T_H9 cells tackle advanced tumours

Therapeutic approaches using adoptive cell therapy (ACT) with tumour-specific T cells have shown some promising results in cancer patients, but they are rarely curative. Now, reporting in Cancer Cell, Qing Yi and colleagues show that ACT with CD4⁺ T helper 9 (T_H9) cells can completely eradicate large established tumours in mouse models of melanoma.

Credit: Simon Bradbrook/Macmillan Publishers Limited

Currently, most anticancer ACT protocols use CD8⁺ cytotoxic T lymphocytes (CTLs), but are hampered by the side effects of IL-2, which needs to be administered systemically to ensure CTL survival. CD4⁺ T cells, which don’t require IL-2, may also hold promise for ACT, and studies so far have focused on tumour-specific T_H1 and T_H17 cells. T_H1 cells can be potently cytolytic, but ex vivo generated tumour-specific T_H1 cells display an exhausted phenotype and low persistence after transfer. T_H17 cells, which have an early memory and/or stem cell phenotype, are less cytolytic than T_H1 cells, but outperform them owing to much higher persistence in vivo. However, there is concern that some T_H17 cells may convert into regulatory T cells.

The authors had previously shown that T_H9 cells have anticancer properties and can promote CTL-mediated anticancer activity. Now they compare the suitability of tumour-specific T_H1, T_H17 and T_H9 cells for ACT in mouse models of melanoma (the B16-OVA and the less immunogenic B16 model, using T_H cells derived from naive OVA-specific or tyrosine-related protein 1-specific CD4⁺ cells, respectively). Similar to ACT protocols in the clinic, mice were treated with cyclophosphamide before transfer to induce temporary lymphopaenia and also received tumour antigen-loaded dendritic cells (DCs) to boost antitumour responses.

Strikingly, in both melanoma models, only T_H9 cells induced significant regression of large established tumours and allowed for long-term survival. They also protected mice against re-challenge with tumour cells. In contrast, ACT with T_H1 and T_H17 cells only led to temporary tumour regression, followed by aggressive regrowth. The antitumour response in mice treated with T_H9 cells did not depend on CTLs, as antitumour responses were only mildly impaired in CD8⁺ T cell-deficient mice, and did not require IL-9 or IFNγ expression, as shown with Il9^–/– and Ifng^–/– T_H9 cells. They did, however, require the co-administration of DCs.

only T_H9 cells induced significant regression of large established tumours and allowed for long-term survival.

To investigate the molecular underpinnings of the superior performance of the T_H9 cells, the transferred cells were retrieved 12 days after ACT and subjected to genetic and molecular analysis. T_H9 cells did not share the ‘exhaustion gene signature’ of T_H1 cells and expressed higher levels of co-stimulatory molecules and much lower levels of inhibitory receptors (such as PD1, LAG3, KLRG1 and CD244) than T_H1 cells. Their gene signature also suggested that T_H9 cells are mature effector T cells, but not terminally differentiated like T_H1 cells.

Interestingly, T_H9 cells expressed high levels of eomesodermin (EOMES), a transcription factor that indicates effector cell development and is also known as a master controller of granzyme expression. Compared with T_H1 and T_H17 cells, T_H9 cells expressed the highest levels of granzymes and showed the most potent tumour-specific cytolytic activity — which depended on the presence of EOMES and granzyme B.

Transferred T_H9 cells showed extraordinary persistence in vivo, equal to or better than T_H17 cells. The persistence of T_H17 cells is attributed to their early memory or stem cell-like features and enhanced resistance to apoptosis. In contrast, T_H9 cells appeared to persist owing to hyperproliferation.

Further analysis revealed that the hyperproliferation was driven by late-phase nuclear factor-κB (NF-κB) hyperactivation, activated by high levels of the ubiquitin ligase TRAF6. The high expression of TRAF6 appeared to be due to epigenetic changes in the Traf6 promoter, opening it up to transcription factors such as PU.1. The authors speculate that accumulated TRAF6 serves as a crucial adaptor that links to the MALT1–CARMA1–BCL-10 complex downstream of the T cell receptor and upstream of NF-κB.

Overall, the study shows that T_H9 cells display an ideal combination of T_H1-like cytolytic and T_H17-like persistence characteristics, which may render them particularly suitable for ACT therapies.