Orthoclone OKT 3 (mOKT3) is a highly effective agent for the reversal of steroid-resistant renal allograft rejection. However, its wider use has been limited by the development of a human anti-mouse antibody response (HAMA) and by the 'cytokine release syndrome' (CRS). CRS has been associated with T cell/monocyte activation and, secondarily, with activation of the complement cascade. These processes are mediated through Abs' Fc regions by their abilities to cross-link T cells and mononuclear cells and to activate complements. To alleviate these problems, a group of five huIgG1- and huIgG4- based OKT3 wild-type antibodies and their corresponding Fc mutants with altered residues at amino acids 234, 235, and 318, reported to be required for FcγRI and FcγRII binding and complement fixation, were constructed. Characterization of these humanized OKT3 Abs, denoted huOKT3γ1, huOKT3γ4, huOKT3γ1(A234, A235), huOKT3γ4(A234, A235), and huOKT3γ1(A318), has demonstrated that huOKT3γ1(A234, A235) and huOKT3γ4(A234, A235), and have at least a 100-fold reduced binding to FcγRI and FcγRII. As expected, they are much less potent in the induction of T cell activation and cytokine release, yet retain in vitro immunosuppressive effects as potent as those of mOKT3. Unexpectedly, while huOKT3γ1(A318) did not show any reduction in its ability to bind C1q and to fix a complement, huOKT3γ1(A234, A235) was completely inactive. The in vitro characteristics of huOKT3γ1(A234, A235) are consistent with recent in vivo studies, in which this Ab showed greatly reduced HAMA and CRS with the retention of its ability to reverse ongoing graft rejection in man. (C) 2000 Academic Press.
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