The localization of the binding site(s) on human IgG1 for the Fc receptors on homologous monocytes and heterologous mouse macrophages.

Two different methods, a rosette assay and a direct binding assay, have been employed in an examination of the binding of human IgG1 to mouse macrophages. In both cases, inhibition of IgG binding was demonstrated by Fc (CH2 + CH3 domains) and pFc' (CH3 domains) fragments of human IgG. In a homologous system, the binding of 125I-human IgG to human peripheral-blood monocytes was inhibited by the Fc fragment whereas the pFc' fragment was inactive. Scatchard plot analysis of the binding data from both the heterologous and homologous systems allowed association constants and numbers of receptors per cell to be calculated. A more thorough examination of the possible location of IgG Fc-receptor binding sites was made using less orthodox proteolytic cleavage fragments of IgG. The site on human IgG1 responsible for binding to mouse macrophage Fc receptors was confirmed to be within the CH3 domains. Human IgG1 binding to homologous monocytes was shown, using a dimeric C gamma 2 domain fragment, to be via the CH2 domains, and was dependent on the integrity of the covalent interaction between the C gamma 2 domains at the hinge region.