Herpes simplex virus glycoproteins gC-1 and gC-2 bind to the third component of complement and provide protection against complement-mediated neutralization of viral infectivity

Dual impacts of a glycan shield on the envelope glycoprotein B of HSV-1: evasion from human antibodies in vivo and neurovirulence

Identification of the mechanisms of viral evasion from human antibodies is crucial both for understanding viral pathogenesis and for designing effective vaccines. Here we show in cell cultures that an N-glycan shield on the herpes simplex virus 1 (HSV-1) envelope glycoprotein B (gB) mediated evasion from neutralization and antibody-dependent cellular cytotoxicity due to pooled γ-globulins derived from human blood. We also demonstrated that the presence of human γ-globulins in mice and immunity to HSV-1 induced by viral infection in mice significantly reduced replication in their eyes of a mutant virus lacking the glycosylation site but had little effect on the replication of its repaired virus. These results suggest that an N-glycan shield on a specific site of HSV-1 envelope gB mediated evasion from human antibodies in vivo and from HSV-1 immunity induced by viral infection in vivo. Notably, we also found that an N-glycan shield on a specific site of HSV-1 gB was significant for HSV-1 neurovirulence and replication in the central nervous system of naïve mice. Thus, we have identified a critical N-glycan shield on HSV-1 gB that has dual impacts, namely evasion from human antibodies in vivo and viral neurovirulence. IMPORTANCE Herpes simplex virus 1 (HSV-1) establishes lifelong latent and recurrent infections in humans. To produce recurrent infections that contribute to transmission of the virus to new human host(s), the virus must be able to evade the antibodies persisting in latently infected individuals. Here, we show that an N-glycan shield on the specific site of the envelope glycoprotein B (gB) of HSV-1 mediates evasion from pooled γ-globulins derived from human blood both in cell cultures and mice. Notably, the N-glycan shield on the specific site of gB was also significant for HSV-1 neurovirulence in naïve mice. Considering the clinical features of HSV-1 infection, these results suggest that the glycan shield not only facilitates recurrent HSV-1 infections in latently infected humans by evading antibodies but is also important for HSV-1 pathogenesis during the initial infection.

Herpes Simplex Virus 1 Infection of Neuronal and Non-Neuronal Cells Elicits Specific Innate Immune Responses and Immune Evasion Mechanisms

Alphaherpesviruses (α-HV) are a large family of double-stranded DNA viruses which cause many human and animal diseases. There are three human α-HVs: Herpes Simplex Viruses (HSV-1 and HSV-2) and Varicella Zoster Virus (VZV). All α-HV have evolved multiple strategies to suppress or exploit host cell innate immune signaling pathways to aid in their infections. All α-HVs initially infect epithelial cells (primary site of infection), and later spread to infect innervating sensory neurons. As with all herpesviruses, α-HVs have both a lytic (productive) and latent (dormant) stage of infection. During the lytic stage, the virus rapidly replicates in epithelial cells before it is cleared by the immune system. In contrast, latent infection in host neurons is a life-long infection. Upon infection of mucosal epithelial cells, herpesviruses immediately employ a variety of cellular mechanisms to evade host detection during active replication. Next, infectious viral progeny bud from infected cells and fuse to neuronal axonal terminals. Here, the nucleocapsid is transported via sensory neuron axons to the ganglion cell body, where latency is established until viral reactivation. This review will primarily focus on how HSV-1 induces various innate immune responses, including host cell recognition of viral constituents by pattern-recognition receptors (PRRs), induction of IFN-mediated immune responses involving toll-like receptor (TLR) signaling pathways, and cyclic GMP-AMP synthase stimulator of interferon genes (cGAS-STING). This review focuses on these pathways along with other mechanisms including autophagy and the complement system. We will summarize and discuss recent evidence which has revealed how HSV-1 is able to manipulate and evade host antiviral innate immune responses both in neuronal (sensory neurons of the trigeminal ganglia) and non-neuronal (epithelial) cells. Understanding the innate immune response mechanisms triggered by HSV-1 infection, and the mechanisms of innate immune evasion, will impact the development of future therapeutic treatments.

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.

Alphaherpesvirus Vaccines

Prophylactic and therapeutic vaccines for the alphaherpesviruses including varicella zoster virus (VZV) and herpes simplex virus types 1 and 2 have been the focus of enormous preclinical and clinical research. A live viral vaccine for prevention of chickenpox and a subunit therapeutic vaccine to prevent zoster are highly successful. In contrast, progress towards the development of effective prophylactic or therapeutic vaccines against HSV-1 and HSV-2 has met with limited success. This review provides an overview of the successes and failures, the different types of immune responses elicited by various vaccine modalities, and the need to reconsider the preclinical models and immune correlates of protection against HSV.

Interactions of viruses and the humoral innate immune response

The innate immune response is crucial for defense against virus infections where the complement system, coagulation cascade and natural antibodies play key roles. These immune components are interconnected in an intricate network and are tightly regulated to maintain homeostasis and avoid uncontrolled immune responses. Many viruses in turn have evolved to modulate these interactions through various strategies to evade innate immune activation. This review summarizes the current understanding on viral strategies to inhibit the activation of complement and coagulation cascades, evade natural antibody-mediated clearance and utilize complement regulatory mechanisms to their advantage.

Herpes simplex encephalitis in adult patients with MASP-2 deficiency

We report here two cases of Herpes simplex virus encephalitis (HSE) in adult patients with very rare, previously uncharacterized, non synonymous heterozygous G634R and R203W substitution in mannan-binding lectin serine protease 2 (MASP2), a gene encoding a key protease of the lectin pathway of the complement system. None of the 2 patients had variants in genes involved in the TLR3-interferon signaling pathway. Both MASP2 variants induced functional defects in vitro, including a reduced (R203W) or abolished (G634R) protein secretion, a lost capability to cleave MASP-2 precursor into its active form (G634R) and an in vivo reduced antiviral activity (G634R). In a murine model of HSE, animals deficient in mannose binding lectins (MBL, the main pattern recognition molecule associated with MASP-2) had a decreased survival rate and an increased brain burden of HSV-1 compared to WT C57BL/6J mice. Altogether, these data suggest that MASP-2 deficiency can increase susceptibility to adult HSE.

Human herpes virus type 1 (HSV-1) infects a large number of individuals during their life, with manifestations usually limited to mild and self-limiting inflammation of the oral mucosa (cold sore). However, HSV-1 can cause a very severe disease of the brain called Herpes simplex encephalitis (HSE) in 1 out of 250’000–500’000 individuals per year. The reasons why HSV-1 can cause such a devastating disease in a very limited number of individuals are unknown. Increasing evidence suggests that susceptibility to HSE in children can results from genetic variations in the immune system, in particular in a viral detection pathway called the Toll-like receptor 3 (TLR3)–interferon (IFN) axis. Fewer data are available to explain HSE in adult patients. Here, we describe two adult patients with HSE who carry mutations in a gene called mannan-binding lectin serine protease 2 (MASP2), which is part of an immune pathway different from the TLR3-IFN axis, called the lectin pathway of the complement system. We demonstrate that MASP2 mutations induce functional defects in immune defense against HSV-1 that prevent viral replication. Mice deficient in the lectin pathway have higher mortality compared to wild-type mice after HSV-1 infection. Altogether, our study suggests that susceptibility to HSE in adults relies of immune deficiencies that are different from those causing HSE in children.

Complement and CD4+ T cells drive context-specific corneal sensory neuropathy

Whether complement dysregulation directly contributes to the pathogenesis of peripheral nervous system diseases, including sensory neuropathies, is unclear. We addressed this important question in a mouse model of ocular HSV-1 infection, where sensory nerve damage is a common clinical problem. Through genetic and pharmacologic targeting, we uncovered a central role for C3 in sensory nerve damage at the morphological and functional levels. Interestingly, CD4 T cells were central in facilitating this complement-mediated damage. This same C3/CD4 T cell axis triggered corneal sensory nerve damage in a mouse model of ocular graft-versus-host disease (GVHD). However, this was not the case in a T-dependent allergic eye disease (AED) model, suggesting that this inflammatory neuroimmune pathology is specific to certain disease etiologies. Collectively, these findings uncover a central role for complement in CD4 T cell-dependent corneal nerve damage in multiple disease settings and indicate the possibility for complement-targeted therapeutics to mitigate sensory neuropathies.

Herpes Simplex Virus Evasion of Early Host Antiviral Responses

Herpes simplex viruses type 1 (HSV-1) and type 2 (HSV-2) have co-evolved with humans for thousands of years and are present at a high prevalence in the population worldwide. HSV infections are responsible for several illnesses including skin and mucosal lesions, blindness and even life-threatening encephalitis in both, immunocompetent and immunocompromised individuals of all ages. Therefore, diseases caused by HSVs represent significant public health burdens. Similar to other herpesviruses, HSV-1 and HSV-2 produce lifelong infections in the host by establishing latency in neurons and sporadically reactivating from these cells, eliciting recurrences that are accompanied by viral shedding in both, symptomatic and asymptomatic individuals. The ability of HSVs to persist and recur in otherwise healthy individuals is likely given by the numerous virulence factors that these viruses have evolved to evade host antiviral responses. Here, we review and discuss molecular mechanisms used by HSVs to evade early innate antiviral responses, which are the first lines of defense against these viruses. A comprehensive understanding of how HSVs evade host early antiviral responses could contribute to the development of novel therapies and vaccines to counteract these viruses.

Interindividual Spread of Herpesviruses

Interindividual spread of herpesviruses is essential for the virus life cycle and maintenance in host populations. For most herpesviruses, the virus-host relationship is close, having coevolved over millions of years resulting in comparatively high species specificity. The mechanisms governing interindividual spread or horizontal transmission are very complex, involving conserved herpesviral and cellular proteins during the attachment, entry, replication, and egress processes of infection. Also likely, specific herpesviruses have evolved unique viral and cellular interactions during cospeciation that are dependent on their relationship. Multiple steps are required for interindividual spread including virus assembly in infected cells; release into the environment, followed by virus attachment; and entry into new hosts. Should any of these steps be compromised, transmission is rendered impossible. This review will focus mainly on the natural virus-host model of Marek's disease virus (MDV) in chickens in order to delineate important steps during interindividual spread.

Complement Opsonization Promotes Herpes Simplex Virus 2 Infection of Human Dendritic Cells

Herpes simplex virus 2 (HSV-2) is one of the most common sexually transmitted infections globally, with a very high prevalence in many countries. During HSV-2 infection, viral particles become coated with complement proteins and antibodies, both present in genital fluids, which could influence the activation of immune responses. In genital mucosa, the primary target cells for HSV-2 infection are epithelial cells, but resident immune cells, such as dendritic cells (DCs), are also infected. DCs are the activators of the ensuing immune responses directed against HSV-2, and the aim of this study was to examine the effects opsonization of HSV-2, either with complement alone or with complement and antibodies, had on the infection of immature DCs and their ability to mount inflammatory and antiviral responses. Complement opsonization of HSV-2 enhanced both the direct infection of immature DCs and their production of new infectious viral particles. The enhanced infection required activation of the complement cascade and functional complement receptor 3. Furthermore, HSV-2 infection of DCs required endocytosis of viral particles and their delivery into an acid endosomal compartment. The presence of complement in combination with HSV-1- or HSV-2-specific antibodies more or less abolished HSV-2 infection of DCs. Our results clearly demonstrate the importance of studying HSV-2 infection under conditions that ensue in vivo, i.e., conditions under which the virions are covered in complement fragments and complement fragments and antibodies, as these shape the infection and the subsequent immune response and need to be further elucidated.

IMPORTANCE During HSV-2 infection, viral particles should become coated with complement proteins and antibodies, both present in genital fluids, which could influence the activation of the immune responses. The dendritic cells are activators of the immune responses directed against HSV-2, and the aim of this study was to examine the effects of complement alone or complement and antibodies on HSV-2 infection of dendritic cells and their ability to mount inflammatory and antiviral responses. Our results demonstrate that the presence of antibodies and complement in the genital environment can influence HSV-2 infection under in vitro conditions that reflect the in vivo situation. We believe that our findings are highly relevant for the understanding of HSV-2 pathogenesis.

Modulation of host CD59 expression by varicella-zoster virus in human xenografts in vivo

Varicella-zoster virus (VZV) is the causative agent of both chickenpox (varicella) and shingles (zoster). VZV survives host defenses, even with an intact immune system, and disseminates in the host before causing disease. To date, several diverse immunomodulatory strategies used by VZV to undermine host immunity have been identified; however, few studies have addressed the complement evasion strategies used by this virus. Here, we show that expression of CD59, which is a key member of host regulators of complement activation (RCA), is significantly upregulated in response to VZV infection in human T cells and dorsal root ganglia (DRG) but not in human skin xenografts in SCID-hu mice in vivo. This is the first report demonstrating that VZV infection upregulates host CD59 expression in a tissue-specific manner in vivo, which may aid VZV in complement evasion and pathogenesis.

Higher Throughput Quantification of Neutralizing Antibody to Herpes Simplex Viruses

We report a rapid, higher throughput method for measuring neutralizing antibody to herpes simplex virus (HSV) in human sera. Clinical isolates and sera from the Herpevac Trial for Women were used in a colorimetric assay in which infection of tissue culture (lack of neutralization) was indicated by substrate metabolism by beta-galactosidase induced in the ELVIS cell line. The neutralization assay was optimized by addition of guinea pig complement, which particularly enhanced neutralizing antibody titers to HSV-2. Higher neutralizing antibody titers were also achieved using virus particles isolated from the supernatant of infected cells rather than lysate of infected cells as the source of virus. The effect of assay incubation time and incubation time with substrate were also optimized. We found that incubating with substrate until a standard optical density of 1.0 was reached permitted a better comparison among virus isolates, and achieved reliable measurement of neutralizing antibody activity. Interestingly, in contrast to results in the absence of complement, addition of complement allowed sera from HSV-2 gD-vaccinated subjects to neutralize HSV-1 and HSV-2 clinical and laboratory isolates with equal potency.

Porcine reproductive and respiratory syndrome (PRRS): an immune dysregulatory pandemic

Porcine reproductive and respiratory disease syndrome (PRRS) is a viral pandemic that especially affects neonates within the “critical window” of immunological development. PRRS was recognized in 1987 and within a few years became pandemic causing an estimated yearly $600,000 economic loss in the USA with comparative losses in most other countries. The causative agent is a single-stranded, positive-sense enveloped arterivirus (PRRSV) that infects macrophages and plasmacytoid dendritic cells. Despite the discovery of PRRSV in 1991 and the publication of >2,000 articles, the control of PRRS is problematic. Despite the large volume of literature on this disease, the cellular and molecular mechanisms describing how PRRSV dysregulates the host immune system are poorly understood. We know that PRRSV suppresses innate immunity and causes abnormal B cell proliferation and repertoire development, often lymphopenia and thymic atrophy. The PRRSV genome is highly diverse, rapidly evolving but amenable to the generation of many mutants and chimeric viruses for experimental studies. PRRSV only replicates in swine which adds to the experimental difficulty since no inbred well-defined animal models are available. In this article, we summarize current knowledge and apply it toward developing a series of provocative and testable hypotheses to explain how PRRSV immunomodulates the porcine immune system with the goal of adding new perspectives on this disease.

Equine herpesvirus 4: recent advances using BAC technology

The equine herpesviruses are major infectious pathogens that threaten equine health. Equine herpesvirus 4 (EHV-4) is an important equine pathogen that causes respiratory tract disease, known as rhinopneumonitis, among horses worldwide. EHV-4 genome manipulation with subsequent understanding of the viral gene functions has always been difficult due to the limited number of susceptible cell lines and the absence of small-animal models of the infection. Efficient generation of mutants of EHV-4 would significantly contribute to the rapid and accurate characterization of the viral genes. This problem has been solved recently by the cloning of the genome of EHV-4 as a stable and infectious bacterial artificial chromosome (BAC) without any deletions of the viral genes. Very low copy BAC vectors are the mainstay of present genomic research because of the high stability of inserted clones and the possibility of mutating any gene target in a relatively short time. Manipulation of EHV-4 genome is now feasible using the power of BAC technology, and should aid greatly in assessing the role of viral genes in the virus-host interaction.

Infections of people with complement deficiencies and patients who have undergone splenectomy

The complement system comprises several fluid-phase and membrane-associated proteins. Under physiological conditions, activation of the fluid-phase components of complement is maintained under tight control and complement activation occurs primarily on surfaces recognized as "nonself" in an attempt to minimize damage to bystander host cells. Membrane complement components act to limit complement activation on host cells or to facilitate uptake of antigens or microbes "tagged" with complement fragments. While this review focuses on the role of complement in infectious diseases, work over the past couple of decades has defined several important functions of complement distinct from that of combating infections. Activation of complement in the fluid phase can occur through the classical, lectin, or alternative pathway. Deficiencies of components of the classical pathway lead to the development of autoimmune disorders and predispose individuals to recurrent respiratory infections and infections caused by encapsulated organisms, including Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae. While no individual with complete mannan-binding lectin (MBL) deficiency has been identified, low MBL levels have been linked to predisposition to, or severity of, several diseases. It appears that MBL may play an important role in children, who have a relatively immature adaptive immune response. C3 is the point at which all complement pathways converge, and complete deficiency of C3 invariably leads to severe infections, including those caused by meningococci and pneumococci. Deficiencies of the alternative and terminal complement pathways result in an almost exclusive predisposition to invasive meningococcal disease. The spleen plays an important role in antigen processing and the production of antibodies. Splenic macrophages are critical in clearing opsonized encapsulated bacteria (such as pneumococci, meningococci, and Escherichia coli) and intraerythrocytic parasites such as those causing malaria and babesiosis, which explains the fulminant nature of these infections in persons with anatomic or functional asplenia. Paramount to the management of patients with complement deficiencies and asplenia is educating patients about their predisposition to infection and the importance of preventive immunizations and seeking prompt medical attention.

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.

A gammaherpesvirus complement regulatory protein promotes initiation of infection by activation of protein kinase Akt/PKB

Background

Viruses have evolved to evade the host's complement system. The open reading frames 4 (ORF4) of gammaherpesviruses encode homologs of regulators of complement activation (RCA) proteins, which inhibit complement activation at the level of C3 and C4 deposition. Besides complement regulation, these proteins are involved in heparan sulfate and glycosaminoglycan binding, and in case of MHV-68, also in viral DNA synthesis in macrophages.

Methodology/Principal Findings

Here, we made use of MHV-68 to study the role of ORF4 during infection of fibroblasts. While attachment and penetration of virions lacking the RCA protein were not affected, we observed a delayed delivery of the viral genome to the nucleus of infected cells. Analysis of the phosphorylation status of a variety of kinases revealed a significant reduction in phosphorylation of the protein kinase Akt in cells infected with ORF4 mutant virus, when compared to cells infected with wt virus. Consistent with a role of Akt activation in initial stages of infection, inhibition of Akt signaling in wt virus infected cells resulted in a phenotype resembling the phenotype of the ORF4 mutant virus, and activation of Akt by addition of insulin partially reversed the phenotype of the ORF4 mutant virus. Importantly, the homologous ORF4 of KSHV was able to rescue the phenotype of the MHV-68 ORF4 mutant, indicating that ORF4 is functionally conserved and that ORF4 of KSHV might have a similar function in infection initiation.

Conclusions/Significance

In summary, our studies demonstrate that ORF4 contributes to efficient infection by activation of the protein kinase Akt and thus reveal a novel function of a gammaherpesvirus RCA protein.

Immunization with HSV-1 glycoprotein C prevents immune evasion from complement and enhances the efficacy of an HSV-1 glycoprotein D subunit vaccine

Herpes simplex virus type 1 (HSV-1) glycoprotein C (gC-1) binds complement component C3b and inhibits complement-mediated immunity. HSV-1 glycoprotein D (gD-1) is a potent immunogen and a candidate antigen for a subunit vaccine. We evaluated whether combined immunization with gD-1 and gC-1 provides better protection against challenge than gD-1 alone based on antibodies to gC-1 preventing HSV-1-mediated immune evasion. IgG purified from mice immunized with gC-1 blocked C3b binding to gC-1 and greatly increased neutralization by gD-1 IgG in the presence of complement. Passive transfer of gC-1 IgG protected complement intact mice against HSV-1 challenge but not C3 knockout mice, indicating that gC-1 antibody activity in vivo is complement-dependent. Immunizing mice with gD-1 and gC-1 provided better protection than gD-1 alone in preventing zosteriform disease and infection of dorsal root ganglia. Therefore, gC-1 immunization prevents HSV-1 evasion from complement and enhances the protection provided by gD-1 immunization.

Role of complement in immune regulation and its exploitation by virus

Complement is activated during the early phase of viral infection and promotes destruction of virus particles as well as the initiation of inflammatory responses. Recently, complement and complement receptors have been reported to play an important role in the regulation of innate as well as adaptive immune responses during infection. The regulation of host immune responses by complement involves modulation of dendritic cell activity in addition to direct effects on T-cell function. Intriguingly, many viruses encode homologs of complement regulatory molecules or proteins that interact with complement receptors on antigen-presenting cells and lymphocytes. The evolution of viral mechanisms to alter complement function may augment pathogen persistence and limit immune-mediated tissue destruction. These observations suggest that complement may play an important role in both innate and adaptive immune responses to infection as well as virus-mediated modulation of host immunity.

Complement is an essential component of the immune response to adeno-associated virus vectors

Adeno-associated virus (AAV) vectors are associated with relatively mild host immune responses in vivo. Although AAV induces very weak innate immune responses, neutralizing antibodies against the vector capsid and transgene still occur. To understand further the basis of the antiviral immune response to AAV vectors, studies were performed to characterize AAV interactions with macrophages. Primary mouse macrophages and human THP-1 cells transduced in vitro using an AAV serotype 2 (AAV2) vector encoding green fluorescent protein did not result in measurable transgene expression. An assessment of internalized vector genomes showed that AAV2 vector uptake was enhanced in the presence of normal but not heat-inactivated or C3-depleted mouse/human serum. Enhanced uptake in the presence of serum coincided with increased macrophage activation as determined by the expression of NF-kappaB-dependent genes such as macrophage inflammatory protein 2 (MIP-2), interleukin-1beta (IL-1beta), IL-8, and MIP-1beta. AAV vector serotypes 1 and 8 also activated human and mouse macrophages in a serum-dependent manner. Immunoprecipitation studies demonstrated the binding of iC3b complement protein to the AAV2 capsid in human serum. AAV2 did not activate the alternative pathway of the complement cascade and lacked cofactor activity for factor I-mediated degradation of C3b to iC3b. Instead, our results suggest that the AAV capsid also binds complement regulatory protein factor H. In vivo, complement receptor 1/2- and C3-deficient mice displayed impaired humoral immunity against AAV2 vectors, with a delay in antibody development and significantly lower neutralizing antibody titers. These results show that the complement system is an essential component of the host immune response to AAV.

Human herpesvirus 7 infection increases the expression levels of CD46 and CD59 in target cells

CD46 (membrane cofactor protein; MCP) is a molecule that functions as either a complement-regulatory protein (CRP) or a receptor for some pathogens, including human herpesvirus 6. DNA microarray analysis suggested that the expression of CD46 was upregulated in T cells infected with human herpesvirus 7 (HHV-7). Northen and Western blot analyses supported this result at both the transcriptional and translational levels. Flow-cytometric analysis revealed that upregulation of CD46 occurred at a late stage of infection in both SupT1 cells and primary CD4+ T cells, and also that expression of another CRP, CD59, was increased at a late stage of infection. Whether these CRPs actually function in HHV-7-infected cells was addressed and it was found that HHV-7-infected cells were more resistant to complement-dependent cytotoxicity than were uninfected cells. This study is the first report demonstrating that HHV-7 infection causes elevation of the CRPs CD46 and CD59, which may be a possible mechanism for HHV-7 to evade humoral immunity via complement.

Herpes simplex virus type 1 and 2 glycoprotein C prevents complement-mediated neutralization induced by natural immunoglobulin M antibody

Glycoprotein C (gC) of herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) binds complement component C3b and protects virus from complement-mediated neutralization. Differences in complement interacting domains exist between gC of HSV-1 (gC1) and HSV-2 (gC2), since the amino terminus of gC1 blocks complement C5 from binding to C3b, while gC2 fails to interfere with this activity. We previously reported that neutralization of HSV-1 gC-null virus by HSV antibody-negative human serum requires activation of C5 but not of downstream components of the classical complement pathway. In this report, we evaluated whether activation of C5 is sufficient to neutralize HSV-2 gC-null virus, or whether formation of the membrane attack complex by C6 to C9 is required for neutralization. We found that activation of the classical complement pathway up to C5 was sufficient to neutralize HSV-2 gC-null virus by HSV antibody-negative human serum. We evaluated the mechanisms by which complement activation occurred in seronegative human serum. Interestingly, natural immunoglobulin M antibodies bound to virus, which triggered activation of C1q and the classical complement pathway. HSV antibody-negative sera obtained from four individuals differed over an approximately 10-fold range in their potency for complement-mediated virus neutralization. These findings indicate that humans differ in the ability of their innate immune systems to neutralize HSV-1 or HSV-2 gC-null virus and that a critical function of gC1 and gC2 is to prevent C5 activation.

The complete genome sequence of herpesvirus papio 2 (Cercopithecine herpesvirus 16) shows evidence of recombination events among various progenitor herpesviruses

We have sequenced the entire genome of herpesvirus papio 2 (HVP-2; Cercopithecine herpesvirus 16) strain X313, a baboon herpesvirus with close homology to other primate alphaherpesviruses, such as SA8, monkey B virus, and herpes simplex virus (HSV) type 1 and type 2. The genome of HVP-2 is 156,487 bp in length, with an overall GC content of 76.5%. The genome organization is identical to that of the other members of the genus Simplexvirus, with a long and a short unique region, each bordered by inverted repeats which end with an "a" sequence. All of the open reading frames detected in this genome were homologous and colinear with those of SA8 and B virus. The HSV gene RL1 (gamma(1)34.5; neurovirulence factor) is not present in HVP-2, as is the case for SA8 and B virus. The HVP-2 genome is 85% homologous to its closest relative, SA8. However, segment-by-segment bootstrap analysis of the genome revealed at least two regions that display closer homology to the corresponding sequences of B virus. The first region comprises the UL41 to UL44 genes, and the second region is located within the UL36 gene. We hypothesize that this localized and defined shift in homology is due to recombination events between an SA8-like progenitor of HVP-2 and a herpesvirus species more closely related to the B virus. Since some of the genes involved in these putative recombination events are determinants of virulence, a comparative analysis of their function may provide insight into the pathogenic mechanism of simplexviruses.

Implications for herpes simplex virus vaccine strategies based on antibodies produced to herpes simplex virus type 1 glycoprotein gC immune evasion domains

Herpes simplex virus type I (HSV-1) glycoprotein gC (gC-1) is an immune evasion molecule that inhibits complement activation by binding C3b. Three assays were used to assess whether IgG antibodies produced by HSV-1 infection in humans block the interaction between C3b and gC-1. In two assays human IgG had no effect, while in one assay IgG partially inhibited C3b binding, which occurred at IgG concentrations approaching the upper limits of those found in human serum. Mice infected with HSV-1 produced antibodies that partially blocked C3b binding at lower IgG concentrations than human IgG. Importantly, gC-1 immunization in mice produced higher titers of gC-1 antibodies than infection. We previously reported that gC-1 immunization in mice totally blocks C3b binding and reduces disease severity. Therefore, gC-1 immunization in humans may also induce blocking antibodies that modify disease, despite the rather limited ability of infection to produce these antibodies.

Pathogenic potentials of glycoprotein C-negative syncytial mutants from rabbit T cells infected persistently with herpes simplex virus type 1

Human T cell lymphotropic virus type I (HTLV-I)-transformed T cells of rabbits were infected persistently with Herpes simplex virus type 1 (HSV-1) strain KOS. These infected cells yielded syncytial mutants, either glycoprotein C (gC)-negative or -positive, which predominated over and replaced the wild-type virus in a long-term culture for 2 years. An alignment of nucleotide sequences showed multiple mutations in glycoprotein B (gB) and gC genes of these mutants, which are or may be responsible for the mutant phenotypes. One of four mutants analyzed produced extensively large syncytia and possessed point mutations within the cytoplasmic domain of gB. All four mutants possessed multiple point mutations in gC and two possessed single insertions which resulted in a frame shift, leading to the premature termination of the gC polypeptide chain. The supernatant of the 2-year culture of cells infected persistently, containing only gC-negative syncytial mutants, induced encephalitic symptoms in B/Jas inbred rabbits, when injected intravenously. One gC-negative syncytial isolate from an encephalitic lesion, together with those from the culture supernatant, were examined for pathogenic potential in vitro and in vivo. All these mutants were more cytotoxic and more susceptible to complement inactivation than the parental virus, and could infect and replicate in adrenal glands when injected intravenously into rabbits. Invasion into the central nervous system appeared to be blocked at the portal of entry, the adrenal gland, i.e., none exhibited neuroinvasive potential by itself. Syncytial gC-negative mutants could thus be pathogenic in rabbits.

Herpes simplex virus: receptors and ligands for cell entry

Entry of herpes simplex virus (HSV) into cells depends upon multiple cell surface receptors and multiple proteins on the surface of the virion. The cell surface receptors include heparan sulphate chains on cell surface proteoglycans, a member of the tumor necrosis factor (TNF) receptor family and two members of the immunoglobulin superfamily related to the poliovirus receptor. The HSV ligands for these receptors are the envelope glycoproteins gB and gC for heparan sulphate and gD for the protein receptors and specific sites in heparan sulphate generated by certain 3-O-sulfotransferases. HSV gC also binds to the C3b component of complement and can block complement-mediated neutralization of virus. The purposes of this review are to summarize available information about these cell surface receptors and the viral ligands, gC and gD, and to discuss roles of these viral glycoproteins in immune evasion and cellular responses as well as in viral entry.

Complement: structure, functions, evolution, and viral molecular mimicry

The complement (C') system has long been recognized as an important mediator of innate immune defense and inflammation. In recent years there is increasing evidence suggesting that complement components may also participate in non-inflammatory and developmental processes. Here we review our current work on the structural-functional aspects of C3-ligand interactions and the rational design of small-sized complement inhibitors. We present a novel, proteomics-based, approach to studying protein-protein interactions within the C' system and discuss our progress in the study of viral immune evasion strategies. Furthermore we discuss the involvement of complement proteins in organ regeneration and hematopoietic development.

Blocking immune evasion as a novel approach for prevention and treatment of herpes simplex virus infection

Many microorganisms encode immune evasion molecules to escape host defenses. Herpes simplex virus type 1 glycoprotein gC is an immunoevasin that inhibits complement activation by binding complement C3b. gC is expressed on the virus envelope and infected cell surface, which makes gC potentially accessible to blocking antibodies. Mice passively immunized with gC monoclonal antibodies prior to infection were protected against herpes simplex virus challenge only if the gC antibodies blocked C3b binding. Mice treated 1 or 2 days postinfection with gC monoclonal antibodies that block C3b binding had less severe disease than control mice treated with nonimmune immunoglobulin G (IgG). Mice immunized with gC protein produced antibodies that blocked C3b binding to gC. Immunized mice were significantly protected against challenge by wild-type virus, but not against a gC mutant virus lacking the C3b binding domain, suggesting that protection was mediated by antibodies that target the gC immune evasion domain. IgG and complement from subjects immunized with an experimental herpes simplex virus glycoprotein gD vaccine neutralized far more mutant virus defective in immune evasion than wild-type virus, supporting the importance of immune evasion molecules in reducing vaccine potency. These results suggest that it is possible to block immune evasion domains on herpes simplex virus and that this approach has therapeutic potential and may enhance vaccine efficacy.

Direct Binding of Herpes Simplex Virus Type 1 Virions to Complement C3

Glycoprotein C (gC) of type 1 herpes simplex virus (HSV-1) binds the human complement C3 as purified proteins, or when expressed on the surface of infected cells. However, it is not clear whether the purified HSV virion binds directly to C3. In this study, direct binding of purified virions, HSV-1(KOS) or HSV-1(hrR3), to C3-coated plate was demonstrated by an enzyme-linked immunosorbent assay (ELISA). Captured virions on C3-coated plates were still infectious as determined by adding Vero cells to allow for infection to occur. The binding of virions to C3 was abolished if C3 was heat-inactivated, confirming a requirement for complement. In addition, the interaction was inhibited by preincubation of purified virions with heparin. In conclusion, a direct interaction of C3 with the HSV-1 virions was demonstrated.

Cloning and expression of the complement receptor glycoprotein C from Herpesvirus simiae (herpes B virus): protection from complement-mediated cell lysis

Simian herpes B virus (SHBV) is the herpes simplex virus (HSV) homologue for the species MACACA: Unlike in its natural host, and unlike other animal herpesviruses, SHBV causes high mortality in accidentally infected humans. SHBV-infected cells, like those infected with HSV-1 and equine herpesvirus types 1 and 4, express complement C3 receptor activity. To study immunoregulatory functions involved in susceptibility/resistance against interspecies transmission, the SHBV glycoprotein C (gC(SHBV)) gene (encoding 467 aa) was isolated. Sequence analysis revealed amino acid identity with gC proteins from HSV-2 (46.9 %), HSV-1 (44.5 %) and pseudorabies virus (21.2 %). Highly conserved cysteine residues were also noted. Similar to gC(HSV-2), gC(SHBV) is less glycosylated than gC(HSV-1), resulting in a molecular mass of 65 kDa if expressed in replication-deficient vaccinia virus Ankara. Stable transfectants expressing full-length gC(SHBV) on the cell surface induced C3 receptor activity and were substantially protected from complement-mediated lysis; no protection was observed with control constructs. This suggests that expression of the gC homologues on infected cell surfaces might also contribute to the survival of infected cells in addition to decreased virion inactivation. Interestingly, soluble gC(SHBV) isolated from protein-free culture supernatants did not interfere with the binding of the alternative complement pathway activator properdin to C3b, which is similar to our findings with gC(HSV-2) and could be attributed to major differences in the amino-terminal portion of the protein with extended deletions in both gC(SHBV) and gC(HSV-2). Binding of recombinant gC(SHBV) to polysulphates was observed. This, together with the heparin-sensitivity of the gC(SHBV)-C3 interaction on the infected cell surface, suggests a role in adherence to heparan sulphate, similar to the gC proteins of other herpesviruses.

Complement regulation by Kaposi's sarcoma-associated herpesvirus ORF4 protein

Kaposi's sarcoma-associated herpesvirus (KSHV) is associated with three types of human tumor: Kaposi's sarcoma, multicentric Castleman's disease, and primary effusion lymphoma. The virus encodes a number of proteins that participate in disrupting the immune response, one of which was predicted by sequence analysis to be encoded by open reading frame 4 (ORF4). The predicted ORF4 protein shares homology with cellular proteins referred to as regulators of complement activation. In the present study, the transcription profile of the ORF4 gene was characterized, revealing that it encodes at least three transcripts, by alternative splicing mechanisms, and three protein isoforms. Functional studies revealed that each ORF4 protein isoform inhibits complement and retains a C-terminal transmembrane domain. Consistent with the complement-regulating activity, we propose to name the proteins encoded by the ORF4 gene collectively as KSHV complement control protein (KCP). KSHV ORF4 is the most complex alternatively spliced gene encoding a viral complement regulator described to date. KCP inhibits the complement component of the innate immune response, thereby possibly contributing to the in vivo persistence and pathogenesis of this virus.

Viral mimicry of the complement system

The complement system is a potent innate immune mechanism consisting of cascades of proteins which are designed to fight against and annul intrusion of all the foreign pathogens. Although viruses are smaller in size and have relatively simple structure, they are not immune to complement attack. Thus, activation of the complement system can lead to neutralization of cell-free viruses, phagocytosis of C3b-coated viral particles, lysis of virus-infected cells, and generation of inflammatory and specific immune responses. However, to combat host responses and succeed as pathogens, viruses not only have developed/adopted mechanisms to control complement, but also have turned these interactions to their own advantage. Important examples include poxviruses, herpesviruses, retroviruses, paramyxoviruses and picornaviruses. In this review, we provide information on the various complement evasion strategies that viruses have developed to thwart the complement attack of the host. A special emphasis is given on the interactions between the viral proteins that are involved in molecular mimicry and the complement system.

Antibody to a lytic cycle viral protein decreases gammaherpesvirus latency in B-cell-deficient mice

While antiviral antibody plays a key role in resistance to acute viral infection, the contribution of antibody to the control of latent virus infection is less well understood. Gammaherpesvirus 68 (gammaHV68) infection of mice provides a model well suited to defining contributions of specific immune system components to the control of viral latency. B cells play a critical role in regulating gammaHV68 latency, but the mechanism(s) by which B cells regulate latency is not known. In the experiments reported here, we determined the effect of passively transferred antibody on established gammaHV68 latency in B-cell-deficient (B-cell(-/-)) mice. Immune antibody decreased the frequency of cells reactivating ex vivo from latency in splenocytes (>10-fold) and peritoneal cells (>100-fold) and the frequency of cells carrying latent viral genome in splenocytes (>5-fold) and peritoneal cells (>50-fold). This effect required virus-specific antibody and was observed when total and virus-specific serum antibody concentrations in recipient B-cell(-/-) mice were <8% of those in normal mice during latent infection. Passive transfer of antibody specific for the lytic cycle gammaHV68 RCA protein, but not passive transfer of antibody specific for the v-cyclin protein or the latent protein M2, decreased both the frequency of cells reactivating ex vivo from latency and the frequency of cells carrying the latent viral genome. Therefore, antibody specific for lytic cycle viral antigens can play an important role in the control of gammaherpesvirus latency in immunocompromised hosts. Based on these findings, we propose a model in which ongoing productive replication is essential for maintaining high levels of latently infected cells in immunocompromised hosts. We confirmed this model by the treatment of latently infected B-cell(-/-) mice with the antiviral drug cidofovir.

Pseudorabies virus (PRV) is protected from complement attack by cellular factors and glycoprotein C (gC)

Swine kidney derived CPK cells were resistant to swine complement attack in vitro while rabbit kidney derived RK13 cells were destroyed by swine complement. To rabbit complement, RK13 cells were resistant but CPK cells were sensitive. This phenomenon was known as homologous restriction (Proc. Natl. Acad. Sci. USA 78 (1981) 5118). The gC deletion mutant of pseudorabies virus (PRVdlgC) grown in CPK cells was resistant to swine complement while the same virus grown in RK13 cells was neutralized by swine complement. PRVdlgC grown in RK13 cells was more resistant to rabbit complement than the virus grown in CPK cells. Hence, the sensitivity of PRVdlgC to swine or rabbit complement was similar to that of the cells in which the virus was grown. It would appear that cell derived factors were present on the virion and they were protective against homologous complement but not against heterologous complement. The expression of gC rendered PRV more resistant to swine or rabbit complement, but the protective effect of gC was much less than that of cell derived factors. The best protection against complement was obtained when gC and cell derived factors were coexistent on the virion.

Kinetic analysis of glycoprotein C of herpes simplex virus types 1 and 2 binding to heparin, heparan sulfate, and complement component C3b

Glycoprotein C (gC) from herpes simplex virus (HSV) facilitates virus entry by attaching the virion to host cell-surface heparan sulfate (HS). Although gC from HSV-1 (gC1) and from HSV-2 (gC2) bind to heparin, gC2 is believed to play a less significant role than gC1 in attachment of virus to cells. This attachment step is followed by the binding of gD to one of several cellular receptors. gC also plays an important role in immune evasion by binding to the C3b fragment of the third component of the host complement system. Yet, although both gC1 and gC2 protect HSV against complement-mediated neutralization, only gC on HSV-1-infected cells acts as a receptor for C3b. We used optical biosensor technology to quantitate the affinities (K(D)) and the stabilities (k(off)) between both serotypes of gC with heparin, HS, and C3b to address three questions concerning gC interactions. First, can differences in affinity or stability account for differences between the contributions of HSV-1 and HSV-2 gC in attachment? Our data show that the gC2-HS complex is highly unstable (k(off) = 0.2 s(-1)) compared to the gC1-HS complex (k(off) = 0.003 s(-1)), suggesting why gC2 may not play an important role in attachment of virus to cells as does gC1. Second, does gC2 have a lower affinity for C3b than does gC1, thereby explaining the lack of C3b-receptor activity on HSV-2 infected cells? Surprisingly, gC2 had a 10-fold higher affinity for C3b compared to gC1, so this functional difference in serotypes cannot be accounted for by affinity. Third, do differences in gC-HS and gD-receptor affinities support a model of HSV entry in which the gC-HS interaction is of lower affinity than the gD-receptor interaction? Our biosensor results indicate that gC has a higher affinity for HS than gD does for cellular receptors HveA (HVEM) and HveC (nectin-1).

Subversion and piracy: DNA viruses and immune evasion

During the co-evolution of viruses with their vertebrate hosts, the DNA viruses have acquired an impressive array of immunomodulatory genes to combat host immune responses and their hosts have developed a sophisticated immune system to contain virus infections. In order to replicate, the viruses have evolved mechanisms to inhibit key host anti-virus responses that include apoptosis, interferon production, chemokine production, inflammatory cytokine production, and the activity of cytotoxic T-cells, natural killer cells and antibody. In addition, some of the viruses encode cytokine or chemokine homologues that recruit or expand cell numbers for infection or that subvert the host cellular response from a protective response to a benign one. The specificity of the viral immunomodulatory molecules reflects the life cycle and the pathogenesis of the viruses. Herpesviruses achieve latency in host cells by inducing cell survival and protecting infected cells from immune recognition. This involves interference with cell signal transduction pathways. Many of the viral immunomodulatory proteins are homologues of host proteins that appear to have been pirated from the host and reassorted in the virus genomes. Some of these have unique functions and indicate novel or important aspects of both viral pathogenesis and host immunity to viruses. The specific example of orf virus infection of sheep is described.

Novel mechanism of antibody-independent complement neutralization of herpes simplex virus type 1

The envelope surface glycoprotein C (gC) of HSV-1 interferes with the complement cascade by binding C3 and activation products C3b, iC3b, and C3c, and by blocking the interaction of C5 and properdin with C3b. Wild-type HSV-1 is resistant to Ab-independent complement neutralization; however, HSV-1 mutant virus lacking gC is highly susceptible to complement resulting in > or =100-fold reduction in virus titer. We evaluated the mechanisms by which complement inhibits HSV-1 gC null virus to better understand how gC protects against complement-mediated neutralization. C8-depleted serum prepared from an HSV-1 and -2 Ab-negative donor neutralized gC null virus comparable to complement-intact serum, indicating that C8 and terminal lytic activity are not required. In contrast, C5-depleted serum from the same donor failed to neutralize gC null virus, supporting a requirement for C5. EDTA-treated serum did not neutralize gC null virus, indicating that complement activation is required. Factor D-depleted and C6-depleted sera neutralized virus, suggesting that the alternative complement pathway and complement components beyond C5 are not required. Complement did not aggregate virus or block attachment to cells. However, complement inhibited infection before early viral gene expression, indicating that complement affects one or more of the following steps in virus replication: virus entry, uncoating, DNA transport to the nucleus, or immediate early gene expression. Therefore, in the absence of gC, HSV-1 is readily inhibited by complement by a C5-dependent mechanism that does not require viral lysis, aggregation, or blocking virus attachment.

Herpes simplex virus types 1 and 2 differ in their interaction with heparan sulfate

Cell surface heparan sulfate (HS) serves as an initial receptor for many different viruses, including herpes simplex virus types 1 and 2 (HSV-1 and 2, respectively). Glycoproteins C and B (gC and gB) are the major components of the viral envelope that mediate binding to HS. In this study, purified gB and gC homologous proteins as well as purified HSV-1 and HSV-2 virions were compared for the ability to bind isolated HS receptor molecules. HSV-1 gC and HSV-2 gC bound comparable amounts of HS. Similarly, HSV-1 gB and its HSV-2 counterpart showed no difference in the HS-binding capabilities. Despite the similar HS-binding potentials of gB and gC homologs, HSV-1 virions bound more HS than HSV-2 particles. Purified gC and gB proteins differed with respect to sensitivity of their interaction with HS to increased concentrations of sodium chloride in the order gB-2 > gB-1 > gC-1 > gC-2. The corresponding pattern for binding of whole HSV virions to cells in the presence of increased ionic strength of the medium was HSV-2 gC-neg1 > HSV-1 gC(-)39 > HSV-1 KOS 321 > HSV-2 333. These results relate the HS-binding activities of individual glycoproteins with the cell-binding abilities of whole virus particles. In addition, these data suggest a greater contribution of electrostatic forces for binding of gB proteins and gC-negative mutants compared with binding of gC homologs and wild-type HSV strains. Binding of wild-type HSV-2 virions was the least sensitive to increased ionic strength of the medium, suggesting that the less extensive binding of HS molecules by HSV-2 than by HSV-1 can be compensated for by a relatively weak contribution of electrostatic forces to the binding. Furthermore, gB and gC homologs exhibited different patterns of sensitivity of binding to cells to inhibition with selectively N-, 2-O-, and 6-O-desulfated heparin compounds. The O-sulfate groups of heparin were found to be more important for interaction with gB-1 than gB-2. These results indicate that HSV-1 and HSV-2 differ in their interaction with HS.

Viral subversion of the immune system

This review describes the diverse array of pathways and molecular targets that are used by viruses to elude immune detection and destruction. These include targeting of pathways for major histocompatibility complex-restricted antigen presentation, apoptosis, cytokine-mediated signaling, and humoral immune responses. The continuous interactions between host and pathogens during their coevolution have shaped the immune system, but also the counter measures used by pathogens. Further study of their interactions should improve our ability to manipulate and exploit the various pathogens.

The role of the UL41 gene of herpes simplex virus type 1 in evasion of non-specific host defence mechanisms during primary infection

The UL41 gene product (vhs) of herpes simplex virus (HSV) is packaged in the virion, and mediates host protein synthesis shutoff at the early stage of the virus replication cycle. In order to clarify the role of vhs in virus replication and virulence, we isolated a completely UL41-deficient mutant (the VRDelta41 strain) and its revertant (the VRDelta41R strain). In the mouse encephalitis model, the replication of strain VRDelta41 was inhibited after 2 days post-infection, resulting in low virulence, by gamma-ray-sensitive cells such as lymphocytes and/or neutrophils. The result suggested that some cytokines, produced in VRDelta41-inoculated brains, activate and induce the migration of gamma-ray-sensitive cells to the infection site. Therefore, cytokines produced by HSV-1-infected human cells were screened, and potent inductions of interleukin (IL)-1beta, IL-8 and macrophage inflammatory protein-1alpha by VRDelta41 infection were observed. Moreover, the VRDelta41 strain showed 20- and 5-fold higher sensitivity to interferon-alpha and -beta compared to the wild-type strain, respectively. These results indicate that one important role of vhs in vivo is evasion from non-specific host defence mechanisms during primary infection through suppression of cytokine production in HSV-infected cells and reduction of the anti-HSV activity of interferon-alpha and -beta.

Complement depletion facilitates the infection of multiple brain tumors by an intravascular, replication-conditional herpes simplex virus mutant

Intravascular routes of administration can provide a means to target gene- and virus-based therapies to multiple tumor foci located within an organ, such as the brain. However, we demonstrate here that rodent plasma inhibits cell transduction by replication-conditional (oncolytic) herpes simplex viruses (HSV), replication-defective HSV, and adenovirus vectors. In vitro depletion of complement with mild heat treatment or in vivo depletion by treatment of athymic rats with cobra venom factor (CVF) partially reverses this effect. Without CVF, inhibition of cell infection by HSV is observed at plasma dilution as high as 1:32, while plasma from CVF-treated animals displays anti-HSV activity at lower dilutions (1:8). When applied to the therapy of intracerebral brain tumors, in vivo complement depletion facilitates the initial infection (assayed at the 2-day time point) by an intra-arterial replication-conditional HSV of tumor cells, located within three separate and distinct human glioma masses. However, at the 4-day time point, no propagation of HSV from initially infected tumor cells could be observed. Previously, we have shown that the immunosuppressive agent, cyclophosphamide (CPA), facilitates the in vivo propagation of an oncolytic HSV, delivered intravascularly, within infected multiple intracerebral masses, by inhibition of both innate and elicited anti-HSV neutralizing antibody response (K. Ikeda et al., Nat. Med. 5:881-889, 1999). In this study, we thus show that the addition of CPA to the CVF treatment results in a significant increase in viral propagation within infected tumors, measured at the 4-day time period. The concerted action of CVF and CPA significantly increases the life span of athymic rodents harboring three separate and large glioma xenografts after treatment with intravascular, oncolytic HSV. Southern analysis of viral genomes analyzed by PCR reveals the presence of the oncolytic virus in the brains, livers, spleens, kidneys, and intestine of treated animals, although none of these tissues displays evidence of HSV-mediated gene expression. In light of clinical trials of oncolytic HSV for malignant brain tumors, these findings suggest that antitumor efficacy may be limited by the host innate and elicited humoral responses.