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1 From the Molecular Pathogenesis of Eye Infection Research Center, Dean A. McGee Eye Institute, Departments of Ophthalmology and 2 Microbiology and Immunology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma.
| Abstract |
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METHODS. HCFs grown from human donor corneas and passaged thrice were infected
with a corneal isolate of Ad19 or mock-infected with virus-free media.
Bioactivity of the cell supernatants was tested by a neutrophil
chemotaxis assay. Supernatants were assayed by enzyme-linked
immunosorbent assay for the neutrophil chemotactants interleukin-8
(IL-8) and GRO-
. Corneal facsimiles were generated with HCFs
and collagen type I, infected with Ad19, and assayed by
immunohistochemistry.
RESULTS. Ad19 infection of HCFs increased neutrophil chemotaxis from a baseline
of 0.4 ± 0.7 cells/high-powered field (hpf; mock-infected) to
21.8 ± 2.3 cells/hpf (Ad19-infected). Chemotaxis was reduced by
the addition of neutralizing antibodies against IL-8 and GRO-
.
Infection of HCFs induced quantities of IL-8 protein 300- and 1000-fold
over mock-infected controls at 4 and 24 hours, respectively (33 versus
11,813 pg/mL at 4 hours, and 57 versus 76,376 pg/mL at 24 hours,
P
0.001 for both). In contrast, GRO-
protein
levels were only sevenfold higher at 24 hours postinfection (118 pg/mL
in mock-infected controls versus 880 pg/mL in Ad19-infected cell
supernatants). Neither chemokine was induced by infection of an
immortalized human corneal epithelial cell line. Immunohistochemistry
of infected corneal facsimiles demonstrated IL-8 in the extracellular
matrix within 3 days after infection.
CONCLUSIONS. Production of chemokines in infected tissues facilitates an early innate immune response to infection, and in the infected corneal stroma represents an elementary defense mechanism. Interleukin-8 may play a role in the development of subepithelial infiltrates in adenovirus keratitis.
| Introduction |
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The cells in residence within the normal human corneal stroma,
the keratocytes, maintain the corneal stroma extracellular matrix in a
precisely organized and transparent state. In addition to their
maintenance functions in the healthy eye, keratocyte responses to
corneal wounding are critical to healing.5
The significant
inflammatory response to stromal infection by a diverse array of
pathogens suggests an additional biological role for keratocytes: the
capacity to amplify acute inflammation in the presence of infection.
Indeed, keratocytes secrete proinflammatory chemokines, such as the
neutrophil chemotactants interleukin-8 (IL-8)6
and
GRO-
,7
in response to a variety of chemical and
infectious stimuli,8
9
10
and may contribute to necrotizing
corneal stromal inflammation due to herpes simplex virus11
and Gram-negative bacteria.12
13
Thus, keratocytes play a
key role in the inflammatory retort to both corneal injury and
invasion.
We hypothesize that the inflammatory cell infiltrates in the adenovirus-infected cornea represent focal areas of stromal infection and manifest due to upregulation of neutrophil chemotactants by keratocytes. We examined adenovirus-infected human corneal cells for their capacity to induce neutrophil chemotaxis by the secretion of chemokines, and used an in vitro model of infection that mimics infection of the human corneal stroma in vivo.
| Methods |
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Corneal facsimiles17 were generated by seeding HCFs at a final concentration of 105 cells/ml in rat tail collagen, type I (Becton Dickinson, Bedford, MA), prepared according to the companys instructions. While still in the fluid phase, the HCF/collagen mixture was plated in individual 6.5-mm Transwell tissue culture inserts (Costar, Cambridge, MA) at 300 [mu]l/insert, and the inserts placed in 12-well tissue culture plates. The facsimiles were allowed to gel briefly at room temperature, then fed with DMEM 10% FBS with antibiotics, and incubated overnight at 37°C in 5% CO2.
Human polymorphonuclear neutrophils were isolated by a technique adapted from that of Harvath et al.18 Twenty-five milliliters of whole blood, harvested from volunteer donors, was overlayed onto 15 ml of Lymphocyte Separation Medium (Life Technologies), followed by centrifugation at 1000g for 20 minutes. The neutrophil/erythrocyte layer was gently mixed with 8 ml of 6% dextran solution, followed by low-speed centrifugation and resuspension in 200 µl calcium/magnesium-free phosphate-buffered saline. The erythrocytes were lysed thrice by the addition of 10 ml ice-cold distilled water and 5 ml of 3.6% NaCl followed by a gentle vortex and low-speed centrifugation. Neutrophil viability was assessed by trypan blue staining.
Adenovirus type 19 (Ad19) cultured directly from the cornea of a patient with EKC was grown in human lung carcinoma cells (A549 cells, CCL 185; American Type Culture Collection, Rockville, MD) in minimum essential medium with 2% FBS, penicillin G sodium, streptomycin sulfate, and amphotericin B. The State of Oklahoma Department of Health confirmed the viral serotype. Typical adenoviral cytopathic effect, positive immunofluorescent staining for adenovirus hexon proteins, and increasing titers of virus within 1 week after infection of human corneal cells with this virus have been previously described.15 Adenovirus stock was purified by cesium chloride gradient. The Tissue Culture Infectious Dose (TCID) of the purified Ad19 preparation was determined, and the virus stored at -80°C.
Adenoviral Infection of Human Corneal Cells
Cells grown to 95% confluence in 48-well plates were washed
gently with OptiMEM (Life Technologies, Gaithersburg, MD). The plates
were infected in duplicate or triplicate with purified Ad19 at a
multiplicity of infection (MOI) of 10 or with OptiMEM without virus as
a control. Virus was adsorbed at 37°C for 1 hour before the addition
of additional OptiMEM to virus- and mock-infected cultures. At 4 and 24
hours after viral adsorption, cell supernatants were aspirated,
centrifuged to remove cellular debris, and stored at -20°C for
subsequent experiments. In viral growth curve studies, cells and
supernatants were removed together at select times post-adsorption with
the assistance of a cell scraper, freeze-thawed, centrifuged, and the
resultant supernatants titered in triplicate in A549 cells. Corneal
facsimiles were infected with purified Ad19 at a MOI of 10 based on the
number of cells seeded in each Transwell tissue culture insert on the
day before infection.
Neutrophil Chemotaxis Assay
A neutrophil chemotaxis assay was performed according to
previously published methods.18
Four-hour infected or
mock-infected HCF supernatants were placed in the bottom well of
blind-well chambers (Poretics, Livermore, CA) and each well covered
with a polyvinyl-pyrrolidone-free 3-µm pore size polycarbonate filter
(Poretics). Monoclonal antibody to IL-8 (10 µg/ml, MAB208; R&D
Systems, Minneapolis, MN) and GRO-
(5 µg/ml, MAB275; R&D) were
added to the viral-infected supernatants in separate experimental
chambers. Concentrations of monoclonal antibodies were chosen to
maximally inhibit neutrophil chemotaxis. Freshly isolated human
neutrophils with greater than 90% viability by trypan blue exclusion
were diluted at 106 viable cells/ml in OptiMEM
and placed above the polycarbonate filter for incubation of 1 hour at
37°C. The membranes were then gently removed, and any neutrophils on
the top of the membrane gently scraped off with a scalpel. The membrane
was then placed bottom side up on a glass slide and fixed in methanol
for 2 minutes, air-dried, and stained with Diff-Quick (Dade
Diagnostics, Aguada, PR). Neutrophils on the bottom of the membrane
were counted in masked fashion in 10 high-powered fields (hpfs) for
each slide with an Axiovert 135 microscope (Zeiss, Thornwood, NY). The
capacity of virus- versus mock-infected cell supernatants to induce
neutrophil chemotaxis was compared by Students t-test. A
value of P < 0.05 was considered significant.
Enzyme-Linked Immunosorbent Assay for Neutrophil Chemotactants
In separate experiments, viral-infected and mock-infected,
triplicate, 4- and 24-hour postinfection HCF and HCEC supernatants were
assayed by enzyme-linked immunosorbent assay (ELISA) for IL-8
(Cytoscreen immunoassay kit; Biosource, Camarillo, CA) and GRO-
(Quantikine Immunoassay, R&D) proteins, according to the
manufacturers instructions. Tumor necrosis factor-
(TNF-
;
Genzyme, Cambridge, MA), diluted in OptiMEM to 500 U/ml, was used as a
positive control to stimulate chemokine production in the absence of
virus.11
The specificity of the ELISA was evaluated by the
addition of anti-IL8 or antiGRO-
at concentrations of 1 or 10
µg/ml (IL-8) and 0.05 µg/ml (GRO-
) before virus adsorption.
Plates were read on an Emax microplate reader (Molecular Devices,
Sunnyvale, CA) and analyzed with SOFTmax analysis software (Molecular
Devices). The means of triplicate ELISA values for each of the virus-
or mock-infected wells were compared by Students t-test.
Immunohistochemistry on Corneal Facsimiles
At various times after infection with Ad19, facsimiles were
removed from the tissue culture inserts and fixed in 10%
neutral-buffered formalin before paraffin embedding and cutting of
5-µm sections. Immunohistochemistry was performed for IL-8 using
polyclonal goat anti-human IL-8 antibody (R&D), and for adenovirus
capsid antigen using monoclonal mouse anti-adenovirus antibody (Cell
Marque, Austin, TX) followed by a biotinylated rabbit anti-goat
antibody (DAKO, Carpinteria, CA) or biotinylated anti-mouse antibody
(BioPath, Oklahoma City, OK), respectively. The reaction was developed
using an Ultra-LINK horseradish peroxidase detection kit (BioPath).
Slides were counterstained with hematoxylin and photographed on an
Axiovert 135 microscope (Zeiss).
| Results |
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together completely inhibited
neutrophil chemotaxis.
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0.01), but the level of IL-8 in infected
HCEC supernatants reached only 80 pg/ml (Fig. 3A
). GRO-
secretion was below the sensitivity of the assay for both
infected and uninfected HCECs (data not shown). Infection of HCFs for 4
and 24 hours induced quantities of IL-8 protein 300- and 1000-fold,
respectively, over mock-infected controls (33 versus 11,813 pg/ml at 4
hours, P
0.001, and 57 versus 76,376
pg/ml at 24 hours, P
0.001; Fig. 3B
). In contrast,
GRO-
protein levels in infected HCF supernatants were only sevenfold
higher than in uninfected controls at 24 hours and achieved much lower
levels than seen for IL-8 (118 pg GRO-
/ml in mock-infected versus
880 pg/mL in Ad19-infected HCF supernatants at 24 hours,
P
0.002; Fig. 4
).
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| Discussion |
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Chemokines are produced by nearly all human cells and cause leukocyte
chemotaxis with a high degree of specificity. Because of existing
evidence that polymorphonuclear neutrophils infiltrate the corneal
stroma in EKC,3
we focused our attention on
neutrophil-specific chemokines. Perhaps the best characterized
chemokine, IL-8 is produced by a variety of human cells and strongly
and selectively induces neutrophil chemotaxis and degranulation with a
long duration of action.6
Tissue infection, ischemia, and
trauma each can lead to the induction of IL-1 and TNF-
, and these
cytokines strongly induce IL-8 production by multiple cell
types,21
including HCFs.8
Herpes simplex
virus infection11
and UV light22
induce IL-8
synthesis by HCFs. These data suggest that IL-8 induction could be a
final common pathway of corneal inflammation for a variety of corneal
tissue insults. In our study, we demonstrated neutrophil chemotaxis
toward supernatants of adenovirus-infected HCFs, with inhibition of
chemotaxis in the presence of antibody against IL-8, and increased
levels of IL-8 protein in infected HCF supernatants. Under our
experimental conditions, infection induced only modest increases in
secretion of GRO-
, another neutrophil chemotactant. Our data also
showed no appreciable induction of neutrophil chemotaxis and minimal
increases in IL-8 production by adenovirus-infected HCECs. Taken
together, these data are consistent with the clinical observation of
significant neutrophil migration into an infected cornea when the
pathogen has breached the stroma.
It remains unclear whether a complete viral replicative cycle is necessary for adenovirus to induce chemokine synthesis by HCFs. Trousdale and coworkers4 demonstrated subepithelial infiltrates in rabbit corneas after intrastromal injection of a replication-deficient adenovirus strain, suggesting that production of infectious virus is not a prerequisite for stromal inflammation. Indeed, Bruder and Kovesdi23 showed IL-8 production by A549 cells in the presence of UV-inactivated but not heat-inactivated adenoviral gene vectors, suggesting that the interaction between the adenovirus and its cellular receptor is necessary and sufficient to induce IL-8 gene transcription. These observations are consistent with one aspect of Jones theory19 of subepithelial infiltrate formation: The presence of nonreplicating viral components in the corneal stroma may be sufficient to induce an inflammatory signal.
Finally, because of a lack of immunologic reagents for available animal models of adenovirus corneal infection,3 24 we used the cornea facsimile model17 to study adenovirus infection of human corneal cells in a simulated tissue microenvironment. Although an oversimplification of the human cornea, this allowed us to test directly the capacity of HCFs to secrete IL-8 into a surrounding extracellular matrix. The demonstration of IL-8 within the "stroma" of an infected corneal facsimile is significant, because IL-8 does not reach appreciable intracellular levels and does not specifically bind to collagen. We propose that the subepithelial location of adenovirus-induced inflammatory infiltrates in the cornea may be due to the presence of potential binding sites for IL-8 in the region of Bowmans and epithelial basement membranes, as has been suggested by investigations in other systems.25
In summary, the induction of chemokines by infected tissues amplifies the innate immune response to the infection. In the infected cornea, chemokine production and subsequent leukocyte infiltration may confine the pathogen and potentially prevent its entrance into the interior of the eye. In adenovirus keratitis, the induction of neutrophil migration into the cornea might serve to limit the extent and duration of infection. Our data support the potential role of IL-8 in the pathogenesis of adenovirus-induced subepithelial corneal infiltrates.
| Footnotes |
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Submitted for publication June 15, 1999; revised October 6, 1999; accepted November 8, 1999.
Commercial relationships policy: N.
Presented at the annual meeting of the Association for Research in Vision and Ophthalmology, Fort Lauderdale, Florida, May, 1999.
Corresponding author: James Chodosh, DMEI-OUHSC, 608 Stanton L. Young Boulevard, Oklahoma City, OK 73104. james-chodosh{at}ouhsc.edu
| References |
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