HuM291, a humanized anti-CD3 antibody, is immunosuppressive to T cells while exhibiting reduced mitogenicity in vitro.

Background. OKT3, a mouse monoclonal antibody (Ab) specific for the human CD3 complex on T cells, is a potent immunosuppressive agent used for the treatment of acute allograft rejection. The utility of the drug has been limited by a neutralizing anti-mouse Ab response and adverse side effects resulting from T cell activation and systemic cytokine release. T cell activation is caused by OKT3-mediatetl cross-linking of T cells and Fc receptor-bearing cells. Studies in the mouse model have shown that global T cell activation is not necessary for immunosuppression, as Fc receptor-nonbinding anti-CDS Abs can suppress graft rejection in the absence of the activation effects seen with Fc receptor-binding Abs. Thus, a humanized anti-CD3 antibody with a low affinity for Fc receptors might improve immunosuppressive therapy by reducing the side effects associated with OKT3, Methods. We developed a mouse monoclonal Ab, M291, which competes with OKT3 for binding to T cells, Humanized, complementary-determining region-grafted versions of M291 featuring various Fc were engineered, including a previously described IgG2 mutant deficient in Fc receptor binding (HuM291), Results. Compared with OKT3 and HuM291-IgG1, HuM291 was significantly less mitogenic to T cells in vitro and induced the release of much lower levels of the cytokines tumor necrosis factor-alpha, interferon-gamma, and interleukin-10. Despite this reduction in T cell. activation, HuM291 retained the ability to modulate the CD3 complex and inhibit the mixed lymphocyte reaction, Conclusions. When evaluated in vivo, HuM291 may be an immunosuppressive agent associated with less of the acute toxicity and immunogenicity seen with OKT3 therapy.