Hemagglutination and proteoglycan binding by the Lyme disease spirochete, Borrelia burgdorferi

The ability of the Lyme disease spirochete to attach to host components may contribute to its ability to infect diverse tissues. We present evidence that the Lyme disease spirochete expresses a lectin activity that promotes agglutination of erythrocytes and bacterial attachment to glycosaminoglycans. Among a diverse collection of 21 strains of Lyme disease spirochete, hemagglutinating activity was easily detected in all but 3 strains, and these three strains were noninfectious. The ability to agglutinate erythrocytes was associated with the ability of the spirochete to bind to the sulfated polysaccharide dextran sulfate and to mammalian cells. Soluble dextran sulfate was a potent inhibitor of both hemagglutination and attachment to mammalian cells, while dextran had no effect on either activity, suggesting that dextran sulfate may inhibit attachment by mimicking host cell glycosaminoglycans. Consistent with this, the spirochete bound to immobilized heparin, and soluble heparin inhibited bacterial adhesion to mammalian cells. The bacterium did not bind efficiently to Vero cells treated with heparinase or heparitinase or to mutant CHO cell lines that are deficient in proteoglycan synthesis. Sulfation of glycosaminoglycans was critical for efficient bacterial recognition, as Vero cells treated with an inhibitor of sulfation, or a mutant CHO cell line that produces undersulfated heparan sulfate, did not mediate maximal spirochetal binding. Binding of the spirochete to extracellular matrix also appeared to be dependent upon this attachment pathway. These findings suggest that a glycosaminoglycan-binding activity which can be detected by hemagglutination contributes to the attachment of the Lyme disease spirochete to host cells and matrix.

Growth inhibition of the intestinal parasite Giardia lamblia by a dietary lectin is associated with arrest of the cell cycle

Giardia lamblia, a cause of diarrheal disease throughout the world, is a protozoan parasite that thrives in the small intestine. It is shown here that wheat germ agglutinin (WGA), a naturally occurring lectin widely consumed in normal human diets, reversibly inhibits the growth of G. lamblia trophozoites in vitro, and reduces infection by G. muris in the adult mouse model of giardiasis. The inhibitory effect was dose related, not associated with cytotoxicity and reversed by N-acetyl-D-glucosamine in accordance with the known specificity of the lectin and in agreement with the presence of GlcNAc residues on the surface membrane of G. lamblia trophozoites. Cell cycle analysis revealed that parasites grown in the presence of WGA are arrested in the G2/M phase, providing an explanation for the lectin-induced inhibition of cell proliferation. Comparison of electrophoretic profiles by lectin blot analysis revealed both glycoprotein induction and suppression in growth-arrested organisms. Our findings raise the possibility that blocking trophozoite growth with naturally occurring dietary lectins may influence the course of giardiasis. In addition, the study of cell cycle arrest by WGA may provide a model to study the regulation of cell division in lower eukaryotes.

Mediation of Trypanosoma cruzi invasion by heparan sulfate receptors on host cells and penetrin counter-receptors on the trypanosomes

Trypanosoma cruzi attaches and invades a large variety of mammalian cells by receptor-mediated interactions, one of them involving the binding of parasite trans-sialidase to host sialyl receptors. Three proteoglycan-deficient mutants of Chinese hamster ovary (CHO) cells were used to probe the role of host heparin and heparan sulfate glycosaminoglycans (GAG) in T. cruzi invasion. All three mutants supported adhesion and infection to a much lower extent than the parental CHO cells. One of the mutants, pgsD-677, did not express heparan sulfate while containing three- to four-fold excess chondroitin sulfate, yet the cell line was a poor substrate for T. cruzi adhesion. Proteoglycan-deficient cells obtained by inhibiting GAG synthesis in parental cells with p-nitrophenyl-beta-D-xyloside, were also poor hosts for T. cruzi invasion. Furthermore, digestion of parental cells with heparinase and heparitinase, two lyases that specifically depolymerize heparin and heparan sulfate, reduced the potential of the cells to support T. cruzi adhesion and growth. Lyases that digested chondroitin sulfate and other GAGs did not affect T. cruzi invasion. These results suggest that heparin/heparan sulfate epitopes are receptors for T. cruzi invasion. The corresponding counter-receptor on T. cruzi appears to be penetrin, a heparin-binding protein that promotes trypanosome penetration into cells. Purified penetrin caused agglutination of red blood cells, and the hemagglutination was exquisitely sensitive to heparin and heparan sulfate. However, sialic acid and sialyl compounds did not inhibit penetrin-induced hemagglutination. Recombinant penetrin competitively inhibited T. cruzi invasion of proteoglycan-containing parental cells, but not of proteoglycan-deficient mutants nor of heparitinase-treated cells. Furthermore, consistent with the sugar specificity of penetrin as a hemagglutinin, recombinant penetrin competed for trypanosome invasion of a CHO cell mutant (Lec2) that expresses heparan sulfate but not sialyl residues. Given that the release of sialic acid from the proteoglycan-deficient mutants further reduced T. cruzi invasion, as did the removal of heparan sulfate from the Lec2 mutant, and given that penetrin does not bind to sialic acid with high affinity, the results indicate that the penetrin-heparan sulfate pathway for T. cruzi invasion is distinct from the trans-sialidase-sialic acid route.

A novel in situ model to study Pneumocystis carinii adhesion to lung alveolar epithelial cells

Pneumocystis carinii, an extracellular parasite thriving in the lungs of immunosuppressed mammals, is a major cause of death in AIDS patients in the USA. As a prelude to growth, the parasite adheres mostly to type I pneumocytes lining the alveolar spaces. The mechanism of adherence remains unknown, largely because of difficulties in isolating type I pneumocytes and maintaining them in vitro. As a first step to understand P. carinii adherence to its natural substrate, we developed an in situ method to directly study parasite binding to lung alveolar cells. We used formaldehyde-fixed paraffin-embedded sections of normal rat lung as substrate for adhesion. As in its binding to the lungs in vivo, P. carinii adhered preferentially to type I pneumocytes. Adherence was saturable, time and dose dependent, and selectively blocked by glycoconjugates, in particular bovine submaxillary mucin, fetuin, and asialofetuin, suggesting that it may be mediated by a lectin type of interaction. Further, IgG of rats with P. carinii pneumonia inhibited adherence, suggesting that it may react with parasite ligands involved in the recognition of type I cell receptors. Our results demonstrate the usefulness of the in situ model for studying the mechanisms of P. carinii adherence to alveolar cells. In addition, this method may be valuable for identifying neutralizing antibodies and drugs potentially useful for controlling the infection in vivo.

Mediation of Trypanosoma cruzi invasion by sialic acid on the host cell and trans-sialidase on the trypanosome

Trypanosoma cruzi attaches and invades a large variety of mammalian cells. The nature of the cell receptors and of the corresponding parasite counter-receptors that mediate T. cruzi-host cell interaction are not known. Three sialic acid-deficient mutants of Chinese hamster ovary (CHO) cells were used to probe the role of host sialyl residues in T. cruzi infection. All three mutants supported adhesion and infection to a much lower extent than the parental CHO cells. One of the mutants, Lec2, contains sugar chains terminating in non-reducing beta Gal residues, which are acceptors for sialylation by the T. cruzi trans-sialidase. Re-sialylation of Lec2 cells restored T. cruzi adhesion and invasion to about the same extent as wild-type cells. Digestion of wild-type cells with bacterial sialidase reduced T. cruzi interaction but after re-sialylation, the cells were almost as good as control, naturally sialylated parental cells. These results suggest that T. cruzi recognizes sialyl residues on the surface of host cells during invasion. On the other hand, affinity-purified trans-sialidase blocked T. cruzi adherence and invasion of sialylated cells, and had no effect on parasite interaction with sialic acid-deficient Lec2 mutant. Furthermore, 2,3-sialyllactose, a substrate for the trans-sialidase, competitively inhibited T. cruzi invasion of sialylated parental K1 cells, but 2,6-sialyllactose, which does not react with the trans-sialidase, was without effect, as were other sugars that do not contain alpha 2,3 sialyl residues. These results suggest that the trans-sialidase functions as a counter-receptor for trypomastigote binding to alpha 2,3-sialyl receptors on host cells as a prelude to T. cruzi invasion.

Identification of a developmentally regulated sialidase in Eimeria tenella that is immunologically related to the Trypanosoma cruzi enzyme

Sporozoites and merozoites of three species of Eimeria, E. tenella, E. maxima, and E. necatrix, that cause diarrhea in chickens worldwide, were examined for their expression of sialidase (SA) activity. The enzyme was found in three species, and the activity of merozoites was 10-20 times higher than that of sporozoites. The enzyme was resistant to degradation by proteases that are normally present in the intestine, a site inhabited by the Eimeria parasites, and it was relatively resistant to heat, with optimum activity being at 40 degrees C, which is within the range of temperature in the chicken intestine (40-43 degrees C). E. tenella SA was immunoprecipitated by monoclonal and polyclonal antibodies raised against the Trypanosoma cruzi SA (TCSA), and enzyme activity was neutralized by these antibodies. E. tenella SA was identified by immunoblots as a doublet of molecular weight 190,000 and 180,000 using, as a probe, anti-TCSA antibodies and antibodies against a synthetic peptide (TR) derived from the long tandem repeat domain of TCSA. Binding of the monoclonal and polyclonal antibodies to E. tenella was completely blocked by TR, but not by an irrelevant peptide (BR). Therefore, E. tenella expresses a developmentally regulated SA that is structurally related to the T. cruzi counterpart. Because of the high SA activity in merozoites, and by analogy with other SA-producing microbes that inhabit mucin-rich epithelia, we suggest that the Eimeria SA plays a role in desialylating intestinal mucins to reduce viscosity of the local environment and thereby facilitate parasite migration. The enzyme could also play a role in host cell-parasite interaction.

Identification of a lectin activity in Pneumocystis carinii

Pneumocystitis carinii is known to adhere to pulmonary alveolar epithelial cells in vivo and to epithelial cell lines in vitro by a mechanism unknown at the molecular level. P. carinii is now found to adhere to rabbit and human red blood cells leading to rosette formation and hemagglutination. P. carinii erythrocyte-adherence was best inhibited by bovine submaxillary mucin and by a polysaccharide from the wall of group A Streptococcus, and to a lesser extent by Streptococcus group C polysaccharide, asialofetuin and fetuin. Among the mono- and oligosaccharides tested, only lactose inhibited hemagglutination. Other glycoconjugates and oligosaccharides tested were inactive. P. carinii also bound to purified glycoproteins coupled to Sepharose or adsorbed to plastic, and the binding was inhibited by soluble bovine submaxillary mucin. These results indicate that P. carinii has a novel surface lectin that may be important in adherence to lung alveolar epithelial cells.

Entry of Trypanosoma cruzi into eukaryotic cells

Trypanosoma cruzi invades a variety of mammalian cells by receptor-ligand interactions. In this review two T. cruzi carbohydrate-binding proteins, neuraminidase/trans-sialidase and penetrin, are discussed as possibly playing a role in parasite entry into mammalian cells.

Mapping of a B-cell epitope present in the neuraminidase of Trypanosoma cruzi

We have previously shown that a polyclonal (rabbit anti-TCNA) and a mouse monoclonal antibody (TCN-2) against the neuraminidase of Trypanosoma cruzi (TCNA) inhibit enzyme activity, immunoprecipitate active enzyme, enhance in vitro infection, and identify a subpopulation of extracellular trypomastigotes. We now report on the identification of a synthetic peptide that contains the epitope recognized by these antibodies. The synthetic peptide (TR) is a dodecamer (D-S-S-A-H-G-T-P-S-T-P-A) deduced from the DNA sequence of the long tandem repeat (LTR) domain present in the TCNA carboxyterminus. By ELISA, rabbit anti-TCNA bound to TR coupled to ovalbumin, and the binding was inhibited by soluble TR but not by BR (Y-S-V-D-D-G-E-T-W-E), a peptide derived from the N-terminal domain of the enzyme. TCN-2 recognized TR, and this reaction as well as TCN-2 binding to endogenous TCNA could be inhibited by soluble TR but not by BR. These results indicate that the rabbit anti-TCNA and TCN-2 react with the LTR region of TCNA. Antibodies to TR reacted by immunoblot with the TCNA of the Silvio X-10/4, MV-13 and Y-H6 strains, identifying the same molecular polymorphism previously observed with the rabbit anti-TCNA and TCN-2. Furthermore, anti-TR antibodies immunoprecipitated active enzyme and immunofluorescence analysis revealed that anti-TR and TCN-2 antibodies detected equally well the differential expression of their epitopes in intra- and extracellular trypomastigotes. Moreover, expression of TR and TCN-2 epitopes on the different stages of T. cruzi paralleled the stage-specificity of TCNA activity. TCN-2 prevented desialylation by TCNA of intact cells but not of soluble glycoconjugates, indicating that TCN-2 epitope is probably not associated with the enzyme catalytic site, in agreement with the predicted sequence of the TCNA gene. Finally, analysis of the humoral response of a Chagasic patient to different areas of the TCNA molecule indicated that the antibody response is predominantly against TR suggesting that the tandem repeat is the immunodominant domain of TCNA.

The neuraminidases of Trypanosoma cruzi and Acanthamoeba castellanii are immunologically related

We have recently reported the presence of neuraminidase (NA) activity in Acanthamoeba castellanii. We now show that the NAs of T. cruzi and A. castellanii share cross-reactive determinants using TCN-2, a monoclonal antibody (mAb) against the T. cruzi NA and a mouse polyclonal Ab (anti-TR) raised against a tandemly repeated dodecapeptide which contains the epitope recognized by TCN-2 (Prioli et al., submitted). This cross-reactivity was demonstrated by the reaction of TCN-2 and anti-TR with A. castellanii parasites using immunofluorescence, immunoblotting and ELISA. Inhibition and immunoprecipitation of enzyme activity confirmed that the A. castellanii antigen recognized by TCN-2 was the NA. Immunoprecipitation of [35S] labeled trophozoite lysates showed the A. castellanii NA to have a molecular weight of 115 kDa. In addition, immunoblot analysis of subcellular fractions obtained by ultracentrifugation showed the A. castellanii NA to be associated with the parasite membrane but not with the cytosol or cytoskeleton fractions. These results suggest that the TCN-2 epitope, contained within a dodecamer tandem repeat unit, is present in T. cruzi and A. castellanii NAs.

Neuraminidase activity in acanthamoeba species trophozoites and cysts

Acanthamoeba species, a widely distributed group of free-living amoeba, can infect humans and spread hematogenously after direct interaction with the mucosal surfaces. The mechanism underlying Acanthamoeba damage to the target cell is unknown. The authors report that trophozoites and cysts of Acanthamoeba species exhibit a neuraminidase activity that is membrane associated and released into the culture medium at the start of the logarithmic phase of growth. The enzyme activity is optimal at pH 5 and at 25-30 degrees C. Live parasites release sialic acid from human cells. Therefore, the neuraminidase of Acanthamoeba species could be relevant in the colonization and damage of the sialic acid-rich corneal epithelium and in the alterations of glycolipids associated with meningoencephalitis.

A novel T. cruzi heparin-binding protein promotes fibroblast adhesion and penetration of engineered bacteria and trypanosomes into mammalian cells

T. cruzi invades mammalian cells in various organs after migrating through the ECM. These activities appear to be mediated by a unique 60 kd protein exposed on the T. cruzi surface, which promotes selective adhesion of trypomastigotes to three ECM components: heparin, heparan sulfate, and collagen. The affinity-purified protein binds to host fibroblasts in a saturable and glycosaminoglycan- and collagen-inhibitable manner. When adsorbed to plastic, it promotes adhesion and spreading of fibroblasts, as does the recombinant protein expressed in E. coli. The endogenous protein, and reactive ECM proteins, are very effective in preventing T. cruzi invasion of culture cells. The recombinant protein localizes on the E. coli surface and induces the bacteria that express it to adhere to and penetrate nonphagocytic Vero cells in a proteoglycan- and collagen-inhibitable manner. Therefore, the protein, named penetrin, could play a critical role in T. cruzi binding to the ECM and to cells, and in host cell invasion.

The Trypanosoma cruzi neuraminidase contains sequences similar to bacterial neuraminidases, YWTD repeats of the low density lipoprotein receptor, and type III modules of fibronectin

Trypanosoma cruzi expresses a developmentally regulated neuraminidase (TCNA) implicated in parasite invasion of cells. We isolated full-length DNA clones encoding TCNA. Sequence analysis demonstrated an open reading frame coding for a polypeptide of 1,162 amino acids. In the N-terminus there is a cysteine-rich domain containing a stretch of 332 amino acids nearly 30% identical to the Clostridium perfringens neuraminidase, three repeat motifs highly conserved in bacterial and viral neuraminidases, and two segments with similarity to the YWTD repeats found in the low density lipoprotein (LDL) receptor and in other vertebrate and invertebrate proteins. This domain is connected by a structure characteristic of type III modules of fibronectin to a long terminal repeat (LTR) consisting of 44 full length copies of twelve amino acids rich (75%) in serine, threonine, and proline. LTR is unusual in that it contains at least 117 potential phosphorylation sites. At the extreme C-terminus is a hydrophobic segment of 35 amino acids, which could mediate anchorage of TCNA to membranes via a glycosylphosphatidylinositol linkage. This is the first time a protozoan protein has been found to contain a YWTD repeat and a fibronectin type III module. The domain structure of TCNA suggests that the enzyme may have functions additional to its catalytic activity such as in protein-protein interaction, which could play a role in T. cruzi binding to host cells.

Identification of developmentally regulated Giardia lamblia cyst antigens using GCSA-1, a cyst-specific monoclonal antibody

GCSA-1, a monoclonal antibody raised against cysts generated in vitro was shown to be Giardia cyst-specific by immunoblot analysis and immunofluorescence. GCSA-1 recognized four polypeptides ranging from 29-45 kD present in the cyst wall. These antigens appeared within eight hours of exposure of trophozoites to encystation medium and were shown to be synthesized by encysting parasites by means of metabolic labelling with [35S]-cysteine. Trophozoites were not stained by the antibody. GCSA-1 also reacted with in vivo cysts obtained from faeces of infected humans, gerbils and mice. These data demonstrate that the determinants recognized by GCSA-1 are early cyst antigens which are developmentally regulated and conserved components of the cyst wall. The actual role of the antigens detected by GCSA-1 in encystation are unknown, but they represent a potential target for strategies directed at inhibiting this process.

N-acetyl-D-glucosamine is present in cysts and trophozoites of Giardia lamblia and serves as receptor for wheatgerm agglutinin

Previously, on the basis of lectin binding and glycosidase digestion assays, we have suggested that N-acetyl-D-glucosamine residues (GlcNAc) are major structural components of both trophozoites and in vivo cysts of the intestinal parasite Giardia lamblia. In this report we confirm that GlcNAc is present both in trophozoites and in vitro cysts as assessed by lectin binding and glycosidase digestion assays, galactosyltransferase labeling, immunochemical analysis using antibodies specific for GlcNAc and its beta 1-4 oligomers, and by gas chromatography/mass spectrometry (GC/MS). The results show that wheatgerm agglutinin (WGA) binds specifically to intact trophozoites and in vitro cysts as well as to SDS-PAGE separated proteins. WGA binding to the separated proteins was markedly reduced after their digestion with N-acetyl-beta-D-glucosaminidase, supporting the conclusion that WGA is reacting with terminal beta-linked GlcNAc residues. Labeling of trophozoites and cysts by 3H-exogalactosylation with galactosyltransferase further confirmed the presence of terminal GlcNAc in both surface and intracellular glycoproteins. The presence of GlcNAc is also supported by microfluorometric analysis using antibodies to (GlcNAc)1, (GlcNAc)2, and (GlcNAc)3, which revealed a sugar-inhibitable binding of the antibody to live trophozoites. Finally, the presence of GlcNAc in both cysts and trophozoites was unequivocally confirmed by GC/MS analysis of detergent-extracted membranes and of glycoproteins isolated by affinity chromatography on WGA-agarose. GC/MS analysis also revealed mannose (Man), N-acetyl-D-galactosamine (GalNAc), fucose (Fuc), galactose (Gal), glucose (Glc) and N-acetylneuraminic acid (NANA) to be present in cysts. All these sugars were also present in trophozoites, except for GalNAc. The glycoproteins isolated by WGA affinity chromatography were 5- to 40-fold enriched in GlcNAc, further supporting the conclusion that WGA reacts with GlcNAc in Giardia. In summary, the data presented here provide biological and chemical evidence for GlcNAc in both cysts and trophozoites of G. lamblia and are consistent with previously published results from this and other laboratories.