title: Transitional B lymphocyte subsets operate as distinct checkpoints in murine splenic B cell development.
abstract: Signaling through the Ag receptor is required for peripheral B lymphocyte maturation and maintenance. Defects in components of the B cell receptor (BCR) signalosome result in developmental blocks at the transition from immature (heat-stable Ag (HSA)(high)) to mature (HSA(low)) B cells. Recent studies have subdivided the immature, or transitional, splenic B cells into two subsets, transitional 1 (T1) and transitional 2 (T2) cells. T1 and T2 cells express distinct surface markers and are located in distinct anatomic locations. In this report, we evaluated the BCR signaling capacity of T1 and T2 B cell subsets. In response to BCR engagement, T2 cells rapidly entered cell cycle and resisted cell death. In contrast, T1 cells did not proliferate and instead died after BCR stimulation. Correlating with these results, T2 cells robustly induced expression of the cell cycle regulator cyclin D2 and the antiapoptotic factors A1/Bfl-1 and Bcl-x(L) and exhibited activation of Akt. In contrast, T1 cells failed to up-regulate these markers. BCR stimulation of T2 cells also led to down-regulation of CD21 and CD24 (HSA) expression, resulting in a mature B cell phenotype. In addition, T2 cells from Bruton's tyrosine kinase-deficient Xid mice failed to generate these proliferative and survival responses, suggesting a requirement for the BCR signalosome specifically at the T2 stage. Taken together, these data clearly demonstrate that T2 immature B cells comprise a discrete developmental subset that mediates BCR-dependent proliferative, prosurvival, and differentiation signals. Their distinct BCR-dependent responses suggest unique roles for T1 vs T2 cells in peripheral B cell selection.
authors.affiliation: Molecular Biology Institute, University of California-Los Angeles, Los Angeles, CA 90095, USA.
authors.forename: Thomas T
authors.initials: TT
authors.lastname: Su
authors.forename: David J
authors.initials: DJ
authors.lastname: Rawlings
mesh_terms: D000077329:Agammaglobulinaemia Tyrosine Kinase; D000818:Animals; D016175:B-Lymphocyte Subsets / Q000276:immunology*; D002454:Cell Differentiation; D002470:Cell Survival; D002478:Cells, Cultured; D056742:Cyclin D2; D016213:Cyclins / Q000096:biosynthesis / Q000235:genetics; D016130:Immunophenotyping; D007700:Kinetics; D008213:Lymphocyte Activation; D051379:Mice; D008807:Mice, Inbred BALB C; D018448:Models, Immunological; D011505:Protein-Tyrosine Kinases / Q000502:physiology; D012333:RNA, Messenger / Q000096:biosynthesis; D011947:Receptors, Antigen, B-Cell / Q000276:immunology; D015398:Signal Transduction; D013154:Spleen / Q000254:growth & development / Q000276:immunology*; D015854:Up-Regulation
chemical_list: C486196:Ccnd2 protein, mouse; D056742:Cyclin D2; D016213:Cyclins; D012333:RNA, Messenger; D011947:Receptors, Antigen, B-Cell; D011505:Protein-Tyrosine Kinases; D000077329:Agammaglobulinaemia Tyrosine Kinase; C000625951:Btk protein, mouse