Vaccines that can prime the adaptive immune system for a quick and effective response against a pathogen or tumor cells, require the generation of antigen (Ag)-specific memory T and B cells. The unique ability of dendritic cells (DCs) to activate naïve T cells, implies a key role for DCs in this process. The generation of tumor-specific CD8(+) cytotoxic T cells (CTLs) is dependent on both T cell stimulation with Ag (peptide-MHC-complexes) and costimulation. Interestingly, tumor cells that lack expression of T cell costimulatory molecules become highly immunogenic when transfected to express such molecules on their surface. Adoptive immunotherapy with Ag-pulsed DCs also is a strategy showing promise as a treatment for cancer. The use of such cell-based vaccines, however, is cumbersome and expensive to use clinically, and/or may carry risks due to genetic manipulations. Liposomes are particulate vesicular lipid structures that can incorporate Ag, immunomodulatory factors and targeting molecules, and hence can serve as potent vaccines. Similarly, Ag-containing plasma membrane vesicles (PMV) derived from tumor cells can be modified to incorporate a T cell costimulatory molecule to provide both TCR stimulation, and costimulation. PMVs also can be modified to contain IFN-gamma and molecules for targeting DCs, permitting delivery of both Ag and a DC maturation signal for initiating an effective immune response. Our results show that use of such agents as vaccines can induce potent anti-tumor immune responses and immunotherapeutic effects in tumor models, and provide a strategy for the development of effective vaccines and immunotherapies for cancer and infectious diseases.