HIV-1 inhibits replication of hematopoietic progenitor cells (HPC) by reducing production of hematopoietic growth factors. HIV-1 infection of mononuclear phagocytes (M) reduces LPS-induced GM-CSF, M-CSF, and IL-1.47 HIV-1 induces expression of IL-10 in mononuclear leukocytes152,153 and IL-10 inhibits LPS induced release of IL-6, IL-1, GM-CSF, and G-CSF.154,155 Infected MVEC inhibit the responsiveness of BM stromal cells to IL-1 such that IL-6 and G-CSF release is inhibited.19 The mechanism of this effect is unknown and may involve release of a repressor factor (marked in the figure as “?”). Finally, infected macrophages release less IL-143 156 and, because IL-1 controls expression of multiple hematopoietic growth factors in auxiliary cells, including endothelial (MVEC) and other stromal cells (S), the responsiveness of HPC to a given set of environmental cues is functionally reduced.