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1 From the Rotterdam Eye Hospital, 2 Institute of Virology, and the 3 Department of Pathology, Erasmus University Rotterdam, The Netherlands.
| Abstract |
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METHODS. T-cell lines (TCLs) were generated from corneal buttons of 12 patients with different clinical stages of HSV-induced necrotizing stromal keratitis (n = 9) or immune stromal keratitis (n = 3). The initiating virus was identified by polymerase chain reaction and immunohistology performed on the corneal buttons. Peripheral blood mononuclear cells (PBMCs) were isolated, and B cell lines (BLCLs) were generated by transformation with Epstein-Barr virus. Proliferative responses of these intracorneal TCLs were determined by culturing T cells with autologous BLCLs infected with HSV-1, HSV-2, wild-type vaccinia virus (VV-WT), or VV expressing HSV-1 UL6 (rVV-UL6). Alternatively, T cells were incubated with PBMCs pulsed with human cornea protein extract.
RESULTS. Irrespective of clinical diagnosis or treatment, T cells were recovered from the corneal buttons of all the 12 HSK patients. The intracorneal TCLs of 9 of the 12 HSK patients showed HSV-specific T-cell reactivity. In none of the TCLs, T-cell reactivity against HSV-1 UL6 or human corneal antigens was detected.
CONCLUSIONS. These data suggest that the potentially immunopathogenic intracorneal T-cell response in HSK patients is directed to the initiating virus and not to a human corneal autoantigen or HSV-1 UL6.
| Introduction |
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Recently, we have demonstrated the presence of HSV-specific CD4+ Th0-like cells in corneas of two patients with necrotizing ulcerative HSK. In this study no reactivity to human corneal antigens could be detected.9 Nevertheless, autoreactive T cells may still be involved in clinically distinct HSK entities. In the present study we determined the antigen specificity of cornea-infiltrating T cells, obtained from 12 patients with different clinical forms of HSK. T-cell reactivity was tested toward the HSV serotypes 1 and 2, recombinant HSV-1 UL6, and a soluble human corneal protein extract.
| Methods |
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DNA Extraction and Polymerase Chain Reaction Analyses
The surplus half of the corneal specimens was minced and treated
with collagenase essentially as described previously.9
DNA
was isolated from one-fourth part of the corneal cell suspension lysed
in a guanidine isothiocyanate buffer using Celite solution
(Jansen Chemika, Beers, Belgium) according to the method of
Boom et al.14
The polymerase chain reaction (PCR) primers
and conditions for detection of HSV type 1 and 2 and varicella zoster
virus specific DNA after Southern blot analysis have been described
previously.15
Cornea-Derived T-Cell Lines
Cornea-derived T-cell lines (TCLs) were generated from the
remaining corneal cell suspension as described
previously.9
After one round of mitogenic stimulation,
using phytohemagglutinin-L (PHA-L; BoehringerMannheim, Mannheim,
Germany) and allogeneic feeder cells, the intracorneal TCLs were frozen
in aliquots at -135°C. Control experiments with corneas
histologically devoid of infiltrating T cells did not result in the
generation of TCLs, indicating that the method applied facilitates the
recovery and outgrowth of T cells compartmentalized to the cornea (data
not shown). The TCLs were characterized for cell surface expression of
CD3, CD4, and CD8 by triple color flowcytometry using fluorescein
isothiocyanate (FITC), RPE-, and R-Phycoerythrin
(RPE)-Cy5conjugated MAbs, respectively (Dako).
T-Cell Proliferation Assays
Autologous BLCLs were infected with HSV-1 and HSV-2 at a
multiplicity of infection (MOI) of approximately 5 at 37°C for 20
hours. The virus- and mock-infected cells were washed and UV-irradiated
(2.5 x 10-2
mW/mm2). Alternatively, BLCLs were infected with
the rVVs at an MOI of approximately 5 for 20 hours and fixed with 1%
paraformaldehyde as described previously.16
The level of
infection of the BLCLs with the respective viruses was determined by
flowcytometry. The expression of HSV-1 UL6 in rVV-UL6infected BLCLs
was demonstrated using a rabbit anti-UL6 serum (2C2)11
and, subsequently, FITC-conjugated swine anti-rabbit serum (Dako). As
for all viruses, approximately 70% to 90% of the BLCLs were shown to
be infected (Fig. 1)
. T cells (3 x 104/well), removed from
culture at days 10 to 12 after one mitogenic stimulation, were cultured
in triplicate together with virus- or mock-infected BLCLs (2 x
104/well) in 96-well round-bottomed plates in 150
µl complete medium at 37°C in a CO2
incubator. Complete medium consisted of RPMI 1640 (GIBCOBRL, Breda,
The Netherlands) supplemented with 10% heat-inactivated pooled human
serum and antibiotics. Because of the limited ability of BLCLs to
process and present exogenous antigens to T cells, T-cell reactivity to
human soluble cornea protein extract (HuSoCo; at final protein
concentrations of 50 and 100 µg/ml) was performed using
105 autologous PBMCs (UV-irradiated) as
antigen-presenting cells (APCs). The cells were cultured for 72 hours
and pulsed with 0.5 µCi [3H]-thymidine over
the last 18 hours of culture. The cells were harvested and the
incorporated radioactivity was determined in a ß-scintillation
counter. Proliferation was considered positive when stimulation indices
(counts per minute [cpm] incorporated in response to antigen/cpm
incorporated in response to control) were more than 4. T-cell
reactivity to all antigens tested for was assayed simultaneously, and
PHA-L (1 µg/ml) was included as positive control for T-cell
proliferation. The assays were performed at least two times, and the SD
was always less than 30% of the mean counts per minute.
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| Results |
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Antigen Specificity of Cornea-Derived TCLs from HSK Patients
We have recently developed a protocol that enables the recovery
and expansion of in vivo activated corneal infiltrating T cells from
corneal buttons of HSK patients.9
This method facilitated
the generation of intracorneal TCLs from all 12 HSK patients studied.
All TCLs consisted predominantly of CD3+ T cells
and the ratio of CD4+and
CD8+ T cells varied interindividually.
Interestingly, the TCLs of 2 patients with quiescent necrotizing
stromal keratitis consisted almost exclusively of
CD4+ T cells (patients 7 and 9; Table 1
). The
reactivity of the cornea-derived TCLs toward the triggering virus was
analyzed in T-cell proliferation assays using mock-, HSV-1, and
HSV-2infected autologous BLCLs as APCs. The intracorneal TCLs of 9 of
12 patients showed HSV-specific T-cell reactivity (Table 2)
. Illustrative for the high sequence homology between the HSV
serotypes, the majority of these TCLs recognized both HSV-1 and
HSV-2infected BLCLs. In the case of patient 8, however, the
HSV-specific intracorneal T-cell response was restricted to HSV-2.
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| Discussion |
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Although the processes orchestrated by cornea-infiltrating CD4+ T cells have been studied extensively, the target antigens recognized remain unclear. Given the involvement of HSV in the etiology of HSK, HSV antigens are the most likely candidates. To address this notion we analyzed the antigen specificity of cornea-infiltrating T cells in 12 patients with HSV-induced stromal keratitis. After one round of mitogenic stimulation, cornea-derived TCLs were successfully generated from corneas of the 12 patients studied. Intracorneal HSV-specific T-cell reactivity, mainly HSV-type common, was observed in 9 of 12 corneas tested. These data indicate that T cells specific for the triggering virus infiltrate corneas of HSK patients. In patient 8, however, the HSV-specific response was solely directed to HSV-2. Possibly, the determinants recognized by these T cells are HSV-type common and are not efficiently processed and presented in HSV-1 compared with HSV-2infected BLCLs. Surprisingly, HSV-reactive T cells could also be detected in TCLs of patients in a quiescent phase, treated with steroids, and even from HSV DNAnegative corneas. These data suggest that HSV-specific T cells can reside for longer periods of time and, even under steroid treatment, in corneas of patients with HSV-induced stromal keratitis.
In contrast to corneas of patients with necrotizing stromal keratitis17 (Fig. 2E) , murine HSK corneas are devoid of HSV antigens.1 2 Nonetheless, HSV-specific T cells have been demonstrated in whole-eye cell suspensions of mice with fulminate HSK.3 Similarly, in 8 of the 9 HSK patients, from which intracorneal HSV-specific T cells were recovered, the corneas were devoid of HSV antigens. On infiltration of the cornea, these HSV-specific T cells may have been activated by viral peptides retained by corneal cells like Longerhans Cell (LC), or the amount of HSV antigens in the corneas is too low to be detected by immunohistochemistry. Alternatively, potential keratogenic CD4+ T cells infiltrating HSV-infected corneas may be activated nonspecifically (e.g., by cytokine-mediated bystander activation)18 or activated on recognition of sequestered corneal autoantigens unmasked or altered after HSV replication in the cornea.2 Recently, studies performed in the murine HSK model have provided evidence for the latter assumption.7 8 HSK could be induced in nude mice after adoptive transfer of HSV-1 UL6 peptidespecific CD4+ T cells8 cross-reacting with an unknown corneal protein.7 We analyzed whether this type of autoimmunity may play a role in human HSK. In none of the TCLs generated from corneas of any of the HSK patients studied here, reactivity to HSV-1 UL6 or a HuSoCo protein extract could be demonstrated. Stimulation of the intracorneal TCLs with PHA-L resulted in high proliferative responses, indicating that this is not due to a low viability of the TCLs tested (Table 2) . The lack of reactivity toward HSV-1 UL6, harboring the cross-reactive epitope, is not surprising given the constraints of major histocompatibility complex allele-specific peptide binding.19 In the case of the HuSoCo protein extract, the negative results could be due to a true lack of autoreactive T cells or an inappropriate corneal antigen preparation used. The putative corneal autoantigen could be located in the buffer-insoluble part of the human cornea extract, it may be genetically polymorph or the intracorneal autoreactive T-cell responses are mediated by CD8+ T cells. Given the nature of the HuCoSo protein extract and the type of assay used (i.e., exogenous antigen preparation in a T-cell proliferation assay), the potential role of CD8+ cornea autoantigenspecific T cells in HSK could not be addressed. In the HSK mouse model, the keratogenic T-cell clone recognized an unknown Trisbuffer soluble cornea-specific antigen7 and was able to induce the disease in HSK-resistant mice, arguing against genetic polymorphism of the autoantigen. The HuSoCo protein extract used here, obtained from 12 human cornea buttons and similarly generated as described in the murine HSK study,7 12 was a heterogeneous protein preparation in which the major soluble cornea protein BCP5412 was predominantly present (sodium dodecyl sulfatepolyacrylamide gel electrophoresis analysis; data not shown). Positive peripheral blood T-cell responses, using similar concentrations of an equivalent HuSoCo protein extract or purified BCP54, have been obtained in patients with inflammatory corneal diseases.20 21 These data suggest that the HuSoCo protein extract used in the present study may be considered immunogenic. Although not formally excluded, the lack of intracorneal T-cell reactivity to HSV-1 UL6 and human corneal antigens does not support the hypothesis that human HSK is an HSV-induced autoimmune disease. The cloning and identification of the putative HSK-related murine cornea autoantigen, and its human homologue will be needed to further address the validity of the molecular mimicry hypothesis at the single antigen level.
In conclusion, the present study demonstrates T cells specific for the
triggering virus in the corneas of the majority of the 12 HSK patients
studied. On antigenic stimulation, the cornea-derived HSV-specific T
cells from HSK patients secrete both interferon gamma (IFN-
) and
interleukin 2 (IL-2)9
(data not shown). In the mouse HSK
model, both cytokines have been shown to be pathologic in the cornea of
HSV-1infected mice.4
5
22
IFN-
has been shown to
facilitate migration of PMNs from the blood into the cornea, and on
activation by IL-2, and perhaps IFN-
secretes proteolytic enzymes
that contribute to destruction of the cornea.1
2
23
We
hypothesize that HSV-specific T cells have an important role in the
local immunopathogenesis of HSK in humans. On entry into the cornea
they are activated by HSV-infected corneal cells or by viral peptides
retained by corneal cells like LC corneal cells and, subsequently,
initiate a cytokine-mediated immunopathogenic response in the cornea.
| Acknowledgements |
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| Footnotes |
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Submitted for publication December 16, 1999; revised April 3, 2000; accepted April 13, 2000.
Commercial relationships policy: N.
Corresponding author: Georges M. G. M. Verjans, Institute of Virology, Erasmus University Rotterdam, PO Box 1738, 3000 DR, Rotterdam, The Netherlands. verjans{at}viro.fgg.eur.nl
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