Episymbiotic Saccharibacteria suppresses epithelial immunoactivation through Type IV pili and TLR2 dependent endocytosis

Saccharibacteria are episymbionts that require host-bacteria to grow. They are positively associated with inflammatory diseases within the human microbiome, yet their mechanisms for interacting with the human host and contributing to diseases remain unknown. This study investigated interactions between a Saccharibacterium (Nanosynbacter lyticus), its host-bacteria (Schaalia odontolytica), and oral epithelial cells. The host-bacteria induced proinflammatory cytokines in epithelial cells, while Saccharibacteria were immune silent. Remarkably, Saccharibacteria dampened cytokine responses to host-bacteria during coinfection. This effect was driven by Saccharibacteria-induced clustering of TLR2 receptors, a process likely facilitated by type IV, ultimately leading to reduced TLR2-mediated cytokine signalling. High resolution imaging showed that Saccharibacteria were endocytosed by oral epithelial cells, and colocalized with endosome markers, eventually trafficking to lysosomes. Moreover, a subset of the Saccharibacteria survive endocytosis long-term, and retains their capability to reinfect host-bacteria, highlighting a mechanism for persistence in the oral microbiome and a vital role in mammalian immune system modulation.