When they recognize a target cell, natural killer (NK) cells mount an attack to kill the target by exerting their cytotoxicity via the exocytosis of cytotoxic granules. Although the details of this process (which includes the movement of cytotoxic granules in the immune synapse and their fusion with the plasma membrane, releasing granzymes and perforin into the synaptic cleft) are relatively better understood, the post-exocytosis regulation of the process is still largely unknown. Here we show that a clathrin-dependent endocytosis stimulated by target cell occurs in NK92 cell line, which is closely correlated with granzyme B recovery. Inhibition of the endocytosis significantly attenuates the cytotoxicity of NK92 cells. The NK cell recovery of its released effector molecules, in turn, suggests that endocytosis may well play a key role in the post exocytosis regulation of immune cells.
"Indeed, a bidirectional trafficking of lytic granule proteins exposed at the plasma membrane on degranulation has been demonstrated; both lysosome-associated membrane protein-1 (LAMP-1, also known as CD107a) (84) and Munc13-4 (83) undergo a rapid endocytosis, leading to the hypothesis that granule exocytosis is coordinated with the retrieval of cytolytic machinery components. Additionally, cytolytic mediators are also recaptured into early endosomes of NK cells via a clathrin-dependent route after target cell stimulation thus contributing to the cytolytic potential (85). "
[Show abstract][Hide abstract] ABSTRACT: Target cell recognition by cytotoxic lymphocytes implies the simultaneous engagement and clustering of adhesion and activating receptors followed by the activation of an array of signal transduction pathways. The cytotoxic immune synapse represents the highly specialized dynamic interface formed between the cytolytic effector and its target that allows temporal and spatial integration of signals responsible for a defined sequence of processes culminating with the polarized secretion of lytic granules. Over the last decades, much attention has been given to the molecular signals coupling receptor ligation to the activation of cytolytic machinery. Moreover, in the last 10 years the discovery of genetic defects affecting cytotoxic responses greatly boosted our knowledge on the molecular effectors involved in the regulation of discrete phases of cytotoxic process at post-receptor levels. More recently, the use of super resolution and total internal reflection fluorescence imaging technologies added new insights on the dynamic reorganization of receptor and signaling molecules at lytic synapse as well as on the relationship between granule dynamics and cytoskeleton remodeling. To date we have a solid knowledge of the molecular mechanisms governing granule movement and secretion, being not yet fully unraveled the machinery that couples early receptor signaling to the late stage of synapse remodeling and granule dynamics. Here we highlight recent advances in our understanding of the molecular mechanisms acting in the activation of cytolytic machinery, also discussing similarities and differences between Natural killer cells and cytotoxic CD8(+) T cells.
Frontiers in Immunology 11/2013; 4:390. DOI:10.3389/fimmu.2013.00390
"In long-term cytotoxicity assays, increased loss of Pfn in NK cells resulted in impaired killing ability. In the process of reuse, the pathway of retrieved Pfn going to lytic granule could be similar to that of granzyme B recycling, from early endosome to LAMP (lysosome-associated membrane protein) 1 + granules (Li et al., 2010) that might take part in an unusual rapid and dramatic de novo formation of nascent secretory lysosomes from as yet unidentified compartments in NK cells after target-cell recognition (Liu et al., 2005). In this report, it was suggested that the nascent secretory lysosomes came from the cytoplasm instead of budding from the trans-Golgi network. "
[Show abstract][Hide abstract] ABSTRACT: When encountering target cells, NK (natural killer) cells exocytose Pfn (perforin) and granzyme B to kill challengers. We previously reported that granzyme B is recycled and reused by NK cells via clathrin-dependent endocytosis. However, whether Pfn, a main secretory vesicle content, indispensible to granzyme B killing, undergoes endocytosis remains unknown. We demonstrate that Pfn is recaptured by early endosomes of NK cells via a clathrin-dependent endocytosis after target cell stimulation. Inhibition of clathrin-dependent endocytosis significantly attenuated the cytotoxicity of NK cells. The data suggest that the recovery of Pfn contributes to the cytotoxicity of NK cells. The assay of endocytosis of lytic molecule presents a particular focus for exploring the mechanism of abnormal cytotoxicity of NK cells.
Cell Biology International 02/2012; 36(2):223-8. DOI:10.1042/CBI20110242 · 1.93 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Natural killer (NK) cells play critical roles in defense against tumors and viral infections. They exert their cytotoxic functions through the secretion of granules containing cytotoxic molecules, such as perforin and granzymes. These cytotoxic molecules are stored within dual-functional organelles, known as secretory lysosomes. Target cell recognition induces the formation of an "immunological synapse", between the NK cell and its target, into which cytotoxic granules release their contents. However the post-exocytosis regulation of the process is still largely unknown. Recent research and the data accumulated therefrom lead to new hypotheses that suggest that, not unlike synaptic vesicle recycling in neuronal terminals, NK cells also recycle not just their secretory lysosome membranes but their correlated cytotoxic molecules (perforin and granzymes). The newly endocytosed vesicles are used to replenish the "reserve pool" of vesicles for continued NK cell serial killings. These hypotheses, if proved to be correct, will significantly improve our understanding of NK cell cytotoxicity mechanisms and might even suggest new NK cell-based therapies that rely on NK serial killing abilities for overcoming tumors.
Medical Hypotheses 11/2010; 76(2):293-5. DOI:10.1016/j.mehy.2010.10.027 · 1.07 Impact Factor
Data provided are for informational purposes only. Although carefully collected, accuracy cannot be guaranteed. The impact factor represents a rough estimation of the journal's impact factor and does not reflect the actual current impact factor. Publisher conditions are provided by RoMEO. Differing provisions from the publisher's actual policy or licence agreement may be applicable.