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From the Department of Microbiology, College of Veterinary Medicine, The University of Tennessee, Knoxville, Tennessee.
| Abstract |
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METHODS. Mice with T-cell specificity to OVA peptide (Tg-RAG mice) as well as control DO11.10 and BALB/c mice were infected with HSV on the scarified cornea and subjected to clinical, histologic, and immunologic analysis. To evaluate involvement of OVA-specific CD4+ T cells in lesion development in Tg-RAG mice, monoclonal antibody to CD4+ T cells was used for in vivo CD4+ T-cell depletion.
RESULTS. Tg-RAG mice were capable of eliciting ocular lesions in the absence of detectable reactivity to viral antigens. Lesion manifestation in Tg-RAG mice was CD4+ T-cell dependent and the cellular infiltrates and their inflammatory products in the HSV-infected cornea were comparable to similarly infected BALB/c and DO11.10 mice.
CONCLUSIONS. The authors conclude that mechanisms other than viral antigen recognition, and hence molecular mimicry, are at play and are sufficient to cause HSV-induced stromal keratitis. The data imply a significant role for nonvirus-specific CD4+ T cells that could become activated by an inflammatory milieu consisting of enhanced accessory molecules and proinflammatory cytokines in the cornea.
| Introduction |
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and Vß T-cell receptor chains that
recognize OVA (ovalbumin peptide, 323339), when back-crossed to
RAG2-deficient mice (Tg-RAG mice),10
fail to respond
immunologically to HSV and are highly susceptible to infection. Mice
usually die of encephalitis by 12 days after infection (p.i.). However,
before death such animals expressed clinical HSK to a magnitude
comparable to lesions evident at the same time after infection in
immunocompetent BALB/c and transgenic parent DO11.10 mice. Such lesions
were essentially identical histologically in all three groups with
CD4+ T cells in the case of the Tg-RAG mice, all
expressing the KJ1-26.1+ TCR (T-cell reactivity) idiotype responsible
for OVA recognition. Our results were interpreted to mean that virus
persistence in the cornea of Tg-RAG mice drives proinflammatory
cytokine expression, which serves to activate invading
CD4+ T cells other than by conventional
TCR-mediated T-cell activation. Such bystander activation of
CD4+ T cells may represent a component of
clinical HSK in immunocompetent animals. | Materials and Methods |
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Virus and Corneal Infections
The HSV-1 RE strain used was propagated on Vero cells and
stored as infectious cell preparations at -70°C. The corneal
surfaces were deeply anesthetized (methoxyflurane; Pittman Moore,
Mondelein, IL) and were scarified with a sterile 27-gauge needle and
5 x 105 pfu. The HSV-1 RE strain was applied in a
4.0-µl volume and was massaged gently on the eye lids.
In Vivo CD4+ T-Cell Depletion
For CD4+ T-cell depletion, ascitic fluid
containing anti-CD4 monoclonal antibody (Ab) (GK 1.5 hybridoma; ATCC
TIB207, Rockville, MD) was used. Ascitic fluid containing rat IgG2b
antibody was used as isotype control. The determination of depletion
dose (0.5 mg/mouse on days -2, 0, +2, and +5) was followed by the
method described previously.11
The ascitic fluids were
titrated for the Ab content using an indirect enzyme-linked
immunosorbent assay (ELISA) as described previously.12
Clinical Evaluation
Mice were scored according to their clinical severity by a person
who was blinded to the experimental design using a slit lamp
biomicroscope (Keeler Instruments, Broomall, PA) as follows; score 0,
normal cornea; score 1, neovascularization at periphery, iris visible;
score 2, partial opacity, iris not visible; score 3, neovascularization
at center, opacity; score 4, bleb formation; score 5, necrosis. The
data were plotted as the mean daily clinical score for all animals in a
particular treatment group.
Immunoglobulin ELISA
Serum collected was analyzed for HSV-specific total IgG, using a
standard ELISA as described previously.13
Basically, ELISA
plates were coated with 100 µl HSV antigen in carbonated buffer (pH
9.8), and after overnight incubation at 4°C, the plates were washed
three times in phosphate-buffered saline (PBS) containing 0.05% Tween
20, pH 7.2 (PBST), and then were blocked using PBS (pH 7.2) with 3%
dehydrated milk for 2 hours at 37°C.
A total of 200 µl serially diluted serum samples (prediluted in PBST) was added in duplicate and washed. Wells coated with goat anti-mouse IgG (0.025 mg/ml) were treated with serially diluted standard mouse IgG. Plates were incubated for 2 hours at 37°C. After three washes, 100 µl goat anti-mouse IgG horseradish peroxidase was added. After three washes, 2,2-azino-bis-3-ethlybenz-thiazoline-6-sulfonic acid substrate (no. A1888; Sigma, St. Louis, MO) was added. The concentration of the antibodies in the serum samples was determined from the standard curve.
Cytokine Assay
For cytokine [interferon gamma (IFN-
)] assay, splenocytes
from mice were suspended in 10% RPMI-1640, and
106 cells in 1 ml were stimulated in vitro with
1.5 MOI (multiplicity of infection, before inactivation) of
UV-inactivated HSV-1 (KOS strain). A similar number of cells were
concanavalin Astimulated (5 µg/106 cells/ml)
in 12-well culture plates. Plates were incubated at 37°C for 72
hours. The supernatant fluid was collected and stored at -20°C until
use. These supernatants were screened for the presence of IFN-
by
ELISA as described previously.14
Histopathology
At the end of each experiment, eyes were enucleated and fixed, and
sections of the eye were prepared for histopathology according to
standard procedures. Briefly, at the termination of experiments whole
eyes were fixed in 10% buffered neutral formalin and embedded in
paraffin. Tissue sections were stained with hematoxylin and eosin.
Sections were observed for the corneal thickness, presence of
inflammatory infiltrates, neovascularization, epithelial erosions,
superficial or deep ulcers, and corneal perforation.
Immunohistochemical Staining
At the termination of experiments, eyes were removed and
snap-frozen in OCT compound (Miles, Elkhart, IN). Six-micron-thick
sections were cut, air-dried, and fixed in cold acetone for 5 minutes.
The sections were then blocked with heat-inactivated goat serum and
stained with biotinylated anti-CD4 (Pharmingen, San Diego, CA). Frozen
sections also were stained for the presence of HSV antigens by the use
of rabbit anti-HSV antiserum (Dako Corp., Carpinteria, CA), which
was followed with biotinylated anti-rabbit Ig (1/20 dilution; Biogenex,
San Ramon, CA). Sections were then treated with horseradish
peroxidaseconjugated streptavidin (1:1000) and 3,3'-diaminobenzidine
(Vector, Burlingame, CA), and counterstained with hematoxylin.
Proliferation Assay
Splenocytes (responders) at day 11 p.i. were collected
and restimulated in vitro with HSV-1 KOS (MOI of 1.5 before UV
irradiation) or OVA peptide (Research Genetics Inc., Huntsville, AL).
The HSV-1specific lymphoproliferation was measured as described
previously.14
Briefly, responders:stimulator ratios tested
ranged from 10:1 to 0.625:1. Responders + stimulator mixtures were
incubated at 37°C for 4 days. [3H]TdR (1.0
µCi/well) was added, harvested 1 hour later, and read using a beta
scintillation counter (Trace 96; Inotech, Lansing, MI). The results
were expressed as means ± SD.
RNA Isolation and Reverse TranscriptionPolymerase Chain Reaction
On day 11 p.i., splenocytes were collected, transferred to
Tri-reagent (Molecular Biology Inc., Cincinnati, OH). The total
cellular RNA was isolated from the Tri-reagent cellular lysate by
adding chloroform and with centrifugation, followed by
ethanol/isopropyl ethanol precipitation of the aqueous RNA solution
according to the manufacturers instructions. The RNA thus obtained
was reverse transcribed using oligo(dT) primers and superscript (Gibco
BRL-Life Technologies, Bethesda, MD) according to standard
protocol.15
The cDNA thus obtained was used as a template
in subsequent qualitative for viral gD mRNA expression and
semi-quantitative polymerase chain reactions (PCRs) for TNF-
and
IFN-
mRNA expression as described previously.14
Preliminary testing established that the number of cycle runs and cDNA
dilutions were within the linear range of amplification for the sets of
primers used. Sequences of primers (5'-3') used were as follows:
TNF-
, ATG AGC ACT GAA AGC ATG ATC (sense) and AAA GCC TAG TAA CGG
GAC ACT (antisense); IFN-
, 5' GCA GCG ACT CCT TTT CCG CTT (sense)
and GCA GCG ACT CCT TTT CCG CTT (antisense); ß actin, 5' GTG GGG CGC
CCC AGG CAC CAG (sense) and TAG CAG GCA CTG TAA TTC CTC (antisense).
Values are expressed as ratios of cytokine to ß-actin to account for
RNA isolation and reverse transcription efficiencies. Viral gD primers
used were as follows: GC CCG AGA CCC CCA ACG CCA (sense) and TG CGC GTG
GAC AAG GCG GAC (antisense).
Flow Cytometric Analysis
Isolated splenocytes or cervical lymph node cell populations were
counted by the trypan blue exclusion method and analyzed for cell
surface markers by flow cytometry. Viable cells were blocked with
heat-inactivated fetal bovine serum, washed three times with
fluorescence activated cell sorter buffer (1x PBS with 1% bovine
serum albumin and 0.05% NaN3). For Tg-RAG mice
characterization, cells from cervical lymph nodes were stained with
antiCD4-fluorescein isothiocyanate (FITC), antiCD8-PE. For
detection of transgenic, TCR-positive T cells, KJ1-26.1 (anti-OVA TCR
Ab, a kind gift from Philippa Marrack, National Jewish Hospital,
Denver, CO) antibody was used and was detected by adding goat
anti-mouse IgG2a-FITC antibody. Events were collected and analyzed
using a Becton-Dickinson FACS analyzer.
Virus Recovery
At various time points after infection, swabs of the corneal
surface were taken using sterile swabs soaked in McCoy medium
containing 100 IU/ml penicillin and 100 µl/ml streptomycin (Life
Technologies, Grand Island, NY). The swabs were then placed in tubes
containing 500 µl of the above-mentioned transport medium and stored
at -80°C. For detection of HSV in swabs, the samples were thawed and
vortexed, and 100 µl of each sample from individually marked mice was
used to measure virus concentration for by standard plaque-forming unit
assay on Vero cell cultures as described elsewhere.16
Statistical Analysis
Wherever specified, data obtained were analyzed for statistical
significance by Students t-test.
| Results |
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responses in splenocytes 10 to 12 days p.i. failed to detect
HSV-specific reactivity. Animals also failed to manifest delayed-type
hypersensitivity reactions to HSV (data not shown). In contrast,
animals showed strong proliferative and cytokine responses to
stimulation by OVA peptide. Depletion of CD4+ T cells from
either Tg-RAG mice or BALB/c mice markedly reduced or abrogated the
proliferative responses (Table 2)
. Whereas BALB/c mice, and in addition DO11.10 mice, infected via the
cornea with HSV-1 RE survived infection, similarly infected
Tg-RAG mice were all dead from encephalitis by 11 to 13 days p.i.
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, IFN-
, and the viral protein (gD) at 10
days p.i. All three species of mRNA were present in RNA samples from
Tg-RAG mice, but only the cytokine mRNA species were present in BALB/c
mice (Figs. 5
6)
. We interpret our experiments to mean that HSK lesions were
orchestrated by CD4+ T cells being activated in
the cornea rich in cytokines as a result of continuous HSV replication.
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| Discussion |
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The mice chosen for study were transgenic for the V
and Vß T-cell
receptor, which recognizes a class IIrestricted OVA peptide. The
animals were repeatedly back-crossed to the RAG-2deficient mouse to
produce Tg-RAG mice that possessed only CD4+ T
cells, >98% of which expressed the KJ1-26.1 idiotype characteristic
of the TCR reactive with the OVA peptide (323339). Such mice were
highly susceptible to HSV infection and, as far as could be determined,
failed to generate HSV-specific T-cell responses after infection or
immunization (Gangappa S, Rouse BT, unpublished observations). The mice
also lacked B cells or CD8+ T cells.
Unexpectedly, however, the Tg-RAG mice still expressed lesions typical,
both clinically and histologically, of HSK. Such lesions did not occur
if the HSV-infected, Tg-RAG mice were depleted of
CD4+ T cells. These observations indicate that
the CD4+ T cells are necessary to orchestrate the
immunoinflammatory lesions but fail to function by specifically
recognizing HSV-encoded antigens. They also serve to argue against the
molecular mimicry hypothesis for HSK pathogenesis. Some have advocated
that autoantigens rather than viral antigens provide the targets for
immunoinflammatory CD4+ T cells in the
cornea.8
Our data cannot formally exclude such a
hypothesis, but with the highly restricted T-cell repertoire expressed
by Tg-RAG mice, the notion is unlikely.
Currently, a mechanistic explanation for the immunopathology observed in Tg-RAG mice is lacking. However, our working hypothesis is that CD4+ T cells enter a cornea rich in proinflammatory molecules, present as a consequence of continuous HSV replication in Tg-RAG mice. Thus, in Tg-RAG mice, virus remained present in the cornea until death and disseminated beyond the epithelial location, typical of the situation in immunocompetent mice. Accordingly, by immunocytochemistry, viral antigen was abundantly evident in the stroma itself. We assume that CD4+ T cells entering the cornea, perhaps after effective angiogenesis has occurred, become activated by a non-TCRmediated process and respond by themselves, producing proinflammatory molecules, and orchestrate the inflammatory lesion. A similar idea recently was proposed to explain the pathogenesis of Coxsackie virusinduced, insulin-dependent diabetes mellitus (IDDM) in transgenic mice incapable of recognizing Coxsackie viral antigen or GAD, an antigen often involved in IDDM.19 We are currently attempting to identify the nature of activators that participate in this bystander activation mechanism of immunopathology. It also will be important to establish what part if any bystander activation plays in HSK in immunocompetent mice or in the natural human disease.
| Acknowledgements |
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| Footnotes |
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Submitted for publication May 21, 1999; revised August 10, 1999; accepted September 15, 1999.
Commercial relationships policy: N.
1 Present address: Department of Pathology and Molecular Microbiology, Washington University School of Medicine, 660 South Euclid Avenue, St. Louis, MO 63110.
Corresponding author: Barry T. Rouse, Department of Microbiology, The University of Tennessee, M409, Walters Life Sciences Building, Knoxville, TN 37996-0845. btr{at}utk.edu
| References |
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and IL-2 are protective in the skin but pathologic in the corneas of HSV-1 infected mice J Immunol 149,3023-3028[Abstract]
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