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CUTTING EDGE |


* Laboratory of Immunology and Hematopoiesis, Department of Pathobiology; Purdue Cancer Center; Bindley Bioscience Center; and Biochemistry and Molecular Biology Program, Purdue University, West Lafayette, IN 47907;
Sagamore Surgical Center, Lafayette, IN 47909; and
Nuffield Department of Clinical Laboratory Sciences, University of Oxford, Oxford, United Kingdom
| Abstract |
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| Introduction |
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It has been unknown whether Foxp3+ Tregs can directly suppress B cells to suppress Ig production or whether they have to suppress Th cells to indirectly suppress the B cell response. In this report, we provide evidence that Foxp3+ Tregs are present in B cell areas where T-B cell interaction and humoral immune responses are believed to occur, and that they can directly suppress B cell Ig production and CSR without having to suppress Th cells.
| Materials and Methods |
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Frozen sections of tonsils were acetone-fixed and stained by polyclonal goat anti-Foxp3 (Novus Biologicals) or monoclonal anti-hFoxp3 mIgG1 Ab (236A/E7; Banham laboratory or Abcam) (15). Anti-goat/mouse-biotin and the VECTASTAIN ABC kit (Vector Laboratories) were used to assess the expression of Foxp3. Frozen sections were separately stained by anti-IgD-FITC or with anti-CD4-FITC (Caltag Laboratories) and then with anti-FITC-AP to visualize follicles and T cell areas, respectively. For confocal microscopy, the frozen tonsil sections were stained with anti-IgD, anti-CD3, and anti-Foxp3 as described previously (16). For flow cytometry analysis, tonsil CD4+ T cells were first stained with surface Abs (anti-CD4, anti-CD25, and anti-CD69), fixed, permeabilized, and then stained with anti-hFoxp3 (PCH101; eBioscience).
Cell isolation, cell culture, and ELISA
CD57CD69CD25+CD4+ Tregs (purity >95%) were isolated by multistep magnetic sorting from human tonsil mononuclear cells as described previously (14). Naive CD19+IgD+CD38 tonsil B cells (purity >99%) and CD19+CD38+ GC-B cells (purity >95%) were isolated as described previously (17). Naive B cells were activated for 2 h at 4°C with Sepharose-conjugated rabbit Ab to human Ig µ-chain and rabbit Ab to human Ig (H + L) chain (Irvine Scientific; mixed 1:1 at 2 µg/ml). Indicated numbers of Tregs were cocultured with B cells in the presence of staphylococcal enterotoxin B (SEB; 1 µg/ml; Sigma-Aldrich) in a 5% CO2 incubator at 37°C for 5 days. For T cell-free B cell stimulation, anti-CD40 (5 µg/ml; BD Pharmingen) and cytokines (0.25 ng/ml IL-2, 20 ng/ml IL-4, 40 ng/ml IL-10; R&D Systems) were used. Concentrations of Igs and CXCL13 in the supernatants were determined by ELISA as previously described (14, 16). When indicated, neutralizing Abs were added at following concentrations: anti-TGF-
1 (R&D Systems; clone 9016.2, 10 µg/ml), anti-CTLA-4 (BD Pharmingen; clone BNI3, 10 µg/ml), and IgG1 isotype Ab (R&D Systems; clone 11711.11, 10 µg/ml).
Analyses of activation-induced cytosine deaminase (AID), Ig-productive and circle transcripts, and switch circles
AID, Ig transcripts, and
-actin were amplified by PCR as previously described (17). Circle transcript analysis was performed using specific primer sets designed to detect only circle transcripts but not productive or germline transcripts (18). Digestion circularization (DC)-PCR was performed as previously described using PCR primers for
3 and
1/2 (19).
| Results and Discussion |
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Foxp3 is a transcriptional repressor (20) mutated in scurfy mice that lack conventional CD4+CD25+ Tregs (21) and in human immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome patients with autoimmune diseases (22). Foxp3 is specifically expressed by T cells with regulatory functions (23) and therefore is an appropriate Ag marker for Tregs. To determine the distribution of Foxp3+ cells in T-zone and B cell areas, we performed immunohistochemistry on human tonsil sections using anti-Foxp3 Abs (Fig. 1A). We also separately stained serial tonsil sections with anti-IgD or anti-CD4 to identify T-zone, B follicle, and T-B border areas. The majority of Foxp3+ T cells were localized in the T cell zone. It was notable that quite a few Foxp3+ cells were found in T-B border areas including mantle zones (a cell layer enriched with IgD+ B lymphocytes surrounding the GCs of lymphoid follicles). Clearly, small but significant numbers of Foxp3+ T cells were seen within the GCs. The presence of Foxp3+ T cells in the T-B border areas was interesting, and we used a three-color confocal microscopy technique to visualize the presence of Foxp3+ cells along with IgD+ B cells and T cells in the T-B border areas. All Foxp3+ cells were T cells (CD3+), and some Foxp3+ T cells were in contact with T as well as IgD+ B cells. When normalized for CD3+ T cell numbers, the frequency of Foxp3+ T cells in T-B border areas exceeded that of T cell or GC areas (Fig. 1B).
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A human CD57+ GC-Th cell subset is specifically present within GCs, and they are highly efficient T helpers in stimulating B cells for production of Igs (3, 17). Upon Ag priming, certain Tregs with the CD4+CD25+CD69 phenotype up-regulate the B cell area homing chemokine receptor CXCR5 and can suppress GC-Th cell-stimulated B cell Ig production (14). However, it is unknown whether these Tregs can suppress Th cells to suppress the B cell response and/or whether they can directly act on B cells to suppress the B cell response. To address this issue, we determined the suppressive capacity of Tregs on T vs B cells separately from each other (Fig. 2, AC). The following two indicators were used to assess the T vs B cell responses: CXCL13 production (a GC-Th cell response) (16, 24) and production of IgG and IgA (B cell responses). Tregs were able to suppress the B cell response induced in a T cell-free activation condition (anti-CD40, IL-2, IL-4, and IL-10) (Fig. 2B). Tregs were also able to suppress the T cell response (GC-Th cell production of CXCL13) induced by phytohemagglutinin (PHA) in a B cell-free activation condition (Fig. 2C). These results suggest that Tregs can directly suppress B cells in addition to T cells.
Generally, Tregs need cell-cell contact with target cells to suppress them. When the Tregs were separated from B cells, B cell response was not suppressed (Fig. 2D), suggesting that the direct B cell suppression activity of Tregs requires cell-cell contact between B cells and Tregs. Although controversial, TGF-
1 and CTLA4 are implicated in Treg suppression of non-Tregs. We used neutralizing Abs to block these molecules in culture. Abs to TGF-
1 and CTLA4 had partial (
30%) blocking effects (Fig. 2E), suggesting possible involvement of these molecules as well as other unidentified molecules in Treg suppression of B cells.
Tregs can directly suppress B cell CSR
We then determined whether Tregs can suppress AID expression, expression of productive Ig transcripts and circle transcripts, and appearance of switch circles to assess their ability to regulate Ig class switch in a T cell-free activation condition (Fig. 3). Tregs were able to suppress the expression of AID, a key enzyme involved in CSR and affinity maturation, in a T cell-free activation condition (Fig. 3A). Tregs suppressed the appearance of
3 and
1/2 switch circles (Fig. 3B), by-products of IgG3 and IgA CSR, respectively, determined by a DC-PCR method. Tregs suppressed the expression of productive transcripts for IgG14 and IgA12 (Fig. 3C) and I
-Cµ and I
-Cµ circle transcripts induced by anti-CD40, IL-2, IL-4, and IL-10 (Fig. 3D). We also investigated whether Tregs can suppress the above-mentioned B cell Ig production events when B cells are stimulated by GC-Th cells. Tregs suppressed generation of switch circles, expression of productive transcripts, and expression of circular transcripts induced by GC-Th cells (Fig. 4). These results suggest that B cell Ig CSR is severely suppressed in the presence of Tregs.
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Through the study, we addressed several important issues regarding Treg suppression of B cell responses. First, our results show that Foxp3+ T cells exist in the T-B area borders and within GCs, the areas where B cells interact with Th cells and undergo Ig production processes. This data is consistent with the fact that CD4+CD25+CD69 Tregs, upon TCR activation, rapidly up-regulate the B cell area homing receptor CXCR5 but down-regulate T cell area chemokine receptor CCR7 (14). Second, it has been unknown whether Foxp3+ Tregs can directly suppress B cells or whether they have to suppress T cells to indirectly inhibit B cell response. We showed evidence that Tregs can directly suppress B cells without needing to suppress Th cells. Third, the direct suppression of B cells by Tregs was accompanied by reduced Ig CSR. Our results suggest that these follicular Tregs can directly suppress B cells independently of Th cells for effective suppression of B cell response. They can also directly suppress Th cells. This versatile suppressive capacity of Tregs is thought to be important to efficiently limit B cell responses and to maintain immune tolerance in Ig response. Our findings are consistent with the fact that Treg number or activity is significantly decreased in a number of autoimmune or infectious diseases with aberrant Ig responses (7, 8, 9).
| Acknowledgments |
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| Disclosures |
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| Footnotes |
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1 This study has been supported, in part, from grants from Leukemia and Lymphoma Society, Sidney Kimmel Foundation for Cancer Research, American Heart Association, American Cancer Society, and National Institutes of Health-National Institute of Allergy and Infectious Diseases (AI063064) to C.H.K. A.H.B. has been supported from Leukaemia Research Fund. ![]()
2 Address correspondence and reprint requests to Dr. Chang H. Kim, Purdue University, VPTH 126, 725 Harrison Street, West Lafayette, IN 47907. E-mail address: chkim{at}purdue.edu ![]()
3 Abbreviations used in this paper: GC, germinal center; CSR, class switch recombination; Treg, regulatory T cell; SEB, staphylococcal enterotoxin B; AID, activation-induced cytosine deaminase; DC-PCR, digestion circularization-PCR; PHA, phytohemagglutinin. ![]()
Received for publication May 11, 2005. Accepted for publication July 27, 2005.
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