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The Journal of Immunology, 2001, 166: 4884-4890.
Copyright © 2001 by The American Association of Immunologists

Unique Functions of CD11b+, CD8{alpha}+, and Double-Negative Peyer’s Patch Dendritic Cells1

Akiko Iwasaki and Brian L. Kelsall2

Immune Cell Interaction Unit, Mucosal Immunity Section, Laboratory of Clinical Investigation, National Institutes of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892


    Abstract
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
We have recently demonstrated the presence of three populations of dendritic cells (DC) in the murine Peyer’s patch. CD11b+/CD8{alpha}- (myeloid) DCs are localized in the subepithelial dome, CD11b-/CD8{alpha}+ (lymphoid) DCs in the interfollicular regions, and CD11b-/CD8{alpha}- (double-negative; DN) DCs at both sites. We now describe the presence of a novel population of intraepithelial DN DCs within the follicle-associated epithelium and demonstrate a predominance of DN DCs only in mucosal lymphoid tissues. Furthermore, we demonstrate that all DC subpopulations maintain their surface phenotype upon maturation in vitro, and secrete a distinct pattern of cytokines upon exposure to T cell and microbial stimuli. Only myeloid DCs from the PP produce high levels of IL-10 upon stimulation with soluble CD40 ligand- trimer, or Staphylococcus aureus and IFN-{gamma}. In contrast, lymphoid and DN, but not myeloid DCs, produce IL-12p70 following microbial stimulation, whereas no DC subset produces IL-12p70 in response to CD40 ligand trimer. Finally, we show that myeloid DCs from the PP are particularly capable of priming naive T cells to secrete high levels of IL-4 and IL-10, when compared with those from nonmucosal sites, while lymphoid and DN DCs from all tissues prime for IFN-{gamma} production. These findings thus suggest that DC subsets within mucosal tissues have unique immune inductive capacities.


    Introduction
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Depending largely on the nature of the applied Ags, mucosal immunization can result in the induction of immunological tolerance or active immunity. For example, low doses of soluble protein Ags given orally result in the generation of Ag-specific T cells capable of secreting TGF-{beta} and IL-10 (1). Upon Ag-specific triggering, these regulatory T cells can have Ag nonspecific suppressive effects at systemic sites. High doses of the same soluble protein Ag given orally can result in nonresponsiveness due to deletion or anergy of Ag-specific T cells (2), similar to what occurs following systemic administration of soluble proteins (3). Not all oral encounters with Ag result in tolerance, however. Local and systemic T cell priming can occur when soluble Ags are administered with an ADP-ribosylating adjuvant, such as cholera toxin, which results in a Th2-dominant response, or following infection with a mucosal pathogen, such as Salmonella typhimurium or Toxoplasma gondii, which results in a Th1-dominant response. Despite recent progress, the mechanisms by which such disparate T cell immune responses occur following the administration of orally administered Ags are still poorly understood. To address this issue, we have focused our studies on the function of dendritic cells (DCs)3 of the Peyer’s patch (PP), because PPs are the primary sites for the induction of immune responses in the intestinal mucosa and are representative of lymphoid follicles present in diffuse mucosal tissues.

It has been postulated that the DC system consists of distinct cellular subsets, which may arise from either a myeloid or lymphoid precursor (4). In the mouse, these two types of DCs have been shown to express either high levels of CD11b (myeloid) or CD8{alpha}{alpha} (lymphoid) molecules, respectively. In addition, we recently described a third subset of DCs that do not express either of these markers (double-negative (DN) DCs) (5). By immunofluorescent staining of tissue sections of PPs, we found that CD11b+/CD8{alpha}- DCs are localized in the subepithelial dome (SED), CD11b-/CD8{alpha}+ DCs in the interfollicular regions, and CD11b-/CD8{alpha}- DN DCs at both sites. Moreover, we have shown that total DC populations, defined as CD11c+/MHC class II+/B220-, isolated from PPs were capable of differentiating naive CD4+ T cells in vitro into Th cells secreting IFN-{gamma}, IL-4, and high levels of IL-10, while total DCs from the spleen primed T cells to secrete predominantly IFN-{gamma} (6). This suggested that DCs from the PP may have a unique intrinsic capacity to induce T cell responses that are particularly capable of providing help for IgA B cell differentiation or for regulating systemic immune responses via the production of IL-4 and IL-10.

In this study, we characterize the phenotype and function of myeloid, lymphoid, and DN DCs from the PP. First, we describe the presence of a unique population of intraepithelial DN DCs within the follicle-associated epithelium (FAE) of the PP. Second, using isolated cells, we show that the three DC subsets from PP and spleen express similar levels of MHC class II and costimulatory molecules. Furthermore, upon overnight in vitro stimulation, all DC subsets up-regulate expression of DEC-205, MHC class II, CD80, and CD86, whereas lineage markers are unchanged. Thus, none of the subsets seem to convert from one type into the other during in vitro maturation. Third, we demonstrate that upon stimulation in vitro, myeloid DCs from the PP have a particular capacity to produce IL-10, while lymphoid and DN DCs from PP produce predominantly IL-12 p70. The ability to secrete high levels of IL-10 by myeloid PP DC was not an attribute of myeloid DCs from other lymph nodes examined (mesenteric, axillary, inguinal, and popliteal). Fourth, we show that only myeloid DCs from the PP stimulated rapid Ag-specific T cell proliferation and the induction of cells producing high levels of IL-4 and IL-10. Myeloid DCs from the spleen induced much lower levels of proliferation, as well as IL-4 and IL-10 secretion, while all DC subpopulations induced the differentiation of T cells capable of secreting IFN-{gamma}.


    Materials and Methods
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Double immunofluorescence staining of PPs

PPs were dissected from small intestine of BALB/c, or C57BL/6 mice were frozen in OCT medium (Sakura Finetek, Torrance, CA). Sections (8 µm) were fixed in cold acetone and stained for DC markers using the tyramide amplification method (TSA-Direct kit; NEN Life Science Products, Boston MA), as previously described (5). Sections were stained with 2 µg/ml anti-CD11c or 10 µg/ml anti-I-A (M5/114 hybridoma supernatant), anti-CD11b (M1/70 hybridoma supernatant), anti-CD8{alpha} (53-6.7; BD PharMingen, San Diego, CA), anti-CD45R (RA3-6B2; BD PharMingen), or anti-CD3 (CT-CD3; Caltag Laboratories, Burlingame, CA). In Fig. 1GoF, section was stained with anti-CD11c and 2 µg/ml of biotinylated UEA I-lectin from Ulex europaeus (Sigma-Aldrich, St. Louis, MO), followed by HRP-conjugated streptavidin and detected with the Tyramide system, as previously described (5). Slides were mounted with Fluoromount G (Southern Biotechnology Associates, Birmingham, AL) and analyzed by confocal microscopy using a Leica TCS-NT/SP confocal microscope using a x63 objective with oil with zoom of 2.0x. For Fig. 1GoA–E, differential interference contrast (DIC) images were collected simultaneously with the fluorescence images using the transmitted light detector confocal laser microscope. For Fig. 1GoF, the FAE was traced using the Trace Contour filter in Adobe Photoshop (San Jose, CA).



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FIGURE 1. DN DCs in the PP within the FAE. Frozen sections of PPs were stained with Ab against CD11c (green) in combination with Abs against a, anti-I-Ad; b, anti-CD8{alpha}; c, anti-CD11b; d, anti-CD3; or e, anti-CD45R (B220; red), and the dome regions were analyzed by confocal microscopy and overlayed with the DIC images to delineate the epithelial layer. The FAE is the one-cell-thick layer between the white and blue arrows, and intraepithelial cells are found in the space between the arrows. The white arrows in each panel indicate the edge of the epithelium. The lumen of the gut is to the right of the white arrows. The basement membrane is indicated by the blue arrows in each panel. Frozen sections of PPs were also stained with anti-CD11c (green) in combination with the lectin UEA-1 (red), which binds specifically to M cells (f). An intraepithelial DN DC within the M cell pocket is indicated by the pink arrow. In this panel, the FAE is delineated by the yellow line tracing instead of the DIC imaging.

 
Flow cytometry

Flow cytometric analysis of DC subsets was conducted using either FITC-, PE-, biotin-, APC-, or CyChrome-conjugated anti-CD11c (HL3), anti-CD11b (M1/70), anti-CD8{alpha} (53-6.7), anti-I-Ad (AMS-32.1), anti-CD40 (HM40-3), anti-CD80 (1G10), anti-CD86 (GL1), anti-CD4 (L3T4), anti-DEC205 (NLDC-145), and anti-CD45R (RA3-6B2). Before staining, Fc receptors (Fc{gamma}RIII/II) were blocked using anti-mouse CD16/CD32 (2.4G2). Isotype-matched control Abs used for staining DCs included rat IgG2a, {kappa} (R35-95); rat IgG2b, {kappa} (R35-38 or A95-1); and hamster IgG (G235-2356). The above Abs were purchased from BD PharMingen, except for NLDC-145, which was purified from hybridoma supernatant, as described previously (5).

Stimulation of DCs in vitro

Six- to 8-wk-old female BALB/c mice were obtained from National Cancer Institute (Frederick, MD). DCs were prepared from spleen (SP) and PP, as previously described (5). Briefly, isolated tissue was digested with collagenase D and DNase, and incubated in the presence of EDTA to dislodge lymphoid DCs (7). Cells were incubated with anti-mouse CD11c-coated magnetic beads (Miltenyi Biotech, Auburn, CA) and selected on MACS separation columns. Cells selected on the basis of CD11c expression were then stained with PE-conjugated anti-CD11c and CyChrome-conjugated anti-B220 Ab and either FITC-labeled anti-CD8{alpha} Ab (for lymphoid DC) or FITC-labeled anti-CD11b Ab (for myeloid DC) or both (for DN DLs). Lymphoid, myeloid, and DN DCs were isolated by flow cytometric sorting performed on a FACStar sorter (Becton Dickinson, Waltham, MA). Sorted DCs were typically between 95% and 98% positive for the surface markers of interest and were all MHC class II+. FACS-purified DC subsets (2 x 105 per well) were incubated overnight in the presence of either recombinant murine CD40L trimer (10 µg/ml; Immunex, Seattle, WA) or with Staphylococcus aureus, Cowan’s (SAC) strain (0.02%; Calbiochem, La Jolla, CA) and IFN-{gamma} (100 ng/ml; BD PharMingen) in a total volume of 200 µl per well of 96-well microtiter plate. Supernatants were collected, and IL-10 and IL-12 p40, p70 levels were measured by ELISA using OptEIA set (BD PharMingen).

Stimulation of TCR transgenic T cells by DCs

Mice transgenic for a TCR that recognizes OVA323–339 peptide in the context of I-Ad (DO11.10TCR-{alpha}{beta} transgenic mice) on a BALB/c background were kindly provided by Dr. Dennis Loh (Washington University, St. Louis, MO). Spleen T cells from TCR-transgenic mice were prepared by negative selection on T cell-enrichment columns (R&D Systems, Minneapolis, MN), followed by isolation of CD4+/MEL-14+ T cells by flow cytometric sorting using FITC anti-CD4 and PE anti-lymphocyte endothelial cell adhesion molecule-1 Abs (BD PharMingen). Sorted T cell populations were typically 99% pure. In vitro T cell differentiation assays were performed as previously described (6). Briefly, primary stimulation cultures were established by coincubation of purified T cells (5 x 104 cells/well) and sorted DCs from spleen or PPs (5 x 103 cells/well) pulsed with the corresponding peptide (3 µM), and 1 ng/ml human rIL-2 (Genzyme, Cambridge, MA) in a 96-well plate at 200 µl/well for 5-7 days. T cells were restimulated with anti-CD3 and anti-CD28. Supernatants from restimulated T cells were collected for detection of IL-4 at 24 h, and IL-10 and IFN-{gamma} at 48 h. Cytokine secretion was assayed by a specific sandwich ELISA for IL-10 and IFN-{gamma} (BD PharMingen), or IL-4 (Endogen, Boston, MA). Proliferation of T cells was assayed by incorporation of [3H]thymidine during the final 8 h of a 48-h incubation.

Statistical analysis

Normally distributed continuous variable comparisons were done using the Student t test.


    Results
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Recently, we identified the presence of CD11c+/MHC class IIhigh cells in the PP that express neither CD8{alpha}+ (lymphoid) nor CD11b+ (myeloid) markers (DN DC). The DN population localizes in the dome as well as in the interfollicular region of the PP (5). In this study, we describe the presence of DN DCs within the FAE, a one-cell-thick layer covering the dome of the PP, depicted by the space between the arrows in Fig. 1Go. The DN DCs (CD11c+/MHC class II+) are clearly present within the FAE (Fig. 1Goa), and they do not express CD8{alpha} or CD11b (green cells in Fig. 1Go, b and c). Interestingly, the MHC class II molecules within these cells appear to be intracellular (Fig. 1GoA), suggesting they are immature DCs (8). Myeloid DCs (yellow cells in Fig. 1Goc) are found to localize in deeper dome areas and not within the FAE. Furthermore, the DN DCs are not intraepithelial T or B lymphocytes because they do not express CD3 or CD45R (B270), respectively (Fig. 1Go, d and e). The intraepithelial lymphocytes (red cells in Fig. 1Go, b, d, and e) are seen adjacent to the DN DCs (green cells) and sometimes can be seen to contact each other (Fig. 1God, upper right corner). Occasionally, DN intraepithelial DCs were found to be associated with M cells within the M cell pocket (Fig. 1Gof).

To determine the surface phenotype of the DN DCs, we had enriched for DC from SP and PP by magnetic selection, and CD11c+/B220- cells were analyzed by flow cytometry. For comparison of the surface phenotype of the three DC subsets, cells were further gated on the CD11b+ (myeloid), CD8{alpha}+ (lymphoid), or CD11b-/CD8{alpha}- (DN) DC subsets and analyzed for the expression of various DC markers (Fig. 2GoA). The DN DCs were found to express comparable levels of MHC class II, CD40, and costimulatory molecules (Fig. 2GoA). DCs expressing CD4 were found within the myeloid and DN populations in the spleen, but not the PP. Next, the proportions of the three DC subsets (all CD11c+/MHC class II+) in the spleen and the PP, mesenteric lymph nodes (MLN), and peripheral lymph nodes (combined axillary, popliteal, and inguinal) were assessed (Fig. 2GoB). There was a significant DN subset in the PP, constituting approximately one-third of the entire DC population in this organ. To our surprise, MLN also contained a similarly substantial proportion (30%) of the DN subset. In contrast, this DN subset constituted only a minor and indistinct population in the spleen (9%) and peripheral lymph nodes (13%).



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FIGURE 2. Surface phenotype of myeloid, lymphoid, and DN PP DCs. A, DC-enriched fractions from SP and PP were gated on CD11c+/B220- cells and either CD8{alpha}+ (lymphoid), CD11b+ (myeloid), or (CD8{alpha}-/CD11b-) (DN) populations. Surface expression of various DC markers is shown as histograms. B, Proportions of the three DC subsets were analyzed from SP, PP, MLN, and peripheral lymph nodes. CD11c+/MHC class IIhigh cells from the respective organs were analyzed for the expression of CD8{alpha} and CD11b. C, The average percentages of the three DC subsets analyzed as in B from five separate experiments are depicted. The p values are indicated for DN DCs in the SP and peripheral LN compared with PP and MLN.

 
Although the three DC subsets can be defined by the markers CD8{alpha} and CD11b on freshly isolated DCs, it was possible that these subsets represented immature and mature stages of the same lineage of DCs. To determine whether the DN DC population represents an immature form of either myeloid or lymphoid DC subsets, we examined the surface phenotype of DN DCs after in vitro stimulation with CD40L3. As we have observed previously for lymphoid and myeloid DC subsets (5), expression of DEC-205 was increased by in vitro activation of DN DCs (Fig. 3Go). In contrast, neither CD8{alpha} nor CD11b was induced upon DN DC maturation, whereas costimulatory molecules and MHC class II levels were substantially up-regulated (data not shown). Thus, the DN DCs, at least during in vitro stimulation, remain DN, and thus do not likely represent an immature form of either CD8{alpha}+ or CD11b+ DC populations. In addition, as previously reported, neither CD8{alpha}+ nor CD11b+ DCs lose expression of these lineage markers upon in vitro maturation (5), indicating that the DN subset does not appear to derive from maturation of these subsets.



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FIGURE 3. DN DCs do not express lineage markers upon maturation. FACS-sorted DN DC populations were activated in vitro in the presence of CD40L3 and assessed for CD8{alpha}, CD11b, and DEC-205 expression. The data are representative of four separate experiments.

 
Next, in an effort to understand the functional relevance of the three DC subsets, purified DC from SP and PP were stimulated in vitro either through CD40 cross-linking (to mimic a mature T cell stimulus) or with SAC and IFN-{gamma} (a microbial stimulus) (Fig. 4Go). Signaling through the CD40 molecule led to the production of IL-10 by PP DC, but not by SP DC. Specifically, only the CD11b+ myeloid DC from the PP secreted IL-10 upon CD40 cross-linking. The ability to secrete high levels of IL-10 by myeloid PP DC was not an attribute of DCs from other lymph nodes because none of the MLN nor peripheral lymph node DC subsets secreted IL-10 upon CD40L3 stimulation (data not shown). When DCs were stimulated with SAC and IFN-{gamma}, the CD11b+ PP DC secreted even higher levels of IL-10, and SP CD11b+ DCs secreted low, but detectable level of IL-10. Myeloid DC subsets from neither PP nor SP secreted IL-12 p70. In contrast, the lymphoid DCs from PP and SP secreted high levels of IL-12 p40 and p70 upon microbial stimulation, but failed to secrete IL-10. The DN PP DC secreted levels of IL-12 p70 comparable with the lymphoid PP DC in response to SAC and IFN-{gamma}, whereas the DN population from SP secreted minimal p70. No significant cytokine secretion was detected from any of the DC subsets in the absence of CD40 or SAC and IFN-{gamma} stimulation (data not shown).



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FIGURE 4. Discrete cytokine secretion from myeloid, lymphoid, and DN DCs from the PP and spleen. Highly purified DC subsets were obtained by FACS sorting CD11c+/MHC class IIhigh CD11b+(myeloid), CD8{alpha}+ (lymphoid), or CD8{alpha}-/CD11b- (DN) cells from PP and SP. DC populations were stimulated overnight with either CD40L3 or SAC + IFN-{gamma}, and supernatants were analyzed for IL-10 (A), IL-12 p40 (B), and IL-12 p70 (C) by ELISA. The p values are indicated for cytokines secreted by the corresponding DC subsets from PP or spleen. The data are the average of seven separate experiments.

 
The observation that the three DC subsets secrete distinct pattern of cytokines upon in vitro stimulation prompted us to examine whether these DCs have different capacities to prime naive T lymphocytes. Thus, naive CD4+ lymphocytes from OVA-specific TCR transgenic mice were isolated and stimulated with the three subsets of DCs from SP and PP pulsed with OVA peptide. When the T cell number was assessed after priming by various DC subsets, myeloid DCs from the PP were found to induce T cell expansion more efficiently than did other DC subsets from SP or PP (Fig. 5GoA). T cells primed with myeloid PP DCs proliferated much more efficiently during secondary stimulation with anti-CD3 and anti-CD28 (Fig. 5GoB). When cytokine secretion from T cells primed with the three DC subsets from SP and PP was assessed following secondary stimulation, comparable levels of IFN-{gamma} secretion were obtained from T cells stimulated with these DC subsets (Fig. 6Go). Interestingly, high levels of the Th2 cytokines (IL-4 and IL-10) were secreted from T cells primed with PP myeloid DCs compared with those stimulated with other DC subsets (Fig. 6Go, B and C). Therefore, myeloid, but not lymphoid or DN DCs in the PP induce naive T cells to proliferate rapidly and to secrete high levels of IL-4 and IL-10, and this function is specific to DCs from the PP.



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FIGURE 5. Proliferation of OVA TCR transgenic T cells during primary and secondary culture with three DC subsets from PP and spleen. A, Naive CD4+/MEL-14+ FACS-sorted OVA TCR transgenic T cells (5 x 104 per well) were coincubated with DC subsets from PP or SP (5 x 103 per well) for 5–6 days. Live cells were counted using hemacytometer. The figure depicts on y-axis the factor by which the number of T cells multiplied during priming. B, OVA TCR transgenic T cells primed with DCs as described in A were restimulated with plate-bound anti-CD3 and soluble anti-CD28 Abs for 48 h. Data are representative of five separate experiments producing similar results.

 


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FIGURE 6. Secretion of cytokines from OVA TCR transgenic T cells primed with three DC subsets from the PP or SP. T cells were restimulated with anti-CD3 and anti-CD28 Abs, as described in Fig. 5Go. Supernatants were harvested, and IFN-{gamma} (A), IL-4 (B), and IL-10 (C) levels were measured by ELISA at 24 h (IL-4) or 48 h (IFN-{gamma} and IL-10). The data are representative of five similar experiments.

 

    Discussion
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
The DC system consists of distinct subsets in both humans and mice. The in vivo function of these DC subsets is largely unknown primarily due to difficulties associated with isolating pure tissue DC populations in sufficient quantity. The clue that DC subsets may possess distinct functions came from two independent reports that described the ability of myeloid and lymphoid splenic DC to prime Th2 and Th1 responses in mice injected with the respective DC subsets (9, 10). In the current study, we describe the function of a novel population of DCs in the PP that do not express either the myeloid or the lymphoid markers, namely the DN DCs. Although the presence of DCs that do not express either CD8{alpha} or CD11b has been previously reported in the lymph nodes (combined mesenteric, aortic, and axillary) (11), upon careful analysis of the peripheral and mucosal lymphoid tissues, we found that the DN DCs are abundant in the gut-associated lymphoid organs (MLN and PP), but not in the peripheral lymph nodes or the spleen. DN DCs constitute almost one-third of the entire PP DC population. In a prior study, we demonstrated that DN DCs localized in the SED and IFR of the PP (5). We now describe the presence of a unique population of DN DCs within the FAE. The bodies of these cells were located above the basement membrane with processes appearing to extend to the luminal surface, and some were found to be associated with M cells within the M cell pocket. In contrast, such DCs in the villus epithelium of the small intestine are rare. However, we have not looked extensively for such cells in the colonic epithelium, a site in which intraepithelial DCs have been identified in the rat (12). In addition to their localization, the fact that the FAE DN DCs expressed high levels of intracellular MHC class II Ags, indicating they are immature DCs, emphasized the possibility that these cells capture luminal Ags either directly or early after transport by M cells. Finally, we have been unsuccessful in demonstrating that DN DCs express CD8{alpha} or CD11b, or that lymphoid or myeloid DCs lose their lineage markers (5) upon maturation in vitro, suggesting that DN DCs do not simply represent a different maturation stage of CD8{alpha}+ or CD11b+ DCs. However, it remains a possibility that the DN DCs would require a specific condition to differentiate into lymphoid or myeloid subset. Growth factors such as IL-3 and GM-CSF have been shown to induce differentiation of lymphoid and myeloid DCs in vitro from human DC precursors, respectively (13).

In our prior study, we determined that total DC purified from the PP secrete IL-10 upon stimulation via CD40 cross-linking, and induce the differentiation of naive CD4+ T cells into helper cells capable of secreting high levels of IL-4 and IL-10 (in addition to IFN-{gamma}). Similar total populations from the spleen did not produce IL-10, nor induce T cells to differentiate into Th2 cells. We now extend these findings to show that the PP DCs that secrete IL-10 and induce Th2 differentiation are restricted to the myeloid DC subset. Consistent with our prior findings, none of the DC subsets from the spleen secreted IL-10 upon CD40 cross-linking; however, the myeloid DCs can produce some IL-10 upon stimulation with SAC and IFN-{gamma}. Neither PP nor spleen myeloid DCs produced biologically active IL-12 p70 upon stimulation. In contrast, the lymphoid and DN DCs from the PP secrete IL-12 p70 and induce the differentiation of Th1 cells. Therefore, DN DCs from the PP are functionally similar to the lymphoid subset, and the myeloid subset is responsible for the capacity of DCs from this organ to induce Th cells producing high levels of IL-4 and IL-10.

In light of these data, one concept that needs to be addressed is whether different subsets of DCs are more apt to induce T cells to differentiate into a Th1 or Th2 pathway, so-called DC1 and DC2, or whether there is significant plasticity in the ability of DCs to function in this regard. The data presented in this work would support the concept that lymphoid, and now DN DCs, are more capable of inducing Th1 differentiation, while myeloid DCs are more capable of inducing Th2 cells (9, 10). In addition, stimulation of DC subsets with SAC and IFN-{gamma} resulted in functional IL-12 p70 production only from lymphoid or DN PP DCs. These data argue that lymphoid and DN DC subsets are intrinsically functionally different from myeloid DCs in the setting of either T cell activation to a protein Ag, or in direct response to a bacterial signal.

That being said, the data presented in this work also support a role for the tissue microenvironment in modulating these functional phenotypes. This is because PP myeloid DCs appear to have a particular ability to produce IL-10 and induce T cells producing IL-4 and IL-10 when compared with myeloid DCs isolated from other organs such as spleen (Figs. 4Go and 5Go). One possibility is that the microenvironment of the PP somehow allows myeloid DC precursors to differentiate into high IL-10-secreting cells. What signals could contribute to such differentiation is unknown; however, cytokines such as TGF-{beta}, IL-10, and PGE2 may influence the differentiation of myeloid DCs in the PP, whereas they may not alter the differentiation or function of PP lymphoid or DN DCs.

A final issue that remains to be addressed is whether the intrinsic phenotype of any PP DC subset can be altered by direct interactions with microbial products or by environmental conditions induced in the PP following infection or administration of mucosal adjuvants, such as cholera toxin. While such alterations are certainly possible, especially given the recent finding that human plasmacytoid DCs can be differentiated in vitro under different conditions to induce either Th1 or Th2 responses (14, 15), to date no in vivo condition has been identified in mice (or in humans) that alters DC function to this extent, i.e., conditions that result in murine myeloid DCs functioning as Th1-inducing, or lymphoid DCs functioning as Th2-inducing cells.

Taken together, we propose the following model of immune induction within the PP. Intestinal Ags gain entry into the PP via M cells specialized in transport of certain types of Ags across the FAE. Once the Ag is transported by the M cells, the first cells to encounter orally delivered Ag are most likely the myeloid and DN DCs present just underneath or within the FAE. If the Ag is a soluble protein Ag, the uptake of the Ag by the SED DCs would not result in the activation or migration of these cells. Under these conditions, the T cells interacting with myeloid DCs would become Th2 or Th3 regulatory cells, whereas those interacting with the DN DCs would secrete IFN-{gamma} and most likely be anergized in the absence of activation signals from the DCs. In support of this possibility, transient IFN-{gamma} secretion by T cells after low dose oral feeding has been observed (16). In addition, Th2/Th3 cells have been argued to be less sensitive than Th1 cells to anergy induction (17). The end result is the induction of suppressor T cells following low dose Ag feeding. If soluble protein Ag is given at high doses, T cell activation occurs in the PP dome, IFR, as well as in the lamina propria (A. Iwasaki, J. Chung, and B. L. Kelsall, unpublished observation). Deletion or anergy of Ag-specific T cells may occur at any of these sites. In the case of microbial infection with intracellular organism such as salmonella, the SED DCs would be the first cells to be infected (18). These DCs, now having been stimulated by the microbial stimuli such as the LPS on the surface of the bacteria, will undergo maturation and migrate to the IFR or to the MLN. There, the DN SED DCs can prime Th1 response either directly, or transfer the microbial Ag to the IFR-resident lymphoid or DN DCs and cross-prime T cells. In the current studies, we found that in addition to CD40 cross-linking, SAC and IFN-{gamma} induced high levels of IL-10 and little to no IL-12 production by PP myeloid DCs. This implies that during an intestinal Th1-inducing bacterial infection of the PP, IL-10 will be produced by myeloid DCs, while the primary source of IL-12 for the induction of Th1 cells is lymphoid or DN DCs. In this regard, IL-10 produced by myeloid DCs may be acting to control the detrimental effects of Th1-induced inflammation.

In conclusion, this study clearly demonstrates the presence of three distinct DC subsets in the PP, and shows that myeloid DCs from the PP have the particular ability to secrete IL-10 and to induce naive CD4+ T cells to differentiate into Th2 cells. Moreover, we define a significant population of DN DCs in the PP that is capable of secreting IL-12 p70 upon recognition of microbial stimuli, and is present within the intraepithelial compartment of the FAE.


    Acknowledgments
 
We thank Ruth Swofford for flow cytometric sorting of DC and T cell populations, Dr. Owen Schwartz for confocal imaging, and Drs. Warren Strober and Charles Dela Cruz for critical review of the manuscript.


    Footnotes
 
1 A.I. is a recipient of Medical Research Council of Canada Fellowship. A.I. is currently at Yale University School of Medicine, Department of Epidemiology and Public Health (New Haven, CT). Back

2 Address correspondence and reprint requests to Dr. Brian L. Kelsall, Immune Cell Interaction Unit, Mucosal Immunity Section, Laboratory of Clinical Investigation, National Institutes of Allergy and Infectious Diseases, National Institutes of Health, 10 Center Drive, Room 11N238, Bethesda, MD 20892-1890. Back

3 Abbreviations used in this paper: DC, dendritic cell; CD40L3, CD40 ligand trimer; DIC, differential interference contrast; DN, double-negative; FAE, follicle-associated epithelium; IFR, interfollicular region; MLN, mesenteric lymph node; PP, Peyer’s patch; SAC, Staphylococcus aureus Cowan’s strain; SED, subepithelial dome; SP, spleen. Back

Received for publication December 11, 2000. Accepted for publication February 13, 2001.


    References
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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