Monocytes direct T lymphocyte stimulation of human peripheral blood granulocyte-macrophage colony formation

The committed granulocyte-macrophage progenitor cell (or colony-forming unit, CFU-GM) differentiates in vitro under the influence of the soluble glycoprotein factor(s) termed colony stimulating activity (GM-CSA), which can be derived from several cellular sources, including normal human peripheral blood mononuclear cells (PBMNC). We have investigated the respective roles of the monocyte and T lymphocyte components of PBMNC in generating GM-CSA using as an assay CFU-GM colony formation by purified peripheral blood null cells. Highly purified unstimulated T-lymphocytes did not themselves stimulate null cell CFU-GM colony formation, but when monocytes were also included, large numbers of colonies were observed, far greater than those stimulated by monocytes alone. This synergism was independent of HLA phenotype, occurring with T-lymphocyte and monocyte fractions from unrelated donors. Media conditioned by unstimulated purified monocytes promoted CFU-GM colonies from null cells only in the presence of T-lymphocytes, suggesting that monocytes produce a factor directing the synthesis and/or release of GM-CSA by T cells.