Poxviral TNFRs: properties and role in viral pathogenesis

Comparative Pathogenesis, Genomics and Phylogeography of Mousepox

Ectromelia virus (ECTV), the causative agent of mousepox, has threatened laboratory mouse colonies worldwide for almost a century. Mousepox has been valuable for the understanding of poxvirus pathogenesis and immune evasion. Here, we have monitored in parallel the pathogenesis of nine ECTVs in BALB/cJ mice and report the full-length genome sequence of eight novel ECTV isolates or strains, including the first ECTV isolated from a field mouse, ECTV-MouKre. This approach allowed us to identify several genes, absent in strains attenuated through serial passages in culture, that may play a role in virulence and a set of putative genes that may be involved in enhancing viral growth in vitro. We identified a putative strong inhibitor of the host inflammatory response in ECTV-MouKre, an isolate that did not cause local foot swelling and developed a moderate virulence. Most of the ECTVs, except ECTV-Hampstead, encode a truncated version of the P4c protein that impairs the recruitment of virions into the A-type inclusion bodies, and our data suggest that P4c may play a role in viral dissemination and transmission. This is the first comprehensive report that sheds light into the phylogenetic and geographic relationship of the worldwide outbreak dynamics for the ECTV species.

Virus-encoded cytokine and chemokine decoy receptors

Poxviruses and herpesviruses encode secreted versions of cytokine receptors as a unique strategy to evade the host immune response. Recent advances in the field have shown the great impact of some of these proteins in immune modulation and viral pathogenesis, and have uncovered unique properties of these viral proteins not found in the cellular counterparts. These modifications inspired by viruses lead to improved immune modulatory activity of the soluble cytokine receptors, information that has been used to develop more efficient therapeutics to treat inflammatory conditions.

Insights into ligand binding by a viral tumor necrosis factor (TNF) decoy receptor yield a selective soluble human type 2 TNF receptor

Etanercept is a soluble form of the tumor necrosis factor receptor 2 (TNFR2) that inhibits pathological tumor necrosis factor (TNF) responses in rheumatoid arthritis and other inflammatory diseases. However, besides TNF, etanercept also blocks lymphotoxin-α (LTα), which has no clear therapeutic value and might aggravate some of the adverse effects associated with etanercept. Poxviruses encode soluble TNFR2 homologs, termed viral TNF decoy receptors (vTNFRs), that display unique specificity properties. For instance, cytokine response modifier D (CrmD) inhibits mouse and human TNF and mouse LTα, but it is inactive against human LTα. Here, we analyzed the molecular basis of these immunomodulatory activities in the ectromelia virus-encoded CrmD. We found that the overall molecular mechanism to bind TNF and LTα from mouse and human origin is fairly conserved in CrmD and dominated by a groove under its 50s loop. However, other ligand-specific binding determinants optimize CrmD for the inhibition of mouse ligands, especially mouse TNF. Moreover, we show that the inability of CrmD to inhibit human LTα is caused by a Glu-Phe-Glu motif in its 90s loop. Importantly, transfer of this motif to etanercept diminished its anti-LTα activity in >60-fold while weakening its TNF-inhibitory capacity in 3-fold. This new etanercept variant could potentially be used in the clinic as a safer alternative to conventional etanercept. This work is the most detailed study of the vTNFR-ligand interactions to date and illustrates that a better knowledge of vTNFRs can provide valuable information to improve current anti-TNF therapies.

Chemokines cooperate with TNF to provide protective anti-viral immunity and to enhance inflammation

The role of cytokines and chemokines in anti-viral defense has been demonstrated, but their relative contribution to protective anti-viral responses in vivo is not fully understood. Cytokine response modifier D (CrmD) is a secreted receptor for TNF and lymphotoxin containing the smallpox virus-encoded chemokine receptor (SECRET) domain and is expressed by ectromelia virus, the causative agent of the smallpox-like disease mousepox. Here we show that CrmD is an essential virulence factor that controls natural killer cell activation and allows progression of fatal mousepox, and demonstrate that both SECRET and TNF binding domains are required for full CrmD activity. Vaccination with recombinant CrmD protects animals from lethal mousepox. These results indicate that a specific set of chemokines enhance the inflammatory and protective anti-viral responses mediated by TNF and lymphotoxin, and illustrate how viruses optimize anti-TNF strategies with the addition of a chemokine binding domain as soluble decoy receptors.

Infection with diverse immune-modulating poxviruses elicits different compositional shifts in the mouse gut microbiome

It is often not possible to demonstrate causality within the context of gut microbiota dysbiosis-linked diseases. Thus, we need a better understanding of the mechanisms whereby an altered host immunophysiology shapes its resident microbiota. In this regard, immune-modulating poxvirus strains and mutants could differentially alter gut mucosal immunity in the context of a natural immune response, providing a controlled natural in vivo setting to deepen our understanding of the immune determinants of microbiome composition. This study represents a proof-of-concept that the use of an existing collection of different immune-modulating poxviruses may represent an innovative tool in gut microbiome research. To this end, 16S rRNA amplicon sequencing and RNAseq transcriptome profiling were employed as proxies for microbiota composition and gut immunophysiological status in the analysis of caecal samples from control mice and mice infected with various poxvirus types. Our results show that different poxvirus species and mutants elicit different shifts in the mice mucosa-associated microbiota and, in some instances, significant concomitant shifts in gut transcriptome profiles, thus providing an initial validation to the proposed model.

A tumour necrosis factor receptor-like protein encoded by Singapore grouper iridovirus modulates cell proliferation, apoptosis and viral replication

It has been demonstrated that tumour necrosis factor receptor (TNFR) homologues encoded by viruses are usually involved in virus immune evasion by regulating the host immune response or mediating apoptotic cell death. Here, a novel TNFR-like protein encoded by Singapore grouper iridovirus (SGIV VP51) was cloned and characterized. Amino acid analysis showed that VP51 contained three cysteine-rich domains (CRDs) and a transmembrane domain at its C terminus. The expression of VP51 in vitro enhanced cell proliferation, and affected cell cycle progression via altering the G1/S transition. Furthermore, VP51 overexpression improved cell viability during SGIV infection via inhibiting virus-induced apoptosis, evidenced by the reduction of apoptotic bodies and the decrease of caspase-3 activation. In addition, overexpression of VP51 increased viral titre and the expression of viral structural protein gene MCP and cell proliferation promoting gene ICP-18. In contrast, the expression of the viral apoptosis inducing gene, LITAF, was significantly decreased. Although all three CRDs were essential for the action of VP51, CRD2 and CRD3 exerted more crucial roles on virus-induced apoptosis, viral gene transcription and virus production, while CRD1 was more crucial for cell proliferation. Together, SGIV TNFR-like products not only affected cell cycle progression and enhanced cell growth by increasing the expression of the virus encoded cell proliferation gene, but also inhibited virus-induced apoptotic cell death by decreasing the expression of the viral apoptosis inducing gene. Our results provided new insights into understanding the underlying mechanism by which iridovirus regulated the apoptotic pathway to complete its life cycle.

Structural Conservation and Functional Diversity of the Poxvirus Immune Evasion (PIE) Domain Superfamily

Poxviruses encode a broad array of proteins that serve to undermine host immune defenses. Structural analysis of four of these seemingly unrelated proteins revealed the recurrent use of a conserved beta-sandwich fold that has not been observed in any eukaryotic or prokaryotic protein. Herein we propose to call this unique structural scaffolding the PIE (Poxvirus Immune Evasion) domain. PIE domain containing proteins are abundant in chordopoxvirinae, with our analysis identifying 20 likely PIE subfamilies among 33 representative genomes spanning 7 genera. For example, cowpox strain Brighton Red appears to encode 10 different PIEs: vCCI, A41, C8, M2, T4 (CPVX203), and the SECRET proteins CrmB, CrmD, SCP-1, SCP-2, and SCP-3. Characterized PIE proteins all appear to be nonessential for virus replication, and all contain signal peptides for targeting to the secretory pathway. The PIE subfamilies differ primarily in the number, size, and location of structural embellishments to the beta-sandwich core that confer unique functional specificities. Reported ligands include chemokines, GM-CSF, IL-2, MHC class I, and glycosaminoglycans. We expect that the list of ligands and receptors engaged by the PIE domain will grow as we come to better understand how this versatile structural architecture can be tailored to manipulate host responses to infection.

Poxvirus-encoded TNF decoy receptors inhibit the biological activity of transmembrane TNF

Poxviruses encode up to four different soluble TNF receptors, named cytokine response modifier B (CrmB), CrmC, CrmD and CrmE. These proteins mimic the extracellular domain of the cellular TNF receptors to bind and inhibit the activity of TNF and, in some cases, other TNF superfamily ligands. Most of these ligands are released after the enzymic cleavage of a membrane precursor. However, transmembrane TNF (tmTNF) is not only a precursor of soluble TNF but also exerts specific pro-inflammatory and immunological activities. Here, we report that viral TNF receptors bound and inhibited tmTNF and describe some interesting differences in their activity against the soluble cytokine. Thus, CrmE, which does not inhibit mouse soluble TNF, could block murine tmTNF-induced cytotoxicity. We propose that this anti-tmTNF effect should be taken into consideration when assessing the role of viral TNF decoy receptors in the pathogenesis of poxvirus.

Strategies of NF-κB signaling modulation by ectromelia virus in BALB/3T3 murine fibroblasts

Nuclear factor κB (NF-κB) is a pleiotropic transcription factor that regulates the expression of immune response genes. NF-κB signaling can be disrupted by pathogens that prevent host immune response. In this work, we examined the influence of ectromelia (mousepox) virus (ECTV) on NF-κB signaling in murine BALB/3T3 fibroblasts. Activation of NF-κB via tumor necrosis factor (TNF) receptor 1 (TNFR1) in these cells induces proinflammatory cytokine secretion. We show that ECTV does not recruit NF-κB to viral factories or induce NF-κB nuclear translocation in BALB/3T3 cells. Additionally, ECTV counteracts TNF-α-induced p65 NF-κB nuclear translocation during the course of infection. Inhibition of TNF-α-induced p65 nuclear translocation was also observed in neighboring cells that underwent fusion with ECTV-infected cells. ECTV inhibits the key step of NF-κB activation, i.e. Ser32 phosphorylation and degradation of inhibitor κBα (IκBα) induced by TNF-α. We also observed that ECTV prevents TNF-α-induced Ser536 of p65 phosphorylation in BALB/3T3 cells. Studying TNFR1 signaling provides information about regulation of inflammatory response and cell survival. Unraveling poxviral immunomodulatory strategies may be helpful in drug target identification as well as in vaccine development.

Comparative Biochemical and Functional Analysis of Viral and Human Secreted Tumor Necrosis Factor (TNF) Decoy Receptors

The blockade of tumor necrosis factor (TNF) by etanercept, a soluble version of the human TNF receptor 2 (hTNFR2), is a well established strategy to inhibit adverse TNF-mediated inflammatory responses in the clinic. A similar strategy is employed by poxviruses, encoding four viral TNF decoy receptor homologues (vTNFRs) named cytokine response modifier B (CrmB), CrmC, CrmD, and CrmE. These vTNFRs are differentially expressed by poxviral species, suggesting distinct immunomodulatory properties. Whereas the human variola virus and mouse ectromelia virus encode one vTNFR, the broad host range cowpox virus encodes all vTNFRs. We report the first comprehensive study of the functional and binding properties of these four vTNFRs, providing an explanation for their expression profile among different poxviruses. In addition, the vTNFRs activities were compared with the hTNFR2 used in the clinic. Interestingly, CrmB from variola virus, the causative agent of smallpox, is the most potent TNFR of those tested here including hTNFR2. Furthermore, we demonstrate a new immunomodulatory activity of vTNFRs, showing that CrmB and CrmD also inhibit the activity of lymphotoxin β. Similarly, we report for the first time that the hTNFR2 blocks the biological activity of lymphotoxin β. The characterization of vTNFRs optimized during virus-host evolution to modulate the host immune response provides relevant information about their potential role in pathogenesis and may be used to improve anti-inflammatory therapies based on soluble decoy TNFRs.

The genome sequence of ectromelia virus Naval and Cornell isolates from outbreaks in North America

Ectromelia virus (ECTV) is the causative agent of mousepox, a disease of laboratory mouse colonies and an excellent model for human smallpox. We report the genome sequence of two isolates from outbreaks in laboratory mouse colonies in the USA in 1995 and 1999: ECTV-Naval and ECTV-Cornell, respectively. The genome of ECTV-Naval and ECTV-Cornell was sequenced by the 454-Roche technology. The ECTV-Naval genome was also sequenced by the Sanger and Illumina technologies in order to evaluate these technologies for poxvirus genome sequencing. Genomic comparisons revealed that ECTV-Naval and ECTV-Cornell correspond to the same virus isolated from independent outbreaks. Both ECTV-Naval and ECTV-Cornell are extremely virulent in susceptible BALB/c mice, similar to ECTV-Moscow. This is consistent with the ECTV-Naval genome sharing 98.2% DNA sequence identity with that of ECTV-Moscow, and indicates that the genetic differences with ECTV-Moscow do not affect the virulence of ECTV-Naval in the mousepox model of footpad infection.

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We describe the genome sequence of two highly virulent ectromelia virus isolates.

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The outbreak of ectromelia virus in USA was caused by Chinese viral isolates.

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We describe a clade of ectromelia virus isolates from China.

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We compare three different sequencing technologies to sequence large DNA viruses.

An orphan viral TNF receptor superfamily member identified in lymphocystis disease virus

Background

Lymphocystis disease virus (LCDV) is a large icosahedral dsDNA-containing virus of the Lymphocystivirus genus within the Iridoviridae family that can cause disease in more than 140 marine and freshwater fish species. While several isolates have been charcaterized and classified into distinct genotypes the complete genomic sequence is currently only available from two species, the LCDV-1, isolated from flounder (Platichtys flesus) in Europe and the LCDV-C, isolated from Japanese cultured flounder (Paralichthys olivaceus) in China. Analysis of the genome of LCDV-C showed it to encode a protein named LDVICp016 with similarities to the Tumour necrosis factor receptor (TNFR) superfamily with immunomodulatory potential.

Findings

We have expressed and purified the recombinant protein LDVICp016 and screened for potential interaction partners using surface plasmon resonance. Commercially available human and mouse members of the TNF superfamily (TNFSF), along with a representative set of fish-derived TNFSF were tested.

We have found the LDVICp016 protein to be secreted and we have identified a second viral TNFR encoded by ORF 095 of the same virus. None of the 42 tested proteins were found to interact with LDVICp016.

Conclusions

We show that LDVICp016 is a secreted protein belonging to the TNF receptor family that may be part of a larger gene family in Lymphocystiviruses. While the ligand of this protein remains unknown, possibly due to the species specific nature of this interaction, further investigations into the potential role of this protein in the blockade of immune responses in its fish host are required.

Subversion of cytokine networks by virally encoded decoy receptors

During the course of evolution, viruses have captured or created a diverse array of open reading frames, which encode for proteins that serve to evade and sabotage the host innate and adaptive immune responses that would otherwise lead to their elimination. These viral genomes are some of the best textbooks of immunology ever written. The established arsenal of immunomodulatory proteins encoded by viruses is large and growing, and includes specificities for virtually all known inflammatory pathways and targets. The focus of this review is on herpes and poxvirus-encoded cytokine and chemokine-binding proteins that serve to undermine the coordination of host immune surveillance. Structural and mechanistic studies of these decoy receptors have provided a wealth of information, not only about viral pathogenesis but also about the inner workings of cytokine signaling networks.