In vitro assay of immunostimulatory activities of plasmid vectors

DNA vaccination represents a novel and potentially important approach to induce immune responses against protein antigens. In this approach, the vaccine is a plasmid DNA vector that can be taken up by cells to produce a protein, encoded by the vector, to be targeted for the induction of humoral or cellular responses. Although the intracellular production of the antigen may promote responses, the vectors themselves may display adjuvant activity because of their intrinsic immunostimulatory properties. These properties reflect sequence motifs, centering on an unmethylated CpG dinucleotide, which can trigger the TLR9 pattern recognition receptor. As shown by studies in vitro, plasmid DNA can stimulate B cells, macrophages, and dendritic cells, and trigger a broad range of pro-inflammatory responses. Because this stimulation results from common sequence motifs, the activity of a plasmid vector can be assessed by the in vitro assay of a limited number of responses, including proliferation of B cells as well as production of cytokines by macrophages or dendritic cells.