Bacterial amyloids: interactions with DNA and pathogenicity

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Summary Amyloids are complex proteins with a conserved beta sheet structure. Bacteria use amyloids to decorate the extracellular matrix of their biofilms, highly structured multicellular communities. Curli are amyloids expressed by enteric bacteria, including Salmonella Typhimurium and Escherichia coli. Our group has shown that curli amyloid fibers are recognized by TLR2/1 as well as the NLRP3 inflammasome. This finding is of broad relevance, because detection of amyloids by the innate immune system drives the pathogenesis of Alzheimer's disease and prion diseases. A recent study demonstrated that an artificially formed amyloid protein can bind DNA in vitro and this complex elicits an autoimmune response in mice. Since the biofilm extracellular matrix naturally harbors amyloids and DNA, we explored the interactions between these two molecules and determined that bacterial DNA released during biofilm formation was irreversibly incorporated into curli fibers. Bacteria expressing curli or purified curli/DNA complex triggered autoimmune responses in vivo. The primary objective of this application is to elucidate the mechanism by which bacterial amyloids are recognized by the immune system, leading to their pathogenic effects in the host. Our central hypothesis is that bacterial amyloid/DNA complexes are pathogenic molecules that act by accessing multiple cellular compartments and engaging several Pattern Recognition Receptors, including, TLR2, TLR9 and NLRP3 resulting in inflammation and autoimmune responses. It is our expectation that successful completion of the proposed studies will identify bacterial amyloids as a novel powerful Pathogen-Associated Molecular Pattern (PAMP) that is recognized by the immune system via multiple receptors and establish a new paradigm that infections with amyloid-expressing bacteria are major environmental triggers not only for SLE but also for several other complex human diseases.