Lactobacillus acidophilus induces a slow but more sustained chemokine and cytokine response in naïve foetal enterocytes compared to commensal Escherichia coli
- Louise H Zeuthen†1,
- Lisbeth N Fink†1Email author,
- Stine B Metzdorff†2,
- Matilde B Kristensen3,
- Tine R Licht3,
- Christine Nellemann3 and
- Hanne Frøkiær2
© Zeuthen et al; licensee BioMed Central Ltd. 2010
Received: 24 July 2009
Accepted: 19 January 2010
Published: 19 January 2010
The first exposure to microorganisms at mucosal surfaces is critical for immune maturation and gut health. Facultative anaerobic bacteria are the first to colonise the infant gut, and the impact of these bacteria on intestinal epithelial cells (IEC) may be determinant for how the immune system subsequently tolerates gut bacteria.
To mirror the influence of the very first bacterial stimuli on infant IEC, we isolated IEC from mouse foetuses at gestational day 19 and from germfree neonates. IEC were stimulated with gut-derived bacteria, Gram-negative Escherichia coli Nissle and Gram-positive Lactobacillus acidophilus NCFM, and expression of genes important for immune regulation was measured together with cytokine production. E. coli Nissle and L. acidophilus NCFM strongly induced chemokines and cytokines, but with different kinetics, and only E. coli Nissle induced down-regulation of Toll-like receptor 4 and up-regulation of Toll-like receptor 2. The sensitivity to stimulation was similar before and after birth in germ-free IEC, although Toll-like receptor 2 expression was higher before birth than immediately after.
In conclusion, IEC isolated before gut colonisation occurs at birth, are highly responsive to stimulation with gut commensals, with L. acidophilus NCFM inducing a slower, but more sustained response than E. coli Nissle. E. coli may induce intestinal tolerance through very rapid up-regulation of chemokine and cytokine genes and down-regulation of Toll-like receptor 4, while regulating also responsiveness to Gram-positive bacteria.
The human gastrointestinal (GI) tract, the largest surface area of the body in contact with the environment, is lined by a single layer of intestinal epithelial cells (IEC). In adults, the GI tract is colonised by more than 1014 microorganisms comprising more than 500 different phylotypes . The gut microbiota is pivotal for the development and maintenance of intestinal immunological homeostasis. The intestinal epithelium plays key roles in maintaining this immune homeostasis in the gut as an active player in maintaining tolerance to the microbiota and food antigens as well as in pathogen combat.
The GI tract of the foetal baby is sterile, but colonisation starts immediately after birth with bacteria from the mother and the environment and, within a few days, it is colonised by numerous bacterial species. These pioneer bacteria have been shown to modulate gene expression in IEC including genes involved in metabolism, absorption, barrier function and IEC maturation . Colonisation at birth by facultative anaerobes, such as enterobacteria, coliforms, lactobacilli and streptococci, creates a reducing environment during the first week of life enabling colonisation by strict anaerobes including bifidobacteria, bacteroides, clostridia and eubacteria . This microbial colonisation contributes to recruitment of immune cells to the GI tract and may furthermore be a major contributor to establishment of the systemic immune system [4, 5]. Thus colonisation in early infancy is crucial in relation to the final composition of the permanent microbiota in adults and also in inducing intestinal and immunological maturation.
IEC sense commensals through expression of pattern recognition receptors (PRRs) recognising conserved microbial structures. The IEC respond by secreting a wide range of chemokines that recruit immune cells to the GI tract, and cytokines that affect the immune cells scattered in the GI tract including DC, macrophages and lymphocytes [6–9]. Due to the heavy bacterial antigen load in the lumen, the expression of PRRs is tightly regulated in IEC. IEC express Toll-like receptor (TLR) 1-9 , nucleotide-binding oligomerisation domain (NOD) 1 and NOD2 . However, contradicting data from cell line studies on the expression of TLRs in IEC exist. Several reports demonstrate that IEC are non-responsive towards lipopolysaccharide (LPS) and express no or very low levels of TLR4 [12, 13], while other groups have reported the presence of TLR4 [10, 14, 15]. This discrepancy may be explained by the finding that IEC gain a cross-hyporesponsive phenotype after stimulation with either LPS or lipoteichoic acid due to decreased signalling through TLR2 and TLR4 . Cario et al. elegantly demonstrated that both TLR2 and TLR4 are constitutively expressed apically in an IEC cell line but traffic to cytoplasmic compartments after ligand stimulation . IEC isolated from intestinal tissue express Tlr2 and Tlr4 mRNA but at low levels both in humans  and mice . Knowledge on IEC responses to microbe-associated molecular patterns (MAMPs) is to a large extent based on cell line studies as cell lines are naïve to MAMP stimulation. However, cell lines may not entirely reflect IEC responses at birth.
Besides playing a role in the recruitment and maturation of immune cells in the GI tract, the bacteria colonising the sterile gut probably induce tolerance dependently on TLR-activation . In this respect, the MAMPs present in the first-coming species might be crucial in tolerance development. It was recently demonstrated that, although both foetal and neonatal IEC express the TLR4/MD-2 receptor complex, they differ dramatically in their responsiveness to LPS, and it was suggested that intestinal bacterial colonisation in the newborn is facilitated by postnatal establishment of IEC tolerance towards LPS stimulation . Moreover, IEC help maintaining the specialised intestinal tolerogenic environment through secretion of different mediators, such as thymic stromal lymphopoietin (TSLP) and transforming growth factor (TGF)-β and commensals differentially affect TSLP and TGF-β production [20, 21]. Thereby the composition of the microbiota indirectly affects immune cells through effects on IEC.
We hypothesized that the very first bacteria encountered by naive IEC influence the signal molecules released to the gut environment, and that Gram-positive (G+) commensals prime IEC differently from Gram-negative (G-) commensals and LPS. Moreover, the developmental state of the IEC may play a role in their responsiveness, and this study is the first to compare foetal and neonatal germfree murine IEC responsiveness to G+ (Lactobacillus acidophilus) and G- (Escherichia coli) commensals in vitro. We present indices that the type of bacterial stimulus indeed affects gene expression in naïve primary IEC, thus suggesting an important role of the first postnatal bacteria for immune cell recruitment and tolerance induction in the GI tract.
L. acidophilus and E. coli strongly induce chemokine gene expression in foetal primary epithelial cells in vitro
L. acidophilus and E. coli up-regulate gene expression of pro-inflammatory and regulatory cytokines in foetal epithelial cells in vitro
Foetal epithelial cells produce cytokines upon in vitro stimulation with Gram-positive and Gram-negative commensals with different kinetics
E. coli is more potent than L. acidophilus in up-regulating Tlr2, Nfkb1, and Nfkb2 gene expression and in down-regulating expression of Tlr4 and Clec7a genes in foetal epithelial cells
Age dependent gut maturation does not influence early responses of epithelial cells towards Gram-positive and Gram-negative commensals
We here demonstrate that L. acidophilus NCFM and E. coli Nissle potently induce pro-inflammatory genes, a number of tolerance related genes, as well as genes involved in recruitment of immune cells to the GI tract. Although G- bacterial stimulation impacts faster than G+ bacteria, G+ bacterial stimulation elicits a more sustained response giving rise to higher production of cytokines and chemokines. Moreover, E. coli induces transcription of Tlr2, a receptor for many G+ bacteria, and down-regulates transcription of the LPS receptor, TLR4. Tlr2 expression was reduced immediately after birth independently of stimulation, but responses to stimulation were similar in IEC isolated from germ-free mice before and after birth.
Upon E. coli stimulation, foetal IEC expressed genes encoding the anti-inflammatory cytokines IL-10 and TSLP, and the chemokines KC, MIP-2 and MCP-1 potently and rapidly when compared to L. acidophilus stimulation. Relating the transcription data to the amount of protein measured in 18 h culture supernatant suggests a fast and transient up-regulation of cytokine and chemokine production induced by E. coli compared to a slower and more sustained up-regulation induced by L. acidophilus. This could be interpreted as fast induction of endotoxin tolerance during the culture period by E. coli. In line with our earlier studies in Caco2 cells , the two distinct bacteria differentially affected Tslp expression: E. coli enhanced Tslp expression, whereas L. acidophilus slightly reduced it. Production of TSLP by IEC is pivotal in maintaining gut homeostasis and indices on lower expression of Tslp in primary IEC from Crohn's disease patients than in healthy individuals have been reported . The first microbial stimuli in neonate life probably affect TSLP production, which impacts on DC in the gut and thereby immune homeostasis. Pro- and anti-inflammatory gene expression as well as Tlr4 downregulation induced by E. coli may be indispensable for balancing the immune system in the gut and, as LPS exhibited similar effects, other G- bacteria may have the same role.
Epithelial cell lines have been shown to develop a cross hypo-responsive phenotype after exposure to TLR2 and TLR4 ligands, probably due to altered signalling through TLR2 and TLR4 . However, no studies have reported how these PRRs are regulated in primary IEC from a sterile gut upon first microbial encounters. We found that E. coli induced up-regulation of Tlr2 and down-regulation of Tlr4. It could be postulated that if the pioneer bacteria colonizing the sterile gut is a G-strain, LPS will down-regulate expression of TLR4 to enable LPS tolerance to be established. However, at 4 h the down-regulation seen at 2 h already approached the basal expression level (non-stimulated). This points towards a transient down-regulation of TLR4 by G- bacteria-derived MAMPs, underlining that the expression of PRRs is tightly regulated in a dynamic fashion in order to initiate a quick response. As expression of Tlr2 was induced by E. coli, G- commensals may induce cross hyper-responsiveness towards G+ commensals in naïve primary IEC, which has not been reported before. Interestingly, Clec7a was down-regulated by E. coli, also pointing towards a yet undescribed cross-regulation of responsiveness to intestinal microorganisms. The induction of tolerance towards both G+ and G- MAMPs has been shown to depend on up-regulation of Tollip, which results in reduced phosphorylation of IL-1 receptor associated kinase (IRAK) and hence reduced NF-κB activation in both primary and immortalized IEC [10, 25]. We measured expression of the genes encoding NFκB1, NFκB2, IKKβ (the IκB kinase), Tollip, and IRAK1. However, only E. coli induced up-regulation of Nfkb1 and Nfkb2 while expression of the other signalling proteins was not modulated by in vitro stimulation. Regulation of signalling relies on phosphorylation of the gene products, protein-protein interactions and protein translocation. Hence, transcriptional regulation is presumably more relevant for the responder cytokine and chemokine genes reported here.
During the first days of life, IEC develop and mature and crypts are formed. It has been demonstrated that germfree rats have impaired formation of crypt cells suggesting that the microbiota supports IEC growth and maturation . E. coli Nissle, but not LPS, up-regulated Tlr2 expression, which may indicate a role for commensals in establishing intestinal integrity . The fact that E. coli Nissle changed the expression of more genes and acted more potently than E. coli-derived LPS reflects that intact E. coli does not, as opposed to LPS, exclusively signal through TLR4. Lotz et al.  report higher secretion of MIP-2 and KC upon in vitro LPS stimulation at day -1 before birth compared with PND1 and PND6 in mice harbouring a conventional microbiota due to tolerance acquisition at birth. However, their study does not take into account that age might influence IEC maturation stage. In order to evaluate how IEC develop with age we studied the IEC responsiveness in germfree pups at Day-1, PND1 and PND6. Except for a transient drop in Tlr2 expression at PND1, we did not find strong age-dependent differences in the IEC response. The drop in Tlr2 expression at PND1 may allow G+ commensals to colonise the gut without concomitant danger signals. Based on these findings, foetal IEC isolated from conventional mice represent an attractive supplement to polarised IEC cell line models for comparison of commensal bacteria as they are naïve to stimulation while being physiologically immature IEC and susceptible to tolerance induction. However, as IEC responses are clearly dose-dependent, in vivo experiments are still required to reveal the extent of contact between IEC and bacteria or bacterial components.
Overall, our data confirm the hypothesis that the concomitant induction of chemokines, pro- and anti-inflammatory cytokines in enterocytes by the first-coming bacteria is indeed genus dependent. We conclude that E. coli and LPS may induce LPS-tolerance partly through very rapid and potent up-regulation of chemokine and cytokine genes and down-regulation of Tlr4, whereas stimulation by L. acidophilus Gram-positive commensals may be potentiated by the up-regulation of Tlr2 by Gram-negative bacteria.
Preparation of UV-killed bacteria
L. acidophilus NCFM was grown anaerobically in de Man, Rogosa, and Sharpe broth (Merck, Darmstadt, Germany) and E. coli Nissle aerobically in Luria-Bertani broth (Merck) overnight at 37 C. The cultures were harvested, washed twice in sterile phosphate-buffered saline (PBS) (Lonza, Basel, Switzerland) and re-suspended in 1/10 the growth volume of PBS. The bacteria used for in vitro stimulation were killed by a 40-min exposure to UV-light and stored at -80 C, as we from earlier studies have concluded that live and UV-killed bacteria elicit similar responses in epithelial cell lines . Concentration was determined by lyophilisation. Endotoxin levels in L. acidophilus NCFM preparations were determined with the Pyrochrome kit (Ass. of Cape Cod, East Falmouth, MA, USA) to < 0.10 EU/ml in the highest concentration of stimuli used in cell culture experiments.
Conventional and germfree Swiss Webster mice were purchased from Taconic (Lille Skensved, Denmark), and housed under either specific pathogen-free conditions or in germfree isolators (as previously described ). Absence of colonising bacteria in germfree mice was confirmed by cultivation of faecal samples. Foetal IEC were isolated from foetuses derived from 4 conventional mothers. Caesarean section was performed on full-term pregnant females at gestation day 19 (referred to as Day -1), foetuses were killed immediately, and subsequently the small intestine was removed. Cells were pooled from 6-10 foetuses. Small intestinal tissue of neonatal mice was obtained from spontaneously delivered pups from germfree mothers at PND1 and PND6. Cells were pooled from 2-3 pups.
Isolation of primary epithelial cells
The small intestine was placed in Hanks buffered saline (HBSS, Lonza) and cut into small pieces. The epithelial cells were detached from the underlying tissue by incubation in fresh HBSS containing 2mM EDTA at 37°C for 10 minutes with vigorously shaking every 3 minutes. Residual tissue was removed by passing the suspension through a 70μm filter. Cells were subsequently washed in cold PBS and re-suspended in culture medium (RPMI 1640 supplemented with 100 U/ml penicillin, 100μg/ml streptomycin, 2 mM L-glutamine, 10% (v/v) heat-inactivated FCS, all from Lonza). Cells were seeded in 48-well tissue culture plates (Nunc, Roskilde, Denmark) at 4x105cells/500 μl/well. Fifty μl/well of bacteria or LPS O26:B6 (Sigma-Aldrich, St. Louis, MO, USA) were then added to obtain final concentrations of 1, 10, 30 or 100 μg/ml as indicated. The stimulus concentrations were chosen based on optimization experiments showing that smaller amounts of E. coli Nissle than L. acidophilus NCFM were required for stimulation of IEC. The cells were incubated for 2 h, 4 h or 18 h at 37°C in 5% CO2 and subsequently harvested by centrifugation and frozen in RNAlater (Qiagen, Hilden, Germany). The purity of the IEC was checked by staining for the lymphocyte marker CD45 (PE-labelled rat anti-mouse CD45 purchased from Abcam, Cambridge, UK) by flow cytometry. IEC contained 0.8 ± 0.4% CD45+ cells at PND6 (staining with a matched isotype antibody (IgG2b) subtracted). Viability and cell numbers of IEC were determined by staining the cell nuclei with propidium iodide before and after cell lysis (reagents from Chemometec, Allerød, Denmark) and analysed with NucleoCounter (Chemometec). Viability of the IEC was evaluated during culture. We found that 23.5 ± 5.0% of freshly purified IEC were dead, 45.3 ± 2.9% after 2 h, 65.7 ± 6.5% after 4 h, 62.3 ± 6.6% after 7 h and 73.3 ± 7.6% after 24 h of culture.
RNA purification and amplification
Samples were spun at 3000 g, 5 min, 4°C to remove RNAlater. RNA was extracted from the cell pellet using Mini Kit from Qiagen following the supplier's protocol for animal cells. The quantity and purity of extracted RNA was evaluated by Nanodrop spectroscopy (Thermo Scientific, Wilmington, DE, USA). cDNA was produced from app. 500 ng total RNA using High-Capacity cDNA Reverse Transcriptase Kit (Applied Biosystems, Foster City, CA, USA) according to the instructions of the manufacturer.
Gene expression analysis by real-time polymerase chain reaction
Genes measured by Taqman low density array and (if bold also) by Taqman gene expression assays
House keeping genes
actin, beta, cytoplasmic
eukaryotic 18S rRNA
chemokine (C-C motif) ligand 2
chemokine (C-C motif) ligand 3
chemokine (C-X-C motif) ligand 1
chemokine (C-X-C motif) ligand 2
transforming growth factor, beta 1
tumor necrosis factor
thymic stromal lymphopoietin
nuclear factor of kappa light chain gene enhancer in B-cells 1
nuclear factor of kappa light chain gene enhancer in B-cells 2
inhibitor of kappaB kinase beta
interleukin-1 receptor-associated kinase 1
toll interacting protein
Pattern recognition receptors
c-type lectin domain family 7, member a
toll-like receptor 2
toll-like receptor 4
lymphocyte antigen 96
Cytokine quantification in culture supernatants
The production of MIP-2, IL-10, -6, TNF-α was analysed using commercially available ELISA kits (R & D systems, Minneapolis, MN, USA).
GraphPad Prism version 4.03 (GraphPad Software, San Diego, CA, USA) was used to perform two-way ANOVA with Bonferroni post-test. Although fold change is plotted in gene expression experiments, statistical analysis was performed on CTvalues as these are assumed normally distributed as opposed to the fold change values.
The authors thank Heidi Letting, Camilla S. Dall, Lisbeth B. Rosholm and Anne ørngren whose expert technical assistance is greatly appreciated. This work was supported by the Globalisation Funds, Technical University of Denmark, through FoodDTU.
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