FcγRIIa Is Expressed on Natural IFN-α-Producing Cells (Plasmacytoid Dendritic Cells) and Is Required for the IFN-α Production Induced by Apoptotic Cells Combined with Lupus IgG

An ongoing production of IFN-alpha may be of etiopathogenic significance in systemic lupus erythematosus (SLE). It may be due to the natural IFN-producing cells (NIPC), also termed plasmacytoid dendritic cells (PDC), activated by immune complexes that contain nucleic acids derived from apoptotic cells. We here examined the role of FcgammaR in the IFN-alpha production in vitro by PBMC induced by the combination of apoptotic U937 cells and autoantibody-containing IgG from SLE patients (SLE-IgG). The Fc portion of the SLE-IgG was essential to induce IFN-alpha production, because Fab fragments or F(ab')(2) were ineffective. Normal, especially heat-aggregated, IgG inhibited the IFN-alpha production, suggesting a role for FcgammaR on PBMC. Using blocking anti-FcgammaR Abs, the FcgammaRIIa,c (CD32) but not FcgammaRI or FcgammaRIII were shown to be involved in the IFN-alpha induction by apoptotic cells combined with SLE-IgG, but not by HSV or CpG DNA. In contrast, the action of all of these inducers was inhibited by the anti-FcgammaRIIa,b,c mAb AT10 or heat-aggregated IgG. Flow cytometric analysis revealed that approximately 50% of the BDCA-2-positive PBMC, i.e., NIPC/PDC, expressed low but significant levels of FcgammaRII, as did most of the actual IFN-alpha producers activated by HSV. RT-PCR applied to NIPC/PDC purified by FACS demonstrated expression of FcgammaRIIa, but not of FcgammaRIIb or FcgammaRIIc. We conclude that FcgammaRIIa on NIPC/PDC is involved in the activation of IFN-alpha production by interferogenic immune complexes, but may also mediate inhibitory signals. The FcgammaRIIa could therefore have a key function in NIPC/PDC and be a potential therapeutic target in SLE.