T Cell Genesis: In Vitro Veritas Est?

Abstract

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 the 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.

Figure 1. Therapeutic applications for T cell generation and expansion

T cell-based therapeutics have uses for a broad range of clinical applications. Antigen-specific regulatory T cells (green) could be useful in preventing tissue damage caused by autoimmunity, or following organ transplantation. Tumor-antigen specific effector T cells (pink) isolated from the tumor and surrounding stroma or from peripheral blood can be expanded in vitro to target and eradicate cancer cells. Similarly, T cells can be engineered to express T cell receptor (TCR) or chimeric antigen receptor (CAR) with tumor eradication ability. In lymphopenia (red), when T cells are absent or insufficient to mount an immune response, as in acquired or primary immunodeficiencies (i.e., AIDS or PIDs), after hematopoietic stem cell transplant, or age-related lymphopenia due to thymic involution, T cell therapy can restore immune system function and prevent infections. While the conventional sources of HSPCs for transplant and research rely heavily on bone marrow and cord blood sources, induced pluripotent stem cells (iPSCs) have emerged as a promising source, bypassing issues of HLA-matching. Work pioneered by many groups has helped to develop different approaches for generation, expansion, and/or manipulation of T cells from HSPC sources in vitro in order to gain a deeper understanding of disease mechanisms and validate therapeutic approaches (blue).

Figure 2. Sources for HSPC or T cell isolation

HSPCs and T cells can be derived from various sites within the human body including peripheral blood, umbilical cord blood and bone marrow. Tumor-infiltrating lymphocytes can be isolated from cancerous tissues and be expanded in vitro. Induced pluripotent stem cells (iPSCs) are reprogrammed from isolated somatic cells, such as skin fibroblast or T cells, then differentiated into hematopoietic cells, and subsequently T cells in vitro.

Figure 3. Human T cell development in the thymus

CD34^+/int multipotent blood-borne progenitors reconstitute the thymus and pass through distinct stages that are distinguished based on cell surface marker expression and state of TCR rearrangements [–13]. The earliest intrathymic cells are double negative for CD4 and CD8 expression. Upon induction of intrathymic Notch signals, human thymocytes up-regulate CD7 followed by CD5. The subsequent acquisition of CD1a marks commitment to the T lineage, and thymocytes lose their capacity to generate non-T lineage cells [12, 13]. Expression of CD4, but not CD8, on the cell surface occurs next, marking CD4 immature single positive (CD4ISP) cells. The CD4ISP then develops into a CD4⁺CD8⁺ double positive (DP) cell, with CD4⁺CD8α⁺β⁻ early DPs preceding CD4⁺CD8α⁺β⁺ late DPs. Following expression of a successful TCRαβ heterodimer, thymocytes undergo positive and negative selection events in the cortex (green) and medulla (blue), respectively. Surviving mature CD4⁺CD8⁻ and CD4⁻CD8⁺ single positive (SP) T cells migrate to the periphery.