PD-L1 Is up-Regulated By EBV-Driven LMP1 through NF-κb Pathway and Correlates with Poor Prognosis in Natural Killer/T-Cell Lymphoma

Background:

Natural killer/T-cell lymphoma (NKTCL) is an EBV-associated, highly aggressive lymphoma. Treatment outcome remains sub-optimal, especially for advanced-stage or relapsed diseases. Programmed cell death receptor 1 (PD-1) and PD ligand 1 (PD-L1) have become promising therapeutic targets for various malignancies, but their role in the pathogenesis and their interactions with Epstein-Barr virus (EBV) in NKTCL remains to be investigated.

Materials and methods:

Expression of PD-L1 was measured in NK-92 (EBV-negative) and SNK-6 (EBV-positive) cells by western blot, quantitative real-time PCR, and enzyme-linked immunosorbent assay, respectively. Latent membrane protein 1 (LMP1)-harboring lentiviral vectors were transfected into NK-92 cells to examine the correlation between LMP1 and PD-L1 expression. Proteins in the downstream pathways of LMP1 signaling were measured in NK-92 cells transfected with LMP1-harboring or negative control vectors, respectively. PD-L1 expression on tumor specimens and serum concentration of soluble PD-L1 were collected in a retrospective cohort of patients with Ann Arbor stage I~II NKTCL and their prognostic significance were analyzed.

Results:

Expression of PD-L1 was significantly higher in SNK-6 cells than in NK-92 cells, at both protein and mRNA levels. Expression of PD-L1was remarkably up-regulated in NK-92 cells transfected with LMP1-harboring lentiviral vectors compared with those transfected with negative control vectors. Proteins in the MAPK/NF-κB pathway were up-regulated in LMP1-expressing NK-92 cells compared with the negative control. Selective inhibitors of those proteins induced significant down-regulation of PD-L1 expression in LMP1-expressing NK-92 cells. Patients with a high concentration of serum soluble PD-L1 (≥ 3.4 ng/ml) or with a high percentage of PD-L1 expression in tumor specimens (≥ 38%) exhibited significantly lower response rate to treatment and remarkably worse survival, compared with their counterparts. A high concentration of serum soluble PD-L1 and a high percentage of PD-L1 expression in tumor specimens were independent adverse prognostic factors among patients with stage I~II NKTCL.

Conclusions:

PD-L1 expression positively correlated LMP1 expression in NKTCL, which was probably mediated by the MAPK/NF-κB pathway. PD-L1 expression in serum and tumor tissues have significant prognostic value for early-stage NKTCL.