MIC expression in renal and pancreatic allografts

Background. MHC class I chain-related antigen A MICA) and MHC class I chain-related antigen B (MICB) are HLA class I related products of polymorphic MHC genes. Constitutive expression in normal tissue is limited to gut epithelium but can be induced in other epithelial cells by stress. Specific antibodies against MICA have been reported in the serum of patients who had rejected kidney allografts, suggesting a potential role for these molecules in transplant immunopathology. However, expression of MICA and MICB in transplanted organs has not been demonstrated. In this study, we report the expression of MICA and MICB in renal and pancreatic allograft biopsies, which were obtained due to clinical signs of rejection. Methods. A monoclonal antibody directed against MICA and MICB was used to perform indirect immunohistochemistry on formalin fixed, paraffin embedded needle biopsies of kidney and pancreas allografts. The results of staining were then compared to the standard light microscopic evaluation of the biopsies for rejection. Results. A total of 53 individual renal transplant biopsies and 19 pancreas transplant biopsies were assayed for expression of MIC. Histologically, renal biopsies were diagnosed as no rejection, acute tubular necrosis (ATN), acute rejection (AR), chronic rejection (CR), and acute and chronic rejection (ACR). No staining was observed in 7 of 10 kidneys showing no rejection. All 11 of the kidney biopsies with AR were positive, as were the 11 ATN cases, 9 of the 11 kidney biopsies with CR, and 7 of the 10 with ACR. The acini of normal, nontransplanted, pancreas, control specimen were consistently negative; however, islets were positive in all specimens. The acini and islets of five histologically normal pancreas biopsies were positive, as were the four biopsies with AR, seven biopsies with CR, and two with ACR. Conclusions. MICA and MICB are expressed in epithelial cells in allografted kidney and pancreas that show histologic evidence of rejection and/or cellular injury. In addition to previous findings of alloantibodies against MICA, expression of these gene products may play a role in allograft rejection.