Interferons play key roles in mediating antiviral and antigrowth responses and in modulating immune response. The main signaling pathways are rapid and direct. They involve tyrosine phosphorylation and activation of signal transducers and activators of transcription factors by Janus tyrosine kinases at the cell membrane, followed by release of signal transducers and activators of transcription and their migration to the nucleus, where they induce the expression of the many gene products that determine the responses. Ancillary pathways are also activated by the interferons, but their effects on cell physiology are less clear. The Janus kinases and signal transducers and activators of transcription, and many of the interferon-induced proteins, play important alternative roles in cells, raising interesting questions as to how the responses to the interferons intersect with more general aspects of cellular physiology and how the specificity of cytokine responses is maintained. CONTENTS

PrefaceSince the discovery of interferons (IFNs), considerable progress has been made in describing the nature of the cytokines themselves, the signalling components that direct the cell response and their antiviral activities. Gene targeting studies have distinguished four effector pathways of the IFNmediated antiviral response: the Mx GTPase pathway, the 2′-5′ oligoadenylate-synthetase-directed ribonuclease L pathway, the protein kinase R pathway and the ISG15 ubiquitin-like pathway. These effector pathways individually block viral transcription, degrade viral RNA, inhibit translation, and modify protein function to control all steps of viral replication. Ongoing research continues to expose additional activities for the effector proteins and has revealed unanticipated functions of the antiviral response. IntroductionInterferon (IFN) was discovered more than 50 years ago as an agent that inhibited the replication of influenza virus 1 . The IFN family of cytokines are now recognized as key components of the innate immune response and the first line of defence against virus infection. Accordingly, IFNs are currently used therapeutically, with the most noteworthy example, to combat Hepatitis C viral (HCV) infection, but also against a range of other disorders, including numerous malignancies and multiple sclerosis (recently reviewed 2 ).Three classes of IFN have been identified, designated types I to III, which are classified according to the receptor complex they signal through (Figure 1). Type II IFN consists of the single IFNγ gene product that binds the IFNGR receptor complex. IFNγ mediates broad immune responses to pathogens other than viruses. The more recently described type III IFNs include three IFNλ gene products that signal via the combined IFNLR1 and interleukin-10 receptor 2 (IL-10R2) receptors. To date little is known about the type III IFNs, although they are known to regulate the antiviral response and have been proposed to be the ancestral type I IFNs 3 . Type I IFNs, which in humans comprise 13 IFNα subtypes, IFNβ, IFNκ, IFNε, IFNo, IFNτ and IFNδ, engage the ubiquitously expressed IFNα receptor (IFNAR) complex that is composed of the two components, IFNAR1 and IFNAR2. The function of type I IFNs is well characterized and they are known to be essential for mounting a robust host response against viral infection. Accordingly, IFNAR-deficient mice have increased susceptibility to numerous viruses but maintain resistance to other microbial pathogens, such as Listeria monocytogenes 4, 5 . Similarly, humans with genetic defects in components of IFN signalling (STAT1, TYK2 or UNC93B) die of viral disease, with the defect in type I IFN (rather than IFNγ) signalling having the more significant role 6-9 .Binding of type I IFNs to IFNAR, with ensuing signal transduction, leads to the induction of more than 300 IFN-stimulated genes (ISGs) 10 . However, relatively few of these ISGs have been directly implicated in instigating the antiviral state. Instead, many of the gene products ISG15One of the most promi...

The pleiotropic activities of interferons (IFNs) are mediated primarily through the transcriptional regulation of many downstream effector genes. The mRNA profiles from IFN-␣, -␤, or -␥ treatments of the human fibrosarcoma cell line, HT1080, were determined by using oligonucleotide arrays with probe sets corresponding to more than 6,800 human genes. Among these were transcripts for known IFN-stimulated genes (ISGs), the expression of which were consistent with previous studies in which the particular ISG was characterized as responsive to either Type I (␣, ␤) or Type II (␥) IFNs, or both. Importantly, many novel IFN-stimulated genes were identified that were diverse in their known biological functions. For instance, several novel ISGs were identified that are implicated in apoptosis (including RAP46͞Bag-1, phospholipid scramblase, and hypoxia inducible factor-1␣). Furthermore, several IFNrepressed genes also were identified. These results demonstrate the usefulness of oligonucleotide arrays in monitoring mammalian gene expression on a broad and unprecedented scale. In particular, these findings provide insights into the basic mechanisms of IFN actions and ultimately may contribute to better therapeutic uses for IFNs.

RNA interference (RNAi) is a powerful tool used to manipulate gene expression or determine gene function. One technique of expressing the short double-stranded (ds) RNA intermediates required for interference in mammalian systems is the introduction of short-interfering (si) RNAs. Although RNAi strategies are reliant on a high degree of specificity, little attention has been given to the potential non-specific effects that might be induced. Here, we found that transfection of siRNAs results in interferon (IFN)-mediated activation of the Jak-Stat pathway and global upregulation of IFN-stimulated genes. This effect is mediated by the dsRNA-dependent protein kinase, PKR, which is activated by 21-base-pair (bp) siRNAs and required for upregulation of IFN-beta in response to siRNAs. In addition, we show by using cell lines deficient in specific components mediating IFN action that the RNAi mechanism itself is independent of the interferon system. Thus, siRNAs have broad and complicating effects beyond the selective silencing of target genes when introduced into cells. This is of critical importance, as siRNAs are currently being explored for their potential therapeutic use.