Transgenic Expression of P1A Induced Thymic Tumor: A Role for Onco-Fetal Antigens in Tumorigenesis Chi-Shan Li1,5., Chong Chen1., Pan Zheng1,3,4*, Yang Liu1,2,3* 1 Division of Immunotherapy, Section of General Surgery, Department of Surgery, School of Medicine, University of Michigan, Ann Arbor, Michigan, United States of America, 2 Division of Molecular Medicine and Genetics, Department of Internal Medicine, School of Medicine, University of Michigan, Ann Arbor, Michigan, United States of America, 3 Program of Molecular Mechanism of Diseases and Comprehensive Cancer Center, School of Medicine, University of Michigan, Ann Arbor, Michigan, United States of America, 4 Department of Pathology, School of Medicine, University of Michigan, Ann Arbor, Michigan, United States of America, 5 Toxicology Program, Department of Environmental Health Sciences, School of Public Health, University of Michigan, Ann Arbor, Michigan, United States of America Abstract P1A is the first known tumor rejection antigen. It is expressed in embryonic stem cells and multiple tumors but is silent in adult tissues except for the testis and placenta. Therefore, P1A represents a prototype for onco-fetal antigens. To test the potential function of P1A in tumorigenesis, we used a transgenic mouse expressing P1A in lymphoid cells. We observed that immunodeficient host P1A transgenic mice developed thymic tumors after 7 months of age and had shorter survival rates compared to control groups. Most of the 7 examined tumors displayed B cell lineage markers. The P1A transgenic bone marrow cells had higher proliferation ability and more potential progenitors compared to control bone marrow cells. To our knowledge, our data provided the first example that onco-fetal antigen can promote tumorigenesis. Citation: Li C-S, Chen C, Zheng P, Liu Y (2010) Transgenic Expression of P1A Induced Thymic Tumor: A Role for Onco-Fetal Antigens in Tumorigenesis. PLoS ONE 5(10): e13439. doi:10.1371/journal.pone.0013439 Editor: Sujit Basu, Ohio State University, United States of America Received July 21, 2010; Accepted September 22, 2010; Published October 15, 2010 Copyright: 2010 Li et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This study is supported by the National Institutes of Health. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: yangl@med.umich.edu (YL); panz@umich.edu (PZ) . These authors contributed equally to this work. Introduction By definition, tumor antigens were identified because cancer- reactive lymphocytes could recognize these proteins [1]. An interesting but still poorly understood phenomenon is that many tumor antigens are also abundantly expressed at early stages of embryogenesis. These antigens are termed onco-fetal antigens. Theoretically, these antigens might be useful for cancer immune therapy, since they are highly expressed on tumor cells but absent in the adult host with the exception of placenta and testis. In addition, the expression of these types of antigens on tumor cells may reflect the fact that cancer may resemble the biological properties of embryonic cells. One key issue is whether the onco- fetal antigens play a significant role in cell differentiation or proliferation, since tumor and embryonic cells share some biological properties. P1A is a prototype of onco-fetal antigens. Only recently, the P1A gene has been mapped to the X-chromosome (Mouse Genome Informatics, ID 98818). Identification of P1A as a tumor rejection antigen marked an important breakthrough in tumor immunology as it was the first tumor antigen known to be recognized by cytotoxic T cells [2]. P1A was first identified in the mastocytoma P815 cell line which was derived from DBA/2 mice after exposure to methylcholantrene [3]. Later, studies also found that P1A was expressed in several tumors such as Meth A sarcoma and J558 plasmacytoma [4]. P1A is highly expressed in the testis and placenta [5]. The levels of P1A mRNA in the testis and placenta are around 10–20% and 150–200% of mastocytoma P815, respectively [5]. In addition, P1A mRNA was detectable in spleen, lung, liver and thymus by reverse transcription- polymerase chain reaction (RT-PCR), although the relative abundance of P1A mRNA in these tissues was much lower than that found in J558 tumor cells [6]. Here, we show that P1A is also expressed in mouse R1 embryonic stem (ES) cells. The function of P1A is unknown to date. Since tumor cells, germ cells and stem cells have the capacity to differentiate and proliferate, it is possible that P1A may be involved in a particular state of cell differentiation and growth regulation. Therefore, to seek the function of onco-fetal antigens, we used P1A as a model. In this paper, we found that our transgenic mouse line, which over-expresses tumor antigen P1A in lymphoid cells, developed a variety of tumors after 7 months. The results from this study suggest that P1A is involved in tumorigenesis. Results P1A express in embryonic stem cells and P1A transgenic (P1A Tg) thymus Our P1A transgene contained the enhancer of the immuno- globulin heavy chain gene (Em), the mb-1 promoter, followed by P1A open reading frame, an ID2 gene, and an SV40 polyadenylation signal (Figure 1A). P1A was under the control of the mb-1 promoter, which is expressed specifically in B cells from the progenitor to mature stages [7]. In addition, the enhancer of Em has been shown to promote transgene expression in B and T lymphoid cells [8,9,10,11,12]. Thus, our P1A Tg mice can express PLoS ONE | www.plosone.org 1 October 2010 | Volume 5 | Issue 10 | e13439