Chemokine profiling of melanoma-macrophage crosstalk identifies CCL8 and CCL15 as prognostic factors in cutaneous melanoma
During cancer evolution, tumor cells attract and dynamically interact with monocytes/macrophages. To find biomarkers of disease progression in human melanoma, we used unbiased RNA sequencing and secretome analyses of tumor-macrophage co-cultures. Pathway analysis of genes differentially modulated in human macrophages exposed to melanoma cells revealed a general upregulation of inflammatory hallmark gene sets, particularly chemokines. A selective group of chemokines, including CCL20, CCL15 and CCL8, was actively secreted upon melanoma-macrophage co-culture. Because we previously described the role of CCL20 in melanoma, we focused our study in CCL8 and CCL15, and confirmed that in vitro both chemokines contributed to melanoma survival, proliferation and 3D invasion, through CCR1 signaling. In vivo, both chemokines enhanced primary tumor growth, spontaneous lung metastasis and circulating tumor cell (CTC) survival and lung colonization in mouse xenograft models. Finally, we explored the clinical significance of CCL8 and CCL15 expression in human skin melanoma, screening a collection of 67 primary melanoma samples, by multicolor staining and quantitative image analysis of chemokine-chemokine receptor content at the single cell level. Primary skin melanomas displayed high CCR1 expression, but there was no difference in its level of expression between metastatic and non-metastatic cases. By contrast, the comparative analysis between these two clinically divergent groups showed a highly significant difference in the cancer cell content of CCL8 (P= 0.025) and CCL15 (P< 0.0001). Kaplan-Meier curves showed that high content of CCL8 or CCL15 in cancer cells correlated with shorter disease-free and overall survival (log-rank test, p< 0.001). Our results highlight the role of CCL8 and CCL15, which are highly induced by melanoma-macrophage interactions in biologically aggressive primary melanomas, and could be clinically applicable biomarkers for patient profiling.