Post‐transplant lymphoproliferative disorders,Epstein‐Barr virus infection,and disease in solid organ transplantation: Guidelines from the American Society of Transplantation Infectious Diseases Community of Practice

These updated guidelines from the American Society of Transplantation Infectious Diseases Community of Practice review the diagnosis, management, and prevention of post‐transplant lymphoproliferative disorders (PTLD) and other Epstein‐Barr virus (EBV) syndromes after solid organ transplantation. PTLD are a heterogeneous spectrum of predominantly B‐cell disorders, often extra‐nodal, with complex distinct pathogeneses and variable clinical presentations determined by pathologic subtype. Recent epidemiologic studies report a decrease in early EBV‐positive (+) PTLD and an increase in late EBV‐negative (?) PTLD. Pre‐transplant EBV‐seronegativity and primary EBV infection, often from donor‐transmitted infection, are an important risk factors for EBV syndromes and early EBV + PTLD. Low‐quality evidence supports preemptive prevention strategies for early EBV + PTLD in EBV‐seronegative recipients that involve EBV DNA measurement in peripheral blood using assays requiring further result harmonization, combined with interventions to lower viral load. Reduction in immunosuppression (RIS) is the best validated intervention. WHO pathology classification of a tissue biopsy remains the gold standard for PTLD diagnosis; optimal staging procedures are uncertain. Treatment of CD20⁺ PTLD with the response‐dependent sequential use of RIS, rituximab, and cytotoxic chemotherapy is recommended. Evidence gaps requiring future research and alternate treatment strategies including immunotherapy are highlighted.     

CD30-Negative Cutaneous T-Cell Lymphomas Other than Mycosis Fungoides

Cutaneous T-cell lymphomas (CTCLs), other than mycosis fungoides/Sézary syndrome and the group of cutaneous CD30⁺ lymphoproliferative disorders, are rare. These include subcutaneous panniculitis–like T-cell lymphoma (SPTCL); extranodal natural killer/T-cell lymphoma, nasal type; primary cutaneous peripheral T-cell lymphoma, not otherwise specified (PTCL, NOS); and rare subtypes of PTCL, NOS. Apart from SPTCL and primary cutaneous CD4-positive small-medium pleomorphic T-cell lymphoma, these lymphomas have in common aggressive clinical behavior and poor prognosis. Differentiation between these different types of CTCL may be difficult and requires integration of histopathologic findings with clinical data and the results of phenotypic and often molecular genetic studies.     

T cells or active epstein-barr virus infection in the development of lymphoproliferative disease in human B cell-injected severe combined immunodeficient mice

Background: Severe combined immunodeficient (SCID) mice develop Epstein-Barr virus (EBV) containing human lymphoproliferative disease (LPD) tumors when reconstituted with human peripheral blood leukocytes (PBLs) from EBV-seropositive donors, but LPD tumors do not develop in the presence of immunosuppressive agents, such as cyclosporine A or corticosteroids. Methods: Therefore, LPD development in SCID mice was used as a model to explore the relationship among B cells, T cells, and EBV in vivo. SCID mice were engrafted with PBLs isolated by leukapheresis from a single EBV-seropositive donor. Purified populations of CD3⁺ lymphocytes (T cells) or CD19⁺ lymphocytes (B cells) were isolated and engrafted into SCID mice. Results: SCID mice engrafted with purified CD3⁺ lymphocytes (T cells) or CD19⁺ lymphocytes (B cells) did not develop LPD. In contrast, mice engrafted with purified B cells developed LPD if they were co-engrafted with purified T cells or if they were inoculated with infections EBV. Conclusions: This study confirms the requirement of T cells or active EBV infection in the development of LPD in animals engrafted with B cells latently infected with EBV. A greater understanding of the cellular and viral interactions leading to transformation and malignancy may allow the development of specific interventional therapies for malignancies in the immunosuppressed host. Presented at the 46th Annual Cancer Symposium of The Society of Surgical Oncology, Los Angeles, California, March 18–21, 1993.     

Syk‐Induced Phosphatidylinositol‐3‐Kinase Activation in Epstein–Barr Virus Posttransplant Lymphoproliferative Disorder

Posttransplant lymphoproliferative disorder (PTLD)‐associated Epstein–Barr virus (EBV)+ B cell lymphomas are serious complications of solid organ and bone marrow transplantation. The EBV protein LMP2a, a B cell receptor (BCR) mimic, provides survival signals to virally infected cells through Syk tyrosine kinase. Therefore, we explored whether Syk inhibition is a viable therapeutic strategy for EBV‐associated PTLD. We have shown that R406, the active metabolite of the Syk inhibitor fostamatinib, induces apoptosis and cell cycle arrest while decreasing downstream phosphatidylinositol‐3′‐kinase (PI3K)/Akt signaling in EBV+ B cell lymphoma PTLD lines in vitro. However, Syk inhibition did not inhibit or delay the in vivo growth of solid tumors established from EBV‐infected B cell lines. Instead, we observed tumor growth in adjacent inguinal lymph nodes exclusively in fostamatinib‐treated animals. In contrast, direct inhibition of PI3K/Akt significantly reduced tumor burden in a xenogeneic mouse model of PTLD without evidence of tumor growth in adjacent inguinal lymph nodes. Taken together, our data indicate that Syk activates PI3K/Akt signaling which is required for survival of EBV+ B cell lymphomas. PI3K/Akt signaling may be a promising therapeutic target for PTLD, and other EBV‐associated malignancies.     

PI3Kδ Inhibition Augments the Efficacy of Rapamycin in Suppressing Proliferation of Epstein−Barr Virus (EBV)+ B Cell Lymphomas

Posttransplant lymphoproliferative disorder (PTLD) continues to be a devastating and potentially life‐threatening complication in organ transplant recipients. PTLD is associated with EBV infection and can result in malignant B cell lymphomas. Here we demonstrate that the PI3K/Akt/mTOR pathway is highly activated in EBV+ B cell lymphoma lines derived from patients with PTLD. Treatment with the mTORC1 inhibitor Rapamycin (RAPA) partially inhibited the proliferation of EBV+ B cell lines. Resistance to RAPA treatment correlated with high levels of Akt phosphorylation. An mTORC1/2 inhibitor and a PI3K/mTOR dual inhibitor suppressed Akt phosphorylation and showed a greater anti‐proliferative effect on EBV+ B lymphoma lines compared to RAPA. EBV+ B cell lymphoma lines expressed high levels of PI3Kδ. We demonstrate that PI3Kδ is responsible for Akt activation in EBV+ B cell lymphomas, and that selective inhibition of PI3Kδ by either siRNA, or a small molecule inhibitor, augmented the anti‐proliferative effect of RAPA on EBV+ B cell lymphomas. These results suggest that PI3Kδ is a novel, potential therapeutic target for the treatment of EBV‐associated PTLD and that combined blockade of PI3Kδ and mTOR provides increased efficacy in inhibiting proliferation of EBV+ B cell lymphomas.     

EBV‐Associated Leukoencephalopathy with Late Onset of Central Nervous System Lymphoma in a Kidney Transplant Recipient

Central nervous system (CNS) lymphoma is a rare posttransplant lymphoproliferative disorder (PTLD), which usually has a poor outcome. To date, no specific conditions predisposing to this complication have been identified. We here describe the case of a renal transplant patient who was initially diagnosed as having Epstein‐Barr virus (EBV)‐associated leukoencephalopathy and ultimately developed EBV‐positive CNS lymphoma. The patient was a young lady who, 2 years after transplantation, presented with focal neurological and electroencephalographic abnormalities and diffuse white matter lesions on brain magnetic resonance imaging. EBV‐DNA was detected in the cerebrospinal fluid (CSF) by polymerase chain reaction. After acyclovir therapy and immunosuppressive drug tapering, the symptoms and electroencephalographic abnormalities subsided, and EBV‐DNA disappeared from the CSF. Ten years later, a bulky cerebral mass was found. After excision, a diagnosis of EBV‐positive, Hodgkin‐like monomorphic B‐cell PTLD was made. This case illustrates the potential pathophysiological relationships between EBV infection, leukoencephalopathy and CNS lymphoma; although a long time elapsed from the initial neurological illness to CNS lymphoma, a link between these two conditions cannot be excluded. Therefore, a careful long‐term follow‐up of EBV‐related encephalopathy is advisable.     

Post-transplant lymphoproliferative diseases in Asian solid organ transplant recipients: late onset and favorable response to treatment

Chan TSY, Hwang Y‐Y, Gill H, Au W‐Y, Leung AYH, Tse E, Chim C‐S, Loong F, Kwong Y‐L. Post‐transplant lymphoproliferative diseases in Asian solid organ transplant recipients: late onset and favorable response to treatment. Abstract: Nineteen consecutive patients with post‐transplant lymphoproliferative disorders (PTLD) in an Asian population were reviewed. The histopathologic diagnoses were monomorphic (CD20‐positive diffuse large B‐cell lymphoma, n = 14); plasmacytic (n = 1); Burkitt‐like (n = 1); natural killer cell lymphoma (n = 1); lymphomatoid papulosis (n = 1); and classical Hodgkin lymphoma (n = 1). Early‐onset (相似文献   

Ex Vivo Priming of Naïve T Cells Into EBV-Specific Th1/Tc1 Effector Cells by Mature Autologous DC Loaded with Apoptotic/Necrotic LCL

Posttransplant lymphoproliferative disorders (PTLDs) represent life‐threatening complications of bone marrow and solid organ transplantation (SOTx). These are B‐cell malignancies triggered by Epstein‐Barr Virus (EBV) infection in chronically immunosuppressed (IS) recipients. Immunosuppressed EBV seronegative (EBV^?) organ recipients are at highest risk of developing PTLD owing to the lack of anti‐EBV memory T cells to control subsequent EBV challenges. Our aim is to establish effective anti‐EBV T‐cell generation protocols for prevention or treatment of PTLD encountered in SOTx. We have used autologous dendritic cells (DCs) loaded with apoptotic/necrotic lymphoblastoid cell lines (LCLs) to evaluate the ability of such an approach to activate naïve T cells in vitro. In EBV^? individuals, both CD8+ and CD4+ T‐cell responses were amplified by this approach, as detected by IFN‐γ ELISPOT and cytotoxicity assays. The CD8+ T cells were poly‐specific anti‐EBNA3 A, ‐LMP2 and ‐BMLF1, with uniform reversion to a CD45RO+/RA‐phenotype, decreased CD62L expression, and up‐regulation of the activation markers CD28 and CD69. Addition of rhIL‐12 improved anti‐viral T‐cell responses and reduced the functional differences observed between EBV⁺ and EBV^? responders. In conclusion, the DC/LCL method promotes cross‐presentation of EBV‐associated epitopes and may serve as an effective protocol for the adoptive immunotherapy of PTLD in EBV^? SOTx patients .     

Rapid Generation of EBV‐Specific Cytotoxic T Lymphocytes Resistant to Calcineurin Inhibitors for Adoptive Immunotherapy

Epstein–Barr virus (EBV)‐associated posttransplant lymphoproliferative disorder (PTLD) remains a major cause of morbidity and mortality after hematopoietic stem cell (HSCT) or solid organ transplant (SOT). Strategies to reconstitute immunity by adoptive transfer of EBV‐specific cytotoxic T lymphocyte (CTL) therapy while highly effective in the HSCT setting where immunosuppression can be withdrawn have been less successful in the SOT setting where continued immunosuppression therapy is necessary. Additionally, the complexity and time taken to generate EBV‐CTLs for adoptive transfer limit the clinical applicability. We have developed a system for the rapid generation of EBV‐CTLs resistant to immunosuppression based on selection of interferon‐gamma (IFN‐γ) secreting EBV‐CTLs and retroviral transduction with a calcineurin B mutant. With this methodology, EBV‐CTLs resistant to the calcineurin inhibitor Tacrolimus (TAC) can be produced in 14 days. These CTLs show high specificity for EBV with negligible alloreactivity in both proliferation and cytotoxicity assays and are able to proliferate and secrete IFN‐γ in response to antigen stimulation in the presence of therapeutic doses of TAC. This strategy will substantially facilitate clinical application of this approach for the treatment of PTLD in SOT recipients.     

A 16‐Year Multi‐Institutional Study of the Role of Age and EBV Status on PTLD Incidence Among Pediatric Heart Transplant Recipients

The objective was to determine the incidence and hazard for posttransplant lymphoproliferative disease (PTLD) in a study of 3170 pediatric primary heart transplants between 1993 and 2009 at 35 institutions in the Pediatric Heart Transplant Study. 147 of 151 reported malignancy events were classified as PTLD. Overall freedom from PTLD was 98.5% at 1 year, 94% at 5 years and 90% at 10 years. Freedom from PTLD was lowest in children (ages 1 to < 10 years) versus infants (<1 year) and adolescents (10 to < 18 years) with children at highest risk for PTLD with a relative risk of 2.4 compared to infants and 1.7 compared to adolescents. Positive donor EBV status was a strong risk factor for PTLD in the seronegative recipient, but risk magnitude was dependent on recipient age at the time of transplantation. Nearly 25% of EBV seronegative recipients of EBV+ donors at ages 4–7 at transplantation developed some form of PTLD. The overall risk for PTLD declined in the most recent transplant era (2001–2009, p = 0.003). These findings indicate that EBV status and the age of the recipient at the time of transplantation are important variables in the development of PTLD in the pediatric heart transplant recipient.     

Dual blockade of the PI3K/Akt/mTOR pathway inhibits posttransplant Epstein‐Barr virus B cell lymphomas and promotes allograft survival

Posttransplant lymphoproliferative disorder (PTLD) is a serious complication of organ transplantation that often manifests as Epstein‐Barr virus (EBV)‐associated B cell lymphomas. Current treatments for PTLD have limited efficacy and can be associated with graft rejection or systemic toxicities. The mTOR inhibitor, rapamycin, suppresses tumor growth of EBV+ B cell lymphoma cells in vitro and in vivo; however, the efficacy is limited and clinical benefits of mTOR inhibitors for PTLD are variable. Here, we show constitutive activation of multiple nodes within the PI3K/Akt/mTOR pathway in EBV+ PTLD‐derived cell lines. Inhibition of either PI3K or Akt, with specific inhibitors CAL‐101 and MK‐2206, respectively, diminished growth of EBV+ B cell lines from PTLD patients in a dose‐dependent manner. Importantly, rapamycin combined with CAL‐101 or MK‐2206 had a synergistic effect in suppressing cell growth as determined by IC₅₀ isobolographic analysis and Loewe indices. Moreover, these combinations were significantly more effective than rapamycin alone in inhibiting tumor xenograft growth in NOD‐SCID mice. Finally, both CAL‐101 and MK‐2206 also prolonged survival of heterotopic cardiac allografts in C57BL/6 mice. Thus, combination therapy with rapamycin and a PI3K inhibitor, or an Akt inhibitor, can be an efficacious treatment for EBV‐associated PTLD, while simultaneously promoting allograft survival.     

EBV Positive Primary Cutaneous CD30+ Large T-Cell Lymphoma in a Heart Transplanted Patient: Case Report

Most post-transplant lymphoproliferative disorders (PTLDs) are of B-cell origin, whereas T-cell lymphomas rarely occur. We detail the clinicopathological features of the first case of Epstein-Barr virus (EBV)-associated primary cutaneous CD30+ anaplastic large cell lymphoma (ALCL) in the setting of heart transplant. A 71-year-old patient, 111 months after transplant, presented with multiple cutaneous lesions on the left thigh; histological and immunohistochemical examinations led to diagnosis of T-cell CD30+ ALCL. In situ hybridization demonstrated the presence of EBV-positive tumour cells. The patient received radiotherapy, but he relapsed at the same cutaneous site with loco-regional nodal spread. Chemotherapy was administered resulting in complete remission; four years later the patient is alive and well. Our findings indicate that primary cutaneous EBV+ CD30+ ALCLs should be included within the T-cell PTLDs spectrum; further studies are required to confirm whether they may be also considered, in transplantation settings, a distinct lymphoma subset with relatively favourable outcome.     

Molecular and Immunologic Mechanisms of Cancer Pathogenesis in Solid Organ Transplant Recipients

The increased risk for the development of malignancies in transplant recipients is generally attributed to the debilitated immune system that results from chronic exposure to potent immunosuppressive drugs required to prevent graft rejection. While impaired immunity is clearly a key determinant, there is strong evidence that a constellation of other factors contribute to the pathogenesis of posttransplant cancers. In this article we discuss the underlying molecular and immunologic mechanisms that contribute to the development of de novo malignancies in transplant recipients, with particular focus on the two leading posttransplant neoplasia, skin cancer and Epstein–Barr virus (EBV)‐associated posttransplant lymphoproliferative disorder (PTLD).     

A Case of Age-Related Epstein–Barr Virus (EBV)-Associated B Cell Lymphoproliferative Disorder, so-called Polymorphous Subtype, of the Mandible, with a Review of the Literature

Epstein–Barr virus (EBV) is known to be associated with the development of lymphomas in immunocompromised patients. Recently, age-related immune impairment has been recognized as a predisposing factor in the development of EBV-driven lymphoproliferative disorders (LPDs) in elderly patients without any known immunodeficiency or prior lymphoma. In approximately 70 % of reported cases, the affected sites have been extranodal, such as the skin, lung, tonsil and stomach. However, age-related EBV-associated B cell (EBV + B cell) LPD is extremely rare in the oral cavity. Here we report a 71-year-old Japanese man who developed an EBV + B cell LPD resembling classical Hodgkin lymphoma (CHL)—so-called polymorphous subtype—of the mandible. Histopathologically, infiltration of large atypical lymphoid cells including Hodgkin or Reed-Sternberg-like cells into granulation tissue with marked necrosis was found in the mandibular bone. Immunohistochemical analysis revealed that the large atypical Hodgkin or Reed-Sternberg-like cells were CD3–, CD15–, CD20+, CD30+ and Epstein–Barr virus (EBV)-latent infection membrane protein-1 (LMP-1)+. In situ hybridization (ISH) demonstrated EBV-encoded small RNA (EBER) + in numerous Hodgkin or Reed-Sternberg-like cells. EBNA-2 was detected by polymerase chain reaction (PCR) using an extract from the formalin-fixed, paraffin-embedded specimen. To our knowledge, this is the first reported case of the polymorphous subtype of age-related EBV + B cell LPD affecting the mandible.     

Circulating Antibody Free Light Chains and Risk of Posttransplant Lymphoproliferative Disorder

Posttransplant lymphoproliferative disorder (PTLD) is a major complication of solid‐organ transplantation. With human immunodeficiency virus infection (an analogous immunosuppressive state), elevated kappa and lambda immunoglobulin free light chains (FLCs) in peripheral blood are associated with increased risk of lymphoma. To assess the role of B‐cell dysfunction in PTLD, we measured circulating FLCs among Canadian transplant recipients, including 29 individuals with PTLD and 57 matched transplant recipients who were PTLD‐free. Compared with controls, PTLD cases had higher kappa FLCs (median 1.53 vs. 1.07 times upper limit of normal) and lambda FLCs (1.03 vs. 0.68). Using samples obtained on average 3.5 months before PTLD diagnosis, cases were more likely to have polyclonal FLC elevations (i.e. elevated kappa and/or lambda with normal kappa/lambda ratio: odds ratio [OR] 4.2, 95%CI 1.1–15) or monoclonal elevations (elevated kappa and/or lambda with abnormal ratio: OR 3.0, 95%CI 0.5–18). Strong FLC‐PTLD associations were also observed at diagnosis/selection. Among recipients with Epstein–Barr virus (EBV) DNA measured in blood, EBV DNAemia was associated with FLC abnormalities (ORs 6.2 and 3.2 for monoclonal and polyclonal elevations). FLC elevations are common in transplant recipients and associated with heightened PTLD risk. FLCs likely reflect B‐cell dysfunction, perhaps related to EBV‐driven lymphoproliferation.     

Preemptive Therapy of EBV-Related Lymphoproliferative Disease after Pediatric Haploidentical Stem Cell Transplantation

The treatment of Epstein-Barr virus (EBV)-related post-transplant lymphoproliferative disease (PTLD) after hematopoietic stem cell transplantation (HSCT) is still unsatisfactory. We conducted a prospective trial to evaluate the impact of routine EBV surveillance and preemptive treatment with the anti-CD20 monoclonal antibody rituximab on the development of PTLD in pediatric recipients of extensively T-cell depleted HSCT from an HLA-haploidentical relative. Twenty-seven patients were included in the surveillance program, 12 developed EBV DNA positivity, with 8 of 12 presenting with sustained viral DNA levels requiring treatment with rituximab. Treatment was well tolerated, and induced clearance of EBV DNA in all patients. However, 4/8 patients showed a new increase in EBV load, coincident with the emergence of CD20(-)/CD19(+) B cells in peripheral blood, accompanied by overt PTLD in 3 patients. The latter cleared PTLD after receiving donor EBV-specific cytotoxic T-lymphocytes (CTLs), and persist in remission at a median 30-month follow-up. EBV-specific T-cell frequency, undetectable at time of EBV DNA positivity, was restored by T-cell therapy to levels comparable with controls. We conclude that preemptive therapy with rituximab is safe, but only partly effective in haplo-HSCT recipients. Patients who progress to PTLD under rituximab treatment can be rescued permanently by infusion of EBV-specific CTLs.     

Epstein–Barr Virus‐Related Post‐Transplant Lymphoproliferative Disorders: Pathogenetic Insights for Targeted Therapy

Post‐transplant lymphoproliferative disorder (PTLD) is a spectrum of major, life‐threatening lymphoproliferative diseases occurring in the post‐transplant setting. The majority of PTLD is of B‐cell origin and is associated with several risk factors, the most significant being Epstein‐Barr virus (EBV) infection. EBV's in vitro transforming abilities, distinctive latency, clonality within the malignant cells and response to targeted therapies implicate a critical role in the biology of PTLD. This minireview focuses on EBV‐related PTLD pathogenesis, in particular the interplay between aspects of the EBV life cycle and latency with nonviral factors resulting in the wide spectrum of histology and clinical presentations encountered in PTLD. With the increased prevalence of transplantation a rise in the incidence of PTLD may be expected. Therefore the importance of laboratory and animal models in the understanding of PTLD and the development of novel therapeutic approaches is discussed.     

Chronic High Epstein-Barr Viral Load State and Risk for Late-Onset Posttransplant Lymphoproliferative Disease/Lymphoma in Children

Increased use of serial EBV-PCR monitoring after pediatric transplantation has led to the identification of asymptomatic patients who carry very high viral loads over prolonged periods. The significance of this high-load state is unknown. We speculated that this state may identify patients at high risk for development of late PTLD/lymphoma. We reviewed data on 71 pediatric heart recipients who had serial viral load monitoring since 1997. Chronic high-load state was defined as the presence of >16 000 genome copies/mL whole blood on ≥50% of samples over at least 6 months. Among 20 high-load carriers (eight following prior PTLD, seven with prior symptomatic EBV infection, five without previous EBV disease), 9 (45%) developed late-onset PTLD 2.5–8.4 years posttransplant (including with four Burkitt's lymphoma). Among 51 controls with low (n = 39) or absent (n = 12) loads, only 2 (4%; p < 0.001 absent/low vs. high load) developed late PTLD/lymphoma. By multivariable analysis, high-load carrier state (OR = 12.4, 95% CI 2.1–74.4) and prior history of PTLD (OR = 10.7, 95% CI 1.9–60.6) independently predicted late PTLD. A chronic high EBV-load state is not benign and is a predictor of de novo or recurrent PTLD.     

The Impact of EBV Status on Characteristics and Outcomes of Posttransplantation Lymphoproliferative Disorder

We examined the associations of Epstein–Barr virus (EBV) status with characteristics and outcomes of posttransplantation lymphoproliferative disorder (PTLD) by studying 176 adult solid organ transplant recipients diagnosed with PTLD between 1990 and 2013 (58 [33%] EBV‐negative; 118 [67%] EBV‐positive). The proportion of EBV‐negative cases increased over time from 10% (1990–1995) to 48% (2008–2013) (p < 0.001). EBV‐negative PTLD had distinct characteristics (monomorphic histology, longer latency) though high‐risk features (advanced stage, older age, high lactate dehydrogenase, central nervous system involvement) were not more common compared to EBV‐positive PTLD. In multivariable analysis, EBV negativity was not significantly associated with worse response to initial therapy (adjusted odds ratio, 0.84; p = 0.75). The likelihood of achieving a complete remission (CR) was not significantly different for EBV‐negative versus EBV‐positive PTLD including when therapy was reduction of immunosuppression alone (35% vs. 43%, respectively, p = 0.60) or rituximab (43% vs. 47%, p = 1.0). EBV negativity was also not associated with worse overall survival (adjusted hazard ratio, 0.91; p = 0.71). Our findings indicate that EBV status is not prognostic or predictive of treatment response in adults with PTLD. The high proportion of EBV‐negative disease diagnosed in recent years highlights the need for new strategies for prevention and management of EBV‐negative PTLD.