Modulatory effects of the colonic milieu on neutrophil oxidative burst: A possible pathogenic mechanism of ulcerative colitis

Abstract

An important hallmark of ulcerative colitis (UC) is mucosal neutrophil (PMN) infiltration associated with mucosal damage. This suggests that colonic chemoattractants such as bacterial products (e.g., N-formyl-methionyl-leucyl-phenylalanine (fMLP), lipopolysaccharide (LPS)) reach systemic circulation and attract PMNs to the colon. PMNs are then activated in the colonic mucosa and release their toxic oxidative metabolites. However, bacterial products are also present in the systemic circulation of healthy subjects. Thus we hypothesized that PMNs develop tolerance to colonic factors in the normal state and that this tolerance is absent in UC. We evaluated the PMN respiratory burst in response to stimulation with fMLP, LPS, or phorbol 12-myristate 13-acetate (PMA) by measuring the production of reactive oxygen species (ROS) with both luminol-enhanced chemiluminescence and a cytochrome C reduction assay. PMNs were obtained from control subjects, inactive UC patients, patients with UC who had undergone colectomies, and non-UC patients with colectomies. All three stimuli induced a significant rise in ROS. PMNs from non-UC colectomy subjects produced significantly higher ROS than PMNs from control subjects with intact colons in response to both fMLP and LPS. In contrast, PMNs from UC colectomy patients produced levels of ROS similar to those produced by PMNs from UC patients with intact colons in response to fMLP and LPS. Colectomy had no effect on PMA-induced ROS production in controls. The observed difference in fMLP-induced ROS production in control subjects with intact colons was not due to fMLP receptor down-regulation because a competition assay performed with the fMLP blocker BMLP showed a similar receptor apparent affinity in all four groups. We conclude the following: (1) the normal colonic milieu modulates the PMN respiratory burst, resulting in hyporesponsiveness of PMNs to “physiologic” but not “pharmacologic” stimulation. This effect is not due to receptor down-regulation. (2) UC colonic milieu does not appear to modulate PMN respiratory burst. This loss of PMN “tolerance” to colonic factors may have a pathogenic role in the sustained inflammation and tissue damage in UC.