Intestinal epithelial claudin-2 upregulation is a primitive defense mechanism that amplifies stress-induced inflammatory responses

The intestinal barrier is formed by epithelial cells and tight junctions. Rather than being impermeant, paracellular channels allow passive transport across the epithelial monolayer. The tight junction proteins claudins 2 and 15 permit selective cation, i.e., Na + and water flux, while excluding larger molecules, e.g., bacterial products. Despite forming functionally similar channels, distinct expression patterns, with claudin -2 or -15 predominating in infants or adults, respectively, suggest that these proteins have unique functions. Consistent with this, claudin-2, but not claudin-15, expression is increased in human and experimental mouse disease. This transcriptional upregulation is beneficial in enteric infection, where claudin-2 enhances fluid efflux and accelerates pathogen clearance. Conversely, claudin-2 upregulation exacerbates experimental sepsis and inflammatory bowel disease. The mechanisms that lead to these divergent effects of claudin-2 expression are unknown. Hypothesis: We hypothesize that claudin-2-dependent Na + and water permeability activate proinflammatory signaling within the intestinal mucosa. Methods: Ileal and colonic immune cells from wildtype (WT), intestinal epithelial-specific EGFP- Cldn2 transgenic ( Cldn2 Tg ), and intestinal epithelial-specific knockout ( Cldn2 ΔIEC ) mice were analyzed. Non-stressed mice were compared to mice with colonic mucosal damage-induced stress (intrarectal TNBS), systemic immune stress (intraperitoneal LPS), or psychological stress (restraint). Results: Weights and behaviors of WT, Cldn2 Tg , and Cldn2 ΔIEC mice were similar. As reported previously, Cldn2 Tg mice have increased fecal Na + and water. However, Cldn2 Tg mice also displayed ~2-fold expansion of CD4 + T cells, Th17 T cells, CD8 + T cells, and neutrophils as well as a ~2-fold reduction in Tregs relative to WT mice. Conversely, Tregs were increased ~2-fold, but Th17 cell numbers were reduced by half in Cldn2 ΔIEC mice relative to WT mice. Therefore, claudin-2 expression increases the number of pro-inflammatory cells, with Cldn2 Tg > WT > Cldn2 ΔIEC , while decreasing Tregs. Relative differences in inflammatory and immunoregulatory cells across the three genotypes were maintained after exposure to each stressor, but absolute numbers of proinflammatory cells increased, and Tregs decreased. Specific stressor-induced changes included increased Th1 cell, ILC1, and ILC2 numbers in Cldn2 Tg mice and reduced ILC3 and NK cell numbers in Cldn2 ΔIEC mice relative to WT mice after damage. In contrast, systemic immune activation increased mucosal Th2 cell and eosinophil numbers in Cldn2 Tg mice but reduced ILC3 ~4-fold in Cldn2 ΔIEC mice relative to WT mice. Psychological stress increased Th2 (ileum), Th1 (colon), NK, and ILC2 cells but dramatically reduced Treg numbers in Cldn2 Tg mice relative to WT mice. Thus, claudin-2 expression primes the mucosa for proinflammatory stress responses, while claudin-2 deletion amplifies immunoregulation. Conclusions: Claudin-2 upregulation and the following immune activation may represent a primitive host-defense response. This is consistent with the beneficial effects of claudin-2 upregulation during infection. However, this host-defense response appears maladaptive in inflammatory disorders, where mucosal immune activation in the absence of a pathogen exacerbates disease. Although further study is needed to determine the mechanisms by which paracellular Na + and water permeability have such profound effects on mucosal immune status, these data highlight the therapeutic potential of claudin-2-modifying agents in a wide range of intestinal and systemic disorders. Funding: NIDDK R01DK061931, R01DK068271, and P30 DK034854. Disclosures: None. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.