The antiviral response to gamma interferon

A role for alpha/beta interferon (IFN-alpha/beta) in the IFN-gamma antiviral response has long been suggested. Accordingly, possible roles for autocrine or double-stranded-RNA (dsRNA)-induced IFN-alpha/beta in the IFN-gamma response were investigated. Use was made of wild-type and a variety of mutant human fibrosarcoma cell lines, including mutant U5A cells, which lack a functional IFN-alpha/beta receptor and hence an IFN-alpha/beta response. IFN-gamma did not induce detectable levels of IFN-alpha/beta in any of the cell lines, nor was the IFN-gamma response per se dependent on autocrine IFN-alpha/beta. On the other hand, a number of responses to dsRNA [poly(I). poly(C)] and encephalomyocarditis virus were greatly enhanced by IFN-gamma pretreatment (priming) of wild-type cells or of mutant cells lacking an IFN-alpha/beta response; these include the primary induction of dsRNA-inducible mRNAs, including IFN-beta mRNA, and, to a lesser extent, the dsRNA-mediated activation of the p38 mitogen-activated protein (MAP) kinase(s). IFN-gamma priming of mRNA induction by dsRNA is dependent on JAK1 and shows biphasic kinetics, with an initial rapid (<30-min) response being followed by a more substantial effect on overnight incubation. The IFN-gamma-primed dsRNA responses appear to be subject to modulation through the p38, phosphatidylinositol 3-kinase, and ERK1/ERK2 MAP kinase pathways. It can be concluded that despite efficient priming of IFN-beta production, the IFN-alpha/beta pathways play no significant role in the primary IFN-gamma antiviral response in these cell-virus systems. The observed IFN-gamma priming of dsRNA responses, on the other hand, will likely play a significant role in combating virus infection in vivo.     

Regulation of synthesis and turnover of an interferon-inducible mRNA.

Regulation of synthesis and turnover of an interferon (IFN)-inducible mRNA, mRNA 561, in HeLa monolayer cells was studied. Cytoplasmic levels of this mRNA were estimated by hybridization analyses with a cDNA clone that we have isolated as a probe. IFN-alpha A induced a high level of this mRNA in a transient fashion, whereas no induction was observed in response to IFN-gamma. Surprisingly little mRNA 561 was induced in cells treated simultaneously with IFN-alpha A and an inhibitor of protein synthesis, suggesting that in addition to IFN-alpha A, an interferon-inducible protein was needed for induction of this mRNA. Apparently this putative protein could be induced by IFN-gamma as well. Thus, although little mRNA 561 was synthesized in cells treated either with IFN-gamma alone or with IFN-alpha A and cycloheximide, a large quantity of this mRNA was induced in cells which had been pretreated with IFN-gamma and then treated with IFN-alpha A and cycloheximide. Once mRNA 561 was induced by IFN-alpha A, it turned over rapidly. This rapid turnover could be blocked by actinomycin D or cycloheximide indicating that another IFN-inducible protein may mediate this process.     

Alpha/Beta interferon and gamma interferon synergize to inhibit the replication of herpes simplex virus type 1

In vivo evidence suggests that T-cell-derived gamma interferon (IFN-gamma) can directly inhibit the replication of herpes simplex virus type 1 (HSV-1). However, IFN-gamma is a weak inhibitor of HSV-1 replication in vitro. We have found that IFN-gamma synergizes with the innate IFNs (IFN-alpha and -beta) to potently inhibit HSV-1 replication in vitro and in vivo. Treatment of Vero cells with either IFN-beta or IFN-gamma inhibits HSV-1 replication by <20-fold, whereas treatment with both IFN-beta and IFN-gamma inhibits HSV-1 replication by approximately 1,000-fold. Treatment with IFN-beta and IFN-gamma does not prevent HSV-1 entry into Vero cells, and the inhibitory effect can be overcome by increasing the multiplicity of HSV-1 infection. The capacity of IFN-beta and IFN-gamma to synergistically inhibit HSV-1 replication is not virus strain specific and has been observed in three different cell types. For two of the three virus strains tested, IFN-beta and IFN-gamma inhibit HSV-1 replication with a potency that approaches that achieved by a high dose of acyclovir. Pretreatment of mouse eyes with IFN-beta and IFN-gamma reduces HSV-1 replication to nearly undetectable levels, prevents the development of disease, and reduces the latent HSV-1 genome load per trigeminal ganglion by approximately 200-fold. Thus, simultaneous activation of IFN-alpha/beta receptors and IFN-gamma receptors appears to render cells highly resistant to the replication of HSV-1. Because IFN-alpha or IFN-beta is produced by most cells as an innate response to virus infection, the results imply that IFN-gamma secreted by T cells may provide a critical second signal that potently inhibits HSV-1 replication in vivo.     

Interleukin-29 functions cooperatively with interferon to induce antiviral gene expression and inhibit hepatitis C virus replication

The interferon (IFN)-related cytokine interleukin (IL)-29 (also known as IFN-lambda1) inhibits virus replication by inducing a cellular antiviral response similar to that activated by IFN-alpha/beta. However, because it binds to a unique receptor, this cytokine may function cooperatively with IFN-alpha/beta or IFN-gamma during natural infections to inhibit virus replication, and might also be useful therapeutically in combination with other cytokines to treat chronic viral infections such as hepatitis C (HCV). We therefore investigated the ability of IL-29 and IFN-alpha or IFN-gamma to cooperatively inhibit virus replication and induce antiviral gene expression. Compared with the individual cytokines alone, the combination of IL-29 with IFN-alpha or IFN-gamma was more effective at blocking vesicular stomatitis virus and HCV replication, and this cooperative antiviral activity correlated with the magnitude of induced antiviral gene expression. Although the combined effects of IL-29 and IFN-alpha were primarily additive, the IL-29/IFN-gamma combination synergistically induced multiple genes and had the greatest antiviral activity. Two different mechanisms contributed to the enhanced gene expression induced by the cytokine combinations: increased activation of ISRE promoter elements and simultaneous activation of both ISRE and GAS elements within the same promoter. These findings provide new insight into the coregulation of a critical innate immune response by functionally distinct cytokine families.