Cell-based
vaccines consisting of
invariant chain-negative
tumor cells transfected with syngeneic MHC class II (MHC II) and costimulatory molecule genes are prophylactic and therapeutic agents for the treatment of murine primary and metastatic
cancers.
Vaccine efficacy is due to direct presentation of endogenously synthesized, MHC II-restricted
tumor peptides to CD4+ T cells. Because the
vaccine cells lack
invariant chain, we have hypothesized that, unlike professional APC, the
peptide-binding groove of newly synthesized
MHC II molecules may be accessible to
peptides, allowing newly synthesized
MHC II molecules to bind
peptides that have been generated in the
proteasome and transported into the endoplasmic reticulum via the TAP complex. To test this hypothesis, we have compared the Ag presentation activity of multiple clones of TAP-negative and TAP-positive
tumor cells transfected with I-Ak genes and the model Ag hen egg white
lysozyme targeted to the endoplasmic reticulum or cytoplasm. Absence of TAP does not diminish Ag presentation of three hen egg white
lysozyme epitopes. Likewise, cells treated with proteasomal and autophagy inhibitors are as effective APC as untreated cells. In contrast, drugs that block endosome function significantly inhibit Ag presentation. Coculture experiments demonstrate that the
vaccine cells do not release endogenously synthesized molecules that are subsequently endocytosed and processed in endosomal compartments. Collectively, these data indicate that
vaccine cell presentation of MHC II-restricted endogenously synthesized
epitopes occurs via a mechanism independent of the
proteasome and TAP complex, and uses a pathway that overlaps with the classical endosomal pathway for presentation of exogenously synthesized molecules.