ANTIGEN-PROCESSING IN-VIVO AND THE ELICITATION OF PRIMARY CTL RESPONSES

CD8(+) T lymphocytes (T-CD8+) play an important role in cellular immun e responses. T-CD8+ recognize MHC class I molecules complexed to pepti des of 8 to 10 residues derived largely from cytosolic proteins. Prote ins are generally thought to be fragmented in the cytoplasm and delive red to nascent class I molecules in the endoplasmic reticulum (ER) by a peptide transporter encoded by the MHC. To explore the extent to whi ch T-CD8+ induction in vivo is limited by proteolysis or peptide trans port into the ER, mice were immunized with recombinant vaccinia viruse s containing mini-genes encoding antigenic peptides (bypassing the nee d for proteolysis), or these peptides with a NH2-terminal ER insertion sequence (bypassing the requirements for both proteolysis and transpo rt). Additionally, mice were immunized with recombinant vaccinia virus es encoding rapidly degraded fragments of proteins. We report that lim itations in induction of T-CD8+ responses vary among Ags: for some, fu ll length proteins are as immunogenic as other forms tested; for other s, maximal responses are induced by peptides or by peptides targeted t o the ER. Most importantly, in every circumstance examined, targeting peptides to the ER never diminished, and in some cases greatly enhance d, the T-CD8+ immune response and provide an important alternative str ategy in the design of live viral or naked DNA vaccines for the treatm ent of cancer and infectious diseases.