How natural killer cells avoid self-destruction when killing their targets

How cytotoxic lymphocytes are protected against their own weapons during close combat with diseased target cells is an important and long-standing question in immunology. A study in this issue provides new insights into the mechanisms by which natural killer (NK) cells avoid self-destruction.

. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests:
The authors have declared that no competing interests exist.
lipid packing density at the presynaptic membrane (i.e., the immune cell side of the immunological synapse) and that interfering with this process by pharmacological disruption of lipid packing is sufficient to make NK cells susceptible to their own cytotoxicity. Using a variety of complementary cell-based and synthetic cell-free assays, Li and Orange provide compelling evidence that densely packed lipid protects the presynaptic membrane from perforation by inhibiting binding of perforin. These data confirm and extend a recent study on CTLs [4] and suggest that assembly of high lipid-order presynaptic membrane is a universal mechanism by which cytotoxic lymphocytes avoid autolysis. A further novelty of Li and Orange investigations lies in the characterization of an extra layer of protection that is dependent on the degranulation process. First, they demonstrate that lytic granules are characterized by remarkably densely packed membranes with higher lipid order as compared to the NK cell presynaptic membrane. Using total internal reflection fluorescence (TIRF) microscopy, a method of choice to image dynamic processes near the cell membrane, they show that degranulation events correlate in time and space with local increase of lipid ordering, suggesting a model where cytotoxic granule fusion with the presynaptic membrane provides enhanced protection at the specific sites where perforin is secreted (Fig 1).
Tumors have evolved multiple immune evasion strategies, which are largely based on preexisting mechanisms used by healthy cells, e.g., to avoid autoimmune reactions during an acute immune response. In addition, impaired binding of perforin on the tumor cell surface has previously been reported as a potential mechanism of resistance to NK cell-mediated cytotoxicity [5]. Accordingly, Li and Orange explored lipid packing density of cancer cell

NK cells assemble a perforin-resistant lipid shield at the immunological synapse.
During directional killing of virus-infected and transformed cells, NK cells establish an immunological synapse to which lytic granules are recruited before they fuse with the presynaptic membrane and release their cytotoxic content, including perforin and death-inducing granzymes, into the synaptic cleft. While perforin is inactive inside lytic granules (due to low pH and calcium concentration), it becomes fully operational in the extracellular environment where it oligomerizes in a calcium-dependent manner and creates pores into the target cell membrane. DAU : PleasecheckwhethertheeditstothesentenceDuri uring target cell engagement, NK cells rapidly assemble a highly lipid-ordered presynaptic membrane, resulting from lipid raft aggregation that potently inhibits perforin membrane binding. Moreover, upon fusion with the NK cell plasma membrane, lytic granules, which have an intrinsically highly ordered membrane, further increase lipid packing density at the degranulation sites and thereby confer an additional layer of protection where the concentration of secreted perforin presumably peaks. NAU : Anabbrevia K, natural killer.
https://doi.org/10.1371/journal.pbio.3001339.g001 membranes during NK cell attack. Very interestingly, they found that highly resistant breast cancer cells accumulate densely packed membranes at the immunological synapse, while this was not observed with more susceptible breast cancer cells. Remarkably, abrogation of postsynaptic lipid membrane packing increased the susceptibility of previously resistant breast cancer cells to NK cell-mediated killing. Therefore, Li and Orange's study does not only expand our knowledge of immune cell biology but also contribute to a better understanding of cancer cell resistance to cytotoxic lymphocyte-mediated killing. These exciting findings lay the groundwork for future investigations and open up important questions. What is the origin of the high-order lipid membranes used to assemble the synaptic shield in cancer cells? How are these membranes recruited to synaptic region? Answering these questions could lead to the development of novel approaches to restore a potent antitumor immune response or improve the efficacy of existing immunotherapies. Recently, fast and prominent accumulation of actin filaments underneath the postsynaptic membrane has been shown to closely correlate with cancer cell resistance to NK cell-mediated lysis [6,7]. Such cytoskeletal changes likely translate into modification of membrane and vesicle trafficking, and, ultimately, result in alteration of the postsynaptic membrane composition, e.g., by promoting lipid raft aggregation and/or recruitment and fusion of vesicles. FAU : PleasecheckwhethertheeditstothesentenceFutureresearchshou uture research should further explore lipid packing at the postsynaptic membrane in relation to actin and membrane and vesicle dynamics.