Chicago - 4:00 PM - 5:00 PM
Title: "Alternative Cell Death Pathways in Antiviral Host Defense"
Special Microbiology-Immunology and Virology Club Seminar
Edward Mocarski, PhD, Emory University
Host: Richard Longnecker, PhD
Herpesviruses and other large DNA viruses elaborate cell death suppressors that reveal how these pathways contribute to host defense. Virus-encoded inhibitors block cell autonomous death pathways in order to sustain infection. Accumulating evidence indicates that programmed cell death pathways in mice, the model mammal, such as caspase-8-mediated apoptosis, caspase-11-dependent pyroptosis, and RIPK3-MLKL-dependent necroptosis all evolved primarily for host defense. Although apoptosis is observed throughout multicellular organisms, pyroptosis and necroptosis are only observed in mammals. Necroptosis is an alternate death pathway that is triggered by caspase-8 compromise and is a potent means of eliminating infected cells. Virus-induced necroptosis depends on the protein kinase activity of RIPK3 to phosphorylate MLKL, a pore-forming protein that executes cell leakage after allosteric activation by binding highly phosphorylated inositol phosphate. Necroptosis is naturally induced by murine cytomegalovirus (MCMV) through the Z-nucleic acid binding protein (ZBP)1 (also called DAI and DLM1), a pathogen sensor that recruits and activates RIPK3 via common RIP homotypic interaction motifs (RHIMs). MCMV M45 encodes the viral inhibitor of RIP activation (vIRA), a RHIM-signaling inhibitor that prevents recruitment of RIPK3 during infection of cells and mice.
Over the course of the last eight years, human CMV, as well as herpes simplex virus (HSV) and vaccinia virus (VACV) have been shown to inhibit ZBP1-RIPK3-MLKL necroptosis, and influenza has been shown to be naturally susceptible to ZBP1-induced necroptosis and apoptosis. The evidence that virus-induced necroptosis is highly potent derives from work on MCMV and VACV, where viral mutants that fail to block activation of RIPK3 cannot infect mice due to the uniform death of cells at the inoculation site. Thus, necroptosis represents the most potent form of cell autonomous innate host defense to prevent infection and reduce the likelihood of dissemination within the host.
HSV1 and HSV2 encode an M45 homolog (RR1, UL39-encoded ICP6) that has been shown to block RHIM signaling and prevent necroptosis in human cells. ICP6 exhibits species specificity and acts to promote necroptosis in mouse cells and in mice by functioning as a RHIM-dependent activator of RIPK3. Recent studies have shown that ICP6 RHIM mutant virus nevertheless retains the ability to induce necroptosis in mice; however, this virus-induced death is dependent on ZBP1 RHIM-dependent recruitment of RIPK3, a pathway strikingly similar to MCMV M45 mutant virus. Thus, HSV1 infection of mouse cells induces necroptosis via two distinct RHIM-dependent mechanisms in mice, one dependent on the ICP6 RHIM-mediated recruitment of RIPK3 and one triggered via the pathogen sensor ZBP1. Like MCMV M45 RHIM mutant virus, ICP6 RHIM mutant HSV1 pathogenesis becomes normalized during infection of Zbp1-/-, Ripk3-/- and Mlkl-/- mice. Importantly, in human HT-29 cells, which are commonly employed for studies on TNF-induced necroptosis (RIPK1-RIPK3-MLKL pathway), lack sufficient ZBP1 to support virus-induced necroptosis. ZBP1-transduced HT-29 cells are susceptible to virus-induced death depending on the Z nucleic acid binding domains, Zalpha1 and Zalpha2. Transcription is necessary for the execution of virus-induced necroptosis, suggesting that, like MCMV, vaccinia and influenza, cell death is initiated by ZBP1 sensing of dsRNA rather than viral DNA, an area that is under active investigation. Thus, virus-induced ZBP1-RIPK3-MLKL necroptosis is likely to be a feature of natural infection in humans but is masked when WT HSV1 is in the non-natural mouse host due to ICP6 RHIM-triggered death.