Natural killer cell

Natural killer (NK) cells are large lymphoid cells with numerous cytoplasmic granules. They are also referred to as large granular lymphocytes. Natural killer cells, like other lymphocyte populations, develop from CD34+ hematopoietic stem cells (HSC), although the site(s) of their maturation (in the bone marrow and/or thymus) and the details of the maturation process are not yet fully understood.

For example, CD56bright NK cells were isolated from lymph nodes and secondary lymphoid tissues, whereas CD56dim NK cells were more abundant in bone marrow, blood and spleen (Fehniger et al. 2003).

NK cell differentiation is associated with the fixed and ordered presentation of NK cell receptors, with CD161, NKp44 and CD171 (c-Kit or stem cell factor receptor) being found on immature NK cells (iNK). As differentiation into mature cytotoxic NK-cells (mNK) progresses, including CD56bright and CD56dim NK-cells, CD117 expression decreases and the receptors NKp46, CD94/NKG2A and NKG2D appear on the cell surface. NKG2C, KIRs, and CD16 appear relatively late in the development of CD56-positive NK cells (Caligiuri 2008).

Once mature, NK cells circulate in the bloodstream. They account for approximately 5-10% of the circulating lymphocyte population. NK cells are larger than B and T lymphocytes and, unlike T and B lymphocytes, do not possess a specific receptor for a particular antigen. They can potentially bind and kill all other body cells. Furthermore, they cannot form memory cells.

NK cells are the major effectors ofinnate lymphoid cells (ILCs) and play an important role in tumor immunosurveillance, defense against viruses, and innate immune responses in general. Thus, NK cells have been used as adaptive immunotherapy in several clinical trials (Becker PS et al. 2016). NK cells have inhibitory effects on regulatory and inducing activities in autoimmunological processes (Zhang C et al. 2017).

NK cells are functionally controlled by, among others, members of the killer cell immunoglobulin-like receptor family (K IRs), which includes both inhibitory (iKIRs) and activating (aKIRs) members. Activation of NK cells further occurs through CD94 and natural cytotoxic receptors, which are able to recognize ligands on virus-infected or on tumor cells. The activity of NK cells increases after contact with the cytokines IFN-alpha, INF-beta, IL-12, IL-15, IL-2.

Labtorically, NK cells are recognized by the markers CD16 and CD56. In blood they are defined as CD3-/CD56+. Overlaps with other surface markers are possible. Monocytes, macrophages, neutrophil granulocytes and eosinophil granulocytes are also capable of this type of target cell recognition.

Based on CD56 expression, two major subsets of NK cells can be distinguished:

• CD56high/CD16dim/neg NK cells with the ability to release cytokines.
• and
• CD56dim/CD16high NK cells with cytotoxic capabilities towards virus-infected or neoplastic cells ( Campbell KS et al (2013).

In blood, NK cells can be recognized by large azurophilic granules, which also led to the name"Large Granular Lymphocytes" (LGL) before their actual function was known. Cytotoxic proteins such as perforin or granzyme B are stored in these granules. Perforin has great similarity to the C9 complement protein and causes channel-like pores in the membrane of cells or bacteria. Granzyme B enters the cytosol of cells through these pores and induces apoptosis.

Normally, tumor cells or infected cells are already recognized and eliminated by cytotoxic T lymphocytes due to foreign antigens they present in complex with MHC-I molecules on their surface. Some viruses, however, are able to suppress the presentation of MHC-I molecules on the surface of their host cells. They thus escape destruction by T lymphocytes. The reduced expression of MHC molecules on tumour cells and on virally infected cells is now in turn noticed by the NK cells. They are activated by this and lyse the target cells (missing-self hypothesis; Ljunggren HG et al. 1990).

This is an essential protection in the early phase of viral infections and seems to activate the antigen-presenting cells (APZ) via the triggered alarm signal in such a way that they can induce specific immune responses.

For a long time it was unclear how NK cells without inhibitory receptors generate self-tolerance. To protect healthy cells from autoaggression by NK cells, these cells are equipped with strong inhibitory receptors(KIRs = Killer Inhibitory Receptors, and NKG2A) that recognize HLA - class -I - molecules on target cells. In vivo, the anti-tumour function mediated by NK cells can be suppressed by inhibitory soluble factors/cytokines or by the involvement of so-called immune checkpoint molecules (e.g. PD1-PDL1) (Beldi-Ferchiou A et al. 2017). The investigation of these immune checkpoints now offers new important possibilities for tumor therapy (Del Zotto G et al. 2017).

1. Becker PS et al (2016) Selection and expansion of natural killer cells for NK cell-based immunotherapy. Cancer Immunol Immunother 65:477-484.
2. Beldi-Ferchiou A et al (2017) Control of NK cell activation by immune checkpoint molecules. Int J Mol Sci 18. pii: E2129. 10.3390/ijms18102129.
3. Caligiuri M A (2008) Human natural killer cells. Blood 112: 461-469.

4. Campbell KS et al (2013) Natural killer cell biology: an update and future directions. J Allergy Clin. Immunol 132:536-544.

5. Cheng M et al (2013) NK cell-based immunotherapy for malignant diseases. Cell Mol Immunol 10:230-252.
6. Del Zotto G et al. (2017) Markers and function of human NK cells in normal and pathological conditions. Cytometry B Clin Cytome 92:100-114.
7. Fehniger T A et al (2003). CD56bright natural killer cells are present in human lymph nodes and are activated by T cell-derived IL-2: a potential new link between adaptive and innate immunity. Blood 101: 3052-3057.
8. Ljunggren HG et al. (1990) In search of the 'missing self': MHC molecules and NK cell recognition. In: Immunology Today 11: 237-244.
9. Zhang C et al (2017) NK cell subsets in autoimmune diseases. J Autoimmune 83:22-30.