Targeting tumor-associated macrophages in an orthotopic murine model of diffuse malignant mesothelioma

Tumors are a mixture of neoplastic and host stromal cells, which establish a microenvironment that contributes to tumor progression. In this study, the contribution of tumor-associated macrophages (TAMs) to tumor growth and metastasis was examined using an orthotopic, immunocompetent murine model of diffuse malignant peritoneal mesothelioma. The expression profile of cytokines and chemokines in solid tumors was consistent with a M2-polarized, TAM-mediated immunosuppressive microenvironment. TAMs were targeted using liposome-encapsulated clodronate (CLIP). Exposure of tumor spheroids to CM-Dil-labeled CLIP in situ confirms targeting of macrophages and not mesothelioma cells. Intraperitoneal (i.p.) delivery of CLIP produced apoptosis in tumor spheroids and solid tumors in contrast to delivery of liposome-encapsulated PBS or PBS. Mice received an i.p. injection of mesothelioma cells with CLIP delivered i.p. every 5 days. This treatment protocol produces a 4-fold reduction in the number of tumors, a 17-fold reduction in the relative tumor burden, and a 5-fold reduction in invasion and metastasis when compared with mice exposed to liposome-encapsulated PBS or PBS. Following transplantation of tumor spheroids and treatment with CLIP, mice showed a 4-fold reduction in the number of tumors and a 15-fold reduction in relative tumor burden. Mice bearing established tumors showed a 2-fold reduction in the number of tumors and relative tumor burden when exposed to half the previous dose of CLIP delivered by repeated i.p. injection. These reductions in tumor burden are statistically significant and identify TAMs as an important host-derived cell that contributes to growth, invasion, and metastasis in diffuse malignant peritoneal mesothelioma.