Figure 1.

DNA vaccines induce adaptive immune responses. A DNA vaccine intended to induce an adaptive immune response needs to encode an antigen and an adjuvant. The according plasmid DNA is applied either systemically or topically, e.g., by intramuscular injection. Transfected keratinocytes or myocytes express transgene and release derived peptide/protein via exosomes or apoptotic bodies. This material is endocytosed by immature dendritic cells (iDC) which subsequently present antigen preferentially via major histocompatibility class (MHCII) to CD4⁺ T cells in draining lymph nodes. Direct transfection of APC including iDC results in endogenous transgene expression, and hence parallel presentation via MHCI and MHCII, yielding CD8⁺ and CD4⁺ T cell responses in parallel. Besides this cellular immune response, a humoral immune response is induced if the B cell receptor recognizes the protein antigen, and acquires help by pre-activated antigen-specific CD4⁺ T cells.