Multiple extrathymic precursors contribute to T-cell development with different kinetics

Abstract

T-cell development in the thymus depends on continuous supply of T-cell progenitors from bone marrow (BM). Several extrathymic candidate progenitors have been described that range from multipotent cells to lymphoid cell committed progenitors and even largely T-lineage committed precursors. However, the nature of precursors seeding the thymus under physiologic conditions has remained largely elusive and it is not known whether there is only one physiologic T-cell precursor population or many. Here, we used a competitive in vivo assay based on depletion rather than enrichment of classes of BM-derived precursor populations, thereby only minimally altering physiologic precursor ratios to assess the contribution of various extrathymic precursors to T-lineage differentiation. We found that under these conditions multiple precursors, belonging to both multipotent progenitor (MPP) and common lymphoid progenitor (CLP) subsets have robust T-lineage potential. However, differentiation kinetics of different precursors varied considerably, which might ensure continuous thymic output despite gated importation of extrathymic precursors. In conclusion, our data suggest that the thymus functions to impose T-cell fate on any precursor capable of filling the limited number of progenitor niches. (Blood. 2010;115:1137-1144) IntroductionT-cell development in the thymus depends on continuous supply of T-cell progenitors from bone marrow (BM) via the circulation. In the thymus T-cell precursors pass through a series of defined developmental stages, with the most immature thymocytes residing in the double-negative (DN) subset, characterized by the absence of the surface markers CD4 and CD8. Thymocyte differentiation then proceeds through the CD4 ϩ CD8 ϩ double-positive (DP) stage, after which thymocytes become either CD4 or CD8 single positive (SP) and leave the thymus to enter the mature T-cell pool. The most immature T-cell progenitors in the thymus are lineage negative (lin Ϫ ), CD44 ϩ , CD25 Ϫ , Sca-1 high , CD117 (c-kit) high (LSK), and CD127 Ϫ/lo (IL-7R␣) early T-lineage progenitors (ETPs), 1 which constitute a subfraction of the CD44 ϩ CD25 Ϫ DN1 population. These cells were shown to have high T but only limited B and some myeloid potential. 2,3 ETPs could be further subdivided according to their expression levels of CD135 (Fms-like tyrosine kinase receptor 3 [Flt3]) 4 and loss of CD135 expression correlated with loss of B-cell potential.Like all hematopoietic lineages T cells are ultimately derived from hematopoietic stem cells (HSCs) residing in BM. HSCs can generate CD135 ϩ multipotent progenitors (MPPs), which are likewise of the LSK phenotype, 5 as well as more committed precursors such as RAG-1-positive early lymphoid progenitors (ELPs) 6 and L-selectin-positive progenitors (LSPs), 7 both of which constitute subsets of MPPs. Common lymphoid progenitors (CLPs), 8 which are lin Ϫ Sca-1 ϩ CD117 ϩ/lo CD127 ϩ CD135 ϩ , and lin Ϫ Sca-1 ϩ CD117 Ϫ CD127 ϩ CD135 ϩ B220 ϩ CLP-2 9 differ from MPP subsets in their lack of myeloid pot...

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

“…23,29,30 The observed CCR7/CCR9 independence of acute post-BMT thymic settling may allow cells outside of this population to directly settle the thymus during this time. 23 Yet because thymic settling signals are normalized by 2 weeks after BMT in nearly all cases ( Figure 1E), we asked whether the identity of thymic settling progenitors at this later time is similarly normalized.…”

“…23,27 Consistently, all thymic settling capacity effectively resides within the BM Lin Ϫ Kit ϩ Flt3 high pool, which includes both Kit high LMPPs and Kit low CLPs but excludes HSCs and MPPs. 23,29,30 These Lin Ϫ Kit ϩ Flt3 high progenitors undergo Notchdependent differentiation into early thymic progenitors (ETPs) on thymic entry. 31,32 ETPs generate CD4/CD8 double-negative 2 (DN2) cells and then DN3 cells.…”

Supply-side economics finds the thymus

“…2 However, mounting evidence suggests that there may actually be multiple progenitors with T lineage and thymic seeding capacity, which all contribute to thymopoiesis. 2,3 Interestingly, while the precise identity of the thymusseeding T-cell progenitor remains contentious, the mechanism by which these cells gain entry to the thymus and differentiate into mature T cells has been surprisingly consistent (see figure). [4][5][6] Expression of several molecules had previously been postulated for their importance in this process; however, only recently have studies emerged, including others by Bhandoola and colleagues, that have offered significant insight into the molecular pathways of thymic importation.…”

Mechanisms Controlling Hematopoiesis

“…Multiple bone marrow-derived hematopoietic precursor populations that belong mainly to the MPP or the CLP subsets are able to enter the thymus (Saran et al, 2010;Serwold et al, 2009), where they represent the population of early thymic progenitors (ETP), the initial source for the development of T cells. At this developmental stage the ETP still retain beside the T cell developmental potential also the capability to develop into B cells, macrophages, granulocytes, dendritic cells, and NK cells.…”