'B1-like' innate B cells present within adult human organs
Innate-like B cells produce natural antibodies, and include marginal-zone (MZ) B cells and B1 cells.  B1 cells have been well-studied in mice where they are pre-natally seeded into serous cavities, and are an important source of regulatory cytokines. A recent cell atlas study described putative 'B1' cells in human prenatal organs, but whether these cells persist in adult humans is unclear. Here, we identified 'B1-like' cells in the IgM+/IgA+ memory compartment in multiple human organs including kidney, lung and spleen. These cells were transcriptionally similar to murine B1 cells and human prenatal 'B1' cells, but distinct from MZ B cells. These adult human 'B1-like' cells expressed several canonical murine B1 cell markers, including AHNAK and BHLHE41. B-cell receptor (BCR) sequencing of paired human kidney and spleen samples showed IgM/IgA clones among adult kidney BCRs with a lower diversity and mutation rate than splenic cells, suggesting seeding independent of splenic B cells. Functionally, adult 'B1-like' cells expressed the regulatory cytokine TGF-beta, and had the highest number of inferred interactions with tissue myeloid cells, with macrophages in turn expressing B cell pro-survival cytokines. Altogether, our study provides evidence of 'B1-like' cells in homeostatic adult human organs. Overall design: Flushed adult human kidneys (n=5) retrieved for transplantation but deemed unsuitable for implantation (e.g., for anatomical reasons) together with matched spleen tissue were promptly homogenized into single cell suspensions. Live single CD45+CD19+ cells were subsequently FACS-sorted from these suspensions and subjected to bulk BCR sequencing to assess BCR repertoire differences between both organs at steady state.