Vaccinia virus complement-control protein prevents antibody-dependent complement-enhanced neutralization of infectivity and contributes to virulence

Understanding the biology of monkeypox virus to prevent future outbreaks

Historically, monkeypox (mpox) was a zoonotic disease endemic in Africa. However, in 2022, a global outbreak occurred following a substantial increase in cases in Africa, coupled with spread by international travellers to other continents. Between January 2022 and October 2023, about 91,000 confirmed cases from 115 countries were reported, leading the World Health Organization to declare a public health emergency. The basic biology of monkeypox virus (MPXV) can be inferred from other poxviruses, such as vaccinia virus, and confirmed by genome sequencing. Here the biology of MPXV is reviewed, together with a discussion of adaptive changes during MPXV evolution and implications for transmission. Studying MPXV biology is important to inform specific host interactions, to aid in ongoing outbreaks and to predict those in the future.

Cross-species transmission and host range genes in poxviruses

The persistent epidemic of human mpox, caused by mpox virus (MPXV), raises concerns about the future spread of MPXV and other poxviruses. MPXV is a typical zoonotic virus which can infect human and cause smallpox-like symptoms. MPXV belongs to the Poxviridae family, which has a relatively broad host range from arthropods to vertebrates. Cross-species transmission of poxviruses among different hosts has been frequently reported and resulted in numerous epidemics. Poxviruses have a complex linear double-strand DNA genome that encodes hundreds of proteins. Genes related to the host range of poxvirus are called host range genes (HRGs). This review briefly introduces the taxonomy, phylogeny and hosts of poxviruses, and then comprehensively summarizes the current knowledge about the cross-species transmission of poxviruses. In particular, the HRGs of poxvirus are described and their impacts on viral host range are discussed in depth. We hope that this review will provide a comprehensive perspective about the current progress of researches on cross-species transmission and HRG variation of poxviruses, serving as a valuable reference for academic studies and disease control in the future.

A comprehensive review of monkeypox virus and mpox characteristics

Monkeypox virus (MPXV) is the etiological agent of monkeypox (mpox), a zoonotic disease. MPXV is endemic in the forested regions of West and Central Africa, but the virus has recently spread globally, causing outbreaks in multiple non-endemic countries. In this paper, we review the characteristics of the virus, including its ecology, genomics, infection biology, and evolution. We estimate by phylogenomic molecular clock that the B.1 lineage responsible for the 2022 mpox outbreaks has been in circulation since 2016. We interrogate the host-virus interactions that modulate the virus infection biology, signal transduction, pathogenesis, and host immune responses. We highlight the changing pathophysiology and epidemiology of MPXV and summarize recent advances in the prevention and treatment of mpox. In addition, this review identifies knowledge gaps with respect to the virus and the disease, suggests future research directions to address the knowledge gaps, and proposes a One Health approach as an effective strategy to prevent current and future epidemics of mpox.

Oncolytic vaccinia virus and cancer immunotherapy

Oncolytic virotherapy (OVT) is a promising form of cancer treatment that uses genetically engineered viruses to replicate within cancer cells and trigger anti-tumor immune response. In addition to killing cancer cells, oncolytic viruses can also remodel the tumor microenvironment and stimulate a long-term anti-tumor immune response. Despite achieving positive results in cellular and organismal studies, there are currently only a few approved oncolytic viruses for clinical use. Vaccinia virus (VACV) has emerged as a potential candidate due to its ability to infect a wide range of cancer cells. This review discusses the mechanisms, benefits, and clinical trials of oncolytic VACVs. The safety and efficacy of different viral backbones are explored, as well as the effects of oncolytic VACVs on the tumor microenvironment. The potential combination of oncolytic VACVs with immunotherapy or traditional therapies is also highlighted. The review concludes by addressing prospects and challenges in the field of oncolytic VACVs, with the aim of promoting further research and application in cancer therapy.

Recombinant Strains of Oncolytic Vaccinia Virus for Cancer Immunotherapy

Cancer virotherapy is an alternative therapeutic approach based on the viruses that selectively infect and kill tumor cells. Vaccinia virus (VV) is a member of the Poxviridae, a family of enveloped viruses with a large linear double-stranded DNA genome. The proven safety of the VV strains as well as considerable transgene capacity of the viral genome, make VV an excellent platform for creating recombinant oncolytic viruses for cancer therapy. Furthermore, various genetic modifications can increase tumor selectivity and therapeutic efficacy of VV by arming it with the immune-modulatory genes or proapoptotic molecules, boosting the host immune system, and increasing cross-priming recognition of the tumor cells by T-cells or NK cells. In this review, we summarized the data on bioengineering approaches to develop recombinant VV strains for enhanced cancer immunotherapy.

Monkeypox: a global health emergency

Over the past 2 years, the world has faced the impactful Coronavirus Disease-2019 (COVID-19) pandemic, with a visible shift in economy, medicine, and beyond. As of recent times, the emergence of the monkeypox (mpox) virus infections and the growing number of infected cases have raised panic and fear among people, not only due to its resemblance to the now eradicated smallpox virus, but also because another potential pandemic could have catastrophic consequences, globally. However, studies of the smallpox virus performed in the past and wisdom gained from the COVID-19 pandemic are the two most helpful tools for humanity that can prevent major outbreaks of the mpox virus, thus warding off another pandemic. Because smallpox and mpox are part of the same virus genus, the Orthopoxvirus genus, the structure and pathogenesis, as well as the transmission of both these two viruses are highly similar. Because of these similarities, antivirals and vaccines approved and licensed in the past for the smallpox virus are effective and could successfully treat and prevent an mpox virus infection. This review discusses the main components that outline this current global health issue raised by the mpox virus, by presenting it as a whole, and integrating aspects such as its structure, pathogenesis, clinical aspects, prevention, and treatment options, and how this ongoing phenomenon is being globally approached.

The pathophysiological and immunological background of the monkeypox virus infection: An update

In addition to the COVID-19 waves, the globe is facing global monkeypox (MPX) outbreak. MPX is an uncommon zoonotic infection characterized by symptoms similar to smallpox. It is caused by the monkeypox virus (MPXV), a double-stranded DNA virus that belongs to the genus Orthopoxvirus (OPXV). MPXV, which causes human disease, has been confined to Africa for many years, with only a few isolated cases in other areas. Outside of Africa, the continuing MPXV outbreak in multiple countries in 2022 is the greatest in recorded history. The current outbreak, with over 10 000 confirmed cases in over 50 countries between May and July 2022, demonstrates that MPXV may travel rapidly among humans and pose a danger to human health worldwide. The rapid spread of such outbreaks in recent times has elevated MPX to the status of a rising zoonotic disease with significant epidemic potential. While the MPXV is not as deadly or contagious as the variola virus that causes smallpox, it poses a threat because it could evolve into a more potent human pathogen. This review assesses the potential threat to the human population and provides a brief overview of what is currently known about this reemerging virus. By analyzing the biological effects of MPXV on human health, its shifting epidemiological footprint, and currently available therapeutic options, this review has presented the most recent insights into the biology of the virus. This study also clarifies the key potential causes that could be to blame for the present MPX outbreak and draw attention to major research questions and promising new avenues for combating the current MPX epidemic.

Virus-Encoded Complement Regulators: Current Status

Viruses require a host for replication and survival and hence are subjected to host immunological pressures. The complement system, a crucial first response of the host immune system, is effective in targeting viruses and virus-infected cells, and boosting the antiviral innate and acquired immune responses. Thus, the system imposes a strong selection pressure on viruses. Consequently, viruses have evolved multiple countermeasures against host complement. A major mechanism employed by viruses to subvert the complement system is encoding proteins that target complement. Since viruses have limited genome size, most of these proteins are multifunctional in nature. In this review, we provide up to date information on the structure and complement regulatory functions of various viral proteins.

Host-virus interactions and defense mechanisms for giant viruses

Giant viruses, with virions larger than 200 nm and genomes larger than 340 kilobase pairs, modified the now outdated perception of the virosphere. With virions now reported reaching up to 1.5 μm in size and genomes of up to 2.5 Mb encoding components shared with cellular life forms, giant viruses exhibit a complexity similar to microbes, such as bacteria and archaea. Here, we review interactions of giant viruses with their hosts and defense strategies of giant viruses against their hosts and coinfecting microorganisms or virophages. We also searched by comparative genomics for homologies with proteins described or suspected to be involved in defense mechanisms. Our search reveals that natural immunity and apoptosis seem to be crucial components of the host defense against giant virus infection. Conversely, giant viruses possess methods of hijacking host functions to counteract cellular antiviral responses. In addition, giant viruses may encode other unique and complex pathways to manipulate the host machinery and eliminate other competing microorganisms. Notably, giant viruses have evolved defense mechanisms against their virophages and they might trigger defense systems against other viruses through sequence integration. We anticipate that comparative genomics may help identifying genes involved in defense strategies of both giant viruses and their hosts.

Buffalopox Virus: An Emerging Virus in Livestock and Humans

Buffalopox virus (BPXV) is the cause of buffalopox, which was recognized by the FAO/WHO Joint Expert Committee on Zoonosis as an important zoonotic disease. Buffalopox was first described in India, later in other countries, and has become an emerging contagious viral zoonotic disease infecting milkers with high morbidity among affected domestic buffalo and cattle. BPXV is a member of the genus Orthopoxvirus and a close variant of the vaccinia virus (VACV). Recent genome data show that BPXV shares a most recent common ancestor of VACV Lister strain, which had been used for inoculating buffalo calves to produce a Smallpox vaccine. Over time, VACV evolved into BPXV by establishing itself in buffaloes to be increasingly pathogenic to this host and to make infections in cattle and humans. Together with the current pandemic of SARS-COV2/COVID 19, BPXV infections illustrate how vulnerable the human population is to the emergence and re-emergence of viral pathogens from unsuspected sources. In view that majority of the world population are not vaccinated against smallpox and are most vulnerable in the event of its re-emergence, reviewing and understanding the biology of vaccinia-like viruses are necessary for developing a new generation of safer smallpox vaccines in the smallpox-free world.

Antibody-mediated complement activation in pathology and protection

Antibody-dependent complement activity is associated not only with autoimmune morbidity, but also with antitumor efficacy. In infectious disease, both recombinant monoclonal antibodies and polyclonal antibodies generated in natural adaptive responses can mediate complement activity to protective, therapeutic or disease-enhancing effect. Recent advances have contributed to the structural resolution of molecular complexes involved in antibody-mediated complement activation, defining the avid nature of participating interactions and pointing to how antibody isotype, subclass, hinge flexibility, glycosylation state, amino acid sequence and the contextual nature of the cognate antigen/epitope are all factors that can determine complement activity through impact on antibody multimerization and subsequent recruitment of complement component 1q. Beyond the efficiency of activation, complement activation products interact with various cell types that mediate immune adherence, trafficking, immune education and innate functions. Similarly, depending on the anatomical location and extent of activation, complement can support homeostatic restoration or be leveraged by pathogens or neoplasms to enhance infection or promote tumorigenic microenvironments, respectively. Advances in means to suppress complement activation by intravenous immunoglobulin (IVIG), IVIG mimetics and complement-intervening antibodies represent proven and promising exploratory therapeutic strategies, while antibody engineering has likewise offered frameworks to enhance, eliminate or isolate complement activation to interrogate in vivo mechanisms of action. Such strategies promise to support the optimization of antibody-based drugs that are able to tackle emerging and difficult-to-treat diseases by improving our understanding of the synergistic and antagonistic relationships between antibody mechanisms mediated by Fc receptors, direct binding and the products of complement activation.

Kaposi's Sarcoma-Associated Herpesvirus and Host Interaction by the Complement System

Kaposi’s sarcoma-associated herpesvirus (KSHV) modulates the immune response to allow the virus to establish persistent infection in the host and facilitate the development of KSHV-associated cancer. The complement system has a central role in the defense against pathogens. Hence, KSHV has adopted an evasion strategy for complement attack using the viral protein encoded by KSHV open reading frame 4. However, despite this defense mechanism, the complement system appears to become activated in KSHV-infected cells as well as in the region surrounding Kaposi’s sarcoma tumors. Given that the complement system can affect cell fate as well as the inflammatory microenvironment, complement activation is likely associated with KSHV pathogenesis. A better understanding of the interplay between KSHV and the complement system may, therefore, translate into the development of novel therapeutic interventions for KSHV-associated tumors. In this review, the mechanisms and functions of complement activation in KSHV-infected cells are discussed.

A chimeric poxvirus with J2R (thymidine kinase) deletion shows safety and anti-tumor activity in lung cancer models

Oncolytic viruses have shown excellent safety profiles in preclinical and clinical studies; however, in most cases therapeutic benefits have been modest. We have previously reported the generation of a chimeric poxvirus (CF33), with significantly improved oncolytic characteristics, through chimerization among different poxviruses. Here we report the sequence analysis of CF33 and oncolytic potential of a GFP-encoding CF33 virus (CF33-GFP) with a J2R deletion in lung cancer models. Replication of CF33-GFP and the resulting cytotoxicity were higher in cancer cell lines compared to a normal cell line, in vitro. After infection with virus, cancer cells expressed markers for immunogenic cell death in vitro. Furthermore, CF33-GFP was safe and exerted potent anti-tumor effects at a dose as low as 1000 plaque forming units in both virus-injected and un-injected distant tumors in A549 tumor xenograft model in mice. Likewise, in a syngeneic model of lung cancer in mice, the virus showed significant anti-tumor effect and was found to increase tumor infiltration by CD8+ T cells. Collectively, these data warrant further investigation of this novel chimeric poxvirus for its potential use as a cancer bio-therapeutic.

A virus-encoded type I interferon decoy receptor enables evasion of host immunity through cell-surface binding

Soluble cytokine decoy receptors are potent immune modulatory reagents with therapeutic applications. Some virus-encoded secreted cytokine receptors interact with glycosaminoglycans expressed at the cell surface, but the biological significance of this activity in vivo is poorly understood. Here, we show the type I interferon binding protein (IFNα/βBP) encoded by vaccinia and ectromelia viruses requires of this cell binding activity to confer full virulence to these viruses and to retain immunomodulatory activity. Expression of a variant form of the IFNα/βBP that inhibits IFN activity, but does not interact with cell surface glycosaminoglycans, results in highly attenuated viruses with a virulence similar to that of the IFNα/βBP deletion mutant viruses. Transcriptomics analysis and infection of IFN receptor-deficient mice confirmed that the control of IFN activity is the main function of the IFNα/βBP in vivo. We propose that retention of secreted cytokine receptors at the cell surface may largely enhance their immunomodulatory activity.

New insights into the immunomodulatory properties of poxvirus cytokine decoy receptors at the cell surface

Poxviruses encode a set of secreted proteins that bind cytokines and chemokines as a strategy to modulate host defense mechanisms. These viral proteins mimic the activity of host cytokine decoy receptors but have unique properties that may enhance their activity. Here, we describe the ability of poxvirus cytokine receptors to attach to the cell surface after secretion from infected cells, and we discuss the advantages that this property may confer to these viral immunomodulatory proteins.

Relative Contribution of Cellular Complement Inhibitors CD59, CD46, and CD55 to Parainfluenza Virus 5 Inhibition of Complement-Mediated Neutralization

The complement system is a part of the innate immune system that viruses need to face during infections. Many viruses incorporate cellular regulators of complement activation (RCA) to block complement pathways and our prior work has shown that Parainfluenza virus 5 (PIV5) incorporates CD55 and CD46 to delay complement-mediated neutralization. In this paper, we tested the role of a third individual RCA inhibitor CD59 in PIV5 interactions with complement pathways. Using a cell line engineered to express CD59, we show that small levels of functional CD59 are associated with progeny PIV5, which is capable of blocking assembly of the C5b-C9 membrane attack complex (MAC). PIV5 containing CD59 (PIV5-CD59) showed increased resistance to complement-mediated neutralization in vitro comparing to PIV5 lacking regulators. Infection of A549 cells with PIV5 and RSV upregulated CD59 expression. TGF-beta treatment of PIV5-infected cells also increased cell surface CD59 expression and progeny virions were more resistant to complement-mediated neutralization. A comparison of individual viruses containing only CD55, CD46, or CD59 showed a potency of inhibiting complement-mediated neutralization, which followed a pattern of CD55 > CD46 > CD59.

Modulating Vaccinia Virus Immunomodulators to Improve Immunological Memory

The increasing frequency of monkeypox virus infections, new outbreaks of other zoonotic orthopoxviruses and concern about the re-emergence of smallpox have prompted research into developing antiviral drugs and better vaccines against these viruses. This article considers the genetic engineering of vaccinia virus (VACV) to enhance vaccine immunogenicity and safety. The virulence, immunogenicity and protective efficacy of VACV strains engineered to lack specific immunomodulatory or host range proteins are described. The ultimate goal is to develop safer and more immunogenic VACV vaccines that induce long-lasting immunological memory.

Combined Proteomics/Genomics Approach Reveals Proteomic Changes of Mature Virions as a Novel Poxvirus Adaptation Mechanism

DNA viruses, like poxviruses, possess a highly stable genome, suggesting that adaptation of virus particles to specific cell types is not restricted to genomic changes. Cowpox viruses are zoonotic poxviruses with an extraordinarily broad host range, demonstrating their adaptive potential in vivo. To elucidate adaptation mechanisms of poxviruses, we isolated cowpox virus particles from a rat and passaged them five times in a human and a rat cell line. Subsequently, we analyzed the proteome and genome of the non-passaged virions and each passage. While the overall viral genome sequence was stable during passaging, proteomics revealed multiple changes in the virion composition. Interestingly, an increased viral fitness in human cells was observed in the presence of increased immunomodulatory protein amounts. As the only minor variant with increasing frequency during passaging was located in a viral RNA polymerase subunit and, moreover, most minor variants were found in transcription-associated genes, protein amounts were presumably regulated at transcription level. This study is the first comparative proteome analysis of virus particles before and after cell culture propagation, revealing proteomic changes as a novel poxvirus adaptation mechanism.

Species Specificity of Vaccinia Virus Complement Control Protein for the Bovine Classical Pathway Is Governed Primarily by Direct Interaction of Its Acidic Residues with Factor I

Poxviruses display species tropism-variola virus is a human-specific virus, while vaccinia virus causes repeated outbreaks in dairy cattle. Consistent with this, variola virus complement regulator SPICE (smallpox inhibitor of complement enzymes) exhibits selectivity in inhibiting the human alternative complement pathway and vaccinia virus complement regulator VCP (vaccinia virus complement control protein) displays selectivity in inhibiting the bovine alternative complement pathway. In the present study, we examined the species specificity of VCP and SPICE for the classical pathway (CP). We observed that VCP is ∼43-fold superior to SPICE in inhibiting bovine CP. Further, functional assays revealed that increased inhibitory activity of VCP for bovine CP is solely due to its enhanced cofactor activity, with no effect on decay of bovine CP C3-convertase. To probe the structural basis of this specificity, we utilized single- and multi-amino-acid substitution mutants wherein 1 or more of the 11 variant VCP residues were substituted in the SPICE template. Examination of these mutants for their ability to inhibit bovine CP revealed that E108, E120, and E144 are primarily responsible for imparting the specificity and contribute to the enhanced cofactor activity of VCP. Binding and functional assays suggested that these residues interact with bovine factor I but not with bovine C4(H₂O) (a moiety conformationally similar to C4b). Mapping of these residues onto the modeled structure of bovine C4b-VCP-bovine factor I supported the mutagenesis data. Taken together, our data help explain why the vaccine strain of vaccinia virus was able to gain a foothold in domesticated animals.IMPORTANCE Vaccinia virus was used for smallpox vaccination. The vaccine-derived virus is now circulating and causing outbreaks in dairy cattle in India and Brazil. However, the reason for this tropism is unknown. It is well recognized that the virus is susceptible to neutralization by the complement classical pathway (CP). Because the virus encodes a soluble complement regulator, VCP, we examined whether this protein displays selectivity in targeting bovine CP. Our data show that it does exhibit selectivity in inhibiting the bovine CP and that this is primarily determined by its amino acids E108, E120, and E144, which interact with bovine serine protease factor I to inactivate bovine C4b-one of the two subunits of CP C3-convertase. Of note, the variola complement regulator SPICE contains positively charged residues at these positions. Thus, these variant residues in VCP help enhance its potency against the bovine CP and thereby the fitness of the virus in cattle.

Complement Evasion Strategies of Viruses: An Overview

Being a major first line of immune defense, the complement system keeps a constant vigil against viruses. Its ability to recognize large panoply of viruses and virus-infected cells, and trigger the effector pathways, results in neutralization of viruses and killing of the infected cells. This selection pressure exerted by complement on viruses has made them evolve a multitude of countermeasures. These include targeting the recognition molecules for the avoidance of detection, targeting key enzymes and complexes of the complement pathways like C3 convertases and C5b-9 formation - either by encoding complement regulators or by recruiting membrane-bound and soluble host complement regulators, cleaving complement proteins by encoding protease, and inhibiting the synthesis of complement proteins. Additionally, viruses also exploit the complement system for their own benefit. For example, they use complement receptors as well as membrane regulators for cellular entry as well as their spread. Here, we provide an overview on the complement subversion mechanisms adopted by the members of various viral families including Poxviridae, Herpesviridae, Adenoviridae, Flaviviridae, Retroviridae, Picornaviridae, Astroviridae, Togaviridae, Orthomyxoviridae and Paramyxoviridae.

Serro 2 Virus Highlights the Fundamental Genomic and Biological Features of a Natural Vaccinia Virus Infecting Humans

Vaccinia virus (VACV) has been implicated in infections of dairy cattle and humans, and outbreaks have substantially impacted local economies and public health in Brazil. During a 2005 outbreak, a VACV strain designated Serro 2 virus (S2V) was collected from a 30-year old male milker. Our aim was to phenotypically and genetically characterize this VACV Brazilian isolate. S2V produced small round plaques without associated comets when grown in BSC40 cells. Furthermore, S2V was less virulent than the prototype strain VACV-Western Reserve (WR) in a murine model of intradermal infection, producing a tiny lesion with virtually no surrounding inflammation. The genome of S2V was sequenced by primer walking. The coding region spans 184,572 bp and contains 211 predicted genes. Mutations in envelope genes specifically associated with small plaque phenotypes were not found in S2V; however, other alterations in amino acid sequences within these genes were identified. In addition, some immunomodulatory genes were truncated in S2V. Phylogenetic analysis using immune regulatory-related genes, besides the hemagglutinin gene, segregated the Brazilian viruses into two clusters, grouping the S2V into Brazilian VACV group 1. S2V is the first naturally-circulating human-associated VACV, with a low passage history, to be extensively genetically and phenotypically characterized.

Attenuation of Vaccinia Virus

Since 1980, in the post-smallpox vaccination era the human population has become increasingly susceptible compared to a generation ago to not only the variola (smallpox) virus, but also other zoonotic orthopoxviruses. The need for safer vaccines against orthopoxviruses is even greater now. The Lister vaccine strain (LIVP) of vaccinia virus was used as a parental virus for generating a recombinant 1421ABJCN clone defective in five virulence genes encoding hemagglutinin (A56R), the IFN-γ-binding protein (B8R), thymidine kinase (J2R), the complement-binding protein (C3L), and the Bcl-2-like inhibitor of apoptosis (N1L). We found that disruption of these loci does not affect replication in mammalian cell cultures. The isogenic recombinant strain 1421ABJCN exhibits a reduced inflammatory response and attenuated neurovirulence relative to LIVP. Virus titers of 1421ABJCN were 3 lg lower versus the parent VACV LIVP when administered by the intracerebral route in new-born mice. In a subcutaneous mouse model, 1421ABJCN displayed levels of VACV-neutralizing antibodies comparable to those of LIVP and conferred protective immunity against lethal challenge by the ectromelia virus. The VACV mutant holds promise as a safe live vaccine strain for preventing smallpox and other orthopoxvirus infections.

Genomic Analysis, Phenotype, and Virulence of the Historical Brazilian Smallpox Vaccine Strain IOC: Implications for the Origins and Evolutionary Relationships of Vaccinia Virus

Smallpox was declared eradicated in 1980 after an intensive vaccination program using different strains of vaccinia virus (VACV; Poxviridae). VACV strain IOC (VACV-IOC) was the seed strain of the smallpox vaccine manufactured by the major vaccine producer in Brazil during the smallpox eradication program. However, little is known about the biological and immunological features as well as the phylogenetic relationships of this first-generation vaccine. In this work, we present a comprehensive characterization of two clones of VACV-IOC. Both clones had low virulence in infected mice and induced a protective immune response against a lethal infection comparable to the response of the licensed vaccine ACAM2000 and the parental strain VACV-IOC. Full-genome sequencing revealed the presence of several fragmented virulence genes that probably are nonfunctional, e.g., F1L, B13R, C10L, K3L, and C3L. Most notably, phylogenetic inference supported by the structural analysis of the genome ends provides evidence of a novel, independent cluster in VACV phylogeny formed by VACV-IOC, the Brazilian field strains Cantagalo (CTGV) and Serro 2 viruses, and horsepox virus, a VACV-like virus supposedly related to an ancestor of the VACV lineage. Our data strongly support the hypothesis that CTGV-like viruses represent feral VACV that evolved in parallel with VACV-IOC after splitting from a most recent common ancestor, probably an ancient smallpox vaccine strain related to horsepox virus. Our data, together with an interesting historical investigation, revisit the origins of VACV and propose new evolutionary relationships between ancient and extant VACV strains, mainly horsepox virus, VACV-IOC/CTGV-like viruses, and Dryvax strain.

IMPORTANCE

First-generation vaccines used to eradicate smallpox had rates of adverse effects that are not acceptable by current health care standards. Moreover, these vaccines are genetically heterogeneous and consist of a pool of quasispecies of VACV. Therefore, the search for new-generation smallpox vaccines that combine low pathogenicity, immune protection, and genetic homogeneity is extremely important. In addition, the phylogenetic relationships and origins of VACV strains are quite nebulous. We show the characterization of two clones of VACV-IOC, a unique smallpox vaccine strain that contributed to smallpox eradication in Brazil. The immunogenicity and reduced virulence make the IOC clones good options for alternative second-generation smallpox vaccines. More importantly, this study reveals the phylogenetic relationship between VACV-IOC, feral VACV established in nature, and the ancestor-like horsepox virus. Our data expand the discussion on the origins and evolutionary connections of VACV lineages.

Complement inhibition prevents oncolytic vaccinia virus neutralization in immune humans and cynomolgus macaques

Oncolytic viruses (OVs) have shown promising clinical activity when administered by direct intratumoral injection. However, natural barriers in the blood, including antibodies and complement, are likely to limit the ability to repeatedly administer OVs by the intravenous route. We demonstrate here that for a prototype of the clinical vaccinia virus based product Pexa-Vec, the neutralizing activity of antibodies elicited by smallpox vaccination, as well as the anamnestic response in hyperimmune virus treated cancer patients, is strictly dependent on the activation of complement. In immunized rats, complement depletion stabilized vaccinia virus in the blood and led to improved delivery to tumors. Complement depletion also enhanced tumor infection when virus was directly injected into tumors in immunized animals. The feasibility and safety of using a complement inhibitor, CP40, in combination with vaccinia virus was tested in cynomolgus macaques. CP40 pretreatment elicited an average 10-fold increase in infectious titer in the blood early after the infusion and prolonged the time during which infectious virus was detectable in the blood of animals with preexisting immunity. Capitalizing on the complement dependence of antivaccinia antibody with adjunct complement inhibitors may increase the infectious dose of oncolytic vaccinia virus delivered to tumors in virus in immune hosts.

Oncolytic tanapoxvirus expressing FliC causes regression of human colorectal cancer xenografts in nude mice

Colorectal cancers are significant causes of morbidity and mortality and existing therapies often perform poorly for individuals afflicted with advanced disease. Oncolytic virotherapy is an emerging therapeutic modality with great promise for addressing this medical need. Herein we describe the in vivo testing of recombinant variants of the tanapoxvirus (TPV). Recombinant viruses were made ablated for either the 66R gene (encoding a thymidine kinase), the 2L gene (encoding a TNF-binding protein), or both. Some of the recombinants were armed to express mouse chemotactic protein 1 (mCCL2/mMCP-1), mouse granulocyte-monocyte colony stimulating factor (mGM-CSF), or bacterial flagellin (FliC). Tumors were induced in athymic nude mice by implantation of HCT 116 cells and subsequently treated by a single intratumoral injection of one of the recombinant TPVs. Histological examination showed a common neoplastic cell type and a range of immune cell infiltration, necrosis, and tumor cell organization. Significant regression was seen in tumors treated with virus TPV/Δ2L/Δ66R/fliC, and to a lesser extent the recombinants TPV/Δ2L and TPV/Δ66R. Our results suggest that oncolytic recombinants of the TPV armed with activators of the innate immune response may be effective virotherapeutic agents for colorectal cancers in humans and should be explored further to fully realize their potential.

Complement component C5 recruits neutrophils in the absence of C3 during respiratory infection with modified vaccinia virus Ankara

Efficient leukocyte migration is important for an effective host response to viral infection and the development of adaptive immunity. The poxvirus strain modified vaccinia virus Ankara (MVA), a safe and efficient viral vector, rapidly induces chemokine expression and respiratory recruitment of leukocytes, which is unique among vaccinia viruses. In addition to chemokines, the complement system contributes to the attraction and activation of different types of leukocytes. Using a murine model of intranasal infection, we show in this study that MVA-induced neutrophil recruitment depends on complement component C5. Remarkably, we find that C5 mediates neutrophil recruitment to the lung, even in the absence of the central complement component C3. Our findings argue for complement C5 activation during MVA infection of the lung via a C3-independent pathway, which enables rapid recruitment of neutrophils.

Glycosylated and nonglycosylated complement control protein of the lister strain of vaccinia virus

The vaccinia virus complement control protein (VCP) is a secreted viral protein that binds the C3b and C4b complement components and inhibits the classic and alternative complement pathways. Previously, we reported that an attenuated smallpox vaccine, LC16m8, which was derived from the Lister strain of vaccinia virus (VV-Lister), expressed a glycosylated form of VCP, whereas published sequence data at that time indicated that the VV-Lister VCP has no motif for N-linked glycosylation. We were interested in determining whether the glycosylation of VCP impairs its biological activity, possibly contributing to the attenuation of LC16m8, and the likely origin of the glycosylated VCP. Expression analysis indicated that VV-Lister contains substrains expressing glycosylated VCP and substrains expressing nonglycosylated VCP. Other strains of smallpox vaccine, as well as laboratory strains of vaccinia virus, all expressed nonglycosylated VCP. Individual Lister virus clones expressing either the glycosylated VCP or the nonglycosylated species were isolated, and partially purified VCP from the isolates were found to be functional equivalents in binding human C3b and C4b complement proteins and inhibiting hemolysis and in immunogenicity. Recombinant vaccinia viruses expressing FLAG-tagged glycosylated VCP (FLAG-VCPg) and nonglycosylated VCP (FLAG-VCP) were constructed based on the Western Reserve strain. Purified FLAG-VCP and FLAG-VCPg bind human C3b and C4b and blocked complement-mediated hemolysis. Our data suggest that glycosylation did not affect the biological activity of VCP and thus may not have contributed to the attenuation of LC16m8. In addition, the LC16m8 virus likely originated from a substrain of VV-Lister that expresses glycosylated VCP.

Viral regulators of complement activation: structure, function and evolution

The complement system surveillance in the host is effective in controlling viral propagation. Consequently, to subvert this effector mechanism, viruses have developed a series of adaptations. One among these is encoding mimics of host regulators of complement activation (RCA) which help viruses to avoid being labeled as 'foreign' and protect them from complement-mediated neutralization and complement-enhanced antiviral adaptive immunity. In this review, we provide an overview on the structure, function and evolution of viral RCA proteins.

Poxviruses and the evolution of host range and virulence

Poxviruses as a group can infect a large number of animals. However, at the level of individual viruses, even closely related poxviruses display highly diverse host ranges and virulence. For example, variola virus, the causative agent of smallpox, is human-specific and highly virulent only to humans, whereas related cowpox viruses naturally infect a broad spectrum of animals and only cause relatively mild disease in humans. The successful replication of poxviruses depends on their effective manipulation of the host antiviral responses, at the cellular-, tissue- and species-specific levels, which constitutes a molecular basis for differences in poxvirus host range and virulence. A number of poxvirus genes have been identified that possess host range function in experimental settings, and many of these host range genes target specific antiviral host pathways. Herein, we review the biology of poxviruses with a focus on host range, zoonotic infections, virulence, genomics and host range genes as well as the current knowledge about the function of poxvirus host range factors and how their interaction with the host innate immune system contributes to poxvirus host range and virulence. We further discuss the evolution of host range and virulence in poxviruses as well as host switches and potential poxvirus threats for human and animal health.

Vaccinia virus immune evasion: mechanisms, virulence and immunogenicity

Virus infection of mammalian cells is sensed by pattern recognition receptors and leads to an innate immune response that restricts virus replication and induces adaptive immunity. In response, viruses have evolved many countermeasures that enable them to replicate and be transmitted to new hosts, despite the host innate immune response. Poxviruses, such as vaccinia virus (VACV), have large DNA genomes and encode many proteins that are dedicated to host immune evasion. Some of these proteins are secreted from the infected cell, where they bind and neutralize complement factors, interferons, cytokines and chemokines. Other VACV proteins function inside cells to inhibit apoptosis or signalling pathways that lead to the production of interferons and pro-inflammatory cytokines and chemokines. In this review, these VACV immunomodulatory proteins are described and the potential to create more immunogenic VACV strains by manipulation of the gene encoding these proteins is discussed.

Inhibitors of C5 complement enhance vaccinia virus oncolysis

Genetically engineered tumor-selective vaccinia virus (VV) has been demonstrated to be a highly effective oncolytic agent, but immune clearance may limit its therapeutic potential. As previously demonstrated, immunosuppression can lead to significant enhancement of viral recovery and therapeutic effect, but the magnitude of complement-mediated viral inactivation has not been fully elucidated and warrants further investigation. Using fluorescent microscopy and quantitative plaque assays, we have determined complement's key role in viral clearance and its multi-faceted means to pathogen destruction. Complement can lead to direct viral destruction and inhibition of viral uptake into cells, even in the absence of anti-vaccinia antibodies. Our data demonstrate C5 to be integral to the clearance pathway, and its inhibition by Staphylococcal superantigen-like protein leads to a 90-fold and 150-fold enhancement of VV infectivity in both the presence and absence of anti-VV antibodies, respectively. This study suggests that complement inhibition may reduce vaccinia viral neutralization and may be critical to future in vivo work.

Unusual features of vaccinia virus extracellular virion form neutralization resistance revealed in human antibody responses to the smallpox vaccine

The extracellular virion form (EV) of vaccinia virus (VACV) is essential for viral pathogenesis and is difficult to neutralize with antibodies. Why this is the case and how the smallpox vaccine overcomes this challenge remain incompletely understood. We previously showed that high concentrations of anti-B5 antibodies are insufficient to directly neutralize EV (M. R. Benhnia, et al., J. Virol. 83:1201-1215, 2009). This allowed for at least two possible interpretations: covering the EV surface is insufficient for neutralization, or there are insufficient copies of B5 to allow anti-B5 IgG to cover the whole surface of EV and another viral receptor protein remains active. We endeavored to test these possibilities, focusing on the antibody responses elicited by immunization against smallpox. We tested whether human monoclonal antibodies (MAbs) against the three major EV antigens, B5, A33, and A56, could individually or together neutralize EV. While anti-B5 or anti-A33 (but not anti-A56) MAbs of appropriate isotypes were capable of neutralizing EV in the presence of complement, a mixture of anti-B5, anti-A33, and anti-A56 MAbs was incapable of directly neutralizing EV, even at high concentrations. This remained true when neutralizing the IHD-J strain, which lacks a functional version of the fourth and final known EV surface protein, A34. These immunological data are consistent with the possibility that viral proteins may not be the active component of the EV surface for target cell binding and infectivity. We conclude that the protection afforded by the smallpox vaccine anti-EV response is predominantly mediated not by direct neutralization but by isotype-dependent effector functions, such as complement recruitment for antibodies targeting B5 and A33.

A survey of host range genes in poxvirus genomes

Poxviruses are widespread pathogens, which display extremely different host ranges. Whereas some poxviruses, including variola virus, display narrow host ranges, others such as cowpox viruses naturally infect a wide range of mammals. The molecular basis for differences in host range are poorly understood but apparently depend on the successful manipulation of the host antiviral response. Some poxvirus genes have been shown to confer host tropism in experimental settings and are thus called host range factors. Identified host range genes include vaccinia virus K1L, K3L, E3L, B5R, C7L and SPI-1, cowpox virus CP77/CHOhr, ectromelia virus p28 and 022, and myxoma virus T2, T4, T5, 11L, 13L, 062R and 063R. These genes encode for ankyrin repeat-containing proteins, tumor necrosis factor receptor II homologs, apoptosis inhibitor T4-related proteins, Bcl-2-related proteins, pyrin domain-containing proteins, cellular serine protease inhibitors (serpins), short complement-like repeats containing proteins, KilA-N/RING domain-containing proteins, as well as inhibitors of the double-stranded RNA-activated protein kinase PKR. We conducted a systematic survey for the presence of known host range genes and closely related family members in poxvirus genomes, classified them into subgroups based on their phylogenetic relationship and correlated their presence with the poxvirus phylogeny. Common themes in the evolution of poxvirus host range genes are lineage-specific duplications and multiple independent inactivation events. Our analyses yield new insights into the evolution of poxvirus host range genes. Implications of our findings for poxvirus host range and virulence are discussed.

Crosstalk between immune cell and oncolytic vaccinia therapy enhances tumor trafficking and antitumor effects

The combination of an oncolytic virus, that directly destroys tumor cells and mediates an acute immune response, with an immune cell therapy, capable of further enlisting and enhancing the host immune response, has the potential to create a potent therapeutic effect. We have previously developed several strategies for optimizing the delivery of oncolytic vaccinia virus vectors to their tumor targets, including the use of immune cell-based carrier vehicles and the incorporation of mutations that increase production of the enveloped form of vaccinia (extracellular enveloped viral (EEV)) that is better adapted to spread within a host. Here, we initially combine these approaches to create a novel therapeutic, consisting of an immune cell (cytokine-induced killer, CIK) preloaded with an oncolytic virus that is EEV enhanced. This resulted in direct interaction between the viral and immune cell components with each assisting the other in directing the therapy to the tumor and so enhancing the antitumor effects. This effect could be further improved through CCL5 expression from the virus. The resulting multicomponent therapy displays the ability for synergistic crosstalk between components, so significantly enhancing tumor trafficking and antitumor effects.

Role of complement and perspectives for intervention in transplantation

The complement cascade is a major contributor to the innate immune response. It has now been well accepted that complement plays a critical role in hyperacute rejection and acute antibody-mediated rejection of transplanted organ. There is also increasing evidence that complement proteins contribute to the pathogenesis of organ ischemia-reperfusion injury, and even to cell-mediated rejection. Furthermore, the chemoattractants C3a and C5a and the terminal membrane attack complex that are generated by complement activation can directly or indirectly mediate tissue injury and trigger adaptive immune responses. Here, we review recent findings concerning the role of complement in graft ischemia-reperfusion injury, antibody-mediated rejection and accommodation, and cell-mediated rejection. We also discuss the current status of complement intervention therapies in clinical transplantation and describe potential new therapeutic strategies for clinical application.

Species selectivity in poxviral complement regulators is dictated by the charge reversal in the central complement control protein modules

Variola and vaccinia viruses, the two most important members of the family Poxviridae, are known to encode homologs of the human complement regulators named smallpox inhibitor of complement enzymes (SPICE) and vaccinia virus complement control protein (VCP), respectively, to subvert the host complement system. Intriguingly, consistent with the host tropism of these viruses, SPICE has been shown to be more human complement-specific than VCP, and in this study we show that VCP is more bovine complement-specific than SPICE. Based on mutagenesis and mechanistic studies, we suggest that the major determinant for the switch in species selectivity of SPICE and VCP is the presence of oppositely charged residues in the central complement control modules, which help enhance their interaction with factor I and C3b, the proteolytically cleaved form of C3. Thus, our results provide a molecular basis for the species selectivity in poxviral complement regulators.

Elucidating the role of the complement control protein in monkeypox pathogenicity

Monkeypox virus (MPXV) causes a smallpox-like disease in humans. Clinical and epidemiological studies provide evidence of pathogenicity differences between two geographically distinct monkeypox virus clades: the West African and Congo Basin. Genomic analysis of strains from both clades identified a ∼10 kbp deletion in the less virulent West African isolates sequenced to date. One absent open reading frame encodes the monkeypox virus homologue of the complement control protein (CCP). This modulatory protein prevents the initiation of both the classical and alternative pathways of complement activation. In monkeypox virus, CCP, also known as MOPICE, is a ∼24 kDa secretory protein with sequence homology to this superfamily of proteins. Here we investigate CCP expression and its role in monkeypox virulence and pathogenesis. CCP was incorporated into the West African strain and removed from the Congo Basin strain by homologous recombination. CCP expression phenotypes were confirmed for both wild type and recombinant monkeypox viruses and CCP activity was confirmed using a C4b binding assay. To characterize the disease, prairie dogs were intranasally infected and disease progression was monitored for 30 days. Removal of CCP from the Congo Basin strain reduced monkeypox disease morbidity and mortality, but did not significantly decrease viral load. The inclusion of CCP in the West African strain produced changes in disease manifestation, but had no apparent effect on disease-associated mortality. This study identifies CCP as an important immuno-modulatory protein in monkeypox pathogenesis but not solely responsible for the increased virulence seen within the Congo Basin clade of monkeypox virus.

Deletion of the monkeypox virus inhibitor of complement enzymes locus impacts the adaptive immune response to monkeypox virus in a nonhuman primate model of infection

Monkeypox virus (MPXV) is an orthopoxvirus closely related to variola virus, the causative agent of smallpox. Human MPXV infection results in a disease that is similar to smallpox and can also be fatal. Two clades of MPXV have been identified, with viruses of the central African clade displaying more pathogenic properties than those within the west African clade. The monkeypox inhibitor of complement enzymes (MOPICE), which is not expressed by viruses of the west African clade, has been hypothesized to be a main virulence factor responsible for increased pathogenic properties of central African strains of MPXV. To gain a better understanding of the role of MOPICE during MPXV-mediated disease, we compared the host adaptive immune response and disease severity following intrabronchial infection with MPXV-Zaire (n = 4), or a recombinant MPXV-Zaire (n = 4) lacking expression of MOPICE in rhesus macaques (RM). Data presented here demonstrate that infection of RM with MPXV leads to significant viral replication in the peripheral blood and lungs and results in the induction of a robust and sustained adaptive immune response against the virus. More importantly, we show that the loss of MOPICE expression results in enhanced viral replication in vivo, as well as a dampened adaptive immune response against MPXV. Taken together, these findings suggest that MOPICE modulates the anti-MPXV immune response and that this protein is not the sole virulence factor of the central African clade of MPXV.

An epitope conserved in orthopoxvirus A13 envelope protein is the target of neutralizing and protective antibodies

Primary immunization of humans with smallpox vaccine (live vaccinia virus (VACV)) consistently elicits antibody responses to six VACV virion membrane proteins, including A13. However, whether anti-A13 antibody contributes to immune protection against orthopoxviruses was unknown. Here, we isolated a murine monoclonal antibody (mAb) against A13 from a mouse that had been infected with VACV. The anti-A13 mAb bound to recombinant A13 protein with an affinity of 3.4 nM and neutralized VACV mature virions. Passive immunization of mice with the anti-A13 mAb protected against intranasal VACV infection. The epitope of the anti-A13 mAb was mapped to a 10-amino acid sequence conserved in all orthopoxviruses, including viriola virus and monkeypox virus, suggesting that anti-A13 antibodies elicited by smallpox vaccine might contribute to immune protection against orthopoxviruses. In addition, our data demonstrates that anti-A13 mAbs are effective for treating orthopoxvirus infection.

Disabling complement regulatory activities of vaccinia virus complement control protein reduces vaccinia virus pathogenicity

Poxviruses encode a repertoire of immunomodulatory proteins to thwart the host immune system. One among this array is a homolog of the host complement regulatory proteins that is conserved in various poxviruses including vaccinia (VACV) and variola. The vaccinia virus complement control protein (VCP), which inhibits complement by decaying the classical pathway C3-convertase (decay-accelerating activity), and by supporting inactivation of C3b and C4b by serine protease factor I (cofactor activity), was shown to play a role in viral pathogenesis. However, the role its individual complement regulatory activities impart in pathogenesis, have not yet been elucidated. Here, we have generated monoclonal antibodies (mAbs) that block the VCP functions and utilized them to evaluate the relative contribution of complement regulatory activities of VCP in viral pathogenesis by employing a rabbit intradermal model for VACV infection. Targeting VCP by mAbs that inhibited the decay-accelerating activity as well as cofactor activity of VCP or primarily the cofactor activity of VCP, by injecting them at the site of infection, significantly reduced VACV lesion size. This reduction however was not pronounced when VCP was targeted by a mAb that inhibited only the decay-accelerating activity. Further, the reduction in lesion size by mAbs was reversed when host complement was depleted by injecting cobra venom factor. Thus, our results suggest that targeting VCP by antibodies reduces VACV pathogenicity and that principally the cofactor activity of VCP appears to contribute to the virulence.

The vaccinia virus A56 protein: a multifunctional transmembrane glycoprotein that anchors two secreted viral proteins

The vaccinia virus A56 protein was one of the earliest-described poxvirus proteins with an identifiable activity. While originally characterized as a haemagglutinin protein, A56 has other functions as well. The A56 protein is capable of binding two viral proteins, a serine protease inhibitor (K2) and the vaccinia virus complement control protein (VCP), and anchoring them to the surface of infected cells. This is important; while both proteins have biologically relevant functions at the cell surface, neither one can locate there on its own. The A56-K2 complex reduces the amount of virus superinfecting an infected cell and also prevents the formation of syncytia by infected cells; the A56-VCP complex can protect infected cells from complement attack. Deletion of the A56R gene results in varying effects on vaccinia virus virulence. In addition, since the gene encoding the A56 protein is non-essential, it can be used as an insertion point for foreign genes and has been deleted in some viruses that are in clinical development as oncolytic agents.

Complement and viral pathogenesis

The complement system functions as an immune surveillance system that rapidly responds to infection. Activation of the complement system by specific recognition pathways triggers a protease cascade, generating cleavage products that function to eliminate pathogens, regulate inflammatory responses, and shape adaptive immune responses. However, when dysregulated, these powerful functions can become destructive and the complement system has been implicated as a pathogenic effector in numerous diseases, including infectious diseases. This review highlights recent discoveries that have identified critical roles for the complement system in the pathogenesis of viral infection.

Smallpox vaccines: targets of protective immunity

The eradication of smallpox, one of the great triumphs of medicine, was accomplished through the prophylactic administration of live vaccinia virus, a comparatively benign relative of variola virus, the causative agent of smallpox. Nevertheless, recent fears that variola virus may be used as a biological weapon together with the present susceptibility of unimmunized populations have spurred the development of new-generation vaccines that are safer than the original and can be produced by modern methods. Predicting the efficacy of such vaccines in the absence of human smallpox, however, depends on understanding the correlates of protection. This review outlines the biology of poxviruses with particular relevance to vaccine development, describes protein targets of humoral and cellular immunity, compares animal models of orthopoxvirus disease with human smallpox, and considers the status of second- and third-generation smallpox vaccines.

The Vaccinia virus complement control protein modulates adaptive immune responses during infection

Complement activation is an important component of the innate immune response against viral infection and also shapes adaptive immune responses. Despite compelling evidence that complement activation enhances T cell and antibody (Ab) responses during viral infection, it is unknown whether inhibition of complement by pathogens alters these responses. Vaccinia virus (VACV) modulates complement activation by encoding a complement regulatory protein called the vaccinia virus complement control protein (VCP). Although VCP has been described as a virulence factor, the mechanisms by which VCP enhances VACV pathogenesis have not been fully defined. Since complement is necessary for optimal adaptive immune responses to several viruses, we hypothesized that VCP contributes to pathogenesis by modulating anti-VACV T cell and Ab responses. In this study, we used an intradermal model of VACV infection to compare pathogenesis of wild-type virus (vv-VCPwt) and a virus lacking VCP (vv-VCPko). vv-VCPko formed smaller lesions in wild-type mice but not in complement-deficient mice. Attenuation of vv-VCPko correlated with increased accumulation of T cells at the site of infection, enhanced neutralizing antibody responses, and reduced viral titers. Importantly, depleting CD8(+) T cells together with CD4(+) T cells, which also eliminated T helper cell-dependent Ab responses, restored vv-VCPko to wild-type levels of virulence. These results suggest that VCP contributes to virulence by dampening both antibody and T cell responses. This work provides insight into how modulation of complement by poxviruses contributes to virulence and demonstrates that a pathogen-encoded complement regulatory protein can modulate adaptive immunity.

Capturing the natural diversity of the human antibody response against vaccinia virus

The eradication of smallpox (variola) and the subsequent cessation of routine vaccination have left modern society vulnerable to bioterrorism employing this devastating contagious disease. The existing, licensed vaccines based on live vaccinia virus (VACV) are contraindicated for a substantial number of people, and prophylactic vaccination of large populations is not reasonable when there is little risk of exposure. Consequently, there is an emerging need to develop efficient and safe therapeutics to be used shortly before or after exposure, either alone or in combination with vaccination. We have characterized the human antibody response to smallpox vaccine (VACV Lister) in immunized volunteers and isolated a large number of VACV-specific antibodies that recognize a variety of different VACV antigens. Using this broad antibody panel, we have generated a fully human, recombinant analogue to plasma-derived vaccinia immunoglobulin (VIG), which mirrors the diversity and specificity of the human antibody immune response and offers the advantage of unlimited supply and reproducible specificity and activity. The recombinant VIG was found to display a high specific binding activity toward VACV antigens, potent in vitro VACV neutralizing activity, and a highly protective efficacy against VACV challenge in the mouse tail lesion model when given either prophylactically or therapeutically. Altogether, the results suggest that this compound has the potential to be used as an effective postexposure prophylaxis or treatment of disease caused by orthopoxviruses.

Virus–complement interactions: an assiduous struggle for dominance

The complement system is a major component of the innate immune system that recognizes invading pathogens and eliminates them by means of an array of effector mechanisms, in addition to using direct lytic destruction. Viruses, in spite of their small size and simple composition, are also deftly recognized and neutralized by the complement system. In turn, as a result of years of coevolution with the host, viruses have developed multiple mechanisms to evade the host complement. These complex interactions between the complement system and viruses have been an area of focus for over three decades. In this article, we provide a broad overview of the field using key examples and up-to-date information on the complement-evasion strategies of viruses.

Ectromelia virus inhibitor of complement enzymes protects intracellular mature virus and infected cells from mouse complement

Poxviruses produce complement regulatory proteins to subvert the host's immune response. Similar to the human pathogen variola virus, ectromelia virus has a limited host range and provides a mouse model where the virus and the host's immune response have coevolved. We previously demonstrated that multiple components (C3, C4, and factor B) of the classical and alternative pathways are required to survive ectromelia virus infection. Complement's role in the innate and adaptive immune responses likely drove the evolution of a virus-encoded virulence factor that regulates complement activation. In this study, we characterized the ectromelia virus inhibitor of complement enzymes (EMICE). Recombinant EMICE regulated complement activation on the surface of CHO cells, and it protected complement-sensitive intracellular mature virions (IMV) from neutralization in vitro. It accomplished this by serving as a cofactor for the inactivation of C3b and C4b and by dissociating the catalytic domain of the classical pathway C3 convertase. Infected murine cells initiated synthesis of EMICE within 4 to 6 h postinoculation. The levels were sufficient in the supernatant to protect the IMV, upon release, from complement-mediated neutralization. EMICE on the surface of infected murine cells also reduced complement activation by the alternative pathway. In contrast, classical pathway activation by high-titer antibody overwhelmed EMICE's regulatory capacity. These results suggest that EMICE's role is early during infection when it counteracts the innate immune response. In summary, ectromelia virus produced EMICE within a few hours of an infection, and EMICE in turn decreased complement activation on IMV and infected cells.