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Activation of the TNF-related apoptosis-inducing ligand (TRAIL) pathway can induce apoptosis in a broad range of human cancer cells. Four membrane-bound receptors have been identified. TRAIL-R1 and TRAIL-R2 contain a functional death domain; TRAIL-R3 and TRAIL-R4 lack a functional death domain and function as decoy receptors. Flow-cytometric analysis revealed that acute myeloid leukemic (AML) blasts expressed significantly more pro-apoptotic receptors compared to normal blasts. However, about 20% of AML patients highly expressed decoy receptor TRAIL-R3, which was strongly correlated to a shortened overall survival. TRAIL-R3 expression was also high on CD34+/CD38- cells, the compartment that harbors the leukemia initiating stem cell. Expression levels of pro-apoptotic TRAIL receptors were not correlated to the susceptibility for soluble TRAIL, which was generally low (mean level of cell death induction 14%). Cell death could be enhanced by down-modulation of TRAIL-R3, confirming its decoy function on AML blasts. Bypassing of TRAIL-R3 by treatment with antibodies directly targeting TRAIL-R2 resulted in higher rates of induced cell death (max. 80%). In conclusion, AML blasts do express pro-apoptotic TRAIL receptors. However, co-expression of decoy receptor TRAIL-R3 results in significant shortened overall survival. AML blasts could be targeted by anti-TRAIL-R2 antibodies, yielding a new therapeutic option for AML patients.

Immune escape in cancer poses a substantial obstacle to successful cancer immunotherapy. Multiple defects in HLA class I antigen presentation exist in cancer that may contribute to immune escape, but less is known about roles for HLA class II antigen presentation. On class II þ leukemic blasts, the presence of class II-associated invariant chain peptide (CLIP) is known to be correlated with poor survival in acute myeloid leukemia (AML). In this study, we evaluated the functional significance of CLIP expression on leukemic blasts of AML patients. CD4 repertoires) that were associated with better leukemia-specific reactivity than with CLIP þ leukemic blasts. Our findings offer a clinical rationale to downmodulate CLIP on leukemic blasts as a strategy to degrade immune escape and improve leukemia-specific T-cell immunity in AML patients. Cancer Res; 71(7); 2507-17. Ó2011 AACR.