Rat IgG subclasses mediating binding and phagocytosis of target cells by homologous macrophages

Attachment and ingestion of 51Cr-labelled TNP-SRBC sensitized by rat IgG1, IgG2a or IgG2b-type antibodies by homologous, elicited peritoneal macrophages were studied. IgG1 was found to be the most efficient isotype in mediating these functions. The antibody doses required for a significant attachment were found to differ with the isotype of Ab, while doses needed for a significant phagocytosis and antibody-dependent cellular cytotoxicity (ADCC) varied between 400-700 Ab/SRBC with all the isotypes studied. Both binding and phagocytosis were also influenced by the degree of hapten conjugation when target cells were sensitized by IgG1. Inhibition of these functions by soluble immune complexes and monomeric immunoglobulins suggests the involvement of two Fc gamma R in binding of the three isotypes. Based on the present work and on previous results we conclude that IgG2a interacts with a receptor binding complexed IgG only (Fc gamma RII), IgG2b binds to a different receptor which appears to bind monomeric ligand as well (Fc gamma RI), while IgG1 seems to interact with both types of receptor. We propose that phagocytosis can be mediated by both Fc gamma RI and Fc gamma RII.