The Leukemia-Associated Antigen PRAME Is Overexpressed in Myeloid Leukemias and Inhibits Cell Differentiation by Blocking the Receptor for Retinoic Acid (RAR)-Signaling in Vitro and Is Therefore a Interesting Candidate for Targeted Immunotherapies.

The leukemia-associated antigen PRAME (preferentially expressed antigen of melanoma) is frequently expressed in several solid tumors. Earlier, we reported (Greiner et al., Blood 2006) that co-expression of leukemia-associated antigens including PRAME constituted a favorable prognostic parameter for AML patients. By microassay analysis and conventional RT-PCR, we detected over-expression of PRAME in more than 60% of AML and of CML patients. In contrast, PRAME is expressed neither in normal CD34-positive hematopoietic stem cells nor in normal tissues (other than testis and placenta). Here, we describe for the first time that retinoic acid-induced cell proliferation and differentiation of several AML cell lines is dependent on PRAME expression: PRAME negative cell lines treated with all-trans retinoic acid (ATRA) in cell culture showed lower cell proliferation than PRAME positive cells as assessed by cell counts and FACS analysis for BrdU, but an increase of cell differentiation detected by FACS analysis for CD66b in contrast with PRAME positive leukemia cell lines. The leukemia-associated antigen PRAME seems to be responsible for the resistance of PRAME-positive AML to ATRA treatment. We detected no differences in induction of apoptosis in PRAME-positive or PRAME-negative cell lines treated with ATRA in FACS analysis for annexin. Over-expression of the antigen and this critical role of PRAME in cell differentiation might be the reason for specific T cell induction against PRAME-derived peptides. To detect specific T cell responses against PRAME-derived peptides we examined samples from healthy volunteers and AML patients. Positive results in ELISPOT assays for IFN-g and Granzyme B secretion were observed in 70% of AML cases analyzed for the peptide PRAME-P3. In chromium release assays, we found a PRAME-specific cell lysis of PRAME-positive AML blasts. Taken together, PRAME is involved in a crucial mechanism for cell growth of leukemic cells, induces specific T cell responses in a high frequency of AML patients, and this constitutes an appropriate target structure for specific immunotherapies or other targeted therapies in AML.