Specificity profile of αGal antibodies in αGalT KO mice as probed with comprehensive printed glycan array: Comparison with human anti-Galili antibodies

BACKGROUND

The α1,3-galactosyltransferase gene-knockout (GalT KO) mice are able to produce natural anti-αGal antibodies apparently without any specific immunization. GalT KO mice are commonly used as a model immunological system for studying anti-αGal responses to Gal-positive xenografts in human. In this study, we compared the specificity of mouse and human αGal antibodies to realize the adequacy of the murine model.

METHODS

Using hapten-specific affinity chromatography antibodies against Galα1-3Galβ1-4GlcNAcβ epitope were isolated from both human and GalT KO mice blood sera. Specificity of isolated antibodies was determined using a printed glycan array (PGA) containing 400 mammalian glycans and 200 bacterial polysaccharides.

RESULTS

The quantity of isolated specific anti-Galα antibodies corresponds to a content of <0.2% of total Ig, which is an order of magnitude lower than that generally assumed for both human and murine peripheral blood immunoglobulin, with a high predominance of IgM over IgG (95% vs 5%). Analysis using a printed glycan array has demonstrated that (a) antibodies from both species bind not only the Galα1-3Galβ1-4GlcNAcβ epitope, but also unrelated glycans; (b) particularly, for human (but not mouse) antibodies the best binders appear to be bacterial polysaccharides; (c) the profile of mouse antibodies is broader, it is noteworthy that they recognize a variety of human blood group B epitopes and even glycans without the α-galactosyl residue.

CONCLUSIONS

We believe that the mouse model should be used cautiously in xenotransplantation experiments when the fine epitope specificity of antibodies is critical.

Human Natural Antibodies Recognizing Glycan Galβ1-3GlcNAc (LeC)

The level of human natural antibodies of immunoglobulin M isotype against Le^C in patients with breast cancer is lower than in healthy women. The epitope specificity of these antibodies has been characterized using a printed glycan array and enzyme-linked immunosorbent assay (ELISA), the antibodies being isolated from donors’ blood using Le^C-Sepharose (Le^C is Galβ1-3GlcNAcβ). The isolated antibodies recognize the disaccharide but do not bind to glycans terminated with Le^C, which implies the impossibility of binding to regular glycoproteins of non-malignant cells. The avidity (as dissociation constant value) of antibodies probed with a multivalent disaccharide is 10⁻⁹ M; the nanomolar level indicates that the concentration is sufficient for physiological binding to the cognate antigen. Testing of several breast cancer cell lines showed the strongest binding to ZR 75-1. Interestingly, only 7% of the cells were positive in a monolayer with a low density, increasing up to 96% at highest density. The enhanced interaction (instead of the expected inhibition) of antibodies with ZR 75-1 cells in the presence of Galβ1-3GlcNAcβ disaccharide, indicates that the target epitope of anti-Le^C antibodies is a molecular pattern with a carbohydrate constituent rather than a glycan.

The Formation of Glycan-Specific Natural Antibodies Repertoire in GalT-KO Mice Is Determined by Gut Microbiota

Gut commensal bacteria are known to have a significant role in regulating the innate and adaptive immune homeostasis. Alterations in the intestinal microbial composition have been associated with several disease states, including autoimmune and inflammatory conditions. However, it is not entirely clear how commensal gut microbiota modulate and contribute to the systemic immunity, and whether circulating elements of the host immune system could regulate the microbiome. Thus, we have studied the diversity and abundance of specific taxons in the gut microbiota of inbred GalT-KO mice during 7 months of animal life by metagenetic high-throughput sequencing (16S rRNA gene, variable regions V3-V5). The repertoire of glycan-specific natural antibodies, obtained by printed glycan array technology, was then associated with the microbial diversity for each animal by metagenome-wide association studies (MWAS). Our data show that the orders clostridiales (most abundant), bacteriodales, lactobacillales, and deferribacterales may be associated with the development of the final repertoire of natural anti-glycan antibodies in GalT-KO mice. The main changes in microbiota diversity (month-2 and month-3) were related to important changes in levels and repertoire of natural anti-glycan antibodies in these mice. Additionally, significant positive and negative associations were found between the gut microbiota and the pattern of specific anti-glycan antibodies. Regarding individual features, the gut microbiota and the corresponding repertoire of natural anti-glycan antibodies showed differences among the examined animals. We also found redundancy in different taxa associated with the development of specific anti-glycan antibodies. Differences in microbial diversity did not, therefore, necessarily influence the overall functional output of the gut microbiome of GalT-KO mice. In summary, the repertoire of natural anti-carbohydrate antibodies may be partially determined by the continuous antigenic stimulation produced by the gut bacterial population of each GalT-KO mouse. Small differences in gut microbiota diversity could determine different repertoire and levels of natural anti-glycan antibodies and consequently might induce different immune responses to pathogens or other potential threats.

Improved spot morphology for printed glycan arrays

Despite considerable success studying glycan-binding proteins using printed glycan arrays (PGAs), unambiguous quantitation of spot intensities by fluorescent readers remains a challenge. The main obstacles are the varying spot shape and size and in-spot fluorescence distribution caused by uneven drying of the printed drops. Two methods have been suggested for solving this problem: using polymeric glycoconjugates, which makes it possible to equalize the physicochemical properties (hydrophobicity, charge, and size) of different glycans, and applying a glycan solution on a slide coated with a thin oil mask, which hinders evaporation of the drop. Both approaches yield spots with similar sizes and an even distribution of the signal across the spot and are likely to be useful for improving the prints of other classes of molecules.