The mucosa-associated invariant T (MAIT) cells are innate-like T cells with restricted T cell receptor (TCR) usage, which are preferentially localized in mucosal tissues and respond to microbial infection by rapidly producing cytokines and cytotoxic effectors. They recognize the non-classical related molecule (MR1). MAIT cells react against a newly identified class of antigens presented by MR1: Riboflavin (Rib) precursors, which are found in most bacteria and yeasts. Currently, very little is known about the ontogeny of MAIT cells in the human, because of the difficulty to follow longitudinally their development. Cross-sectional studies have identified only the initial (cord blood) and final (adult subjects) steps of human MAIT cell maturation program. Moreover, there are no data on relationships between human MAIT cell expansion and maturation, and gut microbiota development. Given the potential importance of MAIT cells in protection from microbial infections at epithelial surfaces, we will investigate the maturation dynamics of MAIT cells in relation with gut microbiota diversity and function in two clinical settings characterized by a high predisposition to severe microbial infections before the establishment of protective adaptive immunity, namely i/ the neonatal period and ii/ the early immune reconstitution period following allogeneic hematopoietic stem cell transplantation (HSCT) in children. Our study will combine multiparametric phenotypic and functional characterization of MAIT cells with the use of new molecular microbiota analytic methodology (high throughput sequencing, metagenomics, Rib microbiology) to determine how the presence or functionality of MAIT cells is influenced by the gut microbiota.