The Impact of CMV-Prophylaxis with Letermovir on Immune Reconstitution after Allogeneic Stem Cell Transplantation: Single-Center Analysis of 363 Transplanted Patients

Introduction. Letermovir (LTV) showed to reduce the incidence of Cytomegalovirus (CMV) reactivations in CMV-seropositive patients after allogeneic stem cell transplantation (allo-SCT). CMV reactivations are known to stimulate polyfunctional T-cell responses, which foster the development of CMV-specific immune reconstitution (IR) and contribute to the advent of the polyclonal IR after allo-SCT. Since IR plays an important role in influencing outcome of transplanted patients, we aimed at investigating the impact of LTV in the advent and timing of polyclonal IR after allo-SCT.

Methods. We conducted a retrospective analysis of CMV-seropositive patients consecutively undergoing allo-SCT at our center. We defined two cohorts according to the administration of LTV: (1) the no-LTV-cohort, encompassing patients who did not receive LTV, but pre-emptive treatment (PET) in case of CMV-reactivation (historical cohort, transplanted between 2013-2018); (2) the LTV-cohort, including patients who received LTV until at least day +100 after allo-SCT (transplanted between 2019-2023). CMV-DNA monitoring was performed weekly using real-time PCR from blood samples for the first 100 days after allo-SCT. Immune monitoring was performed using a flow cytometry-based assay to identify and determine the percentages and absolute counts of T (CD3+), B (CD19+), and natural killer (NK) cells (CD16+CD56+) as well as the CD4 and CD8 subpopulations of T cells in peripheral blood. Immune monitoring was performed monthly during the first year after allo-SCT and afterwards every 3 months until IR, which was defined as the detection in 2 consecutive measurements of CD3+CD4+-T-cells > 200 cells/µl and CD19+-B-cells > 50cells/µl. Statistical analysis included univariate methods using the Mann–Whitney test for continuous variables and the chi-square and Fischer exact test for nominal ones. For multivariable analysis, Cox- and competing risks regressions were used.

Results. The study included 363 CMV-seropositive transplanted patients, with 128 (35%) in the LTV cohort and 235 patients (65%) in the historical no-LTV cohort. Patient characteristics, including age at time of allo-SCT, type of disease, disease status at allo-SCT, EBMT risk score, conditioning regimen and graft versus host (GVHD) prophylaxis, mainly based on ATG, were homogenously balanced between the two groups. The cumulative incidence (CI) of IR at day+ 100 and +180 after allo-SCT showed no significant difference between the two cohorts . Multivariable analysis indicated that LTV did not affect the occurrence of IR 1 year (y) after allo-SCT. Factors such as older age, high EBMT risk score, active disease at the time of allo-SCT and a CMV-seronegative donor significantly negatively impacted the advent of polyclonal IR. We then analyzed the different cellular population in order to assess the dynamics of the shaping of the immune system in the 2 groups. We found that in the no LTV-group there was a significantly higher expansion of the CD3+CD8+-T-cell fraction during the first 6 months after transplantation as compared to the LTV-cohort: at day +100 we detected 228 CD3+CD8+-T-cells/µl in the no LTV cohort vs. 114/µl in the LTV-cohort (p=0.0016). At day +180 the difference was preserved, with 425 CD3+CD8+-T-cells/µl vs. 204/µl respectively, p=0.001. No difference was observed in the CD3+CD8+-T-cells expansion at 1 y after allo-SCT. We also found a significantly higher peak of expansion of NK-cells at day +100 in the LTV-cohort (243 NK-cells/µl vs. 206 NK-cells/µl in the no LTV, p=0.008), but also in this setting no relevant difference was observed in the long-term follow-up (day+180 and +365). No significant differences were observed between the two cohorts considering the dynamic of expansion of CD3+CD4+-T-cells and CD19+-B-cells during the follow-up post allo-SCT.

Conclusions. This is one of the largest retrospective studies evaluating occurrence and dynamic of IR in patients receiving LTV after allo-SCT. We confirmed that LTV could potentially (but transitorily) decrease T-cells (especially in the CD3+CD8+-T-cell fraction), with a peak of expansion of NK-cells in our study occurring at day +100. Despite these differences in the dynamic of polyclonal IR during the early phase after allo-SCT, in the long-term follow-up LTV does not appear to influence the advent, the qualitative composition and the incidence of IR.