Immunity against infection with Listeria monocytogenes is not achieved from innate immune stimulation by contact with killed but requires viable Listeria gaining access to the cytosol of infected cells. It has remained ill-defined how such immune sensing of live Listeria occurs. Here, we report that efficient cytosolic immune sensing requires access of nucleic acids derived from live Listeria to the cytoplasm of infected cells. We found that Listeria released nucleic acids and that such secreted bacterial RNA/DNA was recognized by the cytosolic sensors RIG-I, MDA5 and STING thereby triggering interferon b production. Secreted Listeria nucleic acids also caused RIG-I-dependent IL-1b-production and inflammasome activation. The signalling molecule CARD9 contributed to IL-1b production in response to secreted nucleic acids. In conclusion, cytosolic recognition of secreted bacterial nucleic acids by RIG-I provides a mechanistic explanation for efficient induction of immunity by live bacteria.
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