Enhanced Immunogenicity and Distinct Local Tissue Responses of Porcine Circovirus Type 2 Vaccination in Pigs Using a Needle-Free Jet Injection System

Conventional needle-based vaccination may cause tissue injury and increase the risk of cross-contamination between animals. This study evaluated the immunological effects of delivering an inactivated porcine circovirus type 2 (PCV2) vaccine using a needle-free jet injection system compared with needle injection in pigs. Immune responses were assessed using enzyme-linked immunosorbent assay (ELISA) for PCV2-specific antibody levels on days 7, 14, and 28 post-vaccination. Cellular architecture at injection sites and differential gene expression were analyzed using immunohistochemistry and transcriptome sequencing. Results showed that pigs receiving needle-free vaccination at 0.50 mL and 0.75 mL injection volumes exhibited higher PCV2-specific antibody responses than pigs receiving 1.0 mL needle injection. Needle-free vaccination also preserved dermal collagen fiber architecture, altered antigen-presenting cell distribution, and was associated with distinct activation of immune-related pathways, particularly those involved in leukocyte migration and cytokine-cytokine receptor interactions. These findings indicate that needle-free vaccination was associated with distinct PCV2-specific antibody responses and local immune microenvironment changes, supporting its potential as an alternative vaccine delivery strategy for swine PCV2 vaccination.