Niche-dependent induction of the myeloid gene signature. (A) Intravital imaging of a dormant MM cell (arrow) residing on the endosteal surface and a growing MM colony. Top shows the DiD (red) channel, and bottom shows the eGFP (green) channel. Scale bar = 50 µm. (B) Immunohistochemical staining for CD138 showing an individual 5TGM1 tumor cell (arrow) adjacent to the endosteal bone surface. Scale bar = 50 µm; inset scale bar = 20 µm. (C) Real-time quantitative polymerase chain reaction (RT-qPCR) of myeloid marker genes from 5TGM1 cells cultured with and without MC3T3 cells. (D) Flow cytometric analysis of AXL expression by 5TGM1 cells cultured with and without MC3T3 cells. Symbols denote different experimental replicates. (E) RT-qPCR of myeloid marker genes from 5TGM1 cells cultured alone, with MC3T3 conditioned media (CM), and with MC3T3 cells seeded directly or separated by a transwell. (F) Scatter plot of genes expressed by 5TGM1 cells cultured alone or with MC3T3 cells (left); heatmap of differentially expressed genes (right). (G) Scatter plot of genes expressed by MC3T3 cells cultured alone or with 5TGM1 cells. (C-E) Each point represents an independent experiment. Horizontal lines are the median; box indicates the IQR. Mann-Whitney test: *P < .05, **P < .01, ****P < .0001. ns, not significant.