Specific cytotoxic T-lymphocytes (CTL) are important in the protection against viral and parasitic infections (1 ,2 ), as well as monitoring and even eradicating tumor cells (3 –5 ). CTL normally recognize endogenously processed peptide antigens (Ag) complexed to MHC class I molecules (6 ,7 ). Immunization with virus-infected antigen-presenting cells (APC) or Ag-expressing transfectants, and even APC osmotically loaded with antigens can elicit CD8+ cytotoxic T-cells (8 –10 ). However, APC pulsed with native protein antigens usually do not elicit CTL, unless specialized APC are used (11 ,12 ). Generally, noninfectious forms of viral antigens or other extracellular proteins (“exogenous Ag”) do not efficiently enter the class I processing pathway, whereas proteins synthesizedde novo(“endogenous Ag”) have ready access.
