Ischemia-reperfusion injury is a complex phenomenon that induces cell damage through a bi-phasic process. Ischemia initiates the injury by deprivation of the energy needed to maintain ionic gradients and homeostasis, which may ultimately leads to cellular dysfunction and death. Reperfusion exacerbates this damage triggering an inflammatory reaction in which participates oxygen free radicals, endothelial factors and leukocytes. Ischemia-reperfusion disrupts the delicate balance that maintains homeostasis in the microcirculation with attraction, activation, adhesion and migration of neutrophils causing local tissue destruction by release of proteases and further oxygen free radicals. Mechanisms of ischemic injury are common to all solid organs, but there are some specific characteristics for each one of them. Consequences of ischemia-reperfusion injury on organ function depend on duration of ischemia, temperature and the nature of the organ. On the other hand, kidneys exposed to warm or cold ischemia usually show moderate degree of infiltrating cells, mainly with polymorphonuclears but also with monocytes and T-cells, together with up-regulated expression of MHC class II antigens early after transplantation. In addition, recent evidences suggest that there is also up-regulation of the co-stimulatory molecule B7 and the blockade of T lymphocyte co-stimulation through the B7-CD28 pathway by CTLA4Ig protects against acute as well as chronic consequences of renal ischemia, suggesting that a T-cell immune response emerges in ischemic kidneys. In fact, the change to this " inflammatory-immunological " state of the kidney may accelerates the appearance of early acute rejection in allografts.

As a result of ischemic damage cells may die through two different processes. Apoptosis or programmed cell death ("cell suicide") is a physiological mechanism for removing senescent, damaged or abnormal cells that affects individual cells. Apoptosis is initiated by an endonuclease and is characterised by DNA fragmentation into multiples of 180 - 200 base pairs. Apoptotic cells are ingested by macrophages or neighbouring cells without release of proteolitic enzymes or toxic oxygen species and the process is not accompanied by inflammation. By contrast, necrosis ('cell murder") is a pathological process that affects populations of cells and results in focal tissue destruction, inflammation and often serious systemic consequences. In the kidney, ischemic lesion is mainly identified in the inner stripe of the outer medulla. Nevertheless there is an heterogeneous distribution of the apoptotic and necrotic cells depending on the different response to ischemia between parts of the tubules. Thus, it seems that apoptosis is more evident in the distal tubules meanwhile necrosis occurs in the proximal tubules.

The restoration of renal function following ischemic acute renal failure is dependent in large part on the regeneration of proximal tubule structures and function. Tubular cells proliferate and migrate into the denuded area of the basement membrane to establish a new epithelium. Epidermal growth factor, insulin-like growth factor-1, hepatocyte growth factor and transforming growth factor-ß are known regulatory factors in the restoration of tubular structures following ischemic injury. Temporal appearance of these growth factors has been well identified and the complete repair of tubular integrity may take up to 6 weeks. Nevertheless, depending on the duration and severity of ischemia, and whether other injuries are present, this healing process can prolong indefinitely and turn to a fibrous state of the organ.

Regarding this issue, some studies have been reported referring the development of late renal damage after warm or cold ischemia in native kidney or in allografts mimicking those encountered in chronic rejection. Usually these models need the association of other features, as is the deprivation of renal mass, to completely produce the functional and structural injury. The pathogenic mechanisms that lead to chronic organ dysfunction in this situation remain partially unknown though ischemic organs are progressively infiltrated by T-cells and macrophages, mainly located in glomeruli and around vessels. The presence of these cell populations is associated with the release of biological mediators, such as IL-1, TNF- TGF-ß1, endothelin-1, and the induction of nitric oxide synthase. These mediators guide to a cascade of molecular and cellular events that may ultimately stimulate proliferation of arterial smooth muscle cells, glomerular mesangial cells and fibroblasts. Though TGF-ß1 has a pro-regenerative effect on tubules early after ischemia, it has been shown to be a key fibrogenic cytokine involved in the pathogenesis of chronic nephropathy after ischemia and renal mass reduction. In experimental models of chronic renal injury, an overproduction of TGF-ß1 has been linked to the development of glomerulosclerosis. Strategies designed to block TGF-ß1 bio-activity ameliorate the progression of the disease. Apart from TGF-ß1, other factors have been involved in the development of chronic nephropathy, as is the case of angiotensin-1 and PAF. Our group has recently shown that the administration of an oral PAF receptor antagonist attenuates the long-term effects of ischemia-reperfusion injury in uninephrectomised rats. In this study, animals chronically receiving this compound had a delay in the onset and reduction in proteinuria, did not develop chronic renal failure, and had a decrease in the glomerular TGF-ß1 mRNA expression as well as a lowering in the glomerulosclerosis and interstitial fibrosis, thus suggesting the role of PAF in this pathologic state. Other authors have reported promising results with an inhibitor of macrophages in a model similar to ours. In summary, chronic damage caused by prolonged ischemia-reperfusion may lead to a progressive reduction of functioning kidney mass over time, possibly leading to hyperfiltration and to arteriosclerotic changes in a response-to-injury mode and ultimately to the destruction of the graft.

Presently, there are no consistent effective ways to prevent the damage of ischemia-reperfusion after transplantation. It should be, obviously, desirable to reduce cold ischemia times. Future investigations will need to concentrate on decreasing cellular and molecular events at the endothelial as well as at the effector cell level using specific agents in the preservation solutions or administering them to the recipient before or shortly after engraftment. The reduction of ischemia-reperfusion damage will improve the graft fate. In addition, in renal transplantation, long-term therapeutic interventions must be considered in the future, specially when organs have been severely damaged by initial ischemic injury, for the prevention of chronic nephropathy induced by warm ischemia and mass reduction.

• Clavien PA, Harvey PRC, Strasberg SM. Preservation and reperfusion injuries in liver allografts. Transplantation 53:957-958, 1992.
• Grace PA. Ischemia-reperfusion injury. Br J Surgery 81:637-647, 1994
• Zimmerman BJ, Granger DN. Mechanisms of reperfusion injury. Am J Med Sci 307:284-292, 1994
• Finn WF. Prevention of ischemic injury in renal transplantation. Kidney Int 37:171-182. 1990.
• Kouwenhoven EA, DeBruin RW, Heemann U, Marquet Rijzermans J. Late graft dysfunction after prolonged cold ischemia of the donor kidney. Inhibition by Cyclosporine. Transplantation 68:;1004-1010, 1999.
• Azuma H, Nadeau K, Takada M, Mackenzie HS, Tilney NL. Cellular and molecular predictors of chronic renal dysfunction after initial ischemia-reperfusion injury in a single kidney. Transplantation 64;190-200, 1997.