Involvement of inhibitory NKRs in the survival of a subset of memory-phenotype CD8+ T cells

Inhibitory KIRs decrease HLA class II-mediated protection in Type 1 Diabetes

Inhibitory killer cell immunoglobulin-like receptors (iKIRs) are a family of inhibitory receptors that are expressed by natural killer (NK) cells and late-stage differentiated T cells. There is accumulating evidence that iKIRs regulate T cell-mediated immunity. Recently, we reported that T cell-mediated control was enhanced by iKIRs in chronic viral infections. We hypothesized that in the context of autoimmunity, where an enhanced T cell response might be considered detrimental, iKIRs would have an opposite effect. We studied Type 1 diabetes (T1D) as a paradigmatic example of autoimmunity. In T1D, variation in the Human Leucocyte Antigen (HLA) genes explains up to 50% of the genetic risk, indicating that T cells have a major role in T1D etiopathogenesis. To investigate if iKIRs affect this T cell response, we asked whether HLA associations were modified by iKIR genes. We conducted an immunogenetic analysis of a case-control T1D dataset (N = 11,961) and found that iKIR genes, in the presence of genes encoding their ligands, have a consistent and significant effect on protective HLA class II genetic associations. Our results were validated in an independent data set. We conclude that iKIRs significantly decrease HLA class II protective associations and suggest that iKIRs regulate CD4+ T cell responses in T1D.

NK-like CD8 T cell: one potential evolutionary continuum between adaptive memory and innate immunity

CD8 T cells are crucial adaptive immune cells with cytotoxicity to fight against pathogens or abnormal self-cells via major histocompatibility complex class I-dependent priming pathways. The composition of the memory CD8 T-cell pool is influenced by various factors. Physiological aging, chronic viral infection, and autoimmune diseases promote the accumulation of CD8 T cells with highly differentiated memory phenotypes. Accumulating studies have shown that some of these memory CD8 T cells also exhibit innate-like cytotoxicity and upregulate the expression of receptors associated with natural killer (NK) cells. Further analysis shows that these NK-like CD8 T cells have transcriptional profiles of both NK and CD8 T cells, suggesting the transformation of CD8 T cells into NK cells. However, the specific induction mechanism underlying NK-like transformation and the implications of this process for CD8 T cells are still unclear. This review aimed to deduce the possible differentiation model of NK-like CD8 T cells, summarize the functions of major NK-cell receptors expressed on these cells, and provide a new perspective for exploring the role of these CD8 T cells in health and disease.

Extrinsic and intrinsic drivers of natural killer cell clonality

Clonal expansion of antigen-specific lymphocytes is the fundamental mechanism enabling potent adaptive immune responses and the generation of immune memory. Accompanied by pronounced epigenetic remodeling, the massive proliferation of individual cells generates a critical mass of effectors for the control of acute infections, as well as a pool of memory cells protecting against future pathogen encounters. Classically associated with the adaptive immune system, recent work has demonstrated that innate immune memory to human cytomegalovirus (CMV) infection is stably maintained as large clonal expansions of natural killer (NK) cells, raising questions on the mechanisms for clonal selection and expansion in the absence of re-arranged antigen receptors. Here, we discuss clonal NK cell memory in the context of the mechanisms underlying clonal competition of adaptive lymphocytes and propose alternative selection mechanisms that might decide on the clonal success of their innate counterparts. We propose that the integration of external cues with cell-intrinsic sources of heterogeneity, such as variegated receptor expression, transcriptional states, and somatic variants, compose a bottleneck for clonal selection, contributing to the large size of memory NK cell clones.

The paradox of aging: Aging-related shifts in T cell function and metabolism

T cell survival, differentiation after stimulation, and function are intrinsically linked to distinct cellular metabolic states. The ability of T cells to readily transition between metabolic states enables flexibility to meet the changing energy demands defined by distinct effector states or T cell lineages. Immune aging is characterized, in part, by the loss of naïve T cells, accumulation of senescent T cells, severe dysfunction in memory phenotype T cells in particular, and elevated levels of inflammatory cytokines, or 'inflammaging'. Here, we review our current understanding of the phenotypic and functional changes that occur with aging in T cells, and how they relate to metabolic changes in the steady state and after T cell activation. We discuss the apparent contradictions in the aging T cell phenotype - where enhanced differentiation states and metabolic profiles in the steady state can correspond to a diminished capacity to adapt metabolically and functionally after T cell activation. Finally, we discuss key recent studies that indicate the enormous potential for aged T cell metabolism to induce systemic inflammaging and organism-wide multimorbidity, resulting in premature death.

Polymorphic KIR3DL3 expression modulates tissue-resident and innate-like T cells

Most human killer cell immunoglobulin-like receptors (KIR) are expressed by natural killer (NK) cells and recognize HLA class I molecules as ligands. KIR3DL3 is a conserved but polymorphic inhibitory KIR recognizing a B7 family ligand, HHLA2, and is implicated for immune checkpoint targeting. The expression profile and biological function of KIR3DL3 have been somewhat elusive, so we searched extensively for KIR3DL3 transcripts, revealing highly enriched expression in γδ and CD8+ T cells rather than NK cells. These KIR3DL3-expressing cells are rare in the blood and thymus but more common in the lungs and digestive tract. High-resolution flow cytometry and single-cell transcriptomics showed that peripheral blood KIR3DL3+ T cells have an activated transitional memory phenotype and are hypofunctional. The T cell receptor (TCR) usage is biased toward genes from early rearranged TCR-α variable segments or Vδ1 chains. In addition, we show that TCR-mediated stimulation can be inhibited through KIR3DL3 ligation. Whereas we detected no impact of KIR3DL3 polymorphism on ligand binding, variants in the proximal promoter and at residue 86 can reduce expression. Together, we demonstrate that KIR3DL3 is up-regulated alongside unconventional T cell stimulation and that individuals may vary in their ability to express KIR3DL3. These results have implications for the personalized targeting of KIR3DL3/HHLA2 checkpoint inhibition.

KIR-HLA interactions extend human CD8+ T cell lifespan in vivo

BACKGROUNDThere is increasing evidence, in transgenic mice and in vitro, that inhibitory killer cell immunoglobulin-like receptors (iKIRs) can modulate T cell responses. Furthermore, we have previously shown that iKIRs are an important determinant of T cell-mediated control of chronic viral infection and that these results are consistent with an increase in the CD8+ T cell lifespan due to iKIR-ligand interactions. Here, we tested this prediction and investigated whether iKIRs affect T cell lifespan in humans in vivo.METHODSWe used stable isotope labeling with deuterated water to quantify memory CD8+ T cell survival in healthy individuals and patients with chronic viral infections.RESULTSWe showed that an individual's iKIR-ligand genotype was a significant determinant of CD8+ T cell lifespan: in individuals with 2 iKIR-ligand gene pairs, memory CD8+ T cells survived, on average, for 125 days; in individuals with 4 iKIR-ligand gene pairs, the memory CD8+ T cell lifespan doubled to 250 days. Additionally, we showed that this survival advantage was independent of iKIR expression by the T cell of interest and, further, that the iKIR-ligand genotype altered the CD8+ and CD4+ T cell immune aging phenotype.CONCLUSIONSTogether, these data reveal an unexpectedly large effect of iKIR genotype on T cell survival.FUNDINGWellcome Trust; Medical Research Council; EU Horizon 2020; EU FP7; Leukemia and Lymphoma Research; National Institute of Health Research (NIHR) Imperial Biomedical Research Centre; Imperial College Research Fellowship; National Institutes of Health; Jefferiss Trust.

Negativity begets longevity in T cells

Killer immunoglobulin-like receptors (KIRs) are polymorphic receptors for human leukocyte antigens (HLAs) that provide positive or negative signals controlling lymphocyte activation. Expression of inhibitory KIRs by CD8+ T cells affects their survival and function, which is linked to improved antiviral immunity and prevention of autoimmunity. In this issue of the JCI, Zhang, Yan, and co-authors demonstrate that increased numbers of functional inhibitory KIR-HLA pairs equating to greater negative regulation promoted longer lifespans of human T cells. This effect was independent of direct signals provided to KIR-expressing T cells and was instead driven by indirect mechanisms. Since the long-term maintenance of CD8+ T cells is critical for immune readiness against cancer and infection, this discovery has implications for immunotherapy and the preservation of immune function during aging.

Innate receptors modulating adaptive T cell responses: KIR-HLA interactions and T cell-mediated control of chronic viral infections

Killer-cell immunoglobulin-like receptors (KIRs) are mainly expressed on natural killer (NK) cells and are key regulators of innate immune responses. NK cells are the first responders in the face of infection and help promote placentation during pregnancy; the importance of KIRs in these NK-mediated processes is well-established. However, mounting evidence suggests that KIRs also have a prominent and long-lasting effect on the adaptive immune system. Here, we review the evidence for the impact of KIRs on T cell responses with a focus on the clinical significance of this interaction.

Human CD8+ T-Cell Populations That Express Natural Killer Receptors

CD8⁺ T cells are activated by TCRs that recognize specific cognate Ags, while NK-cell activation is regulated by a balance between signals from germline-encoded activating and inhibitory NK receptors. Through these different processes of Ag recognition, CD8⁺ T cells and NK cells play distinct roles as adaptive and innate immune cells, respectively. However, some human CD8⁺ T cells have been found to express activating or inhibitory NK receptors. CD8⁺ T-cell populations expressing NK receptors straddle the innate-adaptive boundary with their innate-like features. Recent breakthrough technical advances in multi-omics analysis have enabled elucidation of the unique immunologic characteristics of these populations. However, studies have not yet fully clarified the heterogeneity and immunological characteristics of each CD8⁺ T-cell population expressing NK receptors. Here we aimed to review the current knowledge of various CD8⁺ T-cell populations expressing NK receptors, and to pave the way for delineating the landscape and identifying the various roles of these T-cell populations.

Natural Killer T-like Cells: Immunobiology and Role in Disease

CD56+ T cells are generally recognized as a distinct population of T cells and are categorized as NKT-like cells. Although our understanding of NKT-like cells is far from satisfactory, it has been shown that aging and a number of disease situations have impacted these cells. To construct an overview of what is currently known, we reviewed the literature on human NKT-like cells. NKT-like cells are highly differentiated T cells with "CD1d-independent" antigen recognition and MHC-unrestricted cell killing. The genesis of NKT-like cells is unclear; however, it is proposed that the acquisition of innate characteristics by T cells could represent a remodeling process leading to successful aging. Additionally, it has been shown that NKT-like cells may play a significant role in several pathological conditions, making it necessary to comprehend whether these cells might function as prognostic markers. The quantification and characterization of these cells might serve as a cutting-edge indicator of individual immune health. Additionally, exploring the mechanisms that can control their killing activity in different contexts may therefore result in innovative therapeutic alternatives in a wide range of disease settings.

Dissecting CD8+ T cell pathology of severe SARS-CoV-2 infection by single-cell immunoprofiling

Introduction

SARS-CoV-2 infection results in varying disease severity, ranging from asymptomatic infection to severe illness. A detailed understanding of the immune response to SARS-CoV-2 is critical to unravel the causative factors underlying differences in disease severity and to develop optimal vaccines against new SARS-CoV-2 variants.

Methods

We combined single-cell RNA and T cell receptor sequencing with CITE-seq antibodies to characterize the CD8⁺ T cell response to SARS-CoV-2 infection at high resolution and compared responses between mild and severe COVID-19.

Results

We observed increased CD8⁺ T cell exhaustion in severe SARS-CoV-2 infection and identified a population of NK-like, terminally differentiated CD8⁺ effector T cells characterized by expression of FCGR3A (encoding CD16). Further characterization of NK-like CD8⁺ T cells revealed heterogeneity among CD16⁺ NK-like CD8⁺ T cells and profound differences in cytotoxicity, exhaustion, and NK-like differentiation between mild and severe disease conditions.

Discussion

We propose a model in which differences in the surrounding inflammatory milieu lead to crucial differences in NK-like differentiation of CD8⁺ effector T cells, ultimately resulting in the appearance of NK-like CD8⁺ T cell populations of different functionality and pathogenicity. Our in-depth characterization of the CD8⁺ T cell-mediated response to SARS-CoV-2 infection provides a basis for further investigation of the importance of NK-like CD8⁺ T cells in COVID-19 severity.

Donor selection for KIR alloreactivity is associated with superior survival in haploidentical transplant with PTCy

With the continuous increase in the use of haploidentical donors for transplantation, the selection of donors becomes increasingly important. Haploidentical donors have been selected primarily based on clinical characteristics, while the effects of killer cell immunoglobulin-like receptors (KIRs) on outcomes of haploidentical-hematopoietic stem cell transplantation (haplo-HSCT) with post-transplant cyclophosphamide (PTCy) remain inconclusive. The present study aimed to thoroughly evaluate the effect of KIRs and binding ligands assessed by various models, in addition to other patient/donor variables, on clinical outcomes in haplo-HSCT. In a cohort of 354 patients undergoing their first haplo-HSCT, we found that a higher Count Functional inhibitory KIR score (CF-iKIR) was associated with improved progression-free survival (adjusted hazard ratio [HR], 0.71; P = .029) and overall survival (OS) (HR, 0.66; P = .016), while none of the other models predicted for survival in these patients. Moreover, using exploratory classification and regression tree analysis, we found that donor age <58 years combined with cytomegalovirus-nonreactive recipient was associated with the best OS, whereas donor age >58 years was associated with the worst OS. In the rest of our cohort (80%), cytomegalovirus-reactive recipients with a donor <58 years old, a higher CF-iKIR was associated with superior OS. The 3-year OS rates were 73.9%, 54.1% (HR, 1.84; P = .044), 44.5% (HR, 2.01; P = .003), and 18.5% (HR, 5.44; P <.001) in the best, better, poor, and worse donor groups, respectively. Our results suggest that KIR alloreactivity assessed by CF-iKIR score can help optimize donor selection in haplo-HSCT.

Regulatory CD8+ T cells suppress disease

A subset of CD8⁺ T cells regulate chronic inflammation by killing pathogenic CD4⁺ T cells.

Sequential actions of EOMES and T-BET promote stepwise maturation of natural killer cells

EOMES and T-BET are related T-box transcription factors that control natural killer (NK) cell development. Here we demonstrate that EOMES and T-BET regulate largely distinct gene sets during this process. EOMES is dominantly expressed in immature NK cells and drives early lineage specification by inducing hallmark receptors and functions. By contrast, T-BET is dominant in mature NK cells, where it induces responsiveness to IL-12 and represses the cell cycle, likely through transcriptional repressors. Regardless, many genes with distinct functions are co-regulated by the two transcription factors. By generating two gene-modified mice facilitating chromatin immunoprecipitation of endogenous EOMES and T-BET, we show a strong overlap in their DNA binding targets, as well as extensive epigenetic changes during NK cell differentiation. Our data thus suggest that EOMES and T-BET may distinctly govern, via differential expression and co-factors recruitment, NK cell maturation by inserting partially overlapping epigenetic regulations.

Deciphering the biology of KIR2DL3+ T lymphocytes that are associated to relapse in haploidentical HSCT

KIR are mainly expressed on NK cells and to a lesser extent on T lymphocytes. Although the KIR NK cell repertoire was well explored in haploidentical Hematopoietic Stem Cell Transplantation (HSCT), KIR T cell compartment remains to be investigated in this context. In this study, the investigation of NK receptors on T lymphocytes during immune reconstitution after T-cell-replete haploidentical HSCT with Post-Transplant Cyclophosphamide (PTCy) has shown a significant increase of KIR2DL2/3+ T cell frequency at day 25. This was especially observed at day 30 in recipients who relapsed. IL-15 but not IL-12 increased in vitro KIR+ T cell expansion suggesting that the raised IL-15 serum concentration observed after PTCy in haploidentical HSCT might increase KIR+ T cell frequency. Moreover, investigations from healthy blood donors showed a higher inhibiting effect of KIR2DL3 on CMV specific T cell response against allogeneic than autologous C1+ target cells. The association of KIR+ T cell subset with relapse may suggest that inhibitory KIR2DL2/3 limit anti-leukemic effect of specific T lymphocytes at this early step of immune reconstitution. Further phenotypic and mechanistic investigations on this cell subset from a broader cohort of HSCT recipients should clarify its potential implication in relapse occurrence. Our results demonstrate that KIR-HLA interactions known to modulate NK cell functions also modulate T cell immune responses in the context of allogeneic HSCT.

The combinatorial diversity of KIR and HLA class I allotypes in Peninsular Malaysia

Killer cell immunoglobulin-like receptors (KIRs) interact with polymorphic human leucocyte antigen (HLA) class I molecules, modulating natural killer (NK) cell functions and affecting both the susceptibility and outcome of immune-mediated diseases. The KIR locus is highly diverse in gene content, copy number and allelic polymorphism within individuals and across geographical populations. To analyse currently under-represented Asian and Pacific populations, we investigated the combinatorial diversity of KIR and HLA class I in 92 unrelated Malay and 75 Malaysian Chinese individuals from the Malay Peninsula. We identified substantial allelic and structural diversity of the KIR locus in both populations and characterized novel variations at each analysis level. The Malay population is more diverse than Malay Chinese, likely representing a unique history including admixture with immigrating populations spanning several thousand years. Characterizing the Malay population are KIR haplotypes with large structural variants present in 10% individuals, and KIR and HLA alleles previously identified in Austronesian populations. Despite the differences in ancestries, the proportion of HLA allotypes that serve as KIR ligands is similar in each population. The exception is a significantly reduced frequency of interactions of KIR2DL1 with C2+ HLA-C in the Malaysian Chinese group, caused by the low frequency of C2+ HLA. One likely implication is a greater protection from preeclampsia, a pregnancy disorder associated with KIR2DL1, which shows higher incidence in the Malay than in the Malaysian Chinese. This first complete, high-resolution, characterization of combinatorial diversity of KIR and HLA in Malaysians will form a valuable reference for future clinical and population studies.

Functional Characterization of Ly49+CD8 T-Cells in Both Normal Condition and During Anti-Viral Response

The role of Ly49+CD8 T-cells in the immune system is not clear. Previously, several papers suggested Ly49+CD8 T-cells as immunosuppressors, while multiple studies also suggested their role as potent participants of the immune response. The mechanism of Ly49 expression on CD8 T-cells is also not clear. We investigated phenotype, functions, and regulation of Ly49 expression on murine CD8 T-cells in both normal state and during LCMV infection. CD8 T-cells express different Ly49 receptors compared with NK-cells. In intact mice, Ly49+CD8 T-cells have a phenotype similar to resting central memory CD8 T-cells and do not show impaired proliferation and cytokine production. Conventional CD8 T-cells upregulate Ly49 receptors during TCR-induced stimulation, and IL-2, as well as IL-15, affect it. At the same time, Ly49+CD8 T-cells change the Ly49 expression profile dramatically upon re-stimulation downregulating inhibitory and upregulating activating Ly49 receptors. We observed the expression of Ly49 receptors on the virus-specific CD8 T-cells during LCMV infection, especially marked in the early stages, and participation of Ly49+CD8 T-cells in the anti-viral response. Thus, CD8 T-cells acquire Ly49 receptors during the T-cell activation and show dynamic regulation of Ly49 receptors during stimulation.

Hiding in Plain Sight: Virtually Unrecognizable Memory Phenotype CD8+ T cells

Virtual memory T (T_VM) cells are a recently described population of conventional CD8⁺ T cells that, in spite of their antigen inexperience, express markers of T cell activation. T_VM cells exhibit rapid responsiveness to both antigen-specific and innate stimuli in youth but acquire intrinsic antigen-specific response defects in the elderly. In this article, we review how the identification of T_VM cells necessitates a re-evaluation of accepted paradigms for conventional memory T (T_MEM) cells, the potential for heterogeneity within the T_VM population, and the defining characteristics of T_VM cells. Further, we highlight recent literature documenting the development of T_VM cells as a distinct CD8⁺ T cell lineage as well their biological significance in the context of disease.

Ly49R activation receptor drives self-MHC-educated NK cell immunity against cytomegalovirus infection

Natural killer (NK) cells mediate vital control of cancer and viral infection. They rely on MHC class I (MHC I)-specific self-receptors to identify and lyse diseased cells without harming self-MHC I-bearing host cells. NK cells bearing inhibitory self-receptors for host MHC I also undergo education, referred to as licensing, which causes them to become more responsive to stimulation via activation receptor signaling. Previous work has shown that licensed NK cells selectively expand during virus infections and they are associated with improved clinical response in human patients experiencing certain chronic virus infections, including HIV and hepatitis C virus. However, the importance of inhibitory self-receptors in NK-mediated virus immunity is debated as they also limit signals in NK cells emanating from virus-specific activation receptors. Using a mouse model of MHC I-dependent (H-2D^k) virus immunity, we discovered that NK cells depend on the Ly49G2 inhibitory self-receptor to mediate virus control, which coincided with host survival during murine cytomegalovirus infection. This antiviral effect further requires active signaling in NK cells via the Ly49R activation receptor that also binds H-2D^k In tandem, these functionally discordant Ly49 self-receptors increase NK cell proliferation and effector activity during infection, resulting in selective up-regulation of CD25 and KLRG1 in virus-specific Ly49R⁺ Ly49G2⁺ NK cells. Our findings establish that paired self-receptors act as major determinants of NK cell-mediated virus sensing and immunity.

Conservation, Extensive Heterozygosity, and Convergence of Signaling Potential All Indicate a Critical Role for KIR3DL3 in Higher Primates

Natural killer (NK) cell functions are modulated by polymorphic killer cell immunoglobulin-like receptors (KIR). Among 13 human KIR genes, which vary by presence and copy number, KIR3DL3 is ubiquitously present in every individual across diverse populations. No ligand or function is known for KIR3DL3, but limited knowledge of expression suggests involvement in reproduction, likely during placentation. With 157 human alleles, KIR3DL3 is also highly polymorphic and we show heterozygosity exceeds that of HLA-B in many populations. The external domains of catarrhine primate KIR3DL3 evolved as a conserved lineage distinct from other KIR. Accordingly, and in contrast to other KIR, we show the focus of natural selection does not correspond exclusively to known ligand binding sites. Instead, a strong signal for diversifying selection occurs in the D1 Ig domain at a site involved in receptor aggregation, which we show is polymorphic in humans worldwide, suggesting differential ability for receptor aggregation. Meanwhile in the cytoplasmic tail, the first of two inhibitory tyrosine motifs (ITIM) is conserved, whereas independent genomic events have mutated the second ITIM of KIR3DL3 alleles in all great apes. Together, these findings suggest that KIR3DL3 binds a conserved ligand, and a function requiring both receptor aggregation and inhibitory signal attenuation. In this model KIR3DL3 resembles other NK cell inhibitory receptors having only one ITIM, which interact with bivalent downstream signaling proteins through dimerization. Due to the extensive conservation across species, selection, and other unusual properties, we consider elucidating the ligand and function of KIR3DL3 to be a pressing question.

Inhibitory killer cell immunoglobulin-like receptors strengthen CD8+ T cell-mediated control of HIV-1, HCV, and HTLV-1

Killer cell immunoglobulin-like receptors (KIRs) are expressed predominantly on natural killer cells, where they play a key role in the regulation of innate immune responses. Recent studies show that inhibitory KIRs can also affect adaptive T cell-mediated immunity. In mice and in human T cells in vitro, inhibitory KIR ligation enhanced CD8+ T cell survival. To investigate the clinical relevance of these observations, we conducted an extensive immunogenetic analysis of multiple independent cohorts of HIV-1-, hepatitis C virus (HCV)-, and human T cell leukemia virus type 1 (HTLV-1)-infected individuals in conjunction with in vitro assays of T cell survival, analysis of ex vivo KIR expression, and mathematical modeling of host-virus dynamics. Our data suggest that functional engagement of inhibitory KIRs enhances the CD8+ T cell response against HIV-1, HCV, and HTLV-1 and is a significant determinant of clinical outcome in all three viral infections.

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.

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

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

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.

The Inhibitory Receptor NKG2A Sustains Virus-Specific CD8⁺ T Cells in Response to a Lethal Poxvirus Infection

CD8(+) T cells and NK cells protect from viral infections by killing virally infected cells and secreting interferon-γ. Several inhibitory receptors limit the magnitude and duration of these anti-viral responses. NKG2A, which is encoded by Klrc1, is a lectin-like inhibitory receptor that is expressed as a heterodimer with CD94 on NK cells and activated CD8(+) T cells. Previous studies on the impact of CD94/NKG2A heterodimers on anti-viral responses have yielded contrasting results and the in vivo function of NKG2A remains unclear. Here, we generated Klrc1(-/-) mice and found that NKG2A is selectively required for resistance to ectromelia virus (ECTV). NKG2A functions intrinsically within ECTV-specific CD8(+) T cells to limit excessive activation, prevent apoptosis, and preserve the specific CD8(+) T cell response. Thus, although inhibitory receptors often cause T cell exhaustion and viral spreading during chronic viral infections, NKG2A optimizes CD8(+) T cell responses during an acute poxvirus infection.

Coordinated regulation of NK receptor expression in the maturing human immune system

NK cells are responsible for recognizing and killing transformed, stressed, and infected cells. They recognize a set of non-Ag-specific features termed "altered self" through combinatorial signals from activating and inhibitory receptors. These NKRs are also expressed on CD4(+) and CD8(+) T cells, B cells, and monocytes, although a comprehensive inventory of NKR expression patterns across leukocyte lineages has never been performed. Using mass cytometry, we found that NKR expression patterns distinguish cell lineages in human peripheral blood. In individuals with high levels of CD57, indicative of a mature immune repertoire, NKRs are more likely to be expressed on non-NK cells, especially CD8(+) T cells. Mature NK and CD8(+) T cell populations show increased diversity of NKR surface expression patterns, but with distinct determinants: mature NK cells acquire primarily inhibitory receptors, whereas CD8(+) T cells attain a specific subset of both activating and inhibitory receptors, potentially imbuing them with a distinct functional role. Concurrently, monocytes show decreased expression of the generalized inhibitory receptor leukocyte Ig-like receptor subfamily b member 1, consistent with an increased activation threshold. Therefore, NKR expression is coordinately regulated as the immune system matures, resulting in the transfer of "altered self" recognition potential among leukocyte lineages. This likely reduces Ag specificity in the mature human immune system, and implies that vaccines and therapeutics that engage both its innate and adaptive branches may be more effective in the settings of aging and chronic infection.

Functional NK cell repertoires are maintained through IL-2Rα and Fas ligand

Acquisition of a functional NK cell repertoire, known as education or licensing, is a complex process mediated through inhibitory receptors that recognize self. We found that NK cells containing self-killer Ig-like receptors for cognate HLA ligand in vivo were less susceptible to apoptosis. In vitro IL-15 withdrawal showed that uneducated NK cells upregulated Bim and Fas. Conversely, educated NK cells upregulated Fas ligand (FasL) under these conditions. Induction of cell death and Bim expression on uneducated cells correlated with increased IL-2Rα expression. Overexpression and knockdown studies showed that higher IL-2Rα limits NK cell survival in a novel manner that is independent from the role of IL-2 in activation-induced cell death. To study the role of FasL in induction of IL-2Rα(hi) NK cell death, a coculture assay with FasL-blocking Abs was used. IL-15 withdrawal led to FasL-dependent killing of IL-2Rα(hi) NK cells by more educated IL-2Rα(lo) NK cells. Finally, CMV reactivation induces a potent long-lasting population of licensed NK cells with enhanced survival. These findings show that education-dependent NK cell survival advantages and killing of uneducated NK cells result in the maintenance of a functional repertoire, which may be manipulated to exploit NK cells for cancer immunotherapy.

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.

The biochemistry and immunology of non-canonical forms of HLA-B27

HLA-B27 (B27) is strongly associated with the spondyloarthritides. B27 is expressed at the cell surface of antigen presenting cells (APC) both as canonical β2m-associated and non-canonical β2m-free heavy chain (FHC) forms which include B27 dimers (termed B272). B27 FHC forms arise in an endosomal compartment from recycling β2m-associated B27. Formation of cell surface FHC dimers is critically dependent on an unpaired reactive cysteine 67 in the α1 helix of the class I heavy chain. HLA-B27 also form redox-inducible β2m-associated dimers on exosomes and apoptosing cells. By contrast with cell surface expressed cysteine 67-dependent heavy chain dimers these dimers are dependent on a cytoplasmic cysteine 325 for their formation. HLA-B27 binds to immunoregulatory receptors including members of the Killer cell Immunoglobulin-like (KIR) and Leukocyte Immunoglobulin-like receptor family. B27 FHC bind to different but overlapping sets of these immunoreceptors compared to classical β2m-associated HLA-B27. B27 FHC bind more strongly to KIR3DL2 and LILRB2 immune receptor than other β2m-associated HLA-class I ligands. Genetic studies have implicated genes which control production of the important proinflammatory cytokine IL-17 in the pathogenesis of spondyloarthritis. Cell surface HLA-B27 FHC binding to these immune receptors or acting through other mechanisms could impact on the pathogenesis of spondyloarthritis by promoting immune cell production of IL-17. Here we review the literature on these non-canonical forms of HLA-B27 and the immune receptors they bind to and discuss the possible relevance of these interactions to the pathogenesis of spondyloarthropathy.

Multiplex and genome-wide analyses reveal distinctive properties of KIR+ and CD56+ T cells in human blood

Killer cell Ig-like receptors (KIRs) on NK cells have been linked to a wide spectrum of health conditions such as chronic infections, autoimmune diseases, pregnancy complications, cancers, and transplant failures. A small subset of effector memory T cells also expresses KIRs. In this study, we use modern analytic tools including genome-wide and multiplex molecular, phenotypic, and functional assays to characterize the KIR(+) T cells in human blood. We find that KIR(+) T cells primarily reside in the CD56(+) T population that is distinctively DNAM-1(high) with a genome-wide quiescent transcriptome, short telomere, and limited TCR excision circles. During CMV reactivation in bone marrow transplant recipients, KIR(+)CD56(+) T cells rapidly expanded in real-time but not KIR(+)CD56(-) T cells or KIR(+) NK cells. In CMV(+) asymptomatic donors, as much as 50% of CD56(+) T cells are KIR(+), and most are distinguishably KIR2DL2/3(+)NKG2C(+)CD57(+). Functionally, the KIR(+)CD56(+) T cell subset lyses cancer cells and CMVpp65-pulsed target cells in a dual KIR-dependent and TCR-dependent manner. Analysis of metabolic transcriptome confirms the immunological memory status of KIR(+)CD56(+) T cells in contrast to KIR(-)CD56(+) T cells that are more active in energy metabolism and effector differentiation. KIR(-)CD56(+) T cells have >25-fold higher level of expression of RORC than the KIR(+) counterpart and are a previously unknown producer of IL-13 rather than IL-17 in multiplex cytokine arrays. Our data provide fundamental insights into KIR(+) T cells biologically and clinically.

The arthritis-associated HLA-B*27:05 allele forms more cell surface B27 dimer and free heavy chain ligands for KIR3DL2 than HLA-B*27:09

Objectives. HLA-B*27:05 is associated with AS whereas HLA-B*27:09 is not associated. We hypothesized that different interactions with KIR immune receptors could contribute to the difference in disease association between HLA-B*27:05 and HLAB*27:09. Thus, the objective of this study was to compare the formation of β2m-free heavy chain (FHC) including B27 dimers (B272) by HLA-B*27:05 and HLA-B*27:09 and their binding to KIR immunoreceptors.

Methods. We studied the formation of HLA-B*27:05 and HLA-B*27:09 heterotrimers and FHC forms including dimers in vitro and in transfected cells. We investigated HLA-B*27:05 and HLA-B*27:09 binding to KIR3DL1, KIR3DL2 and LILRB2 by FACS staining with class I tetramers and by quantifying interactions with KIR3DL2CD3ε-reporter cells and KIR3DL2-expressing NK cells. We also measured KIR expression on peripheral blood NK and CD4 T cells from 18 HLA-B*27:05 AS patients, 8 HLA-B27 negative and 12 HLA-B*27:05+ and HLA-B*27:09+ healthy controls by FACS staining.

Results. HLA-B*27:09 formed less B27₂ and FHC than HLA-B*27:05. HLA-B*27:05-expressing cells stimulated KIR3DL2CD3ε-reporter T cells more effectively. Cells expressing HLA-B*27:05 promoted KIR3DL2+ NK cell survival more strongly than HLA-B*27:09. HLA-B*27:05 and HLA-B*27:09 dimer tetramers stained KIR3DL1, KIR3DL2 and LILRB2 equivalently. Increased proportions of NK and CD4 T cells expressed KIR3DL2 in HLA-B*27:05+ AS patients compared with HLA-B*27:05+, HLA-B*27:09+ and HLA-B27− healthy controls.

Conclusion. Differences in the formation of FHC ligands for KIR3DL2 by HLA-B*27:05 and HLA-B*27:09 could contribute to the differential association of these alleles with AS.

KIR3DL2 binds to HLA-B27 dimers and free H chains more strongly than other HLA class I and promotes the expansion of T cells in ankylosing spondylitis

The human leukocyte Ag HLA-B27 (B27) is strongly associated with the spondyloarthritides. B27 can be expressed at the cell surface of APC as both classical β2-microglobulin-associated B27 and B27 free H chain forms (FHC), including disulfide-bonded H chain homodimers (termed B27(2)). B27 FHC forms, but not classical B27, bind to KIR3DL2. HLA-A3, which is not associated with spondyloarthritis (SpA), is also a ligand for KIR3DL2. In this study, we show that B27(2) and B27 FHC bind more strongly to KIR3DL2 than other HLA-class I, including HLA-A3. B27(2) tetramers bound KIR3DL2-transfected cells more strongly than HLA-A3. KIR3DL2Fc bound to HLA-B27-transfected cells more strongly than to cells transfected with other HLA-class I. KIR3DL2Fc pulled down multimeric, dimeric, and monomeric FHC from HLA-B27-expressing cell lines. Binding to B27(2) and B27 FHC stimulated greater KIR3DL2 phosphorylation than HLA-A3. B27(2) and B27 FHC stimulated KIR3DL2CD3ε-transduced T cell IL-2 production to a greater extent than control HLA-class I. KIR3DL2 binding to B27 inhibited NK IFN-γ secretion and promoted greater survival of KIR3DL2(+) CD4 T and NK cells than binding to other HLA-class I. KIR3DL2(+) T cells from B27(+) SpA patients proliferated more in response to Ag presented by syngeneic APC than the same T cell subset from healthy and disease controls. Our results suggest that expansion of KIR3DL2-expressing leukocytes observed in B27(+) SpA may be explained by the stronger interaction of KIR3DL2 with B27 FHC.

CD8 T cells express randomly selected KIRs with distinct specificities compared with NK cells

Epistatic interactions between killer cell immunoglobulin-like receptors (KIRs) and their cognate HLA class I ligands have important implications for reproductive success, antiviral immunity, susceptibility to autoimmune conditions and cancer, as well as for graft-versus-leukemia reactions in settings of allogeneic stem cell transplantation. Although CD8 T cells are known to acquire KIRs when maturing from naive to terminally differentiated cells, little information is available about the constitution of KIR repertoires on human CD8 T cells. Here, we have performed a high-resolution analysis of KIR expression on CD8 T cells. The results show that most CD8 T cells possess a restricted KIR expression pattern, often dominated by a single activating or inhibitory KIR. Furthermore, the expression of KIR, and its modulation of CD8 T-cell function, was independent of expression of self-HLA class I ligands. Finally, despite similarities in the stochastic regulation of KIRs by the bidirectional proximal promoter, the specificity of inhibitory KIRs on CD8 T cells was often distinct from that of natural killer cells in the same individual. The results provide new insight into the formation of KIR repertoires on human T cells.

The link between HLA-B27 and SpA--new ideas on an old problem

The strong association of the HLA-B27 with AS was first discovered independently by groups in London and California in 1972 and has recently been confirmed beyond reasonable doubt by fine mapping in the latest and most sophisticated genome-wide association study (GWAS) published this July. Yet, despite nearly four decades of extensive research, the exact role that HLA-B27 plays in pathogenesis remains unknown. However, we believe that recent developments in three fields have allowed us to view this conundrum in a new light and to propose coherent theories of disease pathogenesis. These areas are as follows: (i) GWASs, (ii) studies of B27 biology and (iii) lessons from biologic therapies. In this review we will discuss these recent advances before discussing the current models of AS pathogenesis under investigation.

Differential requirements for IRF-2 in generation of CD1d-independent T cells bearing NK cell receptors

NK cell receptors (NKRs) such as NK1.1, NKG2D, and Ly49s are expressed on subsets of CD1d-independent memory phenotype CD8(+) and CD4(-)CD8(-) T cells. However, the mechanism for the generation and functions of these NKR(+) T cells remained elusive. In this study, we found that CD1d-independent Ly49(+) T cells were reduced severely in the spleen, bone marrow, and liver, but not thymus, in mice doubly deficient for IFN regulatory factor-2 (IRF-2) and CD1d, in which the overall memory phenotype T cell population was contrastingly enlarged. Because a large fraction of Ly49(+) T cells coexpressed NK1.1 or NKG2D, the reduction of Ly49(+) T cells resulted indirectly in underrepresentation of NK1.1(+) or NKG2D(+) cells. Ly49(+) T cell deficiency was observed in IRF-2(-/-) mice additionally lacking IFN-α/βR α-chain (IFNAR1) as severely as in IRF-2(-/-) mice, arguing against the involvement of the accelerated IFN-α/β signals due to IRF-2 deficiency. Rather, mice lacking IFN-α/βR alone also exhibited relatively milder Ly49(+) T cell reduction, and IL-2 could expand Ly49(+) T cells from IFNAR1(-/-), but not from IRF-2(-/-), spleen cells in vitro. These results together indicated that IRF-2 acted in Ly49(+) T cell development in a manner distinct from that of IFN-α/β signals. The influence of IRF-2 deficiency on Ly49(+) memory phenotype T cells observed in this study suggested a unique transcriptional program for this T cell population among other NKR(+) T and memory phenotype T cells.

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.

Th17 cells expressing KIR3DL2+ and responsive to HLA-B27 homodimers are increased in ankylosing spondylitis

CD4 Th cells producing the proinflammatory cytokine IL-17 (Th17) have been implicated in a number of inflammatory arthritides including the spondyloarthritides. Th17 development is promoted by IL-23. Ankylosing spondylitis, the most common spondyloarthritis (SpA), is genetically associated with both HLA-B27 (B27) and IL-23R polymorphisms; however, the link remains unexplained. We have previously shown that B27 can form H chain dimers (termed B27(2)), which, unlike classical HLA-B27, bind the killer-cell Ig-like receptor KIR3DL2. In this article, we show that B27(2)-expressing APCs stimulate the survival, proliferation, and IL-17 production of KIR3DL2(+) CD4 T cells. KIR3DL2(+) CD4 T cells are expanded and enriched for IL-17 production in the blood and synovial fluid of patients with SpA. Despite KIR3DL2(+) cells comprising a mean of just 15% of CD4 T in the peripheral blood of SpA patients, this subset accounted for 70% of the observed increase in Th17 numbers in SpA patients compared with control subjects. TCR-stimulated peripheral blood KIR3DL2(+) CD4 T cell lines from SpA patients secreted 4-fold more IL-17 than KIR3DL2(+) lines from controls or KIR3DL2(-) CD4 T cells. Strikingly, KIR3DL2(+) CD4 T cells account for the majority of peripheral blood CD4 T cell IL-23R expression and produce more IL-17 in the presence of IL-23. Our findings link HLA-B27 with IL-17 production and suggest new therapeutic strategies in ankylosing spondylitis/SpA.

Killer cell Ig-like receptor (KIR) 3DL1 down-regulation enhances inhibition of type 1 diabetes by autoantigen-specific regulatory T cells

Both Foxp3(+) regulatory T cells (Tregs) and antigen-expanded Foxp3(-) Tregs play an important role in regulating immune responses as well as in preventing autoimmune diseases and graft rejection. Molecular mechanisms modulating Treg function remain largely unclear, however. We report here on the expression and function of an inhibitory killer cell Ig-like receptor, KIR3DL1, in a nonobese diabetic (NOD) mouse-derived autoantigen-specific Treg (2D2), which protects from type 1 diabetes (T1D) in adoptive transfer experiments. This gene is not expressed in T1D pathogenic T cells (Tpaths) or non-Tpath T cells. KIR genes are known to play an important role in regulating natural killer (NK) cell function, but their role in Tregs and T1D is unknown. To examine whether KIR3DL1 expression may modulate Treg function, we used shRNA to down-regulate KIR3DL1 expression (2D2-shKIR). We find that KIR3DL1 down-regulation enhances in vitro function, as measured by improved suppression of target cell proliferation. Antibody blockade of IL-10 but not IL-4 partially abrogated suppressive function. In vivo function is also improved. Adoptive transfer of 2D2-shKIR into 10-wk-old NOD mice prevented spontaneous insulitis and T1D, and the inhibitory effect was further improved if the cells were transferred earlier into 6-wk-old NOD mice. These studies indicate that KIR3DL1 expression may negatively regulate Treg function and suggest a previously undescribed target for improving immune tolerance for potential treatment of autoimmune diseases like T1D.

CD8+ T regulatory cells express the Ly49 Class I MHC receptor and are defective in autoimmune prone B6-Yaa mice

The immune system includes a subpopulation of CD8(+) T cells equipped to inhibit the expansion of follicular T helper (T(FH)) cells, resulting in suppression of autoantibody production and associated lupus-like disease. These CD8(+) T regulatory (Treg) cells recognize Qa-1/peptide complexes on target T(FH) cells and depend on the IL-15 cytokine for development and function. Here we show that these CD8(+) Treg cells express a triad of surface receptors--CD44, CD122, and the class I MHC receptor Ly49--and account for <5% of CD8(+) T cells. Moreover, the development of systemic lupus erythematosus-like disease in B6-Yaa mutant mice is associated with a pronounced defect in CD8(+) Treg cell activity, suggesting that this regulatory subset may represent an effective therapeutic approach to systemic lupus erythematosus-like autoimmune disease.

Distribution of killer cell immunoglobulin-like receptor genes in the mestizo population from Venezuela

This study represents the first report on the distribution of KIR genes in 205 unrelated healthy mestizo Venezuelan individuals. Genotyping analysis showed that all KIR genes are present in this population. Frequency of inhibitory killer cell immunoglobulin-like receptors (KIRs) exceeded 0.69, except for KIR2DL2 (0.29) and 2DL5 (0.37). Activating KIRs showed low frequencies (0.11-0.29), except for KIR2DS4 (0.68). Forty-five different KIR genotypes were identified, with a predominance of three genotypes found in 50.7% of the population of which 25.9% were individuals homozygous for haplotype A. The frequencies of KIR genes reflect the ethnic admixture existing in the mestizo Venezuelan population.

Killer cell immunoglobulin-like receptor expression induction on neonatal CD8(+) T cells in vitro and following congenital infection with Trypanosoma cruzi

Major histocompatibility complex (MHC) class I-specific inhibitory natural killer receptors (iNKRs) are expressed by subsets of T cells but the mechanisms inducing their expression are poorly understood, particularly for killer-cell immunoglobulin-like receptors (KIRs). The iNKRs are virtually absent from the surface of cord blood T cells but we found that KIR expression could be induced upon interleukin-2 stimulation in vitro. In addition, KIR expression was enhanced after treatment with 5-aza-2'-deoxycytidine, suggesting a role for DNA methylation. In vivo induction of KIR expression on cord blood T cells was also observed during a human congenital infection with Trypanosoma cruzi which triggers activation of fetal CD8(+) T cells. These KIR(+) T cells had an effector and effector/memory phenotype suggesting that KIR expression was consecutive to the antigenic stimulation; however, KIR was not preferentially found on parasite-specific CD8(+) T cells secreting interferon-gamma upon in vitro restimulation with live T. cruzi. These findings show that KIR expression is likely regulated by epigenetic mechanisms that occur during the maturation process of cord blood T cells. Our data provide a molecular basis for the appearance of KIRs on T cells with age and they have implications for T-cell homeostasis and the regulation of T-cell-mediated immune responses.

Genetic and antibody-mediated reprogramming of natural killer cell missing-self recognition in vivo

Natural killer (NK) cells are lymphocytes of the innate immune system able to recognize and kill tumors lacking self-MHC class I molecules. This "missing-self" recognition is mediated by the lack of engagement of MHC class I-specific inhibitory NK cell receptors that include the killer cell Ig-like receptors (KIR) in humans and Ly49 molecules in mice. A promising immunotherapeutic strategy against MHC class I(+) cancer cells is to block NK cell inhibitory receptors using monoclonal antibodies (mAb). However, interactions between MHC class I molecules and their inhibitory receptors are also required for the acquisition of NK cell functional competence, a process referred as to "education." In addition, inhibitory receptors are involved in self-tolerance on educated NK cells. Here, we developed a preclinical mouse model in which all NK cells are educated by a single transgenic inhibitory receptor, human KIR2DL3, through the engagement with its HLA-Cw3 ligand. This approach revealed that NK cells could be reprogrammed to control the development of mouse syngenic tumors in vivo. Moreover, in vivo anti-KIR mAb treatment induced the killing of HLA(+) target cells without breaking self-tolerance. Finally, the long-term infusion of anti-KIR mAb neither abolished NK cell education nor tumor cell recognition. Therefore, these results strongly support the use of inhibitory receptor blockade in cancer patients.

Umbilical cord blood T cells express multiple natural cytotoxicity receptors after IL-15 stimulation, but only NKp30 is functional

The natural cytotoxicity receptors (NCRs) NKp30, NKp44, and NKp46 are thought to be NK lineage restricted. Herein we show that IL-15 induces NCR expression on umbilical cord blood (UCB) T cells. NCRs were mainly on CD8(+) and CD56(+) UCB T cells. Only NKp30 was functional as demonstrated by degranulation, IFN-gamma release, redirected killing, and apoptosis. Since NCRs require adaptor proteins for function, the expressions of these adaptors were determined. The adaptors used by NKp30 and NKp46, FcepsilonR1gamma and CD3zeta, were detected in UCB T cells. There was a near absence of DAP12, the adaptor for NKp44, consistent with a hypofunctional state. NKp46 was on significantly fewer UCB T cells, possibly accounting for its lack of function. Adult peripheral blood (PB) T cells showed minimal NCR acquisition after culture with IL-15. Since UCB contains a high frequency of naive T cells, purified naive T cells from adult PB were tested. Although NKp30 was expressed on a small fraction of naive PB T cells, it was nonfunctional. In contrast to UCB, PB T cells lacked FcepsilonR1gamma expression. These results demonstrate differences between UCB and PB T cells regarding NCR expression and function. Such findings challenge the concept that NCRs are NK cell specific.

Umbilical cord blood T cells express multiple natural cytotoxicity receptors after IL-15 stimulation, but only NKp30 is functional

The natural cytotoxicity receptors (NCRs) NKp30, NKp44, and NKp46 are thought to be NK lineage restricted. Herein we show that IL-15 induces NCR expression on umbilical cord blood (UCB) T cells. NCRs were mainly on CD8(+) and CD56(+) UCB T cells. Only NKp30 was functional as demonstrated by degranulation, IFN-gamma release, redirected killing, and apoptosis. Since NCRs require adaptor proteins for function, the expressions of these adaptors were determined. The adaptors used by NKp30 and NKp46, FcepsilonR1gamma and CD3zeta, were detected in UCB T cells. There was a near absence of DAP12, the adaptor for NKp44, consistent with a hypofunctional state. NKp46 was on significantly fewer UCB T cells, possibly accounting for its lack of function. Adult peripheral blood (PB) T cells showed minimal NCR acquisition after culture with IL-15. Since UCB contains a high frequency of naive T cells, purified naive T cells from adult PB were tested. Although NKp30 was expressed on a small fraction of naive PB T cells, it was nonfunctional. In contrast to UCB, PB T cells lacked FcepsilonR1gamma expression. These results demonstrate differences between UCB and PB T cells regarding NCR expression and function. Such findings challenge the concept that NCRs are NK cell specific.