PROJECT SUMMARY Infections with enteric pathogens such as Salmonella, Campylobacter, Shigella, or Yersinia are leading causes of morbidity and mortality worldwide. Although in most individuals the infection resolves, approximately 5% of patients subsequently develop a painful chronic inflammatory condition known as Reactive Arthritis (ReA). How Salmonella infections trigger ReA is not well understood. Using Salmonella enterica serovar Typhimurium (STm) as a model organism, we discovered that a Salmonella protein, curli, is a dominant instigator of inflammation following Salmonella infection. Curli is a secreted protein and major component of the STm biofilm in the gastrointestinal tract. Curli fibrils bind extruded bacterial DNA within the biofilm. It is these curli:DNA complexes, rather than curli alone, that are potent triggers of type I interferon, IL-17, and anti-double stranded DNA autoantibody production, leading to ReA. Unknown, however, is why curli:DNA complexes are so inflammatory. We report in this proposal the remarkable discovery that the DNA present within curli:DNA complexes is not solely B-DNA, the classic right-handed (Watson-Crick) double-helix, but includes copious amounts of left-handed Z-DNA as well. Z-form nucleic acids, such as Z-DNA and Z-RNA, were thought not to readily occur in nature, until we showed last year that Z-RNA is indeed produced during virus infections and is a ligand for the necroptosis-activating host sensor protein ZBP1. Our preliminary results now show that the Z-DNA within curli:DNA complexes activates ZBP1 in intestinal epithelial cells (IECs) and fibroblasts, resulting in RIPK3- dependent necroptosis of these cells. These findings allow us to put forward the hypothesis that Z-DNA within curli:DNA fibrils in Salmonella biofilms activates ZBP1 to instigate RIPK3-dependent necroptosis in intestinal epithelial cells (IECs) and other cell types. Necroptosis, in turn, causes cell loss and disrupts gut barrier integrity, releasi