Bacteria， such as Fusobacterium nucleatum， are present in the tumor microenvironment. However， the immunological consequences of intra-tumoral bacteria remain unclear. Here， we have shown that natural killer (NK) cell killing of various tumors is inhibited in the presence of various F. nucleatum strains. Our data support that this F. nucleatum-mediated inhibition is mediated by human， but not by mouse TIGIT， an inhibitory receptor present on all human NK cells and on various T cells. Using a library of F. nucleatum mutants， we found that the Fap2 protein of F. nucleatum directly interacted with TIGIT， leading to the inhibition of NK cell cytotoxicity. We have further demonstrated that tumor-infiltrating lymphocytes expressed TIGIT and that T cell activities were also inhibited by F. nucleatum via Fap2. Our results identify a bacterium-dependent， tumor-immune evasion mechanism in which tumors exploit the Fap2 protein of F. nucleatum to inhibit immune cell activity via TIGIT.
