The innate immune response to viruses is initiated when specialized cellular sensors recognize viral danger signals. Here we show that truncated forms of viral genomes that accumulate in infected cells potently trigger the sustained activation of the transcription factors IRF3 and NF-κB and the production type I IFNs through a mechanism independent of IFN signaling. We demonstrate that these defective viral genomes (DVGs) are generated naturally during respiratory infections in vivo even in mice lacking the type I IFN receptor, and their appearance coincides with the production of cytokines during infections with Sendai virus (SeV) or influenza virus. Remarkably, the hallmark antiviral cytokine IFNβ is only expressed in lung epithelial cells containing DVGs, while cells within the lung that contain standard viral genomes alone do not express this cytokine. Together, our data indicate that DVGs generated during viral replication are a primary source of danger signals for the initiation of the host immune response to infection.

Defective viral genomes (DVGs) are generated during virus replication. DVGs bearing complementary ends are strong inducers of dendritic cell (DC) maturation and of the expression of antiviral and pro-inflammatory cytokines by triggering signaling of the RIG-I family of intracellular pattern recognition receptors. Our data show that DCs stimulated with virus containing DVGs have an enhanced ability to activate human T cells and can induce adaptive immunity in mice. In addition, we describe the generation of a short Sendai virus (SeV)-derived DVG RNA (DVG-324) that maintains strong immunostimulatory activity in vitro and in vivo. DVG-324 induced high levels of IFN-β expression when transfected into cells and triggered fast expression of pro-inflammatory cytokines and mobilization of dendritic cells when injected into the footpad of mice. Importantly, DVG-324 enhanced the production of antibodies to a prototypic vaccine after a single intramuscular immunization in mice. Notably, the proinflammatory cytokine profile induced by DVG-324 was different from that induced by poly I:C, the only viral RNA analogue currently used as an immunostimulant in vivo, suggesting a distinct mechanism of action. SeV-derived oligonucleotides represent novel alternatives to be harnessed as potent adjuvants for vaccination.

Immune evasion is a defining feature of the virus-host relationship. During infection, herpes simplex virus type 1 (HSV-1) utilizes multiple proteins to manipulate the host immune response. In the present study, we investigated the mechanism by which the virion host shutoff (vhs) protein blocks the activation of dendritic cells (DCs). Previously, we found that coinfection of wild-type HSV-1 with a panel of RNA viruses resulted in a block to DC activation that was attributable to vhs. These observations led us to hypothesize that the vhs-mediated inhibition was dependent on signaling through the RIG-I-like receptor (RLR) signaling pathway. By examining DCs generated from MAVS (IPS-1) knockout (KO) mice, we determined that RLR/MAVS signaling is not essential for the DC response to HSV-1. We also evaluated the requirement for the type I interferon ( Herpes simplex virus type 1 (HSV-1) is a highly successful human pathogen belonging to the Alphaherpesviridae subfamily of herpesviruses. Initial exposure to virus results in lifelong infection, and it is estimated that between 60 and 80% of humans are seropositive for the virus (52). Normal HSV infections are characterized by cycling between lytic infection at epithelial surfaces and stages of latency in neuronal cells (reviewed in reference 47). The pathology of HSV infection is greatly influenced by the immune status of the host, which impacts both disease severity and the frequency of reactivation (21,35,42,48,69).Early during primary infection of the epithelium, HSV encounters a specialized type of immune cell, the dendritic cells (DCs). DCs function as a crucial link between innate and adaptive immune responses (reviewed in reference 4). These cells survey peripheral tissues in an immature state and undergo a process referred to as maturation (or activation) upon encounter with virus-associated molecules (5, 32). DC maturation is initially characterized by the secretion of type I and III interferons (IFNs) and proinflammatory cytokines (e.g., interleukin 6 [IL-6], tumor necrosis factor alpha [TNF-␣], and IL-12) and regulation of molecules necessary for migration to peripheral lymph nodes (32). En route to these secondary lymphoid organs, the DCs upregulate several costimulatory markers (CD86 and CD80) and load viral antigen onto major histocompatibility complex (MHC) molecules, which in concert serve to stimulate naïve B and T cells (4).Numerous viral proteins are utilized by HSV to evade the host immune response at all stages of the virus life cycle (7,9,31,33,39,63). Immunomodulatory proteins are either produced during the virus replication cycle or prepackaged in viral particles in the tegument and deposited into the cell immediately following virus envelope-host cell membrane fusion. The virion-host shutoff (vhs) protein is one such tegument-localized viral protein synthesized with late kinetics and packaged into mature virion particles (14,25,51,59). Functionally, vhs is a viral RNase that is known to preferentially degrade both host and viral mRNA species (1...