A fundamental paradigm of IBD pathogenesis is that inflammation results from an inappropriate response to the pathogen-associated molecular patterns (PAMPs) expressed by the gut microbiota. Recent evidence from other immune-mediated disorders indicates that chronic inflammation occurs when PAMP signals are combined with signals derived from tissue damage, the so-called damage-associated molecular patterns (DAMPs). These are released by injured/dead cells and induce a "sterile inflammatory response". DAMPs are numerous, including the high mobility group protein 1 (HMGB1), nucleic acids, uric acid, degradation products of the extracellular matrix, calcium-binding S100 proteins, and many others. DAMPs utilize receptors shared with PAMPs, such as Toll-like receptors (TLRs), but also distinct receptors, co-receptors and accessory molecules to mediate their actions. Some DAMPs have already been identified in IBD, such as HMGB1 and calcium-binding S100 proteins and, therefore, understanding how their signals converge with those coming from PAMPs becomes obviously relevant and will be explored in this proposal. The cytokine IL-1? has been recently identified as a major DAMP released by necrotic cells and is a strong inducer of sterile inflammation. Because the IL-1 cytokine family is involved in IBD, we performed preliminary studies and found that necrotic intestinal epithelial cells release IL-1?, which induces proinflammatory events in the gut. We also found that IL-1? is present in vivo in the epithelium and can trigger experimental colitis. Thus, we propose the novel central hypothesis that epithelial cell-derived IL-1? is a major intestinal DAMP and represents a novel component of IBD pathogenesis. This hypothesis will be tested by 3 specific aims:
Aim 1. Characterize the response of human immune and non-immune cells to necrotic cell-derived IL-1? alone and in combination with other DAMPs and PAMPs.
Aim 2. Determine the mechanisms by which necrotic cell-derived IL-1? regulates the inflammatory response.
Aim 3. Explore the role of necrotic cell-derived IL-1? in induction and modulation of intestinal inflammation in vivo. The innovative concept that epithelial-derived IL-1? can mediate inflammation alone or in association with PAMPs would create a paradigm shift in our current understanding of IBD pathogenesis. Additionally, interfering with the DAMP function of IL-1? may generate new insights into how to modulate intestinal immunity and inflammation.
The proposed studies are aimed at understanding the function of the cytokine IL-1 in intestinal inflammation and how it contributes to the pathogenesis of inflammatory bowel disease (IBD). IL-1 belongs to a group of agents collectively named damage-associated molecular patterns (DAMPs). DAMPs pre-exist in various tissues and organs, but they are normally hidden from the body immune defenses and therefore cause no harm. However, when cells die, DAMPs are released and bind to receptors on a variety of immune and non-immune cells and induce an inflammatory response. The studies proposed in this application are focused on the IL-1? contained in intestinal epithelial cells that is released when these cells are damaged or destroyed. When this occurs IL-1? triggers a strong inflammatory reaction that further damages the intestine and may contribute to perpetuate gut inflammation, as characteristically seen in IBD. In addition to fostering inflammation by itself, I-1? likely interacts with other substances derived from microbes or dead cells, and this combination may amplify or prolong gut inflammation, another aspect that will also be investigated in this grant application.