Suppressing tumour T_reg cells with a histone demethylase inhibitor

Regulatory T (T_reg) cells can be a barrier to anti-cancer immune responses but depleting them has undesirable autoimmune effects. Writing in Nature Immunology, Long et al. report that a histone demethylase JMJDC1 is upregulated in tumour-resident T_reg cells and is critical for their function. An inhibitor of JMJDC1 disrupts T_reg cells in tumours but not in lymphoid organs and retards tumour growth in mouse models.

As a subset of CD4⁺ T cells expressing Foxp3, T_reg cells are important to prevent excessive immune activation and maintain immune tolerance towards self antigens. However, they can be deleterious in cancer by contributing to an immune suppressed tumour microenvironment (TME). Identifying molecules and regulatory pathways that are specific to intratumoural T_reg cells would enable targeted therapeutics that avoid systemic effects on immune homeostasis.

Long et al. investigated the chromatin regulators of intratumoural T_reg cells as potentially druggable targets. They analysed transcriptome datasets from several human cancer types and found increased expression of JMJD1C in tumour T_reg cells compared with peripheral splenic T_reg cells.

JMJDC1 protein levels were increased in mouse tumour T_reg cells and directly impacted by the TME. The authors showed that supernatant from mouse tumour tissue upregulated JMJDC1 in splenic T_reg cells but not in conventional CD4⁺ T cells, an effect that could be mimicked by a combination of cytokines. Furthermore, several inflammatory cytokine signalling pathways were upregulated in tumour T_reg cells, which induced binding of NF-kB and STAT3 transcription factors to the regulatory region of the Jmjd1c gene to activate its transcription.

The authors generated mice with a conditional knockout of Jmjd1c in T_reg cells. The function of T_reg cells in peripheral immune organs was not affected. However, when the mice were challenged with tumours, T_reg cells in the TME were depleted and had reduced immunosuppressive capacity, whereas levels of conventional T cells were increased, resulting in decreased tumour growth. Mice with Jmjd1c deleted in all CD4⁺ T cells had normal immune homeostasis, showing that JMJD1C is dispensable for conventional T cells.

Mechanistically, JMJD1C demethylates histone H3 lysine 9 dimethylation (H3K9me2), a chromatin modification associated with gene repression. In the tumour T_reg cells lacking Jmjd1c, the authors showed that genomic levels of H3K9me2 increased and several molecules critical for T_reg cell function were downregulated, including the cell surface proteins PD1 and NRP1. Signalling through AKT kinase and STAT3 was upregulated, which led to increased IFNγ production, conditions that induce ‘fragility’ of tumour T_reg cells.

To find small molecule inhibitors of JMJD1C demethylase activity, Long et al. performed structure-based virtual screening of about 19,000 drug-like compounds and experimentally evaluated the top candidates in vitro. Compound 193D7 had the greatest effect on JMJD1C inhibition, with an IC50 value of 0.59 μM, and was selective for JMJD1C compared to other histone demethylases. It could be structurally modelled in the active site of JMJD1C and its direct binding was confirmed biochemically.

The inhibitor was then tested in mouse models, where it had similar pharmacokinetics whether administered intraperitoneally or orally. Intraperitoneal doses at 25 mg kg^-1 daily for 2 weeks retarded growth of several types of subcutaneous tumours, without evidence of toxicity. In addition, numbers of intratumoural T_reg cells decreased and effector T cells increased. Similar effects were seen with oral administration of 193D7 at the same dose. As observed in the Jmjd1c knockout mice, the intratumoural T_reg cells had decreased expression of PD1 and NRP1, together with increased STAT3 phosphorylation and IFNγ production.

Therefore JMJD1C regulates several pathways to promote the fitness of tumour T_reg cells and inhibiting this enzyme is a potential therapeutic approach to selectively target these cells. The authors plan to optimize the lead compound and develop derivatives for potential clinical application.