Immunoglobulin G subclasses confer protection against Staphylococcus aureus bloodstream dissemination through distinct mechanisms in mouse models.

Antibodies bind target molecules with exquisite specificity. The removal of these targets is mediated by the effector functions of antibodies. We reported earlier that the monoclonal antibody (mAb) 3F6 promotes opsonophagocytic killing of Staphylococcus aureus in blood and reduces bacterial replication in animals. Here, we generated mouse immunoglobulin G (mIgG) subclass variants and observed a hierarchy in protective efficacy 3F6-mIgG2a > 3F6-mIgG1 ≥ 3F6-mIgG2b >> 3F6-mIgG3 following bloodstream challenge of C57BL/6J mice. This hierarchy was not observed in BALB/cJ mice: All IgG subclasses conferred similar protection. IgG subclasses differ in their ability to activate complement and interact with Fcγ receptors (FcγR) on immune cells. 3F6-mIgG2a-dependent protection was lost in FcγR-deficient, but not in complement-deficient C57BL/6J animals. Measurements of the relative ratio of FcγRIV over complement receptor 3 (CR3) on neutrophils suggest the preferential expression of FcγRIV in C57BL/6 mice and of CR3 in BALB/cJ mice. To determine the physiological significance of these differing ratios, blocking antibodies against FcγRIV or CR3 were administered to animals before challenge. Correlating with the relative abundance of each receptor, 3F6-mIgG2a-dependent protection in C57BL/6J mice showed a greater reliance for FcγRIV while protection in BALB/cJ mice was only impaired upon neutralization of CR3. Thus, 3F6-based clearance of S. aureus in mice relies on a strain-specific contribution of variable FcγR- and complement-dependent pathways. We surmise that these variabilities are the result of genetic polymorphism(s) that may be encountered in other mammals including humans and may have clinical implications in predicting the efficacy of mAb-based therapies.

Catanionic Vesicles as a Facile Scaffold to Display Natural N-Glycan Ligands for Probing Multivalent Carbohydrate-Lectin Interactions.

Multivalent interactions are a key characteristic of protein-carbohydrate recognition. Phospholipid-based liposomes have been explored as a popular platform for multivalent presentation of glycans, but this platform has been plagued by the instability of typical liposomal formulations in biological media. We report here the exploitation of catanionic vesicles as a stable lipid-based nanoparticle scaffold for displaying large natural N-glycans as multivalent ligands. Hydrophobic insertion of lipidated N-glycans into the catanionic vesicle bilayer was optimized to allow for high-density display of structurally diverse N-glycans on the outer membrane leaflet. In an enzyme-linked competitive lectin-binding assay, the N-glycan-coated vesicles demonstrated a clear clustering glycoside effect, with significantly enhanced affinity for the corresponding lectins including Sambucus nigra agglutinin (SNA), concanavalin A (ConA), and human galectin-3, in comparison with their respective natural N-glycan ligands. Our results showed that relatively low density of high-mannose and sialylated complex type N-glycans gave the maximal clustering effect for binding to ConA and SNA, respectively, while relatively high-density display of the asialylated complex type N-glycan provided maximal clustering effects for binding to human galectin 3. Moreover, we also observed a macromolecular crowding effect on the binding of ConA to high-mannose N-glycans when catanionic vesicles bearing mixed high-mannose and complex-type N-glycans were used. The N-glycan-coated catanionic vesicles are stable and easy to formulate with varied density of ligands, which could serve as a feasible vehicle for drug delivery and as potent inhibitors for intervening protein-carbohydrate interactions implicated in disease.

Synthesis and Evaluation of Three Azide-Modified Disaccharide Oxazolines as Enzyme Substrates for Single-Step Fc Glycan-Mediated Antibody-Drug Conjugation.

Antibody-drug conjugates (ADCs) hold great promise for targeted cancer cell killing. Site-specific antibody-drug conjugation is highly desirable for synthesizing homogeneous ADCs with optimal safety profiles and high efficacy. We have recently reported that azide-functionalized disaccharide oxazolines of the Manß1,4GlcNAc core were an efficient substrate of wild-type endoglycosidase Endo-S2 for Fc glycan remodeling and conjugation. In this paper, we report the synthesis and evaluation of new disaccharide oxazolines as enzyme substrates for examining the scope of the site-specific conjugation. Thus, azide-functionalized disaccharide oxazolines derived from Manß1,4GlcNAc, Glcß1,4GlcNAc, and Galß1,4GlcNAc (LacNAc) were synthesized. Enzymatic evaluation revealed that wild-type Endo-S2 demonstrated highly relaxed substrate specificity and could accommodate all the three types of disaccharide derivatives for transglycosylation to provide site-specific azide-tagged antibodies, which were readily clicked with a payload to generate homogeneous ADCs. Moreover, we also found that Endo-S2 was able to accommodate drug-preloaded minimal disaccharide oxazolines as donor substrates for efficient glycan transfer, enabling a single-step and site-specific antibody-drug conjugation without the need of an antibody click reaction. The ability of Endo-S2 to accommodate simpler and more easily synthesized disaccharide oxazoline derivatives for Fc glycan remodeling further expanded the scope of this bioconjugation method for constructing homogeneous antibody-drug conjugates in a single-step manner. Finally, cell-based assays indicated that the synthetic homogeneous ADCs demonstrated potent targeted cancer cell killing.

Synthesis and Immunological Study of N-Glycan-Bacteriophage Qß Conjugates Reveal Dominant Antibody Responses to the Conserved Chitobiose Core.

N-Glycosylation plays an important role in many biological recognition processes. However, very few N-glycan-specific antibodies are available for functional studies and potentially for therapeutic development. In this study, we sought to synthesize bacteriophage Qß conjugates with representative N-glycans and investigate their immunogenicity for raising N-glycan-specific antibodies. An array of Qß glycoconjugates bearing five different human N-glycans and two different chemical linkers were synthesized, and the immunization of the N-glycan-Qß conjugates was performed in mice. We found that the N-glycan-Qß conjugates raised significant IgG antibodies that recognize N-glycans, but, surprisingly, most of the glycan-dependent antibodies were directed to the shared chitobiose core and were nonspecific for respective N-glycan structures. The linker chemistry was found to affect antibody specificity with adipic acid-linked N-glycan-Qß immunogens raising antibodies capable of recognizing both the N-acetylglucosamine (GlcNAc) moieties of the chitobiose core. In contrast, antibodies raised by N-glycan-Qß immunogens with a triazole linker preferentially recognized the innermost N-acetylglucosamine moiety at the reducing end. We also found that sialylation of the N-glycans significantly suppressed the immune response. Furthermore, the N-glycan-Qß immunogens with an adipic acid linker elicited higher glycan-specific antibody titers than the N-glycan-triazole-Qß immunogens. These findings delineate several challenges in eliciting mammalian N-glycan-specific antibodies through the conventional glycoconjugate vaccine design and immunization.

Synthetic Antibody-Rhamnose Cluster Conjugates Show Potent Complement-Dependent Cell Killing by Recruiting Natural Antibodies.

Monoclonal antibodies (mAbs) are one of the most rapidly growing drug classes used for the treatment of cancer, infectious and autoimmune diseases. Complement-dependent cytotoxicity (CDC) is one of the effector functions for antibodies to deplete target cells. We report here an efficient chemoenzymatic synthesis of structurally well-defined conjugates of a monoclonal antibody with a rhamnose- and an αGal trisaccharide-cluster to recruit natural anti-rhamnose and anti-αGal antibodies, respectively, to enhance the CDC-dependent targeted cell killing. The synthesis was achieved by using a modular antibody Fc-glycan remodeling method that includes site-specific chemoenzymatic Fc-glycan functionalization and subsequent click conjugation of synthetic rhamnose- and αGal trisaccharide-cluster to provide the respective homogeneous antibody conjugates. Cell-based assays indicated that the antibody-rhamnose cluster conjugates could mediate potent CDC activity for targeted cancer cell killing and showed much more potent efficacy than the antibody-αGal trisaccharide cluster conjugates for CDC effects.

Synthesis, binding affinity, and inhibitory capacity of cyclodextrin-based multivalent glycan ligands for human galectin-3.

Human galectin 3 (Gal-3) has been implicated to play important roles in different biological recognition processes such as tumor growth and cancer metastasis. High-affinity Gal-3 ligands are desirable for functional studies and as inhibitors for potential therapeutic development. We report here a facile synthesis of ß-cyclodextrin (CD)-based Tn and TF antigen-containing multivalent ligands via a click reaction. Binding studies indicated that the synthetic multivalent glycan ligands demonstrated a clear clustering effect in binding to human Gal-3, with up to 153-fold enhanced relative affinity in comparison with the monomeric glycan ligand. The GalNAc (Tn antigen) containing heptavalent ligand showed the highest affinity for human Gal-3 among the synthetic ligands tested, with an EC50 of 1.4 µM in binding to human Gal-3. A cell-based assay revealed that the synthetic CD-based multivalent ligands could efficiently inhibit Gal-3 binding to human airway epithelial cells, with an inhibitory capacity consistent with their binding affinity measured by SPR. The synthetic cyclodextrin-based ligands described in this study should be valuable for functional studies of human Gal-3 and potentially for therapeutic applications.

One-Pot Conversion of Free Sialoglycans to Functionalized Glycan Oxazolines and Efficient Synthesis of Homogeneous Antibody-Drug Conjugates through Site-Specific Chemoenzymatic Glycan Remodeling.

Antibody-drug conjugates (ADCs) are an important class of therapeutic agents that harness the highly specific antigen targeting property of antibodies to deliver toxic drugs for targeted cell killing. Site-specific conjugation methods are highly desirable for constructing homogeneous ADCs that possess a well-defined antibody-to-drug ratio, stability, ideal pharmacological profile, and optimal therapeutic index. We report here a facile synthesis of functionalized glycan oxazolines from free sialoglycans that are key donor substrates for enzymatic Fc glycan remodeling and the application of an efficient endoglycosidase mutant (Endo-S2 D184M) for site-specific glycan transfer to construct homogeneous ADCs. We found that by a sequential use of two coupling reagents under optimized conditions, free sialoglycans could be efficiently converted to selectively functionalized glycan oxazolines carrying azide-, cyclopropene-, and norbornene-tags, respectively, in excellent yield and in a simple one-pot manner. We further demonstrated that the recently reported Endo-S2 D184 M mutant was highly efficient for Fc glycan remodeling with the selectively modified glycan oxazolines to introduce tags into an antibody, which required a significantly smaller amount of glycan oxazolines and a much shorter reaction time than that of the Endo-S D233Q-catalyzed reaction, thus minimizing the side reactions. Finally homogeneous ADCs were constructed with three different click reactions. The resulting ADCs showed excellent serum stability, and in vitro cytotoxicity assays indicated that all the three ADCs generated from the distinct click reactions possessed potent and comparable cytotoxicity for targeted cancer cell killing.

Evaluation of Two Chemoenzymatic Glycan Remodeling Approaches to Generate Site-Specific Antibody-Drug Conjugates.

Fc-glycosite-specific antibody-drug conjugation represents a promising direction for the preparation of site-specific antibody-drug conjugates (ADCs). In the present research, we conducted a systemic evaluation of two endoglycosidase-catalyzed chemoenzymatic glycoengineering technologies to prepare glycosite-specific ADCs. In the first two-step approach, the antibody was deglycosylated and then reglycosylated with a modified intact N-glycan oxazoline. In the second one-pot approach, antibodies were deglycosylated and simultaneously glycosylated with a functionalized disaccharide oxazoline. For the comprehensive evaluation, we first optimized and scaled-up the preparation of azido glycan oxazolines. Afterwards, we proved that the one-pot glycan-remodeling approach was efficient for all IgG subclasses. Subsequently, we assembled respective ADCS using two technology routes, with two different linker-payloads combinations, and performed systemic in vitro and in vivo evaluations. All the prepared ADCs achieved high homogeneity and illustrated excellent stability in buffers with minimum aggregates, and exceptional stability in rat serum. All ADCs displayed a potent killing of BT-474 breast cancer cells. Moving to the mouse study, the ADCs prepared from two technology routes displayed potent and similar efficacy in a BT-474 xenograft model, which was comparable to an FDA-approved ADC generated from random conjugation. These ADCs also demonstrated excellent safety and did not cause body weight loss at the tested dosages.

Synthetic Site-Specific Antibody-Ligand Conjugates Promote Asialoglycoprotein Receptor-Mediated Degradation of Extracellular Human PCSK9.

Targeted degradation using cell-specific lysosome targeting receptors is emerging as a new therapeutic strategy for the elimination of disease-associated proteins. The liver-specific human asialoglycoprotein receptor (ASGPR) is a particularly attractive lysosome targeting receptor leveraged for targeted protein degradation (TPD). However, the efficiency of different glycan ligands for ASGPR-mediated lysosomal delivery remains to be further characterized. In this study, we applied a chemoenzymatic Fc glycan remodeling method to construct an array of site-specific antibody-ligand conjugates carrying natural bi- and tri-antennary N-glycans as well as synthetic tri-GalNAc ligands. Alirocumab, an anti-PCSK9 (proprotein convertase subtilisin/kexin type 9) antibody, and cetuximab (an anti-EGFR antibody) were chosen to demonstrate the ASGPR-mediated degradation of extracellular and membrane-associated proteins, respectively. It was found that the nature of the glycan ligands and the length of the spacer in the conjugates are critical for the receptor binding and the receptor-mediated degradation of PCSK9, which blocks low-density lipoprotein receptor (LDLR) function and adversely affects clearance of low-density lipoprotein cholesterol. Interestingly, the antibody-tri-GalNAc conjugates showed a clear hook effect for its binding to ASGPR, while antibody conjugates carrying the natural N-glycans did not. Both the antibody-tri-antennary N-glycan conjugate and the antibody-tri-GalNAc conjugate could significantly decrease extracellular PCSK9, as shown in the cell-based assays. However, the tri-GalNAc conjugate showed a clear hook effect in the receptor-mediated degradation of PCSK9, while the antibody conjugate carrying the natural N-glycans did not. The cetuximab-tri-GalNAc conjugates also showed a similar hook effect on degradation of the membrane-associated protein, epidermal growth factor receptor (EGFR). These results suggest that the two types of ligands may involve a distinct mode of interactions in the receptor binding and target-degradation processes. Interestingly, the alirocumab-tri-GalNAc conjugate was also found to upregulate LDLR levels in comparison with the antibody alone. This study showcases the potential of the targeted degradation strategy against PCSK9 for reducing low-density lipoprotein cholesterol, a risk factor for heart disease and stroke.

Site-Specific Chemoenzymatic Conjugation of High-Affinity M6P Glycan Ligands to Antibodies for Targeted Protein Degradation.

Lysosome-targeting chimeras (LYTACs) offer an opportunity for the degradation of extracellular and membrane-associated proteins of interest. Here, we report an efficient chemoenzymatic method that enables a single-step and site-specific conjugation of high-affinity mannose-6-phosphate (M6P) glycan ligands to antibodies without the need of protein engineering and conventional click reactions that would introduce "unnatural" moieties, yielding homogeneous antibody-M6P glycan conjugates for targeted degradation of membrane-associated proteins. Using trastuzumab and cetuximab as model antibodies, we showed that the wild-type endoglycosidase S (Endo-S) could efficiently perform the antibody deglycosylation and simultaneous transfer of an M6P-glycan from a synthetic M6P-glycan oxazoline to the deglycosylated antibody in a one-pot manner, giving structurally well-defined antibody-M6P glycan conjugates. A two-step procedure, using wild-type Endo-S2 for deglycosylation followed by transglycosylation with an Endo-S2 mutant (D184M), was also efficient to provide M6P glycan-antibody conjugates. The chemoenzymatic approach was highly specific for Fc glycan remodeling when both Fc and Fab domains were glycosylated, as exemplified by the selective Fc-glycan remodeling of cetuximab. SPR binding analysis indicated that the M6P conjugates possessed a nanomolar range of binding affinities for the cation-independent mannose-6-phosphate receptor (CI-MPR). Preliminary cell-based assays showed that the M6P-trastuzumab and M6P-cetuximab conjugates were able to selectively degrade the membrane-associated HER2 and EGFR, respectively. This modular glycan-remodeling strategy is expected to find wide applications for antibody-based lysosome-targeted degradation of extracellular and membrane proteins.

General and Robust Chemoenzymatic Method for Glycan-Mediated Site-Specific Labeling and Conjugation of Antibodies: Facile Synthesis of Homogeneous Antibody-Drug Conjugates.

Site-specific labeling and conjugation of antibodies are highly desirable for fundamental research and for developing more efficient diagnostic and therapeutic methods. We report here a general and robust chemoenzymatic method that permits a one-pot site-specific functionalization of antibodies. A series of selectively modified disaccharide oxazoline derivatives were designed, synthesized, and evaluated as donor substrates of different endoglycosidases for antibody Fc glycan remodeling. We found that among several endoglycosidases tested, wild-type endoglycosidase from Streptococcus pyogenes of serotype M49 (Endo-S2) exhibited remarkable activity in transferring the functionalized disaccharides carrying site-selectively modified azide, biotin, or fluorescent tags to antibodies without hydrolyzing the resulting transglycosylation products. This discovery, together with the excellent Fc deglycosylation activity of Endo-S2 on recombinant antibodies, allowed direct labeling and functionalization of antibodies in a one-pot manner without the need of intermediate and enzyme separation. The site-specific introduction of varied numbers of azide groups enabled a highly efficient synthesis of homogeneous antibody-drug conjugates (ADCs) with a precise control of the drug-to-antibody ratio (DAR) ranging from 2 to 12 via a copper-free strain-promoted click reaction. Cell viability assays showed that ADCs with higher DARs were more potent in killing antigen-overexpressed cells than the ADCs with lower DARs. This new method is expected to find applications not only for antibody-drug conjugation but also for cell labeling, imaging, and diagnosis.

A facile synthesis of a complex type N-glycan thiazoline as an effective inhibitor against the antibody-deactivating endo-ß-N-acetylglucosaminidases.

Endo-ß-N-acetylglucosaminidases are a class of endoglycosidases that deglycosylate N-glycans from glycoproteins. We describe here a facile synthesis of a complex type N-glycan thiazoline as a new mechanism-based inhibitor for this class of enzymes. The synthesis started with the readily available sialoglycopeptide (SGP) and its conversion into the glycan thiazoline through several enzymatic and chemical reactions. The synthetic glycan thiazoline showed potent inhibitory activity against several endoglycosidases including the two antibody-deactivating enzymes, Endo-S and Endo-S2, from human pathogen Streptococcus pyogenes, which would be useful as tools for structural and functional studies of these enzymes.

Derivation of Leptomeninges Explant Cultures from Postmortem Human Brain Donors.

Even though great progress has been made in the clinical characterization of Parkinson's disease, several studies report that the diagnosis of Parkinson's disease is not pathologically confirmed in up to 25% of clinically diagnosed Parkinson's disease. Therefore, tissue collected from clinically diagnosed patients with idiopathic Parkinson's disease can have a high rate of misdiagnosis; hence in vitro studies from such tissues to study Parkinson's disease as a preclinical model can become futile. By collecting postmortem human leptomeninges with a confirmed neuropathological diagnosis of Parkinson's disease and characterized by nigrostriatal cell loss and intracellular protein inclusions called Lewy bodies, one can be certain that clinically observed parkinsonism is not caused by another underlying disease process (e.g. tumor, arteriosclerosis). This protocol presents the dissection and preparation of postmortem human leptomeninges for derivation of a meningeal fibroblast culture. This procedure is robust and has a high success rate. The challenge of the culture is sterility as the brain procurement is generally not performed under sterile conditions. Therefore, it is important to supplement the culture media with a cocktail of penicillin, streptomycin, and amphotericin B. The derivation of meningeal fibroblasts from autopsy-confirmed cases with Parkinson's disease provides the foundation for in vitro modeling of Parkinson's disease. Meningeal fibroblasts appear 3-9 days after sample preparation and about 20-30 million cells can be cryopreserved in 6-8 weeks. The meningeal fibroblast culture is homogenous and the cells express fibronectin, a commonly used marker to identify meninges.

The structure of a monomeric mutant Cks protein reveals multiple functions for a conserved hinge-region proline.

Cks (cyclin-dependent kinase subunit) proteins are essential eukaryotic cell cycle regulatory proteins that physically associate with cyclin-dependent kinases (Cdks) to modulate their activity. Cks proteins have also been studied for their ability to form domain-swapped dimers by exchanging ß-strands. Domain swapping is mediated by a conserved ß-hinge region containing two proline residues. Previous structural studies indicate that Cks in its dimer form is unable to bind Cdk, suggesting that the monomer-dimer equilibrium of Cks may have an effect on Cks-mediated Cdk regulation. We present the crystal structure of a proline-to-alanine mutant Saccharomyces cerevisiae Cks protein (Cks1 P93A) that preferentially adopts the monomer conformation but surprisingly fails to bind Cdk. Comparison of the Cks1 P93A structure to that of other Cks proteins reveals that Pro93 is critical for stabilizing a multiple ß-turn structure in the hinge region that properly positions an essential Cdk-binding residue. Additionally, we find that these ß-turn formations, conserved in Cks homologs, have implications for the mechanism and preferentiality of strand exchange. Together, our observations suggest that the conservation of Cks hinge-region prolines reflects their functions in forming a Cdk binding interface and that the ability of these prolines to control partitioning between monomer and dimer is a consequence of the ß-turn networks within the hinge.