Resolution of three nonproliferative immature splenic B cell subsets reveals multiple selection points during peripheral B cell maturation

Although immature/transitional peripheral B cells may remain susceptible to selection pressures before full maturation, the nature and timing of these selection events remain unclear. We show that correlated expression of sur face (s) IgM (sIgM), CD23, and AA4 defines three nonproliferative subpopula tions of immature/transitional peripheral B cells. We designate these popul ations transitional (T) 1 (AA4(+)CD23(-)sIgM(high)), T2 (AA4(+)CD23(+)sIgM( high)), and T3 (AA4(+)CD23(+)sIgM(high)). Cells within all three subsets ar e functionally immature as judged by their failure to proliferate following sIgM cross-linking in vitro, and their rapid rate of turnover in vivo as a ssessed by 5-bromo-2'-deoxyuridine labeling. These labeling studies also re veal measurable cell loss at both the T1-T2 and T2-T3 transitions, suggesti ng the existence of multiple selection points within the peripheral immatur e B cell pool. Furthermore, we find that Btk-deficient (xid) mice exhibit a n incomplete developmental block at the T2-T3 transition within the immatur e B cell pool. This contrasts markedly with lyn(-/-) mice, which exhibit de pressed numbers but normal ratios of each immature peripheral B cell subset and severely reduced numbers of mature B cells. Together, these data provi de evidence for multiple selection points among immature peripheral B cells , suggesting that the B cell repertoire is shaped by multiple unique select ion events that occur within the immature/transitional peripheral B cell po ol.