Assembly of ordered DNA-curli fibril complexes during Salmonella biofilm formation correlates with strengths of the type I interferon and autoimmune responses

Abstract

Deposition of human amyloids is associated with complex human diseases such as Alzheimer’s and Parkinson’s. Amyloid proteins are also produced by bacteria. The bacterial amyloid curli, found in the extracellular matrix of both commensal and pathogenic enteric bacterial biofilms, forms complexes with extracellular DNA, and recognition of these complexes by the host immune system may initiate an autoimmune response. Here, we isolated early intermediate, intermediate, and mature curli fibrils that form throughout the biofilm development and investigated the structural and pathogenic properties of each. Early intermediate aggregates were smaller than intermediate and mature curli fibrils, and circular dichroism, tryptophan, and thioflavin T analyses confirmed the establishment of a beta-sheet secondary structure as the curli conformations matured. Intermediate and mature curli fibrils were more immune stimulatory than early intermediate fibrils in vitro. The intermediate curli was cytotoxic to macrophages independent of Toll-like receptor 2. Mature curli fibrils had the highest DNA content and induced the highest levels of Isg15 expression and TNFα production in macrophages. In mice, mature curli fibrils induced the highest levels of anti-double-stranded DNA autoantibodies. The levels of autoantibodies were higher in autoimmune-prone NZBWxF/1 mice than wild-type C57BL/6 mice. Chronic exposure to all curli forms led to significant histopathological changes and synovial proliferation in the joints of autoimmune-prone mice; mature curli was the most detrimental. In conclusion, curli fibrils, generated during biofilm formation, cause pathogenic autoimmune responses that are stronger when curli complexes contain higher levels of DNA and in mice predisposed to autoimmunity.

Systemic exposure to bacterial amyloid curli alters the gut mucosal immune response and the microbiome, exacerbating Salmonella -induced arthritis

“…Curli was purified as previously described with slight modifications, 36 resulting in a preparation that contains high levels of DNA and it is not cytotoxic. 23 , 36 , 37 The mice were injected intraperitoneally (i.p.) with 100 µg of curli or sterile PBS twice a week for eight weeks.…”

“…The activation of the TLR2/TLR1 receptor complex by curli strongly induces the expression of several cytokines including IL − 6, IL − 12, IL − 17, and TNFα. 13 , 19 , 23 , 24 , 37 Upon TLR2 activation, curli is taken up by immune cells and escapes to the cytosol where it activates the NLRP3 inflammasome leading to the production of IL1β. 20 Additionally, curli and the DNA associated with this amyloid together induce the activation of the type I interferon signaling and the expression of interferon-stimulated genes as well as a strong anti-dsDNA autoantibody response.…”

“… 20 Additionally, curli and the DNA associated with this amyloid together induce the activation of the type I interferon signaling and the expression of interferon-stimulated genes as well as a strong anti-dsDNA autoantibody response. 22 , 23 , 37 …”

“…We have previously shown that purified curli/DNA complexes induce a strong autoantibody response against DNA, even in C57BL/6 mice; 22 , 23 , 37 therefore, we evaluated levels of the anti-dsDNA autoantibodies induced by curli in Jackson Labs and Taconic mice. Regardless of their facility of origin, there was a significant increase in anti-dsDNA IgG production in mice after two weeks of treatment with curli compared to mice injected with PBS, and there were no significant differences in the levels of anti-dsDNA IgG autoantibodies between Jackson Labs mice and Taconic mice ( Figure 6a ).…”

“… 23 , 29 , 47–49 Recent work suggests that chronic systemic exposure to biofilms or to the amyloid/DNA complexes from Gram-negative or Gram-positive bacteria, curli/DNA and PSM/DNA, respectively, are associated with the generation of autoimmune responses both in mice and humans. 22 , 23 , 25 , 31 , 37 , 49 , 50 Curli is produced in the gastrointestinal tract by Gram-negative enteric bacteria 37 and serves as a major biofilm-associated PAMP. 9 , 13 , 19 , 21 , 24 , 51 During invasive S .…”