Why Does the AHR Receptor Make Leukemic Cells More Resistant?

Acute myeloid leukemia (AML) is a blood cancer that mixes genetic mutations, abnormal blood‑cell production, and immune evasion. A key player is the aryl hydrocarbon receptor (AHR), a protein that steers stem‑cell differentiation and immune‑cell development. A retrospective study examined bone‑marrow T and NK cell counts in AML patients before 7+3 chemotherapy and linked them to overall survival. Higher NK percentages correlated with poorer survival and increased HLA‑E on leukemic blasts. AHR was up‑regulated in these cases, fostering an “immune‑resistant” signature: boosted cytokine pathways, interferon‑γ signaling, and MHC I/II expression, which hampered NK function. Treating blasts with an AHR inhibitor (AHRi) before NK‑cell killing reduced checkpoint molecules like HLA‑E and transcription factors STAT1 and IRF1, enhancing NK‑mediated killing across multiple FAB subtypes. These findings suggest that targeting AHR could hit the tumor directly while boosting NK‑cell therapies, opening new avenues against AML.

The aryl hydrocarbon receptor (AHR) is a cytoplasmic‑nuclear receptor that, upon binding to environmental ligands (e.g., dioxins) or endogenous metabolites (tryptophan derivatives), translocates to the nucleus and forms a complex with ARNT (AHR nuclear translocator). This complex binds xenobiotic response elements (XRE) to regulate genes involved in detoxification, cell‑cycle control, and differentiation. In AML, chronic AHR activation appears to enforce an epigenetic program that keeps leukemic cells in a differentiation‑blocked state, typical of monocytic subtypes (FAB M4/M5). Simultaneously, AHR modulates antigen presentation: up‑regulation of HLA‑E, a non‑classical MHC‑I molecule, engages the inhibitory NKG2A receptor on NK cells, delivering a “do not attack” signal. Moreover, AHR activation amplifies the STAT1‑IRF1 pathway, central to interferon‑γ responses, but in this context promotes checkpoint gene expression rather than cytotoxic activity. Pharmacological AHR inhibition (e.g., with CH‑223191) lowers HLA‑E and STAT1/IRF1 transcription, restoring NK cells’ ability to recognize and kill blasts. Therapeutically, combining AHR inhibitors with NK‑based immunotherapies (such as allogeneic NK cells or CAR‑NK) could overcome AML’s intrinsic immune resistance, especially in monocytic subtypes where AHR expression is highest. Nonetheless, systemic side‑effects of AHR blockade must be assessed, given its role in xenobiotic detoxification and intestinal barrier regulation.