CD226 protein is involved in immune synapse formation and triggers Natural Killer (NK) cell activation via its first extracellular domain

Inhibition of CD226 Co-Stimulation Suppresses Diabetes Development in the NOD Mouse by Augmenting Tregs and Diminishing Effector T Cell Function

Aims/hypothesis

Immunotherapeutics targeting T cells are crucial for inhibiting autoimmune disease progression proximal to disease onset in type 1 diabetes. A growing number of T cell-directed therapeutics have demonstrated partial therapeutic efficacy, with anti-CD3 (α-CD3) representing the only regulatory agency-approved drug capable of slowing disease progression through a mechanism involving the induction of partial T cell exhaustion. There is an outstanding need to augment the durability and effectiveness of T cell targeting by directly restraining proinflammatory T helper type 1 (Th1) and type 1 cytotoxic CD8 + T cell (Tc1) subsets, while simultaneously augmenting regulatory T cell (Treg) activity. Here, we present a novel strategy for reducing diabetes incidence in the NOD mouse model using a blocking monoclonal antibody targeting the type 1 diabetes-risk associated T cell co-stimulatory receptor, CD226.

Methods

Female NOD mice were treated with anti-CD226 between 7-8 weeks of age and then monitored for diabetes incidence and therapeutic mechanism of action.

Results

Compared to isotype-treated controls, anti-CD226 treated NOD mice showed reduced insulitis severity at 12 weeks and decreased disease incidence at 30 weeks. Flow cytometric analysis performed five weeks post-treatment demonstrated reduced proliferation of CD4 + and CD8 + effector memory T cells in spleens of anti-CD226 treated mice. Phenotyping of pancreatic Tregs revealed increased CD25 expression and STAT5 phosphorylation following anti-CD226, with splenic Tregs displaying augmented suppression of CD4 + T cell responders in vitro. Anti-CD226 treated mice exhibited reduced frequencies of islet-specific glucose-6-phosphatase catalytic subunit related protein (IGRP)-reactive CD8 + T cells in the pancreas, using both ex vivo tetramer staining and single-cell T cell receptor sequencing (scTCR-seq) approaches. 51 Cr-release assays demonstrated reduced cell-mediated lysis of beta-cells by anti-CD226-treated autoreactive cytotoxic T lymphocytes.

Conclusions/interpretation

CD226 blockade reduces T cell cytotoxicity and improves Treg function, representing a targeted and rational approach for restoring immune regulation in type 1 diabetes.

Research in Context

What is already known about this subject?: The co-stimulatory receptor CD226 is upregulated upon activation and is highly expressed on NK cell subsets, myeloid cells, and effector T cells. A single nucleotide polymorphism in CD226 ( rs763361 ; C>T) results in a Gly307Ser missense mutation linked to genetic susceptibility for type 1 diabetes. Global knockout of Cd226 and conditional Cd226 knockout in FoxP3 + Tregs reduced insulitis severity and diabetes incidence in NOD mice, indicating a crucial role for CD226 in disease pathogenesis. What is the key question?: Can CD226 blockade reduce T cell cytotoxicity and improve Treg function to diminish diabetes incidence in NOD mice?What are the new findings?: Anti-CD226 treatment reduced insulitis, decreased disease incidence, and inhibited splenic CD4 + and CD8 + effector memory T cell proliferation. Pancreatic Tregs from anti-CD226 treated mice exhibited increased CD25 expression; splenic Tregs displayed augmented STAT5 phosphorylation and suppressive capacity in vitro . Anti-CD226 treatment reduced IGRP-specific pancreatic CD8 + T cell frequencies, and reduced autoreactive CD8 + T cell-mediated lysis of beta-cells in vitro . How might this impact on clinical practice in the foreseeable future?: CD226 blockade could reduce autoreactive T cell cytotoxicity, enhance Treg function, and slow disease progression in high-risk or recent-onset type 1 diabetes cases.

Blocking CD226 regulates type 2 innate lymphoid cell effector function and alleviates airway hyperreactivity

BACKGROUND

Type 2 innate lymphoid cells (ILC2s) play a pivotal role in type 2 asthma. CD226 is a costimulatory molecule involved in various inflammatory diseases.

OBJECTIVE

We aimed to investigate CD226 expression and function within human and mouse ILC2s, and to assess the impact of targeting CD226 on ILC2-mediated airway hyperreactivity (AHR).

METHODS

We administered IL-33 intranasally to wild-type mice, followed by treatment with anti-CD226 antibody or isotype control. Pulmonary ILC2s were sorted for ex vivo analyses through RNA sequencing and flow cytometry. Next, we evaluated the effects of CD226 on AHR and lung inflammation in wild-type and Rag2-/- mice. Additionally, we compared peripheral ILC2s from healthy donors and asthmatic patients to ascertain the role of CD226 in human ILC2s.

RESULTS

Our findings demonstrated an inducible expression of CD226 in activated ILC2s, enhancing their cytokine secretion and effector functions. Mechanistically, CD226 alters intracellular metabolism and enhances PI3K/AKT and MAPK signal pathways. Blocking CD226 ameliorates ILC2-dependent AHR in IL-33 and Alternaria alternata-induced models. Interestingly, CD226 is expressed and inducible in human ILC2s, and its blocking reduces cytokine production. Finally, we showed that peripheral ILC2s in asthmatic patients exhibited elevated CD226 expression compared to healthy controls.

CONCLUSION

Our findings underscore the potential of CD226 as a novel therapeutic target in ILC2s, presenting a promising avenue for ameliorating AHR and allergic asthma.

The apoptotic effects of NK-92 cells stimulated with an anti-CD226 antibody on MDA-MB-231 triple-negative breast cancer cells

Research on immunotherapy in breast cancer treatment has recently gained importance. In this context, natural killer (NK) cells have been shown to kill cancer cells without affecting normal cells. Our study used the NK-92 cells that were stimulated with anti-CD226 antibodies (sNK-92) to increase their activity to target MDA-MB-231 triple-negative breast cancer cells. MCF-12A normal breast cells were used as the control in all experiments. The cytotoxic effects of NK-92 and sNK-92 cells on MDA-MB-231 cells were investigated using lactate dehydrogenase tests. The sNK-92 cells were more cytotoxic than NK-92 cells on MDA-MB-231 cells. In contrast, a significant cytotoxic change was not observed in MCF-12A cells cocultured with NK-92 and sNK-92 cells. An increase in granzyme B levels after coculturing with sNK-92 cells was investigated using the granzyme B enzyme-linked immunosorbent assay. The sNK-92 cells secreted more granzyme B than NK-92 cells against MDA-MB-231 cells. This increase was not observed in MCF-12A, indicating that sNK-92 cells specifically target cancer cells. In addition, immunostaining was used to investigate the synthesis level of BAX, CASP3, and CASP9 proteins to determine whether the observed cytotoxic effect was due to apoptosis. These proteins were synthesized more in MDA-MB-231 cells cocultured with sNK-92 than with NK-92 cells. However, no increase in their synthesis was observed in normal breast cells cocultured with NK-92 and sNK-92 cells. In conclusion, NK-92 cells stimulated with anti-CD226 antibodies secrete more granzyme B, resulting in a greater cytotoxic effect by inducing programmed cell death (apoptosis). The fact that the observed effects on breast cancer cells were not observed in normal breast cells indicates that sNK-92 cells specifically target breast cancer cells. These results indicate the potential use of CD226-stimulated NK-92 cells in immunotherapy.

Oncolytic Virus-Driven Biotherapies from Bench to Bedside

With advances in cancer biology and an ever-deepening understanding of molecular virology, oncolytic virus (OV)-driven therapies have developed rapidly and become a promising alternative to traditional cancer therapies. In recent years, satisfactory results for oncolytic virus therapy (OVT) are achieved at both the cellular and organismal levels, and efforts are being increasingly directed toward clinical trials. Unfortunately, OVT remains ineffective in these trials, especially when performed using only a single OV reagent. In contrast, integrated approaches, such as using immunotherapy, chemotherapy, or radiotherapy, alongside OVT have demonstrated considerable efficacy. The challenges of OVT in clinical efficacy include the restricted scope of intratumoral injections and poor targeting of intravenous administration. Further optimization of OVT delivery is needed before OVs become a viable therapy for tumor treatment. In this review, the development process and antitumor mechanisms of OVs are introduced. The advances in OVT delivery routes to provide perspectives and directions for the improvement of OVT delivery are highlighted. This review also discusses the advantages and limitations of OVT monotherapy and combination therapy through the lens of recent clinical trials and aims to chart a course toward safer and more effective OVT strategies.

MD Simulation Reveals Regulation of Mechanical Force and Extracellular Domain 2 on Binding of DNAM-1 to CD155

Two extracellular domains of the adhesive receptor DNAM-1 are involved in various cellular biological processes through binding to ligand CD155, usually under a mechano-microenvironment. The first extracellular domain (D1) plays a key role in recognition, but the function of the second extracellular domain (D2) and effects of force on the interaction of DNAM-1 with CD155 remain unclear. We herein studied the interaction of DNAM-1 with CD155 by performing steered molecular dynamics (MD) simulations, and observed the roles of tensile force and D2 on the affinity of DNAM-1 to CD155. The results showed that D2 improved DNAM-1 affinity to CD155; the DNAM-1/CD155 complex had a high mechanical strength and a better mechanical stability for its conformational conservation either at pulling with constant velocity or under constant tensile force (≤100 pN); the catch-slip bond transition governed CD155 dissociation from DNAM-1; and, together with the newly assigned key residues in the binding site, force-induced conformation changes should be responsible for the mechanical regulation of DNAM-1's affinity to CD155. This work provided a novel insight in understanding the mechanical regulation mechanism and D2 function in the interaction of DNAM-1 with CD155, as well as their molecular basis, relevant transmembrane signaling, and cellular immune responses under a mechano-microenvironment.

Nectin Family Ligands Trigger Immune Effector Functions in Health and Autoimmunity

The superfamily of immunoglobulin cell-adhesion molecules (IgCAMs) is a well-known family of cell-adhesion molecules used for immune-cell extravasation and cell–cell interaction. Amongst others, this family includes DNAX accessory molecule 1 (DNAM-1/CD226), class-I-restricted T-cell-associated molecule (CRTAM/CD355), T-cell-activated increased late expression (Tactile/CD96), T-cell immunoreceptor with Ig and ITIM domains (TIGIT), Nectins and Nectin-like molecules (Necls). Besides using these molecules to migrate towards inflammatory sites, their interactions within the immune system can support the immunological synapse with antigen-presenting cells or target cells for cytotoxicity, and trigger diverse effector functions. Although their role is generally described in oncoimmunity, this review emphasizes recent advances in the (dys)function of Nectin-family ligands in health, chronic inflammatory conditions and autoimmune diseases. In addition, this review provides a detailed overview on the expression pattern of Nectins and Necls and their ligands on different immune-cell types by focusing on human cell systems.

Loss of CD226 protects apolipoprotein E-deficient mice from diet-induced atherosclerosis

CD226 is a costimulatory molecule that regulates immune cell functions in T cells, natural killer cells, and macrophages. Because macrophage-derived foam cell formation is a crucial factor contributing to the development of atherosclerosis, we aimed to evaluate the potential roles of CD226 in the pathogenesis of atherosclerosis. The effects of CD226 on atherosclerosis were investigated in CD226 and apolipoprotein E double-knockout (CD226^-/- ApoE^-/-) mice fed with a high-cholesterol atherogenic diet. CD226 expression in macrophages was evaluated using flow cytometry. Histopathological analysis was performed to evaluate the atherosclerotic lesions. Inflammatory cell infiltration was detected using immunofluorescence staining. Bone marrow-derived macrophages (BMDMs) and peritoneal macrophages (PEMs) were isolated from the mice and used to explore the mechanism in vitro. The in vivo results indicated that CD226 knockdown protected against atherosclerosis in ApoE^-/- mice, evidenced by reduced plaque accumulation in the brachiocephalic artery, aortic roots, and main aortic tree. CD226 gene-deficient macrophages showed reduced foam cell formation under ox-low density lipoprotein stimulation compared with wild-type (WT) cells. CD226 deficiency also decreased the expression of CD36 and scavenger receptor (SR)-A (responsible for lipoprotein uptake) but increased the expression of ATP-binding cassette transporter A1 and G1 (two transporters for cholesterol efflux). Therefore, loss of CD226 hinders foam cell formation and atherosclerosis progression, suggesting that CD226 is a promising new therapeutic target for atherosclerosis.

Conservation of structural and interactional features of CD226 and Necl5 molecules from Nile tilapia (Oreochromis niloticus)

CD226 interacts with its ligand Necl5 as a costimulatory signal. In this study, we cloned a CD226 from Nile tilapia (Oreochromis niloticus, named OnCD226) and a Necl5 (named OnNecl5). The open reading frame of OnCD226 was 1071 bp, encoding a protein of 356 amino acids. Sequence alignment analysis indicated that OnCD226 contained two Ig-like domains in ectodomain. The open reading frame of OnNecl5 was 1155 bp, encoding a protein of 384 amino acids, and there are three lg-like domains in the extracellular domain. In healthy tilapia, OnCD226 was distributed in all tested tissues and relatively higher in the brain, while OnNecl5 was relatively higher in the skin. After Streptococcus agalactiae infection, OnCD226 has the same up-regulated expression pattern as OnNecl5 in different tissues. After HKLs stimulation with S. agalactiae and Poly I:C, respectively. OnCD226 was significantly up-regulated (0.01 < p < 0.05) at 12 h and extremely significant up-regulation was observed (p < 0.01) at 48 h and 96 h, the peak was observed at 96 h after stimulation by S. agalactiae. After stimulation by Poly I:C, OnCD226 expression was extremely significant (p < 0.01) at 72 h and 96 h, the peak was observed at 96 h. After stimulation by Keyhole limpet hemocyanin (KLH), a classical T cell-dependent antigen, the expression of OnCD226 was significantly up-regulated in blood, head kidney, spleen, and thymus. Moreover, when compared with the first challenge, the gene expression of OnCD226 which response to the second challenge was up-regulated earlier. Subcellular co-localization studies showed that OnCD226 and OnNecl5 were distributed mainly in the cytomembrane. Yeast two-hybrid results, indicated a strong interaction between OnCD226 and OnNecl5. These results suggested that OnCD226 plays an important role during pathogens infection, and the interaction between CD226 and Necl5 is conserved in Nile tilapia.

CD155 on Tumor Cells Drives Resistance to Immunotherapy by Inducing the Degradation of the Activating Receptor CD226 in CD8+ T Cells

The activating receptor CD226 is expressed on lymphocytes, monocytes, and platelets and promotes anti-tumor immunity in pre-clinical models. Here, we examined the role of CD226 in the function of tumor-infiltrating lymphocytes (TILs) and resistance to immunotherapy. In murine tumors, a large proportion of CD8⁺ TILs had decreased surface expression of CD226 and exhibited features of dysfunction, whereas CD226^hi TILs were highly functional. This correlation was seen also in TILs isolated from HNSCC patients. Mutation of CD226 at tyrosine 319 (Y319) led to increased CD226 surface expression, enhanced anti-tumor immunity and improved efficacy of immune checkpoint blockade (ICB). Mechanistically, tumor-derived CD155, the ligand for CD226, initiated phosphorylation of Y319 by Src kinases, thereby enabling ubiquitination of CD226 by CBL-B, internalization, and proteasomal degradation. In pre-treatment samples from melanoma patients, CD226⁺CD8⁺ T cells correlated with improved progression-free survival following ICB. Our findings argue for the development of therapies aimed at maintaining the expression of CD226.

CD226 Attenuates Treg Proliferation via Akt and Erk Signaling in an EAE Model

Cluster of differentiation 226 (CD226) molecules play a crucial role in the activation of effector CD4⁺ T cells during the immune response process, but a cell-intrinsic function of CD226 in CD4⁺ T subsets is not clear. In this study, we showed that Cd226^−/− mice were resistant to myelin oligodendrocyte glycoprotein peptide 35-55 (MOG₃₅₋₅₅)-induced experimental autoimmune encephalomyelitis (EAE) with highly expressed IL-10⁺CD4⁺ T cells and downregulated IL-17A⁺CD4⁺ T cells when compared with wild-type (WT) mice. Th17 cell infiltration into the central nervous system (CNS) was largely decreased in the absence of CD226 during EAE. CD226 deficiency facilitated the proliferation of regulatory T cells (Tregs), with increased numbers of Tregs observed in EAE mice, and supported the elevated induced regulatory T cell (iTregs) proliferation in vitro. The Akt and Erk signaling pathways were shown to be involved in Cd226^−/− Treg proliferation and function in vivo and in vitro. These findings collectively indicate that CD226 is a key molecule regulating the Treg-mediated suppression of autoimmune responses by inhibiting Treg proliferation. Thus, the results of this study identify additional mechanisms by which CD226 regulates Treg functions in EAE and supports the potential therapeutic effects of anti-CD226 molecules on autoimmune diseases.

CD226: An Emerging Role in Immunologic Diseases

CD226, a member of the immunoglobulin superfamily, is a functional protein initially expressed on natural killer and T cells. In recent years, the function of CD226 has been increasingly realized and researched. Accumulating evidence shows that CD226 is closely related to the occurrence of autoimmune diseases, infectious diseases, and tumors. Because of the CD226’s increasing importance, the author herein discusses the structure, mechanism of action, and role of CD226 in various pathophysiological environments, allowing for further understanding of the function of CD226 and providing the basis for further research in related diseases.

Co-Stimulatory Receptors in Cancers and Their Implications for Cancer Immunotherapy

Immune checkpoint inhibitors (ICIs), including anti-PD-1 and anti-CTLA-4 therapeutic agents, are now approved by the Food and Drug Administration for treatment of various types of cancer. However, the therapeutic efficacy of ICIs varies among patients and cancer types. Moreover, most patients do not develop durable antitumor responses after ICI therapy due to an ephemeral reversal of T-cell dysfunction. As co-stimulatory receptors play key roles in regulating the effector functions of T cells, activating co-stimulatory pathways may improve checkpoint inhibition efficacy, and lead to durable antitumor responses. Here, we review recent advances in our understating of co-stimulatory receptors in cancers, providing the necessary groundwork for the rational design of cancer immunotherapy.

Cell-Laden Hydrogel as a Clinical-Relevant 3D Model for Analyzing Neuroblastoma Growth, Immunophenotype, and Susceptibility to Therapies

High risk Neuroblastoma (NB) includes aggressive, metastatic solid tumors of childhood. The survival rate improved only modestly, despite the use of combination therapies including novel immunotherapies based on the antibody-mediated targeting of tumor-associated surface ligands. Treatment failures may be due to the lack of adequate in vitro models for studying, in a given patient, the efficacy of potential therapeutics, including those aimed to enhance anti-tumor immune responses. We here propose a 3D alginate-based hydrogel as extracellular microenvironment to evaluate the effects of the three-dimensionality on biological and immunological properties of NB cells. NB cell lines grown within the 3D alginate spheres presented spheroid morphology, optimal survival, and proliferation capabilities, and a reduced sensitivity to the cytotoxic effect of imatinib mesylate. 3D cultured NB cells were also evaluated for the constitutive and IFN-γ-induced expression of surface molecules capable of tuning the anti-tumor activity of NK cells including immune checkpoint ligands. In particular, IFN-γ induced de novo expression of high amounts of HLA-I molecules, which protected NB cells from the attack mediated by KIR/KIR-L matched NK cells. Moreover, in the 3D alginate spheres, the cytokine increased the expression of the immune checkpoint ligands PD-Ls and B7-H3 while virtually abrogating that of PVR, a ligand of DNAM-1 activating receptor, whose expression correlates with high susceptibility to NK-mediated killing. Our 3D model highlighted molecular features that more closely resemble the immunophenotypic variants occurring in vivo and not fully appreciated in classical 2D culture conditions. Thus, based on our results, 3D alginate-based hydrogels might represent a clinical-relevant cell culture platform where to test the efficacy of personalized therapeutic approaches aimed to optimize the current and innovative immune based therapies in a very systematic and reliable way.

Engineering antigen-specific NK cell lines against the melanoma-associated antigen tyrosinase via TCR gene transfer

Introduction of Chimeric Antigen Receptors to NK cells has so far been the main practical method for targeting NK cells to specific surface antigens. In contrast, T cell receptor (TCR) gene delivery can supply large populations of cytotoxic T-lymphocytes (CTL) targeted against intracellular antigens. However, a major barrier in the development of safe CTL-TCR therapies exists, wherein the mispairing of endogenous and genetically transferred TCR subunits leads to formation of TCRs with off-target specificity. To overcome this and enable specific intracellular antigen targeting, we have tested the use of NK cells for TCR gene transfer to human cells. Our results show that ectopic expression of TCR α/β chains, along with CD3 subunits, enables the functional expression of an antigen-specific TCR complex on NK cell lines NK-92 and YTS, demonstrated by using a TCR against the HLA-A2-restricted tyrosinase-derived melanoma epitope, Tyr_368-377 . Most importantly, the introduction of a TCR complex to NK cell lines enables MHC-restricted, antigen-specific killing of tumor cells both in vitro and in vivo. Targeting of NK cells via TCR gene delivery stands out as a novel tool in the field of adoptive immunotherapy which can also overcome the major hurdle of "mispairing" in TCR gene therapy.

NK cells and multiple myeloma-associated endothelial cells: molecular interactions and influence of IL-27

Angiogenesis represents a hallmark of tumor progression in Multiple Myeloma (MM), a still incurable malignancy. Here we analyzed the activity of cytokine-stimulated NK cells against tumor-associated endothelial cells isolated from bone marrow aspirates of MM patients with active disease (MMECs). We show that NK cells activated with optimal doses of IL-15 killed MMECs thanks to the concerted action of multiple activating receptors. In particular, according to the high expression of PVR and Nectin-2 on MMECs, DNAM-1 actively participated in target recognition. Interestingly, in MMECs the surface density of PVR was significantly higher than that detected in endothelium from patients with MM in complete remission or with monoclonal gammopathy of undetermined significance (MGUS). Importantly, IL-27, which unlike IL-15 does not display pro-angiogenic properties, maintained or increased the NK cell functions induced by suboptimal concentrations of IL-15. NK cell properties included killing of MMECs, IFN-γ production as well as a peculiar increase of NKp46 expression on NK cell surface. Finally, IL-27 showed a striking capability of up-regulating the expression of PD-L2 and HLA-I on tumor endothelium, whereas it did not modify that of PD-L1 and HLA-II.

Our results suggest that cytokine-activated endogenous or adoptively transferred NK cells might support conventional therapies improving the outcome of MM patients.

CD226 ligation protects against EAE by promoting IL-10 expression via regulation of CD4+ T cell differentiation

Treatment targeting CD226 can ameliorate experimental autoimmune encephalomyelitis (EAE), the widely accepted model of MS. However, the mechanisms still need to be elucidated. Here we showed that CD226 blockage by anti-CD226 blocking mAb LeoA1 efficiently promoted IL-10 production in human peripheral blood monocytes (PBMC) or in mixed lymphocyte culture (MLC) system, significantly induced the CD4⁺IL-10⁺ T cell differentiation while suppressing the generation of Th1 and Th17. Furthermore, CD226 pAb administration in vivo reduced the onset of EAE in mice by promoting IL-10 production and regulating T cell differentiation. Concomitantly, the onset and severity of EAE were reduced and the serum IL-10 expression levels were increased in CD226 knockout mice than that in control mice when both received EAE induction. These novel findings confirmed that CD226 played a pivotal role in mediating autoimmune diseases such as EAE. Furthermore, to our knowledge, we show for the first time that IL-10 is an important contributor in the inhibitory effects of CD226 ligation on EAE.

Dynamic Regulation of NK Cell Responsiveness

Natural killer (NK) cells deliver cytotoxic granules and immunomodulatory cytokines in response to transformed and virally infected cells. NK cell functions are under the control of a large number of germline-encoded receptors that recognize various ligands on target cells, but NK cells also respond to cytokines in the surrounding environment. The interaction between NK cell receptors and their ligands delivers either inhibitory or activating signals. The cytokine milieu further shapes NK cell responses, either directly or by influencing the way inhibitory or activating signals are perceived by NK cells. In this review, we discuss how NK cell function is controlled by inhibitory receptors and MHC-I molecules, how activating receptors contribute to NK cell education, and finally, how cytokines secreted by the surrounding cells affect NK cell responsiveness. Inputs at these three levels involve different cell types and are seamlessly integrated to form a functional NK cell population.

NK cell education via nonclassical MHC and non-MHC ligands

Natural killer (NK) cell education, a process for achieving functional maturation and self-tolerance, has been previously defined by the interaction between self-major histocompatibility complex class I (MHC-I) molecules and their specific inhibitory receptors. Over the past several years, growing evidence has highlighted the important roles of nonclassical MHC-I and non-MHC-I molecules in NK cell education. Herein, we review the current knowledge of NK cell education, with a particular focus on nonclassical MHC-I- and non-MHC-I-dependent education, and compare them with the classical MHC-I-dependent education theory. In addition, we update and extend this theory by presenting the 'Confining Model', discussing cis and trans characteristics, reassessing quantity and quality control, and elucidating the redundancy of NK cell education in tumor and virus infection.

Overexpression of KIR inhibitory ligands (HLA-I) determines that immunosurveillance of myeloma depends on diverse and strong NK cell licensing

Missing self recognition makes cancer sensitive to natural killer cell (NKc) reactivity. However, this model disregards the NKc licensing effect, which highly increases NKc reactivity through interactions of inhibitory killer cell immunoglobulin-like receptors (iKIR) with their cognate HLA-I ligands. The influence of iKIR/HLA-ligand (HLA-C1/C2) licensing interactions on the susceptibility to and progression of plasma cell (PC) dyscrasias was evaluated in 164 Caucasian patients and 286 controls. Compared to controls, myeloma accumulates KIR2DL1^-L2⁺L3^- genotypes (2.8% vs. 13.2%, p < 0.01, OR = 5.29) and less diverse peripheral repertoires of NKc clones. Less diverse and weaker-affinity repertoires of iKIR2D/HLA-C licensing interactions increased myeloma susceptibility. Thus, the complete absence of conventional iKIR2D/HLA-C licensing interactions (KIR2DL1^-L2⁺L3^-/C2C2, 2.56% vs. 0.35%; p < 0.05; OR = 15.014), single-KIR2DL3⁺/C1⁺ (20.51% vs. 10.84%; p < 0.05; OR = 2.795) and single-KIR2DL2⁺/C1⁺ (12.82% vs. 4.9%; p < 0.01; OR = 5.18) interactions were over-represented in myeloma, compared to controls. Additionally, KIR2DL1^-L2⁺L3^- (20% vs. 83%, p < 0.00001) as well as KIR3DL1^- (23% vs. 82%, p < 0.00001) genotypes had a dramatic negative impact on the 3-y progression-free survival (PFS), particularly in patients with low-tumor burden. Remarkably, myeloma-PCs, compared to K562 and other hematological cancers, showed substantial over-expression of HLA-I ("increasing-self" instead of missing-self), including HLA-C, and mild expression of ligands for NKc activating receptors (aRec) CD112, CD155, ULBP-1 and MICA/B, which apparently renders myeloma-PCs susceptible to lysis mainly by licensed NKc. KIR2DL1^-L2⁺L3^-/C2C2 patients (with no conventional iKIR2D/HLA-C licensing interactions) lyse K562 but barely lyse myeloma-PCs (4% vs. 15%; p < 0.05, compared to controls). These results support a model where immunosurveillance of no-missing-self cancers, e.g., myeloma, mainly depends on NKc licensing.

Critical roles of co-activation receptor DNAX accessory molecule-1 in natural killer cell immunity

Natural killer (NK) cells, which can exert early and powerful anti-tumour and anti-viral responses, are important components of the innate immune system. DNAX accessory molecule-1 (DNAM-1) is an activating receptor molecule expressed on the surface of NK cells. Recent findings suggest that DNAM-1 is a critical regulator of NK cell biology. DNAM-1 is involved in NK cell education and differentiation, and also plays a pivotal role in the development of cancer, viral infections and immune-related diseases. However, tumours and viruses have developed multiple mechanisms to evade the immune system. They are able to impair DNAM-1 activity by targeting the DNAM-1 receptor-ligand system. We have reviewed the roles of DNAM-1, and its biological functions, with respect to NK cell biology and DNAM-1 chimeric antigen receptor-based immunotherapy.

Practical NK cell phenotyping and variability in healthy adults

Human natural killer (NK) cells display a wide array of surface and intracellular markers that indicate various states of differentiation and/or levels of effector function. These NK cell subsets exist simultaneously in peripheral blood and may vary among individuals. We examined variety among selected NK cell receptors expressed by NK cells from normal donors, as well as the distribution of select NK cell subsets and NK cell receptor expression over time in several individual donors. Peripheral blood mononuclear cells were evaluated using flow cytometry via fluorochrome-conjugated antibodies against a number of NK cell receptors. Results were analyzed for both mean fluorescence intensity (MFI) and the percent positive cells for each receptor. CD56(bright) and CD56(dim) NK cell subsets were also considered separately, as was variation in receptor expression in NK cell subsets over time in selected individuals. Through this effort, we provide ranges of NK cell surface receptor expression for a local adult population as well as provide insight into intra-individual variation.

Balancing natural killer cell activation through paired receptors

Natural killer (NK) cells are innate lymphocytes that are crucial for the control of infections and malignancies. NK cells express a variety of inhibitory and activating receptors that facilitate fine discrimination between damaged and healthy cells. Among them, a family of molecules that bind nectin and nectin-like proteins has recently emerged and has been shown to function as an important regulator of NK cell functions. These molecules include CD226, T cell immunoreceptor with immunoglobulin and ITIM domains (TIGIT), CD96, and cytotoxic and regulatory T cell molecule (CRTAM). In this Review, we focus on the recent advances in our understanding of how these receptors regulate NK cell biology and of their roles in pathologies such as cancer, infection and autoimmunity.