Non-canonical Activation of the DNA Sensing Adaptor STING by ATM and IFI16 Mediates NF-κB Signaling after Nuclear DNA Damage.

DNA 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.

cAIMP administration in humanized mice induces a chimerization-level-dependent STING response.

It is well understood that the STING signalling pathway is critical for generating a robust innate immune response to pathogens. Human and mouse STING signalling pathways are not identical, however. For example, mice lack IFI16, which has been proven important for the human STING pathway. Therefore, we investigated whether humanized mice are an appropriate experimental platform for exploring the human STING signalling cascade in vivo. We found that NOG mice reconstituted with human cord blood haematopoietic stem cells (humanized NOG mice) exhibit human STING signalling responses to an analogue of the cyclic di-nucleotide cGAMP. There was an increase in the proportions of monocytes in the lungs of mice receiving cGAMP analogue. The most robust levels of STING expression and STING-induced responses were observed in mice exhibiting the highest levels of human chimerization. Notably, differential levels of STING in lung versus spleen following cGAMP analogue treatment suggest that there are tissue-specific kinetics of STING activation and/or degradation in effector versus inductive sites. We also examined the mouse innate immune response to cGAMP analogue treatment. We detected that mouse cells in the immunodeficient NOG mice responded to the cGAMP analogue and they do so with distinct kinetics from the human response. In conclusion, humanized NOG mice represent a valuable experimental model for examining in vivo human STING responses.

cGAS is activated by DNA in a length-dependent manner.

Cytosolic DNA stimulates innate immune responses, including type I interferons (IFN), which have antiviral and immunomodulatory activities. Cyclic GMP-AMP synthase (cGAS) recognizes cytoplasmic DNA and signals via STING to induce IFN production. Despite the importance of DNA in innate immunity, the nature of the DNA that stimulates IFN production is not well described. Using low DNA concentrations, we show that dsDNA induces IFN in a length-dependent manner. This is observed over a wide length-span of DNA, ranging from the minimal stimulatory length to several kilobases, and is fully dependent on cGAS irrespective of DNA length. Importantly, in vitro studies reveal that long DNA activates recombinant human cGAS more efficiently than short DNA, showing that length-dependent DNA recognition is an intrinsic property of cGAS independent of accessory proteins. Collectively, this work identifies long DNA as the molecular entity stimulating the cGAS pathway upon cytosolic DNA challenge such as viral infections.

Histone deacetylase inhibitor romidepsin inhibits de novo HIV-1 infections.

Adjunct therapy with the histone deacetylase inhibitor (HDACi) romidepsin increases plasma viremia in HIV patients on combination antiretroviral therapy (cART). However, a potential concern is that reversing HIV latency with an HDACi may reactivate the virus in anatomical compartments with suboptimal cART concentrations, leading to de novo infection of susceptible cells in these sites. We tested physiologically relevant romidepsin concentrations known to reactivate latent HIV in order to definitively address this concern. We found that romidepsin significantly inhibited HIV infection in peripheral blood mononuclear cells and CD4(+) T cells but not in monocyte-derived macrophages. In addition, romidepsin impaired HIV spreading in CD4(+) T cell cultures. When we evaluated the impact of romidepsin on quantitative viral outgrowth assays with primary resting CD4(+) T cells, we found that resting CD4(+) T cells exposed to romidepsin exhibited reduced proliferation and viability. This significantly lowered assay sensitivity when measuring the efficacy of romidepsin as an HIV latency reversal agent. Altogether, our data indicate that romidepsin-based HIV eradication strategies are unlikely to reseed a latent T cell reservoir, even under suboptimal cART conditions, because romidepsin profoundly restricts de novo HIV infections.

T-cell derived extracellular vesicles prime macrophages for improved STING based cancer immunotherapy.

A key phenomenon in cancer is the establishment of a highly immunosuppressive tumour microenvironment (TME). Despite advances in immunotherapy, where the purpose is to induce tumour recognition and hence hereof tumour eradication, the majority of patients applicable for such treatment still fail to respond. It has been suggested that high immunological activity in the tumour is essential for achieving effective response to immunotherapy, which therefore have led to exploration of strategies that triggers inflammatory pathways. Here activation of the stimulator of interferon genes (STING) signalling pathway has been considered an attractive target, as it is a potent trigger of pro-inflammatory cytokines and types I and III interferons. However, immunotherapy combined with targeted STING agonists has not yielded sustained clinical remission in humans. This suggests a need for exploring novel adjuvants to improve the innate immunological efficacy. Here, we demonstrate that extracellular vesicles (EVs), derived from activated CD4+ T cells (T-EVs), sensitizes macrophages to elevate STING activation, mediated by IFNγ carried on the T-EVs. Our work support that T-EVs can disrupt the immune suppressive environment in the tumour by reprogramming macrophages to a pro-inflammatory phenotype, and priming them for a robust immune response towards STING activation.

Cellular Requirements for Sensing and Elimination of Incoming HSV-1 DNA and Capsids.

Incoming viruses challenge the cell with diverse foreign molecules, which need to be sensed quickly to initiate immune responses and to remove the viral components. In this study, we investigate the cellular requirements for sensing and degradation of incoming viral DNA and capsids during herpes simplex virus type 1 (HSV-1) infections. Using click chemistry labeling of the viral genome, we found that HSV-1 DNA was released from a subset of capsids into the cytosol early in infection. By next-generation sequencing of cyclic GMP-AMP (cGAMP) synthase (cGAS)-bound DNA from HSV-1-infected cells, we show that HSV-1 DNA was bound by the cytosolic DNA sensor cGAS. Activation of cGAS enzymatic activity by viral DNA did not require proteasomal activity, indicating that viral DNA release into the cytosol is not proteasome-dependent. However, induction of interferon (IFN)-ß expression was blocked by inhibition of the proteasome, suggesting a contribution of the proteasome to IFN-ß induction through the cGAS-stimulator of interferon genes pathway. Viral DNA was cleared from the cytosol within few hours, in a manner dependent on TREX1 and a cGAS-dependent process. Capsid material in the cytoplasm was also degraded rapidly. This was partially blocked by treatment with a proteasome inhibitor. This treatment led to accumulation of DNA-containing viral capsids near the nucleus and reduced nuclear entry of viral DNA. Thus, cells infected with HSV-1 use a panel of mechanisms to eliminate viral DNA and capsids. This represents a barrier for establishment of infection and potentially enables the host to gear the IFN-ß response to a level required for antiviral defense without causing immunopathology.

IFI16 Targets the Transcription Factor Sp1 to Suppress HIV-1 Transcription and Latency Reactivation.

The interferon γ-inducible protein 16 (IFI16) is known as immune sensor of retroviral DNA intermediates. We show that IFI16 restricts HIV-1 independently of immune sensing by binding and inhibiting the host transcription factor Sp1 that drives viral gene expression. This antiretroviral activity and ability to bind Sp1 require the N-terminal pyrin domain and nuclear localization of IFI16, but not the HIN domains involved in DNA binding. Highly prevalent clade C HIV-1 strains are more resistant to IFI16 and less dependent on Sp1 than other HIV-1 subtypes. Furthermore, inhibition of Sp1 by IFI16 or pharmacologically by Mithramycin A suppresses reactivation of latent HIV-1 in CD4+ T cells. Finally, IFI16 also inhibits retrotransposition of LINE-1, known to engage Sp1, and murine IFI16 homologs restrict Friend retrovirus replication in mice. Thus, IFI16 restricts retroviruses and retrotransposons by interfering with Sp1-dependent gene expression, and evasion from this restriction may facilitate spread of HIV-1 subtype C.