The B cell response to Salmonella enteric ser. Typhimurium: promiscuous B cell recruitment for early Ab followed by extrafollicular affinity maturation (MPF6P.743)

Abstract

Abstract We have investigated the unconventional B-cell response to Salmonella typhimurium (Stm). Stm engenders a sustained IgM and IgG extrafollicular plasmablast response, with AFC numbers increasing by >100-fold, but no notable germinal centers. To investigate the properties of this response, we screened large panels of plasmablast-derived mAbs for Stm specificity, using ELISA, flow cytometry, and antigen microarray. To our surprise, the majority of the response (>98%) appeared to be non-specific. Polyclonal B-cell activation via TLRs was an intriguing possibility but excluded based on the identical response in MyD88-deficient mice. Conversely, BCR specificity was important, as infection of BCR transgenic mice with limited repertoire led to a >10-fold loss of AFCs. We thus hypothesized that the initial response was specific, but of such low affinity that Ag-binding could not be detected. Indeed, analysis of the somatic hypermutation (SHM) pattern of specific mAb V regions suggested that SHM was necessary for detectable Ag binding. This was confirmed by reverting mAbs to germline, which caused stepwise loss of reactivity. Strikingly, laser microdissection of extrafollicular plasmablast patches demonstrated robust SHM occurring outside of GCs. These results suggest a revised vision of how clonal selection and affinity maturation operate in the response to bacteria: starting affinities are virtually undetectable, yet eventually improve by SHM occurring during the plasmablast response.