Central to herpesvirus infectivity is the glycoprotein B (gB), an evolutionarily conserved fusion protein that mediates viral entry into host cells. This protein is indispensable across herpesvirus subfamilies, orchestrating membrane fusion events vital for viral replication and spread. The universality of gB among herpesviruses has positioned it as an attractive candidate for the development of broad-spectrum vaccines and therapeutics. However, the feasibility of targeting gB across diverse gammaherpesvirus genera has remained elusive, largely due to the absence of molecular insights that characterize its antigenic profile and functional vulnerabilities on a structural basis.

The research team has now bridged this gap by characterizing a monoclonal antibody named Fab5, which exhibits remarkable cross-genus reactivity by binding a conserved epitope on gammaherpesvirus gB. Fab5’s broad neutralization capacity transcends species boundaries, effectively inhibiting murine gammaherpesvirus 68 (MHV-68), rhesus macaque lymphocryptovirus, and human gammaherpesviruses. This cross-protective efficacy was demonstrated through rigorous in vivo challenges utilizing immune-competent mouse models, non-human primates, and humanized mice, establishing Fab5’s promise as a versatile immunotherapeutic agent for gammaherpesvirus infection.

Employing high-resolution cryogenic electron microscopy (cryo-EM), the investigators elucidated the three-dimensional architecture of the Fab5-gB complex. Structural data revealed that Fab5 targets an epitope exhibited on a highly conserved domain of gB, accessible in both pre-fusion and post-fusion conformations. This epitope is antigenically exposed and structurally constrained, underscoring its vulnerability to antibody engagement. Such dual conformation accessibility amplifies the neutralization breadth of Fab5, suggesting a mechanism whereby the antibody can intercept viral fusion machinery at multiple stages of the entry process.

This discovery carries profound implications for understanding gammaherpesvirus pathogenesis. The conserved nature of the gB epitope signifies an evolutionary pressure maintaining this region’s integrity despite viral diversification, pointing to its essential role in membrane fusion and infectivity. Fab5’s engagement likely disrupts critical conformational rearrangements required for fusion pore formation, thus halting the virus life cycle early and preventing cell-to-cell spread.

Moreover, these insights catalyze the rational design of next-generation broad-spectrum vaccines. By focusing immunogen development on this universal gB epitope, vaccine candidates may elicit robust cross-protective immunity against multiple gammaherpesviruses. This approach contrasts starkly with current vaccine strategies that predominantly target highly variable viral antigens, often resulting in strain-specific responses with limited durability and range.

The translational potential is underscored by the antibody’s efficacy in non-human primate models, which closely recapitulate human immune responses and gammaherpesvirus pathogenesis. Such preclinical validation enhances confidence in advancing Fab5-based therapeutics into clinical evaluation. Furthermore, the antibody’s ability to neutralize both latent and lytic phases of the viral lifecycle could revolutionize treatment paradigms for associated cancers and chronic infections, which often evade conventional antiviral strategies.

This study also opens avenues for exploring analogous fusion proteins in other herpesvirus subfamilies. Given that gB is conserved yet structurally distinct across alphaherpesviruses and betaherpesviruses, the methodological framework established here may guide the search for broadly neutralizing antibodies against these groups. A unified understanding of herpesvirus fusion mechanisms may thus emerge, unlocking pan-herpesvirus vaccination and treatment strategies.

Additionally, the Fab5-gB structural complex provides a template for designing small molecule inhibitors or engineered antibody derivatives with enhanced stability, affinity, and pharmacokinetics. Such modalities could complement active vaccination efforts, offering immediate protection for immunocompromised individuals or in outbreak containment scenarios. The integration of structural biology, immunology, and in vivo validation embodied in this research sets a benchmark for antiviral drug discovery.

In sum, the identification of Fab5 as a broadly reactive antibody against a conserved, vulnerable epitope of gammaherpesvirus gB represents a milestone in herpesvirus biology and therapeutic innovation. It signifies a new horizon wherein structural-guided immunotherapy enables cross-species viral neutralization, mitigating the global health burden posed by gammaherpesvirus infections and associated malignancies. The profound translational promise of this work invites intensified efforts toward clinical development, with the prospect of delivering transformative interventions for patients worldwide.

Subject of Research:
Broad neutralization of gammaherpesviruses via a conserved glycoprotein B (gB)-targeting antibody.

Article Title:
A broadly protective antibody targeting gammaherpesvirus gB.

Article References:
Sun, C., Xie, C., Cheng, BZ. et al. A broadly protective antibody targeting gammaherpesvirus gB. Nature (2026). https://doi.org/10.1038/s41586-026-10192-5

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SARS-CoV-2, the virus responsible for the COVID-19 pandemic, relies heavily on its spike protein to mediate entry into host cells. This spike protein undergoes a series of structural and enzymatic modifications, notably proteolytic priming, which enables the virus to bind to the ACE2 receptor and fuse with the host cell membrane. Proteolytic cleavage of the spike at specific sites, such as the S1/S2 boundary and the S2’ site, is essential for activating the fusion mechanism. Interfering with this cleavage event has been a strategic target for therapeutic intervention.

The research team, led by Milanesi, Urbinati, and Zimmermann, focused on the unique properties of K5 polysaccharides, a class of complex carbohydrate molecules known for their biological activity. These polysaccharides exhibit a structural resemblance to heparan sulfates found on cell surfaces, which are known to interact with various viral proteins. The hypothesis was that K5 polysaccharides might compete with or obstruct interactions vital for the proteolytic processing of SARS-CoV-2 spike protein, thereby preventing viral entry.

Using a comprehensive series of in vitro assays, the scientists demonstrated that K5 polysaccharides effectively bind to the spike protein, particularly near the cleavage sites targeted by proteases such as TMPRSS2 and furin. This binding was shown to sterically hinder access of these proteases to their cleavage targets, resulting in significantly reduced spike priming. Consequently, the virus’s ability to penetrate host cells was markedly diminished, as measured by viral infectivity assays.

To understand the molecular mechanisms underpinning this inhibition, the team employed advanced biophysical techniques, including surface plasmon resonance and cryo-electron microscopy. These analyses revealed that K5 polysaccharides engage with the spike protein in a specific orientation and with high affinity, stabilizing its uncleaved conformation. This stabilization prevents the conformational changes typically triggered by proteolytic cleavage that are necessary for membrane fusion and viral entry.

Importantly, the antiviral effect of K5 polysaccharides extended across multiple SARS-CoV-2 variants, including those harboring mutations in the spike protein that confer increased transmissibility or partial immune escape. This suggests that the mechanism of action — blocking proteolytic activation — is robust and less susceptible to viral evasion compared to neutralizing antibodies that target mutable spike epitopes.

The study also addressed potential concerns regarding the safety and specificity of K5 polysaccharide treatment. In cellular toxicity assays, these compounds exhibited low cytotoxicity, with minimal impact on host cell viability even at concentrations effective against viral infection. Furthermore, their interaction profile indicated a specific targeting of viral components rather than indiscriminate binding to host cell proteins, reducing the risk of unwanted side effects.

Beyond their antiviral activity, K5 polysaccharides may exert additional benefits by modulating host immune responses. Previous literature has implicated heparan sulfate analogs in influencing inflammatory signaling pathways and coagulation cascades; hence, K5 polysaccharides might also contribute to reducing the severity of COVID-19 by tempering pathogen-induced hyperinflammation, although further research is needed to validate these effects.

Therapeutically, the wide-ranging biochemical properties of K5 polysaccharides open doors for various modes of administration, including inhalable formulations that deliver the compound directly to the respiratory tract, the primary site of SARS-CoV-2 entry and replication. This localized delivery could maximize antiviral efficacy while minimizing systemic exposure.

Moreover, the synergy between K5 polysaccharides and existing antiviral drugs or monoclonal antibodies was preliminarily explored, with early data suggesting additive or even potentiated effects. Combining proteolytic priming inhibitors with agents targeting other viral life cycle stages could enhance overall treatment outcomes and prevent resistance development.

The implications of this discovery extend beyond COVID-19. Since many enveloped viruses require proteolytic activation of their fusion proteins, K5 polysaccharides or their derivatives might represent a new class of broad-spectrum antiviral agents. The research team proposes further investigations into their use against influenza, respiratory syncytial virus (RSV), and other coronaviruses.

While these findings herald a promising advancement, the authors emphasize that comprehensive clinical trials are necessary to confirm efficacy and safety in humans. Dose optimization, pharmacokinetics, and long-term impact studies will be essential steps before K5 polysaccharides can enter clinical practice.

In conclusion, the study by Milanesi et al. delineates a novel therapeutic strategy that targets the fundamental mechanism of SARS-CoV-2 spike protein activation. By preventing proteolytic priming with K5 polysaccharides, it is possible to block viral entry effectively, offering hope for new treatments capable of controlling COVID-19 and potentially other viral diseases characterized by similar molecular mechanisms.

Subject of Research: Inhibition of SARS-CoV-2 infection via blockade of spike protein proteolytic priming by K5 polysaccharides.

Article Title: K5 polysaccharides inhibit SARS-CoV-2 infection by preventing spike-proteolytic priming.

Article References: Milanesi, M., Urbinati, C., Zimmermann, L. et al. K5 polysaccharides inhibit SARS-CoV-2 infection by preventing spike-proteolytic priming. npj Viruses 4, 3 (2026). https://doi.org/10.1038/s44298-025-00163-4

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DOI: https://doi.org/10.1038/s44298-025-00163-4

At the heart of arenavirus infectivity lies the spike glycoprotein complex embedded within the viral envelope. This complex orchestrates the initial attachment and subsequent fusion of the virus with host cell membranes—an essential step for viral genome delivery and infection. Unlike many other viral spike proteins that have been extensively studied, the arenavirus glycoprotein complex exhibits a distinctive organization and processing pathway that has until now remained incompletely understood. The recent study utilizes state-of-the-art cryo-electron microscopy (cryo-EM) combined with advanced biochemical techniques to resolve the high-resolution structure of this trimeric complex in its prefusion conformation.

The study reveals that the arenavirus spike complex constitutes three non-covalently linked subunits arranged symmetrically around a central axis. This trimeric architecture exhibits a sophisticated molecular choreography that balances structural stability with conformational flexibility, enabling the transition from receptor binding to membrane fusion. Intriguingly, the glycoprotein complex consists of a stable receptor-binding domain that interfaces with host cell receptors and a metastable fusion machinery poised to undergo dramatic conformational rearrangements upon activation. This interplay ensures that membrane fusion is tightly regulated and occurs only under appropriate cellular conditions.

One of the most fascinating discoveries pertains to the unique cleavage and maturation process of the glycoprotein precursor, which is cleaved into a tripartite complex comprised of the receptor-binding subunit, the transmembrane fusion subunit, and a stable signal peptide that remains associated within the complex. This tripartite assembly departs from canonical viral glycoprotein processing pathways and contributes both to structural integrity and functional regulation. The stable signal peptide, in particular, acts as an intramolecular chaperone and an essential component of the spike complex, a feature that may be exploited for therapeutic intervention.

The structural study details the glycosylation landscape surface of the complex, highlighting how the sugar moieties create a protective shield that impedes neutralizing antibodies. Glycosylation patterns on viral spikes often represent a double-edged sword: they can facilitate immune escape yet potentially present vulnerabilities that immune targeting strategies can exploit. Observed glycan clusters appear to selectively mask vulnerable epitopes without compromising receptor engagement, underscoring the evolutionary fine-tuning of arenaviruses to circumvent host immunity while maintaining infectivity.

Beyond mere structure, the functional implications of the glycoprotein architecture were interrogated through mutational analyses and receptor binding assays. These experiments confirmed that the proper assembly and spatial arrangement of the subunits are critical for viral entry. Mutations disrupting intersubunit interfaces or glycan placements markedly diminished virus-cell fusion efficiency, emphasizing that both structural conformation and post-translational modifications collectively dictate viral fitness. Such mechanistic insights provide essential blueprints to disrupt key viral processes pharmacologically.

Comparative analysis with Old World arenaviruses and other enveloped viruses reveal both conserved and distinctive features. While the general paradigm of trimeric spike assembly and fusion activation is evolutionarily conserved, the New World arenavirus spike complex exploits a notably divergent receptor engagement strategy. This divergence likely mirrors adaptation to distinct receptor repertoires on host cell surfaces and facilitates tissue tropism differences. Hence, therapeutic designs need to be tailored specifically to these structural nuances to achieve broad-spectrum efficacy.

Moreover, the study sheds light on the dynamics of the prefusion-to-postfusion conformational changes, which are energetically demanding yet critical for viral membrane merger. The prefusion spike exists in a metastable state stabilized by strategic molecular contacts, which, upon triggering by receptor interaction and cellular cues such as low pH, rapidly transitions into an extended postfusion state that drives membrane apposition and fusion pore formation. These snapshots captured by cryo-EM not only depict the static architecture but also illuminate the underlying molecular mechanics of viral entry.

The implications of this work extend into vaccine research. Understanding the precise molecular arrangement of the spike glycoprotein allows the rational design of immunogens that mimic the native prefusion conformation, thereby eliciting neutralizing antibody responses more effectively. Stabilizing the spike in its prefusion state might improve the antigenic fidelity of vaccine candidates, a strategy successfully employed against respiratory syncytial virus and coronaviruses. Given the lack of licensed vaccines for many New World arenaviruses, this structural blueprint represents a critical step toward immunoprophylactic solutions.

From a therapeutic standpoint, small molecule inhibitors or monoclonal antibodies targeting the glycoprotein interfaces, glycan shields, or fusion machinery could prove invaluable. The identified allosteric sites and conserved residues essential for conformational changes offer promising targets for drug development. The study forces a reevaluation of arenavirus vulnerability landscapes and encourages investment in targeted antiviral discovery pipelines that exploit these newly mapped molecular architectures.

Furthermore, the research opens avenues to explore how viral evolution shapes glycoprotein structure in response to immune pressure and interspecies transmission barriers. Structural plasticity and glycan remodeling may underpin the zoonotic potential of arenaviruses and their ability to evade pre-existing immunity. Continuous surveillance of glycoprotein sequence variation coupled with structure-function analyses will be essential to anticipate emerging strains and guide public health responses.

In conclusion, the comprehensive molecular elucidation of the New World arenavirus glycoprotein spike presents a cornerstone advancement in our understanding of arenavirus biology. These complex viral machineries, finely tuned through evolution, blend structural ingenuity with functional precision to facilitate infection in hostile host environments. The amalgamation of cutting-edge structural biology with virological experimentation showcased in this study not only fills a critical knowledge gap but also lays a robust framework for translational efforts aiming to mitigate arenavirus-related diseases.

As arenaviruses continue to pose a significant threat to global health, particularly in Latin America where outbreaks remain a persistent concern, advances such as these are invaluable. They provide the detailed molecular targets necessary to steer the next generation of vaccine and antiviral strategies. Moreover, this study exemplifies how multidisciplinary approaches integrating structural and molecular virology yield insights with tangible real-world impacts against emerging viral pathogens.

Looking ahead, future studies may focus on the dynamics of glycoprotein interactions with host receptor variants, immune evasion tactics mediated by glycan variants, and integration of these molecular insights within cellular and animal models of pathogenesis. Continuous efforts to map structural changes under physiological conditions will further enhance the relevance of these findings.

Ultimately, this breakthrough underscores the power of modern structural biology to unravel the complex molecular machines viruses employ. As global health is continually challenged by viral emergence, such detailed molecular portraits remain our most potent tools to design effective countermeasures and safeguard human populations worldwide.

Subject of Research: Molecular architecture and functional organization of the New World arenavirus spike glycoprotein complex.

Article Title: Molecular organization of the New World arenavirus spike glycoprotein complex.

Article References:
Mann, C.J., Yang, P., Olal, D. et al. Molecular organization of the New World arenavirus spike glycoprotein complex. Nat Microbiol (2025). https://doi.org/10.1038/s41564-025-02085-6

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Among coronaviruses, the prototypical example of PCSs’ biological impact is found in SARS-CoV-2 and MERS-CoV. In these pathogens, the acquisition of polybasic cleavage motifs at the spike glycoprotein significantly enhances proteolytic processing by furin-like enzymes within the human respiratory tract. This biochemical modification increases the efficiency of viral entry and augments transmissibility between individuals. Multiple studies have delineated how furin-mediated cleavage exposes fusion peptides, enabling viral membrane fusion at the cell surface or within endosomal compartments. The functional consequences of such processing extend beyond mere infectivity, encompassing expanded cellular tropism and possibly increased pathogenicity.

The influence of polybasic cleavage sites is not confined to coronaviruses. In highly pathogenic avian influenza virus subtypes, specifically H5 and H7, the hemagglutinin (HA) protein harbors similar PCS motifs. These sites permit cleavage by a broader spectrum of proteases beyond the limited trypsin-like enzymes targeting low-pathogenic strains. As a result, viruses bearing multibasic HA cleavage sites attain systemic dissemination capabilities, infecting multiple organs rather than restricting replication to the respiratory or intestinal epithelium. This viral attribute is strongly linked to increased virulence and zoonotic potential, underscoring the PCS as a molecular marker for pathogenicity shifts and pandemic risk.

At the molecular level, proteolytic cleavage of PCSs often exposes a C-terminal sequence motif characterized by a basic amino acid-rich pattern, commonly referred to as the C-end rule (CendR) motif (R/KXXR/K). This motif mediates high-affinity interactions with neuropilin (NRP) receptors, primarily NRP1 and NRP2, which are broadly expressed transmembrane proteins involved in a variety of physiological processes such as angiogenesis, immune modulation, and neuronal guidance. The binding of viral proteins containing CendR motifs to NRPs has emerged as a mechanism facilitating viral internalization, further enhancing infectivity and potentially influencing intracellular trafficking.

Neuropilins’ role as viral entry factors extends beyond coronaviruses, with evidence implicating them in the cellular uptake of Epstein-Barr virus and Kaposi’s sarcoma-associated herpesvirus, among others. In SARS-CoV-2, NRP1 engagement has been shown to augment infectious entry, especially in cell types with low ACE2 receptor availability. However, a comprehensive understanding of whether all viruses harboring PCSs exploit NRPs for entry remains unresolved. It is unclear if neuropilins act alone or as part of a co-receptor complex, and to what extent auxiliary host factors modulate the internalization and subsequent intracellular fate of these viruses following PCS cleavage.

The evolutionary emergence of PCSs raises pressing questions around their role in zoonotic spillover and viral adaptation. Are polybasic motifs representative of convergent evolutionary pressures favoring enhanced protease susceptibility? Do PCS sequences alter viral fitness in the natural reservoir hosts or only upon transmission to humans? Experimental dissection of PCSs’ functions traditionally relies on reverse genetics techniques that introduce or delete cleavage sites within viral genomes. Yet, such manipulations are fraught with challenges due to potential lethality from impaired replication or unexpected gain-of-function phenotypes, amplifying biosafety concerns and ethical debates surrounding pathogen research.

In response, innovative methodologies have been proposed to circumvent these experimental limitations, among which nonviral protein cages (NVPCs) emerge as compelling platforms. These self-assembling proteinaceous nanostructures mimic the size and, to an extent, the geometry of viral capsids while lacking infectious material. By engineering NVPCs to display spike proteins containing native or modified PCSs or presenting isolated CendR motifs, researchers can probe the molecular mechanisms of host receptor binding, protease susceptibility, and internalization pathways in a controlled, biosafe environment. This strategy enables the decoupling of structural and functional viral studies from the risks associated with live pathogen manipulation.

Protein cages endowed with fluorescent or contrasting agents further facilitate high-resolution imaging studies of viral entry and intracellular trafficking routes. Site-specific incorporation of fluorophores or encapsulation of fluorescent proteins within cages permits real-time visualization using confocal or super-resolution microscopy techniques. These visualizations allow kinetic mapping of endocytic pathways, vesicular sorting, and the identification of subcellular compartments involved in the processing of PCS-bearing particles. Such insights are invaluable for understanding the spatiotemporal coordination of viral entry and subsequent steps defining the infectious cycle.

In complement to imaging, proteomics approaches leveraging laser microdissection enable the isolation and molecular profiling of cells that engage with virus-mimicking cages. This permits the identification of host proteins interacting with the virus-like particles during endocytosis or trafficking. Further layer-specific fractionation techniques provide detailed maps of protein distribution across cellular compartments, elucidating potential host receptors, adaptor molecules, or signaling components mediating PCS-driven viral uptake. Subsequent functional investigation utilizing small-molecule inhibitors, RNA interference, or CRISPR-mediated gene editing can validate the roles of candidate host factors in modulating viral internalization.

Structurally, NVPCs offer advantages for cryo-electron microscopy (cryo-EM) investigations due to their uniform size and inherent symmetry. These properties greatly simplify image reconstruction and improve resolution, crucial for dissecting the conformational impacts of PCS insertions or mutations within viral glycoproteins. For example, NVPCs engineered to present mutated influenza HA proteins bearing introduced polybasic sites can reveal structural alterations correlating with enhanced proteolytic accessibility and virulence. Notwithstanding, the lack of a lipid envelope and smaller size compared to native enveloped viruses like influenza may restrict the full recapitulation of native virus-host interactions.

To overcome these inherent limitations, efforts to pseudotype protein cages with viral envelopes are underway. This approach combines the structural precision of proteinaceous scaffolds with the biological complexity of a membrane bilayer, yielding more faithful virus mimics. Such pseudotyped particles could revolutionize the study of PCSs by enabling assays that simultaneously reflect the structural, functional, and phenotypic consequences of cleavage site variants in a safe laboratory context. Integrative analyses combining structural biology, cell biology, and phenotypic assays promise to elucidate the genotype-to-phenotype continuum governing PCS-mediated viral virulence.

Continued exploration of PCS functions facilitated by nonviral protein cage platforms not only enhances fundamental virology but also informs therapeutic strategies. Understanding how PCSs influence neuropilin-mediated entry or protease susceptibility can guide the development of inhibitors targeting critical protease interactions or host receptor binding. Moreover, protein cage technologies may serve as scaffolds for vaccine antigen display or as delivery vehicles for antiviral compounds, leveraging their modularity and safety profile. Thus, the convergence of protein nanotechnology with viral pathogenesis research heralds innovative avenues to combat emerging viral threats.

In essence, polybasic cleavage sites represent molecular fulcrums within viral glycoproteins that modulate host adaptation, transmission dynamics, and pathogenic potential. The strategic employment of nonviral protein cage systems offers unprecedented opportunities to dissect these multifaceted roles without the risks inherent in handling live pathogenic viruses. Through multidisciplinary integration encompassing structural biology, cell imaging, proteomics, and genetic tools, this emerging approach stands to illuminate key viral mechanisms and accelerate the development of countermeasures against current and future viral pandemics.

Subject of Research:
The investigation and characterization of polybasic cleavage sites in viral glycoproteins using nonviral protein cages as biosafe experimental tools to elucidate mechanisms of viral entry, host interactions, and pathogenicity.

Article Title:
Nonviral Protein Cages as Tools to Decipher and Combat Viral Threats

Article References:
Levasseur, M.D. Nonviral protein cages as tools to decipher and combat viral threats.
npj Viruses 3, 45 (2025). https://doi.org/10.1038/s44298-025-00127-8

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