Identification of Mycobacterial α-Glucan As a Novel Ligand for DC-SIGN: Involvement of Mycobacterial Capsular Polysaccharides in Host Immune Modulation

Abstract

Mycobacterium tuberculosis possesses a variety of immunomodulatory factors that influence the host immune response. When the bacillus encounters its target cell, the outermost components of its cell envelope are the first to interact. Mycobacteria, including M. tuberculosis, are surrounded by a loosely attached capsule that is mainly composed of proteins and polysaccharides. Although the chemical composition of the capsule is relatively well studied, its biological function is only poorly understood. The aim of this study was to further assess the functional role of the mycobacterial capsule by identifying host receptors that recognize its constituents. We focused on α-glucan, which is the dominant capsular polysaccharide. Here we demonstrate that M. tuberculosis α-glucan is a novel ligand for the C-type lectin DC-SIGN (dendritic cell-specific ICAM-3-grabbing nonintegrin). By using related glycogen structures, we show that recognition of α-glucans by DC-SIGN is a general feature and that the interaction is mediated by internal glucosyl residues. As for mannose-capped lipoarabinomannan, an abundant mycobacterial cell wall-associated glycolipid, binding of α-glucan to DC-SIGN stimulated the production of immunosuppressive IL-10 by LPS-activated monocyte-derived dendritic cells. By using specific inhibitors, we show that this IL-10 induction was DC-SIGN-dependent and also required acetylation of NF-κB. Finally, we demonstrate that purified M. tuberculosis α-glucan, in contrast to what has been reported for fungal α-glucan, was unable to activate TLR2.

Inorganic Phosphate Limitation Modulates Capsular Polysaccharide Composition in Mycobacteria

“…Interestingly, inactivation of the Pst system is associated with immune modulation and reduced virulence of M. tuberculosis in mice (34,35). This effect might be due to high capsular ␣-glucan levels, which have also been associated with immunomodulation, in these strains (11,13,14,33). Because we identified up-regulation of capsular ␣-glucan production in three different mycobacterial species, M. smegmatis, M. marinum, and M. tuberculosis, upon P i starvation, this phenotype seems to be a conserved feature.…”

“…For example, capsular ␣-glucan was shown to block CD1 expression in dendritic cells, suppress IL-12 production, and stimulate IL-10 production in a CD80-dependent manner (11). Additionally, capsular components have antiphagocytic properties that prevent uptake by macrophages (12) and abolish dendritic cell function, a process dependent on C-type lectin DC-SIGN (13). Furthermore, ␣-glucan interacts with complement receptor 3 (14), and inactivation of glycogen/␣-glucan glycosyltransferase GlgA impairs virulence of M. tuberculosis in mice (10).…”

Broadening Our View About the Role of Mycobacterium tuberculosis Cell Envelope Components During Infection: A Battle for Survival

“…However, the implication of DC-SIGN in triggering the immune response is still controversial. On one hand, engaging of M.tb, mannosylated cell wall components, or -glucan has been shown to induce generation of anti-inflammatory modulators such as IL-10 (Ehlers, 2009;Geurtsen et al, 2009). These findings were supported by in vivo studies using mice expressing human DC-SIGN homologues (McGreal et al, 2005;Park et al, 2001;Powlesland et al, 2006) or transgenic mice expressing human DC-SIGN, showing that DC-SIGN may act damping the immune response, and thus, promote host protection by limiting tissue damage (Schaefer et al, 2008;Tanne et al, 2009;Wieland et al, 2007 be significantly involved in regulating cytokine secretion using an engineered M. marinum strain lacking essential mannosylated components (Appelmelk et al, 2008).…”

Interplay of human macrophages and Mycobacterium tuberculosis phenotypes

“…Attachment of Mtb to macrophages was attributed to the most abundant capsular polysaccharide in Mtb, α-glucan, which binds to the complement receptor 3 (CR3) under non-opsonic conditions [150]. Furthermore, α-glucan modulates immune responses as a ligand of DC-SIGN [149] and alters the differentiation of monocyte-derived DCs and blocks CD1-expression, thereby inhibiting presentation of antigenic lipids of Mtb to CD1-restricted T cells [349]. Furthermore, Mtb mutants deficient in the synthesis of capsular glucan were less virulent and therefore unable to persist in vivo in mice [350,351].…”

“…Like the mannose receptor, DC-SIGN preferentially binds mannosylated LAM from virulent strains through selective binding to the mannose cap [148], indicating that phagocytes are trained to recognize virulent mycobacteria, or alternatively, that virulent strains are better equipped to enter host cells than avirulent mycobacteria. Besides ManLAM, DC-SIGN recognizes α-glucan, a component of the mycobacterial capsule [149]. The complement complement complement complement receptor CR3 receptor CR3 receptor CR3 receptor CR3 recognizes complement-opsonized or unopsonized bacteria, in the latter case through interaction with Mtb polysaccharides [150], and the consequences of CR3 ligation for Mtb are unknown [151].…”