Multifaceted roles of MHC class I and MHC class I-like molecules in T cell activation

Proteomic analysis revealed gender-related differences in the skin mucus proteome of discus fish (Symphysodon haraldi) during the parental and non-parental care periods

The discus fish, Symphysodon spp., a South American cichlid, has a unique parental care behavior where fry bite on parental skin mucus after hatching. In this study, we used LC-MS/MS technique to compare the skin mucus proteome composition of male or female discus fish during parental and non-parental care periods. By multivariate statistical analysis, we found clear separations between different periods and between different sexes of mucus proteome. Compared with non-parental female fish, parental female fish had 283 up-regulated and 235 down-regulated expressed proteins. Compared with non-parental male fish, parental male fish had 169 up-regulated and 120 down-regulated expressed proteins. The differentially expressed proteins for male fish were enriched in sulfur relay system, mucin type O-glycan biosynthesis and antigen processing and presentation pathways, while those for female fish were enriched in sulfur relay system, steroid biosynthesis and complement and coagulation cascades pathways. During the parental care, both male and female discus showed an enhanced lipid metabolism, producing more phospholipids and cholesterol. The difference is that male discus had increased tricarboxylic acid cycle producing more energy during the parental care, while females produced more nucleotides especially guanylic acid. Our study could provide new insights into the understanding of the unique mucus supply behavior of discus fish based on proteomic change.

High-resolution KIR and HLA genotyping in three Chinese ethnic minorities reveals distinct origins

Polymorphism of killer-cell immunoglobulin-like receptors (KIRs) and their HLA class I ligands impacts the effector activity of cytotoxic NK cell and T cell subsets. Therefore, understanding the extent and implications of KIR and HLA class I genetic polymorphism across various populations is important for immunological and medical research. In this study, we conducted a high-resolution investigation of KIR and HLA class I diversity in three distinct Chinese ethnic minority populations. We studied the She, Yugur, and Tajik, and compared them with the Zhejiang Han population (Zhe), which represents the majority Southern Han ethnicity. Our findings revealed that the Tajik population exhibited the most diverse KIR copy number, allele, and haplotype diversity among the four populations. This diversity aligns with their proposed ancestral origin, closely resembling that of Iranian populations, with a relatively higher presence of KIR-B genes, alleles, and haplotypes compared with the other Chinese populations. The Yugur population displayed KIR distributions similar to those of the Tibetans and Southeast Asians, whereas the She population resembled the Zhe and other East Asians, as confirmed by genetic distance analysis of KIR. Additionally, we identified 12.9% of individuals across the three minority populations as having KIR haplotypes characterized by specific gene block insertions or deletions. Genetic analysis based on HLA alleles yielded consistent results, even though there were extensive variations in HLA alleles. The observed variations in KIR interactions, such as higher numbers of 2DL1-C2 interactions in Tajik and Yugur populations and of 2DL3-C1 interactions in the She population, are likely shaped by demographic and evolutionary mechanisms specific to their local environments. Overall, our findings offer valuable insights into the distribution of KIR and HLA diversity among three distinct Chinese ethnic minority populations, which can inform future clinical and population studies.

An Integrated Analysis of Lactation-Related miRNA and mRNA Expression Profiles in Donkey Mammary Glands

Donkey milk is consumed by humans for its nutritional and therapeutic properties. MicroRNAs (miRNAs) and messenger RNAs (mRNAs) have been implicated in the regulation of milk component synthesis and mammary gland development. However, the regulatory profile of the miRNAs and mRNAs involved in lactation in donkeys is unclear. We performed mRNA-seq and miRNA-seq and constructed coexpression regulatory networks for the mammary glands during the lactating and nonlactating period of jennies. We identified 3144 differentially expressed (DE) mRNAs (987 upregulated mRNAs and 2157 downregulated mRNAs) and 293 DE miRNAs (231 upregulated miRNAs and 62 downregulated miRNAs) in the lactating group compared to the nonlactating group. The DE miRNA target mRNA were significantly associated with pathways related to RNA polymerase, glycosphingolipid biosynthesis, mRNA surveillance, ribosome biogenesis in eukaryotes, glycerophospholipid metabolism, Ras signaling, and the fly hippo signaling pathway. The mRNA–miRNA coregulation analysis showed that novel-m0032-3p, miR-195, miR-26-5p, miR-23-3p, miR-674-3p, and miR-874-3p are key miRNAs that target mRNAs involved in immunity and milk lipid, protein, and vitamin metabolism in the jenny mammary gland. Our results improve the current knowledge of the molecular mechanisms regulating bioactive milk component metabolism in the mammary glands and could be used to improve milk production in donkeys.

Comparative analysis of whey proteins in donkey colostrum and mature milk using quantitative proteomics

Donkey milk is attracting increasing attention as a nutritional milk source similar to human milk. In this study, we carried out qualitative and quantitative analysis of the donkey whey proteome using a label-free proteomic approach, combined with parallel reaction monitoring (PRM) as a validation method. A total of 300 whey proteins were identified in donkey colostrum (DC) and donkey mature (DM) milk, of which 18 were differentially expressed (P < 0.05) between the two types of milk. Gene ontology (GO) analysis showed that differentially and uniquely expressed proteins were mainly involved in cellular processes, response to stimulus, metabolic processes, and biological regulation. Their molecular functions included binding, catalytic activity, and molecular functional regulation, and their main annotated areas of origin were the cell, cell-part, and the extracellular region. Most differentially and uniquely expressed proteins were linked with malaria, systemic lupus erythematosus, or antigen processing and presentation. Our results provide insight into the complexity of the donkey whey proteome and molecular evidence for nutritional differences between different lactation stages.

Human Leukocyte Antigen (HLA) and Immune Regulation: How Do Classical and Non-Classical HLA Alleles Modulate Immune Response to Human Immunodeficiency Virus and Hepatitis C Virus Infections?

The genetic factors associated with susceptibility or resistance to viral infections are likely to involve a sophisticated array of immune response. These genetic elements may modulate other biological factors that account for significant influence on the gene expression and/or protein function in the host. Among them, the role of the major histocompatibility complex in viral pathogenesis in particular human immunodeficiency virus (HIV) and hepatitis C virus (HCV), is very well documented. We, recently, added a novel insight into the field by identifying the molecular mechanism associated with the protective role of human leukocyte antigen (HLA)-B27/B57 CD8⁺ T cells in the context of HIV-1 infection and why these alleles act as a double-edged sword protecting against viral infections but predisposing the host to autoimmune diseases. The focus of this review will be reexamining the role of classical and non-classical HLA alleles, including class Ia (HLA-A, -B, -C), class Ib (HLA-E, -F, -G, -H), and class II (HLA-DR, -DQ, -DM, and -DP) in immune regulation and viral pathogenesis (e.g., HIV and HCV). To our knowledge, this is the very first review of its kind to comprehensively analyze the role of these molecules in immune regulation associated with chronic viral infections.

Convergence of Innate and Adaptive Immunity during Human Aging

Aging is associated with profound changes in the human immune system, a phenomenon referred to as immunosenescence. This complex immune remodeling affects the adaptive immune system and the CD8⁺ T cell compartment in particular, leading to the accumulation of terminally differentiated T cells, which can rapidly exert their effector functions at the expenses of a limited proliferative potential. In this review, we will discuss evidence suggesting that senescent αβCD8⁺ T cells acquire the hallmarks of innate-like T cells and use recently acquired NK cell receptors as an alternative mechanism to mediate rapid effector functions. These cells concomitantly lose expression of co-stimulatory receptors and exhibit decreased T cell receptor signaling, suggesting a functional shift away from antigen-specific activation. The convergence of innate and adaptive features in senescent T cells challenges the classic division between innate and adaptive immune systems. Innate-like T cells are particularly important for stress and tumor surveillance, and we propose a new role for these cells in aging, where the acquisition of innate-like functions may represent a beneficial adaptation to an increased burden of malignancy with age, although it may also pose a higher risk of autoimmune disorders.

Suppression of HIV Replication by CD8(+) Regulatory T-Cells in Elite Controllers

We previously demonstrated in the Chinese macaque model that an oral vaccine made of inactivated SIV and Lactobacillus plantarum induced CD8(+) regulatory T-cells, which suppressed the activation of SIV(+)CD4(+) T-cells, prevented SIV replication, and protected macaques from SIV challenges. Here, we sought whether a similar population of CD8(+) T-regs would induce the suppression of HIV replication in elite controllers (ECs), a small population (3‰) of HIV-infected patients with undetectable HIV replication. For that purpose, we investigated the in vitro antiviral activity of fresh CD8(+) T-cells on HIV-infected CD4(+) T-cells taken from 10 ECs. The 10 ECs had a classical genomic profile: all of them carried the KIR3DL1 gene and 9 carried at least 1 allele of HLA-B:Bw4-80Ile (i.e., with an isoleucine residue at position 80). In the nine HLA-B:Bw4-80Ile-positive patients, we demonstrated a strong viral suppression by KIR3DL1-expressing CD8(+) T-cells that required cell-to-cell contact to switch off the activation signals in infected CD4(+) T-cells. KIR3DL1-expressing CD8(+) T-cells withdrawal and KIR3DL1 neutralization by a specific anti-killer cell immunoglobulin-like receptor (KIR) antibody inhibited the suppression of viral replication. Our findings provide the first evidence for an instrumental role of KIR-expressing CD8(+) regulatory T-cells in the natural control of HIV-1 infection.

Ly49 receptors: innate and adaptive immune paradigms

The Ly49 receptors are type II C-type lectin-like membrane glycoproteins encoded by a family of highly polymorphic and polygenic genes within the mouse natural killer (NK) gene complex. This gene family is designated Klra, and includes genes that encode both inhibitory and activating Ly49 receptors in mice. Ly49 receptors recognize class I major histocompatibility complex-I (MHC-I) and MHC-I-like proteins on normal as well as altered cells. Their functional homologs in humans are the killer cell immunoglobulin-like receptors, which recognize HLA class I molecules as ligands. Classically, Ly49 receptors are described as being expressed on both the developing and mature NK cells. The inhibitory Ly49 receptors are involved in NK cell education, a process in which NK cells acquire function and tolerance toward cells that express “self-MHC-I.” On the other hand, the activating Ly49 receptors recognize altered cells expressing activating ligands. New evidence shows a broader Ly49 expression pattern on both innate and adaptive immune cells. Ly49 receptors have been described on multiple NK cell subsets, such as uterine NK and memory NK cells, as well as NKT cells, dendritic cells, plasmacytoid dendritic cells, macrophages, neutrophils, and cells of the adaptive immune system, such as activated T cells and regulatory CD8⁺ T cells. In this review, we discuss the expression pattern and proposed functions of Ly49 receptors on various immune cells and their contribution to immunity.

HTLV-1 infection: what determines the risk of inflammatory disease?

Human T-lymphotropic virus type 1 (HTLV-1) is an exogenous retrovirus that persists lifelong in the infected host. Infection has been linked to a spectrum of diverse diseases: adult T cell leukemia, encephalomyelopathy, and predisposition to opportunistic bacterial and helminth infections. Applications of new technologies and biological concepts to the field have provided new insights into viral persistence and pathogenesis in HTLV-1 infection. Here, we summarize the emerging concepts of dynamic HTLV-1-host interactions and propose that chronic interferon (IFN) production causes tissue damage in HTLV-1-associated inflammatory diseases.

Immunogenetic surveillance of HIV/AIDS

Evolutionary pressure by viruses is most likely responsible for the extraordinary allelic polymorphism of genes encoding class I human leukocyte antigens (HLA) and killer immunoglobulin-like receptors (KIR). Such genetic diversity has functional implications for the immune response to viruses and generates population-based variations in HLA class I allele frequencies and KIR gene profiles. The HIV-1 virus has relatively recently established itself as a major human pathogen, rapidly diversifying into a variety of phylogenetic subtypes or clades (A-G) and recombinants in different populations. HIV-1 clade C is the most common subtype in circulation accounting for 48% of all infections, followed by HIV-1 clades A and B which are responsible for 13% and 11% of infections in the current pandemic, respectively. Candidate gene studies of large cohorts of predominantly HIV-1 clade B but also clades C and A infected patients, have consistently shown significant associations between certain HLA class I alleles namely HLA-B*57, B*58, B*27, B*51 and relatively low viraemia. However, there is evidence that other associations between HLA-B*15, B*18 or B*53 and levels of HIV-1 viraemia are clade-specific. Recent genome-wide association studies of HIV-1 clade B exposed cohorts have confirmed that HLA-B, which is the most polymorphic locus in the human genome, is the major genetic locus contributing to immune control of viraemia. Moreover, the presence of natural killer cell receptors encoded by KIR-3DL1 and 3DS1 genes together with certain HLA class I alleles carrying the KIR target motif Bw4Ile80, provides an enhanced ability to control HIV-1 viraemia in some individuals. It is likely that rapid co-evolution of HIV-1 immune escape variants together with an adjustment of human immune response gene profiles has occurred in some exposed populations. Taken together, immunogenetic surveillance of HIV-1 exposed cohorts has revealed important correlates of natural immunity, which could provide a rational platform for the design and testing of future vaccines aimed at controlling the current AIDS pandemic.

Regulation of self-tolerance by Qa-1-restricted CD8(+) regulatory T cells

Mounting an efficient immune response to pathogens while avoiding damage to host tissues is the central task of the immune system. Emerging evidence has highlighted the contribution of the CD8(+) lineage of regulatory T cells to the maintenance of self-tolerance. Specific recognition of the MHC class Ib molecule Qa-1 complexed to peptides expressed by activated CD4(+) T cells by regulatory CD8(+) T cells triggers an inhibitory interaction that prevents autoimmune responses. Conversely, defective Qa-1-restricted CD8(+) regulatory activity can result in development of systemic autoimmune disease. Here, we review recent research into the cellular and molecular basis of these regulatory T cells, their mechanism of suppressive activity and the potential application of these insights into new treatments for autoimmune disease and cancer.

The efficiency of the human CD8+ T cell response: how should we quantify it, what determines it, and does it matter?

Multidisciplinary techniques, in particular the combination of theoretical and experimental immunology, can address questions about human immunity that cannot be answered by other means. From the turnover of virus-infected cells in vivo, to rates of thymic production and HLA class I epitope prediction, theoretical techniques provide a unique insight to supplement experimental approaches. Here we present our opinion, with examples, of some of the ways in which mathematics has contributed in our field of interest: the efficiency of the human CD8+ T cell response to persistent viruses.

KIR2DL2 enhances protective and detrimental HLA class I-mediated immunity in chronic viral infection

Killer cell immunoglobulin-like receptors (KIRs) influence both innate and adaptive immunity. But while the role of KIRs in NK-mediated innate immunity is well-documented, the impact of KIRs on the T cell response in human disease is not known. Here we test the hypothesis that an individual's KIR genotype affects the efficiency of their HLA class I-mediated antiviral immune response and the outcome of viral infection. We show that, in two unrelated viral infections, hepatitis C virus and human T lymphotropic virus type 1, possession of the KIR2DL2 gene enhanced both protective and detrimental HLA class I-restricted anti-viral immunity. These results reveal a novel role for inhibitory KIRs. We conclude that inhibitory KIRs, in synergy with T cells, are a major determinant of the outcome of persistent viral infection.

Expansions of NK-like αβT cells with chronologic aging: novel lymphocyte effectors that compensate for functional deficits of conventional NK cells and T cells

As the repertoire of αβT cell receptors (TCR) contracts with advancing age, there is an associated age-dependent accumulation of oligoclonal T cells expressing of a variety of receptors (NKR), normally expressed on natural killer (NK) cells. Evidences for differential regulation of expression of particular NKRs between T cells and NK cells suggest that NKR expression on T cells is physiologically programmed rather than a random event of the aging process. Experimental studies show NKRs on aged αβT cells may function either as independent receptors, and/or as costimulatory receptors to the TCR. Considering the reported deficits of conventional αβTCR-driven activation and also functional deficits of classical NK cells, NKR(+) αβT cells likely represent novel immune effectors that are capable of combining innate and adaptive functions. Inasmuch as immunity is a determinant of individual fitness, the type and density of NKRs could be important contributing factors to the wide heterogeneity of health characteristics of older adults, ranging from institutionalized frail elders who are unable to mount immune responses to functionally independent community-dwelling elders who exhibit protective immunity. Understanding the biology of NKR(+) αβT cells could lead to new avenues for age-specific intervention to improve protective immunity.

The role of B27 heavy chain dimer immune receptor interactions in spondyloarthritis

HLA-B27 (B27) is strongly associated with spondyloarthopathy. The classical role of B27 is to present peptides from intracellular pathogens as a heterotrimeric complex with beta2 microglobulin for recognition by the T-cell receptor (TCR) of CD8 T-cells. In addition to heterotrimers, B27 can also be expressed as cell surface beta2-microglobulin (beta2m)-free homodimers (B27(2)). In addition to the TCR, MHC class I molecules bind to immunoregulatory receptors including members of the killer immunoglobulin-like receptor (KIR) and leukocyte immunoglobulin-like receptor (LILR) families. Rodents express the paired immunoglobulin receptor (PIR) family which are related to LILR. B27(2) but not beta2m-associated B27 binds to KIR3DL2 and rodent PIR. NK and T-cells expressing the immune receptor KIR3DL2, which interacts with B27(2), are expanded in B27 AS patients. Ligation of immune receptors by B27(2) promotes the survival of KIR-expressing leukocytes and modulates immune cytokine production. Upregulation ofB272 in spondyloarthritis and differential interaction of beta2m-associated HLA-B27 and B27(2) with immune receptors could be involved in the pathogenesis of B27-associated spondyloarthritis (AS).

Amino-acid sequence motifs for PKC-mediated membrane trafficking of the inhibitory killer Ig-like receptor

Activation-induced upregulation of inhibitory killer Ig-like receptor (KIR) is regulated by protein kinase Cs (PKCs). Conventional PKCs increase KIR expression on the post-transcriptional level by increasing the recycling of surface molecules and endoplasmic reticulum (ER)-Golgi processing. PKCdelta plays a role in the secretion of cytoplasmic KIR through lytic granules. In this study, we identified amino acid sequence motifs associated with PKC-mediated KIR membrane trafficking by systematic mutagenesis. Mutations of Y(398) and HLWC(364) completely inhibited the PMA-induced increase of KIR molecules at surface as well as total protein levels, indicating that these are associated with ER-Golgi processing and sorting to plasma membrane through lytic granules. Mutations of Y-based motif, including Y(398), acidic region (PE(394)), dileucine motif-like region (IL(423)) and PKC-phosphorylatable S(415) caused a blockade of surface KIR endocytosis after PKC stimulation. Mutation of T(145) caused an accumulation of mutant proteins in late endosomes and lysosomes after PKC activation, suggesting that T(145) might be related to the recovery of endocytosed KIR to the surface membrane. We also demonstrated that PKCs could directly phosphorylate the KIR cytoplasmic tail by means of western blot and in vitro kinase assay, implying that phosphorylation status of KIR cytoplasmic tail can direct the fate of surface KIR molecules. Taken together, various sequence motifs are implicated in the PKC-mediated post-transcriptional upregulation of KIR, and each of these motifs work in different steps after PKC activation.

Unusual expression of CD94 on CD8+ TCR-alpha beta T cells in infectious mononucleosis

Infectious mononucleosis, caused by primary Epstein-Barr virus (EBV) infection, is usually a benign, self-limited lymphoproliferative disorder. We report a case of a 21-year-old woman who presented with fever, sore throat, severe neutropenia, and absolute lymphocytosis with atypical lymphocytes. In situ hybridization for EBV-encoded small RNA performed on the marrow aspirate clot specimen demonstrated scattered positive cells. EBV serology was compatible with primary infection. Flow cytometry immunophenotypic studies performed on aspirate material revealed a profoundly expanded population of CD8+ T-cell receptor (TCR)-alphabeta T cells with uniform expression of CD94. No evidence of a monoclonal T-cell population was found as assessed by V(beta) use with flow cytometry and by TCR gamma-chain gene rearrangement using a polymerase chain reaction method. Uniform expression of CD94 in an exuberant reactive proliferation of CD8+ TCR-alphabeta T cells in infectious mononucleosis has not been reported previously, and combined with atypical morphology might be misinterpreted as a malignant neoplasm.

Activation-induced upregulation of inhibitory killer Ig-like receptors is regulated by protein kinase C

Inhibitory killer Ig-like receptor (KIR) expression was upregulated by protein kinase C (PKC) activation in stable Jurkat clones that express KIR or CD8KIR fusion proteins. PKC-induced KIR upregulation was mediated by the cytoplasmic tail of KIR and regulated at the post-transcriptional level. PKC inhibition, metabolic labeling and colocalization studies demonstrated that the activation of the conventional PKCs upregulated surface and cellular KIR levels by stimulating the maturation processes in endoplasmic reticulum-Golgi and by promoting the recycling of surface KIR through sorting endosomes. Similar studies also revealed that KIR was secreted to plasma membrane through lytic granules in a PKCdelta-dependent manner. Consequently, PKCdelta inhibition caused the formation of giant perinuclear granules, which trapped KIR and FasL as well as CPE and Lamp1.

Activation-Induced Expression of CD56 by T Cells Is Associated With a Reprogramming of Cytolytic Activity and Cytokine Secretion Profile In Vitro

A subset of human T lymphocytes expresses the natural killer (NK) cell-associated receptor CD56 and is capable of major histocompatibility complex (MHC)-unrestricted cytotoxicity against a variety of autologous and allogeneic tumor cells. CD56+ T cells have shown potential for immunotherapy as antitumor cytotoxic effectors, but their capacity to control adaptive immune responses via cytokine secretion is unclear. We have examined the inducibility of CD56+ T cells from human blood in vitro and compared the kinetics of Th1, Th2, and regulatory cytokine secretion by CD56+ T cells with those of conventional CD56- T cells. CD56 was induced on CD8+ and CD4- CD8- T cells by CD3/T-cell receptor (TCR)-mediated activation, particularly when grown in the presence of interleukin (IL)-2. Activation-induced CD56+ T cells proliferated less vigorously but displayed enhanced natural cytotoxicity compared with CD56- T cells. CD56+ T cells released interferon-gamma (IFN-gamma) and interleukin-13 (IL-13), but not IL-10, upon TCR stimulation. Flow cytometric analysis demonstrated that, compared with CD56- T cells, elevated proportions of CD56+ T cells expressed IFN-gamma, IL-4, and IL-13 within hours of activation. These acquired cytolytic and cytokine secretion activities of CD56+ T cells make them potential targets for immunotherapy for infectious and immune-mediated disease.

Analysis of KIR2D receptors on peripheral blood lymphocytes from liver graft recipients

KIR2D receptors are killer cell immunoglobulin-like receptors (KIRs) specific for HLA-C epitopes, that are expressed on NK cells as well as on minor peripheral blood T-cell subsets, and are able to control NK and T cells activity. The present work explores NK, and particularly CD8(+) T cells expressing KIR2D2L1/S1 (CD158a) or KIR2D2L2/3/S2 (CD158b) receptors in liver graft alloresponse. Flow cytometry was used to analyse peripheral blood mononuclear cells stained with anti-CD158a and anti-CD158b antibodies from 110 liver recipients and 46 healthy controls, previous to and along the first month after transplantation. Pre-transplantation data shows that both CD158a and CD158b molecules can be detected on NK and T cells from all patients and controls, but both KIR2D(+)NK cells are significantly under-represented in patients respect to controls (P<0.001), and CD3(+)CD8(+)CD158a(+) cells decreased particularly in patients suffering from acute rejection (4.03+/-1.33 cells/microL) compared with controls (7.8+/-2.4 cells/microL). Following transplantation, KIR2D(+)CD8(+) T-cell repertoires increased through the first month, mainly in recipients with a good graft acceptance. In summary, monitoring of KIR2D(+)CD8(+) T cells, particularly KIR2DL1/S1(+)CD8(+) T cells at pre-transplant, and both KIR2DL1/S1(+) and KIR2DL2/3/S2(+) T-cell subsets at early post-transplant period, could offer useful information for clinical follow-up of liver grafts.

Characterization of murine CD160+ CD8+ T lymphocytes

CD160 is an Ig-like glycoprotein expressed on NK, NKT and TCRgammadelta T cells, as well as intestinal intraepithelial T lymphocytes. In addition, a minor subset of CD8(+) but not CD4(+) T cells in the periphery is also known to express CD160, but the subset has not been fully characterized. In this study, we prepared anti-murine CD160 mAbs and investigated the expression profile of CD160 on various subsets of CD8(+) T cells. The amount of CD160 on almost all CD8(+) T cells was increased upon CD3-mediated stimulation in vitro, and soluble CD160 was found to be released. Flow cytometric analysis revealed most CD8(+) T cells expressing CD160 to show a CD44(high) phenotype in vivo. On further analysis, both CD44(high)CD62L(low) effector memory T cells (T(EM)) and CD44(high)CD62L(high) central memory T cells (T(CM)) expressed CD160 at an intermediate level. High levels were evident with recently activated CD8(+) T(EM). Naïve CD8(+) T cells presumably immediately after stimulation (CD44(low)CD62L(low)CD69(+)) also expressed CD160, but only at a low level. Purified CD160(+) CD8(+) T cells from OT-1 transgenic mice expressing TCR against OVA residues 257-264 presented by H-2K(b) produced IFN-gamma more rapidly than CD160(-) CD8(+) T cells upon antigen stimulation. These results together show that CD160 is expressed on the majority of CD8(+) memory T cells as well as recently activated CD8(+) T cells.

The impact of variation at the KIR gene cluster on human disease

Leukocyte behavior is controlled by a balance of inhibitory and stimulatory signals generated on ligand binding to a complex set of receptors located on the cell surface. The killer cell immunoglobulin-like receptor (KIR) genes encode one such, family of receptors expressed by natural killer (NK) cells, key components of the innate immune system that participate in early responses against infected or transformed cells through production of cytokines and direct cytotoxicity. KIRs are also expressed on a subset of T cells, where they contribute to the intensity of acquired immune responses. Recognition of self HLA class I ligands by inhibitory KIR allows NK cells to identify normal cells, preventing an NK cell-mediated response against healthy autologous cells. Activation of NK cells through stimulatory receptors is directed toward cells with altered expression of class I, a situation characteristic of some virally infected cells and tumor cells. The "missing self" model for NK cell activation was proposed to explain killing of cells that express little or no class I, while cells expressing normal levels of class I are spared. Studies performed over the last several years have revealed extensive diversity at the KIR gene locus, which stems from both its polygenic (variable numbers of genes depending on KIR haplotype) and multiallelic polymorphism. Given the role of KIR in both arms of the immune response, their specificity for HLA class I allotypes, and their extensive genomic diversity, it is reasonable to imagine that KIR gene variation affects resistance and susceptibility to the pathogenesis of numerous diseases. Consequently, the evolution of KIR locus diversity within and across populations may be a function of disease morbidity and mortality. Here we review a growing body of evidence purporting the influence of KIR polymorphism in human disease.

NK Cell Receptors as Tools in Cancer Immunotherapy

Natural killer (NK) cells were identified 30 years ago based on their ability to "spontaneously" kill tumor cells. The basis for NK cell recognition and activation is due to a variety of receptors that bind to specific ligands on tumor cells and normal cells. Some of these receptors have the ability to inhibit NK cell function, and other receptors activate NK cell function. Therapeutic strategies for cancer therapy are being developed based on preventing NK cell inhibition or using NK cell receptors to activate NK cells or T cells. There are intriguing clinical data from studies of bone marrow transplantation that support the idea that preventing NK cell inhibition by human leukocyte antigen (HLA) class I molecules can be a means to promote graft-versus-leukemia (GvL) effects and limit graft-versus-host disease (GvHD) in acute myeloid leukemia (AML) and chronic myeloid leukemia (CML) patients. Experimental findings also support the blockade of NK cell inhibitory receptors as a way to protect against leukemia relapse. It may be possible to use our knowledge of NK cell activating receptors and their ligands to immunize patients with modified tumor cells to promote beneficial NK cell responses and development of host antitumor cytotoxic T lymphocytes (CTLs). Finally, new data support the idea of using modified NK cell receptors as a means to target patients' T cells against their own tumor cells and induce long-term immunity against them. Tumors are essentially tissues that have overcome normal regulation mechanisms, and therefore the ability to distinguish normal cells from abnormal cells is a key part of selectively attacking tumor cells. NK cells have various receptor systems designed to recognize infected and abnormal cells. Understanding NK cell receptors and their recognition mechanisms provides new tools for the development of immunotherapies against cancer.

NKG2 receptor-mediated regulation of effector CTL functions in the human tissue microenvironment

NKG2 receptors and their ligands play an essential role in the control of CTL activation in the tissue microenvironment. We discuss the regulation of NKG2 receptor expression by CTL and how uncontrolled activation of NKG2 receptors can lead to organ-specific autoimmune and inflammatory disorders.

Overlap between molecular markers expressed by naturally occurring CD4+CD25+ regulatory T cells and antigen specific CD4+CD25+ and CD8+CD28- T suppressor cells

Alloantigen specific CD8+CD28- T suppressor (TS) cells differ from naturally occurring CD4+CD25+ T-regulatory (natural TR) cells not only by their phenotype but also by their mechanism of action. Natural TR have been extensively studied, leading to the identification of characteristic "molecular markers" such as Forkhead box P3 (FOXP3), glucocorticoid-induced tumor necrosis factor receptor-related protein (GITR) and cytotoxic T lymphocyte-associated antigen 4 (CTLA-4). We have investigated the expression of these genes in alloantigen specific TS and CD4+CD25+ T regulatory (TR) cells and found that they are expressed at levels similar to those observed in natural TR. Furthermore, similar to natural CD4+CD25+ TR, antigen-specific CD8+CD28-CD62L+ TS cells have more suppressive capacity than CD8+CD28-CD62L- TS cells. In spite of these similarities, natural TR are not antigen-specific and inhibit other T cells by T cell-to-T cell interaction, whereas TS are antigen-specific and exert their inhibitory function by interacting with antigen-presenting cells and render them tolerogenic to other T cells. The molecular characterization of TS cells may contribute to a better understanding of mechanisms involved in inhibition of immune responses in autoimmunity, transplantation, and chronic viral infection.

Expression of Leukocyte Immunoglobulin-Like Receptors and Natural Killer Receptors on Virus-Specific CD8+T Cells during the Evolution of Epstein-Barr Virus-Specific Immune Responsesin Vivo

Antigen-primed cytotoxic T lymphocytes (CTL) may express leukocyte immunoglobulin-like receptors (LILRs) and natural killer receptors (NKRs). Published work suggests that expression of some of these receptors confers survival advantage, leading to the idea that cells expressing such receptors may accumulate as an antigen-specific response evolves. Here we tested this hypothesis by analyzing expression of CD85j (also known as LILRB1 or ILT2), KIRs, CD94, and CD161 by Epstein- Barr virus (EBV)-specific CTL during the primary and persistent phases of EBV infection in humans. During primary infection, few EBV-specific CTL expressed these receptors and this proportion was equally low in early persistent infection. Thus, expression of these molecules does not influence capacity to survive downregulation of the primary response. However, in donors persistently infected with EBV for many years, a significantly higher proportion of EBV-specific CTL expressed CD85j and NKRs, suggesting that cells expressing these receptors can accumulate with time. Using FACS analysis, we confirmed, at a single cell level, that expression of CD85j, defined by staining with the antibody VMP55, was associated with reduced capacity of EBV-specific CD8+ T cells to respond to antigen. Thus, in the later stages of persistent infection, protective immunity to EBV may be reduced due to the preferential accumulation of hyporesponsive EBV-specific CD8+ T cells.

Current concepts of tumor-infiltrating lymphocytes in human malignancies

Tumor-infiltrating lymphocytes (TILs) develop as manifestations of the recognition and defense against malignant cells by the host immune system. TILs were literally defined as "tumor-infiltrating lymphocytes", which a posteriori locate within the tumor tissues. Although such cells can be found, they fail to control the growth of tumor. Many have proposed diverse mechanisms for dysfunction of TILs with regard to the roles of immunosurveillance against cancer. However, only a few cancer types, e.g. melanoma, have seen the benefits brought by activating these cells for immunotherapy. Functional defects of TILs have been linked to abnormalities of signaling molecules; however, there is conflicting data. The death of TILs was attributed to expression of cancer-derived FasL, PD-1 and RCAS1, and cancer-induced activation-induced cell death (AICD). Confirmed by studies using TILs and animal models, the compromise of tumor-specific immune responses was thought to result from not only mechanisms of clonal anergy but also exhaustion and/or deletion. Furthermore, functional cytotoxic CD8(+) TILs might be rendered incompetent by cancer-induced up-regulation of inhibitory NK receptors or proximal signaling abnormalities. Additionally, immune privilege was partly attributed to recruitment of regulatory T cells to the tumor sites. The failure of IL-2 signaling, which stands at the center of T cell functionalities, had been linked to the enzymatic activity of cancer-derived matrix metalloproteinases (MMPs). Finally, the exploitation of IDO expression, an important enzyme in pregnancy-related immunosuppression, by cancer cells might play a role in tumor immunity. The disparity of cancer types, origin, developmental stages and individual genetic backgrounds likely account for differences, or even contradictions, which might be the reason why immunotherapy works only on a few cancer types. Delineating the mechanisms behind functional defects of TILs can help not only boost chances of the development of a successful cure but understand the not fully identified roles played by immune system in the face of malignancies.

Human cytolytic T lymphocytes expressing HLA class-I-specific inhibitory receptors

MHC class-1-specific inhibitory receptors were originally described in NK cells, in which they represent an important fail-safe mechanism that induces NK cell tolerance to normal self cells. These inhibitory NK receptors (iNKRs) were subsequently found expressed on different T cell subsets, primarily CD8(+) cytolytic T lymphocytes (CTLs), in which they can inhibit T cell receptor mediated functions. Some iNKR(+) CTLs are HLA-E-restricted, represent oligo- or monoclonal expansions, and can play a defensive role in viral infections. Although T cell activation, in the presence of certain cytokines, can induce the expression of the CD94-NKG2A heterodimeric receptor, the mechanism leading to the expression of killer immunoglobulin-like receptors (KIRs) is still unknown. The expression of iNKRs in T cells might contribute to the prevention of apoptotic cell death, thus allowing their survival and clonal expansion in vivo. In addition, iNKR(+) T cells might contribute to peripheral self-tolerance.

NK cell recognition

The integrated processing of signals transduced by activating and inhibitory cell surface receptors regulates NK cell effector functions. Here, I review the structure, function, and ligand specificity of the receptors responsible for NK cell recognition.

Receptors and lytic mediators regulating anti-tumor activity by the leukemic killer T cell line TALL-104

The major histocompatibility complex nonrestricted cytotoxic leukemic T cell line T acute lymphoblastic leukemia (TALL)-104 is being pursued as a therapeutic agent for cancer. However, the receptors and effector mechanisms responsible for its broad tumoricidal function remain undefined. Here, we examined the roles played by natural cytotoxicity receptors (NCR), killer cell immunoglobulin-like receptors, cytolytic granule components, and tumor necrosis factor (TNF) family members in tumor recognition and lysis by TALL-104 cells. The perforin-granzyme pathway, TNF-related apoptosis-inducing ligand (TRAIL), and Fas were each involved in the lysis of particular tumor targets by TALL-104. Furthermore, phorbol 12-myristate 13-acetate/ionomycin treatment induced surface expression of Fas-L and TRAIL. In addition, supernatants from CD3-stimulated TALL-104 cultures exhibited antiproliferative activity, which was blocked 50-90% by anti-TNF-alpha monoclonal antibody (mAb). Although negative for the NCR natural killer (NK)p44, this cell line was found to express NKp46. An anti-NKp46 antibody strongly blocked TALL-104-mediated lysis of certain targets and directly induced cytokine production, granule release, and redirected lysis responses. Anti-NKG2D and anti-2B4 also stimulated redirected cytotoxicity by TALL-104. By contrast, anti-NKG2A mAb did not stain the cells or inhibit killing responses. Alternatively, KIR3DL2 was detected on TALL-104, and expression of its reported ligand, human leukocyte antigen (HLA)-A, on target cells provided protection from cytotoxicity. Thus, NKp46, NKG2D, and 2B4 are activating receptors, and KIR3DL2 is an inhibitory receptor on TALL-104. The data demonstrate the ability of TALL-104 cells to recognize a wide variety of tumors with NK cell receptors and kill them with a broad arsenal of cytolytic effector mechanisms, including cytolytic granules and TNF family ligands.

MHC class Ib molecules bridge innate and acquired immunity

Our understanding of the classical MHC class I molecules (MHC class Ia molecules) has long focused on their extreme polymorphism. These molecules present peptides to T cells and are central to discrimination between self and non-self. By contrast, the functions of the non-polymorphic MHC class I molecules (MHC class Ib molecules) have been elusive, but emerging evidence reveals that, in addition to antigen presentation, MHC class Ib molecules are involved in immunoregulation. As we discuss here, the subset of MHC class Ib molecules that presents peptides to T cells bridges innate and acquired immunity, and this provides insights into the origins of acquired immunity.

Shaping the human NK cell repertoire: an epigenetic glance at KIR gene regulation

Human NK cells are equipped with arrays of inhibitory and stimulatory KIR receptors, many of them specific for HLA class I molecules on target cells. These NK receptors enable the recognition of virally infected as well as malignantly transformed target cells, which have downregulated the expression of single or multiple HLA class I products. KIR are expressed in clonally distributed ways leading to highly individualized but stable NK cell repertoires. Here, progress is reviewed toward understanding the molecular mechanisms that govern the unusual expression characteristics of KIR genes. Recent results suggest that DNA methylation plays a crucial role in shaping the KIR repertoire and underline the importance of epigenetic mechanisms as regulatory switches in the immune system.

The inhibitory effects of synthetic short peptides, mimicking MICA and targeting at NKG2D receptors, on function of NK cells

NKG2D is an activating receptor expressed on most of human NK cells, one of whose ligands is MICA. Based on the crystal structure of NKG2D-MICA complex, we synthesized three short peptides (P1, P2 and P3), mimicking functional alpha1 and alpha2 domain of MICA. The inhibitory effects of three peptides on NK-92 cells, a human NK cell line against Hela cells were observed and the inhibitory percentage was 38% at maximum for P1+P2+P3 in concentration of 1nM. The same peptides had no effect on NK-92 cell against target cells lacking MICA (K562 cells line). The unrelated peptides as controls had no effect on the system. Two peptides (P2 and P3) were prolonged at one or both ends, and the longer forms of peptides exerted stronger inhibitory effects than their shorter forms. Each combination of two peptides exerted a stronger function than single peptide (P1, P2, P3), indicating that shedding of longer amino acid sequence of alpha1 domain or more domain sites of MICA are better than shorter sequence and fewer sites. P1+P2+P3 revealed the almost same inhibitory rate as the soluble MICA (sMICA). P1+P2+P3 were also able to alleviate the concanavalin A-induced murine autoimmune hepatitis in vivo, conforming the similarity of NKG2D between human and mice. The results demonstrate that MICA-mimicking peptides will be useful to search the specific functional sites for NKG2D-MICA interaction, but also promising in explaining NKG2D-related autoimmunity.

Role of NK and NKT cells in the immunopathogenesis of HCV-induced hepatitis

Natural killer (NK) cells constitute the first line of host defense against invading pathogens. They usually become activated in an early phase of a viral infection. Liver is particularly enriched in NK cells, which are activated by hepatotropic viruses such as hepatitis C virus (HCV). The activated NK cells play an essential role in recruiting virus-specific T cells and in inducing antiviral immunity in liver. They also eliminate virus-infected hepatocytes directly by cytolytic mechanisms and indirectly by secreting cytokines, which induce an antiviral state in host cells. Therefore, optimally activated NK cells are important in limiting viral replication in this organ. This notion is supported by the observations that interferon treatment is effective in HCV-infected persons in whom it increases NK cell activity. Not surprisingly, HCV has evolved multiple strategies to counter host's NK cell response. Compromised NK cell functions have been reported in chronic HCV-infected individuals. It is ironic that activated NK cells may also contribute toward liver injury. Further studies are needed to understand the role of these cells in host defense and in liver pathology in HCV infections. Recent advances in understanding NK cell biology have opened new avenues for boosting innate and adaptive antiviral immune responses in HCV-infected individuals.

Injection of the immuno-modulatory drug alpha-galactosylceramide results in the recruitment of a large population of antigen-presenting cells into the liver of C57BL/6 mice

Injection of the immuno-modulatory drug alpha-galactosylceramide into C57BL/6 mice leads to the already known apoptosis of natural killer T (NKT) cells and to thus far undescribed large changes in the leukocyte populations of the liver. These changes are characterized by the recruitment of neutrophils and that of a population of large monocytic cells. The latter cells display the morphological and immunological features of natural suppressor cells. Their recruitment in the liver depends on the presence of NKT cells, most probably through the local release of cytokines and chemokines by activated NKT cells. We discuss the ubiquitous, long-term effects of alpha-galactosylceramide injection on immuno-pathological processes mediated through the NKT-triggered recruitment of a subset of large macrophages/monocytes.

Direct bacterial protein PAMP recognition by human NK cells involves TLRs and triggers alpha-defensin production

Although human CD56(+)CD3(-) natural killer (NK) cells participate in immune responses against microorganisms, their capacity to directly recognize and be activated by pathogens remains unclear. These cells encode members of the Toll-like receptor (TLR) family, involved in innate cell activation on recognition of pathogen-associated molecular patterns (PAMPs). We therefore evaluated whether the 2 bacterial protein PAMPs, the outer membrane protein A from Klebsiella pneumoniae (KpOmpA) and flagellin, which signal through TLR2 and TLR5, respectively, may directly stimulate human NK cells. These proteins induce interferon-gamma (IFN-gamma) production by NK cells and synergize with interleukin-2 (IL-2) and proinflammatory cytokines in PAMP-induced activation. Similar results were obtained using CD56(+)CD3(+) (NKR-expressing) T cells. NK cells from TLR2(-/-) mice fail to respond to KpOmpA, demonstrating TLR involvement in this effect. Defensins are antimicrobial peptides expressed mainly by epithelial cells and neutrophils that disrupt the bacterial membrane, leading to pathogen death. We show that NK cells and NKR-expressing T cells constitutively express alpha-defensins and that KpOmpA and flagellin rapidly induce their release. These data demonstrate for the first time that highly purified NK cells directly recognize and respond to pathogen components through TLRs and evidence defensins as a novel and direct cytotoxic pathway involved in NK cell-mediated protection against microorganisms.

CTL Are Inactivated by Herpes Simplex Virus-Infected Cells Expressing a Viral Protein Kinase

Numerous cell-to-cell signals tightly regulate CTL function. Human fibroblasts infected with HSV type 1 or 2 can generate such a signal and inactivate human CTL. Inactivated CTL lose their ability to release cytotoxic granules and synthesize cytokines when triggered through the TCR. Inactivation requires cell-to-cell contact between CTL and HSV-infected cells. However, inactivated CTL are not infected with HSV. The inactivation of CTL is sustainable, as CTL function remains impaired when the CTL are removed from the HSV-infected cells. IL-2 treatment does not alter inactivation, and the inactivated phenotype is not transferable between CTL, distinguishing this phenotype from traditional anergy and T regulatory cell models. CTL inactivated by HSV-infected cells are not apoptotic, and the inactivated state can be overcome by phorbol ester stimulation, suggesting that inactivated CTL are viable and that the signaling block is specific to the TCR. HSV-infected cells require the expression of U(S)3, a viral protein kinase, to transmit the inactivating signal. Elucidation of the molecular nature of this signaling pathway may allow targeted manipulation of CTL function.

[Natural killer lymphocyte activation in response to stress]

Function of T and natural killer (NK) lymphocytes is tightly controlled by the balance of activating and inhibitory signals. NK receptors belong to different families: KIRs ("Killer cell Immunoglobulin-like Receptor") and ILTs ("Immunoglobulin-like Transcript"), mainly inhibitory which binds to HLA class I alleles; C-type lectin NK receptors such as CD94/NKG2A which is inhibitory and binds to HLA-E; NCR ("Natural Cytotoxicity Receptors") which directly activate NK cells. These include molecules NKp30, NKp44, NKp46 et NKG2D. Cellular stress (viral and bacterial infections, tumours) may modulate NK function by different mechanisms: decrease in HLA class I molecules expression resulting in the lack of engagement of the inhibitory receptors and ultimately NK cell activation; modulation of CD94/NKG2A inhibitory function through expression of peptides presented by HLA-E as for instance from heat shock proteins; NK activation through NCR expression. Among these, NKG2D is an activating receptor expressed by NK cells and subsets of alphabeta and gammadelta and T cells. Major NKG2D ligands in humans are MIC ("MHC class I related") molecules which are stress-inducible during a viral (CMV) or bacterial infection (M. tuberculosis, E. coli). They may also be expressed by tumors. Therefore, they could play a role in activating NK and/or T lymphocyte responses in these conditions.

Selection of evolutionarily conserved mucosal-associated invariant T cells by MR1

The evolutionary conservation of T lymphocyte subsets bearing T-cell receptors (TCRs) using invariant alpha-chains is indicative of unique functions. CD1d-restricted natural killer T (NK-T) cells that express an invariant Valpha14 TCRalpha chain have been implicated in microbial and tumour responses, as well as in auto-immunity. Here we show that T cells that express the canonical hValpha7.2-Jalpha33 or mValpha19-Jalpha33 TCR rearrangement are preferentially located in the gut lamina propria of humans and mice, respectively, and are therefore genuine mucosal-associated invariant T (MAIT) cells. Selection and/or expansion of this population requires B lymphocytes, as MAIT cells are absent in B-cell-deficient patients and mice. In addition, we show that MAIT cells are selected and/or restricted by MR1, a monomorphic major histocompatibility complex class I-related molecule that is markedly conserved in diverse mammalian species. MAIT cells are not present in germ-free mice, indicating that commensal flora is required for their expansion in the gut lamina propria. This indicates that MAIT cells are probably involved in the host response at the site of pathogen entry, and may regulate intestinal B-cell activity.

Regulation of antiviral CD8+ T cells by inhibitory natural killer cell receptors

Recent evidence indicates that CD8(+) T cells express natural killer cell receptors that constrain the range and magnitude of their activities. For virus-specific CD8(+) T cells, upregulation of these receptors serves to control infection, while concurrently minimizing bystander pathology. Dysregulated expression of these receptors, however, may foster the establishment of persistent virus infection.

Killer cell immunoglobulin-like receptor expression delineates in situ Sézary syndrome lymphocytes

p140/KIR3DL2 has been identified in malignant cell lines isolated from the skin and blood of patients with transformed mycosis fungoides (MF) and Sézary's syndrome (SS). For the first time, the expression of a cell membrane structure appeared to be able to distinguish CD4+ tumour lymphocytes from reactive lymphocytes in these small cutaneous T-cell lymphomas (CTCLs). This study has examined the in vivo expression of this receptor in various CTCL subtypes, which constituted a heterogeneous group. Tumour cells diffusely expressed KIR in SS, in lymphomatoid papulosis (LyP) and in CD4+CD30+ as well as CD8+ large cell pleomorphic CTCL. In contrast, the infiltrating lymphocytes did not express KIR in MF at the patch/plaque stage or in CD4+CD30- large cell pleomorphic CTCL, except for scattered small cells. One quarter of the transformed MF tested exhibited KIR+ tumour cells, suggesting heterogeneity in this subtype. KIR expression was also examined in inflammatory lesions characterized by a dense infiltrate of T cells, such as lupus erythematosus and lichen planus. Only scattered CD8+ cells in lichen planus expressed a significant amount of KIR3DL2. Taken together, these results show for the first time that KIR molecules are expressed in distinct subtypes of malignant CTCL. It is also shown for the first time that SS and MF, which are frequent variants of CTCL with similar histological features, can be distinguished by their KIR3DL2 expression analysis. The identification of this KIR also differentiates between lupus erythematosus and lichen planus, which are both diseases with dense benign lymphocytic infiltrates.

Cell-surface expression and immune receptor recognition of HLA-B27 homodimers

OBJECTIVE

HLA-B27 is capable of forming in vitro a heavy-chain homodimer structure lacking beta(2)-microglobulin. We undertook this study to ascertain if patients with spondylarthritis express beta(2)-microglobulin-free HLA-B27 heavy chains in the form of homodimers and receptors for HLA-B27 homodimers.

METHODS

Expression of HLA-B27 heavy chains by mononuclear cells was analyzed by fluorescence-activated cell sorter staining, Western blotting with the monoclonal antibody HC-10, and 2-dimensional isoelectric focusing. Fluorescence-labeled tetrameric complexes of HLA-B27 heavy-chain homodimers were constructed in which each dimer comprised one His-tagged heavy chain and one biotinylated heavy chain, and were used to stain patient and control mononuclear cells and transfected cell lines.

RESULTS

Patients with spondylarthritis expressed cell-surface HLA-B27 homodimers. Populations of synovial and peripheral blood monocytes, and B and T lymphocytes from patients with spondylarthritis, and controls carried receptors for HLA-B27 homodimers. Experiments with transfected cell lines demonstrated that KIR3DL1 and KIR3DL2, and immunoglobulin-like transcript 4 (ILT4), but not ILT2, are receptors for HLA-B27 homodimers.

CONCLUSION

Patients with spondylarthritis express both HLA-B27 heavy-chain homodimers and receptors for HLA-B27 homodimers. This may be of significance with regard to disease pathogenesis.

TCR specificity dictates CD94/NKG2A expression by human CTL

Activating and inhibitory CD94/NKG2 receptors regulate CTL responses by altering TCR signaling, thus modifying antigen activation thresholds set during thymic selection. To determine whether their expression was linked to TCR specificity, we examined the TCR repertoire of oligoclonal CTL expansions found in human blood and tissues. High-resolution TCR repertoire analysis revealed that commitment to inhibitory NKG2A expression was a clonal attribute developmentally acquired after TCR expression and during antigen encounter, whereas actual surface expression depended on recent TCR engagement. Further, CTL clones expressing sequence-related TCR, and therefore sharing the same antigen specificity, invariably shared the same NKG2A commitment. These findings suggest that TCR antigenic specificity dictates NKG2A commitment, which critically regulates subsequent activation of CTL.