A comprehensive analysis of the binding of anti-KIR antibodies to activating KIRs

Characterization of KIR+ NK cell subsets with a monoclonal antibody selectively recognizing KIR2DL1 and blocking the specific interaction with HLA-C

The phenotypic identification of different NK cell subsets allows more in‐depth characterization of KIR repertoire and function, which are of potential interest in KIR and disease association studies. KIR genes are highly polymorphic, but a great homology exists among the various sequences and few monoclonal antibodies (mAbs) specifically recognize a single KIR. This is the case of HP‐DM1 which was demonstrated by analysis of cell transfectants and epitope mapping to be exclusively KIR2DL1‐specific, covering all allotypes identified to date, except for KIR2DL1*022 and *020, and also to react with KIR2DS1*013. Here, we compared in immunofluorescence analyses the staining of HP‐DM1 with other available mAbs to precisely identify KIR2DL1⁺ NK cells in potential donors for αβT/B‐depleted haplo‐HSCT, with known KIR genotype. HP‐DM1 mAb was used in combination with EB6 or 11PB6 (anti‐KIR2DL1/S1 and anti‐KIR2DL3*005), 143211 (anti‐KIR2DL1/S5), and HP‐MA4 (anti‐KIR2DL1/S1/S3/S5) mAbs, allowing the accurate identification of different KIR⁺ NK cell subsets. These phenotypic evaluations appeared useful to dissect the expression pattern of various KIR2D in NK cells from KIR2DL3*005⁺ individuals, particularly if KIR2DS1 is present. HP‐DM1 mAb remarkably refined NK cell phenotyping of donors carrying KIR2DS5, either in the centromeric or telomeric region. Functional assays with KIR2DL1⁺/S1⁺/S5⁺ NK cells confirmed that only HP‐DM1 exclusively reacts with KIR2DL1. Finally, we demonstrated that HP‐DM1 mAb blocked KIR2DL1 recognition of C2⁺ HLA‐C. Altogether, the data support that HP‐DM1 is a unique reagent valuable for characterizing KIR⁺ NK cell subsets.

Epitope characterization of a monoclonal antibody that selectively recognizes KIR2DL1 allotypes

Killer immunoglobulin‐like receptor (KIR) genes code for a family of inhibitory and activating receptors, finely tuning NK cell function. Numerous studies reported the relevance of KIR allelic polymorphism on KIR expression, ligand affinity, and strength in signal transduction. Although KIR variability, including gene copy number and allelic polymorphism, in combination with HLA class I polymorphism, impacts both KIR expression and NK cell education, only a precise phenotypic analysis can define the size of the different KIR^pos NK cell subsets. In this context, reagents recognizing a limited number of KIRs is essential. In this study, we have characterized the specificity of an anti‐KIR mAb termed HP‐DM1. Testing its binding to HEK‐293T cells transfected with plasmids coding for different KIRs, we demonstrated that HP‐DM1 mAb exclusively reacts with KIR2DL1. Using site‐directed mutagenesis, we identified the four amino acids relevant for HP‐DM1 recognition: M44, S67, R68, and T70. HP‐DM1 mAb binds to a conformational epitope including M44, the residue crucial for HLA‐C K80 recognition by KIR2DL1. Based on the HP‐DM1 epitope characterization, we could extend its reactivity to all KIR2DL1 allotypes identified except for KIR2DL1*022 and, most likely, KIR2DL1*020, predicting that it does not recognize any other KIR with the only exception of KIR2DS1*013. Moreover, by identifying the residues relevant for HP‐DM1 binding, continuously updating of its reactivity will be facilitated.

A CD8+ NK cell transcriptomic signature associated with clinical outcome in relapsing remitting multiple sclerosis

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system (CNS) with the majority of cases characterised by relapsing/remitting (RRMS) attacks of neurologic dysfunction followed by variable resolution. Improving clinical outcomes in RRMS requires both a better understanding of the immunological mechanisms driving recurrent demyelination and better means of predicting future disease course to facilitate early targeted therapy. Here, we apply hypothesis-generating network transcriptomics to CD8⁺ cells isolated from patients in RRMS, identifying a signature reflecting expansion of a subset of CD8⁺ natural killer cells (NK8⁺) associated with favourable outcome. NK8⁺ are capable of regulating CD4⁺ T cell activation and proliferation in vitro, with reduced expression of HLA-G binding inhibitory receptors and consequent reduced sensitivity to HLA-G-mediated suppression. We identify surrogate markers of the NK8⁺ signature in peripheral blood leucocytes and validate their association with clinical outcome in an independent cohort, suggesting their measurement may facilitate early, targeted therapy in RRMS.

Recent Advances in Molecular Mechanisms of the NKG2D Pathway in Hepatocellular Carcinoma

Hepatocellular carcinoma is a common malignant tumor with high mortality. Its malignant proliferation, invasion, and metastasis are closely related to the cellular immune function of the patients. NKG2D is a key activated and type II membrane protein molecule expressed on the surface of almost all NK cells. The human NKG2D gene is 270 kb long, located at 12p12.3-p13.1, and contains 10 exons and 9 introns. The three-dimensional structure of the NKG2D monomeric protein contains two alpha-helices, two beta-lamellae, and four disulfide bonds, and its' signal of activation is transmitted mainly by the adaptor protein (DAP). NKG2D ligands, including MICA, MICB, and ULBPs, can be widely expressed in hepatoma cells. After a combination of NKG2D and DAP10 in the form of homologous two polymers, the YxxM motif in the cytoplasm is phosphorylated and then signaling pathways are also gradually activated, such as PI3K, PLCγ2, JNK-cJunN, and others. Activated NK cells can enhance the sensitivity to hepatoma cells and specifically dissolve by releasing a variety of cytokines (TNF-α and IFN-γ), perforin, and high expression of FasL, CD16, and TRAIL. NK cells may specifically bind to the over-expressed MICA, MICB, and ULBPs of hepatocellular carcinoma cells through the surface activating receptor NKG2D, which can help to accurately identify hepatoma, play a critical role in anti-hepatoma via the pathway of cytotoxic effects, and obviously delay the poor progress of hepatocellular carcinoma.

Activating killer cell immunoglobulin-like receptors: Detection, function and therapeutic use

Killer cell immunoglobulin-like receptors (KIRs) have a central role in the control of natural killer (NK) cell function. The functions of the activating KIRs, as compared to those of the inhibitory KIR, have been more difficult to define due to difficulties in antibody-mediated identification and their apparent low affinities for HLA class I. Immunogenetic studies have shown associations of activating KIRs with the outcome of autoimmune diseases, pregnancy-associated disorders, infectious diseases and cancers. Activating KIR are thus thought to have important roles in the control of natural killer cell functions and their role in disease. In this review, we discuss current knowledge on activating KIR, their ligands and, their roles in the pathogenesis and potential therapy of human diseases.

Natural Killer Cell Education Is Associated With a Distinct Glycolytic Profile

NK cells expressing self-inhibitory receptors display increased functionality compared to NK cells lacking those receptors. The acquisition of functional competence in these particular NK-cell subsets is termed education. Little is known about the underlying mechanisms that lead to the functional differences between educated and uneducated NK cells. An increasing number of studies suggest that cellular metabolism is a determinant of immune cell functions. Thus, alterations in cellular metabolic pathways may play a role in the process of NK-cell education. Here, we compared the glycolytic profile of educated and uneducated primary human NK cells. KIR-educated NK cells showed significantly increased expression levels of the glucose transporter Glut1 in comparison to NKG2A-educated or uneducated NK cells with and without exposure to target cells. Subsequently, the metabolic profile of NK-cell subsets was determined using a Seahorse XF Analyzer. Educated NK cells displayed significantly higher rates of cellular glycolysis than uneducated NK cells even in a resting state. Our results indicate that educated and uneducated NK cells reside in different metabolic states prior to activation. These differences in the ability to utilize glucose may represent an underlying mechanism for the superior functionality of educated NK cells expressing self-inhibitory receptors.

Glioblastoma Stem-Like Cells Are More Susceptible Than Differentiated Cells to Natural Killer Cell Lysis Mediated Through Killer Immunoglobulin-Like Receptors-Human Leukocyte Antigen Ligand Mismatch and Activation Receptor-Ligand Interactions

Glioblastoma (GBM) is the most aggressive brain malignancy in adults, where survival is approximately 14.6 months. Novel therapies are urgently needed and immunotherapy has hailed a new dawn for treatment of solid tumors. Natural killer (NK) cells may be amenable therapeutic effectors against heterogeneous GBM, since they also do not require co-stimulation and antigen specificity. However, it is unclear how culture media routinely used in pre-clinical studies affect GBM cell responses to NK-mediated cytotoxicity. We hypothesized that the culture medium would affect GBM cell phenotype, proliferation, and responses to NK cytotoxicity. We investigated in paired analyses n = 6 patient-derived primary GBM cells propagated in stem cell or serum-containing medium for morphology, proliferation, as well as susceptibility to NK cytolysis and related this to expression of surface and intracellular lineage markers, as well as ligands for NK cell activating and inhibitory receptors. We genotyped the GBM cells for human leukocyte antigen (HLA) as well as the killer immunoglobulin-like receptors (KIR) of the n = 6 allogeneic NK cells used as effector cells. Culture in serum-containing medium induced a switch in GBM cell morphology from suspension neuropsheres to adherent epithelial-mesenchymal-like phenotypes, which was partially reversible. The differentiated cells diminished expression of nestin, CD133 (prominin-1), and A2B5 putative glioma stem-cell markers, attenuated growth, diminished expression of ligands for activating NK cell receptors, while upregulating class I HLA ligands for NK cell inhibitory receptors. When maintained in serum-containing medium, fewer GBM cells expressed intercellular cell adhesion molecule-1 (ICAM-1) and were less susceptible to lysis by NK cells expressing αLβ2 integrin receptor (LFA-1), mediated through combination of inhibitory KIR-HLA ligand mismatch and diminished activation receptor-ligand interactions compared to cells maintained in stem cell media. We conclude that development of preclinical immunotherapy strategies against GBM should not use cells propagated in serum-containing media to avoid misinterpretation of potential therapeutic responses.

Contribution of NK Cell Education to both Direct and Anti-HIV-1 Antibody-Dependent NK Cell Functions

Antibody Fc-dependent functions are linked to prevention and control of HIV-1 infection. Basic NK cell biology is likely key to understanding the contributions that anti-HIV-1 antibody-dependent NK cell activation and cytolysis make to HIV-1 susceptibility and disease progression. The importance of NK cell education through inhibitory receptors specific for self-HLA-I in determining the potency of anti-HIV-1 antibody-mediated NK cell activation and cytolysis is controversial. To address this issue more definitively, we utilized HLA-I genotyping, flow cytometry staining panels, and cytolysis assays to assess the functionality of educated and noneducated peripheral blood NK cells. We now demonstrate that educated NK cells are superior in terms of their capacity to become activated and/or mediate cytolysis following anti-HIV-1 antibody-dependent stimulation. The profiles of activation observed were similar to those observed upon direct stimulation of NK cells with target cells devoid of HLA-I. Noneducated NK cells make significantly lower contributions to total NK cell activation than would be expected from their frequency within the total NK cell population (i.e., they are hypofunctional), and educated NK cells make contributions similar to or higher than their frequency in the total NK cell population. Finally, NK cells educated through at least one killer immunoglobulin-like receptor and NKG2A exhibited the most significant difference between actual and expected contributions to the total NK cell response, based on their frequency within the total NK cell population, suggesting that summation of NK cell education through inhibitory receptors determines overall NK cell functionality. These observations have potential implications for understanding HIV-1 vaccine efficacy and disease progression.IMPORTANCE NK cells are major mediators of anti-HIV-1 antibody-dependent functions, including cytokine production and cytolysis. The mechanisms controlling the capacity of individual NK cells to mediate antibody-dependent functions remain poorly defined. We now show that NK cell education determines the capacity of NK cells to exhibit anti-HIV-1 antibody-dependent activation and mediate antibody-dependent cellular cytotoxicity. These observations suggest that the process of NK cell education could be of importance for understanding HIV-1 pathogenesis and designing immune-based prophylactics or therapeutics.

NK cell dysfunction in chronic lymphocytic leukemia is associated with loss of the mature cells expressing inhibitory killer cell Ig-like receptors

A prospective analysis of natural killer (NK) cell phenotype and function was performed on fresh peripheral blood samples from untreated patients with B-cell chronic lymphocytic leukemia (CLL) or small lymphocytic lymphoma (SLL). Compared to healthy controls, CD56^dim NK cells in CLL patients displayed reduced expression of the NKG2D activating receptor and increased CD27 expression, which indicates declines in mature cells. In addition, NK cells from CLL patients showed reduced degranulation responses toward transformed B cells alone or with rituximab and were more sensitive to activation-induced cell death. We further noted a striking reduction in the frequency and viability of NK cells expressing the inhibitory killer cell Ig-like receptors (KIR)2DL1 and/or KIR3DL1, which progressed over time in most patients. Comparisons between a CLL patient and healthy monozygotic twin were consistent with our results in the larger cohorts. Functional and biomarker alterations were less pronounced on NK cells from SLL patients, which have lower tumor burden in peripheral blood than CLL, but significant reduction in degranulation under ADCC conditions and lower frequency and viability of KIR-expressing NK cells were still evident in SLL. We conclude that mature KIR-expressing NK cells respond to the high circulating B cell tumor burden in CLL, but undergo activation-induced apoptosis. Consequently, CLL patients may benefit from therapies that augment NK cell survival and function.

Activating Killer Immunoglobulin Receptors and HLA-C: a successful combination providing HIV-1 control

Several studies demonstrated a relevant role of polymorphisms located within the HLA-B and -C loci and the Killer Immunoglobulin Receptors (KIRs) 3DL1 and 3DS1 in controlling HIV-1 replication. KIRs are regulatory receptors expressed at the surface of NK and CD8+ T-cells that specifically bind HLA-A and -B alleles belonging to the Bw4 supratype and all the -C alleles expressing the C1 or C2 supratype. We here disclose a novel signature associated with the Elite Controller but not with the long-term nonprogressor status concerning 2DS activating KIRs and HLA-C2 alleles insensitive to miRNA148a regulation. Overall, our findings support a crucial role of NK cells in the control of HIV-1 viremia.

Deciphering the killer-cell immunoglobulin-like receptor system at super-resolution for natural killer and T-cell biology

Killer-cell immunoglobulin-like receptors (KIRs) are components of two fundamental biological systems essential for human health and survival. First, they contribute to host immune responses, both innate and adaptive, through their expression by natural killer cells and T cells. Second, KIR play a key role in regulating placentation, and hence reproductive success. Analogous to the diversity of their human leucocyte antigen class I ligands, KIR are extremely polymorphic. In this review, we describe recent developments, fuelled by methodological advances, that are helping to decipher the KIR system in terms of haplotypes, polymorphisms, expression patterns and their ligand interactions. These developments are delivering deeper insight into the relevance of KIR in immune system function, evolution and disease.

The differential impact of natural killer (NK) cell education via KIR2DL3 and KIR3DL1 on CCL4 secretion in the context of in-vitro HIV infection

Carriage of certain inhibitory natural killer (NK) cell receptor (iNKR)/HLA ligand pairs is associated with protection from infection and slow time to AIDS implicating NK cells in HIV control. NK cells acquire functional potential through education, which requires the engagement of iNKRs by their human leucocyte antigen (HLA) ligands. HIV infection down-regulates cell surface HLA-A/B, but not HLA-C/E. We investigated how NK cell populations expressing combinations of the iNKRs NKG2A, KIR2DL3 (2DL3) and KIR3DL1 (3DL1) responded to autologous HIV infected CD4 (iCD4) cells. Purified NK cells from HIV-uninfected individuals were stimulated with autologous HIV iCD4 or uninfected CD4 T cells. Using flow cytometry we gated on each of the 8 NKG2A^+/- 2DL3^+/- 3DL1^+/- populations and analysed all possible combinations of interferon (IFN)-γ, CCL4 and CD107a functional subsets responding to iCD4 cells. Infected CD4 cells induced differential frequencies of NKG2A^+/- 2DL3^+/- 3DL1^+/- populations with total IFN-γ⁺ , CCL4⁺ and CD107a⁺ functional profiles. 2DL3⁺ NKG2A⁺ NK cells had a higher frequency of responses to iCD4 than other populations studied. A higher frequency of 2DL3⁺ NK cells responded to iCD4 from individuals that were not HLA-C1 homozygotes. These results show that 2DL3⁺ NK cells are mediators of HIV-specific responses. Furthermore, responses of NK cell populations to iCD4 are influenced not only by NK cell education through specific KIR/HLA pairs, but also by differential HIV-mediated changes in HLA expression.

NK Cell Subgroups, Phenotype, and Functions After Autologous Stem Cell Transplantation

High-dose chemotherapy with consecutive autologous stem cell transplantation (autoSCT) is a well-established treatment option for patients suffering from malignant lymphoma or multiple myeloma. Natural killer (NK) cells are an important part of the immune surveillance, and their cell number after autoSCT is predictive for progression-free and overall survival. To improve knowledge about the role of NK cells after autoSCT, we investigated different NK cell subgroups, their phenotype, and their functions in patients treated with autoSCT. Directly after leukocyte regeneration (>1000 leukocytes/μl) following autoSCT, CD56⁺⁺ NK cells were the major NK cell subset. Surprisingly, these cells showed unusually high surface expression levels of CD57 and killer Ig-like receptors (KIRs) compared to expression levels before or at later time points after autoSCT. Moreover, these NK cells strongly upregulated KIR2DL2/3/S2 and KIR3DL1, whereas KIR2DL1/S1 remained constant, indicating that this cell population arose from more immature NK cells instead of from activated mature ones. Remarkably, NK cells were already able to degranulate and produce IFN-γ and MIP-1β upon tumor interaction early after leukocyte regeneration. In conclusion, we describe an unusual upregulation of CD57 and KIRs on CD56⁺⁺ NK cells shortly after autoSCT. Importantly, these NK cells were functionally competent upon tumor interaction at this early time point.

Activating KIRs and NKG2C in Viral Infections: Toward NK Cell Memory?

Natural killer (NK) cells are important players in the immune defense against viral infections. The contribution of activating killer immunoglobulin-like receptors (KIRs) and CD94/NKG2C in regulating anti-viral responses has recently emerged. Thus, in the hematopoietic stem cell transplantation setting, the presence of donor activating KIRs (aKIRs) may protect against viral infections, while in HIV-infected individuals, KIR3DS1, in combination with HLA-Bw4-I80, results in reduction of viral progression. Since, studies have been performed mainly at the genetic or transcriptional level, the effective size, the function, and the "licensing" status of NK cells expressing aKIRs, as well as the nature of their viral ligands, require further investigation. Certain viral infections, mainly due to Human cytomegalovirus (HCMV), can deeply influence the NK cell development and function by inducing a marked expansion of mature NKG2C(+) NK cells expressing self-activating KIRs. This suggests that NKG2C and/or aKIRs are involved in the selective proliferation of this subset. The persistent, HCMV-induced, imprinting suggests that NK cells may display unexpected adaptive immune traits. The role of aKIRs and NKG2C in regulating NK cell responses and promoting a memory-like response to certain viruses is discussed.

KIR2DS5 in the presence of HLA-C C2 protects against endometriosis

Endometriosis is defined as the presence of functional endometrial tissue outside the uterine cavity. Several hypotheses have attempted to explain the etiology and pathogenesis of endometriosis. Recently, it has been suggested that a defect of the natural killer (NK) activity in the recognition and lysis of endometrial cells is one of the crucial points in the development of this disease. Natural killer cells can express killer immunoglobulin-like receptors (KIR), which recognize class I human leukocyte antigens on target cells. We asked whether polymorphisms in KIR, HLA-C, and HLA-B genes are risk factors for endometriosis. We tested 153 women with endometriosis diagnosed on the basis of laparoscopic and histological examination, and 213 control healthy women, who gave birth to at least one child. The frequency of KIR genes in patients was similar to that in controls except for KIR2DS5, which exerted a protective effect only in HLA-C C2-positive individuals. Moreover, KIR2DS5-positive women with endometriosis had 13 times lower chance that the disease would occupy the peritoneum than KIR2DS5- and KIR2DS4del-negative ones (OR = 0.077, P = 0.0061). Similarly, KIR2DS4del-positive endometriotic persons had 11 times lower chance for peritoneal disease (OR = 0.094, P < 0.001). Negative linkage disequilibrium between KIR2DS5 and KIR2DS4del indicates that these genes are mutually exclusive. Our data suggest that KIR2DS5 may be associated with protection from endometriosis, whereas KIR2DS4del seems to be associated with higher disease stages, possibly by exclusion of protective KIR2DS5.

A KIR B centromeric region present in Africans but not Europeans protects pregnant women from pre-eclampsia

In sub-Saharan Africans, maternal mortality is unacceptably high, with >400 deaths per 100,000 births compared with <10 deaths per 100,000 births in Europeans. One-third of the deaths are caused by pre-eclampsia, a syndrome arising from defective placentation. Controlling placentation are maternal natural killer (NK) cells that use killer-cell immunoglobulin-like receptor (KIR) to recognize the fetal HLA-C molecules on invading trophoblast. We analyzed genetic polymorphisms of maternal KIR and fetal HLA-C in 484 normal and 254 pre-eclamptic pregnancies at Mulago Hospital, Kampala, Uganda. The combination of maternal KIR AA genotypes and fetal HLA-C alleles encoding the C2 epitope associates with pre-eclampsia [P = 0.0318, odds ratio (OR) = 1.49]. The KIR genes associated with protection are located in centromeric KIR B regions that are unique to sub-Saharan African populations and contain the KIR2DS5 and KIR2DL1 genes (P = 0.0095, OR = 0.59). By contrast, telomeric KIR B genes protect Europeans against pre-eclampsia. Thus, different KIR B regions protect sub-Saharan Africans and Europeans from pre-eclampsia, whereas in both populations, the KIR AA genotype is a risk factor for the syndrome. These results emphasize the importance of undertaking genetic studies of pregnancy disorders in African populations with the potential to provide biological insights not available from studies restricted to European populations.

Characterization of a weakly expressed KIR2DL1 variant reveals a novel upstream promoter that controls KIR expression

Members of the human KIR (killer cell immunoglobulin-like receptor) class I major histocompatibility complex receptor gene family contain multiple promoters that determine the variegated expression of KIR on natural killer cells. In order to identify novel genetic alterations associated with decreased KIR expression, a group of donors was characterized for KIR gene content, transcripts and protein expression. An individual with a single copy of the KIR2DL1 gene but a very low level of gene expression was identified. The low expression phenotype was associated with a single-nucleotide polymorphism (SNP) that created a binding site for the inhibitory ZEB1 (Zinc finger E-box-binding homeobox 1) transcription factor adjacent to a c-Myc binding site previously implicated in distal promoter activity. Individuals possessing this SNP had a substantial decrease in distal KIR2DL1 transcripts initiating from a novel intermediate promoter located 230 bp upstream of the proximal promoter start site. Surprisingly, there was no decrease in transcription from the KIR2DL1 proximal promoter. Reduced intermediate promoter activity revealed the existence of alternatively spliced KIR2DL1 transcripts containing premature termination codons that initiated from the proximal KIR2DL1 promoter. Altogether, these results indicate that distal transcripts are necessary for KIR2DL1 protein expression and are required for proper processing of sense transcripts from the bidirectional proximal promoter.

Increased frequency and function of KIR2DL1-3⁺ NK cells in primary HIV-1 infection are determined by HLA-C group haplotypes

The acquisition and maintenance of NK-cell function is mediated by inhibitory killer-cell immunoglobulin-like receptors (KIRs) through their interaction with HLA class I molecules. Recently, HLA-C expression levels were shown to be correlated with protection against multiple outcomes of HIV-1 infection; however, the underlying mechanisms are poorly understood. As HLA-C is the natural ligand for the inhibitory receptors KIR2DL1 and KIR2DL2/3, we sought to determine whether HLA-C group haplotypes affect NK-cell responses during primary HIV-1 infection. The phenotypes and functional capacity of NK cells derived from HIV-1-positive and HIV-1-negative individuals were assessed (N = 42 and N = 40, respectively). HIV-1 infection was associated with an increased frequency of KIR2DL1-3(+) NK cells. Further analysis showed that KIR2DL1(+) NK cells were selectively increased in individuals homozygous for HLA-C2, while HLA-C1-homozygous individuals displayed increased proportions of KIR2DL2/3(+) NK cells. KIR2DL1-3(+) NK cells were furthermore more polyfunctional during primary HIV-1 infection in individuals also encoding for their cognate HLA-C group haplotypes, as measured by degranulation and IFN-γ and TNF-α production. These results identify a novel relationship between HLA-C and KIR2DL(+) NK-cell subsets and demonstrate that HLA-C-mediated licensing modulates NK-cell responses to primary HIV-1 infection.

Activating killer cell Ig-like receptors in health and disease

Expression of non-rearranged HLA class I-binding receptors characterizes human and mouse NK cells. The postulation of the missing-self hypothesis some 30 years ago triggered the subsequent search and discovery of inhibitory MHC-receptors, both in humans and mice. These receptors have two functions: (i) to control the threshold for NK cell activation, a process termed “licensing” or “education,” and (ii) to inhibit NK cell activation during interactions with healthy HLA class I-expressing cells. The discovery of activating forms of KIRs (aKIR) challenged the concept of NK cell tolerance in steady state, as well as during immune challenge: what is the biological role of the activating KIR, in particular when NK cells express aKIRs in the absence of inhibitory receptors? Recently it was shown that aKIRs also participate in the education of NK cells. However, instead of lowering the threshold of activation like iKIRs, the expression of aKIRs has the opposite effect, i.e., rendering NK cells hyporesponsive. These findings may have consequences during NK cell response to viral infection, in cancer development, and in the initial stages of pregnancy. Here we review the current knowledge of activating KIRs, including the biological concept of aKIR-dependent NK cell education, and their impact in health and disease.