The ability of natural killer (NK) cells to survive their own toxic enzymes has long puzzled scientists. A new study has found the answer in a lipid ‘shield’ that not only protects NK cells but cancer cells too. Their findings, published in PLOS Biology, have highlighted a potential future target for cancer treatments.
Natural killer cells
NK cells are cytotoxic effector cells fundamental to human health. They contain granules packed full of lytic molecules that are utilised against diseased cells. The contents of the granules are secreted into the lytic immunological synapse, a specialised interface formed between the NK cell and its target. This process, called degranulation, induces death in the target cell.
A key molecule found in lytic granules is perforin, a protein that forms pores in the membrane of target cells. These pores allow for the uptake of proapoptotic serine proteases, which result in the target’s death. Perforin is a key requirement for NK cell cytotoxicity.
Unidirectional killing
Perforin is able to penetrate the target cell membrane through hydrophobic interaction. The interaction is nonspecific, so in theory, the NK cell membrane should be just as vulnerable to perforin as the target. Despite this, NK cells are not generally susceptible to the lytic molecules they release. This unidirectional killing is necessary for the immune system to preserve NK cells. However, the mechanism by which NK cells are able to survive and not target cells has proved elusive – until now.
To uncover the answer, scientists at Columbia University have investigated NK cell membranes for properties that might make them more resistant to perforin upon degranulation.
Lipid packed membranes
The hydrophobic interactions between perforin and the target cell are charge sensitive. The fusion of lipid rafts can greatly affect charge distribution. Therefore, in this study the researchers searched for such features in NK cell membranes.
Lipid packing density of the entire NK cell plasma membrane was measured. This was then compared to the membrane site where contact with the target cells occurred, called the presynaptic membrane. It was found that presynaptic membranes contained more densely packed lipids than the rest of the plasma membrane.
Next, the researchers asked if the observed lipid density served a role in NK cell survival. Lipid packing was disrupted using 7-ketocholesterol (7KC). It was found that 7KC pre-treatment of NK cells did not compromise the destruction of target cells, but it did lead to an increase in NK cell death upon degranulation. This suggests that high lipid density provides NK cells with protection against lytic molecules.
Lipid rafts block perforin binding
The binding of perforin to membranes with differing lipid packing densities was then measured, to discover if it was the lipids that underlied protection against this specific molecule. Liposomes with more fluid and disordered membranes exhibited higher perforin binding. Conversely, more highly ordered membranes tended to be resistant to perforin binding. This suggests that the mechanism of NK cell protection from perforin is likely due to the densely packed presynaptic membranes.
This is further supported by the finding that lytic granule membranes had an even higher lipid order than NK cell presynaptic membranes. It was observed that when lytic granules fused with the NK cell membrane, the higher lipid density was transferred into the presynaptic membrane. This rapid rise of lipid order at fusion sites, where perforin concentration is the highest, provides the NK cell with additional protection.
Reversing natural killer cell protection in cancer cells
NK cells are not the only cells that can survive attack from their lytic molecules. Cells from the breast cancer cell line MB-231 are also known to evade NK cell killing, making it a particularly aggressive cancer. The study evaluated the possibility that this evasion is due to high lipid density preventing perforin binding.
It was found that cancer cell postsynaptic membranes have a similar level of lipid order to that of NK cell presynaptic membranes. This appeared to be characteristic of the MB-231 cells, suggesting it is the lipid density that confers NK cell resistance.
Excitingly, when pre-treated with 7KC the MB-231 cells had disrupted postsynaptic membranes. The loss of lipid shields increased the MB-231 cells’ susceptibility to NK cells. This suggests a future direction for treatments against this cancer.
Conclusion and future work
Until now, it was unknown how NK cells managed to not kill themselves. Previously, protective proteins were proposed, but this study confirms that lipids are the answer! The identified mechanisms of protection can be utilised in understanding how other cells may be resistant to NK cell killing.
“We don’t know yet if this is a general mechanism by which cancer cells resist natural killer cells,” co-author Yu Li said. “If it is generalisable, we can start to think of therapies that disrupt the tumour cell membrane and make it more susceptible to attack by the immune system.”
