Involvement of Fas and FasL in Ectromelia virus-induced apoptosis in mouse brain

A lack of Fas/FasL signalling leads to disturbances in the antiviral response during ectromelia virus infection

Ectromelia virus (ECTV) is an orthopoxvirus (OPV) that causes mousepox, the murine equivalent of human smallpox. Fas receptor-Fas ligand (FasL) signaling is involved in apoptosis of immune cells and virus-specific cytotoxicity. The Fas/FasL pathway also plays an important role in controlling the local inflammatory response during ECTV infection. Here, the immune response to the ECTV Moscow strain was examined in Fas (-) (lpr), FasL (-) (gld) and C57BL6 wild-type mice. During ECTV-MOS infection, Fas- and FasL mice showed increased viral titers, decreased total numbers of NK cells, CD4(+) and CD8(+) T cells followed by decreased percentages of IFN-γ expressing NK cells, CD4(+) and CD8(+) T cells in spleens and lymph nodes. At day 7 of ECTV-MOS infection, Fas- and FasL-deficient mice had the highest regulatory T cell (Treg) counts in spleen and lymph nodes in contrast to wild-type mice. Furthermore, at days 7 and 10 of the infection, we observed significantly higher numbers of PD-L1-expressing dendritic cells in Fas (-) and FasL (-) mice in comparison to wild-type mice. Experiments in co-cultures of CD4(+) T cells and bone-marrow-derived dendritic cells showed that the lack of bilateral Fas-FasL signalling led to expansion of Tregs. In conclusion, our results demonstrate that during ECTV infection, Fas/FasL can regulate development of tolerogenic DCs and Tregs, leading to an ineffective immune response.

Fas/FasL pathway participates in regulation of antiviral and inflammatory response during mousepox infection of lungs

Fas receptor-Fas ligand (FasL) signalling is involved in apoptosis of immune cells as well as of the virus infected target cells but increasing evidence accumulates on Fas as a mediator of apoptosis-independent processes such as induction of activating and proinflammatory signals. In this study, we examined the role of Fas/FasL pathway in inflammatory and antiviral response in lungs using a mousepox model applied to C57BL6/J, B6. MRL-Faslpr/J, and B6Smn.C3-Faslgld/J mice. Ectromelia virus (ECTV) infection of Fas- and FasL-deficient mice led to increased virus titers in lungs and decreased migration of IFN-γ expressing NK cells, CD4+ T cells, CD8+ T cells, and decreased IL-15 expression. The lungs of ECTV-infected Fas- and FasL-deficient mice showed significant inflammation during later phases of infection accompanied by decreased expression of anti-inflammatory IL-10 and TGF-β1 cytokines and disturbances in CXCL1 and CXCL9 expression. Experiments in vitro demonstrated that ECTV-infected cultures of epithelial cells, but not macrophages, upregulate Fas and FasL and are susceptible to Fas-induced apoptosis. Our study demonstrates that Fas/FasL pathway during ECTV infection of the lungs plays an important role in controlling local inflammatory response and mounting of antiviral response.

Immune responses to West Nile virus infection in the central nervous system

West Nile virus (WNV) continues to cause outbreaks of severe neuroinvasive disease in humans and other vertebrate animals in the United States, Europe, and other regions of the world. This review discusses our understanding of the interactions between virus and host that occur in the central nervous system (CNS), the outcome of which can be protection, viral pathogenesis, or immunopathogenesis. We will focus on defining the current state of knowledge of WNV entry, tropism, and host immune response in the CNS, all of which affect the balance between injury and successful clearance.

Fas/FasL and perforin-granzyme pathways mediated T cell cytotoxic responses in infectious bursal disease virus infected chickens

Infectious bursal disease (IBD) is a highly contagious disease of chickens which leads to immunosuppression. In our previous study it was demonstrated that, possibly, CD4(+) and CD8(+) T cells may employ perforin and granzyme-A pathway for the clearance of IBDV-infected bursal cells. In this study, we evaluated the cytotoxic T cell responses involving two independently functioning but complementary mechanisms: Fas-Fas ligand and perforin-granzyme pathways in IBDV-infected chickens. As demonstrated previously, infection of chickens with IBDV was accompanied by influx of CD8(+) T cells in the bursa and spleen. There was an upregulation in the gene expression of cytolytic molecules: Fas and Fas ligand (FasL), perforin (PFN) and granzyme-A (Gzm-A) in bursal and in the splenic tissues of IBDV inoculated chickens. Additionally, for the first time, we detected Fas, Fas ligand, Caspase-3 and PFN producing CD8(+) T cells in the bursa and spleen of IBDV-infected chickens. The infiltration and activation of CD8(+) T cells was substantiated by the detection of Th1 cytokine, IFN-γ. These data suggest that T cells may be involved in the clearance of virus from the target organ bursa and peripheral tissues such as spleen. The findings of these studies provide new insights into the pathogenesis of IBD and provide mechanistic evidence that the cytotoxic T cells may act through both Fas-FasL and perforin-granzyme pathways in mediating the clearance of virus-infected cells.

The role of inflammasome modulation in virulence

Pathogens frequently exist in an immunological balancing act with their host. Pathogens must not only replicate within a host but also transmit effectively between hosts to perpetuate their species. On the other hand, the host seeks to maintain homeostasis by clearing pathogens. The inflammasome is a multi-protein complex that can induce cell death and processes IL-1β and additional proinflammatory substrates. In this review we discuss the pathogen specific modulation of inflammasome activation and the role this plays in virulence and disease pathology.

Oncolytic poxvirus armed with Fas ligand leads to induction of cellular Fas receptor and selective viral replication in FasR-negative cancer

The TNF superfamily members including Fas ligand and TRAIL have been studied extensively for cancer therapy, including as components of gene therapy. We examined the use of FasL expression to achieve tumor selective replication of an oncolytic poxvirus (vFasL) and explored its biology and therapeutic efficacy for FasR− and FasR+ cancers. Infection of FasR+ normal and MC38 cancer cells by vFasL led to abortive viral replication due to acute apoptosis and subsequently displayed both reduced pathogenicity in non-tumor bearing mice and reduced efficacy in FasR+ tumor-bearing mice. Infection of FasR− B16 cancer cells by vFasL led to efficient viral replication, followed by late induction of FasR and subsequent apoptosis. Treatment with vFasL compared to its parental virus (vJS6) led to increased tumor regression and prolonged survival of mice with FasR− cancer (B16), but not with FasR+ cancer (MC38). The delayed induction of FasR by viral infection in FasR− cells provides for potential increased efficacy beyond the limit of the direct oncolytic effect. FasR induction provides one mechanism for tumor selective replication of oncolytic poxviruses in FasR− cancers with enhanced safety. The overall result is both a safer and more effective oncolytic virus for FasR− cancer.

Beclin 1 is involved in regulation of apoptosis and autophagy during replication of ectromelia virus in permissive L929 cells

Several reports have brought to light new and interesting findings on the involvement of autophagy and apoptosis in pathogenesis of viral and bacterial diseases, as well as presentation of foreign antigens. Our model studies focused on the involvement of apoptosis during replication of highly virulent Moscow strain of ectromelia virus (ECTV-MOS). Here, we show evidence that autophagy is induced during mousepox replication in a cell line. Fluorescence microscopy revealed increase of LC3 (microtubule-associated protein 1 light chain 3) aggregation in infected as opposed to non-infected control L929 cells. Furthermore, Western blot analysis showed that replication of ECTV-MOS in L929 cells led to the increase in LC3-II (marker of autophagic activity) expression. Beclin 1 strongly colocalized with extranuclear viral replication centers in infected cells, whereas expression of Bcl-2 decreased in those centers as shown by fluorescence microscopy. Loss of Beclin 1-Bcl-2 interaction may lead to autophagy in virus-infected L929 cells. To assess if Beclin 1 has a role in regulation of apoptosis during ECTV-MOS infection, we used small interfering RNA directed against beclin 1 following infection. Early and late apoptotic cells were analyzed by flow cytometry after AnnexinV and propidium iodide staining. Silencing of beclin 1 resulted in decreased percentage of early and late apoptotic cells in the late stage of ECTV-MOS infection in L929 cells. We conclude that Beclin 1 plays an important role in regulation of both, autophagy and apoptosis, during ECTV-MOS replication in L929 permissive cells.

Central roles of NLRs and inflammasomes in viral infection

The immune response to viral infections is determined by a complex interplay between the pathogen and the host. Innate immune cells express a set of cytosolic sensors to detect viral infection. Recognition by these sensors induces the production of type I interferons and the assembly of inflammasome complexes that activate caspase-1, leading to production of interleukin-1β (IL-1β) and IL-18. Here, I discuss recent progress in our understanding of the central roles of NOD-like receptors (NLRs) and inflammasomes in the immune response during viral infections. This information will improve our understanding of host defence mechanisms against viruses and provide new avenues for interfering in the pathogenesis of infectious diseases.

Fas-mediated apoptotic signaling in the mouse brain following reovirus infection

Type 3 (T3) reovirus strains induce apoptotic neuronal cell death and lethal encephalitis in infected mice. T3 strain Dearing (T3D)-induced apoptosis in primary neuronal cultures occurs by a Fas-mediated mechanism and requires the activation of caspase 8. We now show that Fas mRNA is upregulated in the brains of mice infected with encephalitic reovirus T3D and T3 strain Abney (T3A) but not following infection with nonencephalitic reovirus type 1 strain Lang. Fas is upregulated in regions of the brain that are injured during infection with T3 reovirus strains and colocalizes with virus antigen in individual neurons. In contrast, levels of FasL mRNA induced by encephalitic and nonencephalitic reovirus strains do not differ significantly. Caspase 8, the initiator caspase associated with Fas-mediated apoptosis, is activated in the cortex and hippocampal regions of both T3D- and T3A-infected mice. Furthermore, Bid cleavage and the activation of caspase 9 in the brains of T3D-infected mice suggest that the caspase 8-dependent activation of mitochondrial apoptotic signaling contributes to virus-induced apoptosis. We have previously shown that the inhibition of c-Jun N-terminal kinase (JNK) signaling blocks T3D-induced apoptosis and improves the outcome of virus-induced encephalitis. We now show that the reovirus-induced upregulation of Fas requires JNK signaling, thereby providing a link between reovirus-induced death receptor signaling and mitogen-activated protein kinase pathways and a potential mechanism for the therapeutic action of JNK inhibition.

Fas ligand interactions contribute to CD8+ T-cell-mediated control of West Nile virus infection in the central nervous system

West Nile virus (WNV) is a neurotropic flavivirus that causes encephalitis, most frequently in elderly and immunocompromised humans. Previous studies demonstrated that CD8+ T cells utilize perforin-dependent cytolytic mechanisms to limit WNV infection. Nonetheless, the phenotype of perforin-deficient CD8+ T cells was not as severe as that of an absence of CD8+ T cells, suggesting additional effector control mechanisms. In this study, we evaluated the contribution of Fas-Fas ligand (FasL) interactions to CD8+ T-cell-mediated control of WNV infection. Notably, the cell death receptor Fas was strongly upregulated on neurons in culture and in vivo after WNV infection. gld mice that were functionally deficient in FasL expression showed increased susceptibility to lethal WNV infection. Although antigen-specific priming of CD8+ T cells in peripheral lymphoid tissues was normal in gld mice, increased central nervous system (CNS) viral burdens and delayed clearance were observed. Moreover, the adoptive transfer of WNV-primed wild-type but not gld CD8+ T cells to recipient CD8(-/-) or gld mice efficiently limited infection in the CNS and enhanced survival rates. Overall, our data suggest that CD8+ T cells also utilize FasL effector mechanisms to contain WNV infection in Fas-expressing neurons in the CNS.