Influenza A virus (IAV) enters host cells upon binding of its hemagglutinin glycoprotein to sialylated host cell receptors. Whereas dynamin-dependent, clathrin-mediated endocytosis (CME) is generally considered as the IAV infection pathway, some observations suggest the occurrence of an as yet uncharacterized alternative entry route. By manipulating entry parameters we established experimental conditions that allow the separate analysis of dynamin-dependent and -independent entry of IAV. Whereas entry of IAV in phosphate-buffered saline could be completely inhibited by dynasore, a specific inhibitor of dynamin, a dynasore-insensitive entry pathway became functional in the presence of fetal calf serum. This finding was confirmed with the use of small interfering RNAs targeting dynamin-2. In the presence of serum, both IAV entry pathways were operational. Under these conditions entry could be fully blocked by combined treatment with dynasore and the amiloride derivative EIPA, the hallmark inhibitor of macropinocytosis, whereas either drug alone had no effect. The sensitivity of the dynamin-independent entry pathway to inhibitors or dominant-negative mutants affecting actomyosin dynamics as well as to a number of specific inhibitors of growth factor receptor tyrosine kinases and downstream effectors thereof all point to the involvement of macropinocytosis in IAV entry. Consistently, IAV particles and soluble FITC-dextran were shown to co-localize in cells in the same vesicles. Thus, in addition to the classical dynamin-dependent, clathrin-mediated endocytosis pathway, IAV enters host cells by a dynamin-independent route that has all the characteristics of macropinocytosis. Attachment to and entry into a host cell are the first crucial steps in establishing a successful virus infection and critical factors in determining host cell and species tropism. Influenza A virus (IAV) attaches to host cells by binding of its major surface protein, hemagglutinin, to sialic acids that are omnipresent on the glycolipids and glycoproteins exposed on the surfaces of cells. IAV subsequently enters cells of birds and a wide variety of mammals via receptor-mediated endocytosis using clathrin as well as via (an) alternative uncharacterized route(s). The elucidation of the endocytic pathways taken by IAV has been hampered by their apparent redundancy in establishing a productive infection. By manipulating the entry conditions we have established experimental settings that allow the separate analysis of dynamin-dependent (including clathrin-mediated endocytosis) and independent entry of IAV. Collectively, our results indicate macropinocytosis, the main route for the non-selective uptake of extracellular fluid by cells, as an alternative IAV entry route. As the dynamin-dependent and -independent IAV entry routes are redundant and independent, their separate manipulation was crucial for the identification and characterization of the alternative IAV entry route. A similar strategy might be applicable to the study of endocytic pathways taken by other viruses.

Stress granules (SGs) are protein-mRNA aggregates that are formed in response to environmental stresses, resulting in translational inhibition. SGs are generally believed to play an antiviral role and are manipulated by many viruses, including various alphaviruses. GTPase-activating protein (SH3 domain)-binding protein 1 (G3BP1) is a key component and commonly used marker of SGs. Its homolog G3BP2 is a less extensively studied SG component. Here, we demonstrate that Chikungunya virus (CHIKV) infection induces cytoplasmic G3BP1- and G3BP2-containing granules that differ from bona fide SGs in terms of morphology, composition, and behavior. For several Old World alphaviruses it has been shown that nonstructural protein 3 (nsP3) interacts with G3BPs, presumably to inhibit SG formation, and we have confirmed this interaction in CHIKV-infected cells. Surprisingly, CHIKV also relied on G3BPs for efficient replication, as simultaneous depletion of G3BP1 and G3BP2 reduced viral RNA levels, CHIKV protein expression, and viral progeny titers. The G3BPs colocalized with CHIKV nsP2 and nsP3 in cytoplasmic foci, but no colocalization with nsP1, nsP4, or dsRNA was observed. Furthermore, G3BPs could not be detected in a cellular fraction enriched for CHIKV replication/transcription complexes, suggesting that they are not directly involved in CHIKV RNA synthesis. Depletion of G3BPs did not affect viral entry, translation of incoming genomes, or nonstructural polyprotein processing but resulted in severely reduced levels of negative-stranded (and consequently also positive-stranded) RNA. This suggests a role for the G3BPs in the switch from translation to genome amplification, although the exact mechanism by which they act remains to be explored. IMPORTANCE Chikungunya virus (CHIKV) causes a severe polyarthritis that has affected millions of people since its reemergence in 2004. The lack of approved vaccines or therapeutic options and the ongoing explosive outbreak in the Caribbean underline the importance of better understanding CHIKV replication. Stress granules (SGs) are cytoplasmic protein-mRNA aggregates formed in response to various stresses, including viral infection. The RNA-binding proteins G3BP1 and G3BP2 are essential SG components. SG formation and the resulting translational inhibition are generally considered an antiviral response, and many viruses manipulate or block this process. Late in infection, we and others have observed CHIKV nonstructural protein 3 in cytoplasmic G3BP1- and G3BP2-containing granules. These virally induced foci differed from true SGs and did not appear to represent replication complexes. Surprisingly, we found that G3BP1 and G3BP2 were also needed for efficient CHIKV replication, likely by facilitating the switch from translation to genome amplification early in infection.

Crimean-Congo hemorrhagic fever virus (CCHFV) is a tick-borne pathogen that causes high morbidity and mortality. Efficacy of vaccines and antivirals to treat human CCHFV infections remains limited and controversial. Research into pathology and underlying molecular mechanisms of CCHFV and other nairoviruses is limited. Significant progress has been made in our understanding of CCHFV replication and pathogenesis in the past decade. Here we review the most recent molecular advances in CCHFV-related research, and provide perspectives on future research.

Interferon-stimulated gene product 15 (ISG15) is a key component of host responses to microbial infection. Despite having been known for four decades, grasping the functions and features of ISG15 has been a slow and elusive process. Substantial work over the past two decades has greatly enhanced this understanding, revealing the complex and variable nature of this protein. This has unveiled multiple mechanisms of action that are only now beginning to be understood. In addition, it has uncovered diversity not only between how ISG15 affects different pathogens but also between the function and structure of ISG15 itself between different host species. Here we review the complexity of ISG15 within the context of viral infection, focusing primarily on its antiviral function and the mechanisms viruses employ to thwart its effects. We highlight what is known regarding the impact of ISG15 sequence and structural diversity on these interactions and discuss the aspects presenting the next frontier toward elucidating a more complete picture of ISG15 function.

RIG-I is a cytosolic sensor critically involved in the activation of the innate immune response to RNA virus infection. In the present study, we evaluated the inhibitory effect of a RIG-I agonist on the replication of two emerging arthropod-borne viral pathogens, dengue virus (DENV) and chikungunya virus (CHIKV), for which no therapeutic options currently exist. We demonstrate that when a low, noncytotoxic dose of an optimized 5′triphosphorylated RNA (5′pppRNA) molecule was administered, RIG-I stimulation generated a robust antiviral response against these two viruses. Strikingly, 5′pppRNA treatment before or after challenge with DENV or CHIKV provided protection against infection. In primary human monocytes and monocyte-derived dendritic cells, the RIG-I agonist blocked both primary infection and antibody-dependent enhancement of DENV infection. The protective response against DENV and CHIKV induced by 5′pppRNA was dependent on an intact RIG-I/MAVS/TBK1/IRF3 axis and was largely independent of the type I IFN response. Altogether, this in vitro analysis of the antiviral efficacy of 5′pppRNA highlights the therapeutic potential of RIG-I agonists against emerging viruses such as DENV and CHIKV. IMPORTANCE DENV and CHIKV are two reemerging mosquito-borne viruses for which no therapeutic options currently exist. Both viruses overlap geographically in tropical regions of the world, produce similar fever-like symptoms, and are difficult to diagnose. This study investigated the inhibitory effect of a RIG-I agonist on the replication of these two viruses. RIG-I stimulation using 5′pppRNA before or after DENV or CHIKV infection generated a protective antiviral response against both pathogens in immune and nonimmune cells; interestingly, the protective response against the viruses was largely independent of the classical type I interferon response. The antiviral efficacy of 5′pppRNA highlights the therapeutic potential of RIG-I agonists against emerging viruses such as DENV and CHIKV.

Antiviral responses are regulated by conjugation of ubiquitin (Ub) and interferon-stimulated gene 15 (ISG15) to proteins. Certain classes of viruses encode Ub- or ISG15-specific proteases belonging to the ovarian tumor (OTU) superfamily. Their activity is thought to suppress cellular immune responses, but studies demonstrating the function of viral OTU proteases during infection are lacking. Crimean-Congo hemorrhagic fever virus (CCHFV, family Nairoviridae) is a highly pathogenic human virus that encodes an OTU with both deubiquitinase and deISGylase activity as part of the viral RNA polymerase. We investigated CCHFV OTU function by inactivating protease catalytic activity or by selectively disrupting its deubiquitinase and deISGylase activity using reverse genetics. CCHFV OTU inactivation blocked viral replication independently of its RNA polymerase activity, while deubiquitinase activity proved critical for suppressing the interferon responses. Our findings provide insights into viral OTU functions and support the development of therapeutics and vaccines.