Mechanism of adjuvant activity of cationic liposome: Phosphorylation of a MAP kinase, ERK and induction of chemokines

Cationic liposome has been effectively used as a delivery system for DNA and protein vaccines. Recently, we discovered that strong anti-tumor immunity could be generated when a peptide antigen (E7) was incorporated into 1,2-dioleoyl-3-trimethylammonium-propane (chloride salt) (DOTAP) cationic liposome. Therefore, DOTAP liposome exhibits not only efficient delivery capacity, but also a potent adjuvant activity. In this report, the molecular mechanism of the adjuvanticity was studied both in vitro and in vivo. Microarray of mRNA analysis demonstrated that several chemokine genes are up-regulated by DOTAP liposome, including CCL2, CCL3 and CCL4, upon treatment of dendritic cells (DC) with DOTAP liposomes. CCL2 induction was mediated through extracellular-signal-regulated kinase (ERK) pathway, demonstrated by specific inhibitors of ERK pathway and siRNA approaches. Furthermore, DOTAP-induced CCL2 expression is negatively regulated by the p38 pathway. Consistently, ERK activation by DOTAP is also negatively regulated by p38. Moreover, PI-3 kinase was shown to be involved in both activation of ERK and induction of CCL2 by DOTAP. DOTAP- induced CCL2 release was also confirmed in the draining lymph nodes. More importantly, inhibition of ERK pathway completely abolishes the CCL2 accumulation in the draining lymph nodes and attenuates anti-tumor activity of DOTAP/E7. In conclusion, DOTAP is an active lipid stimulator for DC resulting in ERK activation and CC chemokine induction. Our data elucidated one important mechanism of adjuvant activity of cationic liposome and could facilitate rational design of synthetic lipid based adjuvants.