Systems Biology of Pro- and Anti-Inflammatory Intra- and Inter-Kingdom Signaling in Mammalian Cells

Abstract

Inflammation is a beneficial self-defense mechanism that is usually triggered by injury or infection and is designed to return the body to homeostasis. However, uncontrolled inflammation can be deleterious and has been shown to be involved in the etiology of several diseases, such as inflammatory bowel disorder, rheumatoid arthritis and asthma. Inflammation is counter-balanced by host-derived anti-inflammatory immune responses of the body thereby maintaining immune homeostasis. Millions of bacteria categorized under commensal (non-pathogenic), probiotic (mutualistic) and pathogenic (disease causing) species that inhabit gastro-intestinal (GI) tract of mammalian host have been reported to influence host immune homeostasis. Host immune response is tolerant towards beneficial bacteria and provide them with shelter and food, in turn these bacteria help the host by digesting carbohydrates and proteins and preventing colonization of GI tract by invading pathogens and potentially harmful indigenous bacteria. Intestinal microbiota-derived indole has been shown to exhibit anti-inflammatory properties by down regulating inflammatory markers. A shift in indigenous bacterial population (dysbiosis) has been recently found to initiate and perpetuate chronic inflammation resulting in the etiology of inflammatory bowel diseases (comprising ulcerative colitis and Crohn’s disease). However, the inter-play between pro- and anti-inflammatory signaling as well as molecular mechanism involved in host-microbiota interactions are not fully understood.

In this dissertation, we developed a mathematical model to describe integrated pro- and anti-inflammatory signaling in macrophages. The model incorporates the feedback effects of de novo synthesized pro-inflammatory (tumor necrosis factor α; TNF-α) and anti-inflammatory (interleukin-10; IL-10) cytokines on the activation of the transcription factor named nuclear factor -κ B (NF-κB) under continuous lipopolysaccharide (LPS) stimulation (mimicking bacterial infection). In the model, IL-10 upregulates its own production (positive feedback) and also downregulates TNF-α production through NF-κB (negative feedback). In addition, TNF-α upregulates its own production through NF-κB (positive feedback). We validated the mathematical model predictions by measuring phosphorylated NF-κB, de novo synthesized TNF-α and IL-10 in murine RAW 264.7 macrophages when exposed to LPS. This integrated model was used to incorporate bacteria-derived indole signaling through a potential aryl hydrocarbon receptor (AHR) activated signal transduction pathway in human intestinal epithelial HCT-8 cells. The work described in this dissertation is a step towards modeling integrated pro- and anti-inflammatory immune responses in host (intra-kingdom signaling) and their cross-talk with intestinal microbiota-derived metabolite (inter-kingdom signaling) to maintain homeostasis in the body.