HSV-2 genome recognition by nuclear cGAS instigates IFN-β production and influences inflammasome activation during de novo infection in HFF cells.

INTRODUCTION: Type I interferon response, specifically, the cGAS-cGAMP-STING axis that results in IFN-β response, is well known for its complex roles early during viral infection. Previous reports suggest that HSV-1 DNA in Thp-1 cells and HIV-2 dsDNA in DCs and macrophages could be sensed by cGAS. The nuclear DNA sensor IFI16's viral DNA sensing leads to its acetylation, cytoplasmic translocation and STING activation and inflammasome activation. Although cGAS is known to be associated with IFI16 in the nucleus, however, during HSV-2 infection, the role of nuclear cGAS in viral DNA sensing, inflammasome formation and type I IFN response remains unknown.

METHODS: In the current study, extensive investigation of the complex IFN-β responses elicited early during de novo HSV-2 infections in HFF cells is undertaken. The SiIFI16 and SicGAS treated HFF cells infected with HSV-2 demonstrate that cGAS senses nuclear herpes-viral DNA in an IFI16 dependent manner leading to nuclear cGAMP production.

RESULTS: These results unravel a novel nuclear cooperative role of cGAS and IFI16 and extend the cGAS DNA sensing and its enzymatic activity in the nucleus. IFI16 acetylation required for inflammasome complex formation is cGAS independent. The cGAS-pro-Caspase1 and cGAS-ASC interaction suggests plausible role of cGAS in inflammasome complex for Caspase-1 activation. The activated Caspase-1 interaction with cGAS was also observed. Further, the autophagy and DNA damage responses elicited during de novo HSV-2 infection are suggested.

DISCUSSION: The crosstalk of the type I interferon pathway with the inflammasome, autophagy and DNA damage response pathways suggests an intricate mechanism of inter-regulation at different stages and time points during infection, that might orchestrate a balanced and efficient immune response or facilitate viral immune evasion. Unique and dynamic post translational modifications of cGAS, namely acetylation and K-63 poly-ubiquitination, are observed, and are plausibly involved in cGAS regulation during HSV-2 infection.