Not so long ago
In the early 1970s, researchers observed natural cytotoxicity towards target cells, in non-immunized mice and humans. Once the involvement of macrophages and T and B cells had been excluded, there was only one possible explanation left: the existence of a new cytotoxic cell type. Natural Killer (NK) cells are large granular lymphocytes and have been described as non-B, non-T cells that can rapidly produce interferon (1). NK cells develop in the bone marrow from hematopoietic precursor cells and are indispensable components of the immune system for sensing and destroying virus-infected and malignantly transformed cells (2), but also for eliminating other types of infections (bacteria, fungi, protozoa). A critical feature of the mechanism by which NKs recognize cells to be eliminated was discovered a few years later.
“I‘m missing something”
In 1981 Klas Kärre, a PhD candidate at the time, presented his thesis on the missing self-recognition hypothesis. The idea he proposed was that NK cells recognize the absence, rather than presence, of information in the target cells; in this case, a functional Major Histocompatibility Complex I (MHC I) identical to that of the host (3). This concept was groundbreaking, at a time where molecular recognition was thought to be strictly based on the presence of a pattern, as in the case of T cells who will attack target cells based on the peptides displayed on their surface by MHC I. The formulation of this hypothesis was based on the observation of hybrid resistance to different kinds of grafts (tumor or bone marrow), where rejection was based on the absence of MHC I. The validation of the hypothesis came when mice, inoculated with wild-type vs MHC I-deficient tumor cells, developed tumors, while the MHC I-deficient cells appeared to be less malignant. In the latter case, NK cells took care of business and destroyed the cancerous cells not expressing MHC I (3).
A delicate equilibrium
The activation of NK cells is based on an equilibrium between activating and inhibiting receptors. Different signals (for example inflammatory) can tip the balance one way or the other (1, 2). Inhibitory interactions with native (aka ‘self’) MHC I result in a predictable and quantitative balance between self-recognition and effector potential, a process called NK cell education (4, 5). Importantly, the education of NK cells is what prevents them from engaging in auto-aggression, meaning reacting against themselves (4–6). The exact mechanisms regulating NK education are not understood yet. Moreover, single receptor-ligand interactions are often not sufficient for mediating activation or inhibition of NK cells, hence, a synergistic effect of receptor signaling needs to be considered.
Licence to kill
NKs have a lethal toolkit composed of secreted cytotoxic proteins: perforins and granzymes, that they use for lysing their target cells (6). Interestingly, these cytotoxic molecules do not lyse the NKs themselves whilst being inside the NK cells, thanks to being packaged in granules containing a neutralizing agent called serglycine (7). The formation of the immunological synapse results in reorganization of the actin cytoskeleton of the NKs that allows the lytic granules to travel towards the NK cell membrane with which they fuse and are released to the synapse (8). The granzymes, proteases for lysing the target cell, are delivered to that cell by the perforins which, as you already suspect, perforate the target cell membrane by forming pores (8, 9). Once in the target cell, granzymes will activate the apoptotic pathway, the natural death mechanism of the target cell (8, 10). An alternative mode of action of NKs is the activation of the ‘death receptor’ mechanism of the target cell. In this case, NKs present on their surface ligands that will activate death receptors of the target cells and drive apoptosis through the formation of the ‘death-inducing signaling complex’ (10). The two death pathways function in a complementary mode: perforin and granzyme killing is acute, while the death receptor mechanism is slower and is thought to take place once the NK cell has ‘exhausted’ the perforin/granzyme mechanism (2, 8). After finishing the ‘job’, the NK cell detaches from the former target and proceeds with more killings of neighboring cells, a process referred to as serial killing (8).
The cytotoxicity of NKs needs to be directed towards target cells, to ensure healthy cells are not destroyed. The directional secretion of lytic enzymes is ensured by the formation of an immunological synapse (11); a zone defined by contacts between the NK and the target cell, in which the transmission of molecular signals flows directionally from NKs to its target (2). Interestingly, NKs can switch secretion modes between directional and non-directional, depending on the objective: the directional mode targets specific cells to be executed by lytic granules and the non-directional mode, as is for example the secretion of cytokines, aims at recruiting other immune cells for activating the innate and adaptive immune response.
Lethal weapons for a good purpose
Despite their classification as innate immune cells, in the recent decades NKs have been found to develop memory-like properties after virus infection, exposure to certain molecules (e.g. haptens) or to proinflammatory cytokines (12). Acquiring immunological memory, a feature that is classically reserved to cells of the adaptive immune system, means that NKs are now primed for future response of the immune system (5). This feature of NK cells, together with their direct cytotoxicity and their capacity of immune cell mobilization, makes them attractive agents for cancer immunotherapy. As an example illustrating the NK potential in cancer therapy, studies used NK cells that had developed memory against leukemia cell lines in vitro, for treating myeloid leukemia targets in vitro and in vivo, obtaining promising results (13). Cancer immunotherapy research also focuses on identifying the factors that influence the balance between NK activation and inactivation, as a strategy for using NKs against certain tumors (8, 14). Taken together, NKs are a potent weapon that will continue to present opportunities for future applications in cancer treatment and immunotherapy.
Recognizing and appreciating the labs working in this space:
- NK Cell Recognition- Klas Kärre Group, Karolinska Institutet, Stockholm, Sweden, https://ki.se/en/research/groups/nk-cell-recognition-klas-karre-group.
- Daniel Davis Group, The Lydia Becker Institute of Immunology and Inflammation, University of Manchester, Manchester, UK, https://research.manchester.ac.uk/en/persons/daniel.davis/projects/, X: @dandavis101
- Emily Mace Group, Columbia University Irving Medical Center, Vagelos College of Physicians and Surgeons, New York, USA https://www.vagelos.columbia.edu/profile/emily-m-mace-phd#research, https://www.macelab.nyc/, X: @mace_em
- Malmberg Lab, Karolinska Institutet, Stockholm, Sweden, https://ki.se/en/research/groups/natural-killer-cell-biology-and-cell-therapy-kalle-malmberg-group, https://malmberglab.com/, X: @MalmbergLab
- Strominger Lab, Harvard University, Department of Stem Cell and Regenerative Biology, Cambridge, Massachusetts, USA, https://hscrb.harvard.edu/labs/strominger-lab/
- Whisstock Lab, Monash University, Monash Biomedicine Discovery Institute, Melbourne, Australia, https://www.monash.edu/discovery-institute/whisstock-lab/research
- Prof. Dr. Carsten Watzl Group, Dortmund University, Leibniz Research Center for Working Environment and Human Factors, Dortmund, Germany, https://www.ifado.de/en/research/immunology/team/carsten-watzl
- Prof. Björn Önfelt, KTH Royal Institute of Technology, Stockholm, Sweden https://www.kth.se/profile/onfelt, https://onfeltlab.com/ourlab, X: @OnfeltLab
- The Katharine Hsu Lab, Memorial Sloan Kettering Cancer Center, New York, USA, https://www.mskcc.org/research-areas/labs/katharine-hsu, X: @KathyHsuLab
- Fehniger Lab, NK Cell Research-Basic Immunobiology, Washington University in St Louis, St. Louis, Missouri, USA, https://www.fehnigerlab.org/research/nk-cell-research/, X: @LabFehniger
References
- Basílio-Queirós, Débora, and Eva Mischak-Weissinger. “Natural killer cells- from innate cells to the discovery of adaptability.” Frontiers in immunology vol. 14 1172437. 18 May. 2023, doi:10.3389/fimmu.2023.1172437
- Mace, Emily M. “Human natural killer cells: Form, function, and development.” The Journal of allergy and clinical immunology vol. 151,2 (2023): 371–385. doi:10.1016/j.jaci.2022.09.022
- Kärre, K. “NK cells, MHC class I molecules and the missing self.” Scandinavian journal of immunology vol. 55,3 (2002): 221–8. doi:10.1046/j.1365–3083.2002.01053.x
- Anfossi, Nicolas et al. “Human NK cell education by inhibitory receptors for MHC class I.” Immunity vol. 25,2 (2006): 331–42. doi:10.1016/j.immuni.2006.06.013
- Goodridge, Jodie P et al. “Remodeling of secretory lysosomes during education tunes functional potential in NK cells.” Nature communications vol. 10,1 514. 31 Jan. 2019, doi:10.1038/s41467–019–08384-x
- Ambrose, Ashley R et al. “Synaptic secretion from human natural killer cells is diverse and includes supramolecular attack particles.” Proceedings of the National Academy of Sciences of the United States of America vol. 117,38 (2020): 23717–23720. doi:10.1073/pnas.2010274117
- Metkar, Sunil S et al. “Cytotoxic cell granule-mediated apoptosis: perforin delivers granzyme B-serglycin complexes into target cells without plasma membrane pore formation.” Immunity vol. 16,3 (2002): 417–28. doi:10.1016/s1074–7613(02)00286–8
- Prager, Isabel et al. “NK cells switch from granzyme B to death receptor-mediated cytotoxicity during serial killing.” The Journal of experimental medicine vol. 216,9 (2019): 2113–2127. doi:10.1084/jem.20181454
- Law, Ruby H P et al. “The structural basis for membrane binding and pore formation by lymphocyte perforin.” Nature vol. 468,7322 (2010): 447–51. doi:10.1038/nature09518
- Medema, J P et al. “FLICE is activated by association with the CD95 death-inducing signaling complex (DISC).” The EMBO journal vol. 16,10 (1997): 2794–804. doi:10.1093/emboj/16.10.279
- Davis, D M et al. “The human natural killer cell immune synapse.” Proceedings of the National Academy of Sciences of the United States of America vol. 96,26 (1999): 15062–7. doi:10.1073/pnas.96.26.15062
- O’Sullivan, Timothy E et al. “Natural Killer Cell Memory.” Immunity vol. 43,4 (2015): 634–45. doi:10.1016/j.immuni.2015.09.013
- Gang, Margery et al. “Memory-like natural killer cells for cancer immunotherapy.” Seminars in hematology vol. 57,4 (2020): 185–193. doi:10.1053/j.seminhematol.2020.11.003
- Bhat, Rauf, and Carsten Watzl. “Serial killing of tumor cells by human natural killer cells — enhancement by therapeutic antibodies.” PloS one vol. 2,3 e326. 28 Mar. 2007, doi:10.1371/journal.pone.0000326
About the author:
DR. SEMELI PLATSAKI
Content Editor The League of Extraordinary Cell Types, Sci-Illustrate Stories
Semeli is a biochemist at heart, holding a degree in Chemistry and a PhD in protein biochemistry. After working as a researcher studying the structure-function relationship of protein in a range of biological contexts, from bacterial metalloproteins to synaptic signaling, Semeli moved on to a role in Scientific communication and project management in the European Virus Archive, a collection of virus and virus-derived resources available to researchers worldwide. Semeli is passionate about the creative mix of art, words and science, one of the best ways to make Science impactful.
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NELLY AGHEKYAN
Contributing Artist The League of Extraordinary Cell Types, Sci-Illustrate Stories
Nelli Aghekyan, did a bachelor’s and master’s in Architecture in Armenia, after studying architecture and interior design for 6 years, she concentrated on her drawing skills and continued her path in the illustration world. She works mainly on children’s book illustrations, some of her books are now being published. Currently living in Italy, she works as a full-time freelance artist, collaborating with different companies and clients.
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DR. EMANUELE PETRETTO
Animator The League of Extraordinary Cell Types, Sci-Illustrate Stories
Dr. Petretto received his Ph.D. in Biochemistry at the University of Fribourg, Switzerland, focusing on the behavior of matter at nanoscopic scales and the stability of colloidal systems. Using molecular dynamics simulations, he explored the delicate interaction among particles, interfaces, and solvents.
Currently, he is fully pursuing another delicate interaction: the intricate interplay between art and science. Through data visualization, motion design, and games, he wants to show the wonders of the complexity surrounding us.
About the series:
The League of Extraordinary Cell types
The team at Sci-Illustrate and Endosymbiont bring to you an exciting series where we dive deep into the wondrous cell types in our body, that make our hearts tick ❤.
