HLA-G-Mediated NK Cell Senescence Promotes Vascular Remodeling: Implications for Reproduction

MINI REVIEW

HLA-G-mediated NK cell senescence promotes vascular remodeling: implications for reproduction

Sumati Rajagopalan

The uterus in early pregnancy is a non-lymphoid organ that is enriched in natural killer (NK) cells. Studies to address the role of these abundant human NK cells at the maternal/fetal interface have focused on their response to the major histocompatibility complex (MHC) molecules on fetal trophoblast cells that they contact. The interaction of maternal NK cell receptors belonging to the killer cell immunoglobulin-like receptor (KIR) family with trophoblast MHC class I molecules in pregnancy can regulate NK cell activation for secretion of pro-angiogenic factors that promote placental development. This review will cover the role of KIR at the maternal/fetal interface and focus on KIR2DL4, a KIR family member that is uniquely poised to play a role in pregnancy due to the restricted expression of its ligand, human leukocyte antigen (HLA)-G, by fetal trophoblast cells early in pregnancy. The pathways by which KIR2DL4–HLA-G interactions induce the cellular senescence of NK cells and the role of the resulting senescence-associated secretory phenotype (SASP) in vascular remodeling will be discussed in the context of reproduction. Cellular & Molecular Immunology (2014) 11, 460–466; doi:10.1038/cmi.2014.53; published online 7 July 2014

Keywords: cellular senescence; HLA-G; KIR2DL4; natural killer cells; pregnancy

INTRODUCTION such as samples from first trimester abortions, term human Natural killer (NK) cells are white blood cells that are import- placenta and timed endometrial biopsies from non-pregnant ant for immune defense and reproduction.1,2 Their role in women.5 This is complemented by studies in pregnant rodents immune defense has been well studied. They participate in and humanized mice, where findings can be extrapolated to innate immunity as the first line of defense through their ability human NK cell functions.6 In this regard, studies in mice to kill target cells and secrete cytokines. More recently, the identified spiral artery remodeling as a major uterine NK func- identification of several features of NK cells, such as their capa- tion that results in the conversion of maternal vessels to low- city for memory responses and ability to control adaptive resistance, high-volume conduits to increase blood supply to immune cells, underscores their importance as players in the fetus.7 adaptive immunity.3,4 In contrast to what is known about their role in immune NK CELLS IN PREGNANCY defense, much less is known about their role in reproduction. NK cells are the dominant lymphocyte population that accu- Unlike in the peripheral blood, where NK cells make up 2%– mulates in the decidua basalis of a healthy pregnancy. At this 5% of lymphocytes, they are the most abundant immune cells site, NK cells are CD56bright and CD16neg and phenotypically (50%–70%) at the maternal/fetal interface during implanta- distinct from peripheral blood NK cells that are predominantly tion and early pregnancy. Altered cell numbers and genetic CD56dim and CD161.2 These uterine NK cells exist in lower association studies involving these NK cells in women with numbers in the non-pregnant endometrium and their numbers reproductive disorders have shown that they may play a rel- increase in early pregnancy. The peak human decidual NK cell evant role in reproduction.2 numbers were identified between 8 and 13 weeks of gestation.8 Although systematic manipulation of NK cells in the uterus Recent work has shown a role for the soluble factors secreted by of pregnant women at different times following implantation is these cells during placental development, both in spiral artery not feasible, much has been learnt from alternative approaches, remodeling and in the regulation of trophoblast invasion.8,9

Laboratory of Immunogenetics, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Rockville, MD, USA Correspondence: Dr S Rajagopalan, Laboratory of Immunogenetics, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Rockville, MD 20852, USA. E-mail: [email protected] Received: 1 May 2014; Revised: 4 June 2014; Accepted: 4 June 2014 A role for senescent NK cells in pregnancy S Rajagopalan 461

Decidual NK cells are poorly cytotoxic in a normal preg- activating decidual NK cells to proliferate and secrete soluble nancy, even though these cells contain lytic granules and are factors.16,17 Thus, the view of a tolerogenic HLA-G is being fully capable of cytotoxic activity against classical NK targets, challenged by more recent evidence that points to a more con- such as K562 cells.10 Recent studies have revealed that decidual structive role in promoting changes in placentation. The pres- NK cells have the potential for efficient pathogen control and ence of soluble HLA-G in human embryo culture after in vitro clearance as a result of pathogen mediated signals.11 Coculture fertilization has been associated with better pregnancy rates of human cytomegalovirus-infected decidual fibroblasts with and reduced HLA-G levels in maternal circulation has been decidual NK cells from early gestation resulted in significant reported in disorders of pregnancy such as recurrent spontan- 22 induction of an array of cytokines and chemokines, such as eous abortion and pre-eclampsia. There are important vas- vascular endothelial growth factor-A, IL-6, chemokine (C–X– cular changes in response to MHC-dependent signals that C motif) ligand 1 and macrophage chemoattractant protein appear to contribute to the spiral artery remodeling and con- (MCP)-1.12 A combination of the modulation of expression trol of trophoblast invasion that is critical to a successful preg- of NK receptors and pathogen-induced ligands allows NK cells nancy. This is reflected in pathologies related to defects in this to overcome the inhibition of decidual NK killing seen in nor- vascularization, such as intrauterine growth restriction and mal pregnancy. There are regulatory mechanisms in place to pre-eclampsia. Thus, decidual NK responses to MHC class I prevent killing at the maternal/fetal interface during a healthy molecules including HLA-G on trophoblast cells have import- pregnancy. These include inhibitory receptors on NK cells ant consequences for reproductive fitness down the line. belonging to the killer cell immunoglobulin-like receptor THE INTERPLAY OF KIR AND THEIR MHC LIGANDS IN (KIR), CD94/NKG2 and ILT families of receptors that recog- PREGNANCY nize major histocompatibility complex (MHC) class I molecules on extravillous trophoblast cells and block cytotoxic Several studies have focused on KIR responses to their MHC 2 ligands on trophoblasts and assessed the impact of these res- responses. ponses on successful placentation.23 KIR family members can Of the two types of fetal trophoblast cells at the maternal/ be either inhibitory or activating, and NK cell activation is the fetal interface, villous trophoblast cells, in contact with mater- result of a complex interplay between these different receptors nal blood flowing through the intervillous space, do not express that are stochastically expressed on NK cells.24 any MHC class I molecules and are considered to be immuno- Genetic association studies indicate that the interaction logically inert. Extravillous trophoblast cells that migrate between maternal KIR and fetal HLA-C during pregnancy through the decidua towards the maternal spiral arteries in may influence the delivery of sufficient blood supply for the early pregnancy express HLA-C, HLA-E and HLA-G. In con- fetus.25 Combinations of fetal HLA-C and maternal KIR that trast, the classical, polymorphic MHC molecules (HLA-A and resulted in a potential for increased inhibitory interactions HLA-B) are not present on trophoblasts at the maternal/fetal showed association with pre-eclampsia, a condition of preg- interface. While the inhibitory 2 domain (2D) KIR (KIR2DL1, nancy characterized by inadequate trophoblast invasion and KIR2DL2 and KIR2DL3) recognize HLA-C alleles, HLA-E is impaired vascular remodeling. The strongest association was recognized by the inhibitory CD94/NKG2A and the activating seen with maternal KIR of the AA haplotype (lacking activating CD94/NKG2C heterodimers. Receptors on NK cells for HLA- KIR) in combination with HLA-C2 in the fetus.26 Similar epi- G include the inhibitory ILT2 (LILRB1) and the activating 13 demiological evidence was also reported in women with recur- KIR2DL4. rent miscarriage.27 These association studies underscore the Since all extravillous trophoblast cells express HLA-E (which importance of MHC–KIR interactions to the regulation of pla- relies on the signal peptides of HLA-C and HLA-G for surface centation and show that too much inhibition is detrimental to expression) and all decidual NK cells express CD94/NKG2A, successful placentation. Experimental evidence for a role for this interaction is thought to be responsible for the lack of excessive NK cell inhibition in compromising reproductive killing of trophoblasts by NK cells. In addition, inhibition is success has been elegantly provided in the mouse system. By also provided by interactions of inhibitory KIR with HLA-C using mice that differ in a single additional MHC (expressed by 13 and by ILT2 with HLA-G. either maternal or fetal tissues) that confers additional inhib- What is the role of HLA-G, a non-classical MHC molecule itory potential upon recognition of Ly49 family members, it that is unusual in that it is uniquely restricted to fetal extra- was shown that excess inhibition compromises decidual vas- villous trophoblast cells under normal physiological condi- cular remodeling and results in fetal growth restriction.28 tions? The dogma that HLA-G has a key role in tolerance at In a normal pregnancy, it is well documented that decidual the maternal/fetal interface by protecting the fetus from attack NK cells produce soluble factors that influence placental by NK cells of the mother 14 is losing ground, as recent findings development. These include pro-angiogenic factors, such as do not support such a role.15–18 Earlier reports on the inhib- vascular endothelial growth factor, angiopoietin-1, angiopoie- itory effects of HLA-G used total peripheral blood mononuc- tin-2 and placenta growth factor.9,29,30 The release of these lear cells to study NK cell cytotoxic activity.19–21 The inhibitory soluble mediators is the result of engagement of activating recep- effect of HLA-G was found to be due to T cells and not NK cells, tors (in addition to KIR) that are expressed by decidual NK cells, with HLA-G inhibiting T-cell proliferation and killing while such as NKp30 and NKp46, by ligands on the extravillous

Cellular & Molecular Immunology A role for senescent NK cells in pregnancy S Rajagopalan 462

trophoblasts and decidual stromal cells.9,31 These receptors also but did not bind either ILT2 or KIR2DL4,43 owing to the very induce the secretion of cytokines and chemokines, such as low to negligible surface expression of these two receptors on interferon (IFN)-c, tumor-necrosis factor (TNF)-a, granulo- NK cells. On the cell surface, HLA-G can be expressed as either cyte-macrophage colony-stimulating factor, macrophage monomers or as a disulphide-linked homodimer.41,44 While inflammatory protein-1a and macrophage inflammatory pro- dimeric HLA-G is the preferred ligand of ILT2, whether tein-1b. They also secrete chemokines such as IL-8 and IP10 KIR2DL4 favors monomers or dimers is unknown. From the that interact with chemokine receptors on trophoblast cells.9 crystal structure of dimeric HLA-G and modeling of KIR2DL4– These chemokines favor the migration of extravillous tropho- HLA-G interactions, it is unlikely that KIR2DL4 can bind HLA- blast cells into the decidua basalis where they invade the spiral G in its dimeric state.45 arteries to promote uterine vascular remodeling. The effect of In contrast to all the other KIR family members that are soluble factors, and IFN-c in particular, on mesometrial spiral expressed well at the cell surface, KIR2DL4 is undetectable at artery remodeling, has also been shown in seminal experiments the cell surface of primary, resting NK cells from the peripheral 7,32 in the mouse. blood. Surface levels of this receptor can be transiently The KIR family of receptors contains activating isoforms increased upon IL-2 activation.38,46 In decidual NK cells iso- that recognize specific HLA ligands. Engagement of KIR2DS1 lated from first trimester abortions, there is evidence of low cell on decidual NK cells resulted in the secretion of GM-CSF, a surface expression of KIR2DL4.31 Despite negligible surface 33 factor that enhances trophoblast migration. Likewise, HLA- expression, in resting NK cells, activation of KIR2DL4 with Cw4-mediated activation of KIR2DS4 on decidual NK cells soluble agonists resulted in strong cytokine responses in the resulted in the secretion of IL-8, IP10, vascular endothelial 15 9 absence of killing. This is explained by its intracellular local- growth factor and placenta growth factor. Unlike the rest of ization in Rab51 early endosomes.38 This endosomal targeting the KIR2D receptors that are stochastically expressed on all NK is unique to KIR2DL4 as other KIR family members are cells, KIR2DL4 is an activating receptor that is expressed by all 34,35 expressed at high levels on the cell surface. Endosomal local- NK cells. Its relevance at the maternal/fetal interface is due ization of KIR2DL4 is controlled by its immunoglobulin to the trophoblast-specific expression of its ligand, HLA-G, in domains and not by the cytoplasmic tail, where endocytic sig- healthy individuals. nals normally reside.47 It was recently reported that syndecan 4, a member of the syndecan family of cell surface proteoglycans, KIR2DL4–HLA-G INTERACTIONS AND ENDOSOMAL colocalizes with KIR2DL4 in endosomes and that this inter- SIGNALING action was reduced in the presence of heparin sulfate.48 KIR2DL4 is an unusual KIR family member in many respects, However, its potential role as a partner chain responsible for including its structure, expression, localization and signaling its trafficking to endosomes and signaling there awaits further function. Unlike the other 2D KIR with a D1–D2 domain structure, it has a hybrid D0–D2 domain structure, utilizing study. the D0 domain of the 3D KIR (which have D0, D1 and D2 Analysis of wild-type and chimeric mutants of KIR2DL4 in domains). As the anchor gene in the middle of the KIR com- 293T cells indicated that this receptor needed to be in endosomes to signal, as chimeric receptors targeted to the cell sur- plex, KIR2DL4 is present in all haplotypes and is constitutively 38 expressed by all NK cells. Although it carries a long cytoplasmic face could not signal upon crosslinking at the cell surface. tail characteristic of inhibitory KIR, it carries only a single ITIM Signaling by KIR2DL4 is distinct from that of other KIR as it is and shows weak inhibitory potential.36 It has a charged argi- independent of both ITIM and ITAM-mediated signaling and nine residue in its transmembrane domain that allows it to pair resistant to inhibitors of Src family kinases and phosphatidy- 49 with FceRIc to signal for weak cytotoxicity and cytokine secre- linositide 3-kinase. Unexpectedly, Akt phosphorylation (at tion at the cell surface.37 serine 473) was identified by kinase phosphorylation-profiling The non-classical MHC-I molecule HLA-G is the ligand for upon activation through KIR2DL4. Importantly, this Akt activa- KIR2DL4. The soluble form of HLA-G is likely to be the natural tion upon KIR2DL4 engagement required its endocytosis. Mass ligand as it accumulates into endosomes containing KIR2DL4.38 spectrometry analysis of proteins associated with endosomal Alternate splicing of the transmembrane isoform HLA-G1 KIR2DL4 was used to identify regulators of Akt activation. results in the soluble HLA-G5. Soluble HLA-G can also be This approach identified DNA-PKcs, a DNA damage signaling generated by shedding of the extracellular portion of HLA-G1 kinase, as the kinase responsible for phosphorylation of Akt at 49 from the cell surface due to the action of metalloproteases.39 serine 473. Recombinant KIR2DL4-immunoglobulin fusion proteins KIR2DL4 has a conserved binding motif in its cytoplasmic bound transfectants expressing high levels of HLA-G on their tail for the ubiquitin ligase TNF-receptor-associated factor 6. surface,34,40 and this binding was blocked with antibodies to Its association with TNF-receptor-associated factor 6 results in HLA-G (G233) or KIR2DL4 (#33).38 However, direct binding phosphorylation of the kinase TAK1 and links it to nuclear factor of soluble forms of HLA-G and KIR2DL4 has not been detected kappa B activation pathways.50 The ubiquitin ligase TNF-recep- by surface plasmon resonance,41 possibly due to low intrinsic tor-associated factor 6 is known to couple proximal signals from affinity, as seen with the activating KIR and their MHC endosomal receptors, such as the Toll-like receptors, through ligands.42 Tetrameric HLA-G bound ILT4 on monocytic cells, TAK1 for nuclear factor kappa B-dependent pro-inflammatory

Cellular & Molecular Immunology A role for senescent NK cells in pregnancy S Rajagopalan 463 responses. In this way, KIR2DL4 activates nuclear factor kappa B typical source of decidual NK cells are cells isolated from first responses via the canonical pathway to generate pro-inflammat- trimester abortions, and in these cells, the early stimulation by ory/pro-angiogenic signals. Negative regulation of KIR2DL4 sig- trophoblasts secreting soluble HLA-G has already occurred. naling occurs through a different ubiquitin ligase, Triad3A that To test the hypothesis that decidual NK cells would display a binds the cytoplasmic tail of KIR2DL4 and promotes its degra- senescent phenotype as a result of early responses to soluble dation.51 HLA-G from trophoblasts during pregnancy, a retrospective Thus, endosomal signaling by KIR2DL4 offers the advantage analysis of micro-array data of decidual NK cells from first of sustained signals for the secretion response that involves the trimester abortions (gestational age: 6–12 weeks) compared secretion of cytokines and chemokines, such as IFN-c, TNF-a, to resting peripheral blood NK cells57 was performed. There IL-1a, IL-1b, IL-6 and IL-8.47 Signaling from intracellular was upregulation of genes involved in senescence and SASP in endosomal compartments offers the advantage of escaping the decidual samples, including genes critical for the induction the regulation by inhibitory NK receptors at the NK cell surface. and maintenance of senescence, such as IL-6, IL-8, IL-1b and In addition, endosomes provide a platform that allows optim- CDKN1A (p21).53 It is noteworthy that the cell cycle inhibitor ization of signals that may otherwise be weak due to factors p21, a major determinant of senescence, was upregulated such as low affinity and limiting ligand concentrations.52 throughout the gestational period studied (6–12 weeks).57 This evidence of a senescence signature in decidual NK cells INDUCTION OF CELLULAR SENESCENCE THROUGH was validated by gene set enrichment analysis against the KIR2DL4–HLA-G INTERACTIONS molecular signature of oncogene-induced senescence.53 Thus, It was surprising that signaling from endosomes for a pro- as seen in NK cells from peripheral blood, the increased inflammatory and pro-angiogenic response would involve expression of genes involved in senescence and SASP is also a DNA-PKcs, a kinase involved in DNA damage response signal- feature of NK cells isolated directly from the pregnant uterus in ing pathways. Since DNA damage response signaling occurs the first trimester. during the induction of cellular senescence, the induction of the cyclin kinase inhibitor, p21, a key determinant of sen- A ROLE FOR SENESCENCE IN REPRODUCTION escence was evaluated. Primary NK cells was stimulated with The decidua in early pregnancy is a setting where the senescence the ligand of KIR2DL4, sHLA-G upregulated p21 and, this was and SASP produced by KIR2DL4–HLA-G interactions may con- sensitive to inhibitors of DNA-PKcs.53 These cells underwent tribute to the normal vascular adaptations that occur to support changes characteristic of senescence, such as changes in cell fetal development (Figure 1).58 NK cells at the implantation site shape and size, and staining for senescence-associated b-galac- would sense trophoblast invasion by responding to the soluble tosidase. They also remained arrested at the G0/G1 stage of the HLA-G produced by these invading fetal cells. Endosomal sig- cell cycle and were refractory to apoptosis.53 naling by KIR2DL4 in response to HLA-G would result in the The induction of cellular senescence and the cell cycle arrest reprogramming of NK cells to a senescent state. The resulting associated with it has been implicated in aging, cancer and SASP would include the production of pro-inflammatory fac- tissue repair.54 Although senescent cells do not divide or tors, such as TNF-a,IL-1b and IFN-c, and pro-angiogenic fac- undergo apoptosis, they are metabolically very active and tors, such as IL-6 and IL-8. They also include proteins of the secrete an array of soluble mediators collectively termed the urokinase plasminogen activator system (e.g., uPAR) that con- senescent-associated secretory phenotype (SASP).55,56 The tribute to the extracellular matrix breakdown required for both SASP includes pro-inflammatory cytokines, growth factors trophoblast invasion and spiral artery remodeling. The resulting and proteases. Once a cell becomes senescent, it has altered SASP would shape and remodel the local environment to interactions with its surrounding milieu via the SASP. This increase vascularization required for a growing fetus. In such a SASP can be both detrimental by promoting tumor growth situation, senescence would promote reproductive success. and age-related pathologies and useful as in the case of tissue As neither KIR2DL4 nor HLA-G molecules are present in repair during wound healing. mice, responses generated by their interactions cannot be Micro-array studies of NK cells stimulated via KIR2DL4 studied in mice. However, Croy and colleagues5 have noted, revealed evidence of both senescence markers and SASP. using morphological criteria, the presence of large, post- Moreover, the senescence secretome of such cells was capable mitotic, heavily granulated senescent cells with nuclear changes of inducing both vascular permeability and angiogenesis, using in the mid-gestation decidual basalis of pregnant mice. In the in vitro readouts for permeability induction and tube forma- mouse, normal vascular remodeling occurs after gestation day tion in human umbilical vein endothelial cells.53 Thus, the 8.5 and is completed by gestation day 10.5. At this time, these SASP induced by NK cells in response to sHLA-G could play senescent cells were seen within lumens of the decidual vessels, a favorable role in promoting neovascularization and tissue embedded within arterial walls and intermingled with decidual remodeling at sites of HLA-G expression. These studies used cells. Moreover, studies of mouse uterine NK cells showed an primary, resting NK cells from human peripheral blood to increase in the inhibitory receptor KLRG1 with gestational study responses as a result of KIR2DL4–sHLA-G interactions. age.59 KLRG1 is a marker of senescence in mouse and human It is not possible to study the early events of activation when NK and T cells and is thought to block proliferation by inter- decidual NK cells first encounter fetal trophoblast cells. The fering with Akt activation.60 Thus, there is indirect evidence of

Cellular & Molecular Immunology A role for senescent NK cells in pregnancy S Rajagopalan 464

NON-PREGNANT Fetal trophoblast cell PREGNANT Placenta

sG sG NK sG sG EVT sG

NK cell Endosome

sG Endometrium D0 Decidua D2

Spiral KIR2DL4 artery

NK cell senescence

Senescence associated secretory profile (SASP) e.g. IL-1β, IL-6, IL-8, uPAR

Vascular remodeling

Figure 1 KIR2DL4–HLA-G interactions: relevance to pregnancy. The non-pregnant endometrium (left panel) contains an abundance of NK cells in the secretory phase of the menstrual cycle. Interactions between fetal EVT cells and decidual NK cells cooperate to remodel the spiral arteries that promote increased blood supply to the fetus. Soluble HLA-G secreted by fetal trophoblast cells can be endocytosed into KIR2DL4-containing endosomes. Endosomal signaling leads to the cellular senescence of NK cells and a sustained secretory response termed the SASP. This SASP would remodel the local environment to promote vascularization required for a growing fetus. EVT, extravillous trophoblast; HLA, human leukocyte antigen; KIR, killer cell immunoglobulin-like receptor; NK, natural killer; SASP, senescence-associated secretory profile; uPAR, urokinase plasminogen activator receptor.

a senescent phenotype of the mouse uterine NK cells during the of senescence would be integrated to favor fetal development. key period of remodeling to permit placental blood flow. Senescence of decidual NK cells upon sensing soluble HLA-G Systematic analysis with other markers of senescence on uter- secreted by trophoblast cells would arrest cells, while promoting ine NK cells at different gestational times is needed. the secretion of factors that would favor remodeling at this site.58 During normal placental development, the process of cyto- In this regard, two studies have revealed an unexpected role for trophoblast fusion to form the syncytiotrophoblast continues senescence during normal development.65,66 Markers of sen- throughout pregnancy. Markers of senescence, such as SAbG, escence such as SAbG were used to identify senescent cells at cyclin-dependent kinase inhibitors p21 and p16, and tumor specific stages and in distinct patterns in the developing embryo. suppressor p53, were detected on these cells upon cell fusion.61 The presence of these senescent cells in mouse, chick and human This cell fusion induced senescence would allow the multi- embryos revealed senescence to be a conserved feature of verte- nucleate, postmitotic syncytium to sustain pregnancy, while brate embryonic development. Developmental senescence allowing the maintenance of cell cycle arrest.62 Whether sen- involves p21 upregulation, and expression of a subset of SASP escence occurs in other cells at different stages of placental proteins, but is independent of DNA damage and other character- development and what role, if any it has in placental patholo- istic mediators of classical senescence, such as p53 and p16.65,66 gies merits investigation. The emerging consensus from the latest developments in sen- Historically, cellular senescence has been well studied as a cell escence research is that cellular senescence arose during evolu- fate triggered by different stress stimuli, such as DNA damage or tion as a critical feature of normal development in the removal oncogene activation. The outcomes of senescence induction and of unwanted structures and in patterning of the embryo, and in the resulting SASP have been contradictory and confusing.63 The remodeling the maternal vasculature to support fetal develop- 67 cell cycle arrest associated with senescence is a tumor suppressive ment. This program is being reused later in life as stress or mechanism, but the SASP can paradoxically promote cancer and damage-induced senescence to protect against tumorigenesis also contributes to age-related pathologies. The SASP can have and tissue damage by eliminating potentially harmful cells. apparently opposing effects by promoting proliferation and tumor growth on one hand,56 while having the potential to CONCLUSIONS induce paracrine senescence of neighboring cells.64 It is in the NK cells, abundant at the maternal fetal/interface, contribute context of pregnancy that all the seemingly contradictory features to reproductive success through a host of receptor–ligand

Cellular & Molecular Immunology A role for senescent NK cells in pregnancy S Rajagopalan 465 interactions that are subject to spatial and temporal regulation 16 van der Meer A, Lukassen HG, van Lierop MJ, Wijnands F, Mosselman at this site. Vascular remodeling is carried out by cooperative S, Braat DD et al. Membrane-bound HLA-G activates proliferation and interferon-gamma production by uterine natural killer cells. Mol Hum interactions between NK cells and fetal trophoblast cells. NK Reprod 2004; 10: 189–195. responses to the soluble HLA-G secreted by extravillous tro- 17 van der Meer A, Lukassen HG, van Cranenbroek B, Weiss EH, Braat phoblasts in early pregnancy result in the reprogramming of DD, van Lierop MJ et al. Soluble HLA-G promotes Th1-type cytokine NK cells towards a senescent state. Cellular senescence is a production by cytokine-activated uterine and peripheral natural killer cells. Mol Hum Reprod 2007; 13: 123–133. molecular switch that leads to a regulated and sustained secret- 18 Li C, Houser BL, Nicotra ML, Strominger JL. HLA-G homodimer- ory response upon engagement of KIR2DL4 by HLA-G that induced cytokine secretion through HLA-G receptors on human can promote vascular remodeling. There is a growing appre- decidual macrophages and natural killer cells. Proc Natl Acad Sci ciation that senescence arose during evolution to serve the USA 2009; 106: 5767–5772. 19 Rouas-Freiss N, Goncalves RM, Menier C, Dausset J, Carosella ED. function of remodeling and patterning of the embryo in nor- Direct evidence to support the role of HLA-G in protecting the fetus mal development. In the same manner, NK senescence during from maternal uterine natural killer cytolysis. Proc Natl Acad Sci USA early gestation would favor reproduction through sustained 1997; 94: 11520–11525. 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