Respiratory Syncytial Virus Infection Upregulates NLRC5 and Major Histocompatibility Complex Class I Expression through RIG-I Induction in Airway Epithelial Cells.

UNLABELLED: Respiratory syncytial virus (RSV) is the leading cause of acute respiratory tract viral infection in infants, causing bronchiolitis and pneumonia. The host antiviral response to RSV acts via retinoic acid-inducible gene I (RIG-I). We show here that RSV infection upregulates major histocompatibility complex class I (MHC-I) expression through the induction of NLRC5, a NOD-like, CARD domain-containing intracellular protein that has recently been identified as a class I MHC transactivator (CITA). RSV infection of A549 cells promotes upregulation of NLRC5 via beta interferon (IFN-ß) production, since the NLRC5-inducing activity in a conditioned medium from RSV-infected A549 cells was removed by antibody to IFN-ß, but not by antibody to IFN-γ. RSV infection resulted in RIG-I upregulation and induction of NLRC5 and MHC-I. Suppression of RIG-I induction significantly blocked NLRC5, as well as MHC-I, upregulation and diminished IRF3 activation. Importantly, Vero cells deficient in interferon production still upregulated MHC-I following introduction of the RSV genome by infection or transfection, further supporting a key role for RIG-I. A model is therefore proposed in which the host upregulates MHC-I expression during RSV infection directly via the induction of RIG-I and NLRC5 expression. Since elevated expression of MHC-I molecules can sensitize host cells to T lymphocyte-mediated cytotoxicity or immunopathologic damage, the results have significant implications for the modification of immunity in RSV disease. IMPORTANCE: Human respiratory syncytial virus (RSV) is the leading cause of bronchiolitis and pneumonia in infants and young children worldwide. Infection early in life is linked to persistent wheezing and allergic asthma in later life, possibly related to upregulation of major histocompatibility class I (MHC-I) on the cell surface, which facilitates cytotoxic T cell activation and antiviral immunity. Here, we show that RSV infection of lung epithelial cells induces expression of RIG-I, resulting in induction of a class I MHC transactivator, NLRC5, and subsequent upregulation of MHC-I. Suppression of RIG-I induction blocked RSV-induced NLRC5 expression and MHC-I upregulation. Increased MHC-I expression may exacerbate the RSV disease condition due to immunopathologic damage, linking the innate immune response to RSV disease.

Fungal metabolite myriocin promotes human herpes simplex virus-2 infection.

AIMS: Myriocin is a fungal metabolite with antiviral activity, including influenza, hepatitis B, and hepatitis C viruses. We investigated whether myriocin has activity against human HSV-2, one of the most prevalent pathogens of sexually transmitted disease. MAIN METHODS: Cell culture systems were used to evaluate myriocin effect on HSV-2 infection. Plaque forming assay and immunoblotting studies were used to determine virus production and viral protein expression, respectively. KEY FINDINGS: Myriocin showed no cytotoxic effect at up to 5 µM. Myriocin treatment did not inhibit HSV-2 infection. Instead, the treatment resulted in accelerated replication of HSV-2 and increased titers of infectious virion. The effect was detected at concentrations as low as 3 nM and plateaued at approximately 30 nM. Myriocin at 30 nM increased HSV-2 production by approximately 1.7 logs. Myriocin also promoted HSV-1 infection but required higher concentrations. A time course study revealed that myriocin promoted HSV-2 infection by acceleration of virus replication. Unlike trichostatin A that promotes HSV-2 infection and histone modifications, myriocin treatment did not alter histone modifications. Myriocin is a well characterized inhibitor of sphingolipid biosynthesis pathway. Structurally different inhibitors of the pathway showed no effect on HSV-2 infection. Exogenous sphingolipids did not reverse the effect of myriocin on HSV-2 infection either. SIGNIFICANCE: We found that myriocin promotes HSV-2 replication at nanomolar concentrations with yet unknown mechanisms. Further studies may uncover novel mechanisms regulating HSV replication and targets of myriocin action. This may have potential application in enhancing efficacy of oncolytic HSV for cancer therapy and other diseases.

Identification of OASL d, a splice variant of human OASL, with antiviral activity.

The 2',5'-oligoadenylate synthetases (OASs) are IFN-induced antiviral proteins and are upregulated by infection of viral and some bacterial pathogens. There are at least 2 transcripts of approximately 1.8 and 2.0 kb in interferon-beta treated samples that are recognized by a probe for human OASL in Northern blot assay. By RT-PCR amplification we have isolated a previously undescribed splice variant of human OASL, named OASL d. The new variant was derived from deletion of exons 4 and 5 and encodes a protein of 384 aa residues that shares the N-terminal 219 aa residues with OASL a. Sequence analysis indicates that OASL d also contains the entire ubiquitin-like domain identified in human OASL a. OASL d was strongly induced by IFNγ in THP-1 monocytic cells and in A549 epithelial cells by interferon-beta as detected by immunoblotting assay. Ectopic expression of OASL a or OASL d, but not OASL b that shares the N-terminus with OASL a and d, partially inhibited EV71 and VSV infection. No effect against HSV-2 infection was observed. Therefore, OASL d is a novel isoform of human OASL that possesses antiviral activity against RNA viruses.