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.
The replacement of abnormal hematopoietic stem cells (HSCs) with normal transplanted HSCs can correct a wide range of hematologic disorders. Here, we provide evidence that transplantation of more differentiated progenitor cells can be used to more rapidly correct lymphoid deficiencies in unconditioned immunocompromised mice. Transplantation of flk2+ multipotent progenitors led to robust B and T cell reconstitution that was maintained for at least 16 weeks. Antigenic challenge at 16 weeks post-transplantation revealed that reconstituted lymphocytes maintained a functional repertoire. In contrast to the persistent lymphocytic engraftment, myeloid chimerism was lost by 12 weeks post-transplantation consistent with the fact that flk2+ progenitors are non-self-renewing. Thus, while more differentiated progenitors are capable of rescuing lymphoid deficiencies, transplantation of HSCs must be used for the correction of non-lymphoid disorders, and, we propose, very long-term immune reconstitution. Based on recent evidence, we discuss novel strategies to achieve the replacement of abnormal HSCs without the use of cytotoxic conditioning regimens.
