Liposome-based immunity-inducing systems for cancer immunotherapy

Cancer immunotherapy has gained much attention for next-generation cancer treatment. To conduct cancer immunotherapy, efficient antigen delivery systems must be able to deliver an antigen selectively to antigen-presenting cells, release it at suitable sites for induction of cross-presentation, and simultaneously induce activation of immunocompetent cells. Liposomes are a candidate for use as such multifunctional antigen delivery carriers because of their capability for easy functionalization. This review describes the rational design of liposome-based antigen delivery systems. Surface modification of liposomes by pH-responsive or fusogenic materials can achieve cytoplasmic delivery of antigen, leading to cross-presentation of exogenous antigen via a "cytosolic pathway." In contrast, targeting surface receptors on antigen presenting cells or the selective release of antigen in early endosome induced "vacuolar pathway"-mediated cross-presentation. Introduction of adjuvant molecules such as Toll like receptor agonists, synthetic cationic lipids or bioactive polysaccharides to liposomes improved their immunity-inducing ability. Combination with cancelling systems of immunosuppression in tumor microenvironment enhanced antitumor immunity of antigen delivery systems. Further understanding of immunity-inducing mechanism and molecular basis of tumor immunosuppressive environments and purposeful design of liposome-based antigen delivery systems can provide effective immunity-inducing systems for cancer immunotherapy.