B and T cell responses to pre-erythrocytic R21/Matrix-M and blood-stage RH5.1/Matrix-M malaria vaccines in endemic settings

There is great interest in combining the licensed pre-erythrocytic malaria vaccine R21/Matrix-M ^® and the blood-stage candidate vaccine RH5.1/Matrix-M ^® to maximise efficacy against Plasmodium falciparum malaria. As protection is understood to be antibody-mediated for both vaccines, the success of a multi-stage vaccination strategy will be aided by understanding factors that impact the cellular drivers of humoral immunity, e.g. dosing regimen, exposure to malaria, and age. Prior analyses of adult vaccinees showed higher RH5-specific memory B cell responses with delayed fractional versus monthly booster dosing (50-50-10µg at 0-1-6months and 10-10-10µg at 0-1-2months, respectively), but the B cell impact of delayed booster regimens has not yet been evaluated in the target paediatric population. There have also not yet been NANP-specific B cell immunogenicity analyses with R21/Matrix-M. Here, pre- and post-vaccination PBMC from two Phase 1b clinical trials with R21/Matrix-M ^® ( NCT03580824 ; Kenya) and RH5.1/Matrix-M ^® ( NCT04318002 ; Tanzania) were analysed by flow cytometry for antigen-specific memory B cell and total (R21) or antigen-specific (RH5) circulating Tfh (cTfh) cell responses. For R21, higher frequencies of NANP-specific activated memory IgG⁺ B cells were detected in infants versus adults, and also with a higher Matrix-M ^® dose (50µg versus 25µg). Both NANP-specific IgG⁺ memory B cells and total cTfh cells correlated with anti-NANP serum IgG. For RH5.1, higher previous malaria exposure was associated with increased RH5-specific cTfh and cTfh2 cell frequencies. RH5-specific IgG⁺ memory B cell responses were greatest with delayed booster dosing (10-10-10µg at 0-1-6months) compared to monthly or delayed fractional regimens, and correlated with both peak and late time point serum antibody (2-years after final vaccination). Interestingly, while late time point anti-RH5.1 serum IgG concentrations in delayed booster children vaccinees matched previous reports with RH5.1/AS01 _B delayed fractional dosing in adults ( NCT02927145 ; UK), IgG durability and relationship to RH5-specific B cells differed between these two cohorts. While our conclusions would be strengthened by further analyses in larger clinical trials, our data indicate that age, adjuvant dose, vaccine dose, and timing of final booster vaccination impact antigen-specific cellular responses to malaria vaccines and could thus inform the development of next-generation multi-stage malaria vaccination strategies.