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Tursi SA , Lee EY , Medeiros NJ , Lee MH , Nicastro LK , Buttaro B , Gallucci S , Wilson RP , Wong GCL , Tukel C
Bacterial amyloid curli acts as a carrier for DNA to elicit an autoimmune response via TLR2 and TLR9
PLoS Pathog. 2017 Apr;13(4) :e1006315
PMID: 28410407    PMCID: PMC5406031   
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Bacterial biofilms are associated with numerous human infections. The predominant protein expressed in enteric biofilms is the amyloid curli, which forms highly immunogenic complexes with DNA. Infection with curli-expressing bacteria or systemic exposure to purified curli-DNA complexes triggers autoimmunity via the generation of type I interferons (IFNs) and anti-double-stranded DNA antibodies. Here, we show that DNA complexed with amyloid curli powerfully stimulates Toll-like receptor 9 (TLR9) through a two-step mechanism. First, the cross beta-sheet structure of curli is bound by cell-surface Toll-like receptor 2 (TLR2), enabling internalization of the complex into endosomes. After internalization, the curli-DNA immune complex binds strongly to endosomal TLR9, inducing production of type I IFNs. Analysis of wild-type and TLR2-deficient macrophages showed that TLR2 is the major receptor that drives the internalization of curli-DNA complexes. Suppression of TLR2 internalization via endocytosis inhibitors led to a significant decrease in Ifnbeta expression. Confocal microscopy analysis confirmed that the TLR2-bound curli was required for shuttling of DNA to endosomal TLR9. Structural analysis using small-angle X-ray scattering revealed that incorporation of DNA into curli fibrils resulted in the formation of ordered curli-DNA immune complexes. Curli organizes parallel, double-stranded DNA rods at an inter-DNA spacing that matches up well with the steric size of TLR9. We also found that production of anti-double-stranded DNA autoantibodies in response to curli-DNA was attenuated in TLR2- and TLR9-deficient mice and in mice deficient in both TLR2 and TLR9 compared to wild-type mice, suggesting that both innate immune receptors are critical for shaping the autoimmune adaptive immune response. We also detected significantly lower levels of interferon-stimulated gene expression in response to purified curli-DNA in TLR2 and TLR9 deficient mice compared to wild-type mice, confirming that TLR2 and TLR9 are required for the induction of type I IFNs. Finally, we showed that curli-DNA complexes, but not cellulose, were responsible elicitation of the immune responses to bacterial biofilms. This study defines the series of events that lead to the severe pro-autoimmune effects of amyloid-expressing bacteria and suggest a mechanism by which amyloid curli acts as a carrier to break immune tolerance to DNA, leading to the activation of TLR9, production of type I IFNs, and subsequent production of autoantibodies.
1553-7374 Tursi, Sarah A Lee, Ernest Y ORCID: http://orcid.org/0000-0001-5144-2552 Medeiros, Nicole J ORCID: http://orcid.org/0000-0003-4449-2441 Lee, Michael H Nicastro, Lauren K Buttaro, Bettina Gallucci, Stefania ORCID: http://orcid.org/0000-0003-4737-8003 Wilson, Ronald Paul Wong, Gerard C L Tukel, Cagla ORCID: http://orcid.org/0000-0001-7073-0226 Journal Article United States PLoS Pathog. 2017 Apr 14;13(4):e1006315. doi: 10.1371/journal.ppat.1006315. eCollection 2017 Apr.