Abstract 2738: PD-L1/PD-1 checkpoint inhibition in anaplastic thyroid cancer and enhancement of ICAM1-targeted chimeric antigen receptor (CAR)-T cell tumor lysis

Poorly-differentiated thyroid cancer (PDTC) and anaplastic thyroid cancer (ATC) are associated with a rapidly progressive course and poor median survival. Recently we published promising results in a murine model showing efficient clearance of metastatic tumor burden of human ATC cell line 8505c and an anaplastic patient-derived cell line using healthy donor T cells transduced with a third-generation chimeric antigen receptor (CAR) construct targeting intercellular adhesion molecule-1 (ICAM1). However, patient-derived T cells using an identical ICAM1-targeted CAR construct demonstrated slower tumor-killing in vitro. To investigate the role of immune checkpoint proteins PD-L1 and PD-1 in thyroid cancer and as potential inhibitors of ICAM1-CAR T, we stained formalin-fixed paraffin-embedded archived tissue samples for PD-L1 (papillary thyroid cancer, PTC, n = 17; ATC, n = 13) using immunohistochemistry (IHC) and extracted peripheral blood mononuclear cells (PBMCs) from patient blood samples and healthy donors to evaluate PD-1 expression on CD3+/CD8+ T cells using flow cytometry (healthy donor, n = 2; PTC, n = 7; PDTC/ATC, n = 6). Twelve percent of PTC samples were PD-L1+ (threshold >1% positivity) versus 69% of PDTC/ATC samples (Chi-square, p = 0.012); PD-L1 positivity was associated with circumferential ICAM1 staining on IHC (Chi-square, p = 0.003). Similarly, median percentage of PD-1 expression on PBMC9s was higher in PDTC/ATC than PTC (58.1% versus 5.2%, Wilcoxon rank sum test; p = 0.03). Interestingly, expression of elevated PD-1 was seen in PBMC samples from patients in the absence of PD-L1 positivity of the primary tumor (IHC) in 4 of 5 matched tumor samples. This suggests that PD-1 in circulating CD8+ cells may be an important predictor of response to checkpoint inhibitor therapy. Finally, effector:target assays with patient-derived ICAM1-CAR T effector cells (baseline PD-1 expression 96%) against 8505c (PD-L1 100%/ICAM1 68%) and matched patient-derived cell line (PD-L1 >50%/ICAM1 88%) showed enhanced tumor killing in the presence of anti-PD-1, which approached the level of tumor cell death seen with healthy donor ICAM1-CAR T. Our results show that PD-1 is increased in the peripheral blood of patients with poorly differentiated and anaplastic thyroid cancer and is a potential therapeutic target. Furthermore, anti-PD-1 may be used to counter PD-L1+ tumor-induced T cell suppression (or exhaustion) to increase the efficacy of CAR T cells in ATC. Citation Format: Katherine D. Gray, Yogindra Vedvyas, Olivia Kalloo, Enda Shevlin, Theresa Scognamiglio, Marjan Zaman, Andrew Tassler, Moonsoo M. Jin, Rasa Zarnegar, Thomas J. Fahey, Irene M. Min. PD-L1/PD-1 checkpoint inhibition in anaplastic thyroid cancer and enhancement of ICAM1-targeted chimeric antigen receptor (CAR)-T cell tumor lysis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2738.