Recombinant Toxoplasma gondii expressing FIPV spike protein S1 subunit: A proof-of-concept approach against toxoplasmosis and feline infectious peritonitis.

Feline infectious peritonitis virus (FIPV) is a lethal feline pathogen with no widely effective prophylactic vaccine available. To address this unmet need, we explored the feasibility of using Toxoplasma gondii (whose definitive host is felids) as a live delivery platform to develop a bivalent vector vaccine. We generated a transgenic T. gondii strain engineered to express and secrete the FIPV spike protein S1 subunit into the parasitophorous vacuole. Immunization in mice confirmed the immunogenicity of this recombinant parasite, which induced specific antibody responses targeting both the T. gondii vector and the FIPV S1 antigen. In vitro neutralization assays revealed limited FIPV-neutralizing capacity in immune sera, with only low-level inhibitory activity observed at the lowest serum dilution, which was markedly inferior to the neutralization potency induced by recombinant S1 protein vaccination. Collectively, these data preliminarily verify the potential of T. gondii as a multivalent antigen delivery vector. This work provides a proof-of-concept framework for a dual-target vaccination strategy intended to mitigate the epidemiological burden of both FIPV and T. gondii in cats, and underpins integrated One Health-oriented disease prevention and control efforts.