Mass-tagged self-assembled nanoprobe reveals the transport of PD-L1 from cancer cells to tumor-educated platelets

Background: The expression level of immune checkpoint proteins detected by tissue biopsy is currently used as a predictive biomarker for immune checkpoint blockade (ICB) therapy. However, tissue biopsy is susceptible to invasive sample collection procedures, significant sampling heterogeneity, and the difficulty of repeated sampling. Therefore, liquid biopsy of blood samples is becoming an alternative choice for immune checkpoint protein detection. Among various vesicles in blood, platelets can obtain cancer information to form a specific group called tumor-educated platelets (TEPs). The platelet-derived proteins in TEPs may have a predictive potential in ICB therapy. Results: In this study, a photo-cleavable mass-tagged self-assembled (SAMT) nanoprobe with signal amplification was developed for the quantitative detection of PD-L1. The SAMT probe was assembled by photo-cleavable mass tags, PD-L1 aptamer, and amphiphilic polymer. After binding with PD-L1 on the platelet, the probe can release mass tags with UV light exposure. The amount of the mass tag, representing that of PD-L1, was subsequently determined by mass spectrometry. The assay sensitivity can be greatly improved by up to four orders of magnitude, achieving a detection limit of 10 fM. This assay was subsequently applied to cancer cells and platelet samples from non-small cell lung cancer (NSCLC) patients. The patients with higher tumor stages, higher degrees of lymph node invasion, and better ICB response had higher PD-L1 levels on platelets. Further investigation revealed that PD-L1 on the platelets was transported from cancer cells, providing evidence for the existence of TEPs. Significance: The SAMT probe can amplify the signal of the target molecule into that of multiple mass tags, achieving ultrasensitive ICB protein quantitative detection in platelets. Moreover, the employed SAMT assay not only revealed PD-L1 transport from cancer cells to platelets but also confirmed the presence of TEPs.