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* This work was supported by grants from the European Community “Cluster for Tuberculosis Vaccine Development,” the Institut Pasteur, and the National Institutes of Health (NIAID Contract NO1 AI-75320, “Tuberculosis Research Materials and Vaccine Testing”).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. § A JSPS overseas research fellow. ¶ These authors contributed equally to the work.
Interactions between dendritic cells (DCs) and Mycobacterium tuberculosis, the etiological agent of tuberculosis, most likely play a key role in anti-mycobacterial immunity. We have recently shown that M. tuberculosis binds to and infects DCs through ligation of the DC-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN) and that M. tuberculosis mannose-capped lipoarabinomannan (ManLAM) inhibits binding of the bacilli to the lectin, suggesting that ManLAM might be a key DC-SIGN ligand. In the present study, we investigated the molecular basis of DC-SIGN ligation by LAM. Contrary to what was found for slow growing mycobacteria, such as M. tuberculosis and the vaccine strainMycobacterium bovis bacillus Calmette-Guérin, our data demonstrate that the fast growing saprophytic speciesMycobacterium smegmatis hardly binds to DC-SIGN. Consistent with the former finding, we show that M. smegmatis-derived lipoarabinomannan, which is capped by phosphoinositide residues (PILAM), exhibits a limited ability to inhibit M. tuberculosis binding to DC-SIGN. Moreover, using enzymatically demannosylated and chemically deacylated ManLAM molecules, we demonstrate that both the acyl chains on the ManLAM mannosylphosphatidylinositol anchor and the mannooligosaccharide caps play a critical role in DC-SIGN-ManLAM interaction. Finally, we report that DC-SIGN binds poorly to the PILAM and uncapped AraLAM-containing species Mycobacterium fortuitum and Mycobacterium chelonae, respectively. Interestingly, smooth colony-formingMycobacterium avium, in which ManLAM is capped with single mannose residues, was also poorly recognized by the lectin. Altogether, our results provide molecular insight into the mechanisms of mycobacteria-DC-SIGN interaction, and suggest that DC-SIGN may act as a pattern recognition receptor and discriminate between Mycobacterium species through selective recognition of the mannose caps on LAM molecules.
The interaction between Mycobacterium tuberculosis and host dendritic cells (DCs)
). However, the molecular bases of DC infection by mycobacteria remain poorly understood. We have recently shown that M. tuberculosis, as well as the vaccine strain Mycobacterium bovis bacillus Calmette-Guérin (BCG), use the DC-specific intercellular adhesion molecule-3 (ICAM-3)-grabbing nonintegrin (DC-SIGN) to bind to and enter human DCs (
). DC-SIGN/CD209 is a calcium-dependent (C-type) transmembrane lectin that contains a single carbohydrate recognition domain at its extracellular C-terminal end. It is expressed on DCs as well as on some macrophage (Mφ) subsets, including alveolar Mφs (
). DC-SIGN has been described initially as a receptor for ICAM-3 and ICAM-2, as well as for human and simian immunodeficiency viruses, enabling the trans infection of susceptible CD4+T lymphocytes by these viruses (
), a feature that is consistent with our finding that M. tuberculosis lipoarabinomannan (termed ManLAM; see below), a highly mannosylated surface lipoglycan, might be a key mycobacterial ligand for DC-SIGN (
). Indeed, purified M. tuberculosis-derived ManLAM was found to inhibit the binding ofM. tuberculosis to human monocyte-derived DCs, as well as to recombinant HeLa-derived cells expressing DC-SIGN. LAM is a major component of the mycobacterial cell wall. It contains a carbohydrate backbone composed of d-mannan andd-arabinan (Fig.1). The mannan core is attached to an acylated mannosylphosphatidylinositol (MPI) anchor at its reducing end; the arabinan domain is capped with either mannose residues in so-called ManLAMs or with phosphoinositide motifs in so-called PILAMs (
). So far, ManLAMs have been detected in slow growing mycobacteria only. These include various strains of M. tuberculosis,M. bovis BCG, the leprosy agentMycobacterium leprae, and the opportunistic species Mycobacterium avium (
We have shown that the slow growing mycobacteria M. tuberculosis and M. bovis BCG, which express ManLAM, interact with human DCs through DC-SIGN and that purifiedM. tuberculosis-derived ManLAM inhibitsM. tuberculosis binding to recombinant HeLa-derived cells expressing DC-SIGN and to monocyte-derived DCs. The goal of the present report was to obtain a better understanding of the molecular determinants of the LAM molecule involved in binding to DC-SIGN. Using a DC-SIGN-expressing recombinant cell line as a readout, we first report that DC-SIGN binds poorly to the fast growing species M. smegmatis and that M. smegmatis-derived PILAM, which lacks mannose caps, exhibits a very limited ability to inhibit M. tuberculosis binding to the lectin. Using various chemically or enzymatically generated variants of the ManLAM molecule, we further demonstrate that both the acyl chains on the MPI anchor and the mannose-capping residues play a key role in the ManLAM-DC-SIGN ligation process. Moreover, we show that DC-SIGN does not bind to the PILAM- and AraLAM-containing species M. fortuitum and M. chelonae, respectively. Altogether, our findings provide evidences that DC-SIGN may discriminate between ManLAM-containing slow growers, such as M. tuberculosis, and nonpathogenic PILAM-containing fast growers, such as M. smegmatis, through a high affinity for mannose-capping residues on ManLAM.
Cells and Bacteria
DC-SIGN+ HeLa (HeLa-DC) cells were obtained by transducing HeLa cells with the retroviral vector TRIP-ΔU3 encoding for human DC-SIGN (
). HeLa and HeLa-DC cells were propagated in RPMI 1640 (Invitrogen) supplemented with 10% fetal calf serum (Dutscher, Brumath, France).M. tuberculosis H37Rv, M. smegmatismc2155, and clinical isolates of M. fortuitum,M. chelonae, and M. avium(smooth colony-forming) were propagated in 7H9 medium containing 10% albumin-dextrose-catalase supplement.
Mycobacterial Glycoconjugate Purification and Chemical Degradation
ManLAM from M. bovis BCG Pasteur and PILAM from M. smegmatis were purified as described previously (
). M. tuberculosis H37Rv-purified ManLAM was a kind gift from the Colorado State University. dManLAM was prepared by incubating ManLAM (200 μg) in 200 μl of NaOH 0.1 m for 2 h at 37 °C. After neutralization with 200 μl of HCl, 0.1m, the reaction products were dialyzed against water and freeze-dried. αManLAM was prepared by incubating ManLAM (200 μg) for 6 h at 37 °C in 30 μl of a jack beans α-mannosidase (Sigma) solution (2 mg/ml, 0.1 m sodium acetate buffer, pH 4.5, 1 mm ZnSO4). After a second addition of 50 μl of enzyme solution, the reaction was continued overnight. The reaction products were then dialyzed against 50 mmNH4CO3, pH 7.6. Elimination of α-mannosidase was achieved by denaturation (2 min at 110 °C) followed by overnight tryptic digestion (37 °C, 3.2 μg of trypsin). αManLAM was recovered after dialysis against water, freeze-dried, and analyzed for cap contents by CE-LIF as previously described (
). Briefly, ManLAM or αManLAM (1 μg) was submitted to mild acidic hydrolysis (15 μl of HCl, 0.1 m, for 20 min at 110 °C) in the presence of mannoheptose (100 pmol) as the internal standard. The reaction products were then submitted to 1-aminopyrene-3,6,8-trisulfonate (APTS) tagging and subjected to CE-LIF analysis. Separations were performed using an uncoated, fused silica capillary column (50 μm internal diameter; 40 cm effective length; 47 cm total length; Sigma), and analyses were carried out at a temperature of 25 °C with an applied voltage of 20 kV using acetic acid 1% (w/v), triethylamine 30 mm in water, pH 3.5, as a running electrolyte. The amount of each cap motif was determined relative to the internal standard.
Binding and Inhibition Assay
Cells were infected at a multiplicity of infection of 1 bacterium/cell for 4 h at 4 °C in RPMI 1640, washed extensively in RPMI 1640, and analyzed by scoring colony-forming units after plating on agar and incubation at 37 °C. In binding inhibition experiments, cells were preincubated for 1 h at 4 °C with 10 μg/ml of the indicated components. These components were left in the culture medium during the infection process.
RESULTS AND DISCUSSION
Mycobacterial species can be divided into slow and fast growers. To gain a better understanding of the molecular basis of their ligation to DC-SIGN, we first compared the relative ability of the slow growing pathogenic species M. tuberculosis versus the fast growing saprophytic species M. smegmatis to bind to the lectin. A binding assay was performed on HeLa-derived cells expressing or not DC-SIGN (HeLa-DC and HeLa, respectively). As we reported previously,M. tuberculosis was found to bind to HeLa-DC ∼15 times more than to HeLa cells (Fig.2A), and in a multiplicity of infection-dependent manner (data not shown). Conversely,M. smegmatis binding to HeLa-DC was found to be only ∼2 times more than to HeLa cells (Fig. 2A). Our previous finding that M. tuberculosis ManLAM can inhibit M. tuberculosis binding to DC-SIGN suggests that the reduced ability of M. smegmatis to bind to HeLa-DC cells may be due to the inability of M. smegmatis PILAM to bind to the lectin. To test this hypothesis, we performed a M. tuberculosis binding assay on HeLa-DC cells that had been preincubated or not with LAM molecules from various mycobacterial species. As reported previously, yeast mannan and M. tuberculosis- as well as M. bovis BCG-derived ManLAMs were found to inhibit mycobacterial binding to HeLa-DC cells by as much as ∼90% (Fig. 2B). By contrast, M. smegmatis-derived PILAM was found to inhibit poorly (∼25% inhibition) the binding of M. tuberculosisto HeLa-DC cells. Interestingly, PILAM was found able to fully inhibitM. smegmatis binding to HeLa-DC cells (data not shown). The fact that BCG-derived ManLAM inhibits M. tuberculosis binding to DC-SIGN as well as M. tuberculosis-derived ManLAM does is consistent with our previous result showing that M. bovis BCG binds to DC-SIGN to the same extent as M. tuberculosis and with the known structural similarity between ManLAMs from M. bovis BCG andM. tuberculosis (
Because DC-SIGN is a mannose-binding lectin and because PILAMs are devoid of mannose caps (Fig. 1), we next reasoned that the results described above might indicate that ManLAM capping residues may be the ManLAM subdomains preferentially recognized by the lectin. To test this hypothesis, we treated M. bovis BCG-derived ManLAM with α-exomannosidase to obtain ManLAM devoid of mannose caps (αManLAM). The reaction was assessed by CE-LIF analysis as previously described (
). A typical electropherogram obtained for αManLAM is presented in Fig. 3A. Peaks corresponding to mannooligosaccharide caps, i.e.mono-, α(1→2)-di-, and α(1→2)-tri-mannopyranosides, were almost undetectable. Quantification indicated that more than 95% of cap demannosylation was achieved. The ability of αManLAM to inhibitM. tuberculosis binding to DC-SIGN was evaluated in binding experiments. In contrast to native ManLAM, αManLAM failed to inhibit mycobacterial binding to the lectin (∼10% binding inhibition, Fig. 3B). Similar results were obtained when cells were treated with M. tuberculosis-derived αManLAM prior to the binding assay (data not shown). These results indicate that mannooligosaccharide caps are critical structural features for ManLAM-mediated inhibition of M. tuberculosis binding to DC-SIGN.
Because MPI anchor has been shown previously to be involved in some of the biological activities of ManLAM, particularly their binding to C-type lectins (
), we then evaluated the role of the acyl part of the MPI anchor in ManLAM-DC-SIGN interaction. To this end, M. bovis BCG-derived dManLAM was prepared by alkali treatment. As shown on Fig. 3B, dManLAM failed to inhibit M. tuberculosis binding to HeLa-DC cells, revealing that a native acylated MPI anchor is required for ManLAM-mediated inhibition of mycobacterial binding to DC-SIGN.
Finally, we wished to know whether our finding was still valid in otherMycobacterium species for which the LAM structure was known. In agreement with what we reported above, the PILAM- and AraLAM-containing species M. fortuitum and M. chelonae were poorly recognized by DC-SIGN. Indeed, in a representative binding assay done in triplicate, M. fortuitum and M. chelonae bound to HeLa-DC cells 2.3 and 1.5 times more than to HeLa cells, respectively (data not shown). Interestingly, the ManLAM-containing slow grower M. avium was also found to bind poorly to DC-SIGN-expressing HeLa cells (∼1.7 times more than to HeLa cells; data not shown). This is not surprising because ManLAM from smooth colony-forming M. avium, which is the one used in our assay, has been reported to be capped mainly with single mannose residues instead of the di- and tri-mannopyranoside motifs found in M. tuberculosis- andM. bovis BCG-derived ManLAM (
), it is likely that such mono-mannosylated ManLAM is not recognized by the lectin. These results raise the possibility, currently under investigation, that DC-SIGN may recognize mycobacteria from the tuberculosis complex only.
Altogether, our results demonstrate that the DC-SIGN-ManLAM interaction involves both the MPI anchor acyl chains and the mannose residues from caps of the ManLAM molecule. As established recently for the binding of ManLAM to the human surfactant pulmonary protein A C-type lectin (
), the MPI fatty acids are most likely involved in the supermolecular organization of the ManLAM molecules in aggregates, allowing macromolecular clustering in aqueous solution. Micelle formation probably results in a huge increase in ManLAM valence and increases ManLAM avidity to DC-SIGN. This is likely to explain the poor ability of dManLAM to inhibit M. tuberculosis binding to HeLa-DC cells but does not indicate whether LAM acyl chains, which are likely to be buried within the bacterial cell wall, can interact with the lectin in vivo. However, the latter definitely should be investigated, as our result is reminiscent of the involvement of the acyl chains of the M. tuberculosis 19-kDa lipoprotein antigen in binding to toll-like receptor-2 (TLR2) on phagocytic cells (
Selective recognition of the ManLAM mannose-capping residues by DC-SIGN on the surface of DCs is likely to have important consequences for both the pathogenesis and immunology of tuberculosis and other mycobacterial diseases. LAMs have various effects on phagocytic cells, including Mφs and DCs (
). PILAMs induce the secretion of proinflammatory cytokines, such as tumor necrosis factor-α, interleukin-1 (IL-1), IL-12, and IL-6, and the production of microbicidal radicals, such as NO
, in a much more potent way than do ManLAMs. In addition, ManLAMs, but not PILAMs, inhibit the Mφ activation effect of interferon-γ produced by effector T lymphocytes. PILAMs are thus now considered as proinflammatory molecules, whereas ManLAMs are viewed rather as anti-inflammatory components (
). In particular, our recent results demonstrate that ManLAM inhibits the secretion of IL-12 by human DCs, a process that, like DC-SIGN ligation, requires both the MPI anchor acyl chains and the mannose caps (
), one cannot rule out the possibility that ManLAM could act also through the ligation of DC-SIGN, which is currently under investigation. Indeed, DC-SIGN ligation by ManLAM, either attached to the bacilli or released in the milieu through exocytosis (
). It will be of interest to study the cross-talk between phagocytic cell surface lectins, such as MR and DC-SIGN, and TLRs in response to mycobacterial ligands, including LAMs from various mycobacterial species.
From an evolutionary perspective, it is interesting that DC-SIGN can discriminate between fast growing saprophytic and slow growing virulent or potentially virulent mycobacteria. Until recently, it was proposed that mannose capping of the LAM molecules was a unique feature of virulent mycobacteria. This is unlikely to be the case because the attenuated species M. bovis BCG also contains mannose-capped LAM (
). Even if not virulent stricto sensu, M. bovis BCG can be pathogenic under certain conditions, especially in children and immunocompromised patients, in whom it may cause a variety of effects ranging from local adenitis to disseminated disease (
). One cannot exclude that mannose capping of the ManLAM molecule is a feature of virulent mycobacteria that has been conserved during the recent evolution of M. bovis BCG fromM. bovis. In this context, DC-SIGN could be viewed as a pattern recognition receptor (
) that has evolved to recognize potentially harmful mycobacteria through their specific surface glycosylated moieties. In parallel, mycobacteria could have evolved mechanisms (LAM capping) to resist host immunity (deactivation of the inflammatory response) in contrast with their fast growing, soil-living, harmless ancestors.
We thank L. Tailleux, V. Abadie, O. Schwartz (Institut Pasteur, Paris), and C. Petit (Institut Cochin, Paris) for careful reading of the manuscript and helpful discussions. We thank P. Charneau (Institut Pasteur, Paris) for providing TRIP-ΔU3. We acknowledge Colorado State University for the gift of purified M. tuberculosis H37Rv-derived ManLAM.