Cytomegalovirus inhibition of extrinsic apoptosis determines fitness and resistance to cytotoxic CD8 T cells.

Viral immune evasion is currently understood to focus on deflecting CD8 T cell recognition of infected cells by disrupting antigen presentation pathways. We evaluated viral interference with the ultimate step in cytotoxic T cell function, the death of infected cells. The viral inhibitor of caspase-8 activation (vICA) conserved in human cytomegalovirus (HCMV) and murine CMV (MCMV) prevents the activation of caspase-8 and proapoptotic signaling. We demonstrate the key role of vICA from either virus, in deflecting antigen-specific CD8 T cell-killing of infected cells. vICA-deficient mutants, lacking either UL36 or M36, exhibit greater susceptibility to CD8 T cell control than mutants lacking the set of immunoevasins known to disrupt antigen presentation via MHC class I. This difference is evident during infection in the natural mouse host infected with MCMV, in settings where virus-specific CD8 T cells are adoptively transferred. Finally, we identify the molecular mechanism through which vICA acts, demonstrating the central contribution of caspase-8 signaling at a point of convergence of death receptor-induced apoptosis and perforin/granzyme-dependent cytotoxicity.

Immune Protection by a Cytomegalovirus Vaccine Vector Expressing a Single Low-Avidity Epitope.

Experimental CMV-based vaccine vectors expressing a single MHC class I-restricted high-avidity epitope provided strong, T cell-dependent protection against viruses or tumors. In this study we tested the low-avidity epitope KCSRNRQYL, and show that a mouse CMV (MCMV) vector provides complete immune control of recombinant vaccinia virus expressing the same epitope if KCSRNRQYL is expressed within the immediate-early MCMV gene ie2 The same epitope expressed within the early M45 gene provided no protection, although MCMV vectors expressing the high-avidity epitope SSIEFARL induced protective immunity irrespective of gene expression context. Immune protection was matched by Ag-induced, long-term expansion of effector memory CD8 T cells, regardless of epitope avidity. We explained this pattern by observing regularities in Ag competition, where responses to high-avidity epitopes outcompeted weaker ones expressed later in the replicative cycle of the virus. Conversely, robust and early expression of a low-avidity epitope compensated its weak intrinsic antigenicity, resulting in strong and sustained immunity and immune protection.

IFNß secreted by microglia mediates clearance of myelin debris in CNS autoimmunity.

INTRODUCTION: Multiple sclerosis (MS) is a chronic demyelinating disorder of the central nervous system (CNS) leading to progressive neurological disability. Interferon ß (IFNß) represents a standard treatment for relapsing-remitting MS and exogenous administration of IFNß exhibits protective effects in experimentally induced CNS autoimmunity. Also, genetic deletion of IFNß in mice leads to an aggravation of disease symptoms in the MS model of experimental autoimmune encephalomyelitis (EAE). However, neither the underlying mechanisms mediating the beneficial effects nor the cellular source of IFNß have been fully elucidated. RESULTS: In this report, a subpopulation of activated microglia was identified as the major producers of IFNß in the CNS at the peak of EAE using an IFNß-fluorescence reporter mouse model. These IFNß expressing microglia specifically localized to active CNS lesions and were associated with myelin debris in demyelinated cerebellar organotypic slice cultures (OSCs). In response to IFNß microglia showed an enhanced capacity to phagocytose myelin in vitro and up-regulated the expression of phagocytosis-associated genes. IFNß treatment was further sufficient to stimulate association of microglia with myelin debris in OSCs. Moreover, IFNß-producing microglia mediated an enhanced removal of myelin debris when co-transplanted onto demyelinated OSCs as compared to IFNß non-producing microglia. CONCLUSIONS: These data identify activated microglia as the major producers of protective IFNß at the peak of EAE and as orchestrators of IFNß-induced clearance of myelin debris.