Fc receptor-like 6 (FCRL6) defines pre-BCR dependent and independent pathways of natural antibody selection

B-1a cells produce ‘natural’ antibodies (Abs) to neutralize pathogens and clear neo self-antigens, but the fundamental selection mechanisms that shape their polyreactive repertoires are poorly understood. Here, we identified a B cell progenitor subset defined by Fc receptor-like 6 (FCRL6) expression, harboring innate-like defense, migration, and differentiation properties conducive for natural Ab generation Compared to FCRL6- pro B cells, the repressed mitotic, DNA damage repair, and signaling activity of FCRL6+ progenitors, yielded VH repertoires with biased distal Ighv segment accessibility, constrained diversity, and hydrophobic and charged CDR-H3 sequences. Beyond nascent autoreactivity, VH11 productivity, which predominates phosphatidylcholine-specific B-1a B cell receptors (BCRs), was higher for FCRL6+ cells as was pre-BCR formation, which was required for Myc induction and VH11, but not VH12, B-1a development. Thus, FCRL6 revealed unexpected heterogeneity in the developmental origins, regulation, and selection of natural Abs at the pre-BCR checkpoint with implications for autoimmunity and lymphoproliferative disorders. FCRL6+ and FCRL6- pro B (AA4.1+Lin-CD43+B220+CD19-IgM-) cells derived from the fetal livers of embryonic day 18 and bone marrow of adult 8-12 week old mice were sorted by flow cytometry from BALB/cJ strain mice. Single cell suspensions from leg bones were pooled from 5-10 adult male and female mice and 7-10 fetal livers were pooled from embryonic day 18 fetuses isolated from pregnant mothers according to the timing of vaginal plug formation. Data are from a total of eight samples that were isolated in duplicate from four independent sorts. Each sort yielded two samples per cell type (FCRL6+ and FCRL6- pro B cells).