Lectin interactions with alpha-galactosylated xenoantigens

Urinary nanovesicles captured by lectins or antibodies demonstrate variations in size and surface glycosylation profile

AIM

The use of carbohydrate-binding proteins (lectins) to isolate urinary extracellular vesicles (uEVs) was investigated and the captured subpopulations were characterized.

METHODS

Pooled uEVs from multiple healthy donors were exposed to lectin-conjugated or antibody-conjugated beads. Recovered uEVs were evaluated by protein estimation, transmission electron microscopy, nanoparticle tracking analysis and lectin microarray profiling.

RESULTS

uEVs isolated by lectin- and antibody-based affinity capture exhibited distinct variations in size and surface content. Transmission electron microscopy confirmed similar EV diameters to those established by nanoparticle tracking analysis, but total particle counts did not correlate closely with protein-based quantification. Lectin microarray profiling demonstrated capture-dependent differences in surface glycosylation.

CONCLUSION

Selective, carbohydrate-mediated EV isolation by lectin affinity approaches may prove immediately useful for research and find eventual use in clinical applications.

Xenogeneic acellular conjunctiva matrix as a scaffold of tissue-engineered corneal epithelium

Amniotic membrane-based tissue-engineered corneal epithelium has been widely used in the reconstruction of the ocular surface. However, it often degrades too early to ensure the success of the transplanted corneal epithelium when treating patients with severe ocular surface disorders. In the present study, we investigated the preparation of xenogeneic acellular conjunctiva matrix (aCM) and evaluated its efficacy and safety as a scaffold of tissue-engineered corneal epithelium. Native porcine conjunctiva was decellularized with 0.1% sodium dodecyl sulfate (SDS) for 12 h at 37°C and sterilized via γ-irradiation. Compared with native conjunctiva, more than 92% of the DNA was removed, and more than 90% of the extracellular matrix components (glycosaminoglycan and collagen) remained after the decellularization treatment. Compared with denuded amniotic membrane (dAM), the aCM possessed favorable optical transmittance, tensile strength, stability and biocompatibility as well as stronger resistance to degradation both in vitro and in vivo. The corneal epithelial cells seeded on aCM formed a multilayered epithelial structure and endured longer than did those on dAM. The aCM-based tissue-engineered corneal epithelium was more effective in the reconstruction of the ocular surface in rabbits with limbal stem cell deficiency. These findings support the application of xenogeneic acellular conjunctiva matrix as a scaffold for reconstructing the ocular surface.

Carbohydrate-mimetic peptides for pan anti-tumor responses

Molecular mimicry is fundamental to biology and transcends to many disciplines ranging from immune pathology to drug design. Structural characterization of molecular partners has provided insight into the origins and relative importance of complementarity in mimicry. Chemical complementarity is easy to understand; amino acid sequence similarity between peptides, for example, can lead to cross-reactivity triggering similar reactivity from their cognate receptors. However, conformational complementarity is difficult to decipher. Molecular mimicry of carbohydrates by peptides is often considered one of those. Extensive studies of innate and adaptive immune responses suggests the existence of carbohydrate mimicry, but the structural basis for this mimicry yields confounding details; peptides mimicking carbohydrates in some cases fail to exhibit both chemical and conformational mimicry. Deconvolution of these two types of complementarity in mimicry and its relationship to biological function can nevertheless lead to new therapeutics. Here, we discuss our experience examining the immunological aspects and implications of carbohydrate-peptide mimicry. Emphasis is placed on the rationale, the lessons learned from the methodologies to identify mimics, a perspective on the limitations of structural analysis, the biological consequences of mimicking tumor-associated carbohydrate antigens, and the notion of reverse engineering to develop carbohydrate-mimetic peptides in vaccine design strategies to induce responses to glycan antigens expressed on cancer cells.

Generating novel recombinant prokaryotic lectins with altered carbohydrate binding properties through mutagenesis of the PA-IL protein from Pseudomonas aeruginosa

BACKGROUND

Prokaryotic lectins offer significant advantages over eukaryotic lectins for the development of enhanced glycoselective tools. Amenability to recombinant expression in Escherichia coli simplifies their production and presents opportunities for further genetic manipulation to create novel recombinant prokaryotic lectins (RPLs) with altered or enhanced carbohydrate binding properties. This study explored the potential of the α-galactophilic PA-IL lectin from Pseudomonas aeruginosa for use as a scaffold structure for the generation of novel RPLs.

METHOD

Specific amino acid residues in the carbohydrate binding site of a recombinant PA-IL protein were randomly substituted by site-directed mutagenesis. The resulting expression clones were then functionally screened to identify clones expressing rPA-IL proteins with altered carbohydrate binding properties.

RESULTS

This study generated RPLs exhibiting diverse carbohydrate binding activities including specificity and high affinity for β-linked galactose and N-acetyl-lactosamine (LacNAc) displayed by N-linked glycans on glycoprotein targets. Key amino acid substitutions were identified and linked with specific carbohydrate binding activities. Ultimately, the utility of these novel RPLs for glycoprotein analysis and for selective fractionation and isolation of glycoproteins and their glycoforms was demonstrated.

CONCLUSIONS

The carbohydrate binding properties of the PA-IL protein can be significantly altered using site-directed mutagenesis strategies to generate novel RPLs with diverse carbohydrate binding properties.

GENERAL SIGNIFICANCE

The novel RPLs reported would find a broad range of applications in glycobiology, diagnostics and in the analysis of biotherapeutics. The ability to readily produce these RPLs in gram quantities could enable them to find larger scale applications for glycoprotein or biotherapeutic purification.

The sweets standing at the borderline between allo- and xenotransplantation

Animal cells are densely covered with glycoconjugates, such as N-glycan, O-glycan, and glycosphingolipids, which are important for various biological and immunological events at the cell surface and in the extracellular matrix. Endothelial α-Gal carbohydrate epitopes (Galα3Gal-R) expressed on porcine tissue or cell surfaces are such glycoconjugates and directly mediate hyperacute immunological rejection in pig-to-human xenotransplantation. Although researchers have been able to develop α1,3-galactosyltransferase (GalT) gene knockout (KO) pigs, there remain unclarified non-Gal antigens that prevent xenotransplantation. Based on our expertise in the structural analysis of xenoantigenic carbohydrates, we describe the immunologically significant non-human carbohydrate antigens, including α-Gal antigens, analyzed as part of efforts to assess the antigens responsible for hyperacute immunological rejection in pig-to-human xenotransplantation. The importance of studying human, pig, and GalT-KO pig glycoprofiles, and of developing adequate pig-to-human glycan databases, is also discussed.

Development of a convenient competitive ELISA for the detection of the free and protein-bound nonhuman galactosyl-α-(1,3)-galactose epitope based on highly specific chicken single-chain antibody variable-region fragments

The presence of the nonhuman galactosyl-α-(1,3)-galactose (Gal-α-(1,3)-Gal) carbohydrate epitope on a number of recombinant therapeutic proteins has recently been reported, renewing interest in this immunogenic carbohydrate epitope. It is well-known that this motif is the primary contributing factor in hyperacute rejection of porcine organ xenograft, due to the existence of natural antibodies against this epitope in human serum. Though the number of epitopes on recombinant glycoproteins may be low when compared directly to whole tissue, circulating anti-Gal-α-R immunoglobulins can still induce anaphylaxis. Therefore, there is a need for rapid and convenient methods for detection and monitoring of this epitope in biopharmaceuticals produced in recombinant mammalian systems. To this end, we have generated immune-challenged chicken single-chain antibody variable-region fragment (scFv) libraries targeting the Gal-α-(1,3)-Gal motif and have selected a panel of scFv's that bind the target. We have used one of these antibodies to develop a competitive ELISA for both free and protein-bound Gal-α-(1,3)-Gal and have demonstrated that the ELISA is specific for the target and can be used to determine the loading of the target on glycoproteins. This competitive ELISA will provide a convenient method of detecting and quantifying Gal-α-(1,3)-Gal on therapeutic glycoproteins.

Antigen-binding specificity of anti-αGal reagents determined by solid-phase glycolipid-binding assays. A complete lack of αGal glycolipid reactivity in α1,3GalT-KO pig small intestine

BACKGROUND

αGal-specific lectins, monoclonal and polyclonal antibodies (Abs) are widely used in xenotransplantation research. Immunological assays such as immunohistochemistry, flow cytometry, Western blot and thin layer chromatography are often the only applicable characterization procedures when limited amount of tissue is available and biochemical characterization is impossible. Hence, detailed knowledge of the Ab/lectin carbohydrate-binding specificity is essential.

METHODS

The binding specificity of human blood group AB serum, three different affinity-purified human polyclonal anti-Gal Ab batches, and two anti-Gal mAb clones (TH5 and 15.101) as well as Griffonia simplicifolia isolectin B4 and Marasmius oreades agglutinin were examined for reactivity with glycolipid fractions isolated from human and pig (wild-type and α1,3GalT-KO) tissues using thin layer chromatogram and microtiter well binding assays.

RESULTS

All anti-Gal-specific reagents reacted with the pentaglycosylceramide Galα1,3nLc4, and several 6-12 sugar compounds in wild-type pig kidneys. However, their staining intensity with different αGal antigens varied considerably. Some, but not all, anti-Gal reagents cross-reacted with a pure iGb3 glycolipid reference compound. No reactivity with glycolipids isolated from α1,3GalT-KO pig small intestine or human tissues was found, confirming the specificity of the anti-Gal reagents in those tissues for α1,3Gal-epitopes produced by the α1,3GalT (GGTA1).

CONCLUSIONS

Different anti-Gal reagents vary in their carbohydrate epitope specificity. Mono-/polyclonal Abs and lectins have different carbohydrate epitope fine specificity toward pig glycolipids as well as purified Galα1,3nLc4, and iGb3. Despite the difference in αGal specificity, all reagents were completely non-reactive with glycolipids isolated from α1,3GalT-KO pig small intestine.

Structural biology of carbohydrate xenoantigens

Transplantation of organs across species (xenotransplantation) is being considered to overcome the shortage of human donor organs. However, unmodified pig organs undergo an antibody-mediated hyperacute rejection that is brought about by the presence of natural antibodies to Galalpha(1,3)Gal, which is the major carbohydrate xenoantigen. Genetic modification of pig organs to remove most of the Galalpha(1,3)Gal epitopes has been achieved, but the human immune system may still recognize residual lipid-linked Galalpha(1,3)Gal carbohydrates, new (cryptic) carbohydrates or additional non-Galalpha(1,3)Gal carbohydrate xenoantigens. The structural basis for lectin and antibody recognition of Galalpha(1,3)Gal carbohydrates is starting to be understood and is discussed in this review. Antibody binding to Galalpha(1,3)Gal carbohydrates is predicted to primarily involve end-on insertion of the terminal alphaGal residue, but it is possible that groove-type binding can occur, as for some lectins. It is likely that similar antibody and lectin recognition will occur with other non-Galalpha(1,3)Gal xenoantigens, which potentially represent new barriers for pig-to-human xenotransplantation.

Decreased specific antibody responses to alpha-Gal-conjugated antigen in animals with preexisting high levels of natural antibodies binding alpha-Gal residues

High levels of natural antibodies (NAb) binding the alpha-Gal residue (Galalpha1-3Galbeta1-4GlcNAc) are found in poultry (and humans), which is probably reflected by high levels of natural agglutinating antibodies (Ab) to rabbit red blood cells (RRBC) in plasma from chickens (and humans). Recently, it was shown that alpha-Gal conjugation of proteins induced higher antiprotein Ab responses in alpha-Gal knockout mice, suggesting immune-enhancing features of preexisting Ab binding carbohydrate-protein conjugates. We challenged chickens s.c. with either alpha-Gal-conjugated human serum albumin (HuSA), beta-Gal-conjugated HuSA, or unconjugated ("native") HuSA, respectively, and measured primary and secondary Ab responses to HuSA, including isotype IgM and IgG responses, and cellular immune responses in vitro (lymphoproliferation) to HuSA or concanavalin A. alpha-Gal conjugation, but not beta-Gal conjugation, of HuSA resulted in significantly decreased primary and secondary Ab responses to HuSA, especially IgG isotype responses, as compared with Ab responses to native HuSA. Lymphoproliferation in vitro was also decreased, although not significantly, in birds challenged with alpha-Gal-conjugated HuSA. High levels of agglutinating Ab levels to RRBC and NAb binding porcine thyroglobulin were detected in all birds, as was true for (natural) Ab levels binding alpha-Gal-conjugated HuSA before immunization, whereas low levels of preexisting (natural) antibodies directed to native HuSA were present in plasma before immunization. Levels of RRBC agglutinins and Ab binding thyroglobulin were not affected by immunization with HuSA, alpha-Gal-conjugated HuSA, or beta-Gal-conjugated HuSA. Our data confirm the presence of high levels of (preexisting) NAb in the plasma of chickens directed to the alpha-Gal residue. The decreased responsiveness to alpha-Gal-bearing antigens in the current study shows that, in addition to immune-enhancing features, NAb may also have suppressive effects on specific immune responses, which substantiates the regulatory role of innate immunity (NAb) in mounting specific immune responses.

The bacterial flora of α-Gal knockout mice express the α-Gal epitope comparable to wild type mice

The human genome possesses pseudogenes for the enzyme alpha1,3 galactosyltransferase and hence, human cells and tissues do not express the Galalpha terminated trisaccharide structure Galalpha1-3Galbeta1-4GlcNAc, the so-called alpha-Gal epitope. Circulating antibodies specific for this carbohydrate epitope are, however, present in high amounts in humans. It has previously been hypothesized that the antibody production is induced by the presence of the alpha-Gal epitope in the cell walls of the enteric flora, especially Enterobacteriaceae spp. However, in mice, in which the epitope has been deleted by targeted mutation of the gal-transferase gene, alpha-Gal antibodies do not appear without prior immunization, although the mice through their growth probably have been exposed to a normal bacterial flora of e.g. Enterobacteriaceae spp. It is unknown whether there are different types of immune reactions to antigenic carbohydrate expressing bacteria and whether there are discrepancies in the enteric flora between these knockout mice and their wild type litter mates. In this study the enteric flora of alpha-Gal knockout and wild type mice was compared both in relation to the prevalence of different types of bacteria in the two groups of mice, as well as in relation to the expression of the epitope on the surface of Enterobacteriaceae spp. Our results showed that the enteric flora did not differ significantly between knockout and wild type mice and that it was comparable to the flora known to be present in the intestines of other mice. All Enterobacteriaceae spp. examined expressed the alpha-Gal epitope no matter whether they were isolated from knockout or wild type mice. It is, therefore, discussed whether it is more reasonable to assume that alpha-Gal antibodies in mammals that do not produce alpha1,3 galactosyltransferase such as in the knock mice and in humans are the result of another antigen stimulant than these common representatives of the enteric flora, that we isolated from the two types of mice. Possible candidates for a carrier in humans could be bacteria or viruses not isolated from barrier-bred mice.

Comparison of the binding properties of the mushroom Marasmius oreades lectin and Griffonia simplicifolia I-B isolectin to alphagalactosyl carbohydrate antigens in the surface phase

The binding of two alpha-galactophilic lectins, Marasmius oreades agglutinin (MOA), and Griffonia simplicifolia I isolectin B(4) (GS I-B(4)) to neoglycoproteins and natural glycoproteins were compared in a surface phase assay. Neoglycoproteins carrying various alpha-galactosylated glycans and laminin from basement membrane of mouse sarcoma that contains the xenogenic Galalpha1-3Gal1-4GlcNAc epitope were immobilized in microtiter plate wells and lectin binding determined with an enzyme-linked assay. After 24 h of incubation, MOA had higher affinity for the xenogenic pentasaccharide (Galalpha1-3Gal1-4GlcNAcbeta1-3Galbeta1-4Glc) than for the Galalpha-monosaccharide. The binding properties of MOA and GS I-B(4) to the xenogenic disaccharide (Galalpha1-3Galbeta1) were comparable while the binding of MOA to the xenogenic pentasaccharide was much stronger than the binding of GS I-B(4) to the same epitope. Non-xenogenic disaccharide-coupled neoglycoproteins having galactose end groups linked alpha1-2 or alpha1-4 to Gal or linked alpha1-3 to GalNAc bound very weakly to MOA, whereas GS I-B(4) recognized all of these disaccharides with similarly high affinity. MOA also showed high affinity for laminin. The results indicate that the Marasmius oreades lectin has nearly the same affinities as does GS I-B(4) for the simple xenogenic carbohydrate antigens, but MOA has greater affinity for the pentasaccharide and is far more specific in its binding preferences than the Griffonia lectin.

Solid phase measurements of antibody and lectin binding to xenogenic carbohydrate antigens

OBJECTIVES

In future pig-to-man xenotransplantation it is important to master tools that identify potentially xenogenic alphagalactose (Galalpha) antigens in the doner tissue.

DESIGN AND METHODS

We have measured the binding potentials of Galalpha detecting lectins and antibodies, including a naturally occurring subfraction from human serum, to Galalpha containing neoglycoproteins and mouse laminin that were immobilized on microtiter plates.

RESULTS

Galalpha reactive antibodies with similar monosaccharide specificity have distinct structural preference for sugar ligands. Laminin and neoglycoproteins were treated with alpha-galactosidase and subsequently incubated with antibodies and lectins. The enzyme treatment was more deleterious on antibody binding than on lectin binding.

CONCLUSION

Antibodies and lectins may bind to different galactose determinants on the glycoproteins. Two anti-Galalpha1 antibodies that both have been raised against glycans on rabbit red blood cells may recognize Galalpha-antigens with varying specificities. Binding results obtained after digestion with alpha-galactosidase indicate that some xenoreactive Galalpha groups are not directly accessible for removal by the enzyme.

The resistance of delayed xenograft rejection to α(1,3)-galactosyltransferase gene inactivation and CD4 depletion in a mouse-to-rat model

Critical to the prevention of xenograft loss is the prevention of delayed xenograft rejection (DXR), due to its resistance to conventional immunosuppression. The role of the carbohydrate galactose-alpha1,3-galactose (alpha1,3Gal) has been a matter of great debate and it has been proposed that the reaction between alpha1,3Gal epitopes on donor endothelial cells and recipient anti-alpha1,3Gal antibodies (Abs) may damage the graft during DXR. Recipient anti-alpha1,3Gal Abs are produced by CD4-dependent B cells. To test the above-mentioned hypothesis, hearts from alpha1,3Gal-free mice (GT-Ko mice), generated by alpha1,3-galacto-syltransferase gene disruption, were transplanted to anti-alpha1,3Gal antibody-free Lew/Mol rats. This model consists of an alpha1,3Gal/alpha1,3Gal-antibody-free environment, eliminating a possible influence of this specific system on DXR. A subgroup of recipients were furthermore CD4 depleted in order to inhibit CD4-dependent B-cell antibody production. Rejected hearts were evaluated by light- and immunofluorescence microscopy. Treatment effects on recipient T-cell subsets and cytokine expression were analyzed by flow cytometry, while antibody production was measured by ELISA. All recipients developed DXR with no differences among the groups. DXR was related to thrombosis with IgG and IgM desposition in vessel walls, as well as macrophage and granulocyte accumulation in the myocardium. No complement C3, CD4 cells or NK cells were found. Flow cytometric analysis confirmed peripheral blood CD4 depletion and IFN-gamma suppression in CD4 Ab-treated recipients. Finally, ELISA showed that specific anti-alpha1,3Gal Ab production was absent. However, Ab(s) against an unidentified Galalpha 1 were found among recipients. In our model, DXR is resistant to alpha1,3-galactosyltransferase gene inactivation and CD4 depletion. However, other Galalpha 1 epitopes and antibodies may play a role during DXR. Further studies are needed to elucidate the precise pathways leading to DXR.