Toll-Like Receptor 9 Is Required for Full Host Resistance to Mycobacterium avium Infection but Plays No Role in Induction of Th1 Responses

Abstract

To investigate the role of Toll-like receptor 9 (TLR9) in innate immunity to Mycobacterium avium, TLR9, TLR2, and MyD88 knockout (KO) mice were infected with this bacterium. Bacterial burdens were higher in the spleens, livers, and lungs of infected TLR9 KO mice than in those of C57BL/6 mice, indicating that TLR9 is required for efficient control of M. avium infection. However, TLR9 KO or TLR2 KO spleen cells displayed normal M. avium-induced tumor necrosis factor alpha (TNF-␣) and gamma interferon (IFN-␥) responses. This finding was confirmed by determining the number of splenic CD4؉ T cells producing IFN-␥ by flow cytometry. Furthermore, TLR2 and MyD88, but not TLR9, played a major role in interleukin-12 and TNF-␣ production by M. avium-infected macrophages and dendritic cells (DCs). We also found that major histocompatibility complex class II molecule expression on DCs is regulated by TLR2 and MyD88 signaling but not by TLR9. Finally, lack of TLR9, TLR2, or MyD88 reduced the numbers of macrophages, epithelioid cells, and lymphocytes in M. avium-induced granulomas but only MyD88 deficiency affected the number of liver granulomas. In summary, our data demonstrated that the involvement of TLR9 in the control of M. avium infection is not related to the induction of Th1 responses.Mycobacterium avium is an opportunistic pathogen that infects mostly immunocompromised individuals (15). After entering the host, mycobacteria are internalized by macrophages and survive host killing by preventing phagolysosome fusion, inhibiting the acidification of the phagocytic vacuole, and disrupting the actin cytoskeleton of the cell (1). Similarly to infection with Mycobacterium tuberculosis, the interaction of M. avium with phagocytic cells results in the recruitment of CD4 ϩ T lymphocytes and in the secretion of gamma interferon (IFN-␥) and other proinflammatory cytokines (5). This process is suggested to be at least partially dependent on the recognition of pathogen-associated molecular patterns by Toll-like receptors (TLRs) expressed on innate immunity cells, which in turn leads to MyD88-mediated activation of the cytokine geneassociated transcription factor 20,24).The involvement of TLRs in host resistance to M. avium was suspected given the greatly augmented susceptibility of MyD88-deficient mice to infection with this bacterium (6, 10). Even though TLR2 has been proposed to be a major receptor for the recognition of glycopeptidolipids from M. avium (28), TLR2 knockout (KO) mice were found to be less susceptible to M. avium infection than MyD88 KO mice. Additionally, it has been reported that TLR4 deficiency does not affect the resistance of mice to M. avium (6). Therefore, these findings suggest that, in addition to TLR2, host control of M. avium infection involves other, as-yet-undefined, MyD88-dependent members of the interleukin-1 (IL-1)/TLR superfamily.In the present study, we investigated the role of TLR9 in innate immune recognition of M. avium, since this receptor localizes to the endosomes and phagolysosomes, where i...

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