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Natural killer (NK) cells are CD3-,
sIg-, CD16+ and CD56+ peripheral blood mononuclear cells (large granular
lymphocytes), which are involved in non-specific host defence (CD69 is the
earliest activation marker on T and NK cells). They act as an innate immune
system defence against infected cells (by bacteria, parasites and viruses) and
tumour cells but spare normal cells. They form the first line of defence
especially against viral infections 1-3. They control viral replication during the time required
for activation, proliferation and differentiation of CTL precursors into
functional CTLs at during the first 5 to 7 days of the infection by mediating
direct cytotoxicity and secretion of cytokines such as g-IFN, TNFa and GM-CSF but not IL-2 3-9. The main target for them to attack is a cell that is
missing the self MHC class I molecules 10,11. Unlike cytotoxic T lymphocytes (CTL), NK cells do
not require presentation of a peptide by an MHC molecule. NK cells do not
require activation for cytotoxicity; they always have large granules of
granzymes and perforin in their cytoplasm, which make them constitutively
cytotoxic.
Table
I. Characteristics of NK cell receptor families
|
NK Receptor Family |
Molecular Nature |
Genetic Complex |
Ligands |
Mouse Correspondent |
|
KIR |
Ig-superfamily |
LRC (19q13.4) |
HLA-A, -Bw, -Cw, -G |
gp49 |
|
ILT/LIR |
Ig-superfamily |
LRC |
HLA Class Ia (-G) |
LRC |
|
CD94/NKG2 (KLR) |
C-type lectin-like |
NKC (12p12.3 - 13.2) |
HLA Class Ib (-E) |
NKC / Ly49 |
|
NKG2D (KLRK1) |
C-type lectin-like |
NKC |
MIC and MHC class
I-like |
NKG2D |
|
NCR |
Ig-superfamily |
Various, incl. MHC (NCR3),
LRC |
Viral hemagglutinins
and others |
NCR |
From Refs 12-15
Target cell lysis is controlled by a
number of receptors (Table I). The main ligands for these receptors are MHC
class I and class I-like molecules (more precisely HLA-A, ‑B, ‑Cw, ‑E,
MICA, MICB and others). In general, NK receptors recognise missing self (MHC),
induced self (stress signals) or modified self (stress signals) proteins as
their ligands 16. Crystallographic studies have shown that NK
receptors form an immune synapse directly with the a helices and bound peptide of HLA class I molecules 17. The Ig-like NK receptors interact with the carboxyl
terminus of MHC class I a1 helix 18-20. The peptides within the MHC class I cleft do not
interact with the NK receptors for recognition 21; however, certain side chains at position 7 and 8 of
the nonamer peptide interfere with KIR2DL and KIR3DL binding 22-24. (The amino acid sequence of the peptide in HLA-E is
known to affect the binding by CD94/NKG2A 25 (Sullivan, 2007)).
NK cells recognize conserved epitopes shared by groups of class I molecules rather than individual alleles. In the opposing signals model of NK cell activity, the inhibitory and activating NK receptors may coexist in the same cell but the binding of inhibitory receptors by MHC class I transmits dominant inhibiting signals 26,27. This way, normal cells expressing MHC molecules are protected and this is the basis of the missing self hypothesis of NK cytotoxicity 11. The only exception is the activating killer cell lectin-like receptor (KLR)K1 (NKG2D) which can override inhibitory signals when engaged with its ligands MICA, MICB and other MHC class I-like molecules such as retinoic acid early inducible-1 (RAE-1), H60 minor histocompatibility molecules and CMV UL-binding proteins (ULBP) which are identical to NKDL-1, -2, and -3 in mice and humans 28-42. This stimulatory signal generated by NKG2D, however, is not entirely refractory to inhibitory signals 32. Possible human homologue of the mouse RAE-1 has been cloned recently on human chromosome 6q24.2 43. In its interaction with the activating lectin-like NK cell receptor NKG2D, the MICA polymorphism at amino acid 130 in the a2 domain seems to be functional whereas MICB polymorphism does not have the same effect