Sphingosine-1-phosphate (S1P), a lipid mediator enriched in blood, controls the dynamic migration of osteoclast (OC) precursors (OPs) between the blood and bone, in part via the S1P receptor 1 (S1PR1) which directs positive chemotaxis toward S1P. We show that OPs also express S1PR2, an S1P receptor which mediates negative chemotaxis (or chemorepulsion). OP-positive chemotaxis is prominent in gradients with low maximal concentrations of S1P, whereas such behavior is minimal in fields with high maximal S1P concentrations. This reverse-directional behavior is caused by S1PR2-mediated chemorepulsion acting to override S1PR1 upgradient motion. S1PR2-deficient mice exhibit moderate osteopetrosis as a result of a decrease in osteoclastic bone resorption, suggesting that S1PR2 contributes to OP localization on the bones mediated by chemorepulsion away from the blood where S1P levels are high. Inhibition of S1PR2 function by the antagonist JTE013 changed the migratory behavior of monocytoid cells, including OPs, and relieved osteoporosis in a mouse model by limiting OP localization and reducing the number of mature OCs attached to the bone surface. Thus, reciprocal regulation of S1P-dependent chemotaxis controls bone remodeling by finely regulating OP localization. This regulatory axis may be promising as a therapeutic target in diseases affecting OC-dependent bone remodeling.

A substantial proportion of patients with acute graft‐versus‐host disease (aGVHD) respond to cell therapy with culture‐expanded human bone marrow mesenchymal stromal/stem cells (BM‐MSCs). However, the mechanisms by which these cells can ameliorate aGVHD‐associated complications remain to be clarified. We show here that BM‐MSC‐derived extracellular vesicles (EVs) recapitulated the therapeutic effects of BM‐MSCs against aGVHD. Systemic infusion of human BM‐MSC‐derived EVs prolonged the survival of mice with aGVHD and reduced the pathologic damage in multiple GVHD‐targeted organs. In EV‐treated GVHD mice, CD4+ and CD8+ T cells were suppressed. Importantly, the ratio of CD62L‐CD44+ to CD62L + CD44‐ T cells was decreased, suggesting that BM‐MSC‐derived EVs suppressed the functional differentiation of T cells from a naive to an effector phenotype. BM‐MSC‐derived EVs also preserved CD4 + CD25 + Foxp3+ regulatory T cell populations. In a culture of CD3/CD28‐stimulated human peripheral blood mononuclear cells with BM‐MSC‐derived EVs, CD3+ T cell activation was suppressed. However, these cells were not suppressed in cultures with EVs derived from normal human dermal fibroblasts (NHDFs). NHDF‐derived EVs did not ameliorate the clinical or pathological characteristics of aGVHD in mice, suggesting an immunoregulatory function unique to BM‐MSC‐derived EVs. Microarray analysis of microRNAs in BM‐MSC‐derived EVs versus NHDF‐derived EVs showed upregulation of miR‐125a‐3p and downregulation of cell proliferative processes, as identified by Gene Ontology enrichment analysis. Collectively, our findings provide the first evidence that amelioration of aGVHD by therapeutic infusion of BM‐MSC‐derived EVs is associated with the preservation of circulating naive T cells, possibly due to the unique microRNA profiles of BM‐MSC‐derived EVs. Stem Cells 2018;36:434–445