We have recently shown
in vitro that the chaperone and serine protease HtrA secreted by
C. jejuni exerts a novel pathogenicity factor that is involved in bacterial invasion and transmigration across epithelial cells by cleaving E-cadherin and opening cell-to-cell junctions [
20‐
23]. In the
in vivo study presented here we investigated the impact of the
htrA gene in pathogen-host-interaction and induction of immunopathology upon
C. jejuni infection. To address this, conventionally colonized infant mice were infected either with the
C. jejuni knockout mutant NCTC11168∆
htrA or its syngenic parental WT strain at the age of 3 weeks immediatedly after weaning. Even though only a subset of mice harboured the respective strain in the intestinal tract, about one third of infected mice suffered from bloody diarrhea. In a previous infection study with another
C. jejuni strain (B2), having highly efficient colonizing properties, virtually all infant mice harboured the pathogen at day 7 p.i., whereas up to 90% of mice displayed bloody diarrhea [
30]. However, in our experiments with parental strain NCTC11168, but not ∆
htrA mutant infected infant mice exhibited multi-fold increased numbers of colonic apoptotic cells at day 7 p.i. as compared to naïve controls. Conversely, the number of proliferating cells was significantly increased in ∆
htrA but not parental strain infected mice indicative for up-regulated regenerative properties of intestinal epithelial cells thereby counteracting
C. jejuni induced tissue damage. Less pronounced intestinal immunopathology due to the absence of HtrA was further underlined by lower expression levels of colonic pro-inflammatory cytokines such as TNF-α and IFN-γ, which have been shown to be key cytokines mediating
C. jejuni induced immunopathology in murine infection models with different clinical severity [
24,
25]. Interestingly, less distinct intestinal immunopathology was accompanied by lower colonic expression levels of the matrix-degrading enzyme MMP-2 and its endogenous inhibitor TIMP-1 seven days following ∆
htrA as compared to the parental strain infection. These MMP expression data are in good agreement with previous studies demonstrating that MMP-2 is up-regulated in acute and chronic small as well as large intestinal inflammation in mice and men [
32,
33,
35‐
38]. For the first time we have now presented evidence that MMP-2 might also play an important role in mediating
C. jejuni-induced disease, which is currently further unravelled in ongoing studies.
Surprisingly, rather mild to moderate histopathological sequelae of
C. jejuni infection could be detected as early as one week in extra-intestinal organs such as liver, kidneys and lungs. All organ samples were free of viable
C. jejuni as shown by negative cultures. In our previous study,
C. jejuni B2 strain infected infant mice exhibited histopathological changes in the respective organs more than 100 days p.i. [
30] with inflammatory foci consisting mainly of accumulated CD3-positive T cells [
31]. Strikingly, in the present study, extra-intestinal histopathological changes in kidneys and lungs were less distinct one week following ∆
htrA as compared to parental strain infection. Hence, absence of the HtrA protein is not only associated with less pronounced intestinal but also extra-intestinal inflammation.
In humans, only very few cases of pathogen-associated disease manifestations affecting liver, lungs, heart or spleen have been reported in severely immuno-compromized patients with
C. jejuni bacteremia [
39‐
41]. Fauchere and coworkers showed in isolator-raised germfree mice that
C. jejuni was cleared from extra-intestinal compartments such as liver and spleen and the circulation within 24 hours following infection most likely due to non-specific bactericidal factors such as phagocytes and complement [
42]. Histopathological changes within extra-intestinal organs, however, were not investigated [
42]. In the context with our previous observation that CD3-positive cells accumulate at extra-intestinal locations, it is tempting to speculate that potentially pro-inflammatory immune cell populations might be attracted to the extra-intestinal compartments very early following infection before the subsequent clearing of the pathogen. These immune cells might then further reside in the respective organs and explain the sterile inflammatory responses in extra-intestinal tissue sites observed 7 days p.i. as well as in asymptomatic long-term
C. jejuni carriers more than 100 days p.i. [
30,
31].