Mycobacteria Target DC-SIGN to Suppress Dendritic Cell Function

Mycobacterium tuberculosis represents a world-wide health risk and immunosuppression is a particular problem in M. tuberculosis infections. Although macrophages are primarily infected, dendritic cells (DCs) are important in inducing cellular immune responses against M. tuberculosis. We hypothesized that DCs represent a target for M. tuberculosis and that the observed immuno-suppression results from modulation of DC functions. We demonstrate that the DC-specific C-type lectin DC-SIGN is an important receptor on DCs that captures and internalizes intact Mycobacterium bovis bacillus Calmette-Guérin (BCG) through the mycobacterial cell wall component ManLAM. Antibodies against DC-SIGN block M. bovis BCG infection of DCs. ManLAM is also secreted by M. tuberculosis–infected macrophages and has been implicated as a virulence factor. Strikingly, ManLAM binding to DC-SIGN prevents mycobacteria- or LPS-induced DC maturation. Both mycobacteria and LPS induce DC maturation through Toll-like receptor (TLR) signaling, suggesting that DC-SIGN, upon binding of ManLAM, interferes with TLR-mediated signals. Blocking antibodies against DC-SIGN reverse the ManLAM-mediated immunosuppressive effects. Our results suggest that M. tuberculosis targets DC-SIGN both to infect DCs and to down-regulate DC-mediated immune responses. Moreover, we demonstrate that DC-SIGN has a broader pathogen recognition profile than previously shown, suggesting that DC-SIGN may represent a molecular target for clinical intervention in infections other than HIV-1

Mice lacking SIGNR1 have stronger T helper 1 responses to Mycobacterium tuberculosis

Mycobacterium tuberculosis and the associated disease tuberculosis are health risks causing many deaths worldwide each year in humans. M. tuberculosis targets dendritic cell (DC)-specific intercellular adhesion molecule-3 grabbing non-integrin (DC-SIGN) to induce immunosuppression, since interaction of DC-SIGN with mycobacterial mannose-capped lipoarabinomannan (ManLAM) induces interleukin (IL)-10 and prevents DC maturation. We investigated the role of a murine homolog of DC-SIGN, SIGN Related 1 (SIGNR1), in a model of M. tuberculosis infection using SIGNR1 deficient (KO) mice. Although SIGNR1 is expressed by macrophages and not by DCs, it also interacts with M. tuberculosis similar to DC-SIGN. Peritoneal macrophages from SIGNR1 KO mice produce less IL-10 upon stimulation with ManLAM than those from wild-type mice, suggesting that the interaction of ManLAM with SIGNR1 can result in immunosuppression similar to its human homolog. Indeed, early in infection, we observed increased T cell activity in SIGNR1 KO mice and increased IFNgamma production by splenocytes in SIGNR1 KO mice. However, we did not detect any differences between WT and KO mice in mycobacterial loads in the lungs or distant organs after M. tuberculosis infection resulting in similar survival rates. Moreover, we found that SIGNR1 is not present on alveolar macrophages of uninfected mice nor is it induced on lung macrophages throughout infection. Therefore, our data suggest that although SIGNR1 has a similar binding specificity as DC-SIGN, its role is limited during murine M. tuberculosis infection

Identification of the mycobacterial carbohydrate structure that binds the C-type lectins DC-SIGN, L-SIGN and SIGNR1

Mycobacterium tuberculosis represents a worldwide health risk and although macrophages are primarily infected, dendritic cells (DC) are important in inducing cellular immune responses against M. tuberculosis. Recent studies have demonstrated that M. tuberculosis targets the DC-specific C-type lectin DC-SIGN to inhibit the immuno-stimulatory function of DC through the interaction of the mycobacterial mannosylated lipoarabinomannan (ManLAM) to DC-SIGN, which prevents DC maturation and induces the immuno-suppressive cytokine IL-10. This may contribute to survival and persistence of M. tuberculosis. Here, we have identified the specific pathogen-derived carbohydrate structure on ManLAM that is recognized by DC-SIGN. We have synthesized the mannose-cap oligosaccharides man-ara, (man)2-ara and (man)3-ara, and demonstrate that these neoglycoconjugates are specifically bound by DC-SIGN. Moreover, we demonstrate that the human and murine DC-SIGN homologue L-SIGN and SIGNR1, respectively, also interact with mycobacteria through ManLAM. Both homologues have the highest affinity for the (man)3-ara structure, similar to DC-SIGN. This study provides information about the specific carbohydrate structures on pathogens that are recognized by DC-SIGN, and may provide strategies to develop vaccines against these pathogens. Moreover, the identification of SIGNR1 as a receptor for ManLAM will enable in vivo studies to investigate the role of DC-SIGN in M. tuberculosis pathogenesi