Intact CD3-specific antibody transiently depletes large numbers of T cells and subsequently induces long-term immune tolerance. The underlying mechanisms for the systemic tolerance, however, remain unclear. We show here that treatment of normal mice with intact antibody to CD3 increases systemic transforming growth factor-beta (TGF-beta) produced by phagocytes exposed to apoptotic T cells. Among the phagocytes, macrophages and immature dendritic cells (iDCs) secrete TGF-beta upon ingestion of apoptotic T cells, which induces CD4+Foxp3+ regulatory T cells in culture and contributes to immune tolerance mediated by CD3-specific antibody in vivo. In accordance with these results, depletion of macrophages and iDCs not only abrogates CD3-specific antibody-mediated prevention of myelin oligodendrocyte glycoprotein-induced acute experimental autoimmune encephalomyelitis (EAE), but also reverses the therapeutic effects of antibody to CD3 on established disease in a model of relapsing-remitting EAE. Thus, CD3-specific antibody-induced immune tolerance is associated with TGF-beta production in phagocytes involved in clearing apoptotic T cells, which suggests that apoptosis is linked to active suppression in immune tolerance.

BackgroundOsteoporosis is the most prevalent skeletal disorder, characterized by a low bone mineral density (BMD) and bone structural deterioration, leading to bone fragility fractures. Accelerated bone resorption by osteoclasts has been established as a principal mechanism in osteoporosis. However, recent experimental evidences suggest that inappropriate apoptosis of osteoblasts/osteocytes accounts for, at least in part, the imbalance in bone remodeling as occurs in osteoporosis. The aim of this study is to examine whether aspirin, which has been reported as an effective drug improving bone mineral density in human epidemiology studies, regulates the balance between bone resorption and bone formation at stem cell levels.Methods and FindingsWe found that T cell-mediated bone marrow mesenchymal stem cell (BMMSC) impairment plays a crucial role in ovariectomized-induced osteoporosis. Ex vivo mechanistic studies revealed that T cell-mediated BMMSC impairment was mainly attributed to the apoptosis of BMMSCs via the Fas/Fas ligand pathway. To explore potential of using pharmacologic stem cell based intervention as an approach for osteoporosis treatment, we selected ovariectomy (OVX)-induced ostoeporosis mouse model to examine feasibility and mechanism of aspirin-mediated therapy for osteoporosis. We found that aspirin can inhibit T cell activation and Fas ligand induced BMMSC apoptosis in vitro. Further, we revealed that aspirin increases osteogenesis of BMMSCs by aiming at telomerase activity and inhibits osteoclast activity in OVX mice, leading to ameliorating bone density.ConclusionOur findings have revealed a novel osteoporosis mechanism in which activated T cells induce BMMSC apoptosis via Fas/Fas ligand pathway and suggested that pharmacologic stem cell based intervention by aspirin may be a new alternative in osteoporosis treatment including activated osteoblasts and inhibited osteoclasts.

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