CD32

Abstract: C-reactive protein (CRP) is an acute phase serum protein that shares several functions with immunoglobulin (Ig)G including complement activation and binding to receptors on monocytes and neutrophils. The identity of the receptor for CRP has been the target of extensive research. We previously determined that CRP binds to the high affinity receptor for IgG, FcγRI (CD64). However, this interaction could not account for the majority of binding of CRP to neutrophils or monocytic cells. We now determine that CRP also interacts with FcγRIIa (CD32), the low affinity receptor for IgG on monocytes and neutrophils. COS-7 cells were transfected with a construct containing the human FcγRIIA cDNA. CRP binding and the presence of CD32 were detected by mAb and analyzed by two-color flow cytometry. Cells expressing CD32 bound CRP in a dose-dependent and saturable manner consistent with receptor binding. CRP bound to transfectants and K-562 cells with similar kinetics, and in both cases binding was completely inhibited by aggregated IgG. On monocytic cell lines, treatment with Bt2cAMP increased FcγRII expression and enhanced CRP binding. CRP also specifically precipitated FcγRI and FcγRII from the monocytic cell line, THP-1. It is suggested that the major receptor for CRP on phagocytic cells is FcγRII.

Abstract: 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.

Abstract: Inflammatory processes are important in the pathogenesis of Alzheimer's disease and in response to amyloid-b immunotherapy. We investigated the expression of multiple inflammatory markers in the brains of 28 non-immunized patients with Alzheimer's disease and 11 patients with Alzheimer's disease immunized against amyloid-b42 (AN1792): microglial ionized calcium-binding adaptor Iba-1, lysosome marker CD68, macrophage scavenger receptor A, Fcreceptors I (CD64) and II (CD32); and also immunoglobulin IgG, complement C1q and the T lymphocyte marker CD3 using immunohistochemistry. The data were analysed with regard to amyloid-b and phospho-tau pathology, severity of cerebral amyloid angiopathy and cortical microhaemorrhages. In non-immunized Alzheimer's disease cases, amyloid-b42 correlated inversely with CD32 and Iba-1, whereas phospho-tau correlated directly with all microglial markers, IgG, C1q and the number of T cells. In immunized Alzheimer's disease cases, amyloid-b42 load correlated directly with macrophage scavenger receptor A-positive clusters and inversely with C1q. The severity of cerebral amyloid angiopathy and microhaemorrhages did not relate to any of the analysed markers. Overall, the levels of CD68, macro- phage scavenger receptor A, CD64, CD32 and the number of macrophage scavenger receptor A-positive plaque-related clusters were significantly lower in immunized than non-immunized cases, although there was no significant difference in Iba-1 load, number of Iba-1-positive cells, IgG load, C1q load or number of T cells. Our findings indicate that different microglial populations co-exist in the Alzheimer's disease brain, and that the local inflammatory status within the grey matter is importantly linked with tau pathology. After amyloid-b immunization, the microglial functional state is altered in association with reduced amyloid-b and tau pathology. The results suggest that, in the long term, amyloid-b immunotherapy results in downregulation of microglial activation and potentially reduces the inflammation-mediated component of the neurodegeneration of Alzheimer's disease.

Abstract: Three types of receptor for the Fc (constant) region of human immunoglobulin G have been described1,2; FcRI, a high-affinity (Ka≈108 M−1) receptor expressed on monocytes3–5; FcRII (CD32), a low-affinity (Ka≈106 M−1) receptor expressed on B cells, granulocytes, macrophages and platelets6–9; and FcRIII (CD 16, FcR10), a low-affinity receptor expressed on macrophages, neutrophils, eosinophils, natural killer cells and a subset of T cells believed to comprise the suppresssor cells10–13. Anti-CD 16 anti-bodies block natural killer-cell mediated antibody dependent cellular cytotoxicity (ADCC)14,15. Binding of aggregated IgG to CD 16 on natural killer cells leads to the expression of lymphocyte activation antigens, mediator release, morphological changes and lytic activity16–18. We report here the isolation of a complementary DNA clone encoding CD 16 determinants which gave rise to IgG binding of the expected affinity and subtype specificity in COS cells, and which proved to encode a phospholipid anchored protein. A single messenger RNA transcript was found in all positive RNA samples, and N-glycanase treatment showed the form found in COS cells was identical to the form present on peripheral blood mononuclear cells (PBMCs). We also show that CD 16 is most closely related to the α-form of the murine IgG 2b/l receptor and propose that extracellular contacts mediate the signal initiated by IgG binding.