Human thymus contains multipotent progenitors with T/B lymphoid, myeloid, and erythroid lineage potential

Abstract

It is a longstanding question which bone marrow-derived cell seeds the thymus and to what level this cell is committed to the T-cell lineage. We sought to elucidate this issue by examining gene expression, lineage potential, and self-renewal capacity of the 2 most immature subsets in the human thymus, namely CD34 ؉ CD1a ؊ and CD34 ؉ CD1a ؉ thymocytes. DNA microarrays revealed the presence of several myeloid and erythroid transcripts in CD34 ؉ CD1a ؊ thymocytes but not in CD34 ؉ CD1a ؉ thymocytes. Lineage potential of both subpopulations was assessed using in vitro colony assays, bone marrow stroma cultures, and in vivo transplantation into nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice. The CD34 ؉ CD1a ؊ subset contained progenitors with lymphoid (both T and B), myeloid, and erythroid lineage potential. Remarkably, development of CD34 ؉ CD1a ؊ thymocytes toward the T-cell lineage, as shown by T-cell receptor ␦ gene rearrangements, could be reversed into a myeloid-cell fate. In contrast, the CD34 ؉ CD1a ؉ cells yielded only T-cell progenitors, demonstrating their irreversible commitment to the T-cell lineage. Both CD34 ؉ CD1a ؊ and CD34 ؉ CD1a ؉ thymocytes failed to repopulate NOD/SCID mice. We conclude that the human thymus is seeded by multipotent progenitors with a much broader lineage potential than previously assumed. These cells resemble hematopoietic stem cells but, by analogy with murine thymocytes, apparently lack sufficient self-renewal capacity. IntroductionThe thymic microenvironment is exceptional in its ability to sustain production of T cells. 1 However, hematopoietic stem cells (HSCs) that will eventually give rise to T cells are derived from the bone marrow (BM). The nature of the thymus-seeding cell and its relation to several BM progenitors has remained elusive, despite being the subject of intense investigation. Furthermore, it is controversial whether cells commit to the T-cell lineage prethymically or intrathymically. 2,3 Aided by modern cell-sorting techniques, many studies in the mouse have recently readdressed these issues. Adult murine BM has been demonstrated to contain precursors with a restricted T-/B-lymphoid potential, so-called common lymphoid progenitors (CLPs). 2,4 However, the earliest thymic immigrants were shown to differ from CLPs in several aspects. 5 In peripheral blood, T-lineage potential appeared to be restricted to Lin Ϫ Sca-1 ϩ c-Kit ϩ (LSK) progenitor populations, rather than to CLPs. 6 Recently, two detailed analyses of the earliest subpopulations in the murine thymus showed variable lineage potential of different subsets. 7,8 Together, these studies point toward a model in which a range of BM-derived progenitors colonize the thymus, probably including multipotent progenitors and more lineage-restricted precursor cells. 9 For the human system, comparable experimental data are lacking. Determining lineage potential of early thymocytes will help to unveil the identity of the thymus-seeding cell in humans.In both humans and mice, the most immature ce...

T-, B- and NK-lymphoid, but not myeloid cells arise from human CD34+CD38−CD7+ common lymphoid progenitors expressing lymphoid-specific genes

“…In addition to their T-cell differentiation potential, fetal CD34 hi CD1a À thymocytes possess strong NKcell potential, as well as B-and dendritic cell potential, but lack granulocyte/macrophage potential. 32 Weerkamp et al recently showed that the adult CD34 þ CD1a À thymocyte subset has differentiation potential not only for B, NK and dendritic cells, but also for myeloid and erythroid lineages, 33 leading to the hypothesis that the adult human thymus is seeded by a multipotent stem cell-like progenitor instead of a CLP. Still, the multilineage differentiation capacity of both fetal and adult early thymocytes remains to be shown at the single cell level, thus the possibility exists that, analogously to the corresponding murine DN1 thymocyte stage (CD44 þ CD25 À ), 34 the human CD34 þ CD1a À thymic subset is heterogeneous and contains multiple progenitors with different differentiation potential.…”

Notch and Wnt signaling in T-lymphocyte development and acute lymphoblastic leukemia

“…The exact nature of the progenitors is unclear, but some level of multipotency is now commonly accepted. 6,7 Development into a mature T cell comprises the ordered rearrangement of the different TCR genes and positive and negative selection events that finally lead to the expression of mature TCRab and TCRgd molecules recognizing non-self peptides in context of self-major histocompatibility complex (MHC) or equivalent molecules. These processes take place during a series of discrete phenotypic stages that can be recognized by expression of several key membrane molecules, most notably CD4 and CD8 (Figure 1a).…”

Pretransplant Mobilization with Granulocyte Colony-Stimulating Factor Improves B-Cell Reconstitution by Lentiviral Vector Gene Therapy in SCID-X1 Mice

“…-/ -cells transplanted, T cells increased to normal or near-normal levels, consistent with the selective advantage of the T-cell lineage in these mice and BMindependent homing in the thymus (Weerkamp et al, 2006). In contrast, B-cell reconstitution was strongly dependent on conditioning.…”

“…The donor HSCs and B-cell progenitors may have engrafted poorly in the BM because of the limited accessibility of occupied HSC niches. The relative advantage in reconstitution and marking of T-cell progenitors might be attributed to the higher selective advantage of these cell types, the lack of competing T-cell progenitor cells in the host (Buckley, 2004), or the ability of multipotent progenitors T cells to home directly to the thymus (Weerkamp et al, 2006). Recently initiated gene therapy trials to treat SCID-X1 with self-inactivating (SIN) gammaretroviral vectors adhere to similar protocols without preconditioning (Mukherjee and Thrasher, 2013).…”