Natural immunity to Sindbis virus is influenced by host tissue sialic acid content

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.

The α2,3-sialyltransferase encoded by myxoma virus is a virulence factor that contributes to immunosuppression

Myxoma virus (MYXV) induces a lethal disease called Myxomatosis in European rabbits. MYXV is one of the rare viruses that encodes an α2,3-sialyltransferase through its M138L gene. In this study, we showed that although the absence of the enzyme was not associated with any in vitro deficit, the M138L deficient strains are highly attenuated in vivo. Indeed, while all rabbits infected with the parental and the revertant strains died within 9 days post-infection from severe myxomatosis, all but one rabbit inoculated with the M138L deficient strains survived the infection. In primary lesions, this resistance to the infection was associated with an increased ability of innate immune cells, mostly neutrophils, to migrate to the site of virus replication at 4 days post-infection. This was followed by the development of a better specific immune response against MYXV. Indeed, at day 9 post-infection, we observed an important proliferation of lymphocytes and an intense congestion of blood vessels in lymph nodes after M138L knockouts infection. Accordingly, in these rabbits, we observed an intense mononuclear cell infiltration throughout the dermis in primary lesions and higher titers of neutralizing antibodies. Finally, this adaptive immune response provided protection to these surviving rabbits against a challenge with the MYXV WT strain. Altogether, these results show that expression of the M138L gene contributes directly or indirectly to immune evasion by MYXV. In the future, these results could help us to better understand the pathogenesis of myxomatosis but also the importance of glycans in regulation of immune responses.

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.

Modulation of cellular tropism and innate antiviral response by viral glycans

Arthropod-borne viruses (arboviruses) are a significant cause of human and animal disease worldwide. Multiple interactions between virus and the host innate immune system ultimately determine the pathogenesis and clinical outcome of the infection. Evidence is rapidly emerging that suggests viral glycans play a key role in viral pathogenesis by regulating host cell tropism and interactions with the host innate immune response. Glycan-mediated interactions are especially important for arboviruses which must adapt to variable glycosylation systems and cellular receptors within both vertebrate and invertebrate hosts. This review focuses on emerging evidence which supports a crucial role for viral glycans in mediating host cell tropism and regulating the innate antiviral response.

Glycosyltransferases encoded by viruses

Studies of cellular biology in recent decades have highlighted the crucial roles of glycans in numerous important biological processes, raising the concept of glycomics that is now considered as important as genomics, transcriptomics and proteomics. For millions of years, viruses have been co-evolving with their hosts. Consequently, during this co-evolution process, viruses have acquired mechanisms to mimic, hijack or sabotage host processes that favour their replication, including mechanisms to modify the glycome. The importance of the glycome in the regulation of host–virus interactions has recently led to a new concept called ‘glycovirology’. One fascinating aspect of glycovirology is the study of how viruses affect the glycome. Viruses reach that goal either by regulating expression of host glycosyltransferases or by expressing their own glycosyltransferases. This review describes all virally encoded glycosyltransferases and discusses their established or putative functions. The description of these enzymes illustrates several intriguing aspects of virology and provides further support for the importance of glycomics in biological processes.

The core 2 β-1,6-N-acetylglucosaminyltransferase-M encoded by bovine herpesvirus 4 is not essential for virus replication despite contributing to post-translational modifications of structural proteins

The Bo17 gene of bovine herpesvirus 4 (BoHV-4) is the only virus gene known to date that encodes a homologue of the cellular core 2β-1,6-N-acetylglucosaminyltransferase-mucine type (C2GnT-M). Recently, our phylogenetic study revealed that the Bo17 gene has been acquired from an ancestor of the African buffalo around 1·5 million years ago. Despite this recent origin, the Bo17 sequence has spread to fixation in the virus population possibly by natural selection. Supporting the latter hypothesis, it has been shown by our group for the V. test strain that Bo17 is expressed during BoHV-4 replicationin vitro, and that Bo17 expression product (pBo17) has all three enzymic activities exhibited by cellular C2GnT-M, i.e. core 2, core 4 and I branching activities. In the present study, firstly it was investigated whether encoding a functional C2GnT-M is a general property of BoHV-4 strains. Analysis of nine representative strains of the BoHV-4 species revealed that all of them express the Bo17 gene and the associated core 2 branching activity during virus replicationin vitro. Secondly, in order to investigate the roles of Bo17, its kinetic class of expression was analysed and a deleted recombinant strain was produced. These experiments revealed that Bo17 is expressed as an early gene which is not essential for virus replicationin vitro. However, comparison of the structural proteins, produced by the wild-type, the revertant and the deleted viruses, by 2D gels demonstrated that pBo17 contributes to the post-translational modifications of structural proteins. Possible roles of Bo17in vivoare discussed.

Virus complement evasion strategies

The immune system has a variety of tools at its disposal to combat virus infections. These can be subdivided roughly into two categories: 'first line defence', consisting of the non-specific, innate immune system, and 'adaptive immune response', acquired over time following virus infection or vaccination. During evolution, viruses have developed numerous, and often very ingenious, strategies to counteract efficient recognition of virions or virus-infected cells by both innate and adaptive immunity. This review will focus on the different strategies that viruses use to avoid recognition by one of the components of the immune system: the complement system. Complement evasion is of particular importance for viruses, since complement activation is a crucial component of innate immunity (alternative and mannan-binding lectin activation pathway) as well as of adaptive immunity (classical, antibody-dependent complement activation).

Host recognition and target differentiation by factor H, a regulator of the alternative pathway of complement

Factor H is responsible for recognition of host cells and tissues and mediates discrimination among microbial pathogens during activation of the alternative pathway of complement (AP). Its unique structure of 20 SCR domains arranged in a flexible chain permits a variety of functional sites to interact with complement proteins and surface markers in a biological example of single-molecule combinatorial chemistry. In addition to the complement regulatory site located in the N-terminal four SCR domains, two other sites bind complement protein C3b and three sites appear to recognize a variety of polyanions that serve as host markers. Recent studies indicate that cooperativity among several C3b- and polyanion-binding sites influences the biological functions of factor H and that the degree of influence of each site varies on different cells. The engagement of one or more of the host marker recognition sites enables factor H to control activation of the AP. The absence of host-like markers allows AP activation, but many common pathogens have developed receptors for factor H or mimics of host markers of varying degrees of authenticity allowing them to escape detection by this innate defense system. Organisms using one or more of these evasive techniques include Neisseria gonorrhoeae, Streptococcus pyogenes, Yersinia enterocolitica, Trypanosoma cruzi, and the HIV virus.

The contrasting mechanisms of serum resistance of Neisseria gonorrhoeae and group B Neisseria meningitidis

Neisseria gonorrhoeae and Neisseria meningitidis have evolved intricate mechanisms to evade complement-mediated killing. Sialylation of gonococcal lipooligosaccharide (LOS) results in conversion of previously serum sensitive strains to unstable serum resistance, which is mediated by factor H binding. Porin (Por) is also instrumental in mediating stable serum resistance in gonococci. The 5th loop of certain gonococcal PorlAs binds factor H, which efficiently inactivates C3b to iC3b. Factor H glycan residues may be essential for factor H binding to certain Por1A strains. Por1A strains can also regulate the classical pathway by binding to C4b-binding protein (C4bp) probably via the 1st loop of the Por molecule. Certain serum resistant Por1 B strains can also regulate complement by binding C4bp through a loop other than loop 1. Purified C4b can inhibit binding of C4bp to Por 1B, but not Por1A, suggesting different binding sites on C4bp for the two Por types. Unlike serum resistant gonococci, resistant meningococci have abundant C3b on their surface, which is only partially processed to iC3b. The main mechanism of complement evasion by group B meningococci is inhibition of membrane attack complex (MAC) insertion by their polysaccharide capsule. LOS structure may act in concert with capsule to prevent MAC insertion. Meningococcal strains with Class 3 Por preferentially bind factor H, suggesting Class 3 Por acts as a receptor for factor H.

A novel sialic acid binding site on factor H mediates serum resistance of sialylated Neisseria gonorrhoeae

Factor H (fH), a key alternative complement pathway regulator, is a cofactor for factor I-mediated cleavage of C3b. fH consists of 20 short consensus repeat (SCR) domains. Sialic acid binding domains have previously been localized to fH SCRs 6-10 and 13. To examine fH binding on a sialylated microbial surface, we grew Neisseria gonorrhoeae in the presence of 5'-cytidinemonophospho-N-acetylneuraminic acid, which sialylates lipooligosaccharide and converts to serum resistance gonococci previously sensitive to nonimmune serum killing. fH domains necessary for binding sialylated gonococci were determined by incubating organisms with recombinant human fH (rH) and nine mutant rH molecules (deletions spanning the entire fH molecule). rH and all mutant rH molecules that contained SCRs 16-20 bound to the sialylated strain; no mutant molecule bound to serum-sensitive nonsialylated organisms. Sialic acid was demonstrated to be the fH target by flow cytometry that showed a fourfold increase in fH binding that was reversed by neuraminidase-mediated cleavage of sialic acid off gonococci. Functional specificity of fH was confirmed by decreased total C3 binding and almost complete conversion to iC3b on sialylated gonococci. Sialic acid can therefore bind fH uniquely through SCRs 16-20. This blocks complement pathway activation for N. gonorrhoeae at the level of C3.

The alphaviruses: gene expression, replication, and evolution

The alphaviruses are a genus of 26 enveloped viruses that cause disease in humans and domestic animals. Mosquitoes or other hematophagous arthropods serve as vectors for these viruses. The complete sequences of the +/- 11.7-kb plus-strand RNA genomes of eight alphaviruses have been determined, and partial sequences are known for several others; this has made possible evolutionary comparisons between different alphaviruses as well as comparisons of this group of viruses with other animal and plant viruses. Full-length cDNA clones from which infectious RNA can be recovered have been constructed for four alphaviruses; these clones have facilitated many molecular genetic studies as well as the development of these viruses as expression vectors. From these and studies involving biochemical approaches, many details of the replication cycle of the alphaviruses are known. The interactions of the viruses with host cells and host organisms have been exclusively studied, and the molecular basis of virulence and recovery from viral infection have been addressed in a large number of recent papers. The structure of the viruses has been determined to about 2.5 nm, making them the best-characterized enveloped virus to date. Because of the wealth of data that has appeared, these viruses represent a well-characterized system that tell us much about the evolution of RNA viruses, their replication, and their interactions with their hosts. This review summarizes our current knowledge of this group of viruses.

The alphaviruses: gene expression, replication, and evolution

The alphaviruses are a genus of 26 enveloped viruses that cause disease in humans and domestic animals. Mosquitoes or other hematophagous arthropods serve as vectors for these viruses. The complete sequences of the +/- 11.7-kb plus-strand RNA genomes of eight alphaviruses have been determined, and partial sequences are known for several others; this has made possible evolutionary comparisons between different alphaviruses as well as comparisons of this group of viruses with other animal and plant viruses. Full-length cDNA clones from which infectious RNA can be recovered have been constructed for four alphaviruses; these clones have facilitated many molecular genetic studies as well as the development of these viruses as expression vectors. From these and studies involving biochemical approaches, many details of the replication cycle of the alphaviruses are known. The interactions of the viruses with host cells and host organisms have been exclusively studied, and the molecular basis of virulence and recovery from viral infection have been addressed in a large number of recent papers. The structure of the viruses has been determined to about 2.5 nm, making them the best-characterized enveloped virus to date. Because of the wealth of data that has appeared, these viruses represent a well-characterized system that tell us much about the evolution of RNA viruses, their replication, and their interactions with their hosts. This review summarizes our current knowledge of this group of viruses.

Activation of the alternative complement pathway by mumps infected cells: relationship to viral neuraminidase activity

An inverse relationship exists between the sialic acid content of a particle and its ability to activate the alternative complement pathway. The present studies were performed to determine if the neuraminidase (NANase) activities of different mumps virus strains could influence the ability of mumps virus infected cells to activate the alternative pathway. CV-1 cells were infected with three different mumps virus strains (RW, O'Take, and Kilham) and after 24 hours, 10 percent guinea pig serum (GPS) treated with EGTA/MgCl2 or GPS lacking the 4th component of complement (C4DGPS) was added to the cell monolayers. After 30 minutes, the percentage C3 consumed was determined by a functional hemolytic assay. Cells infected with RW (high NANase) consumed significantly more C3 (23.2 per cent) than cells infected with Kilham (5.7 percent, low NANase). Cells infected with O'Take were intermediate in their ability to activate C3. The degree of C3 deposition on the surface of infected cells, detected by fluorescence microscopy, was also greater for cells infected with the RW than the Kilham strain of mumps virus. These studies suggest that the NANase activity of mumps virus can influence the ability of infected cells to activate the alternative pathway and thereby, the ability of complement to participate in host defense against mumps virus infection.