File:Pharmaceutics-12-00092-g001.png

English: Schematic representation of an mRNA-transfected DC. Endocytosed and phagocytosed material is usually presented in MHC class II to CD4+ helper T cells and can be cross-presented in MHC class I only under certain circumstances (not depicted). Proteins from the cytoplasm are, in contrast, primarily presented in MHC class I. mRNA can be introduced into cells by passive pulsing, relying on intrinsic and yet unknown means of uptake. It can be complexed with lipid reagents that mediate entry into the cytoplasm, or it can be transfected by electroporation. By applying a short electric pulse, pores in the cell membrane temporarily open, allowing entry of the RNA molecules. The transferred mRNA is then translated in the cytoplasm, and the encoded antigens are hence presented in a MHC class I context. By encoding signaling and targeting sequences (green) fused to the antigenic protein, this can also be directed towards the endosomal pathway, resulting in efficient MHC class II presentation.