The Legionella pneumophila replication vacuole: making a cosy niche inside host cells

Abstract

The pathogenesis of Legionella pneumophila is derived from its growth within lung macrophages after aerosols are inhaled from contaminated water sources. Interest in this bacterium stems from its ability to manipulate host cell vesicular-trafficking pathways and establish a membrane-bound replication vacuole, making it a model for intravacuolar pathogens. Establishment of the replication compartment requires a specialized translocation system that transports a large cadre of protein substrates across the vacuolar membrane. These substrates regulate vesicle traffic and survival pathways in the host cell. This Review focuses on the strategies that L. pneumophila uses to establish intracellular growth and evaluates why this microorganism has accumulated an unprecedented number of translocated substrates that are targeted at host cells.

Figure 1. L. pneumophila modulates trafficking of its vacuole to establish a replicative niche

(A) Formation of the replication vacuole. After uptake into target amoebae or macrophages, the “Legionella containing vacuole” (LCV) evades transport to the lysosomal network and is sequestered in a compartment very different from that observed for nonpathogens ,. Within minutes of uptake, vesicles derived from the endoplasmic reticulum (ER; yellow compartments) and mitochondria appear in close proximity to the LCV surface. The identity of the ER-derived vesicles is based on the presence of proteins known to be associated with the early secretory apparatus. The vesicles about the LCV appeared docked and extend out about the surface, and eventually the membranes surrounding the bacterium closely resemble rough ER in appearance, with ribosomes studding them. Within this ER-like compartment, the bacterium replicates to high numbers and eventually lyses the host cell. (B) Default pathway of trafficking nonpathogen. After bacterial uptake, the membrane-bound compartment acquires the character of early endosomes and late endosomes before entering into the lysosomal network.

Figure 2

L. pneumophila proteins secreted via the Dot/Icm translocation system associate with the LCV and recruit host proteins involved in vesicle trafficking through the early secretory pathway. For the sole purpose of simplifying the components displayed in the figure, the Dot/Icm apparatus is depicted as a tube extending from the bacterial cytoplasm into the host cytosol, but this there is no mechanistic support of this simplistic view. Sec22b, involved in docking of ER-derived vesicles at the Golgi, is recruited to the LCV, although the mechanism of recruitment is unclear. Rab1, another vesicle docking and fusion protein is recruited to the LCV by the L. pneumophila protein SidM which functions as both a Rab1 GDF (GDI dissociation factor) and a Rab1 GEF (guanine nucleotide exchange factor). LidA acts in conjunction with SidM to sequester activated Rab1 at the LCV membrane. LepB is a RabGAP, and may be involved in dissociation of Rab1 from the vacuolar membrane. Arf1, involved in vesicle budding and recycling at the Golgi, is recruited to the LCV via RalF which functions as an Arf1 GEF. Host membrane recruitment to the LCV may involve an autophagic process as both the host autophagy proteins Atg7 and Atg8 also localize about the LCV.

Figure 3. The Dot/Icm translocation apparatus

Depicted are the presumed locales and topological relationships of the various Dot/Icm components in the L. pneumophila envelope based on a study of the stability of individual proteins in the presence of defined deletion mutations. Individual letters represent Dot protein names whereas letters preceded by an “i” indicated Icm protein names. See text for further details of the individual Dot/Icm components.

Figure 4. L. pneumophila manipulates host cell death and survival pathways

After uptake into mammalian cells there is a response to L. pneumophila that threatens to terminate intracellular growth by causing host cell death. The cell death pathways have both necrotic as well as apoptotic character, and require the presence of an intact Dot/Icm translocation system. The individual L. pneumophila components or translocated substrates that cause cell death have not been identified. In addition, there are at least two translocated substrates that interfere with host cell death. SdhA is required to inhibit multiple pathways that lead to cell death after L. pneumophila contact with host cells, and its absence causes a defect in intracellular replication within macrophages. L. pneumophila also activates the host transcription factor NFκB to promote expression of anti-apoptotic genes to delay host cell death; however, the mechanism by which this occurs has not yet been determined. At later stages of infection, SidF directly inhibits an apoptotic pathway by interfering with pro-death proteins in the Rambo family. See text.