Microenvironment and Immunology Foxp3þ T Cells Inhibit Antitumor ImmuneMemoryModulated by mTOR Inhibition

Inhibition of mTOR signaling enhances antitumor memory lymphocytes. However, pharmacologic mTOR inhibition also enhances regulatory T-cell (Treg) activity. To counter this effect, Treg control was added tomTOR inhibition in preclinical models. Tregs were controlled with CD4-depleting antibodies because CD4 depletion has high translational potential and already has a well-established safety profile in patients. The antitumor activity of the combination therapy was CD8 dependent and controlled growth of syngeneic tumors even when an adoptive immunotherapy was not used. Lymphocytes resulting from the combination therapy could be transferred into na€ ve 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 FoxP3expressing T lymphocytes was the mechanism underlying immunologic memory formation following CD4 depletion. This was confirmed using transgenic DEREG (depletion of regulatory T cells) mice to specifically remove Foxp3þ T cells. It was further confirmed with reciprocal studies where stimulation of immunologic memory because of CD4 depletion was completely neutralized by adoptively transferring tumor-specific Foxp3þ T cells. Also contributing to tumor control, Tregs that eventually recovered following CD4 depletion were less immunosuppressive. These results provide a rationale for further study of mTOR inhibition and CD4 depletion in patients. Cancer Res; 74(8); 2217–28. 2014 AACR.