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1 From the Laboratory of Immunology, Clinical Immunology Section, National Eye Institute, Bethesda, Maryland; and the 2 Department of Ophthalmology, Academic Medical Center, University of Amsterdam, The Netherlands.
| Abstract |
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METHODS. Patients were evaluated at each visit for signs of ocular inflammation. Peripheral blood leukocytes were harvested and cultured in the presence of bovine S-antigen in a standard culture assay, as well as by limiting dilution using multiple short-term T-cell lines.
RESULTS. Five patients were observed for 2 to 10 months. During follow-up, three patients had episodes of ocular inflammation. No consistent change in proliferative response was observed in standard proliferation assays. However, an increase in established T-cell lines was correlated to the presence of ocular inflammation in all three patients. Ocular activity was associated with an increase of 9- to 30-fold in the frequency of short-term T-cell lines. This increase returned to baseline within 1 to 3 months.
CONCLUSIONS. An increase in S-antigenresponsive lymphocytes is found in the peripheral blood of patients with Behçets disease during episodes of ocular inflammation. This increase cannot be measured using standard proliferation assays but requires the use of techniques exploiting the principles of limiting dilution analysis.
| Introduction |
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Circumventing the qualitative nature of the lymphocyte proliferation assay, limiting dilution assays (LDA),13 allow determination of the number of cells that respond to a specific antigen or antigens (precursor frequency).14 15 LDAs have been used in surveillance of the level of response to the soluble tetanus toxoid antigen over time.16 Using a cell culturebased assay that incorporates elements of a limiting dilution technique, we showed that it is possible to estimate the T-cell precursor frequency to bovine S-Ag in patients with diverse forms of uveitis.17 Using a similar approach, others determined the precursor frequency of T cells responding to fragments of myelin basic protein (MBP) in patients with multiple sclerosis (MS) and after spinal cord injury.18 19 20 Adjusting the culture conditions allowed the demonstration of in vivo clonal expansion of MBP-reactive T cells in both the blood and the cerebrospinal fluid (CSF) of patients with MS, suggesting active trafficking of these lymphocytes across the bloodbrain barrier.18 20 A similar study in Behçets disease would imply the isolation of lymphocytes from the vitreous of a patient with active inflammation, a procedure that is rarely required and is not without risk. However, it is possible to study the influence of ocular inflammation on lymphocytes within the peripheral blood.
Using our modified LDA, we decided to explore the change in S-Ag responsiveness over time in peripheral blood lymphocytes of patients with Behçets disease. We were particularly interested in determining whether fluctuations in responsiveness could be detected after episodes of intraocular inflammation. Bovine S-Ag was used as the stimulating antigen. Because standard lymphocyte proliferation assays have not been carefully studied in this setting, we also wanted to characterize the response of this simpler culture method to ocular inflammation.
| Methods |
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Lymphocyte Isolation and Culture
Peripheral blood samples were collected at several time points, as
indicated in Table 1
. For each time point, 60 ml of peripheral blood was collected.
Peripheral blood mononuclear cells (PBMCs) were separated on
a gradient (Isolymph; GallardSchlesinger, Carle Place, NY). One
fraction was used for both culture assays as indicated below and was
processed immediately. The remaining PBMCs were frozen to be used for
antigen presentation. For the standard proliferation assay, cells were
cultured in flat-bottomed 96-well microplates, at 2 x
105 cells per well, in 0.2 ml RPMI 1640 medium
with HEPES (Cellgro, Herndon, VA), supplemented with 10%
heat-inactivated human AB serum (Biocell Laboratories, Carson, CA). Six
replicate cultures were stimulated with bovine S-Ag at 20 or 100
µg/ml. Phytohemagglutinin (PHA) at 1 µg/ml was used as a control of
adequate proliferation. Cells were cultured for 5 days, pulsed with
[3H]-thymidine (3H-TdR;
NEN, Boston, MA; 2 Ci/mmol, 0.5 µCi/10 µl per well), during the
last 18 hours of culture. Bovine S-Ag was prepared according to the
method of Dorey et al.22
The results are expressed as the
stimulation index (SI = mean counts per minute in cultures
with stimulant/mean counts per minute in control cultures without
stimulant).
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PBMCs were incubated at 37°C, with bovine S-Ag 100 µg/ml for 1 hour at a cell density of 1 x 107/ml. These were then washed twice and resuspended in RPMI 1640 at a concentration of 1 x 106/ml and plated at 200 µl per well into 480 wells on five round-bottomed 96-well microtiter plates (Costar, Cambridge, MA). Beginning on the third day and every 3 days thereafter, half of the medium was replaced with fresh medium containing 5% T-cell stimulant (Collaborative Research, Bedford, MA) and 2 U/ml of human recombinant interleukin (rIL)-4 (Genzyme, Boston, MA).
On day 12 to 14, each well was analyzed for its reactivity to bovine S-Ag. An aliquot from each culture well was split into four aliquots (10,000 cells per aliquot), and placed into fresh, round-bottomed 96-well plates. Two wells were cultured with 105 PBMCs pulsed for 4 hours with S-Ag before irradiation (3000 rads). The two other wells received unpulsed irradiated PBMCs. Wells were considered responsive to bovine S-Ag if the SI was above 3.0 and if the coefficient of variation for each duplicate culture was less than 30%. Results are expressed as the percentage of positively responding wells.
Statistical Analysis
Statistical analysis was performed by computer (Prism; GraphPad
Software, San Diego, CA) without assuming a gaussian distribution using
the MannWhitney test.
| Results |
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Standard Proliferation Assays
A comparison of the proliferative response between control
subjects and patients was performed at study entry. Data on control
subjects are provided in Table 2
. There was no statistically significant difference between the two
groups when tested at the 20-µg/ml concentration of bovine S-Ag (mean
SI: 2.3 control subjects, 2.5 patients). Similarly there was no
statistically significant difference in S-Ag response at 100 µg/ml
(mean SI: 2.2 control subjects, 4.3 patients), although a significant
coefficient of variation was present between the two groups. Reducing
the difference in this variation by logarithmic transformation did not
change the level of significance.
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| Discussion |
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Alternatively, an attempt can be made to demonstrate the presence of an expanded pool of responsive T cells in the peripheral blood after an intraocular inflammation episode. In patients with MS, activated MBP-reactive T cells were shown to undergo in vivo clonal expansion in both the blood and the CSF, indicating the presence of an active traffic of MBP-reactive T cells across the bloodbrain barrier.20 Assuming that a similar process is present in patients with ocular inflammation, we decided to observe a group of patients with Behçets disease prospectively by using two assays to measure T-cell proliferation to S-Ag.
Standard proliferative assays have been used by numerous groups to demonstrate an association with human disease.9 12 24 An often-quoted cutoff for immunologic significance is an SI of 2.0. Using this cutoff value, our present group of patients all showed a positive immune proliferative response during the follow-up period, whereas only three patients had intraocular inflammation. Using a more restrictive cutoff of significance (mean of control subjects + 2 SDs) allowed us to identify two of three patients with ocular inflammation. The heightened response was measured either before or at the time of ocular inflammation. This second approach more appropriately selected patients at risk but missed one individual. In addition, once inflammation developed, subsequent measurements were significantly reduced in intensity. Thus, standard proliferation assays can help predict patients at risk of development of uveitis but cannot be used for surveillance of the immune response during the active phase of inflammation.
In another study, a patient with an unspecified retinal scar, present for more than 30 years, was repeatedly tested over an 18-month period against S-Ag.28 A large variation in lymphocyte responsiveness was attributed to the microenvironment present in the culture well. Cytokines produced by one or more cell populations present in the microwell prevented the expression of the S-Ag phenotype.29
Limiting dilution assays have traditionally been used to quantify cells with observable functional qualities. Appropriate dilution of the seeding cell population, allows the interpretation of data according to "single-hit kinetics" (0-term) of the Poisson distribution. Use of unfractionated cell samples dispersed into limiting dilution cultures yields nonlinear titration curves due to opposing forces generated by suppressor and effector cells.29 30 31 To overcome the suppressor effects requires prolonged cultivation and the supply of needed growth factors such as T-cell growth factor.29 Studies performed in other laboratories have shown that this approach generates CD4 single-cell lineages in the majority of wells that often are responsive to a single peptide determinant.18 20 32 Given the number of wells showing a positive response in these studies, single-hit kinetics apply. Thus, the number of positively responding wells is reflective of the circulating pool of responsive T cells. Of the five patients who were observed, three had a transient ocular inflammatory response. In each of these cases, there was an increase in the number of identifiable T-cell lines concurrent with the ocular inflammation. The expansion was between 9- and 30-fold, and this increase rapidly returned back to baseline between 1 and 3 months.
Assessing limiting dilution using this approach makes certain assumptions. The cell populations within the culture well behave according to single-hit kinetics. Memory T cells, which were probably responsible for the activity observed in these experiments, require multiple hits for activation.33 However, in the presence of IL-2, a major component of the T-cell stimulant, it is converted back to single-hit behavior. Thus, addition of the T-cell stimulant favors cells that are capable of growing in its presence. The adequacy of the technique can be checked by setting up cultures at different dilutions. Increases in the number of cells should lead to a linear increase in the number of positive wells. Deviations from linearity, in particular leveling off in the curve, indicates the presence of suppressive elements in the culture well.19 This phenomenon has been observed in patients with MS. We do not know to what extent this effect was present in the assays performed in our patients, because the chosen cell dilution was based on previous work with patients with uveitis, in whom disease was under control at the time of analysis. It is possible that the observed reduction in the proportion of responding wells was partially due to enhanced suppression provided by other cells present in the culture well. Performing titration experiments on a broad range of concentrations that extend beyond the zone of linearity should help to answer this question.29
In experimental models, both CD8 responses to a viral challenge and CD4 responses to antigen challenge are characterized by three distinct phases.34 35 Initial activation and expansion of the lymphocyte pool lasts for approximately 7 days. It is characterized for a novel agent by a 100- to 5000-fold increase in the number of specific lymphocytes.34 36 37 38 In the case of mice exposed to lymphocytic choriomeningitis virus (LCMV), this expansion was calculated to represent 15 divisions or 1 division every 13 hours for 8 days.37 Rapid expansion is followed by a period of cell death, lasting anywhere from 8 to 30 days and is realized primarily through apoptosis.34 35 36 After 30 days, a stable pool of memory T cells is generated that represents approximately 5% of the initial response. Re-exposure to an antigen or virus leads to a rapid expansion from this pool of memory T cells. Although this response occurs more rapidly than the initial response, the measured lymphocyte expansion in peripheral blood or lymph nodes amounts only to a 5- to 100-fold expansion over baseline.35 36 Our data on patients with Behçets disease closely parallels these experimental findings. Patients were seen within a few days of recurrence and were noted to have a 9- to 30-fold increase in responsive lymphocytes. As seen in experimental models, this increase rapidly returned to the preflare-up level.
This is the first time that a correlation has been observed between peripheral antigen responsiveness and inflammatory disease in humans. Adjusting the culture conditions to favor effector T-cell proliferation has allowed us to unmask this responsiveness. These findings suggest a role for S-Ag in the autoimmune uveitis associated with Behçets disease, but it does not imply causality. As previously observed, patients with established disease respond to a number of autoantigens.5 It is likely that some if not all these antigens contribute or help to perpetuate the inflammatory response.
In conclusion, adjustment in culture conditions that promote clonal expansion of reactive T cells has allowed us to demonstrate a variation in the number of responsive lymphocytes in the peripheral blood of patients with Behcets disease with active intraocular inflammation. Though technically more demanding, this approach can yield considerably more information on the immune status of patients with uveitis than is currently provided by standard lymphocyte culture methods. It is a valuable tool for quantifying cells at a functional level.
| Footnotes |
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Submitted for publication January 31, 2000; revised May 30, 2000; accepted May 31, 2000.
Commercial relationships policy: N.
Corresponding author: Marc D. de Smet, Department of Ophthalmology, University of Amsterdam, Rm G2-217, Meibergdreef 9, Amsterdam 1105 AZ, The Netherlands. m.d.desmet{at}amc.uva.nl
| References |
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