T-Cell Therapy for Viral Infections Following Transplantation: Why Stop at Three Viruses?

The prolonged recovery phase of cell-mediated immunity after allogeneic hematopoietic stem cell transplantation (SCT) brings serious and often fatal risks from opportunistic reactivation or acquisition of common viruses.¹ Post-transplant immunodeficiency particularly involves defective T-cell function, and it is now established that adoptive transfer of viral-specific T cells can prevent and treat viral complications.^2,3,4,5 In this issue, Gerdemann and colleagues describe a technique to generate clinical-grade virus-specific T-cell lines targeting seven viruses that commonly cause morbidity and mortality after SCT.⁶

Many DNA viruses enjoy lifelong latency in the tissues of the individuals they infect. After SCT, reactivation of cytomegalovirus (CMV) causing enteritis and pneumonia, Epstein-Barr virus (EBV) causing post-transplant lymphoproliferative disorder, and reactivation or new infection from adenovirus (Ad) causing enteritis, hepatitis, hemorrhagic cystitis, and pneumonia, account for a significant proportion of the serious complications that follow SCT.⁷ However, other viruses also wreak havoc in the immunodeficient recipient—notably, BK virus causing hemorrhagic cystitis; respiratory syncytial virus (RSV) and influenza, causing often fatal pneumonias; human herpesvirus 6 (HHV6), causing cytopenias and encephalitis; and varicella zoster virus (VZV), causing painful exanthems with long-term consequences.^8,9 Certainly the outcome for viral complications after SCT has been much improved following the introduction into clinical practice of the powerful antiviral agents foscarnet and ganciclovir/valganciclovir for CMV and HHV6, acyclovir/valacyclovir for herpes simplex virus and VZV, and the CD20-specific monoclonal antibody rituximab for post-transplant lymphoproliferative disorder.^10,11,12 Nevertheless, although difficult to compute, the downstream complications of virus prophylaxis and treatment (which include renal failure, cytopenia, and loss of efficacy from viral resistance) contribute significantly to transplant-related mortality. More effective prevention and treatment of the slew of viruses that typically complicate SCT could certainly reduce post-transplant hospital stays and probably improve transplant-related mortality by perhaps as much as 10% in high-risk individuals.

In the 1990s, pioneering trials using adoptive transfer of virus-specific T cells after SCT first addressed the control of CMV and EBV and demonstrated proof of principle that reactivation could be efficiently prevented or treated by transfer of virus-specific T cells generated in the donor.^13,14 Since then, the ability to reliably generate virus-specific T cells to a readily applicable good manufacturing practice–grade quality has improved enormously. Furthermore, the concept of generating multivirus-specific T-cell lines has been developed by the group at Baylor College of Medicine, who have reported extensively on the efficacy of trivirus (EBV, CMV, and Ad) T cells in combating these viruses.¹⁵ The paper by Gerdemann et al. describing production of multivirus-specific T cells covering seven viruses (CMV, EBV, Ad, BK, RSV, VZV, HHV6) is a logical extension to this approach.⁶ To generate multivirus-specific T cells, this group previously used B lymphocytes expanded and immortalized by EBV, transfected with gene-modified Ad capsid incorporating CMV pp65 antigenic DNA, to create a hybrid B cell presenting EBV, CMV, and Ad antigens to T cells.¹⁵ Although this approach is effective, it is limited by the competence of B cells as antigen-presenting cells and by the requirement for stringent regulatory oversight required for gene-modified cells. Their current approach employs dendritic cells (DCs) as reliable professional antigen-presenting cells and avoids the regulatory complications associated with genetic manipulation. For each virus, two or three immunodominant antigens were selected. An overlapping pool of 15mer peptides was generated for each protein. These peptide pools were then mixed together to create a single multimix of antigens to incubate with DCs for subsequent antigen presentation to T cells. A potential problem of this strategy is overcrowding of T-cell responses to particularly powerful antigens (anticipated for CMV and EBV). Fortunately, outcompetition of weaker antigens was not observed in this study.

Critical to successful induction and expansion of T cells was the cytokine mix of interleukins IL-2, IL-5, IL-4, and IL-7, thereby enabling the generation of detectible frequencies of multivirus-specific T helper type 1 polarized T cells within 10 days. Including the time required for generation of DCs, the entire manufacturing process takes 14 days, yielding a clinically useful product expanded 10-fold to 10⁸ cells and enriched 10-fold for virus specificity.

It should be noted that, although donor-derived multivirus-specific T cells have proven efficacious for the three viruses CMV, EBV, and Ad,¹⁵ it has yet to be shown that the new technique will work for the additional four viruses. T-cell immune responses to BK, HHV6, and RSV are not as well characterized. Hence, although the selected antigens chosen are clearly potent inducers of T-cell responses in vitro, it is not certain that they are the key biological targets of immune attack in vivo. A clinical trial demonstrating that T cells specific for the selected BK, HHV6, RSV, and VZV antigens are effective in controlling these viral infections is the next step. A second limitation, which this study does not address, is that the technique is not optimized for generating virus-specific T cells from naive donors (because all donors had immunity to the chosen viruses). This is an important point because the most life-threatening viral complications occur in recipients whose donors have not yet acquired immunity to the reactivating virus. Although strategies exist to generate CMV-specific T cells from CMV-naive donors,^16,17 effective approaches have yet to be developed for the newer viruses described by Gerdemann et al.

These limitations aside, manufacturing universally effective antiviral T cells is an important milestone in the journey toward the wider application of adoptive T-cell therapy. Provided that a good manufacturing practices facility is available, the cost of cell manufacture already compares favorably with the cost of antiviral treatment as well as the expense of prolonged hospitalization for treatment complications or failure. A global approach that promises with a single cell product to comprehensively protect patients against most post-SCT viral complications is of enormous practical importance. It is to be hoped that this research will encourage manufacturers, who normally avoid cell therapy because of its “boutique” patient-specific nature, to consider production of a widely applicable treatment. Before that can happen, we must await the results of clinical trials with these broadly specific multivirus-specific T-cell products. Critical to the more general adoption of this adoptive T-cell therapy approach will be to demonstrate not only efficient prevention of complications from the common post-SCT viruses but also an economic benefit resulting from reduced treatment intensity with fewer post-transplant hospitalizations or intensive care admissions and reduced pharmacy costs. Unfortunately, we have few comprehensive data on the current impact of viral reactivation on these economic parameters. Now is the time to collect the information needed to validate the use of antiviral T-cell therapy through their incorporation into the pipeline of clinical trials.