Similar findings have been repeated with other citrullinated peptides, e.g., citrullinated fibrinogen, -enolase and collagen, where both cross-reactive and monospecific ACPA responses were identified, as illustrated inFigure 5b [115]. degree of cross-reactivity significantly. Clarifying the reactivity pattern of anti-citrullinated protein antibodies may contribute to determining their true nature of origin. Keywords:anti-citrullinated protein antibodies, citrullinated epitopes, rheumatoid arthritis, cyclic citrullinated peptides == 1. Introduction to Anti-Citrullinated Protein Antibodies == == 1.1. Citrullination of Proteins by Peptidylarginine Deiminases == Rheumatoid arthritis (RA) is usually a systemic autoimmune disease characterized by chronic inflammation in the synovial joints, which ultimately may lead to joint destruction and erosion of the underlying bone [1]. One of the most important serological discoveries in rheumatology has been CCT129202 the identification of autoantigens in RA, made up of epitopes with the non-genetically encoded amino acid citrulline (Cit) [2,3]. Cit is usually a post-translationally altered, deiminated derivative of Arg, which originally was named after the watermelonCitrullus vulgaris, which contains CCT129202 a large amount of free Cit. The process of citrullination, or deimination, involves the enzymatic conversion of Arg-containing proteins, which usually occurs during inflammatory processes [4]. Citrullination is usually catalyzed by peptidylarginine deiminase enzymes (PADs), which are calcium-dependent metalloenzymes, belonging to the superfamily of guanidino groupmodifying enzymes [4]. Under physiological conditions, PAD enzymes are normally inactive due to low concentrations of intracellular calcium. Once activated, as a result of elevated calcium levels, the PAD enzymes citrullinate a range of cytoplasmic, nuclear, membrane and mitochondrial proteins [5]. In this process, the positively charged guanidino group of Arg CCT129202 is usually replaced with the neutral ureido group, as depicted inFigure 1. The result of this conversion is usually a very small change in molecular mass and the loss of a positive charge. This apparently small change in the amino acid side chain has a profound effect on intramolecular and intermolecular interactions, as ionic and hydrogen bond interactions are influenced, which changes the isoelectric potential of the proteins leading to structural unfolding, thereby causing more open configurations for the proteins and proteolytic degradation [6]. == Physique 1. == Citrullination (deimination) of peptidylarginine catalyzed by peptidylarginine deiminase in a calcium-dependent process. The positively charged guanidino group is usually replaced with the neutral ureido group. Citrullination of proteins normally occurs in cells undergoing apoptosis, where citrullinated proteins are cleared from the body and not encountered by the immune system. In this process, unlimited influx of extracellular calcium activates PAD enzymes, as calcium levels become higher than the resting physiological concentration [7]. Citrullination marks intracellular proteins for degradation with a complete loss both of the polymerization competence of the Rabbit polyclonal to PLAC1 intermediate filament proteins and of the filament-forming ability [8]. Following citrullination, apoptotic bodies are engulfed by phagocytes, thereby preventing inflammatory reactions. However, in RA, citrullinated proteins are not effectively cleared. In fact, a dysregulation of apoptosis eventually in combination with the ineffective clearance of apoptotic material is usually suspected to be involved in the accumulation of dying cells and the exposition of autoantigens in RA [9]. Consequently, citrullinated proteins are exposed to the immune system, leading to the generation of citrulline-specific antibodies [10]. This is supported by CCT129202 studies describing that this impaired apoptosis of RA fibroblast-like synoviocytes is usually pivotal in the establishment of the inflammatory process [11]. In this process, an increase in PAD2 and PAD4 enzyme levels is usually often observed [7,12]. The role of PAD in the onset of RA has been examined, and recently, a model for PAD infiltration into the synovial joints of RA patients was presented, where the PAD2 and PAD4 enzymes, expressed in monocytes and macrophages, are recruited to the joints [13]. These scholarly research referred to how the triggered cells, including unstimulated PADs, ultimately undergo apoptosis where in fact the PADs are activated mainly because a complete consequence of elevated.