Human T cell activation induced by a monoclonal mouse IgG3 anti-CD3 antibody (RIV9) requires binding of the Fc part of the antibody to the monocytic 72-kDa high-affinity Fc receptor (FcRI)

The mitogenic activity of anti-CD3 mouse monoclonal antibodies (mAb) in cultures of human peripheral blood mononuclear cells (PBMC) depends on the ability of the mAb to interact with CD3 molecules on the T cells, and with Fc receptors (FcR) on monocytes. Two types of FcR with distinct specificity for murine (m) IgG subclasses are involved: a 72-kDa receptor (FcRI) binds mIgG2a and a 40-kDa receptor (FcRII) binds mIgG1. In this study we examined the mitogenic activity of mIgG3 anti-CD3 mAb RIV9. In cultures of human PBMC, the mAb induced T cell proliferation and interleukin 2 production. We found that subjects, unresponsive to mIgG2a anti-CD3 (e.g., OKT3), were also RIV9 nonresponders. In contrast, nonresponders to mIgG1 anti-CD3 (e.g., anti-Leu4) had a normal response to RIV9. Our results therefore suggested that anti-CD3 mAb of the mIgG2a and mIgG3 subclass bind to the same monocytic FcR. Human monomeric IgG, which has been shown to bind to FcRI only, blocked T cell proliferation induced by mIgG2a and mIgG3 anti-CD3, but had no effect on T cell proliferation induced by mIgG1 anti-CD3. In contrast, a mAb (IV.3) to FcRII, which blocks ligand binding of the receptor, blocked the mitogenic activity of mIgG1 anti-CD3 antibodies, but had no effect on T cell proliferation induced by mIgG3 anti-CD3 or by mIgG2a anti-CD3. Binding of RIV9 to FcR of responder monocytes could be demonstrated in immunofluorescence. Monocytes from the RIV9 nonresponder subjects however were unable to bind the Fc portion of this antibody. The binding of fluorescein (FITC)-conjugated mIgG3 or FITC-conjugated mIgG2a to responder monocytes could be inhibited by human monomeric IgG and by mIgG2a and mIgG3, but not by the mAb to FcRII. The results demonstrate that mIgG3 binds to FcRI on human monocytes and that this binding is needed for the mitogenic activity of mIgG3 anti-CD3.