Immunoglobulin gene rearrangement and BAFF responsive maturation defines a novel B cell population undergoing extra-BM development

Transitional type-1 (T1) cells are peripheral immature B cells known to populate the spleen (spl) after completing their BCR assembly in the bone marrow (BM). To advance the understanding of splenic T1 (CD19posCD24hiCD21neg) B cells, we addressed the heterogeneity and biology of these cells using flow cytometry combined with genetically modified mice. Most recent emigrant T1 cells were selected by excluding CD23pos and including CD93high (AA4.1) B cells termed T12123DN. Transcriptomic analysis identified RAG1 and 2 as signature genes for this B cell population. Further separation of T12123DN cells based on surface IgM expression revealed a previously undescribed cell subset with undetectable cell surface IgM (-IgMneg). The spl-IgMneg subsets expresses RAG1/2 and actively undergoes Igk gene rearrangement at levels comparable to BM pre-B cells. Upon in vitro exposure to BAFF or transplantation into immunodeficient hosts, spl-IgMneg cells can give rise to fully mature IgMposIgDpos B cells. Furthermore, BAFF-R and NF-kB pathways are required for their efficient maturation. Our findings suggest that the spl-T1 population encompasses a subset of B cells that resemble but are distinct from the developing B cells in the BM. These spl-IgMneg B cells may represent receptor editing B cells, and/or precursor B cells undergoing BCR assembly and selection in the periphery, possibly providing an opportunity for tolerance induction to tissue restricted self-antigens and microbiota-derived antigens.