Cytomegalovirus (CMV)-Specific Polyfunctional T-Cell Responses after Letermovir (LET) Prophylaxis in Hematopoietic Cell Transplantation (HCT)

Introduction In a randomized, placebo-controlled trial, prophylaxis with LET decreased clinically significant CMV infection after allogeneic HCT; however, there was an increase of CMV infection after discontinuation of prophylaxis at day 100 (NEJM 2017; 377:2433). Objective To determine the influence of LET prophylaxis on CMV-specific immune reconstitution after HCT as a possible mechanism for the observed effect, we compared CMV T-cell responses in a prospective cohort of allogeneic HCT recipients who received LET prophylaxis to CMV T-cell responses in historical controls transplanted prior to the introduction of LET. Methods We included CMV-seropositive adult patients given LET prophylaxis after first allogeneic HCT; controls received PCR-guided preemptive CMV-directed therapy prior to the introduction of LET (2010-17). PBMC samples collected ∼3 months post-HCT were cryopreserved and then analyzed for intracellular expression of TNF-α, IL-2, IFN-γ, CD107a, and Granzyme B in CD4 and CD8 T cells. Samples stimulated with CMV peptide mixes (pp65 and IE-1) and staphylococcal enterotoxin B (SEB) served as positive controls. Clinical data including baseline characteristics were obtained from medical records. Polyfunctional T-cell responses were defined as those expressing IFN-γ plus at least one additional cytokine. Univariable Wilcoxon ranksum tests were used to compare differences between LET patients and controls and multivariate linear regression analysis was performed to adjust comparisons for other factors affecting immune reconstitution, such as donor CMV serostatus, lymphopenia, and GVHD severity. Results Forty-two patients given LET and 95 controls were evaluated; patient characteristics were similar between groups. Polyfunctional CD8 T-cell responses at 3 months were lower in LET patients under all testing conditions while CD4 T-cell responses were decreased after stimulation with IE-1 only (Figures 1-3). After adjusting for CMV donor status, lymphopenia, GVHD and underlying disease, polyfunctional CD8 (Mean difference -5.736, 95% Confidence interval [CI] -11.17, -0.301, p= 0.039) and CD4 (Mean difference -0.138, 95% CI -0.257, -0.019, p= 0.023) T-cell responses to IE-1 remained significantly decreased in patients given LET prophylaxis compared to controls. Conclusion LET prophylaxis appeared to be associated with a delayed CMV-specific cellular immune response. Our study provides a potential mechanism for the observed increase of clinically significant CMV events after discontinuation of LET prophylaxis. Studies are ongoing to evaluate correlations of the observed CMV-specific immunodeficiency with post-day 100 CMV events. Measuring CMV-specific T cell responses at the conclusion of LET prophylaxis could identify patients who might benefit from extended surveillance or prolonged prophylaxis.