Helen Everett

Immunomodulation by viruses: the myxoma virus story.

Summary: Myxoma virus is a poxvirus pathogen of rabbits that has evolved to replicate successfully in the presence of an active immune response by an infected host. To accomplish this, the virus has developed a variety of strategies to avoid detection by or obstruct specific aspects of the antiviral response whose consolidated action is antagonistic to virus survival. We describe two distinct viral strategies carried out by viral proteins with which myxoma virus subverts the host immune response. The first strategy is the production of virus‐encoded proteins known as viroceptors or virokines that mimic host receptors or cytokines. These seek to actively block extracellular immune signals required for effective virus clearance and produce a local environment in the infected tissue that is “virus friendly”. The second strategy, carried out by intracellular viral proteins, seeks to retard the innate antiviral responses such as apoptosis, and hinder attempts by the infected cell to communicate with the cellular arm of the immune system. By studying these viral strategies of immune evasion, the myxoma system can provide insights into virus‐host interactions and also provide new insights into the complex immune system.

The Myxoma Poxvirus Protein, M11L, Prevents Apoptosis by Direct Interaction with the Mitochondrial Permeability Transition Pore

M11L, an antiapoptotic protein essential for the virulence of the myxoma poxvirus, is targeted to mitochondria and prevents the loss of mitochondrial membrane potential that accompanies cell death. In this study we show, using a cross-linking approach, that M11L physically associates with the mitochondrial peripheral benzodiazepine receptor (PBR) component of the permeability transition (PT) pore. Close association of M11L and the PBR is also indicated by fluorescence resonance energy transfer (FRET) analysis. Stable expression of M11L prevents the release of mitochondrial cytochrome c induced by staurosporine or protoporphyrin IX (PPIX), a ligand of the PBR. Transiently expressed M11L also prevents mitochondrial membrane potential loss induced by PPIX, or induced by staurosporine in combination with PK11195, another ligand of the PBR. Myxoma virus infection and the associated expression of early proteins, including M11L, protects cells from staurosporine- and Fas-mediated mitochondrial membrane potential loss and this effect is augmented by the presence of PBR. We conclude that M11L regulates the mitochondrial permeability transition pore complex, most likely by direct modulation of the PBR.

Myxoma Virus M11L Prevents Apoptosis through Constitutive Interaction with Bak

ABSTRACT M11L, a 166-amino-acid antiapoptotic protein of myxoma virus, was previously shown to bind to the peripheral benzodiazepine receptor by hydrophobic interactions at the outer mitochondrial membrane. Here we demonstrate that an additional property of M11L is the ability to constitutively form inhibitory complexes with the proapoptotic Bcl-2 family member Bak in human cells. This binding interaction was identified by both FLAG-tagged pull-down assays and tandem affinity purification from transfected and virus-infected human cells. M11L binds constitutively to human Bak and, under some inducible conditions, to human Bax as well, but not to the other Bcl-2 family members (Bad, Bid, Bcl-2). When stably expressed in human embryonic kidney (HEK293) cells, M11L effectively protects these cells from Fas ligand-induced apoptosis, thereby blocking release of cytochrome c, activation of caspase 9, and cleavage of poly(ADP-ribose) polymerase. We also demonstrate in coexpression studies that M11L can interact with Bak independently of any involvement with Bax. Furthermore, cells stably expressing M11L function to prevent apoptosis that is induced by overexpression of Bak. We conclude that M11L inhibits, in a species-independent fashion, apoptotic signals mediated by activation of Bak.

Poxviruses and apoptosis: a time to die

All known apoptosis modulators in poxviruses have been shown to function as inhibitors. The mechanistic classes of these poxvirus-encoded inhibitors are quite diverse, and indicate that a wide variety of distinct host proteins in cellular apoptotic pathways have been targeted for inhibition by individual poxviruses.

Viral proteins and the mitochondrial apoptotic checkpoint

Regulated cell death by apoptosis constitutes a primary host defense for counteracting invading viral pathogens. In recent years, advances in the field of apoptosis research have revealed that mitochondria and mitochondria-derived factors play a central role in regulating cellular commitment to apoptosis. Here we explore the role of viral proteins in modulating cell death pathways that are relayed via this mitochondrial checkpoint.