Tracing Tumor-derived Exosomal PD-L1 by Dual-aptamer Activated Proximity-induced Droplet Digital PCR.

Tumor-derived exosomal proteins have emerged as promising biomarkers for cancer diagnosis, but the quantitation accuracy is severely hindered by large numbers of normal cell- derived exosomes. Herein, we developed a dual-target-specific aptamer recognition activated in-situ connection system on exosome membrane combined with droplet digital PCR (ddPCR) (TRACER) for precise quantitative analysis of tumor-derived exosomal PD-L1 (Exo-PD-L1). Taking advantage of the high binding affinity of aptamers, excellent selectivity of dual-aptamer recognition and the high sensitivity of ddPCR, this method exhibits significant sensitivity and selectivity for tracing tumor-derived Exo-PD-L1 in a wash-free manner. Due to the excellent sensitivity, the level of tumor-derived Exo-PD-L1 detected by TRACER can effectively distinguish cancer patients from healthy donors, and for the first time was identified as a more reliable tumor diagnostic marker than total Exo-PD-L1. Overall, TRACER strategy holds great potential for analysis of different subtypes of exosomes, unprecedented capability for converting exosomes into reliable clinical indicators and exploring the biological functions of exosomes.