Daniel A. Zlotoff

CCR7 and CCR9 together recruit hematopoietic progenitors to the adult thymus

T lymphopoiesis requires settling of the thymus by bone marrow-derived precursors throughout adult life. Progenitor entry into the thymus is selective, but the molecular basis of this selectivity is incompletely understood. The chemokine receptor CCR9 has been demonstrated to be important in this process. However, progenitors lacking CCR9 can still enter the thymus, suggesting a role for additional molecules. Here we report that the chemokine receptor CCR7 is also required for efficient thymic settling. CCR7 is selectively expressed on bone marrow progenitors previously shown to have the capacity to settle the thymus, and CCR7(-/-) progenitors are defective in settling the thymus. We further demonstrate that CCR7 sustains thymic settling in the absence of CCR9. Mice deficient for both CCR7 and CCR9 have severe reductions in the number of early thymic progenitors, and in competitive assays CCR7(-/-)CCR9(-/-) double knockout progenitors are almost completely restricted from thymic settling. However, these mice possess near-normal thymic cellularity. Compensatory expansion of intrathymic populations can account for at least a part of this recovery. Together our results illustrate the critical role of chemokine receptor signaling in thymic settling and help to clarify the cellular identity of the physiologic thymic settling progenitors.

Cutting Edge: Natural Helper Cells Derive from Lymphoid Progenitors

Natural helper (NH) cells are recently discovered innate immune cells that confer protective type 2 immunity during helminth infection and mediate influenza-induced airway hypersensitivity. Little is known about the ontogeny of NH cells. We report in this study that NH cells derive from bone marrow lymphoid progenitors. Using RAG-1Cre/ROSA26YFP mice, we show that most NH cells are marked with a history of RAG-1 expression, implying lymphoid developmental origin. The development of NH cells depends on the cytokine receptor Flt3, which is required for the efficient generation of bone marrow lymphoid progenitors. Finally, we demonstrate that lymphoid progenitors, but not myeloid–erythroid progenitors, give rise to NH cells in vivo. This work therefore expands the lymphocyte family, currently comprising T, B, and NK cells, to include NH cells as another type of innate lymphocyte that derives from bone marrow lymphoid progenitors.

Hematopoietic progenitor migration to the adult thymus

Although most hematopoietic lineages develop in the bone marrow (BM), T cells uniquely complete their development in the specialized environment of the thymus. Hematopoietic stem cells with long‐term self‐renewal capacity are not present in the thymus. As a result, continuous T cell development requires that BM‐derived progenitors be imported into the thymus throughout adult life. The process of thymic homing begins with the mobilization of progenitors out of the BM, continues with their circulation in the bloodstream, and concludes with their settling in the thymus. This review will discuss each of these steps as they occur in the unirradiated and postirradiation scenarios, focusing on the molecular mechanisms of regulation. Improved knowledge about these early steps in T cell generation may accelerate the development of new therapeutic options in patients with impaired T cell number or function.

T cell development requires constraint of the myeloid regulator C/EBP-α by the Notch target and transcriptional repressor Hes1

Notch signaling induces gene expression of the T cell lineage and discourages alternative fate outcomes. Hematopoietic deficiency in the Notch target Hes1 results in severe T cell lineage defects; however, the underlying mechanism is unknown. We found here that Hes1 constrained myeloid gene-expression programs in T cell progenitor cells, as deletion of the myeloid regulator C/EBP-α restored the development of T cells from Hes1-deficient progenitor cells. Repression of Cebpa by Hes1 required its DNA-binding and Groucho-recruitment domains. Hes1-deficient multipotent progenitor cells showed a developmental bias toward myeloid cells and dendritic cells after Notch signaling, whereas Hes1-deficient lymphoid progenitor cells required additional cytokine signaling for diversion into the myeloid lineage. Our findings establish the importance of constraining developmental programs of the myeloid lineage early in T cell development.

Delivery of progenitors to the thymus limits T-lineage reconstitution after bone marrow transplantation

T-cell production depends on the recruitment of hematopoietic progenitors into the thymus. T cells are among the last of the hematopoietic lineages to recover after bone marrow transplantation (BMT), but the reasons for this delay are not well understood. Under normal physiologic conditions, thymic settling is selective and either CCR7 or CCR9 is required for progenitor access into the thymus. The mechanisms of early thymic reconstitution after BMT, however, are unknown. Here we report that thymic settling is briefly CCR7/CCR9-independent after BMT but continues to rely on the selectin ligand PSGL-1. The CCR7/CCR9 independence is transient, and by 3 weeks after BMT these receptors are again strictly required. Despite the normalization of thymic settling signals, the rare bone marrow progenitors that can efficiently repopulate the thymus are poorly reconstituted for at least 4 weeks after BMT. Consistent with reduced progenitor input to the thymus, intrathymic progenitor niches remain unsaturated for at least 10 weeks after BMT. Finally, we show that thymic recovery is limited by the number of progenitors entering the thymus after BMT. Hence, T-lineage reconstitution after BMT is limited by progenitor supply to the thymus.

Untangling the T branch of the hematopoiesis tree.

T cells develop in the thymus. Previous work suggested an early separation of lymphoid from myeloerythroid lineages during hematopoiesis and hypothesized the thymus was settled exclusively by lymphoid-restricted hematopoietic progenitors. Recent data have instead established the existence of lymphoid-myeloid progenitors, which possess lymphoid and myeloid lineage potentials but lack erythroid potential. Myeloid and lymphoid potentials are present at the clonal level in early thymic progenitors, confirming that progenitors settling the thymus include lymphoid-myeloid progenitors. These results revise our view of the T lineage branch of hematopoiesis and focus attention on the generation, circulation, and homing of lymphoid-myeloid progenitors to the thymus.

The long road to the thymus: the generation, mobilization, and circulation of T-cell progenitors in mouse and man

The majority of T cells develop in the thymus. T-cell progenitors in the thymus do not self-renew and so progenitor cells must be continuously imported from the blood into the thymus to maintain T-cell production. Recent work has shed light on both the identity of the cells that home to the thymus and the molecular mechanisms involved. This review will discuss the cells in the bone marrow and blood that are involved in early thymopoiesis in mouse and man. Understanding the pre-thymic steps in T-cell development may translate into new therapeutics, especially in the field of hematopoietic stem cell transplantation.

Chemokine treatment rescues profound T-lineage progenitor homing defect after bone marrow transplant conditioning in mice

Development of T cells in the thymus requires continuous importation of T-lineage progenitors from the bone marrow via the circulation. Following bone marrow transplant, recovery of a normal peripheral T-cell pool depends on production of naïve T cells in the thymus; however, delivery of progenitors to the thymus limits T-lineage reconstitution. Here, we examine homing of intravenously delivered progenitors to the thymus following irradiation and bone marrow reconstitution. Surprisingly, following host conditioning by irradiation, we find that homing of lymphoid-primed multipotent progenitors and common lymphoid progenitors to the thymus decreases more than 10-fold relative to unirradiated mice. The reduction in thymic homing in irradiated mice is accompanied by a significant reduction in CCL25, an important chemokine ligand for thymic homing. We show that pretreatment of bone marrow progenitors with CCL25 and CCL21 corrects the defect in thymic homing after irradiation and promotes thymic reconstitution. These data suggest new therapeutic approaches to promote T-cell regeneration.

Eliciting the T cell fate with Notch.

Multipotent progenitors arrive at the thymus via the blood. Constraining the non-T cell fates of these progenitors while promoting the T cell fate is a major task of the thymus. Notch appears to be the initial trigger for a developmental program that eventually results in T cell lineage commitment. Several downstream targets of Notch are known, but the specific roles of each are poorly understood. A greater understanding of how Notch and other thymic signals direct progenitors to a T cell fate could be useful for translational work. For example, such work could eventually allow for the generation of fully competent T cells in vitro that could supplement the waning T cell numbers and function in the elderly and boost T cell-mediated immunity in patients with immunodeficiency and after stem cell transplantation.

Progenitor migration to the thymus and T cell lineage commitment

T cells developing in the thymus are ultimately derived from bone marrow (BM) hematopoietic stem cells (HSCs). An understanding of the developmental steps between HSCs and T cells is important for gaining insight into cancers of the T lineage, improving T cell reconstitution after BM transplantation, and also to help ameliorate immunological defects in aging. In this article, we summarize our current understanding of the inter-related fields of early T cell development and thymic aging, and briefly discuss major unresolved questions in this field.