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 hFc gamma RIIb cDNA clones, amongst which are cDNA clones encoding hFc gamma RIIb1 and hFc gamma RIIb2. Two hFc gamma RIIb1 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 hFc gamma RIIb1 variants. This substitution is located at amino acid position 11 of the cytoplasmic tail; a tyrosine in hFc gamma RIIb1 (clone pIP9) was replaced by an aspartic acid in clone pIP14 (encoding hFc gamma RIIb1*). A complication in studying ligand specificity of Fc receptors is the potential co-expression 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 hFc gamma RIIb1* 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, hFc gamma RIIb1* 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 hlgG3 > or = hlgG1 > hlgG4 > hlgG2, as determined at 4 degrees C with hlgG dimeric complexes. However, when assayed at 37 degrees C, the binding of hlgG2 dimers increased significantly. Next, we evaluated whether these transfectants were capable of supporting anti-CD3 induced T cell proliferation.(ABSTRACT TRUNCATED AT 250 WORDS)