HLA-DR-TRIGGERED INHIBITION OF HEMATOPOIESIS 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 d efined, Here we investigated the effect of anti-HLA-DR mAb H81.9 on hu man marrow cells, mAb H81.9 inhibited colony formation from purified C D34(+) marrow cells in long term culture-initiating cell assays, Inhib ition was prevented, however, if c-kit ligand (stem cell factor (SCF)) was added to cultures concurrently with H81.9. DNA histograms from cu ltured untreated marrow mononuclear cells showed 2 +/- 1.2% apoptotic nuclei, whereas 14.1 +/- 5.4% were apoptotic after 12-h exposure to mA b H81.9. The apoptotic peak was reduced to 1.2 +/- 0.8% when SCF was a dded to cultures concurrently with mAb H81.9. The addition of Fas-Ig, a fusion protein that neutralizes Fas ligand (Fas-L), also prevented m Ab 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 induce d by anti-fas mAb was significantly reduced by SCF, After H81.9 exposu re, 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), Thes e findings show that HLA-DR-mediated signals inhibit hemopoiesis in hu man marrow by a mechanism involving Fas/Fas-L-dependent signals that a re blocked by c-kit ligand. These data suggest a possible role for MHC class II molecules in the regulation of hemopoiesis.