Indoleamine 2,3-Dioxygenase-1 (IDO1) Is Expressed by a Subset of Childhood Acute Myeloid Leukemias and Restrains IFN-γ Production by T Cells

Indoleamine 2,3-dioxygenase 1 (IDO1) degrades tryptophan (TRP) into kynurenine (KYN) and other immune suppressive molecules. The IDO1-driven production of KYN promotes the development, stabilization and activation of regulatory T cells (Treg), while suppressing effector T cells, all of which may contribute to immune system impairment in cancer-bearing individuals. It has been previously shown that IDO1 mRNA and protein are detectable in blast cells from 52% of adult patients with newly diagnosed acute myeloid leukemia (AML), in correlation with expanded Treg cells. Importantly, high copy numbers of IDO mRNA may be a negative independent predicting variable for overall and relapse-free survival in adult AML.

We investigated IDO1 expression and function in 21 children with acute leukemia [10 AML (median age 13 years, range 6–16), 9 B-cell precursor (BCP)-ALL, 1 infant acute leukemia with MLL rearrangement and 1 T-cell ALL] and in 1 patient with Ph⁺ chronic myeloid leukemia (CML). Amongst patients with AML, 3 children had secondary AML, 3 patients had core-binding factor (CBF)⁺ AML, 3 patients had FLT3-ITD⁺AML and 1 patient had AML with t(6;9). TRP and KYN levels were measured with reverse phase (RP)-HPLC.

Cells from either BCP-ALL or T-cell ALL expressed IDO1 neither constitutively nor after challenge with 100 ng/ml interferon (IFN)-γ, a prototypical inducer of IDO, whereas they up-regulated both phosphorylated STAT-3 and surface programmed death ligand 1 (PD-L1), an IFN-γ-inducible co-inhibitory receptor also implicated in tumor-induced immune evasion. By contrast, leukemia blast cells from 5 out of 10 AML and from Ph⁺ CML up-regulated IDO1 protein expression after in vitro challenge with IFN-γ (median 20-fold increase, range 11.9–120, compared with unstimulated AML cells). KYN levels significantly increased in supernatants of AML cells treated with IFN-γ for 72h (10.8 μM/L, range 8.15–20.5) compared with unstimulated cultures (1.4 μM/L, range 1.1–2.2), in parallel with TRP consumption (6.3 μM/L, range 3.0–14.1, after challenge with IFN-γ compared with 18.2 μM, range 13.7–20.9, in unstimulated cultures). The IFN-γ-induced increase of IDO expression was significantly inhibited by pre-treatment of leukemia cells with STAT3 inhibitors (median 1.95-fold compared with unstimulated AML cells, range 0.9–27.7), but not with STAT5 inhibitors. In line with these results, STAT3 inhibition prevented the IFN-γ-induced release of KYN in culture supernatants. Western blot runs of immuno-precipitated proteins with specific antibodies suggested a physical interaction between IDO and STAT3 in IFN-γ-challenged leukemia blasts, but not in unstimulated samples. In a mixed tumor cell lymphocyte culture (MTLC), AML blasts primed with IFN-γ significantly inhibited Th1 cytokine production by allogeneic CD8⁺ T cells, while enhancing IL-4 release by CD4⁺ T cells. These effects were potentiated by the supplementation of MTLCs with exogenous KYN. The intracellular levels of IL-17A were unaffected by the exposure of allogeneic T cells to AML blasts. The addition of D,L-1-methyl-tryptophan (1MT), an IDO inhibitor, to the co-cultures of T cells and AML blasts incompletely restored IFN-γ production by CD8⁺ T cells. By contrast, STAT3 inhibitors fully reverted the AML-induced skewing of IFN-γ/IL-4 secretion.

Blast cells from a subset of childhood AML, but not those from BCP-ALL or T-cell ALL, express functional IDO1 and restrain IFN-γ secretion by CD8⁺ T cells, while enhancing IL-4 production by CD4⁺ T cells. From a therapeutic standpoint, STAT3 inhibitors may effectively interfere with IDO1 expression by AML cells, thus tipping the Th1/Th2 balance in favor of anti-leukemia immune responses.

No relevant conflicts of interest to declare.