Induction of M2-macrophages by tumour cells and tumour growth promotion by M2-macrophages: a quid pro quo in pancreatic cancer

Introduction: More effective therapies are required to improve survival of pancreatic cancer. Possible immunologic targets include tumour associated macrophages (TAMs), generally consisting of M1- and M2-macrophages. We have analysed the impact of TAMS on pancreatic cancer in a syngeneic orthotopic murine model.

Methods: 6606PDA murine pancreatic cancer cells were orthotopically injected into C57BL6 mice. Tumour growth was monitored using MRI. Macrophages were depleted by clodronate liposomes. Tumours including microvessel density were evaluated using immunohistochemistry, immunofluorescence and/or cytometric beads assays. Naïve macrophages were generated employing peritoneal macrophages. In vitro experiments included culturing of macrophages in tumour supernatants as well as tumour cells cultured in macrophage supernatants using arginase as well as Griess assays.

Results: Clodronate treatment depleted macrophages by 80% in livers (p = 0.0051) and by 60% in pancreatic tumours (p = 0.0169). MRI revealed tumour growth inhibition from 221.8 mm(3) to 92.3 mm(3) (p = 0.0216). Micro vessel densities were decreased by 44% (p = 0.0315). Yet, MCP-1-, IL-4- and IL-10-levels within pancreatic tumours were unchanged. 6606PDA culture supernatants led to a shift from naïve macrophages towards an M2-phenotype after a 36 h treatment (p < 0.0001), reducing M1-macrophages at the same time (p < 0.037). In vivo, M2-macrophages represented 85% of all TAMs (p < 0.0001). Finally, culture supernatants of M2-macrophages induced tumour growth in vitro by 63.2% (p = 0.0034).

Conclusions: This quid pro quo of tumour cells and M2-macrophages could serve as a new target for future immunotherapies that interrupt tumour promoting activities of TAMs and change the iNOS-arginase balance towards their tumoricidal capacities.