Disclosed are positively-charged, cytotoxic nanoparticle compositions comprising immune modulators (such as the toll-like receptor (TLR)-4 ligand, monophosphoryl lipid (MPL)-A), and Interleukin (IL)-12)), which exhibit enhanced uptake by mammalian cancer cells, and cause increased cancer cell death and/or an increased release of cancer antigens following direct injection to populations of cancer or tumor cells. Also disclosed are nanoparticle-vectored, immunomodulatory compositions that stimulate antigen presenting immune cells and T cells, and support the development of anti-cancer immunity in mammalian hosts. The disclosed cationic liposomes represent an important advance in the area of cancer immunotherapeutics.