Structure, Antigenic Properties, and Highly Efficient Assembly of PCV4 Capsid Protein

Area of Science:

• Virology
• Structural Biology
• Immunology

Background:

• Porcine circovirus type 4 (PCV4) is a novel circovirus identified in 2019.
• Understanding PCV4 capsid protein (Cap) assembly, structure, and antigenicity is crucial for diagnosis and characterization.

Purpose of the Study:

• To express and characterize PCV4 Cap and its self-assembly into virus-like particles (VLPs).
• To investigate the structural and antigenic properties of PCV4 VLPs.
• To explore the potential of PCV4 VLPs for diagnostic development.

Main Methods:

• High-level expression of PCV4 Cap in *Escherichia coli*.
• Purification and characterization of self-assembled VLPs.
• In vitro cell internalization assays using PK15 and 3D4/21 cell lines.
• 3D structural comparison with PCV2 capsids.
• In silico epitope prediction.
• ELISA and Western blotting for cross-reactivity assessment.

Main Results:

• PCV4 Cap successfully self-assembled into VLPs (~20 nm) in vitro.
• PCV4 VLPs were internalized by PK15 and 3D4/21 cells.
• Significant structural differences were observed between PCV4 and PCV2 capsids, particularly in surface loops.
• Five potential B cell epitopes were identified on surface-exposed loops.
• Humoral cross-reactivity between PCV4 and PCV2/PCV3 was unlikely.

Conclusions:

• This study provides novel insights into PCV4 capsid structure and assembly.
• PCV4 VLPs demonstrate potential for the development of serological diagnostic assays.
• Further characterization of PCV4 is facilitated by these findings.