Soluble MICB in Plasma and Urine Explains Population Expansions of NKG2D+CD4 T Cells Inpatients with Juvenile-Onset Systemic Lupus Erythematosus

Lymphocyte subsets and soluble forms of MIC-A and MIC-B are prognostic factors in non-Hodgkin lymphoma patients

MIC-A and MIC-B are the natural ligands for NKG2D, an activator receptor expressed in NK cells. Soluble isoforms of MIC-A and MIC-B (sMICA, sMICB) have been identified in different malignancies, affecting NK cells' cytotoxicity. The study was performed to determine the levels of sMICA, sMICB, the expression of MIC-A, and MIC-B on tumor tissues, and lymphocyte subpopulations (CD4 + , CD8 + , NK, NKT, Tγδ cells, B cells, monocytes) in 94 patients with non-Hodgkin's lymphoma (NHL) and 72 healthy donors.The most frequent lymphoma was diffuse large B cell lymphoma (48%). Patients with NHL had decreased numbers of CD4 T cells, CD8 T cells, B cells, monocytes, NK cells, type 1 dendritic cells, γδ T cells, and increased iNKT cells. Patients showed higher levels of sMIC-A and similar serum levels of sMIC-B.Survival was poorer in patients having higher LDH values and lower numbers of CD4 T cells, type 1 dendritic cells, gamma-delta T cells, and high levels of sMIC-A.In conclusion, high levels of sMIC and decreased numbers in circulating lymphocyte subsets are related to poor outcomes in NHL.

The Role of NKG2D and Its Ligands in Autoimmune Diseases: New Targets for Immunotherapy

Natural killer (NK) cells and CD8+ T cells can clear infected and transformed cells and generate tolerance to themselves, which also prevents autoimmune diseases. Natural killer group 2 member D (NKG2D) is an important activating immune receptor that is expressed on NK cells, CD8+ T cells, γδ T cells, and a very small percentage of CD4+ T cells. In contrast, the NKG2D ligand (NKG2D-L) is generally not expressed on normal cells but is overexpressed under stress. Thus, the inappropriate expression of NKG2D-L leads to the activation of self-reactive effector cells, which can trigger or exacerbate autoimmunity. In this review, we discuss the role of NKG2D and NKG2D-L in systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), multiple sclerosis (MS), type I diabetes (T1DM), inflammatory bowel disease (IBD), and celiac disease (CeD). The data suggest that NKG2D and NKG2D-L play a pathogenic role in some autoimmune diseases. Therefore, the development of strategies to block the interaction of NKG2D and NKG2D-L may have therapeutic effects in some autoimmune diseases.

[Role of CD4+NKG2D+ T cells in the disease activity of juvenile idiopathic arthritis]

OBJECTIVES

To study the expression levels of CD4⁺NKG2D⁺ T cells and NKG2D soluble ligands, the soluble MHC class I chain-related molecules A and B (sMICA/sMICB) in the active stage and stable stage of juvenile idiopathic arthritis (JIA) and their role in the disease activity of JIA.

METHODS

Nineteen children with systemic JIA and 20 children with articular JIA who were diagnosed in Children's Hospital of Chongqing Medical University from November 2019 to December 2021 were enrolled in this prospective study. Six healthy children were enrolled as the control group. After peripheral blood samples were collected, ELISA was used to measure the levels of sMICA and sMICB, and flow cytometry was used to measure the percentage of CD4⁺NKG2D⁺ T cells. Systemic Juvenile Arthritis Disease Activity Score-27 (sJADAS-27)/Juvenile Arthritis Disease Activity Score-27 (JADAS-27) was used to evaluate the disease activity in children with JIA. The Pearson correlation analysis and the receiver operating characteristic (ROC) curve were used to assess the role of CD4⁺NKG2D⁺ T cells, sMICA and sMICB in the disease activity of JIA.

RESULTS

The active systemic JIA and active articular JIA groups had a significant increase in the percentage of CD4⁺NKG2D⁺ T cells compared with the control group and their corresponding inactive JIA group (P<0.05). The JIA groups had significantly higher levels of sMICA and sMICB than the control group (P<0.05), and the active articular JIA group had a significantly higher level of sMICB than the stable articular JIA group (P<0.05). In the children with JIA, the percentage of CD4⁺NKG2D⁺ T cells and the levels of sMICA and sMICB were positively correlated with sJADAS-27/JADAS-27 disease activity scores (P<0.05). The ROC curve analysis showed that sMICB had an area under the curve of 0.755 in evaluating the disease activity of JIA, with a specificity of 0.90 and a sensitivity of 0.64.

CONCLUSIONS

The percentage of CD4⁺NKG2D⁺ T cells and the levels of sMICA and sMICB increase in children with JIA compared with healthy children and are positively correlated with the disease activity of JIA, suggesting that CD4⁺NKG2D⁺ T cells and NKG2D ligands can be used as potential biomarkers for evaluating the disease activity of JIA.

Increased Concentrations of Circulating Soluble MHC Class I-Related Chain A (sMICA) and sMICB and Modulation of Plasma Membrane MICA Expression: Potential Mechanisms and Correlation With Natural Killer Cell Activity in Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is a severe autoimmune disease of unknown etiology. The major histocompatibility complex (MHC) class I-related chain A (MICA) and B (MICB) are stress-inducible cell surface molecules. MICA and MICB label malfunctioning cells for their recognition by cytotoxic lymphocytes such as natural killer (NK) cells. Alterations in this recognition have been found in SLE. MICA/MICB can be shed from the cell surface, subsequently acting either as a soluble decoy receptor (sMICA/sMICB) or in CD4⁺ T-cell expansion. Conversely, NK cells are frequently defective in SLE and lower NK cell numbers have been reported in patients with active SLE. However, these cells are also thought to exert regulatory functions and to prevent autoimmunity. We therefore investigated whether, and how, plasma membrane and soluble MICA/B are modulated in SLE and whether they influence NK cell activity, in order to better understand how MICA/B may participate in disease development. We report significantly elevated concentrations of circulating sMICA/B in SLE patients compared with healthy individuals or a control patient group. In SLE patients, sMICA concentrations were significantly higher in patients positive for anti-SSB and anti-RNP autoantibodies. In order to study the mechanism and the potential source of sMICA, we analyzed circulating sMICA concentration in Behcet patients before and after interferon (IFN)-α therapy: no modulation was observed, suggesting that IFN-α is not intrinsically crucial for sMICA release in vivo. We also show that monocytes and neutrophils stimulated in vitro with cytokines or extracellular chromatin up-regulate plasma membrane MICA expression, without releasing sMICA. Importantly, in peripheral blood mononuclear cells from healthy individuals stimulated in vitro by cell-free chromatin, NK cells up-regulate CD69 and CD107 in a monocyte-dependent manner and at least partly via MICA-NKG2D interaction, whereas NK cells were exhausted in SLE patients. In conclusion, sMICA concentrations are elevated in SLE patients, whereas plasma membrane MICA is up-regulated in response to some lupus stimuli and triggers NK cell activation. Those results suggest the requirement for a tight control in vivo and highlight the complex role of the MICA/sMICA system in SLE.

NKG2D-ligands: Putting everything under the same umbrella can be misleading

NKG2D is a key receptor for the activation of immune effector cells, mainly Natural Killer cells and T lymphocytes, in infection, cancer and autoimmune diseases. Since the detection of ligands for NKG2D in sera of cancer patients is, in many human models, indicative of prognosis, a large number of studies have been undertaken to improve understanding of the biology regulating this receptor and its ligands, with the aim of translating this knowledge into clinical practice. Although it is becoming clear that the NKG2D system can be used as a tool for diagnosis and manipulated for therapy, some questions remain open due to the complexity associated with the existence of a large number of ligands, each one of them displaying distinct biological properties. In this review, we have highlighted some key aspects of this system that differ between humans and mice, including the properties of NKG2D, as well as the genetic and biochemical complexity of NKG2D-ligands. All of these features affect the characteristics of the immune response exerted by NKG2D-expressing cells and are likely to be important factors in the clearance of a tumour or the development of autoimmunity. Implementation of more global analyses, including information on genotype, transcription and protein properties (cellular vs released to the blood stream) of NKG2D-ligands expressed in patients will be necessary to fully understand the links between this system and disease progression.

A subgroup of lupus patients with nephritis, innate T cell activation and low vitamin D is identified by the enhancement of circulating MHC class I-related chain A

The major histocompatibility complex (MHC) class I-related chain A (MICA) is induced upon stress, and labels malfunctioning cells for their recognition by cytotoxic lymphocytes. Alterations in this recognition and also abnormal natural killer (NK) functions have been found in systemic lupus erythematosus (SLE). MICA can be shed from cells, subsequently acting as a soluble decoy receptor (sMICA). Our purpose was to study circulating sMICA levels in relationship with the activation of innate pathways in PBMC in a cohort of lupus patients. NK cells were characterized by flow cytometry. Gene expression of Toll-like receptors (TLR), interferon (IFN)-I sensitive genes and MICA were separately analyzed in monocytes, T cells and B cells. Serum sMICA was measured with enzyme-linked immunosorbent assay (ELISA). In our cohort, NK cell counts dropped in relationship with disease activity. sMICA showed an inverse trend with NK cell counts, as well as a significant association with activity indices, but not with complement decrease. Levels of sMICA associated to proteinuria and active nephritis. A multivariate regression model revealed anti-nuclear antibody (ANA) titres, the up-regulation of TLR-4 in T cells and lower vitamin D as predictors of sMICA enhancement. Interestingly, vitamin D showed an inverse association with proteinuria and a strong correlation with T cell MICA mRNA levels. According to our data, circulating sMICA identifies a subgroup of lupus patients with low vitamin D, innate activation of T cells and nephritis. We propose that lymphocyte shedding could account for the enhancement of sMICA and reflect an immune evasion mechanism driving disease activation in lupus.

NKG2D and Its Ligand MULT1 Contribute to Disease Progression in a Mouse Model of Multiple Sclerosis

NKG2D is an activating receptor expressed on the surface of immune cells including subsets of T lymphocytes. NKG2D binds multiple ligands (NKG2DL) whose expression are differentially triggered in a cell type and stress specific manner. The NKG2D-NKG2DL interaction has been involved in autoimmune disorders but its role in animal models of multiple sclerosis (MS) remains incompletely resolved. Here we show that NKG2D and its ligand MULT1 contribute to the pathobiology of experimental autoimmune encephalomyelitis (EAE). MULT1 protein levels are increased in the central nervous system (CNS) at EAE disease peak; soluble MULT1 is elevated in the cerebrospinal fluid of both active and passive EAE. We establish that such soluble MULT1 enhances effector functions (e.g., IFNγ production) of activated CD8 T lymphocytes from wild type but not from NKG2D-deficient (Klrk1^−/−) mice in vitro. The adoptive transfer of activated T lymphocytes from wild type donors induced a significantly reduced EAE disease in Klrk1^−/− compared to wild type (Klrk1^+/+) recipients. Characterization of T lymphocytes infiltrating the CNS of recipient mice shows that donor (CD45.1) rather than endogenous (CD45.2) CD4 T cells are the main producers of key cytokines (IFNγ, GM-CSF). In contrast, infiltrating CD8 T lymphocytes include mainly endogenous (CD45.2) cells exhibiting effector properties (NKG2D, granzyme B and IFNγ). Our data support the notion that endogenous CD8 T cells contribute to passive EAE pathobiology in a NKG2D-dependent manner. Collectively, our results point to the deleterious role of NKG2D and its MULT1 in the pathobiology of a MS mouse model.

Bystander T Cells: A Balancing Act of Friends and Foes

T cell responses are essential for appropriate protection against pathogens. T cell immunity is achieved through the ability to discriminate between foreign and self-molecules, and this relies heavily on stringent T cell receptor (TCR) specificity. Recently, bystander activated T lymphocytes, that are specific for unrelated epitopes during an antigen-specific response, have been implicated in diverse diseases. Numerous infection models have challenged the classic dogma of T cell activation as being solely dependent on TCR and major histocompatibility complex (MHC) interactions, indicating an unappreciated role for pathogen-associated receptors on T cells. We discuss here the specific roles of bystander activated T cells in pathogenesis, shedding light on the ability of these cells to modulate disease severity independently from TCR recognition.

The Paradoxical Role of NKG2D in Cancer Immunity

The activating receptor NKG2D and its ligands are recognized as a potent immune axis that controls tumor growth and microbial infections. With regards to cancer surveillance, various studies have demonstrated the antitumor function mediated by NKG2D on natural killer cells and on conventional and unconventional T cells. The use of NKG2D-deficient mice established the importance of NKG2D in delaying tumor development in transgenic mouse models of cancer. However, we recently demonstrated an unexpected, flip side to this coin, the ability for NKG2D to contribute to tumor growth in a model of inflammation-driven liver cancer. With a focus on the liver, here, we review current knowledge of NKG2D-mediated tumor surveillance and discuss evidence supporting a dual role for NKG2D in cancer immunity. We postulate that in certain advanced cancers, expression of ligands for NKG2D can drive cancer progression rather than rejection. We propose that the nature of the microenvironment within and surrounding tumors impacts the outcome of NKG2D activation. In a form of autoimmune attack, NKG2D promotes tissue damage, mostly in the inflamed tissue adjacent to the tumor, facilitating tumor progression while being ineffective at rejecting transformed cells in the tumor bed.

Natural Killer Cells: Development, Maturation, and Clinical Utilization

Natural killer (NK) cells are the predominant innate lymphocyte subsets that mediate anti-tumor and anti-viral responses, and therefore possess promising clinical utilization. NK cells do not express polymorphic clonotypic receptors and utilize inhibitory receptors (killer immunoglobulin-like receptor and Ly49) to develop, mature, and recognize “self” from “non-self.” The essential roles of common gamma cytokines such as interleukin (IL)-2, IL-7, and IL-15 in the commitment and development of NK cells are well established. However, the critical functions of pro-inflammatory cytokines IL-12, IL-18, IL-27, and IL-35 in the transcriptional-priming of NK cells are only starting to emerge. Recent studies have highlighted multiple shared characteristics between NK cells the adaptive immune lymphocytes. NK cells utilize unique signaling pathways that offer exclusive ways to genetically manipulate to improve their effector functions. Here, we summarize the recent advances made in the understanding of how NK cells develop, mature, and their potential translational use in the clinic.