Discovery of Glycan-Based Biomarkers for Cutaneous Leishmaniasis and Chagas Disease by Reversed Immunoglycomics
Protozoa are the causative agents of a number of diseases that affect humans and mammalian animals. Several forms of leishmaniasis are caused by different Leishmania species (Leishmania spp.), while Chagas disease (CD) is caused by the parasite Trypanosoma cruzi (T. cruzi). These members of the Trypanosomatidae family have characteristic glycoconjugates broadly distributed on their cell surfaces, which can be useful for diagnosis and follow-up of chemotherapy for the diseases. Leishmania spp. expresses an “exotic” surface glycocalyx mainly composed of a number of glycosyl-phosphatidylinositol (GPI)-anchored proteins, a complex lipophosphoglycan (LPG) and a family of low-molecular mass glycoinositol-phospholipids (GIPLs), some of them containing terminal a-galactopyranosyl (α-Galp) residues. The complete structure of Leishmania major (L. major) GIPLs have been solved in the past.  It was found that L. major express type-2 GIPLs, which are known to be highly immunogenic, most likely due to the presence of unusual αGalp and β-galactofuranosyl (β-Galf) moieties. However, the exact structures of the immunodominant glycotopes responsible for eliciting IgG antibodies (Abs) in Cutaneous Leishmaniasis (CL) patients have not been determined. The identification of these glycotopes are of great interest as potential biomarkers (BMKs) for the accurate diagnosis of leishmaniasis and possibly CD. In Chapters 2 and 3, the chemical synthesis of five terminal oligosaccharides of type-2 GIPLs from L. major ranging from the disaccharide Galfβ1,3Manpα to the more complex tetrasaccharide Galpα1,6Galpα1,3Galfβ1,3Manpα, as well as the tetrasaccharide Galfβ1,3Manpα1,2-[Galfβ1,3Manpα] present in the GIPLs of T. cruzi will be presented. Some of these glycans were then conjugated to bovine serum albumin (BSA) to produce the corresponding neoglycoproteins (NGPs) for their use as antigens in chemiluminescent enzyme-linked immunosorbent assay (CL-ELISA) using CL and CD patient sera, and to evaluate them as BMKs for the diagnosis of CL and CD respectively. In collaboration with Drs. Alvaro Acosta-Serrano (Liverpool School of Tropical Medicine, Dept. of Parasitology) and Igor Almeida (UTEP, Dept. of Biological Sciences) we identified Galpα1,3Galfβ and Galpα1,3Galfβ1,3Manpα as diagnostic BMKs for L. major infections, distinguishable from L. tropica infections and heterologous diseases, respectively. T. cruzi has a cell surface heavily coated by glycoproteins such as GPI-anchored mucins (tGPI-MUC), whose O-glycans are predominantly branched, and often contain highly immunogenic terminal nonreducing α-Galp glycotopes that are entirely absent or cryptic in humans. The immunodominant tGPI-MUC α-Galp glycotope, the trisaccharide Galpα1,3Galpβ1,4GlcNAcα (Galα3LN), elicits high levels of protective T. cruzi-specific anti-αGal Abs in CD patients, in both the acute and chronic phases. Although glycoconjugates are the major parasite glycocalyx antigens, they remain completely unexplored as potential BMKs. In Chapter 4, we searched for a specific α-Gal-based BMK. There, we will show the synthesis of a NGP, consisting of BSA decorated with multiple copies of the branched trisaccharide Galpα(1,6)[Galpα(1,2)]Galpβ and its corresponding CL-ELISA using CD patient sera and Normal Human sera (NHS) of healthy individuals. The biological data obtained in collaboration with Dr. Igor Almeida suggest that Galpα(1,6)[Galpα(1,2)]Galpβ-linker-BSA (KM11b) may be an immunodominant partial structure of terminal T. cruzi glycans or a mimic thereof, and that NGP is suitable as a diagnostic BMK for CD and for drug efficacy assessment in patients.
Montoya, Alba Lucia, "Discovery of Glycan-Based Biomarkers for Cutaneous Leishmaniasis and Chagas Disease by Reversed Immunoglycomics" (2020). ETD Collection for University of Texas, El Paso. AAI28262219.