Hypertension is an independent predictor of delayed graft function and worse renal function only in kidneys with chronic pathological lesions

Abstract

Background.  

Delayed graft function (DGF) has been identified as one of the principal correlates of poor graft survival in cadaveric renal transplantation. However, its risk factors and clinical predictors have been poorly elucidated.

Methods.  

We analyzed the risk factors of DGF with a specific emphasis on the role of histological damage of donor kidney. Then, we also studied the impact of DGF, and donor factors affecting DGF, on kidney graft function over the first year after engraftment in 100 consecutive cadaveric renal transplant (Tx) recipients.

Results.  

The organs displaying DGF (n=48) had a significantly higher degree of glomerular sclerosis and tubular atrophy (P <0.01), as well as of interstitial fibrosis and vascular damage (P <0.02) in time-zero biopsies. In patients who received an “ideal” organ for Tx (total histological score ≤4), DGF showed a strong relationship with Δage D-R (70% increase of risk for donors 10 years older than recipients), and with the histological score (odds ratio 1.34). In contrast, donor hypertension was the most relevant variable independently associated with DGF (odds ratio 19.4) in patients receiving a suboptimal organ (histological score >4). Moreover, DGF and donor hypertension adversely affected graft function at 1 year, but only in Tx patients with a histological score >4 in time-zero biopsy. Of note, both patients with and those without DGF showed a very low incidence of biopsy-proven acute rejection (8.5 and 6.8%, respectively) and a rather short cold ischemia time (<16 hr).

Conclusion.  

Our findings suggest that the quality of the transplanted organ and the occurrence of DGF are strictly related to each other and can influence graft function through apparently nonimmune mechanisms. In addition, long-standing donor hypertension is a strong independent variable affecting both DGF and graft function of suboptimal cadaveric kidneys, at least up to 1 year.

INTRODUCTION

The impressive reduction in acute rejection and early graft loss achieved with modern immunosuppression has focused attention on the long-term outcome of kidney transplants and the factors leading to chronic graft failure. The characteristic interstitial fibrosis, tubular atrophy, vascular damage, and glomerulosclerosis present in chronically failing kidney allografts may have their beginning soon after transplantation (^1,2). Both donor factors and perioperative events are critical to the long-term outcome, with higher rates of failure associated with poor donor organ quality (^3–7), acute rejection (AR) (^8–10), and delayed graft function (DGF) (^11–16).

DGF has long been identified as one of the principal correlates of poor graft survival in cadaveric renal transplantation (^17,18); however, its strength as an independent variable has been questioned. Previous studies had conflicting results over the importance of DGF in the absence of acute rejection, with some groups finding no effect (¹⁴), and other groups finding a DGF effect independent from early acute rejection (^19,20). Indeed, DGF may increase the likelihood of acute rejection, by either the release of cytokines and the up-regulation of major histocompatibility complex class II antigens (²¹) or masking the early diagnosis of rejection. However, ischemia-reperfusion injury, although causing DGF, may also trigger an ongoing injury response involving inflammation, fibrosis, and hyperfiltration injury, eventually leading to chronic allograft nephropathy (^4,22).

The factors that cause some kidney grafts to exhibit delayed function although others function promptly, are poorly understood (²³). In this prospective study, we analyzed the risk factors of DGF with a specific emphasis on the role of histological damage of donor kidney. Then, we also studied the impact of DGF, and donor factors affecting DGF, on kidney graft function over the first year after engraftment, taking graft function at 1 year as a surrogate marker of late graft outcome.

MATERIALS AND METHODS

Donors.

Sixty consecutive cadaveric donors were included in the study. All subjects were Caucasians. Renal specimens were obtained by wedge biopsy performed after the perfusion (time-zero biopsy), fixed by 4% formaldehyde or Bouin, and then used for histological staining (hematoxylin-eosin, periodic acid-Schiff, silver methenamine, and Masson’s trichrome). The histological lesions of the four compartments of renal tissue (glomeruli, tubules, interstitium, and vessels) were scored by light microscopy by two pathologists blinded to the clinical history of the donor. The severity of chronic lesions was evaluated semiquantitatively using in part the criteria suggested by the ‘97 Banff classification on chronic/sclerotic allograft (²⁴), as illustrated in Table 1. To achieve a reliable evaluation, only renal biopsies containing at least 20 glomeruli were considered. Kidneys with more than 30% of glomerulosclerosis were not used for single transplantation. For these reasons, 10 renal donors were excluded from further analysis.

Table 1: Histological grading of the renal biopsy score

At the time of donation request, information on the donor’s medical condition, and particularly on donor hypertension and duration of disease, was obtained from medical records and next of kin. Specifically, arterial hypertension was diagnosed when the patient had been chronically taking antihypertensive therapy to achieve blood pressure control.

Recipients.

Kidneys were allocated to 100 Caucasian recipients of the computerized regional waiting list according to four parameters (blood group, HLA histocompatibility, time on dialysis, and panel reactive antibodies). For donors >55 years, only recipients >45 years were chosen. Immunosuppressive therapy consisted of corticosteroids, cyclosporine A or tacrolimus, and micophenolate mofetil. A chimeric monoclonal antibody toward the α-chain of the IL-2 receptor (Simulect) was given in 2 doses of 20 mg each on day 0 and day 4 after transplantation.

To exclude patients who were dialyzed for reasons other than impaired graft function, DGF was diagnosed if the serum creatinine level (sCr) increased or remained unchanged immediately after surgery during three consecutive days for more than 1 week (¹¹). Moreover, all patients with DGF underwent routine graft biopsy after the first posttransplant week: if it revealed AR, the graft was categorized as primary function. In fact, no such case was observed in the population studied.

Statistical methods.

The results of the quantitative variables were expressed as mean±SD and those of the qualitative variables as proportions. The Mann-Whitney and χ² test were used for testing the differences between the quantitative and qualitative variables, respectively. All tests were two-tailed. P <0.05 was considered statistically significant.

The logistic regression model was used to determine the factors significantly related to DGF. The significant predictors were next fitted in a multivariate model. The risk is expressed as odds ratio (OR) +95% confidence interval (CI). The StatView software package, SAS Inc. Co (5.0 version), was used for all analyses.

RESULTS

The age of donors was 46±16.9 years (range 18–74), recipients age was 46.8±8.7 years (range 22–60). Table 2 shows demographic data of donors and recipients, grouped according to the occurrence of DGF. Cadaveric kidney donors of patients who experienced DGF had an older age, a higher prevalence of long-standing hypertension, and a higher frequency of brain stroke as cause of death. In contrast, the age of renal transplant recipients was superimposable in the two groups, as well as cold ischemia time, number of HLA mismatches, dialytic age, and incidence of AR episodes (Table 2).

Table 2: Clinical features of cadaveric donors and renal transplant recipients divided according to the presence of DGF

We first wondered whether the histological score of donor kidneys would help predict the occurrence of DGF. Indeed, the degree of renal damage in the organs displaying DGF was significantly higher in all the compartments examined (Fig. 1). Noteworthy, all grafts that subsequently developed DGF displayed signs of nonspecific tubular injury, but such histological feature was present also in a percentage of kidneys that showed an immediate graft function. Simple logistic analysis showed that each of the histological variables examined was highly correlated with DGF, but when all variables entered a multiple logistic model, interstitial fibrosis was no longer a risk factor of DGF (Table 3).

Figure 1: Grading of the histologic lesions in renal biopsies of cadaveric donors divided into two groups according to DGF. A, Glomerular sclerosis; B, tubular damage; C, interstitial fibrosis; D, vascular lesions.

Table 3: Histological predictors of DGF in time-zero donor biopsy

Subsequently, we extended the analysis of risk factors to donor hypertension and donor age. In univariate analysis, a history of hypertension (longer than 5 years) in the donor turned out to represent the most relevant independent predictive factor of DGF, followed by the global histological score (Table 4). Then, a donor age higher than that of the recipient, regardless of their absolute values, turned out to be associated with an increased risk of DGF (Table 4).

Table 4: Donor factors predictors of DGF

The policy at our Center is to preferentially allocate organs to recipients with a difference of age between donor and recipient (Δ age) of about 10 years. Thus, to evaluate the impact of donor age on the occurrence of DGF, we chose to use Δ age, rather than the absolute age of donor. Nonetheless, when absolute donor age was examined, statistical analysis displayed closely similar results (not shown).

The number of HLA mismatches and the length of cold ischemia time did not show any significant correlation with DGF (not shown). As a matter of fact, the population studied exhibited rather homogeneous values for both these variables (Table 2), which might help explain the lack of correlation.

When all the independent variables were fitted in a multiple logistic regression model, both total histological score and the presence of hypertension in the donor maintained their relationship with the incidence of DGF, whereas Δ age (as well as absolute donor age) turned out to be statistically irrelevant (Table 4).

Then, we stratified renal transplant recipients in two groups, according to their global histological score (Table 5). In the former group (score≤4, no. 48), whose patients received an apparently “ideal” organ for transplantation, the occurrence of DGF showed a relationship with global histological score and Δ age, but not with donor hypertension. In contrast, in patients of the latter group (score >4, no. 52) donor hypertension and, less, histological score were the most relevant variables independently associated with DGF (Table 5).

Table 5: Predictors of DGF

Graft Outcome

During the first year after kidney transplant, two patients died with a functioning graft (hemorrhagic pancreatitis and sepsis, respectively), and one patient experienced early (1st month) graft loss because of renal vein thrombosis.

One year after transplant, sCr levels were significantly higher in patients who had experienced DGF (Fig. 2 A). Of note, the incidence of AR was strictly superimposable in the two groups (DGF 8.5%; no DGF 6.8%). Similarly, patients receiving an organ from a hypertensive donor showed significantly higher levels of sCr at 12 months (Fig. 2 B). In contrast, the histological score of donor kidney, in itself, failed to influence graft function at 12 months (score <4: sCr 1.52±0.34 mg/dl; score >4: sCr 1.64±0.50 mg/dl;P NS).

Figure 2: One-year serum creatinine levels in renal transplant recipients with or without DGF (A) and in patients with or without donor hypertension (B).

Then, we examined the combined influence of basal histological damage and DGF, or donor hypertension, on 1-year graft function. Interestingly, sCr was significantly higher in recipients with DGF (P <0.001) or donor hypertension (P <0.02) only in the subgroup of patients with a global histological score >4. In contrast, donor hypertension and DGF failed to affect 1-year graft function in patients with a score <4.

DISCUSSION

The purpose of our study was to evaluate whether pretransplant donor biopsy would help predict DGF in kidney transplant recipients. Previously, the quality of the donor organ at implantation had been shown to correlate with graft function and renal histology as early as 3 months after engraftment (²). In contrast, only a few reports have examined the impact of histopathological changes of donor kidney on the incidence of DGF. Lehtonen et al. (²⁵) reported that, though histopathological changes of the donor organ are predictive of subsequent graft function, they did not affect onset of graft function. Oda and coworkers (²⁶) found that tubulointerstitial score was useful to predict the outcome, not the occurrence, of DGF. Finally, Karpinski et al. (⁷) reported that a donor vessel score of three of three was associated with a 100% incidence of DGF, along with a worse 1-year graft function.

In our study, DGF was reliably predicted by glomerulosclerosis, vascular damage, and, though to a lesser extent, tubular atrophy, whereas interstitial fibrosis of the donor organ, when entered a multiple logistic model, turned out to be irrelevant. Interestingly enough, in patients receiving an “ideal organ,” with mild or absent chronic tissue lesions, the risk of DGF was increased by a donor older than the recipient, regardless of their absolute age: a delta value of 10 years increased the risk by almost 70%. This might reflect a renal tissue supply-demand mismatch, and recipient higher metabolic demands would amplify conventional ischemia-reperfusion injury. In contrast, when patients receiving an organ from a suboptimal donor (histological score >4) were examined, delta age did not modify the risk for DGF, whereas donor hypertension was by far the most relevant independent predictor of DGF (OR 19.4), compared with histological score (OR 1.2). It may be argued that arterial hypertension would cause a latent condition of renal ischemia, potentially favoring the onset of DGF in kidneys which exhibit a relevant degree of chronic histological damage.

Then, we examined the influence of DGF, and of the donor factors affecting DGF, on renal function 1 year after engraftment. In our population of 100 renal transplant recipients, DGF and donor hypertension significantly affected graft function, but only in those patients who received a kidney from a suboptimal donor (global score >4). In contrast, the global histological score of time-zero biopsies failed to predict subsequent renal function at 12 months. Taken together, these results would suggest that donor’s medical history and early posttransplant events affect long-term graft outcome only in the presence of a relevant histological damage of donor organ, although the quality of donor organ, in itself, is a poor independent predictor of subsequent graft function.

The above findings confirm that, among donor factors, long-standing hypertension exerts a strong negative impact on allograft outcome (^27,28), although such effect seems to be limited to organs with a remarkable chronic damage. However, controversy exists as to whether DGF could affect cadaveric renal allograft survival independently from acute immune-mediated events (¹⁷). Our study population showed a very low incidence of biopsy-proven AR that was quite comparable in patients with or without DGF, thus supporting the notion that DGF is an independent predictor of long-term graft function. It may be worth mentioning that DGF, but not cellular rejection, has been shown to correlate with the Banff grades of chronic glomerulitis, chronic interstitial fibrosis, and tubular atrophy scores in early (3 months) biopsy specimens (²).

In conclusion, donor biopsy allows to better discriminate the impact of several risk factors on graft outcome. Our findings suggest that the quality of the transplanted organ and the occurrence of DGF are strictly related to each other and can influence graft function through apparently nonimmune mechanisms. In addition, long-standing donor hypertension is a strong independent variable affecting both DGF and graft function of suboptimal cadaveric kidneys, at least up to 1 year.