This project focuses on the interplay of cytomegalovirus (CMV)-encoded immunomodulators and host defense pathways that control the initial stages of viral infection within the natural host. These cell-autonomous pathways dictate parameters of acute infection, such as tissue tropism and dissemination. They also control establishment of a reservior for life-long persistence and latency and constantly defend against host immune clearance mechanisms. Importantly, we have shown that herpesviruses such as human (H) or murine (M) CMV, as well as herpes simplex virus (HSV)1 and HSV2, all employ evolutionarily related cell death suppressors that block programmed cell death. In MCMV, suppressors prolong infection and enhance spread, providing a tractable model to elaborate the contribution of specific cellular compartments to the ancient virus- host arms race that plays out during natural infection. The host pathways we study are conserved in humans as well as mice, providing key insights that cannot be obtained using HCMV or primate CMV models. Macrophages are important partners for all CMVs, starting from the initial encounter with virus inoculum, proceeding with the orchestration of viral dissemination, resulting in the deposition in sites of persistence and latency, and critical for the reappearence of virus sporadically or as a result of immunosuppression. All of the insights into murine CMV pathogenesis and latency from the studies we propose will point relevant directions for studies in human CMV. Our unique approach involves the use of viruses that lack specific cell death suppressors combined with mice that are deficient in the cognate host defense pathways that these suppressors target. We propose to move to a new phase of study and dissect the contribution of apoptosis, necroptosis and other programmed cell death pathways within specific tissue compartments. These studies are therefore relevant to an expanding range of human pathogens, most immediately HSV and other human herpesviruses. The specific host defense value of necroptosis has recently emerged along with the fascinating observation that this and other cell death pathways are interlaced and interdependent. Necroptosis is capable of completely preventing infection of a nave host because all exposed cells die within hours of exposure. In the next period, we will specifically investigate the contribution of serine protease death pathways that were first brought to light in our studies of HCMV in macrophages. Importantly, the project will determine the contribution of cell death in monocytes, macrophages, endothelial cells and epithelial cells during natural infection in mice. As a result, the contribution of specific programmed cell death pathways to all relevant stages of viral pathogenesis will emerge.
Herpesviruses defuse host defense mechanisms to replicate, disseminate and sustain lifelong latency with the potential for reactivation. Murine cytomegalovirus (CMV) provides a tractable mammalian model that has provided significant insights into infection with human CMV as well as with other human herpesviruses such as HSV1. These viruses all share common mechanisms of blocking the antiviral consequences of premature cell death affecting dissemination, immune control and latency. In the past funding period, this model organism has unveiled novel functions of monocytes during viral infection as well as previously unsuspected role of necroptosis in host defense. These transformative discoveries included the fact that the cell death pathways unleashed during virus infection may be dysregulated during development or tissue homeostasis to cause death or inflammatory disease in the animal. Going forward, we have designed experiments to use our existing knowledge and resources to dissect interlinked mitochondrial and cytosolic cell death pathways in host defense, to unveil additional novel caspase-independent pathways that complement apoptosis and are targeted by CMV-encoded cell death suppressors.
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