Recombinant African swine fever virus Protein H108R (Ba71V-117)

What is the H108R protein in African swine fever virus?

H108R is a protein encoded by the H108R gene in the African swine fever virus genome. The protein has been localized to the inner envelope of the virus particle, although its precise function remains unknown . The protein ranges from 108 to 111 amino acids in length depending on the viral isolate and contains a transmembrane region between amino acids 6 and 23 in the N-terminal region . This protein is particularly significant as deletion of the H108R gene from the highly virulent ASFV-Georgia2007 strain drastically reduces virulence in domestic swine, suggesting its critical role in viral pathogenesis .

How is the H108R gene expressed during ASFV infection?

Transcriptional analysis in primary swine macrophages infected with ASFV-Georgia2007 demonstrates that H108R transcription begins at approximately 4 hours post-infection (hpi) and remains stable through 24 hpi . The expression kinetics of H108R overlap with those of the B646L gene (encoding the p72 protein), which is characterized as a late gene . This temporal expression pattern indicates that H108R should be classified as a late gene in the viral replication cycle, suggesting its involvement in later stages of virion assembly or maturation rather than early processes such as host cell takeover .

What is known about the genetic diversity of the H108R gene across ASFV isolates?

Analysis of the H108R gene across multiple ASFV isolates reveals notable genetic diversity. The length of the encoded protein varies between 108 and 111 amino acids across different viral strains . Isolates Malawi Lil-20/1, Ken06.Bus, and Kenya 1950 contain three amino acid insertions, resulting in the longest versions of the protein . Additionally, the Kenya 1950 isolate has a unique deletion at position 47 .

Particularly interesting is the observation that isolates associated with the epidemic Eurasian lineage show 100% sequence identity, indicating strong conservation of this gene throughout the pandemic lineage . A distinctive substitution (S54F) was identified in the Georgia 2007/1 isolate, which represents the pandemic lineage . Phylogenetic analysis suggests the existence of three main genetic groups, with the Georgia 2007/1 isolate belonging to genetic group I .

What evolutionary patterns have been observed in the H108R gene, and what functional implications do they suggest?

Only one amino acid position (position 50) was identified as evolving under positive selection according to the mixed effects model of evolution (MEME) . The substitution N50M appears to be associated with the divergence of specific isolates including Malawi Lil-20/1 and Ken06 . No evidence of recombination was detected when using the genetic algorithm for recombination detection (GARD) .

The concentration of purifying selection in the N-terminal region, which contains the predicted transmembrane domain, strongly suggests functional constraints on this portion of the protein, potentially related to its membrane association or interactions with other viral or host components.

What are the virological and immunological differences between infection with ASFV-G-ΔH108R versus parental ASFV-G?

The deletion of the H108R gene from ASFV-Georgia2007 results in dramatic differences in both virological parameters and host immune responses. In a comparative study, pigs inoculated with 10^2 HAD50 of ASFV-G-ΔH108R displayed strikingly different outcomes from those receiving the same dose of parental ASFV-G .

Virological differences:

• While all animals inoculated with parental ASFV-G developed fatal disease by days 6-7 post-infection, four out of five animals receiving ASFV-G-ΔH108R survived the entire 28-day observation period, with only one animal developing a protracted but ultimately fatal form of the disease .

• ASFV-G inoculated animals developed high-titer viremia (10^4.55 to 10^8.55 HAD50/mL) by day 4 post-infection . In contrast, ASFV-G-ΔH108R produced lower initial viremia, with titers peaking later (around day 11) at approximately 10^6.3 to 10^7.8 HAD50/mL, followed by a gradual decline through day 28 .

Immunological differences:

• Animals surviving ASFV-G-ΔH108R infection developed strong virus-specific antibody responses .

• Most significantly, all surviving animals were completely protected when challenged with the virulent parental ASFV-G strain 28 days after the initial infection . These animals remained clinically normal throughout a 21-day observation period following challenge, while naive control animals developed acute ASF and were euthanized between days 3-7 post-challenge .

These findings demonstrate that ASFV-G-ΔH108R infection induces protective immunity despite its attenuated phenotype, highlighting its potential relevance for vaccine development.

What methodologies are most effective for generating and characterizing H108R gene deletions in ASFV?

The generation and characterization of H108R deletion mutants of ASFV requires specialized techniques across molecular virology, cell biology, and animal studies. Based on published research, the following methodological approach has proven effective:

For mutant generation:

• The recombinant ASFV-G-ΔH108R virus was created using homologous recombination, replacing the H108R gene with a cassette containing the fluorescent reporter gene mCherry under control of the ASFV p72 promoter .

• The recombinant virus was purified through 16 limiting dilution steps based on fluorescence detection .

• Genetic modifications were confirmed by Next Generation Sequencing (NGS), which verified an 86 nucleotide deletion (positions 155,233 to 155,318) from the H108R gene and insertion of the 1226 nucleotide p72mCherry cassette .

For mutant characterization:

• In vitro analysis: Multistep growth curves in primary swine macrophages infected at low MOI (0.01), with samples collected at 2, 24, 48, 72, and 96 hours post-infection .

• In vivo virulence assessment: Intramuscular inoculation of pigs with 10^2 HAD50, followed by monitoring of clinical signs, body temperature, and viremia over a 28-day period .

• Protection studies: Challenge of surviving animals with virulent parental virus, with monitoring for clinical signs and viremia .

This comprehensive approach allows for robust assessment of both the genetic stability of the recombinant virus and its biological properties, including replication kinetics, in vivo virulence, and immunogenicity.

How does H108R compare with other ASFV virulence factors in terms of attenuation potential?

While the available search results focus primarily on H108R, they do provide context for comparing it with at least one other ASFV virulence factor, the A104R gene, which encodes a histone-like protein . Both genes, when deleted from the ASFV-Georgia isolate, produce similar phenotypes characterized by dramatically reduced virulence in domestic pigs .

The deletion of A104R results in a similar outcome to H108R deletion: most inoculated animals survive infection, exhibiting only mild clinical signs, while developing protective immunity against challenge with the virulent parental strain . This parallel suggests that ASFV virulence is multigenic, with multiple proteins contributing to the virus's pathogenic potential.

The similarity in attenuation patterns between these two genes suggests that they may function in distinct but equally critical aspects of viral replication or host interaction. Interestingly, while H108R is a transmembrane protein localized to the inner envelope , A104R is a histone-like protein , indicating that virulence factors with quite different biochemical properties and cellular localizations can produce similar attenuation phenotypes when deleted.

This observation suggests that combination approaches, deleting multiple virulence factors simultaneously, might produce more stable attenuation while maintaining immunogenicity, potentially yielding safer vaccine candidates.

What precautions should be taken when working with recombinant ASFV strains in laboratory settings?

Although not directly addressed in the search results, working with recombinant ASFV strains requires stringent biosafety measures due to the pathogenic potential of these viruses. Based on standard practices for working with Select Agents like ASFV, researchers should implement:

• Containment requirements: Work should be conducted in BSL-3 Ag facilities with appropriate engineering controls, including HEPA filtration, directional airflow, and controlled access.

• Personal protective equipment: Researchers should use appropriate PPE including disposable gowns, gloves, eye protection, and respiratory protection when needed.

• Validation procedures: Recombinant virus stocks should undergo thorough genomic verification using NGS to confirm the intended genetic modifications and absence of unintended changes .

• Inactivation protocols: Validated inactivation methods should be employed for removing samples from containment facilities for further analysis.

• Animal handling: In vivo studies require dedicated animal facilities with appropriate containment features and protocols for managing infected animals and their waste.

• Record-keeping: Detailed documentation of all procedures, including the creation, handling, and disposal of recombinant viruses.

• Regulatory compliance: Research with recombinant ASFV strains typically requires approval from institutional biosafety committees and possibly national regulatory authorities.

These precautions are essential for ensuring both researcher safety and preventing accidental release of recombinant ASFV strains.

What are the optimal parameters for evaluating protective efficacy of H108R-deleted ASFV strains?

Evaluation of the protective efficacy of H108R-deleted ASFV strains should incorporate multiple parameters to comprehensively assess both safety and protection. Based on the methodologies described in the research:

• Initial dose determination: The studies used 10^2 HAD50 for both immunization and challenge, which proved effective for demonstrating both attenuation and protection .

• Observational period: A minimum 28-day period after immunization appears necessary to fully assess attenuation , while a 21-day period post-challenge was sufficient to evaluate protection .

• Clinical assessment:

  • Daily monitoring of body temperature (with temperatures >104°F indicating clinical disease)

  • Systematic recording of clinical signs including anorexia, depression, skin discoloration, and diarrhea

  • Standardized scoring system for quantifying disease severity

• Virological parameters:

  • Regular blood sampling (e.g., days 4, 7, 11, 14, 21, 28 post-inoculation)

  • Quantification of viremia using hemadsorption assays (expressed as HAD50/mL)

  • Monitoring of viral load kinetics throughout both immunization and challenge phases

• Immunological parameters:

  • Assessment of ASFV-specific antibody responses

  • Evaluation of cell-mediated immune responses (though not specifically mentioned in the search results)

• Control groups:

  • Naive animals challenged with virulent virus

  • Animals inoculated with parental virulent virus

This comprehensive approach allows for robust evaluation of both safety (degree of attenuation) and efficacy (level of protection against challenge) of H108R-deleted ASFV strains.

What techniques are available for studying potential H108R protein interactions with host or viral factors?

While the search results don't directly address techniques for studying H108R protein interactions, several approaches would be suitable based on standard practice in molecular virology and the known characteristics of H108R:

• Protein-protein interaction identification:

  • Co-immunoprecipitation using antibodies against H108R or epitope-tagged versions

  • Proximity labeling approaches (BioID, APEX) to identify proteins in close proximity to H108R in infected cells

  • Yeast two-hybrid or mammalian two-hybrid screening to identify potential interaction partners

  • Mass spectrometry-based interactome analysis of purified viral particles or infected cells

• Localization studies:

  • Immunofluorescence microscopy to confirm and refine the localization of H108R to the inner viral envelope

  • Immuno-electron microscopy for high-resolution localization

  • Live-cell imaging using fluorescently tagged H108R to monitor dynamics during infection

• Functional analysis of interactions:

  • Site-directed mutagenesis targeting the conserved regions identified under purifying selection

  • Domain mapping to identify specific regions involved in protein-protein interactions

  • In vitro binding assays with recombinant proteins

  • Competition assays to disrupt potential interactions

• Structural approaches:

  • X-ray crystallography or cryo-electron microscopy of H108R alone or in complex with interaction partners

  • Structural modeling based on the predicted transmembrane domain

These approaches would help elucidate the molecular mechanisms underlying H108R's role in ASFV virulence and potentially identify targets for therapeutic intervention or vaccine development.

How can the H108R gene deletion be utilized in ASFV vaccine development strategies?

The H108R gene deletion shows significant promise for ASFV vaccine development based on several key observations from the research:

• Attenuated virulence with maintained immunogenicity: ASFV-G-ΔH108R demonstrates dramatically reduced virulence while still inducing strong protective immunity against challenge with virulent virus .

• Low-dose efficacy: Protection was achieved with immunization doses as low as 10^2 HAD50, suggesting potent immunogenicity despite attenuation .

• Potential applications in vaccine development:

  • As a stand-alone live attenuated vaccine candidate, though the observation that one animal still developed fatal disease indicates residual virulence that would need to be addressed .

  • As part of a multi-deletion approach to "increase the safety profile of the preexisting vaccine candidate" . Combining H108R deletion with other attenuating deletions could potentially eliminate residual virulence while maintaining immunogenicity.

  • As a platform for introducing additional modifications, such as marker mutations to enable differentiation of infected from vaccinated animals (DIVA).

• Technical considerations:

  • The recombinant virus contains a fluorescent marker (mCherry) , which could facilitate monitoring of vaccine virus replication in research settings.

  • NGS confirmation of genetic stability is essential to ensure consistent attenuation .

Table 1: Comparison of ASFV-G and ASFV-G-ΔH108R characteristics relevant to vaccine development

This data supports the potential utility of H108R deletion in ASFV vaccine development strategies, particularly as part of a multi-deletion approach to optimize the safety-immunogenicity balance.

What structural and functional predictions can be made about H108R based on current evidence?

Although detailed structural and functional characterization of H108R is limited in the available research, several key observations allow for informed predictions:

• Structural features:

  • Protein length of 108-111 amino acids depending on the isolate

  • Predicted transmembrane domain between amino acids 6-23 in the N-terminal region

  • Localization to the inner envelope of the virus particle

  • No homology with other known protein families based on Pfam analysis

• Functional inferences:

  • Expression as a late gene (from 4 hpi onwards) , suggesting involvement in virion assembly, maturation, or egress rather than early events like host cell takeover

  • Delayed replication of deletion mutants in primary macrophages , indicating a role in optimizing viral replication efficiency

  • Strong conservation of the N-terminal region under purifying selection , suggesting functional constraints on this domain

  • Potential parallel with small transmembrane proteins in poxviruses (e.g., I5L) that enhance viral replication and virulence

• Predicted functions based on these observations:

  • Possible role in virion structure or assembly, given its inner envelope localization and late expression

  • Potential involvement in host-virus interactions that facilitate efficient replication

  • Possible contribution to immune evasion or modulation, given its impact on virulence

While these predictions provide a starting framework, definitive structural and functional characterization will require additional experimental approaches including protein crystallography, interaction studies, and detailed mutational analysis targeting the evolutionarily conserved regions.

How might combination approaches using H108R deletion with other attenuating mutations enhance ASFV vaccine development?

The promising results with H108R deletion suggest several strategic directions for combined attenuating approaches:

• Rational selection of complementary deletions: Combining H108R deletion with mutations affecting different aspects of viral replication or host interaction could produce synergistic attenuation. For example, pairing the H108R deletion (affecting a structural protein) with deletion of the A104R histone-like protein might target both structural and genetic aspects of viral replication.

• Safety enhancement: The observation that 20% of animals (1 of 5) still developed fatal disease after ASFV-G-ΔH108R infection indicates that additional attenuating mutations would be necessary to achieve an acceptable safety profile for a vaccine candidate.

• Research approaches to identify optimal combinations:

  • Systematic testing of double or triple deletion mutants in vitro and in vivo

  • Comparison of different combinations for balance between attenuation and immunogenicity

  • Assessment of genetic stability of multi-deletion mutants

  • Evaluation of protection against heterologous challenge with different ASFV strains

• Potential combinations to explore:

  • H108R deletion + MGF360/MGF505 deletions (known to affect host range and interferon suppression)

  • H108R deletion + A104R deletion (targeting both envelope and histone-like proteins)

  • H108R deletion + CD2v deletion (affecting hemadsorption and virulence)

Future research should systematically evaluate these combinations to identify those that produce optimal safety and efficacy profiles for vaccine development.

What techniques would be most valuable for elucidating the molecular mechanism of H108R's contribution to ASFV virulence?

Understanding the precise molecular mechanisms through which H108R contributes to ASFV virulence will require an integrated experimental approach:

• Structural biology:

  • Determination of the three-dimensional structure of H108R, particularly focusing on the transmembrane domain and regions under purifying selection

  • Structural analysis of H108R in the context of the viral particle using cryo-electron tomography

• Interaction mapping:

  • Systematic identification of H108R interaction partners (both viral and host)

  • Temporal analysis of interaction patterns throughout the viral replication cycle

  • Determination of whether these interactions differ between virulent and attenuated strains

• Functional genomics:

  • Transcriptome and proteome analysis comparing host responses to parental and ΔH108R viruses

  • CRISPR screens to identify host factors that differentially affect replication of parental versus ΔH108R viruses

  • Phosphoproteomics to identify signaling pathways affected by H108R

• Comparative virology:

  • Detailed comparison of H108R with functionally analogous proteins in other viruses, such as the I5L protein in poxviruses mentioned in the search results

  • Construction of chimeric viruses expressing H108R variants from different ASFV strains to map virulence determinants

• Advanced imaging:

  • Live-cell imaging to track H108R localization and dynamics during infection

  • Super-resolution microscopy to precisely define H108R distribution within viral particles and infected cells

These approaches would provide complementary insights into how H108R contributes to ASFV virulence, potentially revealing new targets for intervention or improved vaccine design.