Recombinant HBV vaccine platform — The genes encoding HBsAg for the Hepatitis B vaccine were cloned and expressed using which type of vector/host system to mass-produce the antigen?

Introduction / Context:
The landmark Hepatitis B vaccine leverages recombinant expression of HBsAg in non-pathogenic hosts. Identifying the expression system clarifies why large-scale, safe production was feasible and consistent across global manufacturers.

Given Data / Assumptions:

• HBsAg forms self-assembling virus-like particles when expressed in certain systems.
• Yeast provides eukaryotic protein processing and scalable fermentation.
• Vector choice impacts copy number, stability, and yields.

Concept / Approach:
Many licensed HBV vaccines express HBsAg in Saccharomyces cerevisiae using autonomously replicating yeast plasmids (often 2µ origin-based) under strong promoters. Yeast expression yields high titers of properly assembled HBsAg particles suitable for purification and formulation with alum adjuvant.

Step-by-Step Solution:

Verification / Alternative check:
Product monographs and WHO prequalification summaries describe yeast-derived HBsAg production platforms for multiple licensed vaccines.

Why Other Options Are Wrong:

• General bacterial plasmids: early attempts were less successful for VLP assembly compared to yeast.
• Cosmids/phagemids: not standard for licensed HBsAg vaccine production.
• Baculovirus/insect cells: used for some VLPs and proteins, but the classic HBV vaccines are yeast-based.

Common Pitfalls:
Assuming all recombinant vaccines use bacterial expression; eukaryotic hosts often yield better assembly and post-translational features.

Final Answer:
Autonomously replicating plasmid of yeast (e.g., 2µ-based; expression in Saccharomyces cerevisiae).