Tumour immune evasion is promoted by actin cytoskeleton-driven polarization of inhibitory signals to the immunological synapse

Natural killer (NK) cells are innate immune cells that are the first line of defence against infection and malignant transformation. They have the ability to recognize and destroy virally infected or cancerous cells without the need for priming or activation and therefore represent a promising target for new immunotherapeutic approaches against cancer. For their anti-tumour function, NK cells rely on actin cytoskeleton remodelling, in particular during the formation of the lytic immunological synapse (IS) with prospective target cells. The IS is characterized by an extensive assembly of filamentous actin (F-actin) and polarization of the NK cell for directed delivery of lytic granules. However, the IS allows bi-directional exchange of information, and anti-tumour effector functions of NK cells are often impaired through inhibitory signals that are transmitted through killer immunoglobulin-like receptors (KIRs) or the CD94/NK group 2 member A (NKG2A) heterodimeric receptor. Moreover, resistant tumour cells can polarize their own actin cytoskeleton to the IS in a process called actin response, enforcing the formation of an evasion IS or actin response-IS (AR-IS).
This PhD thesis focuses on the evaluation of associated resistance mechanisms that occur at the AR-IS, the conservation of these processes and their translation to in vivo models of cancer. For this purpose, individual cancer-NK cell conjugates were analysed by high-throughput imaging flow cytometry (IFC) to investigate the accumulation of F-actin at the IS and the distribution of inhibitory and activating ligands in relation to the IS. The AR was associated with a wider synaptic cleft, prominent recruitment of predominantly inhibitory ligands and inhibition of NK cell-induced target cell death. Further, the AR could also be applied to cytotoxic T lymphocyte (CTL) attack during antigen specific IS formation. To explore the in vivo relevance of the AR, modified murine cancer cell lines were established that allowed for the evaluation of impact of the AR on tumour progression, the tumour immune landscape, immune cell activation and exhaustion. Reduction of the actin response was associated with a reduction of tumour volume, enhanced infiltration of CTLs and NK cells and higher numbers of effector T cells.
In summary, this study reports a novel, highly conserved immune-escape mechanism that exploits fast remodelling of the actin cytoskeleton of cancer cells to induce clustering of inhibitory ligands at the AR-IS and prevent NK cell activation. This AR is characterized by fine synaptic filopodia-like protrusions (SFPs) that are decorated with inhibitory ligands and probe the surface of the NK cell in addition to providing a steric hindrance for NK cell attachment to target cells. In our pre-clinical mouse model, we could demonstrate that a reduction of the AR results in the restoration of anti-tumour immunity. Understanding the mechanism that enables or initiates the AR and finding new ways to target this mechanism has the potential to improve cancer immunotherapy, especially for NK cell-based approaches.