Immune thrombocytopenia purpura (ITP) is an autoimmune disorder characterized by a reduction in circulating platelet levels, primarily due to generation of autoantibodies to platelet surface antigens followed by their spleen macrophage-mediated clearance. Emerging evidence implicates neuraminidase (sialidase) enzymes including neuraminidase 1 (NEU1) in platelet clearance and ITP severity; however, the underlying cellular mechanisms remain unknown. Using tissue-specific NEU1 knockout mouse models, we studied the contribution of platelet and macrophage NEU1 to ITP pathogenesis and evaluated whether pharmacological inhibition of NEU1 could preserve platelet counts in a murine ITP model. Constitutive and macrophage-specific, but not platelet-specific, NEU1 knockout mice showed a protection against reduction of platelet counts in the passive ITP model suggesting that macrophage, but not platelet, NEU1 promotes platelet clearance. Genetic deletion or pharmacological blockade of macrophage NEU1 also reduced platelet phagocytosis by cultured macrophages in vitro. The selective NEU1 inhibitor CG33301 protected mice against anti-CD41a antibody-induced thrombocytopenia and showed a higher efficacy compared to pan neuraminidase inhibitor oseltamivir phosphate. Our results demonstrate that the macrophage pool of NEU1 plays a central role in platelet clearance by splenocytes during ITP by activating their phagocytosis and suggest that selective NEU1 inhibition may be a promising therapeutic strategy for this disease.

Abstract Sialic acid (Neu5Ac) is a monosaccharide terminating glycoconjugates, serving as ligands for lectins in health and disease. Tools to target sialyltransferases (STs) and neuraminidases (NEUs) are important in the study of sialoglycans. 3FaxNeu5Ac is a common ST inhibitor, while 2,3diFNeu5Ac is a covalent mechanism-based NEU inactivator. Here, we demonstrate the advantages of having two fluorines at C3 for both classes of inhibitors. Motivated by the finding that 3FaxNeu5Ac is transferred by STs slowly, we aimed to synthesize 3,3-difluoro-Neu5Ac (3,3diFNeu5Ac). Two fluorines at C3 successfully prevented ST-mediated transfer. Unexpectedly, CMP-3,3diFNeu5Ac is 5–40-fold more potent than CMP-3FaxNeu5Ac against five human STs. Moreover, CMP-3,3diFNeu5Ac was 6–11-fold more potent toward ST3GAL1 than the four STs tested. While protected 3,3diFNeu5Ac was not very active in cells, CMP-3,3diFNeu5Ac was surprisingly active, showing enhanced inhibition toward ST3GAL1 over ST6GAL1, translating to selective inhibition of Siglec-7 ligands over Siglec-2 ligands. To target NEUs, 2,3,3-trifluoro-Neu5Ac (2,3,3triFNeu5Ac) was synthesized as a mechanism-based covalent NEU inhibitor. For 2,3diFNeu5Ac, a covalent glycosyl-enzyme intermediate was formed that broke down, but covalent inhibition of bacterial and viral NEUs by 2,3,3triFNeu5Ac was more stable. Therefore, through synthetic access to 3,3diFNeu5Ac, more potent and stable inhibitors of STs and NEUs were developed, which can serve as better tools to probe the functions of sialoglycans.

The synthesis of neoglycoconjugates has paved the way for the discovery of novel probes that mimic natural glycoconjugates and can provide designed research tools and therapeutics. In some cases, the target protein may not be amenable to harsh conditions; therefore, semisynthetic or chemical methods must be chosen with care. Here, we present a simple and modular chemoselective coupling strategy between an unprotected sugar and an N,O-disubstituted hydroxylamine under mild acidic conditions. This strategy removes any need for protecting groups on the glycan. The terminal alkene group of the conjugate serves as an effective handle to allow facile conjugation to the protein of interest via thiol-ene coupling (TEC), with proteins bearing a cysteine or free thiol to prepare neoglycoconjugates. We demonstrate that the strategy is compatible with both N- and O-linked glycans using protecting-group free strategies and optimize the TEC conditions using a variety of photocatalysts. Finally, we test the method on an aggregation-prone protein, α-synuclein. We envision that this strategy could allow the construction of complex glycoconjugates for biological testing using isolated glycans, or for generation of conjugates where the protein of interest is sensitive to harsh conditions.

Kidney transplantation from ABO-A2 donors into ABO-O and ABO-B recipients can alleviate inequitable transplant access created by ABO demographics. ABO-A2-incompatible (ABO-A2i) eligibility is determined by anti-A hemagglutination titers. However, titers do not distinguish antibodies specific for A-II glycans, the sole A-antigen subtype in vascular endothelium, from other anti-A antibodies. We examined whether reliance on anti-A titers unnecessarily limited ABO-A2i transplants for candidates with low anti-A-II levels. We created a single-antigen bead immunoassay for ABO antibodies, confirmed the specificity and reproducibility, and demonstrated the ability to detect anti-A and anti-B glycan subtype-specific antibodies in healthy control sera. We then measured subtype-specific anti-A antibodies in original sera from ABO-B and ABO-O candidates who had been previously evaluated for ABO-A2i eligibility. Anti-A-II levels in candidates who had been deemed ineligible (anti-A titers >4) were compared to eligible candidates (anti-A titers ≤4) who had subsequently received ABO-A2i kidneys. Of 141 candidates, 75 (53%) were ineligible; 66 (47%) were eligible and received ABO-A2 kidneys. Retesting original sera, 55% (41/75) of ineligible candidates had anti-A-II levels comparable to eligible candidates. Anti-A titers did not reflect anti-A-II levels. Our ABO antibody assay reproducibly measures graft-specific anti-A-II antibodies, providing information beyond anti-A titers that may increase transplant access for ABO-B and ABO-O candidates.

There is increasing interest in carbohydrate analogs for drug development, and the polar nature of these targets presents a challenge for medicinal chemistry. Multiple substrate hydroxy groups are typically required for enzyme active site recognition. Not all of these polar groups will have the same importance in recognition. A common strategy is to replace or remove these groups and compare the activity of the resulting analogs. If hydroxy groups are non-essential, or if their removal results in increased potency they may form the basis of improved inhibitors or substrates. In our studies of human neuraminidase enzymes (NEU), we have identified modifications at the C5 and C9 positions of the 2-deoxy-2,3-didehydro-N-acetyl neuraminic (DANA) scaffold that provide potent and selective inhibitors. In this study, we sought to test the requirements of each of the four human NEU isoenzymes for the presence of O4, O7, O8, or O9 hydroxy groups found in DANA. We synthesized the corresponding mono- (4, 7, 8, and 9) and di-deoxy (7,9; 7,8; and 8,9) analogs of DANA and tested their potency against human NEU. We found that 8-deoxy compounds increased potency against NEU2 and NEU3. Additionally, several di-deoxy analogs were tolerated by NEU1, NEU2, and NEU3. Finally, we generated known selective inhibitors of NEU3 and NEU4 and tested their 8-deoxy analogs. Combination of these features did not improve overall potency, suggesting deoxygenated analogs will require additional optimization.

Mucopolysaccharidoses (MPS) are lysosomal storage diseases caused by defects in catabolism of glycosaminoglycans. MPS I, II, III, and VII, which are associated with lysosomal accumulation of heparan sulphate (HS), manifest with neurological deterioration and currently lack effective treatments. We report that neuraminidase 1 (NEU1) activity is drastically reduced in brain tissues of patients with neurological MPS and mouse models but not in neurological lysosomal disorders without HS storage. Accumulated HS disrupts the lysosomal multienzyme complex of NEU1 with cathepsin A, β-galactosidase (GLB1), and glucosamine-6-sulfate sulfatase (GALNS), leading to NEU1 deficiency and partial GLB1 and GALNS deficiencies in cortical tissues and induced pluripotent stem cell-derived (iPSC-derived) cortical neurons of patients with neurological MPS. Increased sialylation of N-linked glycans in brains of patients with MPS and mice implicated insufficient processing of sialylated glycans, except for polysialic acid. Correction of NEU1 activity in MPS IIIC mice by lentiviral (LV) gene transfer ameliorated previously identified hallmarks of the disease, including memory impairment, behavioral traits, and reduced levels of excitatory synapse markers VGLUT1 and PSD95. Overexpression of NEU1 also restored levels of VGLUT1/PSD95-positive puncta in cortical iPSC-derived MPS IIIA neurons. Our results demonstrate that HS-induced secondary NEU1 deficiency and aberrant sialylation of brain glycoproteins constitute what we believe is a novel pathological pathway in the neurological MPS spectrum crucially contributing to CNS pathology.

Neuraminidase enzymes (NEU) play a crucial role in many physiological and pathological conditions. Humans have four isoenzymes of NEU, and their specific roles continue to be investigated. Isoenzyme-selective inhibitors are needed as research tools and may lead to future therapeutics. A series of new candidate inhibitors are tested by replacing the C5-amide of 2-deoxy-2,3-dididehydro-N-acetyl neuraminic acid with amide bioisosteres. Design of candidate inhibitors is accomplished using substituents that are components of previously identified NEU inhibitors combined with alternative amide bioisosteres. Compounds are tested for inhibition of the four human NEU, and inhibitory activities are compared to reference amide compounds. 1,4-Disubstituted-1,2,3-triazole is the best bioisostere observed for inhibitors of NEU1. Inhibitor 542 shows high potency for NEU1 (K-i=0.4 +/- 0.1 mu M) and give significant improvement in selectivity compared to the reference amide compound 502. Additionally, compound 542 has improved lipophilic characteristics, which could provide improved pharmacokinetic properties. Screening of these inhibitors also identify a selective NEU2 inhibitor 543 (K-i=2.6 +/- 0.6 mu M), illustrating that amide bioisostere replacement can identify improved inhibitors for multiple NEU isoenzymes.

Human neuraminidases play critical roles in many physiological and pathological processes. These enzymes catalyze the hydrolysis of terminal sialic acids (also known as neuraminic acids) from glycan chains. Humans have four isoenzymes of NEU: NEU1, NEU2, NEU3, and NEU4, making selective inhibitors important tools to investigate the function of individual isoenzymes. A typical scaffold used for the development of NEU inhibitors is 2-deoxy-2,3-dehydro-N-acetylneuraminic acid (DANA), which is a general inhibitor of viral, bacterial, and human neuraminidases. Modifications at the C9 position of DANA have been critical for the potency and selectivity of inhibitors that target human NEU. For example, C9-amides bearing alkyl substituents have been used to develop inhibitors with preference for NEU1, while C9-triazoles with aromatic substituents have higher potency for NEU3 and NEU4. To design improved DANA analogs, we generated a library of compounds with either a short alkyl chain or a biphenyl substituent linked to the C9 position through one of six amide bioisosteres. Bioisostere linkers included triazole, urea, thiourea, carbamate, thiocarbamate, and sulfonamide groups. Within this library, we identified a C9-biphenyl carbamate derivative (963) that showed high potency for NEU3 (K¬i = 0.12 ± 0.01 μM). Additionally, the carbamate analog had enhanced selectivity for NEU3 over all other human isoenzymes. In contrast, NEU1 and NEU4 isoenzymes preferred amide and triazole linkers, respectively. Finally, analogs with urea, sulfonamide, and amide linkers showed enhanced inhibitory activity for a bacterial NEU, NanI from Clostridium perfringens.

Naturally occurring glycans are often found in a multivalent presentation. Cell surface receptors that recognize these displays may form clusters, which can lead to signalling or endocytosis. One of the challenges in generating synthetic displays of multivalent carbohydrates is providing high valency as well as access to heterofunctional conjugates to allow attachment of multiple antigens or payloads. We designed a strategy based on a set of bifunctional linkers to generate a heterobifunctional multivalent display of two carbohydrate antigens to bind BCR and CD22 with four and twelve antigen copies, respectively. We confirmed that the conjugates were able to engage both CD22 and BCR on cells by observing receptor clustering. The strategy is modular and would allow for alternative carbohydrate antigens to be attached bearing amine and alkyne groups and should be of interest for the development of immunomodulators and vaccines.

Glycoconjugates are a vast class of biomolecules implicated in biological processes important for human health and disease. The structural complexity of glycoconjugates remains a challenge to deciphering their precise biological roles and for their development as biomarkers and therapeutics. Human glycoconjugates on the outside of the cell are modified with sialic (neuraminic) acid residues at their termini. The enzymes that install sialic acids are sialyltransferases (SiaTs), a family of 20 different isoenzymes. The removal and degradation of sialic acids is mediated by neuraminidase (NEU; sialidase) enzymes, of which there are four isoenzymes. In this review, we discuss chemical and biochemical approaches for the detection and analysis of sialoglycoconjugate (SGC) structures and their enzymatic products. The most common methods include affinity probes and synthetic substrates. Fluorogenic and radiolabelled substrates are also important tools for many applications, including screening for enzyme inhibitors. Strategies that give insight into the native substrate-specificity of enzymes that regulate SGCs (SiaT & NEU) are necessary to improve our understanding of the role of sialic acid metabolism in health and disease.

Neuraminic acid (Neu5Ac, also known as sialic acid) is an important monosaccharide found in glycoproteins and glycolipids which plays a vital role in regulation of physiological functions and pathological conditions. The study of sialoglycans has benefitted from the development of glycomimetic probes and inhibitors for proteins and enzymes that interact with and modify neuraminic acid in glycan chains. Methods to access sialoside intermediates with high yield are needed to facilitate the design of new targets. Here, we report the synthesis of C5-azido thiosialosides using a mild method to deprotect the C5-acetamido functional group followed by the use of a diazo transfer reagent. We examined two diazo transfer strategies and compared their yields and tolerance of acetate protecting groups. The same methods and comparisons were also performed for the 2,3-didehydro-5-N-acetylneuraminic acid (DANA) scaffold which is commonly used to generate inhibitors of neuraminidase (sialidase) enzymes. We found that C5-azido derivatives of both thiosialosides and DANA could be produced in five or six steps with yields up to 76% and 83%, respectively. Diazo-transfer reagents compared in this study were trifluoromethanesulfonyl azide (TfN3) and imidazole-1-sulfonyl azide (ImzSO2N3). We found that both reagents were compatible with this method and showed comparable yields. Finally, we show that C5-azido derivatives can help to avoid O, N-acyl protecting group migration which was observed in C5-NHAc analogs.

Sialidosis is an ultra-rare multisystemic lysosomal disease caused by mutations in the neuraminidase 1 (NEU1) gene. The severe type II form of the disease manifests with a prenatal/infantile or juvenile onset, bone abnormalities, severe neuropathology, and visceromegaly. A subset of these patients present with nephrosialidosis, characterized by abrupt onset of fulminant glomerular nephropathy. We studied the pathophysiological mechanism of the disease in 2 NEU1-deficient mouse models, a constitutive Neu1-knockout, Neu1ΔEx3, and a conditional phagocyte-specific knockout, Neu1Cx3cr1ΔEx3. Mice of both strains exhibited terminal urinary retention and severe kidney damage with elevated urinary albumin levels, loss of nephrons, renal fibrosis, presence of storage vacuoles, and dysmorphic mitochondria in the intraglomerular and tubular cells. Glycoprotein sialylation in glomeruli, proximal distal tubules, and distal tubules was drastically increased, including that of an endocytic reabsorption receptor megalin. The pool of megalin bearing O-linked glycans with terminal galactose residues, essential for protein targeting and activity, was reduced to below detection levels. Megalin levels were severely reduced, and the protein was directed to lysosomes instead of the apical membrane. Together, our results demonstrated that desialylation by NEU1 plays a crucial role in processing and cellular trafficking of megalin and that NEU1 deficiency in sialidosis impairs megalin-mediated protein reabsorption.

Purpose of reviewThe platelet surface harbors a lush forest of glycans (carbohydrate polymers) attached to membrane proteins and lipids. Accumulating evidence suggests that these glycans may be relevant to the pathophysiology of immune thrombocytopenia (ITP). Here, we critically evaluate data that point to a possible role for loss of sialic acid in driving platelet clearance in ITP, comment on the potential use of neuraminidase inhibitors for treatment of ITP, and highlight open questions in this area.Recent findingsMultiple lines of evidence suggest a role for loss of platelet sialic acid in the pathophysiology of thrombocytopenia. Recent work has tested the hypothesis that neuraminidase-mediated cleavage of platelet sialic acid may trigger clearance of platelets in ITP. Some clinical evidence supports efficacy of the viral neuraminidase inhibitor oseltamivir in ITP, which is surprising given its lack of activity against human neuraminidases.SummaryFurther study of platelet glycobiology in ITP is necessary to fill key knowledge gaps. A deeper understanding of the roles of platelet glycans in ITP pathophysiology will help to guide development of novel therapies.

The processes of activation, extravasation, and migration of immune cells to a site are early and essential steps in the induction of an acute inflammatory response. These events are part of the inflammatory cascade which involves multiple regulatory steps. Using a murine air-pouch model of inflammation with LPS as an inflammation inducer we demonstrate that isoenzymes of the neuraminidase family (NEU1, 3, and 4) play essential roles in this process acting as positive or negative regulators of leukocyte infiltration. Genetically knocked-out (KO) mice for different NEU genes (Neu1 KO, Neu3 KO, Neu4 KO, and Neu3/4 double KO mice) were induced with LPS, leukocytes at the site of inflammation were counted, and the inflamed tissue was analyzed using immunohistochemistry. Our data show that leukocyte recruitment was decreased in NEU1 and NEU3-deficient mice, while it was increased in NEU4-deficient animals. Consistent with these results, systemic levels of pro-inflammatory cytokines and those in pouch exudate were reduced in Neu1 and increased in Neu4 KO mice. We found that pharmacological inhibitors specific for NEU1, NEU3, and NEU4 isoforms also affected leukocyte recruitment. We conclude that NEU isoenzymes have distinct – and even opposing – effects on leukocyte recruitment, and therefore warrant further investigation to determine their mechanisms and importance as regulators of the inflammatory cascade.

Synthetic glycoconjugates are used in the development of vaccines and the design of inhibitors for glycan-protein interactions. The in vivo persistence of synthetic glycoconjugates is an important factor in their efficacy, especially when prolonged interactions with specific cell types may be required. In this study, we applied a strategy for non-covalent association of an active compound with serum proteins for extension of glycoconjugate half-life in serum. The small molecule, AG10, has previously been used to extend the half-life of small molecules through its high affinity for transthyretin (TTR), a serum protein. Using a tetravalent polyethylene glycol (PEG)-based scaffold we developed a synthetic strategy for glycoconjugates that allowed for controlled addition of multiple tags, such as a TTR affinity tag or fluorophore. We designed a version of AG10 modified at the pyrazole core, named GD10, amenable to our conjugation strategy and introduced to glycoconjugates using a tri-functional linker. This approach allowed for attachment of GD10 and fluorophore tags, as well as carbohydrate antigens. We then tested the influence of the GD10 tag on glycoconjugate half-life in vivo using a mouse model. Our results suggest that the combination of the GD10 tag and the PEG scaffold extended the half-life of glycoconjugates by as much as 10-fold when compared to proteins of similar molecular weight. The GD10 tag was able to extend the half-life of similar glycoconjugates by as much as 2-fold. We observed a role for the terminal saccharide residue of the carbohydrate antigen and confirmed that conjugates were able to penetrate multiple compartments in vivo including bone marrow, lymph nodes, and other organs. The introduction of the GD10 tag did not obstruct the ability of conjugates to interact with lectin receptors. We conclude that serum protein binders can be used to extend the persistence of glycoconjugates in vivo.

Cell migration to a site of inflammation is an important step of the immune response. This process is coordinated by cytokines, receptors, and the signal processing machinery of the cell. Many cellular receptors are glycosylated, and their activity can be modulated through changes in glycan structure. Furthermore, glycosylation can be critical to the folding and trafficking of receptors. In this work, we investigated the role of native human neuraminidase enzymes (NEU) in transmigration. We used a cultured T cell line (Jurkat) and a transwell assay with fibronectin (FN) coated wells and cytokines (IL-4 and TNF-α) as chemoattractants in the bottom chamber. We observed that NEU1, NEU3, and NEU4 were positive regulators of transmigration using an siRNA knockdown. Furthermore, we found that pharmacological inhibition of these enzymes inhibited transmigration. We conclude that human NEU isoenzymes NEU1, NEU3, and NEU4 can act as positive regulators of transmigration and should be investigated as targets for anti-inflammatory strategies.

The B cell membrane expresses sialic-acid-binding immunoglobulin-like lectins, also called Siglecs, that are important for modulating immune response. Siglecs have interactions with sialoglycoproteins found on the same membrane (cis-ligands) that result in homotypic and heterotypic receptor clusters. The regulation and organization of these clusters, and their effect on cell activation, is not clearly understood. We investigated the role of human neuraminidase enzymes NEU1 and NEU3 on the clustering of CD22 on B cells using confocal microscopy. We observed that native NEU1 and NEU3 activity influence the cluster size of CD22. Using single-particle tracking, we observed that NEU3 activity increased the lateral mobility of CD22, which was in contrast to the effect of exogenous bacterial NEU enzymes. Moreover, we show that native NEU1 and NEU3 activity influenced cellular Ca2+ levels, supporting a role for these enzymes in regulating B cell activation. Our results establish a role for native NEU activity in modulating CD22 organization and function on B cells.

Background Chronic vascular disease atherosclerosis starts with an uptake of atherogenic modified low‐density lipoproteins (LDLs) by resident macrophages, resulting in formation of arterial fatty streaks and eventually atheromatous plaques. Increased plasma sialic acid levels, increased neuraminidase activity, and reduced sialic acid LDL content have been previously associated with atherosclerosis and coronary artery disease in human patients, but the mechanism underlying this association has not been explored. Methods and Results We tested the hypothesis that neuraminidases contribute to development of atherosclerosis by removing sialic acid residues from glycan chains of the LDL glycoprotein and glycolipids. Atherosclerosis progression was investigated in apolipoprotein E and LDL receptor knockout mice with genetic deficiency of neuraminidases 1, 3, and 4 or those treated with specific neuraminidase inhibitors. We show that desialylation of the LDL glycoprotein, apolipoprotein B 100, by human neuraminidases 1 and 3 increases the uptake of human LDL by human cultured macrophages and by macrophages in aortic root lesions in Apoe −/− mice via asialoglycoprotein receptor 1. Genetic inactivation or pharmacological inhibition of neuraminidases 1 and 3 significantly delays formation of fatty streaks in the aortic root without affecting the plasma cholesterol and LDL levels in Apoe −/− and Ldlr −/− mouse models of atherosclerosis. Conclusions Together, our results suggest that neuraminidases 1 and 3 trigger the initial phase of atherosclerosis and formation of aortic fatty streaks by desialylating LDL and increasing their uptake by resident macrophages.

Regulationof sialic acids by human neuraminidase (hNEU) enzymes is important to many biologicalprocesses. Defining hNEU substrate tolerance can help to elucidate the roles ofthese enzymes in regulating sialosides in human health and disease. Polysialicacid (polySia) is a polyanion of α(2→8) linked sialic acids with roles in nervous,reproductive, and immune systems and is dysregulated in some malignancies andmental disorders. The unique chemical properties of this polymer, which includean enhanced susceptibility to acid-catalyzed hydrolysis, have hampered itsstudy. Herein we describe the first systematic study of hNEU isoenzymeactivity towards polysialic acid in vitro. The experimental design allowedus to study the impact of several factors that may influence polysialic aciddegradation including pH, polymer size, and the relative ionic strength of thesurrounding media. We report that short chains of polysialic acid (degree ofpolymerization, DP 3-8) were substrates of NEU3 and NEU4 at acidic pH, but notat neutral pH. No hNEU-catalyzed hydrolysis of longer polymers (DP 10-20) wasdetected. These findings suggest a neuraminidase-independent mechanism forpolysialic acid turnover such as internalization and degradation in endosomesand lysosomes.

ABO-incompatible (ABOi) transplantation requires preemptive antibody reduction; however, the relationship between antibody-mediated rejection (AMR) and ABO-antibodies, quantified by hemagglutination (HA), is inconsistent, possibly reflecting variable graft resistance to AMR or HA assay limitations. Using an ABH-glycan microarray, we quantified ABO-A antigen-subtype (A-subtype)-specific IgM and IgG in 53 ABO-O recipients of ABO-A kidneys, before and after antibody removal (therapeutic plasma exchange [TPE] or ABO-A-trisaccharide immunoadsorption [IA]) and 1-year posttransplant. IgM binding to all A-subtypes correlated highly (R2  ≥ .90) and A-subtype antibody specificities was reduced equally by IA versus TPE. IgG binding to the A-subtypes (II-IV) expressed in kidney correlated poorly (.27 ≤ R2  ≤ .69). Reduction of IgG specific to A-subtype-II was equivalent for IA and TPE, whereas IgG specific to A-subtypes-III/IV was not as greatly reduced by IA (p < .005). One-year posttransplant, IgG specific to A-II remained the most reduced antibody. Immunostaining revealed only A-II on vascular endothelium but A-subtypes II-III/IV on tubular epithelium. These results show that ABO-A-trisaccharide is sufficient for IgM binding to all A-subtypes; this is true for IgG binding to A-II, but not subtypes-III/IV, which exhibits varying degrees of specificity. We identify A-II as the major, but importantly not the sole, antigen relevant to treatment and immune modulation in adult ABO-A-incompatible kidney transplantation.