Protection or Vulnerability? The Context-Dependent Memory of Intestinal Inflammation

The rising incidence of inflammatory bowel disease (IBD) in industrialized societies has often been framed through the hygiene hypothesis, which links reduced microbial exposure during early life to impaired immune maturation and increased risk of immune-mediated disease.¹ In IBD, however, the relationship between infection and intestinal inflammation is paradoxical: microbial exposure may promote immune tolerance and mucosal homeostasis,² whereas certain enteric pathogens are associated with an increased risk of incident IBD.^3,4 Thus, infection is neither uniformly protective nor harmful, and its effects likely depend on the timing, nature, persistence, and immunologic context of exposure. The mechanisms that determine whether infection promotes mucosal resilience or exacerbates intestinal inflammation remain incompletely understood.

In this issue, Biswas et al report that context-dependent mucosal immune responses can shape host susceptibility to secondary intestinal inflammation.⁵ Using Citrobacter rodentium, a murine model of enteropathogenic and enterohemorrhagic Escherichia coli infections, they show that mice recovered from C rodentium infection are protected from subsequent dextran sulfate sodium (DSS)-induced colitis. Mechanistically, this protection was associated with an expansion of adaptive immune cells, particularly T helper 17 (Th17) cells, which are classically implicated in inflammatory responses. Notably, the protective effect observed in C rodentium–recovered mice was dependent on interleukin17A, indicating that Th17-associated pathways contribute to mucosal resilience after pathogen infection. These findings are consistent with previous work demonstrating that C rodentium infection induces tissue-resident memory Th17 cells in the colon.^6,7 These Th17 cells persist after pathogen clearance and contribute to protection against secondary insult, in part through rapid local interleukin17A-mediated effector responses. Although the mechanisms underlying Th17 expansion after C rodentium infection remain to be fully defined, recent evidence indicates that inflammation-trained microbiota can promote the expansion of tissue-resident Th17 cells.⁷ This finding suggests that crosstalk between the gut microbiota and mucosal immune system may be a key determinant of protection against second inflammatory insult. Together, these studies suggest that prior intestinal inflammation does not inevitably generate pathogenic immune memory. Instead, under defined conditions, infection-induced remodeling of the mucosal immune system and microbiota can establish a protective state that limits subsequent intestinal inflammation.

Conversely, in contrast to the protective phenotype observed in mice recovered from C rodentium infection, Biswas et al found that mice recovered from DSS-induced colitis exhibited increased susceptibility to subsequent C rodentium infection.⁵ Mechanistically, DSS-recovered mice showed an expansion of Th17 cells, similar to that observed after recovery from C rodentium infection; however, their susceptibility to C rodentium was more closely associated with persistent neutrophil accumulation and impaired intestinal barrier integrity. Consistent with this notion, low-virulence C rodentium mutant strains also exacerbated colitis in DSS-recovered mice, suggesting that unresolved epithelial barrier dysfunction is a key determinant of host vulnerability. These findings suggest that sustained barrier dysfunction after DSS-induced injury creates a permissive mucosal environment that heightens susceptibility to enteric infection. Notably, DSS recovery does not uniformly render the host more vulnerable to subsequent inflammatory or infectious challenges. Indeed, mice recovered from DSS-induced colitis have been reported to acquire resistance to a second DSS challenge, and prior DSS-induced colitis can also enhance host resistance to systemic Salmonella infection.^7,8 These observations suggest that the outcome of prior intestinal inflammation is highly context-dependent. One possible explanation is that DSS-induced injury may induce trained innate immunity that enhances resistance to certain infectious challenges,⁹ whereas gut dysbiosis and incomplete intestinal barrier repair may create vulnerability to specific enteric pathogens.^5,10

Collectively, these findings suggest that prior enteric infection and chemically induced epithelial injury can imprint distinct mucosal states that differentially shape mucosal resilience and susceptibility to subsequent intestinal inflammation, depending on the immunologic and epithelial context. Thus, secondary inflammatory outcomes may reflect not only the nature of the subsequent challenge, but also a postinflammatory mucosal memory encoded in the immune, epithelial, and microbial landscape established after the initial insult. Defining the mechanisms that distinguish protective mucosal remodeling from persistent vulnerability will be essential for understanding infection-associated disease initiation and relapse in IBD. These insights may guide strategies to restore barrier integrity, preserve protective host defense, and reduce the risk of recurrent intestinal inflammation.