Abstract LB-259: Interferon-α enhances clinical benefits of regulatory T cell depletion in ovarian cancer through direct T cell effects and by inducing bystander IL-6

Tregs hinder anti-tumor immunity, and depleting them treats cancers effectively in mice. By contrast, Treg depletion in human trials has limited efficacy. In a phase I trial of advanced stage carcinomas (breast, lung, melanoma, ovarian, bladder), we showed that the IL-2/diphtheria fusion toxin denileukin diftitox (DT) significantly depleted functional CD4+CD25hiFoxp3+ Tregs in blood with improved T cell function (increased Ki-67 and interferon-γ) with minimal effects on other blood mononuclear cells. One patient with stage IV ovarian carcinoma experienced significant reduction of metastatic tumor burden with denileukin diftitox at 12 μg/kg. Additional work showed that Treg depletion effects could last up to 4 weeks, and that weekly DT at 12 μg/kg eventually depleted anti-tumor and tumor-specific CD8+ T cells. Based on these data, we conducted a phase II clinical trial of epithelial ovarian carcinomas FIGO stage III-IV failing standard treatments, using DT as a single agent at 12 μg/kg by intravenous infusion every 3-4 weeks. In this trial, DT significantly depleted blood and tumor microenvironmental Tregs with only grade I-II toxicities, but with minimal clinical efficacy in 28 consecutive patients. This trial was halted for futility according to the Simon 2-stage design. Interferon-α alone does not treat ovarian cancer, but we now show that it significantly improves clinical and immune DT-mediated Treg depletion efficacy in human ovarian cancer. In the ID8 mouse ovarian carcinoma model, DT modestly increased survival and anti-tumor immunity. Interferon-α alone did not affect Treg numbers or function, but enhanced CD8+ T cell anti-tumor immunity. Interferon-α plus DT increased mouse survival significantly over either individual drug. In type I IFNR-/- mice unable to mediate interferon-α signals, interferon-α directly increased adoptively transferred IFNR+CD8+ T cell function independent of CD4+ T cell help. When combined with DT, IFN-α reduced Treg function without further numerical Treg reduction by indirect effects on tumor microenvironmental dendritic cells. In vitro studies identified IFN-α-driven dendritic cell IL-6 as a mechanism for reducing Treg function. When three ovarian cancer patients failed DT alone in the phase II trial, two had clinical and immune benefit by adding pegylated interferon-α2a in a separate trial, with acceptable toxicities. This trial was halted due to a lack of further DT. These studies demonstrate that DT depletes Tregs in distinct human carcinomas but is unlikely to be clinically effective as a single agent. We identified novel IFN-α mechanisms that improve Treg depletion effects using FDA-approved agents that can be rapidly translated. More selective Treg depletion agents and rationale combinations with other agents could improve clinical efficacy further. Citation Format: Suzanne Thibodeaux, Vincent Hurez, Shawna Wall, Srilakshmi Pandeswara, Benjamin Daniel, Aijie Liu, Lishi Sun, Leslie Wood, Weiping Zou, Tyler Curiel. Interferon-α enhances clinical benefits of regulatory T cell depletion in ovarian cancer through direct T cell effects and by inducing bystander IL-6. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr LB-259. doi:10.1158/1538-7445.AM2014-LB-259