The Legionella pneumophila type IV secretion system effector proteins LtpD and LtpJ

Legionella pneumophila is the major causative agent of Legionnaires? disease, which is a severe, potentially fatal, form of pneumonia. Legionella pneumophila bacteria are transmitted via inhalation of contaminated water droplets (e.g. from air-conditioning units or swimming pools). The elderly and immunocompromised are most susceptible to infection, making Legionella a major health risk in hospitals and communal facilities (e.g. nursing homes). Somewhat surprisingly, despite the identification of L. pneumophila as a risk factor for community and nosocomial acquired pulmonary infections, and the guidelines for water management in hospitals issued by the UK authorities, no research into the mechanisms by which it causes disease is currently underway in the UK. Following its inhalation, Legionella enters lung cells where it establishes a unique intracellular niche that protects the bacterium from destruction, allowing its own survival and multiplication. The ability of the bacterium to persist within human cells is mediated by special bacterial proteins, known as effectors, which are injected from the bacterial cell into the human cell where they interact with host cell proteins and re-programme cellular processes for the benefit of the bacterium. Understanding the mechanism by which Legionella effectors subvert host cell functions is essential for development of effective prevention and treatment strategies. The aim of this project is to study the role two novel Legionella effectors (LtpD and LtpJ), recently identified as a result of our genome sequencing of an important clinical Legionella isolate, play in subversion of the host cells functions.

Legionella pneumophila is an intracellular, water-borne pathogen and the major causative agent of Legionnaires? disease, a severe form of acute pneumonia. Following inhalation, the bacteria are internalised by alveolar macrophages and epithelial cells where they replicate within a vacuole, known as the LCV. The LCV, which exhibits many characteristics of the endoplasmic reticulum (ER), avoids phagolysosome fusion. Avoidance of lysosomal degradation and recruitment of ER markers require translocation of type IV secretion system effectors, which target specific host cell proteins. A conservative estimation suggests that L. pneumophila encodes over 200 effectors; the function of most remains unknown.

Our initial step in establishing a Legionella project at Imperial College involved sequencing the prototype L. pneumophila serogroup 1 strain 130b. We found that L. pneumophila 130b encodes 146 effectors, 10 of which were not reported before and were named LtpA-J (Schroeder et al., 2010, J. Bacteriol. In press). We have shown that these novel effectors are present, at various frequencies, in clinical and environmental L. pneumophila isolates. The focus of this project is on the novel effectors LtpD and LtpJ. Preliminary studies performed in our laboratory over the last two years revealed that translocated LtpD is phosphorylated and localised to the LCV, while ectopic expression of LtpD altered vesicular trafficking. Unexpectedly, we found that LtpJ appeared to be involved in biogenesis of novel tubular structures, which emanated from and often connected distant LCVs. Based on the resemblance between the LptJ-induced tubular structures and the Salmonella induced filaments (Sifs), which are needed for Salmonella intracellular survival and replication, we named them Legionella induced filaments (Lifs).

The aim of this project is to use a combination of advanced imaging, biochemical and cellular techniques in conjunction with a range of infection models, to investigate the role of LtpD and LtpJ in Legionella pathogenesis. This work will provide new insights into L. pneumophila infection and subversion of host cell processes by intracellular bacterial pathogens of humans.