Epstein-barr virus infection in nasopharyngeal epithelial cells : from early infection to persistent infection

Abstract

Nasopharyngeal carcinoma (NPC) is a highly prevalent type of cancer in Southeast Asia, especially in southern China. NPC is strongly associated with Epstein-Barr virus (EBV) infection and is presented in mostly undifferentiated or poorly differentiated histopathological types in endemic regions. EBV infection in NPC is clonal in origin and occurs early during NPC development. The establishment of latent EBV infection in premalignant nasopharyngeal epithelial (NPE) cells and persistent infection of EBV in NPC is believed to be essential in NPC pathogenesis. However, events controlling EBV infection at early and advanced developmental stages of NPC are largely unclear due to the lack of appropriate cell models. My study is focused on elucidating some of the events involved in early and persistent EBV infection in nasopharyngeal epithelial cells. With the assistance of cell-free EBV infection methods, the dynamics of EBV maintenance, expression of EBV-encoded genes and promoter usage at the early phases post-infection in premalignant NPE cells could be carefully monitored. Using the genome-wide CRISPR/Cas9-mediated screening, the IFI27 was identified as a restriction factor for persistent EBV infection in premalignant nasopharyngeal epithelial cells. The IFI27 was expressed at low level in NPC cells and EBV-stably infected NPE cells while its expression was detected at early phase after EBV infection in NPE cells. Silencing of IFI27 supported stable EBV infection, whereas overexpression of IFI27 promoted loss of EBV-infected cells at early stage post-infection stage in NPE cells. IFI27-mediated suppression of the EBV Qp promoter may be involved. IFI27 directly targeted the Src protein to degradation through the ubiquitination-proteasomal pathway, leading to subsequent suppression of STAT3 phosphorylation. Inhibition of STAT3 suppressed EBV Qp activation, which is critical for establishment of latent and persistent EBV infection. Constitutive activation of STAT3 promoted EBV Qp activation and supported stable EBV infection in NPE cells.

Additionally, EBV latent-lytic switch could be induced by STAT3 inhibition in EBV-infected NPC cells. Interfering with STAT3 signaling resulted in squamous differentiation of EBV-positive NPC cells. KLF4, a differentiation-dependent transcription factor, was a critical effector implicated in lytic EBV reactivation elicited by STAT3 inhibition. KLF4 induction was related to repression of p63. Silencing p63 also triggered lytic EBV reactivation. KLF4 knockdown counteracted the effects of p63 silencing in EBV-positive NPC cells. These finding demonstrated a crucial regulatory role of STAT3/p63/KLF4 in NPC differentiation and lytic EBV reactivation.

The characterization of early EBV infection in the premalignant NPE cells revealed rapid loss of EBV genome and EBV-infected cells. The IFI27-mediated STAT3 inactivation was shown to be unfavourable for persistent EBV infection in premalignant NPE cells. This study provides evidence that STAT3 activation supports persistent EBV infection in premalignant NPE cells; it also revealed the importance of STAT3 in the maintenance of EBV latency and undifferentiated status of NPC cells. Collectively, these findings highlight a central role of STAT3 in persistent EBV infection in nasopharyngeal epithelial cells.