Elevated levels of fibrinogen-derived endogenous citrullinated peptides in synovial fluid of rheumatoid arthritis patients

Endogenous peptides were isolated from the synovial fluid of RA patients and controls by filtration and solid phase extraction. The peptides were identified and quantified using high-resolution liquid chromatography-mass spectrometry.Our data reveal that the synovial fluid of RA patients contains soluble endogenous peptides, derived from fibrinogen, containing significant amounts of citrulline residues and, in some cases, also phosphorylated serine. Several citrullinated peptides are found to be more abundantly present in the synovial fluid of RA patients compared to patients suffering from other inflammatory diseases affecting the joints.The increased presence of citrullinated peptides in RA patients points toward a possible specific role of these peptides in the immune response at the basis of the recognition of citrullinated peptides and proteins by RA patient autoantibodies.Rheumatoid arthritis (RA) is one of the most common autoimmune diseases, for which many autoantigens have been identified over the last decades. The most remarkable but yet most common targets of autoantibodies in RA are proteins containing the amino acid citrulline. This group of proteins includes, among others, histones and fibrin(ogen) [1-3]. Autoantibodies against citrullinated proteins (ACPA) are almost exclusively present in RA and can be present years before disease onset [4]. Detection of ACPA or other autoantibodies is a common part of the clinical diagnosis of RA. ACPA are often detected using the so-called cyclic citrullinated peptide, or CCP test [5].Fibrinogen is a heteromultimeric protein consisting of two copies each of three different peptide chains (alpha, beta and gamma) [6]. It plays an essential role in the clotting cascade, where it is proteolytically cleaved by thrombin to form the clotting protein fibrin [7]. Citrullinated forms of the alpha and beta chains of fibrinogen are known to be present in the serum and synovial fluid (SF) of RA patients and can be targets for A