Clonal Analyses Reveal the Impact of Hematopoietic Stem and Progenitor Cell Aging on T Cell Development

ABSTRACT T cell output from the thymus falls throughout life and is associated with profound remodeling of the thymic stroma. To what extent the decline in T cell output is caused by aging of the hematopoietic stem and progenitor cells (HSPCs) has been difficult to define because of HSPC heterogeneity, the multi‐stage process of HSPC migration, and the cross‐talk between hematopoietic and stromal elements of the thymus. To address the contribution of HSPC aging on T cell development, we interrogated T cell differentiation of phenotypically defined HSPCs from young and aged bone marrow using the Artificial Thymic Organoid (ATO) system, an in vitro model which allows quantification of T cell differentiation from single HSPCs within a controlled microenvironment. Phenotypically, most HSCs from young bone marrow were CD150 lo lymphoid‐biased (Ly‐HSC), whereas aged HSCs were predominantly CD150 hi myeloid‐biased (My‐HSC). Clonal analysis showed Ly‐HSCs had greater T cell potential than My‐HSCs, but aging had little if any impact on T cell output from the same immunophenotypic HSC. Further, clonal studies of early thymic progenitors (ETPs) demonstrated comparable T cell potential in young and aged cells. We conclude that the hematopoietic contribution to thymic insufficiency during aging is likely due to a relative shift in the aged bone marrow to myeloid‐biased HSCs, rather than a per cell loss of T cell potential across HSPC in general. The profound changes that occur in the thymic microenvironment during aging likely also provide a major contribution to defects in thymopoiesis.