In vivo tracking of tumor cell trafficking in a MM mouse model. (A) Depletion of CD138⁺ patient cells from the circulation occurs with the same kinetics as MM.1S cell line. MM.1S (n = 4) or MM patient sample cells (n = 5) were labeled with fluorescent cytoplasmic or membrane dyes, injected into mice, and immediately the proportion of cells remaining in the circulation was measured by in vivo flow cytometry and plotted against time (adapted from Figure 3 of Runnels et al¹⁵⁴  with permission). (B) MM cells position themselves in proximity to the vasculature. 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindodicarbocyanine–stained MM.1S cells were injected intravenously into Col2.3-GFP mice at a dose of 100 000 cells per mouse. Immediately before imaging, the mice were injected with the vascular marker Quantum Dots 800. The mice were imaged within 2, 6, and 72 hours after MM cell injection. Z stacks were acquired from multiple regions in the calvaria of the mice. Distances were measured and tabulated between MM cells and osteoblasts or endosteal surface for the first 6 hours after MM cell injection. (Adapted from Figure 3 of Runnels et al¹⁵⁴  with permission). (C) Imaging at 72 hours after MM cell injection. The image demonstrates the relationship of the MM cells (white) to the vasculature (red), osteoblasts (green), and bone (blue) during the first 72 hours after cell injection. Scale bars represent 100 μm. (Adapted from Figure 3 of Runnels et al¹⁵⁴  with permission). (D) Imaging shows vessel formation around an area of GFP-positive MM cells growing in a cluster in close association to blood vessels. Immediately before imaging, the mice were injected with the vascular marker Quantum Dots 800. The MM1S cells are GFP-positive (green color). Scale bars represent 100 μm. (E) Primary plasma cells injected from a patient with plasma cell leukemia and allowed to engraft and proliferate for 8 weeks. A green-fluorescently labeled anti-CD138 antibody was injected intravenously just before imaging to allow imaging of the cells.