Recent Advances in Adult Post-Transplant Lymphoproliferative Disorder

Simple Summary Post-transplant lymphoproliferative disorder (PTLD) is a potentially life-threatening complication of mainly solid organ and, less frequently, allogeneic hematopoietic stem-cell transplantation, with a reported incidence of 2 to 20%. PTLD has a complex pathogenesis and not all aspects are well understood to date; however, a proportion of cases are strongly related to Epstein–Barr virus. Therapy mainly depends on the histologic subtype; however, the heterogeneity of the disease and lack of clinical trials create gaps in evidence-based management of these patients. In this review, we discuss the pathogenesis, classification, and risk factors of PTLD. We further analyze common treatment strategies and describe the latest advances in disease management and prevention, including novel therapies. Abstract PTLD is a rare but severe complication of hematopoietic or solid organ transplant recipients, with variable incidence and timing of occurrence depending on different patient-, therapy-, and transplant-related factors. The pathogenesis of PTLD is complex, with most cases of early PLTD having a strong association with Epstein–Barr virus (EBV) infection and the iatrogenic, immunosuppression-related decrease in T-cell immune surveillance. Without appropriate T-cell response, EBV-infected B cells persist and proliferate, resulting in malignant transformation. Classification is based on the histologic subtype and ranges from nondestructive hyperplasias to monoclonal aggressive lymphomas, with the most common subtype being diffuse large B-cell lymphoma-like PTLD. Management focuses on prevention of PTLD development, as well as therapy for active disease. Treatment is largely based on the histologic subtype. However, given lack of clinical trials providing evidence-based data on PLTD therapy-related outcomes, there are no specific management guidelines. In this review, we discuss the pathogenesis, histologic classification, and risk factors of PTLD. We further focus on common preventive and frontline treatment modalities, as well as describe the application of novel therapies for PLTD and elaborate on potential challenges in therapy.


Introduction
Post-transplant lymphoproliferative disorder (PTLD) is the most common malignancy in recipients of solid organ transplantation (SOT), excluding nonmelanoma cutaneous and in situ cervical malignancies [1]. In contrast, PTLD occurs in a minority of patients following allogeneic hematopoietic stem-cell transplantation (alloHSCT) [2,3]. Regardless of the setting, PTLD has been associated with significant morbidity and mortality. PTLD mainly arises from B cells but may rarely develop from T or NK cells, which account for 2-15% of all PTLD cases. More than half of B-cell PTLDs are driven by abnormal expansion of Epstein-Barr virus (EBV)-infected B cells. Non-B-cell PTLDs are usually not EBV-related, occur after a long latency period post-transplantation, and are associated with a more aggressive course and poor prognosis, with a median overall survival (OS) of 6 months [4]. Higher PTLD rates have been observed in multiorgan and intestinal (<20%), lung (3-10%), and heart (2-8%) transplants due to the higher level and prolonged need of immunosuppressive therapy (IST) to prevent allograft rejection [1,5,6]. The primary management of PTLD includes reduction or complete cessation of IST. However, this is not always possible considering the risk for allograft loss or dysfunction, especially in cases of vital organ transplantation, such as heart transplant. In this context, a variety of therapeutic options have been proposed including chemo-and immunotherapy, radiation therapy, and various novel treatments, all with variable results [7]. While there are no guidelines for treatment, the type of lesion and EBV status generally drives the type of therapy based on consensus statements [8][9][10]. Further research is necessary to establish concrete treatment recommendations. In this review, we discuss the clinical presentation, pathophysiology, and risk factors of PTLD. We further analyze common treatment strategies, and describe the latest advances in disease management and prevention.

Pathogenesis
The pathogenesis of PTLD is complex and not fully understood. The dominant theory is that the required post-transplant IST negatively impacts the ability of cytotoxic T cells for immune eradication via multiple mechanisms, including inability to produce vital cytokines for immune destruction, such as interferon gamma (IFN-γ), interleukin 2 (IL-2), and tumor necrosis factor-alpha (TNF-α). This dysfunction allows abnormal clones of B lymphocytes (most commonly infected with EBV) to proliferate, ultimately transforming to PTLD.
In most instances, PLTD has been strongly associated with EBV infection [11,12]. EBV can be transmitted from a seropositive donor to a previously EBV-naïve/seronegative recipient and manifest as a primary infection, but PTLD can also be a result of EBV reactivation in a seropositive recipient who had previously acquired EBV via environmental exposure, in the setting of immunosuppression [13]. The virus remains latent in B lymphocytes or progresses with viral replication and B-cell lysis. Primary EBV infections in immunocompetent adults are usually self-limited and do not result in any significant complications because proliferation of B cells is normally suppressed by a virus-specific cytotoxic T-lymphocyte response, eradicating the majority of phenotypically abnormal B cells infected with EBV [13]. Disruption in T-cell surveillance, in the setting of post-transplant iatrogenic immunosuppression, can lead to unchecked B-cell proliferation and transformation, hence developing into PTLD [13].
Less is known about the pathogenesis of EBV-negative PTLD. It is believed to be similar to that of de novo non-Hodgkin's lymphoma (NHL) in immunocompetent hosts based on available molecular and immunohistochemical data, rather than true PTLD [14,15]. Hit-and-run EBV infections, indicating an EBV infection that initiates the pathogenesis of PTLD and then resolves, infections with other herpesviruses (e.g., cytomegalovirus (CMV), human herpesvirus 6 (HHV-6) or other viruses), persistent antigen stimulation by the graft, and long-term IST, have all been proposed as potential mechanisms [13,16]. From a clinical perspective, EBV-negative PTLD tends to occur in older transplant recipients,

Reduction in IST
IST reduction is usually the first step. Initial management should include dose reduction of at least 50% in calcineurin inhibitors and withdrawal of any antimetabolites (e.g., azathioprine and mycophenolate) [79]. This significantly helps the expansion of the cytotoxic T-cell compartment without completely compromising allograft function. In cases of severe PTLD, all immunosuppressive agents should be temporarily discontinued, except steroids. It is crucial to monitor for allograft rejection while patients receive low-dose IST as acute solid organ rejection has been reported at frequencies reaching 37% [48].
Reduction in IST has variable efficacy, which can be as high as 80%. However, cases of EBV-negative PTLD, high burden or disease, or advanced stage are all factors leading to poor response to IST reduction, emphasizing the need for more aggressive approaches in these patient populations [48].

Rituximab
In cases of no or inadequate response to IST reduction, or when IST reduction is not feasible due to a high risk for allograft rejection, the chimeric murine/human IgG1 kappa anti-CD20 mAb, rituximab, is used as frontline therapy [1]. For rare histologic subtypes, treatment strategy is different and will be discussed later. Rituximab binds the CD20 antigen found on the surface of B lymphocytes and eradicates them via complement-or antibody-dependent cell-mediated cytotoxicity. Rituximab can be administered either as a monotherapy or in combination with chemotherapy and has shown adequate clinical

. Reduction in IST
IST reduction is usually the first step. Initial management should include dose reduction of at least 50% in calcineurin inhibitors and withdrawal of any antimetabolites (e.g., azathioprine and mycophenolate) [79]. This significantly helps the expansion of the cytotoxic T-cell compartment without completely compromising allograft function. In cases of severe PTLD, all immunosuppressive agents should be temporarily discontinued, except steroids. It is crucial to monitor for allograft rejection while patients receive low-dose IST as acute solid organ rejection has been reported at frequencies reaching 37% [48].
Reduction in IST has variable efficacy, which can be as high as 80%. However, cases of EBV-negative PTLD, high burden or disease, or advanced stage are all factors leading to poor response to IST reduction, emphasizing the need for more aggressive approaches in these patient populations [48].

Rituximab
In cases of no or inadequate response to IST reduction, or when IST reduction is not feasible due to a high risk for allograft rejection, the chimeric murine/human IgG1 kappa anti-CD20 mAb, rituximab, is used as frontline therapy [1]. For rare histologic subtypes, treatment strategy is different and will be discussed later. Rituximab binds the CD20 antigen found on the surface of B lymphocytes and eradicates them via complement-or antibody-dependent cell-mediated cytotoxicity. Rituximab can be administered either as a monotherapy or in combination with chemotherapy and has shown adequate clinical efficacy up to 60%, based on type, stage, and bulk of disease. The most used chemotherapy regimen is cyclophosphamide, doxorubicin, vincristine, and prednisone (CHOP), which is the typical frontline regimen given for B-cell lymphomas in immunocompetent hosts.
Two phase II trials assessed the administration of four weekly doses of frontline rituximab in post-SOT patients who failed IST reduction. The first trial included 17 patients, with 53% achieving complete response (CR). All responders had EBV-related PTLD. The 3-year OS rate was 56%. All patients who did not benefit from rituximab had EBV-negative Cancers 2022, 14, 5949 9 of 24 and late onset PTLD, and almost all required subsequent chemotherapy, suggesting that EBV-negative PTLD might need combination of rituximab and chemotherapy from the beginning [85]. The second trial evaluated 43 patients, and showed an overall response rate (ORR) of 44% to rituximab monotherapy, with 28% achieving CR. The 1-year PFS and OS rates were 21% and 67%, respectively. [86]. Another phase II trial, assessed the efficacy of upfront rituximab with concomitant IST reduction in 38 patients as a first-line approach. Those who did not achieve CR received a second course of four rituximab infusions. CR and PR rates were 34% and 45%, respectively, after the first course of rituximab. Retreatment of patients achieving PR with rituximab yielded a CR rate of 83%. Those who had no response (38%) to the first course were treated with chemotherapy, and 75% of them achieved CR. At 27.5 months, PFS and OS rates were 42% and 47%, respectively [87]. A long-term follow up of this trial, as well as a real-world cohort of 21 patients treated with the same regimen after the trial was closed (validation cohort), was recently published. For the trial patients (median follow-up of 13 years), the disease-specific survival (DSS) at 10 years was 64.7%; for those that achieved CR (61%), DSS at 5 and 10 years was 94.4% and 88.1%, respectively. For the real-word cohort (median follow-up of 6.5 years), DSS at 5 years was 75.2%; for those that achieved CR (38%), DSS was 87.5%. Authors concluded that PTLD patients in CR after rituximab have an excellent long-term outcome, reproducible in the real-world setting [88].

Rituximab and Chemotherapy
Transplant patients have historically tolerated conventional chemotherapy poorly, with a treatment-related mortality (TRM) of up to 31%, compared to 2% for DLBCL in immunocompetent individuals [86,89,90]. Despite this, patients capable of tolerating chemotherapy may achieve a long-lasting remission as outlined from the PTLD-1 and PTLD-2 trials.
The PTLD-1 trial (NCT01458548) assessed the upfront use rituximab monotherapy followed by CHOP consolidation in 70 patients who were refractory to initial IST reduction. ORR and CR were 60% and 20%, respectively, after rituximab induction, as well as 90% and 68%, respectively, after CHOP consolidation. The 3-and 5-year OS rates were 65% and 57%, respectively, with median PFS and OS of 4 and 6.6 years. Grade 3-4 toxicities were high and 11% had CHOP-TRM [91,92]. Response to rituximab induction was a prognostic factor for improved OS.
Given the high efficacy of rituximab but increased CHOP-TRM, a change in protocol was made (PTLD-1/3, 3rd amendment; NCT00590447) and a response-adapted treatment strategy was introduced to minimize toxicity: rituximab consolidation for complete responders to rituximab induction, and R-CHOP consolidation for all others. Among 152 participants, 25% achieved CR to single-agent rituximab. For those who achieved less than CR, ORR with subsequent R-CHOP was 85%, with a 60% CR rate. ORR and CR for the entire cohort were 88% and 70%, respectively. Median time to progression (TTP) was not reached, and the 3-year estimate without progression was 78%. Median OS was 6.6 years. Response to rituximab induction was a highly significant predictor of better OS, TTP, and PFS (p < 0.001). TRM was lower at 7% [93,94].
A follow-up phase II trial, the PTLD-2 (NCT02042391) (n = 48), applied a slightly modified risk-stratification treatment approach, based again on response to rituximab induction and international prognostic score (IPI) score at diagnosis. Low-risk patients, defined as those achieving CR or RP with IPI < 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96]. defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96]. Newly diagnosed CD20+ PTLD R-induction followed by response assessment to determine consolidation: (response adapted strategy) ▪ CR to R-induction: repeat R ▪ less than CR to R-induction: R-CHOP ▪ R-induction: CR 25% ▪ R-CHOP consolidation: ORR 85%, CR 60% ▪ Entire cohort: final ORR 88%, CR 70% ▪ 3-yr TTP 78%, mOS 6.6 years ▪ TRM 7% NCT02042391 (PTLD-2 trial) [95] II 48 Newly diagnosed CD20+ PTLD R-induction followed by response assessment to determine consolidation: (response adapted strategy) ▪ Low risk (CR or PR w/ IPI 0-2) group: repeat R, n = 21 ▪ High risk (PR w/ IPI 3-5 or SD or PD but not heart or lung recipient) group: R-CHOP, n = 22 ▪ Very high risk (heart, lung or multiorgan recipients with PD) group: alternating R-CHOP and R-DHAOx, n = 5 defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96]. defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96]. defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96]. defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96].   defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96].   defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96].   defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96].   defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96].   defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96]. Newly diagnosed CD20+ PTLD R-induction followed by response assessment to determine consolidation: (response adapted strategy) ▪ Low risk (CR or PR w/ IPI 0-2) group: repeat R, n = 21 ▪ High risk (PR w/ IPI 3-5 or SD or PD but not heart or lung recipient) group: R-CHOP, n = 22 ▪ Very high risk (heart, lung or multiorgan recipients with PD) group: alternating R-CHOP and R-DHAOx, n = 5   defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96].   defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96].   defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96].   defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96].   High risk (PR w/ IPI 3-5 or SD or PD but not heart or lung recipient) group: R-CHOP, n = 22 ER REVIEW 10 of 25 defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96].   defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96].   defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96].   defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96].   defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96]. Newly diagnosed CD20+ PTLD R-induction followed by response assessment to determine consolidation: (response adapted strategy) ▪ Low risk (CR or PR w/ IPI 0-2) group: repeat R, n = 21 ▪ High risk (PR w/ IPI 3-5 or SD or PD but not heart or lung recipient) group: R-CHOP, n = 22 ▪ Very high risk (heart, lung or multiorgan recipients with PD) group: alternating R-CHOP and R-DHAOx, n = 5   defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96]. Newly diagnosed CD20+ PTLD R-induction followed by response assessment to determine consolidation: (response adapted strategy) ▪ Low risk (CR or PR w/ IPI 0-2) group: repeat R, n = 21 ▪ High risk (PR w/ IPI 3-5 or SD or PD but not heart or lung recipient) group: R-CHOP, n = 22 ▪ Very high risk (heart, lung or multiorgan recipients with PD) group: alternating R-CHOP and R-DHAOx, n = 5   defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96]. Newly diagnosed CD20+ PTLD R-induction followed by response assessment to determine consolidation: (response adapted strategy) ▪ Low risk (CR or PR w/ IPI 0-2) group: repeat R, n = 21 ▪ High risk (PR w/ IPI 3-5 or SD or PD but not heart or lung recipient) group: R-CHOP, n = 22 ▪ Very high risk (heart, lung or multiorgan recipients with PD) group: alternating R-CHOP and R-DHAOx, n = 5   defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96]. Newly diagnosed CD20+ PTLD R-induction followed by response assessment to determine consolidation: (response adapted strategy) ▪ Low risk (CR or PR w/ IPI 0-2) group: repeat R, n = 21 ▪ High risk (PR w/ IPI 3-5 or SD or PD but not heart or lung recipient) group: R-CHOP, n = 22 ▪ Very high risk (heart, lung or multiorgan recipients with PD) group: alternating R-CHOP and R-DHAOx, n = 5   defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96].   defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96]. Newly diagnosed CD20+ PTLD R-induction followed by response assessment to determine consolidation: (response adapted strategy) ▪ CR to R-induction: repeat R ▪ less than CR to R-induction: R-CHOP ▪ R-induction: CR 25% ▪ R-CHOP consolidation: ORR 85%, CR 60% ▪ Entire cohort: final ORR 88%, CR 70% ▪ 3-yr TTP 78%, mOS 6.6 years ▪ TRM 7% NCT02042391 (PTLD-2 trial) [95] II 48 Newly diagnosed CD20+ PTLD R-induction followed by response assessment to determine consolidation: (response adapted strategy) ▪ Low risk (CR or PR w/ IPI 0-2) group: repeat R, n = 21 ▪ High risk (PR w/ IPI 3-5 or SD or PD but not heart or lung recipient) group: R-CHOP, n = 22 ▪ Very high risk (heart, lung or multiorgan recipients with PD) group: alternating R-CHOP and R-DHAOx, n = 5  defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96]. Newly diagnosed CD20+ PTLD R-induction followed by response assessment to determine consolidation: (response adapted strategy) ▪ CR to R-induction: repeat R ▪ less than CR to R-induction: R-CHOP ▪ R-induction: CR 25% ▪ R-CHOP consolidation: ORR 85%, CR 60% ▪ Entire cohort: final ORR 88%, CR 70% ▪ 3-yr TTP 78%, mOS 6.6 years ▪ TRM 7% NCT02042391 (PTLD-2 trial) [95] II 48 Newly diagnosed CD20+ PTLD R-induction followed by response assessment to determine consolidation: (response adapted strategy) ▪ Low risk (CR or PR w/ IPI 0-2) group: repeat R, n = 21 ▪ High risk (PR w/ IPI 3-5 or SD or PD but not heart or lung recipient) group: R-CHOP, n = 22 ▪ Very high risk (heart, lung or multiorgan recipients with PD) group: alternating R-CHOP and R-DHAOx, n = 5  defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96]. Newly diagnosed CD20+ PTLD R-induction followed by response assessment to determine consolidation: (response adapted strategy) ▪ CR to R-induction: repeat R ▪ less than CR to R-induction: R-CHOP ▪ R-induction: CR 25% ▪ R-CHOP consolidation: ORR 85%, CR 60% ▪ Entire cohort: final ORR 88%, CR 70% ▪ 3-yr TTP 78%, mOS 6.6 years ▪ TRM 7% NCT02042391 (PTLD-2 trial) [95] II 48 Newly diagnosed CD20+ PTLD R-induction followed by response assessment to determine consolidation: (response adapted strategy) ▪ Low risk (CR or PR w/ IPI 0-2) group: repeat R, n = 21 ▪ High risk (PR w/ IPI 3-5 or SD or PD but not heart or lung recipient) group: R-CHOP, n = 22 ▪ Very high risk (heart, lung or multiorgan recipients with PD) group: alternating R-CHOP and R-DHAOx, n = 5  defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96]. Newly diagnosed CD20+ PTLD R-induction followed by response assessment to determine consolidation: (response adapted strategy) ▪ CR to R-induction: repeat R ▪ less than CR to R-induction: R-CHOP ▪ R-induction: CR 25% ▪ R-CHOP consolidation: ORR 85%, CR 60% ▪ Entire cohort: final ORR 88%, CR 70% ▪ 3-yr TTP 78%, mOS 6.6 years ▪ TRM 7% NCT02042391 (PTLD-2 trial) [95] II 48 Newly diagnosed CD20+ PTLD R-induction followed by response assessment to determine consolidation: (response adapted strategy) ▪ Low risk (CR or PR w/ IPI 0-2) group: repeat R, n = 21 ▪ High risk (PR w/ IPI 3-5 or SD or PD but not heart or lung recipient) group: R-CHOP, n = 22 ▪ Very high risk (heart, lung or multiorgan recipients with PD) group: alternating R-CHOP and R-DHAOx, n = 5   defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96].   defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96].  defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96].   defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96]. defined as those achieving CR or RP with IPI< 3, received a repeat course of rituximab as consolidation. The rest received R-CHOP or more intensifying chemotherapy regimens (Table 3). ORR to rituximab induction was 45% but only 9% achieved CR. The ORR at final staging was 94% and median PFS 3.8 years, with 2-year PFS and OS rate of 56% and 68%, respectively. Given that comparable outcomes to the PTLD-1 were yielded with a less intense regimen for complete responders, a stepwise approach should be followed to avoid unnecessary toxicity in these patients [95]. Rituximab has also been combined with more intense chemotherapy regimens, such as etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) in small case series; however, clear survival benefit has not been observed [22,37,96].

AlloHSCT Recipients
Data for frontline therapy in patients post-alloHSCT is very limited [100]. Reduction in IST is challenging due to the high risk of graft versus host disease and appears to have very limited efficacy in early-onset PLTD. This is because of different mechanisms of the underlying cytotoxic T-cell compartment weakening, compared to post-SOT PTLD. In particular, the donor's immune system needs time to fully recover and expand within the recipient's marrow, with early-onset PTLD being a consequence of this T-cell insufficiency.
Frontline rituximab is typically used for early-onset PTLD displaying the most common histologic subtypes, with efficacy around 60%. Upfront chemotherapy, most frequently CHOP, can also be used along with rituximab if high burden of disease is noted. Alternatively, if inadequate response to rituximab monotherapy is noted, consolidation R-CHOP can be used. Prospective or observational large population studies are not available for this rare entity; therefore, decision making is largely institution-dependent [101].
A retrospective study explored the prognostic factors that affect the outcome of EBVrelated PTLD after rituximab-based therapy in post-alloHSCT patients. Poor response to rituximab was noted in patients with age ≥ 30 years, extra-lymphoid tissue involvement, acute GVHD, and a lack of reduction in IST upon PTLD diagnosis. Notably, IST tapering was associated with a lower PTLD mortality (16% vs. 39%) and a decrease in EBV DNA levels during therapy was predictive of better survival [102]. A recent systematic review reported that the percentage of patients who fail rituximab post-alloHSCT varies greatly (13-67%) but outcomes are overall poor [103].

Treatment of Relapsed Disease
There are no established treatments for relapsed PTLD where all first-line options (IST reduction, rituximab, and CHOP) have been utilized. In this context, decision regarding treatment is institution-dependent based on experience and tumor characteristics. Sometimes based on the histologic type, regimens used for relapsed lymphoma in immunocompetent patients may be considered. These include but are not limited to ifosfamide, carboplatin, and etoposide, or gemcitabine and oxaliplatin; however, these regimens have increased toxicity which PTLD patients have difficulty handling. Enrollment in clinical trials is always encouraged under these circumstances.
A pilot study assessed the efficacy and safety of carboplatin/etoposide in nine patients with relapsed-refractory PTLD post-SOT who were not candidates for intensified salvage regimens. Five patients achieved a CR and one SD, suggesting that carboplatin/etoposide might be a combination with some degree of effect [104]. In another case series of seven patients, some of whom had received salvage with rituximab several times, retreatment with salvage rituximab achieved CR in three patients and PR in one. Median PFS was nine months [105]. These data suggest that rituximab salvage therapy could be effective for intensively pretreated patients [49].
Another available modality is autologous hematopoietic cell transplantation (au-toHSCT); however, PTLD patients have an increased risk for complications and death, making autoHSCT transplant unfeasible in most cases. At present, published data on the efficacy and safety of auto-HSCT in PTLD are limited to case reports and series. A retrospective analysis of 21 patients who received autoHSCT for relapsed PTLD (mainly DLBCL-type, median of two prior lines of therapy) following SOT demonstrated CR and PR of 47% and 38%, respectively. The 3-year PFS and 3-year OS were 62% and 61%, respectively. Overall, 12 deaths were reported, including 4 related to autoHSCT. The 100-day non-relapse-mortality (NRM) and 1-year NRM were 14% and 24%, respectively. The authors concluded that autoHSCT is a feasible option for relapsed PTLD; however, due to the high associated infectious-driven NRM, careful selection is critical [106]. In cases of EBV-positive PTLD, EBV-specific cytotoxic lymphocytes (CTLs) appear capable of inducing a robust EBV-targeted T-cellular immune response. EBV-CTLs can be used for prevention, pre-emptive therapy, and front-line treatment, and they are usually donor or HLA-matched third-party-derived. They can be combined with rituximab, either as a first step or in a stepwise fashion with or without rituximab in patients who have yielded low or no responses to upfront rituximab.
Tabelecleucel, a third-party EBV-CTL product derived from volunteer donors (partially HLA-matched, sharing ≥2 HLA alleles), was evaluated in patients with EBV-positive PTLD. In a phase I/II study (NCT00002663 and NCT01498484) of 46 patients with EBV-positive PTLD refractory to frontline rituximab, tabelecleucel yielded an ORR of 68% (post-HSCT) and 54% (post-SOT), with no significant toxicities. For patients who achieved CR/PR (responders), 1-year OS rate was high at 89% and 82%, respectively [97]. At present, there is one ongoing phase III trial (ALLELE, NCT03394365) assessing the use of tabelecleucel in patients with EBV-positive PTLD who have failed front-line rituximab or both rituximab and chemotherapy. An interim analysis showed an ORR of 50% on both post-SOT and post-HSCT patients, with median overall OS of 18.4 months and 1-year OS rate of 61%. Notably, responders appear to have better survival outcomes compared to non-responders (Table 3) [98,99].
An early phase I study is assessing the efficacy of tacrolimus-resistant EBV-CTLs for PTLD post-SOT (ITREC, NCT03131934). Several other early trials are currently exploring the role of EBV-CTLs (NCT01555892, NCT02580539, and NCT02822495) or their combination with other agents, such as brentuximab vedotin for rituximab-refractory disease.

Antibody Drug Conjugates
Ibritumomab tiuxetan, an anti-CD20 mAb conjugated with a radioactive isotope, was assessed in a series of eight patients post-SOT with relapsed-refractory PTLD (median of two prior lines of therapy), as a single agent (n = 7) or with chemotherapy (n = 1) and yielded an OS rate of 62.5% (all CR) after a median follow-up of 37 months [107].
The anti-CD30 antibody drug conjugate brentuximab vedotin (BV) is currently approved in combination with doxorubicin-based chemotherapy (BV-CHP) as a front-line therapy for CD30+ peripheral T-cell lymphomas (PTCL), after the completion of the phase III pivotal ECHELON-2 trial, where BV-CHP was found to be superior to CHOP. BV is now being investigated in the setting of PTLD, given that there is increasing evidence showing that the CD30+ antigen expression is frequently detected in PTLD [108][109][110]. ECHELON-2 excluded immunocompromised patients, given their poor survival outcomes. However, recently there has been increasing interest in the use of BV-CHP in CD30+ T-cell PTLD patients due to poor responses to rituximab and/or CHOP. One case reported a very prolonged response to BV-CHP in a patient with CD30-positive T-cell PTLD [4], while BV together with EBV-specific cytotoxic T lymphocytes (EBV-CTLs) resulted in CR lasting for >3.5 years in a patient with progressive EBV-positive PTLD post-alloHSCT [111].
A phase I-II trial (NCT01805037) investigated the combination of BV with rituximab as front-line therapy in 22 patients with immunosuppression-associated CD30+ and/or EBVpositive lymphoid malignancies, 16 of which were diagnosed with PTLD. The combination resulted in a CR rate of 60% after a median follow-up of 26 months, with 1-year and 3-year PFS rates of 75% and 67%, respectively; however, the study was terminated due to lack of funding [112]. Another phase II study (NCT04138875) assessing a risk-stratified sequential treatment approach of PTLD, with BV and rituximab plus or minus bendamustine (triplet therapy for high-risk patients), was also withdrawn due to the lack of funding.

Targeted Therapies
The Bruton tyrosine kinase (BTK) inhibitor, ibrutinib, has been studied with rituximab and CHOP prospectively in the phase II TIDaL trial that uses a risk-stratified sequential approach. The combination of ibrutinib and rituximab with or without chemotherapy (based on a risk-stratified strategy) was evaluated in 39 patients with PTLD post-SOT. All patients received ibrutinib daily with four doses of rituximab. Interim response was assessed after~6 weeks and patients achieving CR/PR continued with the same regimen for four further 3-weekly doses of rituximab with concomitant ibrutinib. Patients who did not respond received four cycles of R-CHOP and ibrutinib instead. After the initial rituximab-ibrutinib combination, 29% achieved CR which was not high enough to warrant further investigation [113]. Acalabrutinib, another BTK inhibitor, is currently studied in combination with rituximab for newly diagnosed PTLD in a phase II trial (NCT04337827).

Antiviral Therapies
B cells latently infected with EBV and EBV(+) lymphoproliferative disorders do not express the viral thymidine kinase and thus are unaffected by antiviral agents, such as purine nucleoside analogs. Therefore, monotherapy with nucleoside analogs does not induce any positive responses in EBV-positive PTLD. Pharmacological attempts of inducing the expression of the viral thymidine kinase through the administration of the histone deacetylase inhibitor, arginine butyrate, before administering antivirals, have led to promising results with acceptable toxicity [114]. Similarly, immunomodulatory drugs or proteasome inhibitors appear capable of inducing EBV lytic activation, enhancing the efficacy of antiviral agents [115,116]. There is an ongoing phase Ib/II study assessing the safety and efficacy of the histone deacetylase inhibitor VRx-3996 in combination with valganciclovir in patients with EBV-related lymphomas (NCT03397706). At the same time, novel antivirals are currently being developed. One example is the new antiviral agent, hexadecyloxypropyl-cidofovir (HDP-CDV) that seems to exhibit a significant increase in antiviral activity against various viruses, including EBV in vitro [117]. Further results are eagerly awaited.

Serotype-Dependent Recombinant Adeno-Associated Vector (AAV)
Recombinant adeno-associated virus (rAAV) utilization has raised the prospect of novel, focused, and effective therapy. Although predominantly associated with gene replacement, advances in altering the tropism of viruses, as well as the content and structure of the viral genome have led to an increasing interest in using rAAV for precision cancer therapies [118]. Initial studies erroneously suggested that human B cells are not susceptible to infection with rAAV; however, newer rAAV variants can now be used for the treatment of B-cell tumors and help eradicate focal forms of PTLD [119]. In detail, B-cell infection with EBV increases transduction susceptibility, with rAAV6.2 and its closely related serotypes, rAAV6 and rAAV6TM, displaying the maximal transduction efficiency among 15 examined serotypes. Furthermore, preincubation of rAAV cells with complexed CD40 ligand (CD40L, a member of tumor necrosis factor family involved in B-cell activation and antibody production) or anti-IgM antibody appears to augment rAAV2 transduction capabilities through an unclear mechanism [120].
In this context, apart from the therapeutic potential of rAAV6.2, it has been suggested that it may also be used to introduce nucleic acids to tumor B cells that are difficult to transfect, aiming to uncover oncogenic mechanisms. Unfortunately, rAAV6.2 appears to be capable of transfecting other types of human cells in addition to B cells, indicating that a more specific mutational rAAV6.2 capsid analysis is necessary in order to identify the amino acid substitutions that will guarantee a more selective tropism and, thus, a lower side effect profile at clinical application [119].

T-Cell-Redirecting Bispecific Antibodies
Blinatumomab is a T-cell-redirecting bispecific antibody or bispecific T-cell antigen engager antibody (BiTE) that forms an immunologic bridge between the T cells (via binding to the CD3 receptor) and tumor cells (via binding to the CD19+ antigen), leading to the destruction of the latter. So far, it has been used successfully for early relapsed acute B-cell lymphoblastic leukemia. A recent case report described its use in the context of relapsed EBV-related PTLD post-alloHSCT in a pediatric patient whose tumor displayed DLBCL histology and expressed CD19. After blinatumomab administration, no cytokine release syndrome (CRS) or other side effects were noted and the patient demonstrated rapid improvement with declining EBV titers, with no evidence of relapse after a follow-up of 7 months, suggesting further assessment of blinatumomab in adult PTLD [121].

Chimeric Antigen Receptor (CAR) T-Cell Therapies
The use of CD19 chimeric antigen receptor T-cell (CAR-T) therapy is currently being investigated in the setting of PTLD, after having shown remarkable efficacy in relapsedrefractory DLBCL in immunocompetent patients. However, its application in PTLD has some theoretical barriers. These mainly include the continuous IST that these patients receive, which may compromise the ability for T-cell collection at leukapheresis and the CAR T-cell expansion at the time of infusion [122].
Despite these concerns, CAR T-cell therapy was recently used in a kidney transplant patient with PTLD refractory to immunochemotherapy. IST was discontinued before the infusion to best allow CAR T-cell expansion, and the patient was closely monitored for early signs of organ rejection. CAR T-cell therapy led to CR, and the patient remained off IST thereafter, even 7 months post-infusion. In another report, a kidney transplant patient with high-burden refractory PTLD was enrolled in a clinical trial of CAR T-cell treatment (ChiCTR1800019622) combined with PD-1 inhibitor induction, followed by PD-1 inhibitor maintenance therapy. Here, IST was not discontinued and the patient achieved PR [123].
Recently, further case series have demonstrated the safety and feasibility of CAR T-cell therapy. A case series from MD Anderson reported that three patients were treated with the anti-CD19 CAR T-cell therapy, axicabtagene ciloleucel (axi-cel), for stage IV DLBCL-like PTLD, all 7-10 years post-kidney transplant. All patients were refractory to front-line and/or salvage immunochemotherapy. IST was discontinued 2-4 weeks prior to leukapheresis and remained off post-CAR T-cell infusion. Two patients achieved CR, whereas one patient PR. Two patients eventually relapsed (at week +34 and +12, respectively). Toxicity was variable with CRS only at grade 1; one patient developed neurotoxicity grade 3. One patient had organ rejection, given ongoing discontinuation of IST post-CAR T-cell therapy. Authors concluded that axi-cel is feasible in kidney transplant recipients with DLBCL and suggested that IST can be safely stopped 2 to 4 weeks before leukapheresis and may be reinitiated 4-12 weeks after axi-cel in patients with ongoing remission, with close monitoring of kidney function [124].
Another recent case series described poor outcomes with axi-cel in patients with DLBCL-like PTLD, 10-20 years after SOT (heart, kidney, and pancreas, respectively), which was refractory to first-or second-line chemotherapy. All patients developed complications, such as CRS and neurotoxicity requiring tocilizumab, and 2/3 patients developed severe acute renal injury (AKI) requiring renal replacement therapy. The patient who developed AKI expired after withdrawal of care due to lack of response and toxicity. [125].
Tisagenlecleucel (tisa-cel), another anti-CD19 CAR T-cell product, was used for the management of three patients with refractory EBV-negative DLBCL-like PTLD. All patients continued calcineurin inhibitors throughout the whole course of treatment and responded to a single infusion of tisa-cel, with two achieving CR. Toxicity profile was similar to other patients with non-PTLD DLBCL treated with tisa-cel [126].
Despite the encouraging outcomes, it appears that CAR T-cell therapy can result in high rates of toxicity, and to date there are no reliable predictors of either toxicity occurrence or disease response. Additionally, the impact and timing of restarting IST post-CAR T-cell therapy has yet to be determined. Further prospective studies are warranted to answer these important questions, which will help patient selection with identification of a subgroup of PTLD patients that would receive the greatest benefit and experience the least possible risk for CAR T-related toxicity [127].

Radiation Therapy
Radiation therapy, either alone or in combination with other treatments, is another modality that can be used in PTLD patients who present with localized disease or CNS involvement. Localized forms of PTLD could be efficiently not only treated, but also possibly cured with radiotherapy, given the excellent radio-sensitivity of lymphoid neoplasms. In one report, three adult patients with PTLD were successfully treated with moderate-dose (24-36 Gy) radiotherapy, achieving sufficient local control with minimal toxicity [128]. In another example, ultra-low-dose radiation was used for ocular MALT-subtype PTLD and led to a sustained treatment response [127].

Treatment of Rare Histologic Subtypes
PTLD can rarely display uncommon histologies, such as primary central nervous system (CNS) lymphoma, Burkitt lymphoma, plasmacytic or plasmablastic types, and classic Hodgkin lymphoma, that require different approaches. There is scarce evidencebased information on how to treat these subtypes; hence, guidelines for lymphomas in immunocompetent patients are usually followed. The benefit of reduction in IST is unclear, however, it should be strongly considered as a first step in combination with immunochemotherapy. 9.6.1. CNS PTLD Evens et al. performed a large retrospective analysis (n = 84) of patients with CNS lymphoma, mainly with DLBCL-like histology [129]. First-line regimens included highdose methotrexate (48%), high-dose cytarabine (33%), rituximab (45%), and whole-brain radiation, either alone or in various combinations, with an ORR of 60%. Treatment-related toxicity and mortality were high, with the latter reaching 13%. Most patients (93%) had reduction in IST as part of their management. After a median follow-up of 42 months, 3-year PFS and OS rates were 32% and 43%, respectively. There was a trend for improved PFS for patients who received rituximab and/or high-dose cytarabine. Poor performance status predicted inferior PFS, while increased LDH portended inferior OS. A retrospective study of 25 pediatric patients (84% post-SOT) demonstrated favorable outcomes following systemic and intrathecal chemotherapy and rituximab (4-year PFS and OS were 50% and 74%, respectively) [130]. Another study of 14 adult patients with CNS-PTLD demonstrated that intrathecal rituximab was effective for CNS-PTLD in post-alloHSCT patients who did not respond to frontline intravenous rituximab-based regimens [131]. Recent case reports have described that ibrutinib, with or without third-party EBV-specific CTLs can lead to durable remissions [132]. 9.6.2. Burkitt-like PTLD Burkitt-like PTLD (BL-PTLD) is another rare and aggressive form of PTLD. A retrospective analysis of 23 patients (21 post-SOT) reported that most cases occurred late, after a median of 5.7 years post-transplantation, and were disseminated (stage IV) at diagnosis. Most patients received immunochemotherapy, with 70% achieving CR. All patients treated with rituximab alone required further chemotherapy to achieve CR. The 2-year OS rate was 65%; 75% for patients receiving rituximab-based therapy, and 43% for those without rituximab. Authors concluded that immunochemotherapy combinations (such as R-CHOP or more intensified regimens) yielded good responses, whereas rituximab alone did not, suggesting the use of immunochemotherapy as a first line [133]. Other smaller case series have also described characteristics and outcomes of patients with BL-PTLD, where most patients harboring MYC rearrangement had late onset disease associated with EBV. The first included five patients who were managed with reduction in IST, followed by rituximab and/or intensive chemotherapy [134]. The second also included five patients treated under the PETHEMA ALL-3/97 protocol. Despite the encouraging ORR, toxicity was high, especially to those treated with PETHEMA ALL-3/97 regimen [135]. In the last case series, most patients received sequential immunochemotherapy (rituximab followed by CHOP), resulting in CR of 100%, without treatment-related deaths [136]. One patient with CNS disease received additional intrathecal chemotherapy. Given the rarity of BL-PTLD and the data scarcity with regard to therapy outcomes, there are no established treatment guidelines at present. A major challenge as described by Dierickx et al. is the need for prophylactic intrathecal chemotherapy, especially in cases with aggressive and bulky disease [137].

Plasmacytic PTLD
Plasmacytic PTLD is an uncommon variant that accounts for~4% of all PTLD cases post-SOT. Histologically, it resembles multiple myeloma and frequently presents with extramedullary manifestations. Small case series have reported that reduction in IST and radiotherapy have yielded significant responses in low-burden disease; however, in cases of advanced disease, traditional plasma-cell-directed therapies should be used [138,139]. A small case series evaluated the novel anti-CD38 monoclonal antibody daratumumab [140] in combination with traditional antimyeloma therapy in five post-SOT recipients (4/5 with light-chain amyloidosis), yielding significant responses without organ damage and without always necessitating reduction in IST [141]. 9.6.4. Plasmablastic PTLD Post-transplant plasmablastic lymphoma (PL-PTLD) is an extremely rare type of PTLD. A study of 11 patients revealed morphological and immunophenotypic heterogeneity, with 55% being EBV-positive and 55% harboring MYC rearrangement. Recurrent mutations in epigenetic modifiers, such as the KMT2/MLL family of histone H3 methyltransferases were among the most frequent alterations. Treatment was variable, mainly including rituximab and/or chemotherapy (sometimes with addition of bortezomib) with or without radiation; however, given the heterogeneity, no conclusions can be made [142]. Another case series of eight patients combined IST reduction with systemic chemotherapy; however, five patients died from early progression. Three patients achieved and maintained CR, all of whom were EBV-positive, had no cytogenetic aberrations, and received CHOP-21 along with IST reduction [143].

Classic Hodgkin Lymphoma (HL)-like PTLD
Little is known about this uncommon subtype of PTLD. A comparative analysis of 192 patients with HL-PTLD from the Scientific Registry of Transplant Recipients and 13,847 patients with Hodgkin lymphoma in Surveillance, Epidemiology, and End Result (SEER) revealed that HL-PTLD patients had inferior OS. Treatment with HL-specific chemotherapy was associated with improved OS compared to nontraditional HL regimens. In multivariable analysis, advanced age and elevated creatinine were associated with inferior OS [144]. A case series of 13 patients showed superior responses with Adriamycin, Bleomycin, Vinblastine, and Dacarbazine (ABVD)-like chemotherapy than with rituximab; however, toxicity was significant [145]. Smaller case series and case reports also reported poor outcomes with rituximab or non-HL chemotherapy [146,147].

Conclusions
In summary, PTLD is a rare and severe complication of recipients of alloHSCT or SOT, mainly occurring because of therapeutic IST, and with a large number of cases being strongly associated with EBV infection. Despite recent advances in therapy and prevention, there is still a large gap in evidence-based approaches to treatment given the lack of large prospective clinical trials. Novel therapeutic modalities, increasingly used in immunocompetent hosts with lymphoma, may play a role in the therapeutic landscape of PTLD; however, careful investigation is warranted first given that PLTD has substantial differences compared to lymphoma in immunocompetent hosts, including increased vulnerability and toxicity risk. New insights into the pathophysiology of PTLD are key for the development of a more clinically applicable classification system, as well as development of subtype-specific strategies to improve patient outcomes. Given the rarity of PTLD, multi-institution collaborations are critical to allow the development of phase II and III clinical trials to evaluate treatment interventions in a large number of cases. This will help establish disease-specific guidelines, both preventive and therapeutic, an urgent unmet need in PTLD. Funding: This work received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.