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1 From the Division of Ophthalmology, and the 2 Department of Pathology and Microbiology, University of Bristol, United Kingdom.
| Abstract |
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METHODS. Six weeks after corneal inoculation with HSV-1, the eyes of latently
infected and control mice were UV irradiated and examined for signs of
disease and viral reactivation. The eyes of five mice with recurrent
stromal disease and two controls were processed for
immunohistochemistry on days 4, 7, 10, and 14 after irradiation.
Sections were double stained for viral antigens and one of the
following cytokines: interleukin (IL)-1ß, IL-2, IL-4, IL-6, IL-10,
IL-12, and interferon (IFN)-
.
RESULTS. Fifty percent of mice showed signs of recurrent stromal disease, the
severity of which peaked on day 10 after UV irradiation. There was a
large cellular infiltrate in the stroma of all the corneas with
recurrent disease and the predominant cytokines were IL-1ß, IL-6,
IL-10, IL-12, and IFN-
, all present in large numbers of cells on the
days studied. There were very few cells producing IL-2 and IL-4.
Control eyes had no significant cytokine-producing cells in the stroma.
CONCLUSIONS. These observations suggest that recurrent herpetic stromal keratitis
(HSK) may not be characterized by a classic T-helper (Th)1 or Th2
response. However, the large number of IFN-
+ and
IL-12+ cells and the relative absence of IL-4 favors a Th1
response, and despite the numerous IL-10+ cells, the
overall balance of cytokine production appears to be proinflammatory.
| Introduction |
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It has been shown that T lymphocytes play an essential role in the mouse model of primary infection, and that HSK does not develop in T-celldeficient mice.2 However, the keratitis develops when such mice are given HSV-sensitized T cells.3
Recently, a more complex picture of HSK is emerging and several hypotheses have attempted to explain this disease.4 There is evidence to support a role for both a T-helper (Th)15 6 and a Th27 8 response. In addition, cytotoxic CD8+ T cells9 and an autoimmune response against corneal autoantigens10 have also been implicated.
Some of these contradicting hypotheses may have resulted from different mouse and virus strains used and nearly all the reports investigated the responses after primary infection. However, in humans, HSK is a feature of recurrent corneal infections and occurs in an individual with established viral immunity, a feature absent from the models of primary infection.
Our mouse model of recurrent HSV-1 infection mimics the corneal disease
seen in humans11
12
and using this model, we have
demonstrated a vigorous cellular infiltrate in the corneal stroma,
which consists mainly of neutrophils, but also some
CD4+ T lymphocytes and
macrophages.13
Using this model, it has recently been
shown that both interleukin (IL)-1 and tumor necrosis factor (TNF)-
are important factors in the pathogenesis of recurrent HSK, but little
is known about their source or the involvement of other
cytokines.14
We now report on a wide range of cytokines and the distribution of HSV-1 antigen in recurrent HSK, using a recently developed quantitative immunohistochemical method.15
| Methods |
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Latent Infection and Recurrent Disease
HSV-1 strain McKrae was grown and titrated on Vero cells. Latent
infection and recurrence of HSV-1 infection was induced as described
previously.11
In brief, mice were passively immunized with
an intraperitoneal injection of normal human serum (Chemicon, Temecula,
CA) known to contain HSV-1 antibodies. The serum was diluted in
phosphate-buffered saline (PBS) to give an effective dose
(ED50) of 8000 plaque-forming units
(pfu). After 24 hours, a 5-µl volume containing
104 pfu of HSV-1 was applied to the left cornea
of anesthetized mice, which was subsequently scarified. Control mice
were mock inoculated using the medium of uninfected Vero cells. Only
mice with normal eyes after primary infection were used for
reactivation.
At least 6 weeks after primary infection, mice were anesthetized and their corneas irradiated with UV B light for 90 seconds (UV lamp emitting a peak of 4.03 mJ/cm2 per second at 320 nm; Hanovia, Slough, UK). Mice were examined by slit lamp for signs of recurrent ocular disease using the criteria in Table 1 . Clinical assessments were made immediately before UV irradiation and on days 1 to 7, 10, and 14 after irradiation. Eye washings were taken after each examination for virus isolation.16
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Tissue Fixation and Immunohistochemistry
Tissues were fixed and processed as described
previously.15
In brief, animals were killed and perfused
with periodate-lysine-paraformaldehyde buffer (PLP). Eyes were
enucleated and injected with PLP before fixing overnight at 4°C.
Tissues were then rapidly dehydrated in ethanol and Histoclear
(National Diagnostics, Hull, UK) and infiltrated under vacuum with low
temperature wax. Serial 6-µm sections were cut, and three sections
were transferred to poly-L-lysine precoated glass
microscope slides and stored at -20°C. One hundred fifty serial
sections were cut from each of 20 diseased and 8 control eyes.
Immunohistochemical Staining
For each eye, slides were stained in pairs, and the tissue
sections on the two slides were from two separate areas of the eyes,
approximately 450 µm apart. A pair of slides from each eye was
stained with hematoxylin and eosin (H&E). Separate pairs of slides from
each eye, were also double stained for HSV-1 antigens and one of the
following cytokines: IL-1ß, IL-2, IL-4, IL-6, IL-10, IL-12, and
IFN-
. The clone and dilutions of the primary antibodies are listed
in Table 2
. IL-1ß was obtained from R&D (Oxon, UK); IL-2, IL-10, IFN-
from Harlan Seralab (Loughborough, UK); Il-4 from BD PharMingen (San
Diego, CA); IL-6 from Cambridge Bioscience (Cambridge, UK); and IL-12
from Biosource (Camarillo, CA).
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, and rabbit anti-rat
IgG for the other cytokines. Avidin-biotin peroxidase complex and
diaminobenzidine (DAB; brown stain) reagent were sequentially added
(Vector). Sections were washed in 0.1% PBS-saponin between steps. After staining for cytokines, slides were stained for HSV-1 antigens using a rabbit anti-HSV-1 antiserum, swine anti-rabbit immunoglobulin and rabbit peroxidase anti-peroxidase complex (Dako, Cambridge, UK) before incubation with VIP reagent (purple stain; Vector).
The following control slides were included in each run: experimental eye sections incubated with the appropriate control (isotype) antibody or diluent only, and known cytokine-positive slides were incubated with the appropriate primary antibody or control (isotype) antibody or diluent only (Table 2) . Sections were counterstained with methyl green (BDH, Poole, UK), dehydrated, and mounted.
Microscopy and Quantification
H&E-stained cells and cytokine-positive cells were counted in
the area of greatest infiltrate in the corneal stroma. The cells from
two sections of each slide were counted in a grid area of 0.04
mm2 at x400 magnification, using an image
analysis system (Quantimet; Leica Cambridge, Cambridge, UK). The cell
number was expressed as cells per area (cpa), and a maximum of 255
cells were counted for each area. Data are expressed as mean cell
numbers ± SEM. The MannWhitney test was used for statistical
analysis, and P < 0.05 was considered significant.
| Results |
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The remaining 29 latently infected mice had signs of UV irradiation injury, characterized by corneal epithelial ulceration and uveitis, but they did not show any signs of recurrent HSV-1 infection.
Virus Isolation and Antigen
Of the 74 latently infected mice, 30 shed virus in tears and 22 of
the shedders subsequently showed development of stromal disease (Fig. 1)
. Fifteen mice had clinical signs of recurrent disease, but no virus
was isolated from their tears. Viral shedding occurred between days 2
and 7 after UV irradiation, and the incidence peaked on days 3 and 4
when 16 and 17 mice, respectively, were shedding virus. There was no
correlation between the amount or duration of viral shedding and the
severity of the disease.
All 20 eyes used for immunohistochemistry had stromal disease and 16 had virus detected in the tears at some stage of the disease. Four eyes had signs of recurrent disease in the absence of virus in the tears. Of those 20 eyes, only three showed evidence of HSV-1 antigensone on day 4 and two on day 7. These eyes included the only two that were shedding virus on the day that the eyes were processed. All the other eyes were not shedding virus when the eyes were enucleated. The antigen was seen in small foci in the epithelium on both days, either at the edge of an ulcer or in the deeper layers of the epithelium (Figs. 3H 3L) . Antigen staining was also seen in a few sequential slides in the iris on day 7 in one mouse. No virus was isolated from control mice or latently infected mice before reactivation, and no viral antigen was found in control eyes.
Cytokine Staining
A total of 896 sections were examined under the microscope, and
positively stained cells were counted (Fig. 4)
. In general, cytokine staining was confined to infiltrating cells, and
the relative absence of staining seen in UV-irradiated control animals
was related to the scarcity of infiltrating cells.
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at some time points after UV irradiation (Figs. 3F
4)
. In contrast, corneas with recurrent stromal disease had large numbers of infiltrating cells, particularly in areas underlying epithelial ulcers and near the limbus (Fig. 3E) . They were most numerous in the anterior stromal area but were also seen in the epithelium, particularly in HSV-1 antigenpositive areas and near the edges of ulcers. Although eyes with recurrent disease had increased cellularity of the iris, only one eye had HSV-1 antigen in the iris, and this was associated with a distinct focus of cells. Increased cellularity was also noted in the anterior chamber and between the endothelium and Descemets membrane.
In the corneas with recurrent stromal disease, the number of cells
staining positively for IL-1ß, IL-6, IL-10, IL-12, and IFN-
was
significantly greater than in control corneas (P <
0.05). Examples of such staining can be seen in Figures 3G and 3I
3J
3K
3L
, and the mean cell counts for these cytokines are shown in
Figures 4B
and 4E
4F
4G
4H
. With the exception of IL-1ß, where
there was significantly less staining on day 14 compared with earlier
days (P < 0.05; Fig. 4B
), the mean cell counts for the
other cytokines investigated did not change significantly between
different time points. IL-1ß staining was also seen in central
corneal epithelial cells, in both the infected and the control group,
until day 10 but not on day 14 (data not shown).
There were very few IL-2+ (Fig. 3H) and IL-4+ cells in recurrent stromal disease, and numbers did not vary much with time and were not significantly different from control corneas (P > 0.05; Figs. 4C 4D ). In the corneas with recurrent stromal disease, there was no difference in the pattern of cytokine staining between those 16 mice that shed virus and the 4 mice that did not.
| Discussion |
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The severe ocular signs seen clinically in those mice that had recurrent disease and the large cellular infiltrate that ensued after UV irradiation was not seen in control eyes. This response was likely to be the result of recurrent infection with HSV-1 and not the result of irradiation alone. This observation was supported by the fact that most of the eyes with recurrent disease had shed virus in tears.
However, in eight of the latently infected mice that shed virus, stromal disease did not develop. This may have been due to subclinical disease or an insufficient dose of virus infecting the cornea. Alternatively, recurrent infection of the conjunctiva or eyelid structures may have led to the shedding of small amounts of virus in the tears without the induction of stromal keratitis.
In contrast, the characteristic stromal disease of HSK developed in 15 of the latently infected mice in the absence of detectable virus in their tears. Viral shedding may have occurred between eye washings, or, alternatively, the virus may have infected the deeper layers of the epithelium without detectable surface shedding, as demonstrated in Figure 3L and by others.17
Compared with primary HSV-1 infection, transient and relatively low levels of virus and viral antigen were detected in recurrent disease. Together with the focal nature of the ocular lesions, these observations are consistent with our previous report that such ocular infection originates from reactivation of latency in very few neurons in the TG.18 However, even such a limited presence of virus in the cornea and/or iris appears sufficient to induce HSK, because much of the disease may result from rapid inflammatory responses in an animal with primed antiviral immunity.
Cytokines in Recurrent Infection
It has been shown that UV irradiation of mock-inoculated mice is
associated with an increase in the number of
F4/80+ and CD11b+ cells up
to day 7.13
Furthermore, using the same model, it has been
shown that there is an initial decrease in the number of Langerhans
cells followed by an increase between 7 and 21 days after UV
irradiation.17
In this study, we demonstrated that such
mice had a small number of IL-6+,
IL-10+, IL-12+, and
IFN-
+ cells in the stroma on days 4 and 14
after UV irradiation, as well as IL-1ß staining of the epithelium up
to day 10. The production of IL-1ß by the epithelium and the other
cytokines by perhaps macrophages and dendritic cells in the stroma, may
contribute to the recruitment and activation of antigen-presenting
cells, leading to a rapid innate immune response after recurrent
infection of the cornea.
In contrast to the control animals, there were many more infiltrating
cells in the corneas of mice with recurrent disease. The large numbers
of IFN-
+ and IL-12+
cells in those corneas and the scarcity of IL-4+
cells would suggest a Th1-type response. In addition, the abundance of
IFN-
may exert a local antiviral effect, perhaps accounting for the
low level of HSV-1 antigen. Moreover, this cytokine is recognized as a
powerful potentiator of cell-mediated immune responses,19
particularly those of neutrophils and macrophages, which are known to
be present in this model.13
However, there were very few IL-2+ cells, and IL-2 is a key cytokine in Th1 responses and in T-cell proliferation. Some studies have implicated IL-2 in HSK in the mouse primary infection model but none have demonstrated large quantities of this cytokine in the cornea.5 20 21 22 Therefore, in our study, despite the relative absence of IL-2 in the cornea, T-cell proliferation and IL-2 production could have been occurring in the regional lymph nodes or spleen at the height of recurrent HSK.
Large numbers of IL-1ß+, IL-6+, and IL-10+ cells were also seen in the present study in mice with HSK. Both IL-1ß and IL-6 are produced by many cell types, including lymphocytes and keratocytes, in response to infection, trauma, or immunologic challenge.19 They have both been regarded as proinflammatory cytokines with actions on both Th1- and Th2-type responses. However, the small number of B lymphocytes seen in recurrent HSK13 suggests that both IL-1ß and IL-6 act as proinflammatory cytokines in a cell-mediated response rather than promote a humoral response. The reduction in IL-1ß+ cells on day 14 appeared to correlate with resolution of the clinical signs and thus IL-1ß may be a target for therapeutic intervention in the future.
IL-10, which was also seen in large numbers of cells, is considered to have anti-inflammatory properties and is characteristically associated with Th2 responses. This cytokine has been demonstrated during the resolution of HSK in the primary infection model,5 where it is thought to inhibit Th1 cytokines and produce a shift toward a Th2 type response. Moreover, IL-10 treatment reduces the severity of HSK in the primary model.23 24 25 The early appearance of IL-10 in true recurrent disease may reflect the fact that such disease involves a secondary, and therefore a more rapid, immune response than in primary infection. This immunomodulatory cytokine may contribute to the focal nature of the inflammation, possibly by inhibiting the actions of Th1 type T cells. However, there appears to be an overall proinflammatory cytokine balance.
Our observations suggest that recurrent HSK may not be characterized as a classic Th1 or Th2 response, a feature noted in several other viral infections.26 As with recurrent skin lesions, the relatively short duration, mild severity, and rapid clearance of the virus in recurrent ocular disease is probably the result of a brisk secondary immune response. This may also explain the scarcity of viral antigen compared with primary infection. A broad-spectrum immune response, involving both Th1 and Th2 components, may be advantageous in producing both rapid clearance of the virus and restricted tissue damage. The latter would be particularly important in the cornea, where preservation of its transparency is paramount.
The number, timing, and distribution of the cells staining positive for
IL-1ß, IL-6, IL-10, IL-12, or IFN-
in this study suggests that
these cytokines may be produced by the same cell type.
In a previous study we have shown that the neutrophil is the
predominant and most rapidly infiltrating cell in recurrent
HSK,13
and this was confirmed by H&E staining in the
present study. Because the number, timing, and distribution of these
infiltrating cells was very similar to the cytokine-positive cells, it
seems possible that the neutrophils may be a source of IL-1ß, IL-6,
IL-10, IL-12, and IFN-
, during recurrent HSK. Murine neutrophils
have been reported to produce IL-10 and IL-12,27
and human
neutrophils can produce IFN-
.28
Our observations
therefore support previous suggestions that neutrophils may be
responsible for some of the cytokine production in
HSK.15
29
We are now defining more precisely the role of
such cells in cytokine production.
| Acknowledgements |
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| Footnotes |
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Submitted for publication July 27, 2000; revised October 3, 2000; accepted October 16, 2000.
Commercial relationships policy: N.
Corresponding author: Carolyn Shimeld, Division of Ophthalmology, G43a, School of Medical Sciences, University Walk, Bristol BS8 1TD, UK. c.shimeld{at}bristol.ac.uk
| References |
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are important factors in the pathogenesis of murine recurrent herpetic stromal keratitis Invest Ophthalmol Vis Sci 41,96-102
and IL-2 are protective in the skin but pathologic in the corneas of HSV-1 infected mice J Immunol 149,3023-3028[Abstract]
/ß), in innate and adaptive immune responses to viral infections Semin Immunol 10,383-390[Medline][Order article via Infotrieve]
is produced by polymorphonuclear neutrophils in human uterine endometrium and by cultured peripheral blood polymorphonuclear neutrophils J Immunol 160,5145-5153This article has been cited by other articles:
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