Monkeypox virus infection of human astrocytes causes gasdermin B cleavage and pyroptosis.

Monkeypox virus (MPXV) infections in humans cause neurological disorders while studies of MPXV-infected animals indicate that the virus penetrates the brain. Pyroptosis is an inflammatory type of regulated cell death, resulting from plasma membrane rupture (PMR) due to oligomerization of cleaved gasdermins to cause membrane pore formation. Herein, we investigated the human neural cell tropism of MPXV compared to another orthopoxvirus, vaccinia virus (VACV), as well as its effects on immune responses and cell death. Astrocytes were most permissive to MPXV (and VACV) infections, followed by microglia and oligodendrocytes, with minimal infection of neurons based on plaque assays. Aberrant morphological changes were evident in MPXV-infected astrocytes that were accompanied with viral protein (I3) immunolabelling and detection of over 125 MPXV-encoded proteins in cell lysates by mass spectrometry. MPXV- and VACV-infected astrocytes showed increased expression of immune gene transcripts (IL12, IRF3, IL1B, TNFA, CASP1, and GSDMB). However, MPXV infection of astrocytes specifically induced proteolytic cleavage of gasdermin B (GSDMB) (50 kDa), evident by the appearance of cleaved N-terminal-GSDMB (30 kDa) and C-terminal- GSDMB (18 kDa) fragments. GSDMB cleavage was associated with release of lactate dehydrogenase and increased cellular nucleic acid staining, indicative of PMR. Pre-treatment with dimethyl fumarate reduced cleavage of GSDMB and associated PMR in MPXV-infected astrocytes. Human astrocytes support productive MPXV infection, resulting in inflammatory gene induction with accompanying GSDMB-mediated pyroptosis. These findings clarify the recently recognized neuropathogenic effects of MPXV in humans while also offering potential therapeutic options.

Cytoplasmic factories, virus assembly, and DNA replication kinetics collectively constrain the formation of poxvirus recombinants.

Poxviruses replicate in cytoplasmic structures called factories and each factory begins as a single infecting particle. Sixty-years ago Cairns predicted that this might have effects on vaccinia virus (VACV) recombination because the factories would have to collide and mix their contents to permit recombination. We've since shown that factories collide irregularly and that even then the viroplasm mixes poorly. We've also observed that while intragenic recombination occurs frequently early in infection, intergenic recombination is less efficient and happens late in infection. Something inhibits factory fusion and viroplasm mixing but what is unclear. To study this, we've used optical and electron microscopy to track factory movement in co-infected cells and correlate these observations with virus development and recombinant formation. While the technical complexity of the experiments limited the number of cells that are amenable to extensive statistical analysis, these studies do show that intergenic recombination coincides with virion assembly and when VACV replication has declined to ≤10% of earlier levels. Along the boundaries between colliding factories, one sees ER membrane remnants and other cell constituents like mitochondria. These collisions don't always cause factory fusion, but when factories do fuse, they still entrain cell constituents like mitochondria and ER-wrapped microtubules. However, these materials wouldn't seem to pose much of a further barrier to DNA mixing and so it's likely that the viroplasm also presents an omnipresent impediment to DNA mixing. Late packaging reactions might help to disrupt the viroplasm, but packaging would sequester the DNA just as the replication and recombination machinery goes into decline and further reduce recombinant yields. Many factors thus appear to conspire to limit recombination between co-infecting poxviruses.

Vaccinia virus particles mix inefficiently, and in a way that would restrict viral recombination, in coinfected cells.

It is well established that poxviruses are subjected to genetic recombination, but attempts to map vaccinia virus genes using classical genetic crosses were historically confounded by high levels of experimental noise and a poor correlation between physical and genetic map distances. These virus-by-virus crosses also never produced the 50% recombinant progeny that should be seen in experiments involving distant markers. Poxviruses replicate in membrane-wrapped cytoplasmic structures called virosomes (or factories) and we have developed a method for tracking the development of these structures using live cell imaging and cells expressing phage lambda Cro protein fused to enhanced green fluorescent protein (EGFP). The EGFP-cro protein binds nonspecifically to DNA and permits live cell imaging of developing vaccinia virus factories. Using this method, we see virosomes first appearing about 4 to 5 h postinfection. The early virosomes exhibit a compact appearance and then, after a period of exponential growth lasting several hours, blur and start to dissipate in a process presumably linked to viral packaging. During the growth period, the virosomes migrate toward the nuclear periphery while colliding and fusing at a rate dependent upon the numbers of infecting particles. However, even at high multiplicities of infection (10 PFU/cell), we estimate approximately 20% of the virosomes never fuse. We have also used fluorescence in situ hybridization (FISH) methods to study virosomes formed by the fusion of viruses carrying different gene markers. FISH showed that DNA mixes rather poorly within fused virosomes and the amount of mixing is inversely dependent on the time between virosome appearance and fusion. Our studies suggest that the intracellular movement and mixing of virosomes create constraints that reduce opportunities for forming recombinants and that these phenomena create outcomes reflected in classical poxvirus genetics.

Production and characterization of antibodies against vaccinia virus DNA polymerase.

Poxviruses are large DNA viruses that replicate in discrete locations in the cytoplasm of infected cells called viral factories. Because the host cell DNA replication machinery is located in the nucleus, poxviruses encode many of the proteins required for their own DNA replication, including a DNA polymerase. Although many if not all of the enzymes that are required for viral DNA replication have been identified, the actual mechanism of poxvirus DNA replication remains unclear. Two monoclonal antibodies and a polyclonal antibody against vaccinia virus DNA polymerase were produced and characterized for use as tools to investigate the mechanism of virus DNA replication. Although the monoclonal antibodies were not suitable for Western blotting, the polyclonal antibody was able to detect the protein in infected cell lysates using this method. In contrast, while the polyclonal antibody did not recognize the DNA polymerase when used for immunofluorescence microscopy, the monoclonal antibodies were able to detect the polymerase in vaccinia viral factories. In addition, one of these antibodies also stained viral factories produced by cowpox and ectromelia, two closely related viruses. Finally, all three antibodies were able to immunoprecipitate vaccinia DNA polymerase from infected cell lysates. These antibodies will be useful in experiments designed to describe more fully the role of the viral DNA polymerase in DNA replication of vaccinia virus.

Vaccinia virus DNA ligase recruits cellular topoisomerase II to sites of viral replication and assembly.

Vaccinia virus replication is inhibited by etoposide and mitoxantrone even though poxviruses do not encode the type II topoisomerases that are the specific targets of these drugs. Furthermore, one can isolate drug-resistant virus carrying mutations in the viral DNA ligase and yet the ligase is not known to exhibit sensitivity to these drugs. A yeast two-hybrid screen was used to search for proteins binding to vaccinia ligase, and one of the nine proteins identified comprised a portion (residue 901 to end) of human topoisomerase IIbeta. One can prevent the interaction by introducing a C(11)-to-Y substitution mutation into the N terminus of the ligase bait protein, which is one of the mutations conferring etoposide and mitoxantrone resistance. Coimmunoprecipitation methods showed that the native ligase and a Flag-tagged recombinant protein form complexes with human topoisomerase IIalpha/beta in infected cells and that this interaction can also be disrupted by mutations in the A50R (ligase) gene. Immunofluorescence microscopy showed that both topoisomerase IIalpha and IIbeta antigens are recruited to cytoplasmic sites of virus replication and that less topoisomerase was recruited to these sites in cells infected with mutant virus than in cells infected with wild-type virus. Immunoelectron microscopy confirmed the presence of topoisomerases IIalpha/beta in virosomes, but the enzyme could not be detected in mature virus particles. We propose that the genetics of etoposide and mitoxantrone resistance can be explained by vaccinia ligase binding to cellular topoisomerase II and recruiting this nuclear enzyme to sites of virus biogenesis. Although other nuclear DNA binding proteins have been detected in virosomes, this appears to be the first demonstration of an enzyme being selectively recruited to sites of poxvirus DNA synthesis and assembly.