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Highlights: [•] Evolution of host cell death pathways was driven by virus-encoded cell death suppressors. [•] Caspase 8 regulates extrinsic apoptosis as well as programmed necrosis. [•] RIP3 (also called RIPK3) is the key mediator of programmed necrosis. [•] Programmed necrosis involves RIP3 RHIM-dependent interactions with RIP1, DAI or TRIF. [•] Programmed necrosis is suppressed by RHIM inhibitor vIRA encoded by MCMV. [Copyright &y& Elsevier]

Maturation in herpesviruses initiates in the nucleus of the infected cell, with encapsidation of viral DNA to form nucleocapsids, and concludes with envelopment in the cytoplasm to form infectious virions that egress the cell. The entire process of virus maturation is orchestrated by protein–protein interactions and enzymatic activities of viral and host origin. Viral tegument proteins play important roles in maintaining the structural stability of capsids and directing the acquisition of virus envelope. Envelopment occurs at modified host membranes and exploits host vesicular trafficking. In this review, we summarize current knowledge of and concepts in human cytomegalovirus (HCMV) maturation and their parallels in other herpesviruses, with an emphasis on viral and host factors that regulate this process. [ABSTRACT FROM AUTHOR]

Receptor-interacting protein (RIP) kinase 3 (RIPK3)-dependent necroptosis directs inflammation and tissue injury, as well as anti-viral host defense. In human cells, herpes simplex virus 1 (HSV1) UL39-encoded ICP6 blocks RIP homotypic interacting motif (RHIM) signal transduction, preventing this leakage form of cell death and sustaining viral infection. TNF receptor 1 (TNFR1)-induced necroptosis is known to require the formation of a RIPK1-RIPK3-mixed lineage kinase domain-like pseudokinase (MLKL) signaling complex (necrosome) that we find compartmentalizes exclusively to caveolin-1-associated detergent-resistant membrane (DRM) vesicles in HT-29 cells. Translocation proceeds in the presence of RIPK3 kinase inhibitorGSK'840 orMLKLinhibitor necrosulfonomide but requires the kinase activity, as well as RHIM signaling of RIPK1. ICP6 impedes the translocation of RIPK1, RIPK3, and MLKL to caveolin-1-containing DRM vesicles without fully blocking the activation of RIPK3 or phosphorylation of MLKL. Consistent with the important contribution of RIPK1 RHIM-dependent recruitment of RIPK3, overexpression of RHIM-deficient RIPK3 results in phosphorylation of MLKL, but this does not lead to either translocation or necroptosis. Combined, these data reveal a critical role of RHIM signaling in the recruitment of the MLKL-containing necrosome to membrane vesicle-associated sites of aggregation. A similar mechanism is predicted for other RHIM-containing signaling adaptors, Z-nucleic acid-binding protein 1 (ZBP1) (also called DAI and DLM1), and TIR domain-containing adapter-inducing interferon-β (TRIF). [ABSTRACT FROM AUTHOR]

Abstract: Epstein-Barr virus (EBV) is the major cause of infectious mononucleosis and is associated with several malignancies including nasopharyngeal carcinoma, gastric carcinoma, Hodgkin lymphoma, Burkitt lymphoma, and lymphoma after organ or stem cell transplant. A candidate vaccine containing soluble EBV glycoprotein gp350 protected cottontop tamarins from EBV lymphoma after challenge with EBV. In the only phase 2 trial of an EBV vaccine in humans, soluble gp350 in alum and monophosphoryl lipid A adjuvant reduced the rate of infectious mononucleosis in EBV seronegative adults, but did not affect the rate of EBV infection. A peptide vaccine corresponding to EBV latency proteins has been tested in a small number of adults to prevent infectious mononucleosis. Some of the barriers to development of an EBV vaccine include (a) whether viral proteins in addition to gp350 would be more effective for preventing mononucleosis or EBV malignancies, (b) the difficulty of performing clinical trials to prevent EBV associated malignancies in the absence of good surrogate markers for tumor development, and the long period of time between primary EBV infection and development of many EBV tumors, (c) the lack of knowledge of immune correlates for protection against EBV infection and disease, (d) the limitations in animal models to study protection against EBV infection and disease, and (e) the need for additional information on the economic and societal burden of infectious mononucleosis to assess the cost-benefit of a prophylactic vaccine. [Copyright &y& Elsevier]

Herpesviruses such as cytomegaloviruses encode functions that modulate the innate response in diverse ways to counteract host sensing and delay host clearance during infection. The murine cytomegalovirus M45 protein interacts with receptor-interacting protein (RIP) 1 and RIP3 via a RIP homotypic interaction motif. Cell death suppression by M45 requires RIP homotypic interaction motif-dependent interaction with RIP1. This interaction also underlies the cell tropism role of M45 in preventing premature death of endothelial cells during murine cytomegalovirus infection. Thus, M45 is a viral inhibitor of RIP activation that provides a direct cell type-dependent replication benefit to the virus while modulating other biological processes signaling via the RIP1 adaptor such as activation of Toll-like receptor (TLR)3 as well as other mediators of cell death. [ABSTRACT FROM AUTHOR]

We recently described the induction of noncanonical IL-1β processing via caspase-8 recruited to ripoptosome signaling platforms in myeloid leukocytes. Here, we demonstrate that activated NLRP3βASC inflammasomes recruit caspase-8 to drive IL-1β processing in murine bone marrow-derived dendritic cells (BMDC) independent of caspase-1 and -11. Sustained stimulation (>2 h) of LPS-primed caspase-1-deficient (Casp1/11-/-) BMDC with the canonical NLRP3 inflammasome agonist nigericin results in release of bioactive IL-1β in conjunction with robust caspase-8 activation. This IL-1β processing and caspase-8 activation do not proceed in Nlrp3-/- or Asc-/- BMDC and are suppressed by pharmacological inhibition of caspase-8, indicating that caspase-8 can act as a direct IL-1β-converting enzyme during NLRP3 inflammasome activation. In contrast to the rapid caspase-1-mediated death of wild type (WT) BMDC via NLRP3-dependent pyroptosis, nigericinstimulated Casp1/11-/- BMDC exhibit markedly delayed cell death via NLRP3-dependent apoptosis. Biochemical analyses of WTand Casp1/11-/-BMDCindicated that caspase-8 is proteolytically processed within detergent-insoluble ASC-enriched protein complexes prior to extracellular export during nigericin treatment. Although nigericin-stimulated caspase-1 activation and activity are only modestly attenuated in caspase-8-deficient (Casp8-/-Rip3-/-) BMDC, these cells do not exhibit the rapid loss of viability of WT cells. These results support a contribution [ABSTRACT FROM AUTHOR]

Necroptosis is an alternate programmed cell death pathway that is unleashed by caspase-8 compromise and mediated by receptor-interacting protein kinase 3 (RIP3). Murine cytomegalovirus (CMV) and herpes simplex virus (HSV) encode caspase-8 inhibitors that prevent apoptosis together with competitors of RIP homotypic interaction motif (RHIM)-dependent signal transduction to interrupt the necroptosis. Here, we show that pro-necrotic murine CMV M45 mutant virus drives virus-induced necroptosis during nonproductive infection of RIP3-expressing human fibroblasts, whereas WT virus does not. Thus, M45-encoded RHIM competitor, viral inhibitor of RIP activation, sustains viability of human cells like it is known to function in infected mouse cells. Importantly, human CMV is shown to block necroptosis induced by either TNF or M45 mutant murine CMV in RIP3-expressing human cells. Human CMV blocks TNF-induced necroptosis after RIP3 activation and phosphorylation of the mixed lineage kinase domain-like (MLKL) pseudokinase. An early, IE1-regulated viral gene product acts on a necroptosis step that follows MLKL phosphorylation prior to membrane leakage. This suppression strategy is distinct from RHIM signaling competition by murine CMV or HSV and interrupts an execution process that has not yet been fully elaborated. [ABSTRACT FROM AUTHOR]

Highlights: [•] A January 2012 meeting addressed priorities for development of vaccines to prevent disease caused by cytomegalovirus (CMV). [•] CMV causes serious disease, including congenital CMV and life-threatening infections in immunocompromised patients. [•] Discussions identified appropriate target populations for vaccine use and appropriate endpoints for vaccine studies. [•] Participants at the meeting also made specific suggestions for further research that could facilitate CMV vaccine development. [ABSTRACT FROM AUTHOR]

Toll-like receptor (TLR) signaling is triggered by pathogen-associated molecular patterns that mediate well established cytokine-driven pathways, activating NF-κB together with IRF3/IRF7. In addition, TLR3 drives caspase 8-regulated programmed cell death pathways reminiscent of TNF family death receptor signaling. We find that inhibition or elimination of caspase 8 during stimulation of TLR2, TLR3, TLR4, TLR5, or TLR9 results in receptor interacting protein (RIP) 3 kinase-dependent programmed necrosis that occurs through either TIR domain-containing adapter-inducing interferon-β (TRIF) or MyD88 signal transduction. TLR3 or TLR4 directly activates programmed necrosis through a RIP homotypic interaction motif-dependent association of TRIF with RIP3 kinase (also called RIPK3). In fibroblasts, this pathway proceeds independent of RIP1 or its kinase activity, but it remains dependent on mixed lineage kinase domain-like protein (MLKL) downstream of RIP3 kinase. Here, we describe two small molecule RIP3 kinase inhibitors and employ them to demonstrate the common requirement for RIP3 kinase in programmed necrosis induced by RIP1-RIP3, DAI-RIP3, and TRIF-RIP3 complexes. Cell fate decisions following TLR signaling parallel death receptor signaling and rely on caspase 8 to suppress RIP3-dependent programmed necrosis whether initiated directly by a TRIF-RIP3- MLKL pathway or indirectly via TNF activation and the RIP1- RIP3-MLKL necroptosis pathway. [ABSTRACT FROM AUTHOR]

Abstract: Human cytomegalovirus (HCMV) productively infects CD34+ progenitor-derived, mature Langerhans-type dendritic cells (matLC) and reduces surface expression of MHC class II complexes (MHC II) by increasing intracellular retention of these molecules. To determine whether HCMV also inhibits MHC II expression by other mechanisms, we assessed mRNA levels of the class II transcriptional regulator, CIITA, and several of its target genes in infected matLC. Levels of CIITA, HLA-DRA (DRA) and DRB transcripts, and new DR protein synthesis were compared in mock-infected and HCMV-infected cells by quantitative PCR and pulse-chase immunoprecipitation analyses, respectively. CIITA mRNA levels were significantly lower in HCMV-infected matLC as compared to mock-infected cells. When assessed in the presence of Actinomycin D, the stability of CIITA transcripts was not diminished by HCMV. Analysis of promoter-specific CIITA isoforms revealed that types I, III and IV all were decreased by HCMV, a result that differs from changes after incubation of these cells with lipopolysaccharide (LPS). Exposure to UV-inactivated virus failed to reduce CIITA mRNA levels, implicating de novo viral gene expression in this effect. HCMV-infected matLC also expressed lower levels of DR transcripts and reduced DR protein synthesis rates compared to mock-infected matLC. In summary, we demonstrate that HCMV infection of a human dendritic cell subset inhibits constitutive CIITA expression, most likely at the transcriptional level, resulting in reduced MHC II biosynthesis. We suggest this represents a new mechanism of modulation of mature LC by HCMV. [Copyright &y& Elsevier]

Background: Metabolic and immuno-inflammatory risk factors contribute to cardiac allograft vasculopathy (CAV) pathogenesis. Although systemic inflammation, as detected by C-reactive protein (CRP), predicts CAV development, the relationship between CRP and markers of metabolic abnormalities remains unexplored. Methods: CRP and the entire metabolic panel were evaluated in 98 consecutive heart transplant recipients at the time of annual coronary angiography, 5.8 years after transplant (range, 1–12 years). A ratio of triglycerides (TG) to high-density lipoproteins (HDL) of 3.0 or more was considered a marker of insulin resistance. CAV prevalence was defined by angiography, and subsequent prognosis was evaluated as incidence of major cardiac adverse events. Results: CRP was higher in the 34 patients with angiographic CAV than in those without CAV (1.10 ± 0.20 vs 0.50 ± 0.05 mg/dl, p < 0.001). Patients with insulin resistance had higher CRP concentrations (p = 0.023) and higher CAV prevalence (p = 0.005). High CRP and a TG/HDL of 3.0 or more were independently associated with an increased likelihood of CAV (odds ratio, ≥3.9; p = 0.02) and predicted an increased risk of major cardiac adverse events. The combination of high CRP and a TG/HDL of 3.0 or more identified a subgroup of patients having a 4-fold increased risk for CAV and a 3-fold increased risk for major cardiac adverse events compared with patients with low CRP and normal values for metabolic indicators. Conclusions: Both CRP and insulin resistance, as estimated by TG/HDL, appear to be strong, synergic risk factors for CAV and for major cardiac adverse events. These findings support the hypothesis that in heart transplant recipients, systemic inflammation may be an important mediator of graft vascular injury associated with metabolic syndrome. [Copyright &y& Elsevier]