Ultrasmall oral Saccharibacteria modulate gingival immunoactivation through type IV pili and TLR2-dependent endocytosis

Saccharibacteria are ultrasmall 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 the tripartite interactions between Saccharibacteria (Nanosynbacter lyticus strain TM7x and other strains), their host/non-host-bacteria, and human oral gingival epithelial cells. Both host and non-host-bacteria strongly induce proinflammatory cytokines in epithelial cells, while Saccharibacteria alone elicits limited immune activation. Remarkably, Saccharibacteria dampened proinflammatory cytokine responses to host/non-host-bacteria during coinfection. Mechanistically, this effect results from Saccharibacteria-mediated clustering and endocytosis of surface TLR2 receptor, ultimately leading to reduce TLR2-mediated cytokine signalling. Sacchribacteria type IV pili appendages facilitate epithelial cell binding and subsequent immune dampening via direct interaction between pili adhesins and TLR2. High resolution imaging shows that Saccharibacteria are internalized by epithelial cells through caveolin-mediated endocytosis, subsequently colocalize with endosome markers, and eventually are trafficked to lysosomes for degradation. Moreover, a subset of the Saccharibacteria survives lysosomal degradation and retains the ability to reinfect host-bacteria, highlighting a mechanism for transient persistence in the oral microbiome and a vital role in human immune and microbiome modulation.