The effects of polarized macrophages on melanoma growth characteristics in vitro and in vivo

Abstract

Purpose: To investigate the influence of different tumor microenvironments on melanoma growth characteristics in vitro and in vivo. The effects of age as well as of M1- and M2- polarized macrophages on tumor growth characteristics shall be verified.

Methods: Murine macrophages were polarized in vitro into M1- and M2- macrophages. The result was verified by a multiple cytokine ELISA and immunocytology. The murine melanoma cell line HCmel12 was incubated with the supernatant of M1- or M2- polarized macrophages, respectively. In vitro proliferation of M1- and M2- conditioned HCmel12 cells was examined by a BrdU assay. In vivo, 26 CX₃CR1^+/GFP mice (M1 young n=7, M1 old n=6, M2 young n=7, M2 old n=6; young 8-12 wk, old >10 m) received an intravitreal injection of M1- or M2- conditioned HCmel12 cells. Enucleated eyes were processed for histological (H&E) and immunohistochemical (CD31, collagen 4, laminin, GFP) evaluation of tumor size and different tumor growth characteristics.

Results: In vitro polarization of macrophages was effective and was confirmed by a multiple cytokine ELISA and by immunocytology. Intraocular tumor growth was determined in all mice by H&E stains. By immunohistochemistry, M2- conditioned tumors showed a higher mean vascular density (CD31 positive vessels), more frequent collagen 4- positive structures and an increased infiltration by immune cells, particularly lymphocytes and macrophages, in comparison to M1- conditioned intraocular tumors. Tumor size did not differ between M1- and M2- conditioned tumors. Age did not have any impact on tumor size or on other tumor growth characteristics.

Conclusions: After in vitro M1- or M2- polarization of macrophages and subsequent M1- or M2- conditioning of HCmel12 melanoma cells, tumor growth characteristics revealed a more aggressive phenotype in M2- conditioned tumors since a high mean vascular density, infiltration of inflammatory cells and extracellular matrix structures are described as poor prognostic factors in uveal melanoma in humans. This animal study indicates that a high mean vascular density and increased collagen 4 expression might be attributed to a M2- characteristic local proangiogenic tumor microenvironment and supports the concept that M2 macrophages have a modulating effect on angiogenesis in melanoma.