Effector-host interactome map links type III secretion systems in healthy gut microbiomes to immune modulation.

Pseudomonadota (formerly Proteobacteria) are prevalent in the commensal human gut microbiota, but also include many pathogens that rely on secretion systems to support pathogenicity by injecting proteins into host cells. Here we show that 80% of Pseudomonadota from healthy gut microbiomes also have intact type III secretion systems (T3SS). Candidate effectors predicted by machine learning display sequence and structural features that are distinct from those of pathogen effectors. Towards a systems-level functional understanding, we experimentally constructed a protein-protein meta-interactome map between human proteins and commensal effectors. Network analyses uncovered that effector-targeted neighbourhoods are enriched for genetic variation linked to microbiome-associated conditions, including autoimmune and metabolic diseases. Metagenomic analysis revealed effector enrichment in Crohn's disease but depletion in ulcerative colitis. Functionally, commensal effectors can translocate into human cells and modulate NF-κB signalling and cytokine secretion in vitro. Our findings indicate that T3SS contribute to microorganism-host cohabitation and that effector-host protein interactions may represent an underappreciated route by which commensal gut microbiota influences health.