During early stages of B-lineage differentiation in bone marrow, signals emanating from IL-7R and pre-BCR are thought to synergistically induce proliferative expansion of progenitor cells. Paradoxically, loss of pre-BCR-signaling components is associated with leukemia in both mice and humans. Exactly how progenitor B cells perform the task of balancing proliferative burst dependent on IL-7 with the termination of IL-7 signals and the initiation of L chain gene rearrangement remains to be elucidated. In this article, we provide genetic and functional evidence that the cessation of the IL-7 response of pre-B cells is controlled via a cell-autonomous mechanism that operates at a discrete developmental transition inside Fraction C' (large pre-BII) marked by transient expression of c-Myc. Our data indicate that pre-BCR cooperates with IL-7R in expanding the pre-B cell pool, but it is also critical to control the differentiation program shutting off the c-Myc gene in large pre-B cells.
The retinoblastoma gene (RB1) has been implicated as a tumor suppressor in multiple myeloma (MM), yet its role remains unclear because in the majority of cases with 13q14 deletions, un-mutated RB1 remains expressed from the retained allele. To explore the role of Rb1 in MM, we examined the functional consequences of single- and double-copy Rb1 loss in germinal center B cells, the cells of origin of MM. We generated mice without Rb1 function in germinal center B cells by crossing Rb1(Flox/Flox) with C-γ-1-Cre (Cγ1) mice expressing the Cre recombinase in class-switched B cells in a p107(-/-) background to prevent p107 from compensating for Rb1 loss (Cγ1-Rb1(F/F)-p107(-/-)). All mice developed normally, but B cells with two copies of Rb1 deleted (Cγ1-Rb1(F/F)-p107(-/-)) exhibited increased proliferation and cell death compared with Cγ1-Rb1(+/+)-p107(-/-) controls ex vivo. In vivo, Cγ1-Rb1(F/F)-p107(-/-) mice had a lower percentage of splenic B220+ cells and reduced numbers of bone marrow antigen-specific secreting cells compared with control mice. Our data indicate that Rb1 loss induces both cell proliferation and death in germinal center B cells. Because no B-cell malignancies developed after 1 year of observation, our data also suggest that Rb1 loss is not sufficient to transform post-germinal center B cells and that additional, specific mutations are likely required to cooperate with Rb1 loss to induce malignant transformation.
The mechanisms that mediate the shift from lymphopoiesis to myelopoiesis in response to infectious stress are largely unknown. We show that treatment with granulocyte colony-stimulating factor (G-CSF), which is often induced during infection, results in marked suppression of B lymphopoiesis at multiple stages of B-cell development. Mesenchymal-lineage stromal cells in the bone marrow, including CXCL12-abundant reticular (CAR) cells and osteoblasts, constitutively support B lymphopoiesis through the production of multiple B trophic factors. G-CSF acting through a monocytic cell intermediate reprograms these stromal cells, altering their capacity to support B lymphopoiesis. G-CSF treatment is associated with an expansion of CAR cells and a shift toward osteogenic lineage commitment. It markedly suppresses the production of multiple B-cell trophic factors by CAR cells and osteoblasts, including CXCL12, kit ligand, interleukin-6, interleukin-7, and insulin-like growth factor-1. Targeting bone marrow stromal cells is one mechanism by which inflammatory cytokines such as G-CSF actively suppress lymphopoiesis.
All-lymphoid progenitors (ALPs) yield few myeloid cells in vivo, but readily generate such cells in vitro. The basis for this difference remains unknown. We hypothesized that ALPs limit responsiveness to in vivo concentrations of myeloid-promoting cytokines by reducing expression of the corresponding receptors, potentially through posttranscriptional mechanisms. Consistent with such a mechanism, ALPs express higher levels of CSF1R transcripts than their upstream precursors, yet show limited cell-surface protein expression of colony-stimulating factor 1 receptor (CSF1R). All-lymphoid progenitors and other hematopoietic progenitors deficient in A disintegrin and metalloproteinase domain 17 (ADAM17), display elevated cell surface CSF1R expression. ADAM17(-/-) ALPs, however, fail to yield myeloid cells upon transplantation into irradiated recipients. Moreover, ADAM17(-/-) ALPs yield fewer macrophages in vitro than control ALPs at high concentrations of macrophage colony stimulating factor. Mice with hematopoietic-specific deletion of ADAM17 have normal numbers of myeloid and lymphoid progenitors and mature cells in vivo. These data demonstrate that ADAM17 limits CSF1R protein expression on hematopoietic progenitors, but that compensatory mechanisms prevent elevated CSF1R levels from altering lymphoid progenitor potential.
