Date Published: April 6, 2017
Publisher: Public Library of Science
Author(s): Emmanuel C. Patin, Anna C. Geffken, Sam Willcocks, Christoph Leschczyk, Albert Haas, Falk Nimmerjahn, Roland Lang, Theresa H. Ward, Ulrich E. Schaible, Olivier Neyrolles.
The causative agent of tuberculosis, Mycobacterium tuberculosis (M. tuberculosis), contains an abundant cell wall glycolipid and a crucial virulence factor, trehalose-6,6’-dimycolate (TDM). TDM causes delay of phagosome maturation and thus promotes survival of mycobacteria inside host macrophages by a not fully understood mechanism. TDM signals through the Monocyte-INducible C-type LEctin (Mincle), a recently identified pattern recognition receptor. Here we show that recruitment of Mincle by TDM coupled to immunoglobulin (Ig)G-opsonised beads during Fcγ receptor (FcγR)-mediated phagocytosis interferes with phagosome maturation. In addition, modulation of phagosome maturation by TDM requires SH2-domain-containing inositol polyphosphate 5’ phosphatase (SHP-1) and the FcγRIIB, which strongly suggests inhibitory downstream signalling of Mincle during phagosome formation. Overall, our study reveals important mechanisms contributing to the virulence of TDM.
The mycobacterial glycolipid trehalose-6,6’- dimycolate (TDM) is an abundant component of the cell wall of all Mycobacterium species . In Mycobacterium tuberculosis (M. tuberculosis), the main causative agent of tuberculosis (TB) in humans, TDM is a potent virulence factor involved in establishing the intracellular niche crucial for mycobacterial survival and growth in macrophages . Reconstitution of previously delipidated M. tuberculosis with purified TDM is sufficient to temporarily restore ability to delay phagosome acidification and fusion with lysosomes, thereby promoting bacterial survival . Simplified bead models of phagosome biogenesis revealed the ability of purified mycobacterial and nocardial TDM to decelerate phagosome maturation and delay fusion with lysosomes [2–4]. The germline-encoded patter recognition receptor (PRR) monocyte-inducible C-type lectin (Mincle), a member of the C-type lectin receptor (CLR) family, plays an important role in immunity to mycobacterial and fungal pathogens [5,6]. Mincle has been reported as specific receptor for TDM [7,8]. Upon TDM binding, Mincle triggers the production of pro- and anti-inflammatory mediators such as tumour necrosis factor (TNF)-α, nitric oxide (NO) and IL-10 in murine bone marrow-derived macrophages (BMDM) through the Syk/CARD9 signalling axis [9–12]. However, little is known about its role in phagosome biogenesis. Fcγ receptors (FcγR) are important phagocytic receptors, which mediate phagocytosis of immunoglobulin (Ig)G-opsonised particles. Signal transduction depends on cytosolic domain containing immunoreceptor tyrosine-based activation (ITAM) or immunoreceptor tyrosine-based inhibition (ITIM) motifs for activating or inhibitory FcγR, respectively . Both Mincle and activating FcγR associate with the transmembrane adaptor protein Fcγ chain containing an ITAM and initiating signalling by Syk recruitment . Mycobacteria are potent immunogens inducing specific antibodies to surface molecules, which can be involved in phagocytic uptake of M. tuberculosis . Phagocytes usually rapidly eliminate IgG-opsonised bacteria by FcγR-mediated phagocytosis, which is involved in host defence to M. tuberculosis as mice deficient in activating FcγRs were more susceptible to M. tuberculosis infection than WT ones. In contrast, mice lacking the inhibitory FcγRIIB were more resistant to experimental TB .