Our project aims, first, to show that the post-transplant reconstitution and/or the content of the graft in some immunoregulatory T lymphocytes can early predict the post-transplant clinical outcome and, second, to explore the underlying immunological mechanisms. For that, we propose to analyse, in homogeneous cohorts of allografted patients and their donors, the levels of iNKT cells and other cell populations implicated in the allogeneic immune response (Tregs, mucosa-associated invariant T (MAIT) cells, delta gammaT and anti-viral specific T cells) and correlate the results to the post-transplant outcome (GVHD, infections, relapse, survival). This study will be performed in a cohort of 80 adult allografted patients in four transplant departments in Paris (hospitals of Necker, Saint Antoine, Saint Louis and La Pitié Salpétrière) and in a cohort of 60 allografted paediatric patients (Robert Debré hospital in Paris). Sequential blood samples of patients will be drawn to monitor the immune reconstitution of the different lymphocyte populations of interest by flow cytometry and perform functional studies on iNKT (ex vivo expansion capacities and cytokine production). These analyses will be also performed in the corresponding donors from blood samples collected before the collection process and from the grafts (obtained by the cell therapy departments of Necker, La Pitié Salpétrière and Saint Louis hospitals). In addition, we plan to analyse the effect of, and the mechanisms by which, human CD4- and CD4+ iNKT cells might control the allogeneic response in vitro, particularly via their potential interactions with dendritic cells and Tregs. According to the results of the mechanistic studies, we will test the effect of human subtypes of iNKT cells on the GVH/GVL affects in a preclinical humanized mouse model of allogeneic HSCT.