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From the Departments of 1 Cellular and Structural Biology and 2 Microbiology, University of Texas Health Science Center at San Antonio; and the 3 Department of Ophthalmology, Kobe University School of Medicine, Japan.
| Abstract |
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METHODS. Normal BALB/c mice were inoculated with 4 x 104 plaque-forming units (PFU) of the KOS strain of HSV-1 using the AC route. NK activity was measured in the spleen, the superficial cervical and submandibular lymph nodes, and the inoculated eye by lysis of chromium-labeled, NK-sensitive YAC-1 target cells. Histopathologic scoring and immunohistochemical staining for HSV-1 were performed in NK-depleted (injected intravenously with anti-asialo GM1) or mock-depleted (injected intravenously with normal rabbit serum) mice.
RESULTS. In mock-depleted mice, NK activity in the spleens, superficial cervical and submandibular lymph nodes, and inoculated eyes peaked at postinoculation (pi) day 5 and declined by pi day 7. Treatment with anti-asialo GM1 eliminated NK activity in the eye and at nonocular sites. The histopathologic scores at pi day 5 indicated more damage to the retinas of NK-depleted mice than to those of mock-depleted mice, and immunohistochemical staining for HSV-1 showed spread of the virus to the sensory retina only in NK-depleted mice.
CONCLUSIONS. NK cells were activated within 5 days after AC inoculation of the KOS strain of HSV-1. Activation of NK cells appears to play a role in preventing direct anterior-to-posterior spread of the virus in the inoculated eye which, in turn, protects the retina of this eye and helps to explain why the architecture of the retina of this eye is spared.
| Introduction |
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An early, but nonspecific, mechanism of protection operative in many
types of infections is natural killer (NK) cells.6
NK
cells, a subset of large, granular lymphocytes, are activated by the
cytokines interferon (IFN)-
, IFN-ß, and interleukin (IL)-12 and
can secrete cytokines with antiviral activity such as IFN-
and tumor
necrosis factor (TNF)-
when activated.7
8
NK cells lyse
virus-infected cells after conjugate recognition of those cells by a
major histocompatibility complexunrestricted mechanism. However, the
eye constitutively expresses NK-suppressing factors, including TGF-ß
and macrophage migration inhibitory factor, which have been shown to
diminish NK activity in vitro.9
10
11
12
13
NK cells have been
shown to be important in resistance to a number of primarily
intracellular pathogens, including HSV. Intraperitoneal injection of
HSV increases NK cell activity, and NK cells have been shown to
modulate replication of HSV-1 in the liver and
brain.14
15
16
The purpose of the studies described in this
article was to examine the role of NK cells in the injected eye after
uniocular AC inoculation of HSV-1. The results of these studies suggest
that NK cells prevent direct anterior-to-posterior virus spread of KOS
strain of HSV-1 in the injected eye after uniocular AC inoculation.
| Methods |
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Virus
The KOS strain of HSV-1 was used in this study. Virus stocks were
titered on Vero cell (ATCC, Rockville, MD) monolayers, as described
previously.5
Ocular Injection
Mice were anesthetized by intramuscular injection of a cocktail
containing 0.02 ml Rompun (X-JECT 5.A; Uetus Animal Health, St
Joseph, MO) and 0.03 ml ketamine per 25 g body mass. The AC of the
right eye of each mouse was inoculated with 4.0 x 104
plaque-forming units (PFU) of KOS in a volume of 2 µl.
Chromium Release Assay
One million YAC-1 target cells were labeled with 100 µCi
chromium-51 (Du PontNEN, Wilmington, DE) for 1 hour in 500 µl RPMI
1640 with 5% fetal calf serum (RPMI-5) at 37°C. After labeling, the
target cells were washed three times and seeded into 96-well plates at
a concentration of 104 cells/100 µl per well.
Effector cells were obtained from the spleen, the superficial cervical
and submandibular lymph nodes, or the inoculated eye. Splenic effector
cells were prepared by grinding the spleens between frosted slide
glasses, followed by aspiration through a 22-gauge needle to make a
single-cell suspension. Spleen cells were washed three times in Hanks
balanced salt solution (HBSS), and erythrocytes were lysed by ammonium
chloride treatment; the remaining spleen cells were washed three times
in HBSS and resuspended in RPMI-5. Cells from the superficial cervical
and submandibular lymph nodes ipsilateral to the site of injection were
isolated by teasing the tissue through a 200-µm nylon mesh. The cells
were then washed three times and resuspended in RPMI-5. Eyes were
enucleated after cardiac perfusion, and the conjunctiva and extraocular
muscles were removed under the dissecting microscope. The eyes were
teased through a 70-µm mesh, and the cells were washed three times in
HBSS and resuspended in RPMI-5. To have enough effector cells from the
eye for the assays, three or four eyes were collected at each time
point.
Splenocytes, lymph node cells, and ocular cells were counted, and 100 µl per well of the dilution of effector cells needed to give the appropriate effector-target ratio was plated in triplicate. For spontaneous release, 100 µl RPMI-5 and for maximum release, 100 µl 10% Triton X-100 (Sigma, St. Louis, MO) were added instead of effector cells. Plates were incubated for 4 hours at 37°C, and 100 µl of the supernatant in each well was counted in a gamma counter (Wizard 1470; Wallac, Turku, Finland). Each set of triplicates was averaged, and the percentage of specific lysis was determined by the following formula: % specific lysis = (experimental release - spontaneous release)/(maximum release - spontaneous release) x 100. Spontaneous release in all experiments was less than 10% of the maximum release.
Treatment of Mice with Anti-Asialo GM1 Serum
To deplete NK cell activity in vivo, a 10-fold dilution of
anti-asialo GM1 rabbit serum (Wako Chemicals,
Richmond, VA) was made in sterile phosphate-buffered saline (PBS) and
injected into a tail vein in a volume of 0.1 ml. A single intravenous
injection of this amount of anti-asialo GM1
reduced poly(I-C) activated splenic NK activity to undetectable levels
for 4 days (not shown). This regimen had no effect on the splenic
T-cell populations as determined by flow cytometry (not shown). Control
animals were mock depleted with intravenous injections of the volume
and concentration of normal rabbit serum (Vector Laboratories,
Burlingame, CA). To maintain depletion of NK activity, anti-asialo
GM1 serum was injected intravenously every 4 days
beginning on day -1.
Histopathologic Evaluation of the Retina
Eyes were fixed in buffered formalin, embedded in paraffin, and
sectioned. The sections were then stained with hematoxylin and eosin. A
modification of the semiquantitative histopathologic scoring system
described by Azumi and Atherton5
was used to evaluate the
extent of retinal destruction, inflammatory cell infiltration,
hypercellularity of the choroid, and amount of pyknotic debris as shown
in Table 1
. The maximum possible histopathologic score for a single retina
was 13.
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| Discussion |
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In the normal eye, immune privilege is maintained and inflammatory damage is prevented by immunosuppressive cytokines, such as TGF-ß and macrophage migration inhibitory factor, that are produced within the eye. However, these immunosuppressive ocular cytokines, which have also been shown to suppress NK activity,9 10 11 12 13 do not prevent induction of an NK response after uniocular AC inoculation of the KOS strain of HSV-1. The results from the studies presented in this article demonstrate that after uniocular AC inoculation of the KOS strain of HSV-1, there is both a local (eye and superficial cervical and submandibular lymph nodes) and a systemic (spleen) NK response, shown by cytolytic activity of mononuclear cells isolated from these sites against NK-sensitive YAC-1 targets. The reason ocular factors that normally suppress NK activity do not inhibit NK activation after AC inoculation of HSV-1 may be that the extensive virus replication and inflammation that occur within 1 to 2 days after AC inoculation1 17 outstrip the eyes ability to produce these factors.
However, merely determining that AC inoculation of HSV-1 induces both a local and a systemic NK response does not provide information about the role of such a response during ocular infection or about how NK cells limit virus spread in the injected eye. Results from the immunohistochemistry and histopathologic studies suggest that depletion of NK cells allowed HSV-1 to spread to the ipsilateral retina early after infection and that subsequent virus replication in the retina led to increased retinal damage in NK-depleted mice. Interestingly, virus was observed at pi day 3 in the choroid, but not in the sensory retina, in both NK-depleted mice and mock-depleted mice. In both groups of mice, no virus-infected cells were detected in the choroid or RPE at pi day 5. However, at pi day 5, the retinas of NK-depleted mice had more histopathologic changes than the retinas of non-NK-depleted mice, and the retinal changes were observed coincident with virus infection of the retina. This observation suggests that after AC inoculation of HSV-1, the virus is able to spread to the choroid irrespective of the presence or absence of NK cells. However, NK cells or their products eliminate HSV-1 from the choroid of normal mice by pi day 5, and in so doing, protect the retina of the injected eye from virus infection and destruction.
There is a paucity of information about how NK cells or their products limit virus spread and replication within the eye. NK cells have been reported in the anterior segment (iris and corneal limbus) 5 days after AC inoculation of HSV-1 in BALB/c mice.18 Recently, NK cells have been shown to modulate retinal destruction in a mouse model of cytomegalovirus retinitis.19 It is not clear whether these cells are normally present within the eye and are activated by virus infection within the ocular compartment, whether these cells are activated at an extraocular site and migrate to the eye, or whether there are both ocular and extraocular populations of such cells.
In summary, the results presented in this article support the idea that NK cells are activated after uniocular AC inoculation of the KOS strain of HSV-1 and that these cells are important in preventing spread of this virus from the infected AC to the retina of the inoculated eye. Additional studies will be required, however, to determine the mechanism by which such cells prevent virus spread and the origin and site of activation of these cells.
| Footnotes |
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Submitted for publication April 20, 1999; revised August 27, 1999; accepted September 9, 1999.
Commercial relationships policy: N.
Corresponding author: Sally S. Atherton, Department of Cellular and Structural Biology, Mail Code 7762, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900. atherton{at}uthscsa.edu
| References |
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