Specificity of receptors for IgG on human cytotoxic plaque-forming mononuclear leukocytes. Induction and inhibition of plaque-forming activity.

Intact rabbit IgG antisheep erythrocyte antibodies, and the corresponding F(ab')2 fragments and partially reduced and alkylated IgG were studied for the capacity to induce cytotoxic plaque formation by normal human mononuclear leukocytes in surface monolayers of sheep erythrocytes. The F (ab')2 fragments did not induce plaque formation, whereas partially reduced and alkylated IgG antibodies had a good plaque-inducing capacity compared to untreated IgG anti-SRBC antibodies. The plaque formation was inhibited by human IgG, but not by IgM, IgA, IgD or IgE. Normal and myeloma IgG in aggregated form gave a stronger inhibition than the corresponding proteins. Strong inhibition was observed with IgG1, IgG3, and IgG4, and with IgG2 after aggregation. Both the Fc and pFc' corresponding to the C terminal domain gave a strong inhibition. Thus, the region of the IgG molecule involved in binding to the Fc receptor of the plaque-forming cells appears to be located within the CH3 domain. These observations, therefore, indicate that the plaque-forming cells are of monocytic origin.