Stem cell-like CD8 T cells are at the heart of the anti-tumor response induced by PD-L1/TGFβ dual blockade in immune excluded tumors

In depth analysis of the CD8 T cells infiltrating an immune excluded tumor model revealed the presence of stem cell-like CD8 T cells that expanded in situ upon anti-PD-L1/TGFβ combination therapy fueling an efficient and IFNγ dependent anti-tumor response.

While cancer immunotherapy is a promising strategy for improving patients’ survival and quality of life, currently only a fraction of cancer patients (~25-35%) respond to checkpoint inhibitor therapies (CIT). Patients with inflamed tumors, where CD8 T cells infiltrate the tumor, respond better to CIT compared to those with immune excluded tumors, where CD8 T cells accumulate at the tumor-stroma boundary. 

The excluded immune phenotype is found in more than 50% of all human tumors and is characterized by elevated TGFβ signaling1. Preclinical studies demonstrated that combining CIT with TGFβ inhibition converts immune excluded tumors to inflamed and increases anti-tumor responses2. Unfortunately, this increased anti-tumor efficacy has not been observed in the clinic so far. The lack of efficacy has been accompanied by on target toxicities3. This disconnect between what has been observed in animal models and the clinic warrants additional investigations into the mechanism of action of the CIT/anti-TGFβ combination, with the hope that having a deeper understanding of the biology can help guide future clinical trials. 

Despite the fact that CD8 T cells are the key component of the anti-tumor response, the direct or indirect impact of TGFβ inhibition combined with CIT on CD8 T cells origin and differentiation remains incompletely understood. With this in mind, we aimed to perform an in-depth analysis of the effects of anti-PD-L1/TGFβ combination therapy in a mouse model of T cell exclusion:  the EMT6 breast cancer model. We used a multi-omics approach to analyze how anti-PD-L1/TGFβ combination therapy affects tumor infiltrating CD8 T cells and their interaction with the other main cell types in the tumor microenvironment (TME): tumor cells, myeloid cells and fibroblasts. We show that in immune excluded tumors CD8 T cells are present with three major phenotypes: (1) progenitor-like CD8 T cells (TSCL) with a diverse T cell repertoire, (2) clonally expanded T progenitor exhausted cells (TPEX) characterized by high levels of TGFβ signaling and expression of exhaustion markers like TOX, and (3) different flavors of effector T cells (TEFF) characterized by either high expression of granzymes or IFNγ. Using bioinformatic inferences based both on single-cell RNA sequencing (scRNAseq) and T cell receptor sequencing (scTCRseq) analyses, we show that TSCL are the progenitor cells that can give rise to the different CD8 T cell phenotypes present in the TME. Anti-PD-L1/TGFβ combination therapy expands TSCL within the tumor and pushes CD8 T cell differentiation towards the IFNγhi phenotype, effectively fueling an IFNγ dependent anti-tumor response. IFNγ neutralization or IFNGR1 deletion in EMT6 tumor cells impaired anti-tumor response demonstrating the central role of IFNγ response induced by anti-PD-L1/TGFβ combination therapy. 

Our analysis in an immune excluded mouse model describes how anti-PD-L1 and anti-TGFβ induces a shift towards a more effective CD8 T cell population in the TME and transforms myeloid, stromal, and tumor niches to yield an immune-supportive ecosystem. The data published in this paper improves our understanding of TGFβ biology and might help guide future clinical trials, where the expansion of TSCL could be monitored as a measure of patient’s response to the combination treatment.

  1. Hegde, P. S. & Chen, D. S. Top 10 Challenges in Cancer Immunotherapy. Immunity 52, 17–35 (2020).
  2. Mariathasan, S. et al. TGFβ attenuates tumour response to PD-L1 blockade by contributing to exclusion of T cells. Nature 554, 544–548 (2018).
  3. Robbrecht, D. et al. Safety and efficacy results from the expansion phase of the first-in-human study evaluating TGFβ inhibitor SAR439459 alone and combined with cemiplimab in adults with advanced solid tumors. J. Clin. Oncol. 40, 2524–2524 (2022).

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