# Csep1P protein from Campylobacter concisus induces a chemokine-dominant inflammatory state in macrophages and enhances proinflammatory response to gut bacteria

## TL;DR

A protein from Campylobacter concisus triggers immune cells to become more inflamed when exposed to gut bacteria, potentially linking this bacterium to Crohn’s disease.

## Contribution

The study reveals the 3D structure of Csep1P and shows it primes macrophages to enhance inflammation in response to gut bacteria.

## Key findings

- Csep1P induces a chemokine-dominant inflammatory state in macrophages.
- Csep1P-primed macrophages show heightened proinflammatory response to Escherichia coli.
- DLL4 gene silencing reduces the proinflammatory effect of Csep1P on macrophages.

## Abstract

Translocation of Campylobacter concisus from the oral cavity to the intestinal tract is increasingly recognised as a contributor to inflammatory bowel disease (IBD). The C. concisus secreted protein Csep1 has emerged as a molecular marker of C. concisus strains associated with Crohn’s disease, a form of IBD. However, its structure and role in inflammation remain unknown. Here, we report the X-ray crystal structure of plasmid-encoded Csep1P that reveals a unique α-helical fold with structural similarity to Helicobacter pylori cysteine-rich proteins HcpB and HcpC. Because HcpA, another Hcp family member, is known to affect monocyte differentiation, this structural similarity led us to hypothesise that Csep1P may modulate monocyte differentiation and macrophage function. Transcriptomic analysis revealed that Csep1P induced a chemokine-dominant inflammatory state in macrophages, M1-chem. Protein-level validation in both THP-1-derived and primary human macrophages confirmed this selective chemokine response. While Csep1P alone did not upregulate proinflammatory cytokines, THP-1-derived macrophages pre-incubated with Csep1P produced a higher level of proinflammatory cytokines in response to commensal Escherichia coli, which was validated on primary human macrophages. Furthermore, silencing the delta like canonical notch ligand 4 (DLL4) gene decreased the proinflammatory response of Csep1P-mediated macrophages to E. coli. Collectively, our data demonstrate that the structurally unique Csep1P reprograms macrophage response, which provides a mechanistic link between C. concisus infection and Crohn’s disease pathogenesis, and identifies Csep1P as a potential target for therapeutic intervention.

Crohn’s disease is a form of inflammatory bowel disease (IBD) that is a life-long chronic inflammation of the gastrointestinal tract. The causes of Crohn’s disease are complex and are not fully understood, but certain bacteria are thought to play a role. Our study focuses on Campylobacter concisus, a bacterium commonly found in the human oral cavity that can move to the gut and has been linked to Crohn’s disease. We investigated a C. concisus secreted protein called Csep1P, to better understand how it might affect the immune system. Using X-ray crystallography, we determined the unique 3D structure of this protein. We found that Csep1P drives macrophages, a type of immune cell, into a state where they release specific signals that attract other immune cells. Although Csep1P alone did not trigger inflammation, it “primed” macrophages to react much more strongly to commensal gut bacteria such as Escherichia coli. Our results suggest that Csep1P may help explain how virulent C. concisus strains contribute to gut inflammation and highlight it as a potential target for new treatment for Crohn’s disease.