Resolution of Three Nonproliferative Immature Splenic B Cell Subsets Reveals Multiple Selection Points During Peripheral B Cell Maturation

Abstract

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 surface (s) IgM (sIgM), CD23, and AA4 defines three nonproliferative subpopulations of immature/transitional peripheral B cells. We designate these populations transitional (T) 1 (AA4+CD23−sIgMhigh), T2 (AA4+CD23+sIgMhigh), and T3 (AA4+CD23+sIgMlow). Cells within all three subsets are functionally immature as judged by their failure to proliferate following sIgM cross-linking in vitro, and their rapid rate of turnover in vivo as assessed by 5-bromo-2′-deoxyuridine labeling. These labeling studies also reveal measurable cell loss at both the T1-T2 and T2-T3 transitions, suggesting the existence of multiple selection points within the peripheral immature B cell pool. Furthermore, we find that Btk-deficient (xid) mice exhibit an incomplete developmental block at the T2-T3 transition within the immature B cell pool. This contrasts markedly with lyn−/− mice, which exhibit depressed numbers but normal ratios of each immature peripheral B cell subset and severely reduced numbers of mature B cells. Together, these data provide evidence for multiple selection points among immature peripheral B cells, suggesting that the B cell repertoire is shaped by multiple unique selection events that occur within the immature/transitional peripheral B cell pool.

Tolerance checkpoints in B‐cell development: Johnny B good

“…In Table 1, the markers commonly used to distinguish these subsets are shown. Thus all transitional B cells express CD93, although with different levels, whereas mature B cells are CD93 negative [18][19][20]. T1 cells do not express CD21 and CD23, but express high levels of IgM and low levels of IgD expression.…”

“…Those cells that have made a productive light chain rearrangement will enter the IgM positive immature B-cell pool. These immature B cells can be distinguished from their mature counterparts by the expression of CD93 [18][19][20]. It is at this stage of development that, for the first time, cells are censored for the expression of an auto-reactive BCR.…”

“…Upon down-modulation of surface pre-BCR expression, cells stop proliferating and enter the small pre-BII compartment [3]. At this developmental stage, [18][19][20]. It is at this stage of development that, for the first time, cells are censored for the expression of an auto-reactive BCR.…”

Defining a transcriptional fingerprint of murine splenic B-cell development

“…7,8 The most immature B cells of the spleen, termed transitional 1 cells (T1), 9 are susceptible to B-cell receptor-dependent cell death/arrest. On the basis of the models proposed by Loder, 10 Allman 11 and others, T1 cells lead to another transitional intermediate, the transition 2 (T2) cell. The canonical B-cell differentiation pathway further suggests that marginal zone (MZ) cells and follicular mature (FM) cells are the direct descendents of T2 B cells.…”

“…The canonical B-cell differentiation pathway further suggests that marginal zone (MZ) cells and follicular mature (FM) cells are the direct descendents of T2 B cells. [10][11][12] This highly ordered pathway of marrow and splenic B-cell development relies upon a complex network of various environmental signals and transcription factor regulatory pathways. 13 The products of the mouse Cr2 (CD21) and Fcer2a (CD23) genes are widely used as cellular markers for differentiating splenic B-cell subsets.…”