HHV8 is a DNA virus that belongs to the gamma-herpes virus subfamily. It was first discovered in 1994 as the etiologic agent of Kaposi’s sarcoma (KS) [1]. Four types of KS are distinguished: classic-, endemic-, immunosuppression‐associated-, and AIDS-associated KS [2]. Other HHV8 associated neoplastic disorders include primary effusion lymphoma and multicentric Castleman disease [3, 4].

In SOT recipients, KS is ∼200 fold more frequent than the general population, with cumulative incidence of ∼3%–5% in endemic areas, and <1% in non-endemic areas [5, 6]. Post-transplant KS is a consequence of reactivation of latent infection in seropositive recipients, or a primary donor derived infection in seronegative recipients [7].

Non-neoplastic disorders associated with HHV8 are peripheral cytopenias, hemophagocytic syndromes, acute hepatitis, and KS‐associated herpesvirus inflammatory cytokine syndrome (KICS) [8, 9].

The seroprevalence of HHV8 depends on the geographic region. African countries have the highest seroprevalence rates (>50%), whereas seroprevalence in Europe, North America, South and East Asia is lower [10–14]. In low seroprevalence regions, men who have sex with men (MSM) are at increased risk [15].

Both sexual and non-sexual transmission (including blood transfusion and organ transplantation) of HHV8 occurs. SOT recipients may be infected either before transplantation and reactivate the virus post-transplantation, or acquire the virus as a donor-derived infection. Primary HHV8 infection post-transplant increase the risk for HHV8-associated disease [16].

HHV8 infection in immunocompetent individuals is generally asymptomatic, although occasionally associated with a febrile rash in children [17]. In immunocompromised individuals, HHV8 infection has been associated with fever, splenomegaly, maculopapular rash, lymphadenopathy and cytopenia [18] and rarely causes systemic disease with multi-organ failure post-transplantation [6]. Bone marrow suppression, with or without hemophagocytosis was linked to donor derived HHV8 infection in the early post-transplant period [19–21], and rare cases of sexually transmitted primary HHV8 infections post-transplantation were associated with hemophagocytosis [22].

MCD is characterized by B‐cell transformation to plasmablasts, which subsequently infiltrate multiple lymph nodes and distort their architecture. It typically presents with fever, lymphadenopathy, hepatosplenomegaly, and cytopenia [6]. MCD and PT-KS may occur concomitantly in SOT patients [32–34].

PEL is a non‐Hodgkin lymphoma that rarely develops after SOT and affects serous body cavities (pleura, pericardium, and peritoneum) [6]. The median time of presentation is 8 years after transplant, with a wide range from 5 months to 28 years [35]. It presents as a body cavity effusion in the absence of tumor masses. The prognosis is dismal [35].

KICS is a systemic inflammation that resembles MCD without pathologic findings in lymph nodes. Generally, patients with KICS also have KS and to a lesser extent may have PEL. Patients with KICS have more severe symptoms, and an increased risk of death [36]. Two cases of KICS have been reported in SOT recipients (Table 1) [8, 37].

Recent American society of transplant (AST) guidelines on HHV8 provide a weak recommendation for pre-transplant serological screening of donors and recipients in endemic areas in order to stratify the risk for HHV8 associated disease [6]. In non-endemic areas it is suggested to consider screening for at-risk donors and recipients only (i.e., MSM, people living with HIV or who inject drugs), or for immigrants from endemic countries [8].

The rationale behind recommending serologic screening in endemic settings is the increased risk for KS among seropositive kidney transplant recipients as compared to seronegative recipients (23%–28% vs. 0.7%) [38]. In addition, donor derived post-transplant HHV8 transmission from seropositive donors to seronegative recipients has been described. Nevertheless, HHV8 seropositive individuals are not excluded from organ donation [6]. Preventive reduction of immunosuppression has been suggested in D+R− cases [23].

Lack of standardization of serological assays, variable sensitivity and specificity of these tests, and the absence of an algorithm for management according to serologic findings, result in low rates of pre-transplant screening in practice. In a survey including 51 transplant centers, only one-third performed pre-transplant HHV8 serology. High HHV8 seroprevalence (>6% seropositivity), Italian centers, available protocols for post-transplant viral load monitoring, and having had a recent case of HHV8 disease were associated with screening.

In a study that assessed six different serologic HHV8 assays the Biotrin–DiaSorin IFA and ABI IFA showed the highest agreement with a reference standard of ≥2 concordant positive assays [39].

No AST recommendations are available to direct a schedule of blood viral DNA monitoring in D+R− or R+ SOT recipients, beyond a general recommendation for monitoring in these patients [6]. In a recent survey, 41% of centers reported performing HHV8 PCR monitoring post-transplant, with variable indications including symptomatic patients only, risk-based approaches or universal screening [40]. In case of detectable HHV8 DNAemia, most centers reduced the immunosuppression or changed from calcineurin inhibitors (CNI) to m-TOR inhibitors, with or without addition of antivirals [40]. For viremic patients, guidelines suggest immunosuppression reduction or change to mTOR inhibitors. The rationale for the latter is the antiviral and antiangiogenic effects of sirolimus, though no clinical benefit has been demonstrated in studies [6]. Screening for viral DNA in bronchoalveolar fluid has been suggested for lung transplant recipients and is now under investigation (NCT05081141).

Immune monitoring by HHV8 ELISPOT test is used by some centers as adjunct to HHV8 PCR monitoring in high-risk patients [40]. Absent anti-HHV8 cytotoxic T-cell response has been demonstrated in SOT recipients with KS. It has been suggested that ELISPOT may assist in identifying patients at higher risk for developing KS, and if negative, reducing the immunosuppression may be considered [9].

Table 2 provides a proposed approach for screening or HHV8 pre and post-transplant.

The gold standard for diagnosing KS, MCD and PEL is histopathological examination of tissue. Immunohistochemical staining of HHV8 latency-associated nuclear antigen confirms the diagnosis. Tissue PCR for HHV8 may assist in confirming the diagnosis. There is no established role for peripheral blood PCR in diagnosing KS. Positive PCR supports the diagnosis of KS, however, it may be negative in ∼20% of KS cases [41]. Highest DNAemia levels were reported in MCD, followed by PEL. Hence, PCR may be more sensitive in these cases, and it has been suggested that negative HHV8 PCR may be used to exclude MCD [42]. High HHV8 DNAemia (>10,000 copies/mL) supports the diagnosis of MCD over KS [43]. In patients with PEL, high viral loads have been demonstrated in effusion fluids [42]. Due to limitations of serology discussed above, it is not currently indicated for diagnosis of HHV8 associated disease [44].

Patients with KICS are almost universally DNAemic. The diagnosis of KICS is based on high levels HHV8 DNAemia, exclusion of other possible causes, and possibly detection of HHV8 in involved organs (bone marrow, liver, and others) [9, 36]. A cutoff value of viral load in plasma ≥1,000 copies/mL or ≥100 copies/10⁶ cells in peripheral blood mononuclear cells has been suggested for diagnosis of KICS [36].

(Val)ganciclovir, cidofovir and foscarnet inhibit the replication of human herpes viruses, including HHV8. Among HIV patients, (val)ganciclovir proved to decrease the incidence of KS [45]. However, effectiveness of these drugs as pre-emptive therapy in cases of positive HHV8 PCR has not been demonstrated.

The need for a vaccine to prevent HHV8 associated malignancies in susceptible populations has been recently raised by the National Cancer Institute. Since the HHV8 genome is highly conserved, it is possible that a single vaccine would provide protection worldwide [46].

Treatment of HHV8 associated malignancies and non-malignant conditions in SOT recipients should first include reduction in immunosuppression (RIS) and/or change from CNI to mTOR inhibitors [47, 48]. Older studies demonstrated between 70% and 100% complete response (CR) of KS following a change from cyclosporin to sirolimus, and 20%–50% CR of KS with RIS [5, 38, 49].

In a more recent study, including 145 SOT recipients with KS, immunosuppression reduction with/without switch to mTOR inhibitors, resulted in a response in >80% of patients [5].

Systemic chemotherapy with an anthracycline or paclitaxel is usually required for KS patients with visceral involvement, extensive lymph node or mucocutaneous involvement, and for patients not responding to reduction/change in immunosuppression [6, 9]. Immunomodulatory therapy with interferon-α is avoided in the SOT setting because of the risk for rejection [50]. Specific chemotherapy regimens are routinely used for the management of MCD and PEL, in addition to immunosuppression reduction [9, 45].

Immunological (ELISPOT) and virological (HHV8 PCR) tests are suggested as part of follow up in the management of KS and other HHV8 related diseases [9].

Several antivirals have in-vitro activity against HHV8, including ganciclovir, foscarnet, and cidofovir, while acyclovir is not highly active [51]. Recent NIH guidelines for HIV management do not recommend antivirals as part of KS therapy, based on studies showing limited efficacy [45]. For the treatment of PEL, antiviral drugs may be used as a possible adjunctive therapy, with a CIII level of recommendation [45]. For MCD, two retrospective studies demonstrated remissions using ganciclovir as part of the treatment regimen in HIV patients [45]. This is supported by the rationale of lytic HHV8 infection being present in MCD [52]. There is limited data to support the use of anti-IL6 inhibitors for MCD with no recommendation for general use of these drugs for this indication [45]. Adoptive immunotherapy with cytotoxic T-lymphocytes specific for HHV8 could have a therapeutic role, though there is currently no commercial product available [9].

Epstein–Barr virus (EBV) is a double-stranded DNA virus of the γ-herpesviridae subfamily [53]. The virus was discovered in 1964 from cultured lymphoblasts of Burkitt’s lymphoma biopsies before being identified as the causative agent of mononucleosis in 1968 [54, 55]. EBV was the first known human oncogenic virus and it efficiently transforms human B-lymphocytes [56–58]. Upon infection, EBV establishes life-long latency in memory B-cells [59, 60]. The pathogenesis of EBV-associated oncogenesis is complex and it is related to the ability of the virus to transform and immortalize B-cells and to impede apoptosis of infected cells [53, 61]. EBV is associated with a large spectrum of diseases, including benign diseases (infective mononucleosis, oral hairy leukoplakia), a number of lymphoproliferative disorders (Burkitt’s lymphoma, some Hodgkin lymphomas, EBV-positive diffuse large B-cell lymphomas, natural killer/T-cell lymphoma, nasal type angiocentric lymphomas, chronic active EBV), epithelial cancers (nasopharyngeal carcinoma, some forms of gastric cancer), smooth muscle cell tumors, and diseases related to immune dysfunction (multiple sclerosis, EBV-associated hemophagocytic lymphohistiocytosis) [61, 62].

In SOT patients, EBV is known to play a major role in the development of EBV-positive post-transplant lymphoproliferative disorders (PTLD), one of the most devastating complications of organ transplantation [53, 63, 64].

Seroepidemiologic surveys indicate that >90% of adults are infected with EBV [65, 66]. In developed countries, primary EBV infection tends to occur later nowadays as compared to the past [67–69]. In the transplant setting, donor transmitted EBV infection is common in EBV mismatched (donor EBV+/recipient EBV−) patients. Children are more likely to be EBV-negative, and may acquire the virus from the donor organ or by natural infection, putting them at increased risk for post-transplant primary infection.

The diagnosis of PTLD is based on the histopathological examination of appropriate tissue biopsies. Assessing the presence of latent EBV infection of affected cells by (preferably) RNA-in-situ-hybridization targeting EBV-encoded small RNAs (EBER) or by immunohistochemistry targeting latent membrane protein 1 (LMP1) is essential for the diagnosis of EBV‐associated PTLD [102]. Preceding to tissue sampling, radiographic imaging is a crucial initial step to come to a tentative diagnosis. The radiographic evaluation is similar to that used in the evaluation of suspected lymphoma in the non-transplant population [103]. A computed tomography scan (neck to pelvis) is the first step in most centers. MRI may be the preferred modality for suspected cerebral PTLD [104]. Positron emission tomography‐computerized tomography has emerged as a useful imaging modality for detecting suspicious lymph nodes and extranodal lesions and may be helpful to identify optimal sites for biopsy [105]. Establishing a PTLD diagnosis can be difficult and occasionally multiple attempts for getting conclusive tissue biopsies are necessary (especially for gastrointestinal PTLD). In SOT recipients with persistent gastrointestinal symptoms, PTLD should be part of the differential diagnosis and endoscopy with biopsy of ulcers/lesions should be performed [106].

Studies evaluating the diagnostic test characteristics of EBV DNAemia measurements for diagnosing EBV-positive PTLD are limited. In summary, EBV DNAemia above a specific threshold has good sensitivity (∼90%) for detecting EBV-positive PTLD but lacks specificity [107–109] and EBV PCR is not useful for detection of EBV-negative PTLD.

Several antiviral drugs such as (val)acyclovir, (val)ganciclovir, cidofovir, foscarnet and maribavir inhibit lytic EBV replication [123, 124]. However, these drugs have no effect on latent EBV infection. Since primary EBV infection after transplantation is a major PTLD risk factor, reducing donor‐derived EBV transmission may have an impact on PTLD occurrence. A reduction of primary EBV infection was observed in a cohort of EBV seronegative pediatric KTRs on (val)ganciclovir prophylaxis versus no antiviral prophylaxis [125]. In another cohort of EBV mismatched adult KTRs, antiviral prophylaxis for 3–6 months delayed the rate of EBV primary infection at 100 days post-transplant, but the seroconversion rate 12 months post-transplant was identical with and without prophylaxis (72% vs. 74%) [126]. Recent cohort studies did not find a protective effect of antiviral prophylaxis on PTLD occurrence [72, 127]. These findings are consistent with results of a systematic review published in 2017, concluding that antiviral prophylaxis in high-risk EBV-naive patients has no effect on the incidence of PTLD [128].

The preemptive use of rituximab for prevention of PTLD has become a common strategy in EBV viremic hematologic stem-cell transplant (HSCT) [129, 130]. B-cell depletion before or directly after HSCT, has shown to reduce EBV replication [131, 132] and the incidence of EBV-positive PTLD [133–135] in high-risk patients. The potential effect of rituximab on subsequent PTLD development may be attributable to the depletion of CD20⁺ B-cells, which represent the major reservoir for latent EBV infection. The reduced abundance of these cells at risk for malignant transformation might be linked to a lower PTLD risk [72]. Rituximab use is less well established for prevention of EBV-positive PTLD in SOT recipients. A recent multi-center cohort study reported that rituximab given as part of the induction regimen (mostly in ABO-incompatible kidney transplantation) is associated with a decreased risk for PTLD [72]. A single‐center cohort study reported diminished PTLD rates with rituximab use in heart transplant recipients whose EBV DNAemia did not respond to RIS using a historic control group [75]. Similarly, EBV‐mismatched KTRs with persistent EBV DNAemia or symptomatic EBV infection given rituximab simultaneously with RIS were less likely to develop PTLD compared to contemporaneous controls [114].

The first therapeutic measure in treatment of PTLD is RIS under close monitoring of the graft function. There are no evidence-based guidelines on how to reduce immunosuppression, but in clinical practice, stopping anti-proliferative agents and dose reduction of the CNI is the common approach [90]. Significant RIS may not be feasible in all cases and is especially difficult to achieve in thoracic organ transplant recipients due to the risk of life-threatening graft rejection [136]. RIS eradicates the majority of non-destructive PTLD cases. However, for polymorphic and monomorphic PTLD the response to RIS alone is often insufficient [137, 138]. A radiologic reassessment is performed two to 4 weeks after RIS, and if a CR is achieved no further treatment is needed.

In the following section, we summarize the treatment options for polymorphic PTLD and monomorphic diffuse large B-cell lymphoma (DLBCL) PTLD. Treatment of non-DLBCL monomorphic PTLD depends on the histologic classification of the respect lymphoma and follows the same chemotherapy regimens as for immunocompetent patients, and will not be reviewed here. Immunochemotherapy for treatment of DLBCL PTLD is associated with significant toxicity and many SOT recipients are not fit for highly intensive regimens [139]. Therefore, sequential and risk-stratified treatments are applied for treatment of CD20⁺ monomorphic DLBCL PTLD. The PTLD-1 [140], the PTLD-1 third amended [141] and PTLD-2 [142] phase 2 trials are landmark studies that established sequential, risk-stratified PTLD treatment modalities. The PTLD-1 study proved the efficacy and safety of a sequential treatment of four cycles rituximab monotherapy followed by four cycles of cyclophosphamide, doxorubicin, vincristine, and prednisone (CHOP) for patients who did not achieve complete remission with RIS [140]. The most favorable outcomes were seen in patients who achieved a response to rituximab alone prior to chemotherapy, indicating that these patients may belong to a “good-risk group”. In consequence, the PTLD-1 third amended trial assessed a risk-stratified protocol: patients who achieved a CR after four doses of rituximab, received consolidation with rituximab alone while those who did not achieve CR were treated with four cycles of R-CHOP [141]. This trial proved that withholding chemotherapy and performing a rituximab consolidation in patients with CR to rituximab alone is safe and associated with less toxicity [141]. The PTLD-1 third amended trial identified two subgroups with poor prognosis: thoracic transplant recipients and patients with an International Prognostic Index (IPI) score >2 [141]. The PTLD-2 trial, inter alia, assessed treatment escalation (alternating R-CHOP and R-DHOAx- rituximab, dexamethasone, cytarabine, oxaliplatin) in these patients with poor prognosis [142]. However, the number of patients in this subgroup was low (n = 9), the outcome was poor and the treatment related toxicity was substantial [142].

For further information about novel, less established PTLD treatment options such as infusion of third-party EBV-specific cytotoxic T-lymphocytes, CAR-T cell therapy, proteasome inhibitors, burton-tyrosine kinase inhibitors, and histone deacetylase inhibitors in combination with antiviral nucleoside analogues we refer to the recent review of Atallah-Yunes et al. [143].

The introduction of rituximab, the administration of sequential risk stratified treatment regimens, and optimized supportive care have improved the outcome for patients with PTLD. In the PTLD-1 trial, the median overall survival was 6.6 years [140]. Patients with a CR to rituximab alone have better prognosis as compared to rituximab non-responders [141] and thoracic transplant recipients show less favorable outcome as compared to non-thoracic transplant recipients [141, 142].