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Page 8 of 11 Nakamura et al. J Cancer Metastasis Treat 2018;4:32 I http://dx.doi.org/10.20517/2394-4722.2017.78

PD-L1 expression on CTCs
Immune check point blockade with programmed cell-death protein 1 (PD-L1) inhibitor has recently
attracted attention as a novel anticancer approach for treatment of advanced cancers. Overexpression of PD-
[49]
L1 has been considered a potential mechanism of tumor escape immune elimination . PD-L1 inhibitors
are currently being most actively investigated for clinical use in various cancers. PD-L1 expression has been
evaluated by mainly immunohistochemistry for primary tumor site as a predictive biomarker of response.
[50]
However, recent studies reported tumor heterogeneity in both primary and distant metastatic site .
CTCs survive in the bloodstream by exploiting immune escape mechanisms, including immune check
point molecule. Therefore, it is crucial to understand the interaction of CTCs with the immune system to
[51]
utilize more effective immunotherapies. Mazel et al. demonstrated that PD-L1 frequently upregulated in
[52]
CTCs of metastatic breast cancer patients. Furthermore, Strati et al. showed that the detection of CTCs
overexpressing PD-L1 mRNA at the end of treatment was associated with poor survival, and the absence
of PD-L1 overexpression at the end of treatment was related with complete response in head and neck
squamous cell carcinoma.

CONCLUSION
Although there are many studies focusing on the utility of CTCs for diagnosis, prediction, monitoring, and
choosing therapy, CTCs have not been used yet in clinical practice for gastric cancer. Therefore, further
investigation and clinical studies are necessary to achieve clinical utility of CTC in gastric cancer.

DECLARATIONS
Authors’ contributions
Wrote the initial draft of the manuscript: Nakamura K
Contributed to interpretation of data, and assisted in the preparation of the manuscript: Iwatsuki M
Contributed to data collection and interpretation, and critically reviewed the manuscript: Kurashige J,
Ishimoto T, Baba Y, Miyamoto Y, Yoshida N, Watanabe M, Baba H

Availability of data and materials
Not applicable.

Financial support and sponsorship
None.

Conflicts of interest
There are no conflicts of interest.

Ethical approval and consent to participate
Not applicable.

Consent for publication
Not applicable.

Copyright
© The Author(s) 2018.

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