Antibody interference by a non-neutralizing antibody abrogates humoral protection againstPlasmodiumliver stage

Abstract

Both subunit and attenuated whole sporozoite vaccination strategies against Plasmodium infection have shown promising initial results in malaria-naïve westerners but exhibited less efficacy in malaria-exposed individuals in endemic areas. It has been hypothesized that preexisting immunity to malaria represents a significant roadblock to the development of a protective vaccine. Here, we demonstrate proof-of-concept that non-neutralizing antibodies (nNAb) can directly interfere with protective anti-PyCSP humoral responses. We developed and characterized a novel monoclonal antibody, RAM1, against the P. yoelii sporozoite major surface antigen, circumsporozoite protein (CSP). Unlike the canonical PyCSP repeat domain binding and neutralizing antibody (NAb) 2F6, RAM1 does not inhibit sporozoite traversal or entry of hepatocytes in vitro. Though 2F6 and RAM1 bind non-overlapping regions of the CSP-repeat domain, pretreatment with RAM1 abrogated 2F6’s capacity to block sporozoite traversal and invasion in vitro. Importantly, RAM1 reduced the efficacy of the polyclonal humoral response against CSP in vivo, paralleling the observed reduced efficacy of RTS,S in malaria-exposed populations. Taken together, our data demonstrate the interference of non-neutralizing antibodies with the efficacy of NAbs and may impact the efficacy of anti-CSP vaccines in malaria-exposed individuals.

Germinal center activity and B cell maturation promote protective antibody responses against Plasmodium pre-erythrocytic infection

“…Although RAM1 and RAM2 both target the major repeat domain, they do so with drastically different binding kinetics. RAM2 Fab binds to recombinant CSP with a KD 7.85 × 10 −7 M and with an association and dissociation kinetics values of Kon (1.25 × 10 4 1/Ms) and Koff (9.84 × 10 -3 1/s), whereas RAM1 Fab exhibits little binding to CSP and we were unable to derive a measurable KD (Figure S3; Vijayan et al, 2021). By comparison, the minor repeat NAb 2F6 Fab binds to rec-PyCSP with a KD of 5.24 × 10 −7 M (Vijayan et al, 2021).…”

“…RAM2 Fab binds to recombinant CSP with a KD 7.85 × 10 −7 M and with an association and dissociation kinetics values of Kon (1.25 × 10 4 1/Ms) and Koff (9.84 × 10 -3 1/s), whereas RAM1 Fab exhibits little binding to CSP and we were unable to derive a measurable KD (Figure S3; Vijayan et al, 2021). By comparison, the minor repeat NAb 2F6 Fab binds to rec-PyCSP with a KD of 5.24 × 10 −7 M (Vijayan et al, 2021). This difference in binding affinity between RAM1 and RAM2 is likely a major contributor to their differential functional activity and implies that there may be a minimum threshold for affinity that is necessary to achieve neutralization.…”

“…Previous studies have established that binding affinity to CSP is a key factor in the ability of monoclonal antibodies to neutralize infection (Epstein et al, 2011;Jongo et al, 2018;Sissoko et al, 2017;Vijayan et al, 2021). High affinity antibodies are generated through the process of affinity maturation.…”

“…To evaluate this possibility, we investigated whether the major repeat also could be a target for NAbs, or whether neutralization is solely mediated through the minor repeat motif as observed for mAb 2F6. We previously characterized a non-neutralizing antibody (nNAb), RAM1, which binds to the major repeat with relatively low affinity, does not induce the CSP reaction, nor protects in vitro or in vivo (Vijayan et al, 2021). In this study, we isolated a mAb RAM2, which also binds to an epitope in the major repeat motif like RAM1 (Figure 3A and 3D).…”

“…The repeat region in PyCSP contains two general motifs that make up the longer, N-terminal major repeat region and the shorter, more C-terminal minor repeats (Figure 2B). The minor repeat region is the target of potent α-PyCSP neutralizing antibody activity of mAb 2F6 (Bruna-Romero et al, 2004;Sack et al, 2014;Vijayan et al, 2021) but neutralization targeting the major repeat has not been reported. At week 3, the strains had equivalent IgG titers to the major repeat peptide, but BALB/cJ mice had significantly higher titers to the minor repeat peptide (Figure 2D).…”