Modulation of host innate immune responses by Epstein-Barr virus tegument protein

Abstract

Epstein-Barr Virus (EBV) is present in approximately 95% humans all over the world, causing lymphomas, nasopharyngeal and gastric carcinomas and various other malignancies in a few of the infected people. The virus primarily infects epithelial and B cells and can undergo latent as well as lytic replication in these cells. For survival and maintenance of EBV genome inside the host cells, EBV has developed mechanisms to evade host innate immune responses. This study was aimed to identify major immunomodulatory protein of EBV, with specific focus on interferon (IFN) signaling pathways. EBV itself was found to be a potent inhibitor of type-I/III and type-II IFN signaling pathways. Our research findings show that the Epstein-Barr virus-encoded tegument protein, called BGLF2, plays a significant role in the inhibition of IFN signaling pathways by targeting the signaling intermediates in JAK/STAT pathways. BGLF2 promotes STAT2 degradation and reduces STAT1 phosphorylation inside the cells and therefore, prevents the expression of downstream interferon-stimulated genes (ISGs). BGLF2 was found to interact with STAT2 to promote its degradation via proteasomal degradation pathway by enhancing K48-linked ubiquitination of STAT2. BGLF2 promoted the interaction of STAT2 with Cullin1 suggesting a possible involvement of CRL-ubiquitin ligase complex in STAT2 degradation. BGLF2 was also found to inhibit the tyrosine-phosphorylation of STAT1 by involvement of a tyrosine phosphatase SHP1, without affecting endogenous total protein levels of STAT1. Consequently, BGLF2 could suppress the transcription of downstream ISGs induced by type-I IFNs. Furthermore, BGLF2 could also suppress type-I and type-III IFN production. BGLF2 was also found to act independently of p38-MAPK and JNK pathways to suppress IFN signaling in the cells. Besides targeting the JAK/STAT pathway, BGLF2 was also found to facilitate EBV primary infection and replication in HEK293 cells by downregulating type-I IFN signalling. Additionally, the expression of early and late lytic genes was significantly reduced by IFN-β, even when lytic cycle was induced by 12-O-tetradecanoylphorphol-13-acetate (TPA) treatment, a widely used lytic reactivator, in EBV-positive gastric carcinoma cells suggesting type-I IFN signalling downregulation is required for viral reactivation. The BGLF2-mutant-EBV was created and used to further study the physiological role of BGLF2 in context of viral pathogenesis. BGLF2- mutant-EBV-positive cell lines expressed relatively reduced levels of lytic genes and relatively higher levels of ISGs as compared to the wild type EBV. Our study unveils BGLF2 as a novel modulator of IFN-dependent innate immune responses and its role in EBV lytic infection and reactivation. Supported by HMRF-17160822, AoE/M-06/08 and RGC-C7027-16G.