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Generation of CD4⁺CD25⁺ Regulatory T Cells from Autoreactive T Cells Simultaneously with Their Negative Selection in the Thymus and from Nonautoreactive T Cells by Endogenous TCR Expression¹

Kimito Kawahata^*, Yoshikata Misaki^2,*, Michiko Yamauchi^*, Shinji Tsunekawa, Keigo Setoguchi^*, Jun-ichi Miyazaki and Kazuhiko Yamamoto^*

^* Department of Allergy and Rheumatology, University of Tokyo Graduate School of Medicine, Tokyo, Japan; Medical and Biological Laboratories, Ina, Japan; and Department of Nutrition and Physiological Chemistry, Osaka University Medical School, Suita, Japan

	Abstract

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Normal T cell repertoire contains regulatory T cells that control autoimmune responses in the periphery. One recent study demonstrated that CD4⁺CD25⁺ T cells were generated from autoreactive T cells without negative selection. However, it is unclear whether, in general, positive selection and negative selection of autoreactive T cells are mutually exclusive processes in the thymus. To investigate the ontogeny of CD4⁺CD25⁺ regulatory T cells, neo-autoantigen-bearing transgenic mice expressing chicken egg OVA systemically in the nuclei (Ld-nOVA) were crossed with transgenic mice expressing an OVA-specific TCR (DO11.10). Ld-nOVA x DO11.10 mice had increased numbers of CD4⁺CD25⁺ regulatory T cells in the thymus and the periphery despite clonal deletion. In Ld-nOVA x DO11.10 mice, T cells expressing endogenous TCR chains were CD4⁺CD25^- T cells, whereas T cells expressing autoreactive TCR were selected as CD4⁺CD25⁺ T cells, which were exclusively dominant in recombination-activating gene 2-deficient Ld-nOVA x DO11.10 mice. In contrast, in DO11.10 mice, CD4⁺CD25⁺ T cells expressed endogenous TCR chains, which disappeared in recombination-activating gene 2-deficient DO11.10 mice. These results indicate that part of autoreactive T cells that have a high affinity TCR enough to cause clonal deletion could be positively selected as CD4⁺CD25⁺ T cells in the thymus. Furthermore, it is suggested that endogenous TCR gene rearrangement might critically contribute to the generation of CD4⁺CD25⁺ T cells from nonautoreactive T cell repertoire, at least under the limited conditions such as TCR-transgenic models, as well as the generation of CD4⁺CD25^- T cells from autoreactive T cell repertoire.

	Introduction

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Normal T cell repertoire contains CD4⁺CD25⁺ regulatory T cells that control autoreactive immune responses in the periphery (1, 2, 3, 4). Impairment of the generation of CD4⁺CD25⁺ regulatory T cells results in various organ-specific autoimmune diseases, as demonstrated in mice thymectomized on day 3 of life (5). It has been demonstrated that development of regulatory T cells requires the thymus (6, 7, 8) and the presence of the relevant autoantigen in the periphery (9). However, development of CD4⁺CD25⁺ regulatory T cells remains poorly understood.

Although CD4⁺CD25⁺ T cells are anergic to TCR stimulation and suppress the activation of CD4⁺CD25^- T cells in an Ag-independent manner, it has been demonstrated that regulatory function of CD4⁺CD25⁺ T cells requires their activation via TCR in vitro (10, 11, 12). Seddon and Mason (9) observed that peripheral autoantigen is responsible for the survival of specific regulatory T cells in vivo. These findings suggest the critical role of TCR specificity of CD4⁺CD25⁺ regulatory T cells in their generation, survival, and ability to prevent autoimmunity. Recently, Jordan et al. (13) demonstrated that, in the thymus, self-reactive T cells were positively selected as CD4⁺CD25⁺ regulatory T cells and were not deleted. These results suggest that positive selection of CD4⁺CD25⁺ regulatory T cells requires higher avidity interactions of their TCRs with self ligands, but that the required avidity must not exceed the threshold of the deletion (14). However, it is unclear whether avidity of autoreactive TCRs that induce positive selection as CD4⁺CD25⁺ regulatory T cells is different from avidity of autoreactive TCRs that induce negative selection.

The immune system controls autoreactivity by several mechanisms such as clonal deletion and inactivation. Accumulating evidences suggest that receptor editing or revision, which is induced by autoreactive stimuli and involves endogenous TCR gene rearrangement, is also involved in the generation of nonautoreactive T cell repertoire from autoreactive T cell repertoire (15, 16, 17). Moreover, it is suggested that secondary TCR gene rearrangement occurs to escape not only from clonal deletion, but also from death by neglect during thymic selection (18, 19). Interestingly, disturbance of endogenous TCR gene rearrangement seems to be associated with the impaired development of regulatory T cells (20, 21, 22, 23, 24, 25). Genetic manipulation of TCR gene, as in TCR -chain-deficient (20, 23) and 2B4 TCR -chain transgenic mice (22), sometimes spontaneously induces organ-specific autoimmune diseases, such as inflammatory bowel disease, autoimmune gastritis, and thyroiditis. Itoh et al. (7) found that CD4⁺CD25⁺ T cells in a TCR-transgenic model expressed endogenous TCR chains and disappeared in recombination-activating gene 2 (RAG2)³-deficient TCR-transgenic mice. These findings have led us to consider how endogenous TCR gene rearrangement controls the generation of CD4⁺CD25⁺ T cells or CD4⁺CD25^- T cells, at least, under certain conditions that CD4⁺ T cells lack the CD4⁺CD25⁺ T cells or CD4⁺CD25^- T cells, respectively. However, there are no studies that clearly demonstrate dual roles of endogenous TCR gene rearrangement for the generation of CD4⁺ T cell repertoire in one model.

To examine the ontogeny of CD4⁺CD25⁺ T cells, we used neo-autoantigen-bearing transgenic mice expressing chicken egg OVA systemically in the nuclei (Ld-nOVA) and transgenic mice expressing an OVA-specific TCR (DO11.10). We found that part of autoreactive T cells could be positively selected as CD4⁺CD25⁺ T cells in parallel with their deletion in the thymus. We also found that endogenous TCR gene rearrangement generates autoreactive CD4⁺CD25⁺ regulatory T cells from nonautoreactive T cells and nonautoreactive CD4⁺CD25^- T cells from autoreactive T cells.

	Materials and Methods

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Mice

BALB/c mice were obtained from SLC (Shizuoka, Japan). They were maintained in a temperature- and light-controlled environment with free access to food and water under specific pathogen-free conditions. Female age-matched mice were used in all experiments, and the mice were 7–10 wk old at the start of each experiment. DO11.10 transgenic mice whose T cells express a receptor specific for OVA were kindly provided by T. Watanabe (Medical Institute of Bioregulation, Kyashu University, Fukuoka, Japan). DO11.10 TCR single transgenic mice were kindly provided by S. Koyasu (Department of Microbiology and Immunology, Keio University School of Medicine, Tokyo, Japan). RAG2-deficient BALB/c mice and TCR -chain-deficient C57BL/6 mice were purchased from Taconic Farms (Germantown, NY). Generation of Ld-nOVA transgenic mice has been described in another study (26). Briefly, chicken egg OVA cDNA (kindly provided by P. Chambon, Institut de Genetique et de Biologie Moleculaire et Cellulaire, Universite Louis Pasteur, Strasbourg, France) fused with the nuclear localization signal at the 3' end was subcloned into pLG-Eµ, which had been produced by inserting a human Eµ enhancer into the 5' end of the L^d class I promoter of pLG-2 plasmid (27). This OVA transgene construct was microinjected into the pronuclei of fertilized eggs from C57BL/6 mice. Ld-nOVA BALB/c mice were produced by crossing Ld-nOVA C57BL/6 mice with normal BALB/c mice for more than eight generations. TCR -chain-deficient BALB/c mice were produced by crossing TCR -chain-deficient C57BL/6 mice with normal BALB/c mice for more than six generations.

Preparation of cell populations

Spleen cells were first enriched in T cells by using mouse CD3⁺ T cell enrichment columns (R&D Systems, Minneapolis, MN). T cells were then stained with FITC anti-CD4 mAb (GK1.5; BD PharMingen, San Diego, CA) and biotin anti-CD25 mAb (7D4; BD PharMingen), followed by staining with anti-FITC microbeads. CD4⁺ T cells were purified with MACS using a positive selection column (Miltenyi Biotec, Bergisch Gladbach, Germany). For the purification of CD4⁺CD25⁺ T cells, microbeads of purified CD4⁺ T cells were released by FITC MultiSort kit. CD4⁺ T cells were stained with streptavidin microbeads, followed by separation with MACS using a positive selection column. The purity of CD4⁺CD25⁺ T cells and CD4⁺CD25^- T cells was 88%.

In vitro proliferation assay

CD4⁺ T cells, CD4⁺CD25⁺ T cells, or CD4⁺CD25^- T cells (2 x 10⁴ cells/well) were cultured with irradiated (20 Gy) syngeneic spleen cells (5 x 10⁴ cells/well) in the presence of OVA_323–339 at 0.5 µM for 3 days, followed by a final 16 h of culture in the presence of 1 µCi [³H]TdR per well. Suppressor cell activity was assessed by coculturing CD4⁺CD25⁺ T cells (2 x 10⁴ cells/well) with CD4⁺CD25^- T cells (2 x 10⁴ cells/well) and with irradiated (20 Gy) syngeneic spleen cells (5 x 10⁴ cells/well) in the presence of anti-CD3 mAb (145-2C11) at 10 µg/ml or Con A (Sigma-Aldrich, St. Louis, MO) at 1 µg/ml for 3 days, followed by a final 6 h of culture in the presence of 1 µCi [³H]TdR per well. In some experiments, anti-CTLA-4 mAb (UC10-4F10-11) (100 µg/ml) was added to the culture. Cells were cultured in 96-well round-bottom plates in RPMI 1640 medium supplemented with 2 mM L-glutamine, 100 U/ml penicillin, 100 µg/ml streptomycin, 10% heat-inactivated FCS, and 5 x 10^-5 M 2-ME at 37°C, 5% CO₂. The incorporated radioactivity was counted with a gamma scintillation counter. The proliferative response was expressed as cpm (mean cpm of the test cultures minus the mean cpm of the control cultures without Ag).

Flow cytometry

The following Abs were used for identification and phenotypic analysis of T cell populations: FITC-conjugated or biotinylated KJ1-26; FITC-conjugated or PE-conjugated anti-TCRV2, anti-TCRV6, anti-TCRV8, anti-TCRV14, anti-TCRV2; FITC-conjugated or biotinylated anti-TCR (H57-597); FITC-conjugated anti-CD25; PE-conjugated anti-CD4; PE-conjugated CD8 (all from BD PharMingen); and streptavidin-Tricolor (Caltag Laboratories, Burlingame, CA).

	Results

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The numbers of CD4⁺CD25⁺ regulatory T cells increased in the thymus and the periphery of Ld-nOVA x DO11.10 mice

To investigate immunological tolerance to a systemic nuclear autoantigen, which is perturbed in systemic autoimmune diseases, we generated Ld-nOVA transgenic mice expressing OVA systemically in nuclei. We s.c. immunized Ld-nOVA mice and nontransgenic littermates with 100 µg OVA in CFA at the base of the tail and then cultured the draining lymph node cells with various doses of OVA or OVA_323–339 dominant epitope (OVAp) 10 days after the immunization. The proliferative responses were greatly reduced in the Ld-nOVA mice in comparison with nontransgenic littermates, indicating that Ld-nOVA mice are tolerant to OVA (26).

To address the question as to how autoreactive T cells specific for a nuclear autoantigen were rendered tolerized, we mated Ld-nOVA mice with DO11.10 transgenic mice that express a TCR (V13.1, V8.2) specific for the OVAp bound to I-A^d class II MHC molecules and monitored DO11.10 TCR-bearing cells using an anti-clonotypic Ab, KJ1-26.

Thymocyte numbers in Ld-nOVA x DO11.10 mice were significantly reduced to 55% of those in DO11.10 mice (1 x 10⁸ cells vs 1.8 x 10⁸ cells, p < 0.01) (Table I). In comparison with DO11.10 mice, Ld-nOVA x DO11.10 mice exhibited a reduction in the percentage of CD4 single-positive (SP) (6.7 ± 0.4% vs 9.7 ± 0.9%, p < 0.01) (Fig. 1A) and CD4 CD8 double-positive thymocytes (61 ± 5.9% vs 69 ± 5.8%, p < 0.05), and an increase in the percentage of CD8 SP (2.8 ± 0.4% vs 1.5 ± 0.3%, p < 0.01) and CD4 CD8 double-negative thymocytes (29.5 ± 5.3% vs 19.8 ± 3.5, p < 0.01). These results indicated autoreactive T cells were negatively selected in the thymus. Splenic CD4⁺ T cell numbers in Ld-nOVA x DO11.10 mice were reduced to 45% of those in DO11.10 mice (1.3 x 10⁷ cells vs 2.9 x 10⁷ cells, p < 0.01) (Table I) (Fig. 1A).

View larger version (44K): [in this window] [in a new window]   FIGURE 1. Expression of endogenous TCR - and -chains in DO11.10 mice and Ld-nOVA x DO11.10 mice. A, Thymocyte and splenocyte suspensions prepared from a 2-mo-old DO11.10 mouse and a 2-mo-old Ld-nOVA x DO11.10 mouse were stained with anti-CD4, anti-CD8, and KJ1-26, anti-V8, or anti-TCR. Dot plots show staining of live gated cells with anti-CD4 and anti-CD8. The percentage of cells falling within the indicated region is shown in each dot plot. Histograms show staining of CD4+CD8- cells with KJ1-26, anti-V8, or anti-TCR. The percentage of cells falling within the indicated marker is shown in each histogram. B, T cell-enriched splenocytes from a 2-mo-old DO11.10 mouse and a 2-mo-old Ld-nOVA x DO11.10 mouse were stained with anti-CD4 and anti-V2, anti-V2, anti-V6, or anti-V14. The percentage of CD4+ T cells falling within the indicated region is shown in each dot plot. A representative result of seven independent similar experiments is shown.

We noticed that splenic CD4⁺ T cells, lymph node CD4⁺ T cells, and CD4⁺ SP thymocytes in Ld-nOVA x DO11.10 mice contained a higher percentage of CD4⁺CD25⁺ T cells (12.3, 18.2, and 4%, respectively) than those in DO11.10 mice (3.6, 3.3, and 1.2%, respectively) (Table I). Absolute numbers of CD4⁺CD25⁺ T cells were also increased in Ld-nOVA x DO11.10 mice. Most of clonotypic T cells in DO11.10 mice were CD4⁺CD25^- T cells, whereas most of clonotypic T cells in Ld-nOVA x DO11.10 mice were CD4⁺CD25⁺ T cells (Fig. 2A). We next investigated whether these CD4⁺CD25⁺ T cells were regulatory T cells. CD4⁺CD25⁺ T cell-depleted CD4⁺ T cells from Ld-nOVA x DO11.10 exhibited a more vigorous response to OVAp than CD4⁺ T cells from Ld-nOVA x DO11.10, although they exhibited a lower response than CD4⁺CD25⁺ T cell-depleted DO11.10 CD4⁺ T cells (Fig. 2B). CD4⁺CD25⁺ T cells from Ld-nOVA x DO11.10 mice had the ability to suppress the proliferative responses of CD4⁺CD25^- T cells not only from the Ld-nOVA x DO11.10 mice, but also from nontransgenic BALB/c mice. This inhibitory function was partially blocked by anti-CTLA-4 (Fig. 2C). Because we did not use anti-CTLA4 Fab, the abrogation of inhibition was not so distinguished as demonstrated in the previous reports (28, 29, 30). These results indicate that CD4⁺CD25⁺ T cells generated in Ld-nOVA x DO11.10 mice are regulatory T cells.

View larger version (29K): [in this window] [in a new window]   FIGURE 2. CD4+CD25+ T cells in Ld-nOVA x DO11.10 mice were regulatory T cells. A, CD4+ T cells expressing endogenous TCR chains and CD4+ T cells expressing a clonotypic TCR showed different CD25 expression levels in both DO11.10 mice and Ld-nOVA x DO11.10 mice. T cell-enriched splenocytes from a DO11.10 mouse and a Ld-nOVA x DO11.10 mouse were analyzed for the expression of CD25, CD4, and a clonotypic TCR. Histograms show staining of clonotypic and nonclonotypic CD4+ T cell populations with anti-CD4 and anti-CD25. B, CD4+CD25+ T cell-depleted CD4+ T cells from Ld-nOVA x DO11.10 mice exhibited a vigorous response to OVAp. CD4+ T cells or CD4+CD25+ T cell-depleted CD4+ T cells (2 x 104 cells/well) from three Ld-nOVA x DO11.10 mice and three DO11.10 mice were cultured in 96-well round-bottom plates with irradiated (20 Gy) syngeneic spleen cells (5 x 104 cells/well) in the presence of OVAp for 3 days, followed by a final 16 h of culture in the presence of 1 µCi [3H]TdR per well. C, CD4+CD25+ T cells, CD4+CD25- T cells, and a 1:1 mixture of the two populations were cultured in 96-well round-bottom plates with irradiated (20 Gy) syngeneic spleen cells (5 x 104 cells/well) in the presence of Con A at 1 µg/ml for 3 days, followed by a final 6 h of culture in the presence of 1 µCi [3H]TdR per well. CD4+CD25+ T cells were prepared from four Ld-nOVA x DO11.10 mice, four DO11.10 mice, and four nontransgenic BALB/c mice. CD4+CD25- T cells were prepared from four nontransgenic BALB/c mice. In some wells, an anti-CTLA-4 mAb (100 µg/ml) was added to the culture. A representative result of three independent similar experiments is shown. The proliferative response was expressed as cpm (the mean cpm of the test cultures minus the mean cpm of the control cultures without Ag). DBL, Ld-nOVA x DO11.10 double-transgenic mice.

Clonotypic cells are positively selected into CD4⁺CD25⁺ regulatory T cells in Ld-nOVA x DO11.10 -chain mice

To exclude the possibility that the generation of CD4⁺CD25⁺ regulatory T cells in Ld-nOVA x DO11.10 mice could be attributed to excessive production of autoreactive T cells beyond the capacity for clonal deletion, we crossed Ld-nOVA mice with DO11.10 -chain single transgenic mice and examined CD25 expression of clonotypic T cells in Ld-nOVA x DO11.10 -chain mice. Although lymph node CD4⁺ T cells contained the small population of clonotypic T cells in DO11.10 -chain and Ld-nOVA x DO11.10 -chain mice, we could find clearly different results between these mice. Clonotypic T cells in DO11.10 -chain mice were exclusively CD4⁺CD25^- T cells, whereas clonotypic T cells in Ld-nOVA x DO11.10 -chain mice were exclusively CD4⁺CD25⁺ T cells (Fig. 3). These results are consistent with Fig. 2A and indicate that part of autoreactive T cells is positively selected as CD4⁺CD25⁺ regulatory T cells. Although CD4⁺CD25⁺ T cells in Ld-nOVA x DO11.10 -chain mice contained a higher percentage of clonotypic T cells than CD4⁺CD25^- T cells in DO11.10 -chain mice, as shown in Fig. 3, cell numbers of clonotypic T cells in Ld-nOVA x DO11.10 -chain mice were lower than those in DO11.10 -chain mice.

View larger version (23K): [in this window] [in a new window]   FIGURE 3. Clonotypic cells were positively selected into CD4+CD25+ T cells in Ld-nOVA x DO11.10 -chain mice. Lymph node cells from two DO11.10 -chain mice and two Ld-nOVA x DO11.10 -chain mice were pooled and stained with anti-CD4, anti-CD25, and KJ1-26. Histograms of staining with KJ1-26 were gated for CD4+CD25+ T cells and CD4+CD25- T cells. The percentage of clonotypic cells is shown in each histogram. The scale of histograms is different between CD4+CD25+ T cells and CD4+CD25- T cells (10 vs 100) because cell numbers of CD4+CD25+ T cells were 10% of those of CD4+CD25- T cells. A representative result of three independent similar experiments is shown.

We next investigated the role of endogenous TCR chain expression in the generation of CD4⁺CD25⁺ regulatory T cells and CD4⁺CD25^- T cells. In the thymus and spleen of Ld-nOVA x DO11.10 mice, CD4⁺CD25⁺ T cells were clonotype^high T cells, whereas CD4⁺CD25^- T cells were clonotype^low T cells (Fig. 4A). On the contrary, in the thymus and the spleen of DO11.10 mice, most CD4⁺CD25^- T cells were clonotype^high T cells, whereas CD4⁺CD25⁺ T cells were clonotype^low T cells (Fig. 4A). These clonotype^low T cells in lymph nodes (Fig. 4B) and the thymus (data not shown) expressed not only endogenous Vs, but also endogenous Vs. The percentages of CD4⁺ T cells expressing endogenous Vs in CD4⁺CD25^- T cells and CD4⁺CD25⁺ T cells from the thymus and lymph nodes of Ld-nOVA x DO11.10 mice and DO11.10 mice are summarized in Fig. 4C. The percentage of T cells expressing endogenous -chains was defined as the percentage of TCR C-positive cells in CD4⁺ T cells minus the percentage of V8-positive cells in CD4⁺ T cells. These data and the comparison of MFI of V8, as described above, indicate that CD4⁺CD25⁺ T cells of DO11.10 mice preferentially use endogenous Vs like CD4⁺CD25^- T cells of Ld-nOVA x DO11.10 mice. These results imply that CD4⁺CD25⁺ regulatory T cells are derived from autoreactive T cells, which do not undergo clonal deletion, and that CD4⁺CD25^- T cells are derived from positively selected nonautoreactive T cells.

View larger version (39K): [in this window] [in a new window]   FIGURE 4. Endogenous TCR chain expression in CD4+CD25+ T cells from DO11.10 mice and CD4+CD25- T cells from Ld-nOVA x DO11.10 mice. A, CD4+CD25- T cells were clonotypelow T cells in Ld-nOVA x DO11.10 mice, whereas CD4+CD25+ T cells were clonotypelow T cells in DO11.10 mice. CD8-depleted thymocytes and lymph node cells from a DO11.10 mouse and a Ld-nOVA x DO11.10 mouse were stained with anti-CD4, anti-CD25, and KJ1-26. Dot plots show staining of these cells with anti-CD4 and anti-CD25. The percentage of CD25+ cells in CD4+ cells is shown in each dot plot (the mean percentages are shown in Table I). Histograms of staining with KJ1-26 were gated for CD4+CD25- cells and CD4+CD25+ cells. The percentage of cells falling within the indicated maker is shown in each histogram. A representative result of seven independent similar experiments is shown. B, Clonotypelow T cells expressed endogenous V chains. Lymph node cells from a DO11.10 mouse and a Ld-nOVA x DO11.10 mouse were stained with anti-CD4, anti-CD25, and KJ1-26, anti-V8, or anti-TCR. Histograms of staining with KJ1-26, anti-V8, or anti-TCR were gated for CD4+CD25- cells and CD4+CD25+ cells. The percentage of cells falling within the indicated marker is shown in each histogram. A representative result of seven independent similar experiments is shown. C, The percentages of T cells expressing endogenous V chains in CD4+CD25- T cells and CD4+CD25+ T cells from the thymus and lymph nodes of DO11.10 mice and Ld-nOVA x DO11.10 mice are shown. Seven Ld-nOVA x DO11.10 mice and seven DO11.10 mice were analyzed. The mean percentage of T cells expressing endogenous V chains was defined as the mean percentage of TCR-positive cells in CD4+ T cells minus the mean percentage of V8-positive cells in CD4+ T cells. Statistical comparison is by Student’s t test (*, p < 0.01). DBL, Ld-nOVA x DO11.10 double-transgenic mice.

The contribution of endogenous TCR gene rearrangement to the generation of CD4⁺CD25^- T cells in Ld-nOVA x DO11.10 mice and to the generation of CD4⁺CD25⁺ T cells in DO11.10 mice suggests that endogenous TCR gene rearrangement is used not only for the avoidance of autoreactivity, but also for the generation of CD4⁺CD25⁺ regulatory T cells, probably by creating autoreactive TCRs. To confirm this possibility, we generated RAG2-deficient DO11.10 mice and RAG2-deficient Ld-nOVA x DO11.10 mice, in which endogenous TCR gene rearrangement was impaired. Although the numbers of CD4⁺ T cells in spleens from RAG2-deficient Ld-nOVA x DO11.10 mice were reduced by clonal deletion, most of the CD4⁺ T cells were CD4⁺CD25⁺ T cells (Fig. 5A), supporting the idea that autoreactive T cells were selected as CD4⁺CD25⁺ T cells. We confirmed that these CD4⁺CD25⁺ T cells had the ability to suppress the proliferative responses of CD4⁺CD25^- T cells from nontransgenic BALB/c mice in vitro (Fig. 6). On the contrary, most of CD4⁺ T cells in spleens of RAG2-deficient DO11.10 mice were CD4⁺CD25^- T cells and lacked CD4⁺CD25⁺ T cells. Furthermore, we generated TCR -chain-deficient DO11.10 mice and TCR -chain-deficient Ld-nOVA x DO11.10 mice. These mice had almost the same phenotype as RAG2-deficient DO11.10 mice and RAG2-deficient Ld-nOVA x DO11.10 mice, respectively (Fig. 5B). We also confirmed the suppressive activity of CD4⁺CD25⁺ T cells in TCR -chain-deficient Ld-nOVA x DO11.10 mice (data not shown). These results indicate that endogenous TCR expression, especially endogenous -chain expression, plays an important role in the generation of regulatory T cells in DO11.10 mice.

View larger version (32K): [in this window] [in a new window]   FIGURE 5. Perturbation of endogenous TCR gene rearrangement exclusively generated CD4+CD25+ T cells and CD4+CD25- T cells in Ld-nOVA x DO11.10 mice and DO11.10 mice, respectively. Thymocytes and splenocytes of RAG2-deficient Ld-nOVA x DO11.10 mice and RAG2-deficient DO11.10 mice were analyzed for the expression of CD4 and CD25. B, Thymocytes and splenocytes of TCR -chain-deficient Ld-nOVA x DO11.10 mice and TCR -chain-deficient DO11.10 mice were analyzed for the expression of CD4 and CD25. A representative result of three independent similar experiments is shown.

View larger version (15K): [in this window] [in a new window]   FIGURE 6. CD4+CD25+ T cells in RAG2-deficient Ld-nOVA x DO11.10 mice were regulatory T cells. CD4+CD25+ T cells (2 x 104 cells/well) from RAG2-deficient Ld-nOVA x DO11.10 mice were cultured with CD4+CD25- T cells (2 x 104 cells/well) from nontransgenic BALB/c mice and irradiated (20 Gy) syngeneic spleen cells (5 x 104 cells/well) in the presence of Con A at 1 µg/ml for 3 days, followed by a final 6 h of culture in the presence of 1 µCi [3H]TdR per well. These T cell subpopulations were purified from pooled splenocytes from six RAG2-deficient Ld-nOVA x DO11.10 mice or three nontransgenic BALB/c mice. Means and SDs of triplicate are shown. The data were representative of three independent similar experiments. DBL, Ld-nOVA x DO11.10 double-transgenic mice.

Taken together, these findings indicate that autoreactive T cells are selected as CD4⁺CD25⁺ regulatory T cells and that endogenous TCR gene rearrangement plays a critical role in the generation of CD4⁺CD25⁺ regulatory T cells from nonautoreactive T cells and in the generation of nonautoreactive T cells from autoreactive T cells.

	Discussion

Top Abstract Introduction Materials and Methods Results Discussion References

 
We found that autoreactive T cells were positively selected as CD4⁺CD25⁺ regulatory T cells in parallel with clonal deletion in the thymus. These results indicate that part of autoreactive T cells that have a high affinity TCR enough to lead to clonal deletion could be positively selected as CD4⁺CD25⁺ T cells in the thymus. We also found that endogenous TCR expression generates CD4⁺CD25⁺ regulatory T cells from the nonautoreactive T cells and nonautoreactive CD4⁺CD25^- T cells from the autoreactive T cells.

In Sakaguchi and coworkers’ papers (7, 22), the regulatory T cells in TCR-transgenic models were endogenous TCR -chain-expressing cells, whereas, in our Ld-nOVA x DO11.10 mice, the regulatory T cells did not express endogenous TCRs. However, these findings do not contradict each other. In the case of CD4⁺ T cells expressing high affinity TCRs with peripheral autoantigens or nonself ligands, these T cells could be positively selected as CD4⁺CD25^- T cells without clonal deletion or endogenous TCR chain expression. Because CD4⁺ T cells expressing TCRs specific for autoantigens in the thymus might be selected as CD4⁺CD25⁺ regulatory T cells, in transgenic mice expressing a TCR specific for an exogenous Ag such as DO11.10 mice, CD4⁺CD25⁺ regulatory T cells are selected only from CD4⁺ T cells expressing endogenous TCR chains. Therefore, the impairment of endogenous TCR chain expression led to the disappearance of regulatory T cells. On the contrary, in the case of transgenic mice expressing a TCR specific for a systemic autoantigen such as Ld-nOVA x DO11.10 mice, CD4⁺CD25⁺ regulatory T cells are selected from CD4⁺ T cells expressing a transgenic TCR without endogenous TCR chain expression.

Because endogenous TCR gene rearrangement occurs to escape not only negative selection (15, 16), but also death by neglect during positive selection (18, 19), it is suggested that endogenous TCR gene rearrangement occurs to generate T cells expressing TCRs that have appropriate affinity for a self MHC/self peptide above the selection threshold. Thus, there is a possibility that endogenous TCR gene rearrangement might play an important role in the generation of CD4⁺CD25⁺ T cells from nonautoreactive T cells through creating autospecific TCRs and the generation of CD4⁺CD25^- T cells from autoreactive CD4⁺ T cells. Therefore, it should be addressed whether second TCR gene rearrangement contributes to the generation of CD4⁺CD25⁺ T cells and CD4⁺CD25^- T cells in nontransgenic mice.

Our experiment revealed that CD4⁺CD25⁺ T cells contain a certain T cell repertoire specific for a systemic nuclear autoantigen. Although CD4⁺CD25⁺ T cells are anergic to TCR stimulation and suppress the activation of CD4⁺CD25^- T cells in an Ag-independent manner, it has been demonstrated that regulatory function of CD4⁺CD25⁺ T cells requires their activation via TCR in vitro (10, 11, 12). Autoreactivity of regulatory T cells increases the chance that they encounter their stimulators in periphery. Thus, it is rational that regulatory T cells are specific for autoantigens in the thymus, which may be systemic autoantigens.

Jordan et al. (13) demonstrated that CD4⁺CD25⁺ regulatory T cells are positively selected by a self peptide in the thymus. In their study, autoreactive T cells are positively selected as CD4⁺CD25⁺ regulatory T cells and did not undergo clonal deletion in contrast to our transgenic models. The lack of deletion can probably be attributed either to a lower affinity TCR compared with our transgenic model or to the expression level of the self ligand. These findings suggest that CD4⁺CD25⁺ regulatory T cells comprise a broad autoreactive T cell repertoire.

Our results provide new insight into the ontogeny of regulatory T cells. The thymus has the ability to generate regulatory T cells from autoreactive T cells simultaneously with negative selection, sometimes actively generating autoreactive T cells by endogenous TCR gene rearrangement. The critical contribution of endogenous TCR gene rearrangement to the control of autoreactivity is that it reduces self reactivity in effector precursor T cells by generating nonautoreactive CD4⁺CD25^- T cells from autoreactive T cells and generates autoreactive CD4⁺CD25⁺ regulatory T cells from the nonautoreactive T cells.

	Acknowledgments

 
We are very grateful to Dr. T. Watanabe, Dr. S. Koyasu, and Dr. Pierre Chambon for sharing their valuable transgenic mice and materials with us.

	Footnotes

 
¹ This work was supported by grants from the Ministry of Health, Labor and Welfare; a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan; and the Naito Foundation.

² Address correspondence and reprint requests to Dr. Yoshikata Misaki, Department of Allergy and Rheumatology, University of Tokyo Graduate School of Medicine, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8655, Japan. E-mail address: misaki-tky{at}umin.ac.jp

³ Abbreviations used in this paper: RAG2, recombination-activating gene 2; MFI, mean fluorescence intensity; SP, single-positive.

Received for publication June 25, 2001. Accepted for publication February 22, 2002.

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