Induction of Cytomegalovirus-Specific T Cell Responses in Healthy Volunteers and Allogeneic Stem Cell Recipients Using Vaccination With Messenger RNA–Transfected Dendritic Cells

Infection with human cytomegalovirus (CMV), a member of the β-herpesvirus family, is a significant cause of morbidity and mortality in solid organ and hematopoietic stem cell transplant (HSCT) recipients.1‐5 The virus is present in more than two thirds of donors and recipients before transplantation.6,7 The overall risk of developing clinically relevant CMV disease is mainly determined by baseline CMV-specific serology from donor and recipient as well as the intensity of the immunosuppressive regimen. In CMV-seropositive recipients, CMV infection can be the result of reactivation of latent or persistent virus or superinfection with a different strain of CMV.8 In CMV-seronegative recipients, CMV disease can result from a primary infection when receiving an allograft from a CMV-seropositive donor. After primary infection, CMV persists for the lifetime of the infected carrier. In immunocompetent individuals, this state of latency is effectively controlled by the immune system as evidenced by a low viral load as well as a strong CMV-specific T-cell–mediated cellular immune response against certain immunodominant targets, such as the CMV pp65 protein.9,10 In contrast, given the suppressed T-cell function in immunocompromised patients, there is a significant and unmet need for new immunotherapeutic strategies to reestablish appropriate immune control of CMV. In this perspective, first randomized clinical trials with the Town CMV vaccine, an active vaccination strategy using live-attenuated virus strategies, demonstrated induction of a protective immune response with concomitant protection against CMV disease in renal transplant recipients.11 Despite encouraging clinical results, this strategy was abandoned because of long-term safety concerns associated with the use of live herpes viruses in the transplant population. Subsequent studies primarily focused on the generation of anti-CMV antibody titers in immunocompromised hosts.12,13 In a placebo-controlled phase II study, safety and efficacy of a CMV envelope glycoprotein B (gB)-based vaccine supplemented with MF59 adjuvant was demonstrated in seronegative women of child-bearing age.14 Griffiths and colleagues confirmed that the administration of this vaccine resulted in a significant increase of the gB antibody titer in both CMV-seronegative and CMV-seropositive adults awaiting kidney or liver transplantation.15 However, this finding only translated in a clinical benefit, that is, reduced duration of viremia, in CMV-seronegative recipients transplanted with grafts from CMV-seropositive donors. It was suggested that for long-term control of the virus, CMV-specific T cells are also important for immune protection against CMV.16 Whereas passive immunization by adoptive transfer of CMV-specific T cells has already been successfully applied to HSCT recipients,17,18 the clinical usefulness of this approach is rather limited because of the cumbersome and time-consuming logistics of CMV-specific T-cell cloning and expansion. Moreover, the technique of adoptive T-cell transfer cannot be applied in the context of solid organ transplantation, in which active immunization protocols may be preferable.4,19 Others have designed replication-deficient viral vectors encoding CMV antigens to expand T cells directed against viral-encoded antigens. Indeed, in an attempt to address both humoral and cellular immunities a two-component alphavirus replicon particle vaccine expressing CMV gB or a pp65-IE1 fusion protein was shown to induce CMV-specific T cells as well as neutralizing antibodies in seronegative healthy volunteers.20 However, because this strategy implies the use of virus-like replicon particles based on an attenuated strain of Venezuelan equine encephalitis virus, its use in immunocompromised individuals is limited. Interestingly, in a recent randomized controlled trial with a gB-pp65–based DNA plasmid vaccine in seropositive recipients of an allogeneic HSCT, more time to the first detection of CMV viremia and a shortened duration of viremia was demonstrated in the vaccine group as compared to controls.21 It remains, however, to be established whether this vaccine is able to induce de novo immune responses in seronegative individuals. Given the unique capacity of dendritic cells (DC) to initiate primary T-cell responses against pathogens and tumors, DC-based immunotherapy holds promise to trigger CMV-specific immune responses while circumventing the use of viral vectors. Autologous monocyte-derived DC pulsed with CMV protein have been used to ex vivo stimulate T cells from stem cell donors, which in an adoptive setting have been shown to induce an in vivo CMV-specific immune response in HSCT recipients.22 Feuchtinger et al.23 reported successful induction of a CMV-specific functional T-cell response by vaccination with protein-loaded DC in an allogeneic HSCT (allo-HSCT) recipient receiving a transplantation from a CMV-seronegative donor. In theory, vaccination with protein-loaded DC has a limited capacity to expand antigen-specific CD8+ T cells because this would require cross-priming of the protein by DC after its processing through the exogenous major histocompatibility complex class II pathway. In contrast, we used messenger RNA (mRNA)-transfected DC, which primarily induce CD8+ T-cell immune responses by means of the endogenous major histocompatibility complex class I pathway. To the best of our knowledge, the present phase I-II study was the first to explore the safety, feasibility, and immunogenicity of CMV pp65 mRNA-loaded autologous monocyte-derived DC from healthy volunteers and allo-HSCT recipients as a cellular vaccine for active immunization against CMV.