Blocking the recruitment of tumor promoting macrophages leads to resident macrophages modulating the immune response in pulmonary metastasis of melanoma

Tumour associated macrophages are recruited from the circulation, and show remarkable plasticity in the tumour microenvironment. We performed a PCR array of polarization genes on recruited and resident macrophages sorted in a time course of pulmonary metastasis from B16F10 mouse melanoma. The early recruited (72h after tumour challenge) CD11b+F4/80+ macrophages showed an inflammatory gene expression profile, while the late recruited (21d) upregulated inflammatory and alternative activation related genes alike, displaying a tumour promoting phenotype. The resident CD11c+MR+F4/80+ macrophages showed a significant increase in anti-inflammatory gene expression during the time course. In order to prevent the recruitmentof tumour promoting macrophages, we blocked chemokine receptors CCR1, CCR2 and CCR5 after identifying CCL5 secreted by B16F10 cells. Following lung harvest, the counting and measuring of the surface colonies revealed that CCR1 inhibition reduced the growth of lung colonies, while CCR2 and CCR5 inhibition increased metastasis burden. FACS analysis showed decreased recruitment of tumour promoting macrophages in response to CCR1 inhibition and at the same time a significant increase in the number of resident macrophages secreting IL-15. An increase in NK cells was observed, presumably contributing to the suppression of colony growth beside the blockage of tumour promoting macrophage recruitment via CCR1.