Induction of Membrane Ceramides: A Novel Strategy to Interfere with T Lymphocyte Cytoskeletal Reorganisation in Viral Immunosuppression

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Abstract
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. The ability of measles virus (MV) to impair T cell–dependent immune responses noted more than 100 years ago continues to be central to the severe generalised immunosuppression by this virus. Much has been learned about receptors and mechanisms, which determine the predilection of MV for hematopoetic cells. In contrast, little is known on a molecular level how MV interferes with processes relaying extracellular signals to T cells which translate into reorganisation of their cytoskeleton as required for their migration and cell–cell communication. Our study now shows that MV activates sphingomyelinases and ceramide accumulation in T cell membranes and this severely impairs integrity and stimulated reorganisation of their actin cytoskeleton, morphologically resulting in collapse of actin based protrusions, and functionally in impaired motility. During these studies, we appreciated, however, that cues other than MV eliciting ceramide accumulation in general also caused T cell paralysis. This indicates that ceramide accumulation and its consequences are not only a novel concept for MV-induced T cell silencing, but rather reflects a general strategy which may apply to extracellular ligands including pathogens able to promote plasma membrane ceramide accumulation, thereby preventing T cell activation.