Viral infections are normally controlled by T-cell immunity and are a cause of significant morbidity and mortality during the period of immune recovery after hematopoietic stem cell transplantation (HSCT). Risk for infection is impacted by the degree of tissue mismatch between donor and recipient and the immune status of the donor, including the degree and length of immunosuppression following transplantation. Reactivation of latent viruses such as cytomegalovirus (CMV), Epstein-Barr virus (EBV), and Human Herpesvirus 6B (HHV6) are common and often cause symptomatic disease. Reactivations of the polyomaviruses BK virus and JC virus are also common and frequently cause renal disease including hemorrhagic cystitis and less commonly neurologic disease (pervasive multifocal leukoencephalopathy). Respiratory viruses such as adenovirus and human parainfluenza also frequently cause infection. Antiviral pharmacologic agents are only effective against some of these viruses; their use is costly, and associated with significant toxicities and the outgrowth of drug-resistant mutants. As delay in recovery of virus-specific cellular immune response is clearly associated with viral reactivation and disease in these patients, cellular immunotherapy to restore viral-specific immunity is an attractive option that has already been successfully used to target several of these viruses.

To broaden the specificity of single T cells lines to include the three most common viral pathogens of stem cell recipients, the investigators reactivated CMV and adenovirus-specific T cells by using mononuclear cells transduced with a recombinant adenoviral vector encoding the CMV antigen pp65 (Ad5f35CMVpp65). Subsequent stimulations with EBV-LCL transduced with the same vector both reactivated EBV-specific T cells and maintained the expansion of the activated adenovirus and CMV-specific T cells. This method reliably produced T cells with cytotoxic function specific for all three viruses, which the investigators infused into 14 stem cell recipients in a Phase I prophylaxis study. The investigators observed recovery of immunity to CMV and EBV in all patients but an increase in adenovirus-specific T cells was only seen in patients who had evidence of adenovirus infection pre-infusion. A follow-up study in which the frequency of adenovirus-specific T cells was increased in the infused T cells produced similar results, thus highlighting the importance of endogenous antigen to promote the expansion of infused T cells in vivo. Nevertheless, all patients in both clinical trials with pre-infusion CMV, adenovirus or EBV infection or reactivation were able to clear the infection, including one patient with severe adenoviral pneumonia requiring ventilatory support. T cells recognizing multiple antigens can therefore produce clinically relevant effects against all three viruses.

Recent studies have extended the number of targeted viruses, and included HHV6B, BK virus, and Varicella-zoster virus (VZV). In a recent study, 11 patients were treated with VST targeting 5-viruses (CMV, EBV, Adv, HHV6B, BKV) which were generated using a rapid protocol with overlapping peptides encompassing 12 viral protein. VST infusion resulted in a 94% antiviral response rate in these patients (complete or partial responses against CMV=3/3, EBV=5/5, Adv=1/1, HHV6B=2/2, BKV=6/7). Two of the patients who received 5-virus VST developed transplant-associated microangiopathy, which was deemed secondary to HSCT and unrelated to VST infusion. One of these patients developed grade II skin GVHD, which improved with topical therapy. In another recent study, ten adult patients were prophylactically treated with VST specific for CMV, EBV, Adv, and Varicella (VZV). These VSTs were generated using donor-derived dendritic cells which were infected with either Ad5f35-pp65 or with varivax vaccine, and were then pooled and used to stimulate donor PBMCs. All ten patients were protected against EBV, Adv, and VZV. Six patients developed CMV reactivation, but only one required antiviral therapy. Of these 10 patients, 7 developed acute or chronic GVHD, though compared to a non-treated group at the same institution, the rate of GVHD did not differ significantly. Thus, it has been possible to target an extended panel of viruses with a single VST product.