Background
Human
Campylobacter jejuni infections have been progressively rising during the past decades [
1‐
3]. A plethora of wild and domestic animals harbor
C. jejuni as commensal bacteria within their intestinal microbiota. Upon zoonotic transmission from livestock animals humans become infected via consumption of contaminated meat or surface water [
4‐
6]. Infected individuals may be asymptomatic, present with rather mild malaise and watery diarrhea or with full blown disease characterized by acute enterocolitis with abdominal cramps, fever, and bloody, inflammatory diarrhea lasting for days or even weeks [
1]. Whereas disease is self-limiting in the vast majority of cases, post-infectious sequelae affecting the joints (i.e. reactive arthritis), the nervous system (i.e. Guillain–Barré syndrome, Miller Fisher syndrome and Bickerstaff encephalitis) or the intestinal tract (i.e. irritable bowel syndrome) might arise sporadically [
1,
3,
7]. Susceptibility to pathogenic infection is highly depending on the distinct intestinal microbiota composition of the respective host. Conventionally colonized adult mice, for instance, are protected from
C. jejuni infection and expel the pathogen within a few days even upon peroral high dose infection [
8‐
10]. Modification of the murine microbiota by broad-spectrum antibiotic treatment, however, compromizes physiological colonization resistance and facilitates stable gastrointestinal
C. jejuni infection of the resulting secondary abiotic mice at high loads [
8,
10,
11]. In turn, secondary abiotic mice present with distinct
C. jejuni induced pro-inflammatory immune and colonic apoptotic responses thereby mimicking key features of human campylobacteriosis [
6,
8]. Hence, the secondary abiotic mouse model has been proven well-suitable to dissect enteropathogenic-host interactions in vivo [
10,
11].
Host immune responses are pivotal for combating enteropathogenic including
C. jejuni infections. The nucleotide-binding oligomerization domain (Nod)-like receptors comprize an important family of signaling molecules sensing microbial products and damage-associated factors and thus contributing to innate immunity [
12]. Among these, Nod2 is expressed by innate (such as macrophages and dendritic cells) and adaptive immune cell subsets including T lymphocytes as well as by Paneth cells [
13‐
15]. Upon activation by muramyl dipeptide (MDP), a constituent of bacterial peptidoglycans, Nod2 confers resistance to a broad variety of bacterial species [
12,
16‐
18]. To date it is still unclear, however, whether Nod2 can also sense other microbial molecules or whether it rather acts as a mere signaling partner [
19]. The importance of Nod2 in sensing and elimination of enteropathogens have been demonstrated in vivo, given that Nod2
−/− mice were more susceptible to
Listeria monocytogenes or
Yersinia pseudotuberculosis infection [
20,
21].
In the present study we investigated the impact of Nod2 in C. jejuni infection of secondary abiotic mice. We addressed whether pathogenic colonization properties, potential translocation of viable bacteria to extra-intestinal compartments, infection induced macroscopic and microscopic sequelae, and pro-inflammatory intestinal as well as systemic immune responses were affected by Nod2 signaling in C. jejuni infected secondary abiotic mice.
Discussion
In order to prevent the host from invading pathogens including
C. jejuni and to successfully combat infection, a concerted action of pattern recognition receptors, distinct innate and adaptive immune cells, and evolving signaling pathways are crucial [
8‐
10]. In the present study we shed further light onto the role of the bacterial MDP sensor Nod2 in
C. jejuni infection of secondary abiotic mice. Notably, Nod2 deficiency did not impact gastrointestinal colonization properties of
C. jejuni, given that both secondary abiotic Nod2
−/− and WT mice harbored the pathogen with similar counts of up to 10
9 CFU per g colonic luminal content. This might appear somewhat surprising, given that Nod2 deficiency has been shown to be associated with altered expression of antimicrobial peptides such as defensins leading to an insufficient clearance of the pathogen [
20,
35]. Despite the high pathogenic burden within the intestines, mice were macroscopically uncompromised and did not display overt clinical signs of campylobacteriosis such as wasting or bloody diarrhea. This is well in line with our previous infection studies in secondary abiotic mice that were deficient in matrix metalloproteinase (MMP) -2 or -9 [
24,
36], in cytokines belonging to the IL-23/IL-22/IL-18 axis [
32] or in innate immune receptors including Toll-like receptor (TLR) -4 or -9 and respective WT counterparts [
8,
32]. Nevertheless, in our present and previous studies [
8,
24,
32,
36], distinct
C. jejuni induced microscopic changes within the intestinal tract such as accelerated colonic epithelial apoptosis and pronounced influx of immune cells to the site of infection could be observed. Whereas Nod2 deficiency had no impact on
C. jejuni induced colonic histopathological changes and apoptosis development, infected Nod2
−/− mice exhibited more distinct regenerative properties counteracting potential cell damage than WT controls. Furthermore, adaptive immune cell populations such as T and B lymphocytes were less distinctly abundant in the large intestines of
C. jejuni infected Nod2
−/− as compared to WT mice, which is in line with our very recent investigations in conventionally colonized Nod2
−/− [
37] and Nod2
−/− mice lacking IL-10 [
38]. Our present colonic immune cell data are contrasted by the observed large intestinal secretion of pro-inflammatory mediators, given that
C. jejuni induced increases in nitric oxide, TNF and IFN-γ protein concentrations were even more pronounced in Nod2
−/− mice which also held true for increased colonic TNF expression in
C. jejuni infected conventionally colonized Nod2
−/− versus to WT mice as shown previously [
37]. Overall, standard deviations in pro-inflammatory cytokine concentrations were relative high in
C. jejuni infected mice of either genotype. It is therefore tempting to speculate that the extent of against
C. jejuni directed immune responses might differ individually in both Nod2
−/− and WT mice.
We have recently highlighted the importance of the IL-23/IL-22/IL-18 axis in murine
C. jejuni infection [
25,
30‐
32]. In the present study we could demonstrate that respective cytokines were differentially expressed upon
C. jejuni infection of secondary abiotic Nod2
−/− mice. As for large intestinal pro-inflammatory cytokines,
C. jejuni induced up-regulation of colonic IL-22 mRNA was more pronounced in Nod2
−/− as compared to WT mice, whereas conversely, IL-18 was down-regulated in the former, and both IL-23p19 and IL-18 mRNA were lower in infected Nod2
−/− versus WT mice. IL-22 is a member of the IL-10 cytokine family and exerts dichotomous function which depends on the respective compartment and the surrounding cytokine milieu [
31,
39,
40]. Whereas IL-22 exerts anti-inflammatory properties in the large intestinal tract [
40], it has pro-inflammatory functions in the small intestines [
29,
41,
42]. Given that IL-22 has been proven effective in antimicrobial host defence against
C. jejuni [
31,
32], more pronounced increases in IL-22 mRNA levels observed in
C. jejuni infected Nod2
−/− mice might therefore point towards more distinct anti-pathogenic and anti-inflammatory counter-regulatory measures within the well-orchestrated host immune responses upon
C. jejuni infection. Opposed to IL-22, IL-23p19, the master regulator of IL-22 expression [
43], as well as IL-18 that is known to amplify IL-22 production during intestinal inflammation [
41] were lower in
C. jejuni infected Nod2
−/− as compared to WT mice. This might be due to a potential negative feed-back loop regulation between respective cytokines.
Even though the large intestinal tract constitutes the major predilection site of
C. jejuni infection [
8,
23,
44,
45], we here additionally assessed pro-inflammatory cytokine secretion in the small intestine. Remarkably, as opposed to the colon, ileal nitric oxide, TNF, IFN-γ and IL-6 concentrations were lower in
C. jejuni infected Nod2
−/− as compared to WT mice. It is hence tempting to speculate that Nod2 might have dichotomous functions depending on the respective intestinal tissue, its immunological prerequisites, surrounding cytokines and other intestinal luminal factors.
To date, data are rather conflicting regarding the role of Nod2 in intestinal inflammation including
C. jejuni infection. Depending on the applied in vivo model Nod2 might either prevent from or even enhance colitis development in
C. jejuni infected mice. For instance, in IL-10
−/− mice that had been pre-treated with antibiotics for 1 week, Nod2 was shown to be essential for controlling campylobacteriosis, given that Nod2
−/− IL-10
−/− mice exhibited an exacerbation of
C. jejuni induced large intestinal inflammation [
46]. In another study by Jamontt and colleagues [
47], however, Nod2 was shown to promote IL-10
−/− colitis, given that Nod2
−/− IL-10
−/− mice were protected from colitis development.
Also in non-infection induced intestinal inflammation models data regarding the impact of Nod in disease development are inconclusive so far. Following adaptive transfer of Nod2
−/− T lymphocytes, immunodeficient mice developed less severe chronic colitis, for instance, pointing towards a rather disease-promoting feature of Nod2 signaling [
12]. Conversely, MDP application could sufficiently prevent from 2,4,6-trinitro-benzene-sulfonic acid (TNBS) induced colitis, whereas MDP-mediated prevention of diseases was overruled in Nod2
−/− mice indicative for a protective role of Nod2 signaling [
48]. In an own study we further demonstrated that Nod2 is involved in protection of mice from
Toxoplasma gondii induced acute ileitis [
49].
Epithelial barrier integrity is pivotal for preventing pathogenic translocation from the intestinal tract to extra-intestinal including systemic tissue sites with potentially fatal consequences [
50]. Even though the mucin MUC2 was down-regulated in
C. jejuni infected Nod2
−/− mice, this did not result in increased pathogenic translocation rates to liver, kidney or spleen, whereas viable
C. jejuni could be isolated in comparable numbers from MLN of all infected Nod2
−/− and WT mice. Interestingly, secretion of pro-inflammatory cytokine including TNF, MCP-1 and IL-6 were higher in MLN of
C. jejuni infected Nod2
−/− as compared to WT mice. In the systemic lymphatic compartment, however, virtually no
C. jejuni induced increases in cytokine secretion could be detected.
In conclusion, upon C. jejuni infection of secondary abiotic mice Nod2 signaling is involved in the initiation of a well-orchestrated innate and adaptive immune response but does not limit pathogen colonization. Future studies need to dissect the exact regulatory interactions to improve our understanding of the molecular mechanisms underlying campylobacteriosis.
Authors’ contributions
Conceived and designed the experiments: SB, MMH. Performed the experiments: UG, MEA, AF, MMH. Analyzed the data: SB, UG, MEA, AF, AAK, MMH. Contributed reagents/materials/analysis tools: AAK. Wrote the paper: SB, MMH. All authors read and approved the final manuscript.