Developing a Model for CD8+ T-cell Recognition of Pancreatic Cancer and its Metastases
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Pancreatic ductal adenocarcinoma (PDAC) is a highly aggressive cancer that accounts for 90% of all pancreatic cancers. The 5-year survival rate is 11%, placing it among the most lethal of all cancers. The main contributor to this poor prognosis is the high prevalence of metastasis (seeding of cancer cells in distant organs) at the time of diagnosis. Moreover, less than half of the patients that present with only a localized tumor in the pancreas survive 5 years post diagnosis. This is linked to the ability of PDAC to metastasize early in disease progression, resulting in the presence of undetectable, small metastasis (micrometastasis). These micrometastasis grow out to macrometastasis and are an important factor for the high mortality rate in patients with localized disease. The immune system would be ideally suited to target and clear those metastasis but immunotherapies in PDAC have shown limited success. Therefore, it is essential to understand the role of immune cells during the establishment and outgrowth of pancreatic metastasis, especially T-cells, as they can specifically target and kill cancerous cells. To study metastasis our lab developed a new mouse model. In short, we inject pancreatic tumor cells subcutaneously into the back of a mouse, wait for a primary tumor to develop, then remove this tumor and after a few weeks we can find clear lung or lymph node metastasis. To investigate the role of T-cells specifically during the outgrowth of micrometastasis, we depleted T-cells, including tumor specific T-cells, after resection of the tumor. By doing so, we could test if T-cells are able to prevent outgrowth of micrometastasis. Strikingly, we found that T-cell depletion did not alter the number of metastasis, indicating an absence of T-cell control of micrometastasis. Data from other studies show that the absence of T-cell mediated immune control is due to the lack of tumor specific proteins (tumor antigens). Thus, we worked to generate a pancreatic cancer cell line that expresses the recognizable tumor antigen tyrosinase related protein 1 (TRP1). TRP1 is expressed in skin cancer tumors in both mice and humans and immunologically resembles tumor antigens, because like most tumor antigens, TRP1 does not induce a strong immune response. To use this cell line to study metastasis, we selected for metastatic cells in two subsequent rounds. We collected tumor cells from lung metastasis that have established after intravenous injection then, subcutaneously injected them in our resectable mouse model where, again, we isolated cells from a lung metastasis. Via this way we selected for cells that can both escape from the primary tumor and seed in distant organs. To eventually study T-cell associated immunity in the context of metastasis, we injected these metastatic TRP1 cells in our novel mouse model. We found that the T-cell presence in primary TRP1 tumors is much higher than in tumors without TRP1. Interestingly, when we assessed the metastatic burden of the metastatic TRP1 cells, we found few metastasis. We checked if the tumor cells in those metastasis still expressed TRP1, but could not detect any. Together, these results highlight a role for T-cells in the establishment of pancreatic cancer micrometastasis in the presence of the recognizable tumor antigen TRP1 and pave way for further studies into the involvement of the immune system during pancreatic cancer.