FcgammaRIII (CD16)-deficient mice show IgG isotype-dependent protection to experimental autoimmune hemolytic anemia

In autoimmune hemolytic anemia (AIHA), there is accumulating evidence for an involvement of FcgammaR expressed by phagocytic effector cells, but demonstration of a causal relationship between individual FcgammaRs and IgG isotypes for disease development is lacking. Although the relevance of IgG isotypes to human AIHA is limited, we could show a clear IgG isotype dependency in murine AIHA using pathogenic IgG1 (105-2H) and IgG2a (34-3C) autoreactive anti-red blood cell antibodies in mice defective for FcgammaRIII, and comparing the clinical outcome to those in wild-type mice. FcgammaRIII-deficient mice were completely resistent to the pathogenic effects of 105-2H monoclonal antibody, as shown by a lack of IgG1-mediated erythrophagocytosis in vitro and in vivo. In addition, the IgG2a response by 34-3C induced a less severe but persistent AIHA in FcgammaRIII knock-out mice, as documented by a decrease in hematocrit. Blocking studies indicated that the residual anemic phenotype induced by 34-3C in the absence of FcgammaRIII reflects an activation of FcgammaRI that is normally coexpressed with FcgammaRIII on macrophages. Together these results show that the pathogenesis of AIHA through IgG1-dependent erythrophagocytosis is exclusively mediated by FcgammaRIII and further suggest that FcgammaRI, in addition to FcgammaRIII, contributes to this autoimmune disease when other IgG isotypes such as IgG2a are involved.