Biofilm-derived curli and Z-DNA shape anti-DNA antibody responses during Salmonella infections

Antibodies to Z-DNA, a non-canonical DNA conformation with a left-handed zigzag backbone, are abundant in the serum of patients with systemic lupus erythematosus (SLE), with levels increasing with disease activity and flares. As SLE is associated with bacterial infections, and as extracellular DNA (eDNA) within biofilms of several bacterial species has been shown to adopt the Z-DNA conformation, bacterial Z-DNA may represent a source of immunogenic Z-DNA in SLE and other related autoimmune conditions. In these studies, we investigated whether eDNA in Salmonella biofilms also contained Z-DNA and whether such Z-DNA could elicit an antibody response. Using antibody-based staining approaches, we observed abundant eDNA in Salmonella enterica serovar Typhimurium (STm) biofilms in both the Z- and canonical B-DNA configurations, consistent with the highly Z-prone nature of the GC-rich Salmonella genome. To assess the functional contribution of these DNA conformations to biofilm integrity, biofilms were treated with DNase I, which lacks enzymatic activity against Z-DNA, or with benzonase, a nonspecific nuclease that degrades both B- and Z-DNA. DNase I treatment applied after biofilm maturation was less effective at thinning biofilms than treatment during early biofilm formation, a pattern also observed with benzonase treatment. Purified curli:DNA complexes contained Z-DNA and, when administered intraperitoneally to mice, elicited robust anti-Z-DNA antibody responses. Similarly, infection with invasive STm induced the production of anti-Z-DNA antibodies in vivo. Moreover, STm infection in mice fed a diet that promotes biofilm development was associated with increased Z-DNA levels in the cecal lumen and elevated anti-DNA antibody responses. Collectively, these findings suggest that Z-DNA, likely formed by extruded Salmonella genomic DNA, and embedded within curli:DNA complexes of STm biofilms, triggers a host immune response and drives anti-Z-DNA antibody production. This work provides mechanistic insight into how bacterial infections and diet-dependent modulation of biofilm formation may contribute to anti-Z-DNA antibody responses in autoimmune diseases like SLE.