Cardiovascular, Pulmonary and Renal Pathology The Mannose-Binding Lectin-Pathway Is Involved in Complement Activation in the Course of Renal Ischemia-Reperfusion Injury Bart de Vries,* Sarah J. Walter,* Carine J. Peutz-Kootstra, † Tim G.A.M. Wolfs,* L.W. Ernest van Heurn,* and Wim A. Buurman* From the Department of General Surgery,* Nutrition and Toxicology Research Institute Maastricht (NUTRIM), Academic Hospital Maastricht and Maastricht University, Maastricht; and the Department of Pathology, † Cardiovascular Research Institute Maastricht (CARIM), Academic Hospital Maastricht, Maastricht, the Netherlands Ischemia-reperfusion (I/R) is an important cause of acute renal failure (ARF). The complement system appears to be essentially involved in I/R injury. How- ever , via which pathway the complement system is activated and in particular whether the mannose- binding lectin (MBL)-pathway is activated is unclear. This tempted us to study the activation and regulation of the MBL-pathway in the course of experimental renal I/R injury and in clinical post-transplant ARF. Mice subjected to renal I/R displayed evident renal MBL-depositions , depending on the duration of warm ischemia , in the early reperfusion phase. Renal dep- osition of C3, C6 and C9 was observed in the later reperfusion phase. The deposition of MBL-A and -C completely co-localized with the late complement fac- tor C6, showing that MBL is involved in complement activation in the course of renal I/R injury. Moreover , the degree of early MBL-deposition correlated with complement activation , neutrophil-influx , and or- gan-failure observed in the later reperfusion phase. In serum of mice subjected to renal I/R MBL-A, levels increased in contrast to MBL-C levels , which dropped evidently. In line, liver mRNA levels for MBL-A in- creased , whereas MBL-C levels decreased. Renal MBL mRNA levels rapidly dropped in the course of renal I/R. Finally , in human biopsies , MBL-depositions were observed early after transplantation of ischemi- cally injured kidneys. In line with our experimental data , in ischemically injured grafts displaying post- transplant organ-failure extensive MBL depositions were observed in peritubular capillaries and tubular epithelial cells. In conclusion , in experimental renal I/R injury and clinical post-transplant ARF the MBL- pathway is activated , followed by activation of the complement system. These data indicate that the MBL- pathway is involved in ischemia-induced complement activation. (Am J Pathol 2004, 165:1677–1688) Ischemia-reperfusion (I/R) is an important cause of acute renal failure, associated with a mortality rate of up to 50%. 1,2 Post-transplant renal failure is a common and threatening complication after renal transplantation, in particular when organs of marginal donors, such as non- heart-beating (NHB) donors, are used. 3 Effective treat- ment for I/R injury is currently not available and hemodi- alysis is, though symptomatic, the only treatment available. The pathophysiology of renal I/R injury is com- plicated. Recent studies have shown that the comple- ment system plays a crucial role in pathogenesis of renal injury. Zhou et al 4 demonstrated that complement-defi- cient mice are protected against renal I/R injury. We and others showed that renal I/R injury can be abrogated by treatment with complement inhibitors such as anti-C5 antibodies and C5a receptor antagonists. 5–7 Renal dep- osition of complement has been well described for the complement factors C3, C6, and C9. 4,7 However, via which pathway the complement system is activated in the course of renal I/R is not clear. Park et al 8 demonstrated that renal I/R does not induce IgG or IgM deposition. Moreover, RAG-1 -/- mice subjected to I/R showed renal complement deposition, indicating that renal I/R is not mediated via the classical pathway. Recently, Thurman et al 9 showed that mice lacking a functional alternative com- plement pathway (factor B -/- mice) are partially pro- tected against renal ischemic injury. Whether the alterna- Supported by the Dutch Kidney Foundation, Grant C99.1840 (to W.A.B.) and by AGIKO Stipendium of The Netherlands Organization of Scientific Research (to B.d.V.). Accepted for publication July 20, 2004. Address reprint requests to Dr. W.A. Buurman, Department of General Surgery, Maastricht University, P.O. Box 616, Universiteitssingel 50, 6200 MD Maastricht, the Netherlands. E-mail address: W.Buurman@ah.unimaas.nl. American Journal of Pathology, Vol. 165, No. 5, November 2004 Copyright © American Society for Investigative Pathology 1677