Macrophages are innate immune cells that, when recruited to tumors, can promote tumor progression. Macrophage activity and phenotype can be influenced by molecules secreted by tumor cells. To monitor macrophage behavior, Roh-Johnson et al. used time-lapse imaging of a zebrafish model of melanoma. Unexpectedly, macrophages not only bound to but also transferred their cytoplasm into melanoma cells, which promoted metastasis. Macrophage cytoplasmic transfer to melanoma cells also correlated with metastasis in mice. This unusual mechanism of cell-cell signaling raises questions about how such intercellular communication is regulated and what molecules are transferred to melanoma cells to promote their metastasis.
