T cell regeneration: an update on progress and challenges.

T cells play essential roles in antitumor therapy. Via gene engineering technique to enhance tumor-antigen specificity, patient peripheral blood-derived T cells (PBT) show encouraging clinical outcomes in treating certain blood malignancies. However, the high costs, functionality exhaustion, and disease-condition-dependent availability of PBT prompt the attempts of exploring alternative T cell sources. Theoretically, induced T cells from pluripotent stem cells (PSC) are ideal candidates that integrate plenty of advantages that primary T cells lack, including unlimited off-the-shelf cell source and precision gene editing feasibility. However, researchers are still struggling with developing a straightforward protocol to induce functional and immunocompetent human T cells from PSC. Based on stromal cell-expressing or biomaterial-presenting Notch ligands DLL1 or DLL4, natural and induced blood progenitors can differentiate further toward T lineage commitment. However, none of the reported T induction protocols has yet translated into any clinical application, signaling the existence of numerous technical barriers for regenerating T cells functionally matching their natural PBT counterparts. Alternatively, new approaches have been developed to repopulate induced T lymphopoiesis via in vivo reprogramming or transplanting induced T cell precursors. Here, we review the most recent progress in the T cell regeneration field, and the remaining challenges dragging their clinical applications.