Regulation of T cell function by NK cell receptors for classical MHC class I molecules

Exceptional diversity of KIR and HLA class I in Egypt

Genetically determined variation of killer cell immunoglobulin like receptors (KIR) and their HLA class I ligands affects multiple aspects of human health. Their extreme diversity is generated through complex interplay of natural selection for pathogen resistance and reproductive health, combined with demographic structure and dispersal. Despite significant importance to multiple health conditions of differential effect across populations, the nature and extent of immunogenetic diversity is under-studied for many geographic regions. Here, we describe the first high-resolution analysis of KIR and HLA class I combinatorial diversity in Northern Africa. Analysis of 125 healthy unrelated individuals from Cairo in Egypt yielded 186 KIR alleles arranged in 146 distinct centromeric and 79 distinct telomeric haplotypes. The most frequent haplotypes observed were KIR-A, encoding two inhibitory receptors specific for HLA-C, two that are specific for HLA-A and -B, and no activating receptors. Together with 141 alleles of HLA class I, 75 of which encode a KIR ligand, we identified a mean of six distinct interacting pairs of inhibitory KIR and HLA allotypes per individual. We additionally characterize 16 KIR alleles newly identified in the study population. Our findings place Egyptians as one of the most highly diverse populations worldwide, with important implications for transplant matching and studies of immune-mediated diseases.

Induced CD45 Proximity Potentiates Natural Killer Cell Receptor Antagonism

Natural killer (NK) cells are a major subset of innate immune cells that are essential for host defense against pathogens and cancer. Two main classes of inhibitory NK receptors (NKR), KIR and CD94/NKG2A, play a key role in suppressing NK activity upon engagement with tumor cells or virus-infected cells, limiting their antitumor and antiviral activities. Here, we find that single-chain NKR antagonists linked to a VHH that binds the cell surface phosphatase CD45 potentiate NK and T activities to a greater extent than NKR blocking antibodies alone in vitro. We also uncovered crosstalk between NKG2A and Ly49 that collectively inhibit NK cell activation, such that CD45-NKG2A and CD45-Ly49 bispecific molecules show synergistic effects in their ability to enhance NK cell activation. The basis of the activity enhancement by CD45 ligation may reflect greater antagonism of inhibitory signaling from engagement of MHC I on target cells, combined with other mechanisms, including avidity effects, tonic signaling, antagonism of weak inhibition from engagement of MHC I on non-target cells, and possible CD45 segregation within the NK cell-target cell synapse. These results uncover a strategy for enhancing the activity of NK and T cells that may improve cancer immunotherapies.

MHC/HLA Class I Loss in Cancer Cells

In this chapter I describe Tumour Immune Escape mechanisms associated with MHC/HLA class I loss in human and experimental tumours. Different altered HLA class-I phenotypes can be observed that are produced by different molecular mechanisms. Experimental and histological evidences are summarized indicating that at the early stages of tumour development there is an enormous variety of tumour clones with different MHC class I expression patterns. This phase is followed by a strong T cell mediated immune-selection of MHC/HLA class-I negative tumour cells in the primary tumour lesion. This transition period results in a formation of a tumour composed only of HLA-class I negative cells. An updated description of this process observed in a large variety of human tumors is included. In the second section I focus on MHC/HLA class I alterations observed in mouse and human metastases, and describe the generation of different tumor cell clones with altered MHC class I phenotypes, which could be similar or different from the original tumor clone. The biological and immunological relevance of these observations is discussed. Finally, the interesting phenomenon of metastatic dormancy is analyzed in association with a particular MHC class I negative tumor phenotype.

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.

Study of genetic diversity of killer cell immunoglobulin-like receptor loci in the Tujia ethnic minority

The aim of this study was to analyze the genetic profiles of 14 killer cell immunoglobulin-like receptor (KIR) genes and 2 pseudogenes of 124 individuals from Tujia ethnic minority residing in Enshi Tujia and Miao autonomous prefecture of Hubei province of China and investigate the genetic relationships between the Tujia ethnic minority and other reported groups for the first time. Sequence specific primer amplification (PCR-SSP) methods were used to genotype the 14 KIR genes and 2 pseudogenes. The observed carrier frequencies (OF) and the gene frequencies (GF) of the KIR genes were measured. Neighbor-joining (N-J) tree and the principal component analysis (PCA) plot were constructed. All individuals were typed positive for the three framework loci KIR3DL3, 2DL4 and 3DL2, as well as for pseudogene KIR3DP1. The gene frequencies of the other KIR genes ranged from 9% in KIR2DS2 to 98% in KIR2DP1 and KIR3DL1. The present study of the KIR genes may be a powerful tool for enriching the Chinese ethnical gene information resources of the KIR gene pool, as well as for the anthropological research.

Increased numbers but functional defects of CD56+CD3+ cells in lung cancer

CD56+ T cells were studied in samples of peripheral blood from small-cell lung cancer (SCLC) and non-small-cell lung cancer (NSCLC) patients compared with healthy controls. Relative numbers of CD56+CD3+ cells were increased in NSCLC (P = 0.001) and SCLC (P = 0.002) compared with normal subjects but their ability to respond to activation by up-regulating CD25 or producing IFN-γ were both significantly impaired. Expression of the killer-immunoglobulin-like receptor CD158a was significantly lower on CD56+CD3+ cells in SCLC than controls and also in early stage compared with late stage NSCLC patients. Mean levels of CD158e were higher in NSCLC patients than controls. CD158e levels on CD56+CD3+ cells were increased in the presence of its ligand HLA-Bw4 compared with controls. Although the precise role of CD56+CD3+ cells is not clear, they appear to be functionally impaired in lung cancer, which may have implications for a reduction of direct or indirect anti-tumour responses.

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.

IL-15 induces CD8+ T cells to acquire functional NK receptors capable of modulating cytotoxicity and cytokine secretion

During the last years several authors have described a small population of CD8+ T cells expressing NK receptors (NKRs). Although their origin remains largely unknown, we have recently demonstrated that IL-15 is capable of inducing NKR expression in purified human CD8+CD56- T cells. In this study we show that IL-15-driven NKR induction in CD8+ T cells was linked with CD56 de novo acquisition, consistent with an effector-memory phenotype, increased anti-apoptotic levels, high granzyme B/perforin expression and with the ability of displaying in vitro NK-like cytotoxicity. Interestingly, dissection of NKR functional outcome in IL-15-cultured CD8+ T cells revealed: (i) that NKG2D cross-linking was able per se to upregulate degranulation levels and (ii) that KIR and NKG2A cross-linking upregulated secretion of cytokines such as IFN-γ, TNF-α, IL-1β and IL-10. These results suggest that IL-15 is capable of differentiating CD8+ T cells into NK-like T cells displaying a regulatory phenotype.

Inhibitory and regulatory immune synapses

Cell contact-dependent inhibition and regulation of immune responses play an essential role in balancing the need for rapid and efficient responses to a wide variety of pathological challenges, while at the same time maintaining self-tolerance. Much attention has been given to immune synapses that lead to the activation of, for example, cell-mediated cytotoxicity, and here we compare the supramolecular dynamics of synapses that lead to inhibition or regulatory functions. We focus on natural killer cells where such different synapses have been best studied. An emergent principle is that inhibition or regulatory responses are commonly achieved by selective recruitment of signalling proteins to the synapse and exclusion of membrane-proximal intracellular proteins needed for activation. We also discuss evidence that an inhibitory synapse triggers or maintains effector cells in a migratory configuration, which serves to break the synapse before the steps needed for effector cell activation can be completed. This model implies that the concept of kinetic-proofreading, previously used to describe activation of individual T-cell receptors, can also apply in determining the outcome of intercellular conjugation.

HLA class I molecules regulate IFN-gamma production induced in NK cells by target cells, viral products, or immature dendritic cells through the inhibitory receptor ILT2/CD85j

Recent advances support an important role for NK cells in determining immune responses beyond their cytolytic functions, which is supported by their capacity to secrete several cytokines and chemokines. In particular, NK-derived IFN-gamma has proven to be fundamental in shaping adaptive immune responses. Although the role of inhibitory NK receptors (iNKR) in the regulation of cytotoxicity has been widely explored, their involvement in the control of cytokine production has been scarcely analyzed. Specifically, no data are available referring to the role of the iNKR ILT2/CD85j in the regulation of IFN-gamma secretion by NK cells. Published data support a differential regulation of cytotoxicity and cytokine expression. Thus, formal proof of the involvement of HLA class I in regulating the production of cytokines through binding to ILT2/CD85j has been missing. We have determined the response of human NK-92 and primary human ILT2/CD85j(+) NK cells from healthy donors to target cells expressing or not HLA class I. We found specificities of HLA class I-mediated inhibition of IFN-gamma mRNA expression, protein production, and secretion consistent with the specific recognition by ILT2/CD85j. We also found inhibition of IFN-gamma production by ILT2/CD85j(+) T cells in response to superantigen stimulation. Furthermore, ligation of ILT2/CD85j inhibited the production of IFN-gamma in response to poly(I:C), and blocking of ILT2/CD85j-HLA class I interactions increased the secretion of IFN-gamma in NK/immature dendritic cell cocultures. The data support a role for self HLA class I in the regulation of IFN-gamma secretion at the mRNA and protein levels by interacting with the iNKR ILT2/CD85j.

Antiviral NK cell responses in HIV infection: II. viral strategies for evasion and lessons for immunotherapy and vaccination

As is the case in other viral infections, humans respond to HIV infection by activating their NK cells. However, the virus uses several strategies to neutralize and evade the host's NK cell responses. Consequently, it is not surprising that NK cell functions become compromised in HIV-infected individuals in early stages of the infection. The compromised NK cell functions also adversely affect several aspects of the host's antiviral adaptive immune responses. Researchers have made significant progress in understanding how HIV counters NK cell responses of the host. This knowledge has opened new avenues for immunotherapy and vaccination against this infection. In the first part of this review article, we gave an overview of our current knowledge of NK cell biology and discussed how the genes encoding NK cell receptors and their ligands determine innate genetic resistance/susceptibilty of humans against HIV infections and AIDS. In this second part, we discuss NK cell responses, viral strategies to counter these responses, and finally, their implications for anti-HIV immunotherapy and vaccination.

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.

The microglial networks of the brain and their role in neuronal network plasticity after lesion

Microglia are the resident inflammatory cells of the central nervous system (CNS) extending a network of processes in the CNS parenchyma. Following axon lesion to neurons, most extensively studied in motoneurons, there is a typical retrograde response at the cell body level, including the removal or 'stripping' of synapses from the perikaryon and dendrites of affected cells. Microglia have been attributed a main and active role in this process, although also an involvement of activated astrocytes has been suggested. The signaling pathways for this 'synaptic stripping' have so far been unknown, but recently some classical immune recognition molecules, the MHC class I molecules, have been shown to have a strong influence on the strength and pattern of the synapse elimination response. Since there is an expression of MHC class I in both neurons and glia, in particular microglia, as well as MHC class I related receptors in axons and microglia, there are good reasons to believe that classical immune recognition signaling between neurons and glia underlies part of the 'stripping' response. A role for microglia in an interplay with synapses based on this type of signaling is further exemplified by the fact that, in the absence of some MHC class I related receptors normally found on microglia during development, profound effects on synaptic function and biochemistry have been demonstrated. Such effects may be linked to the development of various disorders of the CNS, such as degenerative disease.

Differential induction of CD94 and NKG2 in CD4 helper T cells. A consequence of influenza virus infection and interferon-gamma?

Influenza A virus causes worldwide epidemics and pandemics and the investigation of memory T helper (Th) cells that help maintain serological memory following infection is important for vaccine design. In this study we investigated CD94 and NKG2 gene expression in memory CD4 T-cell clones established from the spleens of C57BL/10 (H-2(b)) and BALB/c (H-2(d)) mice infected with influenza A virus (H3N2). CD94 and NKG2A/C/E proteins form heterodimeric membrane receptors that are involved in virus recognition. CD94 and NKG2 expression have been well characterized in natural killer (NK) and cytotoxic T cells. Despite CD94 being potentially an important marker for Th1 cells involved in virus infection, however, there has been little investigation of its expression or function in the CD4 T-cell lineage and no studies have looked at in-vivo-generated Th cells or memory cells. We show in this study that in-vivo-generated CD4 Th1 cells, but not Th2 cells, exhibited full-length CD94 and NKG2A gene expression following activation with viral peptide. For NKG2A, a novel 'short' (possibly redundant) truncated isoform was detectable in a Th2 cell clone. Another member of the NK receptor family, NKG2D, but not NKG2C or E, was also differentially expressed in Th1 cells. We show here that CD94 and NKG2A may exist as multiple isoforms with the potential to distinguish helper T-cell subsets.

Amoxicillin conjugates to HLA class I molecules and interferes with signalling through the ILT2/LIR-1/CD85j inhibitory receptor

BACKGROUND

Drugs behave as haptens and are recognized by specific T-cell receptors in the context of major histocompatibility complex (MHC) molecules in allergic subjects. Natural killer cell receptors (NKRs) are MHC class I-specific receptors that modulate the threshold of activation of immunocompetent cells. Amongst them, ILT2/LIR-1/CD85j is an inhibitory NKR widely distributed in several cell lineages and with a broad spectrum of recognition of human leucocyte antigen (HLA) class I ligands.

METHODS

We have evaluated, at the biochemical and cellular level, the ability of amoxicillin (AX) conjugate to HLA class I molecules and to interfere with the inhibitory signal delivered by the HLA class I receptor ILT2/LIR-1/CD85j.

RESULTS

We have detected AX bound to cell membrane proteins and in particular to HLA class I molecules. Preincubation with AX rendered target cells susceptible to NK cell-mediated lysis. In conjugation experiments, target cell-bound AX hampered tyrosine phosphorylation of ILT2/LIR-1/CD85j upon ligand recognition and the subsequent recruitment of SHP-1 phosphatase.

CONCLUSION

Conjugation of AX to HLA class I molecules may mask HLA recognition by inhibitory receptors and attenuate the negative signal delivered by SHP-1 phosphatase, thus lowering the threshold of activation of effector cells.

Lymphocyte activation in response to melanoma: interaction of NK-associated receptors and their ligands

In recent years, studies on the molecular and cellular mechanisms of immune responses against melanoma have contributed to a better understanding of how these tumours can be recognised by cytotoxic cells and the mechanisms they have developed to escape from innate and adaptive immunity. Lysis of melanoma cells by natural killer (NK) cells and cytolytic T cells is the result of a fine balance between signals transmitted by activating and inhibitory receptors. In addition to the T cell receptor, these were initially described as NK cell-associated receptors (NKRs) and were later also found on subsets of T lymphocytes, particularly effector-memory and terminally differentiated CD8 T cells. An increase of NKR(+)CD8(+) T cells has been found in melanoma patients, correlating with the expansion of differentiated effector CD8(+)CD28(null) CD27(null) T cells. NKRs can regulate the lysis of target cells expressing appropriate ligands. Activating receptors recognise ligands on tumours whereas inhibitory receptors are specific for MHC class I antigens and sense missing self. Altered expression of MHC class I antigens is frequently found on melanoma cells, preventing recognition by specific cytolytic T cells but favouring NK cell recognition. Changes in the expression of NKR-ligands in melanoma contribute in explaining the differences in the capacity of cytotoxic immune cells to control melanoma growth and dissemination.

Reverse immunology approach for the identification of CD8 T-cell-defined antigens: advantages and hurdles

One of the challenges of tumour immunology remains the identification of strongly immunogenic tumour antigens for vaccination. Reverse immunology, that is, the procedure to predict and identify immunogenic peptides from the sequence of a gene product of interest, has been postulated to be a particularly efficient, high-throughput approach for tumour antigen discovery. Over one decade after this concept was born, we discuss the reverse immunology approach in terms of costs and efficacy: data mining with bioinformatic algorithms, molecular methods to identify tumour-specific transcripts, prediction and determination of proteasomal cleavage sites, peptide-binding prediction to HLA molecules and experimental validation, assessment of the in vitro and in vivo immunogenic potential of selected peptide antigens, isolation of specific cytolytic T lymphocyte clones and final validation in functional assays of tumour cell recognition. We conclude that the overall low sensitivity and yield of every prediction step often requires a compensatory up-scaling of the initial number of candidate sequences to be screened, rendering reverse immunology an unexpectedly complex approach.

CD8 T cells expressing NK associated receptors are increased in melanoma patients and display an effector phenotype

CD8+ T cells can express NK-associated receptors (NKRs) that may regulate their cytolytic function. We have characterized the expression of several NKRs on peripheral blood CD8+ T cells from melanoma patients and compared them to age-matched healthy donors. The analysis performed includes HLA class I specific receptors (KIRs, LILRB1 and CD94/NKG2) and other NK receptors like CD57, CD56 and CD16. Melanoma patients showed a higher variability in the expression of NKRs on circulating CD8+ T cells than age-matched healthy donors. NKR expression on CD8+ T cells from melanoma patients showed a significant increase of KIR2DL2/L3/S2 (mAb gl183), CD244, CD57, CD56 and CD16. We have also found an increase of CD8+ CD28- CD27- T cells in melanoma patients. This subset represents terminally differentiated effector cells expressing CD244 and high levels of perforin. The expression of NKRs was also mainly restricted to this T cell subset. Altogether, circulating CD8+ T cells from melanoma patients display a distinct phenotype characterized by downregulation of costimulatory molecules and higher expression of NKRs. We suggest that the increased expression of NKRs on T cells may contribute to the final outcome of the immune response against melanoma both stimulating or inhibiting activation and differentiation to effector cells. Blocking inhibitory receptor function and enhancing activating receptors may represent new strategies with therapeutic potential against melanoma.

NK cells play a critical role in the regulation of class I-deficient hemopoietic stem cell engraftment: evidence for NK tolerance correlates with receptor editing

The role that NK cells play in the rejection of hemopoietic stem cell (HSC) and tolerance induction has remained controversial. In this study, we examined whether NK cells play a direct role in the rejection of HSC. Purified HSC from MHC class II-deficient mice engrafted readily in congenic mice, while HSC from class I-deficient donors (beta(2)-microglobulin(-/-) (beta(2)m(-/-))) failed to engraft. Recipient mice lacking CD8(+), CD4(+), or T cells also rejected HSC from class I-deficient donors, pointing directly to NK cells as the effector in rejection of HSC. Recipients, deficient in or depleted of NK cells, engrafted readily with beta(2)m(-/-) HSC. Expression of the activating Ly-49D and inhibitory Ly-49G2 receptors on recipient NK cells was significantly decreased in these beta(2)m(-/-)-->B6 chimeras, and the proportion of donor NK cells expressing Ly-49D was also significantly decreased. Notably, beta(2)m(-/-) chimeras accepted beta(2)m(-/-) HSC in second transplants, demonstrating that NK cells in the chimeras had been tolerized to beta(2)m(-/-). Taken together, our data demonstrate that NK cells play a direct role in the regulation of HSC engraftment, and down-regulation and/or deletion of specific NK subsets in mixed chimeras can contribute to the induction of NK cell tolerance in vivo. Moreover, our data show that bone marrow-derived elements significantly contribute to NK cell development and tolerance.

Negative regulation of T cell receptor signaling by Siglec-7 (p70/AIRM) and Siglec-9

Siglec-7 (p70/AIRM) and Siglec-9 are "CD33"-related siglecs expressed on natural killer (NK) cells and subsets of peripheral T cells. Like other inhibitory NK cell receptors, they contain immunoglobulin receptor family tyrosine-based inhibitory motifs in their cytoplasmic domains, and Siglec-7 has been demonstrated to negatively regulate NK cell activation. Based on reports of the presence of these siglecs on T cells, we sought to determine if they are capable of modulating T cell receptor (TCR) signaling using Jurkat T cells stably and transiently transfected with Siglec-7 or Siglec-9. Following either pervanadate stimulation or TCR engagement, both Siglecs exhibited increased tyrosine phosphorylation and recruitment of SHP-1. Effects of Siglec-7 and -9 were also evident in downstream events in the signaling pathway. Both siglecs reduced phosphorylation of Tyr319 on ZAP-70, known to play a pivotal role in up-regulation of gene transcription following TCR stimulation. There was also a corresponding decreased transcriptional activity of nuclear factor of activated T cells (NFAT) as determined using a luciferase reporter gene. Like all siglecs, Siglec-7 and -9 recognize sialic acid-containing glycans of glycoproteins and glycolipids as ligands. Mutation of the conserved Arg in the ligand binding site of Siglec-7 (Arg124) or Siglec-9 (Arg120) resulted in reduced inhibitory function in the NFAT/luciferase transcription assay, suggesting that ligand binding is required for optimal inhibition of TCR signaling. The combined results demonstrate that both Siglec-7 and Siglec-9 are capable of negative regulation of TCR signaling and that ligand binding is required for optimal activity.

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.

PD-L1/B7H-1 inhibits the effector phase of tumor rejection by T cell receptor (TCR) transgenic CD8+ T cells

Although increased circulating tumor antigen-specific CD8(+) T cells can be achieved by vaccination or adoptive transfer, tumor progression nonetheless often occurs through resistance to effector function. To develop a model for identifying mechanisms of resistance to antigen-specific CTLs, poorly immunogenic B16-F10 melanoma was transduced to express the K(b)-binding peptide SIYRYYGL as a green fluorescent protein fusion protein that should be recognized by high-affinity 2C TCR transgenic T cells. Although B16.SIY cells expressed high levels of antigen and were induced to express K(b) in response to IFN-gamma, they were poorly recognized by primed 2C/RAG2(-/-) T cells. A screen for candidate inhibitory ligands revealed elevated PD-L1/B7H-1 on IFN-gamma-treated B16-F10 cells and also on eight additional mouse tumors and seven human melanoma cell lines. Primed 2C/RAG2(-/-)/PD-1(-/-) T cells showed augmented cytokine production, proliferation, and cytolytic activity against tumor cells compared with wild-type 2C cells. This effect was reproduced with anti-PD-L1 antibody present during the effector phase but not during the priming culture. Adoptive transfer of 2C/RAG2(-/-)/PD-1(-/-) T cells in vivo caused tumor rejection under conditions in which wild-type 2C cells or CTLA-4-deficient 2C cells did not reject. Our results support interfering with PD-L1/PD-1 interactions to augment the effector function of tumor antigen-specific CD8(+) T cells in the tumor microenvironment.

Pure red cell aplasia associated with expansion of CD3+ CD8+ granular lymphocytes expressing cytotoxicity against HLA-E+ cells

T-cell granular lymphocyte-proliferative disorder (T-GLPD) is characterized by the proliferation of cytotoxic T lymphocytes, and is often associated with pure red cell aplasia (PRCA). The mechanism involved in the development of PRCA in T-GLPD is unknown. Peripheral blood mononuclear cells were isolated from 20 patients with T-GLPD. Ten patients had associated PRCA. Granular lymphocytes (GLs) of T-GLPD are positive for CD94, but not NKG2A. To clarify the functional role of CD94 in T-GLPD, we performed a cytotoxicity assay against the trophoblast cell line, BeWo, which is known to express human leucocyte antigen (HLA)-E, a natural ligand of CD94, and is deficient in other HLA class I and class II antigens. GLs isolated from T-GLPD with PRCA patients killed BeWo cells more significantly than GLs from T-GLPD without PRCA patients. Furthermore, GLs from T-GLPD with PRCA were significantly stimulated by a monoclonal antibody against CD94, whereas those of T-GLPD without PRCA were not. Taken together, HLA-E, a ligand of CD94, was suggested to stimulate CD94+ cells to kill HLA-E+ cells in T-GLPD with PRCA. GLs of T-GLPD with PRCA have a potential positive activity against HLA-E+ cells, whereas GLs from T-GLPD without PRCA do not. CD94 may play a key role in the pathogenesis of PRCA in T-GLPD.

A peptide from heat shock protein 60 is the dominant peptide bound to Qa-1 in the absence of the MHC class Ia leader sequence peptide Qdm

The MHC class Ib molecule Qa-1 binds specifically and predominantly to a single 9-aa peptide (AMAPRTLLL) derived from the leader sequence of many MHC class Ia proteins. This peptide is referred to as Qdm. In this study, we report the isolation and sequencing of a heat shock protein 60-derived peptide (GMKFDRGYI) from Qa-1. This peptide is the dominant peptide bound to Qa-1 in the absence of Qdm. A Qa-1-restricted CTL clone recognizes this heat shock protein 60 peptide, further verifying that it binds to Qa-1 and a peptide from the homologous Salmonella typhimurium protein GroEL (GMQFDRGYL). These observations have implications for how Qa-1 can influence NK cell and T cell effector function via the TCR and CD94/NKG2 family members, and how this effect can change under conditions that cause the peptides bound to Qa-1 to change.

Ligand-dependent inhibition of CD1d-restricted NKT cell development in mice transgenic for the activating receptor Ly49D

In addition to their CD1d-restricted T cell receptor (TCR), natural killer T (NKT) cells express various receptors normally associated with NK cells thought to act, in part, as modulators of TCR signaling. Immunoreceptor-tyrosine activation (ITAM) and inhibition (ITIM) motifs associated with NK receptors may augment or attenuate perceived TCR signals respectively, potentially influencing NKT cell development and function. ITIM-containing Ly49 family receptors expressed by NKT cells are proposed to play a role in their development and function. We have produced mice transgenic for the ITAM-associated Ly49D and ITIM-containing Ly49A receptors and their common ligand H2-Dd to determine the importance of these signaling interplays in NKT cell development. Ly49D/H2-Dd transgenic mice had selectively and severely reduced numbers of thymic and peripheral NKT cells, whereas both ligand and Ly49D transgenics had normal numbers of NKT cells. CD1d tetramer staining revealed a blockade of NKT cell development at an early precursor stage. Coexpression of a Ly49A transgene partially rescued NKT cell development in Ly49D/H2-Dd transgenics, presumably due to attenuation of ITAM signaling. Thus, Ly49D-induced ITAM signaling is incompatible with the early development of cells expressing semi-invariant CD1d-restricted TCRs and appropriately harmonized ITIM-ITAM signaling is likely to play an important role in the developmental program of NKT cells.

Prolongation of allogeneic skin graft survival by injection of anti-Ly49A monoclonal antibody YE1/48

Ly49A receptors expressed on NK, NKT, and T cells play inhibitory roles in regulating the immune responses in vivo and in vitro. Whether or not injection of anti-Ly49A monoclonal antibody (mAb) YE1/48 can block allograft rejection has not been evaluated. Balb/c mouse recipients received intraperitoneal injections of YE1/48 mAb (0.5 mg) or control mAb or phosphate-buffered saline on days -1 and 10. On day 0, fully MHC-mismatched allogeneic C57BL/6 (B6) skin grafts were implanted. The skin graft survival and anti-donor humoral responses were observed. Whereas allogeneic B6 skin grafts survived 14 days in isotopy control antibody-treated or nontreated Balb/c mice, injection of YE1/48 mAb significantly prolonged B6 skin graft survival to 19 days (P < 0.0005). Injection of YE1/48 mAb into presensitized Balb/c recipients did not significantly delay B6 skin graft rejection. On the other hand, after depleting recipient NK, NKT, and some cytotoxic T cells by injection of anti-asialo GM1, YE1/48 failed to prolong B6 skin graft survival in Balb/c recipients. The present studies indicate that injection of YE1/48 mAb significantly delays allogeneic skin graft rejection in nonsensitized recipients but not in sensitized recipients. The presence of NK, NKT cells, and some cytotoxic T cells may be essential for YE1/48-mediated immunosuppression in vivo.

Competition between MHC class I alleles for cell surface expression alters CTL responses to influenza A virus

Mammalian cells express up to six different MHC class I alleles, many of which differ in terms of their interaction with components of the Ag presentation pathway and level of cell surface expression. However, it is often assumed in Ag presentation studies that class I alleles function independently of each other. We have compared cell surface expression levels and function of MHC class I molecules in F(1) hybrid mice with those in the homozygous parental strains. The level of cell surface expression of certain alleles in F(1) mice differed significantly from 50% of that found on the same cell type in the corresponding parental strain, suggesting allele-specific competition for cell surface expression, and not expression solely according to gene dosage. The strongest effect was observed in H-2(b) x H-2(k) F(1) mice, in which the H-2(b) class I molecules dominated over the H-2(k) class I molecules. The magnitude of H-2(k)-restricted CTL responses to influenza A virus infection was similar in the F(1) hybrid and parental H-2(k) mice. However, in H-2(k) mice expressing a K(b) transgene, cell surface levels of the endogenous class I molecules were down-regulated to a greater degree than in F(1) hybrid mice, and H-2(k)-restricted CTL responses against influenza A virus were greatly reduced, although the CTL repertoire was apparently present. Therefore, certain MHC class I molecules compete with each other for cell surface expression, and the resulting low cell surface expression of specific alleles can lead to a severe reduction in the ability to generate a CTL response.

Dendritic cells for NK/LAK activation: rationale for multicellular immunotherapy in neuroblastoma patients

Natural killer (NK)/lymphokine-activated killer (LAK) cell-based immunotherapy could be beneficial against major histocompatibility complex class I-negative tumor residual disease such as neuroblastoma (NB), provided that interleukin 2 (IL-2) or surrogate nontoxic NK cell stimulatory factors could sustain NK cell activation and survival in vivo. Here we show that human monocyte-derived dendritic cells (MD-DCs) promote potent NK/LAK effector functions and long-term survival, circumventing the need for IL-2. This study demonstrates (1) the feasibility of differentiating granulocyte colony-stimulating factor-mobilized hematopoietic peripheral blood stem cells (PBSCs) into high numbers of functional MD-DCs and NK/LAK cells in a series of 12 children with stage 4 neuroblastoma (NB); (2) potent DC-mediated NK cell activation in autologous settings; (3) the reciprocal capacity of NK/LAK cells to turn immature DCs into maturing cells electively capable of triggering NK cell functions; and (4) the unique capacity of maturing DCs to sustain NK cell survival, superior to that achieved in IL-2. These data show a reciprocal interaction between DCs and NK/LAK cells, leading to the amplification of NK cell effector functions, and support the implementation of DC/NK cell-based immunotherapy for purging the graft and/or controlling minimal residual disease after autologous stem cell transplantation.

Molecular Mechanism of the Activation-Induced Cell Death Inhibition Mediated by a p70 Inhibitory Killer Cell Ig-Like Receptor in Jurkat T Cells

In this study we investigated the molecular mechanism of the activation-induced cell death (AICD) inhibition mediated by a p70 inhibitory killer cell Ig-like receptor (KIR3DL1, also called NKB1) in Jurkat T cells. Using stable Jurkat transfectants that express KIR or CD8-KIR fusion proteins we have shown for the first time that KIR inhibits, in a ligation-independent manner, the AICD induced by PHA, PMA/ionomycin, or anti-CD3 Ab. The AICD inhibition mediated by KIR appears to result from the blockade of Fas ligand induction upon activation of the Jurkat transfectants. Moreover, the membrane-proximal 20 aa of the KIR cytoplasmic tail were determined to play a crucial role in this process. Since the membrane-proximal portion of the KIR cytoplasmic tail contains a putative protein kinase C (PKC) substrate site, we investigated the molecular interaction between KIR and PKC. Immunoprecipitation analysis demonstrated that KIR constitutively bound both to PKCalpha, a conventional Ca(2+)-dependent PKC, and to PKCtheta, a novel Ca(2+)-independent PKC. Furthermore, an in vitro kinase assay revealed that PKC activation was blocked after PHA stimulation in Jurkat transfectants expressing KIR. These observations were supported by the finding that a recombinant KIR cytoplasmic tail also appeared to inhibit PKCalpha activation in vitro. Taken together these data strongly suggest that KIR inhibits the AICD of T cells by blocking Fas ligand induction upon stimulation, in a process that seems to be accomplished by PKC recruitment to the membrane-proximal PKC binding site and subsequent inhibition of PKC activation against the activating stimuli.

Impaired effector function of hepatitis C virus-specific CD8+ T cells in chronic hepatitis C virus infection

The cellular immune response contributes to clearance of hepatitis C virus (HCV) and persists for decades after recovery from infection. The immunological basis for the inefficiency of the cellular immune response in chronically infected persons is not known. Here, we used four HLA-A2 tetramers, specific for two HCV core and two HCV NS3 epitopes, to investigate at the single-cell level effector function and phenotype of HCV-specific CD8+ T cells in 20 chronically infected and 12 long-term recovered patients. Overall, HCV-specific, tetramer+ T cells were more frequently found in PBMCs of chronically infected patients than in those of recovered patients. However, when compared with HCV-tetramer+ T cells of recovered patients, they displayed an impaired proliferative capacity. As a result of the impaired proliferative capacity, HCV-specific T cell lines derived from chronically infected patients displayed less peptide-specific cytotoxicity than those from recovered patients. In addition, proliferation and ex vivo IFN-gamma production of HCV-tetramer+ cells, but not influenza-virus-specific T cells, were defective in chronically infected patients and could not be restored by in vitro stimulation with peptide and IL-2. At least three distinct phenotypes of HCV-specific CD8+ T cells were identified and associated with certain functional characteristics. In addition, impairment of proliferative, cytokine, and cytotoxic effector functions of tetramer+ T cells in viremic patients was associated with weak ex vivo HCV-specific CD4+ T cell responses. Thus, the defective functions of HCV-specific CD8+ T cells might contribute to viral persistence in chronically infected patients, and knowledge on their reversibility may facilitate the development of immunotherapeutic vaccines.

Viral and bacterial infections induce expression of multiple NK cell receptors in responding CD8(+) T cells

NK cells express several families of receptors that play central roles in target cell recognition. These NK cell receptors are also expressed by certain memory phenotype CD8(+) T cells, and in some cases are up-regulated in T cells responding to viral infection. To determine how the profile of NK receptor expression changes in murine CD8(+) T cells as they respond to intracellular pathogens, we used class I tetramer reagents to directly examine Ag-specific T cells during lymphocytic choriomeningitis virus and Listeria monocytogenes infections. We found that the majority of pathogen-specific CD8(+) T cells initiated expression of the inhibitory CD94/NKG2A heterodimer, the KLRG1 receptor, and a novel murine NK cell marker (10D7); conversely, very few Ag-specific T cells expressed Ly49 family members. The up-regulation of these receptors was independent of IL-15 and persisted long after clearance of the pathogen. The expression of CD94/NKG2A was rapidly initiated in naive CD8(+) T cells responding to peptide Ags in vitro and on many of the naive T cells that proliferate when transferred into lymphopenic (Rag-1(-/-)) hosts. Thus, CD94/NKG2A expression is a common consequence of CD8(+) T cell activation. Binding of the CD94/NKG2A receptor by its ligand (Qa-1(b)) did not significantly inhibit CD8(+) T cell effector functions. However, expression of CD94 and NKG2A transgenes partially inhibited early events of T cell activation. These subtle effects suggest that CD94/NKG2A-mediated inhibition of T cells may be limited to particular circumstances or may synergize with other receptors that are similarly up-regulated.

Expression of CD94/NKG2-A on human T lymphocytes is induced by IL-12: implications for adoptive immunotherapy

NK cell receptors (NKRs) are expressed on a subset of human T cells, predominantly CD8(+), within which they can modulate TCR-mediated functions. In an attempt to identify the mechanisms leading to NKR expression, we analyzed the capacity of IL-12 to modulate the expression by T cells of the components of the CD94/NKG2-A inhibitory receptor, a member of the C-type lectin-like family of NKR. We show that IL-12 induces the expression of NKG2-A and/or CD94 by CD8(+) T cells in culture, and that this induction was mediated neither by IFN-gamma nor by IL-15. We also show, using the redirected killing assay, that IL-12-induced expression of both CD94 and NKG2-A led to the acquisition by T cells of a functional inhibitory receptor. Expression of the CD94/NKG2-A inhibitory receptor was also induced by IL-12 during T cell Ag stimulation so that in the presence of this cytokine a high proportion of melanoma-reactive CTL induced from PBL by melanoma peptide stimulation expressed this receptor. This study emphasizes the implication of IL-12 in the modulation of immune responses through NKR induction.

CD94-NKG2A receptors regulate antiviral CD8(+) T cell responses

CD8(+) T lymphocytes mediate immunosurveillance against persistent virus infections and virus-induced neoplasia. Polyoma virus, a highly oncogenic natural mouse DNA virus, establishes persistent infection, but only a few mice are highly susceptible to tumors induced by the virus. Mature antiviral CD8(+) T cells expand in tumor-susceptible mice, but their cytotoxic effector activity is nonfunctional in vivo. Here we show that the natural killer cell inhibitory receptor, CD94-NKG2A, is up-regulated by antiviral CD8(+) T cells during acute polyoma infection and is responsible for down-regulating their antigen-specific cytotoxicity during both viral clearance and virus-induced oncogenesis.

Structure and function of major histocompatibility complex (MHC) class I specific receptors expressed on human natural killer (NK) cells

Natural killer (NK) cells express receptors that are specific for MHC class I molecules. These receptors play a crucial role in regulating the lytic and cytokine expression capabilities of NK cells. In humans, three distinct families of genes have been defined that encode for receptors of HLA class I molecules. The first family identified consists of type I transmembrane molecules belonging to the immunoglobulin (Ig) superfamily and are called killer cell Ig-like receptors (KIR). A second group of receptors belonging to the Ig superfamily, named ILT (for immunoglobulin like transcripts), has more recently been described. ILTs are expressed mainly on B, T and myeloid cells, but some members of this group are also expressed on NK cells. They are also referred to as LIRs (for leukocyte Ig-like receptor) and MIRs (for macrophage Ig-like receptor). The ligands for the KIR and some of the ILT receptors include classical (class Ia) HLA class I molecules, as well as the nonclassical (class Ib) HLA-G molecule. The third family of HLA class I receptors are C-type lectin family members and are composed of heterodimers of CD94 covalently associated with a member of the NKG2 family of molecules. The ligand for most members is the nonclassical class I molecule HLA-E. NKG2D, a member of the NKG2 family, is expressed as a homodimer, along with the adaptor molecule DAP10. The ligands of NKG2D include the human class I like molecules MICA and MICB, and the recently described ULBPs. Each of these three families of receptors has individual members that can recognize identical or similar ligands yet signal for activation or inhibition of cellular functions. This dichotomy correlates with particular structural features present in the transmembrane and intracytoplasmic portions of these molecules. In this review we will discuss the molecular structure, specificity, cellular expression patterns, and function of these HLA class I receptors, as well as the chromosomal location and genetic organization.

Cutting edge: Regulation of CD8(+) T cell proliferation by 2B4/CD48 interactions

The biological function of 2B4, a CD48-binding molecule expressed on T cells with an activation/memory phenotype, is not clear. In this report, we demonstrate that proliferation of CD8(+) T cells is regulated by 2B4. Proliferative responses of CD8(+) T cells were significantly reduced by anti-2B4 Ab. The effects were not potentiated by anti-CD48 Ab, suggesting that the observed responses were driven by 2B4/CD48 interactions. Surprisingly, the 2B4/CD48-dependent proliferative responses were also observed in the absence of APCs. This suggests that 2B4/CD48 interactions can occur directly between T cells. Furthermore, when activated 2B4(+)CD8(+) T cells were mixed with 2B4(-)CD8(+) TCR-transgenic T cells and specific peptide-loaded APC, the proliferation of the latter T cells was inhibited by anti-2B4 Ab. Taken together, this suggests that 2B4 on activated/memory T cells serves as a ligand for CD48, and by its ability to interact with CD48 provides costimulatory-like function for neighboring T cells.

Neuronal plasticity and cellular immunity: shared molecular mechanisms

It is becoming evident that neurons express an unusual number of molecules that were originally thought to be specific to immune functions. One such molecule, class I major histocompatibility complex, is required in the activity-dependent refinement and plasticity of connections in the developing and adult central nervous system, demonstrating that molecules can perform critical roles in both systems. Recent studies reveal striking parallels between cellular signaling mechanisms in the immune and nervous systems that may provide unexpected insights into the development, function, and diseases of both systems.

T-cell immunity in acute coronary syndromes

Acute coronary syndromes (ACS) are complications of atherosclerotic vascular disease that are triggered by the sudden rupture of an atheroma. Atherosclerotic plaque stability is determined by multiple factors, of which immune and inflammatory pathways are critical. Unstable plaque is characterized by an infiltrate of T cells and macrophages, thereby resembling a delayed hypersensitivity reaction. On activation, T cells secrete cytokines that regulate the activity of macrophages, or the T cells may differentiate into effector cells with tissue-damaging potential. Constitutive stimulation of T cells and macrophages in ACS is not limited to the vascular lesion but also involves peripheral immune cells, suggesting fundamental abnormalities in homeostatic mechanisms that control the assembly, turnover, and diversity of the immune system as a whole. This review gives particular attention to the emergence of a specialized T-cell subset, natural killer T cells, in patients with ACS. Natural killer T cells have proinflammatory properties and the capability of directly contributing to vascular injury.

The relationship between major histocompatibility receptors and innate immunity in teleost fish

Studies of the innate immune system have recently shown that, in addition to its role in producing the primary response that slows down pathogens, it may also play an important role in initiating and directing the type of response that the adaptive immune system makes. These discoveries have shown a complex web of control containing new roles for the innate immune system in organizing responses of T-cell to antigens being presented by major histocompatibility receptors, as well as new roles for those receptors in innate immune responses. Both of these activities are managed through feedback networks involving elements of both the innate and adaptive immune system. This paper will discuss these newly discovered interactions and how they are influencing current theories regarding the initiation of adaptive immune responses. In particular, it will highlight the recent progress that is being made towards understanding these relationships in the immune systems of teleost fish.

Regulation of the natural killer cell receptor repertoire

Natural killer cells express inhibitory receptors specific for MHC class I proteins and stimulatory receptors with diverse specificities. The MHC-specific receptors discriminate among different MHC class I alleles and are expressed in a variegated, overlapping fashion, such that each NK cell expresses several inhibitory and stimulatory receptors. Evidence suggests that individual developing NK cells initiate expression of inhibitory receptor genes in a sequential, cumulative, and stochastic fashion. Superimposed on the receptor acquisition process are multiple education mechanisms, which act to coordinate the stimulatory and inhibitory specificities of developing NK cells. One process influences the complement of receptors expressed by individual NK cells. Other mechanisms may prevent NK cell autoaggression even when the developing NK cell fails to express self-MHC-specific inhibitory receptors. Together, these mechanisms ensure a self-tolerant and maximally discriminating NK cell population. Like NK cells, a fraction of memory phenotype CD8(+) T cells, as well as other T cell subsets, express inhibitory class I--specific receptors in a variegated, overlapping fashion. The characteristics of these cells suggest that inhibitory receptor expression may be a response to prior antigenic stimulation as well as to poorly defined additional signals. A unifying hypothesis is that both NK cells and certain T cell subsets initiate expression of inhibitory receptors in response to stimulation.

Expression and function of NK cell receptors in CD8+ T cells

A wide variety of inhibitory and stimulatory NK cell receptors are expressed by some CD8+ cytotoxic T lymphocytes in mice and humans. Recent data address the induction of these receptors on activated or memory CD8+ T cells and have led to hypotheses addressing their function in the CD8+ T cell response.

Cutting edge: Ly49A inhibits TCR/CD3-induced apoptosis and IL-2 secretion

To evaluate the importance of Ly49A on TCR-induced cellular events, we established clones of the 1F2 T cell hybridoma expressing either Ly49A or a chimeric version, Ly49A/H, where the Ly49A cytoplasmic domain has been replaced by the Ly49H cytoplasmic domain. Ligation of Ly49A, but not Ly49A/H, with its ligand H-2D(d) or anti-Ly49A mAbs caused a specific inhibition of TCR/CD3-induced IL-2 secretion. Moreover, flow cytometry analysis of hypodiploid DNA and annexin V binding revealed that ligation of Ly49A protected cells from apoptosis induced by anti-CD3 mAbs or Ag. In contrast, ligation of the Ly49A/H chimeric receptor had no antiapoptotic effect. In addition, engagement of Ly49A selectively inhibited TCR-induced Fas ligand expression whereas TCR-induced Fas expression was not significantly affected. Expression of Ly49 inhibitory receptors on T cells may represent an important mechanism for the regulation of T cell survival in vivo by inhibiting TCR-induced apoptosis and IL-2 secretion.

MHC Class I-Ly49 Interactions Shape the Ly49 Repertoire on Murine NK Cells

This study aims to determine how the interaction of Ly49 receptors with MHC class I molecules shapes the development of the Ly49 repertoire. We have examined the percentage of NK cells that expressed Ly49A, Ly49G2, and Ly49D in single and double Ly49A/C-transgenic mice on four different MHC backgrounds, H-2(b), H-2(d), H-2(b/d), and beta(2)-microglobulin(-/-). The results show that the total numbers of NK cells were not different among the strains. The prior expression of a Ly49 receptor capable of binding to self MHC class I altered the percentage of NK cells expressing endogenous Ly49A, Ly49G2, and Ly49D even in mice in which no MHC ligand was present for the latter receptors. The NK cells in the Ly49-transgenic mice expressed the same level of endogenous Ly49 receptors as wild-type mice of a similar MHC background. In contrast, the number of NK T cells was reduced in mice in which the Ly49 transgene could bind to a MHC class I molecule. The onset of Ly49 receptor expression on NK cells during ontogeny was not altered in the presence of transgenic Ly49 receptors. These data support a sequential model and argue against a selection model for Ly49 repertoire development on NK cells.