editorial - Advances in Clinical and Experimental Medicine

EDITORIAL
Adv Clin Exp Med 2009, 18, 6, 537–542
ISSN 1230–025X
© Copyright by Wroclaw Medical University
ALEKSANDRA BIELAWSKA−POHL, DANUTA DUŚ
When Killers Become Helpers
– the Ambivalent Role of NK Cells
Gdy zabójcy zaczynają ochraniać – dualistyczna rola komórek NK
Institute of Immunology and Experimental Therapy, Polish Academy of Sciences, Wrocław, Poland
Abstract
The aim of this review is to present current knowledge on natural killer (NK) cell activity. They were originally
known as effector lymphocytes involved in the nonspecific immune response, recognizing and destroying a broad
array of pathological cells without damaging the normal healthy cells of the organism. Recent investigations high−
light the fact that NK cells may also act as regulatory cells engaged in interactions with other cells to limit or exac−
erbate an immune response. New roles for NK cells engaged in autoimmunity and reproduction have indeed been
postulated. All available data suggest that NK cells, depending on their microenvironment, may have antagonistic
functions. Consequently, integration of the interactions with neighboring cells governs the dynamic equilibrium
regulating NK cell activity and dictates whether NK cells are activated to kill target cells or to operate as regula−
tory cells. In summary, certain stimuli may differentially trigger NK cell response and NK cells may therefore act
as a two−edged sword in the human organism, as presented with two examples of NK cell activity: allograft killing
and semi−allograft protection (Adv Clin Exp Med 2009, 18, 6, 537–542).
Key words: natural killer cells, natural cytotoxicity, cellular interactions.
Streszczenie
Celem pracy było przedstawienie obecnego stanu wiedzy na temat aktywności komórek naturalnej cytotoksyczno−
ści (NK). Komórki NK należą do limfocytów efektorowych uczestniczących w niespecyficznej odpowiedzi immu−
nologicznej, odpowiedzialnych za rozpoznanie i zniszczenie patologicznie zmienionych komórek, bez uszkadza−
nia prawidłowych komórek organizmu. Ostatnie doniesienia literaturowe podkreślają, że komórki NK wykazują
także właściwości komórek regulatorowych, zdolnych przez oddziaływania z innymi komponentami układu odpor−
nościowego do aktywacji, lub wręcz przeciwnie, do osłabienia odpowiedzi immunologicznej. Rzeczywiście, opi−
sano nowe funkcje komórek NK postulowane w procesach autoimmunologicznych i rozmnażaniu. Wszystkie do−
stępne obecnie dane wskazują, że limfocyty NK w zależności od mikrośrodowiska mogą wykazywać dwojakie
działanie. Uważa się, że wypadkowa oddziaływań limfocytów NK z sąsiadującymi komórkami reguluje aktywność
komórek NK, decydując, czy nastąpi aktywacja ich cytotoksyczności, czy też będą zdolne do uruchomienia me−
chanizmów regulatorowych. Reasumując, poszczególne bodźce mogą w różnym stopniu regulować aktywność ko−
mórek NK, dlatego też komórki te mogą być swoistą bronią obosieczną, dwojako wykorzystywaną przez organizm
ludzki. Przedstawiono to na przykładzie aktywności limfocytów NK w stosunku do komórek allogenicznych bądź
semiallogenicznych (Adv Clin Exp Med 2009, 18, 6, 537–542).
Słowa kluczowe: komórki naturalnej cytotoksyczności, cytotoksyczność naturalna, oddziaływania międzykomór−
kowe.
Natural killer (NK) cells play an important
role among the natural protective mechanisms.
These first−line immune defense cells have the
ability to recognize and destroy a broad array of
pathologically changed cells, including neoplastic
cells, virus−infected cells, antibody−coated cells,
foreign transplants, as well as “stressed” cells,
without damaging normal healthy “self” cells
belonging to the organism [1]. By virtue of their
different effector functions, NK cells participate in
an early defense system that can directly eliminate
virally infected or transformed cells through direct
538
A. BIELAWSKA−POHL, D. DUŚ
target−cell lysis and/or recruit nonspecific
hematopoietic cells (macrophages), activate den−
dritic cells, and prime adaptive lymphocytes in
order to initiate and execute an inflammatory
response.
What is a Cytotoxic
NK Cell?
Target cell recognition is started by the initial
binding, mediated mainly by cell adhesion mole−
cules, followed by further closer interactions
between NK cell−activating and −inhibitory recep−
tors and their appropriate ligands exposed on the
target cell. The combination of signals transmitted
by these receptors decides whether the NK cell
detaches from the target cell or stays and responds
by final lysing of the target cell [2].
According to the previously accepted “miss−
ing−self” hypothesis, the function of NK cells was
to recognize and eliminate cells that fail to express
self major histocompatibility complex (MHC)
class I molecules [3]. Recent experimental evi−
dence shows that NK cells provide immune sur−
veillance not only of cells that lack MHC class I,
but also of those overexpressing ligands for NK cell−
activating receptors [2]. NK cells whose inhibito−
ry/activatory receptors recognize neither an MHC
class I molecule nor an activating ligand on the tar−
get cell surface are not able to respond (Fig. 1A).
Similarly, no response is observed when inhibitory
NK cell receptors suppress NK cell cytotoxicity
after recognizing their proper ligand (an MHC
class I molecule) on the target cell (Fig. 1B). The
NK cell executes its cytotoxicity towards a target
cell when the NK cell activatory receptor recog−
nizes its ligand on the surface of target cells but
receives no signal from inhibitory NK cell recep−
tors (Fig. 1C). In the case of interaction with a tar−
get cell expressing ligands for both inhibitory and
activating receptors, the outcome is determined by
the sum of the strengths of the antagonistic signals.
This dynamic equilibrium regulates NK cell acti−
vation and dictates whether or not NK cells will be
able to kill the target cell (Fig. 1D) [2, 4].
NK cells, which were initially described as
a population of “naturally occurring killer lym−
phocytes”, are now known as a heterogeneous
population of lymphocytes classified on the base
of their phenotypic and functional features. The
most reliable molecular markers which define
human NK cells are CD16 (low−affinity
immunoglobulin receptor, FcγRIII) and/or CD56
(neural cell−adhesion molecule, N−CAM) and the
absence of CD3 cell surface molecules [5]. Based
on the CD16 and CD56 molecule expression lev−
Fig. 1. Inhibitory and activating signals which deter−
mine NK cell cytotoxicity (adapted from [2])
Ryc. 1. Sygnały aktywujące i hamujące cytotoksyczną
aktywność komórek NK (zmodyfikowane [2])
els, NK cells from normal adults can be subdivid−
ed into two major subsets. The majority of NK
cells circulating in human blood express low lev−
els of CD56 (CD56dim) and high levels of CD16
(CD16bright). These CD56dim/CD16bright NK
cells are highly cytotoxic. The second subset of
NK lymphocytes, located mainly in lymphoid
organs, contains CD56bright cells lacking or
expressing low levels of CD16 molecules. They
are less effective in cytotoxicity, but are very
potent cytokine producers [6, 7].
NK cell functional activation may be triggered
by multiple cytokines, such as the interleukins IL−2,
IL−12, IL−15, and IL−18, type I interferons (IFNs),
and chemokines [5, 8, 9]. Upon cytokine stimula−
tion, NK cells may exert their biological activity by
a triad of functions: direct target−cell killing,
cytokine secretion, and costimulation of other
immunocompetent cells. Consequently, NK cells
can eliminate their target cells using different path−
ways: the perforin/granzyme/granulysin pathway,
the death receptor pathway, and the antibody−de−
pendent cell cytotoxicity (ADCC) pathway (Fig. 2).
Upon activation, NK cells are able to release their
granule components, such as perforin, granzymes,
and granulysin, to induce target−cell lysis. NK cells
also express death receptors which, after recogniz−
ing their cognate ligands on the surface of the tar−
get cell, may induce its death by apoptosis. To
stimulate ADCC, NK cells use CD16 molecules
539
Ambivalent Role of NK Cells
NK cell
target cell
A
TNFα
TNFR1
B
inhibitory
receptors
MHC class I
activatory
receptors
C
Y
D
perforin/
granzyme
CD16 (ADCC)
Fig. 2. Different pathways which may be used by NK
cells to eliminate target cells: A) cytokine−dependent
pathway (activation of death receptors), B) MHC−
dependent pathway (activation of natural cytotoxicity
receptors), C) perforin/granzyme−dependent pathway,
D) antibody−dependent cell cytotoxicity (ADCC) path−
way
Ryc. 2. Różnorodne szlaki zabijania komórek
docelowych przez limfocyty NK: A) szlak zależny od
cytokin (aktywujący receptory śmierci), B) szlak
zależny od cząsteczek MHC (aktywacja receptorów
naturalnej cytotoksyczności, C) szlak zależny od per−
foryn i granzymu, D) cytotoksyczność komórkowa
zależna od przeciwciał (szlak ADCC)
expressed on their surface. CD16 antigen recog−
nizes antibody−coated target cells [10]. After this
recognition, NK cells release their proteolytic
enzymes, which execute target−cell death.
As mentioned above, stimulated NK cells,
demonstrating increased proliferation, themselves
produce their own immunoregulatory cytokines
and/or chemokines, such as interferon gamma
(IFN−γ), tumor necrosis factor (TNF−α), granulo−
cyte macrophage colony−stimulating factor (GM−
CSF), interleukin 10 (IL−10), interleukin 3 (IL−3),
monokine−induced proteins 1 α and β (MIP−1α and
−β), and RANTES. Through cytokine release, NK
cells initiate and amplify other components of both
the innate and adaptive immune systems, i.e. den−
dritic cells and T and B lymphocytes [11].
Mobilization of NK Cells
to an Inflammatory Site –
the Role of the Endothelium
To reach tissue sites of inflammation, infec−
tion, or injury and perform their major role, NK
cells have to leave the blood vessels. This extrava−
sation occurs at specialized postcapillary vascular
sites called high endothelial venules (HEVs). In
humans, HEVs are found in all secondary lym−
phoid organs, including numerous lymph nodes
dispersed throughout the body, tonsils,
Peyer’s patches in the small intestine, appendix,
and small aggregates of lymphoid tissues in the
stomach and large intestine [12]. The extravasa−
tion process is, in principle, divided into three
sequential steps: the recognition of endothelial
cells (ECs) by circulating lymphocytes, their tight
adhesion to the EC monolayer, and finally the
transendothelial migration of lymphocytes.
Endothelial cells, which line the entire vascular
system, play a crucial role in this process.
Circulating lymphocytes learn when and
where they should migrate after receiving certain
activatory signals. Activated lymphocytes move to
the fringe of the blood stream and start to roll on
the surface of EC monolayers. Generally, the
rolling of lymphocytes is mainly mediated by
selectins, which are expressed on the surface of
both lymphocytes and ECs. Inflammatory signals
normally upregulate selectin expressions. In the
second step, lymphocytes tightly attach to ECs.
This tight attachment is mediated mainly by inte−
grins, which are activated by chemokine signaling.
The regulated expression of chemokines and their
receptors seems to be a critical determinant for
lymphocytes’ transendothelial migration. Diape−
desis through the vessel wall is the final step in the
migratory process. Bound lymphocytes can cross
the endothelial barrier. Finally, lymphocytes per−
ceiving and accumulating signals from multiple
chemoattractant sources can migrate to their desti−
nation [13, 14]. During migration, lymphocytes do
not behave aggressively towards ECs.
The Role of NK Cells
in Xenograft Rejection
The most conspicuous activity of NK cells in
immunosurveillance can be observed in xenotrans−
plantation. Xenotransplantation, i.e. the transplan−
tation of tissue between different species, is con−
sidered to be a possible promising treatment for
several illnesses. For medical, practical, as well as
financial reasons, the pig was chosen as the most
suitable donor species. In such a xenograft, human
NK cells exert cytotoxicity against porcine ECs.
These cytotoxic interactions are induced by the
expression of non−self MHC class I molecules by
porcine ECs. Apart from the induction of direct
lysis via the perforin/granzyme pathway, human
NK cells also execute the ADCC pathway [15,
16]. However, in spite of the immune
system’s strong response against xenografts,
a deeper understanding of these interactions may
540
A. BIELAWSKA−POHL, D. DUŚ
bring a solution [17]. To prevent cellular organ
xenograft rejection by inhibiting NK cell cytotox−
icity towards porcine ECs, combinations of differ−
ent blocking antibodies were investigated [16]. It
was demonstrated that interaction between the het−
erodimeric inhibitory receptor (CD94/NKG2A)
expressed by NK cells with its ligand, human
leukocyte antigen (HLA−E), on porcine ECs may
inhibit NK cell−mediated cytotoxicity [18].
However, blocking one adhesion receptor was not
sufficient to protect xenografts from rejection.
Combinations with other measures have been test−
ed. There are also trials to construct transgenic
donor animals (pigs) expressing human MHC
molecules [19].
Pregnancy and NK Cells
– Non−Cytotoxic Uterine
NK Cells?
An opposite example, i.e. of a passive behav−
ior of NK cells, is pregnancy. The maternal
immune system tolerates the semi−allograft fetus.
Under physiological conditions, uterine NK cells
(uNK) do not behave aggressively towards fetal
tissues. The origin of uNK cells, which peak dur−
ing the secretory phase of the luteal cycle and
early pregnancy, is still not well established.
Generally, there are two subsets of uNK cells,
which are distinct from the maternal peripheral
blood NK cells: endometrial NK cells (eNK) and
decidual NK cells (dNK). The first subpopulation,
found in the uterus during the menstrual cycle, has
an antigen expression profile similar to peripheral
blood CD16+CD56+ NK cells. They play a role in
toxicity against viral infections that could disturb
the menstrual cycle. In contrast, dNK cells, found
in the decidua during pregnancy, are NK cells of
the CD16–CD56bright phenotype. These dNK
cells also express the natural cytotoxicity receptors
and may induce lysis of target cells deficient in
major histocompatibility complex presentation.
However, they remain non−cytotoxic to fetal tissue
and even support a normal pregnancy by produc−
ing regulatory cytokines and growth factors.
Among others, dNK cells are an important source
of angiogenic growth factors, including angiopoi−
etins and vascular endothelial growth factor−C
(VEGF−C), which affect the migration and prolif−
eration of endothelial cells, supporting angiogene−
sis, a crucial step in a successful pregnancy [20–22].
Thus in a normal pregnancy, dNK cells behave as
sentinels balancing between attenuated cytotoxici−
ty and normal pregnancy support, producing regu−
latory cytokines and growth factors.
When a decidual macrophage or dendritic cell
recognizes trophoblast debris, uNK cells may
become active and acquire a cytotoxic function
similar to peripheral NK cells. This results in
a decrease in low−cytotoxic uNK cells (CD16
–
CD56+) in the pre−implantation endo−metrium
with a simultaneous increase in cytotoxic uNK cells
(CD16+CD56–). This was observed in women
with recurrent miscarriages who have more acti−
vated cytotoxic uNK cells accumulated at the
endometrium than observed in normal pregnan−
cies. Therefore it may be postulated that sponta−
neous abortion may be caused by insufficient
inhibition of uNK cells’ cytolytic machinery, and
their exact role is still the subject of discussion
[21–23].
Concluding Remarks and
Future Challenges
– Natural Killer Cell−Based
Inhibition of Tumor
Angiogenesis?
For many years, NK cells were considered
exclusively as killer cells. Nowadays it is known
that their functional heterogeneity is more complex
than a simple cytolytic/cytokine−producing dichoto−
my, underscoring the plasticity of NK cell respons−
es in different biological situations. Recent investi−
gations reveal a critical role of the environment and
cytokine milieu in which NK cells are placed. Thus
to play their role correctly, NK cells need to perform
highly specific cross−talk with other cells. Under
these conditions, membrane−bound or soluble fac−
tors may dictate the ultimate effector functions that
dominate during an NK cell response.
As mentioned above, NK cells recognize
pathologically modified tumor cells and, after
recognition, execute their killing program.
However, in most cases this mechanism is insuffi−
cient to prevent tumor growth. The question arose
whether it would be possible to enhance NK cells’
effectiveness inside tumor tissue. Based on the fact
that cytokines may increase NK cells’ cytotoxic
activity, different anti−cancer therapies were inves−
tigated. It is known that interleukin 12 (IL−12)
activates cells of both innate (NK and NKT cells)
and adaptive (CD4+ and CD8+ T cells) immunity.
In particular, IL−12 promotes T−cell, NK−cell, and
NKT−cell effector functions and induces the secre−
tion of IFN−γ, identified as a mediator critically
involved in the cellular immune response [24].
Some reports reveal that IL−12 may even induce
an antiangiogenic program mediated by lympho−
Ambivalent Role of NK Cells
cyte−endothelial cell cross−talk. It has been also
demonstrated that NK cells activated by IL−12 are
able to recognize, adhere to, and even kill endothe−
lial cells [25]. It could be hypothesized that NK−
cell cytotoxicity towards endothelial cells may be
a potential mechanism by which IL−12 can sup−
press tumor neovascularization. Interesting obser−
vations were made by Chouaib et al., who noticed
that in an experimental mouse model, IL−12−treat−
541
ed tumors were characterized by an increase in NK
cell number around existing microvessels [unpub−
lished observation]. It is known that tumor vascu−
lature demonstrates unique features depending on
tumor grade and stage, tissue specificity, the
angiogenic micromilieu, and host immune status
[26, 27]. Therefore it may be concluded that under
certain conditions, NK cells may also be potential
mediators of angiogenesis inhibition.
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542
A. BIELAWSKA−POHL, D. DUŚ
Address for correspondence:
Danuta Duś
Institute of Immunology and Experimental Therapy
Polish Academy of Sciences
Weigla 12
53−114 Wrocław
Poland
E−mail: [email protected]
Conflict of interest: None declared
Received: 26.11.2009
Accepted: 1.12.2009