Introduction Acute inflammation in ulcerative colitis (UC) is protracted. Using an in vivo bacterial challenge model, we characterised the cellular and molecular determinants and consequences of the early inflammatory response in UC, and the functional impacts of 5-ASA on this process.
Method Acute inflammation was provoked in 23 healthy controls (HC) and 26 UC patients off treatment or on 5-ASAs, by intradermal injection with killed E. coli or S. pneumoniae. Local vascular reactions were quantified by laser Doppler. Early and resolving inflammatory exudates were sampled by raising suction blisters over inoculation sites after 4 hour or 48 hour. Cells were characterised by polychromatic flow cytometry; cytokines by multiplex array; and lipid mediators by mass spectrometry. In vivo findings were confirmed by in vitro stimulation of cultured peripheral blood-derived macrophages with killed E. coli.
Results UC patients had enhanced local blood flow within 24 hour of bacterial exposure, and impaired inflammation resolution, to both Gram-negative and Gram-positive bacteria (p=0.01). Neutrophil accumulation (p=0.04) and PGE2 production (p=0.02) were increased within 4 hour, and were not cytokine-driven phenomena. At 48 hour, UC patients had persistent neutrophils (p=0.001) and T lymphocytes (p=0.03). Findings were replicated in cultured macrophages, which also exhibited higher COX-1 up-regulation, supporting the primary impact of these abnormalities. Exaggerated onset was normalised in patients taking 5-ASAs, although these individuals had greater numbers of macrophages at 48 hour (p=0.03). This was accompanied by increased concentrations of the hydroxy fatty acids 9-oxo-octadecadienoic acid (OxoODE) and 13-OxoODE. To characterise their effects, macrophages were co-incubated with E. coli and these mediators. Both led to dose-dependent suppression of TNF-α (p=0.0001 and p=0.01, respectively), at concentrations reflective of those in vivo. These effects were completely reversed by GW9662, a PPARγ antagonist (p=0.006). The in vivo profile of lipid mediators in HC treated with 5-ASAs differed from UC, with subtle differences in resolving inflammatory cellular and cytokine constituents; this has important implications for understanding generation of these hydroxy fatty acids in vivo.
Conclusion Acute inflammation in UC is exaggerated and slow to resolve, associated with perturbed production of inflammatory lipid mediators. 5-ASAs normalise this reaction, partly through generation of hydroxy fatty acids that act through the PPAR-γ receptor.
Disclosure of Interest None Declared
- Acute inflammation
- Inflammation resolution
- Lipid mediators
- Ulcerative colitis