Background Systemic lupus erythematosus (SLE) is a multifactorial autoimmune disease, in which infections are considered to play a pathogenic role, but the cellular and molecular culprits remain unknown. Bacterial biofilms are multicellular bacterial communities important in the establishment of chronic infection by pathogens. Bacteria produce amyloids, complex proteins with a conserved beta sheet structure, that strengthen the extracellular matrix of their biofilms. We have reported that bacterial and eukaryotic DNA is incorporated into curli fibres, functional bacterial amyloids present in Salmonella and E. coli biofilms. We found that curli/DNA complexes from biofilms activate conventional dendritic cells leading to production of pro-inflammatory cytokines, including Type I interferons, in vitro and in vivo, indicating that curli are a new class of danger signals. I.p. infections with curli-expressing S. Typhimurium or E. Coli Nissle, and also systemic administration of curli/DNA complexes purified from Salmonella biofilms accelerated onset in lupus-prone NZBxW/F1 mice and triggered autoantibodies production in non-predisposed mice, suggesting curli/DNA complexes as novel players in SLE pathogenesis.
Materials and methods In order to determine the cellular and molecular players in the acceleration of lupus by curli-expressing infections, bone marrow-derived cultures of conventional dendritic cells and macrophages were generated in vitro and B cells were isolated ex vivo from NZBxW/F1 lupus-prone mice, TLR-deficient mice and wild-type mice and stimulated with curli/DNA complexes purified from Salmonella biofilms. Moreover, lupus prone and control mice were infected i.p. and by oral gavage with curli-expressing or curli-deficient S. Typhimurium and the effects on dendritic cells, macrophages, and B cell activation, and production of autoantibodies were studied. A novel ELISA was developed to measure anti-curli antibodies in SLE patients and infected mice.
Results We found that Curli and DNA synergistically activate innate and adaptive immune cells, in vitro and in vivo, via TLR2 and TLR9, leading to a Type I interferon response. We found that curli/DNA complexes can directly stimulate proliferation and activation of B cells and induce autoantibody production. Natural infections with curli-expressing S. Typhimurium triggered autoantibodies production in NZBxW/F1 mice and control mice, suggesting curli/DNA complexes break tolerance in SLE. Finally, sera from lupus patients during flares showed elevated levels of anti-curli antibodies.
Conclusions Biofilm-derived curli/DNA complexes are potent activators of innate and adaptive immune cells and can mediate the acceleration of lupus by infections. These studies may provide a novel biomarker of flare and suggest targeting biofilms and bacterial infections as new therapeutic tools in lupus.
Acknowledgements We would like to thank the Lupus Research Institute and the NIH NIAID (R21AI119947) for supporting our work.
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