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110 Schroter and Minev
proteins (2,3). Cytosolic peptides are transported across the endoplasmic reticulum
(ER) membrane with the help of the ATP-dependent transporters associated
with antigen processing (TAP) (4,5). Peptides complexed with class I
molecules in the ER are then transported to the cell surface for recognition by
CTL (3,6). The interaction between CTL and the target tumor cells begins with
the binding of the peptide antigen associated with the MHC class I molecule to
the T-cell antigen receptor. Lymphocyte-mediated cytolysis is further enhanced
by accessory molecules, such as lymphocyte function antigen-1 and -3, costimulatory
molecules (CD28, B7), and the intercellular adhesion molecule-1 (7),
among others.
The realization that MHC class I–restricted tumor antigens can act as
targets for CTL (8) promoted the search for tumor antigen genes (9,10). CTL
appear to be among the most direct and effective elements of the immune system
that are capable of generating antitumor immune responses (11). Tumor cells
expressing the appropriate TAA can be effectively recognized and destroyed by
these immune effector cells, which may result in dramatic clinical responses (12–
14). Both the adoptive transfer of tumor-reactive CTL and active immunization
designed to elicit CTL responses have been reported to lead to significant
therapeutic antitumor responses in some patients (12–14). However, currently
there are no human peptide–based vaccines on the market—resulting primarily
from difficulties associated with peptide stability and delivery, and the diversity
of human target antigens. Therefore, further research aimed at enhancing the
stability and immunogenicity of the peptides used for vaccination of patients
with cancer is essential.
TUMOR-ASSOCIATED ANTIGENS
Identification of highly expressed TAA is essential to the development of potent
and specific cancer vaccines. A variety of approaches have been used for the
identification of TAA recognized by CTL, including screening cDNA expression
libraries with tumor-reactive CTL (11), testing of known proteins for recognition
by CTL (15), direct isolation and sequencing of peptides eluted from the tumor
cells (16,17), and serological analysis of recombinant cDNA expression libraries
of human tumors with autologous serum (SEREX) (18). More recently, computer
programs have been used to identify peptide sequences of known proteins based
on their binding affinity for selected HLA molecules. We analyzed the sequence
of human telomerase reverse transcriptase (hTRT) for peptide sequences binding
to the HLA-A2.1 molecule and demonstrated that the hTRT peptide-specific
CTL of normal individuals and patients with cancer specifically lysed a variety
of HLA-A2þ cancer cell lines (19). Using different computer-based algorithms,
we identified six epitopes recognized by human CTL within the sequence of the
new tumor-associated antigen MG50 (20).
Utilizing these approaches, many melanoma target antigens and antigenderived
peptides have been identified, including tyrosinase, MART-1/Melan-A,
gp100, TRP1/gp75, TRP2, MAGE, BAGE, GAGE, RAGE, NY-ESO-1, and