Objectives: Kidney transplant is well accepted as the optimal therapy for children with end-stage renal disease, and new trends suggest using human leukocyte antigen-DR mismatched grafts. The aim of work was to assess the effect of human leukocyte antigen-DR mismatch on the outcome of pediatric renal transplant recipients, regardless of the source of kidney graft.

Materials and Methods: According to human leukocyte antigen-DR matching, 104 pediatric patients were categorized into 3 comparable groups. With optimized immunosuppression protocols, long-term graft and patient outcomes were assessed.

Results: We found that posttransplant complications were comparable in the 3 groups, without significant increase in the risk of infections or malignancies, especially in the full human leukocyte antigen-DR-mismatched group. Moreover, we found no significant difference in the 3 groups regarding long-term graft or patient survival.

Conclusions: With optimization of immuno­suppression, human leukocyte antigen-DR-mis­matched donors can be safely accepted for pediatric kidney transplant with comparable long-term patient and graft survival.

Key words : Children, End-stage renal disease, Renal transplant, Human leukocyte antigen-DR, Outcome

Introduction

Renal transplant is the therapy of choice for children with end-stage renal disease.¹ Compared with dialysis, renal transplant provides better survival rates and also a better quality of life. However, renal graft survival is limited because of immunologic and nonimmunologic factors. Ethnicity affects renal transplant survival rates substantially.² Several papers, mainly from the United States, have reported that adults and African American child recipients had worse patient and transplant survival rates than did white recipients.^3-8 Biological factors such as different major histocompatibility complex antigens, and/or socioeconomic factors (eg, the lack of education), may have determined the unfavorable results in these studies.^9-11 A novel approach has been recommended to ensure well-matched transplants for younger patients, and human leukocyte antigen (HLA) matching is less important for older patients because revision transplant is less likely to be required.¹²

The HLA mismatches may correlate with risk of death with a functioning graft because of the requirement for higher immunosuppression doses and more antirejection therapy. The most frequent causes of death with a functioning graft were infection, cardiovascular disease, and malignancy (32.2%, 30.9%, and 3.6% in year 1; 16.4%, 29.6%, and 15.9% in years 2-5).¹³ In addition, multivariate analyses showed that mismatches for HLA class II were more strongly associated with hospitalization and death with a functioning graft than mismatches for HLA class I.¹³

Another study was performed to determine the effect of HLA-DR mismatch on rejection, graft survival, and sensitization in a local allocation system that emphasized donor quality rather than HLA matching for pediatric patients, and to determine the likelihood of finding an appropriate donor that was based on HLA-DR mismatch.¹⁴ The 1- and 5-year graft survival rates were 97% and 82%; HLA-DR mismatching was not a significant risk factor for either is failing, or will fail; but a history of rejection was an independent predictor of graft failure. However, although avoiding HLA-DRB1 mismatching is beneficial, it was concluded that the likelihood of finding an HLA-DRB1-matched donor also should be considered in donor selection.¹⁴

We aimed to assess the effect of HLA-DR mismatch on the outcome of pediatric renal transplant recipients.

Materials and Methods

Patients
The charts of all children and adolescents who underwent a renal transplant between 1994 and 2011 at the Hamed Al-Essa Organ Transplant Center of Kuwait were reviewed. In these patients, the following data were retrospectively analyzed: age at transplant, sex, underlying renal disease, prior renal replacement therapy, organ donor source, cold ischemia time, and HLA match. According to HLA-DR matching, patients were categorized into 3 groups: group 1, patients with full HLA-DR match; group 2, patients with 1 HLA-DR mismatch; and group 3, patients with full HLA-DR mismatch.

To assess the renal transplant outcome, we analyzed patient and graft survival rates, acute rejection episodes, rejection-free time, allograft function, infection episodes necessitating hospitalization, and number of antihypertensive drugs at transplant and last follow-up. The underlying renal diseases of the examined patients were classified etiologically into 4 groups: glomerulonephritis, chronic tubulointerstitial disease, congenital diseases, and idiopathic disease. Pretransplant complement-dependent cytotoxicity and flow cytometry crossmatches were negative.

Immunosuppressive regimens
All patients received triple immunosuppressive regimens consisting of a calcineurin inhibitor, mycophenolate mofetil, and corticosteroids. All induction therapy was based on our protocol guidelines and transplant risk factors. Patients at high risk for acute rejection, including patients having revision transplant, panel reactive antibodies (PRA) > 20%, or deceased-donor transplant, received rabbit antithymocyte globulin (Thymoglobulin, Genzyme Corp., Cambridge, MA, USA) at 1.0 mg/kg daily (total, 5 doses). Other patients received interleukin 2 receptor antagonist (IL-2RA) induction using basiliximab (Simulect, Novartis Pharmaceuticals, New York, NY, USA) at 10 mg/m² intravenous as a bolus within 2 hours of engraftment on day 0 and the second dose of 10 mg/m² on day 4, or daclizumab (Zenapax, Roche Pharmaceuticals, Nutley, NJ, USA) at 1 mg/kg body weight. The first dose was administered within 2 hours before transplant with subsequent doses given at 2, 4, 6, and 8 weeks after transplant. Patients with zero HLA mismatches received neither rabbit antithymocyte globulin nor IL-2RA induction. Corticosteroids were initiated intraoperatively as methylprednisolone 250 to 500 mg according to body weight (1 mg/kg to a maximum of 60 mg/d from day 1) and tapered down to low-dose prednisone (0.1-0.5 mg/kg/d) by 3 months after transplant.

Target 12-hour whole blood trough concentrations for tacrolimus (Prograf, Astellas Pharmaceuticals, Deerfield, IL, USA) were as follows: 1 to 6 weeks, 10 to 15 ng/mL; 7 to 12 weeks, 8 to 12 ng/mL; 3 to 12 months, 6 to 10 ng/mL; > 1 year, > 5 ng/mL, or as clinically indicated. Target 12-hour whole blood trough concentrations for cyclosporine (Neoral, Novartis Pharmaceuticals) were as follows: 1 to 6 weeks, 200 to 275 ng/mL; 7 to 12 weeks, 175 to 225 ng/mL; 3 to 12 months, 125 to 175 ng/mL; > 1 year, > 70 ng/mL, or as clinically indicated. Doses of calcineurin inhibitors were minimized during antithymocyte globulin induction but were returned to full dose 2 days before discontinuation of the induction regimen. All patients received myco­phenolate mofetil (CellCept, Roche Pharmaceuticals, Nutley, NJ, USA), with initial doses of 600 mg/m² oral twice daily.

Doses were adjusted for efficacy and toxicity. Graft failure was defined as a situation where any other form of renal replacement therapy had to be started. Death with functioning graft was not considered as graft failure. Deaths that were not primarily associated with renal transplant were censored in the Kaplan-Meier analysis. The glomerular filtration rate (GFR) was computed using the Schwartz formula (GFR [mL/min/1.73 m²] = 0.55 × body length [cm]/serum creatinine [mg/dL]).

All cytomegalovirus-positive recipients and cytomegalovirus-negative recipients of a kidney from a cytomegalovirus-positive donors were given prophylaxis with valacyclovir for the first 3 months after transplant. All patients received prophylaxis against Pneumocystis carinii with sulfamethoxazole and trimethoprim for 6 months. All patients had blood polymerase chain reaction for BK virus at 3 months and 1 year after transplant and every year thereafter.

Acute rejection
Acute rejection included biopsy-proven and clinically suspected episodes (defined as the use of high-dose steroids or antibody treatment with subsequent improvement of renal function). Acute rejection was biopsy-proven and the diagnosis was made according to Banff classification 2007 and treated with high-dose corticosteroids. Borderline rejection was included in the analysis if treated as acute rejection. Patients were considered to have delayed graft function if they required dialysis within the first week after transplant.

Statistical analyses
Data were manually collected in a spreadsheet (Excel, Microsoft, Redmond, WA, USA). Statistical analysis was conducted using software (SPSS, version 11.0, SPSS Inc., Chicago, IL, USA). Data were reported as number (%) or mean ± standard deviation. The t test was used to compare means and standard deviations of the 3 groups. Categoric data were compared using the chi-square test. Graft and patient survival were computed using the Kaplan-Meier method. Statistical significance was defined by P ≤ .05.

Results

Most of our pediatric transplant patients were Kuwaiti males with mean ages 13.4 ± 5.4 years, 13.9 ± 3.8 years, and 13.6 ± 4.3 years in the 3 studied groups. Male donors were significantly higher among patients with 1 and full HLA-DR-mismatch groups compared with full HLA-DR-matched group (P = .01) with mean age 35.7 ± 8.5years,34.6 ± 7.4 years, and 30.7 ± 9 years in the same groups (P = .075). We found nonsignificant difference between the 3 groups regarding pretransplant comorbidities, especially anemia; patients who received treatment for tuberculosis, hypertension, or diabetes mellitus; viral profile (especially hepatitis C virus, cytomegalovirus, hepatitis B virus, and herpes viruses); and bone disease (P > .05) (Table 1). The original kidney disease was comparable in all groups (P > .05). Moreover, the number of patients who underwent preemptive renal transplant, and those who were on regular hemodialysis or peritoneal dialysis, were comparable (P = .41) (Table 1).

We observed that the number of patients who received their grafts from living donors (related or unrelated) was significantly higher in groups with ≤ 1 HLA-DR mismatch; the number of patients who received their grafts from deceased donors was significantly higher in the full HLA-DR-mismatched group (P ≤ .001) (Table 1).

The number of patients who received less-induction therapy was significantly higher in the group with 1 HLA-DR mismatch. The number of patients who received more potent induction (antithymocyte globulin) was significantly higher in the group with ≥ 1 HLA-DR mismatch (P ≤ .001). However, the number of patients who were receiving either type of calcineurin inhibitor was comparable in all groups (P > .05) (Table 2). Most studied patients experienced immediate graft function regardless of HLA-DR match (P = .3).

Posttransplant complications and outcome
Most rejection episodes developed during the first posttransplant year. We found that biopsy-proven acute rejection episodes, with different varieties, were comparable in the study groups (P = .16). Regarding nonimmunologic complications, we observed that posttransplant new-onset diabetes, de novo hypertension, and infection episodes that necessitated hospitalization all were comparable in the study groups regardless of the number of HLA-DR mismatches and subsequent induction therapy (P > .05) (Table 2).

We did not observe any evidence of malignancy except 1 case of visceral Burkett lymphoma among patients in the full HLA-DR match group. We observed that there was no significant difference between the 3 groups regarding graft survival (P = .053) or patient survival (P = .84). Graft survival was 95%, 85%, 65%, 72%, and 51.4% in the first group; 93%, 83%, 79%, 70%, and 81% in second group; and 87%, 82%, 82%, 67%. and 24.5% in the third group at 1 , 2 , 3, 5, and 10 years after transplant (P = .31). Patient survival was 93%, 93%, 80%, 80%, and 50.5% in the first group; 95%, 83%, 80%, 70%, and 24% in second group; and 87%, 82%, 78%, 63%, and 24.3% in the third group at 1, 2, 3, 5, and 10 years after transplant (P = .056).

Discussion

Kidney transplant is well-accepted as the optimal therapy for children with end-stage renal disease. By analyzing large databases, many investigators have found that recipients of HLA-matched kidneys experience superior outcomes, as defined by lower rates of rejection and higher rates of graft and patient survival, than recipients of HLA-mismatched kidneys.¹⁵ Furthermore, multifactorial analysis of 1252 pediatric deceased-donor renal transplants performed in the United Kingdom and Ireland between 1986 and 1995 also revealed that HLA matching was 1 of the most important determinants of outcome.¹⁶

A previous retrospective analysis was performed with data from the Organ Procurement and Transplantation Network and the Scientific Registry of Transplant Recipients to examine the effect of HLA-DR matching on allograft survival and sensitization rates in children receiving kidneys from deceased donors.¹⁷ With the aims of shortening the waiting time on dialysis of pediatric renal transplant candidates and minimizing the morbidity of these individuals, it was suggested to use HLA-DR-mismatched grafts. This should be cautiously analyzed. The increased risks of acute rejection, graft loss, and sensitization should be considered and weighed against the problems associated with longer exposure to dialysis.

They analyzed data from 1585 children who had received kidneys between 1996 and 2004 from deceased donors aged ≤ 35 years. However, they reported identical 5-year graft survival rates in the grafts with zero HLA-DR mismatches and in those with 2 HLA-DR mismatches (71%). In addition, the authors showed that the odds of developing PRA > 30% by the time of a second transplant did not increase significantly in the presence of HLA mismatches.

Reduced use of HLA matching in kidney transplant has been advocated recently. In our retrospective study, we included 104 pediatric kidney transplant recipients and compared 3 groups of patients, including deceased- and living-donor transplants, that had different HLA-DR mismatches. The studied groups were matched regarding demographic and pretransplant comorbidities. We observed that the number of pediatric patients with ≤ 1 HLA-DR mismatch who received their graft from living donors (related or unrelated) with their ages < 36 years was significantly higher compared with the other group with full HLA-DR mismatches who received grafts from deceased donors (P ≤ .001). Patients of these groups had received significantly less induction therapy, as IL-2 Ras or no induction at all, compared with the same group (P ≤ .001) (Table 2). In contrast, most patients who had full HLA-DR mismatches received their grafts from deceased donors aged < 35 years (P ≤ .001) (Table 1), and had received significantly more potent induction immunosuppression (P ≤ .001) (Table 2). This was consistent with a previous study that reported that the potential benefit of HLA-matching for the reduction protocol for immunosuppressants may play a role in the withdrawal program.18 It appears unnecessary to pay attention to HLA compatibility in donor selection in living-related liver transplant, except for 1-way HLA matching, or to adjust immuno­suppression according to HLA compatibility.

In the same direction, this issue previously was extrapolated but not confirmed by others who stated in their report that children who received HLA-DR mismatched kidneys probably would undergo more intensive immunosuppressive treatment to overcome the potentially increased risks of acute rejection, graft failure, and sensitization.¹⁹ Ghoneim and Refaie¹⁹ criticized a paper by other workers¹⁷ when referred to a previous study by the Ghoneim group²⁰ to support the author’s contention. They reported that all the kidneys offered to their pediatric recipients were retrieved from living-related donors, and they did not accept any HLA-DR-mismatched grafts. Moreover, mismatches in HLA-A or HLA-B were a significant predictor of graft loss in their series. The frequency of rejection episodes, antirejection treatment, and the adverse effects of such treatment (to which pediatric patients are particularly vulnerable) were not reported.¹⁷ In our study, we observed that most acute rejection episodes developed during the first year after transplant. Moreover, we found that biopsy-proven acute rejection episodes (T-cell-mediated or antibody-mediated) were comparable in the study groups (P = .16), which could be explained by the potent immunosuppressant protocol used. This finding was matched with the report of others²¹ who found that mismatching of HLA-A and -B antigens did not affect frequency of early cellular rejection, but the presence of 2 HLA-DR locus mismatches increased the risk of high-grade rejection in pediatric heart transplant recipients treated with cyclosporine. They added that the potent effects of tacrolimus-based immuno­suppression mitigated the effect of HLA-DR mismatching because patients treated with tacrolimus who had 2 HLA-DR mismatches had less rejection than patients treated with cyclosporine who had 1 HLA-DR mismatch and seemed to be at no greater risk for rejection than patients treated with tacrolimus and who had 1 HLA-DR mismatch. Moreover, there was no evidence that HLA matching was associated with improved kidney or pancreas survival. However, a higher rate of acute rejection was observed with poor HLA match, which may affect long-term survival.²²

We found no significant difference between the 3 groups regarding delayed graft function, because the majority of patients experienced immediate graft function regardless of HLA-DR match (P = .3). However, the notion of comparable success rates has been challenged by a previous study that examined the outcomes of 135 970 deceased-donor transplants performed during 2 decades (1985-1994 and 1995-2004).²³ The data were provided by 363 transplant centers in 41 countries, and 7315 recipients (11%) were aged < 18 years. In that report, the number of mismatches correlated significantly with the rate of graft survival and rejection. Long-term follow up showed that graft outcome was comparable in the study groups (P = .053) (Figure 1), as was patient survival (P = .84) (Figure 2). This was matched with a previous report, despite shorter follow-up, that suggested that the transplant community, especially pediatric transplant programs, should accept HLA-DR-mismatched kidneys from deceased donors aged ≤ 35 years for transplant into children with end-stage renal disease.¹⁷ A previous study noted that if immunosuppression had improved to a degree that enabled transplant of well- and poorly HLA-matched kidneys with identical success rates, then HLA typing and matching would be abolished from kidney allocation algorithms.²⁴

We believe that optimization of induction and maintenance immunosuppression might be responsible for such results among our patients. This observation matched the results of a previous study that included 672 renal transplant recipients with different HLA-DR mismatch status and treated with antithymocyte globulin as induction therapy followed by tacrolimus, prednisone, and myco­phenolate mofetil for maintenance immuno­suppression.²⁵ They concluded that the independent effect of HLA-DR mismatches on adverse graft survival was diminished under potent antibody induction and maintenance immuno­suppression in African Americans. Patients on tacrolimus with 1 or 2 mismatches at the HLA-B or -DR loci may have increased rates of patient and graft survival compared with patients having no mismatches, with the appearance of a protective effect of tacrolimus.²⁶

Some authors have recommended that transplants with 2 HLA-DR mismatches be avoided to reduce the risk of posttransplant non-Hodgkin lymphoma.²⁷ However, we did not observe any evidence of malignancy except 1 case of visceral Burkett lymphoma among patients with full HLA-DR match.

Some authors argued that a 5-year follow-up in another study¹⁷ was insufficient to enable a meaningful analysis of long-term outcomes.^17,28 In our study we tried to show that the long-term outcome of HLA-DR-mismatched children was not significantly different from other groups of patients, especially with the new protocols of immuno­suppression if they are tailored properly.

In conclusion, with optimization of immuno­suppression, HLA-DR-mismatched donors can be safely accepted for pediatric kidney transplant with comparable long-term patient and graft survival.

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