Tu179 - Type IV Pili and TLR2 Dependent Endocytosis of Episybiotic Saccharibacteria Modulates Epithelial Immunity

Abstract Text: The human oral microbiome is a complex and dynamic community of microorganisms that play a crucial role in overall health by preventing pathogenic invasions and contributes to immune system training. Within few inflammatory diseases, recently discovered microbial dark matter, Saccharibacteria, increase their relative abundance. However, their role in these diseases are unclear. Additionally, Saccharibacteria are epibionts that live on the surface of Actinobacteria species and their interactions with the human host and inflammatory disease development are largely unexplored. We aimed to understand the unique immunological interactions of Nanosynbacter lyticus strain TM7x and its cognate host bacteria, Schaalia odontolytica strain XH001, with oral epithelial cells. Our findings demonstrated that TM7x dampen host and non-host bacteria induced innate immune responses. Metatranscriptomic analysis revealed several endocytic and signaling pathways induced by TM7x infection. Conversely, TM7x host bacteria induced TLR2 and NF-kB inflammatory cytokine pathways, while TM7x inhibited this pathway by directly interacting with the epithelial cells. The close interaction between TM7x and oral epithelial cells were mediated by Type IV pili TLR2 receptor interactions, respectively. Furthermore, TM7x clustered TLR2 receptor and endocytosed by the epithelial cells, inhibiting TLR2 activation by the host bacteria. Our studies demonstrate for the first time TM7x binding epithelial cell surfaces and being internalized into epithelial cells via caveolin mediated endocytosis. Upon internalization, TM7x survived within epithelial cells for 3 days before being eliminated by lysosomal degradation. Our results provide mechanistic insights into the largely unknown interactions between Saccharibacteria and their human host.