Engagement of Toll‐like receptor 2 in mouse macrophages infected with Mycobacterium avium induces non‐oxidative and TNF‐independent anti‐mycobacterial activity

Abstract

Toll-like receptor (TLR) 2 plays an important role in the immune response to mycobacterial infections, being required for optimal immunity against certain virulent Mycobacterium avium strains. Here we analyzed the role of TLR2 in the intra-macrophagic growth of M. avium, using macrophages from TLR2-deficient mice. We found that the engagement of TLR2/TLR6 and/or TLR2/TLR1 receptors induced bacteriostasis of M. avium inside bone marrow-derived macrophages in a MyD88-dependent way. Additionally, lipoproteins from the cell envelope of M. avium with a molecular mass of 20-25 kDa triggered this TLR2 pathway, leading to a decrease in the growth of the mycobacteria. Although TLR2 engagement induced the production of TNF, this cytokine as well as nitric oxide and superoxide molecules were not necessary for TLR2-mediated bacteriostasis. Finally, TLR ligation did not induce the expression of the 47-kDa guanosine triphosphatase (LRG-47) but it promoted an increased maturation of the phagosome with regards to acquisition of LAMP1. Our data show that triggering TLR2 inhibited M. avium growth by an as-yetunknown mechanism that may involve increased phagosome maturation.Key words: Immunity Á Nitric oxide Á Phagocytes IntroductionMacrophages initiate the innate immune response by recognizing pathogens, phagocytosing them and producing inflammatory mediators. The initiation of an effective response requires the recognition of conserved molecular structures (pathogen associated molecular patterns) found in a large variety of microbes, and recognized by pattern recognition receptors (PRR) that are expressed on myeloid and other cells [1, 2]. The mammalian Tolllike receptors (TLR) are a structurally conserved family that shares homology to the Drosophila Toll system and represent one group of PRR that plays an important role in the recognition of infecting microbes [1][2][3]. Distinct TLR recognize a variety of chemically diverse agonists. These include lipopolysaccharide (LPS), peptidoglycan, viral coat proteins, lipoproteins, glycolipids, dsRNA and CpG DNA [4].TLR are comprised of two major domains: an extracellular leucine-rich repeats (LRR) domain and an intracellular Toll/IL-1 receptor (TIR) domain. The TIR domain, which is present in all TLR, is responsible for initiating a signaling cascade through interactions with TIR-domain-containing adaptors. Most TLR, including TLR2, recruit the MyD88 adaptor molecule, which leads to the activation of nuclear factor (NF)-jB and mitogen-activated protein (MAP) kinases, resulting in the induction of inflammatory cytokines [5,6]. MyD88-and TLR2-deficient mice have been shown to be more susceptible to several microbial infections, including Mycobacterium avium infections [7,8]. However, there are at least three other adaptor proteins, some of which can be used in a TLR-specific way. Thus, this indicates that the signaling pathways through individual TLR may differ from each other and thereby result in different biological responses [9][10][11]. TLR2 recognizes a wide variety of microbial c...

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