INTERACTION OF A HUMAN FC-GAMMA-RIIB1 (CD32) ISOFORM WITH MURINE AND HUMAN-IGG SUBCLASSES

A group of Fc receptor molecules, classified CD32, recognize the Fc moiety of IgG with low affinity. We report the isolation and identification of different hFcgammaRIIb cDNA clones, amongst which are cDNA clones encoding hFcgammaRIIb1 and hFcgammaRIIb2. Two hFcgammaRIIb1 encoding cDNA clones (pIP9 and pIP14) were isolated, which differed by three nucleotides, probably because of allelic variation. The nucleotide differences result in one amino acid change between the allelic hFcgammaRIIb1 variants. This substitution is located at amino acid position 11 of the cytoplasmic tail; a tyrosine in hFcgammaRIIb1 (clone pIP9) was replaced by an aspartic acid in clone pIP14 (encoding hFcgammaRIIb1*). A complication in studying ligand specificity of Fc receptors is the potential coexpression of different classes, subclasses, or polymorphic forms of FcR on the same cell. We therefore used murine fibroblasts transfected with cDNA clone pIP14, encoding a hFcgammaRIIb1 isoform, as our model system. These fibroblasts were found to interact with erythrocytes sensitized with mIgG2a and mIgG2b in rosetting assays performed at 4 and 37-degrees-C. Interestingly, hFcgammaRIIb1* transfectants bound mIgG1 sensitized erythrocytes only weakly at 4-degrees-C, whereas profound binding was observed at 37-degrees-C. The ligand specificity for human (h) IgG isotypes was found to be hIgG3 greater-than-or-equal-to hIgG1 > hIgG4 > hIgG2, as determined at 4-degrees-C with hIgG dimeric complexes. However, when assayed at 37-degrees-C, the binding of hIgG2 dimers increased significantly. Next, we evaluated whether these transfectants were capable of supporting anti-CD3 induced T cell proliferation. It was found that mIgG1, mIgG2a, mIgG2b, hIgG1, hIgG4, and hIgG4 anti-CD3 mAbs triggered T cell mitogenesis efficiently. hIgG2 anti-CD3 mAb, however, induced T cell mitogenesis to a much lower extent. In conclusion, these analyses revealed a unique isotype specificity of this hFcgammaRII isoform.