Tolerance, which can be defined as allograft acceptance without the need for chronic immunosuppression, is the “holy grail” of transplantation. Various methods are used to induce tolerance-inducing mixed chimerism with allogeneic hematopoietic stem cell transplantation (HSCT). The issue of cytomegalovirus (CMV) reactivation is an important barrier to engraftment of donor stem cells when considering HSCT as part of a tolerance protocol after solid organ transplantation even with nonmyeloablative conditioning.1 Furthermore, strategies for optimal CMV prevention and treatment in such approaches are yet to be studied. In the study by Alonso-Guallart et al2 published in this issue of Transplantation, the authors performed allogeneic HSCT in a cohort of cynomolgus macaques treated subsequently with either cyclosporin A or rapamycin posttransplant. Although the majority of animals developed cynomolgus CMV (CyCMV) DNAemia, there was an approximately 10-day delay in DNAemia onset for those receiving rapamycin compared with cyclosporin A. Although modest, the authors believe that the delay in introduction of CMV treatment allowed for better engraftment of stem cells. A previous report by the same research group showed that failure to induce engraftment was due to CyCMV reactivation, as recipients who did not develop CyCMV had long-term mixed chimerism and graft acceptance. A similar protective effect of mTOR inhibitors on human CMV (HCMV) has already been demonstrated in several clinical studies of organ transplantation.3 In addition, 2 recent clinical studies in allogeneic HSCT showed that rapamycin was able to reduce HCMV DNAemia.4,5 Furthermore, increasing rapamycin concentrations reduced HCMV DNAemia requiring antiviral therapy. At least 2 mechanisms are responsible for this finding. First, it has been demonstrated that mTOR is activated in HCMV-infected cells during the late phase of the viral cycle. In addition, Bak et al6 have demonstrated that rapamycin improves CMV-specific effector memory T-cell function. Unfortunately, rapamycin is generally difficult to use post-HSCT due to its myelosuppressive effects and uncertainty about the development of graft versus host disease. In addition, the rapamycin levels of 20–30 as attained in this study would be toxic to humans.
Opportunistic infections are well-known complications when HSCT is performed to induce tolerance. Cytomegalovirus is one of the most common opportunistic viral infections after both solid organ and allogeneic HSCT. Reactivation of HCMV in the CMV seropositive HSCT population is associated with graft loss and increased mortality.7 For decades, prevention of HCMV in the HSCT population has been through preemptive therapy with weekly monitoring of HCMV viral load and treatment of HCMV once the viral load reaches a predefined threshold or symptoms occur. Unlike the solid organ transplant group, a longstanding issue in the HSCT population has been the reluctance to use universal prophylaxis with (val)ganciclovir to prevent HCMV reactivation due to myelotoxicity. More recently, letermovir, a nonmyelosuppressive CMV antiviral, has been Food and Drug Administration approved for prevention of CMV in HSCT. However, in a randomized trial of letermovir versus preemptive therapy for CMV, 37.5% of patients receiving letermovir still developed CMV DNAemia that required therapy.8 Treatment of CMV post-HSCT is difficult, and recurrent-relapsing DNAemia occurs frequently. The mainstay of antiviral treatment is (val)ganciclovir, and persistent DNAemia or resistant virus is treated with foscarnet, a drug with significant nephrotoxicity. Newer treatment options are being evaluated, including maribavir, a viral kinase inhibitor, and preliminary data show its efficacy in the HSCT population.9 Whether (val)ganciclovir or the newer antivirals have the same pharmacokinetics, pharmacodynamics, and efficacy in a nonhuman primate model of CMV is unclear.
One of the main findings by Alonso-Guallart et al2 was the development of a CyCMV PCR targeting a conserved region of DNA polymerase. Detection of CMV has evolved from antigenemia and conventional cell culture to highly sensitive quantitative molecular methods that detect CMV DNA via nucleic acid testing in plasma, whole blood, or other fluids/tissues. Commercially available PCR assays for HCMV generally target the conserved regions of CMV DNA including immediate early or polymerase genes among others. The CMV virus that infects Cynomolgus macaques (CyCMV) was sequenced by Marsh et al,10 who showed that 137 or 262 open reading frames of CyCMV are homologous to HCMV, including the functional genes that encode terminase, viral kinase, and DNA polymerase. Given the high level of homology between HCMV and CyCMV, it would be interesting to study whether one of the commercially available assays could be used in this setting because this would allow for further standardization of results between investigators using cynomolgus macaque models.
Thus, the study by Alonso-Guallart et al2 shows the importance of CMV in an allogeneic transplant tolerance induction protocol. The animal model and PCR developed by the authors will be helpful in monitoring the dynamics of CMV. Depending on the degree of homology with CMV proteins, the CyCMV model would be useful not only to study tolerance but could also potentially be used for future preclinical studies of antivirals, CMV-specific immune responses, and vaccines in nonhuman primates.
CMV is well known as the “troll of transplantation” but may also become the “troll of tolerance” in the setting of immunosuppressive tolerance protocols. It will be important in future studies to control CMV reactivation by various preventative methods so that graft loss and loss of tolerance do not occur.
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