Proposed mechanisms of Ag presentation. Proposed mechanisms of Ag presentation. Step 1. Somatic cells following plasmid transfection can serve as Ag factories expressing the protein Ag encoded by the gene of interest (GOI). Somatic cells cannot serve as APCs since they lack costimulatory molecules; however, they may regulate immune responses by providing Ag for uptake by professional APCs. Step 2. Immature DCs can be transfected directly and, on maturation, can activate CD4 T cells, CD8 T cells, and B cells. Step 3. Immature DCs can acquire Ag, following uptake of transfected apoptotic muscle cells, and present the Ag to CD4 T cells and CD8 T cells. Alternatively, immature DCs and B cells, following uptake of secreted soluble Ag, can present Ag to CD4 T cells and CD8 T cells. P, promoter; AAA, polyadenylation signal; pcDNA3, plasmid vector. Malavika Giri et al. Clin. Microbiol. Rev. 2004; doi:10.1128/CMR.17.2.370-389.2004