Establishment of an Assay with Ultrahigh Sensitivity for Detecting sEV-Derived PD-L1 as a Serum Biomarker for Lung Cancer—A Pilot Study Using TN-cyclon™

Programmed death-ligand 1 (PD-L1) is an immune checkpoint protein. The soluble form of PD-L1 (sPD-L1) and PD-L1 derived from small extracellular vesicles (sEVPD-L1) are promising cancer biomarkers. While sEVPD-L1 in particular may contribute to immune evasion and is associated with a poor prognosis, it exists only in trace amounts, making it difficult to detect using conventional enzyme-linked immunosorbent assay (ELISA) methods. Therefore, we developed an ultrasensitive detection method, TN-cyclon™. The TN-cyclon™ method combines sandwich ELISA with enzyme cycling amplification. We applied TN-cyclon™ to measure recombinant PD-L1 protein and sEVPD-L1 in serum samples from cancer patients and healthy donors. Recombinant PD-L1 protein was measured with an ultrasensitive detection limit of 0.172 pg/mL. In clinical specimens, sEVPD-L1 levels were significantly higher in lung cancer patients than in healthy donors, whereas sPD-L1 levels measured with a conventional ELISA did not differ significantly between groups. Our results demonstrated that the TN-cyclon™ method exhibits a 20-fold increase in sensitivity compared to a conventional ELISA. Although this is a pilot study, our new assay enables the detection of very low concentrations of sEVPD-L1 in serum that can be used to evaluate the predictive and prognostic performance of sEVPD-L1 in lung cancer patients in future studies.