LnaB: a Legionella pneumophila activator of NF-kappaB.

Legionella pneumophila possesses a large arsenal of type IV translocated substrates. Over 100 such proteins have been identified, but the functions of most are unknown. Previous studies have demonstrated that L. pneumophila activates NF-kappaB, a master transcriptional regulator of the mammalian innate immune response. Activation of NF-kappaB is dependent on the Legionella Icm/Dot type IV protein translocation system, consistent with the possibility that translocated bacterial proteins contribute to this response. To test this hypothesis, an expression library of 159 known and putative translocated substrates was created to evaluate whether ectopic production of a single L. pneumophila protein could activate NF-kappaB in mammalian cells. Expression of two of these proteins, LnaB (Legionella NF-kappaB activator B) and LegK1, resulted in approximately 150-fold induction of NF-kappaB activity in HEK293T cells, levels similar to the strong induction that occurs with ectopic expression of the known activator Nod1. LnaB is a substrate of the Icm/Dot system, and in the absence of this protein, a partial reduction of NF-kappaB activation in host cells occurs after challenge by post-exponential phase bacteria. These data indicate that LnaB is an Icm/Dot substrate that contributes to NF-kappaB activation during L. pneumophila infection in host cells.