The Role of IKKepsilon in JAK/STAT Signaling During the Antiviral Immune Response

Antiviral innate immunity is essential for host defense against virus infection. The rapid expression of the type I interferons (IFNα/β) and subsequent expression of type II IFN (IFNγ) are critical for this response. Once produced, IFN binds to cell surface receptors to activate the Janus kinase and signal transducer and activator of transcription (JAK/STAT) pathway. This leads to the expression of a set of interferon- stimulated genes (ISGs), which collectively establish a non-permissive environment for virus replication. The two types of IFNs regulate the expression of overlapping but distinct sets of ISGs, with the type I IFN response critical for antiviral activity and the type II response mediating the linkage of innate and adaptive immunity. TANK-binding kinase 1 (TBK1) and inhibitor of NF-κB kinase-related kinase epsilon (IKK[varepsilon]) have been implicated in the induction of type I IFNs. However, analysis of TBK1 and IKK[varepsilon] knockout mice reveal that only TBK1 is critical for IFNα/β production. We show that although IKK[varepsilon] deficient mice produce normal levels of IFN in response to virus infection, they are more susceptible to infection compared to wildtype cohorts. In addition, we show that IKK[varepsilon] knockout mice are defective in the type I IFN response and have a reduced ability to upregulate a subset of ISGs, resulting in decreased protection against virus replication. Thus, IKK[varepsilon] plays an essential role in antiviral innate immunity at the level of IFN signaling rather than IFN gene expression. Additional experiments reveal that IKK[varepsilon] regulates IFN signaling by phosphorylation of STAT1, with serine 708 as a major target site. Interestingly, STAT1 phosphorylation by IKK[varepsilon] has different effects on type I and type II IFN signaling. IFNα/β and IFNγ activate distinct transcription factor complexes, interferon-stimulated gene factor 3 (ISGF3) and gamma-activated factor (GAF), respectively. ISGF3 is composed of STAT1, STAT2, and interferon regulatory factor 9 (IRF9), while GAF is a STAT1 homodimer. We demonstrate that phosphorylation of STAT1 by IKK[varepsilon] inhibits GAF formation but does not disrupt ISGF3 assembly on DNA. Thus, it appears that the STAT1:STAT2 interface is different from that of the STAT1 homodimer. IKK[varepsilon], therefore, regulates the incorporation of STAT1 into ISGF3 or GAF. We conclude that IKK[varepsilon] fine-tunes the cellular signaling of type I and II IFNs in order to achieve an optimal antiviral response.