Altered Inflammatory Responses in TLR5-Deficient Mice Infected with Legionella pneumophila

Abstract

Legionella pneumophila (Lp), an important cause of morbidity and mortality from pneumonia, infects alveolar macrophages (AMs) and is recognized by several TLRs as well as Birc1e (NAIP5) and IL-1 converting enzyme-protease activating factor. We examined the role of TLR5 during the murine response to aerosolized Lp infection. At 4 h after infection, Tlr5−/− mice had lower numbers of polymorphonuclear neutrophils (PMNs) in their broncho-alveolar lavage fluid in comparison to wild-type (WT) mice. At 24 and 72 h, the PMN recruitment was similar. WT mice infected with a flagellin-deficient strain (LpFlaA−) also showed an impaired early PMN response at 4 h compared with those infected with the WT strain. There was no consistent difference in bacterial counts at any of the time points when comparing the Tlr5−/− and WT mice. However, at 6 days after infection, the Tlr5−/− mice had increased leukocytic infiltrates in the alveolar and peribronchial interstitial spaces that were consistent with organizing pneumonia. We also examined the role of TLR5 during macrophage infection. In contrast to bone marrow-derived macrophages, AMs secreted TNF-α after stimulation with purified flagellin. In addition, WT, but not Tlr5−/−, AMs produced TNF-α after stimulation with Lp. Live LpFlaA− did not induce TNF-α secretion in AM. These results suggested that AMs recognize Lp flagellin and that a majority of the Lp-induced TNF-α response is TLR5-mediated. Thus, TLR5 mediates recognition of Lp in AMs and performs a distinct role during the in vivo pulmonary immune response through regulation of early PMN recruitment and subsequent later development of pneumonia.

Radioresistant cells expressing TLR5 control the respiratory epithelium's innate immune responses to flagellin

“…Because TLR5 is active when flagellin is administered i.n., we assume that apical/luminal signalling induces the epithelial response, as described previously [37]. It has been suggested that this type of epithelial innate mechanism is essential in mucosal defense against L. pneumophila, Pseudomonas aeruginosa and uropathogenic Escherichia coli [22,26,38]. Furthermore, respiratory pathogens that reach the basolateral side of the epithelial barrier via specific transport mechanisms or following trauma can stimulate either epithelial cells, airway smooth muscle, endothelial cells or any TLR5-expressing haematopoietic cells (such as lung macrophages and dendritic cells).…”

“…The matter of how epithelium and macrophages collaborate on local amplification or sequential activation of innate immunity is still under investigation. Recent observations have revealed that previous exposure to influenza virus determines the functional properties of tissueresiding macrophages in a TLR-dependent manner [38]. Whether other TLR5-expressing haematopoeitic cells like dendritic cells cooperate with epithelium also remains to be further studied.…”

“…Interestingly, alveolar macrophages and epithelial cells display similar TLR5 expression levels. Recently, lung macrophages have been identified as the main source of TNF-a production following flagellin stimulation [27,38]. However, the impact of this type of signalling on the host's defense against pathogens remains to be defined.…”

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

“…Several studies have reported that TLRs contribute to the host response against L. pneumophila during in vitro and in vivo infection (Girard et al, 2003;Kikuchi et al, 2004;Akamine et al, 2005;Hawn et al, 2006Hawn et al, , 2007Fuse et al, 2007;Newton et al, 2007). However, there have been no studies to elucidate immunostimulatory properties of isolated individual proteins from L. pneumophila in terms of engagement of TLRs and linking to elicit the antigen-specific adaptive immune responses.…”

Impaired TLR5 Functionality Is Associated with Survival in Melioidosis

“…We and others have previously shown that TLR2 and TLR4-transmembrane pathogen recognition receptors-regulate host responses in B. pseudomallei infection in vitro and in vivo (3,4). TLR5, a sensor of bacterial flagellin, regulates host defense to a number of lung infections caused by flagellated bacteria (5)(6)(7)(8)(9). The importance of TLR5 in regulating the host response to human melioidosis has not been determined.…”