Type IV pili and TLR2 dependent endocytosis of episymbiotic Saccharibacteria attenuates immunoactivation

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 inflammatory 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 (IL8, GROα, and MCP-1) in epithelial cells, while Saccharibacteria were immune silent. Remarkably, Saccharibacteria dampened cytokine responses to host-bacteria during coinfection. This effect resulted from Saccharibacteria directly interacting with TLR2 receptors via T4P, becoming endocytosed by epithelial cells, and subsequently inhibiting TLR2 activation by the host-bacteria. Super-resolution imaging showed that intracellular Saccharibacteria colocalized with endosome markers, eventually trafficking to lysosomes. Moreover, a subset survives endocytosis, escapes epithelial cells, and reinfects host-bacteria, highlighting a mechanism for persistence in the oral microbiome and a vital role in mammalian immune system modulation.