Antigen-delivery system for CTL vaccine. (1999)

For patients with infectious diseases including AIDS or those with cancer, induction of protective immunity, which can eliminate the cells involved in the respective pathologic conditions, is essential. In particular, induction of cytotoxic T lymphocytes (CTL) becomes important in such patients, since CTL palys a central role in protective immunity. It was reported that endogenous antigens in the cytoplasm are generally presented in association with class I major histocompatibility complex (MHC) molecules, while exogenous antigens enter the endosomal pathway by endocytosis, then presented in association with class II MHC molecules. In recent years, attention has been focused on component vaccines due to their higher safety. However, since component vaccines are exogenous antigens, most component vaccines are endocytosed after administration, then presented in association with class II rather than class I MHC. That is, it is a major drawback of component vaccines that the efficiency of antigen presentation in association with class I MHC molecules, which is essential for CTL induction, is markedly low. Moreover, when short synthetic peptides that contain CTL epitopes are used as CTL vaccines, there is the risk that these peptides may undergo degradation in endosomes during the usual pathway of antigen presentation via endocytosis even when antigen molecules ascaped from endosomes into the cytoplasm. Therefore, antigen presentation via class I MHC molecules is little expected when such vaccines are administered alone. To design effective component vaccines for CTL induction, it is essential to develop a novel technique considering intracellular kinetics of antigens ; that is, an antigen delivery system that directly introduces into the cytoplasm and then efficiently directs into the class I MHC presentation pathway. To date, we have been developing fusogenic liposomes whose membrane fusion capacity was identical to that of Sendai virus. Fusogenic liposomes facilitate direct and efficient induction of inclusion substances into the cytoplasm by membrane fusion without accompanying cytotoxicity. Therefore, we applied fusogenic liposomes to a component vaccine and attempted to solve the above problems. Antigens induced via fusogenic liposomes were presented to the antigen specific CTLs in conjunction with class I MHC molecules in vitro more efficiently than the efficiency of antigen presentation via plane liposomes. In addition, antigen presentation by class I MHC, which was increased by fusogenic liposomes, was not inhibited by endocytosis inhibitors, but was dependent on the activity of envelop proteins derived from Sendai virus. Furthermore, reflecting the above results, when mice were immunized with antigens induced via plane liposomes, antigen-specific CTL was negligibly induced. However, when mice were immunized with antigens induced via fuscgenic liposomes, antigen-specific CTL was induced more potently than in mice immunized with complete Freund's adjuvant (CFA). This observation suggested that when antigen molecules were directly induced into the cytoplasm via fusogenic liposomes and recognized as endogenous antigens, these antigens were efficiently transferred to the class I MHC presentation pathway. Therefore, it is necessary to consider both systemic and intracellular kinetics of antigens to obtain the effects of vaccination efficiently, which suggested the importance of DDS techniques rather than immunomodulator functions, which are to be conferred on CFA, in the development of vaccines. The potential of the present system for clinical use and future of the development of CTL vaccine delivery system is discussed.