Recombinant chimeric OKT3 scFv IgM antibodies mediate immune suppression while reducing T cell activation in vitro

OKT3, a mouse anti-human CD3 monoclonal antibody (mAb), is a potent immunosuppressive agent used in clinical transplantation to treat allograft rejection. Two major drawbacks of this therapy are the systemic release of several cytokines due to cross-linking mediated by the mAb between T cells and FcγR-bearing cells and the human anti-mouse antibody (HAMA) response. To overcome these side effects, three chimeric OKT3 single chain variable fragment (scFv) IgM antibodies, scOKT3-γ ΔIgM wt, scOKT3-γ ΔIgM C575S and scOKT3-γ ΔIgM VAEVD, were generated. They consist of the light and heavy variable binding domains of OKT3 mAb as well as the CH3 and CH4 domains of different human IgM variants linked with a human IgG3 hinge region to provide more flexibility and stability. Like the native IgM, scOKT3-γ ΔIgM antibodies are able to form polymeric structures, which lead to an increase in binding affinity and immunosuppressive potential compared with the parental OKT3 mAb. However, independently of their polymerization, all scOKT3-γ ΔIgM constructs do not induce any significant T cell proliferation or cytokine release (IL-2, TNF-α and IFN-γ) in in vitro assays, while their CD3-modulating properties are retained. These results suggest that the use of scOKT3-γ ΔIgM antibodies may offer significant advantages over the OKT3 mAb in improving clinical immunosuppressive treatment.