Antigen anisotropy: The overlooked geometric determinant of vaccine immunogenicity and fidelity

ABSTRACT

Antigen anisotropy, the directional dependence of protein conformation, epitope exposure, and conformational dynamics, is an under-appreciated determinant of vaccine immunogenicity. Preserving this geometric fidelity may influence outcomes such as neutralization breadth, affinity maturation, and B/T-cell response quality. Native antigens engage B-cell receptors, T-cell receptors, and MHC through oriented interactions that rely on spatial and dynamic constraints for effective immune recognition. This framework is increasingly relevant amid widespread use of nucleoside-modified mRNA vaccines, as recent studies suggest platform-specific deviations in antigen geometry and processing. Platform interventions including chemical inactivation (e.g., formaldehyde and β-propiolactone), formulation and storage conditions, and mRNA design choices (e.g., N¹-methylpseudouridine incorporation, codon optimization) can introduce perturbations that influence folding, glycan shielding, epitope presentation, and hydrodynamic behavior. These effects can generate antigen ensembles that diverge from native forms, and may plausibly contribute to differences in response breadth and quality. Prioritizing anisotropy preservation offers a complementary design principle for next generation vaccines, one that seeks to more closely recapitulate the geometric and dynamic features of natural infection and may improve the predictability and durability of protective immunity.

Introduction

The success of vaccines depends theoretically not only on antigen identity (e.g., the intended amino acid sequence) but also on how the actually produced antigen is presented to the immune system with its particular and actual geometric dynamics. Immune recognition is believed to be inherently anisotropic, shaped by the directional rules governing receptor engagement and protein dynamics.^1–3 Structural studies indicate that B-cell receptor clustering is influenced by preferred geometric constraints, while T-cell receptors typically engage peptide–MHC complexes with conserved docking orientations. Co-translational folding proceeds vectorially from N- to C-terminus, with limited opportunity for rearrangement.^1–8 On authentic virions, glycoprotein epitopes are presented in constrained orientations and repetitive spacings. Viral evolution appears to have shaped these ensembles in ways that frequently favor effective immune engagement, although tradeoffs with immune evasion strategies are also present.^1–3,9 Crystallographic studies of peptide–MHC complexes further illustrate this vectorial logic, suggesting how T-cell receptors engage at conserved docking angles that constrain peptide presentation.^1,3 These angular and spatial rules provide a mechanistic precedent for vaccine platforms that control antigen spacing and orientation, such as computationally designed nanoparticles and germline targeting immunogens. Antigen anisotropy therefore complements existing design principles by emphasizing the geometric syntax by means of which epitopes must be encountered and interpreted by the immune system.

Evidence for geometric determinants of immunogenicity

Experimental studies demonstrate that deliberate control of antigen geometry can enhance immunogenicity. In mice, reducing inter-antigen spacing from ~16 nm to 10–12 nm on computationally designed nanoparticles displaying trimeric influenza hemagglutinin heads enhanced hemagglutination inhibition and neutralizing titers, often yielding ~5-fold increases in potency and breadth against drifted strains, while also shifting epitope specificity and promoting affinity maturation.¹⁰ Similarly, reorienting hemagglutinin on alum using short oligo-aspartate tags placed the antigen in an upside down, stem exposed configuration that redirected antibody responses from the head domain toward the conserved stem.¹¹ These findings illustrate that the immune system interprets not only sequence information but also the spatial syntax, the geometric arrangement and orientation of epitopes.

Systemic perturbations of native anisotropy in platforms

Despite these insights, most licensed vaccine platforms introduce predictable disruptions to native anisotropy (Figure 1):

1. Chemical inactivation (formaldehyde, β-propiolactone) can generate covalent bridges that restrict conformational breadth or alter genome topology.

2. Formulation and solvent effects, including hydration-shell restructuring, interfacial adsorption, and slow deamidation, may progressively modify surface topology during storage.

3. Nucleoside-modified mRNA platforms (e.g., N¹-methylpseudouridine incorporation, codon optimization) can influence ribosomal decoding kinetics, induce +1 frameshifting, and modify glycan processing from the earliest stages of antigen biogenesis. These effects are increasingly documented, although their downstream immunological effects remain an active area of investigation.

Figure 1.

Conceptual overview of three orthogonal axes classes of anisotropy perturbations introduced by licensed platforms: chemical inactivation (post translational covalent modifications), formulation and solvent effects, and mRNA platform biogenesis (early translational and folding perturbations).

Beyond platform-level disruptions, atomic substitutions introduce directional perturbations at the scale of single bonds.

Atomic scale perturbations and functional outcomes

Even subtle chemical modifications can exert measurable effects on molecular recognition. A single methylene bridge (+12 Da from formaldehyde) or an N¹-methyl group (+14 Da in m¹Ψ) may alter bond directionality, steric radii, or vibrational coupling, thereby reshaping antigen dynamics (Figure 5). This “magic methyl” phenomenon, well documented in medicinal chemistry,^12,13 illustrates how small atomic changes can propagate into altered binding energetics. In the vaccine context, such modifications represent directional perturbations that may plausibly influence BCR avidity, T-cell polarization, and the breadth of responses to antigenically variable pathogens.¹⁴

Figure 5.

Molecular convergence versus functional divergence.

(Schematic of uridine (U), Pseudouridine (Ψ), N¹-methylpseudouridine (m¹Ψ)).^{8,14,60,76–78}.

Scope of this review

This review integrates structural, biophysical, and immunological evidence to classify anisotropic disruptions, outline their potential consequences, and propose design principles that prioritize directional fidelity alongside traditional serological endpoints. Relying on the geometric and dynamic language that pathogens evolved to exploit may help close persistent efficacy gaps against influenza, HIV, malaria, and emerging viruses.

Geometric and orientational control of antigen display as a determinant of vaccine immunogenicity

A growing body of work shows that the spatial orientation, geometric arrangement, and conformational dynamics of antigens influence the magnitude, quality, and breadth of antibody responses. These geometric variables may shape epitope accessibility, B-cell receptor (BCR) clustering, and the energetic landscape of receptor engagement.

Native SARS-CoV-2 virions display spike trimers in a highly ordered, predominantly upright geometry. Cryo-electron tomography of authentic virions shows that most spike trimers adopt the prefusion conformation and project at predominantly upright orientations from the viral membrane, with apical exposure of receptor-binding-domain epitopes.⁹ This arrangement provides a consistent directional presentation that supports BCR engagement.

By comparison, current subunit and mRNA-LNP vaccine platforms do not consistently reproduce this ordered geometry. For example, the full-length SARS-CoV-2 spike used in Novavax, when formulated with polysorbate-80 detergent, can assemble into multimeric rosettes where trimers cluster around micellar cores, differing from the relatively dispersed arrangement on authentic virions.¹⁵ Likewise, although mRNA vaccines encode prefusion-stabilized spikes and the expressed trimers are overwhelmingly prefusion, their orientation and inter-antigen spacing are not rigidly controlled after intracellular expression, lacking the rigid, repetitive order seen on authentic virions.¹⁶ These differences demonstrate that geometric fidelity varies across platforms. Their downstream immunological impact remains an area of active investigation rather than a matter of scientifically tested and empirically justified conclusions about efficacy.

It has been empirically demonstrated in mice and rabbits that deliberate control of antigen orientation meaningfully impacts immunogenicity. For example, when Plasmodium falciparum antigens were displayed on self-assembling protein nanoparticles, functional antibody responses were enhanced when orientations preserved proper folding of key epitopes. Inverting the orientation reduced proper folding and immunogenicity despite identical sequence and scaffold.¹⁷ Similarly, site-specific insertion of short oligo-aspartate tracts forced influenza hemagglutinin to bind alum in an inverted configuration, sterically shielding the immunodominant head domain and redirecting the antibody response toward the conserved stem.¹¹ These empirical findings show that geometric orientation is not always immunologically neutral, and it is hypothesized here that it is probably never quite neutral.

Comparable effects emerge when inter-antigen spacing, rather than orientation, is systematically varied on otherwise identical protein nanoparticle scaffolds. Using computationally designed two-component nanoparticles displaying trimeric influenza hemagglutinin (HA) head domains, Ellis and colleagues showed that reducing the distance between adjacent trimeric antigens from ~16 nm to ~10–12 nm enhanced hemagglutination inhibition and neutralizing titers, often yielding ~5-fold increases in potency and breadth against drifted H1 strains, while also redirecting epitope specificity and promoting affinity maturation.¹⁰ These findings support the interpretation that geometric parameters beyond simple valency or orientation, specifically lateral spacing and the resulting B-cell receptor clustering geometry, contribute to the quality of the antibody response.

Taken together, these observations reinforce the hunch, perhaps a physical inevitability, that B-cell recognition of repetitive antigens is inherently anisotropic. Epitope accessibility, receptor engagement, and avidity-dependent signaling are known to be influenced in some experimental studies by how antigens are geometrically arranged. It seems probable that this must always be the case. If so, platforms that preserve native upright geometry, or that engineer orientation and spacing, tend to elicit more focused or broader responses depending on their dynamic geometry (Table 1).^{9–11,15–17}

Table 1.

Impact of antigen geometry and orientation on immunogenicity.*.

Context	Geometry/Orientation	Immunogenic Effect	Reference
Native SARS-CoV-2 virions	Predominantly upright, prefusion spikes	Optimal epitope exposure; maximal apical RBD exposure	9
Novavax CoV2373 subunit	Multimeric rosettes in polysorbate-80	Divergent from upright display on virions	15
mRNA vaccine LNPs	Cellular expression of prefusion preserved	Antigens without rigid geometric control	16
Influenza (Hemagglutinin)	Deliberate inversion via oligo-aspartate tags	Sterically shields head; redirects response to conserved stem	11

*Pre-existing immunity is known often to exert a dominant influence on response specificity, and probably always has some impact that may not have previously been taken into account. Antigen anisotropy should therefore be viewed as an orthogonal design parameter that can modulate, though it certainly does not supersede or cancel out, prior immune history.^18,19.

Although preexisting immunity often dominates response specificity in real world settings, geometric fidelity represents an orthogonal design parameter that almost certainly modulates how antigens are interpreted by the immune system.^18,19 From this perspective platforms that alter native geometric parameters, whether through fixation, folding perturbations, or uncontrolled multimerization, may generate antigen ensembles that differ from those encountered during natural infection, with potential implications for immune response focus and breadth as well as longevity.

Systemic perturbations of native anisotropy

Vaccine manufacturing inevitably perturbs the native structural and dynamic anisotropy of antigens. Some effects of the necessary perturbations are intentional, e.g., to weaken a particular antigen, but other effects are almost certainly unintended, e.g., changing the geometry of the antigen in a way that may negatively influence its impact. Although these manufacturing perturbations are designed to prevent harm while preserving immunogenicity, processes introducing such chemicals as formaldehyde and β-propiolactone (BPL) introduce covalent modifications that may reduce conformational fidelity and subtly influence epitope presentation in immunologically relevant ways. Such perturbations of antigens must in some manner interact with the overwhelming importance of the immunogenic history of the individual.

Formaldehyde Inactivation: crosslinking chemistry and anisotropy perturbation

Formaldehyde (Figure 2) is a small, highly reactive aldehyde widely used in inactivated viral vaccines (e.g., polio, hepatitis A, and some egg-based influenza vaccines) for its ability to covalently modify proteins.^20,21 In aqueous solution, including at refrigerated temperatures, formaldehyde reacts primarily with the ε-amino group of lysine and N-terminal amines, and to a lesser extent with arginine, histidine, cysteine, and tyrosine side chains.^20,21

Figure 2.

Chemical formula and Van der Waals sphere representation of formaldehyde (CH₂O). Atom identities are color coded: oxygen (red), carbon (grey), and hydrogen (white). Each completed methylene bridge adds approximately 12 daltons (one carbon) to the modified structure. Model rendered using molview.com.

The initial reaction forms a reversible hydroxymethyl (methylol) adduct:

$$\mathrm{Protein}-\mathrm{N}{\mathrm{H}}_2+\mathrm{HCHO}\to \mathrm{Protein}-\mathrm{NH}-\mathrm{C}{\mathrm{H}}_2\mathrm{OH}\left(+30\mathrm{Da}\right)$$

Under typical fixation conditions, these adducts progress to stable methylene bridges:

$$\mathrm{Protei}{\mathrm{n}}^1-\mathrm{NH}-\mathrm{C}{\mathrm{H}}_2\mathrm{OH}+{\mathrm{H}}_2\mathrm{N}-\mathrm{Protei}{\mathrm{n}}^2\to \mathrm{Protei}{\mathrm{n}}^1-\mathrm{NH}-\mathrm{C}{\mathrm{H}}_2-\mathrm{NH}-\mathrm{Protei}{\mathrm{n}}^2+{\mathrm{H}}_2\mathrm{O}$$

$$\left(\mathrm{net}+12\mathrm{Da}\mathrm{per}\mathrm{bridge},+1\mathrm{carbon}\mathrm{per}\mathrm{bridge}\right)$$

Each completed bridge incorporates one carbon atom from formaldehyde into the polypeptide chain, increasing molecular weight by ~12 Da, as mentioned earlier, which in biochemistry is always important even if its specific impacts on chemical dynamics cannot be spelled out in advance. It is known that these covalent modifications are not reversed by dialysis, dilution, filtration, or storage.^20,21 Because viral surface proteins typically contain numerous reactive residues, multiple intra- and intermolecular cross-links are common. Mass spectrometric studies of formaldehyde and BPL treated proteins and virions document extensive residue modification and cumulative mass shifts detectable by intact and peptide MS.^21,22 Moreover, once established, these irreversible covalent bridges can constrain bond angles and distances, increase local and global rigidity, and may reduce low-frequency conformational fluctuations (“breadth”) that often contribute to optimal engagement of broadly neutralizing B-cell receptors.

Antigenic outcomes

Comparative studies of formalin inactivated antigens versus native counterparts show altered antibody fine-specificity and reduced recognition of conformational epitopes.^23–26 These findings align with broader principles of immunodominance, in which conformational accessibility shapes epitope hierarchy.²⁷

The resulting reduction in conformational dynamics may:

• mask or distort native conformational epitopes

• generate neo-epitopes arising from the crosslinks

• shift responses from broad, cross-reactive recognition toward narrower specificity

While the immunological effects are known by empirical observation, the mechanistic interpretations derive from established structural principles of protein chemistry and the sensitivity of molecular recognition to minimal covalent modifications.

These observations reflect a general principle in molecular recognition that bears repetition and emphasis: the addition of even a single methyl group (one carbon, contributing +12 Da to the weight of a molecule) may produce substantial shifts in its binding affinity, specificity, toxicity, or biological fate. The “magic methyl/methylene effect,” extensively documented in medicinal chemistry and drug design,^12,13,28,29 provides a conceptual basis for examining closely the permanent single-carbon insertions introduced by formaldehyde crosslinking in vaccine antigens presented to the immune system.

The single methyl group as the quintessential minimal structural perturbation

The single methyl group (+CH₃, Δmass ≈ +14 Da, ΔV ≈ 17 ų van der Waals volume) is among the least of the structural perturbations known to impact molecular recognition.³⁰ Among the established facts of medicinal chemistry is the knowledge that the addition or removal of a single methyl group can modulate binding affinity, enzymatic turnover, toxicity, or biological specificity despite its electronic neutrality and chemical simplicity.^12,13,28,29

Few other covalent modifications of comparable atomic size (e.g., –H → –F, –H → –OH, or –CH₂– → –C=O) match the frequency or magnitude with which a single methyl group elicits substantial functional shifts across diverse systems.

The phenomenon arises in the context of from a confluence of events: the addition of the methyl group brings with it

1. a modest increase in hydrophobic surface area

2. a precise change in steric bulk, often filling small lipophilic pockets or inducing favorable conformational restriction

3. a set of altered rotational barriers or saturated-ring puckering, and

4. the well-known London dispersion interactions.

The foregoing mechanisms are extensively documented in recent compilations.^{12,13,28,29,31} In medicinal chemistry, this is termed the “magic methyl” effect, and late-stage methylation has become a standard optimization tactic in drug-discovery programs worldwide.

Representative examples

Table 2 highlights systems in which the addition or repositioning of a single carbon atom produces functional outcomes, ranging from shifts in toxicity to changes in receptor affinity or genetic stability.

Table 2.

Representative examples of single-methyl perturbations yielding substantial functional shifts.

System	Parent compound/site	Modification	Functional outcome	Magnitude of change	Reference
Alcohol toxicity	Methanol*	→ Ethanol** (one carbon homologation)	Lethal → relatively safe (used as antidote)	Orders of magnitude	32,33
Opioid analgesia	Morphine	→ Codeine***	Analgesic potency ↓; oral activity ↑ markedly	0.1 × potency	28,34
DNA vs RNA identity	Uracil	→ Thymine (5-methyluracil)	Enables excision repair; distinguishes genetic material	Essential for life	35,36

*After Ingestion, methanol is metabolized by hepatic alcohol and aldehyde dehydrogenases to the toxic metabolites’ formaldehyde and formate.

**Ethanol is produced by one carbon homologation of methanol (insertion of a *CH₂- group) rather than simple methylation. The example is retained because it illustrates the large functional effects of adding or repositioning of a single carbon atom.

***Codeine is a naturally occurring low-efficacy opium alkaloid, used for mild-to-moderate pain and cough suppression. Methylation of morphine’s phenolic OH markedly improves oral bioavailability despite reduced receptor affinity.

Across these contexts, the methyl group functions as a context-dependent structural switch, capable of converting toxin to nutrient, agonist to antagonist, or gene silencing to activation.

Relevance to vaccine antigen design

In the context of vaccines, the “magic methyl” principle applies, where minimal covalent modifications can influence folding, dynamics, and epitope presentation. Relevant examples include:

• the evolutionary methylation that distinguishes thymine from uracil and enables DNA repair;

• the N¹-methylpseudouridine substitution used in mRNA platforms;³⁶

• the O-methylation of morphine used to yield codeine (illustrating pharmacological outcomes of single carbon changes); and

• the methylol/methylene adducts and methylene crosslinks (–CH₂-bridges) formed during formalin inactivation.

Hypothesis level relevance

Across these contexts, a ~12 Da increment, one carbon (~14 Da with hydrogens), can shift molecular recognition in ways that plausibly influence immune engagement. While the immunological effects vary by system, these cases cited in Table 2 illustrate how even minimal covalent modifications can alter antigen structure, dynamics, and epitope presentation.

β-Propiolactone inactivation: nucleic-acid alkylation and anisotropy perturbation

β-Propiolactone (BPL, Figure 3) is a strained four-membered lactone that inactivates viruses primarily through rapid ring-opening alkylation of nucleic acids, adding approximately 72 Da per adduct, with strong preference for the N⁷ position of guanine in solvent-exposed single-stranded regions and loops.^37,38 Like formaldehyde, which modifies proteins through methylene crosslinking, BPL introduces covalent mass additions that perturb native molecular anisotropy, though its effects are nucleic acid centric.

Figure 3.

Chemical formula and Van der Waals sphere representation of β-propiolactone.

Atom identities are color-coded: oxygen (red), carbon (grey), and hydrogen (white). Incorporation of a β-propiolactone unit contributes approximately 72.06 daltons per adduct. Model rendered using molview.com.

Its small size and near-planar geometry facilitate penetration into the virion core and reaction at sterically accessible nucleic-acid sites, leading to genome destabilization, helical unwinding, and, in enveloped viruses, internal ribonucleoprotein (RNP) disorder, while largely preserving envelope architecture and surface-protein conformation under optimized conditions.^22,39

However, BPL is not universally protein-sparing. In non-enveloped picornaviruses such as coxsackievirus A16, BPL induces loss of the stabilizing pocket factor, collapse of the VP1hydrophobic pocket, externalization of the VP1 N-terminus, opening of the 2-fold axis, and conversion of both mature virions and procapsids into an expanded 135S-like uncoating intermediate, each modification incorporating +72 Da adducts on nucleophilic residues like cysteine or lysine.^37,40 These structural changes coincide with dose-dependent reductions in receptor (heparan sulfate) binding and altered reactivity with conformation sensitive monoclonal antibodies, indicating that surface exposed lysines, cysteines, and other nucleophilic residues can also be modified, albeit more slowly than nucleic acids.^37,40

Thus, the degree of structural and antigenic perturbation caused by BPL is strongly virus-class dependent. For many enveloped viruses (influenza, rabies, SARS-CoV-2) perturbation is minimal when low concentrations and short incubation times are used. In contrast, non-enveloped viruses, and potentially enveloped viruses under manufacturing conditions, may exhibit more alterations. The presence of a lipid bilayer in enveloped viruses provides a protective barrier, whereas non-enveloped viruses rely on protein capsids, explaining their divergent susceptibility to BPL modification. In all cases, covalent modification, whether of the genome, capsid, or envelope proteins, introduces atomic-level alterations that deviate from native virion anisotropy.

Taken together, these virus-class dependent perturbations illustrate how chemical inactivation may modulate native antigen anisotropy, influencing the directional rules of epitope presentation and receptor engagement that underpin immunotherapeutic fidelity. For viruses like SARS-CoV-2, low concentration BPL inactivation achieves viral inactivation while preserving functional properties, such as COVID-19 odor imprint for scent dog training, implying minimal structural perturbation under optimized conditions.³⁸ Contrasts found in the literature suggest that the extent of perturbation scales with manufacturing parameters. That is, higher BPL concentrations can cause a shift to alkaline pH, and prolonged exposure may amplify covalent modification of both nucleic acids and proteins. All such changes cumulatively add mass in a way that compounds deviations from native virion geometry. Such conditional effects clarify the fact that anisotropy preservation is achievable only under optimized inactivation protocols. All the forgoing, merely underscores the broader principle that vaccine platforms can only benefit by deliberately safeguarding structural fidelity rather than by relying on measures of bulk antigenicity.³⁸

Orthogonal perturbation and functional outcomes

Formaldehyde and β-Propiolactone (BPL) exert largely orthogonal chemical perturbations. Formaldehyde predominantly crosslinks and rigidifies the protein shell (adding approximately 12 Da per linkage),^20,41 whereas BPL irreversibly alkylates viral nucleic acids (adding approximately 72 Da per adduct), fracturing genome topology and ordered RNP (ribonucleoprotein) structure while generally sparing surface protein conformation.^37,42

These mechanistic differences translate into distinct immunological outcomes. Preservation of hemagglutinin conformational dynamics in cell-based vaccines inactivated with BPL has been associated with broader antibody responses and higher effectiveness against H3N2 strains in older adults during some seasons.^24,43,44 Whether the extensive RNA fragmentation produced by BPL also promotes cytosolic antigen release and enhances CD8+ T-cell cross priming remains an attractive but still unproven hypothesis that warrants testing.^45,46

Biophysical synthesis: dual-axis deviation model

The native virion contains two discernible anisotropic components:

1. a flexible, glycosylated protein shell and

2. an ordered genome core characterized by exposed ssRNA loops and RNP chirality.

Formaldehyde primarily perturbs protein shell anisotropy, whereas BPL perturbs genome core anisotropy. Neither method preserves the native ensemble completely; each selectively perturbs fidelity along different structural axes (Figure 1).

Hybrid or tuned hypothetical approaches, such as low-dose formaldehyde following selective nucleic-acid targeting, could in principle modulate the deviation spectrum between protein shell and genome core anisotropy. Rigorous validation would require time resolved cryo-ET, HDX-MS, immunopeptidomics, and functional immunogenicity assays.

Comparative chemical anisotropy: formaldehyde versus β-propiolactone

Formaldehyde and β-propiolactone (BPL) represent complementary inactivation chemistries with orthogonal molecular targets (Table 3). Formaldehyde crosslinks and restricts the dynamics of the glycosylated protein shell, whereas BPL alkylates the viral genome and disrupts RNP topology.³⁷

Table 3.

Comparative parameter of formaldehyde and β‑propiolactone inactivation.

Parameter	Formaldehyde (egg-based)	β‑Propiolactone (cell‑based)	Reference
Target	Protein (Lys, Arg, His, etc)	Nucleic acid (Gua-N7 mainly)	20,37
Geometry	Multi-site crosslinking (Lys-NH2 bridges)	Sterically, ring opening alkylation	12,39,47
Effect on hemagglutinin dynamics	Rigidification	Largely “Preserved”	20,22,39
Effect on viral RNA	Largely “Preserved”	Fragmented, increased disorder	24,25
Entropy loss	High (protein breadth decreased)	High (RNA topology decreased)	8,25,41,48,49
Reported VE* (>65 yo, select H3N2 seasons)	Variable: overall elderly ~31%	Variable, e.g., RVE ~(+12%) vs. egg	44,50

*VE- Vaccine effectiveness, RVE-Relative VE.

Across platforms, chemical inactivation tends to sacrifice fidelity along one structural axis to preserve the other (Figure 1). As a result, this perceived native dual anisotropy, flexible glycoprotein shell plus ordered genome core, is challenging to fully maintain with current licensed approaches.

Tautomerization and immune recognition of chemically modified antigens

Beyond global perturbations to the protein shell and genome, chemical modifications can introduce localized electronic and covalent changes that alter hydrogen bonding networks and local polarity.^20,21,41 Histidine imidazole rings, for example, dynamically interconvert between tautomeric states that modulate β-sheet propensity through altered hydrogen bond patterns and charge distribution.⁵¹ Similar microheterogeneity arises in amide and glycan environments, where water mediated electronic fluctuations and conformational equilibria tune local interactions.^48,49 These fine scale physiochemical variations ultimately influence antigen presentation and recognition.^6,7,10,11

Formaldehyde crosslinking freezes or biases these equilibria, particularly when histidine, asparagine, or glutamine residues are modified. This diminishes the micro-conformational adaptability that allows a single paratope (the antigen-binding site of an antibody) to accommodate slight epitope variation. This suggests a reduction in the subtle electronic anisotropy that supports affinity maturation and cross-reactive recognition.

The influenza virion exemplifies this balance: it requires both a flexible coat and an ordered core. Formaldehyde tends to restrict flexibility in the protein shell, whereas BPL primarily perturbs genome organization. Synthesis: These complementary effects illustrate how different inactivation chemistries modulate distinct axes of native virion anisotropy.

Water as a mediator of antigen structure and stability: a hypothesis

Water comprises 70–90% of most liquid vaccine formulations and functions as far more than an inert solvent (Figure 4). It actively sculpts antigen conformation, stability, and long-term structural dynamics through mechanisms operating across timescales from picoseconds to years. These processes are tightly controlled in licensed products, yet they can introduce subtle, cumulative deviations from the day zero antigen ensemble:

1. Slow non-enzymatic hydrolysis and deamidation

   Asparagine (Asn) and glutamine (Gln) residues undergo spontaneous deamidation in aqueous environments, even at 4°C, converting – CONH₂ side chains to – COOH with introduction of negative charge and an approximately a 1 Da mass shift. Rates are sequence- and conformation-dependent with half-lives ranging from months to several years in formulated vaccines.^52,53

   Observation: isoAsp formation introduces “kinks,” perturbs backbone geometry, and alters surface electrostatics.

   Interpretation: These changes can shift conformational equilibria and epitope presentation over storage time.

2. Interfacial denaturation and adsorption

   In oil-in-water emulsions, aluminum-hydroxide/phosphate adjuvants, lipid nanoparticles (LNPs), or during freeze–thaw cycling, proteins adsorb to phase boundaries.

   Observation: Adsorption promotes partial unfolding, exposure of hydrophobic cores, and irreversible aggregation.^52,54

   Interpretation: Adsorbed antigens may lose native hydration fingerprints and present altered or neo-epitopes with reduced-cell receptor (BCR) avidity.

3. Hydration-shell restructuring and Hofmeister effects

   The first one to two solvation layers surrounding a protein strongly influence conformational selection and receptor engagement. Excipients, salts, buffers, sugars, polyols, and surfactants, modulate these layers in ion- and osmolyte-specific ways (Hofmeister series), altering solubility, aggregation propensity, and preferred conformational sub-states.^48,49 These hydration shell changes can be measured using techniques like neutron scattering (for solvation layer thickness) or calorimetry (for entropy).⁵⁵

   Observation: Hydration shell composition tunes local dynamics and conformational sampling.

   Interpretation: Shifts in these layers may influence epitope accessibility.

4. Water as a stoichiometric reactant: a possible hidden contribution

   While water is traditionally viewed as a passive solvent, emerging perspectives suggest it may act as an active participant in antigen modification.^55,56

Figure 4.

Chemical formula and Van der Waals sphere representation of water.

Atom identities are color-coded: oxygen (red), carbon (gray), and hydrogen (white). Each incorporated water molecule contributes approximately 18 daltons (H₂O molecular weight) to the modified structure. Model rendered using molview.com.

Observations:

• Each covalently incorporated water molecule may add approximately +18 Da, depending on reaction pathway.

• Formaldehyde, supplied as 37–40% w/v formalin, exists in aqueous solution almost entirely as methanediol (methylene glycol, CH₂(OH)₂, 48 Da) due to rapid, near-complete hydration (>99.9% at room temperature).^57,58

  $$\mathrm{C}{\mathrm{H}}_2\mathrm{O}\mathit{\hspace{0.17em}}+\mathit{\hspace{0.17em}}{\mathrm{H}}_2\mathrm{O}\rightleftharpoons \mathrm{C}{\mathrm{H}}_2{\left(\mathrm{OH}\right)}_2$$

Reactions proceed through this hydrated species. Stable covalent crosslinks (methylene bridges) ultimately result in a net +12 Da mass increase per bridge. Water is therefore a stoichiometric participant in this widely used chemical inactivation chemistry.

• Sugars (e.g., sucrose) and polyols can form transient or persistent hydrogen-bonded adducts with surface residues, potentially contributing +18 Da per bound water molecule or promote slow Maillard-type modifications under storage conditions.

• Aluminum adjuvants bind antigens via ligand exchange, displacing surface – OH groups; the resulting coordinative bonds may effectively lock additional water-derived – OH ligands into the antigen – adjuvant complex.

Interpretation: Water, even small amounts of bound or incorporated, can subtly alter mass, conformation, and epitope presentation.

Cumulative effects

Across these mechanisms, water can produce low-level drift in hydration-shell thickness, local electrostatics, epitope exposure, and the entropic cost of antibody binding, which could be measured via HDX-MS or calorimetry. Some changes may be reversible (dynamic hydration fluctuations) while others may be persistent (chemical modification or bound water).

Hypothesis relevance

Because immune recognition is sensitive to directional geometry and conformational dynamics, even subtle hydration induced shifts may influence receptor engagement and the fidelity with which vaccine antigens recapitulate the native virion.

Thus, water, an often-overlooked component of a vaccine, is a silent but persistent mediator of antigenic anisotropy throughout manufacturing, storage, and delivery.

A third class of anisotropy perturbations: nucleoside-modified mRNA

Messenger RNA vaccines and therapeutics rely on synthetic transcripts that differ from native viral mRNA in two principal respects: near-complete replacement of uridine with modified nucleosides, most commonly N¹-methylpseudouridine (m¹Ψ), and extensive codon optimization coupled with stabilizing mutations such as the K986P/V987P “2P” substitutions in the SARS-CoV-2 spike. These alterations can substantially increase transcript stability and protein yield,^59,60 but high-resolution translational studies reveal systematic deviations from the decoding kinetics and fidelity characteristic of natural infection.

Translational dependability and kinetics

Ribosome profiling in mammalian cells shows that full m¹Ψ substitution induces +1 ribosomal frameshifting at slippery sequences, producing out-of-frame polypeptides in approximately 8% of translation events.⁶¹ These frameshifted products create novel peptide junctions that may be immunogenic in vaccinated individuals and elicit T-cell responses not typically observed after natural infection.⁶¹ In addition, m¹Ψ and codon recoding modestly alter codon-specific elongation rates and increase ribosome pausing or density at certain codons without necessarily causing a global slowdown of translation.^62–64 These kinetic shifts suggest influencing co-translational folding trajectories.

Outcomes for co-translational folding and topology

Vectorial N→C folding of nascent glycoproteins is highly sensitive to local translation speed.^63–66 Perturbed elongation kinetics may therefore shift the conformational ensemble sampled during synthesis, increasing the population of metastable intermediates with altered domain pairing or exposure of hydrophobic cores.⁸ For membrane-anchored or secreted glycoproteins, these changes may propagate to post-translational modifications, including N-linked glycosylation. Codon optimization and prefusion-stabilizing mutations in SARS-CoV-2 spike mRNA vaccines have been associated with altered high-mannose glycan content and site-specific occupancy compared with virion-derived spike.^67–69 Although glycosylation sites remain unchanged, the resulting glycan shield could deviate quantitatively and qualitatively from the native viral antigen.^70–72

Molecular convergence versus functional divergence at the ribosomal A-site

Uridine (U), pseudouridine (Ψ), and N¹-methylpseudouridine (m¹Ψ) are sufficiently similar to be accommodated within the ribosomal decoding center, enabling translational incorporation without catastrophic disruption.^73–75 However, atomic-level distinctions, most notably the C5-glycosidic bond in Ψ and the N¹-methyl group in m¹Ψ, modulate tRNA–mRNA pairing stability, hydration dynamics, and vibrational coupling.^63,76 These effects point to substitution density as a factor that can produce measurable changes in elongation kinetics, frameshifting propensity, and innate immune sensing (Figure 5).^60,61,79

Synthesis: mRNA platforms as anisotropic modulators

Nucleoside-modified mRNA represents a distinct class of anisotropy perturbation.³⁶ Whereas chemical inactivation (formaldehyde, β-propiolactone) and formulation effects act post-translationally, m¹Ψ incorporation and codon optimization operate at the earliest stage of antigen biogenesis, during ribosomal decoding and co-translational folding. The resulting spike ensembles often retain high overall structural similarity to native prefusion trimers yet exhibit systematic differences in conformational dynamics, glycan processing, and translational fidelity, including measurable +1 frameshifting in vitro and in cell lines.^61,71 These translational deviations provide a mechanistic basis for interpretation of sequence and conformational heterogeneity in mRNA-derived spike ensembles.

Related hypotheses on fragmentation and encapsulation effects further underscore the potential for platform-specific deviations (Table 4).^79,83 Layered atop altered folding kinetics and glycan processing, these deviations plausibly constrain the directional fidelity of antigenic anisotropy.

Table 4.

Anisotropy perturbations introduced by nucleoside-modified mRNA platforms.

Anisotropy Dimension	Primary Cause	Observed or predicted Effect	Reference
Primary sequence	+1 Frameshifting, pause sites	Novel peptide junctions; off-target T-cell responses	61,64,79,80
Co-translational dynamics	Codon-specific pausing/altered kinetics	Metastable folding intermediates; altered domain pairing	62–64,81,82
Glycan shield	2P mutation + codon optimization	Reduced high mannose glycans; site-specific occupancy changes	64,67–69,83
Local electronic environment	C-glycosidic bond, N1-methyl group	Changes in hydration, vibrational coupling	8,60,76–78

This framework is consistent with observations by Röltgen et al.,¹⁴ who reported that antibody responses following mRNA vaccination are often imprinted on Wuhan Hu1 epitopes and, on average, display narrower variant recognition than responses from natural infection or hybrid immunity.¹⁴

Next steps

Direct biophysical comparison of native viral spike and mRNA-derived material, using polysome-associated cryo-EM, hydrogen–deuterium exchange mass spectrometry, site-specific glycan analysis, and ribosome profiling in cells, will be essential to quantify the functional consequences of these perturbations. Such studies will clarify how closely current mRNA platforms recapitulate the directional fidelity of natural infection and guide next-generation designs that minimize translational infidelity while preserving high expression and innate immune silencing.

Conclusion: embracing antigenic anisotropy as a design principle

The evolving landscape of vaccine development calls for antigens that more faithfully replicate the structural and dynamic features encountered during natural infection. Antigenic anisotropy, the directional and asymmetric presentation of epitopes on native pathogens, represents an important yet underexplored dimension of immune recognition. While many current platforms achieve expression and serological responses, emerging evidence suggests that preserving native geometric and dynamic cues may support broader, more durable, and physiologically relevant immune responses.

Advances in high-resolution structural biology, time-resolved techniques, and AI-driven protein design now make it possible to map anisotropic epitopes at atomic and ensemble scales. Mechanistic insights from these fields can guide the engineering of immunogens that maintain directional fidelity, complementing established strategies such as epitope focusing, germline targeting, and nanoparticle display. Scalable approaches, including orientation-controlled nanoparticle displays, offer practical routes for studying and preserving anisotropy in next generation vaccines.^9–11,15,84 Adoption will require manufacturing processes that safeguard conformational integrity, validated stability metrics, and regulatory frameworks that recognize molecular fidelity as a meaningful quality attribute. Although anisotropy perturbations are hypothesized to influence immunogenicity, further empirical studies in diverse populations are needed to confirm relevance.

Aligning vaccine design with the immune system’s intrinsic sensitivity to directional cues offers a path toward immunogens that not only neutralize present threats, but anticipate emerging ones.

The immune system reads spatial, directional and dynamic detail.⁶ Vaccine manufacturing must increasingly learn to speak the same language.

Biography

Daniel Santiago, RPh, PharmD, is a licensed pharmacist with more than 30 years of experience. He earned a Bachelor of Science with a concentration in molecular and genetic biology from Temple University in 1992, complemented by advanced coursework in molecular biology at the University of Pennsylvania. From 1992 to 1993, he worked at the Wistar Institute in Philadelphia on a federally funded research project under Chin C. Howe, PhD. Dr. Santiago completed his Doctor of Pharmacy at Nova Southeastern University in 1998 and now works as an independent pharmacist and researcher in Florida, dedicated to advancing public health through evidence based analysis and open scientific dialogue.

Funding Statement

The author(s) reported there is no funding associated with the work featured in this article.

Disclosure statement

I declare that I am an Associate Editor for the International Journal of Vaccine Theory, Practice, and Research; no other competing interests are declared. I have no financial ties, consultancy roles, stock ownership, or paid advisory positions related to the content of this manuscript.

AI disclosure

The author used artificial intelligence, assisted tools solely for reference formatting and citation verification. All original ideas, analyses, interpretations, and textual content were conceived and written by the author.