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.