Mast cells promote melanoma colonization of lungs

Mast cells have been implicated in malignant processes, mainly through clinical correlative studies and by experiments performed using animals lacking mast cells due to defective c-kit signaling. However, mast cell-deficient mouse models based on c-kit defects have recently been questioned for their relevance. Here we addressed the effect of mast cells in a tumor setting by using transgenic Mcpt5-Cre(+) R-DTA(+) mice, in which the deficiency of mast cells is independent of c-kit defects. Melanoma cells (B16.F10) were administered either subcutaneously or intravenously into Mcpt5-Cre(+) R-DTA(+) mice or Mcpt5-Cre(-) R-DTA(+) littermate controls, followed by the assessment of formed tumors. In the subcutaneous model, mast cells were abundant in the tumor stroma of control mice but were absent in Mcpt5-Cre(+) R-DTA(+) mice. However, the absence of mast cells did not affect tumor size. In contrast, after intravenous administration of B16.F10 cells, melanoma colonization of the lungs was markedly reduced in Mcpt5-Cre(+) R-DTA(+) vs. Mcpt5-Cre(-) R-DTA(+) animals. Decreased melanoma colonization of the lungs in Mcpt5-Cre(+) R-DTA(+) animals was accompanied by increased inflammatory cell recruitment into the bronchoalveolar lavage fluid, suggesting that mast cells suppress inflammation in this setting. Further, qPCR analysis revealed significant alterations in the expression of Twist and E-cadherin in lungs of Mcpt5-Cre(+) R-DTA(+) vs. control Mcpt5-Cre(-) R-DTA(+) animals, suggesting an impact of mast cells on epithelial-mesenchymal transition. In conclusion, this study reveals that mast cells promote melanoma colonization of the lung.