Grafting of Md5 gC onto herpesvirus of turkeys confers full protection against very virulent Marek's disease virus.

Marek's disease virus (MDV) virulence has progressively compromised the efficacy of current vaccines, including herpesvirus of turkeys (HVT). Glycoprotein C (gC) is conserved among mardiviruses and essential for horizontal transmission, but its immunogenic potential remains underexploited. We developed a "viral grafting" strategy by precisely replacing the HVT gC gene with that of very virulent MDV (vvMDV) strain Md5, generating a recombinant virus rHVT‑gC/Md5. The rHVT‑gC/Md5 replicated similarly to parental HVT in chicken embryo fibroblasts and stably expressed Md5‑gC. Specific-pathogen-free chickens immunized subcutaneously at 1 day old with 2,000 PFU of rHVT‑gC/Md5 and challenged at 6 days old with 500 PFU of vvMDV Md5 showed full protection: 0% mortality, 0% tumor incidence, and a protection index of 100, compared with 28% mortality, 36% tumor incidence, and a protection index of 35 for HVT. rHVT‑gC/Md5 markedly reduced viral loads to near‑undetectable levels in all organs, preserved body weight gain, and maintained bursal and thymic architecture (indices >0.7). Importantly, rHVT‑gC/Md5 markedly reduced viral shedding in feather follicles compared with HVT-vaccinated and unvaccinated groups, indicating reduced potential for horizontal transmission. Mechanistically, rHVT‑gC/Md5 induced gC‑specific T‑cell proliferation, elevated CD8⁺ T‑cell responses, and a Th1‑biased cytokine profile with 11‑fold higher IFN‑γ expression in the thymus versus HVT, while suppressing proinflammatory cytokines. These results establish viral grafting as a novel vaccine design strategy and identify gC as a critical protective antigen against vvMDV.