MAJOR HISTOCOMPATIBILITY COMPLEX CLASS II-MEDIATED INHIBITION OF HEMATOPOIESIS IN LONG-TERM MARROW CULTURES INVOLVES APOPTOSIS AND IS PREVENTED BY C-KIT LIGAND

Expression of major histocompatibility complex (MHC) class II molecule s is developmentally regulated and lineage dependent. Their role in he matopoiesis is not well defined. Previous studies in a canine model sh owed that dogs given 920 cGy of total body irradiation, transplanted w ith autologous marrow, and treated with anti-MHC class II monoclonal a ntibody (MoAb) immediately posttransplant experienced only a transient granulocyte recovery that was followed by graft failure, In the prese nt study, the effect of anti-MHC class II MoAbs on canine in vitro hem atopoiesis was investigated. Anti-MHC class II MoAb H81.9 or B1F6 (bot h recognizing nonpolymorphic determinants) had no inhibitory effect wh en added directly to colony-forming unit-granulocyte-macrophage (CFU-G M) grown in agar, However, the addition of intact MoAb or as F(ab')(2) fragments to long-term marrow cultures (LTMCs) resulted in a dose-dep endent inhibition of the generation of CFU-GM among nonadherent cells, Inhibition was most profound with MoAb added at the time of initiatio n of culture. However, even if MoAb was added 3 weeks after recharging LTMCs, CFU-GM generation rapidly decreased. In addition, the number o f adherent cells in LTMCs decreased; predominantly fibroblast-like cel ls with prominent cytoplasmic vesiculation remained. Acridine orange/e thidium bromide staining and TdT-mediated deoxyuridine triphosphate-di goxigenin nick end labeling (TUNEL) tests showed an increase in the pr oportion of apoptotic cells in both the nonadherent and adherent compa rtments. Binding of anti-MHC class II MoAb to unfractionated marrow ce lls resulted in an increase in free (Ca2+)i; no changes in tyrosine ph osphorylation pattern were observed, The addition of stem cell factor (SCF), but not granulocyte colony-stimulating factor or granulocyte-ma crophage colony-stimulating factor, to LTMCs prevented apoptosis, and the generation of CFU-GM was indistinguishable from controls, Similarl y, a supportive adherent layer was maintained. Thus, anti-MHC class II MoAbs interfere with hematopoiesis both in vitro and in vivo. The mec hanism involves programmed cell death in subpopulations of adherent an d nonadherent cells. Inhibition of hematopoiesis is abrogated by exoge nous SCF. (C) 1995 by The American Society of Hematology.