Moving Successful Virus-specific T-cell Therapy for Hematopoietic Stem Cell Recipients to Late Phase Clinical Trials

Abstract

There are now multiple effective strategies for the activation and expansion, or direct selection of virus-specific T cells that may be able to eliminate a range of virus infections in the immunocompromised host. In this mini-review, we explain how the stage is set for their rigorous evaluation in large-scale clinical trials. Virus infections are the cause of 30% or more of transplant-related deaths in recipients of T-cells depleted, allogeneic hematopoietic stem cell transplants (HSCT).1 Epstein-Barr virus (EBV), cytomegalovirus (CMV), and adenoviruses (AdV) are the most common culprits, but other common viruses like parainfluenza virus, respiratory syncytial virus, influenza viruses, polyomaviruses, and human herpesvirus 6 together contribute significant morbidity and mortality (Figure 1).2,3,4 Small molecule therapies are often ineffectual, always costly and frequently produce significant adverse effects. Figure 1 Relative frequency of viral infections after HSCT. Adv, adenovirus; BK, JC, KI, and WU (polyomaviruses); CMV, cytomegalovirus; H1N1, influenza strain Hemagglutinnin 1 Neuraminidase 1); HHV, human herpes virus; HSCT, hematopoietic stem cell transplant; ... Virus-specific T-cells derived from stem cell donors can prevent and treat post-transplant viral infections, in the recipients for whom they were intended, and also in partially human leukocyte antigen (HLA)-matched, third party recipients.5,6,7 The low toxicity and long-term protection provided by virus-specific T-cells compares favorably with the significant toxicities and short-term effects of most antivirals.5,6,8,9,10 Is it time, therefore, to begin the transfer of T-cell manufacturing from academic phase I/II clinical trials into hospital or industry-supported facilities so that virus-specific T-cells can be made available to all high-risk HSCT recipients? Several barriers prevent the broader use of virus-specific T-cell therapies after stem cell transplantation. While T-cell therapies for EBV, CMV, and AdV have clearly demonstrated their safety and efficacy both as prophylaxis and as therapy, for many other viruses, the antigens that induce protective T-cells have yet to be identified. Moreover, these infections may occur in <5% of patients making it difficult to perform the rigorous comparative effectiveness studies that will be required to show lower overall cost, fewer adverse effects, and equivalent or superior efficacy. Before any of these barriers can be breached effectively by the academic institutions who are the major developers of these T-cells therapeutics, we must select and optimize manufacturing strategies that are robust and scalable and have the lowest possible cost. Finally, the leap to late phase trials cannot be accomplished by academic institutions alone, but requires partnership with industry. This article will largely deal with selection and optimization of virus-specific T-cell manufacturing strategies.