In this report we show that it is possible to induce the expression of HLA-DR antigens on K562 cells, previously reported to be unresponsive to interferon-gamma (IFN-γ). However, only low cell concentrations and a high dose of IFN-γ allowed the induction of HLA-DR antigens. Furthermore, the recombinant glycosylated IFN-γ is 100-fold more efficient than the unglycosylated form. This induction of HLA-DR antigens on K562 was not related to a stage of differentiation or to the presence of cells subsets specifically sensitive to IFN-γ, since repeated sorting of K562 HLA-DR-positive and negative cells did not lead to the selection of a cell subset with a different potential of induction for HLA-DR. The difficulty in obtaining induction is due to the production of a soluble endogeneous inhibitor of proteic nature, whose action is not restricted to the K562 cell line since it operates also on both epithelial and fibroblastic cells. Treatment of normal human epithelial and fibroblastic cells with conditioned medium from K562 cultures caused a marked decrease in the expression of HLA class II antigens (DR and DP) induced by IFN-γ (10,000 U/ml), but had no effect on cell growth; however, it also affected expression of HLA class I antigens. This inhibition is not mediated by prostaglandin or an IFN-α or IFN-β-dependent mechanism. Production of this inhibitor by pluripotent human leukemic cells could cause an unbalance in the complex control exerted by the immunological system during hematopoietic differentiation or leukemic progression.
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