I schemia/reperfusion injury associated with organ retrieval and storage influences the development of chronic graft dysfunction, the major clinical problem in solid organ transplantation. The potential role of mononuclear cells (T cells and monocyte/macrophages) in this type of injury is unknown. Inbred male Lewis rats were uninephrectomized and the left kidney perfused in situ with 10 ml of iced University of Wisconsin solution. Immunohistological studies showed mononuclear cell infiltration of the ischemic organs associated with the upregulation of MHC class II antigen expression. Reverse transcriptase-PCR indicated that T cell associated cytokines and monocyte/macrophage activation markers/products are upregulated early after the ischemic insult. B7 expression occurred within 24 h and peaked at 3 d. Plasma creatinine levels rose transiently with complete recovery of renal function by 5 d. Animals began to develop progressive proteinuria after 8-12 wk, indicative of the long-term functional consequences of early ischemia/reperfusion injury. Blockade of T cell CD28-B7 costimulation with CTLA4Ig resulted in significant inhibition of T cell and macrophage infiltration and activation in situ. Treated animals did not exhibit transient renal dysfunction, nor developed proteinuria over time. This is the first demonstration that blocking T cell costimulatory activation in the absence of alloantigen can prevent the early and late consequences of ischemia/reperfusion injury.