IFN-lambda determines the intestinal epithelial antiviral host defense.

Type I and type III IFNs bind to different cell-surface receptors but induce identical signal transduction pathways, leading to the expression of antiviral host effector molecules. Despite the fact that type III IFN (IFN-λ) has been shown to predominantly act on mucosal organs, in vivo infection studies have failed to attribute a specific, nonredundant function. Instead, a predominant role of type I IFN was observed, which was explained by the ubiquitous expression of the type I IFN receptor. Here we comparatively analyzed the role of functional IFN-λ and type I IFN receptor signaling in the innate immune response to intestinal rotavirus infection in vivo, and determined viral replication and antiviral gene expression on the cellular level. We observed that both suckling and adult mice lacking functional receptors for IFN-λ were impaired in the control of oral rotavirus infection, whereas animals lacking functional receptors for type I IFN were similar to wild-type mice. Using Mx1 protein accumulation as marker for IFN responsiveness of individual cells, we demonstrate that intestinal epithelial cells, which are the prime target cells of rotavirus, strongly responded to IFN-λ but only marginally to type I IFN in vivo. Systemic treatment of suckling mice with IFN-λ repressed rotavirus replication in the gut, whereas treatment with type I IFN was not effective. These results are unique in identifying a critical role of IFN-λ in the epithelial antiviral host defense.

Combined action of type I and type III interferon restricts initial replication of severe acute respiratory syndrome coronavirus in the lung but fails to inhibit systemic virus spread.

STAT1-deficient mice are more susceptible to infection with severe acute respiratory syndrome coronavirus (SARS-CoV) than type I interferon (IFN) receptor-deficient mice. We used mice lacking functional receptors for both type I and type III IFN (double knockout, dKO) to evaluate the possibility that type III IFN plays a decisive role in SARS-CoV protection. We found that viral peak titres in lungs of dKO and STAT1-deficient mice were similar, but significantly higher than in wild-type mice. The kinetics of viral clearance from the lung were also comparable in dKO and STAT1-deficient mice. Surprisingly, however, infected dKO mice remained healthy, whereas infected STAT1-deficient mice developed liver pathology and eventually succumbed to neurological disease. Our data suggest that the failure of STAT1-deficient mice to control initial SARS-CoV replication efficiently in the lung is due to impaired type I and type III IFN signalling, whereas the failure to control subsequent systemic viral spread is due to unrelated defects in STAT1-deficient mice.

Both nonstructural proteins NS1 and NS2 of pneumonia virus of mice are inhibitors of the interferon type I and type III responses in vivo.

Infection of mice with pneumonia virus of mice (PVM) provides a convenient experimental pathogenesis model in a natural host for a human respiratory syncytial virus-related virus. Extending our previous work showing that the PVM nonstructural (NS) proteins were pathogenicity factors in mice, we identify both the NS1 and NS2 proteins as antagonists of alpha/beta interferon (IFN-α/ß) and IFN-λ by use of recombinant PVM (rPVM) with single and combined deletions of the NS proteins (ΔNS1, ΔNS2, and ΔNS1 ΔNS2). Wild-type and NS deletion PVMs were evaluated for growth and pathogenesis by infecting knockout mice that lack functional receptors to IFN-α/ß, IFN-λ, or both. The absence of the receptor to IFN-α/ß (IFNAR) or IFN-λ (interleukin-28 receptor α chain [IL-28Rα]) individually did not reverse the attenuated virulence of the NS deletion viruses although loss of IFNAR partially restored replication efficiency. When both receptors were deleted, replication and virulence were largely rescued for rPVM ΔNS1 and were significantly but not completely rescued for rPVM ΔNS2. As for rPVM ΔNS1 ΔNS2, the effect was mostly limited to partial enhancement of replication. This indicates that both IFN-α/ß and IFN-λ contributed to restricting the NS deletion viruses, with the former playing the greater role. Interestingly, the replication and virulence of wild-type PVM were completely unaffected by the presence or absence of functional receptors to IFN-α/ß and IFN-λ, indicating that both systems are strongly suppressed during infection. However, pretreatment of mice with IFN-α/ß was protective against lethal rPVM challenge, whereas pretreatment with IFN-λ delayed but did not prevent disease and, in some cases, reduced mortality. The fact that virulence of rPVM lacking NS2 was not recovered completely when both interferon receptors were deleted suggests that NS2 may have further functions outside the IFN system.

Transcription factor redundancy ensures induction of the antiviral state.

The transcriptional response to virus infection is thought to be predominantly induced by interferon (IFN) signaling. Here we demonstrate that, in the absence of IFN signaling, an IFN-like transcriptome is still maintained. This transcriptional activity is mediated from IFN-stimulated response elements (ISREs) that bind to both the IFN-stimulated gene factor 3 (ISGF3) as well as to IFN response factor 7 (IRF7). Through a combination of both in vitro biochemistry and in vivo transcriptional profiling, we have dissected what constitutes IRF-specific, ISGF3-specific, or universal ISREs. Taken together, the data presented here suggest that IRF7 can induce an IFN-like transcriptome in the absence of type-I or -III signaling and therefore provides a level of redundancy to cells to ensure the induction of the antiviral state.

Glycine 184 in nonstructural protein NS1 determines the virulence of influenza A virus strain PR8 without affecting the host interferon response.

The nonstructural protein NS1 of influenza A virus counteracts the interferon (IFN) system and thereby promotes viral replication. NS1 has acquired different mechanisms to limit induction of IFN. It prevents double-stranded RNA (dsRNA) and RIG-I-mediated activation of interferon regulatory factor 3 (IRF3), and it blocks posttranscriptional processing of cellular mRNAs by binding to the cleavage and polyadenylation specificity factor (CPSF). Using a mouse-adapted A/PR/8/34 virus and reverse genetics to introduce specific mutations in NS1 which eliminate one or both functions, we determined the relative contributions of these two activities of NS1 to viral virulence in mice. We found that a functional RNA-binding motif was required for IFN suppression and virulence. Restoration of CPSF binding in the NS1 protein of wild-type A/PR/8/34 virus, which cannot bind CPSF due to mutations in the central binding motif at positions 103 and 106, resulted in enhanced virulence. Surprisingly, if CPSF binding was abolished by substituting glycine for arginine at position 184 in the classical NS1-CPSF binding motif, the mutant virus replicated much more slowly in mice, although the mutated NS1 protein continued to repress the IFN response very efficiently. Our results show that a functional RNA-binding motif is decisive for NS1 of A/PR/8/34 virus to suppress IFN induction. They further demonstrate that in addition to its contribution to CPSF binding, glycine 184 strongly influences viral virulence by an unknown mechanism which does not involve the IFN system.

Lambda interferon renders epithelial cells of the respiratory and gastrointestinal tracts resistant to viral infections.

Virus-infected cells secrete a broad range of interferons (IFN) which confer resistance to yet uninfected cells by triggering the synthesis of antiviral factors. The relative contributions of the various IFN subtypes to innate immunity against virus infections remain elusive. IFN-alpha, IFN-beta, and other type I IFN molecules signal through a common, universally expressed cell surface receptor, whereas type III IFN (IFN-lambda) uses a distinct cell-type-specific receptor complex for signaling. Using mice lacking functional receptors for type I IFN, type III IFN, or both, we found that IFN-lambda plays an important role in the defense against several human pathogens that infect the respiratory tract, such as influenza A virus, influenza B virus, respiratory syncytial virus, human metapneumovirus, and severe acute respiratory syndrome (SARS) coronavirus. These viruses were more pathogenic and replicated to higher titers in the lungs of mice lacking both IFN receptors than in mice with single IFN receptor defects. In contrast, Lassa fever virus, which infects via the respiratory tract but primarily replicates in the liver, was not influenced by the IFN-lambda receptor defect. Careful analysis revealed that expression of functional IFN-lambda receptor complexes in the lung and intestinal tract is restricted to epithelial cells and a few other, undefined cell types. Interestingly, we found that SARS coronavirus was present in feces from infected mice lacking receptors for both type I and type III IFN but not in those from mice lacking single receptors, supporting the view that IFN-lambda contributes to the control of viral infections in epithelial cells of both respiratory and gastrointestinal tracts.

Interferon-lambda contributes to innate immunity of mice against influenza A virus but not against hepatotropic viruses.

Virus-infected cells secrete a broad range of interferon (IFN) subtypes which in turn trigger the synthesis of antiviral factors that confer host resistance. IFN-alpha, IFN-beta and other type I IFNs signal through a common universally expressed cell surface receptor, whereas IFN-lambda uses a distinct receptor complex for signaling that is not present on all cell types. Since type I IFN receptor-deficient mice (IFNAR1(0/0)) exhibit greatly increased susceptibility to various viral diseases, it remained unclear to which degree IFN-lambda might contribute to innate immunity. To address this issue we performed influenza A virus infections of mice which carry functional alleles of the influenza virus resistance gene Mx1 and which, therefore, develop a more complete innate immune response to influenza viruses than standard laboratory mice. We demonstrate that intranasal administration of IFN-lambda readily induced the antiviral factor Mx1 in mouse lungs and efficiently protected IFNAR1(0/0) mice from lethal influenza virus infection. By contrast, intraperitoneal application of IFN-lambda failed to induce Mx1 in the liver of IFNAR1(0/0) mice and did not protect against hepatotropic virus infections. Mice lacking functional IFN-lambda receptors were only slightly more susceptible to influenza virus than wild-type mice. However, mice lacking functional receptors for both IFN-alpha/beta and IFN-lambda were hypersensitive and even failed to restrict usually non-pathogenic influenza virus mutants lacking the IFN-antagonistic factor NS1. Interestingly, the double-knockout mice were not more susceptible against hepatotropic viruses than IFNAR1(0/0) mice. From these results we conclude that IFN-lambda contributes to inborn resistance against viral pathogens infecting the lung but not the liver.

What have we learned from the IL28 receptor knockout mouse?

The recently discovered type III interferons (IFNs), also known as IFN-lambda, are part of the early innate immune response against viral infections. The IFN-lambda system closely resembles the type I IFN (IFN-alpha/beta) system in terms of expression after virus infection as well as intracellular signaling and activation of antiviral host factors in susceptible cells. However, in contrast to type I IFN, which signals through a universally expressed cell surface receptor, IFN-lambda uses a distinct receptor complex (IL28R) for signaling, which is expressed on a limited range of cell types. Until recently both the contribution of type III IFN to antiviral resistance as well as the exact nature of IL28R-expressing cells in vivo remained elusive. In this review we discuss data obtained from the experiments with IL28Ralpha(0/0) mice that demonstrated the role of IFN-lambda in viral defense in vivo. We further discuss the experiments that identified the cell types in various organs that express functional IFN-lambda receptors.