Killer Cell Lectin-like Receptor G1 Expression Research Articles

KLRG1 is reduced on NK cells in SLE patients, inversely correlates with disease activity and is modulated by hydroxychloroquine in vitro

Killer cell lectin-like receptor G 1 (KLRG1), a transmembrane receptor with inhibitory capacity expressed in human immune cells, emerged as a novel susceptibility gene for systemic lupus erythematosus (SLE). The aim of this study was to investigate the expression of KLRG1 in SLE patients compared to healthy controls (HC) on both NK and T cells and to evaluate its possible involvement in SLE pathogenesis. Eighteen SLE patients and twelve healthy controls were enrolled. Peripheral blood mononuclear cells (PBMCs) from these patients were phenotypically characterized by immunofluorescence and flow cytometry. The effect of the hydroxychloroquine (HCQ) in vitro on KLRG1 expression and its signaling mediated functions in NK cells were analyzed. KLRG1 expression was significantly reduced on the analyzed immune cell populations in SLE patients compared to HC, especially on total NK cells. Moreover, expression of KLRG1 on total NK cells inversely correlated with the SLEDAI-2K. A direct association between KLRG1 expression on NK cells and patients' treatment with HCQ was observed. In vitro treatment with HCQ increased KLRG1 expression on NK cells. In HC, KLRG1+ NK cells showed reduced degranulation and IFNγ production, while in SLE patient, this inhibition occurred only for the IFNγ production. With this study we revealed a reduced expression and an impaired function of KLRG1 on NK cells in SLE patients. These results suggest a possible role of KLRG1 in the pathogenesis of SLE and as a novel biomarker of this disease.

KLRG1 Depletion Is a Novel Therapeutic Strategy for Patients with Mature T and NK/T-Cell Lymphomas

Outcomes for patients with aggressive T-cell lymphomas (TCLs) remain dismal. Killer cell lectin-like receptor G1 (KLRG1) is a co-inhibitory immune checkpoint receptor expressed on late differentiated CD8+ T effector memory (TEM) and TEMRA cells. Monotherapy with KLRG1 neutralizing antibodiesextended survival in solid tumor models. Here, we report the therapeutic potential of a novel and distinct strategy characterized by depletion of KLRG1+ malignant cells in hematologic malignancies and specifically in TCLs. We defined the molecular epidemiology of KLRG1 by determining tumor cell-specific RNA and protein expression on T cell subsets from 8 healthy donors, 26 TCL lines, 15 patient-derived xenografts (PDXs), and 165 tumor and blood specimens representing various subtypes of T and NK/TCLs. Microarray andRNA-seqdata demonstrated highertranscript levels of KLRG1 in patients with T-LGLL and PDXs of T-PLL and HSTCL relative to other subtypes, which were corroborated by protein expression. We quantified tumor cell-specific protein expression of KLRG1 by IHC by generating H scores (% of KLRG1+ tumor cells × measure of staining intensity) ranging from 0-300 across 141 primary TCL biopsies. PTCL-NOS and AITL had heterogeneous KLRG1 expression, whereas ALCL, CTCL, and EATL demonstrated none. The majority of patients with T-PLL (median 180), δ/γ TCLs (median 140), and NK/TCLs (median 100) exhibited higher H scores relative to other subtypes. Up to 99% of the CD3+/CD8+/CD57+ and up to 97% of the CD3-/CD56+/CD57+/CD16+/CD94+ leukemic cells in the blood of patients with T-LGLL (n=12) and CLPD-NK (n=12) respectively expressed KLRG1 protein (Fig.1). None of the TCL cell lines expressed surface KLRG1. Therefore, SMZ1 cells (TP63 rearranged PTCL-NOS line) were stably transduced to express KLRG1. We utilized two human KLRG1 binding antibodies, 13F12F2 (a commercial mIgG2a Ab) and mAb208 (a humanized afucosylated mIgG2a Ab) for in vitro and in vivo studies. Antibody binding capacity assays confirmed a higher number of KLRG1 receptors per cell on SMZ1 KLRG1+ (175,800), HSTCL (23,586), and CD8+ T cells (16,326) compared with B-cell lymphoma cells (242). Competition binding assays demonstrated that concentrations of 0.1 ug/ml of mAb208 saturated receptors on SMZ1 KLRG1+ cells. mAb208 induced a 2-5-fold dose-dependent increase in antibody-dependent cellular cytotoxicity (ADCC), antibody-dependent cellular phagocytosis (ADCP), and complement-dependent cytotoxicity (CDC), but not apoptosis of SMZ1 KLRG1+ cells relative to SMZ1 KLRG1- WT and isotype control (CTL). This was exhibited across a variety of human and murine effectors such as human PBMCs, human and murine macrophages, and human and murine complements. Reporter-based murine FcγRIV ADCC bioassay demonstrated a dose-dependent 2-6 fold induction of bioluminescence by mAb208 in SMZ1 KLRG1+ cells relative to WT (p=<0.0001). In 4/4 tested T-LGLL patient samples, mAb208 induced ADCC of CD8+/CD57+ LGLs with autologous PBMCs (p=0.03) relative to CTL. Finally, mAb208 induced selective ADCC of KLRG1+ CD8+ TEM and TEMRA cells while sparing KLRG1- naive and central memory T cells in 3 healthy donors (p=0.06). Duvelisib (Duv), a dual PI3K-δ/γ inhibitor (PI3Ki), has demonstrated a high ORR of 50% in phase II trials in TCLs and its ability to reprogram macrophages to an M1 phenotype in TCL PDXs. Thus, we hypothesized that the combination of PI3Ki with anti-KLRG1 mAbs would enhance the clearance of apoptotic and opsonized KLRG1+ TCL cells by proinflammatory macrophages in several PTCL subtypes. We tested this combination in vivo using SMZ1 KLRG1+ subcutaneous xenografts in NSG mice. After 17 days of treatment, combination (mAb208 + Duv)- treated mice had a marked reduction in their mean tumor volumes (159 mm3) relative to mAb208 (1003 mm3 ; p=0.03), Duv (465.6 mm3 ; p=0.03) and CTL (2104 mm3 ; p=0.03) mice resulting in prolonged survival (Fig. 2). Studies in PDX models of T-PLL and HSTCL with varying levels of KLRG1 expression (30% and 80% respectively) have been initiated and will be reported at the meeting. Selective depletion of tumor cells with an anti-KLRG1 depleting mAb in specific subtypes of TCLs, T-LGLL, and CLPD-NK is effective in vitro and in vivo, and its combination with Duv has marked activity in an aggressive PTCL-NOS xenograft model. A multicenter phase I/II trial of the KLRG1 depleting antibody ABC008 (Abcuro) in patients with R/R T-LGLL has been initiated. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

Expression of KLRG1 on subpopulations of lymphocytes in the peripheral blood of patients with locally advanced nasopharyngeal carcinoma and prognostic analysis

AbstractObjectiveWe aimed to investigate the relationship between the expression of killer cell lectin like receptor G1 (KLRG1) in peripheral blood lymphocytes, and the clinical characteristics and prognosis in patients with locally advanced nasopharyngeal carcinoma (NPC), and to dynamically evaluate the changes of KLRG1 expression after treatment.MethodsA total of 82 patients with locally advanced NPC and 44 healthy individuals were recruited in this study. The expression of KLRG1 on T, natural killer, and natural killer T cells in peripheral blood mononuclear cells (PBMCs) was detected by flow cytometry. Its relationship with clinical characteristics and prognosis of patients, as well as changes in KLRG1 expression in PBMCs after treatment, were investigated.ResultsThe expression of KLRG1 in CD8+ T cells in PBMCs of NPC patients was 46.50 (29.10, 69.80)%, significantly higher than 32.50 (17.38, 61.00) % in healthy controls (Z = −2.290, p = 0.022). The expression of KLRG1 in CD3+ CD56+ natural killer T cells in PBMCs of patients was 29.50 (6.21, 78.00) %, significantly higher than 1.47 (0.62, 8.88)% in healthy controls (Z = −5.371, p &lt; 0.001). The expression levels of KLRG1(+)CD8+ T cells in PBMCs of patients were higher in the age ≥50 years (Z = −3.413, p &lt; 0.001), pathology of keratinizing squamous cell carcinoma (H = 10.541, p = 0.005, and EGFR(+) groups (Z = −2.006, p = 0.043). Patients with higher KLRG1(+) expression in CD8+ T cells had a lower progression‐free survival (p = 0.029) and a lower distant metastasis‐free survival (p = 0.023). KLRG1 expression on T cells increased after radiotherapy.ConclusionKLRG1 can be harnessed as a potential target in designing novel therapy to encompass a broader range of NPC patients.

Expression and Clinical Significance of KLRG1 and 2B4 on T Cells in the Peripheral Blood and Tumour of Patients with Cervical Cancer

ABSTRACT Background Killer cell lectin-like receptor G1 (KLRG1) and 2B4 play important roles in the immune regulation and immune tolerance to tumor cells by inhibiting T cell function. However, the clinical relevance of KLRG1 and 2B4 to cervical cancer remains to be understood. Methods We measured the frequency of KLRG1+ or 2B4+ cells in CD4+ or CD8 + T cells derived from peripheral blood or tumour biopsies in cervical cancer patients by flow cytometry. Results Compared with healthy controls, the level of KLRG1 and 2B4 on CD8 + T cells in the blood of the patients increased significantly (P = .0056 and .0441). KLRG1 level on CD8 + T cells and 2B4 level on CD4 + T cells in peripheral blood were significantly higher than in tumor tissues (P < .0001 and P = .0003). Higher KLRG1 level on blood-derived CD8 + T cells was observed in patients older than 54 years (P = .001) or tested to be HPV-negative (P = .026). Tumor-infiltrated CD8 + T cells demonstrated elevated KLRG1 level in patients having pelvic lymph node metastasis (P = .016). Increased 2B4 level on blood-derived CD8 + T cells was also observed in patients older than 54 years (P < .001). KLRG1 expression on both CD4 + T (P = .0158) and CD8 + T (P = .0187) cells in the peripheral blood increased after radiotherapy. Conclusion KLRG1 level on T cells was related to age and HPV in patients with cervical cancer, while 2B4 level on T cells was related to age, underlying their roles in the host immune response to cervical cancer. Radiotherapy can improve the immune function of patients.

CD4+T cells in ageing-associated interstitial lung abnormalities show evidence of pro-inflammatory phenotypic and functional profile

BackgroundInterstitial lung abnormalities (ILA) occur in around 10% of subjects over 60 years, and are associated with a higher rate of all-cause mortality. The pathogenic mechanisms are unclear, and the...

Killer Cell Lectin-like Receptor G1 Inhibits NK Cell Function through Activation of Adenosine 5'-Monophosphate-Activated Protein Kinase.

NK cells are the first line of defense against infected and transformed cells. Defective NK cell activity was shown to increase susceptibility for viral infections and reduce tumor immune-surveillance. With age, the incidence of infectious diseases and malignancy rises dramatically, suggesting that impaired NK cell function might contribute to disease in these individuals. We found an increased frequency of NK cells with high expression of the inhibitory killer cell lectin-like receptor G1 (KLRG1) in individuals >70 y. The role of KLRG1 in ageing is not known, and the mechanism of KLRG1-induced inhibition of NK cell function is not fully understood. We report that NK cells with high KLRG1 expression spontaneously activate the metabolic sensor AMP-activated protein kinase (AMPK) and that activation of AMPK negatively regulates NK cell function. Pre-existing AMPK activity is further amplified by ligation of KLRG1 in these cells, which leads to internalization of the receptor and allows interaction with AMPK. We show that KLRG1 activates AMPK by preventing its inhibitory dephosphorylation by protein phosphatase-2C rather than inducing de novo kinase activation. Finally, inhibition of KLRG1 or AMPK prevented KLRG1-induced activation of AMPK and reductions in NK cell cytotoxicity, cytokine secretion, proliferation, and telomerase expression. This novel signaling pathway links metabolic sensing, effector function, and cell differentiation with inhibitory receptor signaling that may be exploited to enhance NK cell activity during ageing.

Immunosenescence of the CD8(+) T cell compartment is associated with HIV-infection, but only weakly reflects age-related processes of adipose tissue, metabolism, and muscle in antiretroviral therapy-treated HIV-infected patients and controls.

BackgroundDespite effective antiretroviral therapy (ART), HIV-infected patients exhibit systemic inflammation, early onset of age-related diseases, and features of immunosenescence. The role of inflammation in the development of age-related diseases is widely recognized. However, the role of immunosenescence is not well established. Studying immunosenescence in HIV-infection could give insight into its role in ageing processes. In this cross-sectional study, we aimed to investigate whether ART-treated HIV-infected patients exhibit immunosenescence; and whether immunosenescence is associated with age-related processes of inflammation, metabolism, adipose tissue, and muscle. T cell immunosenescence and exhaustion were assessed by flow cytometry analysis of CD8+ cells from 43 ART-treated HIV-infected patients (HIV+) and ten Controls using markers of differentiation: CD27/CD28; maturation: CD27/CD45RA; senescence: killer cell lectin-like receptor G1 (KLRG1); and exhaustion: programmed death-1 (PD-1). Relationships between CD8+ T cell immunosenescence, exhaustion, and age-related processes were assessed using linear regressions.ResultsHIV-infection was strongly associated with more highly differentiated and mature CD8+ T cell phenotypes. PD-1 and KLRG1 expression did not differ between HIV+ and Controls, but depended on differentiation and maturation stages of the cells. CD8+ T cell maturation was associated with age. KLRG1 expression was associated with age, metabolic syndrome, visceral adipose tissue, and high muscle mass. PD-1 expression was not associated with age-related parameters.ConclusionsHIV-infection strongly affected CD8+ T cell differentiation and maturation, whereas age-related processes were only weakly associated with immune parameters. Our findings suggest that, in contrast to inflammation, immunosenescence appears to be highly dependent on HIV-infection and is only to a small extent associated with age-related parameters in well-treated HIV-infection.Electronic supplementary materialThe online version of this article (doi:10.1186/s12865-015-0136-6) contains supplementary material, which is available to authorized users.

TGF-βdownregulates KLRG1 expression in mouse and human CD8(+) T cells.

The inhibitory receptor killer cell lectin-like receptor G1 (KLRG1) and the integrin αE (CD103) are expressed by CD8(+) T cells and both are specific for E-cadherin. However, KLRG1 ligation by E-cadherin inhibits effector T-cell function, whereas binding of CD103 to E-cadherin enhances cell-cell interaction and promotes target cell lysis. Here, we demonstrate that KLRG1 and CD103 expression in CD8(+) T cells from untreated and virus-infected mice are mutually exclusive. Inverse correlation of KLRG1 and CD103 expression was also found in human CD8(+) T cells-infiltrating hepatocellular carcinomas. As TGF-β is known to induce CD103 expression in CD8(+) T cells, we examined whether this cytokine also regulates KLRG1 expression. Indeed, our data further reveal that TGF-β signaling in mouse as well as in human CD8(+) T cells downregulates KLRG1 expression. This finding provides a rationale for the reciprocal expression of KLRG1 and CD103 in different CD8(+) T-cell subsets. In addition, it points to the limitation of KLRG1 as a marker for terminally differentiated CD8(+) T cells if lymphocytes from tissues expressing high levels of TGF-β are analyzed.

Senescence marker killer cell lectin-like receptor G1 (KLRG1) contributes to TNF-α production by interaction with its soluble E-cadherin ligand in chronically inflamed joints

ObjectivesKiller cell lectin-like receptor G1 (KLRG1) is an NK cell marker also expressed on T cells showing an immunosenescent phenotype. KLRG1 binding to its ligand E-cadherin inhibits functional responses. It...

Contribution of Pulmonary KLRG1high and KLRG1low CD8 T Cells to Effector and Memory Responses during Influenza Virus Infection

In response to pathogen insult, CD8 T cells undergo expansion and a dynamic differentiation process into functionally different subpopulations. In this study, we show that during the effector response to influenza virus infection lung CD8 T cell subsets expressing killer cell lectin-like receptor G1 (KLRG1)(high) or KLRG1(low) had similar effector functions and immediate recall efficacy. The KLRG1 expression profile of lung CD8 T cells was not permanent after adoptive transfer and recall. Airway CD8 T cells exhibited a unique phenotype expressing low levels of KLRG1 together with high levels of markers of cellular activation. We investigated the functional characteristics of these cells by analyzing their capacity to survive and to respond to a secondary challenge outside of the airway environment. KLRG1(high) CD8 T cells isolated from the lung during the peak of the effector T cell response could survive for more than a month in the absence of cognate viral Ags after systemic adoptive transfer, and these "rested" CD8 T cells proliferated and participated in a recall response to influenza virus infection. These data highlight the unique phenotype and plasticity of effector CD8 T cell responses in the lung.

Endoplasmic Reticulum Stress Regulator XBP-1 Contributes to Effector CD8+ T Cell Differentiation during Acute Infection

The transcription factor X-box-binding protein-1 (XBP-1) plays an essential role in activating the unfolded protein response in the endoplasmic reticulum (ER). Transcribed XBP-1 mRNA is converted to its active form by unconventional cytoplasmic splicing mediated by inositol-requiring enzyme-1 (IRE-1) upon ER stress. We report activation of the IRE-1/XBP-1 pathway in effector CD8(+) T cells during the response to acute infection. Transcription of unspliced XBP-1 mRNA is up-regulated by IL-2 signals, while its splicing is induced after TCR ligation. Splicing of XBP-1 mRNA was evident during the expansion of Ag-specific CD8(+) T cells in response to viral or bacterial infection. An XBP-1 splicing reporter revealed that splicing activity was enriched in terminal effector cells expressing high levels of killer cell lectin-like receptor G1 (KLRG1). Overexpression of the spliced form of XBP-1 in CD8(+) T cells enhanced KLRG1 expression during infection, whereas XBP-1(-/-) CD8(+) T cells or cells expressing a dominant-negative form of XBP-1 showed a decreased proportion of KLRG1(high) effector cells. These results suggest that, in the response to pathogen, activation of ER stress sensors and XBP-1 splicing contribute to the differentiation of end-stage effector CD8(+) T cells.

Characterization of mouse CD4 T cell subsets defined by expression of KLRG1

The mouse killer cell lectin-like receptor G1 (KLRG1) is an inhibitory receptor known to be expressed on a subset of NK cells and antigen-experienced CD8 T cells. Here, we have characterized expression of KLRG1 on CD4+ T cells from normal mice. While a polyclonal TCR repertoire suggests thymic origin of KLRG1+ CD4+ cells, KLRG1 expression was found to be restricted to peripheral CD4+ T cells. Based on phenotypic analyses, a minority of KLRG1+ CD4+ cells are effector/memory cells with a proliferative history. The majority of KLRG1+ CD4+ cells are, however, bona fide Treg cells that depend on IL-2 and/or CD28 and express both FoxP3 and high levels of intracellular CD152. KLRG1-expressing Treg are contained within the CD38+ subset but are only partially overlapping with the CD25+ CD4+ Treg subset. In functional assays, KLRG1+ CD4+ cells were anergic to TCR stimulation with respect to proliferation, and sorted KLRG1+ CD25+ CD4+ cells were equal or superior to KLRG1+ CD25- CD4+ cells, which were more potent than KLRG1- CD25+ CD4+ cells in suppressing responder cell proliferation. Together, our results demonstrate that KLRG1 expression defines novel and distinctive subsets of senescent effector/memory and potent regulatory CD4+ T cells.

NK Cell Maturation and Peripheral Homeostasis Is Associated with KLRG1 Up-Regulation

NK cells are important for the clearance of tumors, parasites, and virus-infected cells. Thus, factors that control NK cell numbers and function are critical for the innate immune response. A subset of NK cells express the inhibitory killer cell lectin-like receptor G1 (KLRG1). In this study, we identify that KLRG1 expression is acquired during periods of NK cell division such as development and homeostatic proliferation. KLRG1(+) NK cells are mature in phenotype, and we show for the first time that these cells have a slower in vivo turnover rate, reduced proliferative response to IL-15, and poorer homeostatic expansion potential compared with mature NK cells lacking KLRG1. Transfer into lymphopenic recipients indicate that KLRG1(-) NK cells are precursors of KLRG1(+) NK cells and KLRG1 expression accumulates following cell division. Furthermore, KLRG1(+) NK cells represent a significantly greater proportion of NK cells in mice with enhanced NK cell numbers such as Cd45(-/-) mice. These data indicate that NK cells acquire KLRG1 on their surface during development, and this expression correlates with functional distinctions from other peripheral NK cells in vivo.

High-intensity exercise elicits the mobilization of senescent T lymphocytes into the peripheral blood compartment in human subjects

Clonal expansion of T lymphocytes in response to antigenic stimulation is a fundamental process of adaptive immunity. As a consequence of clonal expansion, some T lymphocytes acquire a senescent phenotype, fail to replicate in response to further antigenic stimulation, and express the killer cell lectin-like receptor G1 (KLRG1) and/or CD57. Physical exercise elicits a mobilization of large numbers of T lymphocytes into the bloodstream from peripheral lymphoid compartments, but the frequency of senescent cells in the mobilized population is not known. Eight male runners (age: 29 +/- 9 yr; maximal O2 uptake 62 +/- 6 ml x kg(-1) x min(-1)) performed an intensive treadmill-running protocol at 80% maximal O2 uptake to volitional exhaustion. Blood lymphocytes isolated before, immediately after, and 1 h after exercise were assessed for cell surface expression of KLRG1, CD57, CD28, CD45RA, CD45RO, CD62L, and lymphocyte subset markers (CD3, CD4, CD8, CD56) by flow cytometry. The percentage of all CD3+ T lymphocytes expressing KLRG1 and CD57 increased with exercise (P < 0.01). The change in T-lymphocyte KLRG1 expression was attributed to both CD4+ and CD8 bright T cells, with the relative change being greater for the CD8 bright population (P < 0.01). Mobilized T-lymphocyte populations expressing KLRG1 and CD57 appeared to extravasate the peripheral blood compartment after 1 h of recovery. In conclusion, T lymphocytes with a senescent phenotype are mobilized and subsequently removed from the bloodstream in response to acute high-intensity exercise. This suggests that T lymphocytes contained within the peripheral lymphoid compartments that are mobilized by exercise are likely to be at a more advanced stage of biological aging and have a reduced capacity for clonal expansion than blood-resident T cells.

Analysis of CD127 and KLRG1 Expression on Hepatitis C Virus-Specific CD8 + T Cells Reveals the Existence of Different Memory T-Cell Subsets in the Peripheral Blood and Liver

The differentiation and functional status of virus-specific CD8+ T cells is significantly influenced by specific and ongoing antigen recognition. Importantly, the expression profiles of the interleukin-7 receptor alpha chain (CD127) and the killer cell lectin-like receptor G1 (KLRG1) have been shown to be differentially influenced by repetitive T-cell receptor interactions. Indeed, antigen-specific CD8+ T cells targeting persistent viruses (e.g., human immunodeficiency virus and Epstein-Barr virus) have been shown to have low CD127 and high KLRG1 expressions, while CD8+ T cells targeting resolved viral antigens (e.g., FLU) typically display high CD127 and low KLRG1 expressions. Here, we analyzed the surface phenotype and function of hepatitis C virus (HCV)-specific CD8+ T cells. Surprisingly, despite viral persistence, we found that a large fraction of peripheral HCV-specific CD8+ T cells were CD127+ and KLRG1- and had good proliferative capacities, thus resembling memory cells that usually develop following acute resolving infection. Intrahepatic virus-specific CD8+ T cells displayed significantly reduced levels of CD127 expression but similar levels of KLRG1 expression compared to the peripheral blood. These results extend previous studies that demonstrated central memory (CCR7+) and early-differentiated phenotypes of HCV-specific CD8+ T cells and suggest that insufficient stimulation of virus-specific CD8+ T cells by viral antigen may be responsible for this alteration in HCV-specific CD8+ T-cell differentiation during chronic HCV infection.

Increased Expression of the NK Cell Receptor KLRG1 by Virus-Specific CD8 T Cells during Persistent Antigen Stimulation

The killer cell lectin-like receptor G1 (KLRG1) is a natural killer cell receptor expressed by T cells that exhibit impaired proliferative capacity. Here, we determined the KLRG1 expression by virus-specific T cells. We found that repetitive and persistent antigen stimulation leads to an increase in KLRG1 expression of virus-specific CD8+ T cells in mice and that virus-specific CD8+ T cells are mostly KLRG1+ in chronic human viral infections (human immunodeficiency virus, cytomegalovirus, and Epstein-Barr virus) but not in resolved infection (influenza virus). Thus, by using KLRG1 as a T-cell marker, our results suggest that the differentiation status and function of virus-specific CD8+ T cells are directly influenced by persistent antigen stimulation.

Differential regulation of killer cell lectin-like receptor G1 expression on T cells.

The killer cell lectin-like receptor G1 (KLRG1) is the mouse homologue of the rat mast cell function-associated Ag and contains a tyrosine-based inhibitory motif in its cytoplasmic domain. It has been demonstrated that KLRG1 is induced on activated NK cells and that KLRG1 can inhibit NK cell effector functions. In this study, we show that in naive C57BL/6 mice KLRG1 is expressed on a subset of CD44(high)CD62L(low) T cells. KLRG1 expression can be detected on a small number of V(alpha)14i NK T cells but not on CD8alphaalpha(+) intraepithelial T cells that are either TCRgammadelta(+) or TCRalphabeta(+). We also show that KLRG1 expression is dramatically induced on approximately 50% of the CD8(+) T cells during both a viral and a parasitic infection. Interestingly, during Toxoplasma gondii infection, KLRG1 is up-regulated on CD4(+) T cells. Although KLRG1 expression can be induced on both NK cells and T cells, the molecular mechanism leading to the induction of KLRG1 differs in these two subsets of cells. Indeed, the up-regulation of KLRG1 on NK cells can be driven in vivo by cytokines, whereas KLRG1 cannot be induced on CD8(+) T cells by cytokines. In addition, although induction of KLRG1 on T cells appears to require TCR engagement in vivo, TCR engagement is not sufficient for KLRG1 induction in vitro. Taken together, these data suggest that the expression and induction of KLRG1 on T cells are tightly regulated. This could have important biological consequences on T cell activation and homeostasis.

Lack of proliferative capacity of human effector and memory T cells expressing killer cell lectinlike receptor G1 (KLRG1)

Adaptive immunity necessitates the proliferation of lymphocytes. In the mouse, we have previously shown that antigen-experienced T cells that have lost their proliferative potential express the killer cell lectinlike receptor G1 (KLRG1). By using a newly generated monoclonal antibody specific for human KLRG1, we now demonstrate that expression of KLRG1 also identifies T cells in humans that are capable of secreting cytokines but that fail to proliferate after stimulation. Furthermore, our data show that proliferative incapacity of CD8 T cells correlates better with KLRG1 expression than with absence of the CD28 marker. In peripheral blood lymphocytes (PBLs) from healthy adult donors, KLRG1 was expressed on 44% +/- 14% of CD8 and 18% +/- 10% of CD4 T cells. KLRG1 expression was restricted to antigen-experienced T cells. Here, KLRG1(+) cells were preferentially found in the CCR7(-) effector T-cell pool. Besides T cells, a significant portion (approximately 50%) of human natural killer (NK) cells expressed KLRG1. Interestingly, these KLRG1(+) NK cells were found exclusively in the CD56(dim) NK-cell subset. Thus, the expression of KLRG1 identifies a subset of NK cells and antigen-experienced T cells in humans that lack proliferative capacity.

Viral infections induce abundant numbers of senescent CD8 T cells.

Viral infections are often accompanied by extensive proliferation of reactive CD8 T cells. After a defined number of divisions, normal somatic cells enter a nonreplicative stage termed senescence. In the present study we have identified the inhibitory killer cell lectin-like receptor G1 (KLRG1) as a unique marker for replicative senescence of murine CD8 T cells. KLRG1 expression was induced in a substantial portion (30-60%) of CD8 T cells in C57BL/6 mice infected with lymphocytic choriomeningitis virus (LCMV), vesicular stomatitis virus, or vaccinia virus. Similarly, KLRG1 was found on a large fraction of LCMV gp33 peptide-specific TCR-transgenic (tg) effector and memory cells activated in vivo using an adoptive transfer model. Transfer experiments with CFSE-labeled TCR-tg cells into LCMV-infected hosts further indicated that induction of KLRG1 expression required an extensive number of cell divisions. Most importantly, KLRG1(+) TCR-tg effector/memory cells could efficiently lyse target cells and secrete cytokines, but were severely impaired in their ability to proliferate after Ag stimulation. Thus, this study demonstrates that senescent CD8 T cells are induced in abundant numbers during viral infections in vivo.