The Legionella pneumophila EnhC Protein Interferes with Immunostimulatory Muramyl Peptide Production to Evade Innate Immunity

Successful pathogens have evolved to evade innate immune recognition of microbial molecules by pattern recognition receptors (PRR), which control microbial growth in host tissues. Upon Legionella pneumophila infection of macrophages, the cytosolic PRR Nod1 recognizes anhydro-disaccharide-tetrapeptide (anhDSTP) generated by soluble lytic transglycosylase (SItL), the predominant bacterial peptidoglycan degrading enzyme, to activate NF-kappa B-dependent innate immune responses. We show that L. pneumophila periplasmic protein EnhC, which is uniquely required for bacterial replication within macrophages, interferes with SItL to lower anhDSTP production. L pneumophila mutant strains lacking EnhC (Delta enhC) increase Nod1-dependent NF-kappa B activation in host cells, while reducing SItL activity in a Delta enhC strain restores intracellular bacterial growth. Further, L. pneumophila Delta enhC is specifically rescued in Nod1- but not Nod2-deficient macrophages, arguing that EnhC facilitates evasion from Nod1 recognition. These results indicate that a bacterial pathogen regulates peptidoglycan degradation to control the production of PAR ligands and evade innate immune recognition.