Melanoma LRC disseminate in vivo early from the primary tumor site. (a) Schematic for detecting LRC in vivo. Melanoma cells (WM989 and 451Lu) were labeled with PKH26 and subcutaneously injected into SCID Hairless Outbred mice. Mice were imaged weekly using IVIS (dorsal, ventral and lateral scanning). Between days 0 and 20, the fluorescent signal was detected in the primary tumor at the injection site; after day 20, proliferating tumor cells lost the dye at the primary site. Instead, fluorescent signals indicating the presence of LRC were detected in distant sites. (b) Mice on left: dorsal view of mice scanned 7 days post-tumor injection. For the left two mice of each melanoma, palpable tumors overlapped with fluorescence signals. Graphs on right: tumor growth (black line, left Y-axis) and dilution of PKH26 signal (red line, right Y-axis) at the primary tumor site (n=5). Arrows indicate the cross point of the curves. After reaching ~500 mm3 tumor size, PKH26 signal is rapidly lost. (c) Dorsal (left), ventral (middle), and lateral (right) views of mice scanned when tumors reached ~700 mm3 size. (d) Scatter plots showing quantification of the fluorescent signals for individual mice at days 0–20 (left plots) and 20–60 (right plots) post injection (n=5), for the same two melanoma cell lines as in c. Primary tumors (blue), peritoneal cavities (red), and left flank (black). (e) Bars represent flow cytometry quantification of melanoma LRC (CD146+/PKH26+ positive cells) in primary tumor (gray), mouse lymph nodes (LN, yellow), lungs (blue), liver (green) and spleen (purple); Zebra plot (below) are representative examples from lungs. Upper right quadrant: melanoma LRC double positive for CD146 and PKH26; lower right quadrant: melanoma non-LRC cells (CD146+ positive). Numbers are % of positive cells after subtraction of the percentage detected in the isotype control (left plot). (f) Representative photomicrographs of immunofluorescence performed on frozen tissue from lungs and spleen for a combination of melanoma markers (CD146, β3 integrin, MSCP-1). Cells can be detected as clusters (white square in spleen) or disseminated (white arrows). Bottom panel is the negative antibody control staining.