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From the Departments of 1 Microbiology and 2 Cellular and Structural Biology, The University of Texas Health Science Center at San Antonio.
| Abstract |
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METHODS. BALB/c mice were thymectomized and T celldepleted by injection of monoclonal antibodies to CD4, CD8, or both. Murine cytomegalovirus (9 x 102 plaque forming units [pfu]) was injected into the supraciliary space. Experimental animals received murine cytomegalovirus-specific T cells or subsets of T cells 2 hours before virus injection, whereas control animals received herpes simplex virus type 1specific T cells by tail vein injection. Eight days after virus injection, retinal pathology was scored by histopathologic examination of hematoxylin and eosinstained ocular sections.
RESULTS. CD8+ T cell depletion was sufficient for development of retinitis after supraciliary injection of murine cytomegalovirus. Adoptive transfer of murine cytomegalovirus-specific T cells, but not herpes simplex virus type 1specific T cells, provided protection from retinitis. Additionally, separation of the murine cytomegalovirus-specific T cells into CD8+ and CD4+ subsets before adoptive transfer showed that the CD8+ fraction of the adoptive T cells was responsible for protection.
CONCLUSIONS. These results suggest that adoptive transfer of cytomegalovirus-specific T cells or T cell subsets might be used to treat or prevent cytomegalovirus retinitis in immunosuppressed human patients.
| Introduction |
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To explore the usefulness of adoptive transfer of T cells for prevention of CMV retinitis, a series of depletion and reconstitution experiments was conducted using a mouse model of CMV retinitis. Because members of the cytomegalovirus family are extremely species-restricted, murine cytomegalovirus (MCMV) has been used to study the pathogenesis of cytomegalovirus infections of the retina in BALB/c mice.14 15 16 17 18 Injection of a low dose (5 x 1021 x 103 plaque forming units [pfu]) of MCMV into the supraciliary space of one eye of an immunosuppressed or T celldepleted BALB/c mouse causes a focal necrotizing retinal infection that resembles CMV retinitis in humans.16 18
Depletion of CD8+ T cells from euthymic mice is sufficient to predispose the animals to retinitis after injection of a low-dose of MCMV via the supraciliary route.16 Subsequently, Lu and coworkers determined that protection from MCMV retinitis is provided by adoptive transfer of unfractionated lymph node cells from MCMV-infected mice to thymectomized T celldepleted mice.18 By building on the earlier results of Lu and coworkers, the results of the present study show that the T cell component of the MCMV-specific lymph node cells is responsible for protecting thymectomized T celldepleted mice from retinitis. Furthermore, adoptive transfer of CD8+ T cells alone, but not CD4+ T cells alone, protects against MCMV retinitis.
| Methods |
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Virus and Virus Titrations
Stocks of MCMV (Smith strain) were prepared from salivary gland
homogenates from MCMV-infected BALB/c mice as previously
described.13
Virus stocks and virus recovered from tissues
were titered in duplicate on Swiss Brown mouse embryo fibroblasts
(BioWhittaker, Walkersville, MD).
Thymectomy and T-Cell Depletion
Thymectomies were performed on 6-week-old mice using a
modification of a protocol by Chin.19
20
Thymectomized
mice were rested for 1 week before T cell depletion. T cell depletion
was accomplished by intravenous injection of 500 µg of anti-CD4
(GK1.5) monoclonal antibody and 150 µg of anti-CD8 (2.43) monoclonal
antibody (American Type Culture Collection, Rockville, MD). This
protocol has been shown to deplete >94% of CD4+
T cells and >99% of CD8+ T
cells.21
Mice were then rested for 28 days to allow
catabolization of the rat anti-mouse T cell antibodies before adoptive
transfer.18
Noncross-reactive phycoerythrin
(PE)-labeled antiLy-3.2 (clone 53-5.8; Pharmingen, San Diego, CA) and
fluorescein isothiocyanate [FITC[]labeled anti-L3T4 (clone RM4-4,
Pharmingen) recognizing CD8 and CD4, respectively, were used to
determine the efficiency of T cell depletion.
Ocular Inoculation
Mice were anesthetized by intramuscular injection of a cocktail
containing 0.02 ml rompun and 0.03 ml ketamine per 25 g body mass.
The left eyes of mice were injected with 9.0 x
102 pfu MCMV in a volume of 2 µl via the
supraciliary route as previously described.13
Briefly, a
superficial transscleral entry wound was made parallel and just
posterior to the limbus by introducing the bevel of a sharp 30-gauge
needle into the supraciliary space. Two microliters of virus followed
by 3 µl of air was then injected. The injection was judged successful
if ophthalmic observation using the dissecting microscope showed a
chorioretinal detachment associated with the appearance of air in the
supraciliary space immediately after injection.
Flow Cytometry
Animals were deeply anesthetized before they were killed. Lymph
nodes were harvested, and single cell suspensions were made by grinding
the tissues between frosted glass slides in 5 ml of cold Hanks
balanced salt solution (HBSS) and then aspirating the suspension
through a 22-gauge needle. Cells were washed 3 times in HBSS and
resuspended in FACS buffer (phosphate-buffered saline [PBS] with 3%
fetal bovine serum). Samples were incubated with anti-CD32/CD16 (Fc
receptor block; Pharmingen) according to the manufacturers
recommendations for 15 minutes. Cells were then resuspended in
FITC-labeled anti-L3T4 (CD4; Pharmingen), PE-labeled antiLy-3.2 (CD8;
Pharmingen), FITC-labeled anti-CD3 (GIBCO, Gaithersburg, MD), and/or
PE-labeled anti-CD4 (GIBCO), as required. After 15 minutes, cells were
washed 3 times in FACS buffer and resuspended in FACS buffer for
analysis. Gating was set to count large granular cells including
lymphocytes and macrophages. Percent positive cells = % positive
stained cells minus % positive cells from the same unstained sample.
Adoptive Transfer
Cells for adoptive transfer were prepared by harvesting the
draining lymph nodes from mice 7 days after footpad injection of 5 x 105 pfu of MCMV or herpes simplex virus type-1
(HSV-1). Single cell suspensions were prepared from the lymph nodes as
described above for flow cytometry. Cells were washed 2 times in HBSS
and 1 time in column buffer (prepared to manufacturers
specifications). Cells were then resuspended in column buffer, and T
cells, CD4+ T cells, or
CD8+ T cells were purified by passing the total
cell suspension over the appropriate T cell or T cell subset enrichment
column (R&D Systems, Minneapolis, MN) and eluting the desired cell type
from the column with column buffer. Eluted cells were then washed 3
times in PBS; viability and cell counts were determined by trypan blue
exclusion. Cells were resuspended so that the appropriate number of
cells was contained in 100 µl. Adoptive transfers of cells were made
via tail vein injection using a 27-gauge needle.
Retinitis Scoring
Eyes were fixed in buffered formalin, embedded in paraffin, and
sectioned at 6 levels, 200 µm apart. The sections were then stained
with hematoxylin and eosin (H&E). Changes in the posterior segment of
each section were evaluated microscopically using the following scale,
which was developed by Atherton and coworkers16
and
modified by Bigger and coworkers21
: 0 = normal or
injection artifact; 1/2 = mild atypical retinopathyabsence of
cytomegaly plus retinal folds involving less than three quarters of the
retinal section; 1 = moderate atypical retinopathyabsence of
cytomegaly plus retinal folds involving more than three quarters of the
retinal section plus photoreceptor atrophy or retinal infiltration by
leukocytes involving more than one quarter of the retina; 2 =
retinal infectioncytomegaly of retinal cells plus partial-thickness
retinal necrosis or full-thickness necrosis extending from the ciliary
body, but not beyond a one quarter retinal section from the ciliary
body; 3 = necrotizing retinitiscytomegaly plus full-thickness
retinal necrosis existing further than one quarter of a retinal section
from the ciliary body or full-thickness retinal necrosis extending from
the ciliary body through one quarter of the section; and 4 =
severe necrotizing retinitiscytomegaly with full-thickness necrosis
involving the entire retinal section.
A score of 2 indicates virus infection of the retina adjacent to the site of injection; to differentiate between cytomegalic cells in the retina near the site of injection and the presence of cytomegalic cells and necrosis in the retina remote from the site of injection, a score of 3 or higher was considered positive for retinitis. The highest posterior segment score for each eye was the retinal score, which reflects the focal nature of this infection. KruskalWallis analysis was used to determine whether there was a difference between any of the experimental groups based on the retinal scores. If a significant difference was detected by the KruskalWallis analysis, the Dunns test was used to determine which groups were different from each other.22 Retinitis frequencies were analyzed by two-tailed Fishers exact test using the Bonferroni inequality to adjust probability values to account for multiple tests.
| Results |
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To determine whether thymectomized mice depleted of CD4+ cells, CD8+ cells, or both developed MCMV retinitis, BALB/c mice were thymectomized, depleted of CD4+, CD8+ T cells, or both by injection of monoclonal antibodies, and rested for 28 days. Control mice were thymectomized and mock depleted by injection of normal rat IgG. The mice were then injected with 9.0 x 102 pfu of MCMV via the supraciliary route. Eight days after virus injection, the mice were killed and eyes were removed for histopathologic examination. As shown in Table 1 , 5 of 10 of the CD8-depleted mice and 5 of 10 of the CD4- and CD8-depleted mice developed retinitis compared with 1 of 10 and 2 of 10 of the mock-depleted and CD4-depleted mice, respectively. Analysis of the histopathologic scores of the retinas (Table 1) by the KruskalWallis test showed that there was a significant difference between the groups (P < 0.03). However, Dunns multiple range test was unable to show between which of the groups this difference was found. However, comparison of the retinal scores between the CD8-depleted group and either the mock-depleted group or CD4-depleted group suggests that depletion of CD8+ T cells was sufficient to predispose thymectomized mice to retinitis (Table 1 ; Figs. 1 A and 1B). This finding is similar to the results from experiments conducted previously in T cell-depleted euthymic mice.16
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95% (Fig. 3)
, and these populations contained less than 5% contaminating cells,
which were CD3- (not shown). CD3+,
CD4+, or CD8+ T cells were
then injected into the tail vein of T celldepleted mice. Because
5 x 106 MCMVspecific purified T cells
protected mice from retinitis after supraciliary infection with MCMV
(Table 2) , and because the ratio of CD4+ T
cellstoCD8+ T cells was approximately 2:1 in
unfractionated lymph node cells (Fig. 2)
, the same total number of
CD4+ or CD8+ T cells was
adoptively transferred (i.e., 3.4 x 106
CD4+ T cells or 1.7 x
106 CD8+ T cells). Nine
hundred plaque-forming units of MCMV was then injected into the
supraciliary space; 8 days later, injected eyes were harvested, fixed,
sectioned, stained, and scored for retinal pathology. As shown in Table 3
, adoptive transfer of CD8+ MCMVspecific T cells
decreased retinal pathology compared with the PBS-treated group
(P < 0.05). Additionally, the results in Table 3
again
show that adoptive transfer of unfractionated MCMV-specific
(CD3+) T cells protected T celldepleted mice
from MCMV retinitis.
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| Discussion |
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Adoptive transfer of MCMV-specific CD8+ T cells was sufficient to reduce retinal pathology in susceptible mice (Table 3) , although it was not sufficient to reduce the frequency of retinitis. However, because the retinitis frequency is derived from the scores for retinal pathology, comparisons of the extent of retinal pathology better denote protection against MCMV disease than differences in the frequency of retinitis.
Adoptive transfer of CD4+ T cells did not significantly reduce retinal pathology. Previous T celldepletion studies conducted in the MCMV retinitis model demonstrated that, although a small proportion of CD4+ T celldepleted mice develop retinitis, CD8+ T cells are critical for preventing retinitis.16 In the present studies and in previous studies, small, nonstatistically significant differences were found between CD4+ T celldepleted and mock-depleted mice after supraciliary infection with MCMV. If the sample sizes in these experiments were larger, perhaps these small differences might be statistically significant. In human patients with AIDS, CD4+ T cell counts below 50/mm3 correlate with development of CMV retinitis.26 27 The importance of CD4+ T cells in human CMV retinitis has recently been supported by a series of reports describing the recovery of CD4+ T cells in AIDS patients with CMV retinitis undergoing HAART; as CD4+ T cell counts increase, these patients are able to discontinue anti-CMV therapies without the progression of retinitis.8 9 10 11 Komanduri and coworkers showed that HAART allowed CMV retinitis patients to recover CMV-specific CD4+ T cells.11
The reduced numbers of CD4+ T cells in AIDS patients correlate with reduced numbers of CD8+ T cells and loss of natural killer (NK) cell cytotoxicity.28 29 30 31 32 Low CD8+ T cell counts have been shown to correlate with the development of CMV retinitis. A recent report by Lowder and coworkers demonstrated that patients with CD8+ T cell numbers below 520/mm3 were at risk of developing CMV retinitis.29 The importance of NK cells in CMV retinitis has been less well studied. Natural killer cells lose cytolytic activity in stage IV AIDS patients,28 32 and no studies have been reported in which NK cytolytic activity was correlated with susceptibility to CMV retinitis. Because of the close correlation between the reduction of CD4+ T cells, CD8+ T cells, and NK cell cytotoxicity in patients with stage IV AIDS, it is difficult to determine which, if not all, of these lymphocytes play critical roles in the protection of humans from CMV retinitis. Using the mouse model, our laboratory has shown in the present studies and in previous studies that NK cells and CD8+ T cells are critical for protecting mice from MCMV retinitis and that CD4+ T cells may also contribute to that protection. So, although monitoring CD4+ T cell levels predicts the risk of CMV retinitis, loss of CD4+ T cell activity, CD8+ T-cell activity, and/or NK cell activity may all contribute to susceptibility to CMV retinitis.
The results presented herein show that adoptive transfer of MCMV-specific T cells and of MCMV-specific CD8+ T cells protected T celldepleted mice from MCMV retinitis. Adoptive transfer of CMV-specific CD8+ T cell clones has been shown to prevent CMV pneumonia in human patients after bone marrow transplantation.12 13 Although the pathogenesis of CMV pneumonia is different from CMV retinitis, the results of these adoptive transfer studies in mice suggest that, while CMV retinitis in immunosuppressed transplant patients is a rare event, patients receiving adoptive transfer therapy of CMV-specific CD8+ T cells to prevent CMV pneumonia may also be protected from ocular CMV infection. Furthermore, these experiments suggest that adoptive therapy using CMV-specific T cells might be used to protect immunocompromised patients from retinitis, especially those patients with severe reactions or resistance to current anti-CMV therapies, anti-HIV therapies, or both.
| Acknowledgements |
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| Footnotes |
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Submitted for publication December 28, 1998; revised May 27, 1999; accepted June 23, 1999.
Commercial relationships policy: N.
Corresponding author: Sally S. Atherton, Department of Cellular and Structural Biology, The University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, TX 78229. E-mail: atherton{at}uthscsa.edu
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