IFITM3 Inhibits Influenza A Virus Infection by Preventing Cytosolic Entry

Ragon Institute of Massachusetts General Hospital, Massachusetts Institute of Technology and Harvard University, Charlestown, Massachusetts, United States of America.
PLoS Pathogens (Impact Factor: 8.06). 10/2011; 7(10):e1002337. DOI: 10.1371/journal.ppat.1002337
Source: PubMed

ABSTRACT To replicate, viruses must gain access to the host cell's resources. Interferon (IFN) regulates the actions of a large complement of interferon effector genes (IEGs) that prevent viral replication. The interferon inducible transmembrane protein family members, IFITM1, 2 and 3, are IEGs required for inhibition of influenza A virus, dengue virus, and West Nile virus replication in vitro. Here we report that IFN prevents emergence of viral genomes from the endosomal pathway, and that IFITM3 is both necessary and sufficient for this function. Notably, viral pseudoparticles were inhibited from transferring their contents into the host cell cytosol by IFN, and IFITM3 was required and sufficient for this action. We further demonstrate that IFN expands Rab7 and LAMP1-containing structures, and that IFITM3 overexpression is sufficient for this phenotype. Moreover, IFITM3 partially resides in late endosomal and lysosomal structures, placing it in the path of invading viruses. Collectively our data are consistent with the prediction that viruses that fuse in the late endosomes or lysosomes are vulnerable to IFITM3's actions, while viruses that enter at the cell surface or in the early endosomes may avoid inhibition. Multiple viruses enter host cells through the late endocytic pathway, and many of these invaders are attenuated by IFN. Therefore these findings are likely to have significance for the intrinsic immune system's neutralization of a diverse array of threats.

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    • "In future studies, it will be interesting to explore the relationship between the biological features, steady state of FMDV receptors and sIFITM3 expression. It has further been demonstrated that IFITM3 alters endosomal structure and its acidic milieu (Amini-Bavil-Olyaee et al., 2013; Anafu et al., 2013; Feeley et al., 2011). Therefore, we postulate that virus attachment is not the only step that is targeted by sIFITM3 during FMDV infection. "
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    • "These data demonstrate that IFITM3 is mainly present in late endosomal compartments, where it partly colocalizes with VAPA. Although IFITM3 expression dramatically affects the biogenesis of endosomal compartments , leading to morphological changes of endosomal compartments (Feeley et al., 2011), its expression did not affect endosomal CD63 and LAMP1 protein levels (Figure S3C). Because IFITM proteins block the interaction between VAPA and OSBP that mediates transfer of cholesterol between ER and organelles (Ikonen, 2008), we determined the intracellular cholesterol levels using the histochemical cholesterol markers, filipin and Nile Red dyes. "
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    ABSTRACT: Vesicle-membrane-protein-associated protein A (VAPA) and oxysterol-binding protein (OSBP) regulate intracellular cholesterol homeostasis, which is required for many virus infections. During entry, viruses or virus-containing vesicles can fuse with endosomal membranes to mediate the cytosolic release of virions, and alterations in endosomal cholesterol can inhibit this invasion step. We show that the antiviral effector protein interferon-inducible transmembrane protein 3 (IFITM3) interacts with VAPA and prevents its association with OSBP, thereby disrupting intracellular cholesterol homeostasis and inhibiting viral entry. By altering VAPA-OSBP function, IFITM3 induces a marked accumulation of cholesterol in multivesicular bodies and late endosomes, which inhibits the fusion of intraluminal virion-containing vesicles with endosomal membranes and thereby blocks virus release into the cytosol. Consequently, ectopic expression or depletion of the VAPA gene profoundly affects IFITM3-mediated inhibition of viral entry. Thus, IFITM3 disrupts intracellular cholesterol homeostasis to block viral entry, further underscoring the importance of cholesterol in virus infection.
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