Transforming growth factor (TGF)-β is a critical factor in the regulation of T cell-mediated immune responses and immune tolerance (1–4). In addition to its potent and diverse effects on T cells, TGF-β, in the context of T-cell receptor (TCR) stimulation, is able to convert peripheral CD4+CD25− naïve T cells to CD4+CD25+ T regulatory cells (Tregs) by induction of transcription factor Foxp3 (5–10). Foxp3 is not only a specific marker for CD25+CD4+ Tregs, but also required for their development (9, 11–14). Although TGF-β induction of Foxp3 in CD4+CD25− naïve T cells has been validated, the underlying molecular events are largely unknown. Moreover, it is also not known whether TGF-β induces Foxp3 expression in thymic CD4+CD25− single positive (SP) T cells and therefore converts them to CD4+CD25+ Tregs.
CD28 is the best characterized costimulatory molecule that is not only required for naïve T cell proliferation and differentiation through interleukin (IL)-2 production, but is also implied in the development and homeostasis of natural CD4+CD25+ Tregs (15, 16). CD28−/− mice exhibit profound reduction of CD4+CD25+ Tregs in the peripheral lymphoid tissues, which is responsible at least in part for the autoimmune-like inflammation in the CD28−/− nonobese diabetic (NOD) mice (15–17). Because TGF-β induced CD4+CD25+ Tregs share many phenotypical and functional features with the natural CD4+CD25+ Tregs (5), it is of importance to study whether CD28 signaling is involved in TGF-β induction of Foxp3 and consequently the conversion of CD4+CD25− naïve T cells to CD4+CD25+ Tregs.
In this report, we show that TGF-β and TCR co- stimulation induces Foxp3 expression in thymic CD4+CD25− SP T cells and converts them to CD4+CD25+ Foxp3+ T regulatory cells. We further provide evidence that CD28 signaling is primarily required for the survival/homeostasis of TGF-β converted thymic CD4+CD25+ Tregs, although engagement of CD28 contributes to Foxp3 induction by TGF-β. Importantly, TGF-β induces alloantigen specific CD4+CD25+ Tregs from thymic CD25−SP cells, which also requires CD28 signaling to maintain their survival.
MATERIALS AND METHODS
WT C57BL/6, Rag1−/− (C57BL/6), BALB/c, CD28−/− and B6.PL-Thy1a/CyJ mice were obtained from The Jackson Laboratory and maintained at the National Institute of Dental and Craniofacial Research.
Antibodies and Reagents
The following reagents were from PharMingen (San Diego, CA): FITC- anti-CD45RB, purified anti-CD3 (145-2C11, NA/LETM), PE-anti-murine CTLA-4, hamster IgG isotypic control, FITC- or biotinylated-anti-murine CD25 (Clone 7D4), FITC-rat IgM, PE-, APC- or purified anti-CD25 (Clone PC61, NA/LETM), PE-anti-murine Thy1.1, FITC-anti-murineThy1.2 and anti-FcRII/III (CD16/32). FITC or PE-anti-murine CD4, anti-CD8α, and the respective isotypic control mAbs and streptavidin-FITC, PE or Tricolor were purchased from Caltag (San Francisco, CA). Recombinant human TGF-β1 and murine IL-2 and anti-TGF-β1, 2,3 mAb were purchased from R&D Systems (Minneapolis, MN). 7-amino-actinomycin D (7-AAD) was purchased from Calbiochem. PE- or FITC- anti-mouse/rat Foxp3 Staining Set (clone FJK-16s) was purchased from eBioscience (San Diego, CA).
Thymi were gently minced in complete DMEM containing 10% FBS (Biowhittaker, Walkersville, MD) and CD4+ T cells purified using a mouse CD4+ T Cell Column System (R&D Systems). Whole spleen cells (irradiated 3000 rad) from C57BL/6 mice were used as antigen presenting cells (APCs) as indicated. CD4+CD25− and CD4+CD25+ T cells were isolated as reported before (5) or with mouse CD4+CD25+ T cell isolation kit (Miltenyi Biotec, Auburn, CA) following manufacturer's recommendations. The purity of CD4+CD25− and CD4+CD25+ T cells were usually >98–99% and 80–90%, respectively.
Cell Culture and Induction of Foxp3
For induction of Foxp3, purified thymic CD4+CD25− T cells from C57BL/6 or CD28−/− mice were stimulated with plate-bound anti-CD3 (5 μg/ml) and soluble anti-CD28 (2 μg/ml) in the absence and presence of TGF-β1 (2 ng/ml) or plus IL-2 (2 ng/ml) for 24, 48 and 72 hr. In some experiments, anti-CD3 (10 μg/ml) and anti-CD28 (5 μg/ml) or anti-CD2 (5 μg/ml) were coated onto plate to stimulate T cells. Cells were harvested and RNA was isolated for RT-polymerase chain reaction (PCR) analysis. For induction of T regulatory cells by TCR and TGF-β stimulation, purified CD4+CD25− thymic SP cells from C57BL/6 or CD28−/− mice were cultured with syngeneic splenic APCs and anti-CD3 mAb (0.5 μg/ml)) or with plate-coated anti-CD3 (5 μg/ml) and soluble anti-CD28 (2 μg/ml) without APCs in the presence or absence of TGF-β1 (2 ng/ml) in complete DMEM at 37°C and 5% CO2 for a week. In some cultures, rIL-2 (2 ng/ml) was added as indicated. CD4+ live T cells were then isolated for further analysis.
Co-Culture of CD4+ T Cells
Freshly isolated C57BL/6 thymic or splenic CD4+CD25− responder T cells were cultured in 96-well plates with anti-CD3, splenic APCs, and TGF-β-induced Tregs or control-cultured cells or natural CD4+CD25+Tregs for proliferation assay. In some experiments, co-culture assay was performed with anti-CD3 (coated) and anti-CD28 (soluble) in the absence of APCs to test the suppressive activity of TGF-β-induced Tregs or nTregs. Cells were cultured at 37oC in 5% CO2 for 72 hr and pulsed with 1 μCi 3H thymidine for the last 6–16 hr. Radioactivity incorporated was counted using a faltbed β counter (Wallac). For carboxy-fluorescein diacetate succinimidyl ester (CFSE) labeling assay, CD4+ CD25− T cells (107/ml) were incubated with 2 μM CFSE in PBS at 37oC for 15 min. Cells were then washed three times and resuspended in complete DMEM (without Phenol red) for cell culture.
Flow Cytometry Analysis
T cells were resuspended in PBS containing 1% BSA (Irvine, Santa Ana, CA) and 0.1% sodium azide (Sigma). For the staining of surface antigens, cells were incubated with FITC, PE or Tricolor conjugated mAbs or their negative control antibodies as indicated for 30 min on ice. Intracellular staining of CTLA-4 was performed as reported (5). Intracellular Foxp3 staining was performed with PE anti-mouse/rat Foxp3 staining set following manufacturer's recommendation.
Adoptive Transfer Experiments
TGF-β converted CD4+CD25+ T cells from C57BL/6 and CD28−/− cultures were injected i.p. into nonirradiated syngeneic B6.PL-Thy1a/Cy (Thy1.1+) or Rag1−/− mice. The recipients were bled at days one, two, three, and five post cell transfer. Red blood cells were lysed and the white blood cells were stained with anti-CD4-PE and FITC-anti-Thy1.2 antibodies. The mice were sacrificed on day five and cells from spleens and mesenteric lymph nodes were isolated and stained ex vivo with PE-anti-CD4 and FITC-Thy1.2 mAbs to determine the number of injected TGF-β converted Tregs (CD4+Thy1.2+) in vivo.
Semiquantitative and Real-Time PCR
RT-PCR for Foxp3 and IL-2 expression was performed as described (5, 18). The primer sequences were as follows and synthesized by Invitrogen: GAPDH: 5′-CCATGGAGAAGGCTGGG-3′ and 5′-CAAAGTTGTCATGGATGACC-3′; Foxp3: 5′-CAG CTG CCT ACA GTG CCC CTA G-3′ and 5′-CAT TTG CCA GCA GTG GGT AG-3′; IL-2: 5′-ATGTACAGCATGCAGCTCGCATC-3′ and 5′-GGCTTGTTGAGATGATGCTTTGACA-3′. Normalized values for Foxp3 mRNA expression in each sample were calculated as the relative quantity of Foxp3 divided by the relative quantity of GAPDH. Foxp3 mRNA was also quantified by real-time PCR using the ABI 7500 Real-time PCR system (PE Applied Biosystems, Foster City, CA). Experiments were performed using primers, the fluorescent TaqMan probe specific to Foxp3 and an internal control HPRT gene. Foxp3 primers: 5′-CCC AGG AAA GAC AGCAAC CTT-3′ and 5′-TTC TCA CAA CCA GGC CAC TTG-3′; Foxp3 probe: 5′-FAM-ATCCTA CCC ACT GCT GGC AAA TGG AGT C-3′; HPRT was analyzed according to the protocol of Taqman gene expression assay kit (PE Applied Biosystems). PCR reaction system contained 0.5 μM primers and 0.2 μM TaqMan probe. PCR amplification was preceded by a 10 min denaturation step at 95°C and the amplification step consisted of 40 cycles of 15s at 95°C and 60s at 60°C. The amount of Foxp3 was normalized to the endogenous internal control HPRT using the 2(-Delta Delta C[T]) method (2−ΔΔct), and presented as a relative expression ratio.
Student's t tests were used for the significance of data comparison.
TGF-β Induces Foxp3 in CD4+CD25− Single Positive Thymocytes
We first examined whether TGF-β was able to induce Foxp3 expression in CD4+CD25− SP thymocytes. Stimulation of highly purified C57BL/6 (B6) thymic CD4+CD25− SP cells with anti-CD3 and anti-CD28 in the presence of TGF-β1 induced Foxp3 mRNA expression (Fig. 1, A and B). This TGF-β induction of Foxp3 mRNA was detected within 24 hr (Fig. 1A) and continued to increase dramatically at 48–72 hr (Fig. 1B and data not shown). As in peripheral CD25− T cell conversion (5), neither TCR nor TGF-β stimulation alone could result in Foxp3 expression in CD4+CD25− SP thymocytes. Stimulation of thymic CD4+CD25+ SP natural Tregs with TCR and TGF-β in the presence of high dose IL-2 failed to further enhance their Foxp3 expression (data not shown), consistent with that in peripheral Treg (5). To further explore this TGF-β induction of Foxp3 in thymic SP cells, intracellular Foxp3 protein was stained by anti-Foxp3 mAb and analyzed by flow cytometry. Highly purified CD4+CD25− SP thymocytes (>98–99%) contained 1–3% of Foxp3+ cells (Fig. 1C), whereas majority of natural CD4+CD25+ Tregs expressed Foxp3 (9, 14, 19) (unpublished data). As in natural Tregs, Foxp3 protein of CD4+CD25− thymic SP was also confined to the nucleus. Notably, when thymic CD4+CD25− SP cells were cultured with TCR and TGF-β for only 16–24 hr, 8–10% of the stimulated cells were already Foxp3+ CD25+, whereas only 3–4% of Foxp3+ CD25+ cells was seen in the control cultures (without TGF-β). As expected, there was no detectable T cell proliferation by 24 hr and the total CD25+ T cells were compatible between TGF-β treated and control cultures (40–50%). Strikingly, when cells were continued in culture for 72 hr and stained for Foxp3, 40–50% of TGF-β treated cells became Foxp3+ CD25+, whereas only 0.3–2% of control cultured cells were Foxp3+CD25+(Fig. 1C, 2). This induction of Foxp3 was clearly dependent on the concentration of TGF-β in the cultures (Fig. 2). As low as 80–400 pg/ml of TGF-β1 already resulted in significant CD4+CD25+Foxp3+ Tregs (7–10%)(Fig. 2). When an aliquot of these cells were visualized under the immunofluorescence microscopy, it was found that Foxp3 protein (red) is indeed localized in the nucleus in TGF-β treated cells (Fig. 1D right). In contrast, Foxp3+ cells were hardly seen in the control cultures (Fig. 1D, left). When total Foxp3+ was calculated according to the percentage of Foxp3+ cells and the total number of the viable cells in each well of the plate, it was found that TGF-β treated cultures contained 2.22x105 of 25+Foxp3+ T cells that was about 15-fold higher than that in control cultured cells (Foxp3+: 0.14x105) and more than 55-fold increase compared to that in the freshly isolated CD4+CD25− SP thymocytes (Foxp3+; 0.04x105) (Fig. 1C,D). Thus, TGF-β and TCR costimulation induces Foxp3 expression in thymic CD4+CD25− SP cells. The data strongly argues for the TGF-β de novo induction of the Treg specific gene in the naïve CD4+CD25− SP thymocytes and converts CD25− Foxp3− into CD25+Foxp3+ T cells (9, 20).
TGF-β Converts CD4+CD25− SP Thymocytes to CD4+CD25+hi Suppressor T Cells
To study whether TGF-β-induced Foxp3+ SP thymocytes possessed immunosuppressive ability, thymic CD4+CD25− SP cells were cultured with anti-CD3 and APCs in the presence or absence of TGF-β for 7 days and viable CD4+ T cells were isolated and analyzed. Although CD4+ T cells harvested from both TGF-β treated and control cultures were CD25+ (>90–95%), the pattern of CD25 expression in the two groups was different. While most of CD4+ T cells in control cultures were CD25+low, majority of CD4+ T cells in TGF-β treated cultures were CD25+hi (Fig. 3A). Inclusion of exogenous IL-2 in the primary cultures did not change the expression pattern of CD25 in TGF-β treated cells (Fig. 3A), although it increased total number of recovered CD4+ T cells (Fig. 6A). CD4+CD25+ T cells derived from the TGF-β treated thymic cultures were smaller in size and expressed CD4+low, CD45RB−/low, intracellular CTLA-4+ and GITR+ (Fig. 3A and data not shown) consistent with the TGF-β converted splenic CD4+CD25+ Tregs (5) as well as the natural CD4+CD25+ Tregs (20–24). Strikingly, by real-time PCR analysis, TGF-β converted thymic CD4+CD25+ T cells expressed much higher levels of Foxp3 (30- to 60-fold) than that in CD4+CD25+ T cells obtained from the control cultures at the end of one week (Fig. 3B). Significantly, when co-cultured with freshly isolated thymic or splenic CD4+CD25− SP responder T cells, TGF-β-converted CD4+CD25+ T cells markedly inhibited the proliferation of the responder T cells, whereas the CD4+CD25+ T cells from control cultures had only marginal effect on responder T cells (Fig. 3C). Interestingly, increase in the cell number of TGF-β induced Tregs (CD25− cells:Tregs, 1:2, 1:3) could not further enhance their suppressive ability (Fig. 3D). As expected, neither TGF-β converted Tregs nor natural CD4+CD25+ Tregs (nTregs) could significantly inhibit T cell proliferation (CD4+ CD25− T cells alone: 116666±5773 cpm; plus TGF-β-Tregs [1:1]: 77972±1100 cpm; plus nTregs[1:1]: 72264±15880 cpm) in co-cultures with immobilized anti-CD3 and soluble anti-CD28 without APCs.
We then examined whether APCs were required for induction of CD4+CD25+ Tregs by TGF-β. CD4+CD25− thymocytes were stimulated with immobilized anti-CD3 and soluble anti-CD28 antibodies and TGF-β in the absence of APCs for 6 days. CD4+CD25+ cells were then isolated and tested for their suppressive ability on CD4+CD25− responder T cells in a co-culture assay. Interestingly, TGF-β induced CD25+ Tregs from the APC-free cultures (anti-CD3 and anti-CD28) inhibited CD4+CD25− T cell proliferation (Fig. 2B), which was as potent as natural Tregs (nTreg) (Fig. 2B) and TGF-β converted Tregs from the APC-containing cultures (anti-CD3 and APCs) (Fig. 3C, D). The suppression was observed in the co-cultures in which CD4+CD25− responder T cells were stimulated with anti-CD3 and APCs but not by coated anti-CD3 and soluble anti-CD28 antibodies without APCs. Thus, thymic CD4+CD25− SP T cells can be converted into CD4+CD25+ suppressor T cells by TGF-β through induction of Foxp3. APCs are dispensable for the induction of CD25+ Tregs by TGF-β, but important for the suppressive activity of TGF-β converted Tregs in vitro.
CD28, but not CD2, Contributes to TGF-β Induction of Foxp3 in CD4+CD25− T Cells
Because CD28 signaling has been indicated in homeostasis of natural CD4+CD25+ Treg (15–17), we then sought to determine whether CD28 engagement had an impact on TGF-β induced Foxp3 expression in CD4+CD25− T cells. Thymic CD4+CD25− SP cells were stimulated with immobilized anti-CD3 alone or plus anti-CD28 mAb in the presence of active TGF-β1 and Foxp3 mRNA was then determined by RT-PCR and real-time PCR. Costimulation of CD28 by anti-CD28 mAb enhanced Foxp3 expression induced by TGF-β in thymic CD4+CD25− SP T cells (Fig. 4, A and C). The TGF-β up-regulation was specific for Foxp3, because IL-2 mRNA in the same TGF-β treated cultures was significantly suppressed (data not shown). In contrast to CD28 costimulation, co-engagement of CD2, a molecule required for thymic development of T cells in in vitro models (25), failed to enhance Foxp3 expression in TGF-β treated cultures (Fig. 4A). The same result was observed whether anti-CD3 (Fig. 4A) or anti-TCR antibodies (BD, Clone 3&23, not shown) were used to stimulate TCR in the cultures. As expected, TCR and CD2 stimulation in the absence of TGF-β showed no Foxp3 expression (Fig. 4A). Thus, CD28 engagement enhances the effect of TGF-β on Foxp3 expression. CD2 costimulation, however, plays no role in TGF-β induction of Foxp3 in CD4+CD25− SP thymocytes.
To further explore the role of CD28 signaling in TGF-β induction of Foxp3, CD28−/− CD4+CD25− SP thymocytes were cultured with TCR and TGF-β. Stimulation CD4+CD25− SP thymocytes from CD28−/− mice showed lower levels of Foxp3 mRNA than that in the wildtype thymic CD4+CD25− SP cells during the first 24–48 hr of cultures (Fig. 4, B and C). Because CD28−/− T cells produced lower levels of IL-2, exogenous rIL-2 was added into the cultures. Inclusion of IL-2 however failed to restore the defect in TGF-β induction of Foxp3 in CD28−/− T cells completely (Fig. 4B,C). As in wildtype mice, TGF-β inhibited IL-2 mRNA in the CD28−/− CD4+CD25− SP thymocytes, indicating intact TGF-β signaling in the CD28−/− T cells. Similar results were also observed in CD28−/− splenic CD4+CD25− T cells (data not shown). Interestingly, when the cultures were continued for 60–72 hr, Foxp3 expression induced by TGF-β in the CD28−/− T cells was restored to the levels of TGF-β-treated WT cells. Consequently, there was similar amount of Foxp3 expression in TGF-β treated cells between CD28−/− and WT mice at the end of cultures (day 7) as determined by real-time PCR (Fig. 5B). The data demonstrates that CD28 signaling contributes to, particularly in the early phase of stimulation, but is not essential for TGF-β induced Foxp3 expression in CD4+CD25− SP thymocytes, indicating that other molecules and/or compensatory pathway(s) may also be involved.
TGF-β Converted CD28−/− Thymic CD4+CD25+ Cells Are Suppressor T cells
We then examined whether TGF-β was able to convert CD28−/− thymic CD4+CD25− T cells into CD4+CD25+ Treg in vitro. CD4+CD25− SP thymocytes from CD28−/− and wildtype mice were cultured with TCR and TGF-β for one week and viable CD4+ T cells were then isolated and analyzed. As seen in wildtype cultures, TCR and TGF-β co-stimulation converted CD28−/− thymic CD4+CD25− SP T cells to CD4+CD25+ T cells that expressed high levels of CD25 (Fig. 5A). In contrast, majority of the CD4+ T cells from control cultures (without TGF-β) showed CD25+low (data not shown). Importantly, TGF-β converted CD4+CD25+hi T cells expressed similar levels of Foxp3 compared to that in wildtype mice determined by real-time PCR (Fig. 5B). Significantly, when co-cultured with freshly isolated wildtype thymic CD4+CD25− responder T cells that had been labeled with CFSE, TGF-β converted CD28−/− CD4+CD25+ T cells inhibited TCR mediated responder T cell proliferation (Fig. 5C), which was as efficient as TGF-β induced WT CD4+CD25+ Tregs (Fig. 3C). Addition of exogenous IL-2 in the primary cultures did not change the levels of CD25, Foxp3 expression and suppressive function of CD28−/− CD4+CD25+ Tregs (Fig. 5). Thus, CD28 −/− mice preserve the potential of TGF-β conversion of thymic CD4+CD25− SP cells to CD4+ CD25+ Tregs.
CD28 Is Required for the Homeostasis/Survival of TGF-β Induced CD4+CD25+ Treg In Vitro and In Vivo
Although TGF-β converted CD28−/− CD4+CD25+ Tregs were similar in phenotype and function as compared with wildtype natural and TGF-β induced Tregs, the number obtained was significantly lower. This prompted us to examine whether CD28 deficiency had any impact on homeostasis/survival of TGF-β induced Tregs. Thymic CD4+CD25− SP cells from both CD28−/− and wildtype mice were cultured with anti-CD3 antibody in the presence or absence of TGF-β for one week and viable CD4+ CD25+ T cells were counted. Strikingly, the total number of TGF-β converted CD4+CD25+ Tregs in CD28−/− cultures was profoundly decreased compared to that in the wildtype cultures (Fig. 6A), although the percentage of Tregs were similar between the two groups (Fig. 3A, 5A). The reduction was not specific to TGF-β induced Tregs, because the total number of viable CD4+ T cells in control cultures of CD28−/− mice was also markedly decreased (data not shown). Inclusion of exogenous IL-2 in the primary cultures increased the over all number of CD4+ T cells in both CD28−/− and wildtype mice, but the number of Tregs in CD28−/− cultures still remained significantly lower than that in wildtype cultures (Fig. 6A). Thus, CD28 signaling is required for the maintenance and/or survival of TGF-β induced CD4+CD25+ Tregs in vitro, although it may be dispensable for TGF-β induction of Foxp3.
We then sought to identify whether CD28 deficiency had an impact on homeostasis/survival of TGF-β induced CD4+CD25+ Tregs in vivo. TGF-β converted thymic CD4+CD25+ Tregs from CD28−/− and wildtype mice (Thy1.2+) were adoptively transferred i.p. into syngeneic B6.PL-Thy1a/CyJ (Thy1.1+) without further challenge. The injected Tregs were monitored by FACS staining with PE-CD4 and FITC-Thy1.2 antibodies. On days one, two, three and five after cell transfer, peripheral blood was collected from the recipients and the CD4+ Thy1.2+ donor cells were analyzed. A reduction of CD4+Thy1.2+ CD28−/− Tregs was observed compared to the injected wildtype Tregs (Fig. 6B–E). When the recipients were sacrificed on day 5 after injection, the recovered CD4+Thy1.2+ CD28−/− T cells were lower in spleen (Fig. 6D) and in mesenteric lymph nodes (Fig. 6E). When TGF-β converted CD4+CD25+ Tregs from CD28−/− or WT thymic CD25−cells were injected into the syngeneic Rag1−/− (C57BL/6) mice, similar results were obtained (Fig. 6F.G). The data indicates that CD28−/− Tregs also have low survival capacity than those from wildtype mice in vivo.
Finally, we examined whether TGF-β could induce alloantigen-specific CD25+ Foxp3+ Tregs from CD4+CD25− SP thymocytes and if CD28 deficiency had any effect on such alloantigen-specific Treg generation. Highly purified CD4+CD25− SP thymocytes from CD28−/− and wildtype C57BL/6 mice were cultured with irradiated spleen cells from BALB/c mice in the presence of TGF-β and IL-2 for one week. Cells were then harvested, counted and stained with anti-CD4, CD25 and Foxp3 antibodies to identify Tregs. TGF-β efficiently converted CD25− naïve T cells into CD25+Foxp3+ Tregs upon alloantigen stimulation in the WT cultures (about 80% of CD25+ T cells) (Fig. 7A), although the percentage of alloantigen-specific CD25+Foxp3+ T cells was lower than that in the cultures with anti-CD3 stimulation, CD28 deficiency markedly reduced TGF-β converted CD25+Foxp3+ Tregs, although the ratio between Foxp3+ and Foxp3− T cells within CD25+ population (about 60%) was similar to that in WT mice (Fig. 7A). Consequently, the total number of recovered CD25+Foxp3+ Tregs was significantly lower in CD28−/− cultures than that in the WT controls (Fig. 7B). As expected, the CD25+Foxp3− non-Tregs also decreased in CD28−/− cultures (Fig. 7B). Thus, TGF-β induces alloantigen-specific Foxp3+ Tregs that also require CD28 for survival.
We have shown here that TGF-β and TCR co- stimulation converts thymic CD4+CD25− SP T cells into CD4+CD25+hiFoxp3+ T regulatory cells. The TGF-β converted Tregs are phenotypically and functionally indistinguishable from the TGF-β induced peripheral CD4+CD25+ Tregs (5–10) as well as the natural CD4+CD25+ Tregs (20–24). Several novel conclusions can be drawn from the current study. First, TGF-β can induce the expression of Foxp3 in CD4+CD25− SP thymocytes along with TCR co-stimulation. The TGF-β mediated Foxp3 increase is indeed due to de novo gene induction rather than selective proliferation of a small number of Foxp3+ subset within the CD4+CD25− T cells. This is evident from the fact that Foxp3 expression is clearly seen within 24 hr culture in the absence of obvious T cell proliferation. In addition, immunofluorescent staining clearly demonstrates that TGF-β treatment has already increased Foxp3+ cells in the culture to three- to five-fold within 24 hr. Furthermore, since almost half of TGF-β treated cells express Foxp3+CD25+ and this increase represents about 55-fold increase in total Foxp3+ cells by 72 hr compared to freshly isolated SP thymocytes, it is impossible that TGF-β induces the original minor population of Foxp3+CD25− to proliferate 55-fold within three days. Therefore, CD25+Foxp3+ T cells in TGF-β treated cultures are, at least in great part, converted from Foxp3− SP thymocytes. It is important to point out that as low as 80–400 pg/ml of TGF-β in the culture was already able to induce about 10% CD25+Foxp3+ Tregs, which might have implications in the physiological situation in vivo. Significantly, TGF-β converted thymic Tregs exhibit similar suppressive capacity as the natural Tregs (nTregs) on normal CD4+ T cell proliferation. This suppression seems to require the presence of APCs, since neither TGF-β converted Tregs nor natural CD4+CD25+ Tregs could significantly inhibit T cell proliferation in co-cultures with immobilized anti-CD3 and soluble anti-CD28. Moreover, TGF-β induces Foxp3+CD25+ T cells from CD4+CD25− thymocytes stimulated with anti-CD3 and anti-CD28 antibodies in the absence of APCs. The TGF-β induced Tregs derived from APC-free cultures are equally potent in suppressing normal CD4+ T cell proliferation as those from APC-containing cultures, suggesting that APCs are dispensable in TGF-β induction of Foxp3+ Tregs. Since TGF-β converted CD4+CD25+ Treg share basic features of “natural” Tregs, our data provide additional insight in understanding the development/generation of natural Tregs in the thymus (26–28).
Second, TGF-β converted CD4+CD25+ Tregs express very high levels of CD25 on the surface that are significantly distinct from the low levels of CD25 on CD4+ T cells in the control cultures (without TGF-β). The expression of high levels of CD25 on CD4+ T cells is associated with TGF-β stimulation, but is unlikely to be related to CD28 expression on the T cells or exogenous IL-2 presence in the cultures. This finding has identified an undeniable link between TGF-β signaling and high levels of CD25 surface expression on CD4+ T cells that represents one unique feature of natural murine and human CD4+CD25+ Treg (22, 23, 29). Addition of exogenous IL-2 to the cultures results in an insignificant effect on CD25 expression of TCR stimulated CD4+ T cells, nor did it influence the profile of high levels of CD25 on TGF-β converted CD4+CD25+ Treg, however it indeed increased the total number of viable cells. Although mechanistically elusive, it is likely that TGF-β signaling maintains CD25hi expression by keeping rather than up-regulating its expression. In this regard, the levels of CD25 expression on CD4+ T cells in TGF-β treated cultures are similar and even slightly lower compared to that in the control cultures during initial one, two, and three days of stimulation (5) (Fig. 1 and data not shown). Alternatively, TGF-β may enhance the survival of CD25+hi T cells by preventing them from activation induced cell death after TCR signal induced high expression of CD25, because TGF-β is indispensable in protecting T cell from apoptosis (30, 31).
Third, CD28 signaling contributes to TGF-β induced Foxp3 expression, which is evident by anti-CD28 antibody upregulation of TGF-β mediated Foxp3 in wildtype thymic CD4+CD25− SP cells and by a transient defect in TGF-β induction of Foxp3 in CD28−/− thymic SP cells. The CD28 involvement in TGF-β induction of Foxp3 is however time-dependent and dispensable, because the lower levels of Foxp3 in CD28−/− T cells are only seen in the early phase of cultures (24–48 hr). Furthermore, the early transient defect in Foxp3 induction had no significant impact on TGF-β conversion of CD4+CD25+ Tregs, since TGF-β converted CD4+CD25+ Tregs are phenotypically and functionally indistinguishable between CD28−/− and wild-type mice. The data indicates a limited role for CD28 signaling in the induction and/or conversion phase of Tregs, at least in vitro. Fourth, CD2 signaling is not involved in TGF-β induction of Foxp3 in thymic SP T cells, unlike its requirement, at least in vitro, for the maturation of CD4+CD8+ DP thymocytes toward CD4+or CD8+ SP thymocytes (25). Most importantly, the profound reduction in the number of TGF-β induced CD4+CD25+ Tregs in CD28−/− mice strongly argue for the indispensable role of CD28 in maintaining the homeostasis /survival of T regulatory cells. Although the detailed mechanisms remain to be elucidated, CD28 signaling may function through both increasing proliferation and/or preventing apoptosis (17) of TGF-β induced CD4+CD25+ Tregs, which may not necessarily rely solely on IL-2 production, because exogenous IL-2 fails to restore the number of CD28−/− Tregs to normal levels. CD28 signaling is shown to prevent T cell death by up-regulating anti-apoptotic BCL-XL expression (32), which may also occur in the process of TGF-β mediated conversion of CD4+CD25− T cells to CD4+CD25+ Tregs. Since TGF-β converted CD4+CD25+ Tregs resemble the basic phenotypical and functional features of natural CD4+CD25+ Tregs, our study will help define the molecular events involved in the development of natural Tregs and offer an explanation for the lack of natural CD4+CD25+ Tregs in CD28 as well as B7–1/B7–2 deficient mice.
Finally, TGF-β conversion of naïve CD4+CD25− T cells to CD25+Foxp3+ Tregs upon alloantigen stimulation opens a new avenue to generate alloantigen-specific Tregs from normal cells in vitro, which has implications in the prevention and inhibition of allograft rejection and graft against host reaction. The finding that CD28 is also essential for the maintenance of TGF-β converted alloantigen-specific Foxp3+ Tregs not only confirms our data in pan-TCR stimulation, but also helps to understand the molecular mechanisms for CD28 in immune tolerance in transplantation (33). The fact that natural CD4+CD25+Foxp3+ Tregs are difficult to isolate and expand in vitro makes our approach to induce alloantigen-specific T regs from normal naïve CD4+ T cells particularly attractive for future clinical applications in transplantation.
The authors thank Drs. Sharon M. Wahl, Songtao Shi, and Sylvain Perruche from the NIDCR for critical reading of the manuscript. This work was carried out in the Intramural Division at the National Institute of Dental and Craniofacial Research, National Institutes of Health.
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