T cell activation by anti‐T3 antibodies: Comparison of IgG1 and IgG2b switch variants and direct evidence for accessory function of macrophage Fc receptors

The human T3 antigen is closely associated with the T cell receptor. Some anti‐T3 antibodies cause T cell proliferation in the presence of monocytes which have Fc receptors (FcR) that bind particular antibody subclasses. Such an interaction is thought to determine whether or not an anti‐T3 antibody is mitogenic. We examined the mitogenicity of an IgG1 antibody, UCHT1, and an IgG2b switch variant of identical specificity, UCHT1B. With autologous monocytes, 76% of individuals responded to UCHT1 and 9% to UCHT1B, falling into three patterns of responsiveness. Both antibodies in the absence of monocytes induced responsiveness to recombinant interleukin 2, even for UCHT1B nonresponder T cells. The proliferation induced by UCHT1B, however, was always less than that induced by UCHT1. These findings demonstrate the critical role played by the Fc region for mitogenesis, and suggest a possible role for the hinge region.