Contributions of type 17 immunity and type I interferon signaling to Salmonella-induced autoimmunity
Autoimmune diseases can be broadly categorized into two groups: classical autoimmune disorders, such as systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA), and post-infectious autoimmune conditions that arise as sequelae to microbial exposure. While several bacterial pathogens have been implicated in triggering such post-infectious autoimmunity, Salmonella is particularly well documented for its ability to induce an autoimmune-like condition known as reactive arthritis. While the mechanisms underlying infection-induced autoimmune sequelae remain unclear, type 17 and type I interferon (IFN) signaling play significant roles in the pathogenesis of many classical autoimmune diseases and bacterial infections. Previous research from our lab has shown that Salmonella biofilm-associated amyloid protein, curli, is a potent inducer of systemic inflammation and autoimmune responses. Curli induces immune responses via various receptors, on the cell surface through toll-like receptor (TLR) 2/1 and CD14, and in the cytosol through nucleotide-binding domain leucine-rich repeat family pyrin domain containing 3 (NLRP3). Curli and extracellular DNA (eDNA) form a strong bond that supports the structure of biofilms, resulting in a curli/DNA complex. This complex is recognized by TLR9 in the endosome. Through these receptors, curli induces inflammatory cytokines and chemokines, including interleukin (IL)-6, nitric oxide, IL-8, tumor necrosis factor (TNF)-α, IL-1β, IL-17, and type I IFN-related genes. In addition, systemic presence of curli is documented to induce autoimmune manifestations such as the generation of anti-double-stranded deoxyribonucleic acid (dsDNA) autoantibodies, anti-chromatin autoantibodies, and knee inflammation. Since prior work has demonstrated that curli can induce type 17 immunity and type I IFN signaling, we investigated the role of type 17 and type I IFN responses in Salmonella/curli-driven autoimmunity. We first investigated the role of type 17 immunity in curli-induced inflammation by using C57BL/6 mice from Taconic Farms and Jackson Laboratory, which, due to microbiota differences, have different basal levels of IL-17. When mice were intraperitoneally (i.p.) injected with curli/DNA complexes, we observed no difference in the production of anti-dsDNA autoantibodies in mice from either facility. However, we saw a moderate increase in knee inflammation in Taconic mice compared to their Jackson counterpart. After eight weeks of i.p. injection of curli/DNA complexes, we observed an increase in gut microbiota diversity in Jackson mice, but not in Taconic mice. Interestingly, despite the lack of microbiota changes, Taconic mice displayed elevated systemic and gut mucosal expression and production of IL-1β, a cytokine known to promote IL-17 production, as well as Tnfa, in the gut mucosa, following curli injections. In addition, a significant increase in the expression of c-c motif ligand 3 (Ccl3) in the colon and cecum of Taconic mice injected with curli was detected. In parallel to the cytokine and chemokine data, we observed an increase in macrophage and neutrophil populations in mice i.p. injected with curli/DNA complex, suggesting that microbiota-driven type 17 responses may amplify curli-induced inflammation. To assess the role of type I IFN signaling, we utilized type I IFN receptor knockout (Ifnar-/-) mice. Following three weeks of curli/DNA complex i.p. injections, no differences in the production of anti-dsDNA autoantibodies were detected. To study this further, mice were orally infected with the ΔspiB mutant of Salmonella Typhimurium, which is deficient in Type III secretion system-2 and thus has a defect in intracellular survival. While Ifnar-/- mice were colonized at the same level as the C57BL/6 at 2 days post-infection (dpi), they exhibited significantly reduced colonization at 7 dpi until the end of the experiment (21 dpi). Flow cytometry of mesenteric lymph nodes of infected mice revealed recruitment and egress of B and T cells, respectively, in an IFNAR-dependent manner as early as 3 dpi. We observed no differences in B and T cell responses in the spleen. Interestingly, although Ifnar-/- mice exhibited significantly reduced colonization, anti-dsDNA autoantibody production was equivalent to C57BL/6 mice, indicating that the initial infection itself is sufficient to trigger autoimmune sequelae. Autoantibody and anti-STm antibody production are thought to occur in extrafollicular sites of the spleen. Since we observed no B and T cell response in the spleen, we investigated the role of the spleen in the production of Salmonella/curli-induced autoantibody. Our data show that, unlike infections with WT STm, ΔspiB does not disrupt germinal center formation in the spleen, but infections with WT or ΔspiB still induce anti-dsDNA autoantibodies. To test whether Salmonella-induced autoantibody response was generated in the spleen, we utilized splenectomized mice. Both splenectomized and sham-surgery mice infected with ΔspiB produced equal levels of anti-dsDNA autoantibodies. Our findings suggest that inflammatory responses to curli/DNA complex are potentially amplified in individuals with a gut microbiome that promotes a type 17 response. In contrast, type I IFN signaling does not play a central role in the development of curli- or Salmonella-induced autoimmunity. Importantly, our results suggest that autoimmune responses to curli and biofilms are initiated in the gut, highlighting the potential impact of enteric infections on autoimmunity.
Microbiology, Immunology, Medicine, Biomedical sciences
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