Enhanced replication and immunogenicity of a genome-modified non-replicating vaccinia virus Tiantan strain for robust low-dose protection in mice

A non-replicating Tiantan strain-based vaccinia virus (NTV) holds significant application prospects for vaccination and gene therapy and has recently entered clinical trials as a novel and safer vaccine candidate against monkeypox. However, optimization is still required, particularly regarding its production capacity and immunogenicity. In this study, a recombinant virus was constructed by modifying the F1L and C7L genes in a non-replicating viral backbone using CRISPR/Cas9-mediated gene editing and homologous recombination. The resulting construct, designated NTV-ΔF1L-C7L, exhibited significantly enhanced replication in vaccine production cell lines, with viral yields increasing by more than 680-fold in MRC-5 cells compared to those of the parental NTV. Its pathogenicity in mice was significantly reduced, showing more than a 10-fold decrease compared to the pathogenicity of the vaccinia virus Tiantan strain (VTT). Following two intramuscular doses, NTV-ΔF1L-C7L elicited high titers of orthopoxvirus-specific IgG and neutralizing antibodies against vaccinia and monkeypox viruses, along with a robust cellular immune response exhibiting a Th1 bias, which was significantly stronger than that induced by either parental NTV or VTT vaccination. Complete protection (100%) against a lethal challenge with the vaccinia virus Western Reserve strain was achieved in mice immunized with either a low dose (10 ³ PFU) or a single dose (10⁵ PFU) of NTV-ΔF1L-C7L, comparable to that conferred by VTT. These findings demonstrate that NTV-ΔF1L-C7L combines high safety with enhanced replication and immunogenicity, supporting its value as a novel vaccine vector and its potential application in controlling the current global monkeypox outbreak.