Proteasomal Degradation of Herpes Simplex Virus Capsids in Macrophages Releases DNA to the Cytosol for Recognition by DNA Sensors

Abstract

The innate immune system is important for control of infections, including herpesvirus infections. Intracellular DNA potently stimulates antiviral IFN responses. It is known that plasmacytoid dendritic cells sense herpesvirus DNA in endosomes via TLR9, and that non-immune tissue cells can sense herpesvirus DNA in the nucleus. However, it remains unknown how and where myeloid cells, like macrophages and conventional dendritic cells, detect infections with herpesviruses. Here we demonstrate that the HSV-1 capsid was ubiquitinated in the cytosol and degraded by the proteasome, hence releasing genomic DNA into the cytoplasm for detection by DNA sensors. In this context, the DNA sensor IFI16 is important for induction of IFN-β in human macrophages after infection with HSV-1 and CMV. Viral DNA localized to the same cytoplasmic regions as IFI16, with DNA sensing being independent of viral nuclear entry. Thus, proteasomal degradation of herpesvirus capsids releases DNA to the cytoplasm for recognition by DNA sensors.

The emerging role of human PYHIN proteins in innate immunity: Implications for health and disease

“…In contrast to AIM2, DNA recognition by IFI16 can occur in both the cytosolic and nuclear compartments. For example DNA which has been transfected, lentiviral DNA that has accumulated or herpesvirus DNA which has been exposed in the cytosol can be detected by cytosolic IFI16 [2,46,48] while DNA from nuclear viruses such as HCMV is detected in the nucleus [10,49]. As mentioned earlier, the NLS of IFI16 is required for the nuclear localisation of IFI16.…”

“…Alternatively, in cells such as macrophages, which are less permissive to viral infection, proteasomal degradation of viral capsids allows the release of viral DNA into the cytosol prior to nuclear delivery of the genome. This exposure of viral DNA activates cytosolic IFI16, which also leads to STING/TBK1 activation and IFN-b expression [48]. While STING and TBK1 are widely accepted to be activated following IFI16 stimulation, the details of the molecular events involved in their activation have yet to be unravelled.…”

Innate Nuclear Sensor IFI16 Translocates into the Cytoplasm during the Early Stage of In Vitro Human Cytomegalovirus Infection and Is Entrapped in the Egressing Virions during the Late Stage

“…Recent studies explored these issues in other herpesvirus models, such as HSV-1 (12,13,15,17,51), KHSV (14,16), and EBV (10). Orzalli et al (15,51) found that during HSV-1 infection IFI16, which is required for induction of IRF-3 signaling in these cells, remains nuclear.…”

“…IFI16 displays multifaceted activity due to its ability to bind to various target proteins (i.e., transcription factors, signaling proteins, and tumor suppressor proteins) and to modulate various cell functions (9). In addition, IFI16 has been shown to bind to and function as a pattern recognition receptor (PRR) of virus-derived intracellular DNA and trigger the expression of antiviral cytokines via the STING/TBK1/IRF3 signaling pathway (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20). Although many different functions have been ascribed to IFI16 (and to other proteins of the PYHIN family), its role as an antiviral restriction factor has not yet been fully described.…”

Activation and Regulation of DNA-Driven Immune Responses

“…The focus here is on IFI16 in the STING pathway. IFI16 was demonstrated to associate with microbial DNA during different viral and bacterial infections and to interact with STING (25,31,64,65). In human THP1 cells differentiated with phorbol myristate acetate into a macrophage-like phenotype, it was demonstrated that IFI16, cGAS, and STING were all required for the stimulation of type I IFN expression by either human immunodeficiency virus type 1 (HIV-1) or Listeria monocytogenes (31,65).…”