TIR signaling activates caspase-like immunity in bacteria.

Caspase family proteases and Toll/interleukin-1 receptor (TIR)-domain proteins have central roles in innate immunity and regulated cell death in humans. We describe a bacterial immune system comprising both a caspase-like protease and a TIR-domain protein. We found that the TIR protein, once it recognizes phage invasion, produces the previously unknown immune signaling molecule adenosine 5'-diphosphate-cyclo[N7:1'']-ribose (N7-cADPR). This molecule specifically activates the bacterial caspase-like protease, which then indiscriminately degrades cellular proteins to halt phage replication. The TIR-caspase defense system, which we denote as type IV Thoeris, is abundant in bacteria and efficiently protects against phage propagation. Our study highlights the diversity of TIR-produced immune signaling molecules and demonstrates that cell death regulated by proteases of the caspase family is an ancient mechanism of innate immunity.