T cells, as orchestrators of the adaptive immune response, serve important physiological and potentially therapeutic roles, for example in cancer immunotherapy. T cells are readily isolated from patients; however, the yield of antigen-specific T cells is limited, thus making their clinical use challenging. Therefore, the generation of T lymphocytes from hematopoietic stem/progenitor cells (HSPCs) and human pluripotent stem cells (PSCs) in vitro provides an attractive method for large-scale production and genetic manipulation of T cells. In this review, we discuss recent strategies for the generation of T cells from human HSPCs and PSCs in vitro. Continued advancement in the generation of human T cells in vitro will expand their benefits and therapeutic potential in the clinic.

SummaryHematopoietic stem cells arise from mesoderm-derived hemogenic endothelium (HE) during embryogenesis in a process termed endothelial-hematopoietic transition (EHT). To better understand the gene networks that control this process, we investigated the role of the transcription factor HEB (TCF12) by disrupting the TCF12 gene locus in human embryonic stem cells (hESCs) and inducing them to differentiate toward hematopoietic outcomes. HEB-deficient hESCs retained key features of pluripotency, including expression of SOX2 and SSEA-4 and teratoma formation, while NANOG expression was reduced. Differentiation of HEB−/− hESCs toward hematopoietic fates revealed a severe defect in mesodermal development accompanied by decreased expression of regulators of mesoendodermal fate choices. We also identified independent defects in HE formation at the molecular and cellular levels, as well as a failure of T cell development. All defects were largely rescued by re-expression of HEB. Taken together, our results identify HEB as a critical regulator of human mesodermal and hematopoietic specification.

Tripartite motif protein 22 (TRIM22) is a novel interferon-induced protein that potently inhibits the replication of evolutionarily diverse viruses, including HIV-1. Altered TRIM22 expression is also associated with diseases, such as multiple sclerosis, cancer, and autoimmunity. The factors that influence TRIM22 expression and antiviral activity are largely unknown. In this study, we adopted an evolution-guided functional approach to identify potential genetic determinants of TRIM22 function. Evolutionary analysis of TRIM22 from mammals spanning >100 million years demonstrated that TRIM22 evolution has been shaped by ancient and variable positive selection. We showed that positive selection is operating on multiple TRIM22 residues that cluster in putative functional regions and that some are predicted to be functionally damaging. Interestingly, the second most prevalent TRIM22 SNP in humans (rs1063303) is located at one of these positively selected sites. We showed that the frequency of rs1063303:G>C varies up to 10-fold between ethnicities and that in some ethnicities SNP rs1063303:G>C is being actively maintained in the population. The SNP rs1063303:G>C variant also had an inverse functional impact where it increased TRIM22 expression and decreased the antiviral activity of TRIM22. Taken together, our data characterize the extensive genetic variation in TRIM22 and identify rs1063303:G>C as a highly prevalent SNP that influences its function.