Cytokines of the IL-23/IL-22/IL-18 axis are pivotally involved in host defence and in mediating and regulating inflammatory immune responses upon bacterial and parasitic infection [21, 25, 26]. We here investigated whether IL-23, IL-22 and IL-18 were actors in the orchestrated interplay between C. jejuni, host microbiota and immune system. Interestingly, conventionally colonized mice lacking IL-23p19, IL-22 or IL-18, but not WT control mice were susceptible to C. jejuni infection and could be readily colonized with highest bacterial loads in the terminal ileum and colon. In line with our previous studies, physiological colonization resistance prevented WT mice harboring a conventional intestinal microbiota from pathogenic infection [12, 15]. Modulation of the intestinal microbiota towards elevated luminal commensal enterobacterial (i.e. E. coli) loads, however, was a sufficient way to override colonization resistance, given that C. jejuni infection was facilitated upon feeding conventional adult mice viable E. coli via the drinking water [15] or a Western style diet, for instance [14]. Interestingly, only susceptible IL-22?/?, but neither IL-23p19?/?, nor IL-18?/? mice harbored elevated commensal intestinal E. coli loads when compared to resistant WT mice. This is well in line with a previous report showing an elevated abundance of the phylum Proteobacteria such as commensal E. coli in the intestines of IL-22?/? mice [27]. An altered microbiota composition rendered IL-22?/? mice even more susceptible to colitis development than WT mice and can be explained by the fact that IL-22 plays a critical role in regulating the host microbiota composition due to its important antimicrobial properties including induction of antimicrobial peptides such as ?-defensins, but also of the mucosal barrier forming mucins [27–30]. Hence, other so far unidentified host-related factors might predispose IL-18?/?, but also IL-23p19?/? mice to C. jejuni infection.
We have recently shown that 3-weeks-old conventional infant mice develop self-limiting acute enteritis within 1 week following peroral C. jejuni infection immediately after weaning [15, 31, 32]. Notably, infant mice also harbored higher intestinal commensal E. coli loads in their gastrointestinal tract as compared to adult mice, subsequently facilitating C. jejuni infection [15, 31, 32]. As shown by us previously, colonic IL-23p19, IL-22 and IL-18 mRNA were upregulated in C. jejuni infected infant mice [22]. Moreover, following peroral infection of conventional adult mice with Arcobacter butzleri sharing taxonomic relationships to Campylobacterales, cytokines of the IL-23/IL-22/IL-18 axis were regulated not only in a strain and time course of infection, but also tissue dependent fashion. Whereas in the colon IL-22 and IL-18 were up-regulated upon A. butzleri infection, IL-23p19 and IL-22 mRNA levels increased in the small intestines of infected conventional adult WT mice [33, 34].
In our present study, despite stable C. jejuni infection, mice lacking IL-23p19, IL-22 or IL-18 exhibited even lower colonic epithelial apoptotic cell numbers as compared to WT mice at day 14 p.i., whereas higher numbers of proliferating cells could be observed in the colonic epithelium of infected IL-22?/? mice, thereby counteracting potential C. jejuni induced cell death. These results are in part supported by our recent study in infant mice that were infected with a different C. jejuni strain (namely 81–176): 2 weeks following peroral infection, less pronounced colonic apoptosis and conversely, more distinct proliferative measures could be observed in the large intestines of infant IL-22?/?, but also IL-18?/? as compared to WT mice [35]. In fact it is somewhat surprising that even though the pathogen was expelled from the intestinal tract of infected infant WT mice here, increased numbers of colonic epithelial apoptotic cells could be observed. This observation, however, is well in line with results from our previous studies in different infection models [22, 35, 36]. We hypothesize that the pathogen does not necessarily need to be permanently abundant in the intestinal tract to evoke (early) host responses with subsequent pro-inflammatory sequelae and tissue damage including intestinal apoptosis. Hence, it is rather the initial hit of infection that tips the balance towards immunopathological responses and potential counter-regulatory (i.e. proliferative) measures postinfection [22, 35, 36].
Here, less distinct colonic epithelial apoptosis were accompanied by lower abundance of neutrophilic granulocytes within the large intestinal mucosa and lamina propria of infected IL-23p19?/? and IL-22?/? as compared to WT control mice, and paralleled by lower colonic TNF secretion in IL-22?/? and IL-18?/? mice than in WT animals, which also held true during the early phase (i.e. day 6) of C. jejuni infection of infant mice [35].
The cytokines of the IL-23/IL-22/IL-18 axis appear in fact to be differentially expressed and regulated during murine C. jejuni infection. Our present study revealed that (1) colonic IL-23p19 mRNA expression was lower in infected IL-22?/? mice than WT mice (2) and vice versa, i.e. IL-22 mRNA was lower in IL-23p19?/? vs. WT mice at day 14 p.i., whereas (3) IL-18 mRNA was down-regulated in large intestines of naive and infected IL-22?/? mice. These data are in part supported by results derived from the infant mouse infection model, given that in naive as well as in C. jejuni infected infant IL-22?/? mice, colonic IL-23p19 and IL-18 mRNA were down-regulated, which also held true for IL-18 mRNA expression levels in naive and infected infant IL-23p19?/? as compared to WT control mice [36]. It is most likely that the observed differences in expression data derived from infection studies with infant versus adult mice are due to age-dependent differences in host-factors including intestinal microbiota compositon and subsequently the intraluminal milieu as well as the maturity of immune cell subsets and cells producing antimicrobial peptides for instance. Information about the distinct regulatory pathways within the IL-23/IL-22/IL-18 axis following bacterial (i.e. C. jejuni) infection are very limited. Whereas IL-23 was highlighted as a key regulator of mucosal immune responses following intestinal infection and inflammation [37] including T. gondii induced ileitis [20], IL-22 was shown to be effective in antimicrobial defense directed against C. jejuni [28]. Moreover, in human intestinal ex vivo biopsies IL-22 was upregulated upon C. jejuni infection [38], whereas elevated IL-22 concentrations were observed in the intestines of C. jejuni infected IL-10?/? mice [17]. Data regarding the role of IL-18 in C. jejuni-host interaction, however, are scarce. C. jejuni infection of three different cell lines derived from pre-malignant Barret’s esophagus was accompanied by an up-regulation of IL-18 gene expression [39]. Following infection of differentiated THP-1 macrophages with an adherent and invasive C. concisus strain, genes encoding IL-23 and IL-18, but not IL-22, were regulated as assessed by transcriptomic and proteomic analyses [40]. Our previous A. butzleri infection studies in gnotobiotic IL-10?/? mice further revealed that in the colon IL-18 mRNA levels were elevated during both the early and late phase of infection, whereas colonic IL-22 mRNA was upregulated during the former only [33, 34].
In conclusion, the regulatory pathways within the IL-23/IL-22/IL-18 axis following C. jejuni infection need to be further unraveled in future studies in order to improve our understanding of the distinct molecular mechanisms underlying campylobacteriosis.