Probing the naturally acquired immune response to malaria for broadly reactive antibodies targeting P. falciparum antigens linked with severe disease

Abstract

Severe malaria is caused by the adhesion of Plasmodium falciparum-infected erythrocytes to host receptors on vascular endothelium. Specifically, binding of the parasite variant surface antigen domain CIDRa1 to host endothelial protein C receptor (EPCR) is strongly associated with severe disease. In malaria-endemic regions, children quickly develop immunity against severe malaria, indicative of the development of an effective immune response against CIDRa1 domains. Indeed, serum IgG from naturally immune individuals exhibited reactivity against a diverse panel of CIDRa1 variants. In an effort to analyze CIDRa1 antibody responses at a monoclonal level, we isolated CIDRa1-specific memory B cells from malaria-experienced individuals living in Uganda and expressed the corresponding monoclonal antibodies (mAbs). Our approach has yielded anti-CIDRa1 mAbs ranging in breadth from variant-specific to reactive with the full spectrum of CIDRa1 domain variants, irrespective of their extensive sequence diversity. The broadest antibodies can be separated into two categories, those that bind outside the EPCR binding site and display exceptional breadth with modest inhibition of EPCR binding and those that target the EPCR binding site and show potent EPCR-binding inhibition. Two EPCR-binding site-targeting mAbs isolated from two different donors showed signs of convergent evolution. Our results demonstrate that natural P. falciparum infection can induce a broad and inhibitory antibody response against CIDRa1. Current experiments are aimed at understanding the structural basis of antibody-mediated inhibition of CIDRa1-EPCR binding to inform the design of a vaccine that protects against severe malaria.

Differences in phenotype between long-lived memory B cells againstPlasmodium falciparummerozoite antigens and variant surface antigens

“…Specifically, we showed that VH3-48 was overrepresented among IgG + atypical B cells. Interestingly, we have recently isolated two broadly inhibitory antibodies against CIDRα1 that both used VH3-48 (20).…”

“…This panel included antigen tetramers that allowed for the detection of B cells with specificity to two classes of P. falciparum antigens: (i) merozoite proteins involved in erythrocyte invasion (merozoite surface protein 1 [MSP1] and apical membrane antigen 1 [AMA1]), and (ii) the cysteine-rich interdomain region α1 (CIDRα1) domain of the variant surface antigen P. falciparum erythrocyte membrane protein 1 (PfEMP1) that is expressed on the surface of the infected erythrocyte. These antigens were selected because we have previously used these proteins to isolate monoclonal antibodies with confirmed antigen specificities (10,20). To assess the composition of the B cell compartment irrespective of antigen-specificity, we first determined the relative abundance of major B cell populations during high P. falciparum transmission and after a reduction in parasite exposure.…”