Bacterial protein linked to Crohn's disease primes immune cells for heightened inflammation

Crohn's disease is a lifelong inflammation of the gastrointestinal tract that strikes hardest in adolescents and young adults, though it can emerge at any age. The causes remain incompletely understood, but researchers have long suspected that certain bacteria play a role in triggering or sustaining the disease. A new study published in PLOS Pathogens identifies a specific bacterial protein that appears to prime immune cells for heightened inflammation in response to ordinary gut bacteria—a finding that may explain how some strains of Campylobacter concisus, a bacterium commonly found in the mouth, contribute to Crohn's disease when they migrate to the intestines.

The protein in question is called Csep1P, and it comes from C. concisus strains associated with active Crohn's disease. Using X-ray crystallography, researchers determined the protein's three-dimensional structure for the first time, revealing a unique helical fold that resembles similar virulence factors from Helicobacter pylori, a bacterium known to cause stomach inflammation. This structural similarity suggested that Csep1P might manipulate the immune system in ways comparable to other bacterial proteins. The team, led by researchers at the University of New South Wales, set out to test whether Csep1P could alter how macrophages—key immune cells that initiate and perpetuate inflammation—respond to microbial threats.

What they discovered was unexpected. When exposed to Csep1P alone, macrophages did not mount a full inflammatory response. Instead, they entered a distinct activation state characterized by the production of chemokines—signaling molecules that recruit other immune cells to the site of infection. The researchers designated this state M1-chem, where "chem" denotes the predominant upregulation of chemokines. Notably, Csep1P did not trigger the release of classical proinflammatory cytokines like IL-1β and TNF-α, which are hallmarks of the canonical M1 macrophage activation seen with bacterial lipopolysaccharide. The protein essentially prepared macrophages for action without firing the full inflammatory cascade.

The real significance emerged when the team tested how Csep1P-primed macrophages responded to a common gut bacterium, Escherichia coli. Macrophages that had been exposed to Csep1P for four hours produced dramatically higher levels of proinflammatory cytokines when subsequently challenged with E. coli compared to unprimed cells. In THP-1-derived macrophages, IL-8 production increased from 1,818 pg/ml to 4,369 pg/ml at one bacterial concentration, and IL-1β jumped from 19 pg/ml to 38.6 pg/ml. Similar amplification occurred in primary human macrophages isolated from blood. This priming effect was robust and reproducible across multiple experimental conditions, suggesting that Csep1P fundamentally alters how macrophages perceive and respond to commensal bacteria.

Further investigation revealed that a gene called DLL4, which encodes a ligand for Notch signaling pathways, was significantly upregulated by Csep1P. When researchers silenced DLL4 expression using genetic techniques, the enhanced inflammatory response to E. coli was partially blunted—specifically, production of the chemokine CCL4 and the cytokine TNF-α decreased, though IL-8 production remained elevated. This finding suggests that DLL4-Notch signaling is one mechanism by which Csep1P amplifies macrophage responses, though other pathways are likely involved.

The implications for Crohn's disease are substantial. The disease predominantly affects the terminal ileum, and approximately one to one and a half liters of saliva flow from the mouth to the intestines each day, providing a continuous route for oral bacteria like C. concisus to reach the gut. Virulent strains of C. concisus can invade intestinal epithelial cells, trigger cell death, and produce inflammatory metabolites. The new findings suggest that Csep1P may act as a sensitizer, reshaping the mucosal immune landscape and reprogramming macrophages to mount exaggerated responses to otherwise harmless commensal bacteria. This could explain how infection with specific C. concisus strains might initiate or perpetuate gut inflammation in susceptible individuals. Notably, an earlier study found no increased risk of IBD following C. concisus infection detected by bacterial culture, but the researchers suggest this discrepancy may reflect strain-specific differences—only Csep1-positive strains may pose a genuine threat.

The findings open a new therapeutic avenue. Current anti-inflammatory treatments for Crohn's disease have high relapse rates, possibly because they do not address the microbial triggers underlying the disease. Csep1P is now identified as a potential drug target. Blocking the protein's ability to prime macrophages, or interrupting the DLL4-Notch signaling pathway it activates, could theoretically prevent the immune system from mounting excessive responses to gut bacteria. The research also highlights the importance of distinguishing between different strains of the same bacterial species—a distinction that has often been overlooked in clinical studies of IBD.