Beta2-microglobulin on the cell surface. Specificity of inhibition of the mixed leukocyte reaction and mitogenic properties of antibodies against the two HLA antigen polypeptide chains.

Intact antibodies, and F(ab')2, and Fab' fragments thereof directed against beta2-microglobulin (beta2m) and the HLA alloantigenic polypeptide chain impede the mixed leukocyte culture reaction (MLR). The Fab' fragments are effective only when bound to the responder cell population. The kinetics of their inhibiting action suggest that they interfere with the recognition phase of the MLR. Thus, it seems possible that beta2m may be part of or closely associated with those structures on the responding cells which recognize the stimulatory alloantigens. Antibodies against beta2m or the other HLA antigen polypeptide chain are mitogenic for peripheral blood lymphocytes. The antibodies against beta2m caused greater stimulation than those against the HLA alloantigenic polypeptide chain. Only bivalent antibodies against beta2m were mitogenic and promoted blast transformation of 1 to 2% of the lymphocytes. Although only B lymphocytes were triggered, the mitogenic response of cultures containing various proportions of B and T cells suggests that cooperation of the two cell types is necessary to obtain optimal mitogenic stimulation.