Roles of interferon stimulated genes in Human papillomavirus infection and cancer progression

Abstract

HPVs are potent human pathogens causally associated with multiple malignancies, including cervical, head and neck and anogenital cancers. While it is well known that HPV persistence is a necessary event in cancer development, it is unclear how host cells efficiently clear most infections. Initial detection and response to viral infections is mediated by the innate immune system. This thesis focuses on furthering our understanding of the roles of innate immune system in modulating HPV infections by exploring the roles of interferon (IFN) and IFN stimulated genes (ISGs) during HPV infection and cancer progression. Here, we demonstrate that IFN-β pretreatment of keratinocytes significantly inhibits HPV infection. To narrow down prospective ISGs responsible for reduced HPV infectivity, we performed global gene expression analysis of keratinocytes persistently infected with high-risk HPV16 and HPV18 and HPV-positive clinical specimens. Based on our global gene expression data, two restriction factor families, IFITM and APOBEC3 (A3), were chosen for detailed investigation. First, we show that despite being induced during early stages of HPV persistence, IFITM1 and IFITM3 overexpression consistently enhanced HPV infectivity in various epithelial cell lines and primary keratinocytes. We concluded that HPV, even during high levels of IFITM expression, has evolved mechanisms to evade this innate immune restriction barrier. In contrast, we identified human APOBEC3A (hA3A) as a potent restriction factor of HPV infectivity. Since hA3A is robustly induced by viral oncogene expression, HPV must avoid or modulate hA3A to productively infect keratinocytes. Based on genome analysis of 274 different papillomavirus (PV) genotypes, we revealed that PV genomes have been under strong selective pressure by A3s. By diminishing A3 target sites in their genomes, HPVs may have evolved reduced susceptibility to A3s during the course of natural infection. Finally, given the robust inhibition of HPV infection by IFN-β, we sought to elucidate additional inhibitory ISGs using high-throughput screening of an ISG shRNA library. Through the use of this unbiased screening approach, we have identified several new candidate ISGs with potential anti-HPV activity.