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Background/Aim: We recently reported that renal tubular acidosis (RTA) in Sjögren’s syndrome (SjS) is associated with high titers of an autoantibody against carbonic anhydrase (CA) II, an important enzyme in renal acid-base regulation. The purpose of this study was to determine whether a CA-II antibody could cause RTA in a mouse model of SjS. Methods: PL/J mice were immunized with human CA II to induce CA II antibody formation, whereas controls were injected with phosphate-buffered saline and adjuvant. After 6 weeks, anti-CA-II antibody titers were measured, then ammonium chloride was administered orally for 1 week to detect any acidification defect. Results: CA-II-immunized mice showed higher anti-CA-II antibody titers than control mice. Pathologically, lymphocytic and plasma cell infiltration was seen in the salivary glands and kidneys of CA-II-immunized mice, but not in controls. On acid loading, blood pH and urine pH decreased in both groups of mice, but the slope of urine pH versus blood pH was less steep in the CA-II-immunized mice, suggesting that these mice had an impaired ability to reduce their urine pH in the face of metabolic acidosis. Conclusion: CA-II-immunized mice had a urinary acidification defect, which may be similar to that seen in patients with SjS.

Background The significance of anti-ribonucleoprotein (RNP) antibody and anti-Smith (anti-Sm) antibody remains unclear in patients with systemic lupus erythematosus (SLE) or mixed connective tissue disease (MCTD). Methods Thirty patients were retrospectively enrolled in this study. They were all positive for antinuclear antibody (ANA) and anti-RNP antibody, were diagnosed with SLE(n = 25) or MCTD(n = 5), and underwent renal biopsy at our hospital between January 1990 and December 2014. These 30 patients were classified into 4 groups based on their anti-dsDNA and anti-Sm status. Renal histology was classified into 3 patterns, including pure subepithelial membranous nephropathy (MN), mesangial and/or subendothelial lesions (MES), and MN combined with MES. Results Comparison between groups A [dsDNA(-)and SM(-)] and B༻dsDNA(-)and SM(+)༽ revealed that group A was with pure MN(n = 5) + MN + MES(n = 4) + MES(n = 1) and normocomplementemia, while group B was with MES + MN(n = 2) + MES(n = 5) and hypocomplementemia. Comparison between groups C༻dsDNA(+)and SM(-)༽ and D༻dsDNA(+)and SM(+)༽showed that group C was with pure MES and normocomplementemia, while group D was with MN + MES(n = 4) + pure MES(n = 3) and hypocomplementemia. Among 10 patients in group A, all 5 patients with MCTD were categorized as pure MN, while the 5 patients with SLE were MN + MES or pure MES. Conclusion Among patients showing positivity for anti-RNP antibody alone, those with MCTD seem likely to develop pure MN, while those with SLE tend to have MES as well as MN.