Recombinant Rinderpest virus Hemagglutinin glycoprotein (H)

Advanced Research Questions

• What are the optimal conditions for expressing functional Rinderpest virus H protein in different systems?

  Optimal expression conditions vary by system:

  Baculovirus-Insect Cell System:

  • Optimal MOI (multiplicity of infection): 5-10 for high expression

  • Harvest time: 72 hours post-infection for optimal yield

  • Culture conditions: 27°C, pH 6.2-6.4

  • Benefits: The protein gets incorporated into extracellular baculovirus, forming a particulate antigen that enhances immunogenicity

  E. coli System:

  • Strain optimization: BL21(DE3) typically used

  • Induction conditions: 0.5-1.0 mM IPTG, 25-30°C for 4-6 hours

  • Tag selection: N-terminal His-tag for efficient purification

  • Limitations: Lacks glycosylation, may require refolding protocols

  Vaccinia Virus Vector System:

  • Promoter selection: Strong synthetic promoters increase expression by 3-4 fold compared to natural P7.5 promoters

  • Insertion site: Thymidine kinase (TK) gene preferred for stable expression

  • Co-expression: Combined expression with F protein enhances immunogenicity

  For functional studies requiring properly folded protein with biological activity, the baculovirus-insect cell system offers significant advantages, including preservation of conformational epitopes and enhanced immunogenicity .

• How can the immunogenicity of recombinant Rinderpest virus H protein be enhanced for vaccine development?

  Several strategies have proven effective for enhancing immunogenicity:

  1. Co-expression with F protein: Combining H and F proteins provides superior protection compared to either protein alone, with evidence of sterilizing immunity (no anamnestic response upon challenge)

  2. Optimized expression vectors: Using strong synthetic promoters can increase expression levels 3-4 fold, enhancing immunogenicity. Vaccinia virus vectors with the Copenhagen strain background have shown excellent safety and efficacy profiles

  3. Particulate antigen formation: Expressing H protein in a form that creates virus-like structures or incorporates into extracellular virus particles enhances immunogenicity by:

     • Presenting antigens in a multivalent format

     • Improving antigen uptake by antigen-presenting cells

     • Stimulating both humoral and cell-mediated immunity

  4. Low-dose effectiveness: Remarkably, single administration of low doses (108 PFU) of recombinant vaccinia virus expressing both F and H genes provides long-term sterilizing immunity, suggesting that optimized presentation may be more important than high antigen loads

  These approaches have demonstrated protection with doses as low as 103 PFU for vaccinia-vectored H+F vaccines, highlighting the efficiency of these enhancement strategies .

• What methodologies are most effective for analyzing T-cell responses to recombinant Rinderpest virus H protein?

  Several complementary techniques have been employed to analyze T-cell responses:

  1. Lymphoproliferation assays: Peripheral blood mononuclear cells (PBMCs) from immunized cattle are cultured with recombinant H protein or peptides, and proliferation is measured by [3H]-thymidine incorporation. This approach identified:

     • BoLA class II restricted helper T cell responses

     • T-helper epitope regions (aa 123-137 and 575-583)

  2. Epitope mapping strategies:

     • Expression of truncated H protein fragments in E. coli

     • Synthetic overlapping peptides covering regions of interest

     • Cross-reactive peptides from measles virus H protein

  3. CTL assays: Cytotoxic T-lymphocyte responses are measured using:

     • 51Cr-release assays with effector and target cells

     • Flow cytometry-based killing assays

     • These assays revealed BoLA class I restricted CTL responses to recombinant H protein

  4. In vivo assessment: Challenge studies in vaccinated animals with monitoring of:

     • Clinical protection

     • Absence of anamnestic responses (indicating sterilizing immunity)

     • Prevention of virus shedding

  These methodologies have been crucial for understanding the comprehensive immune responses induced by recombinant H protein and for identifying key epitopes for vaccine development.

• How do post-translational modifications affect the antigenicity and functionality of recombinant Rinderpest virus H protein?

  Post-translational modifications significantly impact H protein functionality:

  1. Glycosylation patterns:

     • Native H protein contains N-linked glycans that influence:

       • Protein folding and stability

       • Receptor binding efficiency

       • Antigenicity and immunogenicity

     • Expression systems differ in glycosylation capabilities:

       • Insect cells provide partial glycosylation (high mannose type)

       • E. coli lacks glycosylation machinery entirely

       • Mammalian cells provide glycosylation closest to native virus

  2. Functional implications:

     • H proteins expressed in insect cells retain receptor binding functionality, indicating that insect cell glycosylation is sufficient for maintaining key antigenic properties

     • H proteins expressed in E. coli are useful for epitope mapping but may lack conformational epitopes dependent on glycosylation

  3. Immunological consequences:

     • H protein expressed in insect cells induces stronger immune responses compared to bacterially expressed protein

     • The particulate nature of H protein incorporated into extracellular baculovirus enhances immunogenicity, partially compensating for differences in glycosylation

  For vaccine development, expression systems that provide appropriate post-translational modifications are preferred to ensure proper folding, antigenicity, and functionality of the recombinant H protein.

• What are the challenges in developing DIVA (Differentiating Infected from Vaccinated Animals) strategies using recombinant Rinderpest virus H protein?

  DIVA strategies are critical for disease eradication programs. Several approaches using recombinant H protein offer potential solutions:

  1. Recombinant vaccinia virus vaccines expressing F and H:

     • Allow serological differentiation between vaccinated and naturally infected animals

     • Vaccinated animals develop antibodies only to F and H proteins

     • Naturally infected animals develop antibodies to all viral proteins

     • Companion diagnostic tests can detect antibodies to non-structural proteins

  2. Epitope deletion/modification approaches:

     • Strategic modification of non-essential epitopes in recombinant H

     • Development of tests detecting antibodies to deleted/modified epitopes

     • Challenge: Maintaining immunogenicity while modifying epitopes

  3. Chimeric H proteins:

     • Incorporation of foreign epitope tags into the H protein structure

     • Animals vaccinated with chimeric H develop antibodies to the tag

     • Companion diagnostics detect tag-specific antibodies

     • An example is the successful incorporation of GFP fusion proteins or influenza hemagglutinin epitopes into recombinant RPV

  4. Implementation challenges:

     • Validating companion diagnostic tests in field conditions

     • Ensuring equivalent protection compared to conventional vaccines

     • Regulatory approval for genetically modified vaccines

     • Training of field personnel in test interpretation

  The recombinant vaccinia virus expressing both F and H genes (v2RVFH) has shown particular promise, as it provides sterilizing immunity while allowing for serological differentiation, making it a candidate for rinderpest eradication programs .

• How can recombinant Rinderpest virus H protein be used to develop cross-protective vaccines against related morbilliviruses?

  The potential for cross-protection stems from structural and antigenic similarities among morbillivirus H proteins:

  1. Conserved T-helper epitopes:

     • The region around aa 123-137 in RPV H is conserved with measles virus and other morbilliviruses

     • This conservation suggests potential for cross-reactive T-cell responses

  2. Experimental approaches for cross-protective vaccines:

     • Chimeric H proteins incorporating conserved epitopes from multiple morbilliviruses

     • Focus on conserved regions while maintaining species-specific neutralizing epitopes

     • Evaluation of cross-neutralization by sera from animals immunized with recombinant H

  3. Potential applications:

     • Protection against Peste-des-petits-ruminants virus (PPRV) in small ruminants

     • A recombinant Bovine Herpesvirus-4 vector expressing PPRV H has shown promise in mice, suggesting a similar approach could work for RPV H

     • Combined vaccines protecting against multiple morbilliviruses

  4. Challenges to address:

     • Balancing breadth and potency of immune responses

     • Ensuring protection against emerging variants

     • Validating cross-protection in relevant animal models

  The high homology between regions of RPV H and other morbillivirus H proteins provides a scientific foundation for developing vaccines with broader protection, potentially addressing multiple morbillivirus threats with a single vaccine formulation.

• What are the most effective techniques for purifying recombinant Rinderpest virus H protein while preserving its immunogenicity?

  Purification strategies must balance yield with preservation of conformational epitopes:

  1. For His-tagged H protein expressed in E. coli:

     • Immobilized metal affinity chromatography (IMAC) using Ni-NTA resins

     • Inclusion body solubilization using 8M urea followed by on-column refolding

     • Size exclusion chromatography for final polishing

     • Challenge: Recovering properly folded protein from inclusion bodies

  2. For H protein expressed in insect cells:

     • Purification of extracellular virus containing H protein by:

       • Ultracentrifugation through sucrose cushions

       • Gradient purification to isolate intact viral particles

     • Advantages: Preserves native conformation and particulate structure

     • This approach showed superior immunogenicity in cattle studies

  3. For vaccinia virus-expressed H protein:

     • Direct immunization with recombinant virus

     • Purification generally not required as the vaccine is used as live recombinant virus

     • Demonstrated excellent efficacy with doses as low as 103 PFU

  4. Quality control parameters:

     • Functional assessment through hemagglutination assays

     • Confirmation of antigenic integrity via ELISA with conformation-dependent monoclonal antibodies

     • SDS-PAGE and Western blot analysis for purity and identity

  For vaccine applications, maintaining the particulate nature of the antigen (either as recombinant virus or virus-like particles) has proven more effective than purified monomeric protein, highlighting the importance of structural presentation for optimal immunogenicity.

• How do mutations in the Rinderpest virus H protein affect receptor binding and cell fusion activities?

  Structure-function relationships in the H protein are critical for viral fitness:

  1. Receptor binding domains:

     • Key regions involved in receptor binding include the beta-sheet propeller structure

     • Mutations in these regions can alter:

       • Host range and tissue tropism

       • Binding affinity to cellular receptors

       • Potential for cross-species transmission

  2. Interaction with F protein:

     • H protein binding to cellular receptors triggers conformational changes

     • These changes activate the F protein, initiating membrane fusion

     • Mutations affecting H-F interactions can impair fusion activity

     • Evidence: Extensive syncytium formation observed with co-expression of F and H proteins under strong promoters

  3. Experimental observations:

     • Recombinant vaccinia virus expressing both F and H (v2RVFH) forms large syncytia containing hundreds of nuclei

     • This extensive fusion activity correlates with enhanced immunogenicity

     • In contrast, lower expression levels result in smaller syncytia with fewer nuclei

  4. Functional implications for vaccine design:

     • Optimizing expression levels of both H and F proteins enhances fusion activity

     • Strong synthetic promoters increase expression 3-4 fold over natural promoters

     • Enhanced syncytium formation may contribute to improved immunogenicity by increasing antigen presentation and spreading

  Understanding these structure-function relationships is crucial for designing optimized vaccine antigens and for predicting the impact of naturally occurring mutations on viral pathogenicity and host range.