[Show abstract][Hide abstract] ABSTRACT: Coxiella burnetii and Legionella pneumophila are evolutionarily related pathogens with different intracellular infection strategies. C. burnetii persists within and is transmitted by mammalian hosts, whereas, L. pneumophila is found primarily in the environment associated with protozoan hosts. Although a type IV secretion system encoded by the defect in organelle trafficking (dot) and intracellular multiplication (icm) genes is a virulence determinant that remains highly conserved in both bacteria, the two pathogens encode a different array of effector proteins that are delivered into host cells by the Dot/Icm machinery. This difference suggests that adaptations to evolutionarily distinct hosts may be reflected in the effector protein repertoires displayed by these two pathogens. Here we provide evidence in support of this hypothesis. We show that a unique C. burnetii effector from the ankyrin repeat (Ank) family called AnkG interferes with the mammalian apoptosis pathway. AnkG was found to interact with the host protein gC1qR (p32). Either the addition of AnkG to the repertoire of L. pneumophila effector proteins or the silencing of p32 in mouse dendritic cells resulted in a gain of function that allowed intracellular replication of L. pneumophila in these normally restrictive mammalian host cells by preventing rapid pathogen-induced apoptosis. These data indicate that p32 regulates pathogen-induced apoptosis and that AnkG functions to block this pathway. Thus, emergence of an effector protein that interferes with a proapoptotic signaling pathway directed against intracellular bacteria correlates with adaptation of a pathogen to mammalian hosts.
Preview · Article · Oct 2010 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: Author Summary
The immune system is designed to identify microbes that enter the body and elicit responses that prevent the replication and dissemination of these organisms. Dendritic cells play an important role in regulating host immunity to pathogens. Their phagocytic capacity enables DCs to internalize and destroy most microbes, and the ability of DCs to migrate to specialized lymphoid organs is important for inducing antigen-specific immunity. Here, we analyzed interactions between DCs and Legionella pneumophila, a bacterial pathogen that can subvert phagocytic host cell functions to create a vacuole that permits intracellular replication. We found that L. pneumophila infection rapidly induced DCs to commit cell death through apoptosis. Rapid apoptosis was not observed after infection of macrophages, which are the phagocytic cells that support L. pneumophila replication in the lungs of infected animals. Using cells derived from knockout mice, we found that DCs deficient in the proteins Bax and Bak, which are essential for induction of the apoptosis pathway, were unable to restrict the intracellular replication of L. pneumophila. Likewise, overproduction of Bcl-2, which is a negative regulator of apoptosis, resulted in DCs that were permissive for L. pneumophila replication. These data indicate DCs have the ability to rapidly undergo apoptosis when infected with a microbe capable of replicating intracellularly, and this response effectively prevents pathogen replication. We hypothesize that this response may be designed to interfere with the migration of infected DCs through the lymphatic system, which would prevent DCs from serving as a “Trojan Horse” that transports pathogenic microbes from peripheral sites to central organs.
[Show abstract][Hide abstract] ABSTRACT: The immune system must discriminate between pathogenic and nonpathogenic microbes in order to initiate an appropriate response. Toll-like receptors (TLRs) detect microbial components common to both pathogenic and nonpathogenic bacteria, whereas Nod-like receptors (NLRs) sense microbial components introduced into the host cytosol by the specialized secretion systems or pore-forming toxins of bacterial pathogens. The host signaling pathways that respond to bacterial secretion systems remain poorly understood. Infection with the pathogen Legionella pneumophila, which utilizes a type IV secretion system (T4SS), induced an increased proinflammatory cytokine response compared to avirulent bacteria in which the T4SS was inactivated. This enhanced response involved NF-kappaB activation by TLR signaling as well as Nod1 and Nod2 detection of type IV secretion. Furthermore, a TLR- and RIP2-independent pathway leading to p38 and SAPK/JNK MAPK activation was found to play an equally important role in the host response to virulent L. pneumophila. Activation of this MAPK pathway was T4SS-dependent and coordinated with TLR signaling to mount a robust proinflammatory cytokine response to virulent L. pneumophila. These findings define a previously uncharacterized host response to bacterial type IV secretion that activates MAPK signaling and demonstrate that coincident detection of multiple bacterial components enables immune discrimination between virulent and avirulent bacteria.