Coronavirus replication does not require the autophagy gene ATG5

Autophagy, antiviral immunity, and viral countermeasures

The autophagy pathway likely evolved not only to maintain cellular and tissue homeostasis but also to protect cells against microbial attack. This conserved mechanism by which cytoplasmic cargo is delivered to the endolysosomal system is now recognized as a central player in coordinating the host response to diverse intracellular pathogens, including viruses. As an endolysosomal delivery system, autophagy functions in the transfer of viruses from the cytoplasm to the lysosome where they are degraded, in the transfer of viral nucleic acids to endosomal sensors for the activation of innate immunity, and in the transfer of endogenous viral antigens to MHC class II compartments for the activation of adaptive immunity. Viruses have, in turn, evolved different strategies to antagonize, and potentially, to exploit the host autophagic machinery. Moreover, through mechanisms not yet well understood, autophagy may dampen host innate immune and inflammatory responses to viral infection. This review highlights the roles of autophagy in antiviral immunity, viral strategies to evade autophagy, and potential negative feedback functions of autophagy in the host antiviral response.

Subversion of the cellular autophagy pathway by viruses

Autophagy is a cellular process that creates double-membraned vesicles, engulfs and degrades cytoplasmic material, and generates and recycles nutrients. A recognized participant in the innate immune response to microbial infection, a functional autophagic response can help to control the replication of many viruses. However, for several viruses, there is functional and mechanistic evidence that components of the autophagy pathway act as host factors in viral replicative cycles, viral dissemination, or both. Investigating the mechanisms by which viruses subvert or imitate autophagy, as well as the mechanisms by which they inhibit autophagy, will reveal cell biological tools and processes that will be useful for understanding the many functional ramifications of the double-membraned vesicle formation and cytosolic entrapment unique to the autophagy pathway.

Autophagosome-independent essential function for the autophagy protein Atg5 in cellular immunity to intracellular pathogens

The physiologic importance of autophagy proteins for control of mammalian bacterial and parasitic infection in vivo is unknown. Using mice with granulocyte- and macrophage-specific deletion of the essential autophagy protein Atg5, we show that Atg5 is required for in vivo resistance to the intracellular pathogens Listeria monocytogenes and Toxoplasma gondii. In primary macrophages, Atg5 was required for interferongamma (IFN-gamma)/LPS-induced damage to the T. gondii parasitophorous vacuole membrane and parasite clearance. While we did not detect classical hallmarks of autophagy, such as autophagosomes enveloping T. gondii, Atg5 was required for recruitment of IFN-gamma-inducible p47 GTPase IIGP1 (Irga6) to the vacuole membrane, an event that mediates IFN-gamma-mediated clearance of T. gondii. This work shows that Atg5 expression in phagocytic cells is essential for cellular immunity to intracellular pathogens in vivo, and that an autophagy protein can participate in immunity and intracellular killing of pathogens via autophagosome-independent processes such as GTPase trafficking.

Autophagy: principles and significance in health and disease

Degradation processes are important for optimal functioning of eukaryotic cells. The two major protein degradation pathways in eukaryotes are the ubiquitin-proteasome pathway and autophagy. This contribution focuses on autophagy. This process is important for survival of cells during nitrogen starvation conditions but also has a house keeping function in removing exhausted, redundant or unwanted cellular components. We present an overview of the molecular mechanism involved in three major autophagy pathways: chaperone mediated autophagy, microautophagy and macroautophagy. Various recent reports indicate that autophagy plays a crucial role in human health and disease. Examples are presented of lysosomal storage diseases and the role of autophagy in cancer, neurodegenerative diseases, defense against pathogens and cell death.

A 'fly-by' killing with a primordial cellular weapon

Autophagy has been suggested—on the basis of in vitro studies—to be involved in defense against bacterial challenge. A study in drosophila now shows the importance of autophagy in vivo and links a pattern recognition receptor to the autophagy pathway.

Eating the enemy within: autophagy in infectious diseases

Autophagy is emerging as a central component of antimicrobial host defense against diverse viral, bacterial, and parasitic infections. In addition to pathogen degradation, autophagy has other functions during infection such as innate and adaptive immune activation. As an important host defense pathway, microbes have also evolved mechanisms to evade, subvert, or exploit autophagy. Additionally, some fungal pathogens harness autophagy within their own cells to promote pathogenesis. This review will highlight our current understanding of autophagy in infection, focusing on the most recent advances in the field, and will discuss the potential implications of these studies in the design of anti-infective therapeutics.

Autophagosome supports coxsackievirus B3 replication in host cells

Recent studies suggest a possible takeover of host antimicrobial autophagy machinery by positive-stranded RNA viruses to facilitate their own replication. In the present study, we investigated the role of autophagy in coxsackievirus replication. Coxsackievirus B3 (CVB3), a picornavirus associated with viral myocarditis, causes pronounced intracellular membrane reorganization after infection. We demonstrate that CVB3 infection induces an increased number of double-membrane vesicles, accompanied by an increase of the LC3-II/LC3-I ratio and an accumulation of punctate GFP-LC3-expressing cells, two hallmarks of cellular autophagosome formation. However, protein expression analysis of p62, a marker for autophagy-mediated protein degradation, showed no apparent changes after CVB3 infection. These results suggest that CVB3 infection triggers autophagosome formation without promoting protein degradation by the lysosome. We further examined the role of the autophagosome in CVB3 replication. We demonstrated that inhibition of autophagosome formation by 3-methyladenine or small interfering RNAs targeting the genes critical for autophagosome formation (ATG7, Beclin-1, and VPS34 genes) significantly reduced viral replication. Conversely, induction of autophagy by rapamycin or nutrient deprivation resulted in increased viral replication. Finally, we examined the role of autophagosome-lysosome fusion in viral replication. We showed that blockage of the fusion by gene silencing of the lysosomal protein LAMP2 significantly promoted viral replication. Taken together, our results suggest that the host's autophagy machinery is activated during CVB3 infection to enhance the efficiency of viral replication.

Autophagic machinery activated by dengue virus enhances virus replication

Autophagy is a cellular response against stresses which include the infection of viruses and bacteria. We unravel that Dengue virus-2 (DV2) can trigger autophagic process in various infected cell lines demonstrated by GFP-LC3 dot formation and increased LC3-II formation. Autophagosome formation was also observed under the transmission electron microscope. DV2-induced autophagy further enhances the titers of extracellular and intracellular viruses indicating that autophagy can promote viral replication in the infected cells. Moreover, our data show that ATG5 protein is required to execute DV2-induced autophagy. All together, we are the first to demonstrate that DV can activate autophagic machinery that is favorable for viral replication.

Autophagy and antiviral immunity

Autophagy is an ancient pathway designed to maintain cellular homeostasis by degrading long-lived proteins and organelles in the cytosol. Recent studies demonstrate that autophagy is utilized by the cells of the innate and adaptive immune systems to combat viral infections. Autophagy plays a key role in recognizing signatures of viral infection, and represents a critical effector mechanism to restrict viral replication. On the other hand, autophagosomes have been exploited by certain viruses as a niche for viral replication. Furthermore, autophagy can be used to deliver endogenous viral antigens to the MHC class II loading compartment, allowing activation of CD4 T cells. In this review, we describe recent advances in the field of autophagy as it relates to innate and adaptive antiviral immune responses.

Autophagy: process and function

Autophagy is an intracellular degradation system that delivers cytoplasmic constituents to the lysosome. Despite its simplicity, recent progress has demonstrated that autophagy plays a wide variety of physiological and pathophysiological roles, which are sometimes complex. Autophagy consists of several sequential steps--sequestration, transport to lysosomes, degradation, and utilization of degradation products--and each step may exert different function. In this review, the process of autophagy is summarized, and the role of autophagy is discussed in a process-based manner.