PD-1 modulates regulatory T-cell homeostasis during low-dose interleukin-2 therapy

Takeru Asano, Yusuke Meguri, Takanori Yoshioka, Yuriko Kishi, Miki Iwamoto, Makoto Nakamura, Yasuhisa Sando, Hideo Yagita, John Koreth, Haesook T. Kim, Edwin P. Alyea, Philippe Armand, Corey S. Cutler, Vincent T. Ho, Joseph H. Antin, Robert J. Soiffer, Yoshinobu Maeda, Mitsune Tanimoto, Jerome Ritz and Ken-ichi Matsuoka

Key Points

  • IL-2 induces expression of PD-1 on Tregs, and PD-1 blockade promotes Treg differentiation and apoptosis.

  • PD-1 regulates IL-2–induced Treg proliferation and prolongs Treg survival in murine models and in patients receiving low-dose IL-2 therapy.


CD4+Foxp3+ regulatory T cells (Tregs) play a central role in the maintenance of immune tolerance after hematopoietic stem cell transplantation. We previously reported that low-dose interleukin-2 (IL-2) therapy increased circulating Tregs and improved clinical symptoms of chronic graft-versus-host-disease (cGVHD); however, the mechanisms that regulate Treg homeostasis during IL-2 therapy have not been well studied. To elucidate these regulatory mechanisms, we examined the role of inhibitory coreceptors on Tregs during IL-2 therapy in a murine model and in patients with cGVHD. Murine studies demonstrated that low-dose IL-2 selectively increased Tregs and simultaneously enhanced the expression of programmed cell death 1 (PD-1), especially on CD44+CD62L+ central-memory Tregs, whereas expression of other inhibitory molecules, including CTLA-4, LAG-3, and TIM-3 remained stable. PD-1–deficient Tregs showed rapid Stat5 phosphorylation and proliferation soon after IL-2 initiation, but thereafter Tregs became proapoptotic with higher Fas and lower Bcl-2 expression. As a result, the positive impact of IL-2 on Tregs was completely abolished, and Treg levels returned to baseline despite continued IL-2 administration. We also examined circulating Tregs from patients with cGVHD who were receiving low-dose IL-2 and found that IL-2–induced Treg proliferation was promptly followed by increased PD-1 expression on central-memory Tregs. Notably, clinical improvement of GVHD was associated with increased levels of PD-1 on Tregs, suggesting that the PD-1 pathway supports Treg-mediated tolerance. These studies indicate that PD-1 is a critical homeostatic regulator for Tregs by modulating proliferation and apoptosis during IL-2 therapy. Our findings will facilitate the development of therapeutic strategies that modulate Treg homeostasis to promote immune tolerance.

  • Submitted September 30, 2016.
  • Accepted January 18, 2017.
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