Anti-regulatory T cells are natural regulatory effector T cells

Pre-existing TGF-β-specific T-cell immunity in patients with pancreatic cancer predicts survival after checkpoint inhibitors combined with radiotherapy

BACKGROUND

Circulating transforming growth factor-β (TGF-β)-specific T cells that recognize TGF-β-expressing immune regulatory cells have been described in patients with cancer. TGF-β-derived peptide vaccination modulates the tumor microenvironment and has shown clinical effects in animal models of pancreatic cancer (PC). TGF-β-expressing regulatory cells are especially elevated in PC and may prevent the clinical response to immune checkpoint inhibitors (ICIs). Thus, in the present study we investigated the significance of TGF-β-specific T-cell immunity in patients with PC treated with ICI combined with radiotherapy in a randomized phase 2 study (CheckPAC).

METHODS

Immune responses to a TGF-β-derived epitope entitled TGF-β-15 as well as epitopes from Clostridium tetani (tetanus) and influenza were measured in peripheral blood mononuclear cells (PBMCs) with interferon-ɣ enzyme-linked immunospot assays. PBMCs were isolated before and after treatment. Correlations between immune response data and clinical data were evaluated with parametric and non-parametric statistical methods. Survival was analyzed with univariate and multivariate Cox-regression. TGF-β-15 specific T cells were isolated and expanded and examined for recognition of autologous regulatory immune cells by flow cytometry.

RESULTS

PBMCs from 32 patients were analyzed for immune responses to the TGF-β-derived epitope entitled TGF-β-15. Patients with a strong TGF-β-specific immune response at treatment initiation had longer progression-free and overall survival, compared with patients with a weak or no TGF-β-specific immune response. This remained significant in multivariate analysis. Patients with weak and strong TGF-β-specific responses displayed similar responses towards viral antigens. Furthermore, we show that TGF-β-specific T cells from a clinical responder specifically reacted to and lysed autologous, regulatory immune cells. Finally, mimicking a TGF-β-15 vaccination, we showed that repeated stimulations with the TGF-β-15 epitope in vitro enhanced the immune response to TGF-β-15.

CONCLUSION

A strong TGF-β-15 specific immune response was associated with clinical benefit and improved survival after ICI/radiotherapy for patients with PC. Importantly, the lack of TGF-β-specific T cells in some patients was not caused by a general immune dysfunction. TGF-β-specific T cells recognized regulatory immune cells and could be introduced in vitro in patients without spontaneous responses. Taken together, our data suggest that combining TGF-β-based vaccination with ICI/radiotherapy will be beneficial for patients with PC.

Cytotoxic T cells isolated from healthy donors and cancer patients kill TGFβ-expressing cancer cells in a TGFβ-dependent manner

Transforming growth factor-beta (TGFβ) is a highly potent immunosuppressive cytokine. Although TGFβ is a tumor suppressor in early/premalignant cancer lesions, the cytokine has several tumor-promoting effects in advanced cancer; abrogation of the antitumor immune response is one of the most important tumor-promoting effects. As several immunoregulatory mechanisms have recently been shown to be targets of specific T cells, we hypothesized that TGFβ is targeted by naturally occurring specific T cells and thus could be a potential target for immunomodulatory cancer vaccination. Hence, we tested healthy donor and cancer patient T cells for spontaneous T-cell responses specifically targeting 38 20-mer epitopes derived from TGFβ1. We identified numerous CD4+ and CD8+ T-cell responses against several epitopes in TGFβ. Additionally, several ex vivo responses were identified. By enriching specific T cells from different donors, we produced highly specific cultures specific to several TGFβ-derived epitopes. Cytotoxic CD8+ T-cell clones specific for both a 20-mer epitope and a 9-mer HLA-A2 restricted killed epitope peptide were pulsed in HLA-A2+ target cells and killed the HLA-A2+ cancer cell lines THP-1 and UKE-1. Additionally, stimulation of THP-1 cancer cells with cytokines that increased TGFβ expression increased the fraction of killed cells. In conclusion, we have shown that healthy donors and cancer patients harbor CD4+ and CD8+ T cells specific for TGFβ-derived epitopes and that cytotoxic T cells with specificity toward TGFβ-derived epitopes are able to recognize and kill cancer cell lines in a TGFβ-dependent manner.