Engineered VP2-NC antigen expressed in an NDV vector elicits enhanced protection against the challenge of the novel variant infectious bursal disease virus.

To enhance the efficacy of Newcastle disease virus (NDV)-vectored bivalent vaccines against novel variant infectious bursal disease virus (nVarIBDV), we engineered recombinant NDV strains expressing nVarIBDV VP2 proteins fused with the viral nucleocapsid NC motif, thereby leveraging the NDV assembly pathway. The resulting vaccine candidates, rDM-VP2NC and rDM-VP2mNC, exhibited markedly enhanced VP2 expression in vitro. Furthermore, the NC motif promoted VP2 co‑localization with the NDV M protein near the cell membrane and facilitated its efficient incorporation into budding virus-like particles (VLPs) and virions. In chickens challenged with a nVarIBDV strain, immunization with rDM‑VP2NC or rDM‑VP2mNC provided superior protection compared to the parental rDM‑VP2 strain, as demonstrated by a drastic reduction in bursal viral load and the absence of detectable pathological lesions. These results establish the NC motif as a key determinant for enhancing antigen assembly and immunogenicity in NDV vectors. Collectively, this work presents a novel strategy for engineering NDV as a vector that directly packages protein cargo within virions, with broad implications for vaccine and oncolytic virotherapy development.