Galactosylation of IgG1 modulates FcγRIIB-mediated inhibition of murine autoimmune hemolytic anemia.

Murine immune effector cells express three different stimulatory FcγRs (FcγRI, FcγRIII and FcγRIV) and one inhibitory receptor, FcγRIIB. Competitive engagement of stimulatory and inhibitory FcγRs has been shown to be critical for the development of immune complex-mediated inflammatory disorders. Because of the previous demonstration that FcγRIIB was unable to inhibit FcγRIII-mediated autoimmune hemolytic anemia induced by 105-2H IgG1 anti-RBC mAb, we reevaluated the regulatory role of FcγRIIB on the development of anemia using two additional IgG1 anti-RBC mAbs (34-3C and 3H5G1) and different 34-3C IgG subclass-switch variants. We were able to induce a more severe anemia in FcγRIIB-deficient mice than in FcγRIIB-sufficient mice after injection of 34-3C and 3H5G1 IgG1, but not 105-2H IgG1. Structural analysis of N-linked oligosaccharides attached to the CH2 domain revealed that 105-2H was poorly galactosylated as compared with the other mAbs, while the extent of sialylation was comparable between all mAbs. In addition, we observed that a more galactosylated 105-2H variant provoked more severe anemia in FcγRIIB-deficient mice than FcγRIIB-sufficient mice. In contrast, the development of anemia induced by three non-IgG1 subclass variants of the 34-3C mAb was not down-regulated by FcγRIIB, although they were more galactosylated than its IgG1 variant. These data indicate that FcγRIIB-mediated inhibition of autoimmune hemolytic anemia is restricted to the IgG1 subclass and that galactosylation, but not sialylation, of IgG1 (but not other IgG subclasses) is critical for the interaction with FcγR, thereby determining the pathogenic potential of IgG1 autoantibodies.