HCMV control in allogeneic stem cell transplant recipients - an analysis of humoral and cellular players beyond antigen-specific T cells in the letermovir era

Abstract Allogeneic hematopoietic stem cell transplant (alloSCT) recipients often experience late-onset human cytomegalovirus (HCMV) reactivations following termination of letermovir prophylaxis. Letermovir prophylaxis extends the window for protective B- and T-cell reconstitution; however, our understanding of humoral responses and their contribution to HCMV immune control remains limited. Combining serological and flow cytometric analyses in 42 HCMV-seropositive alloSCT recipients, we herein provide the first comprehensive longitudinal (days 90-270 post-transplant) characterization of HCMV-specific humoral responses, natural killer (NK)-cell phenotypes, and γδ T cells in the letermovir era. HCMV controllers showed predominantly IgG-driven responses, higher pre-reactivation Vδ1+ γδ T-cell frequencies, and stronger expansion of “memory-like” NK cells than patients with clinically significant CMV infection (csCMVi). In contrast, csCMVi patients showed delayed HCMV-specific IgG production, IgM-skewed humoral responses, and stronger post-reactivation expansion of memory B cells and Vδ1+ γδ T cells. Early γδ T-cell reconstitution by day 90 was predictive of future HCMV control. HCMV-specific IgG levels correlated only weakly with γδ T cells but showed distinct associations with “memory-like” NK-cell reconstitution in HCMV controllers, suggesting synergisms between humoral and cellular immunity. Collectively, these findings highlight a need to study anti-HCMV immune protection beyond type-1 T cells and refine risk stratification models in alloSCT patients by inclusion of novel immune markers such as γδ T-cell frequencies and phenotypes. Leveraging the extended B-cell reconstitution window created by letermovir, novel immunotherapies (e.g., therapeutic antibodies) and future vaccines might boost humoral anti-HCMV immunity and benefit from synergisms with γδ T cells and “memory-like” NK cells in improving HCMV control. Competing Interest Statement The authors have declared no competing interest. Funding Statement This research was funded by a grant from the German Research Council (DFG) within the DFG Research Unit FOR 2830 (KR 5761/1-2, project number 398367752 (to S.K.); EI 269/10-2, project number 398367752 (to H.E.); DO 1275/7-2 (to L.D.); HE2526/9-2 (to H.H.)). S.K. was further supported by a fellowship of the Interdisciplinary Center for Clinical Research (IZKF) and the trust Forschung Hilft. Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: This study was approved by the Ethics Committees of the University of Wuerzburg, Germany (protocol code 17/19-sc). Written informed consent was obtained from all patients. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes Footnotes ↵$ Requests for materials should be addressed to Sabrina Kraus (Kraus_S3{at}ukw.de) Data sharing statement: The datasets generated and analysed in this study are available from the corresponding author upon reasonable request. Please contact Sabrina Kraus (Kraus_S3{at}ukw.de). Individual patient data will not be shared. Data Availability The datasets generated and analysed in this study are available from the corresponding author upon reasonable request.