Natural Killer (NK) Cells
A small fraction (~2%) of the lymphocytes circulating in the blood are neither T cells nor B cells. Most of these are called natural killer (NK) cells because they are already specialized to kill certain types of target cells, especially
- host cells that have become infected with virus;
- host cells that have become cancerous.
Features of NK Cells
- The specificity of the receptors with which NK cells recognize potential targets are NOT diversified like the
- αβ and γδ receptors (TCRs) of T cells and the
- antigen receptors (BCRs) of B cells.
- However, we do inherit a large number of genes for NK receptors. Different subsets of these are expressed in each NK cell producing a diverse repertoire of cell specificities.
- NK cells have
- activating receptors, that activate the NK cell when it binds to a target cell, and
- inhibitory receptors that transmit an inhibitory signal if they encounter class I MHC molecules on a cell surface. (This is in sharp contrast to T cells that can only recognize antigens when presented within a MHC molecule.) [Discussion])
- Because viruses often suppress class I MHC expression in cells they infect, the virus-infected cell becomes susceptible to killing by NK cells.
- Because cancer cells have reduced or no class I MHC expression, they, too, become susceptible to killing by NK cells.
- The killing is done by the exocytosis of granules containing perforin and granzymes.
- Because NK cells are preprogrammed to recognize their targets, they are able to respond rapidly thus providing another arm of innate immunity.
- In addition to killing target cells, NK cells secrete cytokines such as the
- anti-viral cytokine IFN-γ and the
- inflammatory cytokine TNF-α
NKT cells are NOT the same as NK cells.
NKT cells are T cells with an αβ TCR. However, they also express some of the cell-surface molecules of NK cells — hence their name.
They differ from most T cells
NKT cells are able to secrete large amounts of either
- in the sharply limited diversity of their αβ TCRs [Discussion];
- and these respond to glycolipid antigens presented by a cell-surface molecule designated CD1d (rather than to peptide antigens presented by MHC molecules [Discussion]).
Perhaps the function of NKT cells is to provide quicker help for a cell-mediated immune response (IFN-γ) or antibody-mediated response (IL-4) than the several days needed by conventional T-helper cells. If so, NKT cells would represent a link between innate and adaptive immunity.
In addition to defending against some infectious agents, NKT cells have been implicated in protecting against
but have recently been shown to be present in large numbers in the lungs of patients with asthma where they probably play a major role in the lung inflammation characteristic of the disease.
5 April 2012