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Anti-Immunology: Studies on the Interface Between Viruses and the Immune System
Summary: Grant McFadden is investigating the innate antiviral pathways activated during poxvirus infection in mammalian cells. These studies will define better the molecular mechanisms controlling host–virus species barriers and provide insights into host pathways that viruses circumvent to cause human zoonotic infections and will facilitate the development of poxviruses for oncolytic virotherapy.
Poxviruses infect a broad spectrum of vertebrate hosts, but the basis for the host range specificity exhibited by individual virus isolates is poorly understood. However, it is becoming increasingly clear that, unlike many other eukaryotic viruses that depend on specific virus–receptor interactions to initiate infection of a susceptible host cell, poxviruses are relatively promiscuous in that they bind to and enter a wide variety of mammalian cells. Thus, the checkpoint for determining whether a given poxvirus infection will be permissive or nonpermissive is usually downstream from the binding, fusion, and entry events. The poxvirus myxoma virus (MV) causes lethal infection only in rabbits, but the mechanism underlying its strict species tropism is not known. Like all poxviruses, MV expresses a wide array of immunomodulatory proteins, but relatively few of them are specific to the rabbit when tested in vitro. At least one such non-species-specific immunomodulatory protein derived from MV, a secreted virus-encoded serpin called SERP-1, is currently in human phase II clinical trials as an anti-inflammatory drug. In general, large DNA viruses such as members of the poxvirus family often express a broad spectrum of immunomodulatory or host-subversive proteins. We have proposed that, in general, the consortium of such products from microbial sources constitutes a novel discipline that we have designated "anti-immunology."
For the past few years, we have investigated the molecular basis for tropism specificity of poxviruses, using the MV model system, with particular emphasis on how viral proteins mediate host range and pathogenesis. We have shown that MV infection of nonpermissive primary mouse embryo fibroblasts evokes the activation of interferon (IFN) regulatory factor-3 and induces type I IFN, which aborts the virus infection. We discovered that disruption of IFN signaling mediated by signal transducer and activator of transcription 1 (STAT1) breaks the cellular blockade to MV multiplication in nonrabbit cells. Moreover, STAT1 deficiency renders mice highly susceptible to lethal MV infection.
This work prompted us to investigate the use of MV as an oncolytic virus to treat human cancers that exhibit defective IFN responses. Recently, we showed that the ability of human cancer cells to support the productive replication of MV is related to the activation state of the Akt signaling pathway of the cell. Thus, the ability to regulate the host-cell Akt signaling pathway makes it possible to alter the permissiveness of human cancer cells for infection by MV. We are investigating antiviral responses specifically induced in human cells infected with MV, with the intent of better understanding why some cancer cells are nonpermissive, whereas others are productively infected, and also to better understand why the virus is so nonpathogenic outside the rabbit host. Such information will be critical to better evaluate whether MV can be adapted as a viable oncolytic virotherapy for cancer patients.
We are also interested in MV genes that confer host specificity—that is, genes that encode host tropism determinants and thus determine cellular specificity. For example, the MV-encoded M-T5 protein, an ankyrin-repeat protein that regulates tropism of MV for rabbit lymphocytes as well as for a spectrum of human cancer cells, binds to and alters at least two distinct host proteins: Akt and cullin-1. The M13 protein, which regulates MV T-cell tropism, contains a pyrin domain that allows the protein to regulate the cellular structure called the inflammasome. The M11L protein is an apoptosis inhibitor that specifically binds to several host proteins, including Bak, activated Bax, and the mitochondrial peripheral benzodiazepine receptor. Finally, the newly discovered M063 protein is a homologue of host Daxx that controls MV tropism of MV for rabbit cells in general. The various host range factors expressed by MV provide a wealth of insights into the virus–host dynamics that control tropism and virulence.
The study of host tropism by poxviruses thus offers not only the potential for development of novel platforms for replication-restricted vaccine vectors and oncolytic viruses but is also likely to produce novel insights into how and why poxviruses can occasionally leap from a long-term evolutionary host species to cause zoonotic infections in humans. The 2003 human monkeypox outbreak in the United States amply illustrated the need to better understand how poxviruses can leap from one host species to another to cause a new and potentially lethal disease syndrome and the need to better understand how "emerging" viruses in general can occasionally leap into nonevolutionary hosts to cause novel disease. There is a growing consensus that, unlike many other viruses for which tropism depends on the expression of specific cellular receptors, poxviruses can bind and enter almost all mammalian cells; the specific the ability of any individual poxvirus to micromanipulate signaling pathways of the infected cell determines whether the infection will be permissive.
We hope to expand our knowledge of other classes of anti-immune proteins of poxviruses and define their cellular target pathways in greater detail. We also hope to advance MV as an oncolytic virus candidate for virotherapy of human cancer.
This work was also supported by the CIHR, NCIC, the Wellstat Corporation, and VIRON Therapeutics.
Last updated August 2008
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