Antibody production and tolerance to the α-gal epitope as models for understanding and preventing the immune response to incompatible ABO carbohydrate antigens and for α-gal therapies

Abstract

This review describes the significance of the α-gal epitope (Galα-3Galβ1-4GlcNAc-R) as the core of human blood-group A and B antigens (A and B antigens), determines in mouse models the principles underlying the immune response to these antigens, and suggests future strategies for the induction of immune tolerance to incompatible A and B antigens in human allografts. Carbohydrate antigens, such as ABO antigens and the α-gal epitope, differ from protein antigens in that they do not interact with T cells, but B cells interacting with them require T-cell help for their activation. The α-gal epitope is the core of both A and B antigens and is the ligand of the natural anti-Gal antibody, which is abundant in all humans. In A and O individuals, anti-Gal clones (called anti-Gal/B) comprise >85% of the so-called anti-B activity and bind to the B antigen in facets that do not include fucose-linked α1–2 to the core α-gal. As many as 1% of B cells are anti-Gal B cells. Activation of quiescent anti-Gal B cells upon exposure to α-gal epitopes on xenografts and some protozoa can increase the titer of anti-Gal by 100-fold. α1,3-Galactosyltransferase knockout (GT-KO) mice lack α-gal epitopes and can produce anti-Gal. These mice simulate human recipients of ABO-incompatible human allografts. Exposure for 2–4 weeks of naïve and memory mouse anti-Gal B cells to α-gal epitopes in the heterotopically grafted wild-type (WT) mouse heart results in the elimination of these cells and immune tolerance to this epitope. Shorter exposures of 7 days of anti-Gal B cells to α-gal epitopes in the WT heart result in the production of accommodating anti-Gal antibodies that bind to α-gal epitopes but do not lyse cells or reject the graft. Tolerance to α-gal epitopes due to the elimination of naïve and memory anti-Gal B cells can be further induced by 2 weeks in vivo exposure to WT lymphocytes or autologous lymphocytes engineered to present α-gal epitopes by transduction of the α1,3-galactosyltransferase gene. These mouse studies suggest that autologous human lymphocytes similarly engineered to present the A or B antigen may induce corresponding tolerance in recipients of ABO-incompatible allografts. The review further summarizes experimental works demonstrating the efficacy of α-gal therapies in amplifying anti-viral and anti-tumor immune-protection and regeneration of injured tissues.

The human and mouse dendritic cell receptor DCIR binds to LRP1 N-glycans containing terminal galactoses, including the α-Gal antigen

“…35 Of note, tandem mass spectrometry (MS/MS) and antibody staining showed that mouse LRP1 N-glycans exhibit the terminal Galα1-3Galβ1-4GlcNAc (α-Gal) epitope (Figures 4B and S6), a unique carbohydrate antigen abundantly expressed on glycoconjugates of most mammals yet absent in humans. 36, 37 To determine which of these N-glycan structures are ligand(s) for hDCIR and mDCIR1, we performed an enzyme-linked lectin assay using neo-glycoproteins generated by covalent coupling of biantennary complex-type N-glycans with either terminal GlcNAc, Gal, α-Gal or α2,6-linked sialic residues to bovine serum albumin (BSA). We found that both hDCIR-ECD and mDCIR1-ECD strongly interact with biantennary complex-type N-glycans with terminal galactose residues, including those carrying the α-Gal epitope, but poorly or not at all with agalactosylated or sialylated N-glycans (Figure 4C).…”

“…alphagal syndrome, and hypersensitivity reactions to cetuximab, a chimeric mouse-human monoclonal antibody used to treat cancers of the bowel and neck. 36,37,[65][66][67] To our knowledge, hDCIR is the first human lectin described as interacting with the a-Gal epitope carried on glycoproteins, suggesting that this lectin may play an important role in a-Gal-mediated diseases. In agreement with this idea, a recent study established that DCIR on the surface of mast cells mediates the recognition and uptake of cockroach allergen, which contains the a-Gal epitope on N-glycans, and enhances cockroach/IgE-induced mast cell activation and atopic dermatitis-like inflammation.…”

A systematic review reveals conflicting evidence for the prevalence of antibodies against the sialic acid ‘xenoautoantigen’ Neu5Gc in humans and the need for a standardised approach to quantification

“…Neu5Gc-containing glycans are thought to be immunogenic in humans, triggering the development of a polyclonal anti-Neu5Gc humoral immune response that accommodates Neu5Gc in the context of an array of underlying glycan structures (Breimer and Holgersson, 2019). This presentation of Neu5Gc on a diverse range of underlying scaffolds differs from other carbohydrate xenoantibodies, such as anti-αGal antibodies, which are directed against a single Galα1-3Galβ1-4GlcNAc-R epitope and are triggered by early-life exposure to these glycans on commensal bacteria (Padler-Karavani and Varki, 2011;Galili, 2023). This relative simplicity may explain why αGal antibodies are reliably reported across the entire human population, whereas the extent of the anti-Neu5Gc antibody response remains controversial (Galili, 2023).…”