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Diabetes Division, University of Massachusetts Medical Center, Worcester, MA
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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DP-BB rats have severe T cell lymphopenia (5, 6, 7, 8, 9), an abnormality not found in humans with IDDM. T cell subsets present in abnormally low numbers in DP-BB rats include those expressing CD8 (10), RT6 (11), and to a lesser extent CD4 (5). DR-BB rats do not share the lymphopenia of DP animals and have a normal distribution of T cell subsets (11). An autosomal recessive locus, lyp, determines the T cell lymphopenia (12, 13, 14). Lymphopenia and diabetes susceptibility can be independently inherited traits (15, 16), but deficiency in peripheral T cells appears to be tightly linked with the expression of spontaneous IDDM in BB rats (13, 14, 17, 18).
In vivo developmental studies have documented that the abnormal T cell maturation in DP-BB rats due to lyp can be detected intrathymically (19, 20). The defect also affects recent thymic emigrants, naive peripheral T cells (21, 22), and intraepithelial lymphocytes (23, 24). The mechanisms by which the lyp gene affects T cell development remain unknown, in part due to the difficulty of analyzing its intrathymic actions in vivo. To investigate the cell biology of this process in the rat, we have established adult thymus organ cultures (ATOC). We report that cultured DR- and DP-BB thymus fragments, respectively, recapitulate the normal and abnormal T cell development that is observed in these animals in vivo.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Viral Ab-free DP-BB and DR-BB rats were purchased from the colony at the University of Massachusetts Medical Center (Worcester, MA). They were certified as free of Sendai virus, pneumonia virus of mice, sialodacryoadenitis virus, rat corona virus, Kilham rat virus, H1 (Toolan’s virus), GD7, Reo-3, Mycoplasma pulmonis, lymphocytic choriomeningitis virus, mouse adenovirus, Hantaan virus, and Encephalitozoon cuniculi. They were maintained in accordance with the Guide for the Care and Use of Laboratory Animals (Institute of Laboratory Animal Resources, National Research Council, National Academy of Sciences, 1996).
ATOC
Thymi were obtained from adult male or female BB rats 55–66 days of age. Thymi were rinsed three times in sterile HEPES-buffered RPMI 1640 without sodium bicarbonate (H-RPMI). Adherent lymph nodes and fatty tissue were removed. Thymi were placed onto a microscope slide and were cut vertically into eight strips. Thymus strips were moistened with a solution of H-RPMI plus 10% FBS (HyClone, Logan, UT) and kept on ice. Very fine forceps and dissecting scissors were used to cut the thymus strips into 1-mm3 fragments.
Thymus fragments were cultured on filters (0.45 µm HAWP04700 filters, Millipore, Bedford, MA) placed on top of 2 x 6 cm Gelfoam sponges (Upjohn, Kalamazoo, MI). Each Gelfoam sponge was hydrated overnight in a 100 x 25-mm tissue culture dish containing 10 ml HEPES-buffered DMEM with sodium pyruvate (high glucose formulation) supplemented with 100 U/ml penicillin, 0.1 mg/ml streptomycin, 125 ng/ml gentamicin, 2 mM glutamine, 1x nonessential amino acids, 5 x 10-5 M 2-ME, and 20% heat-inactivated FBS (HyClone). Thymus fragments were cultured in petri dishes in a humidified incubator in an atmosphere of 7% CO2 and air at 37°C. Each thymus lobe was divided in two, and each half was cultured separately. Cultures were fed on day 7 with 5 ml of supplemented HEPES-buffered DMEM; cultures were maintained for a maximum of 15 days. Sterile technique was used throughout. At the time of cell harvest, the two cultures comprising one thymus lobe were recombined and analyzed together, except in experiments involving culture in the presence of Z-VAD-FMK (see below).
For harvesting, filters were removed from the cultures, cut in half, and placed tissue side down in sterile 6-well tissue culture dishes containing 3 ml of collagenase solution in sodium phosphate buffer with 0.2 mg/ml disodium EDTA, pH 7.4. Collagenase solutions were prepared just before use with 0.4 mg/ml of collagenase from Clostridium histolyticum Type IV (Sigma, St. Louis, MO) and sterile filtered before use. Tissues were digested for 30 min at 37°C. Digestion was stopped by placing the plates on ice and by adding 0.5 ml of FBS (HyClone) to each well. Thymus fragments were picked from the filters and placed into H-RPMI medium. Residual thymus tissue was flushed off of the filters by pipetting. Thymus fragments were gently extruded through a cell sieve (50-mesh screen), filtered through a 70-µm sterile cell strainer (Falcon #2350), and washed twice in H-RPMI. Viable cell number was determined by trypan blue dye exclusion using a hemocytometer.
In certain experiments, the caspase inhibitor Z-VAD-FMK (Enzyme Systems Products, Livermore, CA) was reconstituted according to manufacturer’s instructions immediately before use and added to cultures on day 0. In these experiments, data analysis was performed on the cells generated by each individually cultured half-thymus lobe.
Design of kinetic studies
Cells from an individual thymus culture plate (i.e., one
half-thymus lobe) could be harvested only once, and for that reason
repetitive sampling was not possible. To perform kinetic analyses,
individual cultures were established for each time point of interest. A
minimum of eight individual cultures, each representing one half of a
thymus lobe, were established for analysis of each time point during
culture. Each data point in Fig. 1
represents the
mean ± SD for 4–12 entire thymus lobes at each time point. Cell
counts are expressed as total cells recovered per individual thymic
lobe.
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Single- and dual-color flow microfluorometry was used to
quantify the expression of cell surface markers (25). Abs to the
TCR-
ß (clone R73), IL-2R -chain (clone OX-39), CD4 (clone
OX-35), CD8 -chain (clone OX-8), and Thy-1.1 (clone OX-7) were
purchased from PharMingen (San Diego, CA). Hybridoma-secreting
anti-RT6.1 (clone DS4.23) is maintained in our laboratory. Isotype
control FITC-conjugated mouse IgG1 and biotin-conjugated mouse IgG1,
mouse IgG2a, and rat IgG2b were also purchased from PharMingen, as was
phycoerythrin (PE)-conjugated streptavidin. Biotin-conjugated
anti-RT6.1 (rat IgG2b isotype) was prepared from affinity-purified
DS4.23 using BIOTIN-X-NHS (Calbiochem, La Jolla, CA). All Abs except
anti-Thy-1.1 were used at a concentration of 1 µg per
106 cells. Anti-Thy-1.1 Abs were used at a concentration of
0.5 µg per 106 cells due to intense labeling of
thymocytes and immature T cells. PE-conjugated streptavidin was used at
a concentration of 0.5 µg per 106 cells. Samples were
fixed in a final concentration of 1% paraformaldehyde in PBS and
analyzed using a FACScan. A minimum of 7500 viable cells in each sample
was analyzed. The lymphocyte fraction was gated on the basis of forward
and side scatter. Results are presented as the percentage of positive
cells in the lymphocyte fraction of the cultures with background
isotype control values subtracted.
Three-color flow microfluorometry was used for analyses of cell proliferation and certain TUNEL assays. ATOC were pulsed for 16–17 h before harvest with bromodeoxyuridine (BrdU) at a final concentration of 25 µg/ml. As a positive control, P815 tumor cells were pulsed in vitro for 3–4 h with BrdU at the same concentration. Cultures were harvested and surface labeled with PE- and biotin-conjugated mAbs followed by Cy Chrome-conjugated streptavidin (PharMingen). Cells were fixed, permeabilized, treated with DNase (Sigma), and labeled with FITC-conjugated anti-BrdU mAb (Becton Dickinson, San Jose, CA) as described (26). Negative control ATOC were incubated in the absence of BrdU, processed as above, and labeled with FITC-conjugated anti-BrdU mAb.
Apoptotic cells were detected by flow microfluorometry using a fluorescein-based in situ detection kit (TUNEL assay, Boehringer Mannheim, Indianapolis, IN). The average background in samples not treated with terminal deoxynucleotidyl transferase was only 1.3%, and this value was not subtracted from experimental readings. TUNEL+ T cell subsets were analyzed by three-color staining. Cells were first labeled with PE- or biotin-conjugated Abs, then incubated with Cy Chrome-streptavidin, fixed, and stained for TUNEL+ cells according to manufacturer’s directions.
With one exception, all analyses of BrdU+ and
TUNEL+ cells were gated on the viable lymphocyte fraction
as determined by forward and side light scatter characteristics. The
single exception was a TUNEL assay of total nucleated cells (see Fig. 4A). A minimum of 25,000 events were analyzed in
triple-staining analyses.
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Thick sections of intact cultured thymic fragments still attached to filters were prepared by fixation in Bouin’s solution for 6 h. Fixed specimens were washed overnight, embedded in paraffin, cut into 5-µm sections, stained with hematoxylin and eosin, and examined by light microscopy. Thin sections of intact cultured thymic fragments still attached to filters were prepared by fixation for 2 h in 3% glutaraldehyde in cacodylate buffer at pH 7.4, rinsed three times in buffer, fixed in 2% osmium tetroxide for 2 h, and rinsed four times in buffer. This procedure dissolved the filter. Fixed specimens were dehydrated and embedded in 1:1 propylene oxide and epoxy resin. Sections 1.5 µm thick were mounted on glass slides and stained with 0.5% toluidine blue. Slides mounted in Permount were examined using a Nikon Eclipse E600 microscope and a Nikon Plan Fluor objectives (Tokyo, Japan). Images were captured using a SPOT color digital camera (Diagnostic Instruments, Sterling Heights, MI).
Statistics
Data are summarized as arithmetic means ± SD. Because thymic organ cultures were not sampled repetitively over time, the number and percentage of cells present at different time points is compared with the average number of cells recovered from uncultured thymic lobes. In experiments involving culture in the presence of varying concentrations of caspase inhibitor, only one time point was used; the number and percentage of cells present at different concentrations of inhibitor represents the mean of three individual half-thymus lobe cultures. Curve fitting was performed using SigmaPlot software (ver. 4.0, SPSS, Chicago, IL). Comparisons of three or more means used one-way analysis of variance and the least significant difference procedure for a posteriori contrasts (27). Comparisons of two means used Student’s t test with separate variance estimates (27). Where appropriate, correction for multiple comparisons was made using the Bonferroni adjustment (28).
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Results |
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The number of viable cells recovered from DR- and DP-BB rat ATOC
at various time points is shown in Fig. 1
. The number of cells
recovered from each freshly isolated DR-BB rat thymus averaged 714
± 79 x 106 per lobe (n = 8). The
number recovered from freshly isolated DP-BB rat thymus was somewhat
lower, 640 ± 93 x 106 per lobe
(n = 9), but the difference between the two strains was
not statistically significant (p = 0.60). The
number of cells recovered declined progressively over the 15-day period
of culture. The greatest decline in cell number occurred during the
first 4 days, during which 
90% of cells were lost. In comparison
with the average number of cells present in freshly isolated thymi,
only 13.2 ± 1.8% of DR- (n = 4) and 10.9 ±
1.8% of DP-BB (n = 4) thymocytes remained after 4 days
of culture. The magnitudes of cell loss in DR- and DP-BB rat ATOC on
day 4 were statistically similar (p = 0.78). By
day 15, only 1.2 ± 0.7% of DR-BB (n = 8) and
0.2 ± 0.04% of DP-BB (n = 4) cells remained;
this difference between DR- and DP-BB rat ATOC cell recovery was
statistically significant (p = 0.006).
Inspection of both DR- and DP-BB rat ATOC kinetics suggested that cell
number over time was determined predominantly by cell death. Consistent
with this inference, the decline in cell number over time exhibited
first order exponential decay kinetics (Fig. 1, inset). The calculated
half-life of DR cells in ATOC was 1.43 days, and the half-life of DP-BB
cells was 1.32 days.
However, first-order exponential decay did not fully describe the
behavior of ATOC at later time points, notably after day 5. This
observation suggested that cell proliferation rates also played a role
in determining cell number over time. To test this hypothesis
quantitatively, the data were refitted using second order regression.
As shown in Fig. 1, the decline in cell number did appear closely to
follow second order kinetics. The coefficients associated with the
first-order term (cell death) were different for DR (-0.26) and DP
(-0.34) ATOC, whereas the second-order terms (cell proliferation) were
similar for DR (+0.010) and DP (+0.012) cultures.
Although cell death was the principal determinant of cell number in BB
rat ATOC over time, the majority of recovered cells were nonetheless
viable as determined by trypan blue exclusion. Representative examples
of the morphology of day 8 DR-BB rat thymus cultures are shown in Fig. 2. Overall, the cultured fragments lack the anatomical
characteristics of normal in vivo thymus, but contain large numbers of
viable lymphoid and thymic stromal cells (Fig. 2A). At
higher magnification (Fig. 2B), viable mononuclear cells
appear to predominate, and thymic stromal cells can also be identified.
However, examination of individual cell types in selected fields did
reveal morphologic evidence of cell death by apoptosis (Fig. 2C). Macrophages could be identified in areas of cultured
fragments that contain ingested cells with condensed nuclear chromatin
and apoptotic bodies.
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Based on our kinetic analysis (Fig. 1), we next directly tested
the hypothesis that differences in the rate of thymocyte apoptosis but
not cell proliferation accounted for the differential recovery of DP
and DR cells in ATOC. We examined the dynamics of cell proliferation by
measuring the uptake of BrdU. We also assessed the activation state of
cells in ATOC by measuring the surface expression of IL-2R -chain.
Few IL-2R+ cells were detected in fresh thymocyte
preparations (data not shown) or on day 1 of culture (Fig. 3A). The percentage of IL-2R+
cells thereafter increased to about 30% on day 5 in both DR- and DP-BB
rat ATOC. The percentage of IL-2R+ cells remained at this
level in DP-BB ATOC through day 9, but declined somewhat in DR-BB rat
ATOC. Consistent with reports that a high frequency of T cells in DP
rats have characteristics of activated T cells (29, 30, 31, 32, 33), we observed
that the percentage of activated IL-2R+ cells was
statistically significantly greater in DP- than in DR-BB rat ATOC at
three of the four time points sampled.
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10–15%, Fig. 3B). However, over time
the percentage of BrdU+ cells did not increase, and in the
case of DR-BB rat ATOC actually declined. By day 9 of culture, the
percentage of BrdU+ cells was significantly greater in DP-
than in DR-BB rat ATOC (Fig. 3B).
We next measured the number of TUNEL+ cells present in
ATOC. In the first experiment (Fig. 4A), all
nucleated cells were analyzed, and the data set therefore includes
viable cells, cells in the process of apoptosis, and dead cells.
Consistent with previous observations (34), the overall percentage of
TUNEL+ apoptotic cells was very low in freshly isolated DR-
(1.4 ± 0.6%) and DP-BB (1.6 ± 0.8%) thymocytes (Fig. 4A). The percentage of TUNEL+ cells then
increased and remained elevated throughout the duration of the study.
As predicted by the kinetic model shown in Fig. 1, the percentage of
TUNEL+ cells was statistically significantly greater in DP
than in DR-BB thymocyte cultures at all time points after day 2.
ATOC of DR- and DP-BB rat thymus yield similar percentages of
TCR-ßhigh and CD4 single-positive, but not CD8
single-positive, T cells over time
To characterize the physiology of rat ATOC, cell subsets present
in both freshly isolated and cultured DR and DP thymi were identified
by flow cytometry using single labeling for TCR-ß and dual
labeling for CD4 and CD8. Fig. 5 shows phenotypic
analyses of percentages of CD4 CD8 double-positive,
TCR-ßhigh, CD4 single-positive, and CD8
single-positive cells over time. The percentage of immature
CD4+ CD8+ cells in freshly isolated DP- and
DR-BB thymocytes was 80%; this percentage declined progressively to
5% during culture (Fig. 5A). The percentage of
double-positive cells in DP and DR ATOC remained similar throughout the
period of observation.
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ßhigh and CD4 single-positive
cells (Fig. 5, B and C). The percentages of
TCR-ßhigh cells in freshly isolated DR- and DP-BB
thymocytes were 19.0 ± 2.5% and 15.6 ± 1.6%,
respectively. These values increased to 93.6 ± 0.8% and
91.2 ± 1.8% on day 11. CD4 single-positive cells increased in
freshly isolated DR- and DP-BB thymocytes from 11.4 ± 0.9% and
9.8 ± 1.8%, respectively, on day 0 to 68.0 ± 2.4% and
72.9 ± 2.3% on day 11. Again, the percentages observed in DP-
and DR-BB rat ATOC were similar throughout the period of observation.
In contrast, the percentages of CD8 single-positive cells in DP and DR
cultures exhibited different patterns. The percentage of
CD8+ cells in freshly isolated thymocytes was higher in DR-
(6.2 ± 1.4%) than in DP-BB (3.9 ± 0.7%) rat ATOC (Fig. 5D). This relatively small difference persisted during the
first 6 days of ATOC; by day 6, the percentage of CD8+ DR
cells had risen to 23.5 ± 0.8% and the percentage of
CD8+ DP cells had risen to 20.0 ± 1.2%.
Subsequently, however, the percentage of CD8+ DR cells
remained stable whereas the percentage of CD8+ DP cells
declined sharply. By day 11, the percentage of CD8+ DR
cells was still 20.6 ± 3.6%, whereas the percentage of
CD8+ DP cells had fallen to 8.2 ± 1.2%
(p < 0.001). The CD4:CD8 ratio on day 7 of
ATOC was 2.5:1 for DR and 3.9:1 for DP rats. By day 11, CD4:CD8 ratios
were still more divergent, 3.3:1 and 9.1:1 for DR and DP cultures,
respectively.
To determine whether the divergence of CD4:CD8 ratios for DR and DP
cultures at later time points was due to differences in
proliferation, we measured the percentage of
BrdU+-proliferating cells that were CD4+,
CD8+, and double positive (Fig. 3, C and
D). In each case, the percentages were statistically similar
for DR- and DP-BB rat ATOC. As expected, the number of
BrdU+-proliferating double-positive cells declined over
time (Fig. 3C), whereas the percentage of
BrdU+-proliferating CD4+ cells increased over
time (Fig. 3D). In contrast, the percentage of
BrdU+-proliferating CD8+ cells remained low and
constant over time (Fig. 3D).
Given the absence of differences in proliferation, we next asked if the
divergence of CD4:CD8 ratios for DR and DP cultures at later time
points was due to differences in rates of cell death. To do so, we
measured the percentage of TUNEL+ cells that were
CD4+, CD8+, and double positive (Fig. 4, B–D). In these experiments, only viable cells were analyzed
so as to include only those cells in the process of apoptosis. This
gating also permitted us to compare the relative rates of apoptosis in
DR- and DP-BB rat ATOC. We observed that percentages of
TUNEL+ but viable cells were initially similar in DR- and
DP-BB rat ATOC but subsequently was much higher in the DP cultures
(Fig. 4B). This result is consistent with the data obtained
when all nucleated cells were analyzed (Fig. 4A). Subset
analysis disclosed additional differences. The percentage of
CD4+ TUNEL+ cells was initially higher in DP-
than in DR-BB rat ATOC, but the percentages were similarly low by day 7
of culture (Fig. 4C). In contrast, the percentage of
CD8+ TUNEL+ cells was initially similar and low
in DP- and DR-BB rat ATOC, but the percentage subsequently became much
higher in the DP cultures (Fig. 4D).
The percentages of Thy1.1+ immature T cells decline at comparable rates in DP- and DR-BB rat ATOC
To analyze further the maturation of T cells in ATOC, we next
measured the expression of the differentiation antigen Thy 1.1. Thy 1.1
expression declines as rat T cells mature (20, 22, 35). Virtually all
rat thymocytes are Thy 1.1bright; recent thymic emigrants
are Thy 1.1low; and peripheral T cells are Thy
1.1- (20, 22, 35). Fig. 6 (upper
panel) shows that the percentages of Thy 1.1+ cells in
both DR- and DP-BB rat ATOC were very similar over time. The percentage
of Thy 1.1+ cells declined from >95% in uncultured
thymocytes to 12–13% on day 9. Thy 1.1 staining was very bright at
early time points and progressively diminished in intensity with time,
an observation that is consistent with progressive T cell maturation.
Representative flow histograms for Thy 1.1 are shown in Fig. 7.
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RT6.1 is an alloantigen that is expressed postthymically on mature
CD4+ and CD8+ T cells and is severely deficient
in the DP strain (11, 36, 37). Mature RT6.1+ T cells, which
are absent (<1%) in the thymus, develop in both DR- and DP-BB ATOC,
reach maximal levels by day 7, and decline thereafter (Fig. 6, lower panel). The generation of substantial numbers of
RT6+ T cells in DP ATOC is striking given the near absence
of these cells in the DP peripheral lymphoid tissues (3). However, in
comparison with DR-BB rat ATOC, DP-BB rat ATOC generated a
statistically significantly smaller percentage and number of
RT6+ cells than did DR-BB rat cultures. On day 7 of
culture, 34.1 ± 1.7% of CD4+ cells in DR-BB rat ATOC
were RT6.1+ (8.1 ± 1.1 x 106
cells). In contrast, 21.3 ± 12.3% of CD4+ cells in
DP-BB rat ATOC were RT6.1+ (1.4 ± 0.8 x
106 cells). On day 7, 87.3 ± 0.6% of
CD8+ cells in DR-BB rat ATOC were RT6.1+
(20.9 ± 2.5 x 106 cells). In contrast,
79.0 ± 6.6% of CD8+ cells in DP-BB rat ATOC were
RT6.1+ (6.0 ± 0.7 x 106 cells).
Representative flow histograms for RT6 are shown in Fig. 8. Both DR and DP cultures generate more Thy
1.1low and RT6.1+ cells than are present in
freshly isolated thymi.
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Our observation of apoptotic cells in morphologic analyses (Fig. 2), and in TUNEL assays (Fig. 4), suggested that a major cell death
pathway in rat ATOC is apoptosis. To test this hypothesis, DR- and
DP-BB rat thymi were cultured in the presence or absence of the
inhibitor of caspase-dependent apoptosis, Z-VAD-FMK, at various
concentrations. Z-VAD-FMK is a general inhibitor of caspases with
broader specificity than the tetra-peptide-based inhibitors YVAD and
DEVD (38, 39, 40). Z-VAD-FMK was chosen for these experiments on the basis
of recent reports documenting the involvement of caspases in negative
selection (41, 42) and anti-CD3-mediated thymocyte apoptosis (40).
Cultures were analyzed by flow cytometry for cell surface expression of
CD4, CD8, and RT6.1.
As shown in Fig. 9, the number of
CD4+CD8+, RT6.1+,
CD4+CD8-, and
CD4-CD8+ cells recovered from DP-BB rat thymi
cultured for 7 days in the presence of Z-VAD-FMK increased in a
dose-dependent manner. Compared with cultures performed in the absence
of the inhibitor, the number of CD4+CD8+,
RT6.1+, CD4+CD8-, and
CD4-CD8+ cells increased 11.0-, 6.5-, 2.5-,
and 4.7-fold, respectively. Culture in the presence of Z-VAD-FMK also
increased the number of immature CD4+CD8+ cells
recovered from DR-BB rat ATOC by a factor of 2 at the highest
concentrations, but Z-VAD-FMK had little or no effect on the number of
mature RT6.1+, CD4+CD8-, or
CD4-CD8+ cells recovered.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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This recapitulation by ATOC exhibits remarkable fidelity to the in vivo
physiology for both the DP and DR cultures. In vivo T cell development
in the DR-BB rat generates normal numbers of T cells with a normal
distribution of phenotypes (3, 11, 20). Ontogenetically, DR-BB T cell
development requires 3–5 days of intrathymic processing during which
3% of double-positive thymocytes survive selection and mature into
single-positive thymocytes. These cells then migrate to the periphery
where they acquire a mature phenotype by down-regulating the expression
of Thy 1.1 and acquiring the expression of RT6 within 3–5 days (20, 22, 35). Each of these kinetic and phenotypic features of normal DR T
cell development was reflected in DR rat ATOC.
The abnormal development of T cells in DP-BB rats also appears to begin
intrathymically; it is intrinsic to the DP-BB rat stem cell and not to
any defect in the DP thymus or peripheral lymphoid compartment (43).
Although subtle defects in DP-BB rat T cells may be detectable at the
double-positive stage of thymocyte development, abnormal
differentiation first becomes readily detectable at the single-positive
stage, at which time there is a major deficiency in CD8+
TCRhigh positively selected thymocytes (19). Thereafter,
there is also reduced export of T cells from the thymus (20, 22) and a
failure of recent thymic emigrants to differentiate into Thy
1.1- RT6+ mature T cells (20, 22). These
cells, which appear to be activated (29, 30, 31, 32, 33), undergo apoptosis and are
removed by the liver (21). Recapitulating the in vivo intrathymic
physiology, DP-BB ATOC was characterized by increased expression of the
activation IL-2R marker, increased apoptosis, deficiencies in
markers of T cell maturation, and reduced overall cell output.
In general, rat ATOC is characterized by progressive cell maturation in the context of extensive cell death. A rapid decline in cell number was observed with both DR- and DP-BB rat cultures. About half of all cells present on day 2 of culture were TUNEL+, either dead or dying of apoptosis. This extensive degree of cell death is consistent with the fact that, in normal rat thymus, >97% of cells fail to mature and emigrate to peripheral tissues (44). In addition, as expected from in vivo studies, the decline in cell number observed in DP-BB ATOC was even greater than that observed with DR rat thymi. Because thymi in ATOC cannot be reseeded with immature bone marrow-derived progenitors, the number of cells recoverable over time declines. The decline is exponential, indicating that the number of cells present over time is principally a function of the number of cells present earlier, consistent with randomly occurring cell death.
The dramatic loss of cells seen during the first 5 days of culture is consistent with the in vivo life span of cells that fail to undergo positive selection (44, 45, 46). We interpret the loss of cells over time in ATOC as due to programmed cell death. This interpretation is supported by our observation that the percentage of TUNEL+ cells is initially low, then increases, and finally plateaus. The presence of apoptotic cells was also confirmed histologically by the observation of apoptotic bodies within phagocytes.
It could be argued that the decline in cell number is due to trauma leading to cell death by necrosis, and we cannot exclude the possibility that some degree of tissue injury occurs during the processing of thymi for culture. However, cell death due to necrosis would be expected to result in the accumulation of cellular debris in the cultures. This was not seen. In addition, we observed proliferation of maturing thymocytes over time. The number of proliferating thymocytes as judged by the percentage of BrdU+ cells was statistically similar in both DP- and DR-BB ATOC at most time points. The only exception was a modestly but significantly higher rate of proliferation in DP-BB rat ATOC on day 9. We conclude that DR rat ATOC recapitulates the development of normal T cell numbers observed in the periphery of these animals in vivo. In contrast, DP-BB ATOC recapitulates the development of lymphopenia that is characteristic of these animals due to homozygosity for the lyp locus (12).
DR- and DP-BB rat ATOC recapitulates not only differences in T cell production, but also strain-specific differences in cell phenotype. Culture of DR-BB rat thymi generates mature CD4 and CD8 single-positive cells with up-regulated TCRs. DR-BB rat cultures also generate mature T cells that down-regulate Thy1.1 and eventually up-regulate RT6. In contrast, DP-BB rat cultures generate smaller numbers and smaller percentages of CD8 single-positive cells than do DR cultures. Like DR-BB rat ATOC, DP-BB cultures initially appear to generate Thy1.1- RT6+ cells, but over time DP cultures yield fewer mature RT6+ cells than do DR cultures. DP-BB rat ATOC does generate a percentage of CD4+ cells that is similar to that observed in DR-BB rat cultures, but due to the overall decreased cellularity of DP cultures, the number of CD4+ cells generated is less. It is likely that these maturing cells are lost due to apoptosis, and it would appear that the process recapitulates the in vivo loss of maturing T cells in the DP rat that occurs by apoptosis in the liver (21).
We hypothesize that the severe deficiency in single-positive cells in DP-BB rats in vivo and in DP-BB rat ATOC in vitro is the result of the failure of double positive thymocytes to be rescued from apoptosis during successful positive selection. This hypothesis in turn predicts that inhibition of apoptosis in DP-BB rat thymi should 1) increase the number of immature double-positive thymocytes and 2) permit their subsequent differentiation into mature single-positive thymocytes. These predictions were confirmed by our observation of increased DP-BB rat cell recoveries in thymus cultures conducted in the presence of the caspase inhibitor Z-VAD-FMK. The increase was greatest for double-positive cells, but recovery of RT6+, CD8+, and CD4+ cells was also enhanced. On the basis of these data, we speculate that the function of the lyp gene, which is responsible for the lymphopenia of the DP-BB rat (12), is to enhance caspase-mediated apoptosis in thymocytes that have successfully undergone positive selection. However, we recognize that these observations may be interpreted in many other ways. Z-VAD-FMK blocks apoptosis initiated by many processes. It is possible that cell death in developing thymocytes in DP-BB rat ATOC is the result of failure to receive or respond to cytokine or stromal cell signals. Alternatively, it may represent increased negative selection or neglect. It is difficult to distinguish among these alternatives in a nontransgenic system.
We observed that the effect of Z-VAD-FMK on DR-BB rat ATOC was much smaller in magnitude than its effect on DP ATOC. DR-BB rats are nonlymphopenic and wild type at the lyp locus (3, 12). Because positive selection of thymocytes in DR-BB rats appears to be normal, it would be predicted that Z-VAD-FMK would not influence the yield of mature T cells generated by DR rat ATOC. This prediction was confirmed. We attribute the modest increase in the yield of double-positive cells generated by DR rat ATOC in the presence of Z-VAD-FMK to the reported general activity of caspases in thymocyte selection (41, 42).
The development of rat ATOC that recapitulate in vivo events holds promise for enhancing our understanding of the processes that lead to the generation of autoreactivity in the rat. Like previously described rodent fetal thymus organ cultures (FTOC), rat ATOC allows for analysis of T cell development in the absence of seeding of the thymus with thymocyte progenitors and in the absence of emigration from the thymus. However, compared with FTOC the development of rat ATOC offers several advantages. Whereas FTOC generates predominantly CD8 single-positive cells, ATOC applied to a phenotypically normal animal like the DR-BB rat generates cells with a normal CD4:CD8 ratio. Applied to the lymphopenic DP-BB rat, ATOC recapitulates defective T cell development. These characteristics suggest that ATOC reflects both normal and abnormal T cell development with reasonable fidelity. In addition, ATOC is technically simpler than FTOC in that it generates larger numbers of cells without requiring timed pregnancies or microdissection of the thymus. Another potential advantage of ATOC over FTOC for the study of autoimmunity may derive from the fact that fetal and adult thymocytes may not be functionally identical (25, 47, 48). Rat ATOC also differs from suspension cultures of thymocytes in that the three-dimensional structure of the thymic microenvironment is disrupted in suspension, and the cells generated in suspension cultures do not develop normally (49, 50).
The availability of ATOC provides the opportunity to analyze in detail the earliest processes that lead to the generation of autoreactive cells. Our laboratory has demonstrated, for example, that autoreactive cells in the BB rat are generated intrathymically. Thymocytes from 8-wk-old DR-BB rats are capable of the adoptive transfer of IDDM (25). However, the mechanisms underlying this observation are difficult to identify and modify in vivo. ATOC should permit investigators to circumvent this problem. In preliminary studies we have observed that cells obtained from day 7 DR-BB rat ATOC transfer diabetes to histocompatible athymic recipients (B.J.W., unpublished observations). ATOC will also permit detailed analysis of the early events that result in enhanced apoptosis in the DP-BB rat and may lead to further insights into the function of the lyp locus (12, 51). Studies to modify the development of autoreactive thymocytes in ATOC by strategies including cytokine polarization, exposure to candidate autoantigens, and coculture with regulatory cells are underway in our laboratory.
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Footnotes |
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2 Address correspondence and reprint requests to Dr. Dale L. Greiner, Diabetes Division, University of Massachusetts Medical School, Two Biotech, 373 Plantation Street, Suite 218, Worcester, MA 01605. E-mail address:
3 Abbreviations used in this paper: BB, BioBreeding; ATOC, adult thymic organ culture; BrdU, bromodeoxyuridine; DP, diabetes-prone; DR, diabetes-resistant; IDDM, insulin-dependent diabetes mellitus; H-RPMI, HEPES-buffered RPMI; PE, phycoerythrin; TUNEL, terminal deoxynucleotidyl transferase-mediated dUTP-nick end labeling; FTOC, fetal thymic organ culture.
Received for publication June 18, 1998. Accepted for publication January 4, 1999.
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