A study has linked psychological stress to Crohn’s disease flare-ups

McMaster-led study links stress to Crohn’s disease flare-ups “The main takeaway is that psychological stress impedes the body’s ability to fight off gut bacteria that may be implicated in Crohn’s disease. Innate immunity is designed to protect us from microbes that do not belong in the gut, like harmful bacteria,” said senior author Brian Coombes, professor and chair of biochemistry and biomedical sciences at McMaster.

“When our innate immune system functions properly, it prevents harmful bacteria from colonizing us, but when it breaks down, it leaves an opening for pathogens to colonize locations they normally cannot and cause illness.”

Below is the introduction and discussion of the study.

Introduction

Crohn’s disease (CD) is an inflammatory disorder of the gastrointestinal tract triggered by microbial and environmental insults^1,2,3. The global burden of disease associated with CD is rising, particularly in developed countries where an upward trend in incident cases has occurred for decades. Emergent disease is now appearing in Asia, Africa, and South America⁴. Current standards of care, including immunomodulatory biologics, are expensive and have high rates of primary and secondary non-responsiveness. Thus, there is an urgent unmet clinical need to better understand the microbial and environmental triggers of CD that will underpin new preventions and therapies.

The CD-associated microbiome has been intensely scrutinized as a source of inflammation in the gut^5,6,7. General microbial features in CD include decreased community diversity, reduced levels of Clostridiales, and increased abundance of Proteobacteria^7,8,9. Clinical observations consistently show bacteria in close association with the mucosal epithelium in Crohn’s patients, particularly members of the Enterobacteriaceae that are enriched in virulence and secretion pathways^10,11,12. Adherent-invasive Escherichia coli (AIEC) is an abundant pathobiont at inflamed sites in the gut^13,14. Numerous studies have confirmed that AIEC are enriched in humans with CD compared to healthy subjects and are often the dominant bacterial species present^15,16. AIEC have a multiphasic lifestyle and can grow as extracellular planktonic cells, in biofilms, and intracellularly in epithelial cells and macrophages, where they induce host inflammatory pathways^17,18,19,20. Thus, understanding the colonisation dynamics of AIEC is expected to yield insights into the progression of CD and may inform therapeutic intervention.

Several components of mucosal immunity are disrupted in CD. These include epithelial barrier integrity, mucous production and cell turnover, the production of antimicrobial peptides and proteins (AMPs), and the release of metal ion scavengers that contribute to nutritional immunity^21,22,23. Nutritional immunity functions to limit bacterial growth by sequestering essential nutrients and metals required for bacterial replication²². Collectively, these elements of mucosal immunity work to spatially restrict bacteria largely to the gut lumen and to reduce the overgrowth of pathogens²⁴. Interleukin (IL)-22 is a cytokine that regulates several aspects of mucosal immunity in the gut, including the production of AMPs and the activation of nutritional immunity^25,26 Some enteric pathogens including Salmonella and Citrobacter can overcome IL-22-dependent host defenses through AMP resistance and metal acquisition systems that bypass the host defense proteins lipocalin (Lcn2) and calprotectin^27,28. Similarly, IL-22 has been implicated in the induction of other iron scavengers, including hemopexin and haptoglobin, that can control the enteric pathogen Citrobacter rodentium²⁹. The role of IL-22 in host control of CD-associated pathobionts has not been explored.

Disease expression in CD patients follows a relapsing and remitting course, where relatively asymptomatic periods are followed by heightened inflammation and disease activity^30,31. A considerable body of clinical literature establishes psychological stress as a disease modifier in CD^{32,33,34,35,36}. Episodes of acute psychological stress are associated with flares, relapse, and increased inflammatory markers in both the serum and mucosa of patients with CD and ulcerative colitis^33,34,37. Rodent models of stress have extended these findings, demonstrating increased inflammation³⁸, barrier disruption^38,39,40, reactivation of disease induced by chemical exposure^38,41, and increased susceptibility to enteric infection⁴². A previous study demonstrated that chronic stress coupled with DSS-induced colitis resulted in a modest alteration in inflammation and microbiome composition in the cecum and colon⁴³. However, the mechanisms driving the mucosal and microbiological dysfunction associated with psychological stress remain unknown, particularly in the ileum.

Here we show in a pre-clinical model, designed to investigate the comorbid effects of psychological stress on microbial composition and immune pathways in the gut, that stress induces a multidimensional loss of host protection, provoking a dysbiotic shift in the ileal microbiome dominated by AIEC and other Enterobacteriaceae, with attendant immunological and barrier defects. These mucosal deficits are traced back to an apoptotic depletion of CD45⁺CD90⁺ cells resulting in the loss of protective IL-22 signaling that blunts AMP responses and barrier repair. The combined effects of nutritional immunity and stress-induced impairment of protective IL-22 responses create a favorable niche for AIEC expansion. These deficiencies are correctable using either exogenous IL-22, or by blocking stress-induced glucocorticoid signaling, which restores mucosal immunity and averts ileal dysbiosis including the expansion of AIEC. These data indicate that immunomodulation can normalize the dysbiotic shifts associated with CD in response to psychological stress, providing avenues for therapeutic interventions.

Discussion

CD patients experiencing psychological stress are more likely to relapse and have increased disease activity, yet the mechanisms underlying this remain obscure^33,65,66,67. Likewise, despite a higher frequency of AIEC colonization in CD patients, the host factors that influence the fluctuation of AIEC load in patients are not understood⁶⁸. Current evidence indicates that AIEC is a pathobiont in the human gut and takes advantage of inflammatory niches derived from host insults, such as antibiotic use⁶⁹, secondary enteric infections²⁰, western diet⁷⁰, and/or genetic predisposition⁷¹. In this study, we determined that psychological stress is associated with impaired IL-22-dependent mucosal antimicrobial defenses that result in profound small intestinal dysbiosis dominated by AIEC expansion.

Previous studies have demonstrated a link between intestinal dysbiosis and inflammation in mice^47,72,73 and in IBD patients⁷. In our study, exposure to acute psychological stress resulted in a profound dysbiosis dominated by outgrowth of Enterobacteriaceae family members and genera commonly found to be enriched in CD patients^7,8,9. For instance, an enrichment of Enterococcus faecalis is strongly associated with disease severity in CD patients⁷⁴, and this species was also enriched following exposure to psychological stress. Bifidobacterium, which are commonly associated with intestinal health and generally have decreased abundance in CD, were also decreased following overnight stress.

Psychological stress has traditionally been considered anti-inflammatory in nature, owing to the production of glucocorticoids^50,75,76. Our findings, in line with a recent publication⁴³, demonstrate that stress induces a mixed inflammatory response. In our model of psychological stress, the pro-inflammatory features appear to play a dominant role in shaping the microenvironment that favors the expansion of AIEC. For example, mice exposed to stress had reduced expression of genes associated with barrier function, including tight junctions and mucins. The encroachment of AIEC towards the epithelium and dissemination to systemic sites that we observed following stress is likely facilitated by these weakened physical barriers, which further provokes the host inflammatory response. Indeed, we observed more pronounced barrier defects, increased pro-inflammatory cytokine expression, and dysbiosis in AIEC-colonized mice exposed to psychological stress compared to AIEC-naive mice exposed to the same stressor. Thus, the combination of inflammation-tolerant bacteria like AIEC, and the mucosal defects brought about by stress, appear to create a tipping point in gut homeostatic balance that favors inflammation.

Interestingly, while nutritional immunity was necessary, it was not sufficient to phenocopy the ileal dysbiosis induced by stress. Indeed, psychological stress independently impaired IL-22-driven protective mucosal immunity against AIEC. A pivotal role for IL-22 was revealed using IL-22 depletion and administration of IL-22-Fc to probe the functional consequences of this pathway during stress. Administration of IL-22-Fc functionally restored the expression of genes downstream of IL-22 signaling including fut2 and antimicrobial proteins which appeared to have a functional effect in preventing stress-induced AIEC expansion and in correcting the ileal dysbiosis that occurred following stress. Interestingly, recent clinical studies are evaluating the use of IL-22 in the treatment of IBD patients (Clinical Trial: NCT03558152, NCT02749630). When ileal explants taken from mice exposed to stress were stimulated with IL-23, these tissues failed to produce IL-22 whereas ileal tissues from unstressed mice showed a robust IL-22 signature, suggesting that the IL-22 producing cell population was depleted following psychological stress. Indeed, the gut of mice exposed to psychological stress had a significant reduction in the number of CD45⁺CD90⁺ cells in the small intestine following psychological stress, which we directly linked to stress because this cell population was restored by blocking glucocorticoid signaling.

Although psychological stress impaired IL-22-dependent host protection, nutritional immunity appeared to remain intact, and in fact was required for AIEC expansion. The host protein calprotectin sequesters zinc, manganese, calcium, and iron under states of inflammation^59,77. In response to host nutritional immune pressure, some Enterobacteriaceae have evolved mechanisms to exploit nutrient limitation to outcompete commensal microbes^27,28,78. For example, Salmonella expresses a high-affinity zinc transporter, ZnuABC, and can thrive in the presence of calprotectin-mediated nutritional immunity⁷⁷. Interestingly, AIEC appears to also thrive in inflammatory environments^13,14 and has acquired the ZnuABC transporter, likely providing a fitness advantage in states of nutritional immunity. Given the essentiality of iron for bacterial replication, the host has multiple mechanisms to limit the availability of iron in its various forms. Iron is typically transported complexed with heme and is a common target of bacterial species^79,80. As such, the host sequesters free-heme in pathophysiological settings^22,29. Accordingly, we found that both HPX and haptoglobin were upregulated during psychological stress exposure, potentially limiting iron availability to bacterial species^22,29,81. Enhanced recruitment of neutrophil-like CD11b⁺Gr1⁺ cells in our model was accompanied by robust induction of neutrophil-derived Lcn2 following stress. The host releases Lcn2 to limit the acquisition of iron-bound enterobactin, preventing its reuptake by commensal strains of E. coli²³. Unlike most commensal strains, many AIEC encode the biosynthetic and secretion machinery for salmochelin, a glycosylated variant of enterobactin that evades Lcn2 sequestration^27,60,82 and provides a competitive advantage to Salmonella^27,82. Indeed, AIEC derived a salmochelin-dependent competitive advantage in stressed mice, but not in unstressed mice, indicating that psychological stress creates a competitive gut environment that appears to benefit microbes that have evolved mechanisms to evade nutritional immunity. Interestingly, we found that neither LPS-mediated induction of iron limitation, nor CD90 depletion, in isolation, was sufficient to phenocopy the AIEC expansion seen during stress. Instead, our data are consistent with a combinatorial effect of psychological stress whereby the combined activation of nutritional immunity and immune cell attrition support the intestinal dysbiosis seen following psychological stress.

Overall, our study shows that psychological stress creates a beneficial environment for AIEC, a CD-associated pathobiont in the gut. Given that the pathological changes observed following psychological stress are augmented in the presence of AIEC, this work establishes a rationale for future studies to dissect the relative contributions of the microbiome and psychological stress on the gut environment. In our current study, AIEC appears to derive this benefit by evading host nutritional immunity while taking advantage of an impaired induction of IL-22-mediated host defenses that rely on antimicrobial proteins and barrier maintenance. Thus, in the presence of pathobionts that efficiently evade nutritional immunity, this aberrant stress-induced host response provides a promiscuous niche for their unregulated expansion. The ability of IL-22 treatment to correct both mucosal host defenses and prevent E. coli-dominated ileal dysbiosis provides compelling rationale for continued investigation of this intervention. This work reveals insight into the role that psychological stress plays in disease expression of CD. Uncovering the interactions between microbes, the host immune system, and epithelial host defenses will lay the biological underpinnings that guide preventions and therapies that address unmet clinical needs for CD management.

Reference: Shaler, C.R., Parco, A.A., Elhenawy, W. et al. Psychological stress impairs IL22-driven protective gut mucosal immunity against colonising pathobionts. Nat Commun 12, 6664 (2021). https://doi.org/10.1038/s41467-021-26992-4

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