Role of Natural Killer cells in the development of HPV-associated pre-neoplastic cervical lesions

Persistent infections with high-risk papillomavirus (HPV) are associated with more than 25% of cancers induced by infectious agents. Nevertheless, the two vaccines preventing HPV infection have no therapeutic efficacy and it has been estimated that there will be no measurable decline of HPV-associated tumours before 2040. The immune system is able to control, at least partially, HPV infection and subsequent tumour development. Around 90% of HPV-infected women will clear the virus within two years, but the immune effectors responsible for this viral clearance are unknown. Hence, the aim of this study was to determine if Natural Killer (NK) cells could play a role in the immune response against HPV infection and related cancers. The first part of this work was focused on the in vitro interaction of NK cells with L1 and L1L2 Virus Like Particles (VLP) of HPV16. We observed that, in the presence of HPV-VLP, NK cells displayed a higher cytotoxic activity against HPV+ cells by increasing the exocytosis of their cytotoxic granules and by secreting TNF-α and IFN-γ. NK cell activation was correlated with a fast entry of HPV-VLP by macropinocytosis and we determined that cell surface CD16 expression was necessary for HPV internalization, but also for degranulation and cytokine production. In the second part, to understand the molecular mechanisms of HPV-VLP stimulation, we investigated the signalling pathways operating in NK cells to trigger their cytotoxic activity in the presence of viral particles. We observed that the MAP kinases ERK and p38 were phosphorylated in the presence of both L1 and L1L2 HPV-VLP. Using specific inhibitors, we demonstrated that phosphorylation of these MAPK was required for degranulation and cytokine secretion by NK cells in the presence of VLP. In conclusion, NK cell activity could be an important player in the immune response contributing to viral clearance and to regression of HPV-induced cervical lesions