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Infection and Immunity, December 2002, p. 6592-6596, Vol. 70, No. 12
0019-9567/02/$04.00+0 DOI: 10.1128/IAI.70.12.6592-6596.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Department of Biochemistry, Universidade Federal de São Paulo/Escola Paulista de Medicina, São Paulo, SP, 04023-900, Brazil
Received 17 May 2002/ Returned for modification 29 June 2002/ Accepted 24 August 2002
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80%) inhibition of promastigote infectivity. These data suggest that a putative new receptor recognizing ß-D-Galf is associated with L. major macrophage infectivity and that GIPL-1 containing a terminal Galf residue is involved in the L. major-macrophage interaction. |
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We analyzed the role of glycosylinositol phospholipid 1 (GIPL-1) of L. major and its galactofuranose (Galf) residue in the interactions of L. major promastigotes and amastigotes with mouse peritoneal macrophages.
Glycoconjugates containing Galf residues have been found in various microorganisms, including trypanosomatids, fungi, and bacteria (1, 5, 6, 11, 12, 14, 18, 26, 29). In Leishmania, Galf can be present as a nonterminal residue, as it is in LPG, the main surface glycoconjugate of promastigotes (31), and in GIPL-2 and GIPL-3 of L. major (14) and as a terminal residue in GIPL-1 of L. major (14). Although the biological role of Galf residues is not known, one intriguing hypothesis is that terminal Galf residues play a central role in the survival of fungi and parasites by blocking action of the host's glycosidases against glycoconjugates of the fungi and parasites. On the other hand, Galf may function as a strong immunogen. The absence of Galf and galactofuranosidases in mammalian host cells makes these molecules potentially useful as specific targets for therapy of parasitic and fungal diseases.
In order to analyze the role of terminal Galf residues in L. major, monoclonal antibody (MAb) MEST-1, which recognizes glycolipids containing terminal Galf residues linked ß-1-3 or ß-1-6 to mannose (26, 27), was used to detect GIPL-1 in Leishmania cells and to analyze its relationship with macrophage infectivity of L. major. MEST-1 reactivity with L. major was analyzed by indirect immunofluorescence analysis and by high-performance thin-layer chromatography (HPTLC) immunostaining of glycolipid fractions. The role of L. major Galf residues in macrophage infectivity was studied by using either MEST-1 or p-nitrophenyl-glycoside as an inhibitor in macrophage infection assays.
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Solid-phase RIA of parasites. Promastigotes of various Leishmania species were adsorbed on 96-well plates precoated with 0.1% poly-L-lysine (molecular weight, 500,000) for 30 min as described by McMahon-Pratt et al. (16). Promastigotes (1 x 106 parasites/well or 2 x 106 parasites in the first well and double-diluted preparations in subsequent wells) were added, the plates were centrifuged for 10 min at 800 x g, and the parasites were fixed for 15 min with 0.5% glutaraldehyde in cold 0.01 M phosphate buffer (pH 7.2) containing 0.15 M NaCl (phosphate-buffered saline [PBS]). The plates were washed with PBS, and unbound aldehyde groups were blocked by adding 0.1 M glycine (pH 8.0) and incubating the preparations for 30 min. Fixed parasites were delipidated by using isopropanol-hexane-water (IHW) (55:20:25, vol/vol/vol; upper phase discarded) as described by Suzuki et al. (26). The plates were washed with PBS, blocked with 1% bovine serum albumin (BSA) in PBS (200 µl) for 2 h, and incubated for 2 h with an MAb for a solid-phase radioimmunoassay (RIA). The amount of MAb bound was determined by reaction with rabbit anti-mouse immunoglobulin G (IgG) (50 µl). The plates were washed three times with PBS, and 50 µl of 125I-labeled protein A (105 cpm) in 1% BSA was added to each well; then the plates were incubated for 1 h and washed five times with PBS. The radioactivity in each well was measured with a gamma counter (24).
Preparation of amastigotes. Promastigotes from the stationary culture phase (2 x 107 parasites) were inoculated into Golden hamster footpads. After 5 to 6 weeks, lesions were surgically removed, and the tissue was minced. Debris was eliminated by Nitex nylon filtration (pore size, 80 µm; Tetko, Monterey Park, Calif.). Infected macrophages were homogenized (20 strokes), and the cell suspension was centrifuged at 1,300 x g for 10 min. To lyse erythrocytes, the pellet was resuspended in an ammonium chloride solution (8.29 g of NH4Cl per liter, 1 g of KHCO3 per liter, 37.2 mg of EDTA per liter), incubated for 10 min, and centrifuged at 1,300 x g, and the pellet containing amastigotes was washed three times with PBS by centrifugation at 1,800 x g. Parasites obtained in this way were used for a macrophage infection assay and for glycolipid purification. All animal experimental procedures were approved by the Institutional Ethical Committee.
Glycolipid extraction. Amastigotes (1 x 108 cells) and promastigotes (1 x 1010 cells) were extracted three times with chloroform-methanol-water (4:8:3), and the extracts were combined and dried in a rotary evaporator. The dried residue was dissolved in chloroform-methanol (95:5) and applied to a Silica Gel 60 column (1 by 15 cm), and the glycolipids were eluted by increasing the concentration of methanol (chloroform/methanol ratios, 95:5, 90:10, 70:30, 60:40, and 10:90). Fractions were dried under a stream of N2 at 37°C, resuspended in chloroform-methanol (2:1), and analyzed by HPTLC.
HPTLC immunostaining. Glycolipids present in fractions obtained from chromatography as described above were detected by HPTLC by using chloroform-methanol-0.02% CaCl2 (60:40:9). Immunostaining was performed by the procedure of Magnani et al. (13). After HPTLC development, the plates were dried and soaked in 0.5% polymethacrylate in hexane, dried, blocked for 2 h with 1% BSA in PBS, incubated with MAb MEST-1 overnight, and incubated sequentially with rabbit anti-mouse IgG and 125I-labeled protein A (4 x 105 cpm/ml) (25).
Preparation of peritoneal macrophages. Peritoneal macrophages were harvested by washing the peritoneal cavities of BALB/c mice with PBS. The macrophages were washed three times with cold PBS by centrifugation at 450 x g, and the final pellet was resuspended in RPMI 1640 medium supplemented with 10% fetal calf serum, 10 mM HEPES, 100 U of penicillin per ml, and 100 µg of streptomycin per ml. Macrophages (5 x 105 cells) were placed on 13-mm-diameter sterile glass coverslips in 24-well plates. Nonadherent cells were removed by several washes with RPMI 1640 medium, and the plates were kept at 37°C in a CO2 incubator (23).
Inhibition of Leishmania infectivity in macrophages by MAbs. Amastigotes and promastigotes (5 x 106 cells/150 µl) were preincubated with different concentrations of Fab fragments (0.03 to 2.5 µg) as indicated below. After 1 h the parasites were washed with RPMI 1640 medium and incubated with peritoneal macrophages (10 promastigotes/macrophage and 5 x 106 parasites/well or 3 amastigotes/macrophage and 1.5 x 106 parasites/well) for 2 h in RPMI 1640 medium without serum at 37°C. Nonadherent parasites were removed by washing monolayers with medium. Infected macrophages were kept in RPMI 1640 medium with 5% fetal calf serum in a CO2 incubator for 24 h. The macrophages were fixed with methanol and stained with Giemsa solution. The phagocytic index was determined by multiplying the percentage of macrophages that phagocytosed at least one parasite by the average number of parasites per infected macrophage (300 cells were examined) as described by Silveira et al. (22). Inhibition was expressed as the percentage of phagocytosis obtained with each MAb compared with the control (Fab fragment of an irrelevant IgG3 MAb).
Inhibition of Leishmania infectivity in macrophages by glycosides.
Twenty-four-well plates containing 5 x 105 macrophages/well were preincubated at room temperature with 0.5-ml portions of various p-nitrophenyl-glycosides (10 mM), including p-nitrophenyl-ß-D-galactofuranoside, p-nitrophenyl-ß-D-galactopyranoside, p-nitrophenyl-
-D-galactopyranoside, and p-nitrophenyl-
-D-mannopyranoside (Sigma). After 1 h, 5 x 106 promastigotes in 0.3 ml of RPMI 1640 medium without serum were added to peritoneal macrophages (10 parasites/macrophage), and the preparations were incubated for 2 h at 37°C. Nonadherent parasites were removed by washing the monolayers with RPMI 1640 medium. Infected macrophages were maintained in RPMI 1640 medium with 5% fetal calf serum in a CO2 incubator for 24 h. Macrophages were fixed with methanol and analyzed as described above.
Effect of p-nitrophenyl-glycosides on L. major growth. Promastigotes (2 x 107 cells) were resuspended in 1.2 ml of RPMI 1640 medium and preincubated with 1.2-ml portions of solutions of various p-nitrophenyl-glycosides as described above (10 mM) for 2 h at 37°C. The suspensions were centrifuged, and the parasites were washed once with medium 199. The parasites were resuspended in 5 ml of medium 199 supplemented with 10% fetal calf serum and kept at 23°C. The number of parasites was determined every 24 h.
Indirect immunofluorescence. Parasites (1 x 108 cells) were fixed with 1% formaldehyde in PBS for 10 min. The cells were washed and resuspended in 1 ml of PBS, and 20-µl portions of the suspension were added to coverslips. Air-dried preparations were delipidated with an IHW mixture, as described by Suzuki et al. (26). The plates were then washed with PBS, flooded for 1 h with PBS containing 5% BSA, and incubated sequentially with MAb culture supernatant for 1 h and with fluorescein isothiocyanate-conjugated goat anti-mouse IgG for 1 h. After each incubation, the coverslips were washed five times with PBS and examined with an epifluorescence microscope. An irrelevant IgG3 MAb was used for control experiments. Alternatively, immunofluorescence of delipidated parasites was examined with sera from BALB/c mice infected with L. major.
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FIG. 1. Reactivity of MAb MEST-1 with promastigotes of Leishmania spp. by solid-phase RIA. (A) Preparation containing 2 x 106 parasites that was serially diluted, adsorbed onto 96-well plates, and incubated with MEST-1. Symbols: , L. major; , L. braziliensis; , L. chagasi; x, L. amazonensis. (B) Preparation containing 1 x 106 L. major promastigotes adsorbed onto 96-well plates that was delipidated ( ) or not delipidatd () with IHW (55:20:25) and incubated with MEST-1, as described in Materials and Methods. (Inset) MEST-1 reactivity with IHW fraction removed from wells and adsorbed on a new 96-well plate.
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FIG. 2. MEST-1 reactivity with Leishmania promastigotes as determined by indirect immunofluorescence. (A and B) L. major promastigotes incubated with MEST-1; (C and D) L. major promastigotes treated with IHW (55:20:25) and incubated with MEST-1; (E and F) L. major promastigotes treated with IHW (55:20:25) and incubated with serum from BALB/c mice infected with L. major; (G and H) L. amazonensis promastigotes incubated with MEST-1; (I and J) L. braziliensis promastigotes incubated with MEST-1. (A, C, E, G, and I) Fluorescence microscopy. (B, D, F, H, and J) Phase-contrast microscopy. Bars = 20 µm.
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FIG. 3. HPTLC pattern of glycolipids from L. major. Glycolipid fractions were applied to an HPTLC plate and developed in chloroform-methanol-0.02% CaCl2 (60:40:9). (A) Staining with orcinol-H2SO4; (B) immunostaining with MEST-1. Lanes 1, glycolipids from promastigotes; lanes 2, glycolipids from amastigotes. OR, origin.
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FIG. 4. Macrophage phagocytic indices for L. major (upper panels) and L. amazonensis (lower panels) preincubated with Fab fragments of MAbs. Fab fragments of MEST-1 (solid bars) or CU-1 (open bars) at different concentrations were incubated at room temperature for 1 h with L. major and L. amazonensis. Next, parasites were incubated with macrophages (10 promastigotes per macrophage or three amastigotes per macrophage) for 2 h at 37°C. Nonadherent parasites were removed by washing monolayers with culture medium. Infected macrophages were kept in a CO2 incubator for 24 h. Monolayers were washed with PBS, fixed with methanol, and stained with Giemsa stain. The phagocytic index (ordinate) was determined by multiplying the percentage of infected macrophages by the average number of parasites per infected macrophage (300 cells were examined). In control experiments, parasites were preincubated with Fab fragments of an irrelevant MAb (anti-Tn antigen CU-1). The values are means ± standard deviations from triplicate experiments. An asterisk indicates that a value is significantly different from the control value, as calculated by Student's t test (P < 0.01).
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-D-mannopyranoside (
65%) and p-nitrophenyl-ß-D-galactofuranoside (
77%) were observed (Fig. 5). On the other hand, when L. amazonensis promastigote forms were used in the infection assay, p-nitrophenyl-ß-D-galactofuranoside did not affect the parasite infectivity (data not shown). In order to demonstrate that the inhibitory effect was not due to a glycoside effect on L. major viability, parasites were cultivated in the presence of glycosides, and no difference in growth rates was observed for up to 8 days of culture (data not shown).
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FIG. 5. Effects of p-nitrophenyl-glycosides on L. major macrophage infectivity. Preparations containing 5 x 105 macrophages were preincubated with four p-nitrophenyl-glycosides (10 mM) for 1 h at room temperature. Next, 5 x 106 promastigotes were incubated with macrophages for 2 h at 37°C. Nonadherent parasites were removed by washing the monolayers with culture medium. Infected macrophages were kept in a CO2 incubator for 24 h. In control experiments, parasites were preincubated in the absence of p-nitrophenyl-glycoside. The values are means ± standard deviations from triplicate experiments. An asterisk indicates that a value is significantly different from the control value, as calculated by Student's t test (P < 0.01). ß-D-galp, p-nitrophenyl-ß-D-galactopyranoside; -D-galp, p-nitrophenyl- -D-galactopyranoside; -D-manp, p-nitrophenyl- -D-mannopyranoside; ß-D-galf, p-nitrophenyl-ß-D-galactofuranoside.
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In indirect immunofluorescence studies, L. major promastigotes showed intense fluorescence with MEST-1. Amastigotes showed a lower degree of fluorescence (data not shown). Treatment of promastigotes with IHW eliminated reactivity with MEST-1, and the reactive material was recovered in the organic fraction, indicating its lipid nature. HPTLC of organic extracts confirmed that both amastigotes and promastigotes presented GIPL-1 as the antigen recognized by MEST-1.
According to McConville and Bacic (15) L. major promastigotes and amastigotes present about 10 x 106 and 4 x 106 molecules of GIPLs/cell, respectively. By performing densitometry with HPTLC plates containing glycolipids of promastigotes (as shown in Fig. 3A), it was determined that GIPL-1 accounts for about 40% of the total GIPLs, which corresponds to about 1.6 x 106 molecules/cell. On the other hand, it was not possible to quantify the GIPL-1 in amastigotes due to the small number of parasites in the hamster lesions, and consequently, it was not possible to determine with accuracy the concentration of GIPL-1 in a mixture that also contained glycolipids from hamster lesions. Nevertheless, it is known that GIPL-1 accounts for 36% of the total GIPLs of LV 39 amastigotes isolated from nude mice lesions, as described by Schneider et al. (21).
The role of GIPL-1 and its Galf residue in the binding and invasion of macrophage monolayers was analyzed by preincubating parasites with MEST-1 Fab fragments, which reduced macrophage infectivity by
80% for promastigotes and 30% for amastigotes. The more modest inhibition of amastigote adhesion than of promastigote adhesion could have been due to GIPL-1 crypticity in amastigotes or due to a lower GIPL-1 concentration in amastigotes than in promastigotes, as observed by immunofluorescence, which detected weaker MEST-1 reactivity with amastigotes than with promastigotes. The substantial inhibition (
80%) of macrophage infection by promastigotes in the presence of p-nitrophenyl-ß-D-galactofuranoside confirmed the importance of the Galf residue. Significant inhibition (
65%) of macrophage infection was also observed when p-nitrophenyl-
-D-mannopyranoside was used, indicating that a mannose receptor is involved in the Leishmania-macrophage interaction. No inhibition was observed when macrophages were preincubated with p-nitrophenyl-ß-D-galactopyranoside. These findings indicate that the galactose receptor present in macrophages associated with L. major infectivity does not recognize isolated galactose residues but requires ligands with a more complex structure. This is consistent with the conclusion of Kelleher et al. (10), who reported that L. major promastigote binding to macrophages requires a minimum structure defined as PO4-6[Gal(ß1-3)Gal(ß1-3)Gal(ß1-3)]Galß1-4Man(
1-), which is present in LPG.
Cell-cell interactions and attachment processes are multistep phenomena, and the present results indicate that one such receptor recognizes terminal Galf residues. The receptor for terminal Galf residues in glycoconjugates of parasites and fungi has not been characterized yet, but it is currently being investigated. Tsuji et al. (30) recently identified a new type of human lectin, termed human intelectin, which binds to pentoses and to D-Galf residues in the presence of Ca2+. A recombinant human intelectin recognizes bacterial arabinogalactan of Nocardia containing D-Galf; thus, this lectin may be involved in recognition of various pathogens containing Galf residues (e.g., Nocardia, Mycobacterium, Streptococcus, Leishmania, and Trypanosoma species). The present results suggest that terminal residues of ß-D-Galf present in GIPL-1 from either promastigote or amastigote forms of L. major are recognized by a previously undescribed macrophage receptor involved in the Leishmania-macrophage interaction.
We thank Stephen Anderson for editing the manuscript.
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