Significance: The host interferon system is a potent antiviral system, responsible for protecting vertebrate organisms against virus infection. In this paper we demonstrate that the N terminus of the vaccinia virus innate immune evasion protein, E3, is necessary to inhibit the action of interferon. Interferon-treated cells infected with virus encoding an E3 deleted of the N-terminal domain die rapidly. This death is dependent on the host RIPK3 protein, which is a mediator of programmed necrotic death, and on DAI, which is a sensor of virus infection. Since both E3 and DAI contain Z-DNA binding domains, we hypothesize that the cellular DAI virus sensor and the viral antiinterferon E3 protein compete for binding to a vaccinia virus-induced Z-nucleic acid, pathogen-associated molecular pattern.

The magnitude of CD8 T cell responses against viruses is checked by the balance of proliferation and death. Caspase-8 (CASP8) has the potential to influence response characteristics through initiation of apoptosis, suppression of necroptosis, and modulation of cell death-independent signal transduction. Mice deficient in CASP8 and RIPK3 (Casp8^−/−Ripk3^−/−) mount enhanced peak CD8 T cell levels against the natural mouse pathogen murine cytomegalovirus (MCMV) or the human pathogen herpes simplex virus-1 compared with littermate control RIPK3-deficient or WT C57BL/6 mice, suggesting an impact of CASP8 on the magnitude of antiviral CD8 T cell expansion and not on contraction. The higher peak response to MCMV in Casp8^−/−Ripk3^−/− mice resulted from accumulation of greater numbers of terminally differentiated KLRG1^hi effector CD8 T cell subsets. Antiviral Casp8^−/−Ripk3^−/− T cells exhibited enhanced proliferation when splenocytes were transferred into WT recipient mice. Thus, cell-autonomous CASP8 normally restricts CD8 T cell proliferation following T cell receptor activation in response to foreign antigen. Memory inflation is a hallmark quality of the T cell response to cytomegalovirus infection. Surprisingly, MCMV-specific memory inflation was not sustained long-term in Casp8^−/−Ripk3^−/− mice even though these mice retained immunity to secondary challenge. In addition, the accumulation of abnormal B220⁺CD3⁺ T cells in these viable CASP8-deficient mice was reduced by chronic MCMV infection. Combined, these data brings to light the cell death-independent role of CASP8 during CD8 T cell expansion in mice lacking the confounding impact of RIPK3-mediated necroptosis. Significance The antiviral immune response in mammals represents a balance of T cell proliferation and death. The autoactivated protease caspase-8 has long been known to mediate cell-extrinsic apoptosis, block unleashed programmed necrosis, and contribute to cell activation state as well as cell growth. Here, caspase-8 is shown to restrict proliferation of CD8 T cells in response to herpesvirus infection without contributing to the contraction of the immune response. Prior investigations into this area were confounded by unleashed necroptosis, masking the role of caspase-8 in restraining proliferation associated with adaptive immunity. Thus, our observations resolve a decades-old debate regarding the cell-autonomous contribution of this protease to initiation of antiviral T cell immunity and establishment of memory.

Mpox disease, caused by the monkeypox virus (MPXV), was recently classified as a public health emergency of international concern due to its high lethality and pandemic potential. MPXV is a zoonotic disease that emerged and is primarily spread by small rodents. Historically, it was considered mainly zoonotic and not likely to sustain human-to-human transmission. However, the worldwide outbreak of Clade IIb MPXV from 2020 to 2022 and ongoing Clade I MPXV epidemics in the Democratic Republic of the Congo and surrounding areas are a warning that human-adapted MPXVs will continually arise. Understanding the viral genetic determinants of host range, pathogenesis, and immune evasion is imperative for developing control strategies and predicting the future of Mpox. Here, we delve into the MPXV genome to detail genes involved in host immune evasion strategies for this zoonotic rodent-borne and human-circulating virus. We compare MPXV gene content to related Orthopoxviruses, which have narrow host ranges, to identify potential genes involved in species-specific pathogenesis and host tropism. In addition, we cover the key virulence factor differences that distinguish the MPXV clade lineages. Finally, we dissect how genomic reduction of Orthopoxviruses, through various molecular mechanisms, is contributing to the generation of novel MPXV lineages with increased human adaptation. This review aims to highlight gene content that defines the MPXV species, MPXV clades, and novel MPXV lineages that have culminated in this virus being elevated to a public health emergency of national concern.

During aging, environmental stressors and mutations along with reduced DNA repair cause germ cell aneuploidy and genome instability, which limits fertility and embryo development. Benevolent commensal microbiota and dietary plants secrete indoles, which improve healthspan and reproductive success, suggesting regulation of germ cell quality. We show that indoles prevent aneuploidy and promote DNA repair and embryo viability, which depends on age and genotoxic stress levels and affects embryo quality across generations. In young animals or with low doses of radiation, indoles promote DNA repair and embryo viability; however, in older animals or with high doses of radiation, indoles promote death of the embryo. These studies reveal a previously unknown quality control mechanism by which indole integrates DNA repair and cell death responses to preclude germ cell aneuploidy and ensure transgenerational genome integrity. Such regulation affects healthy aging, reproductive senescence, cancer, and the evolution of genetic diversity in invertebrates and vertebrates.

Z‐nucleic acid binding protein (ZBP)1 (also known as DAI and DLM1) is a pathogen sensor activated by double‐strand character RNA to recruit receptor‐interacting protein (RIP) kinase via a RIP homotypic interaction motif. The activation of receptor‐interacting protein kinase (RIPK)3 and initiation of virus‐induced necroptosis were initially reported in a landmark publication Upton et al. (Cell Host Microbe 11: 290, 2012) employing the DNA virus murine cytomegalovirus (MCMV). M45‐encoded viral inhibitor of RIP activation prevents virus‐induced necroptosis. Additional virus‐encoded suppressors of necroptosis were then identified, including herpes simplex virus ICP6 and vaccinia virus E3L. Caspase‐8 suppressors encoded by these DNA viruses block apoptosis, unleashing necroptosis mediated through Z‐nucleic acid binding protein 1 (ZBP1) recruitment of RIPK3. These studies all utilized ZBP1‐deficient mice generated by the Akira Lab (Zbp1^{−/− AK}) to bring the significance of virus‐induced necroptosis to light. C57BL/6 mice were chosen as controls based on the assumption that mutant mice were congenic; however, these mice were recently found to display an unexpected innate immune deficit, lacking C57BL/6‐specific NK1.1 and Ly49H natural killer cell subpopulations important in the early control of MCMV infection. Short nucleotide polymorphism analysis of Zbp1^{−/− AK} breeders revealed a mixed genetic background (~ 71% C57BL/6 DNA and ~ 29% 129). Even though this level of 129 strain background does not alter ZBP1 cell‐autonomous function as a sensor and mediator of necroptosis, it confounds innate immune response characteristics. In the future, genetic background must be carefully controlled before implicating ZBP1 function in response characteristics that shape immunity, inflammation, metabolism, and pathogenesis.

Necroptosis, a potent host defense mechanism, limits viral replication and pathogenesis through three distinct initiation pathways. Toll‐like receptor 3 (TLR3) via TIR‐domain‐containing adapter‐inducing interferon‐β (TRIF), Z‐DNA‐binding protein 1 (ZBP1) and tumor necrosis factor (TNF)α mediate necroptosis, with ZBP1 and TNF playing pivotal roles in controlling viral infections, with the role of TLR3–TRIF being less clear. ZBP1‐mediated necroptosis is initiated when host ZBP1 senses viral Z‐form double stranded RNA and recruits receptor‐interacting serine/threonine‐protein kinase 3 (RIPK3), driving a mixed lineage kinase domain‐like pseudokinase (MLKL)‐dependent necroptosis pathway, whereas TNF‐mediated necroptosis is initiated by TNF signaling, which drives a RIPK1–RIPK3–MLKL pathway, resulting in necroptosis. Certain viruses (cytomegalovirus, herpes simplex virus and vaccinia) have evolved to produce proteins that compete with host defense systems, preventing programmed cell death pathways from being initiated. Two engineered viruses deficient of active forms of these proteins, murine cytomegalovirus M45mutRHIM and vaccinia virus E3∆Zα, trigger ZBP1‐dependent necroptosis in mouse embryonic fibroblasts. By contrast, when bone‐marrow‐derived macrophages are infected with the viruses, necroptosis is initiated predominantly through the TNF‐mediated pathway. However, when the TNF pathway is blocked by RIPK1 inhibitors or a TNF blockade, ZBP1‐mediated necroptosis becomes the prominent pathway in bone‐marrow‐derived macrophages. Overall, these data implicate a cell‐type preference for either TNF‐mediated or ZBP1‐mediated necroptosis pathways in host responses to viral infections. These preferences are important to consider when evaluating disease models that incorporate necroptosis because they may contribute to tissue‐specific reactions that could alter the balance of inflammation versus control of virus, impacting the organism as a whole.