The role of sympathetic neurons in modulating resident muscularis macrophage functions during postoperative ileus and mucosal antimicrobial responses in homeostasis

Abstract

<strong>Introduction:</strong> The sympathetic nervous system (SNS) innervates large parts of the gastrointestinal tract. Its projections are densely found in tunica muscularis externa and reach up to the mucosa, the innermost part of the bowel. In the muscularis externa, tyrosine hydroxylase-expressing (TH⁺) sympathetic neurons govern immune modulation by regulating the functions of resident macrophages in their proximity. These macrophages modulate muscularis inflammation during postoperative ileus (POI), a non-infectious, clinically relevant immune motility disorder. However, whether the SNS modulates macrophage functions in vivo during POI is unknown. Notably, the SNS projections are also found in the submucosa, close to the epithelium, where they regulate mucosal immunity. Due to its close association, the role of SNS in epithelial antimicrobial defense is highly likely but has not been elucidated so far. <br /> <strong>Methods:</strong> We tested different sympathectomy (STX) models to ablate the intestinal sympathetic innervation. With the chemical STX (cSTX) approach using 6-OHDA treatment, we abolished TH⁺ sympathetic neurons from the GI tract and interrupted both neuro-immune and neuro-epithelial sympathetic connections to investigate its impact on the gut inflammatory status in homeostasis and a mouse model of POI. <br /> <strong>Results:</strong> In the muscularis externa, the resident CX3CR1⁺ muscularis macrophages anti-inflammatory gene response was reduced after cSTX and the muscularis externa profile was shaped towards a pro-inflammatory status. The cSTX mediated reduction of macrophage anti-inflammatory genes was also observed in the early postoperative phase of POI. At the late and clinically relevant phase of POI, leukocyte infiltration into muscularis was reduced with a quicker recovery of bowel motility. On the mucosal level, cSTX affected the pro-inflammatory status and increased the expression of tight junctions and antimicrobial defense genes, indicating an influence of the SNS in modulating both epithelial cell and immune cell functions in intestinal homeostasis. <br /> <strong>Conclusion:</strong> In summary, our data show that in the muscularis externa, the SNS plays a pivotal role in postoperative inflammation and highlights the role of resident CX3CR1⁺ muscularis macrophages as mediators of sympathetic actions. In the mucosa, the SNS modulates tight epithelial junction and antimicrobial gene expression under homeostasis.