Evidence for interactions between monocytes and natural killer cells in the regulation of in vitro hemopoiesis.

The role of accessory cells in modulation of in vitro hemopoiesis was studied using C-mediated lysis with mAb of differing specificities. One such antibody, 25E11, which bound to all NK cells and a subset of monocytes, consistently led to enhancement of in vitro hemopoiesis when 25E11+ cells were depleted by complement or the FACS. In cultures maximally stimulated by erythropoietin and human placental conditioned medium, depletion of 25E11+ cells from beta-thalassemic PBMC resulted in up to a fourfold enhancement of multi-potential and pure erythroid colonies and a twofold enhancement of non-erythroid colonies when compared with C-only treated cells. This enhancement of in vitro hemopoiesis was also observed with cells from normal bone marrow, cord blood, and peripheral blood. The enhancement of in vitro hemopoiesis was abrogated by removal of either adherent cells or monocytes using Leu M3 antibody and C lysis. The data presented herein suggest that NK cells and a subset of monocytes may exert a negative regulatory control on both granulopoiesis and erythropoiesis. Consequently, the number of progenitor and multipotential cells in cultures of unfractionated cell populations may be greatly underestimated.