Mustelin Cecilia, Einstein Archana, Wang Xiaoxing, Shaikh Farheen, Mustelin Tomas
Division of Rheumatology, Department of Medicine, University of Washington, Seattle, Washington, USA.
Division of Rheumatology, Department of Medicine, University of Washington, Seattle, Washington, USA.
J Biol Chem. 2025 Jul 1;301(8):110438. doi: 10.1016/j.jbc.2025.110438.
Autoantibodies against citrullinated proteins are diagnostic of rheumatoid arthritis (RA), a chronic and systemic autoimmune condition that affects synovial joints. Proteomic studies have revealed that human serum albumin is among the proteins that are citrullinated in RA. Anti-citrullinated protein antibodies reacting with albumin have also been reported. Here, we show that albumin is citrullinated at 11 arginine residues in the blood of both RA patients and, surprisingly, in healthy donors and to a very similar stoichiometry, albeit with some subtle differences. Albumin citrullination exhibited slightly different patterns in synovial fluid from RA patients compared to RA serum-derived albumin, although overall stoichiometry was similar. Incubation of albumin with live neutrophils or recombinant protein arginine deiminases 2 or 4 at 37 °C resulted in its rapid citrullination at multiple sites. Albumin citrullination reduced thyroxin binding in vitro. IgG antibodies in the serum of RA patients and healthy donors displayed comparable reactivities to physiologically citrullinated albumin. Similarly, citrullinated peptides corresponding to 14 citrullination sites were not significantly better recognized by IgG in serum from 86 RA patients than from healthy controls, and surprisingly some were even recognized to a lesser degree in RA. The very few RA patient antibodies exceeding the 95th percentile of the signal in healthy donors may simply represent weak cross-reactivity of antibodies against unrelated citrullinated antigens. Our findings reveal that albumin citrullination is likely physiological and of little interest to the immune system in RA patients, presumably because of persisting immunological tolerance. We discuss potential physiological functions of albumin citrullination.
抗瓜氨酸化蛋白自身抗体是类风湿性关节炎（RA）的诊断标志物，RA是一种影响滑膜关节的慢性全身性自身免疫性疾病。蛋白质组学研究表明，人血清白蛋白是RA中发生瓜氨酸化的蛋白质之一。也有报道称存在与白蛋白发生反应的抗瓜氨酸化蛋白抗体。在此，我们发现，RA患者血液中的白蛋白在11个精氨酸残基处发生瓜氨酸化，令人惊讶的是，健康供体血液中的白蛋白也在相同位置发生瓜氨酸化，且化学计量比非常相似，尽管存在一些细微差异。与源自RA血清的白蛋白相比，RA患者滑液中的白蛋白瓜氨酸化模式略有不同，不过总体化学计量比相似。将白蛋白与活的中性粒细胞或重组蛋白精氨酸脱亚氨酶2或4在37℃下孵育，会导致其在多个位点迅速发生瓜氨酸化。白蛋白瓜氨酸化在体外降低了甲状腺素结合能力。RA患者和健康供体血清中的IgG抗体对生理瓜氨酸化白蛋白的反应性相当。同样，与14个瓜氨酸化位点对应的瓜氨酸化肽在86例RA患者血清中被IgG识别的程度并不比健康对照者显著更高，令人惊讶的是，在RA患者中，有些瓜氨酸化肽甚至被识别的程度更低。极少数超过健康供体信号第95百分位数的RA患者抗体可能仅仅代表针对不相关瓜氨酸化抗原的抗体的弱交叉反应性。我们的研究结果表明，白蛋白瓜氨酸化可能是生理性的，对RA患者的免疫系统影响不大，推测这是由于持续存在的免疫耐受。我们讨论了白蛋白瓜氨酸化的潜在生理功能。
