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Infection and Immunity, June 2006, p. 3325-3333, Vol. 74, No. 6
0019-9567/06/$08.00+0     doi:10.1128/IAI.02049-05
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

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

Kristina A. Archer and Craig R. Roy*

Section of Microbial Pathogenesis, Yale University School of Medicine, Boyer Center for Molecular Medicine, 295 Congress Avenue, New Haven, Connecticut 06536

Received 20 December 2005/ Returned for modification 25 January 2006/ Accepted 9 March 2006

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.

* Corresponding author. Mailing address: Section of Microbial Pathogenesis, Yale University School of Medicine, Boyer Center for Molecular Medicine, 295 Congress Avenue, New Haven, CT 06536. Phone: (203) 737-2408. Fax: (203) 737-2630. E-mail: craig.roy{at}yale.edu.

Editor: J. B. Bliska

Infection and Immunity, June 2006, p. 3325-3333, Vol. 74, No. 6
0019-9567/06/$08.00+0     doi:10.1128/IAI.02049-05
Copyright © 2006, American Society for Microbiology. All Rights Reserved.



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