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1 From the Department of Anatomy/Cell Biology, Wayne State University School of Medicine, Detroit, Michigan.
| Abstract |
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METHODS. Ocular disease was determined by mean clinical score, slit lamp, plate counts, and histopathology, and antigen-specific cellular responses were assessed by immunostaining and measurement of delayed type hypersensitivity (DTH).
RESULTS. Strains of mice favoring Th1 (B6, BL10, and B10.D2) versus Th2 (BALB/c, BALB/cBy, BALB.B, and BALB.K) responsiveness were infected with P. aeruginosa. Mice favoring Th1 response development exhibited a similar course of disease and the infected eyes of all mice perforated by 7 days postinfection (p.i.). Strains (BALB/c, BALB/cBy, BALB.B, and BALB.K) favoring Th2 response development exhibited a milder course of disease, and none of the infected corneas perforated at 7 days p.i. In a Th1-responsive strain (B10.D2), positive immunostaining for CD4+ and CD8+ T cells was observed in the cornea by 3 days p.i. and by 5 days p.i., respectively, some cells stained positively for IL2-R, indicating that the cells were activated. In contrast, in a Th2 responder strain (BALB/c), there was no detectable positive immunostaining in cornea for any of the T-cell markers tested and DTH was significantly elevated in B10.D2 versus BALB/c mice.
CONCLUSIONS. These studies are the first to provide evidence that in P. aeruginosa ocular infection, mouse strains favoring development of a Th1-type response are susceptible (cornea perforates), whereas strains favoring Th2 response development are resistant (no corneal perforation).
| Introduction |
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) was neutralized by MAb before
corneal infection. Similarly, many studies of Th1 versus Th2 responses
have shown that in C57BL/6 and related inbred strains, Th1
responsiveness is predisposed in response to a wide variety of
infectious pathogens and antigens.3
4
5 Recently, the influence of the genetic background of the T-cell on T-helper (Th) phenotype development has been reported.6 Some mouse strains (e.g., B10.D2) were shown to favor Th1, whereas other strains such as BALB/c6 and related strains,7 8 9 favored Th2 T-cell response development. Others have already begun to examine T-cell response profiles as a potential basis for susceptibility to other ocular diseases, such as experimental autoimmune uveitis,5 using some of these naturally resistant versus susceptible animal models. In contrast, except for a recent abstract,10 no other studies have used these natural models to directly test whether T-cell response profiles are important in determination of susceptibility versus resistance to ocular bacterial disease caused by P. aeruginosa. Therefore, the purpose of the present study was to test the hypothesis that after corneal infection with P. aeruginosa, Th1- versus Th2-responsive mouse strains will evidence a susceptibility (cornea perforates) versus resistance (less corneal disease, no perforation) phenotype, respectively.
| Materials and Methods |
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Infection and Ocular Response
To assess the ocular disease response, mice were challenged with
P. aeruginosa strain 19660. Briefly, mice were anesthetized
with Aerrane (Anaquest Inc., Liberty Corner, NJ), and the left cornea
was scarified with a 255/8-gauge needle. Three 1-mm incisions
were made to the corneal surface, which penetrated no deeper than the
superficial stroma. A 5-µl aliquot containing 1.0 x
106 CFU suspension of bacteria was applied to the
wounded cornea, and the ocular disease response was evaluated
macroscopically and microscopically daily for up to 10 days. The
corneal disease response was graded using the following, previously
described11
scale: 0, clear or slight opacity, partially
or fully covering the pupil; +1, slight opacity, fully covering the
anterior segment; +2, dense opacity, partially or fully covering the
pupil; +3, dense opacity, covering the entire anterior segment; +4,
corneal perforation or phthisis. Mean clinical scores were calculated
by summation of the scores for each group (n = 5/group,
repeated 3 times) of mice divided by the total number of mice scored at
each time point.
Quantitation of Viable Bacteria
Individual corneas (n = 3/group) were removed from
the eyes of B6 and BALB/c mice at days 1, 5, and 9 postinfection (p.i.)
using a sterile razor blade and homogenized with glass tissue grinders
(Fisher Scientific, Itasca, IL) in 1 ml of sterile phosphate-buffered
saline (PBS) containing 0.5% bovine serum albumin (BSA). To quantitate
viable bacteria per cornea, a 0.1-ml aliquot of the corneal homogenate
was serially diluted 1:10 in sterile PBS-BSA. Serially diluted samples
were plated in triplicate (0.1 ml/plate) onto Pseudomonas
isolation agar (Difco) plates and incubated for 24 hours in a
water-jacketed CO2 incubator (American Scientific
Products, McGaw Park, IL) at 37°C and 5% CO2.
The lower limit of detection of this method is 100 CFU/cornea. For
corneas with less than 100 CFU of viable bacteria, a value of 0 CFU was
assigned and used to calculate the mean number of CFU. The values of
CFU/cornea are expressed as log10 and shown as
the mean CFU ± the SEM.
Histopathology
Infected eyes from Th1 (B6, BL10, and B10.D2) and Th2 (BALB/c,
BALB.B, and BALB.K) mice (n = 3/strain) were enucleated
on the day before perforation (6 days, p.i.) in the Th1-responsive
mice. The procedure for embedding eyes in resin and sectioning for
light microscopy has been previously described.12
In
brief, mice were killed by cervical dislocation, and the infected and
contralateral (control) eye was enucleated and fixed on ice in a 1:1:1
solution of 2.0% osmium tetroxide, 2.5% glutaraldehyde in 0.1 M
sodium phosphate buffer (pH 7.4), and 0.2 M Sorensons phosphate
buffer (pH 7.4). After 3 hours fixation, eyes were rinsed with 0.1 M
phosphate buffer, dehydrated in graded ethanols and propylene oxide,
and then infiltrated and embedded in epon araldite. Thick (1.5 µm)
sections were cut with glass knives on a Reichert-Jung Ultracut E
ultramicrotome (Cambridge Instruments, Buffalo, NY) and stained with
Richardsons stain13
for histopathologic evaluation.
Immunocytochemistry
Eyes from BALB/c, BALB/cByJ, B10.D2, and BL10 mice
(n = 3/strain/time) were enucleated at 1, 3, 5, 6, and
7 days p.i. Eyes were embedded in Tissue Tek (Miles, Elkhart, IN),
snap-frozen, and stored at -20°C until sectioned. Sections (10 µm)
were cut on a Micron cryostat (Fisher Scientific, Itasca, IL) and
mounted to poly-L-lysinecoated slides (Polysciences Inc.,
Warrington, PA). Before immunostaining, slides were stored at 37°C
overnight and then fixed in cold acetone at -20°C for 2 minutes
After fixation, slides were rinsed several times with 0.01 M PBS. To
block nonspecific binding, each section was covered with 20 µl PBS
containing 1.0% BSA and 0.05% Tween 20 and incubated at room
temperature (RT) in a moist chamber for 30 minutes. Sections were
incubated for 1 hour with primary MAbs specific for CD4 (rat IgG2a,
clone H129.19, 1:10), CD8 (rat IgG2a, clone 53-6.7, 1:25), and CD25
(IL-2R, rat IgM, clone 7D4, 1:50) (all from Pharmingen, San Diego, CA).
Sections were incubated with 0.3% hydrogen peroxide for 30 minutes to
block endogenous peroxidase activity. They were then incubated for 1
hour with the appropriate biotinylated secondary Ab, anti-rat IgG2a
(CD4, 1:25 and CD8, 1:50) and anti-rat IgM (IL-2R, 1:100) (Pharmingen).
Horseradish peroxidaseconjugated avidin (1:25 to 1:100 concentration;
Zymed, San Francisco, CA) was then incubated with the sections for 30
minutes Color was developed with 3,3'-diaminobenzidine
tetrahydrochloride (1015 minutes) containing cobalt and nickel
chloride (Pierce, Rockford, IL) to visualize positively reacted cells.
Control sections were similarly treated after incubation with an
irrelevant rat anti-human MAb (HLA-DR5, clone SFR3-DR5, IgG2b; ATCC).
Delayed Type Hypersensitivity
For this assay, B10.D2 and BALB/cJ mice (n =
5/group, repeated once) were infected ocularly, as described above. At
6 days p.i., 2.0 x 107 CFU of heat-killed
P. aeruginosa cells (10 µl in 0.01 M PBS) were injected SC
into the ear pinna ipsilateral to the infected eye. PBS was injected
into the contralateral ear as a control. Ear thickness was measured
with an engineers micrometer (Mitutoyo, Tokyo, Japan) just before
injection and at 24 and 48 hours after antigen challenge. Delayed type
hypersensitivity (DTH) was calculated using the following equation:
(24- or 48-hour measurement - 0-hour
measurement)Ag challenged ear - (24- or
48-hour measurement - 0-hour measurement)PBS-challenged
ear.14
Statistical Analysis
2 analysis was used to determine the
significance of the ocular disease response (mean clinical score) data
and an unpaired, two-tailed, Students t-test was used to
analyze differences in the plate count and in DTH. Data are presented
as the means ± SEM. A P
0.05 confidence
interval was used to determine the level of significance of
differences.
| Results |
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| Discussion |
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A paradigm for bacterial disease pathogenesis and the role of Th1
versus Th2 responsiveness has evolved from numerous studies using a
mouse leishmaniasis model.28
Data using this model,
although not directly applicable to a pseudomonal infection, are
thought provoking, in that they indicated that the cytokines produced
against the parasite during the primary infection controlled
differentiation of CD4+ T lymphocytes and the
quality of the subsequent immune response.28
According to
this paradigm, CD4+ Th1 (type 1) cell-mediated
immunity confers protection against parasitic infection, whereas the
development of a CD4+ Th2 (type 2) humoral immune
response is associated with disease progression.28
In
BALB/c mice, Leishmania major infection causes a progressive
disease that ultimately results in death, with the predominant
expression of IL-4 and relatively little IFN-
production being
detected. Parasitic infection in other strains of mice such as B10.D2,
C3H, C57BL/6, C57BL/10, and 129 produces self-healing lesions
associated with a strong Th1-cellmediated immune response with
production of high levels of IFN-
and other inflammatory
cytokines.3
In contrast, in the ocular model of infection
used in a previous study2
and in the work reported herein
using the extracellular pathogen, P. aeruginosa, a Th1-type
of inflammatory response eventually may contribute to a decrease in the
number of viable bacteria in the cornea, but the price the susceptible
host pays is dissolution of the corneal stroma and blindness. The
corneas of Th2 responder mice such as the BALB/c had not been tested
before for T-cell subset infiltration into the cornea. Therefore,
immunostaining was used to spatially and temporally test for the
presence of CD4+ and CD8+ T
cells in the corneas of BALB/c mice, as had been done previously in B6
mice.2
Because B6 mice have been described genetically as
Th1 "driven" and BALB/c mice as Th2 "driven" to many
antigens,6
determining if CD4+ or
CD8+ T cells were present in BALB/c corneas was
done as a logical first step before testing for specific type 1 or type
2 cytokine profiles in the cornea. B10.D2 and BL10 corneas were used
for immunostaining, and as reported before,2
for B6 mice,
CD4+ and CD8+ T cells were
first detected in the corneas of both (B10.D2 data shown) these Th1
responder strains of mice at 3 days p.i.; by 5 and 7 days p.i.
immunopositively stained cells were increased and activated, based on
positive staining for IL-2R. In another murine model of disease induced
by infection with Toxoplasma gondii,29
the
presence of CD4+ T cells in B6 mice also has been
demonstrated at early times after infection and has been shown to
predispose these mice to rapid ileal necrosis and death within 7 days
p.i. Quite surprisingly, the corneas of BALB/c or BALB/cBy mice did not
express positive immunostaining for either CD4 or CD8 T-cell surface
markers after infection, despite several repetitions of this experiment
using a sensitive immunostaining technique. In combination, the
immunostaining and the DTH data strongly suggest that resistant BALB/c
(Th2 responder) mice lack a CD4+ Th1-type of
T-cell response to ocular P. aeruginosa challenge. Although
the mechanism of resistance in Th2 responder mice is not yet fully
understood, the data presented herein are provocative and warrant
further testing of the hypothesis that Th2-responsive mice, when
compared with Th1 responder strains, regulate the inflammatory cellular
infiltrate more efficiently. Specifically, resistance may be achieved
by the ability of Th2 responder mice to downregulate the inflammatory
response, resulting in less stromal damage and destruction of the
corneal stroma.
| Footnotes |
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Submitted for publication June 18, 1999; revised September 22 and October 20, 1999; accepted October 27, 1999.
Commercial relationships policy: N.
Corresponding author: Linda D. Hazlett, Wayne State University School of Medicine, Anatomy Department, 540 East Canfield Avenue, Detroit, MI 48201. lhazlett{at}med.wayne.edu
| References |
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-mediated necrosis of the small intestine with genetic susceptibility of mice to peroral infection with Toxoplasma gondii J Exp Med 184,597-607This article has been cited by other articles:
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