Arming CTLs against immunosuppressors

Cell therapy efficacy after organ transplantation is hampered by the detrimental effects of pharmacologic immunosuppression on T-cell expansion and function. In this issue of Blood, 2 independent groups describe genetic engineering of ex vivo expanded EBV-CTLs to induce calcineurin-inhibitor resistance.

The clinical application of potent immunosuppressive regimens has significantly improved solid organ transplantation (SOT) outcome by facilitating grafting across histoincompatibility boundaries. Long-term graft survival, however, has been obtained at the expense of severe impairment in immune surveillance. Consequently, over the past 2 decades, an increasing incidence of infections and secondary malignancies has been observed. Among these, posttransplantation lymphoproliferative disease (PTLD) associated with Epstein-Barr virus (EBV) infection contributes significantly to SOT morbidity and mortality, accounting for as much as 70% of total malignancies in pediatric transplant recipients due to the prevalence of EBV-naiveté in this cohort. Conventional treatment of PTLD, mainly based on strategies aimed at reducing the tumor burden, such as cytotoxic chemotherapy and B cell–directed monoclonal antibodies, is still associated with either toxicity and/or unsatisfactory response rates.¹  Alternative approaches aimed at restoring virus-specific immunity have so far been limited to a reduction in maintenance immunosuppression, a therapeutic strategy that has shown some success only in early polyclonal lesions, and may be burdened with an increased risk of acute graft rejection or chronic allograft damage.

Evidence derived from trials conducted in recipients of hematopoietic stem cell transplants indicate that adoptive transfer of antigen-specific T cells can restore protective immunity and control viral complications, including PTLD.¹  Recently, this strategy has been applied in the setting of organ transplantation. EBV-specific cytotoxic T lymphocytes (EBV-CTLs), derived from patients^2-4  or selected from banked third-party donor CTLs on the basis of the best HLA class I antigen match,⁵  were shown to be effective in preventing PTLD onset or controlling established disease also in SOT recipients receiving life-long maintenance immunosuppression. However, although clinical evidence is only preliminary, response rates are not as encouraging as in the HSCT cohort,⁵  and follow-up data, as might be expected, indicate that the transferred EBV-CTLs do not persist long-term.^3-5 

To achieve clinical efficacy in case of disseminated PTLD, it has been proposed to deliver EBV-CTLs at completion of rituximab/chemotherapy treatment, while patients are temporarily off immunosuppression.⁶  As an alternative elegant approach, in this issue of Blood, 2 independent groups have succeeded in generating EBV-CTLs resistant to calcineurin inhibitors, pharmacologic agents that represent the cornerstone of immunosuppressive therapy used to prevent immune-mediated allograft damage. The drugs cyclosporine-A (CsA) and tacrolimus (FK506) exert their immunosuppressive function by binding to cyclophilin (CyPA) and FK binding protein-12 (FKBP-12), respectively. The complexes thus created prevent the calcium-sensitive phosphatase calcineurin from binding to the transcription factor NFAT, thus inhibiting activation of cytokine genes in T cells. To counteract these immunosuppressive effects, De Angelis and colleagues knocked down the expression of FKBP-12 in EBV-CTLs using a specific small interfering RNA stably expressed from a retroviral vector and found that FKBP-12–silenced EBV-CTLs are FK506-resistant in vitro, and in vivo in a xenograft model.⁷  Brewin and colleagues focused on the interaction between calcineurin and the complexes FK506/CsA-immunophilins. They produced calcineurin mutants at key amino acid residues for binding of either or both FK506–FKBP-12 and CsA-CyPA, or, in an alternative approach, mutated FKBP-12 to disrupt its interaction with calcineurin, and demonstrated that EBV-CTLs expressing such mutants retain the ability to proliferate and secrete interferon-γ in vitro in the presence of therapeutic levels of FK506 and/or CsA.⁸ 

Although these results are promising, hurdles may still stand in the way of successful prophylaxis or treatment of EBV-PTLD after SOT. In addition to limitations common to other clinical settings, such as antigen and cytokine milieu dependency, and tumor escape mechanisms, allograft recipients usually receive combined pharmacologic immunosuppression, including mycophenolate and steroids in addition to calcineurin inhibitors. Therefore, in vivo persistence and expansion of transferred T cells in SOT recipients could require resistance to multiple classes of immunosuppressors. Notwithstanding the envisioned obstacles, the strategy described in this issue of Blood represents a significant technological advance for the field of cellular therapy that could reduce the morbidity and mortality associated with PTLD (and other viral infections) in patients with iatrogenic immune deficiency.