BCR repertoire analysis of extra- and intravascular WT BL6 mouse kidney B cells at homeostasis and following UPEC challenge.

B cells play a central role in humoral immunity but also have antibody-independent functions. Studies to date have focused on B cells in blood and secondary lymphoid organs but whether B cells reside in non-lymphoid organs (NLO) in homeostasis is unknown. Here we identify, using intravenous labeling and parabiosis, a bona-fide tissue-resident B cell population in lung, liver, kidney and urinary bladder, a substantial proportion of which are B-1a cells. Tissue-resident B cells are present in neonatal tissues and also in germ-free mice NLOs, albeit in lower numbers than in specific pathogen-free mice and following co-housing with 'pet-store' mice. They spatially co-localise with macrophages and regulate their polarization and function, promoting an anti-inflammatory phenotype, in-part via interleukin-10 production, with effects on bacterial clearance during urinary tract infection. Thus, our data reveal a critical role for tissue-resident B cells in determining the homeostatic 'inflammatory set-point' of myeloid cells, with important consequences for tissue immunity. Overall design: To assess whether bacterial challenge might influence kidney BCR repertoire, WT C57BL/6 female mice (8-10 week old) had urinary bladders inoculated twice with UPEC (n=5), or PBS (n=6) to induce pyelonephritis on day 0. After 56 days, their intra- and extravascular kidney B cells were FACS sorted and BCR-sequenced (isotype resolved).