HLA-DR-Triggered Inhibition of Hemopoiesis Involves Fas/Fas Ligand Interactions and Is Prevented by c-kit Ligand

The function of MHC class II (HLA-DR) Ags in hemopoiesis is not well defined. Here we investigated the effect of anti-HLA-DR mAb H81.9 on human marrow cells. mAb H81.9 inhibited colony formation from purified CD34⁺ marrow cells in long term culture-initiating cell assays. Inhibition was prevented, however, if c-kit ligand (stem cell factor (SCF)) was added to cultures concurrently with H81.9. DNA histograms from cultured untreated marrow mononuclear cells showed 2 ± 1.2% apoptotic nuclei, whereas 14.1 ± 5.4% were apoptotic after 12-h exposure to mAb H81.9. The apoptotic peak was reduced to 1.2 ± 0.8% when SCF was added to cultures concurrently with mAb H81.9. The addition of Fas-Ig, a fusion protein that neutralizes Fas ligand (Fas-L), also prevented mAb H81.9-induced apoptosis. As determined by terminal deoxynucleotidyl transferase assays, agonistic anti-Fas mAb also induced apoptosis (in 13 ± 4% of cells), and combined treatment with anti-Fas mAb and H81.9 was additive (27% apoptotic nuclei). The extent of apoptosis induced by anti-Fas mAb was significantly reduced by SCF. After H81.9 exposure, Fas was up-regulated on CD34⁺ cells, and Fas-L expression was 2.5-fold higher than in controls or CD34^- cells, particularly within a small cell window with low orthogonal scatter (lymphocyte gate). These findings show that HLA-DR-mediated signals inhibit hemopoiesis in human marrow by a mechanism involving Fas/Fas-L-dependent signals that are blocked by c-kit ligand. These data suggest a possible role for MHC class II molecules in the regulation of hemopoiesis.