We previously demonstrated that murine MS-5 and SI/SI4 cell lines induce the proliferation of human factor-dependent UT-7 cells in the absence of normally required human cytokines and also stimulate the differentiation of CD34+/CD38-LTC-ICs. We report in this study that the effect of MS-5 cells on UT-7 cells can be completely explained by the synergistic action of nerve growth factor (NGF) and stem cell factor (SCF) produced by these murine stromal cells. Purified murine NGF was able to support short-term clone formation and long-term growth of UT-7 cells in suspension cultures as efficiently as rhu-granulocyte- macrophage colony-stimulating factor. NGF action was mediated through the TrkA receptor, in which messenger RNA (mRNA) was easily detected in UT-7 cells by Northern blot. MS-5 cells strongly expressed NGF mRNA in Northern blot, and direct implication of MS-5-derived NGF in the induction of UT-7 cells proliferation was demonstrated in inhibition assays with an anti-NGF monoclonal antibody (MoAb) that neutralized by 84% +/- 4.1% (n = 5) UT-7 clone formation. However, NGF did not act alone, and several arguments demonstrated the synergistic action of MS- 5-derived SCF: (1) an anti-c-kit partially inhibited UT-7 cells clone formation in coculture assays, (2) SCF and NGF synergized in an H3-TdR incorporation assay, and (3) the stimulatory effect of 10x-concentrated MS-5 supernatant was completely inhibited by an anti-c-kit but not by an anti-NGF, and levels of soluble NGF (1.2 ng/mL) detected by enzyme- linked immunosorbent assay in 10x supernatant of MS-5 cells cultures were below the biologically active concentrations. In contrast, although MS-5 cells also promoted the differentiation of very primitive CD34+/CD38- human stem cells both in colony assays and long-term cultures, we could not incriminate MS-5-derived NGF in the observed effect: an anti-NGF MoAb did not inhibit the synergistic effect of MS-5 cells in colony assays or long-term cultures nor did soluble muNGF duplicate MS-5 effect and survival of CD34+/CD38- clonogenic progenitor cells promoted by MS-5 was unaffected by an anti-NGF and was not induced by soluble NGF alone or combined with SCF. In contrast, NGF in synergy with SCF supported the short-term maintenance of high numbers of CD34+/CD38+ mature erythroid progenitors probably through an indirect mechanism implying macrophages. These results suggest that NGF, in which the primary target cells are outside the hematopoietic system, is present in the marrow environment and might act at some steps of hematopoietic stem cell development. These results also underline that the response of cell lines and normal stem cells to stromal cells is mediated by different pathways.