Background and purpose:
Immune-checkpoint blockade with anti-programmed death-1 (PD-1) antibodies is rapidly emerging for the treatment of human malignancies including lung cancer. Although programmed death-ligand 1 (PD-L1) has been studied as a predictive biomarker, detection and evaluation of PD-L1 expression level on tissue samples remain challenging due to its dynamic and unstable expression. Thus the diagnostic tool for real-time monitoring of PD-L1 expression is critically needed. Here, we assessed the expression pattern of PD-L1 on circulating tumor cells (CTCs) by using microcavity array (MCA) system in patients with advanced non-small cell lung cancer (NSCLC) and small cell lung cancer (SCLC).
PD-L1 staining on CTCs was established using NSCLC cell lines H820, H441, A549 and H23 expressing varying levels of PD-L1 spiked in the peripheral blood obtained from healthy donors. For clinical evaluation, 3 ml of peripheral whole blood was collected from 20 advanced lung cancer patients prior to the initiation of chemotherapy and from 10 healthy donors. Cells were captured and immuno-stained by using the automated MCA system (Hitachi Chemical Co., Ltd). CTCs were defined as those positive for DAPI and cytokeratin (CK) and negative for CD45. PD-L1 expression level on CTCs was visualized by addition of PD-L1 immunocytochemistry procedure. High-resolution fluorescent images were obtained using fluorescence microscope (Carl Zeiss Microscopy Co., Ltd).
Characteristics of 20 lung cancer patients enrolled in clinical study were as follows: median age 74 (range, 48 to 84); male 60%; stage III/IV, 10/90%; NSCLC/SCLC, 70/30%. More than 2 CTCs were identified in 14 patients (median 22.5; range, 4 to 71), and PD-L1 positive CTCs were detected in 12 patients (median 5; range, 2 to 15). No correlation was detected between the number of total CTCs and that of PD-L1 positive CTCs in each patient (R2 = 0.05). We found a total of 25 CTC clusters from 20 patients, of which PD-L1 expression was both homogenous and heterogeneous. It is noteworthy that clustered CTCs have larger proportion of PD-L1 positive CTCs per whole clustered CTCs than that of non-clustered CTCs (24/54, 44% versus 51/347, 15%, respectively). We further focused on CTC-interacting white blood cells, which intensively bound with aggregated CTCs rather than single CTC (12/54, 22% versus 43/337, 13%, respectively). Our data implicate that PD-L1 expression on CTC correlates with aggregation of CTCs (p < 0.05).
Our results showed that PD-L1 expression on CTCs was detectable and there is intrapatient heterogeneity of its expression in patients with advanced lung cancer. Further investigation is warranted to better understand the biological importance of the correlation between PD-1 expression and CTC aggregation and CTC bound to white blood cells.
Citation Format: Woong Kim, Yasuhiro Koh, Hiroaki Akamatsu, Satomi Yagi, Ayaka Tanaka, Kuninobu Kanai, Atsushi Hayata, Ryota Shibaki, Masayuki Higuchi, Hisashige Kanbara, Takashi Kikuchi, Keiichiro Akamatsu, Masanori Nakanishi, Hiroki Ueda, Nobuyuki Yamamoto. Differential expression of PD-L1 on circulating tumor cells among patients with advanced lung cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2257.