Autoimmunity is a condition in which loss of immunologic tolerance for specific self-proteins and commonly autoantibody production is observed.1 Rheumatoid arthritis (RA) and Sjögren's syndrome (SjS) are long term systematic autoimmune diseases.2,3 A common feature of autoimmune diseases is the increased production of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in association with an inflammatory response, and this response causes tissue damage.4 Such damage is the result of the chemical reactivity of certain ROS/RNS with biomolecules including DNA, lipids, carbohydrates, and proteins in many autoimmune disorders like RA and SjS, which are associated with autoantibodies that are reactive against self-modified proteins.4,5

High levels of ROS can inflict direct damage to lipids, especially polyunsaturated fatty acids (lipid peroxidation) which produces a wide variety of oxidation end products. Lipid hydroperoxides are the primary products of lipid peroxidation. In addition, many different aldehydes including MDA, propanal, hexanal, and 4-HNE can be formed as secondary products.6,7 Similarly, under oxidative stress, peroxynitrite (a reactive nitrogene species), which is one of the potent oxidant and nitrating species, is formed by the rapid reactions of two excessive free radicals (nitric oxide and superoxide anion). It can modify a variety of biomolecules as proteins like 4-HNE and MDA do, but it has a higher affinity for the tyrosine residues of proteins.8,9 Protein nitration has been observed in many pathological conditions such as atherosclerotic lesions, neurodegenerative disease, and autoimmune and rheumatic diseases.10 The secondary end products of ROS/RNS lead to oxidative modifications. These modifications may alter the protein structure, which usually results in the activation or inhibition of enzymatic and binding capacities, increased susceptibility to aggregation and proteolysis, and a changed uptake rate by cells.8,11, 12, 13 This process eventually leads to neoantigen formation, and self-proteins become immunologically active.14, 15, 16, 17

Carbonic anhydrases (CA; EC 4.2.1.1) are zinc-containing metalloenzymes that catalyze reversible conversion of carbon dioxide and water into bicarbonate and protons.18,19 Sixteen CA isoenzymes have been described in mammals up to now, which are involved in various physiological and pathological processes including CO2 transport, ion secretion, pH regulation, calcification, bone resorption, and tumorigenesis.19 CA I and CA II are both cytosolic enzymes present in significant numbers in erythrocytes. CA I and/or CA II autoantibodies have recently been found in various autoimmune diseases such as systemic lupus erythematosus (SLE), SjS, primary biliary cirrhosis (PBC), Graves’ disease, and RA.20, 21, 22, 23

The presence of autoantibodies is the hallmark of autoimmune diseases, although underlying mechanisms of this immune response have not yet been fully explained. A few studies reported that oxidative stress triggers the formation of autoantibodies against CA; however, there is no study that shows the effects of by-products resulting from oxidative stress on the formation of CA autoantibodies.24 Therefore, in this study, we, first, aimed to investigate the effects of HNE, MDA, and PN modifications on CA antigenicity in mice, and, second, we aimed to investigate the presence of autoantibodies against modified CA isoenzymes in the sera of RA and SjS patients.