Derivation/Development: 

There are NK/T cell progenitors in bone marrow. There is a common T/NK progenitor cell. If this progenitor migrates to thymus, majority will rearrangement and become T cells. However, this same cells, with different environmental factors (stroma, IL7, SCF, IL2) will develop into NK cells. Thus the thymus is not required for development, and NKs do not rearrange as with T cells.

Human NK cells dervie from multipotent CD34_ haematopoietc progenitors in the bone marrow. NK cell maturation occurs in the bone marrow as well as in the lymphoid organs and, unlike for T cells, does not require the thymus. (Simaskaki, “NK cells for cancer immunotherapy” Nature, 19, March 2020).

IL-15 is critical for NK cell development. If components of the IL-15 receptor or components of its pathway, will not get NK cells. (i.e., IL-15R-alpha, IL-2R-y & -B, IRF-1, JAK3, Id2 or IKaros)

NK Activation

NK cell activaiton is managed by a suite of activating, co-stimulatory and inhbititory receptors. The joint signals from these receptors determine whether an adjacent cell is targeted for filling and they also regulate cytokine secretion. A fundamental function of NK cells is the elimination of cells with diminished or absent expression of MHC 9also known ias human leukocyte antigen (HLA)) class I molecules. MHC class I molecules bind a set of inhibitory killer cell immunoglobulin-like receptors (KIRs) which suppress NK cell function and minimize the destruction of healthy self-cells. (Simaskaki, “NK cells for cancer immunotherapy” Nature, 19, March 2020).

Besides downregulating MHC molecules, cancer cells may overexpress ligands for activating NK cell receptors. For example, ligands for the activating NKG2D receptors such as MHC class I polypeptide related sequence A (MICA), MICB and several UL16-binding proteins (ULBPs) are preferentaily expressed in cancer cells on cellular stress, infection or DNA damage. (Simaskaki, “NK cells for cancer immunotherapy” Nature, 19, March 2020).

A separate mechanism for target recognition and NK cell activaiton is mediated by the CD16 recetpor (also known as FCGR3A) which binds the constant region (Fc) of immunoglobulins. CD16 engagement by immunoglobulin opsonized cells (cells with antibodies bound to surface membrane antigens) induces phosphorylation of the immunoreceptor tyrosine-based activaiton motif (ITAM) domasins and initiates a signalling cascade that ultimately results in killing of the antibody-coated cell, a mechanisms termed antibody-ddepednent cell cytoxicity (ADCC) (Simaskaki, “NK cells for cancer immunotherapy” Nature, 19, March 2020).

Where are NK cells: 

They generally comprise 5-20% of peripheral lymphocytes (those present in the spleen, liver and peripheral blood) and are present at lower frequencies in the thymus, lymph nodes and bone marrow. They are abundant in liver. A low frequency of NKs reside in the thymus, BM.

NK Receptors: 

NK cells have 2 types receptors. (1) inhibitory receptor that acts through ITIM and (2) activating receptor which acts through ITAM. So there is a constant battle between these two receptors.

Inhibitory Receptor: These receptors have the motif I/V/L/SxYxxL/V. For example, mice have Ly49 Nk cell receptors which operate through ITIM which bind to MHCI and turn Nk cells off. As another example, a receptor CD94/NKG2A inhibits through recruitment of ITIM in both mice and humans.

Activating Receptor: These receptors have the motif Yxx(L/I)6-8xYxx(L/I). NK cells express several ITAMbearing adaptors (DAP12, FcRI..) ITAM recruits tyrosine kinases (Syk and ZAP70). For example, mice have LY49D NK cell receptors which act through ITAM, recruit ZAP70 kinases and activate NK cells. As an example, the receptor CD94/NKG2C through SAP70 activates NK cells in both humans and mice.

In addition, NK cells have receptors for non-MHC class I ligands.

Functions:

NK cells can rpaidly kill virally infected cells and tumour cells. NK cells can recognize tumour cells by unique mechanisms, which rely on a set of stimulatory and inhibitory recetpors. These receptors can senese whether a proximal cell expresses a profile of corresponding ligands associated with oncogenic transformation: a tumour-associated profiel triggers NK cell activaiton and targeted cell killing. (Simaskaki, “NK cells for cancer immunotherapy” Nature, 19, March 2020). 

Virus protection: Natural killer cells play an important role in eliminating virus infected cells during the first few days of many viral infections. 

NK secrete cytokines: NK cells generate yinterferon production quickly. NK cells also secrete TNFalpha, LTalpha, GM-CSF, IL-5, M-CSF, IL-3, IL-10, IL-13, MIP-1alpha, MIP1beta, RANTES, etc. 

IL-12 augments IFNy production by NK cells, but inhibits IL-5.

IL-4 induces Il-5 and IL13 but inhibits IFNy.

 NK cells do not make IL-2 or IL-4.

After activation, NK cells can secrete various cytokines and chemokines that can directly affect the survival of pathogens (e.g., IFN-y) and the growth and spread of tumours (e.g., IFN and TNF-alpha) as well as the regulation of haematopoietic cell differentiation and the priming of immune effectors that are crucial for subsequent adaptive immunity. Thus, like DCs, NK cells can have an essential role in dynamically defining the environment in which immune interactions occur and immune responses are elicited.

Cytokines such as IL-12 and IL-18 promote optimal cytokine production by NK cells.

NK cells differ from T cells in several ways. First, NK cells do not express antigen specific T-cell receptors or CD3. In addition, recognition of target cells by NK cells is not MHC restricted. 

NK cells recognize stressed, transformed or infected cells by integrating signals transduced by various activating and inhibitory surface receptors. Activated NK cells kill target cells and produce inflammatory cytokines such as IFN-y and TNF thereby acting as primary mediators of innate immunity in peripheral inflamed tissues. NK cells may also participate in adaptive immune response by modulating DC function or by proudcing IFN-y.

Influence Adaptive Immunity: Early IFNy production by NK cells may skew CD4+ T cells towards Th1. IFNy production by NK cells may cause IgG class switching in B cells. 

Upon immunization, NK cells enter the lymph nodes which secrete IFNy. This is believed to promote generation of CTL and Th1 response.

Interact with DCs: NK cells are activated by co-culture with DC. In mouse CMV infection, CD8+ DCs produce IL-12 and IL-18 which activate NK cells in vivo. Depletion of mouse CD8+ DC prevents NK cell activaiton and cotnrol of infection. In the absence of ouse NK cells, CD8+ DCs are lost after MCMV infection (mechanism unkown). 

Natural Killer T cells:

Invariant natural killer T cells (iNKT): is a subfclass of CD1d-restricted natural killer T (NKT) cells that epxress a highly conserved alphabeta-T cell receptor that includes a Valpha24Jalpha18 TCRalpha chain in humans. iNKT cells can be identified by binding with CD1d-multimers like that are loaded with alpha-galactoxylceramide (GalCer), PBS-57, PBS-44 or other natural or synthetic glycolipids adn can be found as tetramers, dendirmers and other structures. Another method of idnetificaiton is an antibody that specifically recognize the Valpha24Jalpha18 region. (US 10,300,090)

Markers:

human: CD-, CD56+

mice CD-, NKR-p1C+

Activation: Interferon-alpha/beta are the most important for NK cell activation. IL-15 is also important for NK activation of the mature resting NK cell.

Both IL-15 and INF-alpha/beta induce the NK to produce IFNy. IL-12 & IL-18 also augments IFNy production. This IFNy can activate CD8+T which will activate TGFbeta which acts as a negative feedback for NK cells.

NK cells can also be activated by IL-2 which induces proliferation of NK cells and increases cytotoxicity.

NK cells do not express antigen specific receptors and appear to attack cells which lack or express lower levels of  as do many virus infected cells. Thus Nk cells like to kill cells lacking MHC class q. F

Many viruses downregulate MHC class I. For example, adenovirus and  encode proteins that block class I MHC gene transcription. Herpes simplex virus and cytomegalovirus block the peptide translocators in the ER membrane that transport proteasome derived peptides from the cytosol into the lumen of the ER. NK cells monitor the level of class I MHC proteins which are expressed on the surface of most vertebrate cells. The presence of high levels of these proteins inhibits the killing activity of NK cells, so that NK cells selectively kill cells expressing low levels like virally infected cells and some cancer cells. However, the process is more complex and in addition to loss of class I, there are activating proteins which are probably required to activate NK cells. 

How do NK cells Kill?: NK cells kill predominantly by the secretion of perforin/granzymes. Perforin acts like complement and punches a hole in the cell. Granzymes probably leak in through these holes which leads to apoptosis.

NK cells produce secreted or membrane TNFalpha, Fas ligand and TRAIL. So if a cell has a receptor for this, they can also kill the cell.

The linking of antibody bound to target cells with the Fc receptor of NK cells can direct the cytotoxic activity of the NK cell against the target cell in a process called antibody-dependent cell-mediated cytotoxicity (ADCC).