T cell paralysis is a main feature of measles virus (MV) induced immunosuppression. to suppress overshooting T cell responses. In line with its suppressive activity exaggerated prolonged NSM2 activation as occurring in co-stimulated T cells following MV exposure was associated with aberrant compartmentalization of ceramides loss of spreading responses interference with accumulation of tyrosine phosphorylated protein species and expansion. Altogether this study for the first time reveals a role of NSM2 in physiological T cell stimulation which is dampening and can be abused by a virus which promotes enhanced and prolonged NSM2 activation to cause pathological T cell suppression. Author Summary Though the ability of measles virus (MV) to impair T cell activation has long been known it is mechanistically not well understood. We have shown earlier that MV can contact dependently trigger activation of sphingomyelinases which is known to affect compartmentalization of membrane lipids and proteins. Because these are particularly important in the activity Ritonavir of the immune synapse (IS) we investigated whether MV-induced sphingomyelinase activity would interfere at that level with T cell activation. Our study for the first time revealed that the neutral sphingomyelinase 2 (NSM2) is transiently activated in primary T cells by co-stimulation through CD3 and CD28 and that this does occur to dampen early T cell responses. The virus appears to exploit this inhibitory activity of the enzyme to suppress T cell activation by promoting an enhanced and prolonged NSM2 activation. These findings do not only assign a hitherto novel role of the NSM2 in regulating T cell responses but also reveal a novel strategy for viral T cell suppression. Introduction Plasma membrane ceramides are released in response to activation of sphingomyelinases and condense into large platforms which alter biophysical properties of the cell membrane. In addition to other stimuli ligation of certain surface molecules also including death receptor family members and viral attachment receptors efficiently activates neutral and/or acid sphingomyelinase (NSM or ASM respectively) followed by Mouse monoclonal to GABPA ceramide release (reviewed in [1]-[3]). Ceramide enriched membrane microdomains act to regulate sorting Ritonavir of membrane proteins and their signalosomes and this affects a variety of biological responses including lateral and vertical receptor segregation as particularly relevant for pathogen uptake apoptosis cell motility and proliferation [3]-[6]. Measles virus (MV) causes profound generalized immunosuppression and interference with T cell viability expansion and function is one of its major hallmarks. A plethora of findings supports the interpretation that MV is acquired and transferred by CD150+ antigen-presenting cells to the secondary lymphatic tissues where it can be transmitted to and deplete CD150+ lymphocytes especially memory T cells [7]-[9]. Though being Ritonavir infected to a very limited extent peripheral blood cells of patients however are generally refractory to expansion driven by polyclonal and antigen-specific stimulation implying they had been paralysed by mechanisms independently of direct infection. In line with this hypothesis exposure of uninfected lymphocytes to UV-inactivated MV or the MV glycoprotein complex (gpc) was sufficient to induce their arrest and deficient mice driven by syngenic superantigen-loaded bone marrow derived DCs was significantly enhanced as compared to that of sufficient littermates (Fig. 3E right panel). In contrast to expansion neither release of cytokines (IL-2 IL-4 IL-5 IL-10 IFN-γ or TNF-α) 4 10 24 or 72 h following α-CD3/CD28 stimulation or intracellular accumulation of IL-2 IL-10 IFN-γ or IL-17α) following a 4 h restimulation were detectably affected by NSM knockdown in human T cells (not shown). Altogether these observations suggest that NSMKD facilitates initiation of T cell activation and therefore NSM activity acts to Ritonavir dampen early T cell activation thresholds. Figure 6 NSM activation Ritonavir contributes to MV interference with T cell early activation and expansion. MV exposure alters ceramide and NSM compartmentalization at stimulatory interfaces If NSM activity regulates the initiation threshold of physiological T cell activation conditions additionally enhancing NSM activity could possibly further dampen T cell activation by promoting timely or spatially aberrant ceramide release..