Nipah Virus V Protein Evades Alpha and Gamma Interferons by Preventing STAT1 and STAT2 Activation and Nuclear Accumulation

Interferons (IFNs) are the primary innate antiviral cytokines in mammalian cells and also regulate aspects of the adaptive immune response (3, 5, 27). Both IFN-α/β and IFN-γ can induce an antiviral state in cells as an end point of signal transduction through the JAK-STAT pathway. The general paradigm for this signaling pathway involves IFN-dependent receptor-mediated tyrosine phosphorylation of latent cytoplasmic STAT proteins to produce DNA-binding oligomers that are competent for nuclear translocation. IFN-α/β responses are mediated primarily by an activated transcription complex, ISGF3, that consists of heterodimeric STAT1 and STAT2 in association with a DNA binding subunit, IRF9. ISGF3 binds to a DNA element, ISRE, in IFN-α/β target gene promoters, inducing their transcription. IFN-γ responses are also mediated by a STAT-containing transcription complex consisting of STAT1 homodimers. The STAT1 homodimer binds to a DNA element termed GAS to induce the transcription of a distinct but overlapping set of IFN-γ target genes. In both cases, these IFN-stimulated gene products produce a general antiviral state in the cell that inhibits the replication of diverse virus species. The Paramyxoviridae family represents a wide variety of enveloped minus-strand RNA viruses including a number of established and emerging human and animal pathogens (12). Several paramyxoviruses, particularly those in the Rubulavirus genus, have been recently demonstrated to interfere with IFN-induced antiviral responses by targeting the STAT1 and/or STAT2 proteins for proteasomal degradation. For example, mumps virus and simian virus 5 (SV5) target STAT1 for degradation, while human parainfluenza virus type 2 (HPIV2) targets STAT2 (7, 11, 20). The exact mechanisms underlying STAT protein targeting by the rubulaviruses are not completely understood, but it is clear that the anti-STAT effects are mediated by a single viral protein, termed V (7). The V protein is not related to any cellular proteins and does not appear to be a protease itself; instead, it acts in concert with cellular factors to mediate STAT degradation. For SV5-induced STAT1 degradation and HPIV2-induced STAT2 degradation, the targeting complex requires at least V, STAT1, and STAT2 to be present in the host cell (21). The role for a nontarget STAT protein in a STAT-targeting complex is not apparent, but available evidence indicates that the requirement for the confederate STAT protein is completely independent of IFN-α/β signal transduction or ISGF3 functions and can be supplied by a STAT protein N-terminal fragment (21). Moreover, the ability of SV5 to induce the degradation of STAT1 and to antagonize IFN signaling is species specific. IFN evasion by SV5 is very efficient in primate cells but restricted in murine cells (6). The cellular basis for the differential ability of murine and human cells to create an innate antiviral response to SV5 was recently uncovered (19). The failure of SV5 to antagonize IFN responses in the mouse system is not due to intrinsic defects in the SV5 proteolytic target, STAT1, since IFN responses that are dependent on murine STAT1 are efficiently blocked in otherwise human cells. Instead, it was found that differences between human and mouse STAT2 proteins provide the molecular basis for SV5 species specificity in IFN antagonism and STAT degradation, providing further support for the critical role of the nontarget STAT2 in V-dependent STAT1 degradation. An outbreak of fatal encephalitis and zoonotic disease in southeast Asia between 1998 and 1999 that resulted in over 100 human deaths was subsequently linked to a previously unidentified paramyxovirus, named Nipah virus (4); reviewed comprehensively in reference 8 and citations therein. Nipah virus is closely related to another recently emerged fatal paramyxovirus, Hendra virus (28). These two viruses differ significantly from the other Paramyxovirus genera, and classification of these viruses in a new paramyxovirus genus, Henipavirus, has been proposed (28, 29). Since these viruses have only recently appeared as human pathogens, little is known about their life cycles or their ability to interact with host cells. Investigation of the mechanisms underlying immune evasion by these viruses and identification of the virus-encoded factors that contribute to their ability to rapidly disseminate in a variety of host species is essential for understanding their pathogenesis. Because V protein-dependent IFN antagonism has been linked to successful paramyxovirus host range determination, replication, and pathogenesis, the Nipah virus V protein is a candidate pathogenesis-determining factor and was therefore tested for IFN inhibition capabilities. Like other paramyxovirus V proteins, expression of the Nipah virus V protein inactivates IFN signaling by direct inhibition of STAT1 and STAT2 function. However, the Nipah V protein differs from other paramyxovirus V proteins in its subcellular distribution and the mechanism used to inhibit cellular IFN responses. Results indicate that Nipah virus V protein accumulates in the cytoplasm as a result of a Crm1-dependent nuclear export mechanism. Nipah virus V protein also associates tightly with both STAT1 and STAT2. As a result, the STAT proteins are retained in the cytoplasm, preventing IFN-induced STAT activation and nuclear translocation, thereby inhibiting cellular responses to either IFN-α or IFN-γ.