Silencing of T cell activation and function is a highly efficient strategy of immunosuppression induced by pathogens. By promoting formation of membrane microdomains essential for clustering of receptors and signalling platforms in the plasma membrane, ceramides accumulating as a result of membrane sphingomyelin breakdown are not only essential for assembly of signalling complexes and pathogen entry, but also act as signalling modulators, e. g. by regulating relay of phosphatidyl-inositol-3-kinase (PI3K) signalling. Their role in T lymphocyte functions has not been addressed as yet. We now show that measles virus (MV), which interacts with the surface of T cells and thereby efficiently interferes with stimulated dynamic reorganisation of their actin cytoskeleton, causes ceramide accumulation in human T cells in a neutral (NSM) and acid (ASM) sphingomyelinase–dependent manner. Ceramides induced by MV, but also bacterial sphingomyelinase, efficiently interfered with formation of membrane protrusions and T cell spreading and front/rear polarisation in response to β1 integrin ligation or αCD3/CD28 activation, and this was rescued upon pharmacological or genetic ablation of ASM/NSM activity. Moreover, membrane ceramide accumulation downmodulated chemokine-induced T cell motility on fibronectin. Altogether, these findings highlight an as yet unrecognised concept of pathogens able to cause membrane ceramide accumulation to target essential processes in T cell activation and function by preventing stimulated actin cytoskeletal dynamics.

Immunosuppression is the major cause of infant death associated with acute measles. Hallmarks of this generalized modulation of immune functions include: (1) lymphopenia, (2) a prolonged cytokine imbalance consistent with suppression of cellular immunity to secondary infections and (3) silencing of peripheral blood lymphocytes that fail to expand in response to ex vivo stimulation. Lymphopenia results from depletion of T cells by mechanisms also involving MV infection, and expression of the major MV receptor CD150 plays an important role for targeting these cells. Virus transfer to T cells is thought to be mediated by dendritic cells (DCs), which are considered as central to the induction of T-cell silencing and functional skewing. MV interaction modulates functional differentiation of DCs, and thereby expression pattern of costimulatory molecules and soluble mediators. Moreover, MV proteins expressed by these cells actively silence T cells by interfering with signaling pathways essential for T-cell activation. As an essential component of this interference, the MV glycoprotein complex activates cellular sphingomyelinases in a contact-dependent manner, and these are effective at preventing stimulated rearrangements of the actin cytoskeleton and thereby morphological and functional polarization and motility of T cells.

Interference with dendritic cell (DC) maturation and function is considered to be central to measles virus (MV)-induced immunosuppression. Temporally ordered production of chemokines and switches in chemokine receptor expression are essential for pathogen-driven DC maturation as they are prerequisites for chemotaxis and T cell recruitment. We found that MV infection of immature monocyte-derived DCs induced transcripts specific for CCL-1, -2, -3, -5, -17 and -22, CXCL-10 and CXCL-11, yet did not induce and CCL-20 at the mRNA and protein level. Within 24 h post-infection, T cell attraction was not detectably impaired by these cells. MV infection failed to promote the switch from CCR5 to CCR7 expression and this correlated with chemotactic responses of MV-matured DC cultures to CCL-3 rather than to CCL-19. Moreover, the chemotaxis of MV-infected DCs to either chemokine was compromised, indicating that MV also interferes with this property independently of chemokine receptor modulation. Hallmarks of measles virus (MV)-induced immuno-suppression include lymphopenia and proliferative unresponsiveness of peripheral blood lymphocytes to mitogens (Griffin et al., 1994;Schneider-Schaulies & ter Meulen, 2002). The ability of myeloid dendritic cells (DCs) to both initiate T cell activation and mediate viral transport to secondary lymphatics has determined that they are central to MV-specific immunity and immunosuppression (Pollara et al., 2005;Steinman et al., 2003). MV targets CD150 + haematopoetic cells (Condack et al., 2007; de Swart et al., 2007), and in experimentally infected macaques, mucosal DCs replicate MV in vivo (de Swart et al., 2007). DCs are generated from precursors and they mature upon MV exposure in vitro (Dubois et al., 2001;Fugier-Vivier et al., 1997;Grosjean et al., 1997;Kaiserlian et al., 1997;Schnorr et al., 1997;Servet-Delprat et al., 2000), this involves toll-like receptor (TLR) signalling and type I interferon (IFN) (Minagawa et al., 2001;Schnorr et al., 1997;Servet-Delprat et al., 2000;Shingai et al., 2007). MV-matured DCs (MV-DCs) fail to promote allogenic T cell expansion, which results from signalling by the viral glycoproteins expressed on the DC surface to T cells (Kerdiles et al., 2006;Schneider-Schaulies & Dittmer, 2006;Schneider-Schaulies et al., 2003;Servet-Delprat et al., 2003).Maturing DCs change their responsiveness from initially pro-inflammatory to lymphoid-type chemokines (reflected by a switch from CCR5 to CCR7 surface expression) which allows egress from peripheral sites where they respond to CCR5-binding chemokines, such as CCL-3, and migration towards secondary lymphatics in response to CCR7-binding chemokines, such as CCL-19 (Caux et al., 2002;Sallusto et al., 1998;Sozzani et al., 1998); alterations in chemokine release also occur (Piqueras et al., 2006). DC chemotaxis is targeted during immune evasion by vaccinia virus, human cytomegalovirus (HCMV), herpes simplex virus (HSV) and influenza A virus (Humrich et al., 2007;Moutaftsi et al., 2004;Prechtel et al., 2005;Salentin e...

In common with most viruses, measles virus (MV) relies on the integrity of the cytoskeleton of its host cells both with regard to efficient replication in these cells, but also retention of their motility which favors viral dissemination. It is, however, the surface interaction of the viral glycoprotein (gp) complex with receptors present on lymphocytes and dendritic cells (DCs), that signals effective initiation of host cell cytoskeletal dynamics. For DCs, these may act to regulate processes as diverse as viral uptake and sorting, but also the ability of these cells to successfully establish and maintain functional immune synapses (IS) with T cells. In T cells, MV signaling causes actin cytoskeletal paralysis associated with a loss of polarization, adhesion and motility, which has been linked to activation of sphingomyelinases and subsequent accumulation of membrane ceramides. MV modulation of both DC and T cell cytoskeletal dynamics may be important for the understanding of MV immunosuppression at the cellular level.