Critical Role of Antimicrobial Peptide Cathelicidin for Controlling Helicobacter pylori Survival and Infection Free

Helicobacter pylori is an important pathogen that colonizes >50% of the world’s population and is associated with chronic gastritis, peptic ulcer, gastric cancer, and MALT lymphoma (1, 2). Despite that the prevalence of H. pylori infection is declining in many developed countries, it remains an important and endemic public health issue in East Asian countries (3), which accounts for ∼65–80% of gastric cancer (4). The standard triple therapy for H. pylori eradication usually consists of a ≥10-d treatment with a proton pump inhibitor, amoxicillin, and clarithromycin (5). Other antibiotics, such as metronidazole and levofloxacin, are also commonly used. However, the development of antibiotic resistance in H. pylori substantially hinders the treatment of H. pylori–associated disorders. Antimicrobial resistance in H. pylori is widespread. The prevalence of H. pylori resistance to clarithromycin, metronidazole, levofloxacin, and multiple antibiotics has reached an alarming rate of 21.5, 95.4, 20.6, and 25.5%, respectively, in southeast China, rendering effective eradication of H. pylori a unique therapeutic challenge (6, 7).

It is evident that bacteria biofilm is an ancient and integral component of the prokaryotic life cycle, and it is a key factor for survival in diverse environments (8). It is critical not only for microbial survival in diverse environments, but also for successful infection by numerous pathogenic bacteria. As biofilm protects bacteria during infections in humans and allows bacterial survival in a hostile environment, inhibition of biofilm formation by novel therapeutics may prevent bacterial colonization (9). For H. pylori, resistance to conventional antimicrobial therapies has been attributed not only to its genetic variability, but also to its ability to form biofilm as a recalcitrant strategy (10). Several recent reports indicate that H. pylori forms biofilm both in vitro (11) and in vivo (12, 13). In this regard, antimicrobial peptides that target either the organization of the bacterial membrane bilayer or bacteria biofilm are promising therapeutic approaches for treating persistent bacterial infection (14).

Cathelicidins are endogenous host defense peptides with a diverse range of antimicrobial activities, which provide the first-line defense against infection by promoting rapid elimination of pathogens. They are protective against Gram-negative and/or Gram-positive bacteria (15, 16), fungi (17), parasites (18), and enveloped viruses (19–21). Notably, cathelicidins have been found to have anti-biofilm activities in various bacteria, including Pseudomonas aeruginosa (16), Burkholderia pseudomallei (22), Fusobacterium nucleatum (15), and Staphylococcus aureus (15, 23). However, it has not yet been demonstrated whether cathelicidins could prevent H. pylori biofilm formation or destruct the existing biofilm formed by the bacteria.

Previously, we found that endogenous cathelicidin acts as a crucial host defense factor against H. pylori colonization by the use of cathelicidin-knockout (Cnlp^−/−) mice (24). In the present study, we report that both endogenous and exogenous cathelicidin display obvious antimicrobial activity on normal and drug-resistant H. pylori infection in vitro and in vivo. Additionally, in reducing bacteria viability, we found that cathelicidin could confer protection by disrupting H. pylori biofilm formation. We also demonstrate the feasibility of CRAMP supplementation by administration of bioengineered Lactococcus lactis that is protective against H. pylori colonization and the associated inflammation and mucus secretion impairment.

H. pylori standard strains Sydney strain 1 (SS1) and 10783 (clarithromycin-resistant strain) were provided by the Department of Microbiology, Prince of Wales Hospital, Chinese University of Hong Kong. The two strains of H. pylori were initially grown on horse blood agar plates (Columbia blood agar base with Dent selective supplements; Oxoid, Basingstoke, U.K.) in anaerobic jar with a microaerophilic environment for 5 d at 37°C. The bioengineered cathelicidin-secreting L. lactis was generated as previously described (24).

The synthetic mouse cathelicidin CRAMP (GLL RKG GEK IGE KLK KIG QKI KNF FQK LVP QPE Q) with purity 95% was purchased from AnaSpec (Fremont, CA). Human cathelicidin LL-37 (LLG DFF RKS KEK IGK EFK RIV QRI KDF LRN LVP RTE S) and shortened LL-37 (FKR IVQ RIK DFL RNL V) with purity of 95% were purchased from Invitrogen (Carlsbad, CA).

H. pylori suspended in Brucella broth with 5% FBS was incubated under microaerophilic conditions at 37°C in 96-well plates. Cultured 10⁸ CFU H. pylori SS1 or 10783 per well were treated with 0.25 μg/ml clarithromycin, CRAMP, LL-37, or sLL-37. PBS was used as a negative control. After 48 h of shaking at 200 rpm at 37°C, H. pylori growth was determined by the OD₅₉₅ value of each well (25). For anti-biofilm assay, H. pylori SS1 (10⁸ CFU) was incubated in 96-well microtiter plates or three-well chamber slides per well under microaerobic conditions for 3 d. Then the supernatants were aspirated and treated with LL-37 for 48 h. At the end of the incubation period, the resident biofilm was measured by crystal violet assay (26, 27) or BacLight Live/Dead stain visualized using confocal microscopy (28).

H. pylori SS1 cells (10⁸ CFU) were treated with CRAMP, LL-37, or sLL-37 for 24 h and then H. pylori was prefixed in a solution of 2.5% (v/v) glutaraldehyde with 0.1 M cacodylate buffer for 1 h and then washed with cacodylate buffer solution. After washing with PBS, the samples were postfixed in 2% (w/v) osmium tetroxide for 2 h and dehydrated with ethanol. All the samples were sputter coated (Emitech K 550) with gold palladium. Scanning electron microscopy (JEOL JSM6301F) was performed using secondary electrons (SE1) at 5 kV (29).

Human gastric epithelial cell line HFE-145 was a gift from D. Smoot (Howard University, Washington, DC). The intracellular H. pylori was determined by gentamicin protection assay as previously described (30).

The expression of LL-37 was lowered using predesigned target-specific small interfering RNA (siRNA) molecules purchased from Santa Cruz Biotechnology (Santa Cruz, CA, USA). Fifty picomoles gene-specific or control siRNA was transfected into cells at 40–60% confluence using jetPRIME transfection kit (Polyplus).

129/SVJ wild-type (Cnlp^+/+) and cathelicidin-knockout (Cnlp^−/−) mice (10- to 13-wk-old male mice) were inoculated with either H. pylori SS1 or H. pylori 10783 (1 × 10⁹ CFU/ml) suspended in sterile brain heart infusion (BHI) or a sterile BHI every other day for a total of three doses. All animal experiments used were performed under Laboratory Animals Ethics Committee approval. L. lactis transformed with control plasmid (N) and CRAMP-secreting L. lactis treated with inducer nisin for 3 h (N₄I) were prepared as described previously (24). Cnlp^−/− and Cnlp^+/+ mice were pretreated with 10¹⁰ CFU probiotics (N or N₄I) for 2 wk before H. pylori challenge. These animals were continuously treated with these probiotics every other day for 2 mo.

Histological sections of stomachs were evaluated and graded according to the updated Sydney classification (31). All sections were evaluated by two experienced pathologists who were blinded to the experimental grouping and had no knowledge of the experimental design. Tissue sections were deparaffinized, rehydrated, and rinsed in distilled water. Ag retrieval was done using a microwave oven with sodium citrate buffer (pH 6.0) for 5 min. The endogenous peroxidase activity was quenched in 3% H₂O₂ for 10 min. Immunofluorescence staining for H. pylori and CRAMP was performed using anti–H. pylori Ab (Abcam, Science Park West, Hong Kong, China) and anti-CRAMP Ab (Santa Cruz Biotechnology) as primary Abs. Alexa Fluor anti-goat 568 and Alexa Fluor anti-rat 488 (Invitrogen, Camarillo, CA) were used as secondary Abs. Additionally, DAPI was used to stain cell nuclei. Sections were evaluated with a laser confocal microscope (Olympus FV1000).

Two hundred microliters whole blood was collected from Cnlp^+/+ and Cnlp^−/− mice using capillary filled with EDTA into 2 ml cold PBS with 15% EDTA. Cells were analyzed by a semiautomatic biochemical analyzer (MS-500A).

Mouse stomach genomic DNA was extracted by a DNA purification kit (Promega, San Luis Obispo, CA) according to the manufacturer’s instruction. Total RNA was isolated from mouse stomachs using TRIzol reagent (Life Technologies, Carlsbad, CA). Reverse transcription and real-time PCR measurement were performed using iQ SYBR Green Supermix (Bio-Rad, Hercules, CA) and a multicolor real-time PCR detection system (Bio-Rad), respectively (Table I). The results were analyzed using the comparative threshold cycle (C_T) method.

Mucus layer was visualized by periodic acid–Schiff (PAS) staining following formalin fixation. After deparaffinization and rehydration, tissue sections were oxidized in 1% (v/v) periodic acid, stained with Schiff’s reagent, and counterstained with Mayer’s hematoxylin. The mucus-containing cells were stained purple-red. The thickness of the mucus-secreting layer was measured perpendicularly to the mucosal surface from the edge of the epithelium to the outermost part of the mucus-secreting layer under microscopy at ×100 magnification. Results were expressed as the ratio of the thickness of the mucus-secreting layer to the thickness of the total mucosa. All analyses were performed blindly.

For detection of anti-CRAMP, microtiter plates were coated with 1 μg/ml CRAMP (Innovagen) for 2 h at room temperature. Sera were diluted 1:500 and incubated for 90 min at room temperature. After several washing steps with PBS/0.05% Tween 20, bound anti-CRAMP was detected using 0.02 μg/ml HRP-conjugated goat anti-human IgG Ab (SouthernBiotech) and substrate solution (eBioscience). Detection was carried out with a Tecan Infinite F200 Pro microplate reader at 450 nm/620 nm.

All data were expressed as the mean ± SEM. To determine statistical significance between groups, comparisons were made using two-tailed t tests. Analyses of multiple groups were done by one-way ANOVA followed by the Tukey t test using GraphPad Prism version 4 (GraphPad Software, San Diego, CA). For all statistical tests, a p value <0.05 was considered statistically significant.

To examine the potential antimicrobial activity of different analogs of cathelicidin against H. pylori, bacterial viability was determined after exposure to a range of concentrations of CRAMP (mouse cathelicidin), LL-37 (human cathelicidin), and sLL-37 (a fragment of LL-37 corresponding to residues 17–32) in the culture media. All three forms of cathelicidin significantly inhibited H. pylori SS1 growth at the micromolar range of concentrations and were comparable to clarithromycin (0.33 μM), a standard drug for H. pylori eradication in humans (Fig. 1A). We further examined the antimicrobial activity of cathelicidins in clarithromycin-resistant H. pylori 10783, a strain derived from clinical isolate. As shown in Fig. 1B, clarithromycin could not inhibit the bacteria growth in a significant manner. In contrast, CRAMP, LL-37, and sLL-37 markedly inhibited H. pylori 10783 growth. The antimicrobial activity of cathelicidin against both drug-sensitive and -resistant strains of H. pylori occurred at concentrations that have been reported to inhibit the growth of other pathogenic bacteria by other investigators (21, 32). Cathelicidins are known to contain an amphipathic α-helical structure that is thought to aggregate and form pores on the bacterial membrane (16). Thus, we performed scanning electron microscopy to observe microscopic alterations in H. pylori SS1 in response to cathelicidin treatment. Herein, CRAMP, LL-37, and sLL-37 were shown to alter the normal morphology of H. pylori, which was characterized by shrinking of the flagella and pore formation on bacteria membranes (Fig. 1C), causing the loss of cell viability ultimately.

Scanning electron microscopy analysis of gastric biopsies of H. pylori–infected patients demonstrated that the bacteria could form biofilm on the gastric mucosa epithelium (10). In this study, we investigated whether H. pylori could form biofilm in the stomachs of Cnlp^+/+ and Cnlp^−/− mice. The scanning electron microscopy images showed that H. pylori SS1 successfully formed biofilm in both types of mouse stomachs compared with the noninfected mice (Supplemental Fig. 1). We next investigated whether cathelicidins could affect biofilm formation by H. pylori in vitro. Consistent with its antimicrobial activity, human cathelicidin LL-37 decreased and disrupted the biofilm formation in a dose-dependent manner as determined by BacLight Live/Dead stain (Fig. 2A) and crystal violet assay (Fig. 2B). These findings supported that cathelicidin not only possesses bactericidal activity but also could disrupt biofilm.

To address the relevance of endogenous cathelicidin in control of H. pylori SS1 colonization in gastric epithelial cells, we transfected HFE-145 cells with control or LL-37–specific siRNA followed by H. pylori infection. Results indicated that LL-37 knockdown significantly promoted the intracellular survival of H. pylori as determined by quantitative culture (Fig. 3Aa) and immunofluorescence staining for H. pylori (Fig. 3Ab, 3B). Moreover, it is noteworthy that this phenomenon was confirmed in Cnlp^−/− mice infected by SS1 or drug-resistant strain 10783 of H. pylori for 3 mo (Fig. 4A, 4B). We found that more H. pylori (SS1 or 10783) was being cultured by colony formation assay and stained in stomachs of Cnlp^−/− mice (Fig. 4C).

To assess the potential role of cathelicidin in H. pylori–associated colonization, we infected Cnlp^+/+ and Cnlp^−/− mice with H. pylori by oral gavage to induce gastritis. Three months after H. pylori SS1 inoculation, mice developed gastritis with altered mucosal architecture and neutrophil infiltration. Inflammation score grading was performed according to the updated Sydney classification. The results showed that the inflammation score was significantly higher in the H. pylori–infected Cnlp^−/− mice (p < 0.05) when compared with the H. pylori–infected Cnlp^+/+ mice (Fig. 5A). To further determine the inflammatory response to H. pylori infection, gastric tissues were collected for evaluation of proinflammatory cytokine expression as determined by real time-PCR whereas the proportion of granulocytes in circulating peripheral leukocyte was measured in mice. Results showed that H. pylori infection induced a more prominent granulocyte expansion (Fig. 5B), and stronger induction of IL-6, IL-1β, and ICAM1 (Fig. 5C) in Cnlp^−/− mice than in Cnlp^+/+ mice. In contrast, the anti-inflammatory cytokine IL-10 mRNA expression was much lower in H. pylori–infected Cnlp^−/− mice (Fig. 5C).

Our laboratory previously bioengineered probiotic bacteria to actively secrete cathelicidin (33–35). In the present study, we determined whether CRAMP (mouse cathelicidin) could be delivered by the bioengineered L. lactis during H. pylori infection. In noninfected wild-type mice, there were low levels of CRAMP expression in gastric epithelial cells (Fig. 6B) whereas no immunoreactivity was observed in the Cnlp^−/− mice (Fig. 6B,′). Upon H. pylori infection, we observed that CRAMP was upregulated in gastric epithelial cells (Fig. 6D). Again, no fluorescence signal was detected in the Cnlp^−/− mice (Fig. 6D,′). Treatment with the control L. lactis did not alter the gastric mucosal expression of CRAMP in both types of mice (Fig. 6F, F,′) whereas exogenous CRAMP could be successfully delivered by the CRAMP-secreting L. lactis to the gastric epithelium as shown by more intense fluorescence signal in the gastric mucosa in both Cnlp^+/+ (Fig. 6H) and Cnlp^−/− (Fig. 6H,′ mice. To quantify the amount of CRAMP produced by cathelicidin-secreting L. lactis, we performed CRAMP ELISA assay for mouse stomachs in different treatment groups. Results showed that 1 × 10¹⁰ CFU CRAMP-transformed L. lactis with 3 h nisin induction could produce 378.521 pg/ml CRAMP peptide in the supernatant of L. lactis culture medium. We also found that the concentrations of CRAMP were lower in the stomachs of control (102.65 ± 5.38 pg/ml) and uninduced L. lactis–inoculated (115.84 ± 19.42 pg/ml) wild-type mice chronically infected with H. pylori when compared with the uninfected mice (152.87 ± 1.78 pg/ml). Inoculation of cathelicidin-secreting L. lactis (140.65 ± 15.99 pg/ml) returned the cathelicidin levels nearly back to those of uninfected mice. In contrast, CRAMP was undetectable in all groups of Cnlp^−/− mice except those inoculated with cathelicidin-secreting L. lactis (99.94 ± 20.76 pg/ml). These results indicated our present regimen could deliver exogenous CRAMP to mouse stomachs and boost CRAMP expression level back to normal (Fig. 7).

The effect of exogenous cathelicidin on H. pylori colonization was measured by determining the levels of H. pylori–specific 16S rDNA in Cnlp^−/− mice. After administration of cathelicidin-secreting L. lactis, the level of H. pylori 16S rDNA was significantly decreased (p < 0.05) in the Cnlp^−/− mice (Fig. 8A). To further determine the anti-inflammatory activity of cathelicidin-secreting L. lactis during H. pylori infection, Cnlp^−/− mice gastric tissues were collected for evaluation of proinflammatory cytokine expression by real time-PCR. Results showed that the mRNA expressions of proinflammatory cytokines IL-6, IL-1β, and ICAM1 were markedly elevated by H. pylori infection in Cnlp^−/− mice, in which cathelicidin-secreting L. lactis significantly reduced their mRNA expression nearly back to the basal levels (Fig. 8B).

The mucus-secreting layer visualized by PAS staining and the ratio of mucus-secreting layer to mucosal thickness were evaluated. As shown by the PAS staining, the thickness of the mucus-secreting layer in both wild-type and Cnlp^−/− mice was clearly reduced during chronic H. pylori infection (Fig. 9A). The mucus-containing cells were drastically depleted in the H. pylori–infected Cnlp^+/+ and Cnlp^−/− mice in the gastric corpus as well as the antrum. Such reduction could be reversed by pretreatment with cathelicidin-secreting L. lactis (Fig. 9B). The gastric MUC1 gene expression was also monitored and compared among groups using real-time PCR. As shown in Fig. 9C, MUC1 expression decreased markedly during inflammation in both H. pylori–infected Cnlp^+/+ and Cnlp^−/− mice, in which cathelicidin supplement by administration of cathelicidin-secreting L. lactis significantly reversed such downregulation. In the present study, the Muc1 expression and mucus layer thickness in the stomachs of Cnlp^−/− mice were the same as those of Cnlp^+/+ mice. This observation indicated that cathelicidin might be required for inducible but not constitutive expression of mucin. It may partially explain why cathelicidin gene knockout had no effect on mucin expression. From these results, the thickness of the mucus-secreting layer seems not to be relevant for the protection of Cnlp^+/+ mice from the colonization of H. pylori in the stomach by cathelicidin.

As occurrence of infection with multidrug-resistant H. pylori is increasing, research into the mechanism by which antimicrobial peptides regulate innate immune responses represents an important direction in formulating novel therapeutic intervention for H. pylori infection. As a prominent member of host defense peptides, cathelicidin controls the fate of many extracellular (36) and intracellular microbes (37). However, our understanding regarding the cathelicidin-related molecular processes on H. pylori infection is still primitive. Hence, we sought to uncover the actions and possible mechanisms of cathelicidin in controlling H. pylori survival and its associated gastritis.

Advanced physiochemical techniques revealed that human cathelicidin, LL-37, uses a toroidal pore carpet-like mechanism commonly used by other cationic antimicrobial peptides to form pores and cause severe leakage on biological membranes (38). Importantly, LL-37 has been shown to inhibit H. pylori growth in vitro with unknown mechanism (39). In the present study, we identified that CRAMP, LL-37, and sLL-37 affected normal as well as drug-resistant H. pylori growth, similar to previous observation against Burkholderia thailandensis (40). Scanning electron microscopy also clearly indicated pore formation on the membrane of H. pylori. Furthermore, disruption of H. pylori flagella may be another mechanism underlying the antimicrobial activities of cathelicidin, which warrants further investigation.

The reliance of H. pylori on biofilm formation for persistent infection has been confirmed in both human gastric mucosa (41) and glass surface in vitro (42). However, the importance of exopolysaccharide matix on Cnlp^+/+ and Cnlp^−/− mice stomach homeostasis and the indirect effect of such matrix on H. pylori colonization are still unknown. Thus, we investigated H. pylori biofilm formation in stomach mucosa from both cathelicidin knockout and wild-type mice by scanning electron microscopy. The results indicated that biofilm existed in mouse stomachs after H. pylori infection and that more exopolysaccharide matrices were deposited by H. pylori in the Cnlp^−/− gastric surface when compared with Cnlp^+/+ mice. In line with our findings, extracellular polysaccharide has been confirmed to promote biofilm in P. aeruginosa (43). Furthermore, we showed evidence of the destruction of H. pylori biofilm formation by LL-37 using in vitro crystal violet assay. Based on the previous antimicrobial findings, we propose that the action of cathelicidin on H. pylori survival in vitro may be attributed to their abilities to form pores on the membrane, weakening the movement of flagella and impairing H. pylori biofilm formation. It has been demonstrated that H. pylori, with the help of urease and flagella, must quickly migrate from the gastric lumen (pH 1–2), through the mucus layer (pH 4–7), to the gastric epithelial surface (pH 7) to maintain survival (44). In the present antimicrobial and anti-biofilm assays, H. pylori were cultured in Brucella broth with 5% FBS (pH 6) under microaerophilic conditions at 37°C. This culture condition should be similar to the in vivo microenvironments, and this condition has been widely used in H. pylori in vitro experiments (11, 45, 46).

To verify whether endogenous cathelicidin could play a role in the clearance of normal and drug-resistant H. pylori, cathelicidin mRNA level was determined in human gastric epithelial cells (Supplemental Fig. 2) and the protein level was determined in mouse stomachs after H. pylori infection (Fig. 9). H. pylori in contact with human gastric epithelial cells resulted in rapid production of cathelicidin, with maximum level detected at 1 h after stimulation. The initial increase was followed by a rapid decrease of the transcript levels, and, after infection for 24 h, the level of cathelicidin mRNA was back to normal (Supplemental Fig. 2). Similar to the in vitro findings, mouse cathelicidin in stomach was found to be substantially increased in the wild-type mice after infection with H. pylori SS1 for 24 h in our previous study (24). This rapid induction hinted at the importance of cathelicidin as the first line of mucosal defense against bacterial infection in the stomach. Indeed, in cathelicidin-deficient (Cnlp^−/−) mice or cathelicidin-silenced (transfected with LL-37 siRNA) gastric epithelial cells, the antimicrobial properties of the gastric epithelium were substantially compromised during either normal or drug-resistant H. pylori infection. These results indicate that absence of endogenous cathelicidin could aggravate both normal and drug-resistant H. pylori colonization.

Cathelicidin is a modulator of mucosal inflammatory cell infiltration (47). Concordantly, our data indicate that cathelicidin could alleviate H. pylori–associated inflammation. To substantiate this anti-inflammatory function, we evaluated the extent of gastritis by histological measurement and cytokine expression in gastric tissues of cathelicidin-deficient mice (Cnlp^−/−) and their normal counterparts (Cnlp^+/+) after 3 mo of H. pylori infection. Histological examination showed that Cnlp^−/− mice had much higher inflammation scores in the gastric mucosa and much more granulocytes in the circulation than did the wild-type mice. Similarly, there was more IL-6, IL-1β,and ICAM1 mRNA expression in the stomachs of cathelicidin-deficient mice after H. pylori challenges. Whether the alleviation of inflammation is caused by reduced bacterial load, however, warrants further investigation.

Previous studies have shown that the probiotic L. lactis could colonize the gastric epithelium (48–50). Using the nisin-inducible system, we have also shown that cathelicidin-encoding vector could be successfully transformed into L. lactis (24, 35) and produce 378.521 pg/ml CRAMP peptide in the supernatant of L. lactis culture medium determined by a CRAMP ELISA kit. Previously, we identified the adherence and colonization of bioengineered L. lactis in the gastric epithelia of Cnlp^−/− mice by immunohistochemical staining (24, 35). In the present study, we performed conventional PCR to measure bioengineered L. lactis in mouse stomachs. Results showed that both control L. lactis and CRAMP-secreting L. lactis could be detected in mouse stomachs as least 2 mo after administration (Supplemental Fig. 3). To determine the levels of CRAMP produced by bioengineered L. lactis, we performed a CRAMP ELISA assay for mouse stomachs in different treatment groups. We found that cathelicidin-secreting L. lactis could supplement cathelicidin in H. pylori–infected mice (Fig. 9). The present data combined with previous findings suggest that this novel gene delivery approach could effectively deliver exogenous cathelicidin to the gastric epithelium and thereby reduce H. pylori survival and H. pylori–associated gastric inflammation.

Epithelial cells lining on mucosal surfaces function as protective barriers against infectious pathogens (51, 52). Mucins play a pivotal role in mucosal barrier and they are secreted to the apical surface of epithelial cells in lungs, stomach, intestines, eyes, and several other organs (53). Increasing evidence also shows that abnormal mucus secretion is a pathogenic hallmark of gastritis (54) and gastric cancer (55, 56). The function of mucins is to protect the body from infection by binding pathogens to oligosaccharides, preventing them from reaching the cell surface (57). To this end, the gastric epithelial cells secrete a mucus layer, which contains glycocalyx and mucin 1 (58). Our previous study has shown that exogenous cathelicidin could regulate mucin expression during experimental colitis (33, 59). During infection with H. pylori, the rate of mucin turnover and the levels of mucin 1 were reduced (54, 53). When H. pylori colonizes in this mucus niche, the decreased turnover may produce a more stable and favorable microenvironment for the bacteria by impairing their clearance by mucus flow (58). In humans, polymorphisms in the MUC1 allele, which result in the short form of this mucin, are associated with an increased susceptibility to gastric cancer and H. pylori–induced gastritis (53, 55). Previous study also showed that deficiency in mucin 1 facilitates a significant increase in H. pylori colonization as well as greater TNF-α and keratinocyte chemoattractant mRNA levels (51, 54). In line with the previous reports (53, 54, 60, 61), H. pylori infection reduced the thickness of the stomach mucus-secreting layer in the present study. Both Cnlp^+/+ and Cnlp^−/− mice showed a significant reduction of the thickness of the mucus layer after gastritis induction, and, more importantly, cathelicidin supplementation by bioengineered L. lactis preserved the mucus-secreting layer. Results from real-time PCR suggest that cathelicidin may stimulate mucus synthesis in the gastric mucosa by upregulating the mRNA expression of MUC1. Consistently, a finding from a recent MUC1 knockout mouse model has confirmed that disruption in mucus synthesis could exacerbate experimental gastritis induced by H. pylori (55).

This work has uncovered the previously unappreciated role of cathelicidin in host defense against H. pylori infection. In this regard, cathelicidin could be regarded as a potential natural antibiotic for promoting the clearance of H. pylori. Our cathelicidin-secreting L. lactis therapy could not only provide a new mechanistic insight into the role of cathelicidin in pathogen infection, but it also represents a novel therapeutic approach to combat infectious diseases, especially those caused by antibiotic-resistant bacteria. By understanding and harnessing the protective mechanisms of cathelicidin against H. pylori infection together with the application of our novel peptide delivery strategy, cathelicidin-secreting probiotics could be effective therapeutic agents for the treatment of H. pylori infection and its associated gastritis in humans.

We thank the Core Facility of the School of Biomedical Sciences of Chinese University of Hong Kong for expert technical help in SEM and confocal imaging.

The authors have no financial conflicts of interest.