Many illnesses, including gastrointestinal diseases, are exacerbated by inflammation contributed by inflammatory monocytes that are recruited to help clear pathogens. However, an increased accumulation of inflammatory monocytes can lead to excessive inflammation and tissue damage. As a result, the recruitment of inflammatory monocytes is tightly regulated. Psychological stressors are known to exacerbate inflammatory diseases, but whether these stressor-induced exacerbations involve inflammatory monocytes is unknown. Recently it has been demonstrated that exposure of mice to the stressor, social disruption (SDR), can significantly enhance Citrobacter rodentium-induced infectious colitis which includes enhanced colonic monocyte/macrophage accumulation, all of which can be significantly reduced by treatment with the probiotic Lactobacillus reuteri. Thus, this study was designed to determine the involvement of inflammatory monocytes in stressor-enhanced infectious colitis, as well as the corollary hypothesis of how probiotic intervention can reduce stressor-enhanced infectious colitis.
Exposure of mice to SDR at the onset of C. rodentium challenge causes significant increases in pathogen-induced colonic histopathology, colonic inflammatory mediator gene expression (i.e., tumor necrosis factor-α, CCL2, inducible nitric oxide synthase), colonic monocyte/macrophage accumulation, and pathogen translocation from the colon to the spleen, all of which are reduced by L. reuteri treatment. The reduction of SDR-enhanced, pathogen-induced CCL2 is of importance as CCL2 is responsible for the recruitment of inflammatory monocytes which are heavily involved in the induction of colitis. In order to test the hypothesis that SDR-enhanced infectious colitis is due to a CCL2-mediated recruitment of inflammatory monocytes, CCL2-deficient mice, which have defective monocyte recruitment, were utilized. Exposing CCL2-deficient mice to SDR during C. rodentium challenge caused an increase in colonic pathogen levels however the resulting pathogen-induced infectious colitis is negated. Also, the addition of monocytes from naive mice to C. rodentium-infected mice alone was not enough to enhance infectious colitis. In determining the source of colonic CCL2 during SDR-exposure in vitro evidence has demonstrated that bacterial byproducts of L. reuteri can significantly reduce pathogen-induced CCL2 and TNF-α gene expression in colonic epithelial cells, but not macrophages. And the infection of mice that are unable to activate the transcription factor NF-κB in either intestinal epithelial cells or myeloid-derived cells (i.e., monocytes and macrophages) with C. rodentium has demonstrated that myeloid-derived, NF-κB-induced upregulation of proinflammatory cytokines is responsible for pathogen-induced colitis.
Lastly, to retest our overarching hypothesis of the efficacy of probiotic intervention on stressor-enhanced infectious colitis, mice were exposed to a different psychological stressor, prolonged physical restraint (RST), at the inception of bacterial challenge with C. rodentium, with and without Lactobacillus reuteri treatment. Probiotic intervention with L. reuteri was able to reduce RST-induced disease severity as evidenced by reductions of pathogen translocation from the colon to the spleen possibly through the fortification of the colonic barrier. However, L. reuteri was unable to reduce RST-enhanced colonic pathogen colonization, colonic histopathology, or colonic inflammatory mediator mRNA expression. Taken together, probiotic intervention with L. reuteri can reduce stressor-exacerbated infectious colitis however the degree to which colitis is reduced is dependent on the severity of colitis.