Prolonged survival of thymoma-bearing mice after vaccination with a soluble protein antigen entrapped in liposomes: a model study.

EG7-OVA cells are mouse thymoma EL4 cells stably transfected with the complementary DNA of chicken ovalbumin (OVA) and thus express OVA epitopes as a unique antigen. Cytotoxic T lymphocytes specific to OVA can be elicited by immunization of mice with OVA osmotically loaded into syngeneic splenocytes or entrapped in liposomes. Cytotoxic T lymphocytes thus induced can specifically cytolyse the EG7-OVA cells in vitro in an antigen-specific and major histocompatibility complex-restricted manner. In the present study, we have examined in this model system whether immunization with liposomal OVA can protect mice against tumors induced by EG7-OVA cells. Vaccination with OVA either entrapped in liposomes or osmotically loaded in the syngeneic splenocytes prolonged the survival of mice which had been challenged with EG7-OVA cells, but not those mice challenged with the parent EL4 cells. The antitumor effect was attributed to the induced OVA-specific cytotoxic T lymphocyte activity, since other forms of acquired immunity such as interaction of tumor cells with specific antibody could not be detected. Our results demonstrate that immunization with antigen incorporated in liposomes could be a useful means of inducing a protective antitumor response.