Differential effects of inhibitors of the PI3K/mTOR pathway on the expansion and functionality of regulatory T cells

- Single mTOR and dual PI3K/mTOR inhibitors support a relative Treg expansion benefit.
- Single mTOR and dual PI3K/mTOR inhibitors promote Treg immunosuppressive activity.
- Single PI3K inhibition results in similar expansion rates of Tregs and Tconv.
- The suppressive activity of Tregs is not altered upon culture with PI3K inhibitors.

The PI3K/mTOR pathway is commonly deregulated in cancer. mTOR inhibitors are registered for the treatment of several solid tumors and novel inhibitors are explored clinically. Notably, this pathway also plays an important role in immunoregulation. While mTOR inhibitors block cell cycle progression of conventional T cells (Tconv), they also result in the expansion of CD4+ CD25hiFOXP3+ regulatory T cells (Tregs), and this likely limits their clinical antitumor efficacy. Here, we compared the effects of dual mTOR/PI3K inhibition (using BEZ235) to single PI3K (using BKM120) or mTOR inhibition (using rapamycin and everolimus) on Treg expansion and functionality. Whereas rapamycin, everolimus and BEZ235 effected a relative expansion benefit for Tregs and increased their overall suppressive activity, BKM120 allowed for similar expansion rates of Tregs and Tconv without altering their overall suppressive activity. Therefore, PI3K inhibition alone might offer antitumor efficacy without the detrimental selective expansion of Tregs associated with mTOR inhibition.