OBJECTIVE: Effective immune stimulation produces long-lasting memory lymphocytes. The mTOR (mechanistic target of rapamycin) pathway is an important checkpoint that governs the formation of CD8 memory cells. However, mTOR inhibition also suppresses the immune system at least in part by enhancing CD4 regulatory T cell (Treg) activity. Therefore we explored a combination therapy targeting the mTOR pathway (temsirolimus) and Tregs.

METHODS: In mice treated with pharmacologic mTOR inhibition, Tregs were controlled with CD4 depleting antibodies because CD4 depletion has high translational potential and already has a well-established safety profile in patients. In mouse models we verified that Treg depletion is the mechanism of immune stimulation mediated by CD4 depletion; Tregs were specifically depleted using DEREG (DEpletion of REGulatory T cells) transgenic mice, which carry a diphtheria toxin receptor transgene under the control of the Foxp3 promoter. In a reciprocal experiment, the beneficial effects of CD4 depletion were neutralized by replacing Tregs from transgenic mice with green fluorescence protein (GFP)-marked Tregs. Immune stimulation was monitored by assessing subcutaneous and metastatic tumor growth, and assessing CD8 T cell function (IFN-γ response and in vivo cytotoxic T lymphocytes assay). Immune memory was assessed using memory markers in CD8 T cells (e.g. Eomes, CD62L, BCL2) and assessing immune recall after secondary immune stimulation.

RESULTS: The antitumor activity of the combination therapy (mTOR inhibition and CD4 depletion) was greater than in all control groups, even when an adoptive immunotherapy was not used, and was CD8 dependent. The timing of CD4 depletion was critical to allow for CD4 helper function during primary immune stimulation. Lymphocytes resulting from the combination therapy could be transferred into naïve mice to inhibit aggressive growth of lung metastases. The combination therapy enhanced CD8 memory formation as determined by memory markers and functional studies of immune recall. Removal of FoxP3 expressing T lymphocytes was the mechanism underlying immune stimulation and memory formation following CD4 depletion. This was confirmed using transgenic DEREG mice to specifically remove Tregs. It was further confirmed with reciprocal studies where immune stimulation due to CD4 depletion was completely neutralized by adoptively transferring tumor-specific Tregs. Also contributing to tumor control, Tregs that eventually recovered following CD4 depletion were less immunosuppressive.

CONCLUSIONS: These results provide a rationale for further study of mTOR inhibition and Treg depletion using anti-CD4 antibodies in cancer patients.

Citation Format: Yanping Wang, Tim L. Sparwasser, Robert Figlin, Hyung L. Kim. Combination of Foxp3+ T cells depletion and mTOR inhibitor enhances immune memory and controls cancer growth. [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-260. doi:10.1158/1538-7445.AM2014-LB-260