A humanized anti-CD3 antibody, HuM291, with low mitogenic activity, mediates complete and reversible T-cell depletion in chimpanzees

Background. An anti-CD3 antibody that reduces cytokine release syndrome (CR S) while maintaining immunosuppression would be a major advance in the trea tment of acute allograft rejection. A humanized (Hu) anti-CD3 IgG2 Ab, HuM2 91 gamma 2 M3 (HuM291; Protein Design Labs, Inc., Mountain View, CA), was e ngineered with mutations in the upper C(H)2 region of the Fc domain. The mu tations were intended to reduce affinity for Fc gamma receptors, thought to be relevant to CRS, Methods. In vitro studies using chimpanzee peripheral blood mononuclear cel ls (PBMCs) were conducted to characterize HuM291 and to establish an animal model. A multidose study was conducted in chimpanzees to evaluate the safe ty, pharmacokinetics, immunomodulatory activity, and immunogenicity of HuM2 91, when administered at doses ranging from 0.1 to 10 mg. Results. HuM291 bound to and effectively downmodulated CD3 from chimpanzee PBMCs and stimulated substantially less cytokine secretion and proliferatio n of chimpanzee PBMCs compared with OKT3 (Orthoclone OKT3; Ortho Pharmaceut ical Corp., Raritan, NJ). Multiple doses of HuM291 (0,1, 1.0, or 10 mg/ dos e) were not associated with adverse events, signs of toxicity, or CRS, desp ite cytokine release. HuM291 exhibited a long elimination t1/2 (81.5 hr) an d, after three 10-mg doses, sustained serum concentrations > 1000 ng/ml wer e maintained for 1 week. Multiple 10-mg doses induced complete depletion of circulating CD2(+)CD3(+) T cells for up to 10 days after the last dose; T cells recovered by Day 28. Anti-HuM291 Abs were observed in only 4 of 12 an imals and were transient in 2 of those animals. Conclusions. In vitro, HuM291 is substantially less mitogenic than OKT3. In chimpanzees, HuM291 effectively depleted peripheral T cells without elicit ing clinical signs of CRS, and recovered T cells were functionally normal.