Paradoxical effects of immune cells on the enteric nervous system in intestinal inflammation
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Inflammatory bowel disease causes structural and functional alterations in the enteric nervous system (ENS). Since the onset of intestinal inflammation involves the activation of resident immune cells as well as rapid influx of infiltrating cells, we proposed that changes in the ENS are a result of the release of toxic inflammatory factors. We hypothesized that early damage to the ENS in inflammation is caused by harmful levels of nitric oxide (NO) generated by the enzyme inducible nitric oxide synthase (iNOS) found in immune cells. This was assessed in the 2, 4, 6-trinitrobenzene sulfonic acid (TNBS)-model of colitis in rats. Large increases in infiltrating granulocytes, particularly neutrophils and blood-derived monocytes were found in the muscularis layers adjacent to the ENS. A rapid increase in iNOS immunoreactivity in the muscularis regions during early stages of inflammation (6 – 24 hr) was observed. Whether high NO levels generated by chemical donors could be toxic to neurons was tested in a co-culture model of myenteric neurons, smooth muscle and glia enzymatically isolated from neonatal rats. Exposure of co-cultures to NO for 48 hr resulted in significant, concentration dependent decrease in neuron survival. We then developed a model that permitted the direct study of immune cell interactions with myenteric neurons. Myenteric neurons were co-cultured with activated peritoneal immune cells that expressed iNOS and generated high NO levels (49 + 6.2µM) for 48 hr. This caused significant neuronal death, reducing neuron number by 19 + 5%, and disruption of axons. Pre-treatment of immune cells with a selective iNOS-inhibitor, L-NIL resulted in neuron numbers that were not significantly different from control (96 + 2%) suggesting that NO played a central role in mediating the damaging effects of immune cells. Lastly, when direct contact between immune cells and neurons was prevented in the previous experiment through use of trans-wells, unanticipated neurotrophic effects were observed. Increased axon outgrowth (282 + 57%) was detected in addition to loss of the neurotoxic effects in spite of similar experimental conditions. We concluded that proximity and contact plays an important role in determining the nature of immune cell mediated alterations in enteric neurons.