Recombinant Rift valley fever virus Envelope glycoprotein (GP)

What are the main envelope glycoproteins of Rift Valley fever virus?

The Rift Valley fever virus (RVFV) genome encodes two primary envelope glycoproteins: the aminoterminal glycoprotein (Gn) and the carboxyterminal glycoprotein (Gc). These proteins are expressed from the M segment of the viral genome, which has multiple translation initiation sites within its single mRNA transcript . The envelope glycoproteins are critical structural components that play essential roles in viral entry and represent major targets for neutralizing antibodies . Both glycoproteins are expressed in glycosylated form, with Gc having four putative N-glycosylation sites, while Gn has one putative N-glycosylation site .

How do RVFV envelope glycoproteins contribute to protective immunity?

RVFV envelope glycoproteins Gn and Gc contain epitopes that elicit the induction of neutralizing antibodies, which are established correlates of protective immunity against RVFV infection . Natural exposure to RVFV generates long-lived protective neutralizing antibodies in both humans and livestock . When passively transferred into mice, human serum samples containing neutralizing antibodies confer protection against RVFV challenge in a dose-dependent manner, supporting the importance of neutralizing antibodies in protection . These neutralizing antibodies target the Gn and Gc glycoproteins that are well conserved across virus strains, thereby providing cross-protective immunity against virus lineages from distant geographical settings .

What expression systems are most effective for producing recombinant RVFV glycoproteins?

The baculovirus expression system has been successfully employed for producing recombinant RVFV glycoproteins with proper post-translational modifications. This approach involves cloning the coding sequences for the glycoproteins (either the ectodomain of Gn or full-length Gc) into baculovirus vectors for expression in insect cells (typically Sf9 cells) .

For Gn expression, researchers have utilized the ectodomain (Gne) rather than the full-length protein in some studies . The expression protocol typically involves:

• Cloning the target sequence into a baculovirus vector

• Transfection into insect cells

• Collection and purification of the expressed proteins

• Verification of expression using Western blot analysis with anti-His antibodies or monoclonal antibodies against the target proteins

In vitro biochemical analysis confirms that the recombinant glycoproteins produced with this system are expressed in glycosylated form, which can be verified through treatment with glycosylation inhibitors such as tunicamycin .

What adjuvants are most effective for RVFV glycoprotein-based subunit vaccines?

For RVFV glycoprotein-based subunit vaccines, oil-based adjuvants have shown promising results in experimental studies. In cattle vaccination studies, recombinant Gn or Gn and Gc glycoproteins were formulated in Montanide ISA-25 VG (Seppic, France), a ready-to-use vaccine adjuvant . This formulation was effective in eliciting protective immune responses when administered subcutaneously.

When designing adjuvant formulations for RVFV glycoprotein vaccines, researchers should consider:

• The ability to enhance both humoral and cellular immune responses

• Safety profile in the target species

• Stability of the vaccine-adjuvant mixture

• Route of administration (subcutaneous administration has shown efficacy in experimental studies)

The selection of appropriate adjuvants is critical as it significantly influences the magnitude and quality of the immune response to the vaccine antigens.

How do single-dose versus prime-boost vaccination strategies compare for glycoprotein-based RVFV vaccines?

Research indicates that both single-dose and prime-boost strategies can be effective for glycoprotein-based RVFV vaccines, though with varying degrees of immunogenicity and protection.

In cattle studies, animals were vaccinated with either one or two doses of vaccines containing Gn only, or Gn and Gc combined . The elicited immune responses by some vaccine formulations (both single and double vaccination approaches) conferred complete protection from RVF within 35 days after the first vaccination .

For sheep, a single subcutaneous vaccination with a glycoprotein-based subunit vaccine elicited high virus neutralizing antibody titers and conferred complete protection, as evidenced by prevention of viremia, fever, and absence of RVFV-associated histopathological lesions .

The determination between single-dose or prime-boost strategies should be based on:

• The target species and its immune response characteristics

• The glycoprotein composition of the vaccine (Gn alone or Gn/Gc combined)

• The adjuvant system being used

• The desired speed of protective immunity development

• Practical considerations for field application in endemic regions

What are the advantages of glycoprotein-based subunit vaccines over other RVFV vaccine approaches?

Glycoprotein-based subunit vaccines offer several distinct advantages over alternative RVFV vaccine approaches:

What are the key immune correlates of protection following RVFV glycoprotein vaccination?

The primary immune correlate of protection following RVFV glycoprotein vaccination is the development of neutralizing antibodies targeting the Gn and Gc envelope glycoproteins . These findings from multiple studies have established neutralizing antibodies as the main correlate of protective immunity against RVFV infection.

In a murine model study, vaccinated mice depleted of T cells were still protected against subsequent challenge, demonstrating that T cells were dispensable in the presence of humoral immunity . Moreover, passive transfer of immune serum from vaccinated animals to naïve animals was also protective, confirming that humoral immunity alone was sufficient for protection .

The timeline of antibody development following vaccination appears to be important:

• In sheep vaccinated with the MP12 strain, recombinant N protein was reactive with day-3 post-vaccination sera

• Both N and NSs proteins showed antibody reactivity with day-10 and day-28 post-vaccination sera

• The Gn ectodomain (Gne) was reactive with some serum samples as early as day 3 post-vaccination, and consistently reactive with all sera obtained on days 10 and 28

• The Gc protein showed no reactivity at day 3, weak reactivity with day 10 sera, but was 90% reactive with day 28 post-vaccination sera

How do cellular immune responses contribute to protection against RVFV following glycoprotein vaccination?

While neutralizing antibodies are considered the primary correlate of protection against RVFV, cellular immunity may also play a role under certain conditions. In a murine model study, animals depleted of B cells and then vaccinated with an attenuated RVFV strain (DelNSsRVFV) were protected against challenge, suggesting a role for cellular immunity in the absence of B cells .

For glycoprotein-based vaccines specifically, the cellular immune response profile may differ from that observed with attenuated virus vaccines, and further research is needed to fully characterize these responses.

What is the durability of immune responses to recombinant RVFV glycoprotein vaccines?

• Natural exposure to RVFV generates long-lived protective neutralizing antibodies in both humans and livestock, suggesting that properly designed vaccines targeting the same epitopes might induce similar durability .

• In experimental studies with sheep, a single subcutaneous vaccination with the glycoprotein-based subunit vaccine elicited high virus neutralizing antibody titers that remained protective through the challenge period (35 days post-vaccination) .

• In cattle studies, vaccine formulations provided complete protection when challenged 35 days after the first vaccination, preventing viremia, fever, and RVFV-associated histopathological lesions .

Factors that may influence durability include:

• Adjuvant selection

• Glycoprotein conformation in the vaccine formulation

• Dosage and vaccination schedule

• Individual variation in immune response

• Species-specific differences in immune memory development

Long-term follow-up studies monitoring neutralizing antibody titers and protection against challenge at extended timepoints (6-12 months or longer) would provide valuable information on the durability of these vaccine-induced responses.

What are the optimal methods for assessing neutralizing antibody responses to RVFV glycoproteins?

For assessing neutralizing antibody responses to RVFV glycoproteins, several methodological approaches can be employed:

• Virus neutralization tests (VNT): This gold standard assay measures the ability of antibodies to prevent virus infection of susceptible cells. For RVFV, this typically involves mixing serial dilutions of test sera with a standardized amount of virus, incubating to allow antibody binding, then adding to susceptible cells to assess inhibition of cytopathic effect .

• Plaque reduction neutralization tests (PRNT): A variation of the VNT where neutralization is measured by reduction in viral plaque formation. This provides a quantifiable measure of neutralizing activity, often expressed as the dilution that reduces plaques by 50% or 80% (PRNT50 or PRNT80).

• Pseudotyped virus neutralization assays: These utilize reporter viruses pseudotyped with RVFV glycoproteins, offering a safer alternative to working with live RVFV for neutralization studies.

• Competitive ELISA: While not directly measuring neutralization, competitive ELISAs using monoclonal antibodies that bind to neutralizing epitopes can serve as a surrogate marker for neutralizing antibody responses.

When conducting these assays, researchers should consider:

• The selection of appropriate cell lines for the assay

• Standardization of virus input

• Inclusion of positive and negative control sera

• Potential variability between different viral strains

• Biosafety considerations when working with RVFV

How can glycosylation patterns of recombinant RVFV glycoproteins be verified and optimized?

Glycosylation patterns of recombinant RVFV glycoproteins can be verified and optimized through several approaches:

• Tunicamycin inhibition assay: Treatment of glycoprotein-expressing cells with varying concentrations of tunicamycin (0.5 μg/mL to 10 μg/mL) inhibits N-linked glycosylation, resulting in a detectable shift in electrophoretic migration. This method has been successfully used to confirm glycosylation of recombinant Gc and Gn proteins. The magnitude of the shift correlates with the number of glycosylation sites (Gc shows a more pronounced shift with its four putative N-glycosylation sites compared to Gn with one site) .

• Enzymatic deglycosylation: Treatment with enzymes such as PNGase F (which removes N-linked glycans) or O-glycosidases can confirm the presence and type of glycosylation.

• Lectin binding assays: Different lectins bind specifically to different glycan structures, allowing detection and characterization of glycosylation patterns.

• Mass spectrometry: For detailed characterization of glycan structures attached to the recombinant proteins.

Optimization strategies include:

• Selection of appropriate expression systems (insect cells vs. mammalian cells)

• Modification of culture conditions

• Co-expression of glycosylation-enhancing enzymes

• Genetic modification of glycosylation sites

• Use of glycoengineered cell lines

The choice of expression system significantly impacts glycosylation patterns; while baculovirus-infected insect cells produce glycosylated proteins, the glycan structures differ from those in mammalian cells. For vaccines, these differences may impact immunogenicity and should be considered in vaccine design.

What heterologous challenge models best evaluate cross-protection of glycoprotein-based RVFV vaccines?

To evaluate the cross-protective potential of glycoprotein-based RVFV vaccines, heterologous challenge models using diverse RVFV strains from different geographical regions and timepoints are most informative. Key considerations for designing such models include:

• Selection of vaccine and challenge strains: In published studies, vaccines based on the 1977 human RVFV isolate ZH548 have been tested against heterologous challenge with the virulent Kenya-128B-15 RVFV strain (isolated from Aedes mosquitoes in 2006) . This represents both a temporal and geographical divergence.

• Target species selection: Both sheep and cattle have been used successfully in heterologous challenge models . The choice depends on research objectives, as different species may show different clinical manifestations and immune responses.

• Challenge parameters:

  • Route of administration (typically mimicking natural infection)

  • Viral dose (sufficient to cause disease in control animals)

  • Timing post-vaccination (typically 28-35 days, but longer intervals may better assess durability)

• Outcome measures:

  • Prevention of viremia (quantified by RT-PCR and/or virus isolation)

  • Prevention of fever and clinical signs

  • Absence of RVFV-associated histopathological lesions

  • Antibody responses (neutralizing antibody titers)

  • Survival rates

• Genomic considerations: Sequencing of both vaccine and challenge strains to quantify genetic divergence, particularly in the glycoprotein-encoding regions, provides context for interpreting cross-protection results.

The advantage of heterologous challenge models is their ability to predict vaccine performance against diverse field strains, which is crucial for vaccines intended for deployment in different geographical regions or during evolving outbreaks.

How do synergistic effects between Gn and Gc antibodies impact protection against RVFV?

Research indicates potential synergistic effects between antibodies targeting Gn and Gc glycoproteins in neutralizing RVFV. While both glycoproteins can independently induce neutralizing antibodies, their combined effect may offer enhanced protection through synergistic neutralization .

Besselaar and Blackburn demonstrated the synergistic neutralization of RVFV by monoclonal antibodies to the envelope glycoproteins . This suggests that vaccine formulations containing both Gn and Gc might provide more robust protection than those containing a single glycoprotein.

The mechanisms behind this synergy may include:

• Binding to complementary neutralizing epitopes

• Enhanced virus aggregation and clearance

• Improved complement activation

• Cooperative blocking of multiple steps in the virus entry process

Further research using defined monoclonal antibodies and structure-based design approaches could help optimize vaccine formulations to maximize this synergistic protection.

What are the challenges in developing multivalent vaccines incorporating RVFV glycoproteins with other pathogens?

Developing multivalent vaccines incorporating RVFV glycoproteins alongside antigens from other pathogens presents several technical and immunological challenges:

• Antigenic interference: Different vaccine components may compete for immune responses, potentially reducing the efficacy against individual pathogens. This requires careful formulation and dosage optimization.

• Adjuvant compatibility: Different antigens may require different adjuvants for optimal immunogenicity, creating formulation challenges for a combined vaccine.

• Stability concerns: Combined antigens may have different stability profiles and storage requirements, complicating vaccine production and distribution.

• Validation complexity: Demonstrating efficacy against multiple pathogens significantly increases the complexity and cost of clinical trials and regulatory approval processes.

• Target species considerations: For veterinary vaccines, different target species may respond differently to combined formulations, requiring species-specific optimization.

• DIVA compatibility across pathogens: Maintaining DIVA capability for multiple pathogens adds another layer of complexity to vaccine design.

Potential approaches to address these challenges include:

• Sequential immunization protocols rather than simultaneous administration

• Compartmentalized delivery systems (e.g., nanoparticles with different antigens)

• Rational antigen selection based on immunodominance profiles

• Chimeric antigen design incorporating epitopes from multiple pathogens

How might structural biology approaches enhance RVFV glycoprotein-based vaccine design?

Structural biology approaches offer significant potential to enhance RVFV glycoprotein-based vaccine design through several strategies:

• Epitope mapping and stabilization: Detailed structural analysis of Gn and Gc can identify neutralizing epitopes that can be stabilized in their native conformation to enhance immunogenicity. This approach could lead to structure-based designed immunogens that more effectively present critical epitopes to the immune system.

• Structure-guided mutations: Introducing specific mutations based on structural information can enhance protein stability, expression levels, or immunogenicity without compromising protective epitopes.

• Glycan engineering: Understanding the role of glycosylation sites in protein folding, stability, and immune recognition can guide modifications to optimize vaccine performance. The differential glycosylation patterns observed between Gn (one putative N-glycosylation site) and Gc (four putative N-glycosylation sites) provide a foundation for such engineering.

• Multimeric presentation platforms: Structural knowledge can inform the design of multimeric presentation platforms that display glycoproteins in virus-like configurations, potentially enhancing B-cell activation and antibody responses.

• Rational adjuvant pairing: Structural understanding of how adjuvants interact with glycoproteins can guide the selection of optimal adjuvant-antigen combinations.

• Nanoparticle display: Designing nanoparticles based on structural data to display RVFV glycoproteins in oriented arrays that mimic viral surfaces could enhance immunogenicity through multivalent presentation.

These approaches could address current limitations in recombinant glycoprotein vaccines by improving stability, immunogenicity, and manufacturing consistency, ultimately leading to more effective and broadly protective RVFV vaccines.