In a recent issue of The Journal of Clinical Investigation, Prockop et al1 report their 10-y experience of the use of “off-the-shelf” third-party Epstein-Barr virus (EBV)-cytotoxic T lymphocytes (CTLs) for the treatment of EBV+ posttransplant lymphoproliferative disorders (PTLD). EBV-CTLs from a cryopreserved cell bank were matched with patients for at least 2 of 10 HLA alleles (HLA-A, B, C, DR, or DQ) and restricted by HLA alleles shared by the EBV+ lymphoma, donor and patient (hematopoietic stem cell transplant [HSCT]), or the patient (solid organ transplant [SOT]). The cells were safely administered at 1 × 106 CTL/kg or 2 × 106 CTL/kg every 3 wk to 46 high-risk recipients of an HSCT (n = 33) or SOT (n = 13) with rituximab-refractory B-cell lymphoma (80% monomorphic diffuse large B cell). With the exception of an isolated case of grade 1 skin graft-versus-host disease, the CTLs were well tolerated.
PTLD, first coined by Starzl et al2, represents a life-threatening complication for 2%–10%3 of transplant recipients and a significant clinical challenge for HSCT and SOT physicians. The use of adoptive T-cell therapy for EBV-associated PTLD is not a new concept and dates back >25 y. Unmanipulated donor-lymphocyte infusions from EBV+ transplant donors were shown to elicit good response rates post HSCT at the risk of significant alloreactivity.4,5 Donor-lymphocyte infusions were then superseded by donor-derived and autologous EBV-CTLs post-HSCT6 and -SOT,7 respectively. Widespread application of this approach was limited by time constraints in patients with rapidly progressive disease, complex manufacturing protocols, and infrastructural requirements. In an attempt to move away from bespoke products, the field has turned to readily available banked and partially matched allogeneic EBV-CTLs (with8 or without9 HLA-restriction). The work by Prockop et al further consolidates understanding of this field, bringing together previously presented work from 2 studies detailing outcomes of patients treated between 2005 and 2015. Detailed analyses of individual cases provide the basis for some interesting hypotheses, which can now be tested more thoroughly.
The authors report an overall response rate of 39% (18 of 46) after a single cycle of allogeneic EBV-CTLs. Notably, best overall response rates of 68% and 54% were ultimately achieved after a median of 2 cycles (range 1–5) for HSCT and SOT patients, respectively. These response rates are comparable to those reported for donor-derived EBV-CTLs,10 albeit slower to achieve. The authors postulate that this may be reflective of brief CTL engraftment and enhanced immunologic clearance of partially matched cells, prompting a repeated dosing schedule. This is also supported by the lower complete remission (CR) rate in SOT recipients (15% versus 58%), a more T-replete cohort. Durable responses despite brief engraftment (a small subset have sustained engraftment) may be attributable to the recruitment of endogenous T-cells, thereby augmenting clinical efficacy. Of those who received a second cycle (n = 25), the same EBV-CTL cell line (n = 16), a different donor with the same shared HLA-restriction (n = 3), or “switch therapy” (n = 6) was administered. “Switch therapy” was defined as the use of an alternate EBV-CTL cell line specific for a different epitope and restricted by another HLA allele shared by the patient’s tumor. Despite the small number of cases, the authors have demonstrated that this approach may prove efficacious for refractory disease, 4 of 6 achieving a CR (n = 1), a partial remission (PR, n = 1), a maintained PR (n = 1), or stable disease (n = 1). Overall survival at 2 y was 57% and 54% for HSCT and SOT patients, respectively, but reaching 83% for those who achieved a CR or PR after the first cycle, identifying an early favorable prognostic marker in a high-risk group.
The article offers valuable insights into the potential mechanisms of allogeneic EBV-CTLs failure. Unsurprisingly, multiple sites of disease, extranodal involvement, prior chemoimmunotherapy, and relatively lower expansion of CTLs in vivo were associated with lower response rates, as previously documented. Poor in vivo cell expansion may be explained by early alloimmune rejection and immune evasion by EBV, mechanisms of which may include silencing of HLA alleles, epitope variation and loss, evasins affecting antigen processing and presentation, and direct inhibition of T-cell function. Contrary to previous reports,9 as the degree of HLA matching did not correlate with response, Prockop et al advocate that prioritizing HLA-restriction (including the selection of EBV-CTLs restricted by >1 HLA allele) may be more appropriate in these patients. Failure to respond to therapy could be detected clinically as early as 8–15 d postinfusion and was associated with a poor overall survival, reflecting the aggressive nature of this disease. Early “switch therapy” can be considered in those with evidence of early disease progression after a single cycle.
Despite the inclusion of a small heterogeneous cohort of high-risk patients, the authors have demonstrated that third-party EBV-CTLs represent a credible and safe therapeutic option for patients who have failed Rituximab for EBV+ PTLD. Like many early experiences in the field of antiviral therapy, definitive proof of efficacy will require appropriate phase 3 evaluation (NCT03392142, NCT03394365) and the results of these studies have the potential to be game changing. Questions raised by this article will likely inform future studies in this field, namely circumventing the mechanisms of immune evasion by EBV, exploring the utility of switch therapy in refractory patients, and understanding the long-term persistence and efficacy of allogeneic EBV-CTLs in selected patients. Gene-modified and edited T-cells may circumvent the well-recognized challenges in these complex patients. Transduction of allogeneic EBV-CTLs with chimeric antigen receptors targeting cluster of differentiation (CD)19 (NCT01430390) or CD30 (NCT04288726) may facilitate the eradication of both EBV+ and CD19+/CD30+ tumor cells. T-cell editing to confer resistance to tacrolimus (NCT03131934) may allow the continuation of immunosuppression, hence minimizing the risk of graft rejection in SOT. Finally, the success of programmed cell death (PD)-1 blockade in solid tumors has prompted the addition of PD-1 inhibitors (NCT02973113) or PD-1 knockout using CRISPR-Cas9 (NCT03044743) to treatment algorithms in the hope of augmenting the antitumor response of EBV-CTLs. Although many questions remain, this work by Prockop et al supports further refinement and evaluation of all these strategies.
1. Prockop S, Doubrovina E, Suser S, et al. Off-the-shelf EBV-specific T cell immunotherapy for rituximab-refractory EBV-associated lymphoma following transplantation. J Clin Invest. 2020; 130:733–747doi:10.1172/JCI121127
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