Rheumatoid arthritis and citrullination

Abstract

Purpose of review: Dysregulated citrullination is a key element that drives the production and maintenance of antibodies to citrullinated proteins, a hallmark in rheumatoid arthritis (RA). This article reviews recent literature on the origin of citrullinated antigens in RA.

Recent findings: The study of synovial fluid from patients with RA has provided important insights into the identity of citrullinated proteins that accumulate in the RA joint (the RA citrullinome) and mechanisms that control their generation.

Summary: Citrullinating enzymes (peptidylarginine deiminases, PADs) are tightly controlled to limit their hyperactivation. Calcium and redox conditions are important regulators of PAD activity. Studies suggest that citrullination is dysregulated both intra- and extracellularly in RA. In neutrophils, host (i.e., perforin and the membrane attack complex) and bacterial (i.e., toxins) pore-forming proteins induce prominent calcium influx, cytolysis, and hyperactivation of PADs. These factors likely drive hypercitrullination in the RA joint and at extraarticular sites of disease initiation, respectively. As oxidizing conditions present in the extracellular environment are known to inactivate PADs, extracellular citrullination in RA probably requires the constant release of active enzymes from dying cells and may be accelerated by autoantibodies that activate PADs.

Figure 1

Normal and dysregulated citrullination in RA. (A) PAD function is limited by transient nanomolar fluctuations in intracellular calcium, oxidizing environments and potentially protein co-factors. By maintaining a suboptimal activity of PADs, these components may increase enzyme specificity, avoiding the abnormal citrullination of non-physiological substrates. Moreover, efficient mechanisms of clearance of citrullinated proteins (likely by degradation and less likely by re-conversion of citrulline residues to arginine residues) are important to prevent their abnormal accumulation in cells. During physiologic forms of cell death, such as NETosis, PADs are likely inhibited by ROS to prevent hyperactivation as result of calcium influx in dying cells. Similarly, oxidation appears to protect against extracellular citrullination by PADs released from activated and dying cells. Through regulated citrullination, the load of immunogenic proteins is insufficient to drive an ACPA response under physiologic conditions. (B, C). Hypercitrullination results from mechanisms that over-activate the PAD enzymes. Membranolytic damage induced by host and bacterial pore-forming proteins are potent inducers of leukotoxic hypercitrullination (LTH). (B) Bacterial pore-forming toxins are potential triggers of LTH and ACPA production in extra-articular sites of diseases initiation (e.g. gums, gut, and lungs, others). (C) Immune-mediated membranolytic pathways, such as perforin and MAC, likely sustain hypercitrullination in the rheumatoid joint. The large number of dying neutrophils in the articular compartment in RA likely maintains a constant release of active PADs for extracellular citrullination. The presence of agonistic antibodies to PADs may enhance extracellular citrullination before the enzymes are inactivated by oxidation. Together, citrullinated proteins from intra- and extracellular sources constitute the RA citrullinome. Uncontrolled hypercitrullination generates suprathreshold amounts of non-tolerized antigens that may initiate an ACPA response and RA in genetically susceptible individuals.