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18 | JANuARY 2021 | volume 20

ORIGINAL ARTIcLe Solomon, I. et al. CD25-Treg- depleting antibodies preserving IL-2 signaling on effector T cells enhance effector activation and antitumor immunity. Nat. Cancer https://doi.org/ 10.1038/s43018-020-00133-0 (2020)ReLATeD ARTIcLe Sharabi, A. et al. Regulatory T cells in the treatment of disease. Nat. Rev. Drug Discov. 17, 823–844 (2018)

In vivo, anti­ CD25NIB showed higher antitumour activity than CD25PC61 in two models of established tumours, with tumour rejection in almost all anti­ CD25NIB­ treated mice

Credit: N. Smith/Springer Nature Limited

Regulatory T (Treg) cells are essential for keeping the immune response under control, but, in cancer, an abundance of Treg cells can result in an immuno- suppressive environment and low antitumour immunity. Thus far, efforts to target Treg cells while maintaining the antitumour activity of effector T cells (Teff cells) have been unsuccessful. Now, Solomon et al. have designed an antibody against the α- subunit of the IL-2 receptor (IL-2Rα, also known as CD25) that can specifically deplete Treg cells while preserving the activity of Teff cells. The antibody showed antitumour efficacy in several mouse models, alone and in combination with immune checkpoint inhibitors.

Although multiple studies have shown Treg cells are potential targets in cancer immunotherapy, there is still limited insight into the most selective approach for Treg cell depletion. CD25 had been considered a target before as it is highly expressed in Treg cells, but not as much on Teff cells.

However, the extent of CD25 expression is not the same on different types of T cells, says Maria Amann, a co- lead author of the study. “This window between high CD25 expression on Treg cells and low- to- absent CD25 on cytotoxic CD8+ T cells can be leveraged with modern antibody engineering capabilities that enable avidity- driven binding,” she notes.

Yet, previous monoclonal antibodies (mAbs) against CD25 (such as clone PC61) were not effective in mouse models, and so the target was dropped.

The team, led by Sergio Quezada at University College London, observed that the lack of activity of previous anti- CD25 mAbs was due to poor binding to activating FcRs on T cells — required for antibody- dependent cell cytotoxicity or phagocytosis (ADCC or ADCP) — which resulted in low depletion of intratumoural Treg cells. Furthermore, these anti- CD25 mAbs also inhibit IL-2R signalling on Teff cells, reducing their antitumour activity, which could potentially be another reason for the poor clinical responses observed. “Engaging of the IL-2 pathway is required for a productive antitumour immune response. Cutting this life- line by using antibodies interfering with IL-2 signalling is like adding a second roadblock after removing the first one,” explains Amann. “Thinking beyond just using a target as a marker for destruction is vital for smart drug design,” she adds.

With these ideas in mind, the authors sought to design an anti- CD25 mAb that would not interfere with IL-2 signalling. Starting with the 7D4 clone — an IgM that binds to mouse CD25 without blocking IL-2 signalling — they cloned its variable region into a mouse IgG2a backbone, generating a Treg cell- depleting but non- IL-2- blocking mAb, which they named anti- CD25NIB. In vivo, anti- CD25NIB showed higher anti-tumour activity than CD25PC61 in two models of established tumours, with tumour rejection in almost all anti- CD25NIB- treated mice. Further analysis showed that, although both mAbs depleted Treg cells, tumours treated with CD25NIB showed greater abundance of proliferating and dif-ferentiated T cells as well as activated CD8+ T cells populations.

When CD25NIB was assessed in combination with an anti- PD1 mAb or GVAX (a vaccine from tumour cells genetically

modified to secrete granulocyte–macrophage colony- stimulating factor) — in mouse models of fibrosarcoma and melanoma, respectively, the combination treatments induced tumour regression in 70% and 90% of animals, delaying tumour progression and significantly increasing overall survival.

Next, the authors generated a homologous human antibody. They assessed 43 candidates for their ability to deplete Treg cells and maintain IL-2–STAT5 signalling, and the best candidate, RG6292, was fucosylated for maximal depleting activity in vivo.

In vitro, treatment with RG6292 did not affect IL-2 signalling in Teff cells, elicited ADCC and ADCP, and selectively depleted Treg cells. Similarly, in humanized mice carrying pancreatic cancer xenografts, treatment with RG6292 showed activation of CD8+ T cells and depletion for at least 14 days of both systemic and intratumoral Treg cells in a dose- dependent manner. Compared with ipilimumab, a mAb that targets CTLA4 — which is also highly expressed in Treg cells — tumours treated with RG6292 showed higher Teff/Treg ratio and activation of infiltrating CD8+ T cells, as well as increased levels of IL-2.

Finally, to assess safety, the authors tested RG6292 in macaques, which tolerated the treatment well.

“We are currently recruiting into an entry- into- human study to evaluate the safety and to identify a recommended phase II dose of RG6292 in patients with solid tumours (NCT04158583),” says Amann. “Ultimately, we are interested in determining in which circumstances Treg cell depletion leads to the best patient outcome,” she concludes.

M. Teresa Villanueva

A N T I c A N c e R T H e R A P Y

Anti-CD25antibodytipstheT cellbalance