| B lymphopoiesis in mouse bone marrow (BM) represents a delicate balance between B cell production and loss. The present study has aimed to examine the differentiation and selection of B lineage cells during their development in BM. A comparison has been made between two major B cell differentiation models, one based on BM cell fractions sorted by immunofluorescence labeling and flow cytometry of cell surface markers and the other using fluorescence microscopy of the B lineage differentiation markers, TdT and mu heavy chains of IgM. The results have revealed a heterogeneity of B cell differentiation stages within each sorted BM fraction, contributing toward a unified model of B cell development. To examine the nature of selection during B cell development, a variety of criteria have been used to demonstrate that B cell loss in BM takes the form of apoptotic cell death. The apoptotic incidences of B cells at various developmental stages ex vivo and the rates of entry into apoptosis analyzed by flow cytometry, have demonstrated that B cells are particularly prone to apoptosis at two differentiation stages, known to follow Ig VH gene rearrangement and antigen receptor expression, respectively. It has been revealed that intracellular levels of Bcl-2 and Bax proteins are developmentally regulated in precursor B cells and are involved in the control of B cell death in both normal and RAG-2 gene-deficient mouse BM. The use of IL-7 transgenic mice, IL-7 gene-deficient mice and stromal cell cocultures demonstrates that the stromal cell-derived cytokine, IL-7, exerts an anti-apoptotic effect, associated with changes in Bax/Bcl-2 ratio. Accelerated apoptosis in op/op mutant mice suggests an augmented B cell loss in the absence of CSF-1. The results demonstrate that the loss of B lineage cells in mouse BM involves programmed cell death at critical quality control checkpoints in B cell development, influenced by both intrinsic cellular mechanisms and microenvironmental factors. |
