Complexes of soluble HLA antigens and anti-HLA autoantibodies in human sera: possible role in maintenance of self-tolerance.

The MHC plays an essential role in regulating the process of self-nonself discrimination. T cells recognize nonself antigens only in the context of self-MHC gene products. It is not clear, however, whether B cells are also endowed with immune receptors for self-MHC antigens. We have explored the existence of antibodies against self-MHC antigens (HLA) in the human by analyzing the specificity of anti-HLA antibodies developed by a population of 727 dialysis patients who had been monitored monthly over a period of 3-78 months. Anti-HLA autoantibodies were identified by serum screening in 119 patients. Twenty-five of these 119 patients had not been exposed to alloantigens before, indicating that the production of anti-HLA autoantibodies is not necessarily stimulated by allogeneic HLA antigens. Cross-matching of sera with autologous lymphocytes, confirmed the autoreactive nature of these anti-HLA antibodies which were of IgM or of IgG isotype in approximately equal numbers of patients. The fine specificities of anti-HLA autoantibodies was ascertained in studies which showed that antibodies can be adsorbed, and the eluted, from the membranes of target cells carrying then relevant HLA antigen. Since the antibodies characterized in our studies may be functionally active in vivo we examined the possibility that their level is controlled by anti-idiotypic antibodies or by soluble HLA antigens. We found that the titer of anti-HLA autoantibodies increased significantly if soluble HLA antigens were depleted from the serum. Our data suggest that circulating HLA antigens form immune complexes with anti-HLA autoantibodies and contribute to the maintenance of self-tolerance by inhibiting antibody binding to membrane HLA antigens. The finding that the immune repertoire includes B cells with receptors for self-MHC opens new perspectives for the study of network perturbations in autoimmune diseases.