MyD88-Dependent Responses Involving Toll-Like Receptor 2 Are Important for Protection and Clearance ofLegionella pneumophilain a Mouse Model of Legionnaires' Disease

Abstract

Legionella pneumophila is a gram-negative facultative intracellular parasite of macrophages. Although L. pneumophila is the causative agent of a severe pneumonia known as Legionnaires' disease, it is likely that most infections caused by this organism are cleared by the host innate immune system. It is predicted that host pattern recognition proteins belonging to the Toll-like receptor (TLR) family are involved in the protective innate immune responses. We examined the role of TLR-mediated responses in L. pneumophila detection and clearance using genetically altered mouse hosts in which the macrophages are permissive for L. pneumophila intracellular replication. Our data demonstrate that cytokine production by bone marrow-derived macrophages (BMMs) in response to L. pneumophila infection requires the TLR adapter protein MyD88 and is reduced in the absence of TLR2 but not in the absence of TLR4. Bacterial growth ex vivo in BMMs from MyD88-deficient mice was not enhanced compared to bacterial growth ex vivo in BMMs from heterozygous littermate controls. Wild-type mice were able to clear L. pneumophila from the lung, whereas respiratory infection of MyD88-deficient mice caused death that resulted from robust bacterial replication and dissemination. In contrast to an infection with virulent L. pneumophila, MyD88-deficient mice were able to clear infections with L. pneumophila dotA mutants, indicating that MyD88-independent responses in the lung are sufficient to clear bacteria that are unable to replicate intracellularly. In vivo growth of L. pneumophila was enhanced in the lungs of TLR2-deficient mice, which resulted in a delay in bacterial clearance. No significant differences were observed in the growth and clearance of L. pneumophila in the lungs of TLR4-deficient mice and heterozygous littermate control mice. Our data indicate that MyD88 is crucial for eliciting a protective innate immune response against virulent L. pneumophila and that TLR2 is one of the pattern recognition receptors involved in initiating this MyD88-dependent response.

Legionella lipoprotein activates toll-like receptor 2 and induces cytokine production and expression of costimulatory molecules in peritoneal macrophages

“…Several studies have identified the role of TLR2 in innate immune responses against L. pneumophila infection, although other TLRS might also contribute to innate immunity against this organism (Akamine et al, 2005;Archer and Roy, 2006;Hawn et al, 2006;Fuse et al, 2007). TLR2 plays a pivotal role in the bacterial recognition and host resistance against the intracellular growth of L. pneumophila (Akamine et al, 2005;Fuse et al, 2007).…”

Airway epithelial cell signaling in response to bacterial pathogens

“…22 The protective role of TLR2-mediated signaling in the clearance of L. pneumophila in the lungs was demonstrated using a murine model of Legionnaires' disease. 23 In vivo growth of wild-type L. pneumophila was enhanced in the lungs of TLR2-deficient mice, resulting in a delay in bacterial clearance whereas L. pneumophila dotA mutants that cannot replicate intracellularly were efficiently cleared. 23 …”

“…23 In vivo growth of wild-type L. pneumophila was enhanced in the lungs of TLR2-deficient mice, resulting in a delay in bacterial clearance whereas L. pneumophila dotA mutants that cannot replicate intracellularly were efficiently cleared. 23 …”

Interferon‐Gamma Reverses the Evasion of Birc1e/Naip5 Gene Mediated Murine Macrophage Immunity by Legionella pneumophila Mutant Lacking Flagellin

“…These cells were then cultured in glass dishes with endotoxin-free RPMI 1640 medium supplemented with 20% L929 cell culture supernatant and 10% fetal calf serum (FCS; Cansera, Rexdate, Ontario, Canada) for 6 days. Adherent cells were referred to as BMMs and suspended at 5ϫ10 4 cells/well in 96-well plates (Falcon Becton Dickinson), followed by incubation at 37 C in 5% CO 2 for more than 12 hr before infection under 5% CO 2 .…”

“…TLR2 is important for the recognition of Legionella and for innate resistance against bacteria in vitro (1) and in vivo (2,10). The Birc1e/Naip5 gene within the Lgn locus was identified as the genetic determinant of permissiveness for the intracellular growth of Legionella within murine macrophages (6,33).…”