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Infection and Immunity, November 2004, p. 6717-6721, Vol. 72, No. 11
0019-9567/04/$08.00+0 DOI: 10.1128/IAI.72.11.6717-6721.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Tapria N. Moody,1,
Vyacheslav A. Furtak,2
Josiah Ochieng,2
Maria F. Lima,1 and
Fernando Villalta1*
Department of Microbiology,1 Department of Biochemistry, School of Medicine, Meharry Medical College, Nashville, Tennessee2
Received 12 May 2004/ Returned for modification 4 July 2004/ Accepted 26 July 2004
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Human galectin-3 is a member of a growing family of ß-galactosidase-binding animal lectins (18, 19). Recent evidence has implicated galectin-3 as a master regulator of inflammation, cell growth, signaling, chemotaxis, cell-matrix interactions, tumor progression, and metastasis (14, 19). Galectin-3 is expressed in a variety of tissues and cell types (3, 4, 6) and is localized in the cytoplasm, nucleus (12), or on the cell surface (5, 22) or is secreted in the extracellular environment by phagocytic and nonphagocytic cells (10, 22, 23). Because galectin-3 is secreted and is known to mediate cell adhesion of some immune cells (25), we have investigated the role of galectin-3 in the process of T. cruzi trypomastigote adhesion to coronary artery smooth muscle (CASM) cells. Here, we report a new mechanism by which T. cruzi uses galectin-3, which is secreted by human CASM cells, to mediate adhesion to these cells.
Galectin-3 binds to the surface of human cells and T. cruzi trypomastigotes in a lectin-like manner. Highly purified endotoxin-free human recombinant galectin-3, expressed as a histidine-tagged protein (15, 26), was labeled with fluorescein isothiocyanate (FITC) (20) for binding assays (27). FITC-labeled galectin-3 (2 µg/ml of phosphate-buffered saline [PBS]) was incubated with either 1% paraformaldehyde-fixed 30% confluent human CASM cell monolayers (Clonetics, San Diego, Calif.) or fixed cultured trypomastigotes (2 x 106) (9), in the presence or absence of lactose (5 mM) or a 100x excess of unlabeled recombinant galectin-3 in PBS supplemented with 0.1% bovine serum albumin for 1 h at 37°C (27). Figure 1A shows that FITC-labeled galectin-3 binds to the surface of CASM cells. This binding is granular, distributed around the cellular membrane, polarized, and more pronounced at the cellular ends (Fig. 1A). These findings suggest that the receptors for human galectin-3 are distributed in patches on the surface of the cells and are more abundant at the terminal regions of the cells. The binding is specific, since a 100x excess of unlabeled galectin-3 completely inhibited the binding of labeled galectin-3 (Fig. 1B). The binding of labeled galectin-3 to the surface of CASM cells is completely inhibited by 5 mM lactose (Fig. 1C) but not by 5 mM sucrose (Fig. 1D), indicating that galectin-3 binds to the surface of CASM cells in a lectin-like manner. FITC-labeled galectin-3 also binds to the surface of trypomastigotes (Fig. 2A); this binding is inhibited by a 100x excess of unlabeled galectin-3 (Fig. 2B), indicating that it is specific. Lactose (Fig. 2C), but not sucrose (Fig. 2D), inhibited this binding, indicating that galectin-3 binds to the surface of trypomastigotes in a lectin-like manner. The binding of galectin-3 to trypomastigotes is also seen as granular, restricted to some areas of the membrane of trypanosomes, and polarized (Fig. 2A).
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FIG. 1. Human galectin-3 binds to the surface of human CASM cells in a lectin-like manner. Human CASM cell monolayers at 30% density were washed with Hanks balanced salt solution (HBSS), fixed with 1% paraformaldehyde, and incubated with FITC-labeled galectin-3 (2 µg/ml) (A), 100x unlabeled galectin-3 plus FITC-labeled galectin-3 (B), 5 mM lactose plus FITC-labeled galectin-3 (C), or 5 mM sucrose plus FITC-labeled galectin-3 (D) for 1 h at 37°C. Cells were washed and examined under fluorescence microscopy. The results shown are from a representative experiment of three experiments performed with the same results.
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FIG. 2. Human galectin-3 binds to the surface of T. cruzi trypomastigotes in a lectin-like manner. Culture trypomastigotes (2 x 106) were washed with HBSS, fixed with 1% paraformaldehyde, and incubated with FITC-labeled galectin-3 (2 µg/ml) (A), 100x unlabeled galectin-3 plus FITC-labeled galectin-3 (B), 5 mM lactose plus FITC-labeled galectin-3 (C), or 5 mM sucrose plus FITC-labeled galectin-3 (D) for 1 h at 37°C. Parasites were washed and examined under fluorescence microscopy. The results shown are from a representative experiment of three experiments performed with similar results.
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FIG. 3. Galectin-3 is expressed on the surface of human CASM cells and is secreted, and exogenous galectin-3 enhances T. cruzi trypomastigote binding to human CASM cells. (A) Galectin-3 is expressed on the surface of human CASM cells and is secreted. Left panel, biotinylated surface proteins of human CASM cells were immunoprecipitated with anti-galectin-3 antibodies (abs) (lane 1) or with preimmune antibodies (lane 2), separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, blotted onto nitrocellulose membranes, and developed by enhanced chemiluminescence (ECL). Right panel, immunoblots of culture supernatants of human CASM cells incubated in DMEM for 6 h (lane 1) or medium alone (lane 2), probed with anti-galectin-3 antibodies, and developed by ECL. (B) Galectin-3 enhances T. cruzi trypomastigote binding to human CASM cells. Triplicate monolayers of human CASM cells were exposed or not exposed to several concentrations of human galectin-3 free of endotoxin and to T. cruzi trypomastigotes at a ratio of 20 parasites per cell for 2 h at 37°C. Unbound parasites were washed out, and trypanosome binding was evaluated by using an immunofluorescence assay as described above. (C) Lactose, but not sucrose, inhibits T. cruzi trypomastigote binding to human CASM cells. Trypomastigote binding assays were performed as described for panel B in the presence of HBSS, with several concentrations of lactose or sucrose, at the ratio of 20 parasites per cell. Unbound trypanosomes were washed out, and binding was evaluated by immunofluorescence as for panel B. The results presented in panels A to C are from one respective representative experiment of three independent experiments performed with similar results. Each point in panels B and C is the mean of results for triplicate samples in one representative experiment (± 1 standard deviation). For panel B, the P value was <0.05 for differences between 0 (control) and 1 µg of galectin-3/ml. For panel C, the P value was <0.05 with Student's t test for differences between lactose and sucrose at all points except 0 µg of sugar/ml.
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Preincubation of either trypomastigotes or CASM cells with galectin-3 enhances trypanosome adhesiveness to cells. To determine which of the two cells, T. cruzi or CASM cells, is the primary facilitator of the galectin-3-mediated parasite-host interaction, the parasites and cells were preincubated with galectin-3 (4 µg/ml) followed by binding assays using a parasite-to-cell ratio of 10:1 (11). Figure 4 shows that the preincubation of either trypomastigotes or CASM cells with galectin-3 significantly enhanced the trypomastigote binding to cells with respect to mock-treated parasites or CASM cells. These results indicate that galectin-3 acts on either T. cruzi or CASM cells or on both types of cells in order to enhance parasite binding to host cells.
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FIG. 4. Trypanosome binding to CASM cells was enhanced when T. cruzi trypomastigotes or CASM cells were preincubated with galectin-3. Monolayers of human CASM cells or trypomastigotes were washed with HBSS and preincubated separately in PBS with 4 µg of galectin-3/ml. Parasites and cells were then incubated together at a ratio of 10 parasites per cell for 2 h at 37°C. Unbound trypanosomes were washed out, and trypanosome binding was evaluated by using an immunofluorescence assay as for Fig. 3B and C. Other controls included human CASM cells incubated with trypanosomes exposed or not exposed to galectin-3. Bars represent the means of results from triplicate samples in one representative experiment (± 1 standard deviation) selected from three experiments with similar results. *, significant difference compared to control values (P < 0.05).
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FIG. 5. Reduced expression of galectin-3 inhibits the attachment of T. cruzi to human cells, but the addition of exogenous galectin-3 restores the initial capacity of these cells to bind to trypanosomes. (A) Blocking galectin-3 expression in human cells by transfection with galectin-3 antisense caused a reduction in T. cruzi adhesion to cells, and adding exogenous galectin-3 restores the initial capacity of these cells to bind to trypanosomes. Monolayers of human 435 cells transfected with vector alone or galectin-3 antisense were washed with HBSS and incubated separately with PBS or with 4 µg of galectin-3/ml in PBS and trypomastigotes at the ratio of 10 parasites per cell for 2 h at 37°C. Unbound trypanosomes were washed off, and trypanosome binding was evaluated by using an immunofluorescence assay as described for Fig. 3B. Bars represent the means of results for triplicate samples in one representative experiment (± 1 standard deviation) selected from three experiments with similar results. The difference between results labeled * and ** and the difference between results labeled 0 and 00 are significant (P < 0.05). (B) Human 435 cells transfected with galectin-3 antisense show significantly reduced expression of galectin-3 as evaluated by immunoblotting. Three µg of 435 cells transfected with either vector alone (lane 1) or galectin-3 antisense (lane 2) were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, blotted onto nitrocellulose membranes, probed with anti-galectin-3 polyclonal antibodies, and developed by ECL. Loading controls were evaluated by stripping the same blots, probing them with anti-ß-actin antibodies, and developing them by ECL. The results shown are from a representative experiment of three experiments performed with the same results. 435/vector alone, 435 cells transfected with pCNC10 vector alone; 435/galectin-3 antisense, 435 cells transfected with pCNC10-galectin-3 antisense.
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Galectin-3 is implicated in the association of T. cruzi with laminin (11), is expressed in B cells from T. cruzi-infected mice (1), and is up regulated by T. cruzi infection of mice (29). The fact that galectin-3 is secreted by macrophages and by other cells, including human CASM cells, as indicated in this report, suggests that released galectin-3 modulates infection. The concentrations of galectin-3 that increase trypanosome adhesion to CASM cells in vitro (Fig. 3B) are similar to the concentrations of galectin-3 present in fluids in vivo (21). Furthermore, the concentrations of galectin-3 in fluids in vivo increase approximately 300-fold during microbial infection (24). These observations suggest that this parasite may have adapted to the host and that it takes advantage of a host inflammatory molecule, galectin-3, to bind to host cells. Furthermore, it has been recently reported that Chagas' disease cardiomyopathy is in part a vasculopathy (17). We also suggest that the findings described in the present study may contribute in part to determining the cause of this pathology. In summary, we report that the expression of galectin-3 is required for T. cruzi infection and that the parasite uses galectin-3 to adhere to and enter human cells.
Y.Y.K. and T.N.M. contributed equally to the present report. ![]()
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binds to Trypanosoma cruzi amastigotes to induce signaling and cellular proliferation. Infect. Immun. 71:4201-4205.
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