Adaptive Admixture of HLA class I Allotypes Enhanced Genetically Determined Strength of Natural Killer Cells in East Asians

Abstract Human natural killer (NK) cells are essential for controlling infection, cancer and fetal development. NK cell functions are modulated by interactions between polymorphic inhibitory killer cell immunoglobulin-like receptors (KIR) and polymorphic HLA-A, -B and -C ligands expressed on tissue cells. All HLA-C alleles encode a KIR ligand and contribute to reproduction and immunity. In contrast, only some HLA-A and -B alleles encode KIR ligands and they focus on immunity. By high-resolution analysis of KIR and HLA-A, -B and -C genes, we show that the Chinese Southern Han are significantly enriched for interactions between inhibitory KIR and HLA-A and -B. This enrichment has had substantial input through population admixture with neighboring populations, who contributed HLA class I haplotypes expressing the KIR ligands B*46:01 and B*58:01, which subsequently rose to high frequency by natural selection. Consequently, over 80% of Southern Han HLA haplotypes encode more than one KIR ligand. Complementing the high number of KIR ligands, the Chinese Southern Han KIR locus combines a high frequency of genes expressing potent inhibitory KIR, with a low frequency of those expressing activating KIR. The Southern Han centromeric KIR region encodes strong, conserved, inhibitory HLA-C specific receptors, and the telomeric region provides a high number and diversity of inhibitory HLA-A and -B specific receptors. In all these characteristics, the Chinese Southern Han represent other East Asians, whose NK cell repertoires are thus enhanced in quantity, diversity and effector strength, likely augmenting resistance to endemic viral infections.

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Genetically Determined Strength of Natural Killer Cells is Enhanced by Adaptive HLA class I Admixture in East Asians

10.1101/2020.07.29.227579 ◽

2020 ◽

Author(s):

Zhihui Deng ◽

Jianxin Zhen ◽

Genelle F. Harrison ◽

Guobin Zhang ◽

Rui Chen ◽

...

Keyword(s):

Natural Selection ◽

Natural Killer ◽

High Frequency ◽

Nk Cell ◽

Hla Class I ◽

Class I ◽

Telomeric Region ◽

East Asians ◽

Cell Functions ◽

Specific Receptors

AbstractHuman natural killer (NK) cells are essential for controlling infection, cancer and fetal development. NK cell functions are modulated by interactions between polymorphic inhibitory killer cell immunoglobulin-like receptors (KIR) and polymorphic HLA-A, -B and -C ligands expressed on tissue cells. All HLA-C alleles encode a KIR ligand and contribute to reproduction and immunity. In contrast, only some HLA-A and -B alleles encode KIR ligands and they focus on immunity. By high-resolution analysis of KIR and HLA-A, -B and -C genes, we show that the Chinese Southern Han are significantly enriched for interactions between inhibitory KIR and HLA-A and -B. This enrichment has had substantial input through population admixture with neighboring populations, who contributed HLA class I haplotypes expressing the KIR ligands B*46:01 and B*58:01, which subsequently rose to high frequency by natural selection. Consequently, over 80% of Southern Han HLA haplotypes encode more than one KIR ligand. Complementing the high number of KIR ligands, the Chinese Southern Han KIR locus combines a high frequency of genes expressing potent inhibitory KIR, with a low frequency of those expressing activating KIR. The Southern Han centromeric KIR region encodes strong, conserved, inhibitory HLA-C specific receptors, and the telomeric region provides a high number and diversity of inhibitory HLA-A and -B specific receptors. In all these characteristics, the Southern Han represent other East Asians, whose NK cell repertoires are thus enhanced in quantity, diversity and effector strength, likely through natural selection for resistance to endemic viral infections.

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High-Resolution Analysis Identifies High Frequency of KIR-A Haplotypes and Inhibitory Interactions of KIR With HLA Class I in Zhejiang Han

Frontiers in Immunology ◽

10.3389/fimmu.2021.640334 ◽

2021 ◽

Vol 12 ◽

Author(s):

Sudan Tao ◽

Yanmin He ◽

Katherine M. Kichula ◽

Jielin Wang ◽

Ji He ◽

...

Keyword(s):

High Resolution ◽

High Frequency ◽

Killer Cell ◽

Hla Class I ◽

Class I ◽

Gene Copy ◽

Human Populations ◽

Telomeric Region ◽

Cell Functions ◽

Hla Class I Molecules

Killer cell immunoglobulin-like receptors (KIR) interact with human leukocyte antigen (HLA) class I molecules, modulating critical NK cell functions in the maintenance of human health. Characterizing the distribution and characteristics of KIR and HLA allotype diversity across defined human populations is thus essential for understanding the multiple associations with disease, and for directing therapies. In this study of 176 Zhejiang Han individuals from Southeastern China, we describe diversity of the highly polymorphic KIR and HLA class I genes at high resolution. KIR-A haplotypes, which carry four inhibitory receptors specific for HLA-A, B or C, are known to associate with protection from infection and some cancers. We show the Chinese Southern Han from Zhejiang are characterized by a high frequency of KIR-A haplotypes and a high frequency of C1 KIR ligands. Accordingly, interactions of inhibitory KIR2DL3 with C1+HLA are more frequent in Zhejiang Han than populations outside East Asia. Zhejiang Han exhibit greater diversity of inhibitory than activating KIR, with three-domain inhibitory KIR exhibiting the greatest degree of polymorphism. As distinguished by gene copy number and allele content, 54 centromeric and 37 telomeric haplotypes were observed. We observed 6% of the population to have KIR haplotypes containing large-scale duplications or deletions that include complete genes. A unique truncated haplotype containing only KIR2DL4 in the telomeric region was also identified. An additional feature is the high frequency of HLA-B*46:01, which may have arisen due to selection pressure from infectious disease. This study will provide further insight into the role of KIR and HLA polymorphism in disease susceptibility of Zhejiang Chinese.

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Carfilzomib Enhances Natural Killer Cell-Mediated Lysis of Myeloma Linked with Decreasing Expression of HLA Class I

Blood ◽

10.1182/blood.v126.23.1854.1854 ◽

2015 ◽

Vol 126 (23) ◽

pp. 1854-1854

Author(s):

Jumei Shi ◽

Guang Yang ◽

Yuanyuan Kong ◽

Minjie Gao ◽

Yi Tao ◽

...

Keyword(s):

Multiple Myeloma ◽

Nk Cells ◽

Natural Killer ◽

Hematological Malignancies ◽

Nk Cell ◽

Killer Cell ◽

Hla Class I ◽

Class I ◽

Dependent Manner ◽

Down Regulation

Abstract Multiple myeloma (MM) is a malignant disorder characterized by uncontrolled monoclonal plasma cell proliferation. It accounts for 10% of all hematological malignancies and causes 15-20% of deaths from hematological malignancies. Although new therapies were introduced and overall survival of MM was improved in the last 10 years, MM still remains an incurable disease due to drug resistance. Natural killer (NK) cell-based treatments are promising therapies for multiple myeloma (MM). Carfilzomib (CFZ), a second-generation proteasome inhibitor, is used to treat patients with MM who are refractory or intolerant to both bortezomib and lenalidomide (or thalidomide). In this study, we determined that CFZ treatment enhanced the sensitivity of MM cells to NK cell-mediated lysis. Here, we report that CFZ decreased the expression of human leukocyte antigen (HLA) class I on MM cell lines and primary MM cells, the mean reduction was 47.7 ± 9.4% and 42.8 ± 12.4%, respectively. The down-regulation caused by CFZ occurred in a dose- and time- dependent manner. We compared the cell surface levels of HLA class I on MM cells in the presence or absence of CFZ after acid treatment. CFZ also down-regulated the expression of newly formed HLA class I on MM cells. CD107a expression levels were used to measure NK-cell degranulation. When NK cells were incubated with MM cells with CFZ treatment, the percentage of NK cells expressing CD107a on the surface greatly increased (mean ± SD: 33.6 ± 2.1%, for treated cells vs 16.7 ± 2.3%, for control cells, P < 0.05). We also showed that CFZ augmented NK-cell cytotoxity by a perforin/granzyme-mediated mechanism, because such enhancement was abolished when CMA, but not anti-TRAIL or anti-Fas-L antibodies, was added. Treatment of MM with CFZ significantly sensitized patients' MM cells to NK cell-mediated lysis (mean ± SD: 43.1 ± 6.4%, for treated cells vs 16.1 ± 4.0%, for control cells at effector/target (E/T) ratio of 10:1, n = 9, P < 0.01). Furthermore, the exogenous HLA-C binding peptides, used in the CFZ treated group rescued the down-regulation of HLA-C and reduced NK cell-mediated lysis to a similar level as in the untreated group. Blocking NKG2D, NCRs and TRAIL did not have a significant impact on NK cell lysis of myeloma cells. These implied the enhancement of NK cell-mediated lysis was mainly linked with the decreased expression of HLA class I. Our findings show a novel activity of CFZ as an immunomodulating agent and suggest a possible approach to therapeutically augment NK cell function in MM patients. Disclosures No relevant conflicts of interest to declare.

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Soluble HLA class I molecules induce natural killer cell apoptosis through the engagement of CD8: evidence for a negative regulation exerted by members of the inhibitory receptor superfamily

Blood ◽

10.1182/blood.v99.5.1706 ◽

2002 ◽

Vol 99 (5) ◽

pp. 1706-1714 ◽

Cited By ~ 55

Author(s):

Grazia Maria Spaggiari ◽

Paola Contini ◽

Roberta Carosio ◽

Marica Arvigo ◽

Massimo Ghio ◽

...

Keyword(s):

Natural Killer ◽

Cell Apoptosis ◽

Fas Ligand ◽

Nk Cell ◽

Killer Cell ◽

Hla Class I ◽

Class I ◽

Inhibitory Receptor ◽

Cell Clones ◽

Soluble Hla

Herein, we show that CD8dull, CD8intermediate, and CD8bright natural killer (NK) cell clones can be identified. Triggering of CD8 with its natural ligand(s), represented by soluble HLA class I (sHLA-I), isolated either from serum of healthy donors or from HLA-I− 721.221 lymphoblastoid cell line transfected with HLA-A2, -Cw4, and -Bw46 alleles, or HLA-G1 leads to NK cell apoptosis. The magnitude of this effect directly correlated with the level of CD8 expression. sHLA-I–induced apoptosis depends on the interaction with CD8, as it was inhibited by masking this molecule with specific monoclonal antibodies (mAbs). Moreover, sHLA-I or CD8 cross-linking with specific mAbs elicited intracellular calcium increases, Fas ligand (FasL) messenger RNA transcription, and FasL secretion, which were needed for delivering the death signal. Indeed, this apoptosis was inhibited by preincubation of NK cell clones with Fas or FasL antagonist mAbs, indicating that the Fas/FasL pathway is involved. Furthermore, members of the inhibitory receptor superfamily, such as CD94/NKG2 complex or killer inhibitory receptors, were shown to exert an inhibitory effect on sHLA-I–mediated apoptosis and secretion of FasL. These findings suggest that interaction between sHLA-I and CD8 evokes an apoptotic signal that is down-regulated by inhibitory receptor superfamily that function as survival receptors in NK cells.

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Natural killer-cell immunoglobulin-like receptors trigger differences in immune response to SARS-CoV-2 infection

PLoS ONE ◽

10.1371/journal.pone.0255608 ◽

2021 ◽

Vol 16 (8) ◽

pp. e0255608

Author(s):

Roberto Littera ◽

Luchino Chessa ◽

Silvia Deidda ◽

Goffredo Angioni ◽

Marcello Campagna ◽

...

Keyword(s):

Immune Response ◽

Natural Killer ◽

Nk Cell ◽

Severe Disease ◽

Killer Cell ◽

Hla Class I ◽

Class I ◽

Disease Course ◽

Group A ◽

The Impact

Background The diversity in the clinical course of COVID-19 has been related to differences in innate and adaptative immune response mechanisms. Natural killer (NK) lymphocytes are critical protagonists of human host defense against viral infections. It would seem that reduced circulating levels of these cells have an impact on COVID-19 progression and severity. Their activity is strongly regulated by killer-cell immuno-globulin-like receptors (KIRs) expressed on the NK cell surface. The present study’s focus was to investigate the impact of KIRs and their HLA Class I ligands on SARS-CoV-2 infection. Methods KIR gene frequencies, KIR haplotypes, KIR ligands and combinations of KIRs and their HLA Class I ligands were investigated in 396 Sardinian patients with SARS-CoV-2 infection. Comparisons were made between 2 groups of patients divided according to disease severity: 240 patients were symptomatic or paucisymptomatic (Group A), 156 hospitalized patients had severe disease (Group S). The immunogenetic characteristics of patients were also compared to a population group of 400 individuals from the same geographical areas. Results Substantial differences were obtained for KIR genes, KIR haplotypes and KIR-HLA ligand combinations when comparing patients of Group S to those of Group A. Patients in Group S had a statistically significant higher frequency of the KIR A/A haplotype compared to patients in Group A [34.6% vs 23.8%, OR = 1.7 (95% CI 1.1–2.6); P = 0.02, Pc = 0.04]. Moreover, the KIR2DS2/HLA C1 combination was poorly represented in the group of patients with severe symptoms compared to those of the asymptomatic-paucisymptomatic group [33.3% vs 50.0%, OR = 0.5 (95% CI 0.3–0.8), P = 0.001, Pc = 0.002]. Multivariate analysis confirmed that, regardless of the sex and age of the patients, the latter genetic variable correlated with a less severe disease course [ORM = 0.4 (95% CI 0.3–0.7), PM = 0.0005, PMC = 0.005]. Conclusions The KIR2DS2/HLA C1 functional unit resulted to have a strong protective effect against the adverse outcomes of COVID-19. Combined to other well known factors such as advanced age, male sex and concomitant autoimmune diseases, this marker could prove to be highly informative of the disease course and thus enable the timely intervention needed to reduce the mortality associated with the severe forms of SARS-CoV-2 infection. However, larger studies in other populations as well as experimental functional studies will be needed to confirm our findings and further pursue the effect of KIR receptors on NK cell immune-mediated response to SARS-Cov-2 infection.

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Evolution and Immunogenetics of Natural Killer Cell Receptors in Health and Disease

Blood ◽

10.1182/blood.v124.21.sci-25.sci-25 ◽

2014 ◽

Vol 124 (21) ◽

pp. SCI-25-SCI-25

Author(s):

Peter Parham

Keyword(s):

Nk Cells ◽

Natural Killer ◽

Mhc Class I ◽

Nk Cell ◽

Killer Cell ◽

Hla Class I ◽

Class I ◽

Mhc Class I Molecules ◽

Kir Gene ◽

Kir Haplotypes

Abstract Natural killer (NK) cells are phenotypically diverse lymphocytes that contribute to innate immunity, adaptive immunity and placental reproduction. Unlike B and T cells, NK cells do not use rearranging genes to make diverse antigen receptors that are clonally expressed. Instead, NK cells express diverse combinations of a variety of receptors that are encoded by conventional non-rearranging genes. Several of these receptors are specific for conserved and variable determinants of major histocompatibility complex (MHC) class I molecules. In humans, the killer-cell immunoglobulin-like receptors (KIR) are a diverse and polymorphic family of NK-cell receptors that recognize determinants of human leukocyte antigen (HLA)-A, B and C, the polymorphic human MHC class I molecules. HLA-A, B and C are the most polymorphic of human genes, and they correlate with susceptibility to a wide range of diseases and clinical outcomes, including allogeneic hematopoietic cell transplantation (HCT). During NK-cell development, interactions between epitopes of HLA class I and KIR educate the NK cells to recognize the normal expression of these epitopes on healthy cells, and to respond to unhealthy cells in which that expression is perturbed. In the context of HCT, certain types of HLA class I mismatch enable donor-derived NK cells to make an alloreactive and beneficial graft-versus-leukemia response. Although it is likely that all placental mammals have NK cells, only a small minority of these species has a diverse KIR family like that in humans. These comprise the simian primates: New World monkeys, Old World monkeys and the great apes. Under pressure from diverse and rapidly evolving pathogens, both the MHC class I and KIR gene families have been driven to evolve rapidly. Consequently, much of their character is species-specific. This is especially true for the human KIR gene family, which is qualitatively different from that of our closest relatives, the chimpanzees. Whereas chimpanzee KIR haplotype diversity represents variations on a theme of genes encoding robust MHC class I receptors, humans have an even balance between group A KIR haplotypes encoding robust HLA class I receptors and group B KIR haplotypes encoding receptors that, to varying degree, have been subject to natural selection for reduced functional recognition of HLA class I. A balance of A and B is present in all human populations and thus appears essential for the long-term survival and competitiveness of human communities. Whereas the A KIR haplotypes correlate with successful defense against viral infection, maternal B KIR haplotypes correlate with reproductive success and donor B KIR haplotypes improve the outcome of allogeneic HCT as therapy for acute myeloid leukemia. Disclosures No relevant conflicts of interest to declare.

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The New Kid on the Block: HLA-C, a Key Regulator of Natural Killer Cells in Viral Immunity

Cells ◽

10.3390/cells10113108 ◽

2021 ◽

Vol 10 (11) ◽

pp. 3108

Author(s):

Sarah Vollmers ◽

Annabelle Lobermeyer ◽

Christian Körner

Keyword(s):

Immune System ◽

Nk Cells ◽

Natural Killer ◽

Viral Infections ◽

Killer Cell ◽

Hla Class I ◽

Class I ◽

Proper Function ◽

Cell Functions ◽

Wide Range

The human leukocyte antigen system (HLA) is a cluster of highly polymorphic genes essential for the proper function of the immune system, and it has been associated with a wide range of diseases. HLA class I molecules present intracellular host- and pathogen-derived peptides to effector cells of the immune system, inducing immune tolerance in healthy conditions or triggering effective immune responses in pathological situations. HLA-C is the most recently evolved HLA class I molecule, only present in humans and great apes. Differentiating from its older siblings, HLA-A and HLA-B, HLA-C exhibits distinctive features in its expression and interaction partners. HLA-C serves as a natural ligand for multiple members of the killer-cell immunoglobulin-like receptor (KIR) family, which are predominately expressed by natural killer (NK) cells. NK cells are crucial for the early control of viral infections and accumulating evidence indicates that interactions between HLA-C and its respective KIR receptors determine the outcome and progression of viral infections. In this review, we focus on the unique role of HLA-C in regulating NK cell functions and its consequences in the setting of viral infections.

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Identification of Natural Killer Cell Receptor Phenotypes Associated with Leukemia.

Blood ◽

10.1182/blood.v104.11.2997.2997 ◽

2004 ◽

Vol 104 (11) ◽

pp. 2997-2997

Author(s):

Sonja J. Verheyden ◽

Michel Bernier ◽

Christian J. Demanet

Keyword(s):

Nk Cells ◽

Natural Killer ◽

Nk Cell ◽

Killer Cell ◽

Hla Class I ◽

Cell Receptor ◽

Innate Immune ◽

Class I ◽

Leukemic Cells ◽

Hla Class I Molecules

Abstract Introduction: Natural Killer (NK) cells play a key role in defense against tumor cells that have the capacity to downregulate Human Leukocyte Antigen (HLA) class I expression. It has been reported that leukemic cells can have down-regulated expression of HLA class I molecules. Apparently, the NK cells of these patients are not able to destroy these leukemic cells and may allow malignant cells to escape from innate immune control. This failure may be due to the fact that NK cells are part of the malignant clone and therefore might have a decreased function. An alternative hypothesis could be that these patients may display a NK cell Receptor (NKR) genotype incapable of destroying leukemic cells with aberrant expression of HLA class I molecules. The polymorphic nature of the NKR genes generates diverse repertoires in the human population, which display specificity in the innate immune response. Materials and Methods: In the present study, 11 Killer cell Immunoglobulin-like Receptor (KIRs) and 2 CD94/NKG2 receptors were genotyped by PCR-SSP in 96 leukemic patients and 148 healthy Caucasians. Results and Conclusion: We report a significant increased frequency of the more inhibitory AB KIR phenotype in leukemic patients compared to the controls (31.1% in healthy controls vs. 51.0% in leukemic patients, Pc = 0.002), which is related to the high prevalence of the inhibitory KIR2DL2 in this population (Pc = 0.007). Moreover, two specific KIR phenotypes AB1 and AB9, including all inhibitory KIRs, were significantly associated with leukemic patients. Our study suggests that an important percentage of leukemic patients express a KIR phenotype in favor of escape from NK cell immunity.

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Natural Killer Cell Activities Against iPSCs-Derived HLA-Knockout Platelets and Megakaryocytes Reveal Perfect Rejection Profiles for Allotransfusion

Blood ◽

10.1182/blood.v128.22.3841.3841 ◽

2016 ◽

Vol 128 (22) ◽

pp. 3841-3841 ◽

Cited By ~ 2

Author(s):

Daisuke Suzuki ◽

Naoshi Sugimoto ◽

Norihide Yoshikawa ◽

Hiroshi Endo ◽

Sou Nakamura ◽

...

Keyword(s):

Cytotoxic Activity ◽

Nk Cells ◽

Natural Killer ◽

Nk Cell ◽

Killer Cell ◽

Annexin V ◽

Hla Class I ◽

K562 Cells ◽

Class I ◽

Target Cells

Abstract Background Platelet transfusion refractoriness (PTR) due to immune factors occurs in 5-15% of thrombocytopenic patients who have received transfusions. The dominant cause of immune PTR is the production of allo-antibodies to human leukocyte antigen (HLA) class I, which is expressed on platelets. In current clinical settings, transfusion of HLA-compatible platelets is the only practical strategy, but their supply is weak due to limited donor source, gives excessive burden on specific donors, and requires increased efforts and costs. To overcome these issues, we plan to produce HLA-knockout platelets from iPSCs-derived megakaryocytes (MKs) as an alternative solution, applicable to all HLA types. However, whether they would be attacked by natural killer (NK) cells has not been well-studied. NK cells are known to show cytotoxic activity against cells downregulated for HLA class I ("missing self" theory). Therefore we assessed the interaction between HLA-knockout platelets derived from induced pluripotent stem cells (iPSCs) and NK cells in allogeneic settings. Methods and Results Immortalized megakaryocyte progenitor cell lines (imMKCLs) were previously established from iPSCs as a source of platelet production with a robust proliferation potential (Nakamura, 2014). Beta 2-microglobulin gene was knocked-out by CRISPR/Cas9 system to obtain HLA-knockout imMKCLs and platelets. NK cells were prepared from peripheral blood of eleven healthy donors. After co-cultures of NK cells and target cells for 6 hours with IL-2, we examined the NK cell cytolytic activity marker CD107, and target cell damage marker Annexin V using flow cytometry. Positive rates of both markers were not enhanced by co-culture with either HLA-expressed or HLA-knockout platelets for all donors. Furthermore, addition of platelets showed minimal effect on high cytotoxic activity of NK cells against K562 cells. In contrast, coculture of imMKCLs with NK cells resulted in higher detection of CD107 and Annexin V staining in some NK cell donors. These data suggested that platelets are immunologically inert for NK cells irrespective of class I HLA expression, while imMKCLs can be potentially attacked. Accordingly, platelets did not express NK cell activating ligands, which were expressed on imMKCLs and K562 cells. To confirm the above-mentioned results in vivo, mice were transfused with NK cells and platelets and MKs together. In our preliminary data, the circulation of platelets was not different between HLA-expressed or HLA-knockout type. In contrast, MKs were shown to be attacked in some cases. Conclusion HLA-knockout platelets evaded attacked from NK cells, while imMKCLs possessed immunogenicity to NK cells. This study provides extended experimental evidence that HLA-knockout platelets produced from a single imMKCL clone are immunologically applicable to all HLA types including majority of patients with PTR. On the other hand, contaminating imMKCLs in imMKCL-derived platelet products can be rejected by NK cells, contributing to their enhanced safety profiles. Taken together, stage of HLA-deficiency in imMKCLs as a starting material of platelet supply shall lead to industrial production of HLA universal platelets. Disclosures No relevant conflicts of interest to declare.

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Decidual natural-killer-cell interaction with trophoblast: cytolysis or cytokine production?

Biochemical Society Transactions ◽

10.1042/bst0280196 ◽

2000 ◽

Vol 28 (2) ◽

pp. 196-198 ◽

Cited By ~ 30

Author(s):

Y. W. Loke ◽

A. King

Keyword(s):

Nk Cells ◽

Natural Killer ◽

Cell Function ◽

Nk Cell ◽

Close Contact ◽

Killer Cell ◽

Hla Class I ◽

Class I ◽

Target Cells ◽

Hla Class I Antigens

At the implantation site, the uterine mucosa (decidua) is infiltrated by large numbers of natural killer (NK) cells. These NK cells are in close contact with the invading fetal trophoblast and we have proposed that they might be the effector cells that control the implantation of the allogeneic placenta. Recent characterization of NK cell receptors and their HLA class I ligands has suggested potential mechanisms by which NK cells might interact with trophoblast. However, what happens as a result of this interaction is not clear. The traditional method for investigating NK cell function in vitro is the protection from lysis of target cells by expression of HLA class I antigens. This might not be an accurate reflection of what happens in vivo. Another function of NK cells is the production of cytokines on contact with target cells. This could be an important outcome of the interaction between decidual NK cells and trophoblast. Decidual NK cells are known to produce a variety of cytokines; trophoblast cells express receptors for many of these cytokines, indicating that they can potentially respond. In this way, decidual NK cells have a significant influence on trophoblast behaviour during implantation.

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