Human Tumour Immune Evasion via TGF-β Blocks NK Cell Activation but Not Survival Allowing Therapeutic Restoration of Anti-Tumour Activity

Abstract

Immune evasion is now recognized as a key feature of cancer progression. In animal models, the activity of cytotoxic lymphocytes is suppressed in the tumour microenvironment by the immunosuppressive cytokine, Transforming Growth Factor (TGF)-β. Release from TGF-β-mediated inhibition restores anti-tumour immunity, suggesting a therapeutic strategy for human cancer. We demonstrate that human natural killer (NK) cells are inhibited in a TGF-β dependent manner following chronic contact-dependent interactions with tumour cells in vitro. In vivo, NK cell inhibition was localised to the human tumour microenvironment and primary ovarian tumours conferred TGF-β dependent inhibition upon autologous NK cells ex vivo. TGF-β antagonized the interleukin (IL)-15 induced proliferation and gene expression associated with NK cell activation, inhibiting the expression of both NK cell activation receptor molecules and components of the cytotoxic apparatus. Interleukin-15 also promotes NK cell survival and IL-15 excluded the pro-apoptotic transcription factor FOXO3 from the nucleus. However, this IL-15 mediated pathway was unaffected by TGF-β treatment, allowing NK cell survival. This suggested that NK cells in the tumour microenvironment might have their activity restored by TGF-β blockade and both anti-TGF-β antibodies and a small molecule inhibitor of TGF-β signalling restored the effector function of NK cells inhibited by autologous tumour cells. Thus, TGF-β blunts NK cell activation within the human tumour microenvironment but this evasion mechanism can be therapeutically targeted, boosting anti-tumour immunity.

The breast tumor microenvironment alters the phenotype and function of natural killer cells

“…Due to the fact that the developmental defect we observed in pyMT mice was specific to the tumor, we believe that the altered NK cell phenotype is directly related to the immunosuppressive cytokines and cells found within that particular microenvironment. Several studies have indicated that TGF-b produced in the tumor can decrease NK cell cytotoxicity and cytokine secretion 12,33 . In human studies, it was found that TGF-b affects the development of NK cells in the blood by inhibiting their development and converting a small fraction of CD56 bright CD161 NK cells into CD56 bright CD162 NK cells 34 .…”

NK cell receptor imbalance and NK cell dysfunction in HBV infection and hepatocellular carcinoma

“…Blocking TGF-b by using small molecule inhibitors of TGF-b or anti-TGF antibodies appears to inhibit HCC cell migration and to attenuate tumor cell-mediated NK-cytotoxicity inhibition. [108][109][110] Interestingly, in human lung cancer or colorectal cancer patients, elevated TGF-b1 secretion is inversely correlated with the expression of NKG2D and cytotoxicity of NK cells. TGF-b1 neutralization by anti-TGF-b1 monoclonal antibodies in vitro can restore NKG2D expression.…”

Blocking oncogenic RAS enhances tumour cell surface MHC class I expression but does not alter susceptibility to cytotoxic lymphocytes

“…The following antibodies were used for flow cytometry (antigen, clone-fluorochrome, [24] using Taqman probe/primer sets from Applied Biosystems/Life Technologies.…”

“…Human NK cells were prepared from blood samples using indirect immunomagnetic separation, using a kit from Miltenyi Biotec, as previously described [24]. NK cell purity, as judged by either the CD56+CD3 neg or NKp46+ cell surface phenotype was routinely >90%.…”

“…For IL-2 stimulation, NK cells were cultured for 5-7 days in 50 units/ml IL-2 (R&D systems). NK cell mediated killing of tumour cells and granule exocytosis assays were performed as we have described previously [24][25][26], including after siRNA transfection of target cells [26,27]. T cells restricted to the HLA-A2…”