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The Journal of Immunology, 2007, 179, 6741 -6748
Copyright © 2007 by The American Association of Immunologists, Inc.
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Shaping of the Autoreactive Regulatory T Cell Repertoire by Thymic Cortical Positive Selection1

Julie Ribot*, Geneviève Enault*, Sylvie Pilipenko{dagger}, Anne Huchenq{dagger}, Maryline Calise{dagger}, Denis Hudrisier{ddagger},§, Paola Romagnoli* and Joost P. M. van Meerwijk2,*,§,¶

* Institut National de la Santé et de la Recherche Médicale Unité 563, Toulouse, France; Université Toulouse III Paul Sabatier, Centre de Physiopathologie de Toulouse Purpan, and Institut Fédératif de Recherche (IFR) 30, Institut Claude de Preval, Toulouse, France; {dagger} Transgenic Mouse Facility, IFR 30, Institut Claude de Preval, Toulouse, France; {ddagger} Institut de Pharmacologie et de Biologie Structurale, Centre National de la Recherche Scientifique, Toulouse, France; § Faculty of Life Sciences University Paul Sabatier, Toulouse, France; and Institut Universitaire de France, Paris, France


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 Disclosures
 References
 
The main function of regulatory T lymphocytes is to keep autoimmune responses at bay. Accordingly, it has been firmly established that the repertoire of CD4+CD25+Foxp3+ regulatory T cells (Tregs) is enriched in autospecific cells. Differences in thymic-positive and/or -negative selection may account for selection of the qualitatively distinct regulatory and conventional T cell (Tconv) repertoires. It has previously been shown that precursors for Tregs are less sensitive to negative selection than Tconv precursors. Studies with TCR/ligand doubly transgenic mice suggested that an agonist ligand might induce positive selection of Treg (but not Tconv) cells. However, massive deletion of Tconv (but not Treg) cell precursors observed in these mice renders interpretation of such data problematic and a potential role for positive selection in generation of the autospecific Treg repertoire has remained therefore incompletely understood. To study this important unresolved issue and circumvent use of TCR/ligand-transgenic mice, we have developed transgenic mice expressing a single MHC class II/peptide ligand on positively selecting thymic cortical epithelial cells. We found that functional Treg (but not Tconv) cells specific for the single ligand were preferentially selected from the naturally diverse repertoire of immature precursors. Our data therefore demonstrate that thymic cortical positive selection of regulatory and Tconv precursors is governed by distinct rules and that it plays an important role in shaping the autoreactive Treg repertoire.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 Disclosures
 References
 
Regulatory T lymphocytes play a critical role in control of immune responses. In experimental settings, these cells inhibit development of autoimmune responses and inflammatory bowel disease, regulate immunity to the fetus and tumors, and also control immunity to various infections (1, 2, 3, 4, 5, 6). Moreover, when appropriately expanded in vitro, regulatory T cells (Treg)3 can prevent allograft rejection as well as graft-vs-host disease (7). Whereas the repertoire of Treg must therefore contain cells specific for nonself-Ags, it has been firmly established that it is enriched in autospecific cells. Our studies using limiting dilution analysis showed that in the CD4+CD25+ Treg repertoire, the frequency of autospecific cells is substantially higher than that of cells proliferating in response to allogeneic APC (8). We therefore concluded that the Treg repertoire is exquisitely shaped to recognize self-Ags. This conclusion was later elegantly confirmed by others (9, 10, 11).

It is thought that most, though not all (12, 13, 14, 15), CD4+CD25+ Treg develop in the thymus (16). Moreover, data from our laboratory as well as from others, indicate that an autoreactive Treg repertoire is shaped in the thymus (11, 17). Given the stringent selection processes developing T cell precursors are submitted to in this organ, it is of utmost importance to evaluate how the autospecific Treg repertoire develops intrathymically. To allow for development of an autoreactive repertoire, Treg precursors must be less susceptible to negative selection than precursors of conventional T cells (Tconv). In mice in which negative selection is generally defective, e.g., in certain transgenic or bone marrow chimeric mice, a strongly autospecific Tconv repertoire develops (18, 19, 20). Development of the autoreactive Treg repertoire may therefore simply result from defective negative selection of Treg precursors. In contrast, positive selection of Treg vs Tconv may also be governed by different rules. Positive selection of Tconv is very MHC/peptide specific and appears to be mediated by nonagonist self-ligands (21). By contrast, Treg precursors with high avidity for self-MHC/peptide ligands may be preferentially selected, thus contributing to shaping of the autoreactive Treg repertoire.

Initially, thymic selection of Treg was studied using TCR/ligand doubly transgenic mice (22, 23, 24, 25, 26, 27). In these mice, strongly increased proportions of CD25+ cells with regulatory potential among mature CD4+ thymocytes were found, and authors concluded that interaction with agonist ligand directs T cell precursors to the Treg lineage. However, it has since been argued that the increased proportion of Treg may largely be due to the massive deletion of conventional (but not regulatory) thymocytes commonly observed in TCR/ligand doubly transgenic mice (28). Understanding thymic selection of Treg therefore required analysis of Treg selection in mice in which T cell precursors have a normally diverse TCR repertoire. We have recently shown that superantigens, when expressed exclusively by thymic epithelial cells, substantially enhanced development of superantigen-specific Treg from precursors expressing a normally diverse TCR repertoire (29). Collectively, these data clearly indicated that developing Treg must be relatively resistant to negative selection induced by agonist ligands.

A common and crucial feature of the above-cited reports was that transgenic or superantigen ligand was expressed by thymic epithelial cells (TEC) only. Using MHC class II transfer from thymic stromal cells to developing thymocytes as a marker for the avidity of interaction between selecting and selected cells, we have recently shown that (in contrast to Tconv precursors) Treg precursors are relatively insensitive to induction of negative selection (i.e., deletion and/or anergy) upon recognition of agonist ligand at the surface of TEC (17). A similar conclusion was reached for TCR-transgenic or superantigen-specific Treg precursors (28, 29). By contrast, APC of bone marrow origin efficiently induce deletion of self-reactive Treg precursors (8, 17, 29, 30, 31).

Therefore, relative resistance of autospecific Treg precursors to thymic-negative selection by TEC (but not APC) may in part explain how the autospecific Treg repertoire is shaped in the thymus. In contrast, positive selection of Treg precursors may or may not preferentially select autoreactive cells. Recently published data indicate that reduced negative selection results in a very cross-reactive T cell repertoire (32, 33, 34). It is therefore essential to study the potential role of thymic-positive selection in shaping of the autoreactive Treg repertoire, but this issue has never been addressed directly. Development of conventional thymocytes requires interaction with MHC expressed by cortical TEC (cTEC) (18, 19, 35, 36, 37), and differentiation of Treg is not an exception to this rule (38). By contrast, whereas some contradictory data exist (Refs. 39 and 40 , but also see Ref. 41), it is now widely admitted that cTEC do not induce negative selection of developing autospecific T cells (18, 19, 42).

In the present report, we directly address the issue of the role of cortical positive selection in shaping of the autoreactive Treg repertoire. We present data on the repertoire of Treg developing in non-TCR-transgenic mice expressing a single MHC class II/peptide ligand on positively selecting cTEC. Our results indicate that functional, single ligand-reactive Treg (but not Tconv) are preferentially selected from the precursor population expressing a normally diverse TCR repertoire in these mice. We conclude therefore that cortical positive selection selectively promotes survival of autoreactive Treg precursors and therefore plays an important role in shaping of the autoreactive Treg repertoire.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 Disclosures
 References
 
Mice

C57BL/6 and DBA/2 mice were purchased from Janvier. B10.Q mice were obtained from The Jackson Laboratory. IAβ° and Ii° C57BL/6 mice were obtained from the Centre de Développement des Techniques Avancées pour l’Expérimentation Animale-Centre National de la Recherche Scientifique. All experiments involving animals were performed in compliance with the relevant laws and institutional guidelines (Institut National de la Santé et de la Recherche Médicale; approval no. 31-13) and were approved by the local ethics committee (Midi-Pyrénees, France; ref. MP/01/31/10/03).

Transgenic mice

Transgenic constructs were generated by RT-PCR amplification of IA{alpha}q and IAβq coding sequences from DBA/1 splenic total RNA. Similarly to a previously published IAβb-peptide transgene (43), a flexible linker and the dominant IAq-associated bovine collagen type II (bCII) epitope 255–272 (44) were subsequently introduced (by PCR) in between amino acids 31 and 32 of the native IAβq chain. The IA{alpha}q and IAβq-bCII sequences were verified by sequencing and cloned into the BamHI site of a K14 expression cassette (45). Linearized IA{alpha}q and IAβq-bCII-transgenic constructs were separately injected or coinjected into fertilized C57BL/6 ovocytes and transgenic mouse lines were established. Transgenic mice were bred to IAβ°Ii° and IAβ°Ii+ mice.

Abs and peptides

The following mAbs were used for phenotypic analysis: anti-HSA-FITC, anti-CD5-FITC, anti-CD69-FITC, anti-H2Kb-FITC, anti-H2Kq-FITC, anti-B220-FITC, anti-CD25-PE, anti-MHC class II-PE (clone M5), anti-MHC class II-biotin (M5), anti-CD4-PE-Cy7, anti-Ly51 (clone 6C3)-FITC (BD Pharmingen), anti-Foxp3-PE, anti-CD8-allophycocyanin, anti-CD25-allophycocyanin, anti-CD8-Pacific Blue, anti-TCR-allophycocyanin, anti-CD45-allophycocyanin-Alexa Fluor 750, and anti-MHC class II-allophycocyanin (eBioscience). For analysis of TECs, the following reagents were used: anti-Ly51 (clone 6C3)-FITC (BD Pharmingen); anti-Ep-CAM (clone G8.8) followed by anti-rat IgG2a (clone RG7/1.30)-biotin (BD Pharmingen) and streptavidin-PE; anti-MHC class II (clone M5/114)-allophycocyanin; and anti-CD45 (clone 104)-allophycocyanin/Alexa 750 (eBioscience).

Peptides used for in vitro experiments were purchased from NeoMPS. The bCII peptide 257–270 sequence is NH2-ELGIAGFKGEQGPK-COOH. Mutants used as control peptides were: G266A (C1), A261I (C2), K264A (C3). The mutant peptides have a similar affinity for IAq as the wild-type peptide (46). We also used the C4 peptide in which all three mutations were combined.

Flow cytometry

Cells were labeled with indicated Abs and analyzed using a FACSCalibur or an LSR II cytometer (BD Biosciences). Intracellular Foxp3 staining was performed according to the instructions of the supplier (eBioscience). Data were analyzed using CellQuest (BD Biosciences) or FlowJo (Tree Star) software. For analysis of MHC class II expression by thymic stromal cells by flow cytometry, thymic stroma was digested and epithelial cells enriched on a discontinuous Percoll density gradient, as previously described (47). After EDTA dissociation, nonspecific staining was blocked by incubation of cells in 2.4G2 (anti-Fc{gamma}R) culture supernatant for 20 min on ice. Cells were then incubated, on ice, with Abs directed to indicated cell surface molecules.

Histological analysis

Thymi were snap-frozen in an isopentane-bath cooled in liquid nitrogen. Cryostat sections (5 µM) were air-dried on frosted microscope slides for 2 h, placed in acetone at room temperature for 10 min and rehydrated in PBS. Blocking was performed with peroxidase block (DakoCytomation) (10 min) and with a biotin-blocking kit (DakoCytomation; 30 min). Sections were then incubated for 30 min with biotinylated M5 (2 µg/ml). After washing with PBS/0.3% BSA, slides were incubated for 30 min with streptavidin-HRP (DakoCytomation), then with diaminobenzidine (DAB; DakoCytomation) for 5 min. Slides were finally counterstained with hematoxylin (DakoCytomation) for 3 min.

Purification of thymic T cell populations

Thymocytes were depleted of CD8+ cells by treatment with anti-CD8 mAb (31M) and complement (HD Supplies). CD25– and CD25+ cells were magnetically sorted (Miltenyi Biotec). CD4+ cells were subsequently magnetically sorted. Cell purity was checked by flow cytometry. Positively sorted CD4+CD25– and CD4+CD25+ T cells were, respectively, always >98% and >87% pure.

Purification of splenic T cell populations

Splenocytes were incubated with a mixture of the following rat mAbs: anti-FcRII/III (2.4G2), anti-CD8 (53.6.7), and anti-MHC class II (M5). Thus, labeled cells were eliminated using Dynabeads (Dynal Biotech). CD25+ and CD25– cells were magnetically sorted. CD4+ cells were magnetically sorted. Cell purity was checked by flow cytometry. Sorted CD4+CD25– and CD4+CD25+ T cells were routinely >95% pure.

In vitro proliferation and inhibition-of-proliferation tests

A total of 105 responder T cells purified from thymus or spleen were cultured in presence of 5.105 irradiated APC (1750 rad gamma) from indicated strains of mice and indicated peptides, in RPMI 1640 supplemented with 10% FCS and 300 U IL-2/ml (EL-4 culture supernatant) for CD4+CD25+ T cells and no IL-2 for CD4+CD25– T cells. Cultures were performed in triplicate for 72 h and 1 µCi [3H]thymidine (Amersham) was added for the last 16 h. [3H]Thymidine incorporation was assessed using the Microbeta Trilux scintillation counter (PerkinElmer).

Functional limiting dilution analysis

Titrated numbers (0–400) of purified CD4+CD25+ thymocytes were cultured in 96-well plates (48 wells/condition) containing 5 x 105 APC pulsed or not with bCII or control peptides (50 µM) in complete RPMI 1640 supplemented with 300 U/ml IL-2. After 14 days, 5 x 105 APC (pulsed or not with bCII or control peptides) and 104 allogenic (DBA/2) CD8+ cells were added. CTL activity was assessed 6 days later by redirected lysis using anti-CD3 mAb (2C11)-coated 51Cr-labeled P815 cells. Cultures were scored as positive for inhibition of lysis if the percentage of lysis was inferior to the mean – 3 SD of 48 control cultures performed in absence of added CD4+CD25+ T cells. Precursor frequencies were calculated as previously described (48).


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 Disclosures
 References
 
Generation and characterization of transgenic mice expressing a single MHC class II/peptide ligand by cTEC

To study the potential role of agonist ligands in positive selection of T lymphocytes from precursors with a normally diverse TCR repertoire, we generated mice in which a single MHC class II/peptide ligand is expressed on positively selecting thymic stromal cells, i.e., cTEC. The chosen ligand was the bCII 255–272 peptide associated with the IAq complex, known to play a major role in an experimental model for rheumatoid arthritis, collagen-induced arthritis (44). A transgenic construct was generated coding for the IAβq chain covalently linked, via a flexible (Gly4Ser)3 linker, to the dominant IAq-associated bCII epitope bCII255–272. Because the IAβq chain very inefficiently associates with the IA{alpha}b chain (R. Holmdahl, unpublished observation), we also generated an IA{alpha}q transgene. Both coding sequences were separately cloned into the human K14 expression cassette that has previously successfully been used to specifically target thymic transgene expression to cTEC (18, 19, 49). Mice carrying the IA{alpha}q and IAβq-bCII transgenes (Tg) were bred to IAβ-deficient (IAβ°) mice to assure exclusive expression of the transgenic IAβq chain. Because in invariant chain expressing (Ii+) cells peptides covalently linked to a transfected I-Aβ chain are proteolytically removed, we crossed these mice to Ii-deficient (Ii°) animals to assure exclusive presentation of the transgenic peptide (43). TgIAβ°Ii+ mice, that express transgenic IAq-presenting endogenous peptides, were used for specificity controls.

Immunohistochemical analysis of thymus sections showed that TgIAβ° mice expressed the transgenic complex on cTEC but not on medullary TEC (mTEC) (Fig. 1A). No transgene expression was found on splenocytes (Fig. 1B). FACS analysis of thymic stromal cells confirmed expression of the single ligand by CD45–EpCAM+Ly51+ cTEC (Fig. 1C). It also showed that <2% of CD45–EpCAM+Ly51– mTEC express very low levels of the transgene (~100-fold lower MHC class II levels than wild-type mTEC, Fig. 1D). IAβ°Ii° mice carrying the IAβq-bCII but not the IA{alpha}q transgene did not express detectable levels of the transgenic β-chain, demonstrating that the endogenous IA{alpha}b chain does not associate with the transgenic IAβq-bCII chain (data not shown).


Figure 1
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  		FIGURE 1. The transgenic IA{alpha}βqbCII complex is expressed by cTEC but not by mTEC or splenic APC. A, Histological analysis of thymi from transgenic and control mice. Thymus sections were stained for MHC class II. The cortical (c) and medullary (m) regions are indicated. B, Flow cytometry analysis of splenocytes from transgenic and control mice detecting indicated surface molecules. Depicted data are representative of three independent experiments. C, Flow cytometry analysis of MHC class II expression by CD45– EpCAM+Ly51+ cTEC of indicated mice in red. D, Flow cytometry analysis of MHC class II expression by CD45–EpCAM+Ly51– mTEC of indicated mice in red. Blue histograms, Staining with isotype-matched control Ab.

 
Treg development in pK14-IA{alpha}βqbCII-transgenic mice

We analyzed the development of Treg in our transgenic mice in which a single MHC class II/peptide ligand is expressed on cTEC. The best marker for Treg is the forkhead/winged helix transcription factor Foxp3 (50, 51, 52, 53, 54) and, as shown in Figs. 2, A and B, substantial percentages of Foxp3+CD4+CD8– (CD4 single-positive (CD4SP)) thymocytes were found in TgIAβ°Ii° and TgIAβ°Ii+, but not in nontransgenic IAβ°Ii°, mice. The percentage of Treg among CD4SP cells in TgIAβ°Ii+ was lower than that in TgIAβ°Ii° mice (Fig. 2B). However, the percentage of CD4SP cells was higher in Ii+ than in Ii° mice and the total number of thymocytes was similar in these mice, and the absolute numbers of Treg developing in Ii+- and Ii°-transgenic mice were similar (Fig. 2B).


Figure 2
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  		FIGURE 2. Tconv and Treg develop in pK14-IA{alpha}βqbCII-transgenic mice. Flow cytometry analysis of CD4 vs CD8 expression by thymocytes (A) and splenocytes (C) from transgenic and control mice (upper panels). TCR and CD25 vs Foxp3 expression on electronically gated CD4+CD8– cells (middle and lower panels). Percentages of CD4SP among total cells and Foxp3+ cells among CD4SP cells using gates as indicated in A thymus (B, upper and middle panels) and in spleen (D, upper and middle panels). Absolute numbers of Foxp3+ cells in thymi (B, lower panel) and spleens (D, lower panel). ***, p < 0.001; Student’s t test. Error bars indicate SD (n = 3).

 
The phenotype of Treg from transgenic mice was similar to that of Treg from control B10.Q animals. Most of the Foxp3+ cells expressed high levels of CD25 (Fig. 2A). They expressed relatively low levels of the maturation marker HSA (CD24) and the activation marker CD69, and expressed high levels of CD5 and MHC class I (Fig. 3). Therefore, transgenic regulatory CD4SP thymocytes had a more mature phenotype than conventional CD4SP cells, similar to that found on wild-type CD4SP CD25+ thymocytes.


Figure 3
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  		FIGURE 3. Tconv and Treg developing in pK14-IA{alpha}βqbCII-transgenic mice display normal phenotypes. A, Expression of CD25 vs HSA, CD69, CD5, and H2-K by electronically gated CD4SP thymocytes. B, Percentages of (most mature) HSAlow, CD69low, CD5high, and H2-Khigh cells among CD25– and CD25high CD4SP populations. ***, p < 0.001; **, p < 0.01; *, p < 0.05; Student’s t test. Error bars indicate SD (n = 3).

 
In transgenic mice, mature CD4SP Treg migrated to the periphery (Fig. 2, C and D). Peripheral-transgenic Foxp3+ Treg mostly expressed high levels of CD25. Similar to what we observed in the thymus, Ii+ mice had lower percentages of Foxp3+ cells among CD4+ splenocytes. However, the percentage of CD4+ splenocytes was higher in Ii+ mice and the absolute numbers of Foxp3+ Treg was similar in transgenic Ii° and Ii+ mice (Fig. 2D).

The repertoire of Treg positively selected by a single MHC class II/peptide ligand expressed by cTEC

We next evaluated the specificity of Treg developing in transgenic mice. Because only minute numbers of Treg (i.e., CD4SP CD25high cells) can be obtained from thymi, we developed a very sensitive functional limiting dilution analysis (LDA). Magnetic bead-sorted CD25high CD4SP thymocytes were seeded at limiting numbers in microtiter plates containing B10.Q APC with bCII or control peptides and high concentrations of IL-2. Two weeks later, the time it took for specific Treg to proliferate, new APC/peptide as well as allogeneic CD8+ responder T cells were added. Alloreactive CTL were allowed to proliferate during one week, after which 51Cr labeled, anti-CD3{epsilon} mAb-coated targets were added. In cultures in which no Treg had proliferated, alloreactive CTL would proliferate and kill targets in the redirected lysis assay. By contrast, in cultures in which Treg had proliferated, these cells would inhibit proliferation of allospecific CTL (55) and thus reduced lysis of targets would be seen in the redirected lysis assay. Using this functional LDA to evaluate the repertoire of regulatory T lymphocytes from TgIAβ°Ii+ mice (in which IAq molecules are expressed by cTEC but in which the transgenic peptide is cleaved off and replaced by endogenous peptides, Ref. 43), we observed similar frequencies of suppressed cultures in presence or in absence of bCII peptide (Fig. 4, A and C, Table I). Presence of autospecific Treg in TgIAβ°Ii+ mice is at least in part due to absence of negative selection in our transgenics. By contrast, when analyzing Treg from TgIAβ°Ii° mice, in which exclusively the transgenic MHC/peptide ligand is expressed by cTEC, we reproducibly observed a significantly, ~2-fold, higher frequency of suppressed cultures in presence of bCII peptide than in presence of control or no peptide (Fig. 4, B and C, Table I). These results showed that an agonist ligand expressed by cTEC positively selects Tregs from a precursor population with a normally diverse TCR repertoire.


Figure 4
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  		FIGURE 4. Treg, but not Tconv, from pK14-IA{alpha}βqbCII mice display increased reactivity toward the IAq/bCII complex. A–C, Functional Treg LDA. Titrated numbers (as indicated) of purified CD4SP CD25+ thymocytes from TgIAβ°Ii+ (A) and TgIAβ°Ii° (B) mice were cultured with irradiated B10.Q splenocytes pulsed or not with peptides, as indicated. After 14 days, same APC/peptide and allogeneic DBA/2 CD8+ cells were added. Seven days later, redirected lysis was assessed. Data shown in A and B are representative of four independent experiments. C, Precursor frequency ratios of Treg specific for bCII vs C1 control peptide observed in the four independently performed experiments. Individual experiments are shown and vertical bars indicate mean values. Raw data from individual experiments are shown in Table I. D and E, In vitro inhibition of proliferation assays. CD4+CD25+ splenic Treg from TgIAβ°Ii+ (D) and TgIAβ°Ii° mice (E) were cocultured, at indicated ratios, with 105 CD4SP CD25– Tconv from B6 mice in the presence of irradiated B10.Q splenocytes pulsed with bCII ({circ}) or C1 control peptides ({diamond}) (50 µM). [3H]Thymidine incorporation was assessed after 5 days. Data are representative of three independent experiments. F–H, In vitro Treg-proliferation assays. CD4+CD25+ splenic Treg from TgIAβ°Ii+ (F) and TgIAβ°Ii° mice (G) were cultured with B10.Q APC pulsed with peptides, in presence of IL-2. [3H]Thymidine incorporation was assessed after 4 days. H, Ratio of proliferation of Treg from indicated mice in response to B10.Q APC pulsed with bCII vs control C1 peptides (50 µM). I–K, In vitro Tconv-proliferation assays. CD4+CD25– Tconv from TgIAβ°Ii° thymi (I) or spleen (J) were cultured with irradiated B10.Q splenocytes pulsed with titrated doses of peptides ({circ}, bCII; {diamond}, C1; {circ}, C2; {triangleup}, C3; and x, C4), in absence of IL-2. [3H]Thymidine incorporation was assayed after 4 days. K, Ratio of proliferation of thymic ({boxtimes}) or splenic ({square}) Tconv in response to B10.Q APC pulsed with bCII vs control C1 peptides (50 µM). In F, G, I, and J, background proliferation, as determined by stimulating T cells with MHC-deficient APC, was inferior to 1000 cpm. **, p < 0.01; Student’s t test.

 

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  		Table I. Treg from pK14-IA{alpha}βqbCII mice show increased reactivity toward the IAq/bCII complexa

 
To consolidate these results, we also performed more conventional in vitro inhibition-of-proliferation assays using splenic Treg from transgenic mice. Titrated numbers of TgIAβ°Ii+ Treg were cultured in presence of B10.Q APC, the bCII or a control peptide, and allogeneic CD4+CD25– responder T cells. Similar inhibition curves were reproducibly obtained in absence and in presence of the bCII peptide (Fig. 4D). Again, inhibition was at least in part due to defective negative selection in our transgenic mice. By contrast, TgIAβ°Ii° Treg inhibited T cell proliferation substantially more efficiently in presence than in absence of specific bCII peptide (Fig. 4E).

We also assessed splenic Treg activation by measuring their proliferation in response to IAq-expressing APC presenting bCII or control peptide in presence of high concentrations of IL-2. As shown in Fig. 4, F and H, Treg from TgIAβ°Ii+ mice proliferated in response to APC expressing IAq (because of defective negative selection), but addition of bCII or control peptide did not modify this proliferation. By contrast, addition of bCII peptide led to increased proliferation of Treg from TgIAβ°Ii° mice, whereas addition of control peptide had no effect (Fig. 4, G and H).

In contrast to the observed preferential specificity of Treg for the positively selecting I-Aq/bCII ligand, in proliferation assays we failed to observe any evidence that the repertoire of (thymic or splenic) Tconv is enriched in cells specific for this ligand (Fig. 4, I–K). Proliferation in these cultures, in which Tconv had been stimulated with syngeneic APC, is due to absence of negative selection in mice expressing MHC molecules on cTEC only (18, 19, 38).

Based on these results, obtained with three distinct experimental setups, we conclude that autoreactive Treg can be positively selected from a precursor population with a naturally diverse TCR repertoire upon interaction with a high-avidity (agonist) ligand expressed by cTEC.


   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
Disclosures
 References
 
In this report, we show that an agonist ligand, when expressed by cTEC only, positively selects autospecific regulatory CD4+ T cells from a precursor population with a naturally diverse TCR repertoire. A significant fraction of Treg positively selected on a single MHC class II/peptide ligand proliferated in response to the same ligand and had suppressive activity in a functional LDA for Tregs we developed, as well as in more classical inhibition assays. By contrast, whereas Tconv developing in mice expressing a single ligand by cTEC are reactive to APC expressing syngeneic MHC class II (because of defective negative selection), no increased reactivity toward the transgenic MHC class II/peptide ligand could be detected in in vitro proliferation assays. Cortical-positive selection of Treg vs Tconv therefore appears to be governed by fundamentally different rules: low-avidity interactions allow for positive selection of Tconv while high(er) avidity interactions are involved in positive selection of Treg. Cortical positive selection of Treg precursors therefore preferentially selects autoreactive cells and thus substantially contributes to the generation of the autospecific Treg repertoire.

The conclusion that agonist ligand can induce selection of Treg has earlier been reached based on TCR/Ag doubly transgenic mice (22, 23, 24, 25, 26, 29). However, in a study in which other TCR/Ag doubly transgenic mice were used it was shown that the increase in the proportion of Treg was accompanied by rather limited increase in their absolute numbers. Moreover, this moderate increase was not limited to transgenic Ag-specific Treg. The relative increase in Treg in TCR/ligand doubly transgenic mice was explained by massive negative selection of Tconv combined with resistance to deletion of Treg precursors (28). This explanation cannot apply to the original experimental system reported here because immature precursors have a naturally diverse TCR repertoire. Rather, positive selection of Treg appears to depend on high-avidity interactions. Given that in the reported experimental model, immature precursors expressed a naturally diverse (as opposed to a monoclonal transgenic) TCR repertoire, we assessed specificity of selected cells using three independent in vitro assays (functional LDA of thymic Treg, in vitro inhibition assays and proliferation assays using splenic Tregs). Moreover, the specificity control was carefully performed using Ii+ vs Ii°, MHC class II/peptide-transgenic mice. Whereas in Ii° mice only the transgenic peptide was presented, in Ii+ mice transgenic peptide is cleaved off the transgenic IAβ chain, and MHC class II molecules therefore presented endogenous peptides (43). In contrast to Treg from Ii° mice, cells from Ii+ mice did not show any biased specificity toward the IAq/bCII ligand. Therefore, our data on selection of Treg from naturally diverse precursors indicate that agonist self-MHC class II/peptide ligands, when expressed on cTEC, induce positive selection of autoreactive Treg.

Due to defective negative selection, in vitro reactivity of transgenic Treg (and Tconv) to APC expressing syngeneic MHC (not presenting transgenic peptide) was quite high, as previously observed (38). When we added transgenic peptide to the functional LDA, a 2-fold increase in the frequency of activated Treg was observed. Given the high level of autoreactivity, this, as compared with TCR/ligand doubly transgenic systems at first sight moderate increase, actually corresponds to a very substantial increase in ligand-specific Treg reactivity. Moreover, in in vitro inhibition-of-proliferation assays, addition of transgenic peptide induced an ~10-fold shift in the inhibition curve. Combined the data showed that cTEC expression of agonist ligand substantially increased the generation of a transgenic MHC/peptide complex-specific Treg repertoire.

Using mice in which expression of transgenic MHC class II was targeted to cTEC, Bensinger et al. (38) earlier showed that cTEC induced development of functional Treg. Treg from these mice were autoreactive, but so were Tconv. These results were therefore explained by defective negative selection and did not show that positive selection of Treg precursors is preferentially induced by agonist ligand. However, these data indicated that interaction of Treg precursors with MHC/peptide ligand in the medulla is not required for their full differentiation and are therefore consistent with the results presented here.

The hypothesis that ligands expressed by mTEC might also be involved in selection of Treg is particularly attractive because mTEC, in contrast to all other thymic stromal cellular compartments, ectopically express tissue-specific Ags (56). The autoimmune syndrome developing in mice deficient in AIRE, a transcription factor involved in ectopic expression of Ags by mTEC, however, does not appear to be due to defective selection of Treg (57). In contrast, using an elegant transgenic mouse model, it was recently shown that interactions with transgenic agonist ligand expressed by mTEC (under control of the AIRE promoter) promoted thymic accumulation of TCR-transgenic Treg in an as yet unidentified manner (27). It was argued that the type of medullary cells expressing self-ligands determines whether an autospecific T cell precursor is deleted or redirected to the Treg lineage. Treg lineage commitment would therefore take place late during T cell development, but (because RAG-sufficient TCR-transgenic animals were used) the nature of the TCR signal involved in positive selection in the cortex remained unclear. Given these two contradictory reports, the precise role of mTEC in selection of a polyclonal repertoire of autospecific Treg remains incompletely understood.

In our transgenic mice, a very small subpopulation of mTEC appears to express very low levels of the transgenic MHC class II/peptide ligand. However, Tconv developing in our transgenic mice react vigorously to stimulation by syngeneic MHC class II-expressing APC, even in absence of added IL-2. This result demonstrates that negative selection by mTEC (via induction of anergy, Ref. 58) is defective, and therefore strongly suggests that the very low MHC class II expression level is functionally not significant. Whereas we formally cannot exclude a minor role of mTEC, we therefore favor the conclusion that cTEC are entirely responsible for selection of the Treg repertoire in our transgenic mice.

The data currently available in the literature support the notion that agonist ligands favor development of Treg. However, in mice expressing a single MHC/peptide ligand by all normally MHC class II-expressing stromal cells, a diverse Treg repertoire develops. In these mice, also negatively selecting dendritic cells express the single ligand, and Treg from such mice do not respond to the selecting ligand (30, 59, 60). Therefore, even if agonist self-ligands expressed by TEC clearly favor development of Treg, they probably are not the only ones to do so. We therefore privilege a model in which positive selection of Treg is mediated by higher avidity interactions than that of Tconv, including but not limited to agonist TCR-ligand interactions.

In conclusion, our data indicate that thymic-positive selection in the cortex substantially contributes to shaping of the autoreactive Treg repertoire. However, they do not exclude a potential role for ligands expressed in the thymic medulla. It will therefore now be important to evaluate how cortical and medullary selection mechanisms collaborate in shaping the autoreactive Treg repertoire.


   Acknowledgments
 
We thank the staff of the l’Institut Fédératif de Recherche 30 Animal Facility for excellent animal husbandry, Dr. Fatima-Ezzahra L’Faqihi-Olive for flow cytometry, Dr. E. Fuchs for the K14 promoter construct, Dr. Talal Al Saati and Florence Capilla for histological analysis, and Drs. Antonio Bandeira, Benoit Salomon, Philippe Naquet, and Roland Liblau for critical reading of the manuscript.


   Disclosures
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Disclosures
References
 
The authors have no financial conflict of interest.


   Footnotes
 
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

1 This work was supported by a grant from the European Community awarded to the EuroThymaide Consortium (Contract No. LSHB-CT-2003-503410). Back

2 Address correspondence and reprint requests to Dr. Joost P. M. van Meerwijk, Institut National de la Santé et de la Recherche Médicale Unité 563, BP 3028, 31024 Toulouse Cedex 3, France. E-mail address: Joost.van-Meerwijk{at}toulouse.inserm.fr Back

3 Abbreviations used in this paper: Treg, regulatory T cell; Tconv, conventional T cell; TEC, thymic epithelial cell; cTEC, cortical TEC; mTEC, medullary TEC; bCII, bovine collagen type II; SP, single positive; LDA, limiting dilution analysis. Back

Received for publication May 21, 2007. Accepted for publication September 3, 2007.


   References
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 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Disclosures
 References
 

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