Ischemic preconditioning improves postischemic acute renal failure

Nitric Oxide in Cardiac Surgery: A Review Article

Perioperative organ injury remains a medical, social and economic problem in cardiac surgery. Patients with postoperative organ dysfunction have increases in morbidity, length of stay, long-term mortality, treatment costs and rehabilitation time. Currently, there are no pharmaceutical technologies or non-pharmacological interventions that can mitigate the continuum of multiple organ dysfunction and improve the outcomes of cardiac surgery. It is essential to identify agents that trigger or mediate an organ-protective phenotype during cardiac surgery. The authors highlight nitric oxide (NO) ability to act as an agent for perioperative protection of organs and tissues, especially in the heart-kidney axis. NO has been delivered in clinical practice at an acceptable cost, and the side effects of its use are known, predictable, reversible and relatively rare. This review presents basic data, physiological research and literature on the clinical application of NO in cardiac surgery. Results support the use of NO as a safe and promising approach in perioperative patient management. Further clinical research is required to define the role of NO as an adjunct therapy that can improve outcomes in cardiac surgery. Clinicians also have to identify cohorts of responders for perioperative NO therapy and the optimal modes for this technology.

Comparison of Direct and Remote Ischaemic Preconditioning of Renal Ischaemia Reperfusion Injury in Rats

Objective

One of the methods that can be used to prevent ischaemia reperfusion (IR) injury is ischaemic preconditioning. The aim of this study was to evaluate and compare the effects of remote and direct ischaemic preconditioning (RIPC and DIPC) histopathologically in the rat renal IR injury model.

Methods

After obtaining an approval from the Dokuz Eylül University School of Medicine Ethics Committee, 28 Wistar Albino male rats were divided into four groups. In Group I (Sham, n=7), laparotomy and left renal pedicle dissection were performed, but nothing else was done. In Group II (IR, n=7), after 45 minutes of left renal pedicle occlusion, reperfusion lasting 4 hours was performed. In Group III (DIPC+IR, n=7), after four cycles of ischaemic preconditioning applied to the left kidney, renal IR was performed. In Group IV (RIPC+IR, n=7), after three cycles of ischaemic preconditioning applied to the left hind leg, renal IR was performed. All rats were sacrificed, and the left kidney was processed for conventional histopathology.

Results

The histopathological injury score of the kidney was significantly lower in the sham group compared with the other groups (p<0.01). The injury scores of the DIPC+IR and RIPC+IR groups were significantly lower than in the IR group (p<0.05). In the RIPC+IR group, the injury score for erythrocyte extravasation was found to be significantly lower than in the DIPC+IR group (p<0.05).

Conclusion

In the present study, it was demonstrated that both DIPC and RIPC decreased renal IR injury, but RIPC was found to be more effective than DIPC. This protective effect requiresfurther detailed experimental and clinical studies.

Role of aldehyde dehydrogenase 2 in ischemia reperfusion injury: An update

Aldehyde dehydrogenase 2 (ALDH2) is best known for its critical detoxifying role in liver alcohol metabolism. However, ALDH2 dysfunction is also involved in a wide range of human pathophysiological situations and is associated with complications such as cardiovascular diseases, diabetes mellitus, neurodegenerative diseases and aging. A growing body of research has shown that ALDH2 provides important protection against oxidative stress and the subsequent loading of toxic aldehydes such as 4-hydroxy-2-nonenal and adducts that occur in human diseases, including ischemia reperfusion injury (IRI). There is increasing evidence of its role in IRI pathophysiology in organs such as heart, brain, small intestine and kidney; however, surprisingly few studies have been carried out in the liver, where ALDH2 is found in abundance. This study reviews the role of ALDH2 in modulating the pathways involved in the pathophysiology of IRI associated with oxidative stress, autophagy and apoptosis. Special emphasis is placed on the role of ALDH2 in different organs, on therapeutic “preconditioning” strategies, and on the use of ALDH2 agonists such as Alda-1, which may become a useful therapeutic tool for preventing the deleterious effects of IRI in organ transplantation.

Which type of conditioning method protects the spinal cord from the ischemia–reperfusion injury in 24 hours?

Objective

This study was designed to test the effects of different types of preconditioning and postconditioning methods on spinal cord protection following aortic clamping.

Methods

The animals (rabbits) were divided into sham-operated, ischemic preconditioning, remote ischemic preconditioning, simultaneous aortic and ischemic remote preconditioning, and ischemic postconditioning groups. After neurological evaluations, ultrastructural analysis and immunohistochemical staining for caspase-3 were evaluated after 24 h following ischemia.

Results

The neurological outcomes of the remote ischemic preconditioning (4.2 ± 0.4) and ischemic postconditioning (4.6 ± 0.8) groups were significantly improved when compared with the ischemia group (2.2 ± 04). The immunohistochemical analysis revealed that the lowest percentage of apoptosis was in-group ischemic preconditioning at 12.5 ± 30.6%. In the comparison of intracellular edema in an ultrastructural analysis, the ischemic preconditioning and ischemic postconditioning groups had significantly lower values than the ischemia group.

Conclusion

The conditioning methods attenuate ischemia–reperfusion injury for spinal cord injury. Ischemic and remote preconditioning and also postconditioning methods are simple to perform and inexpensive.

Fluorescence spectroscopy in renal ischemia and reperfusion: noninvasive evaluation of organ viability

BACKGROUND

Damage provoked by ischemia in renal transplants is difficult to quantify. To determine whether a donated organ is fit for transplantation. We sought to correlate the findings of fluorescence spectroscopy (FS) with histologic evidence of ischemic injury and organ viability.

METHODS

Kidneys of 33 rats were submitted to FS of the upper and lower poles as well as the middle third. Excitation was generated by the laser's wavelengths of 408, 442, and 532 nm. Rats were randomized into groups with the 30, 60, and 120 minutes warm ischemia before analysis by FS, that was repeated at 5 minutes after reperfusion.

RESULTS

FS results in the reperfusion phase correlated with ischemia time and degree of histologic injury. After 60 or 120 minutes of ischemia, the excitation lasers of 532 and 442 nm resented a significant negative correlation coefficient with the histological grade (r = -0.61 and r = -0.73, respectively).

CONCLUSIONS

There was a strong correlation between FS and histologic changes only in the reperfusion phase after renal ischemia. The method was thus unable to assess the viability of organs before transplantation.

Comparison of two models for evaluation histopathology of experimental renal ischemia

Renal ischemia/reperfusion (I/R) injury is one of the frequent causes of acute renal failure (ARF) due to the complex, interrelated sequence of events, that result in damage to and death of kidney cells. Cells of the proximal tubular epithelium are especially susceptible to I/R injury, leading to acute tubular necrosis, which plays a pivotal role in the pathogenesis of ARF. Several models have been explicated to assess morphological changes, including those of Jabonski et al. and Goujon et al. We compared the 2 models for histopathological evaluation of 30- or 120-minute periods of renal ischemia followed by 24-hour reperfusion in rats. Several changes were observed after application of the 2 models: proximal tubular cell necrosis, loss of brush border, vacuolization, denudation of tubular basement membrane as a consequence of flattening of basal cells, and presence of intratubular exfoliated cells in the lumen of proximal convoluted tubules at various stages of degeneration (karyorexis, kariopyknosis and karyolysis). Evaluating tubular lesions after 2 periods of experimental ischemia with light microscopy allowed us to conclude that the Goujon classification better characterized the main changes in cortical renal tubules after ischemia.

Novel pharmacological approaches to the treatment of renal ischemia-reperfusion injury: a comprehensive review

Renal ischemia-reperfusion (I-R) contributes to the development of ischemic acute renal failure (ARF). Multi-factorial processes are involved in the development and progression of renal I-R injury with the generation of reactive oxygen species, nitric oxide and peroxynitrite, and the decline of antioxidant protection playing major roles, leading to dysfunction, injury, and death of the cells of the kidney. Renal inflammation, involving cytokine/adhesion molecule cascades with recruitment, activation, and diapedesis of circulating leukocytes is also implicated. Clinically, renal I-R occurs in a variety of medical and surgical settings and is responsible for the development of acute tubular necrosis (a characteristic feature of ischemic ARF), e.g., in renal transplantation where I-R of the kidney directly influences graft and patient survival. The cellular mechanisms involved in the development of renal I-R injury have been targeted by several pharmacological interventions. However, although showing promise in experimental models of renal I-R injury and ischemic ARF, they have not proved successful in the clinical setting (e.g., atrial natriuretic peptide, low-dose dopamine). This review highlights recent pharmacological developments, which have shown particular promise against experimental renal I-R injury and ischemic ARF, including novel antioxidants and antioxidant enzyme mimetics, nitric oxide and nitric oxide synthase inhibitors, erythropoietin, peroxisome-proliferator-activated receptor agonists, inhibitors of poly(ADP-ribose) polymerase, carbon monoxide-releasing molecules, statins, and adenosine. Novel approaches such as recent research involving combination therapies and the potential of non-pharmacological strategies are also considered.

Ischemic preconditioning in solid organ transplantation: from experimental to clinics

This study reviews the current understanding of ischemic preconditioning (IP) in experimental and clinical setting, and the mechanisms that mediate the complex processes involved as a tool to protect against ischemia and reperfusion (I/R) injury, but is not intended as a complete literature review of preconditioning. IP has been mainly elucidated in cardiac ischemia. Recent reports confirm the efficacy of pre- and postconditioning in cardiac surgery and percutaneous coronary interventions in humans. IP utilizes endogenous as well as distant mechanisms in skeletal muscle, liver, lung, kidney, intestine and brain in animal models to convey varying degrees of protection from I/R injury. Specifically, preconditioned tissues exhibit altered energy metabolism, better electrolyte homeostasis and genetic reorganization, as well as less oxygen-free radicals and activated neutrophils release, reduced apoptosis and better microcirculatory perfusion. To date, there are few human studies, but recent trials suggest that human liver, lung and skeletal muscle acquire protection after IP. Present data address the potential therapeutic application of IP in the prevention of I/R damage specially aimed at clinical transplantation. IP is ubiquitous but more research is required to fully translate these findings to the clinical arena.

Ischemic Preconditioning and Kidney Transplantation: In Vivo Nitric Oxide Monitoring in a Rat Ischemia-Reperfusion Experimental Model

Direct nitric oxide measurement in live tissue would help us to understand its role in ischemia-reperfusion injury and its relationship to ischemic preconditioning (IP). We constructed four experimental groups of ischemia-reperfusion in the rat kidney: G1 were controls; G2, 1 hour of renal ischemia; G3 and G4: one or two 15/10 minute cycles, respectively, of IP prior to 1 hour of ischemia. Real-time in vivo nitric oxide measurements were compared with functional parameters of kidney damage at 24 hours. The peaks of nitric oxide production in the IP periods increased less in the rising curve of nitric oxide production during the 1 hour ischemia time. No improvement in the IP groups was observed based on serum creatinine levels at 24 hours.

Protective effect of erythropoietin on renal ischemia and reperfusion injury

BACKGROUND

Multiple protective effects of erythropoietin (EPO), such as antiapoptotic, antioxidant, angiogenic and neuroprotective effects, against ischemia have been demonstrated in cell culture and animal models. Genistein is also a potent tyrosine kinase inhibitor. The aims of the present study were to evaluate the effects of EPO on renal ischemia/reperfusion injury and to determine the role of the tyrosine kinase pathway on this process.

METHODS

Sprague-Dawley rats were assigned to five groups: (i) sham (Group I); (ii) control with renal ischemia (right nephrectomy and clamping on the left renal pedicle for 45 min and reperfusion; Group II); (iii) EPO + ischemia (Group III); (iv) genistein (an inhibitor of tyrosine kinase) + ischemia (Group IV); and (v) EPO + genistein + ischemia (Group V). Recombinant human EPO (1000 IU/kg) and genistein (10 mg/kg) were given 2 hours before ischemia. Blood samples and the left kidney were obtained after 45 min of reperfusion from half of the rats and after 24 h from the other half.

RESULTS

The blood urea nitrogen, creatinine, tumour necrosis factor-alpha (P < 0.05) and interleukin-2 (P < 0.01) levels, and renal tissue lipid peroxidation (P < 0.05) were significantly lower in Group III than in Group II at 45 min of reperfusion. Following 24 h of reperfusion, EPO decreased tissue peroxidation and histopathological injury, whereas genistein reversed it. The most prominent ischemic injury was observed in Group IV in which genistein was administered. There was no significant difference between Groups II and V.

CONCLUSIONS

These results suggest that EPO is effective in attenuating renal ischemia/reperfusion injury, and this effect may be related to tyrosine kinase activity.

Ischemic preconditioning protects post-ischemic renal function in anesthetized dogs: role of adenosine and adenine nucleotides

AIM

To investigate the effects of renal ischemic preconditioning (IPC) on both renal hemodynamics and the renal interstitial concentrations of adenosine and adenine nucleotides induced by ischemia-reperfusion injury.

METHODS

Renal hemodynamics responses to ischemia-reperfusion injury in mongrel dog models were determined with or without multiple brief renal ischemic preconditioning treatments, as well as the adenosine A1 receptor antagonist (KW-3902), respectively. The renal interstitial concentrations of adenosine and adenine nucleotides in response to ischemia-reperfusion injury, either following 1-3 cycles of IPC or not, were measured simultaneously using microdialysis sampling technology.

RESULTS

One 10-min IPC, adenosine A1 receptor antagonist (KW-3902) also shortened the recovery time of renal blood flow (RBF) and urine flow (UF), as well as mean blood pressure (BP). Advanced renal IPC attenuated the increment of adenosine and adenine nucleotides, as well as recovery time during the 60-min reperfusion which followed the 60-min renal ischemia. All of these recovery times were dependent on the cycles of 10-min IPC. The renal interstitial concentrations of adenosine and adenine nucleotides increased and decreased during renal ischemia and reperfusion, respectively.

CONCLUSION

A significant relativity in dog models exists between the cycles of 10-min renal IPC and the recovery time of BP, UF, and RBF during the 60-min renal reperfusion following 60-min renal ischemia, respectively. Renal IPC can protect against ischemia-reperfusion injury and the predominant effect of endogenous adenosine induced by prolonged renal ischemia; renal adenosine A1 receptor activation during the renal ischemia-reperfusion injury is detrimental to renal function.

Liver ischemic preconditioning: a new strategy for the prevention of ischemia-reperfusion injury

Ischemic preconditioning renders the liver more tolerant to ischemia-reperfusion injury in warm and cold ischemia-reperfusion models. In general, the application of a 5 to 10-minute period of ischemia followed by 10 minutes of reperfusion confers early effective protection to the liver. Mechanisms responsible for this endogenous protective effect include: (1) transient nitric oxide production during liver preconditioning; (2) diminution of toxic reactive species generated on reperfusion; (3) remote effect on extrahepatic organs such as lung, kidney, and pancreas; (4) preservation of energy metabolism during ischemia; and (5) involvement of nuclear transcription factor and others.

Multiparametric monitoring of ischemia-reperfusion in rat kidney: effect of ischemic preconditioning

BACKGROUND

Microelectrode technology is a promising tool for monitoring kidney ischemia and the changes induced by its therapeutic management. Ischemic preconditioning, that is, brief ischemic periods before sustained ischemia, has been shown to protect several organs, including the kidney, from ischemia-reperfusion injury. We tested whether the effect of preconditioning could be appraised by real-time measurement of parameters representative of tissue hypoxia.

METHODS

In a sample of pentobarbital-anesthetized and mechanically ventilated rats, we studied the effect of renal ischemic preconditioning (10-min ischemia and 10-min reflow interval) on subsequent ischemia-reperfusion (45 min and 60 min). Renal tissue electrical impedance, extracellular pH, and potassium concentration [K+] were measured continuously by implanted microelectrodes.

RESULTS

Ischemia induced an early, rapid rise in extracellular potassium and impedance module, followed by a phase of slower increase, whereas pH decreased rapidly, reaching a plateau. Preconditioning treatment did not cause significant changes in interstitial pH and [K+] but increased ischemic tissue impedance. During reperfusion, the three variables recovered progressively; however, after a decline, electrical impedance showed a clear postischemic increase. This rise was suppressed by preconditioning.

CONCLUSIONS

Real-time measurement of any of the three parameters showed capability for early detection of ischemia. In contrast with findings in myocardial tissue, preconditioning in the kidney did not increase potassium cell loss during ischemia or improve ischemic acidosis or tissue impedance. Electrical impedance increased for a second time during reperfusion, indicating the presence of a postischemic cellular edema; concealing this episode was the most noticeable effect of the preconditioning treatment.

Renal protection by brief liver ischemia in rats

BACKGROUND

In this study, we evaluated the beneficial effect of brief ischemia and reperfusion, which was shown to have local effects on liver previously, on kidney as a remote organ in rats.

METHODS

Male Wistar rats were divided into three groups: group I, sham; group II, renal ischemia for 45 min; and group III, 10 min of brief hepatic ischemia and 10 min of reperfusion after 45 min of renal ischemia. Biochemical determination, tumor necrosis factor (TNF)-alpha and tissue thiobarbituric acid-reactive substances (TBARS) levels, and histopathologic findings were evaluated at 45 min and 24 hr of reperfusion.

RESULTS

Although blood urea nitrogen and creatinine levels were similar at 45 min in groups II and III, these levels were lower in group III at 24 hr. Creatine clearance values were higher and fraction excretion of sodium values were lower in group II than in group III at 24 hr. Lactate dehydrogenase levels of groups III and II were similarly elevated at 45 min, whereas group III values decreased more rapidly than those of group II at 24 hr. At 45 min of reperfusion, TNF-alpha and tissue TBARS levels were found lower in group III than in group II. Histopathologic parameters including congestion and tubular vacuolization, tubular cell detachment, and necrosis were significantly reduced in group III as compared with results of group II 45 min after ischemia. All histopathologic parameters were defined as statistically better in group II at 24 hr.

CONCLUSIONS

The beneficial effect of brief ischemia of liver on renal ischemia as a remote organ was confirmed by biochemical, histopathologic, and ultrastructural findings.

Lipoteichoic acid from Staphylococcus aureus reduces renal ischemia/reperfusion injury

BACKGROUND

The aim of this study was to investigate whether in vivo administration of a low, sub-lethal dose of lipoteichoic acid (LTA), a bacterial wall-fragment derived from the Gram-positive bacterium Staphylococcus aureus, protects the kidney against the renal dysfunction and injury caused by ischemia/reperfusion (I/R).

METHODS

Male Wistar rats were administered LTA from S. aureus (1 mg/kg, IP). After 24 hours, rats were subjected to bilateral renal ischemia (45 min) followed by reperfusion (6 h). Serum and urinary markers were measured for the assessment of renal function, tubular and reperfusion-injury. Renal sections were used for histological grading of renal injury and for immunohistochemical localization of P-selectin, inducible nitric oxide synthase (iNOS) and nitrotyrosine (indicative of peroxynitrite formation). Kidney myeloperoxidase (MPO) activity and malondialdehyde (MDA) levels were measured for assessment of polymorphonuclear (PMN) cell infiltration and lipid peroxidation, respectively. Nitric oxide (NO) production was determined by measurement of plasma nitrite/nitrate levels.

RESULTS

LTA pretreatment significantly reduced renal dysfunction, tubular and reperfusion-injury caused by I/R of the kidney as well as histological evidence of renal injury. LTA also reduced the expression of P-selectin and kidney MPO activity associated with renal I/R. MDA levels were significantly reduced by LTA pretreatment suggesting a reduction in the lipid peroxidation and formation of reactive oxygen species (ROS). LTA pretreatment also markedly reduced both the expression of iNOS and the formation of nitrotyrosine associated with renal I/R. Although LTA significantly reduced plasma nitrite/nitrate levels associated with I/R, nitrite/nitrate levels remained at levels significantly higher than that measured from the plasma obtained from Sham-operated animals.

CONCLUSIONS

These data suggest, to our knowledge for the first time, that LTA pretreatment for 24 hours significantly reduces renal I/R injury. We propose that the mechanism of the protective effect involves reduction of the production of NO, ROS and peroxynitrite subsequent to reduced P-selectin and iNOS expression and PMN recruitment. However, although LTA pretreatment resulted in a reduction of iNOS expression and NO production, we hypothesize that the remaining significant levels of NO contribute to the beneficial actions provided by LTA.

Promising effects of ischemic preconditioning in renal transplantation

BACKGROUND

Ischemic preconditioning, a phenomenon induced by brief ischemia and reperfusion periods, renders an organ tolerant to subsequent prolonged ischemia. This study evaluated different schedules of preconditioning the kidney to assess the role of nitric oxide (NO) and determine the effects of preconditioning on kidney transplantation.

METHODS

In study design A, to determine the optimum procedure of preconditioning, one-cycle schedules were assayed by occluding/releasing renal pedicles according to various warm ischemic (5, 10, 15, 20 min) and reperfusion (10, 20, 40 min) windows in Sprague-Dawley rats. Thereafter, warm renal ischemia was induced by clamping both pedicles for 40 minutes. Design B used the most suitable schedule found from the first study to obtain several groups, using either a direct nitric oxide donor (spermine NONOate) or two nitric oxide synthase (NOS) blockers (L-NAME and aminoguanidine), to determine whether NO mediates in renal preconditioning. To establish whether preconditioning reduces cold preservation damage, in Design C the optimum preconditioning schedule was used in syngeneic Lewis rats where preconditioned and non-preconditioned kidneys were transplanted after five hours of cold storage in Euro-Collins solution.

RESULTS

The best preconditioning schedule consisted of 15 minutes of warm ischemia and 10 minutes of reperfusion (Prec 15/10), since it was the only schedule that offered both functional and histological protection. The NO donor reproduced the ischemic preconditioning. Non-selective NOS blockade abolished the preconditioning and exacerbated ischemic damage, which was overcome by the addition of the NO donor. Selective blocking of inducible NOS also abolished the effects of preconditioning. Renal NO increased at the end of preconditioning in the Prec 15/10 group. Prolongation of the reperfusion window (20 or 40 min) abolished the preconditioning protection, although it was associated with a further increase in renal NO. As renal DNA oxidative injury paralleled NO, increasing with prolongation of reperfusion, it may account for the disappearance of preconditioning. Finally, the one-cycle preconditioning schedule offered an effective functional and histological protection against cold preservation damage in rat renal transplantation.

CONCLUSIONS

Fifteen minutes of warm ischemia and 10 minutes of reperfusion in the kidney is the most suitable one-cycle schedule for preconditioning since it protects from both warm and cold ischemia. The beneficial effect of preconditioning is related to the local production of NO, and we believe it has promising therapeutic value in clinical renal transplantation.

Kidney ischemic preconditioning

Ischemic injury to the kidney is associated with high morbidity and mortality. Improving the ability of the kidney to tolerate ischemic injury would have important implications. A significant amount of data now exists to suggest that there may be intrinsic mechanisms brought to bear by the kidney when exposed to a toxic or ischemic insult, which protect it against a subsequent exposure to ischemia. While it is frequently stated that this phenomenon, termed ischemic preconditioning, was first described in the heart, in fact there is almost a century of literature on the kidney that supports the concept that prior injury protects against a second insult. The protective effect of preconditioning is greater than most reported protective effects with pharmacological interventions in animals. There is compelling evidence in other organs that preconditioning occurs in humans. It therefore behooves us to understand the endogenous processes that the kidney has developed to protect itself against an ischemic insult. Armed with this understanding we can then attempt to mimic these processes and thereby prevent and treat ischemic acute renal failure.

Role of adenosine in renal protection induced by a brief episode of ischemic preconditioning in rats

The protective effect of a brief episode of ischemic preconditioning was examined at an early phase of ischemic-reperfusion injury in the rat kidney. Rats were subjected to 50 min of left renal artery occlusion followed by 120 min of reperfusion. Ischemic preconditioned rats were subjected to preconditioning with two cycles of 3-min ischemia and 5-min reperfusion (IPC). Ischemic-reperfusion injury led to a low recovery of the glomerular filtration rate (GFR). Overt morphological changes, consisting of blood trapping and tubular collapse, were seen. IPC improved the recovery of GFR and renal morphology. The IPC effect was not blocked by 8-(p-sulfophenyl)-theophylline (SPT), a non-selective adenosine receptor antagonist, by 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), a selective A1-receptor antagonist, or by 3,7-dimethyl-1-propargylxanthine (DMPX), a selective A2-receptor antagonist. Intravenous infusion of adenosine (30 microg/min per rat, for 5 min) prior to the 50-min occlusion improved the recovery of GFR, and this protection of GFR was blocked by SPT. Thus, both IPC and exogenous adenosine attenuated ischemic-reperfusion injury of the kidney. However, because three adenosine receptor antagonists failed to abolish the protective effect of IPC, there is no evidence to indicate that activation of adenosine receptors contributes to the IPC effect in the kidney.

Mechanisms of chronic rejection

Chronic rejection remains the major obstacle to long-term allograft survival. Detailed understanding of putative etiologic risk factors, both antigen-dependent and -independent, is important for designing effective therapeutic strategies to ameliorate this process. Cell senescence may be an important factor in chronic rejection.