Although the role of Toll-like receptors in extracellular bacterial sensing has been investigated intensively, intracellular detection of bacteria through Nod molecules remains largely uncharacterized. Here, we show that human Nod1 specifically detects a unique diaminopimelate-containing N -acetylglucosamine– N -acetylmuramic acid (GlcNAc-MurNAc) tripeptide motif found in Gram-negative bacterial peptidoglycan, resulting in activation of the transcription factor NF-κB pathway. Moreover, we show that in epithelial cells (which represent the first line of defense against invasive pathogens), Nod1is indispensable for intracellular Gram-negative bacterial sensing.

The mammalian innate immune system recognizes pathogen-associated molecular patterns through pathogen recognition receptors. Nod1 has been described recently as a cytosolic receptor that detects specifically diaminopimelate-containing muropeptides from Gram-negative bacteria peptidoglycan. In the present study we investigated the potential role of Nod1 in the innate immune response against the opportunistic pathogen Pseudomonas aeruginosa. We demonstrate that Nod1 detects the P. aeruginosa peptidoglycan leading to NF-κB activation and that this activity is diminished in epithelial cells expressing a dominant-negative Nod1 construct or in mouse embryonic fibroblasts from Nod1 knock-out mice infected with P. aeruginosa. Finally, we demonstrate that the cytokine secretion kinetics and bacterial killing are altered in Nod1-deficient cells infected with P. aeruginosa in the early stages of infection.

Linking protein misfolding and innate immunity: Multiple innate immune sensors undergo rapid assembly into large complexes known as signalosomes. This is an essential step during cellular responses to microbes and danger signals. How this process is regulated to avoid accumulation of potentially toxic protein aggregates remains poorly understood. Abdel-Nour et al. identified a pathway, dependent on heme-regulated inhibitor, eukaryotic initiation factor 2α, activating transcription factor 4, and heat shock protein B8, which controls the folding and scaffolding of innate immune sensors, allowing optimal proinflammatory signaling (see the Perspective by Pierre). The pathway appears to mirror the endoplasmic reticulum unfolded protein response (UPR), and so was named the cytosolic UPR (cUPR). The cUPR may represent a general mechanism to control protein misfolding in cells. Science , this issue p. eaaw4144 ; see also p. 28