Characterization of glycoprotein C-negative mutants of herpes simplex virus type 1 isolated from a patient with keratitis

"Non-Essential" Proteins of HSV-1 with Essential Roles In Vivo: A Comprehensive Review

Viruses encode for structural proteins that participate in virion formation and include capsid and envelope proteins. In addition, viruses encode for an array of non-structural accessory proteins important for replication, spread, and immune evasion in the host and are often linked to virus pathogenesis. Most virus accessory proteins are non-essential for growth in cell culture because of the simplicity of the infection barriers or because they have roles only during a state of the infection that does not exist in cell cultures (i.e., tissue-specific functions), or finally because host factors in cell culture can complement their absence. For these reasons, the study of most nonessential viral factors is more complex and requires development of suitable cell culture systems and in vivo models. Approximately half of the proteins encoded by the herpes simplex virus 1 (HSV-1) genome have been classified as non-essential. These proteins have essential roles in vivo in counteracting antiviral responses, facilitating the spread of the virus from the sites of initial infection to the peripheral nervous system, where it establishes lifelong reservoirs, virus pathogenesis, and other regulatory roles during infection. Understanding the functions of the non-essential proteins of herpesviruses is important to understand mechanisms of viral pathogenesis but also to harness properties of these viruses for therapeutic purposes. Here, we have provided a comprehensive summary of the functions of HSV-1 non-essential proteins.

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.

Isolation of equine herpesvirus-1 lacking glycoprotein C from a dead neonatal foal in Japan

We isolated a variant equine herpesvirus-1 (EHV-1), strain 5089, from the lung of a dead neonatal foal in Japan and characterized the biological nature of the virus. The virus spread in cultured cells mainly by cell-to-cell infection, unlike wild-type EHV-1, which spreads efficiently as a cell-free virus. The virus titer in cultured supernatant and the intracellular virus titer were low compared to those of wild-type EHV-1. Heparin treatment of the virus had no effect on viral infectivity in cell culture. Glycoprotein C (gC) was not detected by Western blotting and fluorescent antibody tests in 5089 virions and 5089-infected cells, respectively. RT-PCR analysis revealed that the expression level of 5089 gC mRNA was reduced considerably compared to that of wild-type EHV-1. Sequencing analysis of the 5089 gC coding region showed a point mutation in the promoter region of the gC open reading frame. However, the mutation did not affect the promoter activity. These results suggested that the lack of gC in 5089 virions might be one of the reasons for spread of the virus by cell-to-cell infection and that gC mRNA expression might not be activated efficiently due to factors other than the mutation in the gC promoter region.

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.

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.

Detection of cutaneous herpes simplex virus infections by immunofluorescence vs. PCR

BACKGROUND

Detection of cutaneous infections with herpes simplex virus (HSV) has proven difficult, as serum antibody tests sometimes are not sensitive and specific enough for that purpose.

OBJECTIVE

This study was conducted to compare the sensitivity for detection of HSV of an immunofluorescence method (Syva Microtrak) and an internally controlled PCR.

METHODS

Cutaneous swabs from skin lesions were analysed by immunofluorescence separately for HSV types 1 and 2 and by competitive PCR. Detection of PCR products was done by ELISA, if positive additionally by agarose gel electrophoresis.

RESULTS

Of 79 samples 34 were PCR-positive by ELISA (34 = 100%), of which 23 (68%) were also positive on the agarose gel. Eleven samples (32%) were positive by immunofluorescence. No sample was positive by immunofluorescence and negative by PCR.

CONCLUSIONS

These results demonstrate that immunofluorescence using Syva Microtrak is not suitable for exclusion of herpes simplex virus infection as sensitivity was only 32%. However, as immunofluorescence is cheaper and faster than PCR, first screening can be done with immunofluorescence, and negative samples can be investigated by PCR to finally prove or exclude the presence of HSV DNA.

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.

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.

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).

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.

Clinical and epidemiological features of acute follicular conjunctivitis with special reference to that caused by herpes simplex virus type 1

BACKGROUND/AIMS

It is reported by the national surveillance of ocular infectious diseases in Japan that 4.3% of cases of epidemic keratoconjunctivitis (EKC) diagnosed clinically were caused by herpes simplex virus (HSV). Clinical and virological studies of patients with HSV conjunctivitis were carried out.

METHODS

The study population consisted of 478 patients with acute follicular conjunctivitis. Virological analysis was carried out for adenovirus (Ad) and HSV by the cell culture method and fluorescein antibody (FA) method. Polymerase chain reaction for Chlamydia trachomatis was also carried out.

RESULTS

From 23 patients, HSV type 1 was isolated but Ad or C trachomatis was not isolated. 87% of cases were unilateral. Most cases showed clinical resolution within 9 days. Early corneal lesions and preauricular lymphadenopathy were less frequent in HSV conjunctivitis than in adenoviral conjunctivitis, especially that due to subgenus D. No case showed a positive result for HSV by the FA method using conjunctival swabs; however, the FA test was positive in all strains isolated by cell culture.

CONCLUSIONS

These results indicate that it is difficult clinically to differentiate HSV conjunctivitis from adenoviral conjunctivitis in the acute stage, since the clinical features of adenoviral conjunctivitis are similar to those of HSV conjunctivitis. A biological difference may exist between HSV strains causing keratitis and conjunctivitis.

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.

Herpes simplex virus type 2 glycoprotein G-negative clinical isolates are generated by single frameshift mutations

Herpes simplex virus (HSV) codes for several envelope glycoproteins, including glycoprotein G-2 (gG-2) of HSV type 2 (HSV-2), which are dispensable for replication in cell culture. However, clinical isolates which are deficient in such proteins occur rarely. We describe here five clinical HSV-2 isolates which were found to be unreactive to a panel of anti-gG-2 monoclonal antibodies and therefore considered phenotypically gG-2 negative. These isolates were further examined for expression of the secreted amino-terminal and cell-associated carboxy-terminal portions of gG-2 by immunoblotting and radioimmunoprecipitation. The gG-2 gene was completely inactivated in four isolates, with no expression of the two protein products. For one isolate a normally produced secreted portion and a truncated carboxy-terminal portion of gG-2 were detected in virus-infected cell medium. Sequencing of the complete gG-2 gene identified a single insertion or deletion of guanine or cytosine nucleotides in all five strains, resulting in a premature termination codon. The frameshift mutations were localized within runs of five or more guanine or cytosine nucleotides and were dispersed throughout the gene. For the isolate for which a partially inactivated gG-2 gene was detected, the frameshift mutation was localized upstream of but adjacent to the nucleotides coding for the transmembranous region. Thus, this study demonstrates the existence of clinical HSV-2 isolates which do not express an envelope glycoprotein and identifies the underlying molecular mechanism to be a single frameshift mutation.

Herpes simplex virus type 1 glycoprotein gC mediates immune evasion in vivo

Many microorganisms encode proteins that interact with molecules involved in host immunity; however, few of these molecules have been proven to promote immune evasion in vivo. Herpes simplex virus type 1 (HSV-1) glycoprotein C (gC) binds complement component C3 and inhibits complement-mediated virus neutralization and lysis of infected cells in vitro. To investigate the importance of the interaction between gC and C3 in vivo, we studied the virulence of a gC-null strain in complement-intact and C3-deficient animals. Using a vaginal infection model in complement-intact guinea pigs, we showed that gC-null virus grows to lower titers and produces less severe vaginitis than wild-type or gC rescued virus, indicating a role for gC in virulence. To determine the importance of complement, studies were performed with C3-deficient guinea pigs; the results demonstrated significant increases in vaginal titers of gC-null virus, while wild-type and gC rescued viruses showed nonsignificant changes in titers. Similar findings were observed for mice where gC null virus produced significantly less disease than gC rescued virus at the skin inoculation site. Proof that C3 is important was provided by studies of C3 knockout mice, where disease scores of gC-null virus were significantly higher than in complement-intact mice. The results indicate that gC-null virus is approximately 100-fold (2 log10) less virulent that wild-type virus in animals and that gC-C3 interactions are involved in pathogenesis.

Complement and immunity to viruses

Complement is one of the first lines of host defence to be faced and countered by viruses as they struggle to establish an infection. As an important arm of the humoral immune response, the complement system is immediately ready to target and eliminate virus particles and to interact with the surface of virus-infected cells to mark them for destruction by other branches of the immune response. Nevertheless, some viruses are still very successful human pathogens. This article will discuss the role of complement in antiviral immunity, the mechanisms by which complement may be activated by viruses or virus-infected cells, and explore some of the strategies which viruses have evolved to subvert the immune response, including mechanisms by which complement activation may be prevented or aborted.

In vitro selection of variants of herpes simplex virus type 1 which differ in cytopathic changes

To analyze the mechanisms for in vitro emergence of the syncytial variants of herpes simplex virus type 1 (HSV-1), several cell lines were infected with a mixture of equal amounts of two HSV-1 variants, one syncytial and the other non-syncytial, and changes in their relative abundance were monitored during passage. With a combination of two variants of the Miyama strain of HSV-1, the syncytial variant became dominant during passage in Vero, RK-13 and FL cells. On the other hand, the ratios of the two variants remained around 1:1 during the passage in HEp-2, MGC and HEL cells. In another set of variants of the SKO strain of HSV-1, the outcomes were different from those of the Miyama strain in the FL, MGC and HEp-2 cells. The ratios of the two variants remained around 1:1 during passage in FL cells, while the non-syncytial variant became dominant during passage in MGC and HEp-2 cells. In addition, we examined the effects of a complement and interferon-beta (IFN-beta) on the outcome of the selection. As a result, the complement slowed the selection of a syncytial variant, whereas IFN-beta facilitated it.

Disulfide bond structure determination and biochemical analysis of glycoprotein C from herpes simplex virus

A biochemical analysis of glycoprotein C (gC of herpes simplex virus was undertaken to further characterize the structure of the glycoprotein and to determine its disulfide bond arrangement. We used three recombinant forms of gC, gC1(457t), gC1(delta33-123t), and gC2(426t), each truncated prior to the transmembrane region. The proteins were expressed and secreted by using a baculovirus expression system and have been shown to bind to monoclonal antibodies which recognize discontinuous epitopes and to complement component C3b in a dose-dependent manner. We confirmed the N-terminal residues of each mature protein by Edman degradation and confirmed the internal deletion in gC1(delta33-123t). The molecular weight and extent of glycosylation of gC1 (457t), gC1(delta33-123t), and gC2(426t) were determined by treating each protein with endoglycosidases and then subjecting it to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and mass spectrometric analysis. The data indicate that eight to nine of the predicted N-linked oligosaccharide sites on gC1(457t) are occupied by glycans of approximately 1,000 Da. In addition, O-linked oligosaccharides are present on gC1(457t), primarily localized to the N-terminal region (amino acids [aa] 33 to 123) of the protein. gC2(426t) contains N-linked oligosaccharides, but no O-linked oligosaccharides were detected. To determine the disulfide bond arrangement of the eight cysteines of gC1(457t),the protein was cleaved with cyanogen bromide. SDS-PAGE analysis followed by Edman degradation identified three cysteine-containing fragments which are not connected by disulfide linkages. Chemical modification of cysteines combined with matrix-assisted laser desorption ionization mass spectrometry identified disulfide bonds between cysteine 1 (aa 127) and cysteine 2 (aa 144) and between cysteine 3 (aa 286) and cysteine 4 (aa 347). Further proteolysis of the cyanogen bromide-generated fragment containing cysteine 5 through cysteine 8, combined with mass spectrometry and Edman degradation, showed that disulfide bonds link cysteine 5 (aa 386) to cysteine 8 (aa 442) and cysteine 6 (aa 390) to cysteine 7 (aa 419). A similar disulfide bond arrangement is postulated to exist in gC homologs from other herpesviruses.

gp13 (EHV-gC): a complement receptor induced by equine herpesviruses

Equine herpesviruses type 1 (EHV-1) and type 4 (EHV-4) induce a complement receptor protein on the surface of infected cells capable of binding to the third component of complement (C3). The protein mediating the binding to the C3 component of complement was identified as glycoprotein 13 (gp13, EHV-gC), as expression of the cloned viral gene under the control of a CMV promoter induced C3 binding activity at the transfected cell surface. Comparable to glycoprotein C (gC) from herpes simplex virus type 1 (HSV-1-gC), glycoprotein III from pseudorabiesvirus (gIII, PRV-gC) and bovine herpesvirus-1 (gIII, BHV-1-gC), gp13 derived from EHV-infected cell lysates bound to C3 fixed to solid phase, showing preferential binding to the appropriate host complement component. Similar to wild-type isolates, a highly attenuated vaccine EHV-1 strain also displayed complement receptor activity despite apparent differences of the gp13 gene in restriction enzyme digest pattern and reactivity with monoclonal antibodies. In addition, other structural proteins were altered in the vaccine strain as compared to wild-type strains, which might contribute to its attenuated phenotype. In contrast to the situation observed with HSV-1-gC, the interaction of gp13 (EHV-gC) with horse complement was not inhibited by polyanionic substances like heparin or dextran sulfate. These results suggest structural differences in the particular binding mechanism of the respective viral envelope proteins.

The herpes simplex virus type 1 particle: structure and molecular functions. Review article

This review is a summary of our present knowledge with respect to the structure of the virion of herpes simplex virus type 1. The virion consists of a capsid into which the DNA is packaged, a tegument and an external envelope. The protein compositions of the structures outside the genome are described as well as the functions of individual proteins. Seven capsid proteins are identified, and two of them are mainly present in precursors of mature DNA-containing capsids. The protein components of the 150 hexamers and 12 pentamers in the icosahedral capsid are known. These capsomers all have a central channel and are connected by Y-shaped triplexes. In contrast to the capsid, the tegument has a less defined structure in which 11 proteins have been identified so far. Most of them are phosphorylated. Eleven virus-encoded glycoproteins are present in the envelope, and there may be a few more membrane proteins not yet identified. Functions of these glycoproteins include attachment to and penetration of the cellular membrane. The structural proteins, their functions, coding genes and localizations are listed in table form.

Necrotizing chorioretinitis in mice inoculated with herpes simplex virus type 1 with or without glycoprotein C: anterior chamber-associated immune deviation does not persist

BALB/c mice developed contralateral necrotizing retinitis following intracameral inoculation with herpes simplex virus type 1 (HSV-1). The animals showed a positive delayed-type hypersensitivity (DTH) response at 10 days postinoculation, indicating that the anterior chamber-associated immune deviation was transient after HSV-1 inoculation. Since glycoprotein C (gC) of HSV-1 is a major immunogen, we examined DTH and the antibody response induced by a gC-deficient strain TN-1 and compared them with those induced by the recombinant gC-positive mutants. We found that gC was not required for DTH reaction, and that gC was neither necessary for nor protective against the contralateral retinal necrosis. Serial lymphocyte subset analyses of the draining lymph nodes revealed an absolute increase of B cells, CD4-positive T cells, and CD8-positive T cells. CD4-positive T cells but not CD8-positive T cells increased in the contralateral eyes during the inflammation and necrosis. The coincident emergence of the positive DTH and contralateral retinal necrosis of HSV-1-inoculated mice, together with the presence of CD4-positive cells in the retina, indicated that CD4-positive T cells responsible for DTH induction may participate in the retinal necrosis.

Induction of bilateral retinal necrosis in mice by unilateral intracameral inoculation of a glycoprotein-C deficient clinical isolate of herpes simplex virus type 1

Herpes simplex virus can cause acute retinal necrosis, a blinding retinal disease in man. A unilateral intracameral inoculation of herpes simplex virus type 1 (HSV-1) in mice induces retinal necrosis primarily in the contralateral eye and provides an experimental model for the disease. Previous studies suggested that a major envelope glycoprotein of HSV-1, glycoprotein C (gC), is required for retinal necrosis. We studied HSV-1 strain TN-1, a gC-deficient clinical isolated from a lesion of herpetic keratitis, for its pathogenicity in mice with an intracameral inoculation of the virus and found that TN-1 could induce severe necrotizing retinitis in both inoculated and uninoculated eyes of BALB/c mice. Inoculation with a lower dose of TN-1 resulted in a unilateral necrotizing retinitis in the uninoculated eyes. Tissue virus titration of infected mice killed at various times after inoculation detected an infectious virus in various organs including the eyeballs, trigeminal ganglia, brain and adrenal glands. Anterior chamber-associated immune deviation (ACAID) was observed in TN-1-inoculated mice as well as in mice inoculated with gC-positive laboratory strain KOS 7 days postinoculation. Our findings suggested that gC of HSV-1 is not necessary for either the induction of retinal necrosis, neural spread of the virus, or ACAID.

Molecular characterization of naturally occurring glycoprotein C-negative herpes simplex virus type 1

We previously isolated glycoprotein C (gC)-negative herpes simplex virus type 1 (HSV-1) mutants, TN-1, TN-2 and TN-3, from a patient with recurrent herpetic keratitis at one-year intervals. In the present study, the molecular basis for the inability of these clinical isolates to express gC was examined. The nucleotide sequence of the gC gene of the TN-1 strain was compared with that of the HSV-1 KOS strain. In the open reading frame of the gC gene, there were 12 nucleotide differences between the TN-1 and KOS strains, seven of which led to amino acid substitutions. Importantly, one of them was the codon change from CAG for glutamine at position 280 to TAG for the amber termination codon. Accordingly, the TN-1 strain produced a truncated gC with a predicted molecular weight, which was secreted into the extracellular fluid. These results suggest that this amber mutation in the TN-gC gene results in a premature termination of gC translation and is the cause of the gC-negative phenotype of the TN strains. It is expected that these extremely rare HSV-1 strains will provide us with valuable information concerning the in vivo functions of gC, especially in ocular diseases.

Interactions of bovine and caprine herpesviruses with the natural and the foreign hosts

Bovine herpesvirus 1 (BHV1) and caprine herpesvirus 1 (CapHV1) are useful models to study virus-host interactions, as well as pathogenicity and latency, when comparing the outcome of infection in the natural and the foreign hosts. Molecular seroepidemiological analyses revealed that cross-reacting antibodies were mainly induced by glycoprotein gI (gB analogue), by the major capsid protein and by nonstructural proteins, whereas the most virus-specific antibodies were elicited by glycoproteins gIII and gIV. These glycoproteins, especially gIII (gC analogue), might therefore play an important role in the virus-host-interactions. As a basis for further studies, we re-evaluated observations concerning experimental infections with BHV1 and CapHV1 in the natural and the foreign hosts. All parameters indicated that both viruses were able to infect either host, but that the pathogenicity was restricted to the natural host. Latent virus could be reactivated exclusively from cows infected with BHV1. It was possible neither to reactivate BHV1 from goats, nor to reactivate CapHV1 from either species. The experiments indicated that the outcome of infection in the natural and the foreign host is dependent on host and viral factors, whereby gIII is only one important virus component involved. Further investigations in the host and host cell range of BHV1 and CapHV1 will help to clarify the role of factors responsible for virus-host-interactions.

Structural basis of C3b binding by glycoprotein C of herpes simplex virus

Glycoproteins C (gC) from herpes simplex virus type 1 (HSV-1) and HSV-2, gC-1 and gC-2, bind the human complement fragment C3b, although the two glycoproteins differ in their abilities to act as C3b receptors on infected cells and in their effects on the alternative complement pathway. Previously, we identified three regions of gC-2 (I, II, and III) which are important for C3b binding. In this study, our goal was to identify C3b-binding sites on gC-1 and to continue our analysis of gC-2. We constructed a large panel of mutants by using the cloned gC-1 and gC-2 genes. Most of the mutant proteins were transported to the surface of transiently transfected L cells and reacted with one or more monoclonal antibodies to discontinuous epitopes. By using 31 linker insertion mutants spread across the coding region of gC-1, we identified four regions in the ectodomain of gC-1 which are important for C3b binding, three of which are similar in position to C3b-binding regions I, II, and III of gC-2. Region III shares some similarities with the short consensus repeat found in CR1, the human complement receptor. These were, in part, the targets for construction of 20 single amino acid changes in region III of gC-1 and gC-2. These mutants identified similarities and differences in the C3b-binding properties of gC-1 and gC-2 and suggest that the amino half of region III is more important for C3b binding. However, our results do not support the concept of a structural relationship between the short consensus repeat of CR1 and gC, since mutations of some of the conserved residues, including three of four cysteines in region III, had no effect on C3b binding. Finally, we constructed four deletion mutants of gC-1, including one which lacked residues 33 to 123, as well as residues 367 to 449. This severely truncated molecule, lacking four cysteines and five potential N-linked glycosylation sites, was transported to the cell surface and retained its ability to bind monoclonal antibodies as well as C3b. Thus, the four distinct C3b-binding regions of gC-1 and several epitopes within two different antigenic sites are localized within residues 124 to 366.