Reactive oxygen species play a central role in the activity of cationic liposome based cancer vaccine.

Recently, we developed a simple and potent therapeutic liposome cancer vaccine consisting of a peptide antigen and a cationic lipid. The molecular mechanism of the adjuvanticity of cationic liposome was studied and described in the current report. First, cationic DOTAP liposome, but not the neutral liposome DOPC, was shown to generate reactive oxygen species (ROS) in mouse bone marrow-derived dendritic cells (BMDC). ROS generation by DOTAP was required for ERK and p38 activation and downstream chemokine/cytokine induction. Furthermore, ROS were shown to be involved in the expression of the co-stimulatory molecules CD86/CD80 induced by DOTAP. However, as the DOTAP concentration increased from 50 to 800 microM, the apoptotic marker Annexin V and ROS double positive cells increased, suggesting that high dose of DOTAP-generated ROS causes cell apoptosis. In vivo, optimal amount of ROS in the draining lymph nodes (DLN) and anti-tumor (HPV positive TC-1 tumor) activity induced by E7 peptide (antigen derived from E7 oncoprotein of human papillomavirus (HPV) type 16) formulated in 100 nmol DOTAP were attenuated by incorporating DOPC in the formulation, suggesting that ROS are essential for the vaccine induced anti-tumor activity. Moreover, 600 nmol DOTAP/E7 generated huge amount of ROS in the DLN and showed no activity of tumor regression. Interestingly, 600 nmol DOTAP/E7-induced ROS were tuned down to the same level induced by 100 nmol DOTAP/E7 by adding DOPC in the formulation and this formulation showed tumor regression activity. In conclusion, DOTAP is an active DC stimulator resulting in the activation of ERK and p38 and induction of chemokines, cytokines and co-stimulatory molecules mediated by appropriate amount of ROS. Our data elucidated an important mechanism of adjuvant activity of cationic liposome and could facilitate rational design of synthetic lipid based adjuvants and vaccine formulation.