Responsiveness of Developing T Cells to IL-7 Signals Is Sustained by miR-17∼92.

miRNAs regulate a large variety of developmental processes including development of the immune system. T cell development is tightly controlled through the interplay of transcriptional programs and cytokine-mediated signals. However, the role of individual miRNAs in this process remains largely elusive. In this study, we demonstrated that hematopoietic cell-specific loss of miR-17∼92, a cluster of six miRNAs implicated in B and T lineage leukemogenesis, resulted in profound defects in T cell development both at the level of prethymic T cell progenitors as well as intrathymically. We identified reduced surface expression of IL-7R and concomitant limited responsiveness to IL-7 signals as a common mechanism resulting in reduced cell survival of common lymphoid progenitors and thymocytes at the double-negative to double-positive transition. In conclusion, we identified miR-17∼92 as a critical modulator of multiple stages of T cell development.

Extra-thymic physiological T lineage progenitor activity is exclusively confined to cells expressing either CD127, CD90, or high levels of CD117.

T cell development depends on continuous recruitment of progenitors from bone marrow (BM) to the thymus via peripheral blood. However, both phenotype and functional characteristics of physiological T cell precursors remain ill-defined. Here, we characterized a putative CD135(+)CD27(+) T cell progenitor population, which lacked expression of CD127, CD90, and high levels of CD117 and was therefore termed triple negative precursor (TNP). TNPs were present in both BM and blood and displayed robust T lineage potential, but virtually no myeloid or B lineage potential, in vitro. However, TNPs did not efficiently generate T lineage progeny after intravenous or intrathymic transfer, suggesting that a physiological thymic microenvironment does not optimally support T cell differentiation from TNPs. Thus, we propose that physiological T cell precursors are confined to populations expressing either CD127, CD90, or high levels of CD117 in addition to CD135 and CD27 and that TNPs may have other physiological functions.

Chemokine receptor CX3CR1 promotes dendritic cell development under steady-state conditions.

Expression of CX3CR1 is an attribute of myeloid precursors committed to the monocyte/macrophage (Mφ)/DC lineages and is maintained during all stages of DC differentiation. Nevertheless, the exact role of this molecule during developmental progression of myeloid precursors towards the DC lineage remains elusive. To overcome potential compensatory mechanisms and issues of redundancy, we employed competitive adoptive transfer experiments to assess a possible function of CX3CR1 in DC and monocyte/Mφ differentiation in vivo. We show here that expression of CX3CR1 promotes the generation of DCs and monocytes/Mφ under steady-state conditions and during compensatory expansion after selective depletion of DCs, but not under inflammatory conditions evoked by sub-lethal irradiation. Direct administration of CX3CR1-deficient and CX3CR1-sufficient precursors into the spleen or the thymus resulted in a similar competitive advantage of WT over CX3CR1-deficient precursors as i.v. transfer, suggesting that CX3CR1-mediated survival rather than recruitment to lymphoid organs is critical for DC/Mφ differentiation. In conclusion, our data support the hypothesis that CX3CR1 promotes proper development of myeloid precursors into DCs and monocytes/Mφs under steady-state conditions, possibly by providing survival signals or mediating accessibility to organ-specific niches, rather than acting as a mediator of homing to the spleen or the thymus.

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

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.