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The growth-inhibitory effects of type 1 interferons (IFNs) (LFNα/β) are complex, and the role of apoptosis in their antigrowth effects is variable and not well understood. We have examined primary murine interleukin-7-dependent bone marrow-derived pro-B cells, where IFNβ, but not IFNα, induces programmed cell death (PCD). IFNα-stimulated apoptosis is the same in pro-B cells derived from wild type and Stat1-/- mice. However, in pro-B cells from Tyk2-/- mice, where there is normal activation of Stat1 and Stat2, IFNα-stimulated PCD is not observed. Loss of B cells in lymphocytic choriomeningitis virus-infected mice has been shown to be mediated through the expression of IFNα/β (1). In wild type mice infected with lymphocytic choriomeningitis virus, there is a greater loss of B cells in the bone marrow and spleen than in Tyk2-/- mice infected with the virus, suggesting that the expression of this kinase plays an in vivo role in IFNα/β-mediated PCD. In contrast to IFNβ-stimulated tyrosine phosphorylation of Stat 1 and Stat2, Stat3 tyrosine phosphorylation is defective in Tyk2-/- pro-B cells, suggesting that this Stat family member is required for apoptosis. In support of this hypothesis, inhibition of Stat3 activation in wild type B cells reverses the apoptotic effects of IFNβ. Furthermore, expression of a constitutively active form of Stat3 in Tyk2-/- B cells partially restores IFNβ-stimulated PCD. These results demonstrate an important role of Tyk2-mediated tyrosine phosphorylation of Stat3 in the ability of IFNα to stimulate apoptosis of primary pro-B cells. [ABSTRACT FROM AUTHOR]

Linear endocytic motifs of signaling receptors as well as their ubiquitination determine the rate of ligand-induced endocytosis that mediates down-regulation of these receptors and restricts the magnitude and duration of their respective signal transduction pathways. We previously hypothesized that, in the absence of its cognate ligand, type I interferon (IFN), the IFNalpha receptor chain 1 (IFNAR1) receptor chain is protected from basal endocytosis by a hypothetical masking complex that prevents the Tyr-based endocytic motif within IFNAR1 from interacting with components of the adaptin protein complex 2 (AP2). Here we identify a member of the Janus kinase (Jak) family, Tyk2, as a component of such a masking complex. In the absence of ligand or of receptor chain ubiquitination, binding of Janus kinase Tyk2 within the proximity of the Tyr-based linear motif of IFNAR1 is required to prevent IFNAR1 internalization and to maintain its cell surface expression. Furthermore, interaction experiments revealed that Tyk2 physically shields this Tyr-based motif from the recognition by the AP50 subunit of AP2. These data delineate a long-sought ligand- and ubiquitin-independent mechanism by which Tyk2 contributes to both the regulation of total IFNAR1 levels as well as the regulation of the cell surface density of this receptor chain.

Interferons (IFNs) play critical roles in host defense by modulating the expression of various genes via tyrosine phosphorylation of STAT transcription factors. IFN-alpha/beta activates another important transcription factor, nuclear factor-kappaB (NF-kappaB), but its role in IFN-mediated activity is poorly understood. Sensitivity to the antiviral and gene-inducing effects of IFN was examined in normal fibroblasts and in NF-kappaB knockout fibroblasts from p50- and p65-null mice. Antiviral assays demonstrated that NF-kappaB knockout fibroblasts were sensitized to the antiviral action of IFN. Moreover, analysis of IFN-stimulated gene expression by real-time PCR demonstrated selective effects of NF-kappaB on gene expression. Our results demonstrate that a subset of IFN-stimulated genes is regulated through an NF-kappaB-dependent pathway and that NF-kappaB may regulate the sensitivity of cells to IFN-mediated antiviral activity.