Expression of cancer/testis (CT) antigens in lung cancer
Introduction
Lung cancer is the most common neoplasm and is a cause of the largest cancer-related mortality in the world. Although there are three established therapeutic modalities for lung cancer; surgical resection, chemotherapy and radiotherapy, the results remain unsatisfactory so that there is a need for a treatment breakthrough. Immunotherapy is one of the candidates for alternative control of lung cancer.
The number of human tumor antigens known to be recognized by cytotoxic T lymphocytes (CTL) or antibodies has increased over recent years. These antigens are classified into the following categories: (i) differentiation antigens, e.g. Melan A/MART-1 [1], [2], tyrosinase [3] and gp100 [4], [5]; (ii) overexpressed or amplified antigens, e.g. HER2/neu [6] and translation initiation factor-4 gamma [7]; (iii) mutated gene products, e.g. p53 [8], β-catenin [9] and caspase-8 [10]; and (iv) cancer/testis (CT) antigens. CT antigen genes, otherwise termed as “germ cells and cancer expressed genes” or “cancer/germ line genes”, are aberrantly expressed in variable proportions of a wide range of different types of tumors, but not in normal tissues except for cells in testis, ovary and placenta. Since these cells do not express major histocompatibility complex (MHC) class I molecules, CD8(+) T cells cannot recognize CT antigens expressed on these cells, suggesting that CT antigens expressed in tumors are ideal targets for vaccine-based immunotherapy. CTL-based methods have lead to the definition of a series of CT antigens, such as MAGE [11], GAGE [12] and BAGE [13]. Generation of an antibody-based cDNA expression cloning method (SEREX) [14] has also allowed identification of CT antigens including NY-ESO-1 [15], HOM-MEL-40/SSX-2 [16] and HOM-Tes-14/SCP-1 [17]. Recently, in silico surveys of ESTs and subsequent serological expression cloning have been used for identification of OY-TES-1 [18], CT15, CT16 and CT17 [19]. Representational difference analysis (RDA) has also defined several CT antigens, e.g. LAGE-1 [20] and CT7/MAGE-C1 [21], [22]. Several clinical trials using peptides from MAGE-1, MAGE-3 or NY-ESO-1 have already been performed, and tumor regression has been reported in some cases [23], [24], [25], [26], [27].
Over a dozen genes or gene families encode CT antigens and at least eight of these have been mapped to the X chromosome. Although expression frequencies of individual CT antigens in various tumor types have been reported, little is known about their composite expression patterns in lung cancers. It is important to ascertain what proportion of affected patients might be eligible for CT antigen-based vaccination and which CT antigens should be targeted.
In this study, we analyzed the frequency of CT antigen expression in Japanese lung cancer specimens. Furthermore, we attempted to identify new CT antigens by computer-assisted screening using the dbEST database.
Section snippets
Tissues and cell lines
The study was approved by the Institutional Review Boards of Aichi Cancer Center and RIKEN BioResource Center. A total of 46 tumors (22 adenocarcinomas, 13 squamous cell carcinomas, six large cell carcinomas, four small cell carcinomas, and 1 adenosquamous cell carcinoma) were used in this study. Tumor and normal lung tissues from surgical specimens were frozen in liquid nitrogen immediately after surgical removal at the Department of Thoracic Surgery, Aichi Cancer Center Hospital. Tissues were
Expression of CT antigens in lung cancer cell lines
Expression of 12 CT antigens was studied by RT-PCR in 29 lung cancer cell lines and the results are summarized in Table 2. Twenty-three of 29 lung cancer cell lines (79%) were found to express at least one of the CT antigens. The most frequently expressed CT antigen was MAGE-3/6 (15/29, 52%), followed by MAGE-4 (11/29, 38%), SSX-1 (11/29, 38%) and SSX-4 (11/29, 38%). NY-ESO-1, a highly immunogenic tumor antigen, was, however, found to be expressed in only five samples (17%). In small cell lung
Discussion
CT antigens are expressed in variable proportions of a wide range of different types of tumors, such as melanomas, lung and breast carcinomas, but not in normal tissues except for testis, ovary and placenta that express no MHC class I molecules. In addition to this lack of MHC expression, the testis has a blood–testis barrier in the seminiferous tubuli generated by the Sertoli cells, so that it can be considered an immunologically privileged site. Therefore, the CT antigens are promising
Conclusion
We examined the expression of CT antigens in Japanese lung cancers, and found that CT antigens are frequently expressed in both lung cancer cell lines (79%) and primary tumors (74%). Multiple CT antigens were detected in 25 of the 46 primary tumors (54%). 57% expressed MAGE-3 or SSX-1, 65% expressed MAGE-3, SSX-1 or MAGE-4 and 70% expressed MAGE-3, SSX-1, MAGE-4 or SSX-4 (or CT7). Polyvalent CT antigen vaccines targeting MAGE-3+SSX-1, MAGE-3+SSX-1+MAGE-4 or MAGE-3+SSX-1+MAGE-4+SSX-4 (or CT7)
Acknowledgements
The authors thank Dr Motokazu Suyama, Masayuki Shinoda and Shunzo Hatooka, Department of Thoracic Surgery, and Dr T. Sugiura, T. Hida, K. Yoshida and Y. Horio, Department of Thoracic Oncology, Aichi Cancer Center, for providing tumor specimen. The authors also thank Dr Kiyotaka Kuzushima, Yoshiki Akatsuka, Kunio Tsujimura and Eisei Kondo for their valuable suggestions, and Yasue Matsudaira, Keiko Nishida for technical assistance. This work was supported in part by a Grant-in-Aid for Scientific
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