Human Fc gamma RI and Fc gamma RII interact with distinct but overlapping sites on human IgG

Cellular receptors for IgG (FcγR) mediate important protective functions. By using site-specific mutants of a chimeric antibody (mouse V H domain and L chain; human IgG3 C H domains), we have demonstrated that human FcγRI interacts with a site in the lower hinge of human IgG (residues 234 to 237) and that this interaction dictates FcγRI-mediated superoxide generation. Mutations at position 235 resulted in the most profound reductions in FcγRI recognition. We have also mapped an interaction site for FcγRII to the same region; however, mutations at position 234 and 237 resulted in the greatest reductions in FcγRII recognition. The two receptors appear to recognize overlapping but nonidentical sites on the lower hinge of IgG. Deviations from the optimal motif 234-Leu-Leu-Gly-Gly-237 may then explain the human IgG subclass specificity profile for human FcγRI and FcγRII.