Interactions of Fc receptors with antibodies against Ia antigens and other cell surface components

Two Fc receptor dependent tests were investigated to study the question of a relation between Fc receptors and known cell surface antigens, in particular I region associated (Ia) antigens: a rosette assay with antibody coated erythrocytes (EA) as indicator cells and normal mouse lymphoid cells as source of rosette forming cells, and a cytotoxicity test with antibody coated erythrocytes as target cells and normal mouse spleen cells as a source of cytotoxic cells (K cells). EA rosettes were specifically inhibited by antibodies reacting with Ia antigens. Various other antisera reacting with antigens on B lymphocytes, like anti Ly 4.2 (raised in H 2 identical mice), rabbit antimouse B cell serum, or rabbit antimouse immunoglobulin, also specifically inhibited the rosettes. No inhibition occurred in the presence of allogeneic or xenogeneic antisera reacting with T lymphocytes. K cell cytotoxicity was specifically inhibited by each of the antisera (reacting with either B cells or T cells). F(ab')₂ fragments of anti Ia antibodies could still specifically inhibit EA rosettes but they could not inhibit K cell cytotoxicity. Similar results were obtained with F(ab')₂ fragments of anti immunoglobulin antibodies. These results indicate that the mechanism of inhibition of Fc receptors in the 2 tests was different. In neither of the tests could any evidence be found for a unique association between the Fc receptors and Ia antigens. The Fc receptors on K cells did not seem to be associated at all with Ia antigens.