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1 From the Ocular Immunology and Uveitis Service, Massachusetts Eye and Ear Infirmary; 3 Department of Cancer Immunology and AIDS, Dana-Farber Cancer Institute; and 2 Departments of Ophthalmology and 4 Pathology, Harvard Medical School, Boston, Massachusetts.
| Abstract |
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METHODS. Anterior chamber inoculation of the right eye of each mouse with various titers of HSV-1 (KOS strain) was performed. Both eyes of each mouse were enucleated on postinoculation day 15 and processed for histopathologic examination. HSV-1 was inoculated into one cornea of other mice, and the severity of stromal keratitis was scored.
RESULTS. Contralateral destructive chorioretinitis developed in susceptible Balb/cByj mice (19/23); ipsilateral chorioretinitis did not occur (0/23). Stromal keratitis developed in susceptible C.AL-20 mice (15/16). None of the C57BL/6 (0/10 for keratitis or 0/20 for retinitis) developed inflammation. Neither did B6.SMN.C3H.gld (FasL deficient; 0/12 or 0/28) or B6.MRL.lpr (Fas deficient; 0/11 or 0/34) mice (keratitis or contralateral chorioretinitis). Minimal scattering of inflammatory cells in the contralateral retina but not destructive chorioretinitis was observed in two C57BL/6, three B6.SMN.C3H.gld, and five B6.MRL.lpr mice. Few inflammatory cells were also found in the ipsilateral vitreous and vitreoretinal interface (but not destructive chorioretinitis) of all C57BL/6, two gld, and three lpr mice.
CONCLUSIONS. Immune dysregulation secondary to deficiency in Fas or FasL system does not influence the resistance of the C57BL/6 mice to develop herpes simplex keratitis or destructive herpes simplex chorioretinitis.
| Introduction |
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We have identified, using Igh-1 disparate congenic murine strains, a profound influence of gene products from the Igh-1 or closely associated loci on susceptibility to development of the destructive inflammatory response to HSV in von Szily retinitis. Thus, Balb/cByj mice (Igh-1a) are susceptible, whereas their Igh-1 congenic relatives C.B-17 (Igh-1b) and C57BL/6 mice (Igh-1b) are resistant to contralateral retinitis.17 The Igh-1associated gene products control both humoral and cellular aspects of the immune response,18 and we and others have demonstrated that T cells,18 19 20 21 B cells,22 and natural killer cells and macrophages23 play a role in the pathogenesis of or protection from contralateral HSR and ipsilateral retinal sparing.16 20 23 The kinetics of the immune response, virus specific delayed-type hypersensitivity reactions and virus neutralizing antibodies, has been shown to differ in the resistant and susceptible mice.21
Resistance to a second HSV-1induced ocular disease, herpes simplex keratitis (HSK), is also controlled by the IgH locus.24 Inbred mouse strains that carry the Ighd/e alleles (e.g., C.AL-20 [Ighd]) are susceptible to necrotizing stromal keratitis induced by infection with HSV-1, whereas congenic strains that carry the Ighb allele (C.B-17) are highly resistant.25 The resistance to HSK in mice is associated with allotypic variation in immunoglobulin genes,26 possibly because circulating immunoglobulin-derived peptides can cross-tolerize T cells specific for corneal tissue autoantigens.27
Fas (CD95) is a 36-kDa cell surface protein that induces apoptosis in appropriate target cells. Fas is expressed on a variety of cell types, including hemopoietic, epithelial, and other cells.28 The ligand for Fas (FasL) has a more restricted pattern of expression. Expression of FasL is induced on mature CD4+ and CD8+ T lymphocytes after their activation, but it is not expressed on any other hemopoietic cells.29 30 FasL is also constitutively expressed in two immunologically privileged tissues: the eye and the testis.31 Recently, Fas and FasL have been implicated in regulating the normal immune responses by influencing various immune effector cell functions. FasFasL interactions have been shown to limit the magnitude of helper T-celldependent B-cell activation.28 The constitutive expression of FasL in ocular tissue has been reported to play an important role in curtailing HSV-induced inflammatory response.32 However, these studies did not establish a role of FasFasL-dependent apoptosis in corneal and retinal destruction after HSV-1 infection of resistant mouse strains.
In this study, we ask if FasFasL interactions are responsible for prevention of ocular inflammatory damage after corneal or anterior chamber inoculation of HSV in HSK- and HSR-resistant mice. Therefore, we studied the effects of immune dysregulation secondary to Fas or FasL deficiency on lpr (Fas deficient) and in gld (FasL deficient) mice, on the development of von Szily HSR and HSK in resistant C57BL/6 mice.
| Methods |
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Animals
Male and female HSV-1susceptible Balb/cByJ
(Igh-1a), C.AL-20 (Igh-1d),
HSV-1resistant C57BL/6 (Igh-1b), B6.SMN.C3H.gld
(Fas-ligand deficient), and B6.MRL.lpr (Fas deficient) mice, aged 7 to
9 weeks, were obtained from the Jackson Laboratories (Bar Harbor, ME).
Mice were housed in microisolators mounted in ventilated animal racks
(VR-1; Laboratory Products, Inc., Rochelle Park, NJ). All mice were
handled in accordance with the ARVO Statement for the Use of Animals in
Ophthalmic and Vision Research.
HSK Model
Inoculation.
Animals were anesthetized with a 0.1 ml intraperitoneal injection of a
1:1:2 mixture of 100 mg/ml ketamine hydrochloride, 20 mg/ml xylazine,
and sterile water. The right eye of each mouse was scratched with a
23-gauge needle, and 5 µl 4 x 105 plaque-forming
units (PFU) of HSV-1 (strain KOS) in minimum essential media (MEM;
GIBCO Laboratories, Grand Island, NY) was deposited onto the scarified
cornea. Ten C57BL/6, 16 C.AL-20, 11 B6.SMN.C3H.gld, and 12 B6.MRL.lpr
mice per group were used.
Clinical Observation and Grading.
The severity of clinical stromal keratitis was graded at day 14 using a
Zeiss binocular surgical microscope (Thornwood, NY) and a scoring
system, as previously described.25
33
Briefly, the
geographic area of the active stromal keratitis was scored from 0 to
4+. Active stromal keratitis was defined as a stromal white blood cell
infiltration of sufficient density to obscure the underlying
intraocular structures and/or definite stromal ulceration. A clinical
score of 1+ was consistent with a corneal involvement of < 25%;
2+ between 25% and 50%; 3+ between 50% and 75%; and 4+ > 75% of
the cornea.
HSR Model
Inoculation.
Animals were anesthetized with a 0.1 ml intraperitoneal injection of a
1:1:2 mixture of 100 mg/ml ketamine hydrochloride, 20 mg/ml xylazine,
and sterile water. An anterior chamber paracentesis was performed in
the right eye using a glass micropipette, and aqueous was drained to
decompress the globe. The anterior chamber of the right eye was then
inoculated with HSV using a 33-gauge needle on a 50-µL Hamilton
syringe under binocular microscopy. The mice were divided into two
groups. The first group consisted of 18 Balb/cByJ, 15 C57BL/6, 13
B6.SMN.C3H.gld, and 19 B6.MRL.lpr mice; each received an anterior
chamber inoculum of 4 x 105 PFU of HSV-1 in 5 µl of
MEM. The second group consisted of 5 Balb/cByJ, 5 CBL/6, 15
B6.SMN.C3H.gld, and 15 B6.MRL.lpr mice; each received 6.2 x
108 PFU of HSV-1 in 5 µl of MEM.
Histopathology.
Mice were killed by overdose with the anesthetic solution described
above on the 15th day postinoculation. Both eyes were enucleated and
processed for histopathologic examination. The globes were fixed in
Karnovskys fixative (1% paraformaldehyde, 1.25% glutaraldehyde in
0.2 M sodium cacodylate buffer) for 24 hours at 4°C, rinsed in buffer
solution, and dehydrated using serial ascending concentrations of
ethanol. The globes were then infiltrated with glycol methacrylate
solution overnight and then embedded in LKB Historesin (LKB Produker
AB, Bromma, Sweden). Five-micrometer sections were prepared using a
JB-4 microtome (Sorval; E. I. DuPont, Wilmington, DE) and stained
with hematoxylin-eosin. Ipsilateral chorioretinitis was defined as the
presence of focal or diffuse chorioretinal inflammation or destruction;
contralateral chorioretinitis was defined as the presence of
chorioretinal necrosis, vitreous cellular infiltration, or retinal
edema.
All experiments were replicated, and all evaluations were conducted in a masked manner.
| Results |
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| Discussion |
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HSK resulting from HSV-1 (KOS) infection of C.AL-20 mice represents a virally induced autoimmune reaction against corneal tissues initiated by T cells. HSK can be mediated by T-cell clones specific for corneal selfantigens, which also recognizes an allotype-bearing peptide derived from IgG2a, and exposure of HSK-susceptible mice to a soluble form of this peptide (tolerization to it) confers resistance to HSK.27 Most recently, UL6, a coat protein of HSV-1 (KOS), has been shown to be recognized by these autoreactive T cells, which target corneal antigens, suggesting that this model of autoimmune disease may be exacerbated by viral mimicry.34 No such virion-associated protein has been identified in the HSR model.
Several studies have implicated immunologic processes in the pathogenesis of ipsilateral retinal sparing and the contralateral HSR after uniocular anterior chamber HSV-1 inoculation. The contributions by various participants of the immune system, such as T cells,5 16 17 18 19 20 21 B cells,22 and natural killer cells and macrophages,23 have been reported. The role of T cells in this interesting model has been elucidated by studies involving T-cell depletion in euthymic BALB/c mice using anti-CD4 (helper/inducer) and -CD8 (cytotoxic/suppressor) antibodies20 and adoptive transfer of selective T-cell subsets in athymic BALB/c and SCID mice.19 In athymic BALB/c (nude) mice, bilateral necrotizing retinitis develops after unilateral anterior chamber injection with HSV. Adoptive transfer studies have revealed that CD4+ cells can mediate destruction of the contralateral retina,15 16 18 20 whereas CD8 cells have been implicated in preventing contralateral retinitis.16
Because depletion of B cells in resistant CB-17 mice results in bilateral chorioretinitis, B cells may indirectly contribute to inhibition of contralateral retinitis.22 We have also shown that the contralateral retina is protected in susceptible mice after intravitreal injection of anti-CD11, an antibody against immune effector cells, macrophages, and natural killer cells.23 Together, these studies suggest a role for immune-mediated protection of the ipsilateral retina and for immune-mediated destruction of the contralateral retina.
Recently, the FasFasL system has been demonstrated to limit the magnitude of helper T-celldependent B-cell activation.28 FasL is expressed constitutively on ocular tissue and has been suggested to play an important role in curtailing the virus-induced inflammatory response.32 We therefore studied the effect of immune dysregulation in Fas- and FasL-deficient mice on the development of retinitis in resistant C57 BL/6 mice.
Griffith and colleagues32 reported the results of their studies of the inflammatory response resulting from HSV-1 infection in the anterior chamber of the eye in C57BL/6 mice and in their FasL-deficient congenics. In the C57BL/6 mice, the response was contained within the anterior segment, with only minimal spread of inflammatory cells into the vitreous cavity and no invasion of the retina by inflammatory cells or virus. In contrast, in gld mice (FasL deficient), there were numerous inflammatory cells in the vitreous cavity of ipsilateral eyes, with cells attached to and invading the retina and the optic nerve. In addition, there were numerous inflammatory cells attaching to and invading the cornea, inducing keratitis. The authors, on the basis of these results, concluded that the induction of apoptosis by FasFasL interactions is a potent mechanism of "immune privilege" and that the consequences of defective FasFasL interactions for the eye can be the spread of tissue-destructive inflammatory responses.
The results of our studies indicate that defects in the Fas or FasL system do not influence the resistance of C57BL/6 to von Szily HSR nor to the development of HSK. Despite an enhanced inflammatory response in the ipsilateral vitreoretinal interface and the infiltration of inflammatory cells in the inner retinal layers of the lpr and gld mice, compared with the wild-type C57BL/6 mice, there was no evidence of retinal architectural destruction.
Griffith et al. did not indicate whether they had examined the contralateral retina in their gld and lpr mice.32 Our definition of retinitis is based on the von Szily model, which requires destruction of retinal architecture, in addition to the presence of inflammatory cells on the retinal surface (which is a minor criterion in our description, but is used by Griffith et al. as evidence to show the role of Fas and FasL). A very small percentage of our gld and lpr mice showed scattered vitreous inflammatory cells, which may be similar to the findings that Griffith et al. reported. However, ipsilateral or contralateral destructive retinitis was not seen. Thus, we cannot conclude that Fas and/or FasL system influences the resistance (to the full extent of keratitis or retinitis) of C57BL/6 to von Szily HSR or to the development of HSK, as suggested by Griffith and colleagues. It has been shown that in the von Szily model, herpes virus is transmitted from the central nervous system to the retina of the contralateral eye by retrograde axonal transport through the optic nerve along the endocrineoptic pathway between the retina and the suprachiasmatic nucleus of the hypothalamus.5 6 The Fas or FasL system may play a partial role in the resistance of certain murine strains (e.g., C57BL/6) to keratitis or retinitis. However, there may be other more dominant factors along the optic nerve transmission route, which maintain most of the HSV-1 resistance, even in the absence of the Fas or FasL system. Although resistance to HSK and herpes retinitis in mice may, to some degree, involve activation-induced cell death, the presence or absence of Fas and FasL does not play a significant role in genetic resistance to HSK and necrotizing retinitis.
| Footnotes |
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Commercial relationships policy: N.
Presented in part at the annual meeting of the Association for Research in Vision and Ophthalmology, Fort Lauderdale, Florida, May 1998.
The publication costs of this article were defrayed in part by page
charge payment. This article must therefore be marked
"advertisement" in accordance with 18 U.S.C.
1734
solely to indicate this fact.
Corresponding author: C. Stephen Foster, Ocular Immunology and Uveitis Service, Massachusetts Eye and Ear Infirmary, 243 Charles Street, Boston, MA 02114. fosters{at}helix.mgh.harvard.edu
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