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

The low affinity IgG receptor FcgammaRII (CD32) represents the most widely distributed class of human FcgammaR. To analyze the biologic functions of different FcgammaRII isoforms, we stably transfected FcgammaRIIb1, IIb1*, IIb2, IIa, and a IIa tail- mutant to the mouse IIA1.6 B lymphoma cell line. Of these, FcgammaRIIb1* represents a receptor variant that is identical to IIb1 except for a single amino acid difference in the cytoplasmic tail (amino acid position 11) where a tyrosine (IIb1) is replaced by an aspartic acid (IIb1*). Evaluation of capping ability showed the FcgammaRIIb1 molecules to cap effectively, which was even more apparent with IIb1*. None of the FcgammaRIIa, IIa tail-, or IIb2 isoforms capped significantly. Internalization of FcgammaR-antibody complexes proved very efficient for both the FcgammaRIIa and IIb2 isoforms, whereas the IIb1 molecules internalized moderately compared with IIb1*, which internalized less efficiently. Notably, human IgG aggregates were internalized effectively by FcgammaRIIa and moderately by IIb2. Neither FcgammaRIIb1 nor IIb1* proved capable of internalizing such IgG aggregates. Cross-linking of the different FcgammaR molecules showed FcgammaRIIa capable of triggering increases in [Ca2+]i. FcgammaR expressed on B cells were able to down-regulate [Ca2+]i ion co-cross-linking with sIgG. Notably, all three FcgammaRIIb receptors proved active in this respect, in contrast to FcgammaRIIa. The cell distribution 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 expressed both at message and protein levels in all tested human B cell lines. In the pre-B cell lines evaluated, no FcgammaRIIb molecules were 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 functional differences between these B cell molecules.