Immune Responses Induced by Recombinant Bacillus subtilis Expressing the PEDV Spike Protein Targeted at Microfold Cells

Bacillus subtilis (B. subtilis), a probiotic bacterium and feeding additive, is widely used for heterologous antigen expression and protective immunisation. Porcine epidemic diarrhoea virus (PEDV) invades swine via mucosal tissue. To enhance the mucosal immune response to PEDV, we modified B. subtilis to express a PEDV antigen and used it as a mucosal vaccine delivery system. Initially, we constructed a recombinant B. subtilis strain (B.s-RCL) that expressed the PEDV spike protein and L-Lectin-β-GF, with the goal of inducing mucosal secretory immunoglobulin A (sIgA) and anti-PEDV serum immunoglobulin G (IgG) production, as well as to increase the number of microfold cells (M cells). Following the oral administration of B.s-RCL to mice, the small intestinal PEDV-specific sIgA expression levels significantly increased, as well as the increased number of B.s-RCL adhered to M cells. Moreover, we found that mice administered B.s-RCL exhibited markedly higher percentages of CD4(+) and CD8(+) T cells in the mesenteric lymph nodes and spleen compared to the control mice. Furthermore, we found that intestinal mucosa sIgA and serum anti-PEDV IgG levels were higher in mice orally immunised with B.s-RCL, suggesting that the mice could be more resistant to PEDV. In this study, we developed a novel oral vaccine to prevent porcine diarrhoea epidemics.