Human CD8 T cells generated in vitro from hematopoietic stem cells are functionally mature

HSC/OP9-DL1 cocultures supported the differentiation of CD8 T cells, which were TCR/CD3hi CD27hi CD1aneg and thus phenotypically resembled mature functional CD8 single positive thymocytes. These in vitro-generated T cells also appeared to be conventional CD8 cells, as they expressed high levels of Eomes and low levels of Plzf, albeit not identical to ex vivo UCB CD8 T cells. Consistent with the phenotypic and molecular characterization, upon TCR-stimulation, in vitro-generated CD8 T cells proliferated, expressed activation markers (MHC-II, CD25, CD38), secreted IFN-γ and expressed Granzyme B, a cytotoxic T-cell effector molecule.Taken together, the ability to direct human hematopoietic stem cell or T-progenitor cells towards a mature functional phenotype raises the possibility of establishing cell-based treatments for T-immunodeficiencies by rapidly restoring CD8 effector function, thereby mitigating the risks associated with opportunistic infections.T lymphocytes develop within the thymus, undergoing distinct differentiation events based on phenotype, while following spatial-temporal cues. The most immature human thymocytes are triple negative for CD3, CD4 and CD8, but express CD34. CD7 is one of the earliest markers to be expressed during human T cell ontogeny, followed by the upregulation of CD1a, which marks T cell commitment. Following this stage, human thymocytes progress to a CD4 immature single positive (CD4ISP) stage, at which point CD4 is expressed in the absence of CD8. Thereafter, a subset of CD4ISP cells are thought to complete TCRβ rearrangement leading to β-selection and differentiation to the CD4+ CD8+ double positive (DP) stage. Finally, following successful TCRα rearrangement, TCRαβ expressing DP thymocytes undergo positive and negative selection events, which result in the production of CD4+ CD8- and CD4- CD8+ single positive (SP) T cells, which then emigrate to the periphery [1].Cytotoxic CD8 SP T cells play a critical role in adaptive immunity b