STEM CELL-LIKE PROPERTIES OF MEMORY T CELLS IN HUMAN IMMUNE RECONSTITUTION

ABSTRACT The memory T cell compartment comprises a plethora of subsets that are diverse at the functional, transcriptional and metabolic level. T memory stem cells (TSCM) are the least differentiated subset endowed with superior immune reconstitution capacity, anti-tumor activity and long-term persistence compared to more differentiated progeny in preclinical models. However, their role in human T cell recovery remains undetermined. In the first part of my work, I investigate the dynamics of T cell reconstitution in human haploidentical bone marrow transplantation (haplo-BMT) with post-transplant cyclophosphamide (pt-Cy) and provide evidence that donor antigen-specific naïve (TN)- derived TSCM cells survive pt-Cy and are the most abundant circulating T cell population in the early days after transplantation. I further demonstrate that these cells later contribute to peripheral reconstitution by differentiating into effector cells. With these experiments, I define the cellular basis of human T cell reconstitution following haploidentical transplantation with pt-Cy and propose to employ naive-derived TSCM cells in the clinical setting to overcome immunodeficiency. However, the clinical use of these cells is limited by their paucity ex vivo. Therefore, I dedicate the second part of this work to investigate the biological and molecular mechanisms involved in the generation and expansion of TSCM cells from CD8+ TN precursors. I show that curtailed T-cell receptor stimulation curbs effector CD8+ T-cell differentiation and allows the generation of CD45RO– CD45RA+CCR7+CD27+CD95+ -phenotype cells from highly purified TN precursors, resembling naturally-occurring TSCM. These cells proliferate extensively in vitro and in vivo, express low amounts of effector-associated genes and transcription factors and undergo considerable self-renewal in response to IL-15 while retaining effector differentiation potential. Such a phenotype is associated with a lower number of mitochondria compared to highly-activated effector T cells committed to terminal differentiation. These results shed light on the molecular signals that are required to generate high numbers of long-lived memory T cells with potential application in adoptive cell transfer immunotherapy.