Bacteria and their interaction with macrophages in Crohn's disease pathogenesis

Abstract

Crohn’s disease (CD) is an inflammatory bowel disease (IBD) of unknown aetiology. It affects up to 0.2% of the population and causes significant morbidity and increased mortality. Medical and surgical treatments are available but neither are curative. Functional and genome-wide association studies suggest that defective luminal bacterial handling by the gut innate immune system contributes to pathogenesis. Pathogenic bacteria may also play a role and there has been particular focus on gastrointestinal (GI) mucosal adherent-invasive E.coli (AIEC) which are prevalent in CD.

The purpose of this thesis was to characterise GI mucosal E.coli strains and examine the interaction of E.coli and other bacteria with macrophages in CD.

A case-control study of cultured GI mucosal biopsies from patients with CD, ulcerative colitis (UC) and healthy controls illustrated that mucosal E.coli were most prevalent in CD and were cultured from both inflamed and uninflamed CD mucosa. There was substantial variety of E.coli genotype and phenotype in IBD patients and healthy controls. AIEC were identified only in mucosa from CD patients and adherent CD strains shared characteristics with extraintestinal pathogenic E.coli.

A subsequent study demonstrated that E.coli could be identified in lamina propria macrophages commonly in CD, rarely in UC and not in healthy controls. There was a clear pattern of macrophage cytokine polarisation in relation to the presence or absence of intracellular E.coli. The CD autophagy risk allele ATG16L1 was associated with higher macrophage cytokine mRNA expression in CD.

In vitro studies demonstrated prolonged survival of E.coli but not S.aureus in monocytes from patients with CD. This possible bacterial killing defect did not appear to relate to respiratory burst dysfunction as this was normal in CD monocytes. Finally, a rapid in vitro monocyte intracellular survival assay for a luminescent Mycobacterium avium ssp paratuberculosis (MAP) strain was developed and validated. MAP survived in monocytes from both patients with CD and healthy controls for at least 2 weeks.

In conclusion, this thesis provides evidence for a potential role of an abnormal interaction between lamina propria macrophages and E.coli in CD pathogenesis.