Identification of derepressed autoimmunocompetent B lymphoid cells in NZB mice.

Plaque assays for derepressed autoimmunocompetent B lymphoid cells with specificities for two murine erythrocyte surface autoantigens have been developed. Using purified anti-HB and anti-X anti-erythrocyte autoantibodies, fluid-phase complement fixation reactions were performed to determine the influence of pH and ionic strength on the efficiency of autoantibody-mediated cytolysis of murine erythrocytes. Anti-HB autoantibody exhibited optimal cytolysis of bromelin-treated erythrocytes at pH 7·0, ionic strength 0·15; haemolysis of intact erythrocytes by anti-X autoantibody was observed over a broad range with maximum sensitivity and specificity at pH 6·6, ionic strength 0·14. NZB spleen cells secreting anti-HB autoantibody were detected in modified direct haemolysis-in-gel assays, were neutralized by soluble HB autoantigen, and appeared to represent derepressed B lymphoid cells of anti-HB type. Anti-X plaque-forming cells, not demonstrable using conventional assays, were detected under conditions corresponding to the pH and ionic strength optima determined for complement-dependent cytolysis using purified autoantibody. NZB spleen cells secreting anti-X autoantibody were detected in indirect assays employing monolayers of erythrocytes, were neutralized by soluble X autoantigen, and appeared to represent derepressed B lymphoid cells of anti-X type. The distribution of observed plaque diameters in both HB and X assays suggested that more than 95% of all cells secreting autoantibody were detected.