THU0098 ANTIBODIES AGAINST THREE POST-TRANSLATIONAL PROTEIN MODIFICATIONS IN PATIENTS WITH RHEUMATOID ARTHRITIS

Abstract

Background:Autoantibodies to some post-translational modifications (PTMs) of proteins have a relevant role in rheumatoid arthritis (RA). Antibodies to other PMTs such as carbonylation (aCarbo) [1], nitration (aNitra) [2] and homocysteinylation (aHCis) [3] have also been described but they have been scarcely studied.Objectives:We aimed to evaluate the presence of antibodies against three poorly studied PTMs in patients with RA and explore their value as biomarkers.Methods:Serum samples from 196 healthy controls and RA patients meeting the 1987 ACR classification criteria that had either early RA (< 2 years from symptom onset) (n=182) or established RA (n=143) were analyzed. The antibodies were evaluated by indirect ELISA following the described protocols [1-3]. They include as antigens: HOCl- or ribose- carbonylated bovine type II collagen (HOCl-CII and GLI-CII, respectively) for aCarbo antibodies; homocysteinylated human serum albumin (Hcy-HSA) for aHCis antibodies; andin vitronitrated synovial proteins and synthetic nitrated peptides (3-NT-PS and 3-NT-pep, respectively) for aNitra antibodies. The efficiency of the reactions to produce these PTMs was verified. The study was approved by the CEIC of Galicia. The results were analyzed as differences between the optical densities (OD) obtained without correction, or the specific OD after subtracting reactivity to the native protein.Results:The aCarbo antibodies were studied in patients with early RA, as reported [1]. Despite this, uncorrected ODs against the two carbonylated forms of CII were not greater in the patients than in the controls (Table 1). Consequently, the corrected reactivity to GLI-CII that was significantly higher in patients than controls did not reflect aCarbo antibodies but differences in reactivity to native bovine CII.The other antibodies were analyzed in patients with established RA as previously done [2,3]. However, none of them showed specific reactivity (Table 1). In effect, although an excess of uncorrected reactivity to 3-NT-pep was observed in the patients with RA, it disappeared when corrected (Table 1). Also, the vast majority of patients and controls showed no reactivity to 3-NT-SP (not shown). In turn, the aHCis antibodies were significantly elevated in patients, but only in the uncorrected analysis because the difference disappeared when corrected for reactivity against native HSA (Table 1).Table 1.Antibodies against three post-translational protein modifications in RA patients and controlsRAControlsAntigenCorrectionMedian DOIQRMedian DOIQRPHOCl-CIIno0.1410.127/0.1730.1600.141/0.1880.0001HOCl-CIIyes-0.003-0.011/0.008-0.004-0.020/0.0050.02GLI-CIIno0.2340.171/0.2850.2220.163/0.3050.5GLI-CIIyes0.0720.016/0.1150.028-0.010/0.1000.00053-NT-pepno0.2500.201/0.3290.2090.190/0.2300.023-NT-pepyes0.1370.097/0.1850.1130.100/0.1330.1Hcy-HSAno0.1600.145/0.1780.1450.130/0.1550.00011Hcy-HSAyes-0.041-0.074/-0.015-0.038-0.069/-0.200.5Conclusion:The presence in RA patients of specific antibodies against the three PTMs has not been replicated. None of the observed reactivities were attributable to the PTMs. If confirmed, these results would indicate that only particular PTMs are relevant for RA and that carbonylation, nitration or homocysteinylation are not among them.