Inhibition of the mixed leucocyte reaction by histocompatibility antibodies requires the Fc part.

We have examined the effect in mixed leucocyte culture (MLC) with rat cells of the IgG fraction of alloantibodies against the major histocompatibility complex (anti-BN major) and against some of the minor histocompatibility antigens. Anti-BN major specifically inhibited the MLC response, both when BN cells were responding and stimulating cells. Absorption with BN erythrocytes completely removed the haemagglutinating antibodies but had no effect of MLC inhibition, while absorption with BN lymphocytes removed MLC inhibition as well as haemagglutinating antibodies. The F(ab')2 fragments of anti-BN major did not inhibit MLC, although they still had the capacity to combine specifically with BN lymphocytes. We therefore suggest that the inhibition of MLC responses by alloantibodies is either because of a cytotoxic effect (even if cytotoxicity could not be demonstrated in unmixed cultures) or a blocking of antibody-coated lymphocytes by Fc receptor-bearing cells. Antibodies against minor histocompatibility antigens did not inhibit MLC.