Using Super-Resolution Microscopy to Watch Immune Cells Kill

Human Natural Killer (NK) cells can directly kill diseased cells by secretion of cytolytic granules across an immune synapse. The molecular choreography that leads to assembly of the synapse and the secretion of granules has widely been studied. However, a long-standing gap in our understanding of this process is how disassembly of the synapse occurs, allowing immune cells to dissociate from target cells. We report that shedding of an activating receptor increased NK cell motility and facilitated detachment of NK cells from target cells. Disassembly of the immune synapse caused by receptor shedding aided NK cell survival and boosted serial engagement of target cells. Thus, counter-intuitively, shedding of receptors can positively impact immune responses, by allowing immune cells to sequentially move from one cell to another. In a separate line of research, using super-resolution microscopy, we have found that inhibitory Killer Ig-like receptors (KIR) encoded by different genes and alleles organise differently at the surface of primary human NK cells. KIR which are expressed at a low level at the cell surface assemble in smaller clusters than KIR which are highly expressed. Upon receptor triggering, lowly expressed receptors generate more phosphorylated Crk than highly expressed receptors. Thus, genetic variation modulates the nanoscale organisation of inhibitory KIR, which in turn impacts receptor signalling. This identifies a new way in which genetic diversity can impact immune responses. Finally, NK cells contribute to immune surveillance against cancer, but the impact of commonly used treatments on their activity is poorly understood. Radiotherapy, one of the most broadly used treatments for solid tumours, is known to influence many aspects of anti-tumour immunity, but little is still known about its effect on the interaction between NK cells and cancer cells. Surprisingly, treatment with three doses of 8Gy radiation on consecutive days reduced NK cell cytotoxicity dramatically. Moreover, irradiated cancer cells were found to require a 2-4-fold greater concentration of purified perforin protein to induce lysis. Thus, radiotherapy induces a profound reduction in the susceptibility of cancer cells to NK cell cytotoxicity, which is important to consider for cancer patients treated with a combination of radiotherapy and immunotherapy.