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

Murine cytomegalovirus downregulates ERAAP and induces an unconventional T cell response to self

The endoplasmic reticulum aminopeptidase associated with antigen processing (ERAAP) plays a crucial role in shaping the peptide-major histocompatibility complex (MHC) class I repertoire and maintaining immune surveillance. While murine cytomegalovirus (MCMV) has multiple strategies for manipulating the antigen processing pathway to evade immune responses, the host has also developed ways to counter viral immune evasion. In this study, we find that MCMV modulates ERAAP and induces an interferon γ (IFN-γ)-producing CD8+ T cell effector response that targets uninfected ERAAP-deficient cells. We observe that ERAAP downregulation during infection leads to the presentation of the self-peptide FL9 on non-classical Qa-1b, thereby eliciting Qa-1b-restricted QFL T cells to proliferate in the liver and spleen of infected mice. QFL T cells upregulate effector markers upon MCMV infection and are sufficient to reduce viral load after transfer to immunodeficient mice. Our study highlights the consequences of ERAAP dysfunction during viral infection and provides potential targets for anti-viral therapies.

An updated review of the immunological mechanisms of keloid scars

Keloid is a type of disfiguring pathological scarring unique to human skin. The disorder is characterized by excessive collagen deposition. Immune cell infiltration is a hallmark of both normal and pathological tissue repair. However, the immunopathological mechanisms of keloid remain unclear. Recent studies have uncovered the pivotal role of both innate and adaptive immunity in modulating the aberrant behavior of keloid fibroblasts. Several novel therapeutics attempting to restore regulation of the immune microenvironment have shown variable efficacy. We review the current understanding of keloid immunopathogenesis and highlight the potential roles of immune pathway-specific therapeutics.

A molecular signature of lung-resident CD8+ T cells elicited by subunit vaccination

Natural infection as well as vaccination with live or attenuated viruses elicit tissue resident, CD8⁺ memory T cell (Trm) response. Trm cells so elicited act quickly upon reencounter with the priming agent to protect the host. These Trm cells express a unique molecular signature driven by the master regulators-Runx3 and Hobit. We previously reported that intranasal instillation of a subunit vaccine in a prime boost vaccination regimen installed quick-acting, CD8⁺ Trm cells in the lungs that protected against lethal vaccinia virus challenge. It remains unexplored whether CD8⁺ Trm responses so elicited are driven by a similar molecular signature as those elicited by microbes in a real infection or by live, attenuated pathogens in conventional vaccination. We found that distinct molecular signatures distinguished subunit vaccine-elicited lung interstitial CD8⁺ Trm cells from subunit vaccine-elicited CD8⁺ effector memory and splenic memory T cells. Nonetheless, the transcriptome signature of subunit vaccine elicited CD8⁺ Trm resembled those elicited by virus infection or vaccination. Clues to the basis of tissue residence and function of vaccine specific CD8⁺ Trm cells were found in transcripts that code for chemokines and chemokine receptors, purinergic receptors, and adhesins when compared to CD8⁺ effector and splenic memory T cells. Our findings inform the utility of protein-based subunit vaccination for installing CD8⁺ Trm cells in the lungs to protect against respiratory infectious diseases that plague humankind.

Shared and distinct biological circuits in effector, memory and exhausted CD8+ T cells revealed by temporal single-cell transcriptomics and epigenetics

Naïve CD8+ T cells can differentiate into effector (Teff), memory (Tmem) or exhausted (Tex) T cells. These developmental pathways are associated with distinct transcriptional and epigenetic changes that endow cells with different functional capacities and therefore therapeutic potential. The molecular circuitry underlying these developmental trajectories and the extent of heterogeneity within Teff, Tmem and Tex populations remain poorly understood. Here, we used the lymphocytic choriomeningitis virus model of acute-resolving and chronic infection to address these gaps by applying longitudinal single-cell RNA-sequencing (scRNA-seq) and single-cell assay for transposase-accessible chromatin sequencing (scATAC-seq) analyses. These analyses uncovered new subsets, including a subpopulation of Tex cells expressing natural killer cell-associated genes that is dependent on the transcription factor Zeb2, as well as multiple distinct TCF-1+ stem/progenitor-like subsets in acute and chronic infection. These data also revealed insights into the reshaping of Tex subsets following programmed death 1 (PD-1) pathway blockade and identified a key role for the cell stress regulator, Btg1, in establishing the Tex population. Finally, these results highlighted how the same biological circuits such as cytotoxicity or stem/progenitor pathways can be used by CD8+ T cell subsets with highly divergent underlying chromatin landscapes generated during different infections.

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.

The Yin and Yang of Targeting KLRG1+ Tregs and Effector Cells

The literature surrounding KLRG1 has primarily focused on NK and CD8⁺ T cells. However, there is evidence that the most suppressive Tregs express KLRG1. Until now, the role of KLRG1 on Tregs has been mostly overlooked and remains to be elucidated. Here we review the current literature on KLRG1 with an emphasis on the KLRG1⁺ Treg subset role during cancer development and autoimmunity. KLRG1 has been recently proposed as a new checkpoint inhibitor target, but these studies focused on the effects of KLRG1 blockade on effector cells. We propose that when designing anti-tumor therapies targeting KLRG1, the effects on both effector cells and Tregs will have to be considered.

Differential expression of inhibitory receptor NKG2A distinguishes disease-specific exhausted CD8+ T cells

Exhausted CD8+ T (Tex) cells are caused by persistent antigenic stimulation during chronic viral infection or tumorigenesis. Tex cells upregulate and sustain the expressions of multiple immune inhibitory receptors (IRs). Blocking IRs of Tex cells, exemplified by PD-1, can partially restore their effector functions and thus lead to viral suppression or tumor remission. Tex cells derived from chronic viral infections share the expression spectrum of IRs with Tex cells derived from tumors; however, whether any IRs are selectively expressed by tumor-derived Tex cells or virus-derived Tex cells remains to be learnt. In the study, we found that Tex cells upregulate IR natural killer cell lectin-like receptor isoform A (NKG2A) specifically in the context of tumor but not chronic viral infection. Moreover, the NKG2A expression is attributed to tumor antigen recognition and thus bias expressed by tumor-specific Tex cells in the tumor microenvironment instead of their counterparts in the periphery. Such dichotomous NKG2A expression further dictates the differential responsiveness of Tex cells to NKG2A immune checkpoint blockade. Therefore, our study highlighted NKG2A as a disease-dependent IR and provided novel insights into the distinct regulatory mechanisms underlying T cell exhaustion between tumor and chronic viral infection.

An NK-like CAR T cell transition in CAR T cell dysfunction

Chimeric antigen receptor (CAR) T cell therapy has achieved remarkable success in hematological malignancies but remains ineffective in solid tumors, due in part to CAR T cell exhaustion in the solid tumor microenvironment. To study dysfunction of mesothelin-redirected CAR T cells in pancreatic cancer, we establish a robust model of continuous antigen exposure that recapitulates hallmark features of T cell exhaustion and discover, both in vitro and in CAR T cell patients, that CAR dysregulation is associated with a CD8+ T-to-NK-like T cell transition. Furthermore, we identify a gene signature defining CAR and TCR dysregulation and transcription factors, including SOX4 and ID3 as key regulators of CAR T cell exhaustion. Our findings shed light on the plasticity of human CAR T cells and demonstrate that genetic downmodulation of ID3 and SOX4 expression can improve the efficacy of CAR T cell therapy in solid tumors by preventing or delaying CAR T cell dysfunction.

Human cytomegalovirus expands a CD8+ T cell population with loss of BCL11B expression and gain of NK cell identity

[Figure: see text].

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.

Memory CD8+ T cell heterogeneity is primarily driven by pathogen-specific cues and additionally shaped by the tissue environment

Factors that govern the complex formation of memory T cells are not completely understood. A better understanding of the development of memory T cell heterogeneity is however required to enhance vaccination and immunotherapy approaches. Here we examined the impact of pathogen- and tissue-specific cues on memory CD8⁺ T cell heterogeneity using high-dimensional single-cell mass cytometry and a tailored bioinformatics pipeline. We identified distinct populations of pathogen-specific CD8⁺ T cells that uniquely connected to a specific pathogen or associated to multiple types of acute and persistent infections. In addition, the tissue environment shaped the memory CD8⁺ T cell heterogeneity, albeit to a lesser extent than infection. The programming of memory CD8⁺ T cell differentiation during acute infection is eventually superseded by persistent infection. Thus, the plethora of distinct memory CD8⁺ T cell subsets that arise upon infection is dominantly sculpted by the pathogen-specific cues and further shaped by the tissue environment.

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Heterogeneous subsets of both circulating and tissue-resident memory CD8⁺ T cells exist

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Memory CD8⁺ T cell heterogeneity is profoundly sculpted by pathogen-specific cues

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Memory CD8⁺ T cell heterogeneity is additionally shaped by the tissue environment

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Viral persistance supersedes memory CD8⁺ T cell differentiation after acute infection

Multimodal Mapping of the Tumor and Peripheral Blood Immune Landscape in Human Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDA) is characterized by an immune-suppressive tumor microenvironment that renders it largely refractory to immunotherapy. We implemented a multimodal analysis approach to elucidate the immune landscape in PDA. Using a combination of CyTOF, single-cell RNA sequencing, and multiplex immunohistochemistry on patient tumors, matched blood, and non-malignant samples, we uncovered a complex network of immune-suppressive cellular interactions. These experiments revealed heterogeneous expression of immune checkpoint receptors in individual patient's T cells and increased markers of CD8+ T cell dysfunction in advanced disease stage. Tumor-infiltrating CD8+ T cells had an increased proportion of cells expressing an exhausted expression profile that included upregulation of the immune checkpoint TIGIT, a finding that we validated at the protein level. Our findings point to a profound alteration of the immune landscape of tumors, and to patient-specific immune changes that should be taken into account as combination immunotherapy becomes available for pancreatic cancer.

Qa-1-Restricted CD8+ T Cells Can Compensate for the Absence of Conventional T Cells during Viral Infection

The role of non-classical T cells during viral infection remains poorly understood. Using the well-established murine model of CMV infection (MCMV) and mice deficient in MHC class Ia molecules, we found that non-classical CD8⁺ T cells robustly expand after MCMV challenge, become highly activated effectors, and are capable of forming durable memory. Interestingly, although these cells are restricted by MHC class Ib molecules, they respond similarly to conventional T cells. Remarkably, when acting as the sole component of the adaptive immune response, non-classical CD8⁺ T cells are sufficient to protect against MCMV-induced lethality. We also demonstrate that the MHC class Ib molecule Qa-1 (encoded by H2-T23) restricts a large, and critical, portion of this population. These findings reveal a potential adaptation of the host immune response to compensate for viral evasion of classical T cell immunity.

Anderson et al. describe a heterogenous population of non-classical CD8⁺ T cells responding to MCMV. Importantly, this population can protect mice from MCMV-induced lethality in the absence of other adaptive immune cells. Among the MHC class Ib-restricted CD8⁺ T cells responding, Qa-1-specific cells are required for protection.

Immune Exhaustion: Past Lessons and New Insights from Lymphocytic Choriomeningitis Virus

Lymphocytic choriomeningitis virus (LCMV) is a paradigm-forming experimental system with a remarkable track record of contributing to the discovery of many of the fundamental concepts of modern immunology. The ability of LCMV to establish a chronic infection in immunocompetent adult mice was instrumental for identifying T cell exhaustion and this system has been invaluable for uncovering the complexity, regulators, and consequences of this state. These findings have been directly relevant for understanding why ineffective T cell responses commonly arise during many chronic infections including HIV and HCV, as well as during tumor outgrowth. The principal feature of exhausted T cells is the inability to elaborate the array of effector functions necessary to contain the underlying infection or tumor. Using LCMV to determine how to prevent and reverse T cell exhaustion has highlighted the potential of checkpoint blockade therapies, most notably PD-1 inhibition strategies, for improving cellular immunity under conditions of antigen persistence. Here, we discuss the discovery, properties, and regulators of exhausted T cells and highlight how LCMV has been at the forefront of advancing our understanding of these ineffective responses.

Anti-NKG2A mAb Is a Checkpoint Inhibitor that Promotes Anti-tumor Immunity by Unleashing Both T and NK Cells

Checkpoint inhibitors have revolutionized cancer treatment. However, only a minority of patients respond to these immunotherapies. Here, we report that blocking the inhibitory NKG2A receptor enhances tumor immunity by promoting both natural killer (NK) and CD8⁺ T cell effector functions in mice and humans. Monalizumab, a humanized anti-NKG2A antibody, enhanced NK cell activity against various tumor cells and rescued CD8⁺ T cell function in combination with PD-x axis blockade. Monalizumab also stimulated NK cell activity against antibody-coated target cells. Interim results of a phase II trial of monalizumab plus cetuximab in previously treated squamous cell carcinoma of the head and neck showed a 31% objective response rate. Most common adverse events were fatigue (17%), pyrexia (13%), and headache (10%). NKG2A targeting with monalizumab is thus a novel checkpoint inhibitory mechanism promoting anti-tumor immunity by enhancing the activity of both T and NK cells, which may complement first-generation immunotherapies against cancer.

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Blocking NKG2A unleashes both T and NK cell effector functions

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Combined blocking of the NKG2A and the PD-1 axis promotes anti-tumor immunity

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Blocking NKG2A and triggering CD16 illustrates the efficacy of dual checkpoint therapy

A lipidated bi-epitope vaccine comprising of MHC-I and MHC-II binder peptides elicits protective CD4 T cell and CD8 T cell immunity against Mycobacterium tuberculosis

Background

The clinical trials conducted at Chingleput India suggest that BCG fails to protect against tuberculosis (TB) in TB-endemic population. Recent studies advocate that non-tuberculous mycobacteria and latent Mycobacterium tuberculosis (Mtb) infection interferes in the antigen processing and presentation of BCG in inducing protective immunity against Mtb. Thereby, indicating that any vaccine that require extensive antigen processing may not be efficacious in TB-endemic zones. Recently, we have demonstrated that the vaccine candidate L91, which is composed of lipidated promiscuous MHC-II binder epitope, derived from latency associated Acr1 antigen of Mtb is immunogenic in the murine and Guinea pig models of TB and conferred better protection than BCG against Mtb.

Methods

In this study, we have used a multi-stage based bi-epitope vaccine, namely L4.8, comprising of MHC-I and MHC-II binding peptides of active (TB10.4) and latent (Acr1) stages of Mtb antigens, respectively. These peptides were conjugated to the TLR-2 agonist Pam2Cys.

Results

L4.8 significantly elicited both CD8 T cells and CD4 T cells immunity, as evidenced by increase in the enduring polyfunctional CD8 T cells and CD4 T cells. L4.8 efficiently declined Mtb-burden and protected animals better than BCG and L91, even at the late stage of Mtb infection.

Conclusions

The BCG-L4.8 prime boost strategy imparts a better protection against TB than the BCG alone. This study emphatically denotes that L4.8 can be a promising future vaccine candidate for controlling active and latent TB.

Electronic supplementary material

The online version of this article (10.1186/s12967-018-1653-x) contains supplementary material, which is available to authorized users.

KIR3DL1-Negative CD8 T Cells and KIR3DL1-Negative Natural Killer Cells Contribute to the Advantageous Control of Early Human Immunodeficiency Virus Type 1 Infection in HLA-B Bw4 Homozygous Individuals

Bw4 homozygosity in human leukocyte antigen class B alleles has been associated with a delayed acquired immunodeficiency syndrome (AIDS) development and better control of human immunodeficiency virus type 1 (HIV-1) viral load (VL) than Bw6 homozygosity. Efficient CD8 T cell and natural killer (NK) cell functions have been described to restrain HIV-1 replication. However, the role of KIR3DL1 expression on these cells was not assessed in Bw4-homozygous participants infected with HIV-1 CRF01_A/E subtype, currently the most prevalent subtype in China. Here, we found that the frequency of KIR3DL1-expressing CD8 T cells of individuals homozygous for Bw6 [1.53% (0–4.56%)] was associated with a higher VL set point (Spearman r_s = 0.59, P = 0.019), but this frequency of KIR3DL1⁺CD8⁺ T cells [1.37% (0.04–6.14%)] was inversely correlated with CD4 T-cell count in individuals homozygous for Bw4 (r_s = −0.59, P = 0.011). Moreover, CD69 and Ki67 were more frequently expressed in KIR3DL1⁻CD8⁺ T cells in individuals homozygous for Bw4 than Bw6 (P = 0.046 for CD69; P = 0.044 for Ki67), although these molecules were less frequently expressed in KIR3DL1⁺CD8⁺ T cells than in KIR3DL1⁻CD8⁺ T cells in both groups (all P < 0.05). KIR3DL1⁻CD8⁺ T cells have stronger p24-specific CD8⁺ T-cell responses secreting IFN-γ and CD107a than KIR3DL1⁺CD8⁺ T cells in both groups (all P < 0.05). Thus, KIR3DL1 expression on CD8 T cells were associated with the loss of multiple functions. Interestingly, CD69⁺NK cells lacking KIR3DL1 expression were inversely correlated with HIV-1 VL set point in Bw4-homozygous individuals (r_s = −0.52, P = 0.035). Therefore, KIR3DL1⁻CD8⁺ T cells with strong early activation and proliferation may, together with KIR3DL1⁻CD69⁺NK cells, play a protective role during acute/early HIV infection in individuals homozygous for Bw4. These findings highlight the superior functions of KIR3DL1⁻CD8⁺ T cells and KIR3DL1⁻CD69⁺NK cells being a potential factor contributing to delayed disease progression in the early stages of HIV-1 infection.

Recombinant Listeria promotes tumor rejection by CD8+ T cell-dependent remodeling of the tumor microenvironment

Agents that remodel the tumor microenvironment (TME), prime functional tumor-specific T cells, and block inhibitory signaling pathways are essential components of effective immunotherapy. We are evaluating live-attenuated, double-deleted Listeria monocytogenes expressing tumor antigens (LADD-Ag) in the clinic. Here we show in numerous mouse models that while treatment with nonrecombinant LADD induced some changes in the TME, no antitumor efficacy was observed, even when combined with immune checkpoint blockade. In contrast, LADD-Ag promoted tumor rejection by priming tumor-specific KLRG1⁺PD1^loCD62L^- CD8⁺ T cells. These IFNγ-producing effector CD8⁺ T cells infiltrated the tumor and converted the tumor from an immunosuppressive to an inflamed microenvironment that was characterized by a decrease in regulatory T cells (Treg) levels, a proinflammatory cytokine milieu, and the shift of M2 macrophages to an inducible nitric oxide synthase (iNOS)⁺CD206^- M1 phenotype. Remarkably, these LADD-Ag-induced tumor-specific T cells persisted for more than 2 months after primary tumor challenge and rapidly controlled secondary tumor challenge. Our results indicate that the striking antitumor efficacy observed in mice with LADD-based immunotherapy stems from TME remodeling which is a direct consequence of eliciting potent, systemic tumor-specific CD8⁺ T cells.

Costimulatory and Coinhibitory Receptor Pathways in Infectious Disease

Costimulatory and inhibitory receptors play a key role in regulating immune responses to infections. Recent translation of knowledge about inhibitory receptors such as CTLA-4 and PD-1 into the cancer clinic highlights the opportunities to manipulate these pathways to treat human disease. Studies in infectious disease have provided key insights into the specific roles of these pathways and the effects of their manipulation. Here, recent studies are discussed that have addressed how major inhibitory and costimulatory pathways play a role in regulating immune responses during acute and chronic infections. Mechanistic insights from studies of infectious disease provide opportunities to further expand our toolkit to treat cancer and chronic infections in the clinic.

Characterization of Mycobacterium tuberculosis-Specific Cells Using MHC Class II Tetramers Reveals Phenotypic Differences Related to HIV Infection and Tuberculosis Disease

A major challenge for the development of an effective vaccine against tuberculosis (TB) is that the attributes of protective CD4+ T cell responses are still elusive for human TB. Infection with HIV type 1 is a major risk factor for TB, and a better understanding of HIV-induced alterations of Mycobacterium tuberculosis-specific CD4+ T cells that leads to failed host resistance may provide insight into protective T cell immunity to TB. A total of 86 participants from a TB-endemic setting, either HIV-infected or uninfected and with latent or active TB (aTB), were screened using M.tuberculosis-specific MHC class II tetramers. We examined the phenotype as well as function of ex vivo M. tuberculosis-specific tetramer+CD4+ T cells using flow cytometry. The numbers of M. tuberculosis-specific tetramer+CD4+ T cells were relatively well maintained in HIV-infected persons with aTB, despite severe immunodeficiency. However, although HIV-uninfected persons with latent TB infection exhibited ex vivo M. tuberculosis-specific CD4+ T cells predominantly of a CXCR3+CCR6+CCR4- (Th1*) phenotype, aTB or HIV infection was associated with a contraction of this subset. Nevertheless, in individuals with aTB and/or HIV infection, circulating ex vivo M. tuberculosis-specific CD4+ T cells did not display defects in exhaustion or polyfunctionality compared with healthy HIV-uninfected individuals with latent TB infection. Collectively, these data suggest that increased susceptibility to TB disease could be related to a loss of circulating Th1* CD4+ T cells rather than major changes in the number or function of circulating CD4+ T cells.

T Cells Going Innate

Natural killer (NK) cell receptors (NKRs) play a crucial role in the homeostasis of antigen-experienced T cells. Indeed, prolonged antigen stimulation may induce changes in the receptor repertoire of T cells to a profile that features NKRs. Chronic antigen exposure, at the same time, has been shown to trigger the loss of costimulatory CD28 molecules with recently reported intensified antigen thresholds of antigen-experienced CD8(+) T cells. In transplantation, NKRs have been shown to assist allograft rejection in a CD28-independent fashion. We discuss here a role for CD28-negative T cells that have acquired the competency of the NKR machinery, potentially promoting allorecognition either through T cell receptor (TCR) crossreactivity or independently from TCR recognition. Collectively, NKRs can bring about innate-like T cells by providing alternative costimulatory pathways that gain relevance in chronic inflammation, potentially leading to resistance to CD28-targeting immunosuppressants.

CCR7 expression alters memory CD8 T-cell homeostasis by regulating occupancy in IL-7- and IL-15-dependent niches

C-C receptor 7 (CCR7) is important to allow T cells and dendritic cells to migrate toward CCL19- and CCL21-producing cells in the T-cell zone of the spleen and lymph nodes. The role of this chemokine receptor in regulating the homeostasis of effector and memory T cells during acute viral infection is poorly defined, however. In this study, we show that CCR7 expression alters memory CD8 T-cell homeostasis following lymphocytic choriomeningitis virus infection. Greater numbers of CCR7-deficient memory T cells were formed and maintained compared with CCR7-sufficient memory T cells, especially in the lung and bone marrow. The CCR7-deficient memory T cells also displayed enhanced rates of homeostatic turnover, which may stem from increased exposure to IL-15 as a consequence of reduced exposure to IL-7, because removal of IL-15, but not of IL-7, normalized the numbers of CCR7-sufficient and CCR7-deficient memory CD8 T cells. This result suggests that IL-15 is the predominant cytokine supporting augmentation of the CCR7(-/-) memory CD8 T-cell pool. Taken together, these data suggest that CCR7 biases memory CD8 T cells toward IL-7-dependent niches over IL-15-dependent niches, which provides insight into the homeostatic regulation of different memory T-cell subsets.

CD57 in human natural killer cells and T-lymphocytes

The CD57 antigen (alternatively HNK-1, LEU-7, or L2) is routinely used to identify terminally differentiated 'senescent' cells with reduced proliferative capacity and altered functional properties. In this article, we review current understanding of the attributes of CD57-expressing T-cells and NK cells in both health and disease and discuss how this marker can inform researchers about their likely functions in human blood and tissues in vivo. While CD57 expression on human lymphocytes indicates an inability to proliferate, these cells also display high cytotoxic potential, and CD57(pos) NK cells exhibit both memory-like features and potent effector functions. Accordingly, frequencies of CD57-expressing cells in blood and tissues have been correlated with clinical prognosis in chronic infections or various cancers and with human aging. Functional modulation of senescent CD57(pos) T-cells and mature CD57(pos) NK cells may therefore represent innovative strategies for protection against human immunological aging and/or various chronic diseases.

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.

T cell exhaustion during persistent viral infections

Although robust and highly effective anti-viral T cells contribute to the clearance of many acute infections, viral persistence is associated with the development of functionally inferior, exhausted, T cell responses. Exhaustion develops in a step-wise and progressive manner, ranges in severity, and can culminate in the deletion of the anti-viral T cells. This disarming of the response is consequential as it compromises viral control and potentially serves to dampen immune-mediated damage. Exhausted T cells are unable to elaborate typical anti-viral effector functions. They are characterized by the sustained upregulation of inhibitory receptors and display a gene expression profile that distinguishes them from prototypic effector and memory T cell populations. In this review we discuss the properties of exhausted T cells; the virological and immunological conditions that favor their development; the cellular and molecular signals that sustain the exhausted state; and strategies for preventing and reversing exhaustion to favor viral control.

Mammalian non-classical major histocompatibility complex I and its receptors: Important contexts of gene, evolution, and immunity

The evolutionary conserved, less-polymorphic, nonclassical major histocompatibility complex (MHC) class I molecules: Qa-1 and its human homologue human leukocyte antigen-E (HLA-E) along with HLA-F, G and H cross-talk with the T-cell receptors and also interact with natural killer T-cells and other lymphocytes. Moreover, these nonclassical MHC molecules are known to interact with CD94/NKG2 heterodimeric receptors to induce immune responses and immune regulations. This dual role of Qa-1/HLA-E in terms of innate and adaptive immunity makes them more interesting. This review highlights the new updates of the mammalian nonclassical MHC-I molecules in terms of their gene organization, evolutionary perspective and their role in immunity.

Expression of coinhibitory receptors on T cells in the microenvironment of usual vulvar intraepithelial neoplasia is related to proinflammatory effector T cells and an increased recurrence-free survival

Human papillomavirus-induced usual-type vulvar intraepithelial neoplasia (uVIN) are infiltrated by immune cells but apparently not cleared. A potential explanation for this is an impaired T cell effector function by an immunesuppressive milieu, coinfiltrating regulatory T cells or the expression of coinhibitory molecules. Here, the role of these potential inhibitory mechanisms was evaluated by a detailed immunohistochemical analysis of T cell infiltration in the context of FoxP3, Tbet, indoleamine 2,3-dioxygenase, programmed cell death 1, T cell immunoglobulin mucin 3 (TIM3), natural killer cell lectin-like receptor A (NKG2A) and galectins-1, -3 and -9. Paraffin-embedded tissues of primary uVIN lesions (n=43), recurrent uVIN lesions (n=20), vulvar carcinoma (n=21) and healthy vulvar tissue (n=26) were studied. We show that the vulva constitutes an area intensely surveyed by CD8+, CD4+, Tbet+ and regulatory T cell populations, parts of which express the examined coinhibitory molecules. In uVIN especially, the number of regulatory T cells and TIM3+ T cells increased. The expression of the coinhibitory markers TIM3 and NKG2A probably reflected a higher degree of T cell activation as a dense infiltration with stromal CD8+TIM3+ T cells and CD3+NKG2A+ T cells was related to the absence of recurrences and/or a prolonged recurrence-free survival. A dense coinfiltrate with regulatory T cells was negatively associated with the time to recurrence, most dominantly when the stromal CD8+TIM3+ infiltration was limited. This notion was sustained in vulvar carcinoma's where the numbers of regulatory T cells progressively increased to outnumber coinfiltrating CD8+TIM3+ T cells and CD3+NKG2A+ T cells.

Complexity and Diversity of the NKR-P1:Clr (Klrb1:Clec2) Recognition Systems

The NKR-P1 receptors were identified as prototypical natural killer (NK) cell surface antigens and later shown to be conserved from rodents to humans on NK cells and subsets of T cells. C-type lectin-like in nature, they were originally shown to be capable of activating NK cell function and to recognize ligands on tumor cells. However, certain family members have subsequently been shown to be capable of inhibiting NK cell activity, and to recognize proteins encoded by a family of genetically linked C-type lectin-related ligands. Some of these ligands are expressed by normal, healthy cells, and modulated during transformation, infection, and cellular stress, while other ligands are upregulated during the immune response and during pathological circumstances. Here, we discuss historical and recent developments in NKR-P1 biology that demonstrate this NK receptor–ligand system to be far more complex and diverse than originally anticipated.

KLRG1 impairs CD4+ T cell responses via p16ink4a and p27kip1 pathways: role in hepatitis B vaccine failure in individuals with hepatitis C virus infection

Coinfection of hepatitis B virus (HBV) with hepatitis C virus (HCV) is quite common, leading to an increase in morbidity and mortality. As such, HBV vaccination is recommended in HCV-infected individuals. However, HBV vaccine responses in HCV-infected individuals are often blunted compared with uninfected populations. The mechanism for this failure of vaccine response in HCV-infected subjects remains unclear. In this study, we investigated the expression and function of an inhibitory receptor, killer cell lectin-like receptor subfamily G member 1 (KLRG1), in the regulation of CD4(+) T cells and HBV vaccine responses during HCV infection. We demonstrated that KLRG1 was overexpressed on CD4(+) T cells from HCV-infected, HBV vaccine nonresponders compared with HBV vaccine responders. The capacity of CD4(+) T cells to proliferate and secrete IL-2 cytokine was inversely associated with the level of KLRG1 expression. Importantly, blocking KLRG1 signaling resulted in a significant improvement in CD4(+) T cell proliferation and IL-2 production in HCV-infected, HBV vaccine nonresponders in response to TCR stimulation. Moreover, blockade of KLRG1 increased the phosphorylation of Akt (Ser(473)) and decreased the expression of cell cycle inhibitors p16(ink4a) and p27(kip1), which subsequently enhanced the expression of cyclin-dependent kinase 2 and cyclin E. These results suggest that the KLRG1 pathway impairs CD4(+) T cell responses to neoantigen and induces a state of immune senescence in individuals with HCV infection, raising the possibility that blocking this negative-signaling pathway might improve HBV vaccine responses in the setting of chronic viral infection.

KLRG1 negatively regulates natural killer cell functions through the Akt pathway in individuals with chronic hepatitis C virus infection

In this study, we demonstrate that killer cell lectin-like receptor subfamily G member 1 (KLRG1), a transmembrane protein preferentially expressed on T cells, is highly expressed on CD56(+) NK cells, which are significantly reduced in their numbers and functions in the peripheral blood of patients with chronic hepatitis C virus (HCV) infection compared to subjects without infection. KLRG1 expression is also upregulated on healthy NK cells exposed to Huh-7 hepatocytes infected with HCV in vitro. Importantly, the expression levels of KLRG1 are inversely associated with the capacity of NK cells to proliferate and to produce gamma interferon (IFN-γ) but positively associated with apoptosis of NK cells in response to inflammatory cytokine stimulation. KLRG1(+) NK cells, including CD56(bright) and CD56(dim) subsets, exhibit impaired cell activation and IFN-γ production but increased apoptosis compared to KLRG1(-) NK cells, particularly in HCV-infected individuals. Importantly, blockade of KLRG1 signaling significantly recovered the impaired IFN-γ production by NK cells from HCV-infected subjects. Blockade of KLRG1 also enhanced the impaired phosphorylation of Akt (Ser473) in NK cells from HCV-infected subjects. Taken together, these results indicate that KLRG1 negatively regulates NK cell numbers and functions via the Akt pathway, thus providing a novel marker and therapeutic target for HCV infection.

Intratumoral treatment of smaller mouse neuroblastoma tumors with a recombinant protein consisting of IL-2 linked to the hu14.18 antibody increases intratumoral CD8+ T and NK cells and improves survival

Hu14.18-IL2 is an immunocytokine (IC) consisting of human IL-2 linked to hu14.18 mAb, which recognizes GD2 disialoganglioside. Phase II clinical trials of intravenous-hu14.18-IL2 (IV-IC) in neuroblastoma and melanoma are underway, and have already demonstrated activity in neuroblastoma. In our Phase II trial, lower neuroblastoma burden at the time of treatment was associated with a greater likelihood of clinical response to IV-IC. We have previously shown that intratumoral-hu14.18-IL2 (IT-IC) compared to IV-IC results in enhanced local and systemic antitumor activity in tumor-bearing mice. We utilized a mouse model to investigate the impact of tumor burden on hu14.18-IL2 treatment efficacy in IV- versus IT-treated animals. Studies presented here describe the analyses of tumor burden at the initiation of treatment and its effects on treatment efficacy, survival, and tumor-infiltrating leukocytes in A/J mice bearing subcutaneous NXS2 neuroblastoma. We show that smaller tumor burden at treatment initiation is associated with increased infiltration of NK and CD8+ T cells and increased overall survival. NXS2 tumor shrinkage shortly after completion of the 3 days of hu14.18-IL2 treatment is necessary for long-term survival. This model demonstrates that tumor size is a strong predictor of hu14.18-IL2-induced lymphocyte infiltration and treatment outcome.

Bystander-activated memory CD8 T cells control early pathogen load in an innate-like, NKG2D-dependent manner

During an infection the antigen-nonspecific memory CD8 T cell compartment is not simply an inert pool of cells, but becomes activated and cytotoxic. It is unknown how these cells contribute to the clearance of an infection. We measured the strength of T cell receptor (TCR) signals that bystander-activated, cytotoxic CD8 T cells (BA-CTLs) receive in vivo and found evidence of limited TCR signaling. Given this marginal contribution of the TCR, we asked how BA-CTLs identify infected target cells. We show that target cells express NKG2D ligands following bacterial infection and demonstrate that BA-CTLs directly eliminate these target cells in an innate-like, NKG2D-dependent manner. Selective inhibition of BA-CTL-mediated killing led to a significant defect in pathogen clearance. Together, these data suggest an innate role for memory CD8 T cells in the early immune response before the onset of a de novo generated, antigen-specific CD8 T cell response.

Intratumoral hu14.18-IL-2 (IC) induces local and systemic antitumor effects that involve both activated T and NK cells as well as enhanced IC retention

hu14.18-IL-2 (IC) is an immunocytokine consisting of human IL-2 linked to hu14.18 mAb, which recognizes the GD2 disialoganglioside. Phase 2 clinical trials of i.v. hu14.18-IL-2 (i.v.-IC) in neuroblastoma and melanoma are underway and have already demonstrated activity in neuroblastoma. We showed previously that intratumoral hu14.18-IL-2 (IT-IC) results in enhanced antitumor activity in mouse models compared with i.v.-IC. The studies presented in this article were designed to determine the mechanisms involved in this enhanced activity and to support the future clinical testing of intratumoral administration of immunocytokines. Improved survival and inhibition of growth of both local and distant tumors were observed in A/J mice bearing s.c. NXS2 neuroblastomas treated with IT-IC compared with those treated with i.v.-IC or control mice. The local and systemic antitumor effects of IT-IC were inhibited by depletion of NK cells or T cells. IT-IC resulted in increased NKG2D receptors on intratumoral NKG2A/C/E⁺ NKp46⁺ NK cells and NKG2A/C/E⁺ CD8⁺ T cells compared with control mice or mice treated with i.v.-IC. NKG2D levels were augmented more in tumor-infiltrating lymphocytes compared with splenocytes, supporting the localized nature of the intratumoral changes induced by IT-IC treatment. Prolonged retention of IC at the tumor site was seen with IT-IC compared with i.v.-IC. Overall, IT-IC resulted in increased numbers of activated T and NK cells within tumors, better IC retention in the tumor, enhanced inhibition of tumor growth, and improved survival compared with i.v.-IC.

Inhibitory receptors on lymphocytes: insights from infections

Costimulatory and inhibitory receptors are critical regulators of adaptive immune cell function. These pathways regulate the initiation and termination of effective immune responses to infections while limiting autoimmunity and/or immunopathology. This review focuses on recent advances in our understanding of inhibitory receptor pathways and their roles in different diseases and/or infections, emphasizing potential clinical applications and important unanswered mechanistic questions. Although significant progress has been made in defining the influence of inhibitory receptors at the cellular level, relatively little is known about the underlying molecular pathways. We discuss our current understanding of the molecular mechanisms for key inhibitory receptor pathways, highlight major gaps in knowledge, and explore current and future clinical applications.

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.

Natural killer cells and their role in rheumatoid arthritis: friend or foe?

Rheumatoid arthritis (RA) is a long-term disease that leads to inflammation of the joints and surrounding tissues. Natural killer (NK) cells are an important part of the innate immune system and are responsible for the first line of defense against pathogens during the initial immune challenge before the adaptive immune system eventually eliminates the infectious burden. NK cells have the capacity to damage normal cells or through interaction with other cells such as dendritic cells, macrophages, and T cells cause autoimmune diseases, such as RA. NK cells isolated from the joints of patients with RA suggest that they may play a role in this disease. However, the involvement of NK cells in RA pathology is not fully elucidated. Both protective and detrimental roles of NK cells in RA have recently been reported. A better understanding of NK cells' role in RA might help to develop new therapeutic strategies for treatment of the RA or other autoimmune diseases. We have decided in this paper to focus on the NK cell biology, and attempt to bring the interested readership of this Journal up to date on the NK cell, specifically its possible relation to RA.

Phenotypic changes to the endogenous antigen-specific CD8+ T cell response correlates with the development and resolution of allergic airway disease

The role of CD8(+) T cells in the pathogenesis of asthma remains controversial, as both pro- and anti-inflammatory functions have been suggested. This study was designed to examine the endogenous CD8(+) T cell response in a biphasic ovalbumin (OVA)-induced model of allergic airway disease (AAD) and its subsequent resolution with the development of local inhalational tolerance (LIT). We observed increases in OVA-specific CD8(+) T cell numbers in the local lung compartments (bronchoalveolar lavage, lung tissue, hilar lymph node) at AAD and LIT; systemic compartments (spleen, inguinal lymph node) displayed no such increases in CD8(+) T cell numbers. OVA-specific CD8(+) T cells appeared to exhibit plasticity both phenotypically and functionally. They possessed pro-inflammatory characteristics at AAD, with high phenotypic expression of CD11a and increased functional expression of granzyme B and interferon-γ. In contrast, at LIT they showed increased phenotypic expression of the inhibitory marker NKG2A and functionally did not produce granzyme B or interferon-γ. In addition, in a discontinuous model the OVA-specific CD8(+) T cells could be recalled on re-exposure to OVA, demonstrating memory. Finally, confocal microscopy results showed that OVA-specific CD8(+) T cells at AAD are associated with B cell aggregates in lung tissue. These B cell aggregates resembled tertiary ectopic lymphoid tissue and may thus provide a local environment for the salient cellular interactions that contribute to the development of LIT.

Natural killer T cells subsets in cancer, functional defects in prostate cancer and implications for immunotherapy

Natural killer T cells are T lymphocytes with unique activation and effector properties. The majority of NKT cells, termed type-I or iNKT cells, recognize lipid antigens presented on MHC-like CD1d molecules. Type-I NKT cells have the capacity to rapidly secrete various cytokines upon activation, thereby regulate immune responses exerts dominant anti-tumor and anti-microbial effector functions. Specific activation of type-I NKT cells in mouse models boosts immunity and prevents metastasis, which has led to a number of phase I-II clinical trials. Since the discovery of NKT cells other subsets with different specificities and effector functions have been described. This article briefly reviews the physiological functions of NKT cell subsets, their implications in cancer and the attempts that have been made to employ NKT cells for immune therapy of cancer.

Calcineurin-dependent negative regulation of CD94/NKG2A expression on naive CD8+ T cells

Immune responses lead to expression of immunoregulatory molecules on T cells, including natural killer (NK) receptors, such as CD94/NKG2A on CD8+ T cells; these receptors restrain CD8+ responses, thereby preventing T-cell exhaustion in chronic infections and limiting immunopathology. Here, we examined the requirements for inducing CD94/NKG2A on T cells responding to antigen. In vitro, moderate induction of CD94/NKG2A expression occurred after exposure of naive CD8+ (but not CD4+) cells to CD3 ligation or specific peptide. Surprisingly, expression was inhibited by CD28/B7 costimulation. Such inhibition applied only to CD94/NKG2A and not other NK receptors (NKG2D) and was mediated by IL-2. Inhibition by IL-2 occurred via a NFAT cell-independent component of the calcineurin pathway, and CD94/NKG2A induction was markedly enhanced in the presence of calcineurin blockers, such as FK506 or using calcineurin-deficient T cells, both in vitro and in vivo. In addition to CD28-dependent inhibition by IL-2, CD94/NKG2A expression was impaired by several other cytokines (IL-4, IL-23, and transforming growth factor-β) but enhanced by others (IL-6, IL-10, and IL-21). The complex interplay between these various stimuli may account for the variable expression of CD94/NKG2A during responses to different pathogens in vivo.

E-cadherin expression on human carcinoma cell affects trastuzumab-mediated antibody-dependent cellular cytotoxicity through killer cell lectin-like receptor G1 on natural killer cells

Trastuzumab is a recombinant antibody drug that is widely used for the treatment of HER2-overexpressing breast carcinoma. Despite encouraging clinical results, many HER2-overexpressing carcinomas are primarily resistant to trastuzumab. We attempted to explain trastuzumab resistance and search for solutions. Since the killer cell lectin-like receptor G1 (KLRG1), an inhibitory receptor expressed on subsets of natural killer (NK) cells recognizes E-cadherin as ligands and may inhibit immune responses by regulating the effector function of NK cells, we used HER2-overexpressing carcinoma cells which were expressing E-cadherin to investigate the role of antibody-dependent cellular cytotoxicity (ADCC) through KLRG1 on NK cells in vitro and vivo. The results indicated that HER2-overexpressing carcinoma cells were killed by trastuzumab-mediated ADCC and the ADCC activity was reflected the degree of E-cadherin expression on carcinoma cells. We found that expression of E-cadherin was shown to be a predictor of response to trastuzumab-based treatment for HER2-overexpressing carcinomas, furthermore, trastuzumab-mediated ADCC was markedly enhanced by KLRG1-negative peripheral blood mononuclear cells (PBMCs(KLRG1(-))).

NK cells suppress experimental cholestatic liver injury by an interleukin-6-mediated, Kupffer cell-dependent mechanism

BACKGROUND & AIMS

Natural killer (NK) cells are innate immune effector cells first characterized by their ability to lyse susceptible tumor cells. Recent studies demonstrated their role in initiating and modulating adaptive immunity. NK cells represent a larger percentage of the lymphoid population in liver than other organs, suggesting that hepatic NK cells express some unique function. Here, we examined the response of NK cells to liver injury that occurs in a mouse model of biliary obstruction.

METHODS

Bile duct ligations (BDL) were performed in mice previously depleted or not depleted of NK cells. NK cell activation, interleukin (IL)-6 mRNA expression and protein production by Kupffer cells, and the ability of exogenous IL-6 to ameliorate liver injury in NK cell-depleted mice, were determined.

RESULTS

The number of activated hepatic NK cells increased markedly following BDL. Activation was suppressed in mice rendered Kupffer cell-depleted prior to ligation. Increased liver injury occurred in NK cell-depleted mice correlating with a reduction in IL-6 production. Purified Kupffer cells, obtained from NK cell-depleted or anti-interferon (IFN)-γ monoclonal antibody-pretreated mice following BDL, produced less IL-6 in culture than did Kupffer cells derived from control animals. In culture, hepatic NK cells derived from BDL mice stimulated IFN-γ-dependent IL-6 production by Kupffer cells; splenic NK cells obtained from the same animals had a negligible effect. Treatment with recombinant murine IL-6 reduced liver injury in BDL, NK cell-depleted mice.

CONCLUSIONS

Hepatic NK cells suppress cholestatic liver injury by stimulating Kupffer cell-dependent IL-6 production.

KLRG1+NKG2A+ CD8 T cells mediate protection and participate in memory responses during γ-herpesvirus infection

Functional CD8 T cell effector and memory responses are generated and maintained during murine γ-herpesvirus 68 (γHV68) persistent infection despite continuous presentation of viral lytic Ags. However, the identity of the CD8 T cell subpopulations that mediate effective recall responses and that can participate in the generation of protective memory to a γ-herpesvirus infection remains unknown. During γHV68 persistence, ∼75% of γHV68-specific CD8 T cells coexpress the NK receptors killer cell lectin-like receptor G1 (KLRG1) and NKG2A. In this study, we take advantage of this unique phenotype to analyze the capacity of CD8 T cells expressing or not expressing KLRG1 and NKG2A to mediate effector and memory responses. Our results show that γHV68-specific KLRG1(+)NKG2A(+) CD8 T cells have an effector memory phenotype as well as characteristics of polyfunctional effector cells such us IFN-γ and TNF-α production, killing capacity, and are more efficient at protecting against a γHV68 challenge than their NKG2A(-)KLRG1(-) counterparts. Nevertheless, γHV68-specific NKG2A(+)KLRG1(+) CD8 T cells express IL-7 and IL-15 receptors, can survive long-term without cognate Ag, and subsequently mount a protective response during antigenic recall. These results highlight the plasticity of the immune system to generate protective effector and proliferative memory responses during virus persistence from a pool of KLRG1(+)NKG2A(+) effector memory CD8 T cells.

The descent of memory T cells

Our T cell repertoire is shaped by antigen encounter. From a naive T cell pool that contains millions of different T cells with unknown specificities, pathogen infection leads to selection of those T cells that can detect pathogen-derived antigens. Following clearance of infection, a population of memory T cells remains and protects the individual from severe reinfection. A central question in the field has been how the generation of long-lived memory T cells, versus short-lived ("terminally differentiated") T cells, is controlled. In this review we discuss the models that have been put forward to explain the generation of memory T cells after infection and the experimental evidence supporting these hypotheses. Based on the available data we propose a new model that stipulates that during immune responses T cells do not acquire different fates that determine their subsequent long-term survival but rather T cells assume different states that simply reflect the likelihood of future survival, states that can still be modulated by external signals.

Tumor-induced disruption of proximal TCR-mediated signal transduction in tumor-infiltrating CD8+ lymphocytes inactivates antitumor effector phase

The presence in cancer tissue of Ag-specific, activated tumor infiltrating CD8(+) T cells proves that tumors express Ags capable of eliciting immune response. Therefore, in general, tumor escape from immune-mediated clearance is not attributable to immunological ignorance. However, tumor-infiltrating lymphocytes are defective in effector phase function, demonstrating tumor-induced immune suppression that likely underlies tumor escape. Since exocytosis of lytic granules is dependent upon TCR-mediated signal transduction, it is a reasonable contention that tumors may induce defective signal transduction in tumor infiltrating T cells. In this review, we consider the biochemical basis for antitumor T cell dysfunction, focusing on the role of inhibitory signaling receptors in restricting TCR-mediated signaling in tumor-infiltrating lymphocytes.

Phenotypic and functional profiling of malaria-induced CD8 and CD4 T cells during blood-stage infection with Plasmodium yoelii

It is widely accepted that antibodies and CD4 T cells play critical roles in the immune response during the blood stage of malaria, whereas the role of CD8 T cells remains controversial. Here, we show that both CD8 and CD4 T cells robustly responded to an acute self-limiting blood-stage infection with Plasmodium yoelii. Similar to antigen-specific T cells, both CD8 and CD4 T cells showed dynamic expression of the surface proteins interleukin (IL)-7R and programmed death-1 (PD-1). Additionally, activated CD8 T cells showed differences in the expression of Killer cell lectin-like receptor G1, L-selectin and B cell lymphoma-2 and produced granzyme B, indicating cytotoxic activity, and the initially high expression of T-box transcription factor TBX21 in malaria-activated CD4 T cells indicated an early T helper type 1 (Th1)-skewed immune response. Our data demonstrate that blood-stage malaria infection results in a striking T-cell response and that activated CD8 and CD4 T cells have phenotypic and functional characteristics that are consistent with conventional antigen-specific effector and memory T cells. Therefore, a better understanding of the CD8 and CD4 T-cell response induced by blood-stage infection may prove to be essential in the development of a vaccine that targets the erythrocytic stage of the malarial parasite.

CD4 T-cell help programs a change in CD8 T-cell function enabling effective long-term control of murine gammaherpesvirus 68: role of PD-1-PD-L1 interactions

We previously showed that agonistic antibodies to CD40 could substitute for CD4 T-cell help and prevent reactivation of murine gammaherpesvirus 68 (MHV-68) in the lungs of major histocompatibility complex (MHC) class II(-/-) (CII(-/-)) mice, which are CD4 T cell deficient. Although CD8 T cells were required for this effect, no change in their activity was detected in vitro. A key question was whether anti-CD40 treatment (or CD4 T-cell help) changed the function of CD8 T cells or another cell type in vivo. To address this question, in the present study, we showed that adoptive transfer of CD8 T cells from virus-infected wild-type mice or anti-CD40-treated CII(-/-) mice caused a significant reduction in lung viral titers, in contrast to those from control CII(-/-) mice. Anti-CD40 treatment also greatly prolonged survival of infected CII(-/-) mice. This confirms that costimulatory signals cause a change in CD8 T cells enabling them to maintain effective long-term control of MHV-68. We investigated the nature of this change and found that expression of the inhibitory receptor PD-1 was significantly increased on CD8 T cells in the lungs of MHV-68-infected CII(-/-), CD40(-/-), or CD80/86(-/-) mice, compared with that in wild-type or CD28/CTLA4(-/-) mice, correlating with the level of viral reactivation. Furthermore, blocking PD-1-PD-L1 interactions significantly reduced viral reactivation in CD4 T-cell-deficient mice. In contrast, the absence of another inhibitory receptor, NKG2A, had no effect. These data suggest that CD4 T-cell help programs a change in CD8 T-cell function mediated by altered PD-1 expression, which enables effective long-term control of MHV-68.

Immunoregulatory functions of KLRG1 cadherin interactions are dependent on forward and reverse signaling

KLRG1 is an inhibitory receptor expressed on a subset of mature T and NK cells. Recently, E-, N-, and R-cadherin have been identified as ligands for KLRG1. Cadherins are a large family of transmembrane or membrane-associated glycoproteins that were thought to only bind specifically to other cadherins to mediate specific cell-to-cell adhesion in a Ca(2+)-dependent manner. The consequences of cadherin KLRG1 molecular interactions are not well characterized. Here, we report that the first 2 extracellular domains of cadherin are sufficient to initiate a KLRG1-dependent signaling. We also demonstrate that KLRG1 engagement inhibits cadherin-dependent cellular adhesion and influences dendritic cell secretion of inflammatory cytokines, thereby exerting immunosuppressive effects. Consistent with this, engagement of cadherin by KLRG1 molecule induces cadherin tyrosine phosphorylation. Therefore, KLRG1/cadherin interaction leads to the generation of a bidirectional signal in which both KLRG1 and cadherin activate downstream signaling cascades simultaneously. Taken together, our results provide novel insights on how KLRG1 and E-cadherin interactions are integrated to differentially regulate not only KLRG1(+) cells, but also E-cadherin-expressing cells, such as dendritic cells.