Effects of different mucosal adjuvants on the immune responses induced by porcine deltacoronavirus in mice

Abstract:

[Objective] The aim of this study is to screen an ideal adjuvant for an inactivated porcine deltacoronavirus (PDCoV) vaccine to induce mucosal immunity and reduce the side effect of the vaccine. We used different mucosal adjuvants to prepare the inactivated PDCoV vaccines. We then used mouse model to evaluate the humoral, cellular and mucosal immune responses induced by the inactivated vaccines via different immunization routes. [Methods] The adjuvants IMS1313 and GEL02 were respectively combined with polyactin A (PA), CpG ODN2395, and monophosphoryl lipid A (MPLA) to prepare the inactivated PDCoV vaccines, which were then used to immunize BALB/c mice intranasally. The inactivated PDCoV vaccine prepared with ISA201 adjuvant was used to immunize BALB/c mice subcutaneously. The inactivated PDCoV vaccine without adjuvant was used as a control to immunize BALB/c mice intranasally. The mice were immunized once again at the same doses in the same ways 14 days post the primary immunization. Enzyme-linked immunosorbent assay (ELISA) was employed to determine the expression levels of IgG, IgG1, IgG2a, IL-4, and IFN-γ in serum and bronchial lavage fluid (BALF) samples, as well as the levels of sIgA in feces and BALF samples. Methyl thiazolyl tetrazolium (MTT) assay was used to detect the proliferation of spleen lymphocytes. We observed and recorded the clinical manifestations of immunized mice. Meanwhile, we observed the pathological changes of major organs and tissues of immunized mice via hematoxylin-eosin (HE) staining to evaluate the safety of the vaccines. [Results] The ISA201 group had high expression levels of antibodies (IgG and IgG1) and IL-4 in BALF and serum and low expression levels of IgG2a, IFN-γ, and sIgA in feces. GEL02, GEL02+2395, GEL02+PA, and GEL02+MPLA groups showed higher expression levels of antibodies (IgG, IgG1, and IgG2a), IL-4, and IFN-γ in BALF and serum and higher expression level of sIgA in feces than IMS1313, IMS1313+2395, IMS1313+PA, and IMS1313+MPLA groups. In particularly, the expression levels of IgG2a in BALF and serum samples and IFN-γ and sIgA in BALF and feces samples of GEL02+2395 group were significantly higher than those of other groups. The vaccines prepared with the adjuvant combinations GEL02+2395 and IMS1313+2395 promoted T lymphocyte proliferation. No obvious pathological changes were observed in the main organs and tissues of mice. The mice immunized with the vaccines prepared with the adjuvants GEL02 and GEL02+2395 had the mildest adverse reactions. [Conclusion] The inactivated PDCoV vaccine prepared with GEL02 and CpG ODN2395 can not only enhance the humoral immunity but also improve the cellular immunity and mucosal immunity in mice, which provides basic information for the research and development of novel mucosal adjuvants for PDCoV vaccines.