Sensitive Detection of Exosomal Proteins via a Compact Surface Plasmon Resonance Biosensor for Cancer Diagnosis

Abstract

Exosomes are small extracellular vesicles released by cells for cell-cell communication. They play important roles in cancer development, metastasis, and drug resistance. Exosomal proteins have been demonstrated by many studies as promising biomarkers for cancer screening, diagnosis, and monitoring. Among many detection techniques, surface plasmon resonance (SPR) is a highly sensitive, label-free, and real-time optical detection method. Commercial prism-based wavelength/angular-modulated SPR sensors afford high sensitivity and resolution, but their large footprint and high cost limit their adaptability for clinical settings. Recently, a nanoplasmonic exosome (nPLEX) assay was developed to detect exosomal proteins for ovarian cancer diagnosis. However, comparing with conventional SPR biosensors, the broad applications of nanoplasmonic biosensors are limited by the difficult and expensive fabrication of nanostructures. We have developed an intensity-modulated, compact SPR biosensor (25 cm × 10 cm × 25 cm) which uses a conventional SPR sensing mechanism and does not require nanostructure fabrication. Calibration from glycerol showed that the compact SPR biosensor offered sensitivity of 9.258 × 10%/RIU and resolution of 8.311 × 10 RIU. We have demonstrated the feasibility of the compact SPR biosensor in lung cancer diagnosis using exosomal epidermal growth factor receptor (EGFR) and programmed death-ligand 1 (PD-L1) as biomarkers. It detected a higher level of exosomal EGFR from A549 nonsmall cell lung cancer (NSCLC) cells than BEAS-2B normal cells. With human serum samples, the compact SPR biosensor detected similar levels of exosomal EGFR in NSCLC patients and normal controls, and higher expression of exosomal PD-L1 in NSCLC patients than normal controls. The compact SPR biosensor showed higher detection sensitivity than ELISA and similar sensing accuracy as ELISA. It is a simple and user-friendly sensing platform, which may serve as an in vitro diagnostic test for cancer.

Exosomal PD-L1: New Insights Into Tumor Immune Escape Mechanisms and Therapeutic Strategies

“…To completely surmount the disadvantages of ELISA, a nanoplasmonic exosome (nPLEX) assay has been established, which involves modified surface plasmon resonance (Enderle et al, 2015) with a compact SPR biosensor (Liu et al, 2018b). Notably, the nPLEX assay is able to detect ExoPD-L1 in 50 µl serum samples in real-time, which is undetectable by ELISA (Table 3; Liu et al, 2018b). Chen G. et al, 2018;Liu et al, 2018b;Theodoraki et al, 2018aTheodoraki et al, ,b, 2019Fan et al, 2019;Li et al, 2019a;Lux et al, 2019;Cordonnier et al, 2020;Huang et al, 2020…”

“…However, the levels of soluble PD-L1 were not different between NSCLC patients and healthy donors (Li et al, 2019a). Moreover, soluble PD-L1 did not correlate with disease progression in patients with metastatic gastric cancer, HNSCC, or NSCLC (Liu et al, 2018b;Fan et al, 2019;Li et al, 2019a;Pang et al, 2020). On the contrary, circulating ExoPD-L1 in plasma is an independent biomarker to predict poor prognosis in patients with metastatic gastric cancer (Fan et al, 2019).…”

“…However, there are pitfalls of using cellular PD-L1 such as traumatic biopsy, missing small tumors, heterogeneity of PD-L1 expression within tumors, unavailability of dynamic observation, and limited sensitivity ( Kaunitz et al, 2017 ; Bassanelli et al, 2018 ; Teixido et al, 2018 ; Davis and Patel, 2019 ; Stovgaard et al, 2019 ; Xu G. et al, 2019 ). Recent studies indicate that circulating ExoPD-L1 is emerging as a non-invasive and readily available biomarker, and is more easily detectable and reliable than both tissue and soluble PD-L1 in plasma ( Liu et al, 2018b ; Cordonnier et al, 2020 ; Huang et al, 2020 ; Pang et al, 2020 ).…”

“…However, in melanoma, the level of ExoPD-L1 in the blood, rather than the number and total protein content of exosomes, is elevated in metastatic melanoma patients compared with healthy subjects ( Chen G. et al, 2018 ). Furthermore, patients with NSCLC and adenocarcinoma also exhibit higher levels of circulating ExoPD-L1 compared with healthy controls ( Liu et al, 2018b ; Li et al, 2019a ; Huang et al, 2020 ; Pang et al, 2020 ). Therefore, circulating ExoPD-L1 may be a potential diagnostic marker.…”

Label-free detection of exosomes using a surface plasmon resonance biosensor

“…The approach involved the use of periodically placed antibody-functionalized nanohole arrays and exosome profiling was performed by using transmission surface Plasmon resonance system. To avoid the nanostructure fabrication complexity for nano-plasmonic sensor development, Liu et al developed a compact SPR biosensor for exosomal protein detection [30].…”