Evaluation of a peptidomimetic ribonucleotide reductase inhibitor with a murine model of herpes simplex virus type 1 ocular disease

The ribonucleotide reductase (RR) of herpes simplex virus type 1 (HSV-1) is an important virulence factor, being required for neurovirulence, ocular virulence, and reactivation from latency. The RR activity requires the association of two distinct homodimeric subunits, and the association of the subunits is inhibited in the presence of a peptide homologous to the carboxy terminus of the small subunit. A structural analog of the inhibitory peptide (BILD 1263) has been shown to inhibit the replication of HSV-1 at micromolar concentrations in vitro. We used a mouse model of HSV-1 ocular infection to determine the in vivo efficacy of topical BILD 1263. Treatment of HSV-1 KOS-infected mice resulted in significant reductions in the severity and incidence of stromal keratitis and corneal neovascularization. At higher concentrations (5%) BILD 1263 reduced the severity but not the incidence of blepharitis. Treatment with 5% BILD 1263 also reduced viral shedding from the cornea by 10- to 14-fold (P < 0.001). In uninfected mice treated with 5% BILD 1263, we found no evidence of corneal epithelial damage, conjunctivitis, or blepharitis, and histopathological studies revealed no changes in the corneas of these mice. These results show that the peptidomimetic RR inhibitor BILD 1263 is effective in preventing disease, has an antiviral effect in vivo, and has little or no toxicity.

Thymidine kinase and susceptibility to interferon are not involved in the increased virulence of recombinant viruses isolated following mixed ocular infection with HSV strains OD4 and CJ394

We previously described the isolation of herpes simplex virus type 1 (HSV-1) intratypic recombinants that had increased ocular and neurovirulence. To better understand the mechanism of increased virulence, we have characterized two virulence-related factors, viral thymidine kinase (TK) activity and interferon (IFN) sensitivity, in the parental and recombinant viruses. The parental and recombinant viruses had comparable TK activities and were equally resistant to IFN-alpha, IFN-gamma and to a combination of the two IFNs. These results indicate that these factors are not involved in the increased virulence of the recombinant viruses.

Evidence for an interaction of herpes simplex virus with chondroitin sulfate proteoglycans during infection

In a previous study, a mouse L cell mutant was isolated which is 90% resistant to HSV-1 infection (S. Gruenheid, L. Gatzke, H. Meadows, and F. Tufaro. J. Virol. 67, 93-100, 1993). This cell line, termed gro2C, failed to express heparan sulfate (HS)-glycosaminoglycans on the cell surface, which normally act as initial receptors for HSV-1 attachment to cultured cells. In this report, we extended the characterization of gro2C cells to explore the possibility that cell-surface chondroitin sulfate (CS) facilitates virus attachment to gro2C cells in the absence of HS. We found that soluble CS types A, B, and C strongly interfere with adsorption of HSV-1 to the surface of gro2C cells in a dose-dependent manner, and CS type B (dermatan sulfate) inhibited adsorption to parental (control) L cells by up to 10%. Moreover, gro2C cell infection was hypersensitive to inhibition by HS in comparison to control L cell infection. In all cases, a decrease in adsorption resulted in a decrease in infection. By contrast, the highly-sulfated glycosaminoglycan analog dextran sulfate was a relatively poor inhibitor of gro2C cell infection, indicating that the inhibitory effects of CS were related to its carbohydrate structure and not solely to its strong negative charge. By using a mutant virus strain which does not express the heparin-binding glycoprotein gC, we show that gC was not required for infection of gro2C cells, and was not required for the inhibition by HS or CS. Thus, the characterization of gro2C cell infection has revealed that one or more components of the HSV-1 particle can interact with cell-surface CS as well as HS to mediate infection of susceptible cells.

The effect of viral inoculum level and host age on disease incidence, disease severity, and mortality in a murine model of ocular HSV-1 infection

It has been previously shown that the strain of virus, immune competence of the host, and innate resistance of the host have an effect on the severity of ocular disease induced by topical infection with herpes simplex virus type 1 (HSV-1). This study has expanded on earlier work by examining the effect of virus inoculum and host age on mortality, incidence of ocular disease, and severity of ocular disease. BALB/c mice were infected with inocula ranging from 2 x 10(3) to 1 x 10(6) pfu of HSV-1 strain CJ394. The most significant effect of variation in the inoculum was on the percent of mice developing disease. Increasing the inoculum resulted in significantly increased disease incidence, but at 5 x 10(3) pfu/mouse or higher, there was little difference in disease severity in those animals exhibiting symptoms. Decreasing host age also resulted in a significant increase in the incidence of ocular disease, but the dependence of disease severity on host age varied with the symptom being scored. In animals exhibiting disease, the peak severity of stromal keratitis and vascularization of the cornea were unaffected by host age. However, the severity of blepharitis was significantly reduced in older mice. Increasing host age also resulted in increased resistance to encephalitis. Three to four-week old mice were very susceptible to encephalitis (100% mortality), while only 20% of 4-5 week old mice died by day 15 post-infection. Mice older than 5 weeks were completely resistant to lethal encephalitis after corneal infection.

Recombinants are isolated at high frequency following in vivo mixed ocular infection with two avirulent herpes simplex virus type 1 strains

Mixed infections with different strains of herpes simplex virus type 1 (HSV-1) may result in more severe disease than infection with either strain alone. This phenomenon is important because it may facilitate the identification of virulence genes through the transfer of virulence determinants between complementing strains, and it may pose a problem in the use of attenuated HSV strains for vaccines and gene delivery vectors. In this study, we have compared the percentage of recombinants present after mixed infection with HSV-1 strains OD4 and 994 in vitro and in vivo. After corneal inoculation, we found that 74% of randomly picked isolates from the trigeminal ganglia were recombinants, compared with 59% from the cornea. Twenty-six percent of randomly picked isolates were recombinant following mixed infection of Vero cells in vitro. Seventeen recombinant strains isolated from the in vivo mixed infections were assayed for ocular virulence, and they were found to exhibit a wide range of virulence phenotypes. The presence of virulent recombinants suggests that recombination plays a role in the increased disease observed in this mixed infection, and the broad range of virulence indicates that there may be multiple genetic factors involved in the increased virulence observed after mixed infection with these two strains. The recombinants were also tested for their ability to grow in NIH 3T3 fibroblasts, and though some correlation was observed between growth in vitro and ability to cause ocular disease, improved growth in murine cells does not sufficiently explain the increased virulence observed in some recombinants.

IFN-alpha induces MxA gene expression in cultured human corneal fibroblasts

Treatment of certain human cells with Interferon-alpha (IFN-alpha) induces the synthesis of a 76 kDa protein designated MxA that is involved in resistance to viral infection. We have used a specific cDNA clone and monoclonal Ab to show that MxA is induced in IFN-alpha treated human corneal fibroblast cultures. Mx RNA was increased 23-fold and 45-fold after 5 and 9 h of IFN-alpha treatment, respectively. The MxA protein was detectable by immunoblotting at 5 hr after IFN treatment and peaked at 17 hr. Concentrations of IFN-alpha as low as 1 U/ml induced detectable amounts of MxA, and expression was maximal at 1 x 10(3) U ml-1. These results confirm that MxA synthesis is induced in human corneal fibroblasts treated with IFN-alpha.

Renin mRNA is synthesized locally in rat ocular tissues

Components of the Renin Angiotensin System (RAS) have been detected in ocular tissues and fluids. The source of the ocular RAS proteins is unknown but possibilities include diffusion or leakage from the systemic circulation, specific uptake from the blood, or local synthesis. We have used RT-PCR and in situ hybridization (ISH) to show that renin mRNA is present in ocular tissues from 3 strains of rats. By RT-PCR, we found 10 of 15 ciliary body samples, 13 of 16 iris samples, and 1 of 3 retina samples were positive for renin mRNA. Also, 6 of 6 brain and 7 of 8 kidney samples were positive. Using ISH, we found renin mRNA in the ciliary muscle adjacent to the sclera extending into the choroid. Tissue near the outflow channels of the anterior chamber angle also labeled. Retinal labeling was weak but present in the nerve fiber layer. Clusters of grains, possibly representing blood vessels, were also seen in the ciliary body, iris, and retina using ISH. These results suggest the presence of a local ocular RAS.

Rapid small-scale isolation of herpes simplex virus DNA

A method has been developed for the rapid isolation of herpes simplex virus DNA analogous to miniprep methods for bacterial plasmid isolation. Infected Vero cells are lysed with three freeze-thaw cycles, and the nuclei are removed by centrifugation. DNA is released from the virions in the supernatant by proteinase K digestion. Then the DNA is extracted with phenol/chloroform and precipitated with ethanol. This method requires only small amounts of infected cells as a source of viral DNA, does not use radioactivity, and routinely produces DNA of sufficient purity to be used for restriction fragment length polymorphism (RFLP) analysis on ethidium-stained gels.

Glycoprotein C-independent binding of herpes simplex virus to cells requires cell surface heparan sulphate and glycoprotein B

Previous studies have shown that the initial interaction of herpes simplex virus (HSV) with cells is binding to heparan sulphate and that HSV-1 glycoprotein C (gC) is principally responsible for this binding. Although gC-negative viral mutants are impaired for binding and entry, they retain significant infectivity. The purpose of the studies reported here was to explore the requirements for infectivity of gC-negative HSV-1 mutants. We found that absence or alteration of cell surface heparan sulphate significantly reduced the binding of gC-negative mutant virus and rendered cells resistant to infection, as shown previously for the wild-type virus. We isolated a recombinant double-mutated HSV strain that produces virions devoid of both of the known heparin-binding glycoproteins, gB and gC. The drastically impaired binding of these mutant virions to cells, relative to gC-negative and wild-type virions, indicates that gB mediates the binding of gC-negative virions to cells. Thus at least two HSV glycoproteins can independently mediate the binding of HSV to cell surface heparan sulphate to start the process of viral entry into cells.

Interferon-alpha and interferon-gamma induced modulation of proteins in human corneal fibroblasts

Little is known about the effects of interferon (IFN) on cell function in the eye. We have analyzed the effect of INF-alpha and IFN-gamma on the expression of proteins in cultured human corneal fibroblasts. Treatment with IFN-alpha increased the synthesis of proteins of 84, 76, 52, and 28 kD and decreased the synthesis of a 72-kD protein. Treatment with IFN-gamma increased the synthesis of proteins of 83, 66, 64, 54, and 47 kD. The effect of IFN-alpha and IFN-gamma were first detected at 5-9 h and 9 h, respectively, after the addition of the IFNs and were maximal at 17 and 24 h, respectively. Most of the changes were seen at doses of 1 x 10(1) to 1 x 10(2) U/ml of IFN-alpha or IFN-gamma and were maximal at 1 x 10(2) to 1 x 10(3) U/ml. Thus, each IFN induced distinct proteins based on apparent molecular weight and isoelectric point. These results show that IFN-alpha and IFN-gamma affect the synthesis of small groups of distinct proteins in human corneal fibroblasts.

The HSV-1 UL45 18 kDa gene product is a true late protein and a component of the virion

Previously we constructed a null mutation in the HSV-1 UL45 gene, showed that the UL45 gene was not required for growth in Vero cells, and confirmed that it coded for an 18 kDa protein (R.J. Visalli and C.R. Brandt, Virology 185:419-423, 1991). In this study, we have continued our characterization of the UL45 gene and the 18 kDa protein. Analysis of UL45 RNA revealed that the gene was expressed late and was inhibited in the presence of phosphonoacetic acid (paa), indicating it is a gamma 2 class gene. Using a specific polyclonal antiserum, we found that the 18 kDa UL45 gene product was also expressed late and was inhibited in the presence of paa. The 18 kDa protein was present in purified virions and was substantially enriched in the envelope-tegument fraction of virions disrupted with NP-40 detergent. The 18 kDa protein is thus a structural protein of the virus and appears to be associated with the viral envelope. A 20 kDa protein that cross-reacted with a polyclonal HSV-1 UL45 antiserum was also detected in cells infected with HSV-2 strain 333.

IFN-gamma-mediated antimicrobial response. Indoleamine 2,3-dioxygenase-deficient mutant host cells no longer inhibit intracellular Chlamydia spp. or Toxoplasma growth

The role of indoleamine 2,3-dioxygenase (IDO) in IFN-gamma-mediated inhibition of intracellular parasite growth has been examined previously, although earlier work has been largely correlative. In this study, we defined more completely the role of IDO in the IFN-antimicrobial response. Two mutant cell lines, derived from ME180 cells and exhibiting reduced IDO activity (IR3B6A, IR3B6B) were characterized to determine if they retained the capacity to inhibit intracellular Chlamydia and Toxoplasma growth. Mutant cells treated with IFN-gamma exhibited reduced capacity to suppress pathogen growth. The expression of several IFN-regulated genes also was measured to confirm that the inability to inhibit pathogen growth was because of the lack of IDO. The expression of class II MHC, intracellular adhesion molecule-1, MxA, and P68 kinase genes was induced in the IFN-gamma-treated wild type ME180 cells, but was variable in the mutant cell lines, supporting the hypothesis that IFN-gamma-induced production of IDO is a key IFN-gamma-mediated antimicrobial mechanism.

IFN-gamma-mediated antimicrobial response. Indoleamine 2,3-dioxygenase-deficient mutant host cells no longer inhibit intracellular Chlamydia spp. or Toxoplasma growth

The role of indoleamine 2,3-dioxygenase (IDO) in IFN-gamma-mediated inhibition of intracellular parasite growth has been examined previously, although earlier work has been largely correlative. In this study, we defined more completely the role of IDO in the IFN-antimicrobial response. Two mutant cell lines, derived from ME180 cells and exhibiting reduced IDO activity (IR3B6A, IR3B6B) were characterized to determine if they retained the capacity to inhibit intracellular Chlamydia and Toxoplasma growth. Mutant cells treated with IFN-gamma exhibited reduced capacity to suppress pathogen growth. The expression of several IFN-regulated genes also was measured to confirm that the inability to inhibit pathogen growth was because of the lack of IDO. The expression of class II MHC, intracellular adhesion molecule-1, MxA, and P68 kinase genes was induced in the IFN-gamma-treated wild type ME180 cells, but was variable in the mutant cell lines, supporting the hypothesis that IFN-gamma-induced production of IDO is a key IFN-gamma-mediated antimicrobial mechanism.

Heterogeneity of type I collagen expression in human corneal keratoconus fibroblasts

Corneal fibroblast cultures were established from 3 normal and 4 keratoconus (KC) corneas. Type I collagen protein synthesis and steady-state RNAs were analyzed in these cultures by metabolic labeling studies, slot blots and immunofluorescent microscopy. Type I collagen proteins and steady-state RNAs were reduced in cells from 3 of the 4 KC cultures when compared with normal fibroblasts, suggesting that collagen expression is heterogenous in KC and that the heterogeneity is expressed at the cellular and molecular level. The type I collagen matrix synthesized by the KC fibroblasts appeared normal when analyzed by immunofluorescence, suggesting that the incorporation of type I collagen into the extracellular matrix is not affected in KC fibroblasts.

Enhanced inhibition of herpes simplex virus type 1 growth in human corneal fibroblasts by combinations of interferon-alpha and -gamma

Trials testing the topical treatment of herpes simplex virus (HSV) ocular infections with single interferons (IFN) have provided mixed results. To determine if combination therapy with IFN may be more effective, the ability of combinations of IFN-alpha and IFN-gamma to inhibit HSV growth in human corneal fibroblasts (HCF) was assessed. Virus yields were reduced 282-fold and 37-fold, respectively, in HCF treated with either IFN-alpha or IFN-gamma (10(3) units/mL each). In cells treated with a combination of IFN-alpha and IFN-gamma (10(3) units/mL each), an average reduction of 5.1 x 10(5)-fold in the yield of infectious virus was achieved. Combinations of IFN-alpha and IFN-gamma considerably enhanced the antiviral effect in HCF, suggesting that combination treatment may be efficacious against ocular HSV infections; these findings provide a possible explanation at the cellular level for the poor results achieved in previous clinical trials.

An ocular renin-angiotensin system. Immunohistochemistry of angiotensinogen

The circulating renin-angiotensin system (RAS) is an important determinant in maintaining adequate systemic blood pressure, and it also may modify organ-specific blood flow. All recognized RAS components have been identified in the eye. In this study, angiotensinogen (ANG) was localized using an affinity-purified antibody and paraffin sections of seven human eyes. An antibody for human serum albumin was used for comparison. The ANG was present selectively in the cytoplasm of the nonpigmented ciliary epithelium (NPCE), more prominently in the pars plana than in the pars plicata. Both ANG and albumin were present in the blood vessel lumina of the uvea and retina. Both antibodies also stained perivascular tissue in the uvea, but not in the retina, reflecting the relative tightness of blood-tissue barriers. The detection of ANG in the NPCE may be significant in view of previous descriptions localizing prorenin and angiotensin-converting enzyme in the same cell layer.

A murine model of herpes simplex virus-induced ocular disease for antiviral drug testing

Previous studies to determine the efficacy of new antiviral compounds in treating HSV ocular infections have used rabbit models. However, rabbits are expensive to purchase and maintain, and require the use of substantial amounts of test compounds. We have used the currently licensed antiviral trifluorothymidine in a murine model of HSV-induced ocular infection to demonstrate that the less expensive murine model can be used for the in vivo evaluation of potentially useful antiviral compounds. Treatment with TFT reduced the severity of blepharitis, vascularization of the cornea, stromal keratitis, and the percentage of animals developing symptoms. TFT treatment did not reduce the peak titers of infectious virus in the eyes of the infected animals, but did enhance clearance of virus from the tissues in a dose-dependent manner. Treatment with 1.0% TFT prevented the establishment of reactivatable latent infections. However, treatment with 0.01% or 0.1% TFT did not affect latency. The ED50 values for blepharitis, vascularization, and stromal keratitis ranged between 0.007% and 0.023%. These results are very similar to results obtained in rabbits and establish baseline data for comparing rabbit and murine models. This murine model provides a potentially less expensive alternative for in vivo drug testing.

Activation of NK cells in mice following corneal infection with herpes simplex virus type-1

Natural killer (NK) cells are large granular lymphocytes that mediate antigen nonspecific, non-major histocompatibility complex (MHC) restricted lysis of virus infected cells. They are thought to play a role in innate resistance to herpes simplex virus (HSV) infections. In most animal studies reported to date, the virus was injected intraperitoneally, not a natural route of infection. Using a murine model of acute HSV-1 ocular infection, we demonstrate that increased splenic NK activity is induced in BALB/c mice following corneal infection with four different strains of HSV-1. The kinetics of NK cell activation depended on the strain of virus and was associated with virulence of the strain and with the ability of the viruses to grow in vivo. We also assessed the role of interferon-alpha/beta, IFN-gamma, and interleukin-2 (IL-2) in the HSV-1 induced NK cell activation by treating mice with antisera against these lymphokines prior to infection. Treatment with anti-IFN-alpha/beta or anti-IFN-gamma significantly reduced NK cell cytotoxicity, suggesting that these lymphokines were involved in the activation of NK cells following HSV-1 ocular infection. Treatment with anti-IL-2 resulted in increased NK cell activity, suggesting that in vivo, IL-2 is involved in the suppression of NK cell activity in infected mice. Treatment with a combination of anti-IL-2 and anti-IFN also increased NK cytotoxicity. Despite the induction of high levels of NK activity, mice developed severe ocular disease or died of encephalitis.

Susceptibility of +/+, +/nu and nu/nu BALB/c mice to ocular herpes simplex virus infection

Athymic (nude) mice have played an important role in defining the function of the immune system and its role in infectious diseases. In the majority of these studies, heterozygous +/nu mice have been used as normal controls for the nu/nu mice, and it has been assumed that +/nu mice have essentially normal immune systems. We have compared the response of +/+, +/nu and nu/nu BALB/c mice following ocular infection with HSV-1 and have found that +/nu mice develop significantly more severe blepharitis, vascularization of the cornea, stromal keratitis and extraocular disease (herpetiform spread) than +/+ BALB/c mice. The extraocular disease was particularly severe in the +/nu mice, suggesting that factors regulating herpetiform spread of the virus are deficient in these mice. Susceptibility to lethal encephalitis did not differ between +/+ and +/nu mice. These results suggest that significant differences exist in the response to ocular HSV infection between +/+ and +/nu mice.

Mixed ocular infections identify strains of herpes simplex virus for use in genetic studies

Studies on the genetic mechanisms involved in the ocular virulence of herpes simplex virus (HSV) require the careful selection of parental strains. We used the technique of mixed ocular infection in vivo to identify strains of HSV for use in genetic studies. A pair of viruses (OD4 and 994) were identified that cause significantly more severe ocular disease when mixed together and used to infect the eyes of Balb/c mice compared to each strain when used alone. The mixed infection with OD4 and 994 did not result in increased neurovirulence. The technique of mixed ocular infections provides a sensitive screen to identify strains of virus that can act synergistically to cause more severe disease. Marker transfer can then be used to map the genes involved.

The HSV-1 UL45 gene product is not required for growth in Vero cells

We have constructed a HSV-1 UL45 null mutant (UL45 delta) by inserting a TK-lacZ cassette into a BclI site near the 5' end of the UL45 gene. A polyclonal antiserum produced to an Escherichia coli trpE:UL45 fusion protein was used to show that an 18-kDa polypeptide corresponding to the predicted UL45 gene product was produced in HSV-1 strain KOS-infected Vero cells but was not detected in UL45 delta-infected Vero cells. The absence of the 18-kDa protein had only a slight effect on viral growth in cell culture, indicating that the UL45 gene product is not essential for growth in Vero cells. However, the burst size of UL45 delta was smaller than HSV-1 KOS in Vero and HeLa cells. UL45 delta also had a smaller plaque size and an altered plaque morphology.

The herpes simplex virus ribonucleotide reductase is required for ocular virulence

We used a herpes simplex virus (HSV) type 1 ribonucleotide reductase (RR) null mutant (ICP6 delta) to study the role of HSV-1 RR in ocular HSV infections. We found that ICP6 delta was unable to induce vascularization of the cornea or stromal keratitis following inoculation into the cornea of BALB/c mice, but was able to induce a transient mild blepharitis. The parental strain (HSV-1 KOS) and a revertant of ICP6 delta, ICP6 delta+3.1, both caused severe ocular disease, indicating that HSV-1 RR is required for ocular virulence in mice. ICP6 delta grew poorly in vitro (Vero and BALB/c 3T3 fibroblasts) and in vivo (eye, trigeminal ganglia and brain) compared to ICP6 delta+3.1 and HSV-1 KOS, suggesting that the avirulence of ICP6 delta is due to poor growth in the host. ICP6 delta also grew less well in primary human corneal fibroblasts, suggesting that RR may be required for virulence in humans. These results indicate that drugs inhibiting the function of RR might be effective in treating ocular HSV infections.

Mixed infection with herpes simplex virus type 1 generates recombinants with increased ocular and neurovirulence

The authors used the method of mixed ocular infection and subsequent in vivo selection to isolate Herpes simplex virus type 1 intratypic recombinants with increased ocular virulence and neurovirulence. Four recombinants were studied in some detail (DRG1A3, DRG2A2, DRG3A3, and DRG4A1). The recombinants had lethal doses in 50% of animals tested (LD50) at least 2-3 log units lower than either parent virus (OD4 and CJ394) and caused significantly more severe stromal keratitis, vascularization of the cornea, and blepharitis than either parent. Studies on the ability of DRG1A3 and DRG4A1 to replicate in the eye, trigeminal ganglia, and brain showed that these recombinants replicated to higher titers (1-3.5 log units) than the parents in all three tissues. One of the parents, OD4, spread to the central nervous system with the same kinetics as CJ394, DRG1A3, and DRG4A1 but had a restricted ability to replicate in all tissues, which may account for its lack of virulence. The other parent, CJ394, was nonneurovirulent but replicated to titers which were only 1-1.5 log units lower than the neurovirulent recombinants. These recombinants should be useful in studying virulence determinants in herpetic ocular infections.

In vivo induction of Ia expression in murine cornea after intravitreal injection of interferon-gamma

Intravitreal injection of interferon gamma (IFN-gamma) induces increased expression of Class II major histocompatibility complex (Ia) antigen expression on corneal endothelial cells and stromal fibroblasts. In contrast, IFN-gamma has no detectable effect on Ia antigen expression in epithelium. Induction of Ia antigen expression was rapid with increases detectable as early as 6-12 hours after a single injection of 1 x 10(5) units. Expression peaked at 24-48 hours and decreased to background levels by 120 hours. The Ia antigen expression increased in a dose-dependent manner, and IFN-gamma treatment also induced the synthesis of increased amounts of a 65-kilodalton (kD) protein in the cornea. Increased levels of this 65-kd protein are seen as early as 12 hours after treatment and can be induced with as little as 1 x 10(2) units of IFN-gamma. The function of the 65-kd protein is unknown. This model should be useful in studies on in vivo modulation of Ia antigen expression.

Herpes simplex virus stromal keratitis is not titer-dependent and does not correlate with neurovirulence

We developed a murine model of ocular herpes simplex virus (HSV) disease which is particularly suited for testing stromal keratitis because most animals show some evidence of infection. Using this model, we characterized the ocular disease patterns caused by ten recent low-passage clinical isolates of HSV-1, as well as those caused by the established laboratory strains HSV-1 KOS and HSV-2 333. Viral strains were evaluated for their ability to cause stromal keratitis, blepharitis, vascularization of the cornea, and mortality. The model was not useful for scoring epithelial keratitis. The ocular disease caused by the recent isolates ranged from very mild disease to severe stromal keratitis. Some of the recent isolates caused disease as severe as the two laboratory strains. A comparison of the virulence characteristics expressed by various HSV strains indicated that the ability to cause stromal disease was correlated with vascularization of the cornea (correlation coefficient = 0.797, P less than 0.001) and was not correlated with the neurovirulence of the strains (correlation coefficient 0.045, P greater than 0.05). The severity of stromal keratitis was not dependent on the amount of inoculum over the range tested and a strain causing severe stromal keratitis caused severe ocular disease even when mixed with a nonstromal strain at ratios of 10:1, 100:1, and 1000:1.