Time course and cellular localization of inducible nitric oxide synthases expression during cardiac allograft rejection

Background. We have demonstrated that inhibition of inducible nitric oxide synthase (NOS) ameliorated acute cardiac allograft rejection. This study de termined the time course and cellular localization of inducible NOS express ion during the histologic progression of unmodified acute rat cardiac allog raft rejection. Methods. Tissue from syngeneic (ACI to ACI) and allogeneic (Lewis to ACI) t ransplants were harvested on postoperative days 3 through 10 and analyzed f or inducible NOS mRNA expression (ribonuclease protection assay), inducible NOS enzyme activity (conversion of L-[H-3]arginine to nitric oxide and L-[ H-3]citrulline), and nitric oxide production (serum nitrite/ nitrate levels ). Inducible NOS mRNA and protein expression were localized using in situ h ybridization and immunohistochemistry. Results. Inducible NOS mRNA and enzyme activity were expressed in allograft s during mild, moderate, and severe acute rejection (postoperative days 4 t hrough 10), but were not detected in normals, isografts, or allografts befo re histologic changes of mild acute rejection (postoperative day 3). induci ble NOS expression resulted in increased serum nitrite/nitrate levels durin g mild and moderate rejection (postoperative days 4 through 6). Inducible N OS mRNA and protein expression localized to infiltrating mononuclear inflam matory cells in allograft tissue sections during all stages of rejection bu t were not detected in allograft parenchymal cells or in normals or isograf ts. Conclusions. Inducible NOS expression and increased nitric oxide production occurred during the early stages of acute rejection, persist-ed throughout the unmodified rejection process, and localized to infiltrating inflammato ry cells but not allograft parenchymal cells during all stages of acute rej ection. (Ann Thorac Surg 1999;67:716-22) (C) 1999 by The Society of Thoraci c Surgeons.