Recognition of the class Ib molecule Qa-1(b) by putative activating receptors CD94/NKG2C and CD94/NKG2E on mouse natural killer cells

A novel NKG2A alpaca nanobody targeting immune checkpoint blockade for the treatment of malignant melanoma

Alpacas belong to the Camelidae family. Antibodies produced through alpaca immunization are called nanoantibodies. Compared to traditional antibodies, nanoantibodies have several characteristics, including smaller molecular weight, stable structure, high homology with human antibodies, and suitability for prokaryotic expression. Malignant melanoma (MM) is a severe and aggressive form of cancer that affects both humans and animals. It commonly arises in the mucous membranes of the skin, nose, mouth, anus, and digestive tract, as well as in the choroid of the eyes. Multiple factors contribute to melanocyte carcinogenesis, including ultraviolet (UV) radiation, endocrine disorders, viral infections, immune deficiencies, and chemical carcinogens. At present, surgical resection remains to be the primary treatment for MM, although the prognosis is generally poor. However, targeted therapy and immune checkpoint inhibitors (ICIs) are increasingly employed in the clinical treatment of melanoma. NKG2A is an inhibitory receptor protein found on the surface of CD8+ T cells and natural killer (NK) cells. HLA-E ligands expressed on the surface of malignant melanoma cells can facilitate immune evasion by binding to the NKG2A receptor complex on immune cells. This interaction suppresses immune responses, enabling tumor cells to escape immune surveillance. Immunosuppressive antibody drugs work by blocking this recognition mechanism, thereby reactivating immune cells to target and destroy tumor cells. As such, NKG2A has emerged as a novel target for immunotherapeutic intervention. In this study, an alpaca-derived nanoantibody targeting NKG2A with high affinity was obtained from a melanoma-specific phage library display. Through induced expression and protein purification, a monoclonal nanobody strain expressing NKG2A was successfully isolated. This NKG2A-targeting nanobody demonstrates the potential for application in both the detection and treatment of MM.

Temporal dynamics of immune cell transcriptomics in brain metastasis progression influenced by gut microbiome dysbiosis

Interactions between metastatic cancer cells and the brain microenvironment regulate brain metastasis (BrMet) progression. Central nervous system (CNS)-native and peripheral immune cells influence the BrMet immune landscape, but the dynamics and factors modulating this microenvironment remain unclear. As the gut microbiome impacts CNS and peripheral immune activity, we investigated its role in regulating immune response dynamics throughout BrMet stages. Antibiotic-induced (ABX) gut dysbiosis significantly increased BrMet burden versus controls but was equalized with fecal matter transplantation, highlighting microbiome diversity as a regulator of BrMet. Single-cell sequencing revealed a highly dynamic immune landscape during BrMet progression in both conditions. However, the timing of the monocyte inflammatory response was altered. Microglia displayed an elevated activation signature in late-stage metastasis in ABX-treated mice. T cell and microglia perturbation revealed involvement of these cell types in modulating BrMet under gut dysbiosis. These data indicate profound effects on immune response dynamics imposed by gut dysbiosis across BrMet progression.

NK cell-based immunotherapy for hepatocellular carcinoma: Challenges and opportunities

Hepatocellular carcinoma (HCC) remains one of the most challenging malignancies globally, characterized by significant heterogeneity, late-stage diagnosis, and resistance to treatment. In recent years, the advent of immune-checkpoint blockades (ICBs) and targeted immune cell therapies has marked a substantial advancement in HCC treatment. However, the clinical efficacy of these existing therapies is still limited, highlighting the urgent need for new breakthroughs. Natural killer (NK) cells, a subset of the innate lymphoid cell family, have shown unique advantages in the anti-tumour response. With increasing evidence suggesting the crucial role of dysfunctional NK cells in the pathogenesis and progression of HCC, considerable efforts have been directed toward exploring NK cells as a potential therapeutic target for HCC. In this review, we will provide an overview of the role of NK cells in normal liver immunity and in HCC, followed by a detailed discussion of various NK cell-based immunotherapies and their potential applications in HCC treatment.

HLA-E: Immune Receptor Functional Mechanisms Revealed by Structural Studies

HLA-E is a nonclassical, nonpolymorphic, class Ib HLA molecule. Its primary function is to present a conserved nonamer peptide, termed VL9, derived from the signal sequence of classical MHC molecules to the NKG2x-CD94 receptors on NK cells and a subset of T lymphocytes. These receptors regulate the function of NK cells, and the importance of this role, which is conserved across mammalian species, probably accounts for the lack of genetic polymorphism. A second minor function is to present other, weaker binding, pathogen-derived peptides to T lymphocytes. Most of these peptides bind suboptimally to HLA-E, but this binding appears to be enabled by the relative stability of peptide-free, but receptive, HLA-E-β2m complexes. This, in turn, may favor nonclassical antigen processing that may be associated with bacteria infected cells. This review explores how the structure of HLA-E, bound to different peptides and then to NKG2-CD94 or T-cell receptors, relates to HLA-E cell biology and immunology. A detailed understanding of this molecule could open up opportunities for development of universal T-cell and NK-cell-based immunotherapies.

Targeting the aminopeptidase ERAP enhances antitumor immunity by disrupting the NKG2A-HLA-E inhibitory checkpoint

The aminopeptidase, endoplasmic reticulum aminopeptidase 1 (ERAP1), trims peptides for loading into major histocompatibility complex class I (MHC class I), and loss of this activity has broad effects on the MHC class I peptidome. Here, we investigated the impact of targeting ERAP1 in immune checkpoint blockade (ICB), as MHC class I interactions mediate both activating and inhibitory functions in antitumor immunity. Loss of ERAP sensitized mouse tumor models to ICB, and this sensitivity depended on CD8+ T cells and natural killer (NK) cells. In vivo suppression screens revealed that Erap1 deletion inactivated the inhibitory NKG2A-HLA-E checkpoint, which requires presentation of a restricted set of invariant epitopes (VL9) on HLA-E. Loss of ERAP altered the HLA-E peptidome, preventing NKG2A engagement. In humans, ERAP1 and ERAP2 showed functional redundancy for the processing and presentation of VL9, and loss of both inactivated the NKG2A checkpoint in cancer cells. Thus, loss of ERAP phenocopies the inhibition of the NKG2A-HLA-E pathway and represents an attractive approach to inhibit this critical checkpoint.

CD94 deficiency or blockade unleashes the anti-tumor immunity in mice and humanized murine models

NKG2 family members have emerged as promising targets in tumor immunotherapy. CD94 can dimerize with both inhibitory and activating NKG2 proteins, while the overall effect and value of targeting CD94 on anti-tumor immunity are unclear. Here, it is shown that the expression of CD94 is upregulated on tumor-infiltrating natural killer (NK) cells and CD8+ T cells, and is related to their exhausted characteristics. Tumor-bearing CD94 knockout (CD94-KO) mice exhibit delayed tumor growth, decreased lung metastases, and prolonged survival. Single cell RNA-seq reveals a remodeled tumor microenvironment in CD94-KO mice, with a reduction in immunosuppressive cells and an increase in anti-tumor immune cells. Moreover, NK cells and CD8+ T cells become proliferative and strongly tumoricidal in CD94-KO mice, thus contributing to the tumor inhibition effect of CD94 deficiency. Treatment with a humanized anti-CD94 blocking antibody (h15C10) alone, in tumor-bearing humanized mouse, delays tumor progression, and improves the therapeutic efficacy of PD-L1 blockade through combination therapy. Our study indicates that CD94 may work as a candidate target in checkpoint immunotherapy.

Innate and adaptive immune responses that control lymph-borne viruses in the draining lymph node

The interstitial fluids in tissues are constantly drained into the lymph nodes (LNs) as lymph through afferent lymphatic vessels and from LNs into the blood through efferent lymphatics. LNs are strategically positioned and have the appropriate cellular composition to serve as sites of adaptive immune initiation against invading pathogens. However, for lymph-borne viruses, which disseminate from the entry site to other tissues through the lymphatic system, immune cells in the draining LN (dLN) also play critical roles in curbing systemic viral dissemination during primary and secondary infections. Lymph-borne viruses in tissues can be transported to dLNs as free virions in the lymph or within infected cells. Regardless of the entry mechanism, infected myeloid antigen-presenting cells, including various subtypes of dendritic cells, inflammatory monocytes, and macrophages, play a critical role in initiating the innate immune response within the dLN. This innate immune response involves cellular crosstalk between infected and bystander innate immune cells that ultimately produce type I interferons (IFN-Is) and other cytokines and recruit inflammatory monocytes and natural killer (NK) cells. IFN-I and NK cell cytotoxicity can restrict systemic viral spread during primary infections and prevent serious disease. Additionally, the memory CD8+ T-cells that reside or rapidly migrate to the dLN can contribute to disease prevention during secondary viral infections. This review explores the intricate innate immune responses orchestrated within dLNs that contain primary viral infections and the role of memory CD8+ T-cells following secondary infection or CD8+ T-cell vaccination.

LAG-3 sustains TOX expression and regulates the CD94/NKG2-Qa-1b axis to govern exhausted CD8 T cell NK receptor expression and cytotoxicity

Exhausted CD8 T (Tex) cells in chronic viral infection and cancer have sustained co-expression of inhibitory receptors (IRs). Tex cells can be reinvigorated by blocking IRs, such as PD-1, but synergistic reinvigoration and enhanced disease control can be achieved by co-targeting multiple IRs including PD-1 and LAG-3. To dissect the molecular changes intrinsic when these IR pathways are disrupted, we investigated the impact of loss of PD-1 and/or LAG-3 on Tex cells during chronic infection. These analyses revealed distinct roles of PD-1 and LAG-3 in regulating Tex cell proliferation and effector functions, respectively. Moreover, these studies identified an essential role for LAG-3 in sustaining TOX and Tex cell durability as well as a LAG-3-dependent circuit that generated a CD94/NKG2+ subset of Tex cells with enhanced cytotoxicity mediated by recognition of the stress ligand Qa-1b, with similar observations in humans. These analyses disentangle the non-redundant mechanisms of PD-1 and LAG-3 and their synergy in regulating Tex cells.

ITAM-based receptors in natural killer cells

The ability of cells of the immune system to acquire features such as increased longevity and enhanced secondary responses was long thought to be restricted to cells of the adaptive immune system. Natural killer (NK) cells have challenged this notion by demonstrating that they can also gain adaptive features. This has been observed in both humans and mice during infection with cytomegalovirus (CMV). The generation of adaptive NK cells requires antigen-specific recognition of virally infected cells through stimulatory NK receptors. These receptors lack the ability to signal on their own and rather rely on adaptor molecules that contain ITAMs for driving signals. Here, we highlight our understanding of how these receptors influence the production of adaptive NK cells and propose areas in the field that merit further investigation.

Intratumoral co-injection of NK cells and NKG2A-neutralizing monoclonal antibodies

NK-cell reactivity against cancer is conceivably suppressed in the tumor microenvironment by the interaction of the inhibitory receptor NKG2A with the non-classical MHC-I molecules HLA-E in humans or Qa-1b in mice. We found that intratumoral delivery of NK cells attains significant therapeutic effects only if co-injected with anti-NKG2A and anti-Qa-1b blocking monoclonal antibodies against solid mouse tumor models. Such therapeutic activity was contingent on endogenous CD8 T cells and type-1 conventional dendritic cells (cDC1). Moreover, the anti-tumor effects were enhanced upon combination with systemic anti-PD-1 mAb treatment and achieved partial abscopal efficacy against distant non-injected tumors. In xenografted mice bearing HLA-E-expressing human cancer cells, intratumoral co-injection of activated allogeneic human NK cells and clinical-grade anti-NKG2A mAb (monalizumab) synergistically achieved therapeutic effects. In conclusion, these studies provide evidence for the clinical potential of intratumoral NK cell-based immunotherapies that exert their anti-tumor efficacy as a result of eliciting endogenous T-cell responses.

The prospective outcome of the monkeypox outbreak in 2022 and characterization of monkeypox disease immunobiology

A new threat to global health re-emerged with monkeypox's advent in early 2022. As of November 10, 2022, nearly 80,000 confirmed cases had been reported worldwide, with most of them coming from places where the disease is not common. There were 53 fatalities, with 40 occurring in areas that had never before recorded monkeypox and the remaining 13 appearing in the regions that had previously reported the disease. Preliminary genetic data suggest that the 2022 monkeypox virus is part of the West African clade; the virus can be transmitted from person to person through direct interaction with lesions during sexual activity. It is still unknown if monkeypox can be transmitted via sexual contact or, more particularly, through infected body fluids. This most recent epidemic's reservoir host, or principal carrier, is still a mystery. Rodents found in Africa can be the possible intermediate host. Instead, the CDC has confirmed that there are currently no particular treatments for monkeypox virus infection in 2022; however, antivirals already in the market that are successful against smallpox may mitigate the spread of monkeypox. To protect against the disease, the JYNNEOS (Imvamune or Imvanex) smallpox vaccine can be given. The spread of monkeypox can be slowed through measures such as post-exposure immunization, contact tracing, and improved case diagnosis and isolation. Final Thoughts: The latest monkeypox epidemic is a new hazard during the COVID-19 epidemic. The prevailing condition of the monkeypox epidemic along with coinfection with COVID-19 could pose a serious condition for clinicians that could lead to the global epidemic community in the form of coinfection.

Engineering Human Pluripotent Stem Cell Lines to Evade Xenogeneic Transplantation Barriers

Allogeneic human pluripotent stem cell (hPSC)-derived cells and tissues for therapeutic transplantation must necessarily overcome immunological rejection by the recipient. To define these barriers and to create cells capable of evading rejection for preclinical testing in immunocompetent mouse models, we genetically ablated β2m, Tap1, Ciita, Cd74, Mica, and Micb to limit expression of HLA-I, HLA-II, and natural killer cell activating ligands in hPSCs. Though these and even unedited hPSCs readily formed teratomas in cord blood-humanized immunodeficient mice, grafts were rapidly rejected by immunocompetent wild-type mice. Transplantation of these cells that also expressed covalent single chain trimers of Qa1 and H2-Kb to inhibit natural killer cells and CD55, Crry, and CD59 to inhibit complement deposition led to persistent teratomas in wild-type mice. Expression of additional inhibitory factors such as CD24, CD47, and/or PD-L1 had no discernible impact on teratoma growth or persistence. Transplantation of HLA-deficient hPSCs into mice genetically deficient in complement and depleted of natural killer cells also led to persistent teratomas. Thus, T cell, NK cell, and complement evasion are necessary to prevent immunological rejection of hPSCs and their progeny. These cells and versions expressing human orthologs of immune evasion factors can be used to refine tissue- and cell type-specific immune barriers, and to conduct preclinical testing in immunocompetent mouse models.

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.

Adaptive NK cell response to human cytomegalovirus: Facts and open issues

Human cytomegalovirus (HCMV) infection exerts broad effects on the immune system. These include the differentiation and persistent expansion of a mature NK cell subset which displays a characteristic phenotypic and functional profile hallmarked by expression of the HLA-E-specific CD94/NKG2C activating receptor. Based on our experience and recent advances in the field, we overview the adaptive features of the NKG2C+ NK cell response, discussing observations and open questions on: (a) the mechanisms and influence of viral and host factors; (b) the existence of other NKG2C- NK cell subsets sharing adaptive features; (c) the development and role of adaptive NKG2C+ NK cells in the response to HCMV in hematopoietic and solid organ transplant patients; (d) their relation with other viral infections, mainly HIV-1; and (e) current perspectives for their use in adoptive immunotherapy of cancer.

Monkeypox: disease epidemiology, host immunity and clinical interventions

Monkeypox virus (MPXV), which causes disease in humans, has for many years been restricted to the African continent, with only a handful of sporadic cases in other parts of the world. However, unprecedented outbreaks of monkeypox in non-endemic regions have recently taken the world by surprise. In less than 4 months, the number of detected MPXV infections has soared to more than 48,000 cases, recording a total of 13 deaths. In this Review, we discuss the clinical, epidemiological and immunological features of MPXV infections. We also highlight important research questions and new opportunities to tackle the ongoing monkeypox outbreak.

The nonclassical MHC class I Qa-1 expressed in layer 6 neurons regulates activity-dependent plasticity via microglial CD94/NKG2 in the cortex

During developmental critical periods, circuits are sculpted by a process of activity-dependent competition. The molecular machinery involved in regulating the complex process of responding to different levels of activity is now beginning to be identified. Here, we show that the nonclassical major histocompatibility class I (MHCI) molecule Qa-1 is expressed in the healthy brain in layer 6 corticothalamic neurons. In the visual cortex, Qa-1 expression begins during the critical period for ocular dominance (OD) plasticity and is regulated by neuronal activity, suggesting a role in regulating activity-dependent competition. Indeed, in mice lacking Qa-1, OD plasticity is perturbed. Moreover, signaling through CD94/NKG2, a known cognate Qa-1 heterodimeric receptor in the immune system, is implicated: selectively targeting this interaction phenocopies the plasticity perturbation observed in Qa-1 knockouts. In the cortex, CD94/NKG2 is expressed by microglial cells, which undergo activity-dependent changes in their morphology in a Qa-1–dependent manner. Our study thus reveals a neuron–microglial interaction dependent upon a nonclassical MHCI molecule expressed in L6 neurons, which regulates plasticity in the visual cortex. These results also point to an unexpected function for the Qa-1/HLA-E (ligand) and CD94/NKG2 (receptor) interaction in the nervous system, in addition to that described in the immune system.

A Single-Domain TCR-like Antibody Selective for the Qa-1b/Qdm Peptide Complex Enhances Tumoricidal Activity of NK Cells via Blocking the NKG2A Immune Checkpoint

The NKG2A/HLA-E axis is an immune checkpoint that suppresses immune effector activity in the tumor microenvironment. In mice, the ligand for the NKG2A/CD94 inhibitory receptor is the nonclassical MHC molecule Qa-1^b, the HLA-E ortholog, which presents the peptide AMAPRTLLL, referred to as Qdm (for Qa-1 determinant modifier). This dominant peptide is derived from the leader sequences of murine classical MHC class I encoded by the H-2D and -L loci. To broaden our understanding of Qa-1^b/Qdm peptide complex biology and its tumor protective role, we identified a TCR-like Ab from a single domain VHH library using yeast surface display. The TCR-like Ab (EXX-1) binds only to the Qa-1^b/Qdm peptide complex and not to Qa-1^b alone or Qa-1^b loaded with control peptides. Conversely, currently available Abs to Qa-1^b bind independent of peptide loaded. Flow cytometric results revealed that EXX-1 selectively bound to Qa-1^b/Qdm-positive B16F10, RMA, and TC-1 mouse tumor cells but only after pretreatment with IFN-γ; no binding was observed following genetic knockdown of Qa-1^b or Qdm peptide. Furthermore, EXX-1 Ab blockade promoted NK cell-mediated tumor cell lysis in vitro. Our findings show that EXX-1 has exquisite binding specificity for the Qa-1^b/Qdm peptide complex, making it a valuable research tool for further investigation of the Qa-1^b/Qdm peptide complex expression and regulation in healthy and diseased cells and for evaluation as an immune checkpoint blocking Ab in syngeneic mouse tumor models.

Epigenetic regulation of natural killer cell memory

Immunological memory is the underlying mechanism by which the immune system remembers previous encounters with pathogens to produce an enhanced secondary response upon re-encounter. It stands as the hallmark feature of the adaptive immune system and the cornerstone of vaccine development. Classic recall responses are executed by conventional T and B cells, which undergo somatic recombination and modify their receptor repertoire to ensure recognition of a vast number of antigens. However, recent evidence has challenged the dogma that memory responses are restricted to the adaptive immune system, which has prompted a reevaluation of what delineates "immune memory." Natural killer (NK) cells of the innate immune system have been at the forefront of these pushed boundaries, and have proved to be more "adaptable" than previously thought. Like T cells, we now appreciate that their "natural" abilities actually require a myriad of signals for optimal responses. In this review, we discuss the many signals required for effector and memory NK cell responses and the epigenetic mechanisms that ultimately endow their enhanced features.

Natural Killer Cells and Type 1 Innate Lymphoid Cells in Hepatocellular Carcinoma: Current Knowledge and Future Perspectives

Natural killer (NK) cells and type 1 innate lymphoid cells (ILC1) are specific innate lymphoid cell subsets that are key for the detection and elimination of pathogens and cancer cells. In liver, while they share a number of characteristics, they differ in many features. These include their developmental pathways, tissue distribution, phenotype and functions. NK cells and ILC1 contribute to organ homeostasis through the production of key cytokines and chemokines and the elimination of potential harmful bacteria and viruses. In addition, they are equipped with a wide range of receptors, allowing them to detect “stressed cells’ such as cancer cells. Our understanding of the role of innate lymphoid cells in hepatocellular carcinoma (HCC) is growing owing to the development of mouse models, the progress in immunotherapeutic treatment and the recent use of scRNA sequencing analyses. In this review, we summarize the current understanding of NK cells and ILC1 in hepatocellular carcinoma and discuss future strategies to take advantage of these innate immune cells in anti-tumor immunity. Immunotherapies hold great promise in HCC, and a better understanding of the role and function of NK cells and ILC1 in liver cancer could pave the way for new NK cell and/or ILC1-targeted treatment.

The nonclassical immune surveillance for ERAAP function

The peptide repertoire presented by MHC class I molecules on the cell surface is essential for the immune surveillance of intracellular pathogens and transformed cells. The generation of this peptide repertoire is critically dependent on the endoplasmic reticulum aminopeptidase associated with antigen processing (ERAAP). Loss of ERAAP function leads to the generation of a profoundly disrupted peptide repertoire including many novel and immunogenic peptides. Strikingly, a large fraction of these novel peptides on ERAAP-KO cells are presented by the nonclassical MHC Ib molecule, Qa-1^b. One immunodominant Qa-1^b-restricted novel peptide is recognized by a unique CD8⁺ T cell population showing features of both conventional cytotoxic T cells and unconventional innate-like T cells. While much remains to be uncovered, here we summarize the latest discoveries of our lab on the important immune surveillance of ERAAP function mediated by nonclassical MHC Ib molecules and their unusual cognate T cells.

Viral infection modulates Qa-1b in infected and bystander cells to properly direct NK cell killing

Natural killer (NK) cell activation depends on the signaling balance of activating and inhibitory receptors. CD94 forms inhibitory receptors with NKG2A and activating receptors with NKG2E or NKG2C. We previously demonstrated that CD94-NKG2 on NK cells and its ligand Qa-1b are important for the resistance of C57BL/6 mice to lethal ectromelia virus (ECTV) infection. We now show that NKG2C or NKG2E deficiency does not increase susceptibility to lethal ECTV infection, but overexpression of Qa-1b in infected cells does. We also demonstrate that Qa-1b is down-regulated in infected and up-regulated in bystander inflammatory monocytes and B cells. Moreover, NK cells activated by ECTV infection kill Qa-1b-deficient cells in vitro and in vivo. Thus, during viral infection, recognition of Qa-1b by activating CD94/NKG2 receptors is not critical. Instead, the levels of Qa-1b expression are down-regulated in infected cells but increased in some bystander immune cells to respectively promote or inhibit their killing by activated NK cells.

Clinical Relevance of CD4 Cytotoxic T Cells in High-Risk Neuroblastoma

Neuroblastoma is the most common extracranial childhood solid tumor. The majority of high-risk neuroblastoma is resistant/refractory to the current high intensity therapy, and the survival of these patients remains poor for the last three decades. To effectively treat these extremely unfavorable neuroblastomas, innovative immunotherapy approaches would be the most promising. In this article, we discuss the identity of tumor-infiltrating effector cells and immunosuppressive cells in high-risk neuroblastoma. Neuroblastoma is unique in that it expresses little or no classical HLA Class I and II. In contrast, high-risk neuroblastomas express the stress-responsive non-classical Class I, HLA-E molecule. HLA-E is the ligand of activating receptors NKG2C/E that are expressed on memory NK cells, CD8+T cells and CD4 CTLs. By examining a comprehensive RNA-seq gene expression dataset, we detected relatively high levels of CD4 expression in high-risk neuroblastoma tissues. The majority of CD4+ cells were CD3+, and thus they were likely tumor-associated CD4+T cells. In addition, high-level of both CD4 and NKG2C/E expression was associated with prolonged survival of the high-risk neuroblastoma patients, but CD8 levels were not, further suggesting that the CD4+ NKG2C/E+ T cells or CD4 CTL conferred cytotoxicity against the neuroblastoma cells. However, this T cell mediated- "protective effect" declined over time, in part due to the progressive formation of immunosuppressive tumor microenvironment. These observations suggest that to improve survival of high-risk neuroblastoma patients, it is essential to gain insights into how to enhance CD4 CTL cytotoxicity and control the immunosuppressive tumor microenvironment during the course of the disease.

The CD94/NKG2A inhibitory receptor educates uterine NK cells to optimize pregnancy outcomes in humans and mice

The conserved CD94/NKG2A inhibitory receptor is expressed by nearly all human and ∼50% of mouse uterine natural killer (uNK) cells. Binding human HLA-E and mouse Qa-1, NKG2A drives NK cell education, a process of unknown physiological importance influenced by HLA-B alleles. Here, we show that NKG2A genetic ablation in dams mated with wild-type males caused suboptimal maternal vascular responses in pregnancy, accompanied by perturbed placental gene expression, reduced fetal weight, greater rates of smaller fetuses with asymmetric growth, and abnormal brain development. These are features of the human syndrome pre-eclampsia. In a genome-wide association study of 7,219 pre-eclampsia cases, we found a 7% greater relative risk associated with the maternal HLA-B allele that does not favor NKG2A education. These results show that the maternal HLA-B→HLA-E→NKG2A pathway contributes to healthy pregnancy and may have repercussions on offspring health, thus establishing the physiological relevance for NK cell education.

Video Abstract

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CD94/NKG2A educates uterine NK cells

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NKG2A-deficient dams display reduced utero-placental hemodynamic adaptations

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Asymmetric growth restriction and abnormal brain development in NKG2A-deficient dams

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Non-functional HLA-B→HLA-E→NKG2A pathway exposes women to greater pre-eclampsia risk

Non-KIR NK cell receptors: Role in transplantation of allogeneic haematopoietic stem cells

Natural killer (NK) cells are of major significance in patients after allogeneic haematopoietic stem cell transplantation (HSCT). They are the first subset of lymphocytes to appear in peripheral blood after transplantation and play an important role in the immune responses against cancer and viral infections. The function of NK cells is controlled by various surface receptors, of which type I integral proteins with immunoglobulin-like domains (killer-cell immunoglobulin-like receptors, KIRs) have been the most extensively studied. The present review focuses on less studied NK cell receptors, such as type II integral proteins with lectin-like domains (CD94/NKG2, NKG2D), natural cytotoxicity receptors (NCRs), immunoglobulin-like transcripts (ILTs) and their ligands. Their potential role in patients with haematological disorders subjected to HSC transplant procedure in the context of post-transplant complications such as viral reactivation and acute graft-versus-host disease (GvHD) will be presented and discussed.

Emerging immunotherapies for metastasis

Major advances in cancer immunotherapy have dramatically expanded the potential to manipulate immune cells in cancer patients with metastatic disease to counteract cancer spread and extend patient lifespan. One of the most successful types of immunotherapy is the immune checkpoint inhibitors, such as anti-CTLA-4 and anti-PD-1, that keep anti-tumour T cells active. However, not every patient with metastatic disease benefits from this class of drugs and patients often develop resistance to these therapies over time. Tremendous research effort is now underway to uncover new immunotherapeutic targets that can be used in patients who are refractory to anti-CTLA-4 or anti-PD-1 treatment. Here, we discuss results from experimental model systems demonstrating that modulating the immune response can negatively affect metastasis formation. We focus on molecules that boost anti-tumour immune cells and opportunities to block immunosuppression, as well as cell-based therapies with enhanced tumour recognition properties for solid tumours. We also present a list of challenges in treating metastatic disease with immunotherapy that must be considered in order to move laboratory observations into clinical practice and maximise patient benefit.

Protective Vaccination Reshapes Hepatic Response to Blood-Stage Malaria of Genes Preferentially Expressed by NK Cells

The role of natural killer (NK) cells in the liver as first-line post infectionem (p.i.) effectors against blood-stage malaria and their responsiveness to protective vaccination is poorly understood. Here, we investigate the effect of vaccination on NK cell-associated genes induced in the liver by blood-stage malaria of Plasmodium chabaudi. Female Balb/c mice were vaccinated at weeks 3 and 1 before being infected with 10⁶P. chabaudi-parasitized erythrocytes. Genes preferentially expressed by NK cells were investigated in livers of vaccination-protected and non-protected mice on days 0, 1, 4, 8, and 11 p.i. using microarrays, qRT-PCR, and chromosome landscape analysis. Blood-stage malaria induces expression of specific genes in the liver at different phases of infection, i.e., Itga1 in expanding liver-resident NK (lrNK) cells, Itga2 in immigrating conventional NK (cNK) cells; Eomes and Tbx21 encoding transcription factors; Ncr1, Tnfsf10, Prf1, Gzma, Gzmb, Gzmc, Gzmm, and Gzmk encoding cytolytic effectors; natural killer gene complex (NKC)-localized genes encoding the NK cell receptors KLRG1, KLRK1, KLRAs1, 2, 5, 7, KLRD1, KLRC1, KLRC3, as well as the three receptors KLRB1A, KLRB1C, KLRB1F and their potential ligands CLEC2D and CLEC2I. Vaccination enhances this malaria-induced expression of genes, but impairs Gzmm expression, accelerates decline of Tnfsf10 and Clec2d expression, whereas it accelerates increased expression of Clec2i, taking a very similar time course as that of genes encoding plasma membrane proteins of erythroblasts, whose malaria-induced extramedullary generation in the liver is known to be accelerated by vaccination. Collectively, vaccination reshapes the response of the liver NK cell compartment to blood-stage malaria. Particularly, the malaria-induced expansion of lrNK cells peaking on day 4 p.i. is highly significantly (p < 0.0001) reduced by enhanced immigration of peripheral cNK cells, and KLRB1F:CLEC2I interactions between NK cells and erythroid cells facilitate extramedullary erythroblastosis in the liver, thus critically contributing to vaccination-induced survival of otherwise lethal blood-stage malaria of P. chabaudi.

Dynamic Natural Killer Cell and T Cell Responses to Influenza Infection

Influenza viruses have perplexed scientists for over a hundred years. Yearly vaccines limit their spread, but they do not prevent all infections. Therapeutic treatments for those experiencing severe infection are limited; further advances are held back by insufficient understanding of the fundamental immune mechanisms responsible for immunopathology. NK cells and T cells are essential in host responses to influenza infection. They produce immunomodulatory cytokines and mediate the cytotoxic response to infection. An imbalance in NK and T cell responses can lead to two outcomes: excessive inflammation and tissue damage or insufficient anti-viral functions and uncontrolled infection. The main cause of death in influenza patients is the former, mediated by hyperinflammatory responses termed “cytokine storm.” NK cells and T cells contribute to cytokine storm, but they are also required for viral clearance. Many studies have attempted to distinguish protective and pathogenic components of the NK cell and T cell influenza response, but it has become clear that they are dynamic and integrated processes. This review will analyze how NK cell and T cell effector functions during influenza infection affect the host response and correlate with morbidity and mortality outcomes.

Natural killer cells in cancer biology and therapy

The tumor microenvironment is highly complex, and immune escape is currently considered an important hallmark of cancer, largely contributing to tumor progression and metastasis. Named for their capability of killing target cells autonomously, natural killer (NK) cells serve as the main effector cells toward cancer in innate immunity and are highly heterogeneous in the microenvironment. Most current treatment options harnessing the tumor microenvironment focus on T cell-immunity, either by promoting activating signals or suppressing inhibitory ones. The limited success achieved by T cell immunotherapy highlights the importance of developing new-generation immunotherapeutics, for example utilizing previously ignored NK cells. Although tumors also evolve to resist NK cell-induced cytotoxicity, cytokine supplement, blockade of suppressive molecules and genetic engineering of NK cells may overcome such resistance with great promise in both solid and hematological malignancies. In this review, we summarized the fundamental characteristics and recent advances of NK cells within tumor immunometabolic microenvironment, and discussed potential application and limitations of emerging NK cell-based therapeutic strategies in the era of presicion medicine.

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.

The murine CD94/NKG2 ligand, Qa-1b, is a high-affinity, functional ligand for the CD8αα homodimer

The immune co-receptor CD8 molecule (CD8) has two subunits, CD8α and CD8β, which can assemble into homo or heterodimers. Nonclassical (class-Ib) major histocompatibility complex (MHC) molecules (MHC-Ibs) have recently been identified as ligands for the CD8αα homodimer. This was demonstrated by the observation that histocompatibility 2, Q region locus 10 (H2-Q10) is a high-affinity ligand for CD8αα which also binds the MHC-Ib molecule H2-TL. This suggests that MHC-Ib proteins may be an extended source of CD8αα ligands. Expression of H2-T3/TL and H2-Q10 is restricted to the small intestine and liver, respectively, yet CD8αα-containing lymphocytes are present more broadly. Therefore, here we sought to determine whether murine CD8αα binds only to tissue-specific MHC-Ib molecules or also to ubiquitously expressed MHC-Ib molecules. Using recombinant proteins and surface plasmon resonance-based binding assays, we show that the MHC-Ib family furnishes multiple binding partners for murine CD8αα, including H2-T22 and the CD94/NKG2-A/B-activating NK receptor (NKG2) ligand Qa-1b We also demonstrate a hierarchy among MHC-Ib proteins with respect to CD8αα binding, in which Qa-1b > H2-Q10 > TL. Finally, we provide evidence that Qa-1b is a functional ligand for CD8αα, distinguishing it from its human homologue MHC class I antigen E (HLA-E). These findings provide additional clues as to how CD8αα-expressing cells are controlled in different tissues. They also highlight an unexpected immunological divergence of Qa-1b/HLA-E function, indicating the need for more robust studies of murine MHC-Ib proteins as models for human disease.

Chronic Lymphocytic Choriomeningitis Infection Causes Susceptibility to Mousepox and Impairs Natural Killer Cell Maturation and Function

Chronic viral infections. like those of humans with cytomegalovirus, human immunodeficiency virus (even when under antiretroviral therapy), and hepatitis C virus or those of mice with lymphocytic choriomeningitis virus (LCMV) clone 13 (CL13), result in immune dysfunction that predisposes the host to severe infections with unrelated pathogens. It is known that C57BL/6 (B6) mice are resistant to mousepox, a lethal disease caused by the orthopoxvirus ectromelia virus (ECTV), and that this resistance requires natural killer (NK) cells and other immune cells. We show that most B6 mice chronically infected with CL13 succumb to mousepox but that most of those that recovered from acute infection with the LCMV Armstrong (Arm) strain survive. We also show that B6 mice chronically infected with CL13 and those that recovered from Arm infection have a reduced frequency and a reduced number of NK cells. However, at steady state, NK cells in mice that have recovered from Arm infection mature normally and, in response to ECTV, get activated, become more mature, proliferate, and increase their cytotoxicity in vivo Conversely, in mice chronically infected with CL13, NK cells are immature and residually activated, and following ECTV infection, they do not mature, proliferate, or increase their cytotoxicity. Given the well-established importance of NK cells in resistance to mousepox, these data suggest that the NK cell dysfunction caused by CL13 persistence may contribute to the susceptibility of CL13-infected mice to mousepox. Whether chronic infections similarly affect NK cells in humans should be explored.IMPORTANCE Infection of adult mice with the clone 13 (CL13) strain of lymphocytic choriomeningitis virus (LCMV) is extensively used as a model of chronic infection. In this paper, we show that mice chronically infected with CL13 succumb to challenge with ectromelia virus (ECTV; the agent of mousepox) and that natural killer (NK) cells in CL13-infected mice are reduced in numbers and have an immature and partially activated phenotype but do respond to ECTV. These data may provide additional clues why humans chronically infected with certain pathogens are less resistant to viral diseases.

Towards Clinical Translation of CD8+ Regulatory T Cells Restricted by Non-Classical Major Histocompatibility Complex Ib Molecules

In central lymphoid tissues, mature lymphocytes are generated and pathogenic autoreactive lymphocytes are deleted. However, it is currently known that a significant number of potentially pathogenic autoreactive lymphocytes escape the deletion and populate peripheral lymphoid tissues. Therefore, peripheral mechanisms are present to prevent these potentially pathogenic autoreactive lymphocytes from harming one's own tissues. One such mechanism is dictated by regulatory T (Treg) cells. So far, the most extensively studied Treg cells are CD4+Foxp3+ Treg cells. However, recent clinical trials for the treatment of immune-mediated diseases using CD4+ Foxp3+ Treg cells met with limited success. Accordingly, it is necessary to explore the potential importance of other Treg cells such as CD8+ Treg cells. In this regard, one extensively studied CD8+ Treg cell subset is Qa-1(HLA-E in human)-restricted CD8+ Treg cells, in which Qa-1(HLA-E) molecules belong to a group of non-classical major histocompatibility complex Ib molecules. This review will first summarize the evidence for the presence of Qa-1-restricted CD8+ Treg cells and their regulatory mechanisms. Major discussions will then focus on the potential clinical translation of Qa-1-restricted CD8+ Treg cells. At the end, we will briefly discuss the current status of human studies on HLA-E-restricted CD8+ Treg cells as well as potential future directions.

Multiple receptors converge on H2-Q10 to regulate NK and γδT-cell development

Class Ib major histocompatibility complex (MHC) is an extended family of molecules, which demonstrate tissue-specific expression and presentation of monomorphic antigens. These characteristics tend to imbue class Ib MHC with unique functions. H2-Q10 is potentially one such molecule that is overexpressed in the liver but its immunological function is not known. We have previously shown that H2-Q10 is a ligand for the natural killer cell receptor Ly49C and now, using H2-Q10-deficient mice, we demonstrate that H2-Q10 can also stabilize the expression of Qa-1b. In the absence of H2-Q10, the development and maturation of conventional hepatic natural killer cells is disrupted. We also provide evidence that H2-Q10 is a new high affinity ligand for CD8αα and controls the development of liver-resident CD8αα γδT cells. These data demonstrate that H2-Q10 has multiple roles in the development of immune subsets and identify an overlap of recognition within the class Ib MHC that is likely to be relevant to the regulation of immunity.

A herpesvirus encoded Qa-1 mimic inhibits natural killer cell cytotoxicity through CD94/NKG2A receptor engagement

A recurrent theme in viral immune evasion is the sabotage of MHC-I antigen presentation, which brings virus the concomitant issue of ‘missing-self’ recognition by NK cells that use inhibitory receptors to detect surface MHC-I proteins. Here, we report that rodent herpesvirus Peru (RHVP) encodes a Qa-1 like protein (pQa-1) via RNA splicing to counteract NK activation. While pQa-1 surface expression is stabilized by the same canonical peptides presented by murine Qa-1, pQa-1 is GPI-anchored and resistant to the activity of RHVP pK3, a ubiquitin ligase that targets MHC-I for degradation. pQa-1 tetramer staining indicates that it recognizes CD94/NKG2A receptors. Consistently, pQa-1 selectively inhibits NKG2A⁺ NK cells and expression of pQa-1 can protect tumor cells from NK control in vivo. Collectively, these findings reveal an innovative NK evasion strategy wherein RHVP encodes a modified Qa-1 mimic refractory to MHC-I sabotage and capable of specifically engaging inhibitory receptors to circumvent NK activation.

NKG2A Blockade Potentiates CD8 T Cell Immunity Induced by Cancer Vaccines

Tumor-infiltrating CD8 T cells were found to frequently express the inhibitory receptor NKG2A, particularly in immune-reactive environments and after therapeutic cancer vaccination. High-dimensional cluster analysis demonstrated that NKG2A marks a unique immune effector subset preferentially co-expressing the tissue-resident CD103 molecule, but not immune checkpoint inhibitors. To examine whether NKG2A represented an adaptive resistance mechanism to cancer vaccination, we blocked the receptor with an antibody and knocked out its ligand Qa-1b, the conserved ortholog of HLA-E, in four mouse tumor models. The impact of therapeutic vaccines was greatly potentiated by disruption of the NKG2A/Qa-1b axis even in a PD-1 refractory mouse model. NKG2A blockade therapy operated through CD8 T cells, but not NK cells. These findings indicate that NKG2A-blocking antibodies might improve clinical responses to therapeutic cancer vaccines.

Evidence of functional Cd94 polymorphism in a free-living house mouse population

The CD94 receptor, expressed on natural killer (NK) and CD8+ T cells, is known as a relatively non-polymorphic receptor with orthologues in humans, other primates, cattle, and rodents. In the house mouse (Mus musculus), a single allele is highly conserved among laboratory strains, and reports of allelic variation in lab- or wild-living mice are lacking, except for deficiency in one lab strain (DBA/2J). The non-classical MHC-I molecule Qa-1b is the ligand for mouse CD94/NKG2A, presenting alternative non-americ fragment of leader peptides (Qa-1 determinant modifier (Qdm)) from classical MHC-I molecules. Here, we report a novel allele identified in free-living house mice captured in Norway, living among individuals carrying the canonical Cd94 allele. The novel Cd94^LocA allele encodes 12 amino acid substitutions in the extracellular lectin-like domain. Flow cytometric analysis of primary NK cells and transfected cells indicates that the substitutions prevent binding of CD94 mAb and Qa-1b/Qdm tetramers. Our data further indicate correlation of Cd94 polymorphism with the two major subspecies of house mice in Europe. Together, these findings suggest that the Cd94^LocA/NKG2A heterodimeric receptor is widely expressed among M. musculus subspecies musculus, with ligand-binding properties different from mice of subspecies domesticus, such as the C57BL/6 strain.

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

Lymphocytes Negatively Regulate NK Cell Activity via Qa-1b following Viral Infection

NK cells can reduce anti-viral T cell immunity during chronic viral infections, including infection with the lymphocytic choriomeningitis virus (LCMV). However, regulating factors that maintain the equilibrium between productive T cell and NK cell immunity are poorly understood. Here, we show that a large viral load resulted in inhibition of NK cell activation, which correlated with increased expression of Qa-1b, a ligand for inhibitory NK cell receptors. Qa-1b was predominantly upregulated on B cells following LCMV infection, and this upregulation was dependent on type I interferons. Absence of Qa-1b resulted in increased NK cell-mediated regulation of anti-viral T cells following viral infection. Consequently, anti-viral T cell immunity was reduced in Qa-1b- and NKG2A-deficient mice, resulting in increased viral replication and immunopathology. NK cell depletion restored anti-viral immunity and virus control in the absence of Qa-1b. Taken together, our findings indicate that lymphocytes limit NK cell activity during viral infection in order to promote anti-viral T cell immunity.

The Molecular Mechanism of Natural Killer Cells Function and Its Importance in Cancer Immunotherapy

Natural killer (NK) cells are innate immune cells that show strong cytolytic function against physiologically stressed cells such as tumor cells and virus-infected cells. NK cells show a broad array of tissue distribution and phenotypic variability. NK cells express several activating and inhibitory receptors that recognize the altered expression of proteins on target cells and control the cytolytic function. NK cells have been used in several clinical trials to control tumor growth. However, the results are encouraging only in hematological malignancies but not very promising in solid tumors. Increasing evidence suggests that tumor microenvironment regulate the phenotype and function of NK cells. In this review, we discussed the NK cell phenotypes and its effector function and impact of the tumor microenvironment on effector and cytolytic function of NK cells. We also summarized various NK cell-based immunotherapeutic strategies used in the past and the possibilities to improve the function of NK cell for the better clinical outcome.

Uterine Natural Killer Cells: Functional Distinctions and Influence on Pregnancy in Humans and Mice

Our understanding of development and function of natural killer (NK) cells has progressed significantly in recent years. However, exactly how uterine NK (uNK) cells develop and function is still unclear. To help investigators that are beginning to study tissue NK cells, we summarize in this review our current knowledge of the development and function of uNK cells, and what is yet to be elucidated. We compare and contrast the biology of human and mouse uNK cells in the broader context of the biology of innate lymphoid cells and with reference to peripheral NK cells. We also review how uNK cells may regulate trophoblast invasion and uterine spiral arterial remodeling in human and murine pregnancy.

Modular Activating Receptors in Innate and Adaptive Immunity

Triggering of cell-mediated immunity is largely dependent on the recognition of foreign or abnormal molecules by a myriad of cell surface-bound receptors. Many activating immune receptors do not possess any intrinsic signaling capacity but instead form noncovalent complexes with one or more dimeric signaling modules that communicate with a common set of kinases to initiate intracellular information-transfer pathways. This modular architecture, where the ligand binding and signaling functions are detached from one another, is a common theme that is widely employed throughout the innate and adaptive arms of immune systems. The evolutionary advantages of this highly adaptable platform for molecular recognition are visible in the variety of ligand-receptor interactions that can be linked to common signaling pathways, the diversification of receptor modules in response to pathogen challenges, and the amplification of cellular responses through incorporation of multiple signaling motifs. Here we provide an overview of the major classes of modular activating immune receptors and outline the current state of knowledge regarding how these receptors assemble, recognize their ligands, and ultimately trigger intracellular signal transduction pathways that activate immune cell effector functions.

NKG2C/E Marks the Unique Cytotoxic CD4 T Cell Subset, ThCTL, Generated by Influenza Infection

CD4 T cells can differentiate into multiple effector subsets, including ThCTL that mediate MHC class II-restricted cytotoxicity. Although CD4 T cell-mediated cytotoxicity has been reported in multiple viral infections, their characteristics and the factors regulating their generation are unclear, in part due to a lack of a signature marker. We show in this article that, in mice, NKG2C/E identifies the ThCTL that develop in the lung during influenza A virus infection. ThCTL express the NKG2X/CD94 complex, in particular the NKG2C/E isoforms. NKG2C/E⁺ ThCTL are part of the lung CD4 effector population, and they mediate influenza A virus-specific cytotoxic activity. The phenotype of NKG2C/E⁺ ThCTL indicates they are highly activated effectors expressing high levels of binding to P-selectin, T-bet, and Blimp-1, and that more of them secrete IFN-γ and readily degranulate than non-ThCTL. ThCTL also express more cytotoxicity-associated genes including perforin and granzymes, and fewer genes associated with recirculation and memory. They are found only at the site of infection and not in other peripheral sites. These data suggest ThCTL are marked by the expression of NKG2C/E and represent a unique CD4 effector population specialized for cytotoxicity.

Class I HLA haplotypes form two schools that educate NK cells in different ways

Natural killer (NK) cells are lymphocytes having vital functions in innate and adaptive immunity, as well as placental reproduction. Controlling education and functional activity of human NK cells are various receptors that recognize HLA class I on the surface of tissue cells. Epitopes of polymorphic HLA-A,-B and -C are recognized by equally diverse killer cell immunoglobulin-like receptors (KIR). In addition, a peptide cleaved from the leader sequence of HLA-A,-B or -C must bind to HLA-E for it to become a ligand for the conserved CD94:NKG2A receptor. Methionine/threonine dimorphism at position -21 of the leader sequence divides HLA-B allotypes into a majority having -21T that do not supply HLA-E binding peptides and a minority having -21M, that do. Genetic analysis of human populations worldwide shows how haplotypes with -21M HLA-B rarely encode the KIR ligands: Bw4⁺HLA-B and C2⁺HLA-C KIR. Thus there are two fundamental forms of HLA haplotype: one preferentially supplying CD94:NKG2A ligands, the other preferentially supplying KIR ligands. -21 HLA-B dimorphism divides the human population into three groups: M/M, M/T and T/T. Mass cytometry and assays of immune function, shows how M/M and M/T individuals have CD94:NKG2A⁺ NK cells which are better educated, phenotypically more diverse and functionally more potent than those in T/T individuals. Fundamental new insights are given to genetic control of NK cell immunity and the evolution that has limited the number of NK cell receptor ligands encoded by an HLA haplotype. These finding suggest new ways to dissect the numerous clinical associations with HLA class I.

Expression of bovine non-classical major histocompatibility complex class I proteins in mouse P815 and human K562 cells

Major histocompatibility complex class I (MHC-I) proteins can be expressed as cell surface or secreted proteins. To investigate whether bovine non-classical MHC-I proteins are expressed as cell surface or secreted proteins, and to assess the reactivity pattern of monoclonal antibodies with non-classical MHC-I isoforms, we expressed the MHC proteins in murine P815 and human K562 (MHC-I deficient) cells. Following antibiotic selection, stably transfected cell lines were stained with H1A or W6/32 antibodies to detect expression of the MHC-I proteins by flow cytometry. Two non-classical proteins (BoLA-NC1*00501 and BoLA-NC3*00101) were expressed on the cell surface in both cell lines. Surprisingly, the BoLA-NC4*00201 protein was expressed on the cell membrane of human K562 but not mouse P815 cells. Two non-classical proteins (BoLA-NC1*00401, which lacks a transmembrane domain, and BoLA-NC2*00102) did not exhibit cell surface expression. Nevertheless, Western blot analyses demonstrated expression of the MHC-I heavy chain in all transfected cell lines. Ammonium-sulfate precipitation of proteins from culture supernatants showed that BoLA-NC1*00401 was secreted and that all surface expressed proteins where shed from the cell membrane by the transfected cells. Interestingly, the surface expressed MHC-I proteins were present in culture supernatants at a much higher concentration than BoLA-NC1*00401. This comprehensive study shows that bovine non-classical MHC-I proteins BoLA-NC1*00501, BoLA-NC3*00101, and BoLA-NC4*00201 are expressed as surface isoforms with the latter reaching the cell membrane only in K562 cells. Furthermore, it demonstrated that BoLA-NC1*00401 is a secreted isoform and that significant quantities of membrane associated MHC-I proteins can be shed from the cell membrane.

The role of MHC class Ib-restricted T cells during infection

Even though major histocompatibility complex (MHC) class Ia and many Ib molecules have similarities in structure, MHC class Ib molecules tend to have more specialized functions, which include the presentation of non-peptidic antigens to non-classical T cells. Likewise, non-classical T cells also have unique characteristics, including an innate-like phenotype in naïve animals and rapid effector functions. In this review, we discuss the role of MAIT and NKT cells during infection but also the contribution of less studied MHC class Ib-restricted T cells such as Qa-1-, Qa-2-, and M3-restricted T cells. We focus on describing the types of antigens presented to non-classical T cells, their response and cytokine profile following infection, as well as the overall impact of these T cells to the immune system.

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.

The natural killer cell dysfunction of aged mice is due to the bone marrow stroma and is not restored by IL-15/IL-15Rα treatment

Immune dysfunctions in the elderly result in increased susceptibility to infectious diseases, cancer, and autoimmune diseases. Natural killer (NK) cells are bone marrow-derived lymphocytes crucial for host defense against several infections and cancer. We have previously shown that compared to young, aged C57BL/6 mice have decreased numbers of mature NK cells in the blood, spleen, and bone marrow, resulting in susceptibility to mousepox, a lethal disease caused by ectromelia virus. Here, we describe further age-related defects in NK cells including reduced proliferation in vivo, additional signs of immaturity, and dysregulated expression of activating and inhibitory receptors. Aging also alters the expression of collagen-binding integrins in conventional NK cells and the frequency and phenotype of liver tissue-resident NK cells. We additionally show that the defect in NK maturation is the consequence of deficient maturational cues provided by bone marrow stromal cells. Moreover, we demonstrate that in aged mice, treatment with complexes of the cytokine IL-15 and IL-15Rα induce massive expansion of the NK cells, but most of these NK cells remain immature and are unable to restore resistance to mousepox. The use of rodent model to understand immunosenescence may help the development of treatments to improve the immune fitness of the aged. Our work with NK cells should contribute toward this goal.

Functional characterization of a conserved pair of NKR-P1 receptors expressed by NK cells and T lymphocytes in liver and gut

Natural killer cell receptor protein 1 (NKR-P1) molecules are C-type lectin-like receptors modulating cellular responses toward target cells expressing C-type lectin-like related (Clr) molecules. Although the function of the prototypic rat NKR-P1A receptor and its inhibitory counterpart NKR-P1B are known, little is known about NKR-P1F and NKR-P1G apart from their promiscuity for Clr ligands. Here we generated mAbs against both receptors for phenotypic and functional analyses in rat tissues. NKR-P1F induced redirected lysis and robust Ca(2+) signaling in NK cells, which were prevented by simultaneous engagement of NKR-P1G. NKR-P1G also inhibited NK-cell lysis of Clr transfectants. NKR-P1F was expressed by most NK cells and NKR-P1A(+) T cells in all tissues analyzed, and by many NKR-P1A(-) intestinal T cells, while NKR-P1G was expressed by subsets of these cells with highest prevalence in gut and liver. In the intraepithelial compartment, the proportion of NKR-P1A(+) and NKR-P1F(+) cells was high at birth and thereafter declined, while NKR-P1B(+) and NKR-P1G(+) cells increased with age. Expression levels were also modulated by cytokines, with an increase of NKR-P1B and NKR-P1G induced by inflammatory cytokines, and a reduction of NKR-P1A by TGF-β. The physiological impact of NKR-P1 receptors might thus be dependent on age, tissue, and inflammatory status.