Higher antigen occupancy of high avidity CSP serum antibodies associates with protection against human malaria

R21, a next generation malaria vaccine candidate, demonstrated 74-77% efficacy among children living in a malaria endemic area. In a phase IIa controlled human malaria infection (CHMI) study, R21 demonstrated 48.6% to 78.2% vaccine efficacy. The immune mechanisms responsible for protection by the R21 immunogen are unknown. We hypothesized that both magnitude and avidity of R21-induced polyclonal antibodies (pAbs) contribute to protection status. The biophysical properties of plasmodium falciparum (Pf) circumsporozoite protein (CSP) specific pAbs of participants from CHMI study were measured by Biolayer Interferometry, including antigen binding response and dissociation rate constant (kd) when binding to recombinant CSP, central repeat-peptides (NANP)6 and (NPNA)3, and the virus like particle R21 vaccine immunogen presenting 19 repeats of NANP motif, with the C-terminal region of CSP fused to the hepatitis B surface antigen. An innovative polyclonal antibodies avidity resolution tool (PAART) was used to derive kd values reflecting different avidities of pAbs and the fraction of their contribution to total binding (antigen occupancy). We found that protected vaccinees from all groups combined displayed higher median binding response (p<0.05 for R21, CSP and (NPNA)3, Mann-Whitney U test) and slower median kd values (p<0.01 for (NANP)6 and (NPNA)3, Mann-Whitney U test) compared to non-protected vaccinees. For the majority of participants, further dissecting the heterogeneity of avidities of the R21 immunogen-induced antibodies revealed two kd components, 10−4 s−1 (higher avidity component) and >10−2 s−1 (lower avidity component). Interestingly, the median antigen occupancy of higher avidity component for CSP, (NANP)6 and (NPNA)3 binding, i.e., the dominant fraction (82-99% occupancy, mean value 95%), was greater in the protected than non-protected vaccinees. These results suggest that high magnitude and avidity pAbs contribute to the R21 vaccine protection against Pf sporozoite infection.