Co-expression of PD1/PD-L1 on Tumor Cells Is Involved in the Regulation of Cell Proliferation

Immune checkpoint inhibitors have recently been applied as anticancer agents for various thoracic malignancies. However, the mechanisms underlying their efficacy remain insufficiently understood. We therefore conducted a study to investigate the role of programmed cell death protein 1 (PD1) molecules expressed on the surface of cancer cell lines.

We assessed PD1 expression on the surface of nine cell lines using flow cytometry. To explore the involvement of the PD1/programmed cell death ligand 1 (PD-L1) axis in cell proliferation, we inhibited PD1 function by adding nivolumab to PD1-positive tumor cell lines. We then analyzed changes in cell proliferation signaling pathways of a small-cell lung cancer cell line, SBC-3, and changes in PD-L1 expression and associated signaling pathways by adding cobalt chloride and interferon-γ, known to influence PD-L1 expression in vivo.

Among the nine cell-lines, SBC-3 showed strong PD1 expression, whereas the other eight cell lines exhibited minimal or no expression. Significant increases in cell proliferation were observed following treatment of SBC-3 cells with nivolumab at concentrations of 4 and 40 μg/ml. Immunoblotting analysis revealed enhanced phosphorylation of AKT serine/threonine kinase 1 and extracellular-regulated kinase, along with a concentration-dependent decrease in COP9 signalosome subunit 5 (CSN5) expression. Although cobalt chloride treatment increased expression of hypoxia-inducible factor 1α, it did not affect PD-L1 expression. In contrast, treatment with interferon-γ resulted in marked inhibition of cell proliferation and a clear decrease in PD-L1 expression.

Co-expression of PD1 and PD-L1 may play a role in regulating proliferation of tumor cells, particularly in cases with strong PD1 expression on tumor cells.