The generation of a humanized, non‐mitogenic CD3 monoclonal antibody which retains in vitro immunosuppressive properties

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Abstract
CD3 antibodies are proven immunosuppressants capable of reversing transplant rejection episodes. Their general application has been limited both by their immunogenicity and, in particular, by the “first‐dose” cytokine‐release syndrome experienced by patients after the initial administration of antibody. We have produced a set of variants of the humanized YTH 12.5 CD3 monoclonal antibody (mAb) (Routledge et al., Eur. J. Immunol. 1991. 21: 2717) bearing different human heavy (H) chain constant regions, with the intention of finding a form of the antibody that is not able to activate T cells. Comparison of the variants having γl, γ2, γ3 and γ4 H chains in a competitive binding assay showed that antibody avidity was not affected by IgG subclass. Using a sensitive indicator of FcR binding activity (the capacity of the CD3 mAb to redirect cytotoxic T cells to kill the monocytic cell line U‐937) we demonstrated a functional hierarchy of γl = γ4 α2 = γ3 mb > γ2. An aglycosyl version of the γl CD3 mAb, produced by site‐directed mutagenesis (Asn297 to Ala), still had considerable activity in this assay (intermediate to the γl and α2 CD3 mAb), albeit at a level approximately 10‐fold lower than that of the parental γl form. When we tested their ablity to stimulate T cell proliferation in vitro in the presence of 5% human serum, all of the wild‐type immunoglobulin isotypes were found to be active, although there were T cell donor‐dependent variations in the extent of the responses. The aglycosyl γl mAb was, however, completely non‐mitogenic in all of ten donors tested, unless the assay was performed in IgG‐free medium. Despite being non‐stimulatory, this mAb was also able to inhibit the mixed lymphocyte reaction responses of both naive and primed T cells. Comparison of the yl and aglycosyl γ1 mAb in an experimental mouse model for CD3 mAb‐induced cytokine release indicated that removal of the carbohydrate moiety from the γl constant region reduced the in vivo tumor necrosis factor‐a response by a factor of at least 16‐fold. These data suggest that the aglycosyl γl CD3 mAb is a promising candidate for immunosuppressive therapy without “first dose” side effects.

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