Download Activating and inhibitory receptors and their role in Natural Killer cell

Indian Journal of Biochemistry & Biophysics
Vol. 40, October 2003, pp. 291-299
Minireview
Activating and inhibitory receptors and their role in Natural Killer cell function
Raghvendra M Srivastava, B Savithri and Ashok Khar*
Centre for Cellular & Molecular Biology, Uppal Road, Hyderabad 500 007, India
Received 19 May 2003; revised 30 July 2003
Last decade has seen a significant advancement in our understanding of the Natural Killer (NK) cell biology and
function. Several receptors present on NK cells have been identified, which are involved in either their activation or
inhibition. Similarly, a large number of interacting ligands have been identified on the target cells that upon interaction
transmit either activating or inhibitory signals. This review is oriented towards understanding the role of these receptors on
the NK cells, which are considered as the first line of defense.
Keywords: NK cells, receptors, ligands, function, inhibition, activation, cytotoxicity
Successful host defense relies on the concerted action
of innate and adaptive immune response. Natural
killer cells (NK) are an integral component of the
innate immunity both in the production of cytokines
(IFN-γ, TNF-α) to stimulate other cells of the immune
system and in direct destruction of the infected or
transformed cells. “Natural killer cells” named first by
Herberman et al1, are bone marrow-derived large
granular lymphocytes that share a common progenitor
with T-cells. They represent a lymphoid population
——————
*Author for correspondence
Fax: +91-40-27160591, 27160311
Email: [email protected]
Abbreviations used: NK, natural killer cells; MHC, major
histocompatibility complex; ITAM, immunoreceptor tyrosine
based activation motif; ITIM, immunoreceptor tyrosine based
inhibitory motif; Ig, immunoglobulin; ADCC, antibody dependent
cellular cytotoxicity; TCR, T cell antigen receptor; NCRs; natural
cytotoxicity receptors, SHP-1, Src homology-1 (SH1) containing
phosphatase; CTLD, c-type lectin-like domain, NKR-P1, natural
killer cell surface protein 1; KIRs, killer cell immunoglobulin-like
receptors; CXCP motif, (cysteine-X-cysteine-proline motif);
YXXM motif, (tyrosine-X-X-methionine); where X denotes any
amino acid; MICA, [major histocompatibility complex (MHC)
class I chain] related gene A; and MICB, [major
histocompatibility complex (MHC) class I chain] related chain B;
DAP, death associated protein; CTLs, cytotoxic T lymphocytes;
HLA,
human
leukocyte
antigen;
MCMV,
murine
cytomegalovirus; XLP, X-linked lymphoproliferative disease;
EBV, Epstein barr virus; RAG, recombination activating gene;
Rae-1, retenoid acid early transcript 1; ULBPS, UL16 binding
proteins; H-60, histocompatibility antigen; m157, a viral gene
product; ILT, immunoglobulin-like transcripts; LIR, leukocyte Iglike receptors; MIR, monocyte/macrophage Ig-like receptors;
Mac-1, complement receptor type 3; ICAM, intercellular adhesion
molecule; LFA, leukocyte function associated antigen; CD,
cluster designation
that, in contrast to T or B-lymphocytes, does not
express clonally distributed receptors for antigens.
Unlike T or B cells, NK cells develop normally in
scid mice2 or in mice with disrupted RAG-1 or RAG-2
genes3, indicating that gene rearrangement is not
required for their differentiation or function and are
thus considered as a key performer of innate immune
system4.
NK cell recognition of pathogen-infected cells
(viral or bacterial infection) and tumour is based on
the expression of multiple cell surface receptors that
bind to either MHC-I (major histocompatibility
complex) for inhibitory and or non-MHC ligands to
transduce activating signals. Thus, NK cells provide a
unique defense mechanism to eliminate cells with
abnormal MHC class I expression (“Missing self”
hypothesis of tumour immunosurvillence)5,6. The
mechanisms that regulate NK cell function against
tumour and infection have exceeded beyond our
expectations in terms of their sophistication and
complexity and are based on the regulation of
activation/inhibition signals for NK cells.
Two-Receptor Model
NK cells express a number of cell surface receptors
that fall into two major categories, namely activation
and inhibitory receptors. Activation receptors have a
short cytoplasmic tail and a positively charged (basic)
amino acid in transmembrane region. It is through this
basic amino acid region that the receptor associates
with signal transducing molecule such as DAP127,
which also have a negatively charged (acidic) amino
acid in their transmembrane domain. These signal
292
INDIAN J. BIOCHEM. BIOPHYS., VOL. 40, OCTOBER 2003
transducing molecules have a series of amino acids in
their cytoplasmic domain known as “ITAM”
(immunoreceptor tyrosine based activation motif).
Another adapter protein DAP10 acts like co-receptor
possessing YXXM motif in its cytoplasmic domain
instead of ITAM can co-stimulate NK cells8. On
engagement of activation receptors with ligand, a
signal is sent to the cell that activates a series of
kinases (enzymes that add charged groups),
eventually leading to activation of the cell’s cytotoxic
effector function and IFN-γ production. In contrast to
activating receptors, inhibitory receptors have a long
cytoplasmic tail that contains an “ITIM”
(immunoreceptor tyrosine based inhibitory motif).
Upon engagement of ligand, a tyrosine residue in the
ITIM is phosphorylated by a kinase, and the
phosphotyrosine in turn attracts a phosphatase known
as SHP-1, which removes, charged phosphate group.
The function of SHP-1 is to antagonize the kinase of
activation signal. Because of the requirement for
ITAM- associated kinases to phosphorylate the ITIM,
and the presence of SHP-1 near the same kinases for
their dephosphorylation, the inhibitory signal can only
be delivered when the activating and inhibitory
receptors are colligated9. This is shown schematically
in Fig. 1. Thus, when both activating and inhibitory
receptors are cross-linked by their ligands on the
surface of a potential target cell, the inhibitory signal
delivered to the NK cell overrides the activating
signal, and the target cell is spared. If only the
activating receptor interacts or if more activating
receptors than inhibitory receptors are engaged, then
the target is killed. Structural and functional
characterization of these receptors is currently the
most demanding aspect of NK cell biology. This
review focuses briefly on the activation and inhibitory
receptors on NK cells and their ligands.
Activation Receptors involved in Cytotoxicity
CD16 (FcγγRIII)
The most extensively studied membrane receptor
on majority of NK cells is the transmembrane
~70 kDa glycoprotein of the immunoglobulin (Ig)
superfamily, which posseses low affinity for IgG,
CD16 (FcγRIII)10. CD16 is non-covalently associated
with the γ subunit of the high affinity IgE receptor
(FcεRI-γ) in mouse NK cells and with FcεRI-γ or the
ζ subunit of T cell antigen receptor (TCR) complex in
human NK cells11. Upon CD16 mediated activation,
NK cells secrete cytokines12, mediate ADCC
(antibody dependent cellular cytotoxicity)13 and
induce apoptosis as a consequence of Fas ligand
(CD95L) interaction14. While CD16 is responsible
and necessary for ADCC, this receptor has not been
implicated in other modes of NK cell-mediated
cytotoxicity.
CD2 (LFA-2, E-Rosette Receptor)
CD2 is a ~50 kDa membrane glycoprotein of Ig
superfamily expressed on NK15 and T cells. The
biochemical events accompanying CD2 activation are
similar to those initiated by CD16 ligation. In
particular, CD2 stimulation of NK cells results in the
activation of src-family kinases and the generation of
inositol-triphosphates (IP3). The predominant ligand
of human CD2 is a membrane glycoprotein, CD58
(LFA-3, lymphocyte function associated antigen-3).
However, expression of CD58 alone is not sufficient
to confer cells sensitive to NK mediated lysis
suggesting that CD2 may serve as co-stimulatory
receptor that augments, but does not initiate the
primary activation of NK cells16.
Fig. 1—Schematic representation of NK cell-target cell
interaction involving different receptors and ligands [Activation
receptors associate with adapter molecules, which possess
signaling motifs in their cytoplasmic domains. DAP10 (YXXM)
motif provides co-stimulation, whereas DAP12 (ITAM) activates
NK cells. Tyrosine phosphorylation by Syk or ZAP-70 kinase at
ITAM/YXXM and dephosphorylation by SHP-1 phosphatase,
modulate the cytotoxicity of NK cells in majority of cases]
CD28
The CD28 receptor/B7 ligand system is the
dominant co-stimulatory pathway affecting T cell
immune responses. Although CD28 is expressed on
human fetal NK cells, it is lost upon maturation and is
absent on peripheral blood NK cells in adults. YT
SRIVASTAVA et al.: NATURAL KILLER CELL RECEPTORS
(immature NK) cell line17, which expresses CD28,
killed target cells expressing either CD80 or CD86.
Splenic NK cells express CD28, and ligation of CD28
with B7 ligand augments NK cell proliferation, and
tumour cell lysis18.
NKR-P1 (CD161)
NKR-P1, first identified in the rat is expressed by
almost all NK cells as a disulfide-linked homodimer.
The polypeptide is a type II integral membrane
protein with a deduced external carboxy-terminal
domain homologous to Ca2+-dependent (C-type) lectin
superfamily19. The rat NKR-P1 cDNA was cloned by
using a monoclonal antibody (mAb) selected for its
ability to activate rat NK cells20. Three highly related
genes NKR-P1A, B and C have been identified in
mice and rats. These genes display allelic
polymorphism and the C57BL/6 and BALB/c allelic
forms differ by 1-10%21. The prototype mouse NK
cell antigen, NK1.1 molecule, long considered to be
most specific serological determinant on murine NK
cell, is encoded by Mus NKR-P1C in C57 BL/6
mice22. A single human NKR-P1 homolog has been
found as disulfide-linked homodimer of C-type lectin
superfamily on the subsets of NK and T-cells.
The NKR-P1 genes are present on rat chromosome
4, mouse chromosome 6, and human chromosome
12p12-p13 in a region designated as the “NK gene
complex”. The mouse NKR-P1 genes are clustered
and genetically linked to the Ly-49 gene family in the
NK gene complex23. The NKR-P1 receptor is more
promiscuous in its carbohydrate specificity than other
C-type lectins and there are several known NKR-P1
genes each with potential allelic forms. These
observations may explain the confusing array of
effects that different carbohydrates have on natural
killing24. Interestingly, expression of NKR-P1 in the
rat has also been detected on activated peripheral
blood granulocytes, activated monocytes and splenic
dendritic cells which killed NK-sensitive YAC-1 cells
in vitro mediated by Ca2+-dependent pathways25.
NKR-P1 proteins in all rodents express the CXCP
motif, which is also found in the cytoplasmic domains
of CD4 and CD8 that interacts with phosphorylated
p56 lck; however, human NKR-P1 lacks this motif.
Another remarkable feature is the presence of ITIM in
the cytoplasmic domain of rat and mouse NKR-P1B,
but not in other NKR-P1 isoforms. Because of ITIM
motif some isoforms have been found to be inhibitory
rather than having activation effector function26.
NKR-P1 crosslinking and inhibition studies suggest a
293
role for this receptor in target cell recognition and in
cytotoxicity. However, NKR-P1 has not been found
essential for NK cell development and function27.
CD94/NKG2
CD94/NKG2 receptors are expressed on human
NK cells and on subset of T-cells. NKG2 are
members of type II, C-type lectin superfamily, which
heterodimerize with CD94, except NKG2D, which is
an activation receptor. NKG2A and CD94 form a
heterodimer28 (70 kDa) composed of ~30 kDa CD94
glycoprotein disulfide bonded to a 43 kDa NKG2A
glycoprotein; it can recognize the MHC non-classical
class I HLA-E ligand coexpressed with peptides
derived from the leader sequences of certain HLA-A,
B, C and non-classical HLA-G chains29. Upon ligand
engagement, inhibition signal is sent to NK cell
through ITIM motif on cytoplasmic tail of NKG2A.
Whereas CD94:NKG2C heterodimer, upon interaction with activation molecule can activate NK
cells30. Even after transfection of NKG2A/B with
strong promoters, the proteins are unable to be
expressed on the cell membrane unless they are
disulfide bonded to CD94 glycoprotein. Thus, CD94
might assist in the transport of NKG2 receptors to the
cell surface31. Human NKG2A possess two ITIMs
that have been shown to be capable of interacting with
SHP-1 and SHP-2; like KIR (Ig superfamily), ITIMs
are required for mediating the maximal inhibitory
signal, but opposite to KIR, the membrane distal
ITIM is of primary importance rather than the
membrane-proximal ITIM, for inhibitory function32.
CD94 (Kp43) essentially lacks a cytoplasmic
domain, thus deficient in intrinsic signal transduction
capacity. CD94 is a single gene with limited or no
allelic polymorphism, The NKG2 family comprises
four genes, designated as NKG2, -A, -C, -E, -D/F.
CD94 and the four NKG2 genes are closely linked on
human chromosome 12P12.3-P13.1 in the ‘Human NKgene complex’. NKG2D/F is an unusual gene, which
shows homology with the other NKG2 genes only in
the 5′ region33,34. NKG-2D is a recently characterized
potent activation molecule conserved among human,
mouse and rat (NKR-P2) NK and CTLs, with a
C-terminal lectin-like domain (CTLD) and is the most
characterized receptor till date on NK cells. Unlike
other activating NK receptors, NKG-2D does not
appear to signal through ITAM containing signaling
polypeptide. Instead, it selectively pairs with
KAP/DAP10
(killer
cell
activating–receptor
associated polypeptide/DNAX activating protein,
294
INDIAN J. BIOCHEM. BIOPHYS., VOL. 40, OCTOBER 2003
transmembrane adapters), which can recruit the p85
subunit of PI3-kinase through YXXM motif present in
its cytoplasmic domain. This signaling motif is related
to a similar functional motif contained in the
cytoplasmic domain of CD28 (co-stimulatory
receptor), thereby suggesting that NKG2D may costimulate rather than directly activate effector
function35.
Recently, two splice variants of NKG-2D have
been identified: a long form (NKG-2DL) that has a
13-amino acid extension at the amino terminus, and a
short form (NKG-2DS). Resting NK cells express
mRNA encoding NKG2D-L, and after activation,
mRNA encoding NKG2D-S is upregulated.
Immunoprecipitation studies showed that NKG2D-L
associates only with DAP10, whereas NKG2D-S can
associate with both DAP10 and DAP12. Differential
signaling ability of NKG2D depends both on the cell
type and on the activation state, which is determined
by the differential expression of DAP10 versus
DAP12, on the two different NKG-2D isoforms.
Thus, NKG-2D has been found to deliver a direct
stimulatory signal to NK cells, but only a costimulatory signal to T cells, because the specificity to
T cell signaling would be compromised by direct
stimulation of T cells by NKG2D. NKG2D is unique
in the sense that it is relatively non-polymorphic. It
forms a homodimer, whereas the other NKG2
molecules (NKG2, -A, -C, E) heterodimerize with
CD94. NKG-2D shows minimal amino acid sequence
differences between four mouse strains. These
molecules most likely represent alleles with the same
function because of conservation of transmembrane
and cytoplasmic domains. Arginine at residue 69 is
conserved in all four-mouse strains, which is
responsible for NKG-2D interaction with DAP1036,37.
The limited polymorphism of NKG-2D is
somewhat surprising because the ligands for NKG-2D
are numerous and heterogenous and display
significant allelic polymorphism. A number of
NKG-2D target ligands have been identified. The
most interesting of these are a pair of closely related
hNKG2D ligands called MICA and MICB [major
histocompatibility complex (MHC) class I chainrelated]38. MICA/B are structurally similar to MHCclass I and possess α1, α2 and α3 domains, but they
share only 27% amino acid identity with human MHC
class I. The transcriptional control regions of MICA
and MICB genes contain sequences that are highly
homologous to heat shock elements of hsp70 genes.
Expression of MIC proteins is upregulated under
conditions of “stress” such as heat shock, viral
infection and allograft rejection. ULBPs (UL16
binding proteins) are another group of hNKG2D
ligands39, distantly related to MHC class I in their α1
and α2 domains, and are anchored to the membrane
via a GPI (glycophosphatidyl inositol)-linkage,
whereas UL16 bears no sequence or structural
similarity to NKG-2D.
Natural Cytotoxicity Receptors
A true NK receptor involved in natural cytotoxicity
should mediate NK cell triggering independent of the
engagement of other surface molecules. In addition,
its surface expression may be expected to be mostly
restricted to NK cells. The triggering surface
molecules with both of these properties have been
identified in humans, based upon the screening of
monoclonal antibodies capable of inducing NKmediated killing of Fc receptor positive tumour target
cells (in a redirected killing assay). This approach led
to the identification of several molecules, which
mediate NK cell activation and are referred to as
natural cytotoxicity receptors (NCRs)40.
NKp46 is a 46 kDa type I transmembrane
glycoprotein belonging to the Ig superfamily and has
been found in immature NK cells undergoing in vitro
maturation starting from CD34+ cells. Cross-linking
of NKp46 resulted in increased cytotoxicity, Ca2+
mobilization, and cytokine production41. Analysis of
cell distribution indicates that NKp46 is expressed by
all NK cells including the CD56bright CD16− subset. Its
cytoplasmic modules are coupled to intracytoplasmic
transduction machinery by their association with
CD3ζ and FcεRIγ adapter proteins that contain
ITAM42. Recent work by Tomasello et al43 indicated
that in double knock-out mice lacking CD3ζ and
FcεRIγ chains the cytolytic activity against most
target cells (including YAC, BW15.02, RMA,
RMA-S and C4.4.25) is profoundly reduced. These
findings indicate that in mice NKp46 (or other, still
undefined, murine receptors associated with ζ/γ
polypeptide) may also play a prominent role in natural
cytotoxicity mediated by NK cells44.
NKp30,
another
triggering
receptor
(Ig
superfamily) of 30 kDa, functionally similar to
NKp46 and CD16 has been found to be associated
with CD3ζ chain that is phosphorylated at tyrosine
following treatment of cells with pervanadate and thus
takes part in activation45. Similar to NKp46 and
SRIVASTAVA et al.: NATURAL KILLER CELL RECEPTORS
NKp30, NKp44 also triggers NK-mediated cytotoxicity upon cross-linking by specific monoclonals.
Expression of NKp44 alone in IL-2 activated NK
cells, gave it a status to be considered as the first
marker for activated human NK cells. NKp44 is a
glycoprotein of 29 kDa that associates with the
ITAM-bearing KARAP/DAP12 signal transducing
molecule46. A direct correlation has been found
between the surface density of NCRs and the ability
of NK cells to kill various tumours47. NKG2D
(another potent triggering receptor) plays either a
complementary or a synergistic role with NCRs48.
Several other membrane receptors have been
implicated in NK cell activation based on the ability
of specific monoclonal antibodies to permit lysis of
Fc-receptor bearing target cells (mAb redirected
cytoxicity assay). Structures implicated with this idea
include: Ly 6, mouse and human CD69, mouse and
human 2B4, human, CS1 (CRACC)49, DNAM-150 and
CD4451. The ligands for various activation receptors
are listed in Table 1.
Inhibitory Receptors
The inhibitory receptors superfamily (IRS)34,52
describes an expanding group of receptors that
attenuate activation of a number of different cell types
in the immune system. By definition, IRS members
must act in trans, be able to recruit phosphatase (such
as SHP-1) through an ITIM. Inhibitory receptors
mediate this function only upon their clustering with
an activating counter part on the cell surface.
Generally, the activating and inhibitory receptors
recognize different ligands; however, under certain
circumstances, cells simultaneously express pairs of
activating and inhibitory receptors with closely
related extracellular domains binding similar ligands.
When both activating and inhibitory receptors are coengaged by their respective ligands, the net outcome
is determined by the relative strength of these
opposing signals. Thus, a central paradigm has
emerged in which pairing of activation and inhibition
is necessary to modulate immune response53.
Ly-49 Receptors
The Ly-49 receptors are type II transmembrane, Ctype lectin-like molecules that are expressed as
homodimers. Mouse NK cells express at least seven
inhibitory Ly-49 receptors54. These are responsible for
the recognition of polymorphic MHC-I molecules on
potential target cells and successful recognition of
MHC class I by Ly-49 requires α1 and α2 domains of
295
Table 1—Activating receptors and their ligands in human and
mouse NK cells
Receptor
Ligand
Shared between human
and mice
CD11b (Mac-1, αMβ2, C3R) CD54 (ICAM-1), CD102 (ICAM-2)
IgG
CD16 (FcγIIIa)
CD28
CD44
CD69
CD244 (2B4)
B7-1, B7-2
Hyaluronic Acid
CD48
IFN-αβR
NKG2C, NKG2E
NKG2D
IFN-α, IFN-β
HLA-E (human), Qa1b (mouse)
ULBP1, ULBP2, ULBP3 (human)
MICA, MICB (human)
Rae-1, H-60 (mouse)
Tumour Ligands (?), Influenza virus
Hemagglutinin (human), ligands on
normal cells (?) (human)
NKp46
Human
CS1(CRACC)
ILT2(LIR-1)
KIR3DS
NKp44
NKp30, NKp80
Mouse
gp49A
LY49D
LY49H
HLA class I
HLA class I
Tumour ligands (?), Influenza virus
Hemagglutinin
Tumour ligands (?), ligands on normal
cells (?)
H-2 class I
m157
MHC class I as well as the presence of a peptide in
the peptide binding groove. Expression of Ly-49
family of receptors is a relatively late event in NK cell
development. Seven well described Ly-49 family
members containing inhibitory motifs in their
cytoplasmic domains (Ly49A, -B, -C, -E, F, G2 and
–I) are known to be expressed at least at the mRNA
level by NK cells from C57BL/6 (B6) mice55.
Diversity within Ly-49 family is provided by alternate
mRNA splicing and allelic polymorphism. Expression
of Ly-49 genes is monoallelic and each NK cell can
co-express several Ly-49 receptors at one time56.
Acquisition of Ly-49 receptor is thought to be a
stochastic process in which NK cells will continue to
express different Ly-49 receptors in a cumulative
manner until they express one that recognizes self
296
INDIAN J. BIOCHEM. BIOPHYS., VOL. 40, OCTOBER 2003
MHC-class I. The resulting diverse array of receptors
displayed by each NK cell creates the divergent
repertoire that monitors cells from the selective loss
of class I molecules57. Based on the functional studies,
Ly-49A was initially attributed to be specific for Dd,
but not Kd, Ld, Kb or Db. Ly-49A also binds to H-2K
cells, but binding of Ly-49A to H-2d was consistently
higher than H-2K. Ly-49A transfectants bound
significantly to H-2S cells, albeit weakly, and to
H-2f,q,3,v. These results indicate that Ly-49A exhibits a
broad and often strong MHC reactivity58. Ly-49B is
unique in displaying by far the least homology to the
other Ly-49 family members59 raising the possibility
that it has a distinct function or specificity. Ly49C
recognizes the Kb molecule. In cell adhesion
experiments, Ly49C has been found to possess
affinity with H-2b, H-2d, H-2k and H-2s gene products.
Ly49C exhibits allelic diversity, differing in four
amino acids between the B6 and BALB/c strains.
However, later reactivity difference between Ly46CB6
and Ly49CBALB could not be detected showing no
evidence for difference in the specificity of these
allelic Ly49C receptors. Ly49C shares sequence
homology with Ly49I and Ly49F. By the tetramer
binding studies, Ly49C has been found to exhibit
little, if any, capacity to discriminate peptides bound
to Kd or Db 60.
Two of the Ly49 receptors Ly49D and Ly49H
appear to be activating rather than inhibitory
receptors. Ly49D shows activation in response to
target cells expressing the Dd class I-molecule61.
Ly49D and Ly49H lack ITIM and associated noncovalently with ITAM bearing membrane adaptor
protein DAP12 via a charged residue in the
transmembrane region62. A viral gene product (m157)
which is expressed on MCMV (murine
cytomegalovirus) infected cells can bind to Ly49I and
gives an inhibitory signal in MCMV susceptible
strains, whereas m157 has been found to give
activation signal in MCMV resistant strain after its
binding to Ly49 H on NK cells63. Ly49E is very
similar in sequence to other MHC-reactive Ly49
receptors (Ly49C, I and F), suggesting a role in MHC
recognition. Weak binding of Ly49E to H-2r has been
reported in cell-cell adhesion assays. Ly49F binds
weakly but consistently to MHC molecules of the
(H-2d haplotype. Strong inhibitory action of Ly49G2
with H-2d has been reported despite weak binding;
though comparatively equal extent of inhibitory effect
was found to be associated with Ly49A after strong
binding because of avidity differences or strong
signaling in the former54.
Ly49I binds to H-2b,d,k,r,s,v with variable strength
and can discriminate peptides presented by Kd . In
humans, members of Ly49 multigene family have
been identified; however the encoded transcript is
probably not functional. The human Ly-49L gene is
located approximately 150 kb from NKG-2A gene64.
Interacting ligands (HLA subgroup) for various Ly 49
receptors are shown in Table 2.
Killer Cell Immunoglobulin (Ig)-like Receptors
(CD158)
Killer cell immunoglobulin-like receptors (KIRs)
are type I transmembrane molecules with either two
or three Ig domains. In humans, the KIR genes encode
glycoprotein of Ig superfamily that bind HLA class-I
as ligands and can inhibit or activate NK cells, which
is dependent on the presence of ITIM or ITAM motifs
in their cytoplasmic domain65. KIR genes are located
on human chromosome 19q 13.4, where about 12
genes are estimated to be located based on the
analysis of existing cDNA sequences. Two
subfamilies of the KIR are identified, depending on
the number of Ig-like domains in the extracellular
regions of the molecules that are KIR2D (2 Ig-like
domains) and KIR3D (3 Ig-like domains). Within
both subfamilies, there is diversity throughout the
extracellular, transmembrane and intracellular
domains. Like Ly49, KIRs are encoded by about 12
polymorphic genes, recognize polymorphic epitope
on human leukocyte antigen classes HLA-A, HLA-B
and HLA-C and are expressed on overlapping subsets
of NK cells and memory T cells. Half of the KIR
molecules have two ITIMs in their cytoplasmic
domain that bind predominantly SHP-1 and inhibit
cell mediated cytotoxicity and cytokine secretion66.
Table 2—Ly49 receptors and their ligands
Receptor isotype
Ligand
Function
Ly49A
Ly49B
Ly49C
Ly49D
Ly49E
Ly49F
Ly49G2
Ly49H
Ly49I
H2d, H2k, H2s
H2b, H2d, H2k, H2s
H2k, H2dd
H2r
H2d
H2d
m157
H2b,d,r,s,v
Inhibitory
Inhibitory
Activation
Inhibitory
Inhibitory
Inhibitory
Activation
Inhibitory
SRIVASTAVA et al.: NATURAL KILLER CELL RECEPTORS
Activating isoforms in KIR, lacking ITIM,
associate with the ITAM-bearing DAP-12 adaptor
protein. Simultaneous expression of activating and
inhibitory KIRs that recognize different HLA-class II
ligands would provide signal for elimination of cells
that had lost the expression of ligands for the
inhibitory receptor, but retained expression of the
ligands for the activating receptor. Xenocompatibility
(NK cell receptors of one species functionally
recognize MHC class I ligands of other) has been
detected in chimpanzee and human67. The KIR
families are divergent, with only three conserved
between chimpanzee and humans. The receptors for
polymorphic class I being divergent, whereas those
for non-polymorphic class I are conserved. Although
chimpanzee and human NK cells exhibit identical
receptor specificities for MHC-C, they are mediated
by non-orthologous KIR. This evolutionary progress
demonstrates the rapid evolution of NK cell receptor
systems and implies that “catching up” with class I is
not the only force driving this evolution68.
Mouse NK cell gp49 (type I) is the first example of
an Ig-related molecule with the ability to inhibit
mouse NK cell activity and is closely related to a
newly identified human gene than it is to KIR,
indicating that gp49 and KIR are not immediate
homologs gp49A and gp49B cloned from murine
mast cells. The identification of gp49 as a receptor
with inhibition potential in mast cells and NK cells
suggests that inhibition may be a more wide spread
means of regulation in the lymphoid and myeloid
system than was previously recognized69.
All NK cells do not express all the inhibitory
receptors, but it has been suggested that all NK cells
must express at least one of the inhibitory receptors to
prevent the killing of autologous cells. This
interaction between the MHC and its receptor would
bring about the shutdown of the NK cells70.
In vivo function of NK Receptors
As the NK receptor biology is in progress, its
relation to the pathological conditions is proving its
significance; one of the severe disorders, X-linked
lymphoproliferative disease (XLP) is characterized by
abnormal immune responses to EBV (Epstein barr
virus). After exposure to EBV, XLP males either
succumb to fulminant infectious mononucleosis or
develop lymphoma, hypo or dys-gammaglobulinemia,
and or histiocyte infiltration leading to marrow
aplasia. This disease is associated with the altered
function of 2B4 molecule, owing to the lack of
297
SH2D1A/Sap, which is an intracytoplasmic
molecule71.
In bone marrow transplantation, elimination of
parental bone marrow cells by subsets of NK cells due
to lack of inhibitory receptor for parental H-2 antigen
proves its involvement in hybrid histocompatibility
phenomenon. Recent clinical data suggest that
mismatch of NK receptors and ligands during
allogenic bone marrow transplantation may be used to
prevent leukemia relapse72.
In placental decidual tissue, the number of NK
cells have been found to be more than T cells, which
is the site of exposure to alloantigen. The population
of NK cells in the decidual tissue differs from those in
circulation with respect to the repertoire of NK cell
receptors on these cells. The biological significance of
NK cells in placenta is an intriguing issue, which may
redefine the maternal-fetal interactions73,74. However,
the most important role of NK cells is the host
defense in antiviral and cancer immunity.
In conclusion, NK cells have shown great promise
for the immunotherapy of cancer for several decades.
However, till now only self-effacing clinical success
has been achieved by manipulating the NK cell
compartment in patients with malignancies. Progress
in the field of NK receptors has revolutionized our
concept of how NK cells selectively recognize and
lyse tumours and virally infected cells while sparing
normal cells. Major families of cell surface receptors
that inhibit and activate NK cells to lyse target cells
have been characterized, including KIR, NCR, and
C-type lectins. Further identification of NK receptor
ligands and their expression on normal and
transformed cells is needed to begin development of
rational
clinical
approaches
to
manipulate
receptor/ligand interaction for clinical benefits.
Identification of diverse receptors regulating the NK
cell function may help us to understand its
physiology, thereby offering novel therapeutic tools.
Acknowledgement
Authors are thankful to Mrs. T Hemalatha for
secretarial help.
References
1 Herberman R B, Nunn M E & Lavrin D H (1975) Int J
Cancer 16, 216-229
2 Hackett J J, Bosma G C, Bosma M J, Bennet M & Kumar V
(1986) Proc Natl Acad Sci USA 83, 3427-3431
3 Shinkai Y, Rathbun G, Lam K P, Oltz E M, Stewart V,
Mendelsohn M, Charron J, Stall A M & Alt F W (1992) Cell
68, 855-867
298
INDIAN J. BIOCHEM. BIOPHYS., VOL. 40, OCTOBER 2003
4 Trinchieri G (1989) Adv Immunol 47, 187-376
5 Karre K, Ljunggren H G, Piontek G & Kiessling R (1986)
Nature (London) 319, 675-678
6 Ljunggren H G & Karre K (1990) Immunol Today 11, 237244
7 Lanier L L, Corliss B C, Wu J, Leong C & Philips J H (1998)
Nature (London) 391, 703-707
8 Wu J, Cherwinski H, Spies T, Philips J H & Lanier L L
(2000) J Exp Med 192, 1059-1068
9 Leibson P J (1997) Immunity 6, 655-661
10 Ravetch J V & Perussia B (1989) J Exp Med 170, 481-497
11 Kurosaki T, Gander I & Ravetch J V (1991) Proc Natl Acad
Sci USA 88, 3837-3841
12 Anegon I, Cuturi M C, Trinchieri G & Perussia B (1988) J
Exp Med 167, 452-472
13 Perussia B, Trinchieri G, Jackson A, Warner N L, Faust J,
Rumpold H, Kraft D & Lanier L L (1984) J Immunol 133,
180-189
14 Eischen C M, Schilling J D, Lynch D H, Krammer P H &
Leibson P J (1996) J Immunol 156, 2693-2699
15 Lanier L L, Yu G & Philips J H (1985) Nature (London) 342,
803-805
16 Sewell W A, Brown M H, Dunne J, Owen M J & Crumpton
M J (1986) Proc Natl Acad Sci USA 83, 8718-8722
17 Sanchez M J, Spits H, Lanier L L & Philips J H (1993) J Exp
Med 178, 1857-1866
18 Nandi D, Gross J A & Allison J P (1994) J Immunol 152,
3361-3369
19 Giorda R, Rudert W A, Vavassori C, Chambers W H,
Hiserodt J C & Trucco M (1990) Science 249, 1298-1300
20 Chambers W H, Vujanovic N L, DeLeo A B, Olszowy M W
& Herberman R B (1989) J Exp Med 169, 1373-1389
21 Yokoyama W M, Ryan J C, Hunger J J, Smith H R C, Stark
M & Seeman W E (1991) J Immunol 147, 3229-3236
22 Ryan J C, Truck J, Niemi E C, Yokoyama W M & Seaman
W E (1992) J Immunol 149, 1631-1635
23 Lanier L L, Corliss B & Philips J H (1997) Immunol Rev
155, 145-154
24 Bezouska K, Yuen C T, O’Brien J, Childs R A, Chai W,
Lawson A M, Drbal K, Fiserova A, Pospisil M & Feizi T
(1994) Nature 372 (6502) 150-157
25 Josien R, Heslan M, Soulillou J P & Cuturi M C (1997) J
Exp Med 186, 467-472
26 Lanier L L, Chang C & Philips J H (1994) J Immunol 153,
2417-2428
27 Giorda R, Weisberg E P, Ip T K & Trucco M (1992) J
Immunol 149, 1957-1963
28 Lazetic S, Chang S C, Houchins J P, Lanier L L & Philips J
H (1996) J Immunol 157, 4741-4745
29 Brooks A G, Borregof F, Posch P E, Patamawenu A,
Scorzelli C J, Matthias U, Weiss E H & Coligan J E (1999) J
Immunol 162, 305-313
30 Houchins J P, Lanier L L, Niemi E C, Philips J H & Ryan J
C (1997) J Immunol 158, 3603-3609
31 Carretero M, Cantoni C, Bellon T, Bottino C, Biassoni R,
Rodriguez A, Perez Villar J J, Moretta L & Moretta A (1997)
Eur J Immunol 27, 563-575
32 Wagtmann N, Blassoni R, Cantoni C, Verdiani S, Malnati M
S, Vitale M, Bottino C, Moretta L, Moretta A & Long E O
(1995) Immunity 2, 439-449.
33 Brooks A G, Posch P E, Scorzelli C J, Borrego F, Coligan J
E (1997) J Exp Med 185, 795-800
34 Lanier L L (1998) Annu Rev Immunol 16, 359-393
35 Wu J, Song Y, Bakker A B H, Bauer S, Spies T, Lanier L L
& Philips J H (1999) Science 285, 730-732
36 Gilfillan S, Ho E L, Cella M, Yokoyama W M & Colonna M
(2002) Nat Immunol 3, 1150-1155
37 Diefenbach A, Tomasello E, Lucas M, Jamieson M A, Hsia J
K, Vivier E & Raulet D H (2002) Nat Immunol 3, 1142-1149
38 Bauer S, Groh V, Wu J, Steinle A, Philips J H, Lanier L L &
Spies T (1999) Science 285, 727-729
39 Cosman D, Mullberg J, Sutherland C L, Chin W, Armitage
R, Fanslow W, Kubin M & Chalupny J N (2001) Immunity
14, 123-133
40 Moretta A, Biassoni R, Bottino C, Mingari M C & Moretta L
(2000) Immunol Today 21, 228-234
41 Tomasello E, Desmoulins P O, Chemin K, Guia S, Cremer
H, Ortaldo J, Lore P, Kaiserlian D & Vivier E (2000)
Immunity 13, 355-364
42 Sivori S, Vitale M, Morelli L, Sanseverino L, Augugliaro R,
Bottino C, Moretta L & Moretta A (1997) J Exp Med 186,
1129-1136
43 Pessino A, Sivori S, Bottino C, Malaspina A, Morelli L,
Moretta L, Biassoni R & Moretta A (1998) J Exp Med 188,
953-960
44 Moretta A, Bottino C, Vitale M, Pende D, Cantoni C,
Minagari M C, Biassoni R & Moretta L (2001) Annu Rev
Immunol 19, 197-273
45 Pende D, Parolini S, Pessino A, Sivori S, Augugliazo R,
Morelli L, Marcenaro E, Acame L, Malaspina A, Biassoni R,
Bottino C, Moretta L & Moretta A (1999) J Exp Med 190,
4745-4754
46 Vitale M, Bottino, C, Sirori S, Sanseverino L, Castriconi R,
Marcenaro R, Augugliaro R, Moretta L & Moretta A (1998)
J Exp Med 187, 2065-2072
47 Bouchon A, Cella M, Griersen H L, Cohn J I & Colonna M
(2001) J Immunol 167, 5517-5521
48 Sivoris S, Pende D, Bottino C, Marcenaro E, Pessino A,
Biassoni R, Moretta L & Moretta A (1999) Eur J Immunol
29, 1656-1666
49 Pende D, Cantoni C, Rivera P, Vitale M, Castriconi R,
Marcenaro S, Nanni M, Biassoni R, Bottino C, Moretta A &
Moretta L (2001) Eur J Immunol 4, 1076-1086
50 Colucci F, Santo D P J & Leibson P J (2002) Nat Immunol 3,
807-813
51 Shibuya K, Lanier L L, Phillips J H, Ochs H D, Shimizu K,
Nakayama E, Nakauchi H & Shibuya A (1999) Immunity 5,
615-623
52 Kanki K, Torigoe T, Hirai I, Sahara H, Kamiguchi K,
Tamura Y, Yogihashi A, Sato N (2000) Microbiol Immunol
44, 1051-1061
53 Yokoyama W M & Seaman W E (1993) Ann Rev Immunol
11, 613-635
54 Hanke T, Takizawa H, Christopher W, McMahon, Busch D
H, Pamer E G, Miller J D, Altman J D, Liu Y, Cado D,
Lemonnier F A, Bjorkman P J & Raulet D H (1999)
Immunity 11, 67-77
55 Thomas M L (1995) J Exp Med 181, 1953-1956
56 Natarajan K, Dimasi N, Wang J, Margulies D H & Mariuzza
RA (2001) Mol Immunol 38, 1023-1027
SRIVASTAVA et al.: NATURAL KILLER CELL RECEPTORS
57 Karlhofer F M, Ribaudo R K & Yokoyama W M (1994) J
Immunol 153, 2407-2416
58 Dorfman J R & Raulet D H (1998) J Exp Med 187, 609-618
59 Wong S, Freeman J D, Kelleher C, Mager D & Takei F
(1991) J Immunol 147, 1417-1423
60 Renedo M, Arce I, Rodriguez A, Carretero M, Lanier L L,
Lopez-Botet M & Fernandez-Ruiz E (1997) Immunogenetics
46, 307-311
61 Arase H, Mocarski E S, Campbell A E, Hill A B & Lanier L
L (2002) Science 296, 1323-1326
62 Westgaard I H, Berg S F, Orstavik S, Fossum S & Dissen E
(1998) Eur J Immunol 28, 1839-1846
63 George T C, Mason L H, Ortaldo J R, Kumar V & Bennett M
(1999) J Immunol 162, 2035-2043
64 Gosselin P, Mason L H, Willett-Brown J, Ortaldo J R,
McVicar D W & Anderson S K (1999) J Leukoc Biol 66,
165-171
65 Long E O, Barber D F, Burshtyn D N, Faure M, Peterson M,
Rajgopalan S, Renard V, Sandusky M, Stebbins C C,
Wagtamann N & Watz C (2001) Immunol Rev 181, 223-233
299
66 Uhrberg M, Valiante N M, Shum B P, Shilling H G, LienertWeidenbach K, Corliss B, Tyan D, Lanier L L & Parham P
(1997) Immunity 7, 753-763
67 Lanier L L (2001) Nat Immunol 2, 23-27
68 Khakoo S I, Rajalingam R, Shum B P, Weidenbach K,
Flodin L, Muir D G, Canavez F, Cooper S L, Valiante N M,
Lanier L L & Parham P (2000) Immunity 12, 687-698
69 Rojo S, Burshtyn D N, Long E O & Wagtmann N (1997) J
Immunol 158, 9-12
70 Dhillon S, Das A & Saxena R K (1999) Proc Natl Acad Sci
India 69, 229-243
71 Nichols K E, Harkin D P, Levitz S, Krainer M, Kolquist K
A, Genovese C, Bernard A, Ferguson M, Zuo L, Snyder E,
Buckler A J, Wise C, Ashley J, Lovett M, Valentine M B,
Look A T, Gerald W, Housman D E & Haber D A (1998)
Proc Natl Acad Sci USA 95, 13765-13770
72 Sherif S F, Todd A F, Loredana R, Velardi A & Caligiuri M
A (2002) Blood 100,1935-1947
73 King M A (2002) Nat Rev Immunol 2, 656-664
74 Ashkar A A & Croy B A (2001) Semin Immunol 13, 235-241