Cervical cancer, the fourth most frequent malignancy in the world, is caused by infection with high-risk human papillomaviruses (HPVs). Recently, we have shown that natural killer (NK) cells are directly activated by virus-like particles (VLPs) formed by the major capsid protein L1 of HPV-16, responsible for more than 50% of uterine cervical cancers. These VLPs are licensed as the first prophylactic vaccine against this cancer. Since NK cells collaborate with dendritic cells (DCs) to induce an immune response against viral infections and tumors, we studied the impact of this crosstalk in the context of HPV vaccination.
We have established autologous co-cultures of monocyte-derived DCs with negatively sorted NK cells at a ratio of 1:1. These cells were incubated during 24h with HPV-16 VLPs obtained by self-assembly of L1 proteins produced in baculovirus/insect cell systems and purified on cesium chloride gradient. Lysate of insect cells infected with wild type baculovirus were used as a negative control.
NK cells in the presence of VLPs enhanced DC maturation as attested by an upregulation of CD86 and HLA-DR and an increased production of IL-12p70, but not of the immunosuppressive cytokine IL-10. The killing of immature DCs by NK cells was not increased in the presence of VLPs and this could be due to a decreased expression of NKp30L on DCs in the DC-NK-VLP condition.
This activation was bi-directional. Indeed, in the presence of VLPs, DCs further activated NK cells by inducing the upregulation of cell surface activation markers such as CD69 and HLA-DR, but not NK cell receptors (NCRs) such as NKp30, NKp44 and NKp46. The function of NK cells was improved as shown by an increase in IFN-γ secretion and cytotoxic activity against HPV+ cell line. This crosstalk between NK cells and DCs needed CD40 interaction and IL-12p70 secretion, whereas NKG2D and IFN-γ seemed not to be implicated.
Our results provide insight into how VLPs interact with innate immune cells and how NK cells and DCs play a role in the immune response induced by this vaccine agent.