Abstract: Objective To investigate the expression of programmed death ligand 1 (PD-L1) in serum exosomes of patients with hepatocellular carcinoma ( HCC ) and its clinical significance. Methods Exosomes from the serum samples of 75 HCC patients and 15 healthy controls were extracted by using ExoQuick kit, and were further identified through the methods of transmission electron microscopy, NanoSight and Western blotting. RT-qPCR assay was employed to detect the differential expression of PD-L1 mRNA in serum exosomes. Chi-square test was used to analyze the relationship between the expression level of serum exosomal PD-L1 mRNA and the clinicopathological features of HCC patients. Furthermore, Kaplan-Meier curve combined with the Cox regression analysis were used to investigate the prognostic value of the mRNA expression of serum exosomal PD-L1 in HCC. Results The transmission electron microscopy and NanoSight results showed that the particle sizes of most exosomes in the serum of HCC patients and healthy controls were about 100 nm. Western blotting analysis showed that the exosomes expressed the marker CD9 and TSG101 proteins, but did not express the cytoskeleton protein-Calnexin. RT-qPCR results showed that the expression level of PD-L1 mRNA in the serum exosomes of HCC patients was significantly higher than that of healthy controls (t = 11. 670, P< 0. 001). Further analyses exhibited that its high expression level was associated with the tumor diameter, tumor numbers, Child-Pugh grade, AFP levels, and portal vein tumor thrombin (all P< 0. 005). Kaplan-Meier survival analysis showed that the overall survival of HCC patients with higher mRNA expression level of serum exosomal PD-L1 was significantly shorter than that of those with lower expression level of serum exosomal PD-L1 (P = 0. 004). Moreover, univariate and multivariate Cox regression analysis demonstrated that the high expression of serum exosomal PD-L1 was an independent risk factor for the shortened overall survival in HCC patients (HR = 2. 013, 95 % CI: 1. 012-3. 669, P = 0. 015). Conclusion The expression level of PD-L1 in the serum exosomes of HCC patients is significantly up-regulated, which is closely related to tumor malignant progression and patient’s poor prognosis, indicating that the serum exosomal PD-L1 has a great potential to be a prognostic marker for HCC.

Key words: hepatocellular carcinoma, exosome, PD-L1, prognosis