INTRODUCTION
Antimicrobial peptides secreted by intestinal immune and epithelial cells are important effectors of innate immunity. These endogenous peptides are induced during exposure to enteric pathogens in an attempt to protect the host from infection (
1). It is increasingly apparent that antimicrobial peptides also play an essential role in the maintenance of intestinal homeostasis by limiting microbial-epithelium interactions and preventing unnecessary microbe-driven inflammation (
2). This is particularly important in the distal gut where microbiota load and density are high.
The intestinal microbiota consists of a complex community of bacteria with various physiological and immune-modulating capacities (
3). A balanced composition of symbionts and pathobionts is thought to stimulate homeostatic responses in the host (
3), while shifts in this balance (dysbiosis) have been associated with inflammatory disorders of the gut, such as inflammatory bowel disease (IBD) (
4). Recently, it has been shown that the intestinal microbiota provides pivotal stimuli and cues necessary for the induction of antimicrobial peptides (
5).
Regenerating islet-derived III (REGIII) proteins, which belong to the family of C-type lectins, are one class of antimicrobials that are expressed in the intestine. In mice, three distinct classes of
RegIII,
RegIII-α, -β, and -γ, have been identified. In contrast, only
REGIII-α and -γ have been identified in humans. Human
REGIII-α, also known as hepatocarcinoma-intestine-pancreas/pancreatic-associated protein (HIP/PAP), shares 67% homology with murine
RegIII-γ, while human
REGIII-γ shares 68% homology with murine
RegIII-β. REGIII proteins bind to the peptidoglycan moieties of bacteria and induce damage to the bacterial cell wall (
6–8). Different intestinal cell types express REGIII proteins (
6,
9). We have previously shown that modulating intestinal microbiota composition, either by colonizing with microbiota devoid of pathobionts or supplementation with a probiotic bacteria, affects the expression of
RegIII-γ in mice lacking intracellular microbial recognition receptors (
10). However, it is unclear whether a specific component(s) of the intestinal microbiota differentially and directly regulates the expression of REGIII proteins by various intestinal cell types.
Here, we sought to determine whether different components of the intestinal microbiota and specific probiotics differ in their capacities to stimulate the expression of antimicrobial peptide REGIII by intestinal epithelial cells (IECs). For this, we investigated the effects of colonization with both diverse communities and specific components of the microbiota on REGIII-γ expression by ileal and colonic epithelial cells. REGIII-γ levels were quantified in whole ileal and colonic tissue of germfree (GF) mice and in mice colonized with specific-pathogen-free (SPF) microbiota, altered Schaedler flora (ASF), commensal Escherichia coli JM83, or the probiotic Bifidobacterium breve NCC2950. The importance of Toll-like receptor (TLR) signaling in this process was also investigated using GF MyD88−/−; Ticam1−/− mice. Furthermore, REGIII-α expression in human colonic epithelial cells was also quantified after stimulation with E. coli or B. breve.
DISCUSSION
Antimicrobial REGIII proteins play an important role in maintaining gut homeostasis through spatially segregating bacteria, preventing potentially harmful immune responses, and protecting the host from infection (
16–18). In parallel with previous findings (
19–22), this study shows that the intestinal microbiota affects the level of REGIII expression in the intestine. However, the level of expression differs depending on the region of the gut examined and the nature of microbiota to which it is exposed. We demonstrated that the probiotic
Bifidobacterium breve NCC2950 but not the commensal
Escherichia coli JM83 significantly induced REGIII expression
in vivo in mice and
in vitro in a human intestinal cell line, and this upregulation was independent from the metabolic activity of the strain and mediated through MyD88-Ticam1 signaling. Collectively, these results indicate that regulation of REGIII depends on the richness and specific components of the intestinal microbiota.
Colonization with the community of eight strains of bacteria that compose the ASF did not induce the same level of REGIII-γ expression as observed in SPF mice. Colonization with ASF has previously been shown to effectively reverse GF-related phenotypes (
23); however, ASF-induced phenotypes are not always identical to those found in SPF mice (
11,
24). This may be related to variability in SPF composition; the SPF mice used in this study came from Taconic and, unlike SPF mice from other suppliers, contain segmented filamentous bacteria (SFB), a potent inducer of T-helper 17 (Th17) cells (
25). IL-22 produced by Th17 cells (
26) induces the expression of REGIII-γ in both murine and human colonic epithelial cells (
17,
27). In fact, monocolonization with SFB led to increased REGIII-γ production, comparable to that of ASF-colonized BALB/c mice (
28). It has been shown that reducing microbiota diversity with broad-spectrum antibiotics decreases REGIII-γ expression (
16,
22). Thus, in conjunction with previous work, our results support the notion that microbiota composition, especially with regard to the presence of pathobionts, is an important factor in REGIII-γ regulation.
We have previously shown that supplementation of
B. breve upregulated REGIII-γ in
Nod1−/−;
Nod2−/− mice with lower baseline expression of this antimicrobial peptide (
10), but the mechanisms underlying this specific probiotic stimulation were unclear. In this study, we monocolonized wild-type C57BL/6 mice with
B. breve or
E. coli and found that
B. breve but not
E. coli significantly increased REGIII-γ levels in the ileum and colon. Although ileal bacterial counts were not performed, fecal and cecal content showed similar bacterial loads of
B. breve and
E. coli, suggesting that the effects on REGIII-γ expression were not due to differential capacity of these two strains to colonize the gut. It should be noted that although
B. breve stimulated REGIII-γ, the level of induction was lower than in SPF mice. In accordance with our findings, monocolonization with
Bacteroides thetaiotaomicron, but not with noninvasive
Listeria innocua, resulted in increased REGIII-γ expression in the small intestine that did not reach the level seen in SPF mice (
6). Other investigators have also determined the effect of specific mono- or dicolonizations on REGIII-γ production in the colon and cecum, with variable results (
28,
29). Overall, the data support the conclusion that the net effect of intestinal bacteria on REGIII-γ expression will be modulated by the presence of specific strains in the microbiota that include both commensals and potential pathobionts. One important aspect in colonization studies relates to variability in experimental design and the time point of tissue sampling. Dynamic changes in the microbiota load and diversity, as well as in immune responses, occur immediately after colonization (
30). We chose to evaluate REGIII-γ at a steady state (21 days) postcolonization (
31) since we were interested in defining REGIII responses under stable conditions. Indeed, REGIII levels have been shown to peak at 4 days postcolonization in the small intestine and to stabilize by day 16. Therefore, in addition to strain specificity, the time point chosen to determine colonization effects in antimicrobial peptides should be carefully defined.
Many cell types in the gastrointestinal tract are capable of producing REGIII proteins, including intestinal epithelial cells (IECs) and γδ intraepithelial lymphocytes (
6,
32). Studies have proposed that IECs, particularly enterocytes, are producers of REGIII-γ in the colon (
28,
33). In this study, we used immunofluorescence to investigate the main source of REGIII-γ after microbial exposure
in vivo. We found that IECs are the cell types that predominantly express REGIII-γ in both the ileum and colon. Furthermore, our
in vitro studies confirmed that incubation of human colonic epithelial cells with
B. breve, but not
E. coli, induced expression of REGIII-α, the human ortholog and homolog of REGIII-γ.
There is evidence to suggest REGIII-γ expression in the intestine is regulated by MyD88-mediated TLR signaling (
18,
22,
34–36).
B. breve has previously been shown to stimulate TLR2/MyD88 responses in CD103
+ dendritic cells (DC). Thus, it is probable that
B. breve potentially induces responses in epithelial cells through TLR2/MyD88 signaling as well (
37). We found that REGIII-γ expression in
MyD88-Ticam1 double knockout mice monocolonized with
B. breve was low and comparable to that of GF mice, indicating that
B. breve-induced REGIII-γ production requires TLR signaling. Direct signaling of
B. breve through epithelial TLRs is consistent with the epithelial cell-autonomous model of REGIII-γ expression (
38). The results may also explain our previous results in
Nod1−/−;
Nod2−/− mice in which
B. breve led to normalization of REGIII-γ expression, likely through preserved TLR signaling in these mice. Recently, a new model of REGIII-γ production that involves IL-22 has been proposed in which luminal bacteria interact with TLR expressed by DC, leading to the release of cytokines which then prime innate lymphoid cells (ILC) to release the cytokine IL-22 (
38). In our previous study, we did not detect an increase in IL-22 after
B. breve-induced REGIII-γ expression (
10). We propose that
B. breve-induced REGIII-γ expression may occur in the absence of IL-22 supplementation through an epithelial cell-autonomous manner that involves the
MyD88-Ticam1 pathway although, in this case, induction may be more moderate than in the presence of IL-22.
A number of studies have demonstrated that anti-inflammatory effects of probiotics can be elicited without live bacteria (
4,
39,
40). Likewise, induction of antimicrobial human β-defensin 2 can be mediated by either live
Escherichia coli Nissle 1917 or its structural flagellum protein (
41). Here, we examined different probiotic preparations on
REGIII-α expression by colonocytes. We found that live and heat-inactivated
B. breve increased
REGIII-α expression, whereas the spent culture medium did not induce any changes. These results suggest that a specific component of the structure of
B. breve, and not its secreted metabolites, is responsible for
REGIII-α epithelial expression.
In conclusion, we demonstrated that the effects of the microbiota on REGIII expression in the intestine correlate with microbial composition and that the effect is strain and formulation specific. We determined that the probiotic
B. breve NCC2950 upregulates REGIII-γ expression through MyD88-Ticam1 signaling. We have previously shown that preventive administration of
B. breve NCC2950 to genetically susceptible mice not only increased REGIII-γ expression but also ameliorated the severity of subsequent colitis (
10). Based on these findings, we hypothesize that treatment with
B. breve may regulate REGIII-γ production in a controlled manner that enhances barrier integrity and protects from inflammation. Our results support the use of microbiota-modulating strategies to target homeostatic regulation of antimicrobial peptides. This could be of benefit for IBD patients, their first-degree relatives, and patients undergoing chemotherapy or radiation therapy to prevent intestinal injury.