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1 From the Casey Eye Institute and the 4 Departments of Medicine and 5 Cell and Developmental Biology, Oregon Health & Science University, Portland, Oregon; and 3 National Eye Institute, Bethesda, Maryland.
| Abstract |
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METHODS. Intravital microscopy was used to record the movement and location of leukocytes in the irises of mice with uveitis induced by intravitreal injection of 250 ng Escherichia coli endotoxin. Each mouse concurrently received an intraperitoneal injection of monoclonal neutralizing antibodies for ICAM-1, LFA-1, or both or control irrelevant antibodies.
RESULTS. Mice treated with endotoxin and control antibodies had an inflammatory response that was clearly present at the 6- and 24-hour time points and was mostly resolved by 48 hours. Mice that received anti-ICAM-1 or anti-LFA-1 had significantly fewer cells infiltrating their irises at 6 and 24 hours. Detailed analysis of the 6-hour time point recordings revealed that neither anti-ICAM-1 nor anti-LFA-1 significantly reduced the number of leukocytes rolling on venule endothelial surfaces, but the treatments reduced the number of firmly adherent cells.
CONCLUSIONS. These data confirm previous reports that ICAM-1 and LFA-1 are important mediators of EIU. The dynamic in vivo images clearly support the hypothesis that integrin-mediated cell adhesion is more critical for the firm adhesion of sticking cells than for leukocyte rolling.
| Introduction |
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In the present study, we used intravital microscopy to extend our knowledge about the role of cell-adhesion molecules, specifically ICAM-1 and one of its counterreceptors, LFA-1, in murine EIU. By visualizing leukocytes in and around iris venules of living mice that have been treated with neutralizing antibodies directed against a specific cell-adhesion molecule, we can clarify the role of these molecules in specific steps of the dynamic process of leukocyte migration.
| Materials and Methods |
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Induction of Uveitis and Experimental Design
Baseline intravital microscopy recordings were made of vessels
on the day before induction of uveitis. Uveitis was induced by
intravitreal injection of 2 µl containing 250 ng endotoxin
(lipopolysaccharide; LPS) from Escherichia coli 055:B5 (List
Biological Laboratories, Campbell, CA) and 0.25% human serum albumin
(Baxter Healthcare, Glendale, CA) in saline.13
At the same
time, the mice received an intraperitoneal injection of experimental
monoclonal or control polyclonal antibodies. One group of mice received
200 µl with 200 µg of clone YN-1 rat IgG2b antibody against mouse
ICAM-1.14
A second group received 200 µl with 200 µg
of clone M17 rat IgG2a antibody against mouse LFA-1.15
The
third group received 200 µg of both anti-ICAM-1 and anti-LFA-1
antibodies. Similar doses of these antibodies have been reported to
inhibit experimental autoimmune uveitis, systemic EIU, and experimental
allergic conjunctivitis.10
16
17
Control mice received 200
µl with 200 µg polyclonal rat IgG2b (PharMingen, San Diego, CA).
Experimental intravital microscopy recordings were made 6, 24, and 48
hours after LPS injection.
Intravital Microscopy
Details of the microscopy technique have been published
previously.13
Approximately 10 minutes before each time
point, mice received an intravenous injection of 35 mg/kg rhodamine 6G
(Sigma Chemical Co, St. Louis, MO) in phosphate-buffered saline to
label circulating leukocytes. Mice were anesthetized with 1 l/min 1.7%
isoflurane (Ohmeda, Liberty Corner, NJ) in oxygen and placed on the
stage of an epifluorescence microscope. Pupils were constricted with
topical pilocarpine. A viscous gel (Vidisic Gel, Dr. Mann
Pharma, Berlin, Germany) was applied between the cornea and aqueous
immersion objectives. Images were captured (NTSC video format)
with a black-and-white camera coupled to an image intensifier (CF 8/4
NNIR; Kappa, Gleichen, Germany) and recorded by computer (Power PC;
Apple, Cupertino, CA) with a video board (Ignitor; Aurora Video
Systems, Shelby Township, MI). For each eye, five regions of interest
containing one to three vessels were recorded for 20 seconds. To avoid
operator bias, these regions were determined by a fixed grid pattern
beginning at the first branching of an iris arteriole from the major
iridal circle at 9 oclock on the iris periphery. At the baseline time
point, vessels were mapped to ensure that the operator returned to
exactly the same segments for subsequent recordings. At least 10
vessels were recorded for each eye, but some were excluded from
analysis if off-line analysis was compromised because of eye movement.
Quantification of Visual Data
Details of the quantitative analysis have been published
previously.13
Measurements were made within 100- to
300-µm venule segments that did not branch or appreciably change
diameter. The mean of five measurements equally spaced over the length
of the vessel was used as the vessels internal diameter for surface
area calculations. Rolling leukocytes were defined as cells whose
contact with the vessel wall made the cell move considerably slower
than the mean blood velocity and are reported as the number of rolling
cells per square millimeter of endothelial surface per minute. Sticking
leukocytes were defined as cells firmly attached to vessel walls and
immobile for the 20-second observation period. Sticking cells are
reported as the number of cells per square millimeter of endothelial
surface. The categories of rolling and sticking cells are mutually
exclusive. Unstained, freshly emigrated leukocytes were clearly seen as
round, dark dots in the perivascular iris tissue and are reported as
the number of infiltrating cells per square millimeter of iris tissue.
Statistical Analysis
The groups of values for rolling and sticking leukocytes in each
venule were subjected to the Mann-Whitney rank sum test, because they
did not usually follow a normal distribution. The Mann-Whitney rank sum
test was also used to evaluate the numbers of infiltrating cells and
the
2 test was used for comparing the
percentages of venules with rolling or sticking leukocytes. Differences
with P < 0.05 were considered statistically
significant.
| Results |
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Applying the same analysis to venules with sticking cells again
demonstrates the effectiveness of the anti-ICAM-1 and anti-LFA-1
antibodies in blocking leukocyte sticking. At baseline, only 2% to 5%
of venules had at least one sticking cell (Table 2)
. By 6 hours after
endotoxin, 66% of venules in the control group had sticking cells. The
experimental groups all had significantly fewer venules with sticking
cells (P
0.005).
| Discussion |
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The anti-ICAM-1 and anti-LFA-1 antibodies may block leukocyte infiltration to a greater extent than leukocyte sticking. All the eyes from mice receiving either of these antibodies had fewer infiltrating cells per square millimeter of iris than the median count in the control animals, and most were less than the 25th percentile in the control animals. In contrast, 11% to 24% of the venules in the experimental groups had more sticking leukocytes per square millimeter of venule surface than the control median. This discrepancy raises the question of whether these two adhesion molecules have critical roles in addition to trapping circulating cells on the endothelium in areas of active inflammation. Some possibilities are participation in the extravasation process itself and reduction of leukocyte migration from the perivascular tissues. ICAM-1 is expressed on leukocytes as well as the vascular endothelium.18 19 Direct binding of anti-ICAM-1 to the leukocytes may have an effect on cell migration.
In summary, intravital microscopy is useful in defining the role of adhesion molecules in endotoxin-induced eye inflammation. Intravital microscopy is superior to other techniques, such as conventional histology. For example, in a previous publication we showed that ß2-integrin and perhaps ICAM-1 gene knockout mice had reduced cell infiltration in intravitreal EIU.12 The present study goes further and allows the stepwise process of leukocyte migration to be dissected by demonstrating a major effect on sticking and not on rolling. In designing optimal pharmacotherapy for anterior uveitis, combining an inhibitor of rolling with an inhibitor of sticking may be synergistic, whereas two drugs that each blocked sticking may not show an additive response. Dynamic visualization of the inflammatory response is an invaluable tool in clarifying the process of inflammation.
| Acknowledgements |
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| Footnotes |
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Supported by National Institutes of Health Grants EY06484, EY06477, and EY13093; Research to Prevent Blindness (SRP, JTR); the Casey Eye Institute; and Deutsche Forschungsgemeinschaft (MDB).
Submitted for publication February 23, 2001; revised May 31, 2001; accepted June 11, 2001.
Commercial relationships policy: N.
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: Stephen R. Planck, Casey Eye Institute, Oregon Health Sciences University, 3375 SW Terwilliger Boulevard, Portland, OR 97201. plancks{at}ohsu.edu
| References |
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