Diet induced changes in gut microbial composition and dysregulated immune responses are key players in the progression of metabolic syndrome (MetSyn) and obesity.The lymphatics that are directly exposed to the gut microbiota in the mesenteric bed are central players in inflammatory and immune responses and are impaired during MetSyn. However, the mechanisms remain unknown.Endogenous tryptophan derived gut microbiota metabolites such as indole, significantly limits inflammation and immune response. We hypothesize that microbiota influence the lymphatic mesenteric microenvironment via such discrete metabolites to impact metabolic disease. We evaluated the effects of indole, tryptamine and indole-3-acetate (I-3A) on inflammatory mechanisms in vitro and in vivo using cultured models of lymphatic endothelium (HDLECs) and rat models of high fructose diet induced MetSyn and LPS induced acute inflammation. HDLECs were treated with varying doses of indole, tryptamine and I-3A (100uM-500uM) in presence or absence of LPS (20ng/ml) and its effects on inflammatory cytokines were evaluated. Rats were injected with LPS (10mg/kg, 24hrs) with or without indole (20mg/kg) to assess for inflammation. Further, rats were also subjected to a 7-week high fructose (60%) diet regimen and gavaged with indole for 4 weeks prior to completion of the diet regimen and metabolic parameters were measured. Our results show that indole decreases levels of inflammatory cytokines as cfos, cjun, IL1, MCP1, JAK1, SOCS3 and SOCS1 induced by LPS in the HDLECs and in the mesenteric arcades and lymph nodes. Similar effects are also observed with tryptamine and I-3A. Flow cytometric analysis of tissues show that indole significantly suppresses the population of inflammatory MHCII + /cd45 + cd11b + cells (p<0.02) and augments Foxp3+/cd4+ cells (p<0.003) in the mesenteric arcades, lymph nodes and spleen that are induced in LPS treated animals. In MetSyn animals, indole reduces levels of glucose, triglyceride and epididymal fat content that is also associated with concomitant decrease in expression of inflammatory cytokines. Taken together, we show that indole and other tryptophan metabolites limit lymphatic inflammatory pathways and also decreases metabolic parameters associated with MetSyn and thus can be a viable therapeutic intervention for metabolic disease.
