OBJECTIVE—In this study, we asked whether a possible quantitative or qualitative deficiency in naturally occurring Foxp3+CD4+ regulatory T-cells (nTreg), which display potent inhibitory effects on T-cell functions in vitro and in vivo, may predispose to the development of type 1 diabetes.
RESEARCH DESIGN AND METHODS—We assessed the frequency and function of Foxp3+ nTreg cells in primary and secondary lymphoid tissues in the NOD animal model of type 1 diabetes.
RESULTS—We show that the cellular frequency of Foxp3+ nTreg cells in primary and secondary lymphoid tissues is stable and does not decline relative to type 1 diabetes–resistant mice. We show that thymic and peripheral CD4+CD25+ T-cells are fully functional in vivo. We also examined the functional impact of CD4+Foxp3+ nTreg cells on the development of autoimmune diabetes, and we demonstrate that nTreg cells do not affect the initial priming or expansion of antigen-specific diabetogenic T-cells but impact their differentiation in pancreatic lymph nodes. Moreover, CD4+Foxp3+ nTreg cells also regulate later events of diabetogenesis by preferentially localizing in the pancreatic environment where they suppress the accumulation and function of effector T-cells. Finally, we show that the nTreg cell functional potency and intra-pancreatic proliferative potential declines with age, in turn augmenting diabetogenic responses and disease susceptibility.
CONCLUSIONS—This study demonstrates that Foxp3-expressing nTreg cells in NOD mice regulate diabetogenesis, but temporal alterations in nTreg cell function promote immune dysregulation and the onset of spontaneous autoimmunity.
NOD mice are characterized by spontaneous development of several autoimmune diseases, including type 1 diabetes, which occurs progressively through a T-cell–dependent infiltration and destruction of insulin-producing β-islet cells of Langerhans, leading to insulin deficiency (1–4). The extended time interval between establishment of insulitis and onset of disease suggests a progressive loss of peripheral regulatory mechanisms in pre-diabetic NOD mice before disease progression (1).
CD4+ naturally occurring Treg (nTreg) cells have emerged as the predominant regulatory population mediating peripheral self-tolerance (5–7). The majority of these cells constitutively express the interleukin (IL)-2R α-chain (CD25) and represent 1–10% of thymic or peripheral CD4+ T-cells in mice and man (8,9). Functional abrogation of nTreg cells increases immunity to tumors, allografts, and pathogens and results in the development of multiorgan-specific autoimmunity by an as-yet undefined mechanism (6). Recent studies show that nTreg cells constitutively and specifically express the Foxp3 transcription factor marker, a critical molecular switch for nTreg cell development and function (10,11). Targeted deletions or natural mutations of the foxp3 gene leads to a deficiency of nTreg cells and provokes the development of severe autoimmunity in scurfy in mice and immune dysregulation, polyendocrinopathy, enteropathy, and X-linked inheritance (IPEX) patients (10–13).
Several studies have implicated nTreg cells in prevention of type 1 diabetes. Depletion of CD25-expressing T-cells or disruption of the B7/CD28 pathway in NOD mice has been shown to decrease CD4+CD25+ nTreg cell frequency and ultimately leads to an accelerated type 1 diabetes onset (14,15). Other studies have correlated type 1 diabetes resistance in aged NOD mice with the expansion of CD25-expressing CD4+ T-cells with regulatory activity within inflamed pancreatic lymph nodes (pancLN) (16,17). Although Treg cell function has been examined in lymphoid tissues of pre-diabetic and insulitic NOD mice, it is unclear whether these CD4+CD25+ T-cells are thymus-derived nTreg or inflammation-induced Treg cells (18). Recently, Chen et al. (19) reported that Foxp3-deficient NOD mice display a significantly increased incidence and earlier onset of type 1 diabetes compared with normal NOD mice, strongly implying a role for Foxp3+ nTreg cells in the control of type 1 diabetes pathogenesis. Some central questions remain: How and where do nTreg cells mediate tolerance to β-islet antigens and disease protection? Also, does the spontaneous onset of type 1 diabetes in NOD mice reflect developmental or functional deficiencies in nTreg cells, consequently tipping the balance toward the activity of diabetogenic T-cells and clinical type 1 diabetes (18,20,21)?
In this study, we examined whether quantitative or qualitative deficiencies in CD4+Foxp3+ nTreg cells lead to a failure to control the onset of type 1 diabetes. We show that the frequency of nTreg cells in primary and secondary lymphoid tissues is stable and does not decline relative to type 1 diabetes–resistant mice. We demonstrate that thymic and peripheral nTreg cells from neonatal NOD mice are fully functional in vivo and dramatically halt the onset of primary and established type 1 diabetes. We show that nTreg cells affect neither the priming nor the expansion of antigen-specific diabetogenic T-cells but suppress their differentiation in pancLN. Moreover, CD4+Foxp3+ nTreg cells also regulate disease by localizing in draining lymph nodes and pancreatic lesions, where they suppress the accumulation and function of effector T-cells (Teff cells). We also show that the nTreg cell functional potency and intra-pancreatic proliferative potential declines with age, in turn augmenting diabetogenic responses and disease susceptibility. In summary, we show that qualitative and not quantitative alterations in Foxp3+ nTreg cells in NOD mice drive immune dysregulation and the spontaneous onset of type 1 diabetes.
RESEARCH DESIGN AND METHODS
NOD, C57BL/6, NOD.TCRα−/−, and NOD.BDC2.5 mice were bred and maintained in pathogen-free conditions at the McGill University animal facility. NOD.TCRα−/− and NOD.BDC2.5 TCR mice were a gift from Dr. Christophe Benoist (Harvard University, Boston, MA).
Phenotypic analysis of CD4+ T-cells.
T-cells were stained with a variety of fluorochrome-conjugated or biotinylated monoclonal antibodies (mAbs), as previously described (22,23): anti-CD4 (clone RM5), anti-CD25 (clone PC61.5) (eBioscience, San Diego, CA), anti-Vβ4 (clone CTVB4), anti-CD69 (clone H1.2F3), anti-CD44 (clone IM7), anti-CD62L (clone MEL14) (BD Bioscience, Mississauga, ON, Canada), and anti-Foxp3 (eBioscience). Stained cells were acquired on a FACSCalibur flow cytometer (BD, San Jose, CA). In adoptive transfer experiments, pancreata were digested with collagenase-V (Invitrogen, Burlington, ON, Canada), and T-cells were separated from the digested tissue by centrifugation on a Lympholyte-M gradient (Cederlane) and then stained accordingly.
Purification of CD4+ T-cell subsets.
Intracellular cytokine production.
Cells from the spleen and pancLN were stimulated 4–5 h with PMA and ionomycin and were treated with monensin (2 μmol/l) for the last 2–3 h of culture. After surface staining, cells were fixed and permeabilized, and then intracytoplasmic staining was performed using anti-mouse IL-2 mAb (JES6–5H4), anti-mouse tumor necrosis factor-α (TNF-α) mAb (MP6), anti-mouse IL-17 (17B7), or phycoerythrin-labeled rat IgG1 isotype control (eBioscience). Stained cells were analyzed on a FACSCalibur flow cytometer (Becton Dickinson) using CellQuest software.
Adoptive cell transfers.
Purified CD4+ T-cell subsets were transferred intravenously, either alone or in combination, into recipient mice (1–3 × 105/mouse). In some experiments, donor T-cells were labeled with carboxyfluorescein succinimidyl ester (CFSE) to assess cell proliferation in vivo, as previously shown (24).
Diagnosis of diabetes.
Recipient mice were tested every 2–3 days for diabetes, as previously described (25). Overt diabetes was defined as glycemia >300 mg/dl. Hemoglukotest kits were provided by Roche Diagnostics (Laval, Canada).
Hematoxylin-eosin (H-E)-stained histological slides of pancreatic tissues were prepared and graded for insulitis as previously described (14,25,26). Three randomly obtained levels of pancreas were analyzed in double-blind fashion, and 12–15 islets per section were scored per time point. Statistical analysis was performed with the χ2 test.
Ki-67 cell proliferation analysis.
Pancreata were harvested, washed with PBS, and treated with a digestion buffer (Hanks’ balanced salt solution [HBSS], 1 mg/ml collagenase V, and 1 μg/ml DNase I) for 25 min at 37°C. The digestion was quenched with fetal bovine serum (Invitrogen). Cell suspensions were then extensively washed in HBSS, resuspended in nonenzymatic dissociation solution (Invitrogen), and incubated for 10 min at 37°C. The cells were resuspended in PBS, stained with Pe-Cy7-conjugated CD4 and antigen-presenting cell (APC)-conjugated CD25 mAbs, and then stained intracellularly with phycoeryhrin-conjugated Foxp3 and fluorescein isothiocyanate-conjugated Ki-67 (clone B56) (BD Bioscience). Data were analyzed on a FACSCalibur flow cytometer (BD) using the Flowjo software.
Normal thymic CD4+ nTreg cell frequency and function in NOD mice.
Some reports suggested that the ability of nTreg cells to suppress type 1 diabetes is either absent or poorly developed, thus promoting disease onset (14,20,21). To determine whether NOD mice developed CD4+Foxp3+ nTreg cells in their thymus, the frequency of thymic CD4+Foxp3+ nTreg cells was assessed in CD4SP T-cells in pre-diabetic NOD and BDC2.5 mice relative to type 1 diabetes–resistant C57BL/6 (B6) mice. We show that the proportion of CD4+Foxp3+ nTreg cells represents ∼1 and 5% of CD4SP T-cells in BDC2.5 and NOD mice, respectively, and does not decrease in adult mice (Fig. 1A). Total numbers of CD4+Foxp3− or CD4+Foxp3+ T-cells were stable between 10 and 25 days of age and seemingly increased between 25 to 50 days of age in NOD mice only to reach levels not significantly different from C57BL/6 mice (data not shown). No differences were observed in CD25+ or CD25− Foxp3+ Treg cells from thymi of pre-diabetic mice compared with C57BL/6 mice (data not shown).
To assess NOD thymic Treg cell function, we transferred 5 × 105 thymic CD4+CD25− Teff cells in the presence or absence of 5 × 104 thymic CD4+CD25+ nTreg cells from neonatal NOD mice into NOD.scid recipient mice. CD4+CD25− Teff cells from pre-diabetic mice transferred diabetes at a 50% incidence 5–6 weeks after transfer, reaching a total incidence of 80–90% at 10 weeks after transfer (Fig. 1B). However, when thymic CD4+CD25+ nTreg cells were co-transferred, type 1 diabetes onset and incidence were dramatically reduced (Fig. 1B) and β-islet cells were preserved (data not shown), suggesting that thymic nTreg cells, which are devoid of potential peripherally activated CD25+ Teff cells, are protective in NOD mice. To further assess thymic nTreg function in a synchronous, antigen-specific system, similar experiments were performed in the BDC2.5 TCR transgenic transfer model of type 1 diabetes (26). To this end, we transferred thymic CD4+CD25− Teff cells from pre-diabetic, 3- to 4-week-old BDC2.5 mice, either alone or in the presence of thymic CD4+CD25+ Treg cells, into NOD.TCRα−/− recipients. Thymic CD4+CD25− Teff cells transferred diabetes as early as 11 days after transfer, with an incidence as high as 60–75% at 30 days after transfer, and correlated with a prominent infiltration of mononuclear cells into the islets and general loss of islet morphology (Fig. 1B–D). In contrast, the presence of thymic nTreg cells in transferred CD4+ T-cells completely abrogated the onset of disease and protected from type 1 diabetes for at least 3–4 weeks after T-cell transfer (Fig. 1B). Although not completely preventing the immune cell infiltration of the islets, nTreg cells nonetheless reduced the severity of insulitis with a greater proportion of islets with preserved morphology (Fig. 1C–D). Thus, nTreg cells develop in the NOD thymus and can control the onset of type 1 diabetes.
Peripheral CD4+ nTreg cells maintain tolerance to β-islet cells in pre-diabetic NOD mice.
We hypothesized that alterations in the numbers or function of peripheral Treg cells may precipitate type 1 diabetes. We first enumerated Foxp3+ T-cells in lymph nodes and spleen of pre-diabetic NOD and BDC2.5 mice of various ages. In NOD mice, Foxp3+ Treg cell represented an average of 16–20% of CD4+ T-cells in the spleen and 7–10% in the pancLN with variations between ages not significantly different from type 1 diabetes–resistant B6 mice (Fig. 2A). Similarly, nTreg cells accounted for 9–11% and 4–7% of CD4+ T-cells from spleen and pancLN BDC2.5 mice, respectively (Fig. 2C). The total pool of nTreg cells in pancreatic sites gradually increased with age in both NOD and BDC2.5 mice but was comparable with age-matched B6 mice (Fig. 2B; data not shown), suggesting that a quantitative defect in peripheral Treg cells does not precede type 1 diabetes onset.
We then assessed the function of nTreg cells from neonatal NOD mice (7–10 days old). To this end, we transferred 5 × 105 CD4+CD25− Teff cells from pre-diabetic or diabetic mice into NOD.scid mice either alone or in the presence of titrated doses of CD4+CD25+ nTreg cells from neonatal, pre-diabetic NOD mice. CD4+CD25− T-cells transferred diabetes at a 50% incidence 5–6 weeks after transfer, reaching an incidence of 80–90% at 10 weeks after transfer (Fig. 2D), and correlated with a prominent infiltration of mononuclear cells into the islets and general loss of islet morphology (Fig. 2E and F). However, when CD4+CD25+ nTreg cells were cotransferred with either pre-diabetic (Fig. 2D, left panel) or diabetic (Fig. 2D, right panel) CD4+CD25− Teff cells, type 1 diabetes onset and incidence were dramatically reduced at every nTreg-to-Teff ratio transferred. Moreover, the severity of insulitis was greatly reduced, with a greater proportion of β-islets demonstrating preserved morphology (Fig. 2E and F), albeit to a lesser extent than thymic nTreg cells. Transfer of CD4+CD25+ T-cells from diabetic mice with pre-diabetic or diabetic CD4+CD25− Teff cells did not prevent type 1 diabetes onset and incidence (data not shown). Therefore, neonatal nTreg cells are functional and have the potential of protecting NOD mice from primary and established type 1 diabetes.
Temporal decline in the function of peripheral CD4+Foxp3+ nTreg cells in BDC2.5 mice.
We reasoned that functional changes in the periphery might nevertheless disrupt nTreg function and precipitate type 1 diabetes over time. We evaluated the potential for an age-dependent variation in nTreg cell suppressor function in our BDC2.5 transfer model. CD4+CD25+ Treg cells were isolated from young or adult BDC2.5 mice and cotransferred with CD4+CD25− Teff cells from BDC2.5 mice into NOD.TCRα−/− mice. We show that whereas nTreg cells from young BDC2.5 mice completely suppressed type 1 diabetes induced by young or old Teff cells (Fig. 3A), nTreg cells isolated from older or overtly diabetic BDC2.5 animals were completely ineffective at suppressing type 1 diabetes induced by young or old Teff cells (Fig. 3A). It was possible that the lack of regulation of type 1 diabetes was due to the increased frequency of activated T-cells among CD4+ T-cells. The expression of CD25, CD69, CD44hi, or CD62Llo activation markers on T-cells was not significantly different between 3- and 4-week-old and 6- and 8-week-old donors, and the reduced ability of old Treg cells to control type 1 diabetes was not due to a reduced proportion of Foxp3+ T-cells (Fig. 2C) or to an increased cellular frequency of activated or TNF-α–secreting Foxp3− T-cells, suggesting that the waning nTreg function in older donors was not attributable to an increased contamination of activated Teff cells within Treg preparations (Figs. 2C and 3B and C). CD4+CD25+ T-cells from overtly diabetic donor mice were completely inefficient in controlling the onset of type 1 diabetes in our transfer system (Fig. 3A). Furthermore, recipients of 6- to 8-week-old donor CD4+CD25− T-cells did not demonstrate a higher diabetes incidence compared with 3- to 4-week-old donor cells, excluding the possibility of reduced pathogenicity of younger Teff cells (Fig. 3A). Interestingly, regulation of type 1 diabetes could be restored if an additional bolus of purified CD4+CD25+ Treg cells from 6- to 8-week-old donor mice were infused in recipient mice (data not shown), suggesting that an increase in the circulating pool of nTreg cells could ultimately control type 1 diabetes in older NOD mice. Our results do not exclude the possibility that alterations in the pathogenic potential of Teff cells from older mice may also contribute to type 1 diabetes onset by heightening their resistance to regulation. Overall, these results suggest that while nTreg cells function normally in the periphery of neonatal NOD mice, the progression beyond insulitis is seemingly due to a time-dependent waning in the functional potency of Treg cells, allowing self-reactive Teff cells to escape regulation and initiate a destructive infiltration of the islets.
CD4+ nTreg cells do not affect antigen-induced priming of diabetogenic CD4+ T-cells in lymphopenic and nonlymphopenic hosts.
The functional impact of nTreg cells on diabetogenic T-cells during the pre-diabetic period is not thoroughly understood. CD4+ Treg cells may potentially control diabetes progression by inhibiting the activation or clonal expansion of islet-specific CD4+ T-cells in the pancLN, altering Teff cell trafficking to the pancreas, or suppressing effector functions in situ (18,19,27). To determine whether nTreg cells influence the priming of diabetogenic T-cells, NOD.TCRα−/− recipient mice were transferred intravenously with 106 CFSE-labeled CD4+CD25− Teff cells in the presence or absence of 105 CD4+CD25+ isolated by fluorescence-activated cell sorter (FACS) from peripheral lymph nodes of 2- to 4-week-old BDC2.5 mice. At 3 days after transfer, we examined the frequency of activated and proliferating islet-specific CD4+ T-cells in pancLN and nondraining lymph nodes by FACS and in the presence or absence of nTreg cells. In this system, transferred CD4+ Teff cells migrate to the pancLN (Fig. 4A), and the activation of CD4+Vβ4+ Teff cells (CD69 early marker expression) (Fig. 4B; 22–26 vs. 3% in nondraining lymph nodes), as well as the frequency of proliferating T-cells (CFSE dilution) (Fig. 4C; 32–35 vs. 7% in nondraining lymph nodes), could only be detected in pancLN, confirming that T-cell activation is antigen specific. The frequency of CD4+Vβ4+CD69+ T-cells (21.6 vs. 26%) and the frequency of dividing CD4+Vβ4+ T-cells (32.4 vs. 35.2%) in the pancLN remained unaffected in the presence of nTreg cells compared with recipient mice transferred with Teff cells alone, suggesting that antigen presentation and proximal TCR signals are not inhibited in Treg cell-protected mice (Fig. 4A).
Previous studies have suggested that lymphopenic environments might impair the functional dissection of the mechanism of nTreg cells. To circumvent the possible confounding effect of homeostatic proliferation in our system, similar transfer experiments were also conducted in nonlymphopenic, wild-type NOD recipient mice. Our results show that the frequency of CD4+Vβ4+ T-cells (0.41 vs. 0.71%) and Ag-driven proliferation of Teff cells in the pancLN were unaffected by the presence of BDC2.5 Treg cells (53.1 vs. 53.1%) (Fig. 4D and E), suggesting that Treg cells do not control the diabetogenicity of Teff by altering their homing to draining pancLN, early activation, or antigen-driven proliferation. However, the frequency of TNF-α and IL-17 secreting CD4+ T-cells in pancLN was significantly suppressed in the presence of BDC2.5 Treg cells, suggesting that while priming and expansion of CD4+ Teff cell remain unaffected, the differentiation of proinflammatory cytokine producing diabetogenic T-cells is suppressed by Treg cells (Fig. 4F).
Protection from type 1 diabetes correlates with an increased expansion of CD4+Foxp3+ nTreg cells in pancreatic sites.
Recent studies show that nTreg cells localize in sites of inflammation to mediate their protective effect (16,18,19,27). To determine whether nTreg cells home to and expand within pancLN, NOD.TCRα−/− or wild-type NOD recipient mice were transferred intravenously with 105 CFSE-labeled CD4+CD25+ nTreg cells in the presence of 106 CD4+CD25− Teff cells isolated from 2- to 4-week-old BDC2.5 mice, and the frequency of proliferating Treg cells in pancLN and nondraining lymph nodes was examined 3 days after T-cell transfer. The frequency of BDC2.5 Foxp3+ nTreg cells in recipients of CD25+/CD25− cells was found to be similar in the pancLN and the nondraining lymph nodes of both recipients (Fig. 5A and C). The proportion of proliferating Foxp3+ nTreg cells was significantly greater in pancLN than in nondraining lymph nodes in immunodeficient (43 vs. 13%, respectively; Fig. 5B) and immunocompetent (52.7 vs. 20.5%, respectively; Fig. 5D) hosts, suggesting that nTreg cells actively expand in pancreatic sites.
Because altered early T-cell priming events cannot directly explain the protective effect of nTreg cells, we then hypothesized that nTreg cells accumulating in pancreatic sites may have a functional impact on Teff cells at later stages of the diabetogenic process. To this end, CD4+ T-cell subsets from thymus of BDC2.5 mice were adoptively transferred into NOD.TCRα−/− mice as described above, and 30 days after transfer, the frequency of CD4+Foxp3− Teff and CD4+Foxp3+ nTreg cells was examined in various lymph nodes and pancreas. Although our results do not show an influence of nTreg cells on the initial rounds of proliferation of diabetogenic T-cells within pancLN (Figs. 4 and 5), our results indicate that the absence of CD4+ nTreg cells favors the infiltration/expansion of Teff cells in spleen (data not shown), pancLN, and pancreas, with a two- to threefold increase in the number of CD4+Vβ4+ T-cells in these sites compared with recipient mice cotransferred with Teff and nTreg cells (Fig. 6A). This increased accumulation of Teff cells correlated with the onset of diabetes and may suggest a role for nTreg cells in the control of antigen-driven recruitment or proliferation of Teff cells in pancreatic sites. Our result show that protection from type 1 diabetes correlates with a significantly increased proportion of Foxp3+ nTreg cells in these sites and mirrors the decline in Teff cell numbers, suggesting that a crucial part of the protective role of nTreg cells resides in their ability to migrate to or expand in sites of autoimmune attack (Fig. 6A). Although not entirely preventing the infiltration of Foxp3− Teff cells into the islets, nTreg cells nonetheless reduced the severity of insulitis, with a greater proportion of islets with preserved morphology (Fig. 2E and F). Interestingly, examination of the few recipient mice receiving only CD4+CD25− Teff cells and that never developed type 1 diabetes revealed that the proportion of Foxp3+ cells was similar to that observed in recipient mice co-injected with thymic nTreg cells, suggesting that this protective Foxp3+ T-cell subset originated from the thymic CD25− T-cell donor fraction, possibly expanded/differentiated within these sites, and ultimately ensured disease protection (Fig. 6A).
Because young and adult nTreg cells differed in their protective effect, we then compared the differential ability of Foxp3+ nTreg cells from 3- to 4-week-old or 6- to 8-week-old donors to accumulate in inflamed pancreatic sites. In recipients of 3- to 4-week-old cells, the frequency of nTreg cells in pancLN and pancreas was significantly greater than recipients receiving Teff alone and correlated with type 1 diabetes protection (Fig. 6B and C). In stark contrast, nTreg cells from 6- to 8-week-old donor mice accumulated less efficiently in the pancLN and pancreas and correlated with the onset of type 1 diabetes in the majority of recipient mice (Fig. 6B and C). In the few type 1 diabetes–free mice receiving nTreg cells from 6- to 8-week-old donor mice, we observed that the frequency of nTreg cells in the pancreas was comparable with recipient mice receiving 3- to 4-week-old nTreg cells (Fig. 6C).
We were unable to find any difference in the homing of nTreg and Teff cells in pancLN over time (data not shown). To directly determine whether this reduced frequency of BDC2.5 Treg cells in pancreatic sites was due to a loss of nTreg cell expansion in these sites over time, we examined by FACS the cellular frequency of cycling CD4+Foxp3+ T-cells in the periphery of young and old BDC2.5 mice, as determined by the Ki-67 proliferation marker (Fig. 6D and E). We show that the total proportion of cycling Foxp3+ Treg cells and the fraction of cycling cells among the total pool of CD4+Foxp3+ nTreg cells are significantly reduced in pancLNs and pancreata of old BDC2.5 mice compared with young mice (Fig. 6D and E). Collectively, our results show that nTreg cells expand within inflamed pancreatic sites where they constrict the size of the Teff cell pool and reduce the histopathological consequences of a destructive infiltration. Thus, a possible waning with age of nTreg expansion within pancreatic sites could explain the onset of an uncontrolled Teff cell infiltration of the pancreas and type 1 diabetes induction.
Foxp3+ nTreg cells have been implicated as a central control point in type 1 diabetes progression, and defects in their development or function may represent a major predisposing factor for spontaneous autoimmunity in NOD mice (2,28,29). Here, we show that thymic and peripheral CD4+CD25+ nTreg cells can suppress disease in both normal NOD and BDC2.5 antigen-specific model of type 1 diabetes. We also show that CD4+CD25+Foxp3+ nTreg cells do not affect the priming of antigen-specific Teff cells in pancLN but localize within insulitic lesions, where they suppress the infiltration of Teff cells. The cellular potency of nTreg cells, although fully operative in neonatal mice, declines with age despite a stable cellular frequency of Foxp3+ nTreg cells in primary and secondary lymphoid tissues.
Recent studies stipulate that defective or reduced CD4+CD25+ T-cell frequencies in autoimmune-prone hosts, including NOD mice, represent the primary predisposing factor to spontaneous autoimmunity(20,21,30,31). In most studies, the CD25 surface marker is frequently used for the monitoring of nTreg cell frequencies, albeit at a time point when pancreatic inflammation is well engaged. CD25 is an unreliable marker because activated CD4+ T-cells upregulate CD25, thus precluding its use as a tracking biomarker of nTreg cells in NOD mice. We observed that Foxp3+CD4+ nTreg cells, irrespective of CD25 expression, represent a stable pool in thymocytes, lymph nodes, or spleen of neonatal and adult NOD mice and are comparable with type 1 diabetes–resistant C57BL/6 mice, thus refuting the view that NOD mice have a developmental defect in nTreg cells. However, it is possible that a functional deficiency in nTreg cells may not be visible as a sudden decline in the frequency of these cells in peripheral tissues and may conceivably be resultant to gaps in the TCR repertoire or gene polymorphisms modulating various effector functions (32). Consistently, we show that nTreg cell function wanes with time as evidenced by their inability to expand efficiently and to prevent Teff cell infiltration in pancreatic sites, suggesting that the loss of Ag-driven homing or expansion of nTreg cells in pancreatic environments may represent an essential checkpoint in the progression to type 1 diabetes. We cannot exclude the possibility that time-dependent changes in Teff cells may contribute to type 1 diabetes onset (33).
An unresolved question relates to the location of nTreg cell-mediated tolerance induction in vivo. The ability of nTreg cells to localize within tissues to dampen the magnitude of Teff cell responses and prevent the histopathological consequences has been observed in models of infectious disease, inflammatory bowel disease, and tumors (34,35). Our results show that nTreg preferentially home and expand within inflamed pancLN and islets of type 1 diabetes–protected mice and that nTreg cells control the effector functions of infiltrating diabetogenic CD4+ T-cells in these sites, albeit not completely preventing insulitis. Interestingly, a more significant reduction in the degree of insulitis was observed with thymic nTreg cells compared with peripheral nTreg cells (Fig. 1D vs. 2F), suggesting that an increased functional potency may exist in the thymic nTreg cell compartment, as well as a potential waning of this functional potency in peripheral nTreg cells (21,29). It is conceivable that the thymic microenvironment provides the necessary developmental and homeostatic signals that may be lacking in the periphery of NOD mice. One study showed that pancreatic BDC2.5 CD4+CD25+ Treg cells abrogated disease induced by pancreatic CD4+CD25− Teff cells in NOD.scid recipients, and the majority of Treg cells were actively suppressing in the pancreas, rather than affecting the initial priming of the autoreactive T-cells in the pancLN (19). Similarly, in vitro expanded BDC2.5 CD4+CD25+ Treg cells suppressed type 1 diabetes induced after transfer of diabetic NOD splenocytes into NOD.scid recipients, despite the fact that insulitis was nonetheless apparent in protected mice (36,37). Interestingly, the gene expression profile of islet-infiltrating nTreg cells differ from nTreg cells residing in the pancLN, suggesting that the target tissue engages unique transcriptional programs in nTreg cells, which might relate to their regulation in these sites (19). It is unclear whether these distinct nTreg gene signatures occur as a result of their tissue localization or as a consequence of their own suppression. Target organs may confer unique regulatory pressures on infiltrating Teff cells and may shape the type of regulation needed for disease resolution. Alternatively, different Foxp3+ nTreg cell subsets may exist to operate in tissue-specific fashions, or chemokine receptors like CCR5 or CCR6 may endow nTreg cells with a competitive advantage to enter more efficiently in pancLN (38–40).
CD4+ nTreg cells may potentially suppress anti-islet T-cell responses by affecting their activation and clonal expansion in pancLN. In our system, the initial activation of islet-specific CD4+ T-cells is unaffected in the presence of nTreg cells, suggesting that antigen presentation and TCR signals are not inhibited by nTreg cells in draining pancLN, a finding consistent with those from Chen et al. (19). This is also in agreement with our observation that nTreg cells suppress type 1 diabetes mediated by diabetic T-cells, which likely traffic directly to islets, circumventing priming in the pancLN. In addition, we were unable to detect any changes in the frequency of proliferating diabetogenic CD4+ T-cells in the pancLN, either in the presence or absence of nTreg cells, suggesting that antigen-induced priming of autoreactive T-cells is not directly affected by nTreg cells. Paradoxically, the absolute number of Teff cells in pancreatic sites is dramatically increased in the absence of nTreg cells, suggesting that nTreg cells may restrain T-cell clonal expansion, survival, or homing at later events of diabetogenesis. Consistently, transfer of BDC2.5 T-cells into thymectomized NOD.B7–2−/− recipients in conjunction with in vivo depletion of CD25+ T-cells resulted in an increased accumulation of Teff cells in the pancLN compared with control mice (41). Moreover, the cellular frequency of TNF-α–and IL-17–secreting antigen-specific T-cells is significantly suppressed in the functional presence of CD4+Foxp3+ Treg cells, suggesting that although Treg cells remain unable to suppress the activation and priming of diabetogenic T-cells, they remain fully capable to inhibit the differentiation of Teff cells in sites. Interestingly, Treg cells were shown to control the pathogenicity of islet-specific, CD8+ Teff cells by inhibiting dendritic cell maturation in the pancLN (42), and in vitro expanded BDC2.5 Treg cells have been shown to stably interact with dendritic cells and potentially disrupt BDC2.5 T/dendritic cell cellular interactions in pancLN, suggesting that nTreg/APC interactions may be, in part, responsible for nTreg cell-mediated protection (27). Lastly, we cannot formally exclude a more subtle effect on Teff cells in these sites such that Teff cells are now imprinted with a reduced pathogenic potential, which would reveal itself once they traffic to islets.
A recent study by Chen et al. (19) used NOD mice harboring the scurfy mutation of the foxp3 gene (FoxP3sf) to examine the functional role of nTreg cells in type 1 diabetes. Because the mutation of FoxP3 impairs the development of nTreg cells, NOD.FoxP3sf displayed a significantly advanced onset of type 1 diabetes compared with normal NOD mice, implying a role for Foxp3+ nTreg cells in the control of type 1 diabetes pathogenesis. However, this study did not address the possibility that the injection of wild-type, antigen-specific Treg cells, while rescuing Foxp3-deficient NOD mice from the early onset of type 1 diabetes, were compensating for the primary deficit in nTreg cells believed to exist in these mice or whether such injection was actually suppressing the global inflammation that likely arose as a secondary consequence of Foxp3 deficiency. Although the NOD.FoxP3sf model provides a system devoid of nTreg cells, the Scurfy mice also possess multiple immune defects, which likely have consequences on the physiopathology of type 1 diabetes. Chang et al. (43) have shown that a T-cell extrinsic defect may contribute to the Scurfy and IPEX syndrome because the Foxp3sf mutation in nonhematopoietic, thymic stromal cell leads to an ErbB2-dependent defective thymopoiesis. Furthermore, it is unclear whether NOD.FoxP3sf mice possess aberrant antigen presentation or costimulation, which could combine to reduce the activation thresholds for Foxp3sf Teff cells and render them resistant to suppression (44,45).
In conclusion, nTreg cells represent a master switch regulating disease onset and progression in NOD mice because abrogation of nTreg function can break T-cell tolerance to β-islet antigens. Despite nTreg cells actively suppressing anti-islet T-cell responses in the neonatal immune system, this suppression is ultimately insufficient to maintain tolerance to pancreatic antigens because autoimmunity ultimately ensues in these mice, as shown in recent studies (46,47). These studies may provide insights into the cellular basis of type 1 diabetes susceptibility and may lead to the development of novel approaches to potentiate nTreg activity in autoimmune-prone hosts.
Published ahead of print at http://diabetes.diabetesjournals.org on October 2007. DOI: 10.2337/db06-1700.
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.
M.T., E.S., and E.d'H. have received fellowships from the Canadian Institutes for Health Research Training Grant in Neuroinflammation. C.A.P. has received a Canada Research Chair. This study has been supported by Canadian Institutes for Health Research Grant CIHR MOP 67211 and Canadian Diabetes Association Grant GA-3-05-1898-CP.
We thank Michal Pyzik, Ekaterina Yurchenko, and Valerie Hay for advice and technical assistance.