The introduction of letermovir (LTV) in prophylaxis after allogeneic-hematopoietic stem cell transplantation (allo-HSCT) has reduced the incidence of Cytomegalovirus (CMV) clinically relevant reactivations (CRE), but these events increase after LTV cessation. CMV-specific T cells protect the patients against CRE, but the mechanisms stimulating their emergence during or immediately after LTV treatment need to be further explored, especially in the setting of post-transplant cyclophosphamide (PT-Cy). In this study, we analyzed 42 CMV-seropositive adult patients with hematological malignancies undergoing allo-HSCT with PT-Cy and calcineurin inhibitor (CNI)-free GvHD prophylaxis in a single-center observational study. Fifteen patients received LTV as prophylaxis in the first 100 days after transplant. CMV-specific CD8+ T cells were quantified by flow cytometry using Dextramer® CMV-Kit (IVD, Immudex) in PBMC frozen 90 (D90) and 180 (D180) days after allo-HSCT, and protective anti-viral immunity was defined based on the threshold that we had previously identified of 0.5 CMV-specific cells/ul. Our findings reinforce the protective role of LTV against clinically relevant CMV-CRE and confirm that LTV prophylaxis is associated with a delayed reconstitution of CMV-specific CD8⁺ T cells compared to no-LTV patients. Importantly, our data underscore the pivotal role of antigen exposure—even transient and at low levels, such as during CMV blips—in promoting the expansion of protective levels of CMV-specific T lymphocytes. This suggests that minimal antigenic stimulation is sufficient to boost protective CMV-specific immune responses in the context of ongoing reconstitution.

The introduction of letermovir (LTV) in prophylaxis after allogeneic-hematopoietic stem cell transplantation (allo-HSCT) has reduced the incidence of Cytomegalovirus (CMV) clinically relevant reactivations (CRE), but these events increase after LTV cessation. CMV-specific T cells protect the patients against CRE, but the mechanisms stimulating their emergence during or immediately after LTV treatment need to be further explored, especially in the setting of post-transplant cyclophosphamide (PT-Cy). In this study, we analyzed 42 CMV-seropositive adult patients with hematological malignancies undergoing allo-HSCT with PT-Cy and calcineurin inhibitor (CNI)-free GvHD prophylaxis in a single-center observational study. Fifteen patients received LTV as prophylaxis in the first 100 days after transplant. CMV-specific CD8+ T cells were quantified by flow cytometry using Dextramer® CMV-Kit (IVD, Immudex) in PBMC frozen 90 (D90) and 180 (D180) days after allo-HSCT, and protective anti-viral immunity was defined based on the threshold that we had previously identified of 0.5 CMV-specific cells/ul. Our findings reinforce the protective role of LTV against clinically relevant CMV-CRE and confirm that LTV prophylaxis is associated with a delayed reconstitution of CMV-specific CD8⁺ T cells compared to no-LTV patients. Importantly, our data underscore the pivotal role of antigen exposure—even transient and at low levels, such as during CMV blips—in promoting the expansion of protective levels of CMV-specific T lymphocytes. This suggests that minimal antigenic stimulation is sufficient to boost protective CMV-specific immune responses in the context of ongoing reconstitution.