Cytotoxic T lymphocyte induction by plasmid DNA immunization, mechanism and applications

grantor: University of TorontoPlasmid DNA vaccines represent a powerful new method of expressing antigen ' in vivo' for the generation of both humoral and cellular responses. Since its first discovery in 1992, DNA immunization has proven efficacious in a number of pre-clinical disease models. A DNA immunogen consists of a bacterial plasmid encoding the antigen of interest and can either be needle-injected into skeletal muscle, or may be coated onto gold particles and inoculated into skin epidermis using a biolistic device known as a "gene gun". The precise mechanism underlying the remarkable ability of DNA to induce immune responses remains largely unknown. A particular advantage of DNA vaccines is that they provide an infection-independent means of inducing cytotoxic T lymphocytes (CTL). Because CTL responses can play an important role in anti-viral and anti-tumor defenses, options for enhancing plasmid DNA-induced CTL responses were investigated. Co-expression of the costimulatory molecule B7-2, but not B7-1, was found to enhance CTL induction. As well, co-administration of a plasmid expressing the cytokines GM-CSF and IL-12 had a synergistic effect on CTL stimulation. To understand the mechanism underlying this vaccination strategy, the cell types responsible for CTL priming were assessed in bone marrow chimeric mice. This study unequivocally demonstrated that bone marrow derived cells, not transfected muscle cells or skin keratinocytes, were responsible for presenting antigen to naive CTL. Furthermore, an assessment of the importance of the DNA injection sites revealed that gene gun inoculated skin cells, but not needle-injected muscle cells, play a central role in DNA-induced immune responses. Based on these studies, possible models of immune induction by DNA immunization are discussed. Finally, the use of DNA vaccines to block 'in vivo' tumor growth was explored using a minimally antigenic CTL epitope as a surrogate tumor antigen. Epitope-specific CTL induced by DNA immunization protected immune mice from challenge with tumor cells expressing the epitope. Moreover, these mice subsequently developed protective immune responses against endogenous tumor antigens, and were able to resist challenge with parental tumor cells. Thus, DNA immunization strategies designed to induce CTL responses hold considerable promise for both the prevention and treatment of disease.Ph.D