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RNA virus infection results in expression of type 1 IFNs, especially IFN-α/β, which play a crucial role in host antivirus responses. Type 1 IFNs are induced in a cell type-specific manner through TLR and RIG-I–like receptor pathways, both of which activate IFN regulatory factors (IRFs) and NF-κB transcription factors. Although NF-κB activation and association with the IFN-β promoter after RNA virus infection is well documented, our previous work showed that, surprisingly, NF-κB is not essential for IFN-β gene expression. Thus, the actual function of NF-κB in IFN-β expression and virus replication is not clear. In this study, we found Newcastle disease virus and vesicular stomatitis virus replication is enhanced in mouse embryonic fibroblasts (MEFs) lacking the NF-κB RelA subunit. Increased virus replication was traced to a specific requirement for RelA in early virus-induced IFN-β expression. At these time points, when IFN-β expression is ~100-fold less than peak levels, impaired IFN-β production delayed IFN-induced gene expression, resulting in increased virus replication in RelA−/− MEFs. Importantly, our results show that RelA requirement is crucial only when IRF3 activation is low. Thus, high levels of activated IRF3 expression are sufficient for induction of IFN-β in RelA−/− MEFs, transcriptional synergism with the coactivator CREB-binding protein, and rescue of susceptibility to virus. Together, these findings indicate that NF-κB RelA is not crucial for regulating overall IFN-β production, as previously believed; instead, RelA is specifically required only during a key early phase after virus infection, which substantially impacts the host response to virus infection.

Reverse transcription, an essential step in the life cycle of all retroelements, is a complex, multistep process whose regulation is not yet clearly understood. We have recently shown that reverse transcription in the pararetrovirus duck hepatitis B virus is associated with complete dephosphorylation of the viral core protein, which forms the nucleocapsid wherein reverse transcription takes place. Here we present a genetic study of the role of this dynamic nucleocapsid phosphorylation in regulating viral reverse transcription. Detailed analyses of the reverse transcription products synthesized within nucleocapsids composed of core phosphorylation site mutants revealed that alanine substitutions, mimicking the nonphosphorylated state, completely blocked reverse transcription at a very early stage. In contrast, aspartate substitutions, mimicking the phosphorylated state, allowed complete first-strand DNA synthesis but were severely defective in accumulating mature doublestranded DNA. The latter defect was due to a combination of mutant nucleocapsid instability during maturation and a block in mature second-strand DNA synthesis. Thus, the reversible phosphorylation of the nucleocapsids regulates the ordered progression of reverse transcription.Reverse transcription, the synthesis of a double-stranded (DS) DNA copy of an RNA, is an essential step in the life cycle of all reverse-transcribing elements, including classical retroviruses, retrotransposons, and pararetroviruses, such as the hepadnaviruses. The family Hepadnaviridae, which includes the global human pathogen Hepatitis B virus (HBV) and closely related animal viruses, such as Duck hepatitis B virus (DHBV), is a group of small hepatotropic DNA viruses (7,8). All hepadnaviruses replicate their DNA genomes via an RNA intermediate termed the pregenomic RNA (pgRNA) (31). Viral assembly begins with the packaging of the pgRNA, along with the virally encoded reverse transcriptase (RT), into an immature nucleocapsid (NC), followed by reverse transcription of the pgRNA within the NC into the characteristic, partially DS relaxed circular (RC) DNA genome. The DNA-containing mature NC is then enveloped and secreted extracellularly as a complete virion (10,26,31,36).The icosahedral NC consists of 180 or 240 copies of a single viral protein, the capsid or core protein (2,6,38). The Nterminal two-thirds of the core protein forms the capsid shell (1), while its basic, arginine-rich C-terminal domain is dispensable for capsid assembly but required for pgRNA packaging and reverse transcription (30,39). Both the HBV and DHBV core proteins are known to be phosphorylated at the C-terminal domain in the host cells, which has been shown to modulate core protein functions, including pgRNA packaging and DNA synthesis (9,18,20,22,24,28,30,40,41).Recently, we reported that the phosphorylation state of the DHBV NC undergoes a dynamic change during reverse transcription such that the phosphorylated, immature NCs become dephosphorylated as they mature (27). The immature NCs consist of co...

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The C-terminal domain (CTD) of hepadnavirus core protein is involved in multiple steps of viral replication. In particular, the CTD is initially phosphorylated at multiple sites to facilitate viral RNA packaging into immature nucleocapsids (NCs) and the early stage of viral DNA synthesis. For the avian hepadnavirus duck hepatitis B virus (DHBV), CTD is dephosphorylated subsequently to facilitate the late stage of viral DNA synthesis and to stabilize NCs containing mature viral DNA. The role of CTD phosphorylation in virion secretion, if any, has remained unclear. Here, the CTD from the human hepatitis B virus (HBV) was found to be dephosphorylated in association with NC maturation and secretion of DNA-containing virions, as in DHBV. In contrast, the CTD in empty HBV virions (i.e., enveloped capsids with no RNA or DNA) was found to be phosphorylated. The potential role of CTD dephosphorylation in virion secretion was analyzed through mutagenesis. For secretion of empty HBV virions, which is independent of either viral RNA packaging or DNA synthesis, multiple substitutions in the CTD to mimic either phosphorylation or dephosphorylation showed little detrimental effect. Similarly, phospho-mimetic substitutions in the DHBV CTD did not block the secretion of DNA-containing virions. These results indicate that CTD dephosphorylation, though associated with NC maturation in both HBV and DHBV, is not essential for the subsequent NC-envelope interaction to secrete DNA-containing virions, and the CTD state of phosphorylation also does not play an essential role in the interaction between empty capsids and the envelope for secretion of empty virions.IMPORTANCE The phosphorylation state of the C-terminal domain (CTD) of hepatitis B virus (HBV) core or capsid protein is highly dynamic and plays multiple roles in the viral life cycle. To study the potential role of the state of phosphorylation of CTD in virion secretion, we have analyzed the CTD phosphorylation state in complete (containing the genomic DNA) versus empty (genome-free) HBV virions. Whereas CTD is unphosphorylated in complete virions, it is phosphorylated in empty virions. Mutational analyses indicate that neither phosphorylation nor dephosphorylation of CTD is required for virion secretion. These results demonstrate that while CTD dephosphorylation is associated with HBV DNA synthesis, the CTD state of phosphorylation may not regulate virion secretion.

Production of type I interferons (IFNs; prominently, IFN-␣/␤) following virus infection is

Receptor-interacting protein kinase 1 (RIPK1) regulates cell fate and proinflammatory signaling downstream of multiple innate immune pathways, including those initiated by TNF-α, TLR ligands, and IFNs. Genetic ablation of Ripk1 results in perinatal lethality arising from both RIPK3-mediated necroptosis and FADD/caspase-8–driven apoptosis. IFNs are thought to contribute to the lethality of Ripk1-deficient mice by activating inopportune cell death during parturition, but how IFNs activate cell death in the absence of RIPK1 is not understood. In this study, we show that Z-form nucleic acid binding protein 1 (ZBP1; also known as DAI) drives IFN-stimulated cell death in settings of RIPK1 deficiency. IFN-activated Jak/STAT signaling induces robust expression of ZBP1, which complexes with RIPK3 in the absence of RIPK1 to trigger RIPK3-driven pathways of caspase-8–mediated apoptosis and MLKL-driven necroptosis. In vivo, deletion of either Zbp1 or core IFN signaling components prolong viability of Ripk1−/− mice for up to 3 mo beyond parturition. Together, these studies implicate ZBP1 as the dominant activator of IFN-driven RIPK3 activation and perinatal lethality in the absence of RIPK1.

A mass vaccination campaign in India seeks to control and eventually eradicate foot-and-mouth disease (FMD). Biosanitary measures along with FMD monitoring are being conducted along with vaccination. The implementation of the FMD control program has drastically reduced the incidence of FMD. However, cases are still reported, even in regions where vaccination is carried out regularly. Control of FMD outbreaks is difficult when the virus remains in circulation in the vaccinated population. Various FMD risk factors have been identified that are responsible for FMD in vaccinated areas. The factors are discussed along with strategies to address these challenges. The current chemically inactivated trivalent vaccine formulation containing strains of serotype O, A, and Asia 1 has limitations including thermolability and induction of only short-term immunity. Advantages and disadvantages of several new-generation alternate vaccine formulations are discussed. It is unfeasible to study every incidence of FMD in vaccinated animals/areas in such a big country as India with its huge livestock population. However, at the same time, it is absolutely necessary to identify the precise reason for vaccination failure. Failure to vaccinate is one reason for the occurrence of FMD in vaccinated areas. FMD epidemiology, emerging and re-emerging virus strains, and serological status over the past 10 years are discussed to understand the impact of vaccination and incidences of vaccination failure in India. Other factors that are important in vaccination failure that we discuss include disrupted herd immunity, health status of animals, FMD carrier status, and FMD prevalence in other species. Recommendations to boost the search of alternate vaccine formulation, strengthen the veterinary infrastructure, bolster the real-time monitoring of FMD, as well as a detailed investigation and documentation of every case of vaccination failure are provided with the goal of refining the control program.