A critical point in cancer progression is evading recognition by the immune system. Cancer cells accomplish this by stimulating immune checkpoint signals on effector T-cells. In patients with advanced melanoma treated with immune checkpoint inhibitors, 3-year survival increased by 20%. While immune checkpoint therapies are the new first treatment option for advanced melanoma in over a decade, their efficacy is limited because resistance often develops. Understanding the molecular mechanisms of immune checkpoint inhibitor resistance is critical to develop combinatorial drug therapy to potentiate therapeutic responsiveness. The unfolded protein response (UPR) is an endoplasmic reticulum (ER) stress pathway activated when unfolded/misfolded proteins accumulate within the ER. Highly secretory cell types, such as T-cells, have larger ER cell compartments and elevated UPR components to deal with the increased protein synthesis/folding required by these cell types. Therefore, these cell types may be highly sensitive to ER stress. Our data demonstrates elevated UPR signaling components as a driver of T-cell exhaustion/dysfunction. Using a previously established T-cell exhaustion protocol, we stimulated naïve T-cells with antibodies to CD3/CD28 for 5 days and co-cultured them with MDA-MB-231 breast cancer cells, Mun2b melanoma cell line, or CMI patient-derived melanoma cell line. Exhausted T-cells displayed an increased PD-1 and PERK expression, suggesting that UPR signaling is activated during T-cell exhaustion. Treatment of naïve T-cells with DTT, a chemical agent that stimulates ER stress, also induced PD-1 and PERK compared with vehicle-treated T-cells. Gene expression analysis of T-cells indicate that co-culture with cancer cells, not CD3/CD28 activation, elevates T-cell UPR gene expression. Furthermore, induction of ER stress through low-dose DTT treatment decreased cytotoxic T-cell mediated cancer cell death, further supporting our hypothesis of ER stress inducing T-cell exhaustion. Inhibition of PERK by RNAi in TALL-104, a human cytotoxic T-cell line, enhanced T-cell mediated cancer cell clearance when exposed to ER stress-inducing agents, suggesting that PERK may represent a novel target to prevent T-cell exhaustion or restore T-cell effector capabilities. PERK inhibition in the patient-derived melanoma cells did not negatively affect T-cell-mediated killing, suggesting that systemic PERK inhibition may be an effective therapeutic strategy to enhance anti-tumor immune responses. Matched PBMC from melanoma patients before treatment or after ipilimumab therapy resistance indicated increased UPR signaling components in PBMC samples from patients after ipilimumab resistance when compared with PBMC samples before therapy, supporting a novel role UPR signaling in anti-CTLA4 therapy resistance.

Citation Format: Yismeilin R. Feliz Mosquea, David R. Soto Pantoja, Adam Wilson, Pierre L. Triozzi, Katherine L. Cook. UPR signaling promotes T-cell dysfunction to prevent immune-mediated cancer cell killing and immune checkpoint therapy resistance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1704. doi:10.1158/1538-7445.AM2017-1704