We describe 15 children of various racial and ethnic backgrounds with T cell-EBV-HLH. East Asian populations have previously been reported to have a high incidence of T cell-EBV-HLH and T/NK cell-mediated EBV-lymphoproliferative disorder (LPD)[10, 13, 17, 18]. In our cohort, nine of the 15 patients were Hispanic suggesting that T cell-EBV-HLH may be under recognized in this group. Limited reports exist on the prevalence of T and NK cell-based EBV-LPD disorders in patients of Central and South American backgrounds [19-21]. Beutel et al. described seven German patients with T cell-EBV-HLH . Our study demonstrates that T cell-EBV-HLH can be found in Caucasian and African American patients as well, albeit at a lesser frequency. Recent reports of Hydroa vacciniforme-like LPD in Caucasians are suggestive that T cell-EBV-LPD can also be seen in Caucasians . The clinical spectrum of T/NK-mediated EBV-LPD is broad [18, 21]. In our study, majority had HLH as the presenting manifestation of underlying EBV-driven LPD. Only one had history of NK/T cell lymphoma, and none had history of CAEBV.
The reason why T cell-EBV-HLH is more common in East Asian and Hispanic populations is not apparent and the genetic basis is not well defined. Though monogenic diseases associated with increased EBV susceptibility are identified, most tend to involve EBV affecting the B cell compartment [23-26]. In our cohort, evaluation for classical p-HLH genetic defects was negative in most patients. One patient had two RAB27a variants in trans along with a decrease in CD107a degranulation, suggesting there might have been a component of CTL defect similar of p-HLH. Though the vast majority of atypical T/NK cell-based EBV-LPD has no known monogenetic defects, case reports allude to the presence of a classical cytotoxic pathway defect in some patients [8, 27].
NK function was noted to be decreased in five of the six patients tested. This is despite having a higher expression of perforin and granzyme B in the majority of the tested patients. Degranulation was also noted to be decreased in some patients, suggesting there could be an acquired degranulation defect due to EBV infection of T cells in some patients with T cell-EBV-HLH. The findings of decreased NK cytotoxicity and decreased degranulation could be due to EBV-induced alterations such as immune exhaustion, acquired defects in degranulation, co-stimulation and cytotoxicity . Interestingly, we found that in two of the three tested patients, CTL cytotoxicity assessed against EBV-immortalized B cells showed normal cytotoxicity. In our limited patient data, we have shown both T cell-EBV-HLH and p-HLH have high PD-1 expression, which might partly be the basis for decreased cytotoxicity.
Utilizing T cell activation biomarker, sIL2R, and direct T cell activation markers such as HLA-DR and CD38, we have shown that the amplitude of T cell activation in patients with T cell-EBV-HLH was comparable to patients with p-HLH . When compared to other forms of secondary HLH, such as other infection associated HLH and MAS, the extent of T cell immune activation is significantly higher. These findings suggest that despite the absence of classical genetic defects in granule-mediated cytotoxicity, the amplitude of T cell activation in T cell-EBV-HLH is similar to that seen in p-HLH. Higher acuity, morbidity, and mortality in T cell-EBV-HLH when compared to other infection-associated HLH or MAS could be in part due to higher T cell activation noted in this form of HLH. Transient suppression of EBV-induced NK and T cell cytotoxic functions could lead to a hyper-inflammatory state similar to that seen in p-HLH.
EBV PCR flow in sorted cell populations is a more definitive modality to identify T cell-EBV-HLH. However, an alternate strategy, such as dual staining for CD3 and EBER and EBER flow FISH  could also help establish T cell-EBV. The findings of persistently high EBV viral load despite empiric rituximab could also offer a clue to practitioners that EBV may not be predominantly in the B cell compartment. Differentiating this from B cell-EBV-HLH is critical, as most B cell-EBV-HLH responds to rituximab-based treatment regimens . In contrast, T cell-EBV-HLH usually has a poor response to rituximab. In our cohort, EBV clearance in many patients was achieved with T cell directed therapy alone without using rituximab. As rituximab holds a possible risk of post-treatment persistent hypogammaglobulinemia and recurrent infections , its use could be restricted in cases of T cell-EBV-HLH.
Alemtuzumab, a monoclonal anti-CD52 antibody which causes mature T cell depletion, was suggested as a reasonable salvage therapy for HLH . Two patients in our cohort had clinically deterioration while receiving alemtuzumab therapy. The mechanism for this observation remains unknown. A recent report by Liu et al. demonstrated the efficacy of nivolumab in patients with relapsed/ refractory T cell EBV HLH . It is important to note that only two of the seven patients in that cohort had sIL2R>2000 U/mL. Similarly ferritin of more than 2000 ng/mL was noted in only two of the seven patients . Based on this observation, it appears that Liu et al. used nivolumab in EBV-HLH patients with only modest immune activation. In our patients, relapse or recurrence was characterized by a rapid increase in sIL2R to more than 20,000 U/mL, and in many, ferritin was >100,000 ng/mL. The overwhelming immune activation might be the reason why nivolumab did not have a favorable response in our patient.
In our cohort, patient’s refractory to etoposide responded to combination chemotherapy regimens consisting of cyclophosphamide and doxorubicin. It is hard to predict which T cell-EBV-HLH patients need aggressive therapy, including etoposide and/or a combination chemotherapy followed by HSCT, and which patients could receive a milder immune modulation and expectant management. It is worth noting that unlike most patients with CAEBV who require HSCT for a definitive cure, a significant proportion of patients with T cell-EBV-HLH can remain in remission without proceeding to HSCT. The finding that a proportion of patients with T cell-EBV-HLH can do well without needing HSCT aligns with similar findings by Sawada et al. in Japanese patients . The biological basis for the difference in long-term management of T-cell EBV HLH and CAEBV is still poorly understood. However, it is known that in-T cell-EBV-HLH, the cellular target for EBV is usually CD8+ T cells compared to CD4+ T cells in CAEBV [34, 35]. Thus, the management of all T/NK cell-based EBV-LPD is not the same.
In our study, the acuity of presentation seems to be an important predictor of long-term outcome in T cell-EBV-HLH. Patients who are not critically ill at presentation needed less immune modulation and had an excellent long-term survival without needing HSCT. Additionally, even among those presenting with high disease acuity and critical illness, a significant proportion of patients are long-term survivors without HSCT. It is important to note that one patient had a relapse of EBV-HLH despite having 100% donor cells in T cell compartment.
Despite this being one of the largest cohorts of T cell-EBV-HLH in children in non-Asians, the single-center experience and retrospective nature of the study limits its generalizability. Larger, prospective multi-institutional studies are needed to understand the biological basis, risk factors and define treatment paradigms for this potentially fatal disorder.
In conclusion, this study provides a detailed description of non-Asian patients with T cell-EBV-HLH in the United States, highlighting the presence of this disease in patients from a variety of racial and ethnic backgrounds. We also show that the amplitude of T cell activation in patients with T cell-EBV-HLH is similar to patients with p-HLH. Therefore, early diagnosis of T cell-EBV-HLH is critical. Staged management based on the acuity of initial presentation and response-based escalation of therapy, including HSCT, is also warranted. Non-critically ill patients, who responded well to initial immunosuppressive therapy, may not need HSCT. Patients, who are refractory to combination chemotherapy or other salvage therapies, experience high mortality, and emergent allogeneic HSCT may be indicated. Further prospective studies to enhance our understanding of immunobiology, genetic basis and risk stratification-based therapies for patients with T cell-EBV-HLH patients are needed to validate our results and improve outcome for this potentially fatal disorder.