DNA-based fluorescent probes of NOS2 activity in live brains

Significance When a pathogen is engulfed by an immune cell, it is trapped in a transient organelle called the phagosome. The host cell recognizes the pathogen through receptors called Toll-like receptors (TLRs) that each recognize specific pathogen-associated molecular pattern (PAMP). When a TLR recognizes its cognate PAMP, it activates inducible nitric oxide synthase (NOS2), filling the phagosome with NO and destroying the pathogen. By developing a fluorescent reporter that directly images PAMP-triggered NOS2 activity in real time, we show that single-stranded RNA acts as a PAMP in zebrafish by activating its cognate TLR. Such a technology can help identify new PAMPs, pinpoint ligands for TLRs of unknown function, and suggest how PAMPs might synergize to help bugs evade the phagosomal deathtrap.