Markers that reliably identify cancer stem cells (CSC) in ovarian cancer could assist prognosis and improve strategies for therapy. CD133 is a reported marker of ovarian CSC. Aldehyde dehydrogenase (ALDH) activity is a reported CSC marker in several solid tumors but it has not been studied in ovarian CSC. Here we report that dual positivity of CD133 and ALDH defines a compelling marker set in ovarian CSC. All human ovarian tumors and cell lines displayed ALDH activity. ALDH+ cells isolated from ovarian cancer cell lines were chemoresistant and preferentially grew tumors compared to ALDH− cells, validating ALDH as a marker of ovarian CSC in cell lines. Notably, as few as 1000 ALDH+ cells isolated directly from CD133(−) human ovarian tumors were sufficient to generate tumors in immunocompromised mice, whereas 50,000 ALDH− cells were unable to initiate tumors. Using ALDH in combination with CD133 to analyze ovarian cancer cell lines we observed even greater growth in the ALDH+CD133+ cells compared to ALDH+CD133− cells, suggesting a further enrichment of ovarian CSC in ALDH+CD133+ cells. Strikingly, as few as 11 ALDH+CD133+ cells isolated directly from human tumors were sufficient to initiate tumors in mice. Like other CSC, ovarian CSC exhibited increased angiogenic capacity compared to bulk tumor cells. Lastly, the presence of ALDH+CD133+cells in debulked primary tumor specimens correlated with reduced disease-free and overall survival in ovarian cancer patients. Taken together, our findings define ALDH and CD133 as a functionally significant set of markers to identify ovarian CSCs.

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Background: Murine studies suggest that myeloid cells such as vascular leukocytes (VLC) and Tie2 + monocytes play a critical role in tumor angiogenesis and vasculogenesis. Myeloid cells are a primary cause of resistance to anti-VEGF therapy. The elimination of these cells from the tumor microenvironment significantly restricts tumor growth in both spontaneous and xenograft murine tumor models. Thus animal studies indicate that myeloid cells are potential therapeutic targets for solid tumor therapy. Abundant VLC and Tie2 + monocytes have been reported in human cancer. Unfortunately, the importance of VLC in human cancer growth remains untested as there are no confirmed therapeutics to target human VLC.