Fresh sera from mice immunized by bearing an immunogenic tumor or by repeated injections of allogeneic spleen cells or xenogeneic erythrocytes powerfully suppress cytolytic T cell responses in one-way mixed lymphocyte cultures. Suppression is not antigen specific, though is mediated by immunoglobulin (Ig)G specific for the immunizing antigen. Suppression caused by IgG mimics that caused by active transforming growth factor beta (TGF-beta). IgG associates with or carries latent TGF-beta; however, suppression caused by the complex of IgG-TGF-beta requires macrophages (M phi), whereas active TGF-beta alone does not. Also, IgG dissociated from TGF-beta does not cause suppression, suggesting that M phi may take up Ig-TGF-beta, process the complex, and deliver active TGF-beta to lymphocytes. Indeed, suppression by immune serum was prevented by antibody to Fc receptors, by saturating Fc receptors with heterologous IgGs, and by antibodies against TGF-beta. The overall findings reveal a previously unrecognized regulatory circuit whereby IgG produced in response to one antigen nonspecifically downregulates cytolytic T lymphocyte responses to unrelated antigens. The findings introduce the intriguing possibility that TGF-beta delivered by IgG and processed by M phi may mediate important biological effects in processes such as wound healing, tumor growth, and some autoimmune diseases.

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