Tunable dynamics of B cell selection in gut germinal centres

Abstract

Germinal centers (GCs), structures normally associated with B cell immunoglobulin (Ig) hypermutation and development of high-affinity antibodies upon infection or immunization, are present in gut-associated lymphoid organs of humans and mice under steady state. Gut-associated (ga)GCs can support antibody responses to enteric infections and immunization 1 . However, whether B cell selection and antibody affinity maturation can take place in face of the chronic and diverse antigenic stimulation characteristic of steady-state gaGCs is less clear 2 – 8 . Combining multicolor “Brainbow” fate-mapping and single-cell Ig sequencing, we find that 5–10% of gaGCs from specific pathogen-free (SPF) mice contained highly-dominant “winner” clones at steady state, despite rapid turnover of GC B cells. Monoclonal antibodies (mAbs) derived from these clones showed increased binding to commensal bacteria compared to their unmutated ancestors, consistent with antigen-driven selection and affinity maturation. Frequency of highly-selected gaGCs was markedly higher in germ-free (GF) than in SPF mice, and winner B cells in GF gaGCs were enriched in public IgH clonotypes found across multiple individuals, indicating strong B cell receptor (BCR)-driven selection in the absence of microbiota. Vertical colonization of GF mice with a defined microbial consortium (Oligo-MM 12 ) did not eliminate GF-associated clonotypes, yet induced a concomitant commensal-specific, affinity-matured B cell response. Thus, positive selection can take place in steady-state gaGCs, at a rate that is tunable over a wide range by the presence and composition of the microbiota.

Developmental changes in the rules for B cell selection

“…Third, GF mice still harbor gut‐associated chronic GCs containing a distinct representation of canonical public clonotypes. This finding suggests that, like for B‐1 cells, self‐antigens may be able to support both the developmental selection and peripheral expansion of chronic GC B cell clonotypes 81,82 . Finally, in line with an increased tolerance threshold early in life, recent reports have demonstrated an enrichment for polyreactive specificities among terminally differentiated lamina propria IgA PCs in mice and human 83 .…”

“…Several unique features in the repertoire of chronic GC B cells suggest that their developmental selection may have occurred under the relaxed tolerance constraints of early ontogeny 81,82 . First, gut‐associated GC B cells display a high level of clonal dominance and the enrichment for public clonotypes recurrent in mice housed under SPF conditions.…”

“…For example, B‐1 cells are largely excluded from GC reactions which are required for IgA responses 86 . Furthermore, public clonotypes that have been described in the GC and B‐1 of GF mice do not overlap suggesting that despite similar characteristics, a divergence occurs prior to peripheral antigen‐driven expansion 3,81,82 . Lastly, since the PerC B‐1 cell pool are by all measures a highly clonal dominant memory subset, 3 it retains limited potential to productively contribute to gut‐associated GCs where the chronic antigenic landscape is distinct.…”

B Cells and Microbiota in Autoimmunity

“…In lymphoid organs associated with the gut, however, germinal centers are invariably present [ 43 , 44 ]. In this context, the differentiation into memory B cells is primarily generated through T-independent responses [ 45 ].…”

“…These clonotypes seem to be replaced by commensal specific clones upon colonization with fecal bacteria or by a consortium of bacteria that shapes the B-cell repertoire. In summary, GF mice have antibodies against microbiota components, and these antibodies are the starting repertoire that supports the construction of the gut IgA repertoire against commensals [ 44 , 75 ].…”