Immune-Mediated Pore-Forming Pathways Induce Cellular Hypercitrullination and Generate Citrullinated Autoantigens in Rheumatoid Arthritis

Abstract

Autoantibodies to citrullinated protein antigens are specific markers of rheumatoid arthritis (RA). Although protein citrullination can be activated by numerous stimuli in cells, it remains unclear which of these produce the prominent citrullinated autoantigens targeted in RA. In these studies, we show that RA synovial fluid cells have an unusual pattern of citrullination with marked citrullination of proteins across the broad range of molecular weights, which we term cellular hypercitrullination. Although histone citrullination is a common event during neutrophil activation and death induced by different pathways including apoptosis, NETosis, and necroptosis/autophagy, hypercitrullination is not induced by these stimuli. However, marked hypercitrullination is induced by two immune-mediated membranolytic pathways, mediated by perforin and the membrane attack complex (MAC), which are active in the RA joint and of importance in RA pathogenesis. We further demonstrate that perforin and MAC activity on neutrophils generate the profile of citrullinated autoantigens characteristic of RA. These data suggest that activation of peptidylarginine deiminases during complement and perforin activity may be at the core of citrullinated autoantigen production in RA. These pathways may be amenable to monitoring and therapeutic modulation.

Two rheumatoid arthritis–specific autoantigens correlate microbial immunity with autoimmune responses in joints

“…Moreover, there is hypercitrullination of a range of proteins in the joints of many patients with RA (40). In our study, the GNS protein appeared to be citrullinated in vivo, and antibodies against it correlated with ACPA levels, whereas levels of antibodies against the FLNA protein, which did not appear to be citrullinated, did not correlate with ACPA levels.…”

Autoantibodies in systemic autoimmune diseases: specificity and pathogenicity

“…Complement activation by autoantibodies has been shown to occur through both classical and alternative pathways (89,90) and can lead to direct cell lysis and damage as well as recruitment of leukocytes to further enhance inflammatory responses. Complement-mediated cell lysis has also been shown to serve as a source of antigen, such as DNA, RNP, and citrullinated proteins (91).…”

Crossreactivity to vinculin and microbes provides a molecular basis for HLA-based protection against rheumatoid arthritis

“…Interestingly, the DERAA sequence is also found in many microbes and in the self-protein vinculin. Vinculin is expressed in the synovium and was recently shown to be citrullinated in the synovial fluid of an RA patient 4,23 . Likewise, T cells directed to vinculin are found under certain infectious conditions, indicating that T cell tolerance to vinculin is not absolute 24,25 .…”