Sébastien Trop

Low Activation Threshold As a Mechanism for Ligand-Independent Signaling in Pre-T Cells

Pre-TCR complexes are thought to signal in a ligand-independent manner because they are constitutively targeted to lipid rafts. We report that ligand-independent signaling is not a unique capability of the pre-TCR complex. Indeed, the TCRα subunit restores development of pTα-deficient thymocytes to the CD4+CD8+ stage even in the absence of conventional MHC class I and class II ligands. Moreover, we found that pre-TCR and αβTCR complexes exhibit no appreciable difference in their association with lipid rafts, suggesting that ligand-independence is a function of the CD4−CD8− (DN) thymocytes in which pre-TCR signaling occurs. In agreement, we found that only CD44−CD25+ DN thymocytes (DN3) enabled activation of extracellular signal-regulated kinases by the pre-TCR complex. DN thymocytes also exhibited a lower signaling threshold relative to CD4+CD8+ thymocytes, which was associated with both the markedly elevated lipid raft content of their plasma membranes and more robust capacitative Ca2+ entry. Taken together these data suggest that cell-autonomous, ligand-independent signaling is primarily a property of the thymocytes in which pre-TCR signaling occurs.

Inhibition of T Cell Protein Tyrosine Phosphatase Enhances Interleukin-18-Dependent Hematopoietic Stem Cell Expansion†‡§

The clinical application of hematopoietic progenitor cell‐based therapies for the treatment of hematological diseases is hindered by current protocols, which are cumbersome and have limited efficacy to augment the progenitor cell pool. We report that inhibition of T‐cell protein tyrosine phosphatase (TC‐PTP), an enzyme involved in the regulation of cytokine signaling, through gene knockout results in a ninefold increase in the number of hematopoietic progenitors in murine bone marrow (BM). This effect could be reproduced using a short (48 hours) treatment with a pharmacological inhibitor of TC‐PTP in murine BM, as well as in human BM, peripheral blood, and cord blood. We also demonstrate that the ex vivo use of TC‐PTP inhibitor only provides a temporary effect on stem cells and did not alter their capacity to reconstitute all hematopoietic components in vivo. We establish that one of the mechanisms whereby inhibition of TC‐PTP mediates its effects involves the interleukin‐18 (IL‐18) signaling pathway, leading to increased production of IL‐12 and interferon‐gamma by progenitor cells. Together, our results reveal a previously unrecognized role for IL‐18 in contributing to the augmentation of the stem cell pool and provide a novel and simple method to rapidly expand progenitor cells from a variety of sources using a pharmacological compound. STEM CELLS2013;31:293–304