SummaryDNA damage can be sensed as a danger-associated molecular pattern by the innate immune system. Here we find that keratinocytes and other human cells mount an innate immune response within hours of etoposide-induced DNA damage, which involves the DNA sensing adaptor STING but is independent of the cytosolic DNA receptor cGAS. This non-canonical activation of STING is mediated by the DNA binding protein IFI16, together with the DNA damage response factors ATM and PARP-1, resulting in the assembly of an alternative STING signaling complex that includes the tumor suppressor p53 and the E3 ubiquitin ligase TRAF6. TRAF6 catalyzes the formation of K63-linked ubiquitin chains on STING, leading to the activation of the transcription factor NF-κB and the induction of an alternative STING-dependent gene expression program. We propose that STING acts as a signaling hub that coordinates a transcriptional response depending on its mode of activation.

The interleukin-receptor-associated kinase (IRAK) family are involved in regulating Toll-like receptor (TLR) and interleukin-1 (IL-1) signalling pathways.TLRs are pattern recognition receptors of the innate immune response that are responsible for sensing pathogens and initiating immunity, while IL-1 is one of the key cytokines that mediates inflammation.

Toll-like receptors (TLRs) are pattern-recognition receptors that recognize microbial ligands and subsequently trigger intracellular signaling pathways involving transcription factors such as NFB and MAPKs such as p38. TLR signaling can regulate both transcriptional and post-transcriptional events leading to altered gene expression and thus appropriate immune responses. The interleukin-1 receptor-associated kinase (IRAK) family comprises four kinases that regulate TLR signaling. However, the role of IRAK-2 has remained unclear, especially in human cells. Recent studies using cells from in-bred Irak2 ؊/؊ mice showed that murine IRAK-2 was not required for early TLR signaling events but had a role in delayed NFB activation and in cytokine production. IRAK-2 in mice has four splice variants, two of which are inhibitory, whereas human IRAK-2 has no splice variants. Thus IRAK-2 in mice and humans may function differently, and therefore we analyzed the role of IRAK-2 in TLR responses in primary human cells. siRNA knockdown of IRAK-2 expression in human peripheral blood mononuclear cells showed a role for human IRAK-2 in both TLR4-and TLR8-mediated early NFB and p38 MAPK activation and in induction of TNF mRNA. These data conflict with findings from the in-bred Irak2 ؊/؊ mice but concur with what has been seen in wild-derived mice for TLR2. Moreover, human IRAK-2 was required for regulating MyD88-dependent TNF␣ mRNA stability via the TNF 3UTR. Collectively, these data demonstrate for the first time an essential role for IRAK-2 in primary human cells for both transcriptional and post-transcriptional TLR responses.The innate immune system detects the presence of pathogens through a variety of pattern-recognition receptors, which recognize conserved pathogen-associated molecular patterns on invading pathogens (1). One important family of such receptors is the Toll-like receptors (TLRs), 2 of which there are 10 members in the human and 13 members in the mouse (2). For example, the pathogen-associated molecular pattern for TLR4 is LPS, TLR9 responds to CpG DNA, whereas TLR7 and TLR8 both recognize viral single strand RNA and synthetic imidazoquinoline-like molecules (3). TLRs are transmembrane proteins, which contain a cytoplasmic TIR (Toll/IL-1 receptor) domain and leucine-rich repeats located extracellularly (2). Detection of a pathogen-associated molecular pattern by a TLR results in receptor dimerization, which allows recruitment of the TIR-domain-containing adaptor proteins: MyD88 is required for all TLR signaling pathways except for TLR3 and MyD88-independent TLR4 pathway (4): MyD88-adaptor-like (Mal) is required for TLR2 and TLR4 signaling to recruit MyD88 to the receptor complex (5); TIR domain-containing adaptor inducing IFN-␤ (TRIF) is required for TLR3 signaling and for a MyD88-independent TLR4 pathway (6), while TRIFrelated adaptor molecule is also required for the MyD88-independent TLR4 pathway (7). The formation of these receptoradaptor complexes results in the activation of various signaling pathways of the i...

Patients carrying very rare loss‐of‐function mutations in interleukin‐1 receptor–associated kinase 4 (IRAK4), a critical signaling mediator in Toll‐like receptor signaling, are severely immunodeficient, highlighting the paramount role of IRAK kinases in innate immunity. We discovered a comparatively frequent coding variant of the enigmatic human IRAK2, L392V (rs3844283), which is found homozygously in ∼15% of Caucasians, to be associated with a reduced ability to induce interferon‐alpha in primary human plasmacytoid dendritic cells in response to hepatitis C virus (HCV). Cytokine production in response to purified Toll‐like receptor agonists was also impaired. Additionally, rs3844283 was epidemiologically associated with a chronic course of HCV infection in two independent HCV cohorts and emerged as an independent predictor of chronic HCV disease. Mechanistically, IRAK2 L392V showed intact binding to, but impaired ubiquitination of, tumor necrosis factor receptor–associated factor 6, a vital step in signal transduction. Conclusion: Our study highlights IRAK2 and its genetic variants as critical factors and potentially novel biomarkers for human antiviral innate immunity. (Hepatology 2015;62:1375–1387)

Background: Activation of p38 MAPK by poxviral A52 is TRAF6-dependent, but the mechanism is unknown. Results: Disruption of either an identified TRAF6-binding motif or of A52 dimerization prevents TRAF6 oligomerization, TAK1 recruitment, and p38 activation. Conclusion: A52 activates p38 MAPK activation by causing TRAF6 oligomerization, leading to TAK1 recruitment. Significance: This work reveals the molecular basis for poxviral activation of p38 MAPK.

Background: Interleukin-1 receptor-associated kinases (IRAKs) play a critical role in TLR signaling and thus innate immunity. Results: A coding IRAK2 variant, rs35060588, affects TLR signaling and colorectal cancer survival. Conclusion: IRAK2 rs35060588 is a functional, disease-relevant variant. Significance: IRAK2 and its variant rs35060588 may serve as a point of therapeutic intervention and predictive biomarker, respectively.

Within innate immune signaling pathways, Myeloid differentiation (MyD) 88 and Interleukin-1 receptor associated kinases (IRAKs) fulfill pivotal roles downstream of multiple Toll-like receptors (TLR). Somatic MyD88 mutations in B cells were reported to affect lymphomagenesis by constitutive hyperactivation of NF-κB. We here show that oncogenic mutants augment spontaneous TIR domain oligomerization into so-called Myddosome signaling complexes driving NF-κB activation. Blocking of MyD88 oligomerization consequently induced the death of MyD88-mutated but not unmutated lymphoma cells. In line with the involvement of the MyD88 axis in lymphomagenesis we detected a genetic association of a coding germline variant of IRAK2, a MyD88 interactor, with increased risk (OR = 1.43, 95% confidence interval 1.06-1.93, p=0.018) to develop lymphoma in a German cohort (n=387). This variant is frequently found in 15-44% of individuals of different ethnic groups. Unexpectedly, this variant was associated with reduced NF-κB activation and cytokine responses to TLR agonists in healthy donors. Mechanistically, the IRAK2 variant retained intact binding to but impaired ubiquitination of TRAF6, a vital step in signal transduction. Thus our findings confirm the involvement of MyD88/IRAK in lymphoma but suggest that MyD88 and IRAK2 may contribute differently to oncogenic signaling. Additionally we highlight IRAK2 genetic variants as a critical factor and potentially novel biomarker for human lymphomagenesis.

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