FUNCTIONAL-ANALYSIS OF HUMAN FC-GAMMA-RII (CD32) ISOFORMS EXPRESSED IN B-LYMPHOCYTES

The low affinity IgG receptor FcgammaRII (CD32) represents the most wi dely distributed class of human FcgammaR. To analyze the biologic func tions of different FcgammaRII isoforms, we stably transfected FcgammaR IIb1, IIb1, IIb2, IIa, and a IIa tail- mutant to the mouse IIA1.6 B l ymphoma cell line. Of these, FcgammaRIIb1 represents a receptor varia nt that is identical to IIb1 except for a single amino acid difference in the cytoplasmic tail (amino acid position 11) where a tyrosine (II b1) is replaced by an aspartic acid (IIb1). Evaluation of capping abi lity showed the FcgammaRIIb1 molecules to cap effectively, which was e ven more apparent with IIb1. None of the FcgammaRIIa, IIa tail-, or I Ib2 isoforms capped significantly. Internalization of FcgammaR-antibod y complexes proved very efficient for both the FcgammaRIIa and IIb2 is oforms, whereas the IIb1 molecules internalized moderately compared wi th IIb1, which internalized less efficiently. Notably, human IgG aggr egates were internalized effectively by FcgammaRIIa and moderately by IIb2. Neither FcgammaRIIb1 nor IIb1 proved capable of internalizing s uch IgG aggregates. Cross-linking of the different FcgammaR molecules showed FcgammaRIIa capable of triggering increases in [Ca2+]i. Fcgamma R expressed on B cells were able to down-regulate [Ca2+]i ion co-cross -linking with sIgG. Notably, all three FcgammaRIIb receptors proved ac tive in this respect, in contrast to FcgammaRIIa. The cell distributio n of these FcgammaRII isoforms was analyzed in a panel of human B cell lines to complement the IIA1.6 B cell model. FcgammaRIIa was found ex pressed both at message and protein levels in all tested human B cell lines. In the pre-B cell lines evaluated, no FcgammaRIIb molecules wer e detectable, whereas both FcgammaRIIb1 and IIb2 molecules were found present in more mature B cell lines. These data support both a complex expression pattern of FcgammaRII isoforms in B cell lines and functio nal differences between these B cell molecules.