Gamma interferon induces colony-forming cells of the human monoblast cell line U937 to respond to inhibition by lactoferrin, transferrin, and acidic isoferritins.

Human gamma interferon (HuIFN gamma) was assessed for its capacities to induce MHC class-II antigens on U937 cells and to induce responsiveness of U937 colony-forming cells (CFC) to the suppressive influences of lactoferrin (LF), transferrin (TF), and acidic isoferritins (AIF). U937 cells grown in suspension culture for many years demonstrated variable percentages of MHC class-II antigen+ cells (6%-42%) as determined by analysis with monoclonal anti-MHC class-II and the FACS IV when checked at different times. The percentage of U937 cells positive for MHC class-II antigens, as well as the density distribution of MHC class-II antigens on these cells, was increased by preincubating the cells for 72 h in the presence of 10(-6) M indomethacin and increasing concentrations of natural HuIFN gamma up to 20-40 U/ml. Colony formation by cells preincubated in control medium plus indomethacin for 72 h was not decreased by treating cells with monoclonal anti-MHC class-II plus complement (C'), high specific activity tritiated thymidine (3HTdr), LF, TF, or AIF. After preincubation of U937 cells with natural HuIFN gamma plus indomethacin in suspension culture for 72 h, colony formation in semisolid medium was reduced 40%-50% by treating the cells with anti-MHC class-II plus C', 3HTdr, LF, TF, or AIF. Colony formation was not reduced further by LF, TF, or AIF, after cells were pretreated with anti-MHC class-II (1:200 dilution) plus C' or 3HTdr. Increasing concentrations of HuIFN gamma up to 20 U/ml increased the percentage of MHC class-II antigen+ U937 CFC as well as the sensitivity of U937 CFC to suppression by LF, TF, and AIF. The inducing activities of natural HuIFN gamma were due to the IFN gamma itself since the inducing activity of natural HuIFN gamma was inactivated by pretreatment with a monoclonal antibody against natural HuIFN gamma. Also the inducing effects were mimicked by recombinant HuIFN gamma. The suppressive effects of LF, TF, and AIF on colony formation were blocked by treating the cells with monoclonal anti-MHC class-II (1:50 dilution, but not 1:200 dilution) in the absence of C'. The suppressive effect of TF only was blocked by pretreating cells with a monoclonal antibody against the TF receptor. U937 cells can be used as a model to study the regulatory mechanisms of action of HuIFN gamma, LF, TF, and AIF.