Fidelity and infidelity in commitment to B-lymphocyte lineage development

During B-lymphocyte development in mouse fetal liver and bone marrow, a pre-B I cell stage is reached in which the cells express B-lineage-specific genes, such as CD19, Ig alpha and Ig beta and V-preB and lambda 5, which encode the surrogate light (SL) chain. In these pre-B I cells both alleles of the immunoglobulin heavy (IgH) chain locus are D(H)J(H) rearranged. Transplantation of pre-B I cells from wild-type (e.g. C57Bl/6) mice in histocompatible RAG-deficient hosts: leads to long-term reconstitution of some of the mature B-cell compartments and to the establishment of normal IgM levels. a third of the normal serum IRA levels, and Ige levels below the rlrtectinn limit, Neither T-lineage nor myeloid cells of donor origin can be detected in the transplanted hosts, indicating that the pre-B I cells are committed to B-lineage differentiation. Consequently, the B-cell-reconstituted hosts respond to T-cell-independent antigens but not to T-cell-dependent antigens. Responses to T-cell-dependent antigens can be restored in die pre-B I-cell-transplanted, RAG-deficient hosts by the concomitant transplantation of mature CD4(+) T cells. The transplanted wild-type pre-B I cells do nor home back to the bone marrow and become undetectable shortly after transplantation. B-lymphocyte development in Pax-5-deficient mice becomes arrested at the transition of pre-B I to pre-B II cells i.e. at the stage when V-H to D(H)J(H) rearrangements occur and when the pre-B-cell receptor, complete with mu H chains and SL chains. is normally formed. T-lineage and myeloid cell development in these mice is normal. Pre-B I cells of Pax-5-deficient mice have a wild-type pre-B I-cell-like phenotype: while they do not express Pax-5-controlled CD19 gene, and express Ig alpha to a lesser extent, they express Ig beta, V-preB and lambda 5, and proliferate normally in vitro on stromal cells in the presence of interleukin (IL)-7. Clones of these pre-B I cells carry characteristic D(H)J(H) rearrangements: on both IgH chain alleles. However, removal of IL-7 from the tissue cultures, unlike wild-type pre-B I cells, does not induce B-cell differentiation to surface IRM-expressing B cells, but induces macrophage differentiation. This differentiation into macrophages requires either the presence of stromal cells or addition of macrophage colony-stimulating factor (M-CSF). Addition of M-CSF followed by granulocyte-macrophage colony-stimulating factor induces the differentiation to MHC class II-expressing, antigen-presenting dendritic cells. In vitro differentiation to granulocytes and osteoclasts can also be observed in the presence of the appropriate cytokines. Moreover transplantation of Pax-5-deficient pre-B I clones into RAG-deficit nt hosts, while not allowing B-cell differentiation. leads to the full reconstitution of the thymus with all stages of CD4(-) CD8(-) and CD4(+) CD8(+) thymocytes, to normal positive and negative selection of thymocytes in the thymus, and to the development of normal, reactive mature CD4(+) and CD8(+) T-cell compartments in the peripheral lymphoid tissues, all carrying the done-specific D(H)J(H), rearrangements. On the other hand, Ig alpha, Ig beta V-preB and lambda 5 are turned off in the thymocytes, demonstrating that die expression of these genes does not commit cells irreversibly to the B lineage. Furthermore, Pax-5-deficient pre-B I cells are long-term reconstituting cells. They home back to the bone marrow of the RAG-deficient host, can be reisolated and regrown in tissue culture, and can be retransplanted into a secondary RAG-deficient host. This again develops thymocytes and mature T cells and allows the transplanted clonal pre-B I cells to home to the bone marrow.