Platelets induce apoptosis via membrane-bound FasL.

After tissue injury, both wound sealing and apoptosis contribute to restoration of tissue integrity and functionality. Although the role of platelets (PLTs) for wound closure and induction of regenerative processes is well established, the knowledge about their contribution to apoptosis is incomplete. Here, we show that PLTs present the death receptor Fas ligand (FasL) on their surface after activation. Activated PLTs as well as the isolated membrane fraction of activated PLTs but not of resting PLTs induced apoptosis in a dose-dependent manner in primary murine neuronal cells, human neuroblastoma cells, and mouse embryonic fibroblasts. Membrane protein from PLTs lacking membrane-bound FasL (FasL(△m/△m)) failed to induce apoptosis. Bax/Bak-mediated mitochondrial apoptosis signaling in target cells was not required for PLT-induced cell death, but increased the apoptotic response to PLT-induced Fas signaling. In vivo, PLT depletion significantly reduced apoptosis in a stroke model and an inflammation-independent model of N-methyl-d-aspartic acid-induced retinal apoptosis. Furthermore, experiments using PLT-specific PF4Cre(+) FasL(fl/fl) mice demonstrated a role of PLT-derived FasL for tissue apoptosis. Because apoptosis secondary to injury prevents inflammation, our findings describe a novel mechanism on how PLTs contribute to tissue homeostasis.

Cyclophilin A affects inflammation, virus elimination and myocardial fibrosis in coxsackievirus B3-induced myocarditis.

Extracellular cyclophilin A (CyPA) and its receptor Extracellular Matrix Metalloproteinase Inducer (EMMPRIN, CD147) modulate inflammatory processes beyond metalloproteinase (MMP) activity. Recently, we have shown that CyPA and CD147 are upregulated in patients with inflammatory cardiomyopathy. Here we investigate the role of CyPA and CD147 in murine coxsackievirus B3 (CVB3)-induced myocarditis. CVB3-infected CyPA(-/-) mice (129S6/SvEv) revealed a significantly reduced T-cell and macrophage recruitment at 8 days p.i. compared to wild-type mice. In A.BY/SnJ mice, treatment with the cyclophilin-inhibitor NIM811 was associated with a reduction of inflammatory lesions and MMP-9 expression but with enhanced virus replication 8 days p.i. At 28 days p.i. the extent of lesion areas was not affected bei NIM811, whereas the collagen content was reduced. Initiation of NIM811-treatment on day 12 (after an effective virus defense) resulted in an even more pronounced reduction of myocardial fibrosis. In conclusion, in CVB3-induced myocarditis CyPA is important for macrophage and T cell recruitment and effective virus defense and may represent a pharmacological target to modulate myocardial remodeling in myocarditis.

Platelets contribute to the pathogenesis of experimental autoimmune encephalomyelitis.

RATIONALE: Multiple sclerosis (MS) and its mouse model, experimental autoimmune encephalomyelitis (EAE), are inflammatory disorders of the central nervous system (CNS). The function of platelets in inflammatory and autoimmune pathologies is thus far poorly defined. OBJECTIVE: We addressed the role of platelets in mediating CNS inflammation in EAE. METHODS AND RESULTS: We found that platelets were present in human MS lesions as well as in the CNS of mice subjected to EAE but not in the CNS from control nondiseased mice. Platelet depletion at the effector-inflammatory phase of EAE in mice resulted in significantly ameliorated disease development and progression. EAE suppression on platelet depletion was associated with reduced recruitment of leukocytes to the inflamed CNS, as assessed by intravital microscopy, and with a blunted inflammatory response. The platelet-specific receptor glycoprotein Ibα (GPIbα) promotes both platelet adhesion and inflammatory actions of platelets and targeting of GPIbα attenuated EAE in mice. Moreover, targeting another platelet adhesion receptor, glycoprotein IIb/IIIa (GPIIb/IIIa), also reduced EAE severity in mice. CONCLUSIONS: Platelets contribute to the pathogenesis of EAE by promoting CNS inflammation. Targeting platelets may therefore represent an important new therapeutic approach for MS treatment.