Engineering a dimeric porcine deltacoronavirus S-RBD subunit vaccine for enhanced immunogenicity and neutralizing antibody responses

Abstract

Porcine deltacoronavirus (PDCoV) is an emerging swine enteric coronavirus that has caused global epidemics and substantial economic losses, posing a potential risk of cross-species transmission, including to humans. However, commercial vaccines are currently scarce. The receptor binding domain (RBD) of the spike (S) protein is a key target for inducing neutralizing antibodies, but the RBD monomer typically exhibits low molecular weight and weak immunogenicity. This study aimed to develop a tandem dimeric RBD subunit vaccine and evaluate its immunogenicity and protective efficacy in comparison with the RBD monomer and an inactivated vaccine. A PDCoV RBD dimer protein was successfully expressed and purified using a mammalian expression system. In mice, the RBD dimer vaccine elicited significantly higher RBD specific IgG levels, and induced a mixed Th1/Th2 type immune response, while simultaneously inducing numerically higher neutralizing antibody titers compared with the monomer and inactivated vaccines, despite non-significant differences. Post-challenge histopathological analysis revealed that the intestinal villi of mice in the dimer group remained largely intact, with significantly less damage than those in the control groups. In piglets, the peak neutralizing antibody titer induced by the RBD dimer was approximately 2.8 times that induced by the inactivated vaccine, and antibody levels remained relatively high until day 72, accompanied by significantly stimulated lymphocyte proliferation. In mice, the RBD dimer subunit vaccine demonstrated superior antibody titers and protective efficacy compared with both the monomer and inactivated vaccines. In piglets, the dimer vaccine elicited higher and more durable antibody and neutralizing activity than the inactivated vaccine. These findings indicate the considerable potential of this candidate vaccine and warrant further in‑depth evaluation of its clinical protective efficacy in piglets.