Natural killer (NK) cells are essential in tumor monitoring and defense against the virus-infected cells. They are valuable for eliminating the intracellular pathogens that have evaded recognition by T cells. Unlike T cells, NK cells don’t require activation and prior sensitization to the pathogen to mount an immune response. Their role has been evident in tumors, viral infections, autoimmunity, and inflammatory disorders. Therefore, different aspects of NK cells are currently under investigation for the development of novel therapies.
Natural Killer (NK) Cells
NK cells belong to the innate arm of the immune system. However, their development from the common progenitor of NK cells and T cells has also categorized them as lymphocytes. Therefore, Natural Killer cells are also referred to as cytotoxic lymphocytes of the innate immune system. Their target cells include stressed cells which are infected or transformed into tumors. They differentiate from the hematopoietic stem cells (HSCs) in bone marrow and can mature in bone marrow or secondary lymphoid tissues. NK cells can also be found and extracted from the peripheral blood of the patients. NK cells constitute almost 2-18% of lymphocytes in peripheral blood.
NK Cell Subsets
NK cells are large granular leukocytes defined by the positive expression of CD56 and CD16 and the negative expression of CD3 and T cell receptor (TCR). NK cells are broadly classified into two subsets based on the expression of CD56 marker- CD56bright and CD56dim subsets. CD56dim cells also express CD16 and contain perforin, required for cytotoxic activity. CD56bright cells produce cytokines and lack perforin and CD16 expression. CD56dim subset constitute 90% of peripheral blood Natural Killer cells. Presently, several more subpopulations have also been determined. For example, CD56negative subpopulation present during HIV and Hepatitis B virus infection and the tissue-resident NK cell subsets in the lungs and liver. The tissue-resident cells have a differential set of identification markers like CD103, CD69, and CD49a. These tissue-resident NK cells also differ in phenotype and function from the NK cells in circulation.
Memory NK Cells
Just like adaptive immune cells, NK cells also possess immunological memory. Memory NK cells are primed for an antigen and exhibit a long survival period. NKG2C+ cells are an example of memory NK cells. Interestingly, not only antigens but even the cytokines can induce the immunological memory in NK cells. The research has indicated that cytokine-induced memory NK cells have higher reactivation potential and respond to multiple stimuli in comparison to viral or hapten-induced memory cells.
Recognition of Target Cells
NK cells have a distinct process of recognizing their target. T cells identify the antigen when presented via antigen-presenting cells, whereas other immune cells recognize the evolutionarily conserved regions on pathogen. However, NK cells have the unique ability to identify the target cells by the levels of self-molecules. Two different receptors, inhibitory receptors and activating receptors, tightly regulate the target cell recognition and the subsequent lysis of the abnormal cells.
Inhibitory Receptors
The healthy cells have a constitutive expression of certain self-molecules like major histocompatibility complex (MHC) I. The inhibitory receptors sense the levels of these molecules. The interaction of receptors and self-molecules signals the immunoreceptor tyrosine inhibitory motif (ITIM) of the receptor to activate phosphatases SHP1 and SHP2. They suppress the kinase-based activating signals, thus sending an inhibitory signal to the cell lysis process (Fig 1A). As a result, NK cells disengage with the healthy cells. However, NK cells lose the inhibitory signal when these receptors sense the downregulation of self-molecules on the target cells. Killer cells immunoglobulin-like receptors (KIR), NKG2A (C-type lectin receptors), and leukocyte inhibitory receptor 1 (LIR1) are a few examples of these receptors. KIR recognizes MHC I on cells.
The Activating Receptors
Abnormal cells, including infected cells and tumor cells, evade T cell recognition by downregulating MHC I expression, but Natural Killer cells recognize the downregulation. This is referred to as induced self-recognition. The loss of inhibitory signals due to sub-threshold interaction with the self-molecules on the target cells prevents the suppression of activating receptors, which, in turn, stimulates the cell lysis process (Fig 1B). This is also known as the missing-self hypothesis. Several activating receptors such as natural cytotoxicity receptors (NCRs), NK2D, CD16, etc., are present on NK cells. Research has suggested that NK cells require more than one activation signal to initiate cell lysis, except for CD16 receptor signaling.
Immune Response
NK cells mediate immune response by two different manners (Fig 1B):
Cytotoxicity: After activation, NK cells release granules containing perforin, which disrupts the membrane, and granzymes (proteases) in the direction of the target cells. Together they induce the apoptosis pathways in the target cells. Additionally, NK cells also carry Fas ligand (FasL) and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) which have their counter receptors on the target cells. Their interaction results in caspase-dependent apoptosis.
Effector functions: NK cells also demonstrate effector functions by secreting cytokines such as, IFNγ, TNFα, IL10, etc. These cytokines assist NK cells to recruit more immune cells to the infection site and upregulate MHC I expression on target cells for the activation of T cells.
Antibody-dependent Cellular Cytotoxicity (ADCC)
ADCC entails the bridging of pathogen or pathogen-infected cells and effector cells by an antibody. The Fab fragment of the antibody binds to the antigen, while its Fc fragment interacts with immune cells. These immune cells express receptors for the Fc region of antibody and include Nk cells, macrophages, dendritic cells, eosinophils, etc. NK cells participate as effector cells in ADCC via its activating receptor FcγRIII receptor (CD16). After binding, NK cells initiate the lysis of the target cells (Fig 1C).
Clinical Implications
As innate immune cells, NK cells provide the first line of defense against pathogens that invade the cells. The studies on murine models demonstrated that NK cell depletion or deficiency increases the susceptibility towards tumors and viral infections, particularly herpes virus. Scientists have also correlated low NK cell activity with increased risk of cancer. A research study even showed that a mismatch in KIR between donor and recipient during transplantation could eliminate leukemia, graft vs. host disease, and immune rejection of the graft.
CAR NK Cell Therapy
Similar to chimeric antigen receptor (CAR) T cell therapy, scientists have altered the receptors of NK cells to formulate CAR NK cell therapy against cancer cells. The modified receptor combines the intracellular region of NK cells and the extracellular region of the scFv fragment, which is an antigen-binding domain from antibody. NK cells don’t need prior sensitization to an antigen like that in CAR T cell therapy, thus enabling the NK cell preparation in advance. They also have a lower risk of immune rejection that allows their extraction from different sources and tissues. The initial results with CAR NK cell therapy have been promising in regards to safety and efficacy. Research has been continuing in this direction to improve the structure of modified receptors and to evade the suppression of NK cell activity by tumor microenvironment (TME).
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Conclusion
NK cells are the most distinct cells of the immune system for recognizing the levels of self-molecules on cells instead of that of the pathogen. NK cells don’t require activation, and therefore, they can mount an immune response rapidly without prior stimulation. Scientists have been harnessing these abilities for therapeutic interventions. The ongoing research has also been revealing the involvement of NK cells in various disorders. Kosheeka offers NK cells derived from the bone marrow and peripheral blood of different species. Our team performs robust tests on these cells to ensure their purity, sterility, viability, and functionality.
FAQs
Q: How do natural killer (NK) cells recognize the target?
NK cells sense the levels of self-molecules like MHC I on the cells via inhibitory receptors. Nk cells interact with other cells via their receptors. The suitable levels of these molecules on healthy cells initiate inhibitory signals to prevent the NK cell cytotoxicity. But intracellular pathogens and tumor cells downregulate these molecules, thus the low levels of self-molecules on the cells lead to the loss of inhibitory signals. The activating signals are no more suppressed, resulting in the NK cell cytotoxicity.
Q: What is the difference between NK cells and T cells?
NK cells do not possess the antigen specificity of T cells. Unlike T cells, NK cells do not require prior interaction with antigen. They don’t recognize antigenic molecules like T cells, instead NK cells identify the downregulation of constitutive markers. The immune response of NK cells is more rapid than T cells as they don’t require activation.
Q: What are the target cells for NK cells?
NK cells act on the cells infected with pathogens and also on other abnormal cells like tumor cells. These cells downregulate the MHC I molecules to evade T cell response. However, NK cells identify the downregulated molecules and mount an appropriate immune response.
Q: How do NK cells act on the target cells?
NK cells demonstrate cytotoxicity and effector functions against the target cells. They destroy the target cells via enzyme and ligand-mediated pathways. Additionally, they also secrete cytokines to activate other immune cells.
