A common feature of B cell chronic lymphocytic leukemia (CLL) is
chromosomal loss of 13q14, containing the miR15a/16-1 locus controlling B cell
proliferation. However, CLL etiology remains unclear. CLL is an adult leukemia
with an incidence that increases with advancing age. A unique feature of CLL is
biased B cell antigen receptor (BCR) usage, autoreactivity with polyreactivity,
and CD5 expression, all suggest a role for the BCR in driving CLL pathogenesis.
Among human CLLs, BCRs autoreactive with non-muscle myosin IIA (AMyIIA) are
recurrent. Here we identify an unmutated AMyIIA BCR in mouse, with distinctive
CDR3 segments capable of promoting leukemogenesis. B cells with this AMyIIA BCR
are generated by BCR-dependent signaling during B-1 fetal/neonatal development
with CD5 induction, but not in adults. These early-generated AMyIIA B1 B cells
self-renew, increase during aging, and can progress to become monoclonal B cell
lymphocytosis, followed by aggressive CLL in aged mice, often with loss of a
chromosomal region containing the miR15a/16-1 locus of varying length, as in
human CLL. Thus, the ability to generate this defined autoreactive BCR by B1 B
cells is a key predisposing step in mice, promoting progression to chronic
leukemia.