NK cells are lymphocytes that constitute 5-10% of cells in human peripheral blood. They play an important role in the innate immune response by killing infected or abnormal cells. NK cells are divided into two main subpopulations, CD56dim NK cells which are highly cytotoxic and mediate antibody-dependent cellular cytotoxicity, and CD56bright NK cells which rapidly produce cytokines and chemokines upon activation. A specialized NK cell population in the uterus, uterine NK cells, help form the placenta during pregnancy by enlarging blood vessels to supply the growing fetus.

1. By: Kanchan Rawat
M.Sc. Biotechnology
Jamia Hamdard University

2.  Natural killer (NK) cells are large granular lymphocytes
derived from the common lymphoid progenitor cells
(lymphoblasts).
 NK cells constitute 5%-10% of lymphocytes in human
peripheral blood.
 NK cells are cytotoxic; small granules in
their cytoplasm contain proteins such as perforin
and proteases known as granzymes, that will trigger
either apoptosis or cell lysis in an abnormal cell.
 Defensins, an antimicrobial secreted by NK cells, directly kills
bacteria by disrupting their cell walls.
 NK cells differentiate and mature in thymus, spleen, bone
marrow, lymph nodes and tonsils.

3. The innate and adaptive arms of the immune response:
Recent observations of memory like characteristics suggest a role for
NK cells in the adaptive immune response

4. NK cells provide an early response to virus
infection

5. On the basis of the abundance of the CD56
glycoprotein on the cell surface and their cytotoxic
potential NK cells are divided into two subpopulations:
1) CD56dim NK cells:
 It comprises upto 90%, of NK cells and is considered
the most cytotoxic subset.
 It binds to the FC portion of IgG antibodies and
mediate antibody dependant cell-mediated
cytotoxicity (ADCC) of antibody-bound target cells.
 Expression of granzyme B and perforin is also high
in this subset compared with CD56bright NK cells.

6. 2) CD56bright NK cells:
 This subtype comprises up to 10% of NK cells.
 This subset is known as the cytokine producing
subset of NK cells, and rapidly produces cytokines
and chemokines including IFNγ, TNFα, GM-
CSF after activation.
 A specialized population of NK cells are found in
woman’s uterus, called uterine NK cells (uNK).
 They are essential for forming the placenta.
 The role of the uterine NK cells is not to kill cells
and develop inflammation but to cooperate with fetal
trophoblast cells in enlarging maternal blood vessels
so that they have the capacity to supply the
placenta, and thus the growing fetus, with sufficient
oxygen and nourishment throughout pregnancy.

7.  NK cells kill cells infected with virus by blocking
replication of virus.
 It maintains and even increase the inflammation in
the infected tissue by secreting inflammatory
cytokines such as interleukin (IL)-2, IL-12, IL-15 and
interferon gamma.
 NK cells also kills tumor cells, they recognizes cells
that lack expression of major histocompatibility
complex (MHC) class I antigens.

8.  herpes simplex viruses (HSVs) inhibit the lytic
activity of natural killer cells and interleukin 2-
activated killer cells.
 Chediak-Higashi syndrome is an autosomal
recessive disorder in which mutations in CHS1,
gene,which leads to the downregulation of immune
response that includes neutrophils, macrophages,
and NK cells.

9.  Silke Paust, Balimkiz Senman, and Ulrich H. von Andrian,
Adaptive Immune Responses Mediated by Natural Killer Cells.
Immunol Rev. 2010 May; 235(1): 286–296.
 http://technical.sanguinebio.com/natural-killer-cell-subtypes-
and-markers-in-human-pbmc/
 Peter Parham, The Immune System, 4th edition.
 https://ojrd.biomedcentral.com/articles/10.1186/s13023-014-
0132-6