Comparison of GK1.5 and chimeric rat/mouse GK1.5 anti-CD4 antibodies for prolongation of skin allograft survival and suppression of alloantibody production in mice

GK1.5 is a rat mAb that recognizes the mouse CD4 Ag. It has been shown to deplete CD4⁺ cells in vivo and to be immunosuppressive. To evaluate the effect of the C region of this antibody in achieving cell depletion, chimeric antibodies, each having the rat GK1.5 V regions and one of the four mouse IgG C region isotypes, were compared with the native rat antibody. The chimeric antibodies and the native antibody were tested for their ability to mediate in vitro C-dependent cytotoxicity, in vivo cell depletion, and prolongation of allogeneic skin graft survival and suppression of alloantibody production. In vitro C-dependent cytotoxicity assays revealed that rat IgG_2b and the chimeric antibodies containing mouse IgG_2a, mouse IgG_2b, and mouse IgG₃ were effective in lysing CD4⁺ lymphocytes whereas mouse IgG₁ was ineffective. In vivo studies of CD4⁺ cell depletion showed that mouse IgG_2a, rat IgG_2b, and mouse IgG_2b were effective isotypes, mouse IgG₁ was less effective, and mouse IgG₃ did not deplete CD4⁺ cells. A correlation was found between the ability of an isotype to deplete CD4⁺ cells in vivo and its ability to prolong the survival of skin allografts and to suppress alloantibody production. The nondepleting mouse IgG₃ was ineffective in these assays. Overall the most effective mouse isotype was IgG_2a which was as effective as rat IgG_2b. These results indicate 1) that syngeneic isotypes of mAb can cause cell depletion and consequently the prolongation of allograft rejection and suppression of alloantibody production; 2) that not all isotypes are equally effective; and 3) that the ability of a given isotype to deplete cells in vivo does not correlate with its ability to mediate C-dependent lysis in vitro. Our results are consistent with the hypothesis that in vivo depletion of cells is mediated by opsonization and binding through the FcR.