Identification of a B-1 B cell–specified progenitor

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Abstract
The B-1 subpopulation of B lymphocytes differs phenotypically and functionally from conventional B-2 B cells. B-1 B cells are proposed to derive from a distinct progenitor, but such a population has not been isolated. Here we identify and characterize a B-1 B cell progenitor whose numbers peaked in fetal bone marrow but were less abundant in postnatal bone marrow. These Lin−CD45Rlo–negCD19+ cells responded to thymic stromal lymphopoietin and 'preferentially' reconstituted functional sIgMhiCD11b+CD5lo–neg B-1 B cells, but not sIgM+CD11b− B-2 B cells, in vivo. These data indicate that the CD45Rlo–negCD19+ population includes B-1 B cell–specified progenitors and support models proposing distinct developmental pathways for B-1 B cells.