Anti-carbonic anhydrase II antibody reflects urinary acidification defect especially in proximal renal tubules in patients with primary Sjögren syndrome.

Primary Sjögren syndrome (pSS) is a systemic autoimmue disease featured by excessive autoantibody production. It has been demonstrated that anti-carbonic anhydrase II (anti-CA II) antibody is correlated with renal tubular acidosis in pSS; however, no further details about urinary acidification defect have been reported, and the antibody's relationship with other organ impairments remains unknown. This case-control study aimed to examine anti-CA II antibody levels in relation to various systemic complications in pSS, and evaluate its potential role as a organ-specific biomarker in a Chinese cohort. Serum anti-CA II antibody levels were determined using ELISA in 123 patients with pSS and 72 healthy controls. The medical records of the patients were collected, and the correlation between serum anti-CA II antibody and clinical/immunological parameters was investigated. Serum anti-CA II antibody level and its positive rate were significantly increased in pSS patients compared with controls, and ANA-positive patients presented even higher titers of the antibody. In anti-CA II positive group, remarkably higher urine pH and bicarbonate, as well as lower urine titratable acid and serum potassium were observed, which indicated renal tubular acidification dysfunction both involving bicarbonate reabsorption and acid secretion. In addition, platelet count and complement 3, complement 4 levels decreased, whereas serum IgG, IgA and γ-globulin levels increased notably in accord with a higher EULAR SS disease activity index score in these patients. Further analysis showed that anti-CA II antibody was most elevated in patients with defect in bicarbonate reabsorption, reflecting proximal renal tubular injury, rather than in patients with distal renal tubular acidosis as previously reported. In conclusion, anti-CA II antibody reflects renal (especially proximal renal tubular) and hematologic impairment as well as increased disease activity in pSS. It may act as a serum biomarker of systemic damage of pSS.