Immunosenescent characteristics of T cells in young patients following haploidentical haematopoietic stem cell transplantation from parental donors

Objectives. Paediatric and adolescent patients in need of allogeneic haematopoietic stem cell transplantation (HSCT) generally receive stem cells from older, unrelated or parental donors when a sibling donor is not available. Despite encouraging clinical outcomes, it has been suggested that immune reconstitution accompanied by increased replicative stress and a large difference between donor and recipient age may worsen immunosenescence in paediatric recipients. Methods. In this study, paired samples were collected at the same time from donors and recipients of haploidentical haematopoietic stem cell transplantation (HaploSCT). We then conducted flow cytometry-based phenotypic and functional analyses and telomere length (TL) measurements of 21 paired T-cell sets from parental donors and children who received T-cell-replete HaploSCT with post-transplant cyclophosphamide (PTCy). Results. Senescent T cells, CD28 or CD57⁺ cells, were significantly expanded in patients. Further, not only CD4⁺CD28 T cells, but also CD4⁺ CD28⁺ T cells showed reduced cytokine production capacity and impaired polyfunctionality compared with parental donors, whereas their TCR-mediated proliferation capacity was comparable. Of note, the TL in patient T cells was preserved, or even slightly longer, in senescent T cells compared with donor cells. Regression analysis showed that senescent features of CD4⁺ and CD8⁺ T cells in patients were influenced by donor age and the frequency of CD28 cells, respectively. Conclusion. Our data suggest that in paediatric HaploSCT, premature immunosenescent changes occur in T cells from parental donors, and therefore, long-term immune monitoring should be conducted.