Entrapment of Viral Capsids in Nuclear PML Cages Is an Intrinsic Antiviral Host Defense against Varicella-Zoster Virus

Abstract

The herpesviruses, like most other DNA viruses, replicate in the host cell nucleus. Subnuclear domains known as promyelocytic leukemia protein nuclear bodies (PML-NBs), or ND10 bodies, have been implicated in restricting early herpesviral gene expression. These viruses have evolved countermeasures to disperse PML-NBs, as shown in cells infected in vitro, but information about the fate of PML-NBs and their functions in herpesvirus infected cells in vivo is limited. Varicella-zoster virus (VZV) is an alphaherpesvirus with tropism for skin, lymphocytes and sensory ganglia, where it establishes latency. Here, we identify large PML-NBs that sequester newly assembled nucleocapsids (NC) in neurons and satellite cells of human dorsal root ganglia (DRG) and skin cells infected with VZV in vivo. Quantitative immuno-electron microscopy revealed that these distinctive nuclear bodies consisted of PML fibers forming spherical cages that enclosed mature and immature VZV NCs. Of six PML isoforms, only PML IV promoted the sequestration of NCs. PML IV significantly inhibited viral infection and interacted with the ORF23 capsid surface protein, which was identified as a target for PML-mediated NC sequestration. The unique PML IV C-terminal domain was required for both capsid entrapment and antiviral activity. Similar large PML-NBs, termed clastosomes, sequester aberrant polyglutamine (polyQ) proteins, such as Huntingtin (Htt), in several neurodegenerative disorders. We found that PML IV cages co-sequester HttQ72 and ORF23 protein in VZV infected cells. Our data show that PML cages contribute to the intrinsic antiviral defense by sensing and entrapping VZV nucleocapsids, thereby preventing their nuclear egress and inhibiting formation of infectious virus particles. The efficient sequestration of virion capsids in PML cages appears to be the outcome of a basic cytoprotective function of this distinctive category of PML-NBs in sensing and safely containing nuclear aggregates of aberrant proteins.

Author Summary

Many DNA viruses, including varicella-zoster virus (VZV), a herpesvirus that causes varicella (chickenpox) and zoster (shingles), replicate in the host cell nucleus. Here, we have identified an intrinsic antiviral mechanism that specifically targets newly assembled VZV capsids and contains these essential viral structures in a nuclear “safe house”. Using immuno-electron microscopy, PML (promyelocytic leukemia) protein fibers that formed filamentous spherical cages were shown to trap virion capsids very efficiently, preventing their transport out of the nucleus and inhibiting the formation of infectious virus particles. PML cages containing virion capsids were found in VZV-infected neurons and satellite cells in human sensory ganglia and in skin cells, which are major targets during VZV pathogenesis. Similar PML nuclear bodies that sequester abnormal proteins have been reported in neurodegenerative disorders, like Huntington's disease. We found that cages formed by PML isoform IV sequestered both the virion capsids of VZV, which is a neurotropic herpesvirus, and the mutant Huntington's disease protein. This work provides the first evidence that PML, which is abundant in mammalian cell nuclei, can function both to contain potentially damaging cellular protein aggregates and as an intrinsic host defense against a herpesvirus during nuclear virion assembly.