Exosomes released by EBV-infected nasopharyngeal carcinoma cells convey the viral Latent Membrane Protein 1 and the immunomodulatory protein galectin 9

BACKGROUND: Nasopharyngeal carcinomas (NPC) are consistently associated with the Epstein-Barr virus (EBV). Their malignant epithelial cells contain the viral genome and express several antigenic viral proteins. However, the mechanisms of immune escape in NPCs are still poorly understood. EBV-transformed B-cells have been reported to release exosomes carrying the EBV-encoded latent membrane protein 1 (LMP1) which has T-cell inhibitory activity. Although this report suggested that NPC cells could also produce exosomes carrying immunosuppressive proteins, this hypothesis has remained so far untested. METHODS: Malignant epithelial cells derived from NPC xenografts – LMP1-positive (C15) or negative (C17) – were used to prepare conditioned culture medium. Various microparticles and vesicles released in the culture medium were collected and fractionated by differential centrifugation. Exosomes collected in the last centrifugation step were further purified by immunomagnetic capture on beads carrying antibody directed to HLA class II molecules. Purified exosomes were visualized by electron microscopy and analysed by western blotting. The T-cell inhibitory activities of recombinant LMP1 and galectin 9 were assessed on peripheral blood mononuclear cells activated by CD3/CD28 cross-linking. RESULTS: HLA-class II-positive exosomes purified from C15 and C17 cell supernatants were containing either LMP1 and galectin 9 (C15) or galectin 9 only (C17). Recombinant LMP1 induced a strong inhibition of T-cell proliferation (IC50 = 0.17 nM). In contrast recombinant galectin 9 had a weaker inhibitory effect (IC50 = 46 nM) with no synergy with LMP1. CONCLUSION: This study provides the proof of concept that NPC cells can release HLA class-II positive exosomes containing galectin 9 and/or LMP1. It confirms that the LMP1 molecule has intrinsic T-cell inhibitory activity. These findings will encourage investigations of tumor exosomes in the blood of NPC patients and assessment of their effects on various types of target cells

Expression of the DNase encoded by the BGLF5 gene of Epstein-Barr virus in nasopharyngeal carcinoma epithelial cells.

In contrast with most Epstein-Barr virus (EBV)-infected healthy carriers, nasopharyngeal carcinoma patients frequently have increased serum levels of antibodies directed against EBV-DNase. These antibodies are potentially interesting serological markers for the diagnosis and the follow-up of nasopharyngeal carcinoma (NPC). In this context, it is important to determine whether malignant EBV-infected cells are the source of significant amounts of EBV-DNase contributing to antigenic stimulation. Therefore EBV-DNase expression has been investigated in several NPC specimens. A significant expression of this viral enzyme was demonstrated in both fresh biopsies and transplanted tumor lines. The DNase isolated from tumor has a molecular weight varying between 52 and 60 kDa and its activity eluted from a single-stranded DNA affinity column was specifically inhibited by both NPC sera and the rabbit polyclonal antibody against EBV-DNase. The enzyme activity was functional in the presence of 300 mM KCl, with which cellular DNases are completely inhibited. The DNase was mainly localized in epithelial tumor cells of both NPC biopsies and nude mice-derived NPC cells