Immunity to vacuolar pathogens: What can we learn from Legionella?

Abstract

SummaryIntracellular pathogens can manipulate host cellular pathways to create specialized organelles. These pathogen-modified vacuoles permit the survival and replication of bacterial and protozoan microorganisms inside of the host cell. By establishing an atypical organelle, intracellular pathogens present unique challenges to the host immune system. To understand pathogenesis, it is important to not only investigate how these organisms create unique subcellular compartments, but to also determine how mammalian immune systems have evolved to detect and respond to pathogens sequestered in specialized vacuoles. Recent studies have identified genes in the respiratory pathogen Legionella pneumophila that are essential for establishing a unique endoplasmic reticulum-derived organelle inside of mammalian macrophages, making this pathogen an attractive model system for investigations on host immune responses that are specific for bacteria that establish vacuoles disconnected from the endocytic pathway. This review will focus on the host immune response to Legionella and highlight areas of Legionella research that should help elucidate host strategies to combat infections by intracellular pathogens.

Legionella pneumophila-induced IκBζ-dependent expression of interleukin-6 in lung epithelium

“…In agreement, we observed reduced IL-6 mRNA expression at later time points in cells infected with Legionella strain JR32 lacking DotA, an integral part of the type-IVB system in these bacteria [8]. Overall, extracellular and possibly intracellular recognition of L. pneumophila seems to be important for IL-6 expression in lung epithelium.…”

“…As published previously, FlaA-deficient Legionella replicated twice as well as wildtype bacteria over 24 h in lung epithelial cells but did not alter cell death as determined by lactate dehydrogenase release (data not shown and VINZING et al Type-IVB Dot/Icm secretion system-related activity is known to be important for cytosolic recognition of Legionella flagellin and subsequent cell activation [38]. Upon infection of A549 cells with a L. pneumophila JR32 mutant deficient for dotA, an integral part of the Legionella type-IVB system [8], we observed a similar induction of IkBf, IL-6, and IL-8 mRNA at early time points compared with infection of cells with JR32 wildtype, but a clear reduction after 5 h ( fig. 3a and b) as well as reduced liberation of IL-6 ( fig.…”

“…With respect to L. pneumophila pathogenesis, essential results were obtained by analysing infection of protozoans or immune cells like macrophages [8,9]. However, although Legionella replicates efficiently within lung epithelial cells and recent studies pointing to the lung epithelium as an important sentinel and effector of innate immunity [7,[10][11][12][13][14][15][16], little is known of the consequences of pulmonary epithelial cell infection with Legionella.…”

Legionella pneumophila-induced NF-κB- and MAPK-dependent cytokine release by lung epithelial cells

“…Cianciotto, Northwestern University Medical School, Chicago, IL, USA [13]), Philadelphia 1 (ATCC 33152, kindly provided by B. Neumeister, Tü bingen University, Tü bingen, Germany [14]), JR32 wildtype [15] and JR32 dotA mutant (LELA 3118, both kindly provided by H. Shuman, Columbia University, New York, NY, USA [16]), and Corby wildtype and a Corby flaA, defective in flagellin, mutant (both kindly provided by K. Heuner, Wü rzburg University, Wü rzburg, Germany) were routinely grown on buffered charcoal-yeast extract (BCYE) agar for 2-3 days at 37uC [17] and subsequently inoculated into plain RPMI medium at an optical density at 660 nm of 0.2-0. 4 In order to verify intracellular infection, A549 cells were incubated with the added bacteria for 2 h with or without kinase inhibitors, gentamicin (100 mg?mL -1 ) was added for a further 2 h and then the cells were washed three times with plain medium, to remove unbound bacteria, and treated with 10% (weight/volume) saponin (Sigma Chemical Company, Munich, Germany) to lyse the host cells. Serial dilutions were plated on BCYE agar.…”

“…L. pneumophila-containing phagosomes initially do not fuse with lysosomes and the bacteria induce remodelling of their membranebound compartment into an endoplasmic reticulum-like organelle [3]. This remodelling depends on the defect in organelle trafficking (Dot)/intracellular multiplication (Icm) type IVB secretion apparatus of L. pneumophila [4]. Besides delivery of proteins by the type IVB secretion system, L. pneumophila contains a battery of additional virulence factors, including a type II secretion apparatus [3].…”