Purine and pyrimidine-based bacterial cyclic dinucleotides egress the phagosome and activate the innate immune sensor STING

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Toll-like receptors (TLRs) are considered general sensors of bacterial encounters. Here, we examined whether other pattern recognition receptors are commonly activated during bacterial infection. TLR-independent interferon (IFN) responses were induced in macrophages in response to diverse bacterial encounters. Of the cytoplasmic receptor families examined, the cyclic dinucleotide (CDN) sensor STING was required for IFN responses to evolutionarily diverse bacteria. Various bacterial CDNs were present in murine tissues; these activated stimulator of interferon genes (STING) after bacteriolysis in phagolysosomes in a manner requiring two CDN transporters. Importantly, bacterial CDNs were increased in colonic biopsies from patients with inflammatory bowel disease. Systemic delivery of dead, CDN-laden bacteria promoted anti-tumor immunity in mice. Detection of diverse CDNs, including pyrimidine-based CDNs, was an evolutionarily conserved feature of STING, with distinct binding modes for purine- and pyrimidine-based CDNs. Thus, a phagocytosis-CDN-STING connection places cytoplasmic sensing as a common outcome of host-bacteria interactions that set the immune tone of a tissue, with implications for host defense.

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