C-Reactive Protein Inhibits Plasmacytoid Dendritic Cell Interferon Responses to Autoantibody Immune Complexes

Plasmacytoid dendritic cells (pDC) play an important role in host defense against viral infection by producing large quantities of type I IFN (including IFN-α, IFN-β and other subtypes that bind to the IFN-α receptor, IFNAR) (1, 2). Production of type I IFN by pDC is triggered by recognition of viral nucleic acids through intracellular sensors located in the cytoplasm and in endosomes. In the pDC endosomal pathway TLR7 recognizes RNA and TLR9 recognizes DNA resulting in MyD88-dependent signaling that culminates in the phosphorylation of interferon regulatory factor (IRF) 7, and transcription of type I IFN and other genes (2, 3). The intracellular sequestration of TLR7 and TLR9 prevents activation of this pathway by self RNA and DNA. However, activation can occur when IC containing nucleoprotein antigens derived from apoptotic cells are taken up by pDC through FcγRIIa (4, 5). The generation of autoantibodies against nucleoprotein autoantigens and subsequent formation of IC is characteristic of SLE. The majority of patients with SLE show an IFN signature of gene expression in their peripheral blood cells as the result of chronic overproduction of type I IFN and activation of IFN-inducible genes (6, 7). The IFN signature of gene expression is associated with recent onset of SLE and with a more severe clinical picture including renal and hematological disease (8). Type I IFN stimulates other immune cells including monocytes, dendritic cells, NK cells, T and B lymphocytes and it has been postulated that the activation of pDC by SLE IC may set up a self-perpetuating cycle of autoimmunity (7, 9). This makes the type I IFN pathway an attractive therapeutic target for SLE and several other autoimmune diseases (10). CRP is a soluble pattern recognition molecule and a major acute phase serum protein (11). Serum levels of CRP increase from less than 5 μg/ml at baseline to several hundred μg/ml following trauma, infection or inflammation. The majority of circulating CRP is produced in the liver in response to IL-6. Structurally CRP is a cyclic pentamer, which binds to microbial and host ligands through five identical calcium-dependent binding sites on one face. The other face interacts with the immune system through FcγR and FcαRI on cells (12) or C1q of the classical complement pathway. CRP ligands include apoptotic cells and protein components of the major SLE autoantigens, snRNPs and chromatin (13-15), as well as microbial polysaccharides and phospholipids. We and others have proposed that CRP contributes to the non-inflammatory removal of dead and damaged cells and prevents immunization with nucleoprotein autoantigens. Although there are no known CRP deficiencies or variations in amino acid sequence, a number of polymorphisms affect levels of CRP and single nucleotide polymorphisms (SNPs) in the CRP gene have been associated with SLE risk (16-19). In addition, IFN-α suppresses CRP synthesis (20) and CRP levels are inappropriately low in SLE flares in the absence of serositis or infection (21, 22). CRP administered by injection was first shown to have therapeutic benefit in chromatin-accelerated disease in the (NZB × NZW) F1 female mouse (23). Subsequently CRP expressed from a transgene or injected was found to prolong survival and decrease renal disease in the spontaneous (NZB × NZW) F1 female and the MRL/MpJ-Faslpr mouse models of SLE (24-26). The most pronounced effect of CRP in these models was the prevention and suppression of proteinuria and glomerulonephritis. Experiments in other autoimmune and IC disease models including nephrotoxic nephritis, immune thrombocytopenia and experimental autoimmune encephalomyelitis implicate macrophages, FcγR, and IL-10 as key elements in CRP suppression of autoimmune disease (24, 27, 28). Effects of CRP on the type I IFN pathway have not been reported previously. In this study we examined the effect of CRP on the IFN-α response of human peripheral blood mononuclear cells (PBMC) to IC containing nucleoprotein autoantigens. The results demonstrate a regulatory role for CRP in the type I IFN pathway and provide an additional mechanism for the therapeutic effects of CRP in autoimmune disease models.