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* Channing Laboratory, Department of Medicine,
Department of Neurology, and
Laboratory of Immunogenetics and Transplantation, Brigham and Womens Hospital and Harvard Medical School, Boston, MA 02115; and
Department of Microbiology and Molecular Genetics, and
¶ School of Public Health, Department of Immunology and Infectious Disease, Harvard Medical School, Boston, MA 02115
| Abstract |
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TCR-bearing T cells or CD4+ T cells fail to develop abscesses following challenge with B. fragilis or abscess-inducing zwitterionic polysaccharides, compared with CD8-/- or wild-type animals. Transfer of CD4+ T cells from wild-type mice to 
TCR-/- animals reconstituted this ability. The induction of abscesses required T cell costimulation via the CD28-B7 pathway, and T cell transfer experiments with STAT4-/- and STAT6-/- mice demonstrated that this host response is dependent on STAT4 signaling. Significantly higher levels of IL-17, a proinflammatory cytokine produced almost exclusively by activated CD4+ T cells, were associated with abscess formation in Th2-impaired (STAT6-/-) mice, while STAT4-/- mice had significantly lower levels of this cytokine than control animals. The formation of abscesses was preceded by an increase in the number of activated CD4+ T cells in the peritoneal cavity 24 h following bacterial challenge. Confocal laser-scanning microscopy analysis revealed that CD4+ T cells comprise the abscess wall in these animals and produce IL-17 at this site. Administration of a neutralizing Ab specific for IL-17 prevented abscess formation following bacterial challenge in mice. These data delineate the specific T cell response necessary for the development of intra-abdominal abscesses and underscore the role of IL-17 in this disease process. | Introduction |
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Relatively little is known regarding the biology of T cell interactions with carbohydrate-based Ags. Recently, we have shown that the zwitterionic charge motif associated with Zps confers the ability of these polymers to activate human and rat CD4+ T cells in vitro (5, 6). T cell activation by Zps is dependent on the presence of free amino and carboxyl groups on these repeating unit structures. Zps that have been chemically modified to remove either of these charged groups do not stimulate T cells. T cell activation by these polymers requires MHC class II on APCs and T cell costimulation via the CD28-CD86 pathway (6, 7). T cell hybridomas specific for one type of Zps respond to other, structurally distinct Zps, but not to polysaccharides that lack this charge motif (unpublished data). Zps-activated T cells are functional, as they induce abscess formation when transferred to the peritonea of rats (5, 7). These data provide a framework for understanding how bacterial pathogens such as B. fragilis and S. aureus that display Zps on their cell surface predominate in clinical cases of abscess formation and the role of T cells in this process. In the present study, we sought to characterize the specific T cell response that mediates abscess formation in vivo. These data reveal that CD4+ T cells have an essential role in the development of intra-abdominal abscesses. These cells home to and predominate at the site of infection and comprise the wall of these fibrinous structures. This disease process is mediated, at least in part, by the T cell-derived proinflammatory cytokine IL-17.
| Materials and Methods |
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TCR-/-, CD4-/-, CD8-/-, CD28-/-, C57BL/6, and BALB/c mice were purchased from The Jackson Laboratory (Bar Harbor, ME). STAT4-/- and STAT6-/- mice were generated by Grusby and colleagues (8, 9), as described previously, and backcrossed into a C57BL/6 background for at least 10 generations. Animals were maintained according to the Harvard Medical School animal management program, which is accredited by the American Association for the Accreditation of Laboratory Animal Care.
Bacterial strains and polysaccharide preparation
B. fragilis NCTC 9343 was obtained from the Channing Laboratory stock culture collection. CP1 was obtained from the American Type Culture Collection (Manassas, VA) and purified, as described previously (7).
Animal model for intra-abdominal abscess formation
An intra-abdominal sepsis model was used for these studies (10, 11). In brief, mice were injected i.p. with B. fragilis (1 x 108 CFU/animal) or CP1 (50 µg/animal) mixed with sterile cecal contents (SCC; 1:1 v/v, 0.2 ml total volume). SCC is a required adjuvant for the development of abscess formation by intact bacteria or purified polysaccharide in this model and is administered to reflect the spillage of colonic contents that occurs during the onset of intra-abdominal sepsis in humans (5, 11). Administration of SCC alone does not induce abscess formation in animals. Six days later, animals were examined at necropsy and for the presence of one or more abscesses within the peritoneal cavity.
T cell depletion
For 
TCR+ T cell depletion, C57BL/6 mice were treated with 300 µg of the TCR
-chain-specific mAb (H57-597; BD PharMingen, San Diego, CA) or isotype-matched control Abs via the i.p. route 4 days before B. fragilis challenge. For depletion of CD4 or CD8 T cells, C57BL/6 mice were treated with 0.2 mg of CD4-specific mAb (GK1.5; BD PharMingen) or CD8-specific mAb (53-6.7; BD PharMingen) via the i.p. route 48 h before challenge. Depletion of the targeted cell type was confirmed by subsequent FACS analysis, which showed >93% depletion of the respective cell type.
Blockade of the CD28-B7 pathway
Murine CTLA4Ig and control L6Ig were obtained from Bristol-Myers Squibb (Princeton, NJ). CTLA4Ig binds specifically to CD80 and CD86. L6Ig was used as a control Ig fusion protein. This molecule has the same Ig H chain fused to an irrelevant protein. Fusion proteins (500 µg) were administered to animals via the intracardiac route at the time of challenge.
T cell transfer studies
Splenic CD4+ T cells from STAT4-/-, STAT6-/-, or wild-type mice were purified on a nylon wool column and negative selection with magnetic beads, as previously described (7). T cells were transferred to 
TCR-/- mice at 3 x 106 cells per mouse via the intracardiac route 24 h before challenge. All recipient animals were challenged via the i.p. route with 1 x 108 CFU of B. fragilis and SCC.
Immunohistochemistry and confocal laser-scanning microscopic imaging
Paraffin-embedded tissue sections of intra-abdominal abscess from B. fragilis-challenged C57BL/6 mice were stained with H&E or dewaxed with EZ-DeWax deparaffinization solution (InnoGenex, San Ramon, CA), according to the manufacturers protocol. The sections were permeabilized with 0.05% saponin in dH2O and incubated with 10% goat serum in PBS to block nonspecific binding sites. Intracytoplasmic IL-17 was stained with rabbit anti-mouse IL-17 IgG (Santa Cruz Biotechnology, Santa Cruz, CA) and then stained with goat anti-rabbit IgG conjugated to Alexa Fluor 568 (Molecular Probes, Eugene, OR). Staining for CD4 was performed with hamster anti-mouse CD4 IgG (BD PharMingen) and goat anti-hamster IgG conjugated to Alexa Fluor 488 (Molecular Probes). These sections were compared with those stained with irrelevant isotype control primary Abs and the respective Alexa Fluor conjugates. Confocal microscopy images were collected with a Bio-Rad (Hercules, CA) scanning confocal microscope equipped with a krypton-argon laser. Images were analyzed with Bio-Rad Confocal Assistant and Adobe Photoshop 5.5 (Adobe Systems, San Jose, CA). For colocalization studies, red channel fluorescence was plotted against red channel fluorescence to correlate the number of cells that stain for both CD4 and IL-17.
Measurement of IL-17 in the peritoneal cavity
IL-17 levels in the peritoneal cavities of STAT4-/-, STAT6-/-, and wild-type mice were assessed after B. fragilis infection. Four mice in each group were exsanguinated, and peritoneal lavage was performed with 1 ml of PBS. Supernatant fluids were centrifuged at 12,000 x g, and assayed for IL-17 using specific ELISA kit (R&D Systems, Minneapolis, MN), according to the manufacturers protocols.
Kinetics of cellular influx into the peritoneal cavity
Mice were challenged with B. fragilis (1 x 108 CFU), and the cellular influx into the peritoneal cavity was assessed. Animals (n = 5) underwent peritoneal lavage with 1 ml of PBS at 6, 24, and 48 h following challenge. Lavage fluid from each animal (25 µl) was treated with NH4Cl to lyse RBCs, and a total cell count was performed with a hemocytometer. Each sample was then analyzed by flow cytometry. After preincubation with rat anti-mouse CD16/CD32 (BD PharMingen) to block Fc receptors, cells were stained with FITC- or PE-labeled isotype control Abs or mAbs to CD4, CD16 (polymorphonuclear neutrophils (PMNs)), Mac-1 (macrophages), CD25 (IL-2R), and CD69 (T cell activation marker). Stained cells were analyzed on a Coulter EPICS XL cytometer (Beckman Coulter, Fullerton, CA), the CellQuest (BD Biosciences, San Jose, CA), and WinMDI 2.8 analysis software (Scripps Research Institute, La Jolla, CA; http://facs.scripps.edu). The absolute number of peritoneal cells collected was determined by trypan blue staining and counted with a hemocytometer. The absolute number of each respective cell type present was calculated by taking the proportion of each cell type (as determined by FACS analysis) and multiplying it by the total number of cells obtained from each mouse.
Production of IL-17-specific Ab
Polyclonal Ab to murine IL-17 was produced by immunizing rabbits at multiple intradermal sites with mouse rIL-17 (R&D Systems) mixed with CFA, as previously described (10). The IgG fraction was purified by HiTrap protein G affinity chromatography, according to the manufacturers instructions (Amersham Pharmacia Biotech, Piscataway, NJ). Fractions eluted from the column were concentrated with the Centriprep YM-10 (Millipore, Bedford, MA) and buffer exchanged to PBS on PD-10 columns. Specific Ab was obtained after additional IL-17 affinity chromatography. Briefly, purified rIL-17 was coupled to Sepharose beads (Amersham Pharmacia Biotech) and used for affinity column chromatography. IL-17-specific IgG that bound to the column was eluted with 0.1 M glycine, neutralized, dialyzed, and concentrated for use. The concentration of purified Ab was determined by a standard protein assay. This Ab was specific for IL-17, as determined by ELISA, but did not cross-react IL-2, IFN-
, IL-4, IL-10, IL-5, or IL-13. The control Ab used in these experiments was the IgG fraction from normal rabbit serum purified by HiTrap protein G affinity chromatography, as described above.
Effect of IL-17-specific neutralizing Ab on abscess formation
For neutralization experiments, C57BL/6 mice were injected with affinity-purified IL-17 Abs (100 µg of Ab/animal) via the i.p. route at the time of challenge and 6 h thereafter. Control groups were given 100 µg of affinity-purified rabbit IgG. All groups of mice were sacrificed after 6 days and assessed for abscess formation.
Statistical analyses
Evaluation of differences between groups in abscess induction studies was performed by
2 analysis (InStat, GraphPad Software, San Diego, CA). The results shown are a compilation of at least two separate experiments. Comparison of means from IL-17 ELISA experiments was made by the unpaired t test.
| Results |
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Specific mAbs were used to deplete mice of 
TCR+, CD4+, or CD8+ T cells, respectively. When challenged with B. fragilis, animals depleted of 
TCR+ T cells or CD4+ T cells showed significantly lower abscess rates as compared with sham-depleted animals (Table I), whereas depletion of CD8+ T cells did not impair abscess formation. This result was confirmed with the use of genetic knockout mice. 
TCR-/- and CD4-/- mice showed significantly lower abscess rates as compared with wild-type control littermates, whereas the abscess rate in CD8-/- mice was comparable to that in control animals (Table I).
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We investigated the role of T cell costimulation via the CD28-B7 pathway in T cell-mediated abscess formation induced by B. fragilis. Mice deficient in CD28 showed a significantly lower abscess rate (20%) as compared with wild-type mice (90%) (Table II, p = 0.006). Furthermore, treatment with the fusion protein CTLA4Ig, which inhibits CD28-mediated signaling, reduced the incidence of abscess formation to 30% as compared with treatment with a fusion protein control (L6Ig), which resulted in a 90% abscess rate (p = 0.02 compared with CTLA4Ig-treated animals).
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The role of Th1 and Th2 responses in the development of intra-abdominal abscesses was studied in STAT4-/- and STAT6-/- mice (Table III). Challenge with B. fragilis resulted in a significantly lower abscess rate in STAT4-/- (15%) than in wild-type littermate control mice (81%), whereas the abscess rate in STAT6-/- mice (94%) was comparable to that in control animals. Challenge of these mouse strains with a known abscess-inducing Zps, CP1, yielded similar results (Table III).
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TCR-/- mice, previously shown to be genetically impaired in their ability to develop abscesses (Table I), were used as recipients. In these studies, the transfer of purified CD4+ T cells from STAT6-/- or wild-type mice reconstituted abscess formation in 
TCR-/- mice, whereas transfer of these cells from STAT4-/- mice did not have this effect (Table IV). The transfer of CD4+ T cells from STAT6-/- to 
TCR-/- mice resulted in abscess formation in recipient animals (90% abscess rate), a rate comparable to that in 
TCR-/- mice that received CD4+ T cells from wild-type littermate controls (78%, Table IV). However, the transfer of CD4+ T cells from STAT4-/- mice to 
TCR-/- mice resulted in a significantly lower rate of abscess formation (17%, p = 0.009 compared with control animals). These data clearly show that the development of abscesses is dependent on STAT4 signaling, and indicate that Th1 cells are responsible for this host response.
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The cellular response subsequent to challenge with B. fragilis was studied to determine the types of cells that home to the peritoneal cavity as a prelude to the induction of abscesses. After bacterial challenge, PMNs accumulated very rapidly (within 6 h) in the peritoneal cavity, becoming a dominant cell type at this site within 24 h (Fig. 1). Macrophages are the primary resident host cell in the peritoneal cavity of naive mice. In animals challenged with B. fragilis, the number of these cells declined at the 6-h time interval, but increased by nearly 2-fold 24 h after challenge. A striking increase in the number of T cells in the peritoneal cavity was seen at the 24-h time interval, in which the mean number of these cells per mouse increased from 1.5 x 105 cell/ml to
4 x 106 cells/ml. This corresponded to an increase in the number of T cells that expressed the activation markers CD25 and CD69 at this time point. The number of T cells decreased by 48 h, with the number of activated T cells falling to baseline levels. These data indicate that the initial influx of PMNs to the challenge site is soon followed by a large increase in the number of activated T cells.
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IL-17 is a proinflammatory cytokine produced almost exclusively by activated CD4+ T cells (12, 13). To determine whether this cytokine plays a role in the development of abscesses and whether its release is associated with a particular type of Th response, we compared the IL-17 levels following challenge with B. fragilis in STAT-/- and littermate control mice. The level of IL-17 in the peritoneal fluid of STAT4-/- mice was significantly lower than in control animals 4 and 8 h after the challenge (Fig. 2, p < 0.0001 and p < 0.001, respectively). In contrast, the level of IL-17 in STAT6-/- mice was significantly higher than wild-type animals 4 h after the challenge (p = 0.02). In both wild-type and STAT-/- mice, IL-17 was barely detectable 24 h postchallenge.
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Histologic analysis of H&E-stained sections of intra-abdominal abscesses harvested from mice previously challenged with B. fragilis revealed the cellular organization typical of this host response (Fig. 3A). Abscesses had a defined fibrinous wall surrounding a dark-staining purulent focus of PMNs and bacteria. Confocal microscopic analysis of these sections after staining with a CD4-specfic Ab showed the presence of CD4+ T cells within the fibrinous wall of the abscess (Fig. 3B, green-stained cells). However, very few of these cells were found within abscesses (not shown). Staining with an IL-17-specific Ab revealed that many of the cells found in the abscess wall produce this cytokine (Fig. 3C, red-stained cells). Two-color colocalization analysis revealed that the CD4+ T cells found within the walls of abscesses produce IL-17 (red + green = yellow-stained cells). A plot of cells fluorescing green (T cells) vs the number of cells fluorescing red (IL-17) revealed a linear relationship (Fig. 3D), suggesting that the majority of CD4+ T cells present in the abscess wall express this cytokine.
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To demonstrate the role of IL-17 in the development of abscesses, we performed in vivo neutralization experiments. Animals were administered 100 µg of an affinity-purified Ab specific for IL-17 via the i.p. route at t = 0 and 6 h relative to challenge with B. fragilis. Control animals were similarly treated with a control Ab. Animals treated with the IL-17-specific Ab had a significantly lower abscess rate (10%) than animals treated with the control Ab (90%, p = 0.001, Table V).
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| Discussion |
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We previously have shown that the capsular polysaccharides of B. fragilis and other abscess-inducing bacteria such as S. aureus activate human and rat CD4+ T cells in vitro. The transfer of these activated T cells along with SCC to naive rats promotes abscess formation (5, 7, 14, 15). In the present study, we used a mouse model of intra-abdominal sepsis to take advantage of the availability of different knockout mice to characterize the T cell response that governs this host response in vivo. These studies demonstrate that CD4+ T cells have a definitive role in this disease process and support the concept that T cells can be pathogenic in certain inflammatory tissue disorders, such as experimental autoimmune encephalomyelitis, idiopathic pulmonary fibrosis, progressive systemic sclerosis, experimental colitis, and granuloma formation (16, 17, 18, 19, 20). More recently, we have shown that CD4+ T cells promote the development of surgical adhesions, another type of fibrotic tissue response that ensues following surgical trauma (10).
CD4-/- animals had a significantly lower abscess rate than wild-type control animals. However, it should be pointed out that the few abscesses found in CD4-/- mice were abnormal, as determined by histologic examination (data not shown). CD4+ T cells can be classified into Th1 and Th2 subsets, according to the types of cytokines they produce. To determine the Th subset responsible for mediating abscess formation, we used mice deficient in the transcriptional activators STAT4 or STAT6. STAT4-/- and STAT6-/- mice are genetically impaired in the ability to generate Th1 and Th2 responses, respectively, and have been widely used in investigations of the role of Th1 and Th2 subsets in different immune responses. Experiments with STAT4 and STAT6 knockout mice show that Th1 cells are most likely responsible for the development of abscesses induced by B. fragilis or an abscess-inducing Zps, CP1. The finding that the transfer of CD4+ T cells from STAT6-/- mice reconstitutes this ability in 
TCR-/- mice, which do not develop abscesses, supports this result. Based on these data, we hypothesize that the hallmark Th1 cytokine, IFN-
, may have a predominant role in this inflammatory response. The impact of this proinflammatory cytokine in this host response is currently under investigation.
Characterization of the cellular response that leads to abscess formation revealed, in addition to the expected increase in the number of PMNs that infiltrate the peritoneal cavity, an increase in the number of T cells expressing activation markers. These data correlate with our finding that T cell costimulation via the CD28-B7 pathway is required for abscess formation in mice, and support our previous studies showing that activation of human and rat T cells by abscess-inducing Zps in vitro is mediated by the CD28-B7-2 pathway (7).
Perhaps the most striking result is the characterization of the singular role that IL-17 plays in abscess formation. This recently described cytokine is a product of activated CD4+ T cells and has been implicated as a mediator of tissue inflammation (14, 15). IL-17 can selectively recruit neutrophils into the peritoneal cavity through the release of neutrophil-specific chemokines, such as KC and macrophage-inflammatory protein-2, from the peritoneal mesothelium (12). Because PMNs are the major cellular components of abscesses and T cells play a critical role in abscess formation, we hypothesized that IL-17 could be one of the soluble factors that mediate this host response. The finding of low levels of IL-17 in the peritoneal fluid of STAT4-/- mice and high levels in STAT6-/- mice suggests a close relationship between IL-17 production and Th1 cells. However, this point is controversial, as there have been reports of a role for IL-17 in inflammatory processes controlled by both Th1 and Th2 CD4+ T cells (21, 22, 23, 24).
The importance of IL-17 was corroborated by the demonstration that CD4+ T cells localize in the walls of intra-abdominal abscesses and produce IL-17 at this site. This finding, considered together with the FACS analysis of the cellular response elicited by B. fragilis challenge, suggests that activated CD4+ T cells play a critical role in the development of intra-abdominal abscesses. We hypothesize that following activation of these cells, IL-17 and other soluble mediators are produced and regulate the recruitment of inflammatory host cells to the peritoneal cavity. This series of events ultimately lead to the development of abscesses at this site. The ability of an IL-17-specific Ab to abrogate abscess formation in this setting supports this concept.
The development of abscesses associated with intra-abdominal sepsis is a common infectious complication that can have severe clinical outcomes. In this study, we show that CD4+ T cells mediate abscess formation associated with intra-abdominal sepsis by a mechanism that is dependent on the T cell-derived cytokine IL-17. These results delineate a major component of the cellular and subcellular host response that leads to this inflammatory tissue disorder.
| Footnotes |
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2 Address correspondence and reprint requests to Dr. Arthur O. Tzianabos, Channing Laboratory, 181 Longwood Avenue, Boston, MA 02115. E-mail address: atzianabos{at}channing.harvard.edu ![]()
3 Abbreviations used in this paper: Zps, zwitterionic polysaccharide; CP1, S. pneumoniae type 1 capsule; PMN, polymorphonuclear neutrophil; SCC, sterile cecal contents. ![]()
Received for publication September 17, 2002. Accepted for publication December 11, 2002.
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