TLR9 Regulates Lung Macrophage Phenotype and Host Immunity in Murine Legionella Pneumonia

Departments of Internal Medicine, Division of Pulmonary and Critical Care Medicine, and Pathology, University of Michigan Medical Center, Ann Arbor, MI 48109; and Coley Pharmaceutical Group, Wellesley, MA. 02481

^* To whom correspondence should be addressed. Email: tstandif{at}umich.edu.

	Abstract

Experiments were performed to determine the contribution of TLR9 to the generation of protective immunity against the intracellular respiratory bacterial pathogen Legionella pneumophila. In initial studies, we found that the i.t. administration of L. pneumophila to mice deficient in TLR9 (TLR9^-/-) displayed significantly increased mortality, which was associated with an approximately 10-fold increase in lung CFU, as compared to wildtype BALB/c mice. Intrapulmonary bacterial challenge in TLR9^-/- resulted in reduced accumulation of myeloid DC and activated CD4+ T cells. Lung macrophages isolated from Legionella-infected TLR9^-/- mice displayed impaired internalization of bacteria and evidence of alternative rather than classical activation, as manifest by markedly reduced expression of nitric oxide and type 1 cytokines, whereas expression of FIZZ-1 and arginase was enhanced. The adoptive transfer of bone marrow-derived DC from syngeneic WT, but not TLR9^-/- mice administered i.t. reconstituted anti-legionella immunity and restored macrophage phenotype in TLR9^-/- mice. Finally, the i.t. but not i.p. administration of the TLR9 agonist molecule CpG oligodeoxynucleotide stimulated protective immunity in Legionella-infected mice. In total, our findings indicate that TLR9 is required for effective innate immune responses against the intracellular bacterial pathogen L. pneumophila, and approaches to maximize TLR9-mediated responses may serve as a means to augment antibacterial immunity in pneumonia.