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*
Department of Pathology, University of Cambridge, Cambridge, United Kingdom; and the
Medical Research Council Laboratory of Molecular Biology, Cambridge, United Kingdom.
| Abstract |
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| Introduction |
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Here we describe the effect of IL-13 up-regulation on the development of immune responses in the context of Th2-inducing antigenic challenge. In experiments using transgenic mice expressing IL-13 in a lymphoid-specific manner (15), we find that exposure to Ag induces rapid and highly elevated production of Ag-specific IgE and that subsequent exposure to Ag induces fatal anaphylaxis with attendant mast cell degranulation and histamine release. In addition, this predisposition to immediate hypersensitivity is associated with a pulmonary pathology highly characteristic of asthma. Thus, IL-13 plays an important role in the systemic response to allergen challenge, in addition to regulating local tissue modification.
| Materials and Methods |
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IL-13-transgenic (IL-13Tg), IL-13Tg x IL-4-/-, and wild-type littermates were prepared on a C57BL/6 background (15). Eight- to 10-wk-old female mice were used in this study. All mice were tagged s.c. with electronic chips, and to maintain objectivity, these were only scanned upon completion of the experiment. Synchronous pulmonary granulomas were induced by i.v. injection of mice with Schistosoma mansoni eggs. Mice were sensitized to schistosome eggs by i.p. injection of 5000 eggs. Two weeks later, sensitized and naive mice, four to six mice/group, were injected i.v. with 5000 eggs. Serum and plasma were recovered and stored at -20°C. Data presented are from four separate experiments.
Immunological analysis
Serum levels of IL-4, IL-5, IL-13, and IFN-
were determined
by ELISA (11). Levels of histamine in plasma were assayed
using a commercial histamine enzyme immunoassay kit (Immunotech,
Marseille, France) according to the manufacturers instructions .
Total serum IgE was determined by ELISA using purified IgE as standard
(BD PharMingen, San Diego, CA). Ag-specific IgE was detected using
digoxigenin-labeled egg Ag (16).
Rat basophilic leukemia cells (RBL.2H3) were passively sensitized by
incubation for 2 h at 37°C with a 1:20 dilution of mouse serum.
To inactivate IgE, sera were heated at 56°C for 2 h. After
gentle washing of the cells, egg Ag (25 ng/ml) was added for 1 h
at 37°C. Degranulation of cells was determined by quantification of
-hexosaminidase activity in culture supernatants using the substrate
p-nitrophenyl-N-acetyl-
-D-glucosamide
(17). Total cellular enzymatic activity was measured in
cell lysates (Triton X-100-treated). Because schistosome egg Ags elicit
low levels (<5%) of spontaneous degranulation of RBL cells in the
absence of sera, background Ag-induced degranulation was subtracted
from serum + Ag-mediated degranulation. Data are expressed as the
percentage of the mediator released relative to total cellular
-hexosaminidase activity. Data presented are mean and SD values from
more than six mice per group and are representative of a minimum of
three experiments performed.
Pulmonary histopathology
Following termination of mice, lungs were perfused via the right ventricle with 5 ml of formalin saline. Lungs were fixed in formalin saline, then paraffin-embedded, and 4-µm sections were cut. During preparation of histological sections, all slides were numerically coded and analyzed blind. Sections were stained with hematoxylin and eosin (cellular infiltration), toluidine blue (mast cells), Martius scarlet blue (collagen), periodic acid Schiff (PAS) (goblet cells), Giemsa (eosinophils), and Papanicolaou stain (eosinophilic crystals). Pulmonary fibrosis was quantified by differential staining of histological sections, with data expressed as micrograms of collagen per milligram of protein (18). The numbers of toluidine blue-stained mast cells were enumerated by counting the number of positive-stained cells per x100 field of view, with 20 fields counted per mouse. Mast cells were classified as either intact or degranulated, i.e., mast cells showing extrusion of >10% of cell granules. To quantify tissue eosinophilia, the numbers of peribronchial eosinophils in a high-field view (x1000 magnification) were counted; a minimum of 50 fields were counted per mouse. PAS-stained cells were counted using an arbitrary scoring system to quantify mucus secretion (12). PAS-stained cells in airway epithelia were measured using a numerical scoring system (0: <5% goblet cells; 1: 525%; 2: 2550%; 3: 5075%; 4: >75%). The sum of airway scores from each lung was divided by the number of airways examined, 2050 airways per mouse, and expressed as mucus score in arbitrary units (AU). Autofluorescent eosinophilic crystals were examined on a Leica (Deerfield, IL) confocal microscope. Data are presented as mean and SD values from more than four mice per group, and are representative of three experiments performed.
| Results |
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Allergic reactions are characterized by Th2 cytokine production
and are believed to represent an inappropriate immune response that
would normally be beneficial in the development of immunity to helminth
parasite infection (9, 19, 20). Therefore, we used a
pulmonary granuloma model in which egg Ags from the helminth parasite
S. mansoni specifically produce a Th2 response in mice. This
Th2 cytokine-mediated inflammation model evokes pulmonary changes that
are characteristic of allergies/asthma, including induction of Th2
cytokines, elevated IgE, pulmonary eosinophilia, goblet cell
hyperplasia, and airway remodeling (11, 16, 21).
Unexpectedly, i.v. injection of eggs elicited immediate anaphylaxis in
IL-13Tg mice that were presensitized to egg Ags; in four separate
experiments, 075% of IL-13Tg mice died within 10 min of Ag challenge
(Fig. 1
). The symptoms observed in
IL-13Tg mice after i.v. egg challenges were characteristic of fatal
anaphylaxis in mice: bronchospasm, piloerection, tachypnea, and,
ultimately, death. In contrast, comparably treated wild-type mice
developed no symptoms (Fig. 1
), and in all mouse strains tested with
this model to date there have been no reports of fatal anaphylaxis.
Pre-exposure to the Ag was essential for fatal anaphylaxis because egg
challenge of naive transgenic mice did not cause symptoms (Fig. 1
).
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RI) on mast
cells is one of the triggers that elicits mast cell degranulation
(1), we quantified serum levels of Ag-specific IgE in
wild-type and IL-13Tg mice. We found that even within the short-term
(14-day) challenge the IL-13Tg mice generated
10-fold higher levels
of egg Ag-specific IgE than wild-type control mice (Fig. 2
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Because the lungs represent the site where schistosome eggs lodge
following i.v. challenge, and because pulmonary pathology is causally
related to fatal anaphylaxis, we performed histological analysis on
lungs from wild-type and transgenic mice. This analysis demonstrated
profound perturbation of the pulmonary architecture of the IL-13Tg mice
following Ag sensitization, resulting in fibrosis, epithelial
hypertrophy, goblet cell hyperplasia, and eosinophil recruitment.
Sensitized IL-13Tg mice displayed marked peribronchial fibrosis, with
collagen deposition (blue stain) strikingly associated with airway
epithelium throughout the pulmonary parenchyma, resulting in
considerable thickening of the alveolar epithelium (Fig. 4
a). In contrast, pulmonary
collagen deposition in naive and sensitized wild-type mice as well as
naive IL-13Tg animals was similar (Fig. 4
a). Quantification
of collagen deposition demonstrated a 3-fold elevation in pulmonary
fibrosis in the sensitized IL-13Tg mice, whereas naive IL-13Tgs
displayed only marginally elevated levels of collagen deposition
compared with wild-type animals (Fig. 4
b). The lungs of
sensitized IL-13Tg mice also developed marked airway epithelial
hypertrophy and goblet cell hyperplasia within the bronchioles (Fig. 4
, c and d), which were absent from sensitized
wild-type mice (Fig. 4
, c and d). The combined
epithelial hypertrophy and mucus secretion caused occlusion of airways
in sensitized IL-13Tg mice (Fig. 4
, e and f).
Within occluded airways were mucus, cellular infiltrate, and
extracellular needle-like crystal structures (Fig. 4
, e and
f). These structures were confirmed to be eosinophilic
crystals (the murine equivalent of Charcot-Leyden crystals in humans)
by their fluorescence on Papanicolaou-stained sections
(22) (Fig. 4
g). The crystals were up to 150
µm in length and formed lattices that contributed to the blockage of
the airways. In mice, comparable eosinophilic crystals have been
observed in a number of conditions where they are often associated with
eosinophilia (23). Indeed, in contrast to the naive
IL-13Tg and wild-type animals, sensitization of IL-13Tg mice evoked
marked pulmonary eosinophilia associated with the hypertrophic
bronchiolar epithelium (Fig. 4
h). It is noteworthy that
small numbers of eosinophilic crystals were also observed in the lungs
of some naive IL-13Tg mice in the absence of overt eosinophilia.
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| Discussion |
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The cross-linking of IgE Fc
R on mast cells, leading to mast cell
degranulation, is probably the most important factor in generating the
pathogenesis of atopic disorders. The inflammatory mediators released
from the mast cell, including histamine, lead to the onset of symptoms
such as bronchoconstriction, edema, and the recruitment of inflammatory
cells. Although a number of studies have shown that IgE and mast cells
are not essential for the generation of anaphylaxis, with both mast
cell-deficient (24) and IgE-deficient mice developing
anaphylaxis (25), it is clear that they play an extremely
important role in the native immune reaction. We have shown that
IL-13Tg mice produce highly elevated levels of Ag-specific IgE and that
this can induce mast cell/basophil degranulation in vivo and in vitro.
Although this pathway appears to represent the main route of immediate
hypersensitivity displayed in the IL-13Tg mice, it is also possible
that other cell types and IgG1 also play a role in this process.
The IgE-mediated allergic responses in Ag-sensitized IL-13Tg animals is associated with elevated serum levels of IL-13, but also increased expression of IL-4 and IL-5. Because IL-4 and IL-13 display biological overlap with respect to the induction of allergic responses (11), the elevated IL-4 production in IL-13Tg mice may represent an important contributing factor to the anaphylaxis in these animals. We have previously used IL-13Tg mice that do not express IL-4 (IL-13Tg x IL-4-/-) to demonstrate that IL-13 can elicit IgE expression independently of IL-4 (15). Following schistosome egg sensitization, IL-13Tg x IL-4-/- mice develop anaphylaxis responses at an order of magnitude comparable to that of IL-13Tg mice (mean ± SD of egg Ag-specific IgE 0.62 ± 0.2 OD in IL-13Tg x IL-4-/- mice compared with 0.567 ± 0.1 OD in IL-13Tg mice). Serum from IL-13Tg or IL-13Tg x IL-4-/- mice elicited 2024% mediator release from RBL cells in the presence of schistosome egg Ags, whereas serum from sensitized wild-type mice elicited <4% basophil degranulation. In this experiment, no mortality was observed in any of the groups, and we are unable to comment on the importance of IL-4 in this process (data not shown). Our data further highlight that overexpression of IL-13 acts independently of IL-4 in the induction of certain allergic responses.
A number of studies on humans have identified a correlation between the expression of IL-13 and the development of atopy (26, 27). Furthermore, IL-13 expression and IL-13 gene polymorphisms have been found to associate with increased IgE expression in atopic patients (5, 28). A direct role for IL-13 in regulating lung epithelial cell function has also been reported, indicating that IL-13 may differentially modulate subsets of lung fibroblasts, inducing recruitment and activation of inflammatory cells (29). Only recently have experimental mouse models been used to investigate the potential roles of IL-13 in allergic responses. Such studies have highlighted the importance of IL-13 in the modulation of goblet cell hyperplasia and as an uncharacterized effector molecule in AHR (12, 13). In addition, a recent study by Zhu et al. (14) demonstrated that overexpression of IL-13 using a lung tissue-specific promoter resulted in morphological changes similar to those observed in our study. However, these effects were not Ag-specific and failed to show the systemic alteration in circulating IgE, increased Th2 responses, or the onset of anaphylaxis, probably due to restricted expression of IL-13 to the lung tissue. In contrast, our study clearly shows that in a more physiological model, where IL-13 expression is regulated by the IL-13 promoter and a T cell enhancer and requires Ag stimulation, IL-13 is a potent modifier of IgE and fibrotic responses to allergen and plays a causal role in the generation of immediate hypersensitivity reactions.
Thus, IL-13 is an extremely important regulator of allergic responses, influencing a broad spectrum of Th2-driven effector functions and predisposing to atopy. Furthermore, these IL-13Tg mice represent an important model for studying the induction of such responses following allergen challenge and demonstrate that the therapeutic modulation of IL-13 may prove beneficial in regulating both tissue remodeling and systemic IgE responses to allergens.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Andrew McKenzie, Medical Research Council Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, U.K. ![]()
3 Abbreviations used in this paper: AHR, airway hyperresponsiveness; IL-13Tg, IL-13 transgenic; RBL, rat basophilic leukemia cells; PAS, periodic acid Schiff. ![]()
Received for publication August 1, 2000. Accepted for publication November 21, 2000.
| References |
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-deficient and IL-4-deficient mice reveal a role for IL-13 in the regulation of Th2 responses. Curr. Biol. 8:669.[Medline]
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