While 65–80% of adult patients with acute myeloid leukemia (AML) will enter complete remission (CR) with anthracycline based induction chemotherapy, only 20–30% of patients will maintain durable remission with standard consolidation therapies.1 Novel treatment strategies for AML patients might include more specific immunostimulatory elements. The immune system plays an important role in the control of proliferating leukemic cells: interleukin 2 (IL-2), a T cell stimulating cytokine, was shown to be effective in inducing complete remission in a patient with AML.2 For younger patients with AML or CML (chronic myeloid leukemia), allogeneic hematopoietic stem cell transplantation (HSCT) constitutes an optional treatment.1, 3 The therapeutic concept of HSCT employs not only the effects of total body irradiation and high dose chemotherapy, but also the induction of the so-called graft-vs.-leukemia (GVL) effect.4 Patients with CML and an increasing number of patients with AML who relapse from leukemia after HSCT were administered donor lymphocyte infusions (DLI), often resulting again in a complete remission.1, 5, 6, 7 This fact demonstrates the efficacy of the GVL reaction and suggests that also leukemia-associated antigens (LAA) are recognized by the donor lymphocytes in the framework of the GVL reaction. For the amplification and expansion of the GVL effect in the context of HSCT, specific immunotherapies using LAA as targets might be a successful approach. Moreover, these immunotherapies might prolong the CR in patients achieved by polychemotherapy.
To define antigens for such specific immunotherapies, we examined in our study the mRNA expression of tumor/leukemia-associated antigens (TAA/LAA) with expected or known expression in AML and with already described immunogenicity. Candidates are at first cancer/testis (CT) antigens, now called cancer/germline (CG) antigens, like BAGE,8 GAGE,9 MAGE,10 NY-ESO-111 and SSX2,12, 13 being members of families of TAA expressed in tumor cells and in normal testis tissue but not in other normal tissues. The CG antigen PRAME is expressed in different cancer tissues and also at a very low level in some other tissues like adrenals, endometrium and ovary.14 TAA were hitherto characterized in solid tumors; some of these antigens like PRAME and WT1 are known to also be expressed in leukemias.14, 15, 16, 17, 18, 19, 20
A further candidate antigen is proteinase 3. This gene is expressed in AML patients, and a cytotoxic T lymphocyte (CTL) epitope designated PR1 has been characterized.21, 22, 23 By the method of serologic screening of recombinant expression libraries (SEREX), several LAA in AML have been identified, such as RHAMM,24 MPP11 and the heat-shock proteins HSJ2 and HSP70,25 showing humoral immune responses only in tumor patients but not in healthy volunteers (HV). Humoral immune responses to MAZ were found in AML patients but also in HV.20 The SEREX defined CML-antigens RBPJκ and RFAFTK showed humoral immune responses in CML patients.26
Significant mRNA expression of these antigens would be a prerequisite for immunotherapies in AML patients. Simultaneous expression of different antigens might a basis for the efficacy of polyvalent vaccines. To investigate their specificity, the expression of TAA/LAA was evaluated in samples from AML patients in contrast to peripheral blood mononuclear cells (PBMN) from HV, CD34 positive cells and a panel of normal tissues.