MM tumor cells stimulate tube-like structure formation and Flk-1 staining in freshly obtained CD14⁺ cells that are blocked by anti-PTN antibodies; PTN and M-CSF induce tube-like structure formation and Flk-1 staining in fresh human monocytes in vitro that is blocked by anti-PTN antibodies. (A) BMMCs from a MM patient containing more than 90% plasma cells and a healthy control subject and U937 cells were stained with either anti-PTN or isotype-matched control antibodies (40×/objective lens, Olympus BX51; Olympus, San Diego, CA). (B top row) CD14⁺ cells were cocultured with BMMCs from a healthy donor or MM patient containing more than 90% tumor cells with and without anti-PTN antibody using Transwell culture plates for 14 days. Light microscopy of CD14⁺ cells stained with an anti–Flk-1 antibody is shown. (Middle row) CD14⁺ cells were exposed to M-CSF alone, PTN and M-CSF together, or the combination of PTN, M-CSF, and VEGF. The cells were stained with anti–Flk-1 antibody using immunohistochemical (IHC). (Bottom row) In a separate experiment, CD14⁺ cells were exposed to M-CSF alone or the combination of M-CSF and PTN with or without anti-PTN antibody.