On the induction of cytotoxic T lymphocytes in vitro with soluble proteins and peptides

Effective immunity to many infectious agents, particularly viruses, requires a CD8+ cytotoxic T lymphocyte (CTL) response. Understanding how to achieve CTL induction with soluble antigens is important for vaccine development, since such antigens are usually not processed appropriately to induce CTL. To induce cytotoxic responses almost invariably requires endogenous protein synthesis, as occurs with replicating agents (viruses). Studies in vitro reveal a similar scenario and it seems that attaining primary CTL induction with non-replicating antigens has rarely been reported. This study demonstrates that potent, primary CTL responses against a soluble protein (ovalbumin) can be achieved by delivering antigen via pH-sensitive liposomes to dendritic cells (DC) either in vivo or in vitro. The observation was extended to HSV-1 proteins (glycoprotein B) and peptides (glycoprotein B and α27 protein) and primary, HSV-1-specific cytotoxic responses were generated in vitro using DC as stimulators. Macrophages (Mø) treated in a similar manner were not effective stimulators for primary CTL induction. Peptide prediction schemes were used to determine CTL epitopes and CTL inducing activity of the peptide was assessed using the primary CTL induction approach. An important aspect of vaccine development is the development of an efficient antigen delivery system to induce protective responses. This requires a thorough understanding of antigen processing and presentation in vivo. The pH-sensitive liposomes were used to study the importance of the splenic environment and cellular interactions in the initiation of cytotoxic reponses. The hypothesis was based on the assumption that processing and presentation are distinct events involving specialized cells. in vivo depletion of Mø, with dichloromethylene diphosphonate in liposomes, was used to study cellular interactions between DC and Mø. Based on these studies a model for in vivo cellular interaction between DC and Mø is proposed, wherein Mø process antigen encapsulated in liposomes and the preprocessed antigen fragments are transferred to DC for presentation and activation of naive T cells.