Cytokines play a critical role in modulating the innate and adaptive immune systems. Here, we have identified from the human genomic sequence a family of three cytokines, designated interleukin 28A (IL-28A), IL-28B and IL-29, that are distantly related to type I interferons (IFNs) and the IL-10 family. We found that like type I IFNs, IL-28 and IL-29 were induced by viral infection and showed antiviral activity. However, IL-28 and IL-29 interacted with a heterodimeric class II cytokine receptor that consisted of IL-10 receptor beta (IL-10Rbeta) and an orphan class II receptor chain, designated IL-28Ralpha. This newly described cytokine family may serve as an alternative to type I IFNs in providing immunity to viral infection.

Type III IFNs (IFN-λ/IL-28/29) are cytokines with type I IFN-like antiviral activities, which remain poorly characterized. We herein show that most cell types expressed both types I and III IFNs after TLR stimulation or virus infection, whereas the ability of cells to respond to IFN-λ was restricted to a narrow subset of cells, including plasmacytoid dendritic cells and epithelial cells. To examine the role of type III IFN in antiviral defense, we generated IL-28Rα-deficient mice. These mice were indistinguishable from wild-type mice with respect to clearance of a panel of different viruses, whereas mice lacking the type I IFN receptor (IFNAR−/−) were significantly impaired. However, the strong antiviral activity evoked by treatment of mice with TLR3 or TLR9 agonists was significantly reduced in both IL-28RA−/− and IFNAR−/− mice. The type I IFN receptor system has been shown to mediate positive feedback on IFN-αβ expression, and we found that the type I IFN receptor system also mediates positive feedback on IFN-λ expression, whereas IL-28Rα signaling does not provide feedback on either type I or type III IFN expression in vivo. Finally, using bone-marrow chimeric mice we showed that TLR-activated antiviral defense requires expression of IL-28Rα only on nonhemopoietic cells. In this compartment, epithelial cells responded to IFN-λ and directly restricted virus replication. Our data suggest type III IFN to target a specific subset of cells and to contribute to the antiviral response evoked by TLRs.

T he superfamily of human Class II cytokines contains interleukin-10 (IL-10), the IL-10 -related interleukins , the interferons (IFN-␣, -␤ -, -, -, and -␥) and the interferon-like molecules IL-28A, IL-28B, and IL-29 (also referred to as lambda interferons). 1 Collectively, these molecules modulate innate and adaptive immune responses to environmental pathogens and protect the host against diseases such as cancer. The best-characterized class II cytokines are the type I interferons, whose expression is tightly regulated by viral infection. 2 After binding, these proteins induce a large set of interferonstimulated genes (ISGs) that inhibit viral replication and activate numerous downstream cellular responses involving dendritic cells, lymphocytes, and macrophages. 3 In addition to the type I interferons, viral infection also stimulates the rapid production of IL-28 and IL-29, a related, but distinct subset of the class II cytokine superfamily. 4,5 These proteins also possess potent antiviral activity; however, in contrast to the type I interferons, they bind a heterodimeric receptor consisting of the IL-28R␣ 4,5 subunit and the IL-10R␤ subunit, a receptor subunit that is also shared by IL-10, Chronic viral hepatitis is the leading cause of liver disease and may play a role in the pathogenesis of lesions characteristic of cirrhosis, hepatocellular carcinoma, and end-stage liver failure. The two major causes of chronic viral hepatitis are hepatitis B virus (HBV), a DNA-containing member of the Hepadnaviridae family that infects approximately 350 million people worldwide, 7 and hepatitis C virus (HCV), an RNA virus of the Flaviviridae family that infects approximately 170 million individuals worldwide. 8 IFN-␣ is an approved treatment for both types of chronic viral hepatitis and has demonstrated considerable clinical success. [9][10][11] However, this cytokine is ineffective for a substantial percentage of infected individ-

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-α, IFN-β and other type I IFNs signal through a common universally expressed cell surface receptor, whereas IFN-λ uses a distinct receptor complex for signaling that is not present on all cell types. Since type I IFN receptor-deficient mice (IFNAR10/0) exhibit greatly increased susceptibility to various viral diseases, it remained unclear to which degree IFN-λ 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-λ readily induced the antiviral factor Mx1 in mouse lungs and efficiently protected IFNAR10/0 mice from lethal influenza virus infection. By contrast, intraperitoneal application of IFN-λ failed to induce Mx1 in the liver of IFNAR10/0 mice and did not protect against hepatotropic virus infections. Mice lacking functional IFN-λ receptors were only slightly more susceptible to influenza virus than wild-type mice. However, mice lacking functional receptors for both IFN-α/β and IFN-λ 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 IFNAR10/0 mice. From these results we conclude that IFN-λ contributes to inborn resistance against viral pathogens infecting the lung but not the liver.

Ovules play a central role in plant reproduction, generating the female gametophyte within sporophytic integuments. When fertilized, the integuments differentiate into the seed coat and support the development of the embryo and endosperm. Mutations in the AINTEGUMENTA (ANT) locus of Arabidopsis have a profound effect on ovule development. Strong ant mutants have ovules that fail to form integuments or a female gametophyte. Flower development is also altered, with a random reduction of organs in the outer three whorls. In addition, organs present in the outer three floral whorls often have abnormal morphology. Ovules from a weak ant mutant contain both inner and outer integuments but generally fail to produce a functional female gametophyte. We isolated the ANT gene by using a mutation derived by T-DNA insertional mutagenesis. ANT is a member of a gene family that includes the floral homeotic gene APETALA2 (AP2). Like AP2, ANT contains two AP2 domains homologous with the DNA binding domain of ethylene response element binding proteins. ANT is expressed most highly in developing flowers but is also expressed in vegetative tissue. Taken together, these results suggest that ANT is a transcription factor that plays a critical role in regulating ovule and female gametophyte development.