Signaling via the MyD88/IRAK pathway in T cells is indispensable for cell survival; however, it is not known whether this pathway functions in the progression of T acute lymphoblastic leukemia (T-ALL). Here, we determined that compared with thymic and peripheral T cells, T-ALL cells from patients have elevated levels of IRAK1 and IRAK4 mRNA as well as increased total and phosphorylated protein. Targeted inhibition of IRAK1 and IRAK4, either with shRNA or with a pharmacological IRAK1/4 inhibitor, dramatically impeded proliferation of T-ALL cells isolated from patients and T-ALL cells in a murine leukemia model; however, IRAK1/4 inhibition had little effect on cell death. We screened several hundred FDA-approved compounds and identified a set of drugs that had enhanced cytotoxic activity when combined with IRAK inhibition. Administration of an IRAK1/4 inhibitor or IRAK knockdown in combination with either ABT-737 or vincristine markedly reduced leukemia burden in mice and prolonged survival. IRAK1/4 signaling activated the E3 ubiquitin ligase TRAF6, increasing K63-linked ubiquitination and enhancing stability of the antiapoptotic protein MCL1; therefore, IRAK inhibition reduced MCL1 stability and sensitized T-ALL to combination therapy. These studies demonstrate that IRAK1/4 signaling promotes T-ALL progression through stabilization of MCL1 and suggest that impeding this pathway has potential as a therapeutic strategy to enhance chemotherapeutic efficacy.
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