Ph.D. Student Researcher Harvard Univ. Cambridge, Massachusetts, United States
Disclosure(s):
Natalie Caputo: No financial relationships to disclose
Introduction/Rationale: Developing B cells undergo V(D)J recombination, generating a B cell receptor (BCR) repertoire capable of recognizing diverse antigens (Ags). Upon BCR contact with cognate Ag, B cells enter the germinal center (GC) reaction and undergo somatic hypermutation (SHM). This leads to affinity maturation, a process by which the BCR acquires mutations that can increase affinity of the BCR for its cognate Ag, thus generating higher affinity antibodies. A traditional model of B cell activation requires BCR recognition of cognate Ag for initiation of SHM; however, evidence suggests this model may be incomplete. Multiple studies measuring the percentage of Ag+ GC B cells in response to immunization have described B cells in the mature GC with no measurable BCR recognition of immunizing Ag. In addition, some Abs generated against tumors and viral pathogens do not measurably bind Ag when reverted back to germline sequence. Here, we investigate whether non-cognate B cells participate in GC reactions and undergo SHM to gain specificity for non-cognate antigen.
Methods: We developed bone marrow chimeric mouse models with monoclonal B cell repertoires to pre-define non-cognate B cells prior to Ag exposure. We serially immunized these mice with a diverse panel of non-cognate Ags and used ELISAs, flow cytometry, and single-cell RNA-sequencing to track the mutational paths of these non-cognate BCRs.
Results: Pre-defined non-cognate B cells participated in GC reactions and underwent SHM in every mouse model and Ag tested. Phylogenetic analysis revealed diverse BCR mutational pathways resulting in de novo Ag recognition.
Conclusion: Our results expand upon a traditional model of B cell activation and reveal that specific BCR Ag recognition is not required for GC entry and initiation of SHM. In the absence of B cell competition, SHM can expand BCR Ag recognition beyond the primary repertoire, a phenomenon we term ‘affinity birth via SHM’. Current studies in the lab are exploring the role of CD4+ T cells in this phenomenon.
