foster the desired responses can provide guidance in development of acceptable alternatives for human use. Insight from these animal studies will be used to guide immunological studies and vaccine design for early stage human vaccine trials using malarial antigens. For FY19, we highlight here our published results relevant to adjuvantation of Pfs230 vaccine: Scaria et al. Outer membrane protein complex as a carrier for malaria transmission blocking antigen Pfs230. We have observed that AS01-type adjuvants induce an isotype shift toward complement-fixing antibody that enhances the functional transmission blocking activity of Pfs230 vaccines. However, AS01 may not be available for future commercialization and deployment of a TBV, therefore alternative adjuvants and platforms are needed that might induce similar responses. 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. 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, thereby enhancing Pfs230 antibody functional activity which depends on complement activation. In unpublished work, we extended our studies under the Research Collaboration Agreement with the Walter Reed Army Institute of Research (WRAIR) that are examining potent adjuvants with clinical potential. We are using these Army Liposome Formulation (ALF)-based adjuvants in preclinical models to optimize our leading vaccine candidate, Pfs230, and to understand the mechanistic basis for the high activity of the AS01 adjuvant class (liposomal adjuvant incorporating TLR4 ligand and saponin faction QS21). In mouse vaccination studies with PfS230-EPA formulated in ALF-Q (an AS01 biosimilar), we demonstrated that CD4 T cell help was required to boost antibody titers after the 2nd, but not after the 3rd vaccination. Interestingly, we did not observe an increase in antibody titers after the 3rd vaccination compared to the 2nd vaccination. Furthermore, the results suggest that the carrier protein was not required after the third dose, as Pfs230 monomer alone was sufficient to induce peak titers. We also compared full and fractional doses in this model and concluded that the fractional dose does not induce greater antibody titers or transmission reducing activity. Lastly, we showed that the addition of QS21 and MPL-A to liposomal adjuvant contributed to immunogenicity of the vaccine construct, but this was not enhanced with the addition of Alhydrogel. We also conducted vaccination studies in Rhesus macaques that may be a more accurate predictor of human responses. However, similar to the mouse studies, a third dose did not increase PfS230 titers over dose 2, a finding that is in contrast to the results obtained in our clinical trial. The studies in the Rhesus model did show that vaccination with PfS230 induced greater transmission blocking activity than Pfs25. The functional activity was maintained for more than three months similar to the findings in humans. Interestingly, we observed an enhancement of the avidity of vaccine induced antibodies after each vaccination even though no increases in titers were observed.
