Viral glycopeptide mapping of antigenic proteins allows determination of the carbohydrate structures associated with specific amino acid residues. This allows us to determine precisely how glycosylation affects immunogen structure and vaccine effectiveness at particular sites. This knowledge can be used to design glycopeptide mapping vaccines. The presence of glycans can change how epitopes are presented, along with protein structure, its ability to remain stable, and its immunogenicity. Unlike the broad view provided by bulk analysis, glycopeptide mapping pinpoints glycan placement across a viral structure.
Viral antigens are commonly glycosylated. Viral envelope proteins tend to be very glycosylated. In some glycoproteins carbohydrates can represent up to 50% of the protein's molecular weight. Glycosylation plays important roles in viral protein folding and evasion from host immunity. Glycosylation plays a key role in immunogenicity of vaccines. Determining site specific glycosylation will allow for researchers to understand how glycosylation alters antigenicity and host response.
Table 1 Functional Roles of Viral Glycosylation in Vaccine Contexts
| Glycan Function | Mechanistic Impact | Vaccine Design Implication |
| Epitope shielding | Steric hindrance of antibody binding | Target selection for unshielded regions |
| Structural stability | Protein folding and conformation maintenance | Preservation of native-like immunogens |
| Immune evasion | Mimicry of host glycans | Evasion of anti-carbohydrate responses |
| Receptor binding | Modulation of host cell entry | Balancing immunogenicity and function |
Functionally, glycans decorate viral envelope proteins and glycan shielding creates a physical barrier between antibodies and antigenic sites by steric hindrance. Glycans have both physical and chemical properties that mask sites on viruses that antibodies can recognize and bind to known as neutralization-sensitive sites from antibodies involved in the humoral immune response. Neutralization-sensitive sites are frequently occupied by oligomannose and complex glycans. Glycans can help fold proteins in the endoplasmic reticulum and stabilize viral glycoproteins. Viruses employ host glycosylation, displaying carbohydrates resembling those of the host to sidestep immune recognition, thus hindering immune reactions against carbohydrates and hiding protein antigens.
Potential impact of under glycosylation on the immunogenicity of a plant-produced viral glycoprotein.1,5
Site-specific glycosylation will play a role in vaccines since glycans can shield conserved epitopes from neutralization-sensitive regions. Knowing which glycosylation sites shield important epitopes and which do not will allow for antigen design that purposely displays conserved epitopes without conferring immunogenicity towards variable or non-neutralizing epitopes. Site specific glycosylation will play a role in breadth as broad neutralizing antibodies will not recognize epitopes shielded by glycans. Site-specific glycosylation information can be used for structure-based antigen optimization. Antigens can be altered in various ways such as having no glycans or extra glycans at certain positions to allow better immune recognition of conserved epitopes.
Site information is lost when performing released glycan profiling, therefore precluding the use of this approach for vaccine studies. Released glycan profiling refers to enzymatic release of glycans from protein which produces bulk population glycan information. Site-specific information is lost when performing released glycan profiling, therefore it is impossible to know where on the protein certain glycoforms are localized. Site information would allow structural based inference about which glycans might be sterically protecting an epitope. For vaccine design it can be useful to know whether glycans are clustered around a receptor binding site or neutralization important region. Released glycan profiling cannot provide this information so it cannot be used to rationally engineer immunogens or compared recombinant vaccine antigens.
Mass spectrometry-based glycopeptide mapping analysis of viral antigens allows for analysis of intact glycopeptides released from viral envelope proteins, retaining site-specific information between carbohydrate structures and amino acid sequences. This allows visualization of glycan shields as well as how these glycans spatially relate to neutralization sensitive epitopes, offering structural data that cannot be obtained through traditional glycan analysis.
In mapping viral antigen glycopeptides, purified viral envelope proteins are digested enzymatically into peptides, which include glycosylation sites. Trace enrichment of glycopeptides from viral proteins may be performed prior to analysis by methods such as hydrophilic interaction chromatography or lectin enrichment. The glycopeptides are fragmented by tandem mass spectrometry to produce ions that can provide structural information. For example, fragmentation using collision-induced dissociation can determine glycan composition while fragmentation using electron-transfer dissociation can retain glycan-peptide connectivity allowing for direct localization of the glycosylation site.
Site mapping reveals which, out of all the possible N-glycosylation sequons displayed on a viral antigen, are actually utilized and occupied by glycans. This information shows the macroheterogeneity distribution on the protein. Quantification determines what percentage of each glycoform present at each site are oligomannose, hybrid, complex, etc. This determines the microheterogeneity that results in a heterogeneous mixture of glycoproteins. Quantification allows for researchers to differentiate between dominant glycoforms present and minor glycoforms that may play a larger role in epitope exposure/receptor recognition.
Site-specific glycopeptide mapping offers much higher structural resolution of modification occupancy than intact mass approaches. Intact mass can only determine overall mass shifts of proteins without localization of these shifts to amino acid residues. Additionally, site-specific glycopeptide mapping maintains the topological relationship of protein-carbohydrate interactions that are destroyed during released glycan characterization approaches. As a result of maintaining spatial relationship glycopeptide mapping can associate observed glycosylation at certain sites with functional outcomes such as resistance or susceptibility to neutralization or receptor binding.
The analysis of glycopeptide maps is applicable to many vaccine platforms. Site-specific glycosylation information is critical for understanding antigenicity, immunogen stability, and accessibility of protective epitopes. Glycopeptide mapping provides these important insights which allow for rational immunogen design. Glycopeptide mapping has been used to characterize viral glycoproteins including coronavirus, HIV, and influenza glycoproteins. Rational vaccine design requires evaluation of vaccine candidates for epitope exposure as well as structural similarity to native proteins to induce neutralizing antibodies.
Research on the SARS-CoV-2 virus revealed a heavily glycosylated glycoprotein, with N-linked glycans that effectively mask the receptor binding domain. These studies mapped sterically inaccessible epitopes hidden from antibody contact as well as vulnerable sites able to be targeted for vaccine development. Vaccine development can be aided to use stable variants of spike proteins with alterations to N-linked glycans that optimally alter exposure of neutralizing epitopes while maintaining protein folding and antigenicity.
Both HIV gp120 and gp41 are studded with dense arrays of glycans, mainly oligomannose glycans, which protect epitopes from neutralizing antibodies. Site specific glycosylation mapping of viral glycoproteins is crucial to identify open sites where antibodies can potentially bind. Glycosylation of influenza hemagglutinin evolves under immune selective pressure. Glycans inserted at specific sites on HA can mask antigenic sites. Site knowledge enables strategic design of creating glycan stripped sites at epitopes or targeting conserved glycan-protein interactions for bnAbs vaccine development.
For recombinant subunit vaccines, it is important to evaluate how the expression system alters glycosylation of the protein-of-interest, as glycosylation patterns differ depending on the host cell system and can impact conformation and antigenicity of proteins. Glycosylation sites can be mapped to ensure structural consistency within lots and across production-scale and small-scale versions, as well as fidelity to how the protein is glycosylated on the virus. This allows one to make educated choices about expression systems/process development to alter glycosylation as desired to improve presentation of protective epitopes and limit immunogenic non-human carbohydrates.
The viral vector and mRNA approaches are associated with host-cell-dependent glycosylation differences. Since the glycans decorating antigens produced in vaccine recipients are reliant on the cells that are transfected or transduced with these vaccines, side-by-side comparison allows platform comparison. With side-by-side analysis, researchers can compare antigens generated by each platform and determine if glycosylation patterns are similar to what is seen on viral proteins or a recombinant antigen of interest. Understanding these glycosylation patterns allows for researchers to determine if antigens created with vaccines will contain relevant epitopes necessary for immune detection and exclude modifications that would deter neutralizing antibodies or vaccine effectiveness.
The expression host that a protein is produced in will determine what types of glycans are added onto recombinant viral proteins. Glycans from host cells like CHO or HEK will more closely resemble those found on viruses that infect humans. Insect derived glycosylation can also be highly divergent from humans and can cause immunogenicity. Therefore, when you are selecting your expression system for vaccine production, attempting to engineer glycans to look more human, or evaluating regulatory concerns for novel glycans it is important to know what glycosylation signature your producer cell displays.
CHO, HEK, and insect cells also process antigens differently with respect to glycosylation, which can also affect immunogenicity and protective efficacy. CHO cells tend to yield glycoproteins with increased sialic acid content and branching, and HEK systems have increased glycosyltransferase activities with different processing motifs. A defining characteristic of protein glycosylation in insect cells is the presence of paucimannose glycans without terminal sialic acid. When selecting a cell line to produce a particular antigen, consider any glycosylation differences that may exist, if glycosylation is important to the antigen function or if regulatory authorities require a defined glycosylation profile for the vaccine.
Examples of host-cell-specific glycan signatures comprise foreign carbohydrate structures whose presence could impact vaccine safety or efficacy by either eliciting unwanted immunogenic responses or modifying antigen presentation. Expression systems from non-mammalian origins can lead to the generation of unique epitopes that are foreign to humans including core alpha-linked fucose found in plants and yeast expression systems or high mannose glycans found in yeast. Expression of these glycans on vaccine antigens could result in immunogenicity against the vaccine antigen. Manufacturing guidelines/regulations require structural characterization of these features and confirmation that glycosylation does not vary significantly from batch to batch. Risks considered include analysis of potential immunogenic epitopes as well as implications of foreign glycans on antigen uptake/presentation by antigen-presenting cells.
Monitoring glycopeptides over multiple lots as part of a longitudinal study will also provide assurance of continued manufacturing reproducibility. Changes in site-specific glycan distributions from lot to lot may identify subtle process drift that can be corrected before there is any impact on vaccine quality attributes. Setting the range of expected variation in glycoform distributions at antigen sites will help to assure that lots will continue to produce similar immune responses. As with comparisons between preclinical and clinical batches, analytical results will help support regulatory filings by showing that the expression system is stably producing proteins with the same glycosylation machinery and that product purification is not introducing any bias in antigen glycosylation. Thorough documentation of batch-to-batch consistency will be needed for regulatory approval as well as pharmacovigilance.
Glycosylation has been shown to modify vaccine efficacy. Glycosylation patterns may hide or display epitopes for antibody binding or antigen presenting cell binding. Deliberate engineering of glycosylation patterns to appropriately mask or display antigens can allow for better vaccine design by creating more efficacious vaccines. For example, glycans can block certain antibody binding sites. Changing where glycans are attached to an antigen can either hide or expose an epitope that is recognized by neutralizing antibodies.
Table 2 Glycosylation Impact on Vaccine Immunogenicity
| Mechanism | Glycan Effect | Vaccine Design Implication |
| Epitope shielding | Steric hindrance of neutralizing sites | Target unshielded conserved regions |
| Antibody recognition | Modulation of B-cell epitope accessibility | Engineer glycan-deficient immunogens |
| Structural stability | Maintenance of native conformation | Preserve essential glycosylation sites |
A common immune evasion strategy observed in viruses is glycan shielding. Dense glycan coverage can physically prevent access of antibodies to epitopes on conserved viral proteins. Simulations reveal glycans to be highly flexible which leads to an effective shielding cloud that hides around 40-50% of the surface area on viral glycoproteins from antibody interactions. In comparison to rigid structures, glycans are able to move aside to shield sensitive areas while still allowing proteins to function properly. Removal of certain glycans can unveil hidden epitopes that are normally inaccessible to antibodies.
Glycoconjugates that formed by carbohydrates are covalently bonded to proteins and lipids on mammalian cell membranes.2,5
Recognition by neutralizing antibodies is highly glycosylation dependent, certain glycans can either aid or obstruct antibody binding. Antibodies that target regions dense in glycans are more likely to be non-neutralizing as these glycans are easily mutated by the virus to avoid immune pressure. Antibodies with strong neutralization potential tend to recognize conserved protein epitopes that are often partially obscured. Binding of high-mannose glycans can also change antigen presentation and processing, thus indirectly affecting antibody response. Determining glycosylation on a site specific manner allows identification of glycans that need to be maintained for structural purposes versus glycans that need to be removed for increased accessibility to neutralizing epitopes.
Correlation of structure and function allows for quantitative linkages between glycosylation sites and immune endpoints so that vaccine engineering can be rationalized. Analyses are done using site-specific glycopeptides so that the carbohydrates present adjacent to neutralizing epitopes are known. The structure of these glycans can be directly correlated to binding affinities and neutralization strengths of antibodies. Glycan positioning can also be placed onto solved protein structures of the antigen to visualize how these alterations will affect epitope exposure. Having this mechanistic data allows for rational glycoengineering of antigens so that structure is maintained and optimized to induce immune responses.
Rationally designing vaccine antigens with structural glycans in mind can allow for alteration of the immunological properties of the antigen. Such manipulation can take the form of glycan shielding of immunodominant variable regions to shift immune recognition to conserved regions, removal of glycans to reveal unprotected sites on the virus that are inaccessible during natural infection, or incorporation of selected glycoforms with improved antigen presentation and/or germinal center responses. Tailoring the glycan expression at specific sites may avoid elicitation of undesirable antibodies against non-functional regions while inducing broadly neutralizing antibodies against conserved functionally epitopes involved in viral entry.
Guidances for glycosylation characterization of vaccines have defined regulatory requirements that are designed to assure that glycosylation of viral antigens will remain controlled, safe, and effective throughout production. Requirements include extensive characterization of structure so that glycosylation patterns are matched to reference substances and do not pose any antigenic risks to ensure safety for approval and continued quality.
Documentary evidence for vaccines submitted as Chemistry, manufacturing and controls (CMC) information needs significant structural glycosylation analysis to show that the glycan structures are suitable for the desired immune action. Agencies require characterization of N-linked glycans to viral proteins including occupancy and microheterogeneity. Characterization should rule out unintended glycan epitopes that may be immunogenic such as non-human glycans and confirm that glycosylation patterns of the production system are consistent with the native virus protein or humanized sufficiently for therapeutic use.
To ensure manufacturing consistency during development and beyond, extensive comparability studies are needed. Glycosylation must be demonstrated to be consistent not only from batch to batch but also across scales of manufacturing. Assessing glycosylation at the site-specific glycopeptide level allows manufacturers to pinpoint drifts in glycoform distribution that could potentially alter immunogenicity or efficacy. Changes to the manufacturing process must be thoroughly evaluated via glycosylation comparability studies to confirm that these changes do not structurally affect epitopes responsible for protection or introduce glycan forms that negatively impact vaccine performance in the intended demographic.
INDS and BLAs include glycosylation information used in the risk analysis of immunogenicity or lot-to-lot variability. The CMS include information on glycan profiling ensuring that products manufactured have consistent glycosylation patterns that fall within set specifications. Risk evaluations question if certain glycoforms have potential to induce antibodies against carbohydrate structures or interact with carbohydrate binding lectin receptors in the innate immune system and that risks have been addressed with specifications and controls.
The need for glycosylation as a quality attribute (QA) should be decided based on its impact on potency, stability or safety of the vaccine. If carbohydrates contribute to these attributes of vaccine, glycosylation may be assigned as a critical quality attribute. Setting up of thresholds (e.g., quantitative specification ranges of glycoforms at particular sites) would require justification through clinical or nonclinical data that shows lack of immunogenicity or emergence of safety concern outside of defined ranges. Any changes that occur during commercial production must be tracked according to regulatory requirements.
High throughput analysis of viral glycopeptide maps include the use of mass spectrometry platforms along with sample preparation and bioinformatics techniques to elucidate glycosylation profiles of glycoproteins found on virus surfaces. Site-specific structural information obtained from glycopeptide mapping can be used in vaccine development for rational immunogen design and submission for regulatory approval.
Typical mass spectrometers for glycopeptide analysis are Orbitrap or quadrupole-time-of-flight instruments which provide sufficient mass accuracy and resolution to differentiate glycopeptide species that differ by one monosaccharide or are isobaric. Glycan composition and peptide identity can be accurately determined by such instruments. Fragmentation techniques like higher-energy collisional dissociation or electron-transfer dissociation can provide complementary structural information as the former tends to break the glycosidic bonds providing information on the carbohydrate sequence while the latter maintains peptide-glycan connectivity allowing for confident site localization.
Sample preparation that is optimized for viral glycoproteins includes detergent-based solubilization and denaturation steps which help to break apart the hydrophobic transmembrane domains commonly found in envelope proteins. Enzymatic digestion with proteases produces glycopeptides that can be more easily chromatographed, and enrichment of glycopeptides using hydrophilic interaction chromatography (HILIC) or lectin affinity can help focus in on trace-level enrichment. This helps with identifying glycosylation sites that are low in abundance but may play a significant role in antigenicity.
Absolute quantitation of glycoforms measures the relative abundances of glycans at a given glycosylation site. This generates profiles of microheterogeneity across glycoprotein molecules. These relative abundances are typically determined by integration of extracted ion chromatograms for various glycoforms. The relative quantities of oligomannose, hybrid and complex glycans can be determined semiquantitatively. Comparison between batches can be made to show consistency between production runs. Samples can also be compared to show equivalence of overall structure between vaccine product and comparator.
Automated bioinformatics programs interpret glycopeptide fragmentation spectra. They compare experimental fragmentation patterns with expected patterns derived from glycopeptide structure to produce confident assignments. Bioinformatic algorithms are designed to accommodate the complexity of heavily glycosylated antigens with large microheterogeneity. Expanded search spaces for calculating theoretical fragmentation patterns are considered with advanced algorithms. Spectral matches are manually verified to confirm automated assignments. Rare glycoforms may have unexpected fragmentation patterns that are missed by automated analysis software. Multiple fragmentation datasets can be used in combination and analyzed to generate statistical confidence in identification prior to vaccine decision making points and submission.
Technical limitations for glycopeptide characterization include the heterogeneity of glycosylation on envelope proteins, molecular conformation of glycans, isomers and need for standardization between instruments used for carbohydrate analysis. Mapping carbohydrates on viral glycoproteins is required for site-specific analysis.
Very high densities of glycosylation present significant analytical difficulties because many viral envelope proteins have multiple (>5) N-linked glycosylation sites within an immunodominant region, producing complex mixtures of glycopeptides that often have very similar physiochemical properties. Dense carbohydrate content makes these peptides challenging to resolve by chromatography and identify by mass spectrometry because several glycoforms can elute together or generate complex spectra. The complex glycan structure requires extensive mapping utilizing multiple separation methods to account for all sites and differentiate between highly glycosylated peptides and those with simpler modifications.
Glycans that have the same molecular weight but differ in terms of glycosidic bondages, branching structures or monosaccharide sequences are known as glycan isomers. Common mass spectrometry cannot differentiate between isomers that share the same mass, for example alpha versus beta linkage of sialic acid, diverse mannose unit linkage, or alpha1-2 versus alpha1-3 fucoses may have differences in immune cell response, but would be indistinguishable by common assays. Isomers can be distinguished through fragmentations techniques like electron transfer dissociation or tandem mass spectrometry, or with ion mobility spectrometry which distinguishes between ions based on their shape in the gas phase.
Rare glycans may be difficult to observe because of interference from abundant species and limited quantities available for analysis. Some rare glycoforms, such as certain high mannose or misprocessed glycans, can have a large effect on epitope exposure or immunogenicity even if they are only present at low levels of the total glycans expressed. Rare glycans may require enrichment techniques such as HILIC or lectin enrichment methods, as well as mass spectrometry methods capable of detecting peptides with low stoichiometry glycosylation..
Cross-platform and cross-laboratory reproducibility is difficult to achieve due to differences in ionization and fragmentation efficiencies, differences in data processing software algorithms, and other technical aspects. Standard operating procedures for sample preparation, instrument settings, and glycan assignment rules should be followed as much as possible so differences in results can be attributed to biological differences instead of technical issues. Cross-laboratory comparisons using the same reference samples can determine expected range of performance between different labs and allow comparison between different studies for vaccine advancement.
Such outputs include datasets of viral glycopeptides intended to report mass spectrometry findings in an organized manner. Documents record identified N-linked glycans at known sites which can be used as decision-support material for vaccine candidates. These decisions can include establishing antigen design and manufacture as well as structural selection for continued preclinical and clinical progression.
Table3 Deliverables from Viral Glycopeptide Analysis
| Deliverable Type | Content Description | Strategic Application |
| Site-specific maps | Glycan distribution per antigen site | Epitope shielding assessment |
| Quantification data | Relative glycoform abundance | Batch consistency monitoring |
| Spectral annotations | Fragment ion assignments | Structural verification |
| Technical reports | Comprehensive documentation | Regulatory submission support |
Site-specific glycosylation maps graphically depict the distribution of glycan structures at the level of individual amino acids along viral envelope proteins. They allow visualization of which glycoforms reside at each glycosylation sequon and provide information about site occupancy and microheterogeneity that may impact epitope exposure. Because glycans are localized to specific sequence positions within these maps, shielding can be quickly evaluated in the context of neutralization sensitive sites allowing for informed vaccine design choices.
Relative quantification identifies the ratio of glycans present at glycosylation sites. This allows for determination of microheterogeneity between carbohydrate structures within a population of viral antigens. The relative levels of oligomannose, hybrid, and complex glycans can be determined and quantified to identify predominant glycoforms. This information can be used to determine the specifications for glycosylation based quality attributes for manufacturing consistent antigens with the correct carbohydrate structures to elicit desired immune responses.
Glycopeptide assignments are unequivocally validated using Labeled tandem mass spectra. These files contain the list of fragment ions seen, but also annotate each fragment indicating which ions support a glycan composition vs. which ions support peptide amino acid sequence and site assignment. Spectra with full interpretation display the thought process that leads to the structure validation and serve as a searchable document for audits requiring confirmation of glycan attachment site and peptide identities.
Technical reports generated from glycopeptide analysis projects that are prepared for regulatory submission purposes are typically organized for inclusion in chemistry, manufacturing and controls sections of INDs and BLAs. Validation of the analytical methodology as well as sample preparation and acceptance criteria are described. Results are typically summarized in tables describing site specific glycoform distributions, representative chromatograms and batches passing/failing from statistical analyses testing batch-to-batch consistency. Structural data such as these are used by Regulatory Agencies to assess vaccine antigen quality as well as control of the manufacturing process.
Viral antigens used in vaccine development often exhibit dense and heterogeneous glycosylation patterns that directly influence antigen structure, epitope exposure, and immune recognition. Accurate site-specific glycopeptide mapping requires not only advanced LC-MS/MS capability, but also a deep understanding of viral protein biology, expression platform variability, and vaccine CMC requirements. We combine analytical precision with vaccine-focused scientific insight to deliver glycosylation data that supports rational antigen design, manufacturing consistency, and regulatory readiness.
Vaccine antigens—such as viral spike proteins, envelope glycoproteins, and recombinant subunits—often contain multiple N-linked and O-linked glycosylation sites with complex microheterogeneity. Understanding how these glycans affect antigen conformation and immunogenicity is critical for successful vaccine development. Our expertise includes:
By integrating high-resolution glycopeptide LC-MS/MS with structural biology insights, we generate data that informs antigen optimization and immune response evaluation.
Viral antigens may be produced in a variety of expression systems, each introducing distinct glycosylation signatures. Differences between CHO, HEK293, insect cells, yeast, or other platforms can significantly impact glycan composition and immunogenicity. We support vaccine developers by:
Our experience across multiple production systems enables accurate interpretation of glycosylation variability and supports risk-based decision-making during vaccine development.
Each vaccine program presents unique analytical needs depending on antigen type, development stage, and regulatory strategy. We design tailored glycopeptide mapping studies aligned with your scientific objectives and project milestones. For early-stage research, we focus on:
For late-stage or commercial programs, we provide:
Our flexible approach ensures analytical rigor while aligning with program-specific goals and timelines.
Regulatory authorities expect robust structural characterization of vaccine antigens, particularly when glycosylation may influence immunogenicity or safety. We generate glycopeptide mapping reports structured to support inclusion in CMC documentation and regulatory submissions. Our deliverables include:
Throughout each engagement, we maintain transparent project timelines, defined milestones, and consistent technical communication. Clients receive proactive updates and scientifically grounded insights that facilitate confident decision-making and regulatory preparedness. By combining vaccine-specific expertise with advanced glycopeptide LC-MS capabilities, we serve as a reliable analytical partner for viral antigen characterization across the full vaccine development lifecycle.
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