Vγ9Vδ2 T cells are a subset of human T lymphocytes activated by phosphoantigens in a T cell receptor-dependent manner to fight microbial invaders or kill transformed cells. Phosphoantigens are low molecular weight nonpeptidic pyrophosphate containing metabolites produced both endogenously (upregulated in transformed cells) and by microbes. Vγ9Vδ2 T cells are the first T cells generated in the foetus and have programmed functions before encountering the post-partum environment.In this PhD thesis, the aim was to assess the origin of Vγ9Vδ2 T cells in early versus adult life and to evaluate their T cell receptor repertoire and effector potential in the neonatal and infant period. First, human Vγ9Vδ2 T cells were characterised coming from foetal blood and generated by the foetal thymus and then similarities and differences with adult blood Vγ9Vδ2 T cells were identified. The data showed that there is a post-natal thymic output of Vγ9Vδ2 T cells which are different from their foetal counterparts. This finding could help guide the development of cancer immunotherapy strategies aiming to improve the resistance and tenacity of Vγ9Vδ2 T cells which enter an exhaustion state after long encounter with the antigen.Furthermore, human Vγ9Vδ2 T cells were studied early after birth regarding their T cell receptor repertoire and function. At 10 weeks after birth, Vγ9Vδ2 T cells had expanded, and a big part of the Vγ9Vδ2 T cell repertoire was foetal-derived. Additionally, Vγ9Vδ2 T cells had undergone significant functional polarisation toward potent killer effector cells. The expansion and shift in effector functions were not influenced by neonatal BCG vaccination, highlighting the role of environmental exposure upon birth. The data gathered here highlight the unique properties of this innate-like lymphocyte population which can act as a first wave of protection in early life while conventional αβ T cells are not yet optimal. Later in life, another wave of Vγ9Vδ2 T cells arrives from the thymus to expand and populate the adult periphery, providing a possible avenue of new and robust cancer cell killers in the scope of immunotherapy. / Doctorat en Sciences biomédicales et pharmaceutiques (Pharmacie) / info:eu-repo/semantics/nonPublished