Non FcR-binding murine antihuman CD3 monoclonal antibody is capable of productive TCR signalling and induces proliferation in the presence of costimulation

CLB T3/4.A is a non FcR-binding CD3 mAb of the murine IgA isotype, which ma y be used as an alternative for the mitogenic OKT3 mAb in the treatment of acute cellular rejection after organ transplantation. We studied TCR signal ling and T cell activation in response to T3/4.A in normal human PBMC in vi tro. T3/4.A induced a rapid rise in free cytoplasmic Ca2+, not different fr om the response to mitogenic CD3 mAb. However, protein tyrosine phosphoryla tion and, particularly, MAPK activation, were reduced as compared to mitoge nic CD3 mAb. T3/4.A enhanced expression of both CD69 and CD25, but prolifer ation and detectable cytokine production did not occur. Addition of either CD28 mAb or IL-2 induced a strong proliferative response, which was accompa nied by cytokine production. At higher mAb concentrations, T cell activatio n decreased, which correlated with TCR downmodulation. To exclude the possi bility that activation by T3/4.A depends on interaction of murine IgA Fc wi th as yet unknown FcR, we showed that also with CD3 mAb F(ab')(2) fragments upregulation of activation molecules occurred, as well as proliferation in the presence of costimulation. We conclude that the non FcR-binding murine IgA mAb T3/4.A acts as a partial agonist and leads to proliferation and cy tokine production only in the presence of appropriate costimuli. These find ings may explain the mitigated cytokine release syndrome observed in vivo w ith some nonmitogenic CD3 mAbs.