Recombinant Coxiella burnetii Succinate dehydrogenase hydrophobic membrane anchor subunit (sdhD)
Description
Introduction to Coxiella burnetii and the Succinate Dehydrogenase Complex
Coxiella burnetii is a highly infectious, intracellular, Gram-negative bacterial pathogen responsible for Q fever, a zoonotic disease with worldwide distribution . Initially classified within the Rickettsia genus, C. burnetii was later reclassified into its own genus, Coxiella . This bacterium was first discovered in 1935 during an outbreak of febrile illness among abattoir workers in Brisbane, Australia, and was first isolated two years later following the injection of patient blood and urine samples into guinea pigs .
The pathogen establishes infection by creating a unique lysosome-derived intracellular niche termed the Coxiella-containing vacuole (CCV) . This specialized compartment enables the bacterium to survive and replicate within host cells, particularly alveolar macrophages, by manipulating host cell functions through a sophisticated type IV secretion system (T4SS) . The disease manifestations of Q fever are polymorphic, ranging from acute flu-like symptoms to potentially lethal chronic infections, primarily endocarditis and vascular infections .
Succinate dehydrogenase (Sdh), also known as Complex II in the electron transport chain, plays a crucial role in both the tricarboxylic acid (TCA) cycle and aerobic respiration. In C. burnetii, the Sdh enzyme complex consists of four protein subunits encoded by the sdhCDAB gene cluster . Among these, SdhD functions as a hydrophobic membrane anchor subunit, essential for the proper assembly and function of the entire complex within the bacterial membrane.
Protein Characteristics of C. burnetii SdhD
The SdhD protein from C. burnetii strain RSA 493 / Nine Mile phase I has been cataloged in the UniProt database under accession number P51057 . Commercial recombinant versions of this protein typically achieve purities exceeding 85% as determined by SDS-PAGE analysis . The recombinant protein is often produced in E. coli expression systems, which provide an efficient platform for generating sufficient quantities for research purposes.
A comparative analysis of SdhD amino acid composition across multiple bacterial species reveals several conserved hydrophobic domains that are essential for membrane integration. These domains are punctuated by charged residues that likely participate in interactions with other subunits of the Sdh complex.
Genetic Organization and Expression of the sdhD Gene
The genetic architecture surrounding the sdhD gene in C. burnetii provides valuable insights into its regulation and functional relationships. The sdhCDAB gene cluster, which includes sdhD, is positioned 3326 base pairs upstream from the citrate synthase-encoding gene (gltA) start codon and is transcribed in the opposite direction . This arrangement suggests coordinated regulation of TCA cycle components.
Notably, an open reading frame encoding the N-terminal 280 amino acids of 2-oxoglutarate dehydrogenase (SucA) begins just 24 base pairs downstream from the stop codon of the gene specifying the iron-sulfur subunit (sdhB) of Sdh . This proximity indicates a potential operon structure that encompasses multiple components of central metabolic pathways.
Primer extension studies have identified transcription start points for both sdh and sucA genes. While the region upstream from the sdh transcription start point displays homology to promoter consensus sequences of E. coli, no such homology was observed for the sucA transcription start point . This finding suggests differential regulation of these genes despite their physical proximity.
Further evidence for independent regulation comes from experiments demonstrating that transcription of sucA can occur independently of sdh transcription. A TnphoA insertion disrupting sdhB had no effect on the production of SucA by an E. coli cell-extract-directed in vitro transcription/translation system . This independence may allow for metabolic flexibility under varying environmental conditions.
Role in Energy Metabolism
As part of the succinate dehydrogenase complex, SdhD contributes to two essential metabolic processes:
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In the TCA cycle, the Sdh complex catalyzes the oxidation of succinate to fumarate.
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In the electron transport chain, the complex transfers electrons from succinate to ubiquinone (coenzyme Q), contributing to the proton gradient that drives ATP synthesis.
The hydrophobic nature of SdhD enables proper anchoring of the complex in the membrane, positioning the catalytic components optimally for interaction with other components of the respiratory chain.
Production and Purification of Recombinant SdhD
Recombinant production of C. burnetii SdhD provides a valuable tool for structural, functional, and immunological studies. The protein is typically produced in heterologous expression systems, with E. coli being the most common host .
Commercial recombinant C. burnetii SdhD (such as product CSB-EP344689DXP1-B) is available with purities exceeding 85% as determined by SDS-PAGE . These preparations often include tag systems to facilitate purification and detection, though the specific tag type may vary depending on the manufacturing process and intended application.
Expression Systems for SdhD Production
The following table summarizes common expression systems used for recombinant SdhD production:
| Expression System | Advantages | Limitations | Typical Yield | Purity |
|---|---|---|---|---|
| E. coli | High yield, rapid growth, cost-effective | May lack proper folding for complex proteins | 10-50 mg/L | >85% |
| Insect cells | Better folding for eukaryotic proteins | Higher cost, longer production time | 5-20 mg/L | >90% |
| Mammalian cells | Superior post-translational modifications | Highest cost, complex media requirements | 1-10 mg/L | >95% |
Among these options, E. coli remains the predominant system for SdhD production due to its balance of yield, cost-effectiveness, and relative simplicity .
Applications of Recombinant SdhD in Research and Diagnostics
Recombinant C. burnetii SdhD has several important applications in both basic research and applied sciences:
Immunological Applications
Recombinant SdhD serves as an antigen for antibody production, enabling the development of immunodetection methods for C. burnetii. Such antibodies can be used in techniques including Western blotting, immunofluorescence microscopy, and enzyme-linked immunosorbent assays (ELISAs). These tools facilitate research into C. burnetii pathogenesis and may contribute to improved diagnostic methods for Q fever.
Drug Discovery and Therapeutic Development
As a component of a vital metabolic enzyme, SdhD represents a potential target for antimicrobial development. Recombinant protein enables high-throughput screening of compound libraries to identify molecules that specifically inhibit C. burnetii Sdh function. Such inhibitors could form the basis for novel therapeutics against Q fever, addressing concerns about antibiotic resistance in C. burnetii .
Role of SdhD in Coxiella burnetii Pathogenesis
The contribution of SdhD to C. burnetii pathogenesis must be considered within the context of central metabolism and bacterial survival within host cells. As C. burnetii establishes infection within a specialized vacuole derived from host lysosomes, it must adapt its metabolism to the unique conditions of this niche. The Sdh complex, including SdhD, likely plays a crucial role in this metabolic adaptation.
Recent studies exploring the genomic features of C. burnetii have revealed that the bacterium possesses an open pangenome, with pathogenicity islands detected across all analyzed genomes . While specific virulence roles for SdhD have not been directly demonstrated, the protein's function in energy metabolism suggests its importance for bacterial survival and replication within host cells.
Notably, genes encoding the type IVB secretion system, essential for C. burnetii pathogenesis, were found in all 75 C. burnetii genomes analyzed in a recent pangenomic study . This secretion system enables the bacterium to manipulate host cell functions, facilitating establishment and maintenance of the Coxiella-containing vacuole.
Potential Therapeutic Targeting of SdhD
The emergence of antibiotic-resistant strains of C. burnetii highlights the need for new therapeutic approaches . Given its essential role in energy metabolism, the Sdh complex, including SdhD, represents a potential target for antimicrobial development.
Selective inhibition of bacterial Sdh function could disrupt C. burnetii metabolism while minimizing effects on host cells. This approach aligns with current interest in host-directed antimicrobial drugs (HDADs) that target host processes required for intracellular bacterial growth . While not directly targeting SdhD, these approaches demonstrate the potential of metabolic targeting as a therapeutic strategy.
A recent screening of FDA-approved or late-stage clinical trial compounds identified 88 that significantly inhibited C. burnetii growth within human THP-1 macrophage-like cells . Many of these compounds target components of neurotransmitter systems and have been used to treat psychosis and mood-related disorders. These findings open new avenues for drug repurposing in Q fever treatment, potentially including compounds that affect metabolic pathways involving the Sdh complex.
Comparative Analysis of SdhD Across Bacterial Species
The SdhD protein belongs to a conserved family of membrane anchor subunits found across diverse bacterial species. Comparative genomic analyses provide insights into evolutionary relationships and functional conservation of this protein.
The C. burnetii SdhD protein shares approximately 36.6% amino acid identity with its E. coli homologue, representing a moderate level of conservation . This degree of similarity is sufficient to enable functional complementation, as demonstrated by the ability of C. burnetii sdhCDAB genes to rescue an E. coli sdhA mutant .
Conservation Across C. burnetii Strains
The following table presents a comparative analysis of selected genomic features across C. burnetii strains that may influence SdhD expression and function:
| Genomic Feature | Range Across Strains | Significance |
|---|---|---|
| Genome size | 1.910 - 2.158 Mb | Reflects strain-specific gene content |
| G+C content | 42.4 - 42.9% | Indicates evolutionary stability |
| Number of contigs | 1 - 282 | Reflects assembly quality |
| MST genotypes | 22 different types | Indicates genetic diversity |
| Pathogenicity islands | Present in all genomes | Suggests conserved virulence mechanisms |
This genomic consistency, particularly in core metabolic functions, underscores the potential importance of SdhD across all C. burnetii strains.
Product Specs
Note: We prioritize shipping the format currently in stock. However, if you have specific format requirements, please indicate them during order placement. We will accommodate your request.
Note: All proteins are shipped with standard blue ice packs. If you require dry ice shipping, please communicate this in advance as additional fees will apply.
Generally, the shelf life of liquid form is 6 months at -20°C/-80°C. The shelf life of lyophilized form is 12 months at -20°C/-80°C.
Target Background
KEGG: cbu:CBU_1402
STRING: 227377.CBU_1402
Q&A
What is the genomic organization of the succinate dehydrogenase (sdh) gene cluster in Coxiella burnetii?
The succinate dehydrogenase enzyme complex in Coxiella burnetii is encoded by a gene cluster designated as sdhCDAB. This cluster begins 3326 base pairs upstream from the citrate synthase-encoding gene (gltA) start codon and is transcribed with opposite polarity. The organization includes four genes encoding the protein subunits that comprise the succinate dehydrogenase enzyme complex. An open reading frame encoding the N-terminal portion of 2-oxoglutarate dehydrogenase (SucA) begins 24 base pairs downstream from the stop codon of the gene specifying the iron-sulfur subunit (sdhB) of Sdh .
How does C. burnetii sdhD compare to homologous proteins in other bacteria?
The deduced amino acid sequence of C. burnetii sdhD reveals significant, though relatively lower, amino acid identity with its Escherichia coli homologue. Specifically, SdhD shows approximately 36.6% amino acid identity with the E. coli counterpart, which represents the lowest identity among the Sdh subunits. In comparison, SdhA and SdhB demonstrate higher conservation with approximately 61.2% amino acid identity to their E. coli homologues . This varying degree of conservation may reflect functional constraints on different components of the enzyme complex.
What is known about the transcriptional regulation of the sdh gene cluster in C. burnetii?
Primer extension analysis has identified specific transcription start points (tsp) for the sdh gene cluster. The region upstream from the sdh transcription start point displays homology to promoter consensus sequences found in E. coli, suggesting conservation of regulatory elements. Transcriptional studies have demonstrated that the C. burnetii sdhCDAB coding and upstream regulatory regions can be functionally expressed in heterologous systems, as evidenced by successful complementation of an E. coli sdhA mutant (MOB252) .
What are the challenges in designing expression systems for recombinant C. burnetii membrane proteins like sdhD?
Expressing recombinant membrane proteins from C. burnetii presents several distinctive challenges. The hydrophobic nature of membrane anchor subunits like sdhD often leads to protein misfolding, aggregation, or toxicity to the host cells. Based on methodologies applied to other C. burnetii proteins, successful expression strategies typically involve: (1) careful selection of expression vectors with tightly regulated promoters; (2) fusion partners that enhance solubility (his-tags are commonly used); (3) optimization of induction conditions with lower temperatures (16-25°C) and reduced inducer concentrations; and (4) screening multiple E. coli host strains specialized for membrane protein expression . Expression in cell-free systems may also be considered for particularly challenging membrane proteins.
How might mutations in the sdhD gene affect C. burnetii metabolism and virulence?
Mutations in sdhD would likely disrupt the assembly and function of the succinate dehydrogenase complex, which plays a crucial role in the tricarboxylic acid (TCA) cycle and the electron transport chain. Such disruptions could have profound effects on C. burnetii's energy metabolism, particularly relevant given its intracellular lifestyle and adaptation to the acidic environment of the phagolysosome. While direct evidence for sdhD's role in virulence is not available in the search results, metabolic perturbations are known to affect pathogen persistence and replication capabilities . Comparative studies with sdhD mutants under different growth conditions would be valuable to determine the relationship between central carbon metabolism and virulence in this obligate intracellular pathogen.
How could a lysine auxotrophy-based selection system be adapted for genetic manipulation of the sdhD gene in C. burnetii?
The recently developed nutritional selection system based on lysine auxotrophy for C. burnetii could be specifically adapted for sdhD genetic studies. This approach would involve: (1) creating a lysine auxotrophic C. burnetii strain through deletion or inactivation of a lysine biosynthesis gene; (2) constructing a complementation plasmid containing both the lysine biosynthesis gene as a selectable marker and either wild-type sdhD for complementation studies or mutated sdhD for functional analysis; (3) transforming the auxotrophic strain with this construct; and (4) selecting transformants on media lacking lysine supplementation . This system enables precise genetic manipulation without antibiotic selection, which is particularly valuable for membrane proteins where antibiotic resistance markers might interfere with membrane insertion.
What are the optimal conditions for overexpression and purification of recombinant C. burnetii sdhD protein?
Based on methodologies applied to other C. burnetii recombinant proteins, the following protocol would likely be effective for sdhD:
Expression System:
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Vector: pET-based expression system with his-tag fusion
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Host: E. coli strains specialized for membrane proteins (C41(DE3) or C43(DE3))
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Culture conditions: LB medium, 0.1-0.5 mM IPTG induction at OD600 0.6-0.8, followed by 16-18°C incubation for 16-20 hours
Purification Protocol:
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Cell lysis using mild detergents (e.g., n-dodecyl-β-D-maltoside)
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Membrane fraction isolation via differential centrifugation
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Solubilization with appropriate detergent mixtures
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Immobilized metal affinity chromatography (IMAC) using Ni-NTA resin
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Size exclusion chromatography for final purification
This approach has successfully been used for other C. burnetii proteins, with most recombinant proteins showing good antigenicity when properly purified .
What techniques are most effective for assessing the functional activity of recombinant sdhD in vitro?
To evaluate whether recombinant sdhD is functionally active, researchers should consider:
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Reconstitution assays: Combining purified recombinant sdhD with other Sdh subunits to reconstitute the enzyme complex, followed by activity measurements using succinate oxidation assays with artificial electron acceptors like dichlorophenolindophenol (DCIP)
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Complementation studies: Transformation of E. coli sdhD mutants with the C. burnetii sdhD gene, followed by growth assessment on media with succinate as the sole carbon source
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Membrane integration analysis: Blue-native PAGE combined with western blotting to verify incorporation into intact Sdh complexes
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Enzyme activity measurements: Spectrophotometric assays measuring electron transfer from succinate to ubiquinone analogues
Similar complementation approaches with the full sdhCDAB gene cluster have demonstrated functional expression, with a cell extract of E. coli sdhA mutant (MOB252) transformed with C. burnetii genes showing sixfold greater Sdh enzyme activity compared to wild-type E. coli .
How should researchers design immunization experiments to evaluate sdhD as a potential vaccine candidate?
When evaluating sdhD as a potential vaccine candidate, researchers should consider the following experimental design elements based on previous studies with other C. burnetii proteins:
Animal Model Selection:
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BALB/c mice have been successfully used in C. burnetii vaccine studies
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Group size should be statistically significant (typically 8-10 animals per group)
Immunization Protocol:
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Prepare highly purified recombinant sdhD protein (>95% purity)
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Formulate with appropriate adjuvants (aluminum hydroxide or other TLR-activating adjuvants)
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Administer 3 doses at 2-week intervals
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Include control groups: negative control (adjuvant only), positive control (licensed Q-Vax vaccine)
Efficacy Evaluation:
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Challenge with C. burnetii Nine Mile RSA493 strain (1.8 × 10^8 organisms) intraperitoneally
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Monitor clinical signs, weight changes, and spleen/liver weights
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Assess bacterial burden in tissues via qPCR and culture
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Analyze humoral and cellular immune responses
This approach mirrors previous immunization studies with recombinant C. burnetii proteins, though researchers should note that earlier recombinant protein mixtures failed to provide protective immunity compared to the whole-cell Q-Vax vaccine .
What are the most informative assays for characterizing the immunogenicity of recombinant sdhD protein?
To comprehensively characterize the immunogenicity of recombinant sdhD protein, researchers should employ the following assays:
Humoral Immunity Assessment:
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ELISA to detect sdhD-specific IgG, IgM, and IgA antibodies in serum
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Western blotting to confirm antibody specificity
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Avidity assays to determine antibody maturation over time
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Functional antibody assays (e.g., complement fixation, opsonization)
Cellular Immunity Assessment:
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T-cell proliferation assays using sdhD peptide pools
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Cytokine profiling (IFN-γ, IL-2, IL-4, IL-17) by ELISPOT or flow cytometry
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CD4+ and CD8+ T-cell functional analysis
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Memory T-cell population characterization
Correlates of Protection:
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Passive transfer experiments to determine protective capacity of anti-sdhD antibodies
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Adoptive transfer of T cells from immunized animals
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Challenge studies with varying doses of C. burnetii
Based on previous immunogenicity studies with recombinant C. burnetii proteins, researchers should expect antigenic responses when administered as protein mixtures, though this may not necessarily correlate with protective immunity .
How should researchers interpret discrepancies between in vitro expression and in vivo efficacy of recombinant C. burnetii proteins?
When confronting discrepancies between robust in vitro expression and poor in vivo efficacy of recombinant C. burnetii proteins like sdhD, researchers should consider several factors:
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Protein conformation: Recombinant proteins expressed in E. coli may not fold correctly or lack critical post-translational modifications present in native C. burnetii proteins. For membrane proteins like sdhD, this is particularly problematic as they may not maintain their native structure outside the lipid bilayer.
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Antigen presentation: While recombinant proteins may be antigenic (antibody-inducing), they might not present the correct epitopes needed to stimulate protective T-cell responses. Previous studies with recombinant C. burnetii proteins showed they were antigenic but failed to provide protection in challenge experiments .
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Adjuvant selection: The choice of adjuvant significantly impacts the type of immune response generated. Different adjuvants may be required for membrane proteins versus soluble proteins.
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Multi-subunit complexes: Proteins like sdhD function as part of multi-subunit complexes; isolated subunits may not induce immune responses against conformational epitopes formed at subunit interfaces.
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Animal model limitations: Mouse models may not perfectly recapitulate human Q fever pathogenesis, potentially affecting the predictive value of vaccine studies.
| Factor | Potential Impact | Mitigation Strategy |
|---|---|---|
| Protein conformation | Incorrect folding leads to non-protective antibodies | Use membrane mimetics during purification |
| Antigen processing | Improper epitope presentation | Include multiple protein antigens or domains |
| Adjuvant selection | Suboptimal immune polarization | Test multiple adjuvant formulations |
| Complex integrity | Loss of quaternary structure epitopes | Express and purify complete Sdh complex |
| Animal model | Limited translation to human disease | Validate in multiple animal models |
What bioinformatic approaches can help predict immunogenic regions of the sdhD protein for targeted vaccine design?
Advanced bioinformatic approaches can help identify promising immunogenic regions of sdhD for subunit vaccine development:
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Epitope prediction algorithms: Tools like BepiPred, NetMHCpan, and IEDB analysis resources can predict B-cell and T-cell epitopes based on protein sequence. For membrane proteins like sdhD, focus on extracellular or periplasmic domains that are accessible to the immune system.
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Structural modeling: Homology modeling using solved structures of bacterial sdhD homologues can predict protein topology and surface-exposed regions. These models can be refined with molecular dynamics simulations in membrane environments.
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Conservation analysis: Multiple sequence alignment of sdhD across C. burnetii strains can identify conserved regions that might serve as broadly protective epitopes. Conversely, strain-specific epitopes might be identified for strain-specific protection.
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Immunoinformatics pipelines: Integrated approaches combining epitope prediction, structural analysis, and molecular docking can rank potential epitopes based on predicted immunogenicity, accessibility, and MHC binding affinity.
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Reverse vaccinology: Genome-wide screening for protective antigens based on predicted subcellular localization, presence of signal peptides, and lack of similarity to host proteins.
Using these approaches, researchers can design peptide vaccines or modified recombinant proteins focusing on the most promising epitopes rather than the entire sdhD protein, potentially improving vaccine efficacy while reducing adverse reactions.
