Differential Roles of Toll-Like Receptors 2 and 4 in In Vitro Responses of Macrophages to Legionella pneumophila

Abstract

ABSTRACT The role of Toll-like receptors (TLRs) in innate immunity to Legionella pneumophila , a gram-negative facultative intracellular bacterium, was studied by using bone marrow-derived macrophages and dendritic cells from TLR2-deficient (TLR2 −/− ), TLR4 −/− , and wild-type (WT) littermate (C57BL/6 × 129Sv) mice. Intracellular growth of L. pneumophila was enhanced within TLR2 −/− macrophages compared to WT and TLR4 −/− macrophages. There was no difference in the bacterial growth within dendritic cells from WT and TLR-deficient mice. Production of interleukin-12p40 (IL-12p40) and IL-10 after infection with L. pneumophila was attenuated in TLR2 −/− macrophages compared to WT and TLR4 −/− macrophages. Induction of IL-12p40, IL-10, and tumor necrosis factor alpha secretion from macrophages by the L. pneumophila dotO mutant, which cannot multiply within macrophages, and heat-killed bacteria, was similar to that caused by a viable virulent strain. There was no difference between the WT and its mutants in susceptibility to the cytopathic effect of bacteria. An L. pneumophila sonicated lysate induced IL-12p40 production by macrophages, but that of TLR2 −/− macrophages was significantly lower than those of WT and TLR4 −/− macrophages. Treatment of L. pneumophila sonicated lysate with proteinase K and heating did not abolish TLR2-dependent IL-12p40 production. Our results show that TLR2, but not TLR4, is involved in murine innate immunity against L. pneumophila , although other TLRs may also contribute to innate immunity against this organism.