Vaccine antigens are predominantly processed as exogenous antigens and are presented on MHC class II molecules stimulating CD4⁺ T cell immune responses. However in some cases, for example cancer, CD8⁺ cytotoxic immune responses would be preferable. In order for this to occur, exogenous antigen must be delivered into the cytoplasm of antigen presenting cells (APCs) to take advantage of the endogenous, and well-studied, MHC-I loading pathway.

To achieve cytoplasmic delivery we developed a membrane-modified delivery system and combined this with antigen complexed with polyethyleneimine (PEI), a commonly used transfection agent. PEI is thought to facilitate endosomal lysis and delivery of antigen into the cytoplasm in response to endosomal acidification through the ‘proton sponge effect’. The membrane-modified delivery system consisted of liposomes where the liposomal membrane was modified with cyclodextrin-based ion channels to promote the movement of ions into the aqueous interior of the liposome where they could protonate the PEI, resulting in swelling and lysis of the liposomes and release of antigen.

Liposome uptake and release was investigated in vitro through the delivery of a self-quenching fluorescent dye, calcein, by flow cytometry and fluorescence microscopy. In-vitro APC stimulation was assessed by measuring the upregulation of CD80/CD86 in murine bone marrow derived dendritic cells. An in-vivo murine melanoma model was used to determine the efficacy of the membrane-modified liposomes as vaccine carriers.