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Infection and Immunity, May 2008, p. 2235-2239, Vol. 76, No. 5
0019-9567/08/$08.00+0     doi:10.1128/IAI.01481-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Macrophages Are Mediators of Gastritis in Acute Helicobacter pylori Infection in C57BL/6 Mice

Maria Kaparakis,^1, Anna K. Walduck,¹ Jason D. Price,^1,2 John S. Pedersen,³ Nico van Rooijen,⁴ Martin J. Pearse,^2,5 Odilia L. C. Wijburg,^1,2,* and Richard A. Strugnell^1,2,

Department of Microbiology and Immunology and the NHMRC Bacterial Pathogenesis Group, The University of Melbourne, Parkville, Victoria 3010, Australia,¹ CRC for Vaccine Technology,² Department of Anatomical Pathology, Alfred Hospital, Prahran, Victoria 3181, Australia,³ Department of Molecular Cell Biology, Vrije Universiteit VUMC, Amsterdam, The Netherlands,⁴ CSL Limited, Poplar Rd., Parkville, Victoria 3052, Australia⁵

Received 6 November 2007/ Returned for modification 2 January 2008/ Accepted 26 February 2008

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Helicobacter pylori is the etiological agent of human chronic gastritis, a condition seen as a precursor to the development of gastrointestinal ulcers or gastric cancer. This study utilized the murine model of chronic H. pylori infection to characterize the role of macrophages in the induction of specific immune responses and gastritis and in the control of the bacterial burden following H. pylori infection and vaccination. Drug-loaded liposomes were injected intravenously to deplete macrophages from C57BL/6 mice, and effective removal of CD11b⁺ cells from the spleens and stomachs of mice was confirmed by immunofluorescence microscopy. Transient elimination of macrophages from C57BL/6 mice during the early period of infection reduced the gastric pathology induced by H. pylori SS1 but did not affect the bacterial load in the stomach. These data suggest that macrophages are important to the severity of gastric inflammation during H. pylori infection.

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Helicobacter pylori is the causative agent of human gastroduodenal disease, resulting in acute gastric inflammation (16), ulcers, gastric adenocarcinoma, or mucosa-associated lymphoid tissue lymphoma (18, 24). H. pylori resides extracellularly within the gastric mucus layer of the human stomach, with only 1% of the organisms attached to host epithelial cells (11). The normal gastric mucosa of H. pylori-negative adults and children is populated by very few macrophages (12). Macrophages and neutrophils enter gastric tissue in response to H. pylori infection (20) and increase in number with the severity of gastritis and the duration of infection (12). H. pylori can activate macrophages and elicit interleukin-1 (IL-1), tumor necrosis factor alpha, IL-6, IL-8, MIP-1, and GRO- production (5, 6, 13), which induce the recruitment and activation of inflammatory cells, including macrophages and T cells.

Murine Helicobacter infection mimics many aspects of human H. pylori disease, including the development of chronic inflammation characterized by an influx of T cells expressing Th1 cytokines (14). The murine model therefore provides an excellent opportunity to analyze the genesis and adaptive immune regulation of Helicobacter-induced chronic gastritis (19). Moreover, the murine model is seen as an important aid in understanding vaccination against H. pylori infection (3).

While extensive studies have been performed to define the role of the acquired immune response during H. pylori infection and vaccination, the role of the innate immune response is not well understood. Very few in vitro studies have investigated the role of macrophages and their interaction with Helicobacter (1, 8, 17). To date, there is no reported study examining in vivo interactions of macrophages as antigen-presenting cells, and the initiator of H. pylori inflammation, during infection or vaccination. In this study, the role of macrophages in driving the inflammatory response and in controlling levels of H. pylori colonization in C57BL/6 mice during infection was examined, identifying the macrophage as a key cell in the induction of gastritis during H. pylori infection.

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Mice. Six-week-old female C57BL/6 mice were bred and housed under specific-pathogen-free conditions at the Department of Microbiology and Immunology Animal Facility, The University of Melbourne. Mice were fed sterile food and H₂O ad libitum and euthanized by CO₂ asphyxiation. All experiments were approved by The University of Melbourne Animal Ethics Committee.

In vivo macrophage depletion. Macrophages were depleted in vivo by using the well-characterized liposome-mediated macrophage "suicide" technique (21). Mice were intravenously (i.v.) injected with 200 µl of dichloromethylene diphosphonate (Cl₂MDP)-loaded liposomes. Cl₂MDP was a kind gift from Roche Diagnostics GmbH (Mannheim, Germany). Macrophage depletion was maintained, when required, by i.v. injecting 50 µl of Cl₂MDP-loaded liposomes every 4 to 5 days for the indicated periods.

H. pylori culture and infection of mice. H. pylori SS1 was grown in brain heart infusion broth (Oxoid, England) supplemented with 5% (vol/vol) fetal calf serum (10). Mice were anesthetized with Penthrane (Abbot Laboratories) and received approximately 10⁸ bacteria in 200 µl of phosphate-buffered saline (PBS) by oral gavage. The animals were infected three times within 5 days for infection studies or challenged after prophylactic immunization with a single dose (10⁸ CFU) of bacteria.

H. pylori colonization and gastritis levels of infected mice. Mouse stomachs were opened along the greater curvature, washed in PBS, and cut in half to include the greater curvature. One half was used to quantify H. pylori colonization by determining the number of CFU per gram of stomach tissue (10). The remaining half was embedded in Tissue-tek O.C.T. compound (Sakura Finetech) and frozen over liquid nitrogen. Hematoxylin-and-eosin-stained cryosections (7 µm) were used to grade the inflammatory response as previously described (10). The following six-point scale was used to define mononuclear cell infiltration: 1, mild multifocal; 2, mild widespread or moderate multifocal; 3, mild widespread and moderate or severe multifocal; 4, moderate widespread; 5, moderate widespread and severe multifocal; 6, severe widespread. All sections were graded blindly.

H. pylori-specific ELISA. Mouse serum H. pylori-specific antibody levels were determined by enzyme-linked immunosorbent assay (ELISA). In brief, MaxiSorp immunoplates (NUNC, Roskilde, Denmark) were coated with 10 µg/ml H. pylori sonicate and diluted sera were subsequently incubated in the plates. Bound antibodies were detected with horseradish peroxidase-conjugated sheep anti-mouse immunoglobulin (Silenus Laboratories, Hawthorn, Australia). Enzymatic activity of bound antibodies was detected with Immunopure O-phenylenediamine (Pierce) with H₂O₂ as the substrate. Absorbances were read by determining optical density at 492 nm with a Titertek Multiskan (Titertek, Finland). Serum endpoint titers are expressed as the reciprocal of the dilution of the specific antibody that gave an optical density at 492 nm of five times the value of horseradish peroxidase-conjugated antibody alone. Positive and negative control sera with known Helicobacter antibody titers were included in all ELISA experiments.

Immunohistochemistry. Acetone-fixed cryosections of spleen and stomach tissues were washed in PBS and blocked for 30 min with 20% (vol/vol) goat serum-2% mouse serum-FcRIII antibody (eBiosciences) in PBS. Sections were incubated with anti-CD11b clone M1/70 (BD Biosciences) and/or rabbit anti-H. pylori serum diluted in 5% (vol/vol) goat serum-PBS overnight at 4°C. Bound antibody was detected with fluorescein isothiocyanate-conjugated anti-rat antibody and/or phycoerythrin-conjugated anti-rabbit antibody (Jackson ImmunoResearch). Slides were mounted with Mowiol 4-88 (Merck, Australia) and examined with a UV epifluorescence microscope.

Statistics. The nonparametric two-tailed Mann-Whitney U test was used for statistical analysis of the results. One stomach section removed from along the greater curvature per animal was analyzed for pathology and colonization by H. pylori bacteria. Antibody levels below the detection limit of the assay were assigned the minimum level of detection of the assay to enable statistical analysis. Differences were considered statistically significant at P < 0.05.

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Macrophage elimination by treatment with Cl₂MDP-liposomes. The role of macrophages in the control of H. pylori colonization and immunopathology were examined by transiently depleting mice of macrophages with Cl₂MDP-liposomes. Mice were injected i.v. with Cl₂MDP-liposomes, and macrophage depletion was maintained by i.v. injection of Cl₂MDP-liposomes every 4 to 5 days, from 8 days prior to the first dose of infection to 10 days after the final dose of infection with H. pylori (a total of six injections). Control mice received PBS instead of Cl₂MDP-liposomes. Three days after the final dose of bacteria, mice were euthanized and the spleens and stomachs of the mice were frozen and used for histological analysis. Cryostat sections were stained with anti-CD11b antibody to determine if macrophages were depleted from Cl₂MDP-liposome-treated mice. Figure 1 shows that CD11b⁺ cells were easily distinguished in the spleens of control naïve and H. pylori-infected animals. Cl₂MDP-liposome treatment resulted in successful elimination of CD11b⁺ cells from the red pulp and marginal zone area of the spleens of naïve and H. pylori-infected animals. There were very few detectable CD11b⁺ cells in the gastric tissue of PBS-treated mice and no detectable CD11b⁺ cells in Cl₂MDP-liposome-treated naïve mice. Analysis of the presence of CD11b⁺ cells (green) and H. pylori bacteria (red) detected in the stomachs of Cl₂MDP-liposome-treated and control H. pylori-infected animals demonstrated that infection with H. pylori resulted in an influx of CD11b⁺ cells into the gastric tissue and that treatment with Cl₂MDP-liposomes before and during administration of the bacteria resulted in a substantial reduction in the number of detectable CD11b⁺ cells in the stomach.

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FIG. 1. Absence of CD11b+ cells in the spleens and stomachs of Cl2MDP-liposome-treated mice. C57BL/6 mice received Cl2MDP-liposomes or PBS via i.v. injection every 4 or 5 days from 8 days prior to the first dose of H. pylori until 10 days after the final dose of H. pylori. Three days after the final dose of bacteria, mice were euthanized and their spleens and stomachs were frozen for analysis by histology. Shown are cryostat sections of the spleens and stomachs of PBS-treated or Cl2MDP-liposome-treated, H. pylori-infected and naïve animals. Tissues were stained with rat anti-CD11b and anti-rat-fluorescein isothiocyanate (green) and rabbit anti-H. pylori antibodies and anti-rabbit-Texas Red (red, H. pylori-infected stomach only), and nuclei are stained with DAPI. Negative control slides were stained with fluorochrome-conjugated second antibody only. Representative sections from each treatment group are shown (n = 5 for each group). Data are representative of two independently performed experiments. Original magnifications: spleens, x20; stomachs, x10.

Cl₂MDP-liposome treatment during early H. pylori infection does not alter gastric colonization levels. Cl₂MDP-liposome-treated and PBS-treated control mice were euthanized 3 days, 10 days, or 3 months after the final dose of H. pylori infection, and gastric H. pylori colonization levels were determined by culture (Fig. 2A). H. pylori was isolated from the gastric tissue of all infected mice at all time points, and colonization levels of Cl₂MDP-liposome-treated mice were comparable to those of PBS-treated control mice during the 3-month infection period (at 3 days [data not shown; n = 5]; P = 0.31 [n = 5] and P = 0.10 [n = 10] at both 10 days and 3 months, respectively).

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FIG. 2. Cl2MDP-liposome treatment during H. pylori infection does not affect colonization levels or H. pylori-specific antibody responses. C57BL/6 mice were treated with either PBS (white symbols) or Cl2MDP-liposomes (black symbols) from 8 days prior to the first dose of H. pylori until 10 days after the final dose of H. pylori. (A) At 10 days (n = 5) and 3 months (n = 10) after infection, mice were euthanized and stomach homogenates were plated onto selective media to determine the gastric colonization levels. (B) The H. pylori-specific serum endpoint titer was determined by ELISA 3 months after infection. The median levels of the groups in panels A and B are indicated by dashes, and each symbol represents an individual animal. Data are representative of two independently performed experiments.

The H. pylori-specific antibody response is not affected by Cl₂MDP-liposome treatment. H. pylori-specific antibody responses in the sera of all infected mice were measured by ELISA 3 months after infection (Fig. 2B). H. pylori-specific antibodies were detectable in the sera of all infected mice, and Cl₂MDP-liposome treatment did not affect the levels of H. pylori-specific antibodies in serum.

Reduced gastric pathology in H. pylori-infected mice as a result of treatment with Cl₂MDP liposomes. Gastric mononuclear cellular infiltrate in Cl₂MDP-liposome-treated and PBS-treated control H. pylori-infected mice was scored 3 months after infection. Figure 3A shows an immunohistological analysis of the inflammatory infiltrate of a representative sample from each stomach region from each group of mice. The average inflammatory score of each group of mice is shown in Fig. 3B. Overall, Cl₂MDP-liposome-treated mice had lower inflammation scores, with significantly reduced levels of chronic inflammation in the lower regions of the stomach than the PBS-treated control mice (P = 0.035 [n = 10] in the antrum and lower-body regions, respectively). The average inflammation in the mid- and upper-body regions of Cl₂MDP-liposome-treated mice (1.8 ± 0.41 [mean ± standard deviation; n = 10] and 1.3 ± 0.5 [n = 10], respectively) was reduced compared with that of PBS-treated control mice (2.4 ± 0.53 and 2.1 ± 1.0, respectively), although the differences were not statistically significant (P = 0.068 in the mid-body region and P = 0.09 in the upper-body region). Gastric tissue from all mice was negative for acute gastric inflammation (influx of neutrophils) and atrophy.

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FIG. 3. Cl2MDP-liposome treatment during H. pylori infection reduces gastric inflammation. C57BL/6 mice were treated with either PBS (white symbols) or Cl2MDP-liposomes (black symbols) from 8 days prior to the first dose of H. pylori until 10 days after the final dose of H. pylori. (A) Sections of gastric tissue from mice infected H. pylori for 3 months were stained with hematoxylin and eosin and examined for gastritis by light microscopy. Values for mononuclear cell infiltration, described as chronic inflammation, were estimated by using a six-point scale. Shown are representative images of PBS-treated (a to d) and Cl2MDP-liposome-treated (e to h) mice showing inflammatory infiltrate (arrows) in the upper-body (a [score = 2.5] and e [score = 1.0]), mid-body (b [score = 2.0] and f [score = 1.5]), lower-body (c [score = 4.0] and g [score = 2.5]), and antrum (d [score = 3.0] and h [score = 2.0]) regions of the stomach. Original magnification, x20. (B) Box plot showing the gastric inflammatory scores of Cl2MDP-liposome (n = 10, hatched boxes) and PBS-treated (n = 10, white boxes), H. pylori-infected animals determined by histology 3 months after infection. Data are representative of two independently performed experiments.

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Macrophages are bone marrow-derived mononuclear phagocytes that reside within tissues, act as scavengers of immunogenic debris, and eliminate foreign particles by phagocytosis (7). Macrophages are thought to have a central function in controlling and regulating the immune response; once activated, they produce proinflammatory cytokines and chemokines (7). Activated macrophages also express major histocompatibility complex class II and can therefore activate antigen-specific CD4⁺ T cells (4).

In this study, macrophages were transiently depleted in vivo by the well-characterized liposome-mediated macrophage suicide technique (21). Van Rooijen et al. demonstrated that Cl₂MDP affects cellular metabolism once intracellular concentrations of Cl₂MDP exceed a threshold and macrophages are depleted by apoptosis. It should be noted that elimination of macrophages by Cl₂MDP-loaded liposomes is transient, as macrophage numbers return to almost normal levels in the spleen 4 weeks after Cl₂MDP-liposome treatment ceases (22).

The depletion of macrophages from the spleens of Cl₂MDP-liposome-treated C57BL/6 mice was confirmed by immunohistochemistry and reproduced previously published results (21). We also demonstrated the removal of macrophages from the stomachs of naïve animals and a substantial reduction in the number of CD11b⁺ cells in the stomachs of Cl₂MDP-liposome-treated, H. pylori-infected animals.

This paper describes the effect of in vivo macrophage depletion during experimental infection of mice with H. pylori. Mice were injected with Cl₂MDP-liposomes prior to infection with H. pylori until 10 days postinfection. While the H. pylori colonization levels were unaltered by the liposome treatment over the 3-month period following infection, the transient elimination of macrophages from the spleen and stomach significantly reduced the H. pylori-induced chronic gastric inflammation in C57BL/6 mice.

There are several potential explanations for the observation that macrophages modulate H. pylori-mediated gastritis during the establishment of infection. The accumulation of phagocytic cells in the gastric tissue during H. pylori infection correlates with the development of gastritis (25). Macrophages accumulated in the gastric mucosa are stimulated by H. pylori proteins, which results in the production of IL-1, tumor necrosis factor alpha, IL-6, IL-8, MIP-1, and GRO (5, 6, 13). These chemokines and inflammatory molecules produced during H. pylori infection may play a key role in the regulation, recruitment, and activation of inflammatory cells, such as macrophages themselves, in the gastric mucosa. For example, MIP-1 is implicated in the continuation of mucosal immune and inflammatory reactions of H. pylori-positive patients (13). Also, inducible nitric oxide synthase is important in the pathogenesis of H. pylori gastritis, as it is up-regulated in patients with chronic active gastritis or gastritis with intestinal metaplasia (23). Ihrig et al. reported that iNOS^–/– mice infected with H. felis had decreased antibody levels and less gastritis than wild-type mice (9). Therefore, cytokine production by mucosal macrophages may be central to the genesis and severity of H. pylori-induced inflammation and malignancy.

Furthermore, macrophages express Toll-like receptors that recognize H. pylori antigens and induce MyD88 and NF-B signaling pathways (15) that may aid in the development of H. pylori-induced gastritis. Moreover, products generated by macrophages can also have adverse effects on host tissue. Matrix metalloproteinase 9 and 2 levels are increased in the gastric mucosa of H. pylori-infected individuals, contributing to the destruction of gastric tissue during infection (2).

This study demonstrates that macrophages have a central role in Helicobacter infection-induced gastritis but do not affect H. pylori-specific antibody responses. This effect is most probably due to cytokine secretion and/or antigen presentation, and this hypothesis is the subject of ongoing studies. In identifying a role for macrophages in the initiation of gastritis during H. pylori infection, this study may assist in future studies targeting the inhibition of gastritis in the host and provide a stimulus to study the capacity of macrophage-modifying drugs to reduce the gastritis associated with Helicobacter disease.

 
These studies were supported by the National Health and Medical Research Council of Australia and CSL Limited. O.L.C.W. is an NH&MRC R. D. Wright Fellow.

 
* Corresponding author. Mailing address: Department of Microbiology and Immunology, The University of Melbourne, Melbourne, VIC 3010, Australia. Phone: 61 3 8344 9919. Fax: 61 3 9347 1540. E-mail: odilia{at}unimelb.edu.au

Published ahead of print on 10 March 2008.

Editor: F. C. Fang

Present address: Department of Microbiology, Monash University, Melbourne, Australia.

Contributed equally as senior authors.

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Infection and Immunity, May 2008, p. 2235-2239, Vol. 76, No. 5
0019-9567/08/$08.00+0     doi:10.1128/IAI.01481-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Reprints and Permissions Copyright Information Books from ASM Press MicrobeWorld Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Kaparakis, M. Articles by Strugnell, R. A. Search for Related Content PubMed PubMed Citation Articles by Kaparakis, M. Articles by Strugnell, R. A.