ABSTRACT
The spleen plays a pivotal role in the immune defense against encapsulated bacteria such as Streptococcus pneumoniae. Murine splenic marginal zone macrophages express the C-type lectin SIGNR1, which is crucial for the capture of S. pneumoniae from blood. In this study, we demonstrate that SIGNR1 is able to interact in vitro with the juxtaposing marginal zone B cell population, which is responsible for the production of the early IgM response against the S. pneumoniae-epitope phosphorylcholine. Strikingly, SIGNR1-deficient mice display a reduction in the marginal zone B cell population. In addition, ex vivo B cell stimulation assays demonstrate a decrease in phosphorylcholine specificity in the splenic B cell population derived from SIGNR1-deficient mice, whereas the total IgM response is unaffected. Therefore, we hypothesize that antigens are presented by SIGNR1 expressed by marginal zone macrophages to the developing marginal zone B cell population thereby influencing the repertoire of this B cell population, which is pivotal for the early immune response against encapsulated bacteria such as S. pneumoniae.
Subject(s)
B-Lymphocytes/immunology , B-Lymphocytes/microbiology , Cell Adhesion Molecules/immunology , Immunoglobulin M/immunology , Lectins, C-Type/immunology , Macrophages/immunology , Macrophages/microbiology , Receptors, Cell Surface/immunology , Streptococcus pneumoniae/immunology , Animals , B-Lymphocytes/cytology , Blood-Borne Pathogens , CHO Cells , Cell Adhesion Molecules/deficiency , Cell Count , Cricetinae , Cricetulus , Lectins, C-Type/deficiency , Mice , Phosphatidylcholines/immunology , Polysaccharides, Bacterial/immunology , Protein Binding , Receptors, Cell Surface/deficiencyABSTRACT
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.
Subject(s)
Cell Adhesion Molecules/deficiency , Cell Adhesion Molecules/physiology , Lectins, C-Type/deficiency , Lectins, C-Type/physiology , Mycobacterium tuberculosis/immunology , Receptors, Cell Surface/deficiency , Receptors, Cell Surface/physiology , T-Lymphocytes/immunology , Tuberculosis/immunology , Animals , Cell Adhesion Molecules/genetics , Colony Count, Microbial , Disease Models, Animal , Histocytochemistry , Humans , Interferon-gamma/biosynthesis , Interleukin-10/biosynthesis , Lectins, C-Type/genetics , Lipopolysaccharides/immunology , Lipopolysaccharides/metabolism , Liver/microbiology , Lung/microbiology , Lung/pathology , Macrophages/immunology , Mice , Mice, Inbred C57BL , Mice, Knockout , Microscopy , Receptors, Cell Surface/genetics , Spleen/immunology , Spleen/microbiology , SurvivalABSTRACT
The dendritic cell-specific ICAM-3-grabbing nonintegrin (DC-SIGN) homolog, SIGN-related 1 (SIGNR1) is a pathogen receptor expressed by splenic marginal zone and peritoneal macrophages, and is essential for clearance of Streptococcus pneumoniae by phagocytosis after intraperitoneal infection. Here, we identified an important in vivo function for SIGNR1 in S. pneumonia infection induced via its natural entrance route. Upon intranasal infection with S. pneumoniae, SIGNR1-deficient mice did not clear bacteria from lung and blood, and displayed severely enhanced inflammatory parameters compared to the wild-type mice. However, SIGNR1 is not expressed by alveolar macrophages, suggesting that another mechanism than a decrease in phagocytosis is responsible for this difference. Natural anti-phosphorylcholine IgM produced by marginal zone B cells is essential for protection against infection with S. pneumoniae. Strikingly, during infection, SIGNR1-deficient mice failed to produce a rapid anti-phosphorylcholine IgM response. Marginal zone macrophages have been suggested to capture antigens for presentation to marginal zone B cells. We demonstrate that marginal zone macrophages from SIGNR1-deficient mice in contrast to wild-type mice are not able to capture pneumococci from blood, suggesting that SIGNR1 on marginal zone macrophages captures S. pneumoniae for antigen presentation to and activation of marginal zone B cells, resulting in an anti-phosphorylcholine IgM response.
Subject(s)
Cell Adhesion Molecules/immunology , Lectins, C-Type/immunology , Lung/immunology , Macrophages/immunology , Pneumococcal Infections/immunology , Receptors, Cell Surface/immunology , Animals , Cell Adhesion Molecules/deficiency , Fluorescent Antibody Technique , Immunoglobulin M/biosynthesis , Immunoglobulin M/immunology , Lectins, C-Type/deficiency , Lung/microbiology , Mice , Phosphorylcholine/immunology , Receptors, Cell Surface/deficiency , Streptococcus pneumoniaeABSTRACT
C-type lectins are important receptors expressed by antigen presenting cells that are involved in cellular communications as well as in pathogen uptake. An important C-type lectin family is represented by DC-SIGN and its homologues in human and mouse. Here we have investigated the carbohydrate specificity of cellular mSIGNR1 and compared it with DC-SIGN and L-SIGN. mSIGNR1 has a similar specificity as human DC-SIGN for high mannose-containing ligands present on both cellular and pathogen ligands. However, the DC-SIGN molecules differ in their recognition of Lewis antigens; mSIGNR1 interacts not only with Le(x/y) and Le(a/b) antigens similar to DC-SIGN, but also with sialylated Lex, a ligand for selectins. The differential recognition of Lewis antigens suggests differences between mSIGNR1 and DC-SIGN in the recognition of cellular ligands and pathogens that express Lewis epitopes.
Subject(s)
Carbohydrate Metabolism , Cell Adhesion Molecules/metabolism , Lectins, C-Type/metabolism , Receptors, Cell Surface/metabolism , Animals , Binding Sites , Carbohydrates/chemistry , Humans , Lewis X Antigen/metabolism , Mice , Substrate SpecificityABSTRACT
The Gram-positive bacterium Streptococcus pneumoniae is the leading causative pathogen in community-acquired pneumonia. The ever-increasing frequency of antibiotic-resistant S. pneumoniae strains severely hampers effective treatments. Thus, a better understanding of the mechanisms involved in the pathogenesis of pneumococcal disease is needed; in particular, of the initial interactions that take place between the host and the bacterium. Recognition of pathogens by dendritic cells is one of the most crucial steps in the induction of an immune response. For efficient pathogen recognition, dendritic cells express various kinds of receptors, including the DC-specific C-type lectin DC-SIGN. Pathogens such as Mycobacterium tuberculosis and HIV target DC-SIGN to escape immunity. Here the in vitro binding of DC-SIGN with S. pneumoniae was investigated. DC-SIGN specifically interacts with S. pneumoniae serotype 3 and 14 in contrast to other serotypes such as 19F. While the data described here suggest that DC-SIGN interacts with S. pneumoniae serotype 14 through a ligand expressed by the capsular polysaccharide, the binding to S. pneumoniae serotype 3 appears to depend on an as yet unidentified ligand. Despite the binding capacity of the capsular polysaccharide of S. pneumoniae 14 to DC-SIGN, no immunomodulatory effects on the dendritic cells were observed. The immunological consequences of the serotype-specific capacity to interact with DC-SIGN should be further explored and might result in new insights in the development of new and more potent vaccines.
Subject(s)
Cell Adhesion Molecules/immunology , Lectins, C-Type/immunology , Receptors, Cell Surface/immunology , Streptococcus pneumoniae/genetics , Streptococcus pneumoniae/immunology , Animals , Cell Adhesion , Cell Adhesion Molecules/metabolism , Cell Differentiation , Dendritic Cells/cytology , Dendritic Cells/immunology , Dendritic Cells/metabolism , Humans , Lectins, C-Type/metabolism , Receptors, Cell Surface/metabolism , Serotyping , Streptococcus pneumoniae/metabolismABSTRACT
Antigen presenting cells express C-type lectins that are involved in pathogen capture, processing and antigen presentation to induce immune responses against these pathogens. However, it is becoming clear that pathogens have evolved to subvert the function of some C-type lectins to escape immune surveillance. An important C-type lectin family is represented by DC-SIGN and its homologues in human and mouse. Here we discuss the structure in relation to the pathogen binding specificity of the SIGN receptors and the function of these receptors in mouse and humans.
Subject(s)
Antigens, CD/immunology , Cell Adhesion Molecules/immunology , Lectins, C-Type/immunology , Receptors, Cell Surface/immunology , Animals , Antigen-Presenting Cells/immunology , Antigens, CD/biosynthesis , Cell Adhesion Molecules/biosynthesis , Humans , Lectins, C-Type/biosynthesis , Mice , Receptors, Cell Surface/biosynthesisABSTRACT
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 pathogenesis.
Subject(s)
Antigens, CD/chemistry , Cell Adhesion Molecules/chemistry , Dendritic Cells/immunology , Lectins, C-Type/chemistry , Mannose-Binding Lectins/chemistry , Receptors, Cell Surface/chemistry , Animals , Antigens, CD/metabolism , Binding Sites , Carbohydrate Sequence , Cell Adhesion Molecules/metabolism , Dendritic Cells/microbiology , Humans , Lectins, C-Type/metabolism , Ligands , Mannose-Binding Lectins/metabolism , Mice , Molecular Sequence Data , Mycobacterium tuberculosis/chemistry , Mycobacterium tuberculosis/metabolism , Oligosaccharides/chemistry , Plasmids , Receptors, Cell Surface/metabolism , TransfectionABSTRACT
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.
