02.03.2013 Views

Target Discovery and Validation Reviews and Protocols

Target Discovery and Validation Reviews and Protocols

Target Discovery and Validation Reviews and Protocols

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

320 Jäger<br />

Furthermore, this immune response seems to be effective, resulting in a better<br />

prognosis <strong>and</strong> prolonged survival of the individual patient. A number of different<br />

cloning strategies have been developed over the past 15 yr to identify tumor<br />

antigens recognized by T-cells (see Chapter 14, Volume 1): the T-cell epitope<br />

cloning technique introduced by T. Boon’s group (5) was successful in identifying<br />

tumor antigens based on spontaneous T-cell (cytotoxic T-lymphocyte<br />

[CTL]) responses. The first tumor-associated antigen identified with this technique<br />

was the melanoma antigen MAGE-1. During the following years, new<br />

tumor-associated antigens such as the BAGE <strong>and</strong> GAGE gene family were<br />

identified using the same cloning strategy (6,7). These antigens share some<br />

characteristic features: their expression in normal tissues is restricted to germ<br />

cells in the testis <strong>and</strong> the ovary, whereas these antigens are widely expressed in<br />

tumor tissues. Therefore, this group of antigens was recognized as a seperate<br />

category, named “cancer-testis” antigens (8,9). A second category of antigen<br />

comprises the differentiation antigens that are expressed in normal cells <strong>and</strong> tissue,<br />

such as melanocytes <strong>and</strong> the malignant counterpart, the melanoma. Using<br />

the same T-cell epitope cloning technique, Melan-A (10), tyrosinase (11),<br />

gp100 (12), <strong>and</strong> gp75 (13) were identified as CTL-defined tumor antigens. Tumorspecific<br />

mutated antigens (CDK4 <strong>and</strong> p53) also can be targets for CTL (14,15).<br />

Mutated antigens are a category of individual antigens; typically, the mutation<br />

is not shared by other individuals, with few exceptions such as the k-ras oncogene<br />

mutated in pancreatic cancer. Viral <strong>and</strong> retroviral antigens are frequently<br />

expressed in cancers such cervical carcinoma, <strong>and</strong> they also can be recognized by<br />

T-lymphocytes (16).<br />

A different cloning strategy analyzes peptide sequences eluted from separated<br />

major histocompatibility complex (MHC) class I complexes isolated from<br />

tumor cells lysates. By comparing the peptide sequence to protein databases,<br />

the relevant tumor antigen can often be identified (17).<br />

The identification of tumor antigens that elicited spontaneous T-cell<br />

responses in cancer patients <strong>and</strong> the discovery of the mechanisms of antigen<br />

expression, processing, <strong>and</strong> MHC class I-restricted presentation of antigenderived<br />

peptides made it possible to design peptide-based immunotherapy protocols.<br />

Patients with antigen-positive tumors were vaccinated with the relevant<br />

peptides that are naturally processed <strong>and</strong> presented in the context of the respective<br />

MHC class I molecule (18). It was shown in several phase I <strong>and</strong> II clinical<br />

studies that peptide-specific cellular immune responses (CD4 <strong>and</strong> CD8) can be<br />

induced by immunizing patients with the respective peptide (19–25). Objective<br />

clinical remissions of single metastases <strong>and</strong> long-term clinical stabilizations<br />

were observed in some immunized patients, whereas progression of antigennegative<br />

lesions was frequently documented. This tumor escape phenomenon<br />

occurring in the presence of potent antigen-specific T-cell responses reflects the

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!