Mechanisms of immune tolerance induction by H. pylori and of systemic immunomodulation
One of the most exciting discoveries of our recent work was that the outcome of the Helicobacter/host interaction differs depending on the age of the host at the time of first exposure. If we infect mice during the neonatal period (i.e. at a time when H. pylori is typically transmitted from mothers to their babies in regions where H. pylori is endemic), they fail to control the infection (i.e. they are colonized at 50-100 fold higher levels), but are completely protected against the gastric immunopathology that is a hallmark of adult-infected mice (Arnold, 2011c). This relative resistance to H. pylori-associated disease despite heavy colonization is maintained for at least one year, i.e. it is not restricted to the newborn period. A closer examination of the mechanism of protection revealed that neonatally infected mice preferentially generate H. pylori-specific Treg- over T-effector responses, and develop immune tolerance to the infection (Arnold, 2011c). The systemic depletion of Tregs breaks this tolerance, and leads to clearance of the bacteria and severe gastric pathology (Arnold, 2011c). We believe that similar processes may be operative in humans infected as children, and may explain the findings of high Treg/Teff ratios in the gastric mucosa of infected children (Harris, 2008), and the lack of H. pylori-associated stomach problems in certain areas of the world where H. pylori is endemic (and presumably transmitted early in life).
Based on a series of papers describing an inverse epidemiological association between H. pylori infection and asthma and other allergic disease manifestations, especially in children and young adults (Blaser, 2008; Chen, 2007; Chen, 2008; Reibman, 2008), we hypothesized that immune tolerance to H. pylori might cross-protect against allergen-specific, pathogenic T-cell responses. Using an experimental model of allergic airway disease induced by ovalbumin-specific sensitization and challenge, we found that H. pylori infection protects mice against the clinical and histopathological symptoms of asthma, i.e. airway hyper-responsiveness, tissue inflammation and goblet cell metaplasia, and prevents the infiltration of eosinophils, Th2- and Th17-cells into the bronchoalveolar fluid and lungs (Arnold, 2011a). As predicted, the protection is largely restricted to neonatally infected mice; the results could further be reproduced also with an allergen (house dust mite antigen) relevant in humans (Arnold, 2011a). Most strikingly, asthma protection could be transferred from neonatally infected to naive mice via small numbers of highly purified regulatory T-cells, which we isolate from the gut-draining lymph nodes of the infected donors. Conversely, the depletion of Tregs abrogates asthma protection (Arnold, 2011a). Taken together, the results indicate that neonatally acquired immune tolerance to H. pylori not only prevents the gastric immunopathology that underlies and precedes H. pylori-associated gastric disease, but may also be beneficial in preventing asthma (summarized in Figure 1).
Figure 1. Schematic representation of the current model of H. pylori-induced immune tolerance and asthma protection.Tolerogenic dendritic cells and H. pylori-induced regulatory T-cells act in concert to prevent adaptive Th1/Th17-driven immunity to the infection and to inhibit allergen-specific Th2 responses. In chronically infected humans, H. pylori resides exclusively in the gastric mucosa, where it is presumably encountered and detected by tissue-resident DC populations extending dendrites into the gastric lumen. H. pylori-experienced DCs migrate to the gut-draining mesenteric lymph nodes, where they act as potent inducers of TGF-b-dependent FoxP3+ regulatory T-cells, but fail to prime H. pylori-specific Th1 and Th17 responses. Induced Tregs may further perpetuate the tolerogenic effects of H. pylori-experienced DCs by retaining mesenteric lymph node DCs in a semi-mature state and by directly suppressing H. pylori-specific gastric Th1 and Th17 responses, thereby protecting the host from excessive gastric immunopathology. Newly induced Tregs further migrate to the lung, where they suppress allergen-specific Th2 and Th17 responses involved in the pathogenesis of asthma. The generation of allergic T-cell responses may be blocked either through the tolerogenic effects of Tregs on DCs (retaining DCs in a semi-mature state) or directly through suppression of Th2 and Th17 responses via Treg/T-effector cell contact or via soluble cytokines, in particular IL-10. The ultimate outcome of gastric H. pylori infection on the allergen-challenged lung is reduced eosinophilia, mucus production and airway hyper-responsiveness. The involvement of the tracheal lymph nodes in H. pylori-induced asthma suppression is likely, but currently not well understood.
To elucidate the mechanisms involved in the induction and maintenance of immune tolerance to H. pylori, we are focussing primarily on dendritic cells (DCs), as these cells are known to exhibit tolerogenic (as well as immunogenic) properties in the gut (Maldonado, 2010). Indeed, we found that H. pylori has evolved to effectively re-program DCs towards a tolerance-promoting state; contact of DCs with H. pylori generates so-called “semi-mature” DCs that express high levels of MHCII, but no or low amount of co-stimulatory molecules such as CD80 and CD86 (Oertli, 2012). H. pylori-experienced DCs also do not express T-cell-activating cytokines such as IL-12, but preferentially produce IL-10 (Oertli, 2012). DCs that have been exposed to H. pylori in vitro or in vivo furtherfail to induce T-cell effector functions, and instead efficiently induce expression of the Treg lineage-defining transcription factor FoxP3 in naive T-cells. The experimental depletion of DCs breaks H. pylori-specific, neonatally acquired tolerance and results in improved control of the infection, but also in aggravated immunopathology. DCs infiltrating the gastric mucosa of human H. pylori carriers exhibit a semi-mature DC-SIGN+HLA-DRhighCD80loCD86lo phenotype, indicating that a human cell counterpart exists for our observations in the experimental model (Oertli, 2012). Interestingly, the tolerogenic activity of H. pylori-experienced DCs requires interleukin-18 in vitro and in vivo; DC-derived IL-18 acts directly on T-cells to drive their conversion to Tregs. The adoptive transfer of CD4+CD25+ T-cells from infected wild type, but not IL-18-/- or IL18R-/- animals, prevents airway inflammation and hyper-responsiveness in the above-mentioned experimental model of asthma (Oertli, 2012) (see schematic in Figure 2).
Figure 2. Schematic representation of the effects of H. pylori exposure on DCs and the DC/T-cell interaction. Exposure to H. pylori induces semi-mature MHCIIhighCD80loCD86lo DCs. Inflammasome activation by H. pylori through as yet uncharacterized cytoplasmic Nod-like receptors (NLRs) leads to caspase-1 activation and the processing and secretion of IL-1b and IL-18. IL-1b promotes Th17 differentation, whereas IL-18 is required for Th1 and Treg differentation. H. pylori-experienced DCs actively induce the conversion of naive T-cells to FoxP3+ Tregs in a process that requires IL-18, TGF-b, and possibly IL-10. In contrast, H. pylori-experienced DCs are poor inducers of Th17 and Th1 differentiation. The documented lack of H. pylori TLR ligands in conjunction with efficient inflammasome activation by the bacteria suggests that the relative availability of pro-IL-b (low level expression due to lack of transcriptional activation) and pro-IL-18 (high levels due to constitutive expression) for caspase-1 processing may dictate the outcome of the DC/T-cell interaction.
The requirement for DC-derived IL-18 in the process of H. pylori-specific Treg differentation is confirmed by the phenotypes of the respective gene-targeted mouse strains. IL-18-/- or IL18R-/- mice fail to develop neonatally-acquired immune tolerance to the infection, and as a consequence are significantly better able to control the infection (Oertli, 2012). A similar phenotype is seen in adult-infected IL-18-/- mice (Hitzler et al., 2012). We could further show that caspase-1 is activated, and IL-1b and IL-18 are processed in vitro and in vivo as a consequence of Helicobacter infection (Hitzler et al., 2012). Interestingly, caspase-1-/- mice phenocopy IL-18-/- animals with respect to their hypersusceptibility to H. pylori-induced gastric disease. The results thus suggest an important regulatory function of caspase-1 and the inflammasome in H. pylori pathogenesis, which we are currently following up in more detail by screening for the responsible H. pylori PAMPs and identifying the pattern recognition receptors involved.