Triggering of viral RNA sensors induces CD55 expression on synovial fibroblasts

Background and objectives CD55 (decay-accelerating factor) is a complement-regulatory protein, which is highly expressed by fibroblast-like synoviocytes (FLS). CD55 is also a ligand for CD97, an adhesion-type G protein-coupled receptor abundantly present on leucocytes. We recently showed that lack of either CD55 or CD97 ameliorates disease in murine collagen-induced and K/BxN serum transfer models of arthritis.1 Little is known regarding the regulation of CD55 expression in FLS. We therefore investigated the effect of toll-like receptors ligation and pro-inflammatory cytokines on CD55 expression. Materials and methods Synovial fibroblasts, obtained from biopsy samples of arthritis patients, were cultured and stimulated with cytokines (TNF, IFNγ, IL-1β, IL-6, IFNα) or TLR ligands (LTA, poly (I:C), LPS, imiquimod, CpG). Expression of CD55 was measured by flow cytometry using domain-specific monoclonal antibodies and recombinant CD97-loaded fluorescent beads. Chloroquine was used to inhibit TLR3 activity. Upregulation and functionality of dsRNA sensors in response to poly (I:C) or 5'-triphosphate RNA was analysed by PCR. Apoptosis was measured by PI/annexinV staining and was blocked with the pan-caspase inhibitor Q-VD-OPH. Results Cultured synovial fibroblasts of patients with rheumatoid arthritis (RA), osteoarthritis, psoriatic arthritis, and spondylarthritis express equal amount of CD55. Stimulation of RA-FLS with IL-1β (p=0.02) and poly (I:C) (p=0.001) induced a significant upregulation of CD55. Engagement of TLR3 by the dsRNA analog poly (I:C) was confirmed using chloroquine, an inhibitor of endosomal acidification that impairs TLR3 signaling. Synovial fibroblasts also expressed the cytoplasmic dsRNA sensors melanoma differentiation-associated gene 5 (MDA5) and retinoic acid-inducible gene I (RIG-I). Stimulation of these receptors with either poly (I:C) or 5'-triphosphate RNA induced CD55 expression, but, in case of MDA5, also induced significant cell death (p