It is believed that memory CD8+ T cells are maintained in secondary lymphoid tissues, peripheral tissues, and BM by homeostatic proliferation. Their survival has been shown to be dependent on IL-7, but it is unclear where they acquire it. Here we show that in murine BM, memory CD8+ T cells individually colocalize with IL-7+ reticular stromal cells. The T cells are resting in terms of global transcription and do not express markers of activation, for example, 4-1BB (CD137), IL-2, or IFN-γ, despite the expression of CD69 on about 30% of the cells. Ninety-five percent of the memory CD8+ T cells in BM are in G0 phase of cell cycle and do not express Ki-67. Less than 1% is in S/M/G2 of cell cycle, according to propidium iodide staining. While previous publications have estimated the extent of proliferation of CD8+ memory T cells on the basis of BrdU incorporation, we show here that BrdU itself induces proliferation of CD8+ memory T cells. Taken together, the present results suggest that CD8+ memory T cells are maintained as resting cells in the BM in dedicated niches with their survival conditional on IL-7 receptor signaling.

Osteoclasts are derived from hematopoietic precursor cells belonging to the monocyte/macrophage lineage. Osteoclast development has been reported to be regulated by several molecules such as macrophage colony-stimulating factor (M-CSF), receptor activator of nuclear factor (NF)-B ligand (RANKL), and a decoy receptor of RANKL, osteoprotegerin (OPG). Recently, it was demonstrated that the Notch signaling pathway regulates myeloid differentiation and antagonizes cell fate determination, however, the effect of Notch signaling on the osteoclast lineage has not been reported. In this study, we examined the effect of signaling via Notch receptors on the differentiation into osteoclasts by using cells from the bone marrow, spleen, and peritoneal cavity, and a cloned macrophagelike cell line IntroductionThe control of cell fate by Notch signaling was first described for Drosophila melanogaster neural/epidermal precursors as a mechanism involving lateral inhibition. 1,2 At present, 4 Notch receptors, Notch-1, -2, -3, and -4, and their ligands, Delta-1, -3, and -4, and Jagged-1 and -2 have been identified in mammals. [3][4][5][6][7][8][9][10][11] Notch ligands bind to Notch receptors through their extracellular domain, trigger proteolytic processing, and release the Notch intracellular domain (NIC) from the cell membrane. Cleaved NIC interacts with the DNA-binding transcription factor CBF1/RBP-J, and the complex is translocated into the nucleus. 12,13 This complex binds to the Hairy enhancer of split (Hes1) promoter and stimulates the transcription of the Hes1 gene. 14,15 Overexpressed NIC acts as a constitutively active form of Notch receptor. 16 Recently, several studies have reported the roles of Notch signaling in hematopoietic cell development. 17-20 NIC transgenic mice have defective B-cell development. 21 Notch signaling influences the decision of differentiation into ␣␤ versus ␥␦T cells, 22 as well as that of differentiation into CD4 versus CD8 T cells. 23 Delta-1 blocks differentiation into the B-cell lineage, while it promotes the emergence of T/natural killer (NK) cell precursors. 24 In myeloid cell lineages, an immobilized form of the extracellular domain of Delta-1 induces monocytes to undergo apoptosis when treated with macrophage colony-stimulating factor (M-CSF) 25 and inhibits the differentiation of monocytes into mature macrophages when treated with granulocyte/macrophage (GM)-CSF, but permits their differentiation into dendritic cells in the presence of GM-CSF and interleukin-4 (IL-4). 26 Osteoclasts are also included in the myeloid lineage and are derived from hematopoietic precursor cells shared with the macrophage and dendritic cell lineages. 27,28 Osteoclast differentiation is a multistep process that eventually leads to expression of tartrateresistant acid phosphatase (TRAP), multinucleation, and boneresorbing activity. [29][30][31][32][33] Osteoclastogenesis is dependent on stromal cells that support hematopoiesis, [34][35][36] and it has been demonstrated that the critical molecules produced by s...

Osteoclasts are derived from hemopoietic stem cells and play critical roles in bone resorption and remodeling. Multinucleated osteoclasts are attached tightly to bone matrix, whereas precursor cells with the potential to differentiate into osteoclasts in culture are widely distributed. In this study, we assessed the characteristics of osteoclast precursors in bone marrow (BM) and in extramedullary organs as indicated by their responsiveness to ligands for Toll-like receptors (TLRs) and to TNF-α. Development of osteoclasts from precursor cells in the BM was inhibited by CpG oligonucleotides, a ligand for TLR9, but not by LPS, a ligand for TLR4. BM osteoclasts were induced by TNF-α as well as receptor activator of NF-κB ligand in the presence of M-CSF. Splenic osteoclast precursors, even in osteoclast-deficient osteopetrotic mice, differentiated into mature osteoclasts following exposure to TNF-α or receptor activator of NF-κB ligand. However, splenic osteoclastogenesis was inhibited by both LPS and CpG. Osteoclastogenesis from peritoneal precursors was inhibited by not only these TLR ligands but also TNF-α. The effects of peptidoglycan, a ligand for TLR2, were similar to those of LPS. BM cells precultured with M-CSF were characterized with intermediate characteristics between those of splenic and peritoneal cavity precursors. Taken together, these findings demonstrate that osteoclast precursors are not identical in the tissues examined. To address the question of why mature osteoclasts occur only in association with bone, we may characterize not only the microenvironment for osteoclastogenesis, but also the osteoclast precursor itself in intramedullary and extramedullary tissues.

The tooth, composed of dentin and enamel, develops through epithelium-mesenchyme interactions. Neural crest (NC) cells contribute to the dental mesenchyme in the developing tooth and differentiate into dentin-secreting odontoblasts. NC cells are known to differentiate into chondrocytes and osteoblasts in the craniofacial region. However, it is not clear whether the dental mesenchymal cells in the developing tooth possess the potential to differentiate into a lineage(s) other than the odontoblast lineage. In this study, we prepared mesenchymal cells from E13. STEM CELLS 2007;25:78 -87