Targeted Liposomal Delivery of TLR9 Ligands Activates Spontaneous Antitumor Immunity in an Autochthonous Cancer Model

Accessibility of tumors for highly effective local treatment represents a major challenge for anticancer therapy. Immunostimulatory oligodeoxynucleotides (ODN) with CpG motifs are ligands of TLR9, which prime spontaneous antitumor immunity, but are less effective when applied systemically. We therefore developed a liposome-based agent for selective delivery of CpG-ODN into the tumor environment. A peptide that specifically targets angiogenic endothelial cells in a transgenic tumor model for islet cell carcinogenesis was engrafted into CpG-ODN containing liposomes. Intravenous injection of these liposomes resulted in specific accumulation around tumor vessels, increased uptake by tumor-resident macrophages, and retention over time. In contrast, nontargeted liposomes did not localize to the tumor vasculature. Consequently, only vascular targeting of CpG-ODN liposomes provoked a marked inflammatory response at vessel walls with enhanced CD8(+) and CD4(+) T cell infiltration and, importantly, activation of spontaneous, tumor-specific cytotoxicity. In a therapeutic setting, 40% of tumor-bearing, transgenic mice survived beyond week 45 after systemic administration of vascular-directed CpG-ODN liposomes. In contrast, control mice survived up to 30 wk. Therapeutic efficacy was further improved by increasing the frequency of tumor-specific effector cells through adoptive transfers. NK cells and CD8(+) T cells were major effectors which induced tumor cell death and acted in conjunction with antivascular effects. Thus, tumor homing with CpG-ODN-loaded liposomes is as potent as direct injection of free CpG-ODN and has the potential to overcome some major limitations of conventional CpG-ODN monotherapy.