We recently identified defective monocyte accessory function as the cause of T cell unresponsiveness to the mitogenic activity of OKT3 antibody in cultures of peripheral blood mononuclear cells (PBMC) from five healthy subjects, members of one family. We now report that the underlying abnormality in nonresponders is at the level of monocyte Fc gamma receptors for murine IgG2a. T cell unresponsiveness was not restricted to the signal provided by OKT3 but occurred also for two other anti-T3 antibodies of the IgG2a subclass, in contrast to a normal proliferative response to IgG1 anti-T3 antibodies in one of the OKT3 nonresponders. By using cytofluorography, we found that monocytes from responders but not from nonresponders bound OKT3-FITC to their membrane. The binding could be blocked by mouse IgG2a and by human IgG, but not by mouse IgG1 nor by serum albumin. The data suggest that, through specific Fc gamma receptors for murine IgG2a, monocytes bind the Fc portion of OKT3 during T cell activation. The function of this Fc gamma receptor binding was further studied by culturing PBMC from nonresponders on plates coated with affinity-purified goat anti-mouse IgG antibodies as a substitute for monocyte Fc gamma receptors. The addition of OKT3 to nonresponder PBMC, cultured on such plates, resulted in T cell activation, as evidenced by thymidine incorporation, IL 2 production, and expression of IL 2 receptors. Soluble anti-mouse IgG was not able to substitute for monocyte Fc gamma receptors. The results demonstrate the existence of polymorphism in monocyte Fc gamma receptors for murine IgG2a. They also substantiate that an essential helper function of monocytes in T cell activation by anti-T3 is to provide a matrix for multimeric binding of the Fc portion of the anti-T3 antibodies in order to cross-link T3 molecules.

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