Figure 1.

Mechanism of gene delivery, mRNA amplification, and initiation of immune response

Upon oral administration, Salmonella is translocated from the luminal surface to submucosa by specialized M cells in the gut epithelium. Bacteria then invade APCs such as macrophages and DCs and are spread to different organs, like the liver and spleen, through lymphatics and the bloodstream. The vector encoding SFV replicon (nsp1-4) and SARS-CoV-2 immunogens is released within the host cell cytoplasm through bacterial lysis. Transcription of the delivered plasmid takes place in the cell nucleus and, following in situ translation, the nsp1-4 proteins form an RNA-dependent RNA polymerase (RdRp) complex. The RdRp complex then recognizes the sub-genomic promoter and flanking conserved sequence elements (CSEs) leading to enhanced mRNA amplification of vaccine genes. The resulting mRNAs are translated to produce immunogenic proteins. The APCs process and present antigen to CD8 and CD4 T cells via the MHC I and MHC II molecules, respectively, leading to the elicitation of T cell response. DCs can present antigen directly to B cells or follicular DCs (FDCs). FDCs store antigen for a longer time, periodically displaying the antigen to cognate B cells. B cells then differentiate to specific antibody-secreting plasma cells and memory B cells. nsp, non-structural protein; CD, cluster of differentiation; CTL, cytotoxic T cell. The figure was created with the help of the BioRender online tool (https://app.biorender.com/).