Treg Depletion with Interleukin-2-Diphteriatoxin (IL-2DT) Is Sufficient to Induce Tumor Clearance in Acute Myeloid Leukemia (AML)

Regulatory T-cells impede efficient immuno-surveillance and tumor clearance. Therapeutic Treg reduction with anti-CD25Ab can lead to improved survival in murine tumor models, but due to its long half-life, anti-CD25mAb may deplete anti-tumor reactive T-cells that are generated or expanded in Treg-depleted recipients. Tregs can also be targeted using an immunoconjugate consisting of IL-2 linked to diptheria toxin (IL- 2DT), that has a shorter half-life. Clinical trials with IL-2DT have had variable results in stimulating anti-tumor immune responses in the context of large tumor burdens. To determine whether responses with IL-2DT can be achieved in AML patients with minimal residual disease after induction chemotherapy, C57BL/6 (B6) mice were injected i.v. with moderately immunogenic luciferase- and DsRed-transduced murine AML cell line (C1498) that is MHC class I+II-. IL-2DT had no effects on in vitro tumor growth. After in vivo tumor injection, cohorts were given 1ug IL-2DT on days 0, 2 and 4 and monitored for tumor burden by Xenogen^® luciferase imaging and survival. Untreated mice all died by d35. Whereas mice treated with IL-2DT vs. untreated controls initially showed no difference in tumor growth, tumor burden started to decrease by day 10 in treated mice. No tumor was observed in treated mice by d28, and all mice survived long term without relapse (p<0.001). The IL-2DT conferred survival advantage was observed regardless as to whether the tumor was injected i.v., s.c. or directly into the bone marrow, indicating that typical sites of AML disease were not refractory to IL2DT therapy. Long-term survivors after IL-2DT therapy survived rechallenge with a lethal AML cell dose. Consistent with these data, T-cell subset depletion experiments showed that tumor rejection was critically dependent upon CD8 T-cells and only partially dependent on CD4 T-cells. Experiments using gene knockout mice were used to determine whether cytokines implicated in anti-tumor CTL responses were essential for the efficacy of IL-2DT. These data indicate that the protective effect of IL-2DT was IL-17 independent but critically dependent on interferon gamma (p<0.01). To determine effects of IL-2DT on the cellular infiltrate at the tumor site of s.c. injected mice, mice were injected with C1498 s.c. and analyzed 14 days later. The primary tumor of s.c. injected untreated mice showed an intratumoral infiltrate rich in CD4+FoxP3+ Tregs. In contrast the s.c. tumor site in IL-2DT treated mice were infiltrated predominately by CD8-positive cells. These data suggest that a short treatment with IL-2DT abrogates tumor-induced tolerance mediated by FoxP3-expressing Tregs and allows the influx of CD8 T-cells that ultimately reject the tumor via an interferon gamma dependent mechanism. In summary, the use of IL-2DT in this model of AML results in tumor clearance, long term immunity and survival.