The role of interleukin-18 in vesicular stomatitis virus infection of the CNS

How is inflammation initiated? Individual influences of IL-1, IL-18 and HMGB1

Pro-inflammatory cytokines are crucial for fighting infection and establishing immunity. Recently, other proteins, such as danger-associated molecular patterns (DAMPs), have also been appreciated for their role in inflammation and immunity. Following the formation and activation of multiprotein complexes, termed inflammasomes, two cytokines, IL-1β and IL-18, along with the DAMP High Mobility Group Box 1 (HMGB1), are released from cells. Although these proteins all lack classical secretion signals and are released by inflammasome activation, they each lead to different downstream consequences. This review examines how various inflammasomes promote the release of IL-1β, IL-18 and HMGB1 to combat pathogenic situations. Each of these effector molecules plays distinct roles during sterile inflammation, responding to viral, bacterial and parasite infection, and tailoring the innate immune response to specific threats.

VSV oncolytic virotherapy in the B16 model depends upon intact MyD88 signaling

We show here, for the first time to our knowledge, that the antitumor therapy of oncolytic vesicular stomatitis virus (VSV) in the B16ova model depends upon signaling through myeloid differentiation primary response gene 88 (MyD88) in host cells. VSV-mediated therapy of B16ova tumors was abolished in MyD88(-/-) mice despite generation of antigen-specific T cell responses similar to those in immune-competent mice. Mice defective in only toll-like receptor 4 (TLR4), TLR7, or interleukin 1 (IL-1) signaling retained VSV-induced therapy, suggesting that multiple, redundant pathways of innate immune activation by the virus contribute to antitumor immune reactivity. Lack of MyD88 signaling was associated with decreased expression of proinflammatory cytokines and neutrophil infiltration in response to intratumoral virus, as well as decreased infiltration of draining lymph nodes (LN) with plasmacytoid dendritic cells (pDCs) (CD11b(-)GR1(+)B220(+)) and myeloid-derived suppressor cells (CD11b(+)GR1(+)F4/80(+)). MyD88 signaling in response to VSV was also closely associated with a type I interferon (IFN) response. This inhibited virus replication within the tumor but also protected the host from viral dissemination from the tumor. Therefore, the innate immune response to oncolytic viruses can be, simultaneously, protherapeutic, antioncolytic, and systemically protective. These paradoxically conflicting roles need to be carefully considered in future strategies designed to improve the efficacy of oncolytic virotherapy.

Gene expression profiling of nonhuman primates exposed to aerosolized Venezuelan equine encephalitis virus

Host responses to Venezuelan equine encephalitis viruses (VEEV) were studied in cynomolgus macaques after aerosol exposure to the epizootic virus. Changes in global gene expression were assessed for the brain, lungs, and spleen. In the brain, major histocompatibility complex (MHC) class I transcripts were induced, while the expression of S100b, a factor associated with brain injury, was inhibited, as was expression of the encephalitogenic gene MOG. Cytokine-mediated signals were affected by infection, including those involving IFN-mediated antiviral activity (IRF-7, OAS, and Mx transcripts), and the increased transcription of caspases. Induction of a few immunologically relevant genes (e.g. IFITM1 and STAT1) was common to all tested tissues. Herein, both tissue-specific and nontissue specific transcriptional changes in response to VEEV are described, including induction of IFN-regulated transcripts and cytokine-induced apoptotic factors, in addition to cellular factors in the brain that may be descriptive of the health status of the brain during the infectious process. Altogether, this work provides novel information on common and tissue-specific host responses against VEEV in a nonhuman primate model of aerosol exposure.

Role of MyD88 in Route-Dependent Susceptibility to Vesicular Stomatitis Virus Infection

TLRs are important components of the innate immune response. The role of the TLR signaling pathway in host defense against a natural viral infection has been largely unexplored. We found that mice lacking MyD88, an essential adaptor protein in TLR signaling pathway, were extremely sensitive to intranasal infection with vesicular stomatitis virus, and this susceptibility was dose dependent. We demonstrated that this increased susceptibility correlates with the impaired production of IFN-alpha and defective induction and maintenance of neutralizing Ab. These studies outline the important role of the TLR signaling pathway in nasal mucosae-respiratory tracts-neuroepithelium environment in the protection against microbial pathogen infections. We believe that these results explain how the route of infection, probably by virtue of activating different cell populations, can lead to entirely different outcomes of infection based on the underlying genetics of the host.

Gene expression contributing to recruitment of circulating cells in response to vesicular stomatitis virus infection of the CNS

During acute Vesicular Stomatitis Virus (VSV) infection of the mouse central nervous system, neutrophils, natural killer (NK) cells, macrophages, and CD4+ and CD8+ T cells are recruited from the circulation in response to chemokines and cytokines. This study elucidated the production of these factors and infiltration of these peripheral cells. Chemokines that were observed included CCL1, CXCL10 (IP-10), CCL5 (RANTES), CCL3 (MIP-1alpha), CCL4 (MIP-1beta), CXCL1 (MIP-2), CCL2 (MCP-1), and CCL11 (eotaxin). Cytokines produced in response to the infection include IL-1 and interferon-gamma, but not type I interferons. Neutrophils are the first recruited cell type, appearing as early as 24 h after intranasal application of the virus. NK cells follow, but T cells are not detected until 6 days postinfection.

IL-18: a key player in neuroinflammation and neurodegeneration?

Interleukin (IL)-18 is a potent inflammatory cytokine of the IL-1 family. It is synthesized as an inactive precursor (pro-IL-18), which is cleaved into its functionally active form by caspase-1. Resident cells of the CNS express IL-18 and caspase-1 constitutively, thus providing a local IL-18-dependent immune response. Recent studies have highlighted a crucial role for IL-18 in mediating neuroinflammation and neurodegeneration in the CNS under pathological conditions, such as bacterial and viral infection, autoimmune demyelinating disease, and hypoxic-ischemic, hyperoxic and traumatic brain injuries. This review provides a synopsis of the current knowledge of IL-18-dependent mechanisms of action during acute neurodegeneration in immature and adult brains.

IL-12 and IL-18 act in synergy to clear vaccinia virus infection: involvement of innate and adaptive components of the immune system

Development of a protective host response against intracellular pathogens requires innate and cell-mediated immune responses, with cytokines playing an important role in host defences. Different studies in mice have shown that IL-12 can promote protective immunity to a variety of viruses but, during virus infection, little is known about the in vivo function of IL-18 alone or in combination with IL-12. Using recombinant vaccinia viruses (rVVs) expressing IL-12 and IL-18, the antiviral role of both cytokines in mice has been analysed. The specific anti-VV immune response elicited and the persistence of the virus in target tissues were compared in BALB/c mice inoculated with rVVs expressing IL-12 and IL-18 either singly or in combination. Delivery of IL-12 and IL-18 by rVVs in mice induced a significant enhancement in virus clearance from ovaries and spleen, greater than that expected from the sum of action of both cytokines. Virus clearance involved NK and T cells, as demonstrated in mice depleted of NK cells and in immunodeficient SCID animals. Th1 parameters (CD8(+) T cell response and IgG2a : IgG1 ratios) were increased in mice inoculated with rVVs expressing both IL-12 and IL-18 as compared to those animals receiving a single cytokine. These findings indicate that when IL-12 and IL-18 are delivered by rVVs, different mechanisms involving both the innate and specific arms of the immune system act as mediators in the synergistic action of IL-12 and IL-18, leading to VV clearance. These results are of interest for the design of prophylactic as well as therapeutic VV-based strategies.

IL-12, while beneficial, is not essential for the host response to VSV encephalitis

In this report, the role of STAT4 and local production of interleukin (IL)-12 in the central nervous system (CNS) were examined during experimental vesicular stomatitis virus (VSV) encephalitis. We have previously shown that IL-12 treatment is beneficial both in vitro and in vivo during experimental VSV infection. This inhibition of VSV replication was dependent on the production of nitric oxide (NO) by the neuronal isoform of nitric oxide synthase (NOS-1). In vitro, IL-12 induces the phosphorylation and nuclear localization of STAT4 in neuroblastoma cell lines. STAT4 expression was not required for host survival or clearance of virus during experimental VSV encephalitis. Taken together, these data suggest that while neurons can respond directly to IL-12 in vitro by signaling through STAT4, STAT4 is not required for survival. It is likely that redundant innate host inflammatory cytokine responses compensate for the absence of IL-12 signaling.

Elevated intracranial IL-18 in humans and mice after traumatic brain injury and evidence of neuroprotective effects of IL-18-binding protein after experimental closed head injury

Proinflammatory cytokines are important mediators of neuroinflammation after traumatic brain injury. The role of interleukin (IL)-18, a new member of the IL-1 family, in brain trauma has not been reported to date. The authors investigated the posttraumatic release of IL-18 in murine brains following experimental closed head injury (CHI) and in CSF of CHI patients. In the mouse model, intracerebral IL-18 was induced within 24 hours by ether anesthesia and sham operation. Significantly elevated levels of IL-18 were detected at 7 days after CHI and in human CSF up to 10 days after trauma. Published data imply that IL-18 may play a pathophysiological role in inflammatory CNS diseases; therefore its inhibition may ameliorate outcome after CHI. To evaluate the functional aspects of IL-18 in the injured brain, mice were injected systemically with IL-18-binding protein (IL-18BP), a specific inhibitor of IL-18, 1 hour after trauma. IL-18BP-treated mice showed a significantly improved neurological recovery by 7 days, accompanied by attenuated intracerebral IL-18 levels. This demonstrates that inhibition of IL-18 is associated with improved recovery. However, brain edema at 24 hours was not influenced by IL-18BP, suggesting that inflammatory mediators other than IL-18 induce the early detrimental effects of intracerebral inflammation.

Innate immune responses in viral encephalitis

The innate immune system is multifaceted, comprised of preformed factors, cells, and many proteins and lipid mediators produced by those cells. In the CNS these are critical in initiation and amplification of the inflammatory response and in the subsequent elicitation of the specific T cell response to viral encephalitis. Cells that are resident in brain parenchyma and peripheral cells that are recruited both play key roles in the hosts's responses. Unlike the peripheral compartments, in the CNS, non-cytolytic means of eliminating viral infections have been critical, since, in contrast to columnar epithelial cells, neurons are non-renewing. When the innate immune responses are inefficient or absent in viral encephalitis, pathology is more likely. Much more work remains to elucidate all of the critical cells and their mediators, as well as to develop new therapies for infections of the CNS.