Donor antigen-specific regulatory T-cell function affects outcome in kidney transplant recipients

Chronic transplant dysfunction, a major impediment to long-term allograft survival, is caused by several factors including an ongoing alloimmune response termed chronic rejection. To define some of these factors further, we selected 107 patients mismatched to their donors from 623 patients transplanted at a single center. Patients were categorized according to their immunosuppressive treatment and further divided into those with stable or chronic allograft dysfunction. Donor human lymphocyte antigen allopeptide-specific T-cell lines were then generated from stable patients and those with biopsy-proven chronic allograft nephropathy. Increased amounts of CD4+CD25+ regulatory T cells (Tregs) and Treg-associated gene expression profiles were found in cell lines derived from the patients with stable compared with those with chronic allograft dysfunction. Furthermore, a higher percentage of Tregs was found in patients with stable graft function on tacrolimus-based compared with cyclosporine-based immunosuppression protocols. Patients with stable graft function had a significantly higher expression of interleukin (IL)-4 and IL-10, whereas the cytokines IL-2, IL-17, and interferon-γ were significantly higher in patients with allograft dysfunction in vitro. Thus, enhancing the operational role of naturally occurring donor-specific Tregs in allograft recipients by adjusting the immunosuppression protocol may be advantageous particularly for patients with ongoing chronic rejection.     

Tuberculosis in Iranian kidney transplant recipients: a single-center experience

INTRODUCTION: Renal transplantation recipients are at a high risk of developing tuberculosis (TB) following transplantation, especially in developing countries, with high incidences of morbidity and mortality. In this report, we examined the risk factors and impact of TB on the outcome of kidney transplantation. PATIENTS AND METHODS: Among 1350 living donor Iranian kidney transplantations, 52 (3.9%) had TB diagnosed in various organs. Of these, 7 (13.5%) had TB pretransplantation and 40 (76.9%) were men. The overall mean age was 32.6 +/- 10.5 years. RESULTS: The interval between transplantation and diagnosis was 54.6 +/- 48.23 (range 4 to 140) months. In 34 (65.6%) patients TB was diagnosed after the first year posttransplantation. Pleuro/pulmonary TB was the most common form (68%). All posttransplant TB patients received a quadriple antituberculosis therapy; pyrazinamide, rifampicin, ethambutol, and isoniazide). Hepatotoxicity was seen in 16 (30%) patients, including 12 mild cases with normalization after temporary withdrawal of isoniazide and rifampicin, and four were severe, but mortality was not attributable to hepatocellular failure. Twelve patients (23%) died. Chronic allograft dysfunction occurred in 34 (65%) patients, 19 (37%) with graft loss. Pre-TB patients showed comparable posttransplant courses. CONCLUSION: TB is a common infection among renal transplant recipients in developing countries. The peak incidence is after the first year of transplantation and mortality is considerable. Hepatoxicity is a considerable risk of treatment, possibly as a result of additive toxic effects of immunosuppressive drugs. Chronic allograft nephropathy is a serious complication that has a negative impact on the graft survival.     

Infectious complications in kidney transplant recipients: a single-center experience

To determine the patterns of infectious complications in renal transplant recipients in our center, we evaluated 48 patients (29 men and 19 women) who were transplanted between 1994 and 2003. The average age of the patients was 29 years. Thirty (62.5%) and 18 (37.5%) transplants were from living related and cadaveric donors, respectively. Posttransplant immunosuppression consisted of azathioprine or mycophenolate mofetil (MMF), prednisone, antithymocyte globulin (ATG), and cyclosporine or tacrolimus. The acute rejection episodes were treated with pulse doses of methylprednisolone; steroid-resistant rejection was treated with ATG or muromonab (OKT3). All patients received prophylaxis with sulfadoxine-pyrimethamine; none received prophylaxis against cytomegalovirus (CMV) infection. Thirty-nine (81%) recipients developed 77 confirmed episodes of infection; 35 (46%) episodes occurred in the early postoperative period, 28 (36%) in the first month and 14 (18%) after 6 months. According to the type of infection, there were 24 urinary tract, 16 CMV, seven herpetic, nine general septic, six fungal, four pneumonia, one disseminated nocardial, and 10 miscellaneous episodes. All 26 (100%) patients who had acute rejection episodes developed infections compared with 13/22 (59%) who did not have rejection (P < .01). There was a significant correlation between CMV disease and acute rejection and/or tacrolimus or MMF use. CMV infection occurred after the additional immunosuppressive treatment for acute rejection in 10 patients or during the use of tacrolimus or MMF in six patients. We conclude that CMV infection was the most frequent opportunistic pathogen in our renal transplant population and related to the intensive antirejection therapy, followed by urinary tract infections within 3 months after surgery.     

髂内动脉钙化对肾移植受者移植物功能延迟恢复和近期预后的影响

目的  分析髂内动脉钙化与肾移植受者移植物功能延迟恢复（DGF）及近期预后的相关性。方法  回顾性分析222例肾移植受者的临床资料。依据肾功能恢复情况分为DGF组（50例）和移植物功能正常恢复（IGF）组（172例）,根据是否合并髂内动脉重度钙化将DGF组和IGF组分为DGF高危组（22例）、DGF低危组（28例）、IGF高危组（41例）以及IGF低危组（131例）。比较两组供受者临床资料,总结肾移植术后DGF及髂内动脉钙化发生情况,分析肾移植术后发生DGF的危险因素、髂内动脉钙化与临床指标的相关性以及DGF合并髂内动脉重度钙化受者近期预后。结果  本研究中DGF发生率为22.5%（50/222）。肾移植受者中28.4%（63/222）合并髂内动脉重度钙化,DGF组中44%（22/50）合并髂内动脉重度钙化,高于IGF组中的23.8%（41/172）（P < 0.05）。单因素分析结果显示供者终末血清肌酐（Scr）高、男性供者,受者甘油三酯水平高和髂内动脉重度钙化是肾移植术后发生DGF的危险因素（均为P < 0.05）。多因素logistic回归分析显示供者Scr≥143 μmol/L及受者髂内动脉重度钙化是肾移植术后发生DGF的独立危险因素（均为P < 0.05）。相关性分析结果显示髂内动脉钙化与受者年龄和肾动脉吻合方式均呈弱相关（均为P < 0.05）。DGF组受者术后1个月的Scr高于IGF组,估算肾小球滤过率（eGFR）低于IGF组（均为P < 0.05）;DGF高危组受者术后12个月的eGFR低于DGF低危组、IGF高危组以及IGF低危组（均为P < 0.05）。结论  髂内动脉钙化不仅是影响移植肾功能恢复的危险因素,也对移植肾功能的近期预后造成不良影响。     

Association between acute graft pyelonephritis and kidney graft survival: A single-center observational study

The association between acute graft pyelonephritis (AGPN) and graft failure in kidney transplant recipients (KTR) remains controversial. In this single-center observational study, we aimed to assess the incidence of AGPN as a time-dependent posttransplantation event. We also examined the association between the diagnosis of AGPN and graft outcomes. In total, we evaluated 1480 patients who underwent kidney transplantation between January 2007 and December 2017. During a median follow-up of 5.04 years, we observed 297 AGPN episodes that occurred in 158 KTR. To evaluate the association between AGPN and clinical outcomes, we performed Cox proportional hazards regression analyses in which AGPN was entered as a time-dependent covariate. AGPN was independently associated with an increased risk of graft loss (hazard ratio = 1.66; 95% confidence interval [CI]: 1.05−2.64, p < .03) and a persistently decreased eGFR (fixed effect on intercept: −2.29 ml/min/1.73 m²; 95% CI: from −3.23 to −1.35, p < .01). However, neither mortality nor biopsy-proven acute rejection was found to correlate with AGPN. Moreover, recurrent AGPN episodes did not appear to have an additive detrimental impact on graft loss. These data represent a promising step in understanding whether AGPN prevention may decrease the risk of graft loss in KTR.     

肾移植受者新型冠状病毒肺炎的流行病学特征：单中心回顾性研究

目的  探讨肾移植受者新型冠状病毒肺炎（新冠肺炎）的流行病学特征,分析新型冠状病毒（新冠病毒）感染重型/危重型的危险因素及保护因素。 方法  回顾性分析感染新冠病毒的468例肾移植受者的临床资料,按感染严重程度分为新冠病毒感染轻型受者（439例）和新冠肺炎组（29例）。将439例新冠病毒感染轻型受者按性别、年龄、移植时间与新冠肺炎组以3∶1进行随机配比的87例受者分为新冠病毒感染轻型组。将29例新冠肺炎组受者分为新冠肺炎中型组（21例）及新冠肺炎重型/危重型组（8例）。收集受者一般资料,分析肾移植受者新冠病毒感染的危险因素及保护因素。 结果  新冠肺炎组受者合并症种类2～3种的比例高于新冠病毒感染轻型组,新冠肺炎组受者采用他克莫司（Tac）+咪唑立宾+糖皮质激素免疫抑制方案的比例低于新冠病毒感染轻型组,差异均有统计学意义（均为P<0.05）。29例新冠肺炎组肾移植受者患新冠肺炎后白细胞、淋巴细胞绝对值、嗜酸性粒细胞绝对值、总T细胞绝对值、CD4⁺T细胞绝对值、CD8⁺T细胞绝对值及血尿酸较患新冠肺炎前明显下降,铁蛋白水平升高,差异均有统计学意义（均为P<0.05）。与新冠肺炎中型组比较,重型/危重型组受者低氧血症的比例更高,采用Tac/环孢素（CsA）+霉酚酸酯+糖皮质激素免疫抑制方案的受者比例更高,接种2～3针新型冠状病毒疫苗（新冠疫苗）者比例更少,差异均有统计学意义（均为P<0.05）。 结论  肾移植受者合并症多、使用含霉酚酸酯的免疫抑制方案是新冠病毒感染的危险因素,接种新冠疫苗、使用含咪唑立宾的免疫抑制方案可能是降低新冠病毒感染率的保护因素,炎症因子水平与新冠肺炎的严重程度相关。     

Calcineurin and mTOR inhibitors have opposing effects on regulatory T cells while reducing regulatory B cell populations in kidney transplant recipients

BackgroundRegulatory B (Breg) and T (Treg) cells represent a biomarker for tolerance in transplant patients. Despite the importance of Treg and Breg in transplantation and the suggested crosstalk between both suppressive cell populations, little is known on how they are influenced by long-term immunosuppressive treatment. The aim of the present study was to investigate the effect of different immunosuppressive drugs used in routine clinical practice on Treg and Breg cell numbers.MethodsThirty-six kidney transplant recipients with stable graft function were recruited and classified according to their concomitant therapy: 22 patients received calcineurin inhibitors (CNI) and 14 patients received mammalian target of rapamycin (mTOR) inhibitors. A group of 8 healthy untreated subjects was included as control. Absolute numbers of peripheral blood-derived IL10-producing B cells (CD19⁺IL10⁺), CD19⁺CD24^hiCD38^hi transitional B cells and Treg cells (CD4⁺CD25⁺FOXP3⁺) were quantified in all KT patients and controls by flow cytometry.ResultsCD19⁺CD24^hiCD38^hi transitional B cells increased over time and seem to be related with long-term therapeutic graft survival irrespective of the treatment regimen. CNI and mTOR inhibitors significantly reduced numbers of Breg cells when compared with healthy individuals, whereas mTOR inhibitors expanded Treg cells in comparison with CNI drugs.ConclusionsBridging the drug-mediated reduction of Breg cell numbers in vivo with the requirements of Breg cells for long-term transplant success remains an as yet unresolved task for therapeutic intervention. Further larger cohort studies that evaluate the effect of different treatment regimen on defined lymphocyte subpopulations are warranted.     

A prospective multicenter observational study of cell‐mediated immunity as a predictor for cytomegalovirus infection in kidney transplant recipients

T cell immunity is essential for the control of cytomegalovirus (CMV) infection after transplantation. We evaluated a CMV‐specific peptide‐based enzyme‐linked immunosorbent spot (ELISPOT) assay to determine whether assay results could predict subsequent CMV events. Adult kidney transplant recipients at 43 centers underwent ELISPOT testing to enumerate interferon gamma (IFN‐γ) binding spot‐forming units (sfu) after stimulation of cells with an overlapping peptide pool of CMV phosphoprotein 65 (pp65) and immediate early‐1 (IE‐1) protein at the end of antiviral prophylaxis (EOP) and various time points thereafter. The primary outcome was a CMV event in the first posttransplant year. In 583 kidney transplant recipients (260 seropositive donor [D+]/seronegative recipient [R?] and 277 R+), CMV events occurred in 44 of 368 eligible patients (11.8%) at a median of 227 days (range 92‐360) posttransplant. A cutoff value of >40 sfu/2.5 × 10⁵ cells for either IE‐1 or pp65 was derived as a threshold for positivity, with a negative predictive value of >97% for CMV events. CMV events were significantly lower in assay positive vs assay negative patients (3.0% vs 19.5%, P < .0001 for pp65). Time to CMV event post‐EOP was significantly greater in those with sfu >40 at EOP (P < .0001). In this large, multicenter trial of kidney transplant recipients, we show that an assessment of CMV‐specific immunity using a novel ELISPOT assay is able to predict protection from CMV infection.     

A single-center experience of retransplantation for liver transplant recipients with a failing graft

With the accumulation of orthotopic liver transplantation (OLT) recipients, an increased number of patients with graft failure need retransplantation (re-OLT). This study was undertaken to examine our clinical experience of re-OLT for patients with poor graft function after primary transplantation at a single center. We analyzed retrospectively, the clinical data of 32 re-OLTs in 31 patients at our center from January 2004 to February 2007, including indications and causes of death, timing of retransplantation, and surgical techniques. The indications included bile leak (2 cases), biliary stricture (16 cases), recurrence of hepatocellular carcinoma (HCC) (5 cases), hepatic artery stenosis (4 cases), hepatic artery thrombosis (HAT) (2 cases), and hepatitis B recurrence (3 cases). The rate of re-OLT was 4.29%. All patients underwent modified piggyback liver transplantations with cadaveric allografts. No intraoperative mortality and acute rejection occurred. Overall, 17 of 31 patients (54.8%) died after re-OLT with survival times ranging from 2 weeks to 28 months. Another 14 patients were cured with survival times of 4 to 32 months. The perioperative mortality rate of patients who underwent re-OLT between 8 and 30 days after their initial transplantation was highest (66.7%). The most common cause of death after re-OLT was sepsis (47.1%), multiple-organ failure (17.6%), and recurrence of HCC (17.6%), whereas the majority of deaths posttransplantation were sepsis-related (54%) within 1 year. Re-OLT is the only therapeutic option for a failing liver graft. Proper indications and optimal operative time, advanced surgical procedures, reasonable individual immunosuppression regimens, and effective perioperative anti-infection treatments contribute to the improved survival of patients after re-OLT.     

Ketoconazole-tacrolimus coadministration in kidney transplant recipients: two-year results of a prospective randomized study

BACKGROUND/AIMS: In developing countries, kidney transplantation is greatly hindered by financial problems, especially due to costly newer immunosuppressive medications. Ketoconazole increases blood levels of tacrolimus and cyclosporine through inhibition of cytochrome P450 microsomal enzymes. We previously reported on the 6-month safety and the outstanding impact on treatment costs of the ketoconazole-tacrolimus combination in kidney transplant recipients. Data of this combination are still lacking in the literature. We hereby report on the 2-year results of our trial. METHODS: This prospective, randomized study included 70 live-donor kidney transplant recipients receiving tacrolimus (age 16-45 years, 54 males and 16 females). Patients were randomized into two equal groups: group 1, where ketoconazole 100 mg/day was added, and group 2 (control group). RESULTS: After 2 years, group 1 (ketoconazole) patients still showed a highly significant reduction of the tacrolimus dose (by 53.8%) and cost (by 52.9%) compared with the control group (p < 0.001) and a significant improvement in graft function in comparison to their own initial graft function (p = 0.002). Throughout the 2 years, no side effects of ketoconazole were noted. CONCLUSION: We conclude that the long-term ketoconazole-tacrolimus combination therapy in kidney transplant recipients during the 2 years is safe, has an outstanding impact on treatment costs and improves graft outcome.     

Nosocomial infection in kidney transplant recipients: a retrospective analysis of a single-center experience

Posttransplant bacterial infections are important because of their influence on patient and graft outcomes. Therefore, prevention of infection as well as prompt diagnosis and appropriate treatment are crucial. In this retrospective analysis, we reviewed all posttransplant bacterial infections occurring during the admission of kidney transplant patients from January 2000 to May 2004. Of our patients, 25% had at least one episode of infection. Patients with immunosuppression based on an mTOR inhibitor showed the highest rate of wound infections compared to those receiving a calcineurin inhibitor (odds ratio 5.6, P < .001). Patients with renal failure caused by a urologic disease revealed a increased risk of a urinary tract infections (odds ratio 5.9, P < .001). Although infection complications are an important cause of morbidity in renal transplantation, the extensive use of antibiotics should be avoided in favor of a strict policy for infection prevention and control.     

肾移植受者新发高甘油三酯血症的危险因素：单中心分析

目的  探讨肾移植受者新发高甘油三酯血症（HTG）的危险因素。 方法  回顾性分析149例肾移植受者的临床资料,根据术后血清甘油三酯（TG）水平,分为无HTG组（TG≤1.7 mmol/L,60例）与新发HTG组（TG>1.7 mmol/L,89例）。比较两组受者的一般资料,采用广义估计方程（GEE）分析肾移植受者发生HTG的危险因素,并应用多个回归方程验证。 结果  两组受者一般资料差异均无统计学意义（均为P>0.05）。多因素分析结果显示,与低浓度Tac组受者相比,中浓度Tac组和高浓度Tac组的HTG发生率升高[中浓度Tac组比值比（OR）3.11,95%可信区间（CI）1.22～7.93,P=0.018;高浓度Tac组OR 5.11,95%CI 1.31～19.98,P=0.019]。与A型血受者相比,O型血受者新发HTG的风险增加（OR 2.77,95%CI 1.14～6.71,P=0.024）。随着术前球蛋白水平升高,受者新发HTG的风险降低（OR 0.93,95%CI 0.87～0.99,P=0.043）。术后3个月时,新发HTG组Tac血药浓度高于无HTG组,差异有统计学意义（P<0.05）。多个回归方程验证了O型血肾移植受者较A型血受者新发HTG的风险增加,中浓度Tac组和高浓度Tac组肾移植受者较低浓度Tac组新发HTG的风险增加（均为P<0.05）。 结论  O型血肾移植受者更易发生HTG,加强血脂的术后监测和控制十分重要。Tac血药浓度对肾移植受者术后新发HTG可能存在一定影响,维持适宜血药浓度可能有利于降低HTG发生率。     

Regulatory CD25+ T cells in human kidney transplant recipients

Recent evidence suggests that a population of professional regulatory cells, which limit immune responsiveness, exist in rodents and healthy human subjects. However, their role in disease states remains unclear. A proportion of renal transplant recipients do not demonstrate in vitro reactivity toward their mismatched donor-derived HLA-DR antigens; it was therefore hypothesized that this may be due to such regulatory cells. A cohort of 23 renal transplant recipients was studied at a single institution. In patients with no history of acute rejection, 6 (40%) of 15 demonstrated regulation toward the mismatched HLA-DR allopeptides by CD25(+) cells. By contrast, only one (12.5%) in eight of those with a history of acute rejection demonstrated regulation. Interestingly, if the patient assays were stratified according to initial in vitro immune responsiveness toward the mismatched allopeptides, 8 (47.1%) of 17 of patient assays with low allopeptide responsiveness (alloreactive T cell frequencies less than 60/million) demonstrated regulation of indirect pathway alloresponses by CD25(+) cells, whereas 0 of 8 with higher responses (frequencies greater than 60/million) demonstrated no such regulation (P < 0.05 by chi(2) test). The regulatory cells are present in the circulation as early as 3 mo after transplantation and persist for a number of years, despite conventional immunosuppression. Furthermore, induction treatment with anti-IL-2R mAb did not prevent the development of these regulatory CD25(+) cells. Data from two patients suggest that these cells may also play a role in preventing epitope shifting, implicated in the ongoing immune activation contributing to chronic rejection, and that loss of regulation in a given patient may precede an episode of rejection.     

Hyperuricemia in kidney transplant recipients with intact graft function

Objectives

The aim of this study was to investigate the prevalence of hyperuricemia and factors predicting its occurrence, and to establish the relationship over time between serial changes in estimated glomerular filtration rate (eGFR) and uric acid (UR) concentration in kidney transplant (KT) recipients with eGFR >60 mL/min/1.73 m².

Methods

Adult patients who underwent KT at the Asan Medical Center between 1990 and 2008 and maintained eGFR >60 mL/min/1.73 m² were retrospectively assessed. Clinical and laboratory data were obtained from inpatient and outpatient charts and from the hospital electronic database.

Results

Of 356 patients, 301 (84.55%) had normal UR levels and 55 (15.45%) had hyperuricemia. After multivariate adjustment, transplant duration, male gender, eGFR, diabetes mellitus (DM), and calcium level were associated with higher mean UR levels. Mean UR level increased significantly and mean eGFR decreased significantly during the first year after transplantation, but there were no significant differences over the next 4 years. Serial UR and eGFR levels changed almost simultaneously.

Conclusions

Transplantation duration, male gender, eGFR level, DM, and serum calcium level were risk factors for hyperuricemia in kidney recipients with intact graft function. Increased uric acid after KT did not significantly affect graft function.     

Predicting long-term graft survival in adult kidney transplant recipients

The ability to accurately predict a population's long-term survival has important implications for quantifying the benefits of transplantation. To identify a model that can accurately predict a kidney transplant population's long-term graft survival, we retrospectively studied the United Network of Organ Sharing data from 13,111 kidney-only transplants completed in 1988- 1989. Nineteen-year death-censored graft survival (DCGS) projections were calculated and compared with the population's actual graft survival. The projection curves were created using a two-part estimation model that (1) fits a Kaplan-Meier survival curve immediately after transplant (Part A) and (2) uses truncated observational data to model a survival function for long-term projection (Part B). Projection curves were examined using varying amounts of time to fit both parts of the model. The accuracy of the projection curve was determined by examining whether predicted survival fell within the 95% confidence interval for the 19-year Kaplan-Meier survival, and the sample size needed to detect the difference in projected versus observed survival in a clinical trial. The 19-year DCGS was 40.7% (39.8-41.6%). Excellent predictability (41.3%) can be achieved when Part A is fit for three years and Part B is projected using two additional years of data. Using less than five total years of data tended to overestimate the population's long-term survival, accurate prediction of long-term DCGS is possible, but requires attention to the quantity data used in the projection method.     

Altered balance between effector T cells and FOXP3+HELIOS+ regulatory T cells after thymoglobulin induction in kidney transplant recipients

This study examined the effect of thymoglobulin induction therapy on leukocyte population dynamics in kidney transplant patients. Patients receiving standard immunosuppression were compared with those who received additional thymoglobulin at the time of kidney transplantation. Thymoglobulin induction led to an immediate and significant decrease of all T cells and NK cells, but not B cells or monocytes. CD8⁺ T cells recovered to near pretransplant level by 4 weeks post‐transplant. CD4⁺ T cells remained at less than 30% of pretransplant level for the entire study period of 78 weeks. Both CD4⁺ and CD8⁺ T cells showed reduced cytokine production after recovery. Deletion of CD4⁺FOXP3⁺HELIOS⁺ regulatory T cells (Tregs) was less profound than that of CD4⁺FOXP3^? cells, thus the relative percentage of Tregs elevated significantly when compared with pretransplant levels in thymoglobulin‐treated patients. In contrast, the percentages of Tregs and their expression of FOXP3 in the standard immunosuppression group decreased steadily and by 12 weeks after transplant the average percentage of Tregs was 56% of the pretransplant level. Thus, thymoglobulin‐induced deletion of T cells led to significant and long‐lasting alterations of the T‐cell compartment characterized by a preservation of Tregs and long‐lasting reduction in CD4⁺, and potentially pathogenic, T cells.     

Frailty and delayed graft function in kidney transplant recipients

The ability to predict outcomes following a kidney transplant is limited by the complex physiologic decline of kidney failure, a latent factor that is difficult to capture using conventional comorbidity assessment. The frailty phenotype is a recently described inflammatory state of increased vulnerability to stressors resulting from decreased physiologic reserve and dysregulation of multiple physiologic systems. We hypothesized that frailty would be associated with delayed graft function, based on putative associations between inflammatory cytokines and graft dysfunction. We prospectively measured frailty in 183 kidney transplant recipients between December 2008 and April 2010. Independent associations between frailty and delayed graft function were analyzed using modified Poisson regression. Preoperative frailty was independently associated with a 1.94-fold increased risk for delayed graft function (95% CI, 1.13-3.36; P = .02). The assessment of frailty may provide further insights into the pathophysiology of allograft dysfunction and may improve our ability to preoperatively risk-stratify kidney transplant recipients.