Tumor-educated macrophages promote tumor growth and peritoneal metastasis in an orthotopic nude mouse model of human pancreatic cancer

Macrophages promote tumor growth by stimulating tumor-associated angiogenesis, cancer-cell invasion, migration, intravasation, and suppression of antitumor immune responses. Ten transgenic nude mice, ubiquitously expressing green fluorescent protein (GFP), were injected subcutaneously with the human pancreatic cancer cell line, BXPC3, stably expressing red fluorescent protein (RFP). GFP-expressing macrophages from the GFP mice with the subcutaneous BxPC3-RFP tumor were harvested and defined as "tumor-educated macrophages". Macrophages were also harvested from transgenic GFP mice (n=10) without tumors and identified as "naïve macrophages." The tumor-educated and naïve macrophages were then implanted into BxPC-3-RFP tumor-bearing non-transgenic nude mice and compared for their ability to enhance tumor progression. In the control group, without macrophage injection, the average primary tumor weighed 668 mg and only three mice (30%) developed peritoneal metastases, which averaged 72 mg. The naïve-macrophage group had an average tumor weight of 823 mg (p=0.51) and 50% developed peritoneal metastases, whose weight averaged 975 mg (p=0.029). The group treated with tumor-educated macrophages had an average primary tumor weight of 2095 mg (p=0.001) and 75% of mice developed peritoneal metastases, whose weight averaged 2135 mg (p=0.008). These results suggest that macrophages influence tumors, and tumors influence macrophages, and tumor-educated promote tumor progression. Tumor-educated macrophages may be a target for therapy of metastatic cancer.