BCAP inhibits proliferation and differentiation of myeloid progenitors in the steady state and during demand situations.

B-cell adaptor for phosphatidylinositol 3-kinase (BCAP) is a signaling adaptor expressed in mature hematopoietic cells, including monocytes and neutrophils. Here we investigated the role of BCAP in the homeostasis and development of these myeloid lineages. BCAP^-/- mice had more bone marrow (BM) monocytes than wild-type (WT) mice, and in mixed WT:BCAP^-/- BM chimeras, monocytes and neutrophils skewed toward BCAP^-/- origin, showing a competitive advantage for BCAP^-/- myeloid cells. BCAP was expressed in BM hematopoietic progenitors, including lineage^-Sca-1⁺c-kit⁺ (LSK), common myeloid progenitor, and granulocyte/macrophage progenitor (GMP) cells. At the steady state, BCAP^-/- GMP cells expressed more IRF8 and less C/EBPα than did WT GMP cells, which correlated with an increase in monocyte progenitors and a decrease in granulocyte progenitors among GMP cells. Strikingly, BCAP^-/- progenitors proliferated and produced more myeloid cells of both neutrophil and monocyte/macrophage lineages than did WT progenitors in myeloid colony-forming unit assays, supporting a cell-intrinsic role of BCAP in inhibiting myeloid proliferation and differentiation. Consistent with these findings, during cyclophosphamide-induced myeloablation or specific monocyte depletion, BCAP^-/- mice replenished circulating monocytes and neutrophils earlier than WT mice. During myeloid replenishment after cyclophosphamide-induced myeloablation, BCAP^-/- mice had increased LSK proliferation and increased numbers of LSK and GMP cells compared with WT mice. Furthermore, BCAP^-/- mice accumulated more monocytes and neutrophils in the spleen than did WT mice during Listeria monocytogenes infection. Together, these data identify BCAP as a novel inhibitor of myelopoiesis in the steady state and of emergency myelopoiesis during demand conditions.

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