Germline Encoded B Cell Receptor-Antigen Recognition

Abstract

B cells recognize foreign material through their B cell receptors (BCRs), which are highly diverse to accommodate essentially any antigen. Within the BCR, substrate recognition is primarily mediated by the CDRH3 loop, a hypervariable complementarity determining region (CDR) that is stochastically assembled during B cell development. However, not all epitopes are created equal and CDRH3-dominated antigen recognition can fail to engage certain antibody targets, resulting in immunological subdominance. This is problematic for vaccines that must focus antibody attention upon such targets. However, in some cases, the gene encoded CDRs surrounding the hypervariable CDRH3 can hardwire for specificity to these sites, which may provide a foundation for reproducible targeting and amplification of these otherwise subdominant antibody responses. In this dissertation, we explore whether the principle of gene encoded BCR-antigen recognition can be harnessed to ‘pathway amplify’ otherwise low-titer human antibody responses to conserved sites of vulnerability on microbial pathogens. To experimentally test this, we deployed a humanized mouse model where BCRs are constrained to single VH-genes of interest yet bear human-like CDRH3 diversity. In Chapter 2, we demonstrate that the human antibody VH-gene, IGHV1-69, endows for polyclonal germline BCR specificity to the immuno-subdominant broadly neutralizing antibody (bnAb) epitope on the hemagglutinin (HA) stalk region of influenza virus. Moreover, this encoded specificity can be harnessed to amplify high titer and broadly protective stalk-directed influenza bnAbs upon vaccination with a rationally designed HA immunogen. In Chapter 3, we identify how human allelic polymorphism within IGHV1-69 modulates elicitation of the encoded bnAb response through peripheral immune tolerance, which we find is differentially gated by subtle changes within the IGHV1-69 sequence. Lastly in Chapter 4, we investigate B cell recognition of bacterial antigens and demonstrate that a single antibody VH-gene can naturally tune for BCR recognition of a conserved substructure within bacterial lipopolysaccharide (LPS) across diverse strains of gram-negative bacteria. Collectively we highlight the existence, regulation, and utility of non-conventional antigen recognition by human B cells.