Interferon Induction In Cells Research Articles

RIG-I Detects Viral Genomic RNA during Negative-Strand RNA Virus Infection

RIG-I is a key mediator of antiviral immunity, able to couple detection of infection by RNA viruses to the induction of interferons. Natural RIG-I stimulatory RNAs have variously been proposed to correspond to virus genomes, virus replication intermediates, viral transcripts, or self-RNA cleaved by RNase L. However, the relative contribution of each of these RNA species to RIG-I activation and interferon induction in virus-infected cells is not known. Here, we use three approaches to identify physiological RIG-I agonists in cells infected with influenza A virus or Sendai virus. We show that RIG-I agonists are exclusively generated by the process of virus replication and correspond to full-length virus genomes. Therefore, nongenomic viral transcripts, short replication intermediates, and cleaved self-RNA do not contribute substantially to interferon induction in cells infected with these negative strand RNA viruses. Rather, single-stranded RNA viral genomes bearing 5'-triphosphates constitute the natural RIG-I agonists that trigger cell-intrinsic innate immune responses during infection.

Translational inhibition and increased interferon induction in cells infected with C protein-deficient measles virus.

In addition to the phosphoprotein, the P gene of measles virus (MV) also encodes the V and C proteins by an RNA editing process and by alternative initiation of translation in a different reading frame, respectively. Although the MV C protein is required for efficient MV replication in vivo and in some cultured cells, its exact functions in virus infection are currently unclear. Here, we report that a recombinant MV lacking the C protein (MVDeltaC) grew poorly in a human cell line possessing the intact interferon (IFN) pathway and that this growth defect was associated with reduced viral translation and genome replication. The translational inhibition was correlated with phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2. Moreover, increased IFN induction was observed in MVDeltaC-infected cells. The NS1 protein of influenza virus, which binds to double-stranded RNA (dsRNA) and consequently inhibits IFN induction and dsRNA-dependent protein kinase activation, complemented the growth defect of MVDeltaC. These results indicate that the MV C protein inhibits IFN induction and modulates host antiviral responses, thereby ensuring MV growth in host cells.

The mechanism of interferon induction in mouse spleen cells stimulated with HVJ

This study showed that functional viral RNA and the penetration of virus into the cell are needed for interferon induction in L cells, while simple contact of the viral glycoprotein with the cell surface appears to be sufficient for interferon induction by the same HVJ in mouse spleen cells. Thirty minutes of uv irradiation resulted in complete loss of the interferon-inducing ability of HVJ in mouse L cells. In contrast to this result, HVJ irradiated for 2 hr could induce interferon in mouse spleen cells as efficiently as untreated HVJ. These findings showed that the actual inducer of interferon in mouse spleen cells was not viral nucleic acid, but some other viral component. When HVJ was treated with potassium periodate at 37° for 1 hr, infectivity for eggs and the hemolytic and neuraminidase activities of the virus were not detectable, but a considerable portion of its hemagglutinating activity was retained. The binding to erythrocytes of this inactivated HVJ, which showed no interferon-inducing ability in both L and mouse spleen cells, was restored in mouse spleen cells but not in L cells. The results indicated that hemolytic and neuraminidase activities are not essential for interferon induction in mouse spleen cells and that hemagglutinating activity might be closely related to interferon induction in the cells, although the presence of hemagglutinating activity alone is not sufficient for interferon induction in the cells. It seems that structural integrity of hemagglutinin on the erythrocyte surface may be important for interferon induction. HeLa cell-grown HVJ, which is characterized by its inability to penetrate into tissue culture cells, was found to stimulate interferon production in mouse spleen cells but not in L cells. This suggests that the process of virus penetration may be essential for induction of interferon in L cells. Interferon was produced in mouse spleen cells incubated with membranous particles with HVJ glycoproteins, but interferon activity could not be detected in the culture fluids of L cells. Aggregation of the glycoproteins by an antibody enhanced its interferon-inducing ability in mouse spleen cells. These results showed that the actual inducer of interferon in mouse spleen cells is not viral nucleic acid, but viral glycoproteins of HVJ, and that the size of its membranous structure is related to interferon inducibility. The mechanism of interferon induction by influenza virus in mouse spleen cells is similar to that of interferon induction by HVJ.