A. D. Michaels1, S. J. Adair2, M. G. Mullen1, S. Nagdas2, J. B. Persily1, N. Jha1, J. T. Parsons2, T. W. Bauer1 1University Of Virginia,Surgery,Charlottesville, VA, USA 2University Of Virginia,Microbiology, Immunology, And Cancer Biology,Charlottesville, VA, USA
Introduction: Despite a margin-negative (R0) resection, most patients with localized pancreatic adenocarcinoma (PDAC) eventually die from metastatic disease and therefore likely harbor occult metastases at the time of resection. The ability to inhibit growth of occult metastases could greatly improve survival of patients with PDAC. Prior studies suggest a role of the innate immune system in suppressing the progression of PDAC hepatic micrometasases. To test this hypothesis, we utilized a murine model of micrometastatic PDAC with patient-derived xenografts (PDX).
Methods: Patient-derived PDAC cells expressing luciferase were injected into the spleens of two strains of mice resulting in liver metastases, followed by splenectomy to remove the primary tumor. The mouse strains used were athymic nude mice which lack cell-mediated immunity but have functioning innate immunity and NOD scid gamma (NSG) mice which lack both cell-mediated immunity as well as functional macrophages, dendritic cells, and NK cells. Metastatic tumor burden was evaluated by serial bioluminescent imaging. When disease burden threatened life, mice were sacrificed and necropsied. Gene expression profiling (GEP) was performed on matched hepatic and peritoneal metastases from NSG mice.
Results: The spleens of 12 nude and 12 NSG mice were injected with PDX 449 cells. There was faster and more extensive clearance of tumor cells in nude mice. By post-injection day 42, the majority of the NSG mice had extensive abdominal disease whereas only three nude mice had mild-to-moderate tumor burden. Figure 1 displays representative bioluminescent imaging at days 2 and 42. At necropsy, gross liver metastases were present in 5/12 (42%) NSG mice and 100% had peritoneal metastases. By bioluminescent imaging through 78 days, 4/12 (33%) nude mice had peritoneal metastasis and there was no evidence of any hepatic metastasis. For both metastatic sites, the NSGs had significantly more disease (liver, p=0.04; peritoneum, p=0.001). GEP of metastases from three NSG mice demonstrated significant differences between tumors from the different sites. Hierarchical clustering by innate immunity-related genes revealed that hepatic tumors cluster separately from peritoneal tumors with the most substantial differences in the TLR, VEGFA, and MAPK signaling pathways.
Conclusion: The innate immune system significantly inhibits progression of occult metastatic PDAC cells in the liver and peritoneum. Immunotherapy approaches which augment the innate immune system could greatly improve survival in patients with PDAC.
