TLR2-dependent eosinophil interactions with mycobacteria: role of α-defensins

Abstract

Peripheral blood and tissue eosinophilia are a prominent feature in allergic diseases and during helminth infections. Eosinophil recruitment also frequently occurs upon mycobacterial infections, particularly in lung granuloma. However, the mechanism by which eosinophils interact with mycobacteria remains largely unknown. Because eosinophils recently have been shown to be involved in innate immune responses, we investigated the direct interactions of eosinophils with Mycobacterium bovis BCG as a study model. We show that live BCG attracts human eosinophils and induces reactive oxygen species (ROS) synthesis, granule protein release, and tumor necrosis factor (TNF)-␣ secretion. Using anti-TLR2 neutralizing antibodies before exposure of eosinophils to BCG, we showed a critical role of TLR2 signaling in ROS and eosinophil peroxidase release. BCG-induced eosinophil activation is mediated through the p38 mitogen-activated protein (MAP) kinase and nuclear factor (NF)-B pathways. In addition, a mycobacterial wall component, lipomannan, induced a TLR2-dependent eosinophil activation. In addition, we showed that eosinophils express and produce ␣-defensins upon stimulation with BCG and lipomannan and that ␣-defensins could inhibit mycobacterial growth in synergy with eosinophil cationic protein. These results suggest a role for human eosinophils as direct effectors in TLR2-mediated innate immunity against mycobacteria and confer to these cells potent cytotoxic functions through defensin and eosinophil cationic protein production. (Blood. 2009;113:3235-3244) IntroductionTuberculosis is the most prevalent infectious disease worldwide, accounting for 3 million deaths annually. Control of Mycobacterium tuberculosis bacillus (MTB) infection requires the development of a Th1-type CD4 T-cell response and activation of alveolar macrophages, leading to the formation of lung granuloma. 1 Initial recognition of mycobacteria by the innate immune system through pathogen recognition receptors (PRRs) such as Toll-like receptors (TLRs) contributes to triggering this adaptive immune response. 2 Indeed, cooperation between TLR2, TLR4, and TLR9 may contribute to resistance against mycobacteria, 3 and the authors of several studies have shown that mycobacterial components act as TLR agonists. Indeed, lipomannans (LM) and lipoarabinomannans (LAM), lipoglycans that ubiquitously are found in the wall of mycobacteria, are involved in the release of proinflammatory or anti-inflammatory cytokines. 4 Whereas mycobacterial infections are rather associated to Th1 responses, the occurrence of Th2 cytokine responses also has been reported in human tuberculosis. It is suggested that these Th2 responses might depress macrophage immunity and lead to increased susceptibility to tuberculosis. 5 Th2-driven immunity is associated with cell types that seldom are studied during mycobacterial infections. Among them, eosinophils, which are now considered as multifunctional leukocytes, are involved in inflammatory processes as well as in modulation of innate and ad...

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