Secreted Bacterial Effectors That Inhibit Host Protein Synthesis Are Critical for Induction of the Innate Immune Response to Virulent Legionella pneumophila

The intracellular bacterial pathogen Legionella pneumophila causes an inflammatory pneumonia called Legionnaires' Disease. For virulence, L. pneumophila requires a Dot/Icm type IV secretion system that translocates bacterial effectors to the host cytosol. L. pneumophila lacking the Dot/Icm system is recognized by Toll-like receptors (TLRs), leading to a canonical NF-κB-dependent transcriptional response. In addition, L. pneumophila expressing a functional Dot/Icm system potently induces unique transcriptional targets, including proinflammatory genes such as Il23a and Csf2. Here we demonstrate that this Dot/Icm-dependent response, which we term the effector-triggered response (ETR), requires five translocated bacterial effectors that inhibit host protein synthesis. Upon infection of macrophages with virulent L. pneumophila, these five effectors caused a global decrease in host translation, thereby preventing synthesis of IκB, an inhibitor of the NF-κB transcription factor. Thus, macrophages infected with wildtype L. pneumophila exhibited prolonged activation of NF-κB, which was associated with transcription of ETR target genes such as Il23a and Csf2. L. pneumophila mutants lacking the five effectors still activated TLRs and NF-κB, but because the mutants permitted normal IκB synthesis, NF-κB activation was more transient and was not sufficient to fully induce the ETR. L. pneumophila mutants expressing enzymatically inactive effectors were also unable to fully induce the ETR, whereas multiple compounds or bacterial toxins that inhibit host protein synthesis via distinct mechanisms recapitulated the ETR when administered with TLR ligands. Previous studies have demonstrated that the host response to bacterial infection is induced primarily by specific microbial molecules that activate TLRs or cytosolic pattern recognition receptors. Our results add to this model by providing a striking illustration of how the host immune response to a virulent pathogen can also be shaped by pathogen-encoded activities, such as inhibition of host protein synthesis.
Author Summary In animals, the innate immune system senses infection primarily through detection of conserved microbial molecules. It has been suggested, but not clearly established, that the immune system may also respond to pathogen-associated activities—i.e., the manipulations of host cell processes that a pathogen employs to survive and replicate in its host. Previous studies have established that macrophages infected with the bacterial pathogen Legionella pneumophila can discriminate between virulent wildtype bacteria and an avirulent, nonreplicating mutant. Here we show that a unique host transcriptional response to virulent L. pneumophila is due to the activity of secreted bacterial proteins that inhibit host translation. Furthermore, we show that multiple bacterial toxins or chemicals that inhibit host translation can cooperate with host sensors of microbial molecules to induce the unique transcriptional response, even in the absence of bacterial infection. By demonstrating that the host mounts a response to a pathogen-encoded activity, we provide evidence for a novel mechanism of innate immune sensing that may aid in distinguishing pathogenic microbes from non-pathogens.