Secrets of a successful pathogen: Legionella resistance to progression along the autophagic pathway

PERSPECTIVE ARTICLE published: 28 June 2011 doi: 10.3389/fmicb.2011.00138 Secrets of a successful pathogen: Legionella resistance to progression along the autophagic pathway Amrita D. Joshi and Michele S. Swanson* Department of Microbiology and Immunology, University of Michigan Medical School, Ann Arbor, MI, USA Edited by: Carmen Buchrieser, Pasteur Institute, France Reviewed by: Elizabeth L. Hartland, The University of Melbourne, Australia Patrice Coodgno, INSERM, France Serge Mostowy, Institut Pasteur, France *Correspondence: Michele S. Swanson, Department of Microbiology and Immunology, University of Michigan Medical School, 6733 Medical Science Building II, 1150 West Medical Center Drive, Ann Arbor, MI 48109-5620, USA. e-mail: To proliferate within phagocytes, Legionella pneumophila relies on Type IV secretion to modulate host cellular pathways. Autophagy is an evolutionarily conserved degradative pathway that captures and transfers a variety of microbes to lysosomes. Biogenesis of L. pneumophila-containing vacuoles and autophagosomes share several features, including endoplasmic reticulum (ER)-derived membranes, contributions by the host GTPases Rab1, Arf1 and Sar1, and a final destiny in lysosomes. We discuss morphological, molecular genetic, and immunological data that support the model that, although A/J mouse macrophages efficiently engulf L. pneumophila within autophagosomal membranes, the Type IV effector proteins DrrA/SidM, LidA, and RalF prolong association with the ER. By inhibiting immediately delivery to lysosomes, the bacteria persist in immature autophagosomal vacuoles for a period sufficient to differentiate into an acid-resistant, replicative form. Subsequent secretion of the Type IV effector LepB releases the block to autophagosome maturation, and the adapted progeny continue to replicate within autophagolysosomes. Accordingly, L. pneumophila can be exploited as a genetic tool to analyze the recruitment and function of the macrophage autophagy pathway. Keywords: Legionella pneumophila, Type IV secretion system, autophagy, vacuole maturation, Rab conversion INTRODUCTION Legionella pneumophila is an accidental respiratory pathogen that can cause pneumonia in people whose immune defenses are compromised. The natural hosts of L. pneumophila are different species of protozoa that are abundant in aquatic environments (Lau and Ashbolt, 2009). L. pneumophila thrives in natural ecosystems such as ponds, rivers and moist soil, and also in man-made water systems, including cooling towers, whirlpools, and vegetable misters. Although protozoa routinely ingest bacteria as a food source, they can be parasitized by some species of Legionella. Evolutionary pressure to survive and replicate in professional phagocytes of water and soil has led to the emergence of virulence traits that also equip L. pneumophila to proliferate in a similar eukaryotic host, the macrophage. Protozoa and macrophages possess similar anti-microbial defenses, such as production of reactive oxygen and nitrogen species and delivery of invading microbes to the acidic, hydrolytic lysosomes via phagocytosis. Indeed, prior growth in ameba augments subsequent replication in both macrophages and mouse models of infection (Cirillo et al., 1994, 1999; Neumeister et al., 2000). Upon inhalation within contaminated aerosols, L. pneumophila are phagocytosed by alveolar macrophages. However, the nascent L. pneumophila phagosome avoids the endocytic pathway and instead forms a unique replication vacuole that interacts with particular organelles, including mitochondria and endoplasmic reticulum (ER; Horwitz, 1983a, 1983b; Swanson and Isberg, 1995). After a few rounds of replication in permissive A/J mouse macrophages, the L. pneumophila vacuole acquires lysosomal components, and the progeny continue to replicate in a www.frontiersin.org phagolysosomal compartment (Sturgill-Koszycki and Swanson, 2000). Its mode of entry and replication in host cells require a Type IV secretion system named defect in organelle t rafficking/intracellular multiplication (Dot/Icm; Hilbi et al., 2001; Bandyopadhyay et al., 2004, 2007; Hubber and Roy, 2010). To establish a replication niche, intracellular L. pneumophila exploit Type IV secretion to deliver to the host cytosol a large number of effectors predicted to modulate cellular pathways that are highly conserved in ameba and macrophages (Ensminger and Isberg, 2009). Here we focus on interactions between L. pneumophila and the autophagy pathway, an alternate route to the lysosomes of macrophages and amebae. Autophagy is best known as a catabolic process in which cellular cytoplasm and organelles are degraded as a means to cope with starvation. More than 30 autophagy (Atg) genes in yeast and at least 20 in mammals regulate autophagosome formation and maturation (Mehrpour et al., 2010). Autophagy begins when a double-membraned structure called an isolation membrane, or phagophore, forms around cytoplasm or organelles destined for degradation. The phagophore expands and closes on itself to form a double-membraned vacuole, or autophagosome. In a series of tightly controlled events, the phagophore fuses with vesicles from the endocytic pathway. Maturation is complete when the autophagosome merges with lysosomal vacuoles to form an autophagolysosome, wherein contents of the vacuole are degraded. In addition to its long-established role as a non-selective response to starvation and more recent recognition as a selective mechanism for disposal of damaged organelles or misfolded proteins marked by ubiquitin (Pankiv et al., 2007; Kirkin et al., 2009; Thurston June 2011 | Volume 2 | Article 138 | 1 Joshi and Swanson et al., 2009), autophagy is also recruited by the innate and adaptive immune systems (Levine et al., 2011). AUTOPHAGY, AN INNATE DEFENSE MECHANISM AGAINST INTRACELLULAR PATHOGENS By capturing cytosolic invaders and delivering them to lysosomes, autophagy acts as a barrier against a variety of microbes. When Streptococcus pyogenes, Salmonella enterica, Listeria monocytogenes, or Mycobacterium tuberculosis damage or escape from their phagosomes, some of the microbes are ubiquitinated, recognized by the autophagic surveillance system and trafficked to lysosomes for degradation (Nakagawa et al., 2004; Perrin et al., 2004; Birmingham et al., 2006; Thurston et al., 2009; Yoshikawa et al., 2009; Zheng et al., 2009; Ponpuak et al., 2010). A recent study identified another pathway to capture cargo for autophagy: the cytoskeletal protein Septin traps Shigella flexneri within a meshwork that targets the intracellular bacterium for autophagic degradation (Mostowy et al., 2010). Bacterial pathogens that reside in endosomal compartments also face death by autophagy, and some of the host regulatory factors have been identified. For example, IFN-γ stimulated cells deliver vacuoles containing M. tuberculosis to lysosomes via autophagy (Gutierrez et al., 2004). Similarly, Chlamydia trach (...truncated)