Anti-carbonic anhydrase II antibody in autoimmune pancreatitis and tubulointerstitial nephritis

Sir, Autoimmune pancreatitis (AIP) has been characterized by diffuse enlargement of the pancreas, irregular narrowing of the pancreatic duct, high levels of serum IgG4, and lymphoplasmacytic infiltration in the pancreatic parenchyma [ 1 ]. Accumulating evidence suggests that this autoimmune disease could present with diffuse infiltration of IgG4-positive plasmacytes in multiple organs, resulting in sclerosing cholangitis, sialadenitis and retroperitoneal fibrosis [ 1 ]. In 2004, tubulointerstitial nephritis (TIN) associated with AIP was independently reported by two groups [ 2 , 3 ]. Renal biopsy revealed tubulointerstitial inflammatory infiltrates with IgG4-positive plasma cells. However, it remains unknown to which antigenic peptides in the kidney these antibodies bind. A 66-year-old male was referred to our department because of a rise in serum creatinine concentrations from 0.83 to 1.75 mg/dl during the previous 6 months. The diagnosis of AIP had been made a year previously by high serum IgG4 level of 670 mg/dl, and by the findings of imaging studies such as computed tomography (CT), magnetic resonance imaging (MRI), and endoscopic retrograde cholangiopancreatography (ERCP). On admission, his laboratory data showed serum IgG4 of 1830 mg/dl. Renal biopsy demonstrated TIN with plasma-cell infiltration, although the glomeruli appeared almost normal ( Figure 1 A). Immunohistochemistry revealed that IgG4 was positive in the infiltrating plasma cells and also in the distal tubular cells ( Figure 1 B). After 8 weeks of treatment with oral prednisolone, his serum creatinine and IgG4 levels decreased to 0.87 mg/dl and 694 mg/dl, respectively. ( A ) Renal biopsy demonstrated interstitial plasma-cell infiltration (inset) forming pseudolymphoid follicle (arrow). ( B ) Immunohistochemistry for IgG4 showed positive immunoreactivity in DCT and CCD. PT, proximal tubule; DCT, distal convoluted tubule; CCD, cortical collecting duct. Bar indicates 50 µm (A) and 100 µm (B). This case showed clinical findings characteristic of AIP-associated TIN, such as plasma-cell infiltration in the interstitium, high serum level of IgG4, and a favourable response to steroid therapy. Moreover, renal biopsy demonstrated IgG4 staining, not only in the infiltrated plasma cells in the interstitium, as already described in other reports [ 2 , 3 ], but also in renal tubular cells of the distal tubules and collecting ducts. Recently, autoantibodies against a number of endogenous proteins, such as carbonic anhydrase II (CA II), lactoferrin [ 4 ] and pancreatic secretory trypsin inhibitor [ 5 ], have been implicated in the pathogenesis of AIP. Among them, CA II is one of the key enzymes that regulate acidification of the urine. Thus, we hypothesized that there was an autoantibody, presumably of the IgG4 class, to CAII in the patient's serum, which may be involved in the positive staining IgG4 in renal tubular cells. To verify this, we purified IgG4 from his serum and examined its immunoreactivity with CA II, using Western blotting analysis [ 4 , 6 ]. The result was that anti-CAII antibody of IgG4 class existed in the serum, and levels significantly decreased after treatment ( Figure 2 ). The autoantibodies appeared to target the CAII in the distal tubules, although it remained to be clarified why the positive staining of IgG4 was observed predominantly in the distal segments of the nephron. Thus, it is speculated that renal tubular damages by TIN cause a release of intracellular CAII into the bloodstream, coupled with disordered immune tolerance, leading to the production of specific anti-CAII antibodies, which then may react with the intracellular CAII of the tubules. In summary, we describe a patient with TIN showing high serum concentrations of IgG4 during the course of untreated AIP. Further investigations will be needed to delineate the link between an IgG4 class of autoantibodies and the pathogenesis of TIN in AIP. IgG4 was purified from the serum of healthy control (lane 1) and from the patient before (lane 2) and after treatment (lane 3); the same concentration of IgG4 (10 μg/ml) was reacted to a membrane blotted with 2.5 µg of carbonic anhydrase II (29 kDa) by a standard western blotting technique. Conflict of interest statement . None declared.