Purpose: The CD52-targeted antibody alemtuzumab induces major clinical responses in a group of patients with myelodysplastic syndromes (MDS). The mechanism underlying this drug effect remains unknown. Experimental Design: We asked whether neoplastic stem cells (NSC) in patients with MDS (n ¼ 29) or acute myelogenous leukemia (AML; n ¼ 62) express CD52. Results: As assessed by flow cytometry, CD52 was found to be expressed on NSC-enriched CD34 þ /CD38 À cells in 8/11 patients with MDS and

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... d del(5q). In most other patients with MDS, CD52 was weakly expressed or not detectable on NSC. In AML, CD34 þ /CD38 À cells displayed CD52 in 23/ 62 patients, including four with complex karyotype and del(5q) and one with del(5q) and t(1;17;X). In quantitative PCR (qPCR) analyses, purified NSC obtained from del(5q) patients expressed CD52 mRNA. We were also able to show that CD52 mRNA levels correlate with EVI1 expression and that NRAS induces the expression of CD52 in AML cells. The CD52-targeting drug alemtuzumab, was found to induce complement-dependent lysis of CD34 þ /CD38 À /CD52 þ NSC, but did not induce lysis in CD52 À NSC. Alemtuzumab also suppressed engraftment of CD52 þ NSC in NSG mice. Finally, CD52 expression on NSC was found to correlate with a poor survival in patients with MDS and AML. Conclusions: The cell surface target Campath-1 (CD52) is expressed on NSC in a group of patients with MDS and AML. CD52 is a novel prognostic NSC marker and a potential NSC target in a subset of patients with MDS and AML, which may have clinical implications and may explain clinical effects produced by alemtuzumab in these patients. Clin Cancer Res; 20(13); 1-14. Ó2014 AACR. represent mean AE SD from all mice per group in three independent experiments (3 donors). Engraftment level reduction by alemtuzumab: #73: 66%; #79: 44%; #49: 47%). Asterisk, P < 0.05. Blatt et al.