Bacterial flagellin is a known ligand for Toll-like receptor 5 (TLR5). However, several recent papers have now revealed that in addition to the TLR5 pathway, which responds to extracellular flagellin, host macrophages can respond to cytosolic flagellin through members of the NOD-like receptor (NLR) family.

The recognition of pathogen-associated molecular patterns by host TLRs is a key component of innate immunity, and much has been learned about TLRs and their signalling pathways over the past decade. More recently, attention has turned to the role of non-TLR pattern-recognition receptors in innate immunity, including the cytoplasmic NLR family. Details of the NLR-signalling pathways are beginning to emerge, and NLRs are known to be involved in secretion of the pro-inflammatory cytokine interleukin-1β (IL-1β) by macrophages. IL-1β is produced initially as a zymogen that is activated for secretion by caspase-1.

In Salmonella enterica serovar Typhimurium (S. typhimurium) infection, the NLR protein ICE-protease activating factor (IPAF; also known as CARD12 and CLAN) was known to be involved in caspase-1 activation and IL-1β secretion, but until now, the S. typhimurium ligand for IPAF was unknown. Two independent groups led by Gabriel Núñez and Alan Aderem investigated the nature of the innate immune response to S. typhimurium infection. Both groups confirmed that IPAF was required for IL-1β production and caspase-1 activation by macrophages. Additionally, they both found that S. typhimurium mutants that either lack or have mutated flagella did not stimulate caspase-1 activation or IL-1β secretion, suggesting that flagellin is the S. typhimurium ligand for IPAF.

As flagellin is also a known ligand for TLR5, the involvement of TLRs was examined. Both groups found that S. typhimurium could stimulate caspase-1 activation and IL-1β secretion by TLR5-deficient macrophages and by wild-type macrophages, and in addition, Franchi et al. found normal levels of caspase-1 activation and IL-1β secretion by tolerant macrophages that are refractory to TLR stimulation. Taken together, these results suggest that macrophages sense flagellin through a TLR5-independent pathway that relies on the cytoplasmic sensor IPAF.

Further confirmation that IPAF senses flagellin in the cytosol independently of TLR5 comes from the fact that both groups also demonstrated that purified flagellin delivered to the cytosol triggered caspase-1 activation in wild-type but not IPAF-deficient macrophages. The mechanism by which flagellin accesses the cytosol during infection remains to be completely elucidated. However, genetic evidence presented by Miao et al. suggests that it is transferred directly into the eukaryotic cytoplasm by the virulence-associated type III secretion system.

These results are echoed by results published recently in two independent papers, one in PLoS Pathogens and one in The Journal of Experimental Medicine, which indicate that an NLR is involved in cytosolic sensing of Legionella pneumophila flagellin through a TLR5-independent, caspase-1-dependent pathway.