ExoHCR: a sensitive assay to profile PD-L1 level on tumor exosomes for immunotherapeutic prognosis

Cancer immunotherapy has made recent breakthrough, including immune checkpoint blockade (ICB) that inhibits immunosuppressive checkpoints such as programmed cell death protein 1 (PD-1) and programmed death-ligand 1 (PD-L1). However, most cancer patients do not durably respond to ICB. To predict ICB responses for patient stratification, conventional immunostaining has been used to analyze the PD-L1 expression level on biopsied tumor tissues but has limitations of invasiveness and tumor heterogeneity. Recently, PD-L1 levels on tumor cell exosomes showed the potential to predict ICB response. Here, we developed a non-invasive, sensitive, and fast assay, termed as exosome-hybridization chain reaction (ExoHCR), to analyze tumor cell exosomal PD-L1 levels. First, using αCD63-conjugated magnetic beads, we isolated exosomes from B16F10 melanoma and CT26 colorectal cancer cells that were immunostimulated to generate PD-L1-positive exosomes. Exosomes were then incubated with a conjugate of PD-L1 antibody with an HCR trigger DNA (T), in which one αPD-L1-T conjugate carried multiple copies of T. Next, a pair of metastable fluorophore-labeled hairpin DNA (H1 and H2) were added, allowing T on αPD-L1-T to initiate HCR in situ on bead-conjugated exosome surfaces. By flow cytometric analysis of the resulting beads, relative to αPD-L1-fluorophore conjugates, ExoHCR amplified the fluorescence signal intensities for exosome detection by 3–7 times in B16F10 cells and CT26 cells. Moreover, we validated the biostability of ExoHCR in culture medium supplemented with 50% FBS. These results suggest the potential of ExoHCR for non-invasive, sensitive, and fast PD-L1 exosomal profiling in patient stratification of cancer immunotherapy.