Natural Killer Cells

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.

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)

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 ITAM bearing 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.

Function:

• 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 CTLs 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).

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 Class I MHC molecules 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 HIV 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 F_c 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).