Age-related immune cell dynamics influence outcomes after allogeneic haematopoietic cell transplantation.

An efficient immunological reconstitution construes the pillar for the success of allogeneic haematopoietic cell transplantation (HCT) in haematological disorders. Factors influencing post-transplant immune recovery have been largely investigated across multiple cohorts issuing heterogeneous results. Differences in outcomes in adult and paediatric populations suggest an age-related contribution to post-transplant immune reconstitution; however, it is unclear how recipient and donor age may affect the dynamics of single immune cells. Here, we retrospectively collected and analysed immunological data of 174 patients (58 children and 116 adults) consecutively transplanted for haematological disorders in our centre. We show that trajectories of specific immune cells were strictly dependent on recipient age and pretransplant virus exposure, with the strongest effect seen on T CD4+ and B-cell counterparts, while donor age and transplant platforms had a minimal impact. This mirrored different kinetics of immune reconstitution in adult and paediatric patients, with major divergences in immune cell composition in late post-transplant phases, featuring better survival, relapse-free survival and cumulative incidence of pathogen-specific infections in younger patients. Altogether, these findings underpin the importance of recipient age on post-transplant immune cell recovery and define the basic dynamics of the immune reconstitution in paediatric and adult populations as a benchmark for future studies.