Expression of the CAMPATH-1 Antigen (CD52) on CD34+/CD38- Progenitor Cells in Patients with 5q- MDS and in a Subset of AML

Abstract 1728

The target antigen CAMPATH-1 (CD52) is widely expressed in various hematopoietic lineages inlcuding lymphocytes, basophils, and blood monocytes. The anti-CD52 antibody Alemtuzumab is used successfully to treat patients with chemotherapy-refractory chronic lymphocytic leukemia. Based on its strong immunosuppressive effects, Alemtuzumab has also been considered for patients with aplastic anemia and hypoplastic myelodysplastic syndromes (MDS). Indeed, more recently, Alemtuzumab was found to induce major hematologic responses in a group of patients with MDS. Although the immunosuppressive effect was considered to play a role, the exact mechanisms underlying this drug effect remained speculative. In the current study, we asked whether CD34+ bone marrow (BM) progenitor cells in MDS and acute myeloid leukemia (AML) express the CAMPATH-1 antigen. Twelve patients with MDS (5 females, 7 males; median age: 70 years), 25 patients with AML (16 females, 9 males; median age: 62 years), and 34 control cases (normal reactive BM, n=12; idiopathic cytopenia of unknown significance, n=11; chronic myeloid leukemia, CML, n=4; chronic myelomonocytic leukemia, CMML, n=3; JAK2 V617F+ myeloproliferative neoplasms, MPN, n=4) were examined. Surface expression of CD52 on CD34+/CD38+ and CD34+/CD38- BM progenitor cells was analyzed by monoclonal antibodies and multicolor flow cytometry. In the group of MDS, CD52 was detectable on CD34+/CD38- stem cells in 3/4 patients with isolated 5q-. In most of the other MDS patients, CD52 was weakly expressed or not detectable on CD34+/CD38- cells. In AML, CD34+/CD38- cells displayed CD52 in 12/25 patients, namely 3 with complex karyotype including 5q-, 2 with inv(3), one with t(8;21), one with inv(16), one with del13q, one with trisomy 8, one with monosomy 7, and 2 with normal karyotype. Expression of CD52 mRNA in CD34+/CD38- AML stem cells was confirmed by qPCR in all patients tested (n=14). In addition, a good correlation was found between surface CD52 expression and CD52 mRNA expression in AML progenitor fractions. In patients with normal hematopoiesis (n=12) or idiopathic cytopenia (n=11), CD34+/CD38- cells stained weakly positive or negative for CD52. Almost in all cases tested, blood monocytes and blood basophils stained positive for CD52. Together, our data suggest that the target antigen CAMPATH-1 (CD52) is expressed on primitive CD34+/CD38- progenitor cells in MDS, preferentially in 5q- patients, and in a subset of patients with AML. These observations may have clinical implications and explain recently described effects of Alemtuzumab in patients with MDS. Our data also suggest that Alemtuzumab may be an interesting targeted drug in patients with refractory or relapsed AML in whom neoplastic stem cells express the target antigen CD52.