Arsenic Trioxide and Interferon-Alpha Eradicate Adult T Cell Leukemia Initiating Cells through PML/p53 Senescence and Activation of Innate Immunity

Adult T cell leukemia (ATL) is an aggressive chemo-resistant T cell malignancy secondary to chronic HTLV-I infection. The viral oncoprotein Tax initiates ATL in transgenic mice. We previously showed that arsenic trioxide (As) and interferon-α (IFN) combination, known to trigger proteasome mediated Tax degradation and apoptosis in ATL cells, cures murine ATL through clearance of leukemia initiating cells (LIC). We also showed that the triple combination of As, IFN and zidovudine is highly effective in the treatment of chronic ATL patients, inducing a shift from an immunosuppressive Treg/Th2 profile at diagnosis (high IL-10; low IL2; low IFN-γ) towards a Th-1 immunocompetent profile (high IFN-γ; high IL2; low IL-10).

To dissect the molecular basis of As/IFN-induced ATL LIC eradication, primary ATL SCID mice were treated with As/IFN for 3 days. Spleen derived ATL cells were injected into secondary recipient SCID mice left untreated and sacrificed on a weekly basis. Whereas, secondary mice derived from untreated primaries succumbed from ATL between 3 and 4 weeks after transplantation, we observed a biphasic growth intersected by a spontaneous regression phase in untreated secondary mice transplanted from As/IFN treated primary mice. This process was associated with telomere shortening, DNA damage, activation of the PML/P53 axis, and senescence in tumor cells. Interestingly, injection of spleen derived ATL cells from secondary SCID sacrificed before ATL senescence, into untreated tertiary and quaternary SCID, resulted in normal ATL development, indicating that senescence of ATL cells in secondary SCID mice resulted in a change of the microenvironment that culminated in ATL cell apoptosis and loss of ATL LIC activity. Furthermore, injection of spleen derived ATL cells from untreated secondary SCID sacrificed before or after ATL senescence, into untreated tertiary NSG mice, resulted in normal ATL development. These NSG mice, contrary to SCID mice, lack natural killer (NK) cells and have an impaired macrophage activity. In agreement with these results, injection of spleen derived ATL cells from treated primary SCID into untreated secondary NSG mice, resulted in normal ATL development. RT-PCR and ELISA analysis of sorted CD25 positive (ATL) or negative (microenvironment) cells revealed treatment-induced upregulation of MIP1, RANTES, and IFN-γ in the CD25 negative fraction in untreated secondary SCID, following ATL senescence, and concomitant with apoptosis and loss of LIC activity. Conversely, and as previously seen in treated ATL patients, IL-10 expression was rapidly and dramatically downregulated in the CD25 positive fraction in untreated secondary SCID mice. Addition of the proteasome inhibitor PS-341 to AS/IFN treatment of primary mice reverted all above results, presumably indicating the role of As/IFN-induced Tax degradation in the observed phenotypes.

Collectively, these results demonstrate that As/IFN-induced, PML/p53 dependent, senescence of ATL cells resulted in activation of the innate immunity that culminated in apoptosis and loss of ATL LIC activity. The striking similarity of these results with patients' data suggests a critical role for innate immunity in ATL cure.