Delayed kidney graft function: from mechanism to translation

In as many as 50% of cases the immediate post-kidney transplant course is complicated by delayed graft function that is most commonly related to ischemia and reperfusion injury. In addition to the acute complications related to renal failure and the associated economic impact of prolonged hospitalization, the development of delayed graft function is associated with an increased risk of chronic allograft nephropathy and shortened allograft survival. Challenges in understanding its mechanisms include the complexity, as contributors are derived from both the donor and the recipient. This acute kidney injury is modulated and caused by a complex interplay of events that lead to hypoxic and ischemic injury as well as to altered repair mechanisms. New therapies primarily seek to suppress the inflammatory homing of adaptive immune cells to the kidney, limit cell death, and/or interrupt detrimental signaling of necrosis. Although there are several promising novel targets and innovative therapeutics available, many challenges remain in their translation from bench to bedside. Identifying organs at risk and clearly defined end points will be critical in designing interventional trials. In as many as 50% of cases the immediate post-kidney transplant course is complicated by delayed graft function that is most commonly related to ischemia and reperfusion injury. In addition to the acute complications related to renal failure and the associated economic impact of prolonged hospitalization, the development of delayed graft function is associated with an increased risk of chronic allograft nephropathy and shortened allograft survival. Challenges in understanding its mechanisms include the complexity, as contributors are derived from both the donor and the recipient. This acute kidney injury is modulated and caused by a complex interplay of events that lead to hypoxic and ischemic injury as well as to altered repair mechanisms. New therapies primarily seek to suppress the inflammatory homing of adaptive immune cells to the kidney, limit cell death, and/or interrupt detrimental signaling of necrosis. Although there are several promising novel targets and innovative therapeutics available, many challenges remain in their translation from bench to bedside. Identifying organs at risk and clearly defined end points will be critical in designing interventional trials. Delayed graft function (DGF) is a common early complication following deceased-donor kidney transplantation. The incidence of DGF varies across studies and is, to a large extent, definition dependent. The straightforward United Network for Organ Sharing definition of DGF is the need for at least one dialysis in the first week after transplantation.1.Yarlagadda S.G. Coca S.G. Formica Jr., R.N. et al.Association between delayed graft function and allograft and patient survival: a systematic review and meta-analysis.Nephrol Dial Transplant. 2009; 24: 1039-1047Crossref PubMed Scopus (539) Google Scholar Once hyperacute rejection, vascular and urinary tract complications, and hyperkalemia are excluded as causes for early graft dysfunction or indication for dialysis, DGF is primarily a consequence of ischemia and reperfusion (IR) injury resulting in post-ischemic acute tubular necrosis. The degree of IR injury is dependent on a complex interplay of pre-transplant injury and on subsequent innate and adaptive immune responses after reperfusion.2.Cheung K.P. Kasimsetty S.G. McKay D.B. Innate immunity in donor procurement.Curr Opin Organ Transplant. 2013; 18: 154-160Crossref PubMed Scopus (17) Google Scholar As with all therapies for acute kidney injury (AKI), clinically feasible interference to attenuate organ injury may not be as effective or even possible by the time a clinical diagnosis is made. This rapid progression of signaling can most effectively be intervened in when these events are anticipated, as in the case of ischemia related to surgery or organ transplantation. The advantage of the predictable timing of key events and the significant clinical consequences make the study of DGF attractive. In addition to the acute complications related to renal failure and the associated costs of prolonged hospitalization, the magnitude of the association between DGF and subsequent chronic allograft dysfunction is fairly strong in most studies but it is not clear whether DGF directly affects long-term graft survival.1.Yarlagadda S.G. Coca S.G. Formica Jr., R.N. et al.Association between delayed graft function and allograft and patient survival: a systematic review and meta-analysis.Nephrol Dial Transplant. 2009; 24: 1039-1047Crossref PubMed Scopus (539) Google Scholar Only if treatment-induced reduction in the DGF rate also translates to better long-term graft function can a direct causal relationship be present. Several new drugs show promise in animal studies in preventing or ameliorating IR injury and were or are currently being tested in clinical trials (Tables 1 and 2). The FDA recently held an open workshop to summarize the current state of the translational science related to IR injury on outcomes in kidney transplantation.3.Cavaille-Coll M. Bala S. Velidedeoglu E. et al.Summary of FDA workshop on ischemia reperfusion injury in kidney transplantation.Am J Transplant. 2013; 13: 1134-1148Crossref PubMed Scopus (110) Google Scholar Here, we review the translational science investigating the principal mechanisms of IR injury, detail the definitions of DGF, outline the clinical risk factors and consequences of DGF, describe the traditional and novel tools to detect DGF, and summarize the clinical trials regarding the prevention or management of DGF after transplantation.Table 1Selected recently published randomized and controlled trials in DGFaExcluding trials testing preservation solutions and machine perfusion.InterventionTargetPopulationEnd point and outcomeRef.Epoetin-ßMultipleN=104No difference in DGF, SGF, GFR at 1–3 months48.Martinez F. Kamar N. Pallet N. et al.High dose epoetin beta in the first weeks following renal transplantation and delayed graft function: Results of the Neo-PDGF Study.Am J Transplant. 2010; 10: 1695-1700Crossref PubMed Scopus (8) Google ScholarEpoetin-aMultipleN=72No difference in DGF, SGF, and urine NGAL/IL-1847.Sureshkumar K.K. Hussain S.M. Ko T.Y. et al.Effect of high-dose erythropoietin on graft function after kidney transplantation: a randomized, double-blind clinical trial.Clin J Am Soc Nephrol. 2012; 7: 1498-1506Crossref PubMed Scopus (47) Google ScholarYSPSL (rPSGL-Ig)P-E-L selectinsN=59No difference in DGF; lower serum creatinine 5 days after transplant29.Gaber A.O. Mulgaonkar S. Kahan B.D. et al.YSPSL (rPSGL-Ig) for improvement of early renal allograft function: a double-blind, placebo-controlled, multi-center Phase IIa study.Clin Transplant. 2011; 25: 523-533Crossref PubMed Scopus (49) Google ScholarDopamine (donor treatment)MultipleN=264Decreased dialysis requirements, no difference in graft failure at 3 years45.Schnuelle P. Gottmann U. Hoeger S. et al.Effects of donor pretreatment with dopamine on graft function after kidney transplantation: a randomized controlled trial.J Am Med Assoc. 2009; 302: 1067-1075Crossref PubMed Scopus (200) Google ScholarAbbreviations: DCD, donation after cardiac death; DGF, delayed graft function; GFR, glomerular filtration rate; SGF, slow graft function.a Excluding trials testing preservation solutions and machine perfusion. Open table in a new tab Table 2Selected registered randomized and controlled DGF trials in ClinicalTrials.gov (accessed 3 September 2013)aExcluding trials testing preservation solutions and machine perfusion.InterventionTargetPrimary end point(s)Stage and estimated enrollmentClinicalTrials.gov identifierI5NPsiRNA inhibiting p53Safety and incidence of delayed kidney graft functionPhase 2BN=366NCT00802347EculizumabComplement C5aHemodialysis (7 days post-transplantation)Phase 2N=24NCT01919346OPN-305TLR2Hemodialysis (7 days post-transplantation)Phase 2N=278NCT01794663BB3Hepatocyte growth factor/scatter factorDifference in creatinine clearance over timePhase 2N=36NCT01561599Remote ischemic preconditioningMultipleNumber of organs recovered per donorPhase 3N=320NCT01515072HypothermiaMultipleFeasibility/safety; recipient organ functionPhase 2N=60NCT01544530AlteplaseDissolution of microthrombi by ex-vivo treatment of DCD organs with rTPADelayed kidney graft function and primary liver graft non-functionN=135NCT01197573EtanerceptTNF-a inhibitor to the perfusion fluidHemodialysis (7 days post-transplantation)Phase 2N=100NCT01731457Abbreviations: DGF, delayed graft function; siRNA, small interfering RNA; TLR, Toll-like receptor; TNF, tumor necrosis factor.a Excluding trials testing preservation solutions and machine perfusion. Open table in a new tab Abbreviations: DCD, donation after cardiac death; DGF, delayed graft function; GFR, glomerular filtration rate; SGF, slow graft function. Abbreviations: DGF, delayed graft function; siRNA, small interfering RNA; TLR, Toll-like receptor; TNF, tumor necrosis factor. It is unlikely that AKI induced by clamping of the renal vessels for 30–45min in experimental animals mimics the events in human kidneys after transplantation. In addition to the timing of the intervention relative to injury, some of the limitations arise from different susceptibility to AKI of the animal species, strain, and gender. In addition, the use of various techniques to accurately reflect and distinguish the effects of warm versus cold ischemia and isograft versus allograft vary. Nevertheless, a large quantity of excellent science has been generated identifying a wide range of pathological processes that contribute to hypoxic and IR-associated injury (reviewed in detail by Eltzschig and Eckle4.Eltzschig H.K. Eckle T. Ischemia and reperfusion–from mechanism to translation.Nat Med. 2011; 17: 1391-1401Crossref PubMed Scopus (2115) Google Scholar). We will focus here on cell death and survival programs, the innate and adaptive immune activation, as these are potentially amenable to innovative therapeutic approaches. IR activates various programs of cell death, which can be categorized as necrosis, apoptosis, or autophagy-associated cell death. These events are critical stimulators of a subsequent repair response. Knockdown of p53 in proximal tubular cells by synthetic small interfering RNA inhibited apoptotic signaling and decreased kidney injury in clamp ischemia and transplant models.5.Molitoris B.A. Dagher P.C. Sandoval R.M. et al.siRNA targeted to p53 attenuates ischemic and cisplatin-induced acute kidney injury.J Am Soc Nephrol. 2009; 20: 1754-1764Crossref PubMed Scopus (259) Google Scholar Important regulators (e.g., p53, PI3K/Akt axis, Bcl-2, ER stress) can control and trigger apoptosis and autophagy. Recently, ‘necroptosis’ was coined as a regulated form of cell death, and members of the receptor-interacting protein kinase family (RIPK1,3) were determined to be key members in its regulation.6.Lau A. Wang S. Jiang J. et al.RIPK3-mediated necroptosis promotes donor kidney inflammatory injury and reduces allograft survival.Am J Transplant. 2013; 13: 2805-2818Crossref PubMed Scopus (160) Google Scholar Inhibition of caspase 8 in the donor kidneys increased necroptosis, enhanced HMGB1 release, and reduced renal function when transplanted into major histocompatibility complex-mismatched recipient mice. Inhibition of necroptosis by using RIPK3−/− donor kidney prevented necrosis and was protective in IR injury.6.Lau A. Wang S. Jiang J. et al.RIPK3-mediated necroptosis promotes donor kidney inflammatory injury and reduces allograft survival.Am J Transplant. 2013; 13: 2805-2818Crossref PubMed Scopus (160) Google Scholar Autophagy is a general term for pathways by which cytoplasmic material is delivered to lysosomes for degradation7.Huber T.B. Edelstein C.L. Hartleben B. et al.Emerging role of autophagy in kidney function, diseases and aging.Autophagy. 2012; 8: 1009-1031Crossref PubMed Scopus (205) Google Scholar (Figure 1). The main purposes of autophagosome formation are quality control and removal of defunct organelles, to provide an energy source during starvation, to regulate cell survival and cell death, and, more recently described, to serve as an important effector and regulator of innate and adaptive immunity.7.Huber T.B. Edelstein C.L. Hartleben B. et al.Emerging role of autophagy in kidney function, diseases and aging.Autophagy. 2012; 8: 1009-1031Crossref PubMed Scopus (205) Google Scholar Autophagy in tubular cells is upregulated following IR injury in murine models and in tubule cells obtained from human transplanted kidney biopsy specimens (reviewed in the study by Huber et al.7.Huber T.B. Edelstein C.L. Hartleben B. et al.Emerging role of autophagy in kidney function, diseases and aging.Autophagy. 2012; 8: 1009-1031Crossref PubMed Scopus (205) Google Scholar). Using autophagy reporter mice (CAG-RFP-EGFP-LC3), it was determined that autophagic activity peaked after 1 day and returned to baseline 3 days after IR. Interestingly, most tubuli with activated mechanistic target of rapamycin showed no signs of autophagy, whereas inhibition of mTROC1 induced autophagy and limited regenerative cell proliferation.8.Li L. Wang Z.V. Hill J.A. et al.New autophagy reporter mice reveal dynamics of proximal tubular autophagy.J Am Soc Nephrol. 2013Google Scholar This suggests a role of the mechanistic target of rapamycin complex in autophagy regulation and renal repair and supports reports that rapamycin delayed renal recovery after ischemic insult.9.Lieberthal W. Fuhro R. Andry C. et al.Rapamycin delays but does not prevent recovery from acute renal failure: role of acquired tubular resistance.Transplantation. 2006; 82: 17-22Crossref PubMed Scopus (56) Google Scholar Consistent with this data are several definite studies that clearly show autophagy as being reno-protective during IR injury. By using two different tubular-specific ATG5 and ATG7 knockout mouse models, IR injury in autophagy-deficient tubular cells was found to be more severe compared with that in wild-type mice. In addition, ATG-deficient kidneys showed rapid accumulation of p62 (also known as SQSTM1) a key autophagy substrate, more ROS markers, and increased tubular apoptosis.10.Jiang M. Wei Q. Dong G. et al.Autophagy in proximal tubules protects against acute kidney injury.Kidney Int. 2012; 82: 1271-1283Abstract Full Text Full Text PDF PubMed Scopus (360) Google Scholar,11.Liu S. Hartleben B. Kretz O. et al.Autophagy plays a critical role in kidney tubule maintenance, aging and ischemia-reperfusion injury.Autophagy. 2012; 8: 826-837Crossref PubMed Scopus (202) Google Scholar The observation that autophagy precedes apoptosis suggests that it is an early response to cell stress and not a result of apoptosis. It is still not well understood how and under what circumstances autophagy is protective and which signaling pathways lead to tubular autophagy. It will be critical to identify nexuses where innate sensing receptors (e.g., Toll-like receptors (TLRs)) and cell survival or cell death pathways intersect and the mechanisms by which they do so. Although there is enormous clinical interest in the mechanisms of cell death (e.g., Crohn’s disease, cancer, aging, and Alzheimer’s disease), the lack of validated clinical markers and the absence of selective inducers and inhibitors of autophagy are challenges for successful translational research and clinical trials. The innate immune system should not be seen as a separate system, but rather as an overlapping response to disturbed tissue integrity. Important components that have been well studied in animal models of IR injury are TLRs, the inflammasomes, and the complement system. TLRs are expressed on immune as well as nonimmune cells, and endogenous, cell-derived ligands (so-called damage-associated molecular patterns) can signal through specific TLRs12.Leventhal J.S. Schroppel B. Toll-like receptors in transplantation: sensing and reacting to injury.kidney Int. 2012; 81: 826-832Abstract Full Text Full Text PDF PubMed Scopus (82) Google Scholar (Figure 2). Among the list of damage-associated molecular patterns that have been described to be induced or upregulated after IR, only HMGB1 was so far mechanistically linked to the pathogenesis of IR injury.12.Leventhal J.S. Schroppel B. Toll-like receptors in transplantation: sensing and reacting to injury.kidney Int. 2012; 81: 826-832Abstract Full Text Full Text PDF PubMed Scopus (82) Google Scholar,13.Li J. Gong Q. Zhong S. et al.Neutralization of the extracellular HMGB1 released by ischaemic damaged renal cells protects against renal ischaemia-reperfusion injury.Nephrol Dial Transplant. 2011; 26: 469-478Crossref PubMed Scopus (91) Google Scholar HMGB1 is a nuclear protein that binds DNA and modulates transcription and chromatin modeling. Dependent on its redox state, HMGB1 functions as an extracellular signaling molecule during sterile inflammation, providing a chemotactic as well as an activation signal to inflammatory cells.14.Andersson U. Tracey K.J. HMGB1 is a therapeutic target for sterile inflammation and infection.Ann Rev Immunol. 2011; 29: 139-162Crossref PubMed Scopus (1071) Google Scholar Our group found that TLR4 expression was upregulated and tubular HMGB1 was detectable in deceased-donor kidneys when compared with living-donor kidneys. In addition, kidneys carrying the loss-of-function TLR4 variants (Asp299Gly and Thr399Ile), known to diminish ligand-receptor binding, were linked with better function immediately after kidney transplantation.15.Kruger B. Krick S. Dhillon N. et al.Donor Toll-like receptor 4 contributes to ischemia and reperfusion injury following human kidney transplantation.Proc Natl Acad Sci USA. 2009; 106: 3390-3395Crossref PubMed Scopus (287) Google Scholar The injury-promoting role of TLR4 in particular is evident in most solid organs, as demonstrated by the protection of TLR4-deficient mice after hepatic, renal, cardiac, and cerebral IR injury. Chimeric mice with deficiency in renal-associated TLR2 and TLR4 had less renal damage and dysfunction when compared with wild-type mice. On comparing single TLR2−/− and TLR4−/− mice with the TLR2/4−/− double knockout mice, no increased protection was seen, indicating that ligands prime TLR2 and TLR4 during IR injury.16.Rusai K. Sollinger D. Baumann M. et al.Toll-like receptors 2 and 4 in renal ischemia/reperfusion injury.Pediatr Nephrol. 2010; 25: 853-860Crossref PubMed Scopus (75) Google Scholar In addition to targeting the receptors directly, modulating signaling molecules downstream of TLR may present an alternative for interventions (Figure 2). Inflammasomes are intracellular multiprotein complexes expressed in both parenchymal and non-parenchymal cells in the kidney. Inflammasomes respond to damage-associated molecular patterns making them optimal sentinels for cellular stress and injury. The NOD leucine–rich repeat pyrin domain containing NLRP, named NLRP1, assembles and oligomerizes into a common structure that collectively activates the caspase-1 cascade, thereby leading to the production of pro-inflammatory cytokines, especially interleukin-1β and interleukin-18. The NLRP1 multimolecular complex was coined the ‘inflammasome’. Other inflammasomes include NLRP3, which was found to protect mice from IR injury.17.Shigeoka A.A. Mueller J.L. Kambo A. et al.An inflammasome-independent role for epithelial-expressed Nlrp3 in renal ischemia-reperfusion injury.J Immunol. 2010; 185: 6277-6285Crossref PubMed Scopus (196) Google Scholar Although the absence of other NLRP3 inflammasome components such as ASC failed to protect mice from renal IR injury in one study, others reported that ASC-deficient kidneys were largely resistant against IR injury.17.Shigeoka A.A. Mueller J.L. Kambo A. et al.An inflammasome-independent role for epithelial-expressed Nlrp3 in renal ischemia-reperfusion injury.J Immunol. 2010; 185: 6277-6285Crossref PubMed Scopus (196) Google Scholar,18.Iyer S.S. Pulskens W.P. Sadler J.J. et al.Necrotic cells trigger a sterile inflammatory response through the Nlrp3 inflammasome.Proc Natl Acad Sci USA. 2009; 106: 20388-20393Crossref PubMed Scopus (539) Google Scholar Studies in small and large animals revealed that the alternative pathway of complement activation is a key proximal mediator of kidney IR injury. Small animal studies found increased C3 deposition along the tubular basement membrane after IR. The mechanisms of how the alternate pathway of complement system is activated at the site of renal injury are not well understood. Possible causes or mediators include ammonia, an acidic environment, and the binding of natural antibodies to neo-epitopes. Nevertheless, the kidney is known to be susceptible to the effects of alternative pathway complement dysregulation and complement proteins can further promote local complement activation and injury (reviewed in the study by McCullough et al.19.McCullough J.W. Renner B. Thurman J.M. The role of the complement system in acute kidney injury.Seminars Nephrol. 2013; 33: 543-556Abstract Full Text Full Text PDF PubMed Scopus (63) Google Scholar). Data obtained using genetic alterations of the complement system support the pathogenic role of this pathway in response to ischemia. IR injury is abrogated in animals that are deficient in C3 (and factor B) and is exacerbated in animals that are deficient in DAF, a situation that results in unregulated complement activation.20.Damman J. Daha M.R. van Son W.J. et al.Crosstalk between complement and Toll-like receptor activation in relation to donor brain death and renal ischemia-reperfusion injury.Am J Transplant. 2011; 11: 660-669Crossref PubMed Scopus (67) Google Scholar,21.Yamada K. Miwa T. Liu J. et al.Critical protection from renal ischemia reperfusion injury by CD55 and CD59.J Immunol. 2004; 172: 3869-3875Crossref PubMed Scopus (147) Google Scholar Interestingly, using chimeric mice, it was found that the complement receptors (C3aR and C5aR) expressed on renal cells as well as on leukocytes contribute to IR injury.22.Peng Q. Li K. Smyth L.A. et al.C3a and C5a promote renal ischemia-reperfusion injury.J Am Soc Nephrol. 2012; 23: 1474-1485Crossref PubMed Scopus (150) Google Scholar Data obtained in humans were similar to those reported in rodent models. In clinical kidney transplantation, several key complement transcripts were upregulated in deceased-donor kidneys compared with kidneys from living donors. The complement induction correlated inversely with early graft function (post-operative days 2–3).23.Naesens M. Li L. Ying L. et al.Expression of complement components differs between kidney allografts from living and deceased donors.J Am Soc Nephrol. 2009; 20: 1839-1851Crossref PubMed Scopus (109) Google Scholar On the basis of these strong preclinical data the C5a inhibitor Eculizumab is currently being tested in kidney transplant recipients who are at high risk for DGF (Table 2). At this point, Eculizumab is the only Food and Drug Administration-approved complement inhibitor but there are a large number of complement inhibitory drugs in development. Brain death has well-known effects on hemodynamic stability, on hormone regulation, and on the inflammatory reactivity of islet, kidney, liver, and heart grafts. This brain death–triggered local inflammation is believed to be in part responsible for the inferior results of kidneys retrieved from brain-dead donors compared with living donor kidneys in terms of DGF, acute rejection, and long-term allograft survival. In kidneys retrieved from brain-dead donors compared with kidneys from living donors, systemic generation of C5a mediates renal inflammation via tubular C5a–C5aR interaction.24.van Werkhoven M.B. Damman J. van Dijk M.C. et al.Complement mediated renal inflammation induced by donor brain death: role of renal C5a-C5aR interaction.Am J Transplant. 2013; 13: 875-882Crossref PubMed Scopus (50) Google Scholar Of note, the inhibition of systemic complement activation in rat brain-dead donors was able to improve renal function after transplantation.25.Damman J. Seelen M.A. Moers C. et al.Systemic complement activation in deceased donors is associated with acute rejection after renal transplantation in the recipient.Transplantation. 2011; 92: 163-169Crossref PubMed Scopus (71) Google Scholar Future studies need to define where the specific components of the complement cascade that mediates the injury are produced and activated, and identify the tissue triggers exposed by brain death or ischemia-induced stress. With eculizumab, a C5-inhibitor, agents are available and clinical trials in DGF are ongoing (Figure 3, Table 2). Therapeutic interventions at the time of brain death may be needed for optimal post-transplant effects on graft outcome. IR injury elicits a robust adaptive immune response. T cells (CD4 and CD8) accumulate during IR injury and mediate injury.26.Day Y.J. Huang L. Ye H. et al.Renal ischemia-reperfusion injury and adenosine 2A receptor-mediated tissue protection: the role of CD4+ T cells and IFN-gamma.J Immunol. 2006; 176: 3108-3114Crossref PubMed Scopus (184) Google Scholar The specific mechanisms underlying T-cell activation in the absence of specific exogenous antigen remain to be elucidated, but data indicate antigen-specific and antigen-independent mechanisms of action.4.Eltzschig H.K. Eckle T. Ischemia and reperfusion–from mechanism to translation.Nat Med. 2011; 17: 1391-1401Crossref PubMed Scopus (2115) Google Scholar In warm liver IR injury, CD4 T cells function without the requirement of de novo antigen-specific activation, and are dependent on CD154-CD40.27.Shen X. Wang Y. Gao F. et al.CD4 T cells promote tissue inflammation via CD40 signaling without de novo activation in a murine model of liver ischemia/reperfusion injury.Hepatology. 2009; 50: 1537-1546Crossref PubMed Scopus (76) Google Scholar In contrast, Tregs appear to have a protective role and are able to suppress innate immune responses in IR injury.28.Kinsey G.R. Huang L. Jaworska K. et al.Autocrine adenosine signaling promotes regulatory T cell-mediated renal protection.J Am Soc Nephrol. 2012; 23: 1528-1537Crossref PubMed Scopus (108) Google Scholar The early graft dysfunction especially using deceased donors is often classified into immediate, slow (SGF), delayed (DGF), or, in the most severe cases, primary nonfunction (PNF) (Figure 4). Owing to the complexity of its pathophysiology, it is both difficult and simplistic to find one simple definition of early graft dysfunction, which explains why currently more than 18 definitions coexist.1.Yarlagadda S.G. Coca S.G. Formica Jr., R.N. et al.Association between delayed graft function and allograft and patient survival: a systematic review and meta-analysis.Nephrol Dial Transplant. 2009; 24: 1039-1047Crossref PubMed Scopus (539) Google Scholar The most frequent definition is based on post-transplant dialysis requirements (at least one dialysis session during the first 7 days after transplantation).1.Yarlagadda S.G. Coca S.G. Formica Jr., R.N. et al.Association between delayed graft function and allograft and patient survival: a systematic review and meta-analysis.Nephrol Dial Transplant. 2009; 24: 1039-1047Crossref PubMed Scopus (539) Google Scholar Although useful for data reporting, this definition suffers from many pitfalls including clinical-dependent decision, dialysis required for potassium or fluid overload, residual renal function, or preemptive transplantation, which may lead to misclassification or large variations in DGF rates that were observed in multicenter trials.29.Gaber A.O. Mulgaonkar S. Kahan B.D. et al.YSPSL (rPSGL-Ig) for improvement of early renal allograft function: a double-blind, placebo-controlled, multi-center Phase IIa study.Clin Transplant. 2011; 25: 523-533Crossref PubMed Scopus (49) Google Scholar Other definitions that may rely on urine output (with pitfalls such as residual renal function, preemptive transplantation, non-oliguric AKI), creatinine reduction (with drawbacks such as time to diagnosis and pre-transplant dialysis), and analysis of urine biomarkers such as interleukin-18, KIM1, NGAL, which appear early after AKI, are noninvasive but so far not used outside research protocols. Still others include analysis of renal blood flow using MAG3 renal scan or gadolinium-enhanced magnetic resonance imaging (costly, time consuming, and with risk of nephrogenic fibrosing sclerosis) and pre-implantation kidney biopsy analyzed with light microscopy or gene transcripts (tumor necrosis factor-α). These heterogeneous definitions reflect the complexity of the processes leading to clinical syndrome of DGF. Indeed, in order to advance in the prevention and/or treatment of DGF, it is important to isolate the diagnosis of IR-induced AKI to dissect and evaluate the influence of factors related to the donor (age, donor management), to the recipient (age, quality of arteries, surgical procedure, anti-human leukocyte antigen immunization), to the organ allocation (leading to various cold ischemia times), and finally to other causes of renal failure, such as surgical complications, drug nephrotoxicity, and rejection (Figure 5). Overall, our current inability to define and hence to accurately diagnose DGF leads to misclassification of patients in ‘real life’ but more importantly to misclassification in clinical trials devoted to preventing or treating DGF. The main donor factors increasing the risk of DGF are increasing donor age, donor type, and quality of pre-kidney procurement care (Figure 5). The risk of DGF (and subsequent graft failure) augments from living donor kidneys to deceased donors (SCD