P1A as an Oncogene Figure 3. Heterogeneity of tumors in individual mouse. The thymic tumor from different mice was stained by the following monoclonal antibodies, CD4, CD8, B220, CD11c, Gr1, and CD11b. After antibody staining, thymic tumor cells were subsequently analyzed by flow cytometry. A. Mouse #931 had thymic tumors with B220+ and B220+CD11c+CD11b+ cell populations of tumor cells. B. Mouse #946 had thymic tumor of CD4+CD8+ cell population of tumor cells. C. Mouse #947 had B220+CD11c+ and B220+CD11c+Gr1+CD11b+ cell populations of tumor cells. D. Mouse #954 had B220+, CD8+B220+CD11c+, Gr1+CD11b+ cell populations of tumor cells. E. Mouse #956 thymic tumor had B220lo cell population of tumor cells. F. Mouse #962 had tumor with B220+ cell population of tumor cells. G. Mouse #963 showed CD8+CD11c+CD11b+, B220+CD11c+ and CD11b+CD11c+ cell populations of tumor cells. doi:10.1371/journal.pone.0013439.g003 Several clinical studies have indicated that the frequency of cancer in immunodeficient background is more frequent [20,21,22]. For example, children with immunodeficiency are at a higher risk of malignancy, such as lymphomas and leukemias [23]. Therefore, the immune system, including innate and adaptive immunity, is able to inhibit the tumor development and eradicate the tumors already established [21]. A similar phenomenon was also shown in animal models. For example, after subcutaneous injection of the chemical carcinogen methylcholan- threne, Rag22/2 background mice developed higher frequency of sarcomas compared to the strain-matched controls [24]. Our data indicated that thymic tumors occurred more frequently in the Rag2-deficient background and the immune system of a normal immunocompetent mouse may be able to eliminate the P1A- induced tumor. Taken together, P1A induced tumorigenesis and resulted in various thymic tumor types in mice. Our present work demonstrated that onco-fetal antigen P1A was associated with the transformed phenotype in P1A transgenic mice. Therefore, expression of onco-fetal antigens may be biologically relevant to tumorigenesis. Further studies are required to dissect the mechanisms of the process of oncogenesis by P1A. Materials and Methods Cell lines and experimental animals Mouse plasmacytoma J558 from ATCC were cultured in RPMI 1640 medium containing 5% fetal bovine serum (FBS) and 100 mg/ ml penicillin and streptomycin. Murine R1 ES cells were co-cultured with mouse embryonic fibroblast (MEF) cells in DMEM medium PLoS ONE | www.plosone.org 4 October 2010 | Volume 5 | Issue 10 | e13439