The Nod-like receptor family member Naip5/Birc1e restricts Legionella pneumophila growth independently of caspase-1 activation

Similar to Ipaf and caspase-1, the Nod-like receptor protein Naip5 restricts intracellular proliferation of Legionella pneumophila, the causative agent of a severe form of pneumonia known as Legionnaires' disease. Thus, Naip5 has been suggested to regulate Legionella replication inside macrophages through the activation of caspase-1. In this study, we show that cytosolic delivery of recombinant flagellin activated caspase-1 in A/J macrophages carrying a mutant Naip5 allele, and in C57BL/6 (B6) macrophages congenic for the mutant Naip5 allele (B6-Naip5(A/J)), but not in Ipaf(-/-) cells. In line with these results, A/J and B6-Naip5(A/J) macrophages induced high levels of caspase-1 activation and IL-1beta secretion when infected with Legionella. In addition, transgenic expression of a functional Naip5 allele in A/J macrophages did not alter Legionella-induced caspase-1 activation and IL-1beta secretion. Notably, defective Naip5 signaling renders B6-Naip5(A/J) macrophages permissive for Legionella proliferation despite normal caspase-1 activation. These results indicate that the restriction of intracellular Legionella replication is more complex than previously appreciated and requires both Ipaf-dependent caspase-1 activation as well as functional Naip5 signaling.