Indoleamine 2,3-dioxygenase (IDO) degrades the amino acid tryptophan which is essential for T cells. Tryptophan depletion causes T cell cycle arrest and solid tumors that express high levels of IDO can create immune suppression. Recently, blasts of acute myeloid leukemic (AML) patients were shown to express the IDO protein. We now determined INDO (the encoding gene for IDO) mRNA expression in myeloid leukemic blasts of 285 AML patients by gene expression profiling. Results were validated by quantitative PCR (qPCR) analysis in blasts of an independent cohort of 71 patients. Correlation of INDO expression to relevant known prognostic factors and survival identified high INDO expression as a strong negative independent predicting variable for overall and relapse free survival of AML patients. Inhibition of IDO may result in breaking tumor induced immune tolerance and offers new therapeutic options for AML patients.

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