Study of the hen immune system led to seminal contributions to basic immunological principles . Recent studies of spontaneous ovarian cancer in the laying hen suggest it would be a valuable model for studies of ovarian tumor immunology. The laying hen spontaneously develops ovarian tumors with numerous similarities to human tumors [2–8]including similar tumor histology and tumor types . The incidence of tumors increases with age as in human ovarian cancer and tumors are fully progressive and in late stages metastasize to distant sites [2, 9]. Hen ovarian tumors show similar alterations in gene expression profiles compared to human tumors . Moreover, multiple proteins are similarly expressed in hen and human ovarian tumors  and include CA125 , Selenium Binding Protein 1 , COX-1 [6, 13], E-cadherin , VEGF [15, 16] and CYP1B1 . In addition, we showed that similar to human ovarian cancer  hens produce anti-ovarian and anti-tumor antibodies in response to ovarian tumors . However, antigen specific responses are unexplored in the hen model.
Mesothelin is a well characterized biomarker for ovarian cancer in human. Mesothelin is a 40 kDa cell-surface differentiation antigen that is normally expressed at low levels and is restricted to tissues such as the mesothelial cells lining some body cavities and epithelial cells of kidney, tonsil, trachea, and fallopian tube [20, 21]. However mesothelin is highly expressed in ovarian cancer, mesotheliomas and to a lesser extent in other cancers such as pancreatic, lung, and stomach . Increased mesothelin protein expression was reported in 70% of ovarian epithelial tumors and up to 100% of serous papillary ovarian cancer [21–28]. Indeed, mesothelin is shed into the circulation  and is one of a few specific serum markers for ovarian cancer [30–32]. In addition, mesothelin autoantibodies were detected in the sera of patients whose tumors were positive for mesothelin in ovarian cancer . The frequently elevated expression of mesothelin in cancer cells compared to normal cells and the immune response to mesothelin [32, 34–37] have led to exploration of mesothelin as a therapeutic target for ovarian cancers [26, 31, 38, 39].
The biological function of mesothelin remains speculative. Studies of the mouse mesothelin gene show that it is not critical for development or reproduction in normal mice . In ovarian cancer it is thought to have an effect on heterotypic cell-adhesion and cell-to-cell recognition and signaling by binding to another tumor antigen, CA125 (MUC16) to facilitate the cell invasiveness and metastasis [40–43].
Ovarian cancer has the highest mortality rate of the gynecological cancers. This is primarily due to a lack of symptoms and early detection tests. Therefore, the diagnosis of ovarian cancer primarily occurs at stage III/IV . When ovarian cancer is detected early survival is greater than 80% [45, 46], suggesting that earlier detection could significantly increase survival. The increased tissue expression and the presence of circulating mesothelin in human ovarian cancer is relatively specific and mesothelin shows promise as a specific marker and a target of immunotherapy for ovarian cancer. Efforts to understand the trajectory of biomarker expression and to validate early markers in pre-clinical studies are facilitated by the use of animal models. There are several models of ovarian cancer in rodents that were produced by genetic manipulation [9, 47–50]. The observation that specific genetic alterations lead to specific histologic sub-types of ovarian tumors [48, 50] is informative and is congruent with the concept that the different sub-types rise by different mechanisms. However, few of these models develop spontaneous ovarian tumors with the hallmarks of human tumors.
In order to use the laying hen as a preclinical model to study spontaneous immunological responses to ovarian tumor antigens and to investigate the potential of mesothelin as a therapeutic target for cancer vaccine, our objective was to determine if the hen expresses mesothelin, if mesothelin expression is increased in ovarian tumors and if circulating mesothelin autoantibody is associated with ovarian tumors.