Regulatory T cells—an important target for cancer immunotherapy

A recent article by Drake et al. (Breathing new life into immunotherapy: review of melanoma, lung and kidney cancer. Nat. Rev. Clin. Oncol. 11, 24–37 [2014])¹ reviewed how monoclonal antibodies against the immune checkpoint molecules cytotoxic T-lymphocyte antigen-4 (CTLA-4), programmed death-1 (PD-1) and PD-ligand 1 (PD-L1) are effective in mediating tumour shrinkage in several cancer types. Drake et al.¹ also highlighted ongoing phase III clinical trials and discussed the increased antitumour activity of the combinations of these antibodies compared with blocking either checkpoint alone. However, we would like to note that regulatory T cells (T_REG) might represent another important immunological checkpoint to target in cancer immunotherapy.^2,3 In fact, T_REG that express CD4, CD25 and the forkhead protein 3 (FoxP3) can inhibit the antitumour immune response, thereby limiting the power of cancer immunotherapies.²

Although Drake et al.¹ did not discuss the effects of CTLA-4 and PD-1 or PD-L1 blockade on CD4⁺CD25⁺FoxP3⁺ T_REG, this needs to be considered. PD-1 blockade negatively regulates intracellular FoxP3 expression in T_REG.^4,5 Specifically, Wang et al.⁴ showed that PD-1 blockade leads to the downregulation of intracellular FoxP3 expression in T_REG of patients with melanoma, suggesting that PD-1 is implicated in the regulation of T_REG function. Furthermore, Sharma et al.⁵ demonstrated that the ability of T_REG to suppress target T-cell proliferation is abrogated by PD-1 and PD-L1 antibodies. The effect of CTLA-4 blockade on T_REG is less clear.^6,7 CTLA-4 is constitutively expressed on CD4+ T_REG^8,9 and Wing et al.⁸ reported that T_REG-specific CTLA-4 deficiency impaired in vivo and in vitro suppressive function of T_REG, and also produced potent tumour immunity. Regarding the effect of CTLA-4 blockade on T_REG, Kavanagh et al.⁶ showed that treatment with CTLA-4 antibodies in patients with metastatic prostate cancer induces an increase in the number of activated effector CD4⁺ T cells and CD4⁺CD25⁺FoxP3⁺ T_REG, suggesting that CTLA-4 antibodies enhance antitumour immunity by the activation of effector T cells rather than by depleting CD4⁺CD25⁺FoxP3⁺ T_REG in vivo.⁶ Conversely, Simpson et al.⁷ reported that treatment with an anti-CTLA-4 antibody induces a selective depletion of T_REG within the tumour lesions in a mouse model of melanoma.

On the basis of the different effects that immune checkpoint blockade can have on T_REG function, it is possible that circumventing the activity of T_REG might represent an important step to overcome some of the obstacles that, so far, have prevented the complete exploitation of the immunotherapy potential for the successful treatments of many cancers, including melanoma.³ A substantial number of T_REG, in tumour tissues and peripheral blood specifically express C-C chemokine receptor 4 (CCR4), therefore, treatment with anti-CCR4 monoclonal antibody can evoke and augment antitumour immunity in patients with melanoma by selectively depleting or inhibiting T_REG from the tumour tissue.^10,11 The combinations of CTLA-4 and PD-1 or PD-L1 blockade showed an increased antitumour immunity compared with using each antibody alone;¹ however, an alternative combination strategy could include both immune checkpoint blocking antibodies and T_REG-depleting molecules. For example, depletion of T_REG by intraperitoneal administration of interleukin-2 diphtheria toxin followed by sequential PD-1 or PD-L1 blockade showed superior efficacy for eradication of acute myeloid leukaemia in a mouse model than did PD-1 or PD-L1 blockade alone.¹² Furthermore, Goding et al.¹³ demonstrated that either blockade of the PD-1 pathway with anti-PD-L1 antibodies or depletion of tumour-specific T_REG alone did not prevent tumour recurrence in a mouse model of melanoma. However, the combination of PD-L1 blockade and in-tumour T_REG depletion via administration of interleukin-2 diphtheria toxin effectively mediated melanoma regression.¹³ These results indicate that primary and relapsing cancer might have different characteristics and the use of combined immunotherapy approaches that specifically target T_REG cells, could be required for highly resistant recurrent disease.

Overall, we believe that the effect of immune checkpoint blockade (via CTLA-4, PD-1 or PD-L1) on T_REG should be considered when evaluating the efficacy of cancer-immunotherapy and that the available data clearly warrant clinical studies of combined immune checkpoint blockade and a T_REG-targeting strategy in primary and relapsed cancers.