Delayed fractional dose regimen of the RTS, S/AS01 malaria vaccine candidate enhances an IgG4 response that inhibits serum opsonophagocytosis

A recent study of the RTS, S malaria vaccine, which is based on the circumsporozoite protein (CSP), demonstrated an increase in efficacy from 50-60% to 80% when using a delayed fractional dose regimen, in which the standard 0-1-2 month immunization schedule was modified to a 0-1-7 month schedule and the third immunization was delivered at 20% of the full dose. Given the role that antibodies can play in RTS, S-induced protection, we sought to determine how the modified regimen alters IgG subclasses and serum opsonophagocytic activity (OPA). Previously, we showed that lower CSP-mediated OPA was associated with protection in an RTS, S study. Here we report that the delayed fractional dose regimen resulted in decreased CSP-mediated OPA and an enhanced CSP-specific IgG4 response. Linear regression modeling predicted that CSP-specific IgG1 promote OPA, and that CSP-specific IgG4 interferes with OPA, which we subsequently confirmed by IgG subclass depletion. Although the role of IgG4 antibodies and OPA in protection is still unclear, our findings, combined with previous results that the delayed fractional dose increases CSP-specific antibody avidity and somatic hypermutation frequency in CSP-specific B cells, demonstrate how changes in vaccine regimen alone can significantly alter the quality of antibody responses to improve vaccine efficacy.