Specificity of Receptors for IgG on Human Lymphocyte-Like Cells

Some human lymphocyte-like cells (EA-RFC) have receptors for IgG demonstrable by their ability to form rosettes with human Rh-positive O erythrocytes sensitized with anti-CD isoantibodies (Ripley). The specificity of these receptors for the various Ig classes, IgG subclasses, and fragments of the IgG molecule was determined by studying the inhibitory capacity of the corresponding immunoglobulins in the rosette assay. The receptors showed specificity only for IgG among the Ig classes and about equal affinity for IgG1 and IgG3, but only weak binding of IgG2 and IgG4 was obtained. Whereas no inhibition was obtained with Fab and F(ab')₂ fragments prepared from IgG, the Fc fragment showed strong inhibitory capacity, which was even surpassed by the IgG3 Fch fragment, containing an extension from the N-terminal part of Fc. The inhibitory capacity of the Fc and Fch fragments was considerably reduced by partial reduction and alkylation. The pFc' fragment of IgG, which corresponds to the C_γ3 region, did not inhibit rosette formation. These data indicate that mainly the C_γ2 region is involved in the binding of IgG to EA-RFC. Inhibition studies did not show any differences in the relative inhibitory capacity of monomerie, dimeric, or highly polymerized (heat-aggregated) IgG. However, antibodies of rabbit origin complexed with antigen (ferritin) gave stronger inhibition than the corresponding native Ig.