Circulating tumor cells (CTCs) are cells that have shed from the tumor tissue into the bloodstream, and detection and enumeration of CTCs have been recognized as a useful test for personalized cancer therapy. Moreover, characterization of CTCs including epithelial-mesenchymal transition, stemness, and immune evasion phenotype is essential for a deeper understanding of cancer biology. To assess the immunologic characteristics of CTCs, we investigated the expression profile of immune-regulatory molecules, PD-L1, PD-L2, and CD47 in CTCs derived from patients with head and neck squamous cell carcinoma (HNSCC). Furthermore, the PD-L1 expression in the CTCs was compared with that in the tumor tissues assessed by immunohistochemistry. CTCs were isolated from 26 HNSCC patients with recurrent and/or distant metastasis by depletion of CD45-positive cells, and the expression of four epithelial markers (EpCAM, CK-19, EGFR, and c-Met) was first analyzed by qRT-PCR with a low concentration of RNA from the CTC population. Twenty (76.9%) of 26 patients tested were positive for at least one epithelial-related gene, indicating the existence of CTCs in those patients. Among 20 CTC-positive patients, 16 (80%), 13 (65%), and 15 (75%) patients were positive for CD47, PD-L1, and PD-L2, respectively. Of note, the mRNA expression level of each immune-regulatory molecule correlated positively with each other (PD-L1 vs PD-L2, p = 0.0028; PD-L1 vs CD47, p = 0.0015; PD-L2 vs CD47, p < 0.0001). Although the PD-L1 expression in the tumor tissues was positive in 12 (60%) of the 20 patients, concordance of the PD-L1 expression between the tumor tissues and the CTCs was observed in only 11 (55%) patients. Our findings suggest that, in some patients, CTCs may facilitate their evasion of innate and acquired antitumor immunity mediated via effector T cells, NK cells, and macrophages by expressing multiple immune-regulatory molecules. Currently, precise evaluation of PD-L1 expression are indispensable for the development of more effective and personalized therapeutic approaches using immune checkpoint inhibitors; therefore, the evaluation of PD-L1 expression in CTCs may enable real-time assessment as well as continuous monitoring of the PD-L1 status in tumor cells within tumor microenvironment.

Citation Format: Hiroe Tada, Hideyuki Takahashi, Hiroki Ishii, Yuki Kuwabara-Yokobori, Shota Ida, Masato Shino, Kazuaki Chikamatsu. Immunologic characteristics of circulating tumor cells in patients with head and neck squamous cell carcinoma [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2018 Nov 27-30; Miami Beach, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(4 Suppl):Abstract nr B74.