Regulation of natural cytotoxicity by the adaptor SAP and the Src-related kinase Fyn

Hippo kinases Mst1 and Mst2 maintain NK cell homeostasis by orchestrating metabolic state and transcriptional activity

Natural killer (NK) cells play a crucial role in immune response against viral infections and tumors. However, further investigation is needed to better understand the key molecules responsible for determining the fate and function of NK cells. In this study, we made an important discovery regarding the involvement of the Hippo kinases Mst1 and Mst2 as novel regulators in maintaining mouse NK cell homeostasis. The presence of high Mst1 and Mst2 (Mst1/2) activity in NK cells is essential for their proper development, survival and function in a canonical Hippo signaling independent mode. Mechanistically, Mst1/2 induce cellular quiescence by regulating the processes of proliferation and mitochondrial metabolism, thereby ensuring the development and survival of NK cells. Furthermore, Mst1/2 effectively sense IL-15 signaling and facilitate the activation of pSTAT3-TCF1, which contributes to NK cell homeostasis. Overall, our investigation highlights the crucial role of Mst1/2 as key regulators in metabolic reprogramming and transcriptional regulation for mouse NK cell survival and function, emphasizing the significance of cellular quiescence during NK cell development and functional maturation.

Receptors of immune cells mediates recognition for tumors

Over the last few decades, the immune system has been steered toward eradication of cancer cells with the help of cancer immunotherapy. T cells, B cells, monocytes/macrophages, dendritic cells, T-reg cells, and natural killer (NK) cells are some of the numerous immune cell types that play a significant part in cancer cell detection and reduction of inflammation, and the antitumor response. Briefly stated, chimeric antigen receptors, adoptive transfer and immune checkpoint modulators are currently the subjects of research focus for successful immunotherapy-based treatments for a variety of cancers. This chapter discusses ongoing investigations on the mechanisms and recent developments by which receptors of immune cells especially that of lymphocytes and monocytes/macrophages regulate the detection of immune system leading to malignancies. We will also be looking into the treatment strategies based on these mechanisms.

The hyperinflammatory spectrum: from defects in cytotoxicity to cytokine control

Cytotoxic lymphocytes kill target cells through polarized release of the content of cytotoxic granules towards the target cell. The importance of this cytotoxic pathway in immune regulation is evidenced by the severe and often fatal condition, known as hemophagocytic lymphohistiocytosis (HLH) that occurs in mice and humans with inborn errors of lymphocyte cytotoxic function. The clinical and preclinical data indicate that the damage seen in severe, virally triggered HLH is due to an overwhelming immune system reaction and not the direct effects of the virus per se. The main HLH-disease mechanism, which links impaired cytotoxicity to excessive release of pro-inflammatory cytokines is a prolongation of the synapse time between the cytotoxic effector cell and the target cell, which prompts the former to secrete larger amounts of cytokines (including interferon gamma) that activate macrophages. We and others have identified novel genetic HLH spectrum disorders. In the present update, we position these newly reported molecular causes, including CD48-haploinsufficiency and ZNFX1-deficiency, within the pathogenic pathways that lead to HLH. These genetic defects have consequences on the cellular level on a gradient model ranging from impaired lymphocyte cytotoxicity to intrinsic activation of macrophages and virally infected cells. Altogether, it is clear that target cells and macrophages may play an independent role and are not passive bystanders in the pathogenesis of HLH. Understanding these processes which lead to immune dysregulation may pave the way to novel ideas for medical intervention in HLH and virally triggered hypercytokinemia.

Allosteric inhibition of SHP2 rescues functional T-cell abnormalities in SAP deficiency

BACKGROUND

X-linked lymphoproliferative disease (XLP) is a primary immunodeficiency arising from SH2D1A mutations leading to loss of SLAM-associated protein (SAP). SAP is an intracellular adaptor protein that binds to SLAM family receptors and is expressed in specific lymphoid lineages. In T cells, SAP relays activatory signals from the T-cell receptor but in its absence SH2 containing protein tyrosine phosphase-1 (SHP1), SH2 containing protein tyrosine phosphase-2 (SHP2), and SH2 containing inositol 5'-phosphatase proteins (SHIP) induce T-cell inhibitory signals leading to abnormal T-cell responses. This results in severe clinical manifestations including immune dysregulation, dysgammaglobulinemia, lymphoma, and hemophagocytic lymphohistiocytosis. Current treatment relies on supportive therapies including immunoglobulin replacement and symptom-directed therapy, with hematopoietic stem cell transplant offering the only curative option.

OBJECTIVES

As most XLP symptoms are due to defective T-cell function, this study investigated whether inhibition of SHP2 can restore cellular function in the absence of SAP.

METHODS

Healthy donor and XLP patient T cells were activated with anti-CD3/CD28 in T-cell media supplemented with a SHP2 inhibitor (RMC-4550 in vitro for 24 hours) and functional assays were performed to assess follicular T_H (T_FH) cell function, CD8 cytotoxicity, and sensitivity to restimulation-induced cell death. Additionally, SAP-deficient (SAP^y/-) mice were treated with RMC-4550 before T-cell mediated challenge with 4-hydroxy-3-nitrophenylacetly conjugated chicken gammaglobulin and subsequent assessment of humoral immunity analyzing T_FH cell population, germinal center formation, and antigen-dependent immunoglobulin secretion.

RESULTS

This study shows that the use of RMC-4550 restores T-cell function in XLP patient cells and a SAP^y/- model, demonstrating restoration of T_FH cell function through immunoglobulin and cytokine secretion analysis alongside rescue of cytotoxicity and restimulation-induced cell death.

CONCLUSIONS

These data suggest that SHP2 inhibitors could offer a novel and effective targeted treatment approach for patients with XLP.

A novel 2B4 receptor leads to worse pregnancy outcomes by facilitating TNF-α and IFN-γ production in dNK cells during Toxoplasma gondii infection

Background

Infections are a major threat to human reproductive health because they can induce pregnancy failure, including recurrent abortion, stillbirth, and preterm birth. Toxoplasma gondii (T. gondii) infection can result in adverse pregnancy outcomes by affecting certain immune molecules and cytokines. However, the detailed mechanisms behind T. gondii-induced pregnancy failure are poorly understood.

Methods

Toxoplasma gondii-infected wild-type (WT) pregnant mice and 2B4 knockout (2B4−/−) pregnant mice were established for in vivo study. Human decidual natural killer (dNK) cells were cultured for in vitro study. Abnormal pregnancy outcomes were observed, and the expression of 2B4, functional molecules (CD69, CD107a, tumor necrosis factor alpha [TNF-α], interferon gamma [IFN-γ]), and signaling molecules (SHP-2, Fyn, p-ERK, p-P38) in dNK cells were detected by flow cytometry, Western blot, reverse transcriptase polymerase chain reaction (RT-PCR), and/or immunofluorescence. The direct interactions (2B4 interacts with SHP-2 and Fyn; SHP-2 interacts with p-P38 and 2B4; Fyn interacts with p-ERK and 2B4) were verified by co-immunoprecipitation (co-IP) in NK-92 cells.

Results

Here, results showed that 2B4 was significantly downregulated after T. gondii infection. Subsequently, infected 2B4−/− pregnant mice displayed worse pregnancy outcomes compared with infected WT pregnant mice. Also, increased TNF-α and IFN-γ expression and elevated dNK cell cytotoxicity were found in 2B4−/− pregnant mice during T. gondii infection. In contrast, reduced TNF-α and IFN-γ expression and decreased human dNK cell activity were found following 2B4 activation during T. gondii infection. Interestingly, results showed that 2B4 binds to adaptor SHP-2 or Fyn, which then triggers different signaling pathways to regulate TNF-α and IFN-γ expression in dNK cells during T. gondii infection. Further, SHP-2 binds 2B4 and p-P38 directly after 2B4 activation, which generates an inhibitory signal for TNF-α and IFN-γ in NK-92 cells. In addition, Fyn can bind to 2B4 and p-ERK after activation of 2B4, thereby inhibiting TNF-α and IFN-γ expression in NK-92 cells following T. gondii infection.

Conclusions

These data suggest that 2B4 may be a novel danger-signaling molecule that is implicated in pregnancy failure during T. gondii infection. Unraveling the mechanism by which 2B4 regulates dNK cell activity will provide novel insights to aid our understanding of T. gondii-induced adverse pregnancy outcomes.

Graphical Abstract

Analysis of gut microbiome profiles in common marmosets (Callithrix jacchus) in health and intestinal disease

Chronic gastrointestinal (GI) diseases are the most common diseases in captive common marmosets. To understand the role of the microbiome in GI diseases, we characterized the gut microbiome of 91 healthy marmosets (303 samples) and 59 marmosets diagnosed with inflammatory bowel disease (IBD) (200 samples). Healthy marmosets exhibited "humanized," Bacteroidetes-dominant microbiomes. After up to 2 years of standardized diet, housing and husbandry, marmoset microbiomes could be classified into four distinct marmoset sources based on Prevotella and Bacteroides levels. Using a random forest (RF) model, marmosets were classified by source with an accuracy of 93% with 100% sensitivity and 95% specificity using abundance data from 4 Prevotellaceae amplicon sequence variants (ASVs), as well as single ASVs from Coprobacter, Parabacteroides, Paraprevotella, Phascolarctobacterium, Oribacterium and Fusobacterium. A single dysbiotic IBD state was not found across all marmoset sources, but IBD was associated with lower alpha diversity and a lower Bacteroides:Prevotella copri ratio within each source. IBD was highest in a Prevotella-dominant cohort, and consistent with Prevotella-linked diseases, pro-inflammatory genes in the jejunum were upregulated. RF analysis of serum biomarkers identified serum calcium, hemoglobin and red blood cell (RBC) counts as potential biomarkers for marmoset IBD. This study characterizes the microbiome of healthy captive common marmosets and demonstrates that source-specific microbiomes can be retained despite standardized diets and husbandry practices. Marmosets with IBD had decreased alpha diversity and a shift in the ratio of Bacteroides:Prevotella copri compared to healthy marmosets.

SLAMF7 selectively favors degranulation to promote cytotoxicity in human NK cells

NK cells play an important role in immunity by recognizing and eliminating cells undergoing infection or malignant transformation. This role is dependent on the ability of NK cells to lyse targets cells in a perforin-dependent mechanism and by secreting inflammatory cytokines. Both effector functions are controlled by several cell surface receptors. The Signaling Lymphocyte Activation Molecule (SLAM) family of receptors plays an essential role in regulating NK cell activation. Several studies have demonstrated that SLAMF7 regulates NK cell activation. However, the molecular and cellular mechanisms by which SLAMF7 influences NK effector functions are unknown. Here, we present evidence that physiological ligation of SLAMF7 in human NK cells enhances the lysis of target cells expressing SLAMF7. This effect was dependent on the ability of SLAMF7 to promote NK cell degranulation rather than cytotoxic granule polarization or cell adhesion. Moreover, SLAMF7-dependent NK cell degranulation was predominantly dependent on PLC-γ when compared to PI3K. These data provide novel information on the cellular mechanism by which SLAMF7 regulates human NK cell activation. Finally, this study supports a model for NK cell activation where activated receptors contribute by regulating specific discrete cellular events rather than multiple cellular processes.

An ENU mutagenesis approach to dissect "self"-induced immune responses: Unraveling the genetic footprint of immunosurveillance

The immune system exerts a critical function as it recognizes and eliminates transformed or neoplastic cells, a process also referred to as immunosurveillance. NK cells play a particularly important role in that they are able to recognize tumor cells via “missing-self”—i.e., the absence of major histocompatibility complex Class I on target cells. Moreover, recent studies suggest that NK cells also participate in the onset and regulation of adaptive immune responses. The exact molecular pathways by which this occurs, however, remain poorly understood. To obtain further insight into the genes that are required for self-induced immune responses via NK cell-mediated cell death, our laboratory initiated a forward genetic approach using N-ethyl-N-nitrosourea (ENU) as a mutagen. Specifically, we tested the ability of NK cells from G3 ENU germline mice to recognize missing-self target cells and induce CD8+ T-cell responses following immunization with irradiated tumor cells. Here we present two ENU germline mutants, designated Ace and Chip, that are defective in the recognition of β-2 microglobulin-deficient target cells, yet exhibit improved clearance of B16 melanoma cells in vivo. Coarse mapping and whole genome sequencing of the Chip mutation revealed a missense mutation causing a T’A amino acid substitution in the highly conserved third immuno-receptor tyrosine-based switch motif of CD244 (2B4). The forward genetic approach described here promises to reveal important insight into critical genes that are required for host responses involved in anticancer immunity.

CD244 represents a new therapeutic target in head and neck squamous cell carcinoma

BACKGROUND

Developing novel strategies to overcome the immunosuppressive tumor microenvironment is a critically important area of cancer therapy research. Here, we assess the therapeutic potential of CD244 (2B4/signaling lymphocyte activation molecule family 4), an immunoregulatory receptor found on a variety of immune cells, including exhausted CD8+ T cells, dendritic cells (DCs), and myeloid-derived suppressor cells (MDSCs).

METHODS

Using de-identified human tumor and blood samples from patients with head and neck squamous cell carcinoma (HNSCC) and HNSCC models in WT and CD244-/- mice, we assessed the therapeutic potential of CD244 using flow cytometry, RT-PCR, Luminex immunoassays and histopathological analyses.

RESULTS

Compared with healthy tissues, tumor infiltrating CD8+ T cells from HNSCC patients and a HNSCC mouse model showed significant increased expression of CD244 expression that correlated with PD1 expression. Moreover, CD244 was increased on intratumoral DC and MDSC and high CD244 expression correlated with PD-L1 expression and increased spontaneous expression of immune-suppressive mediators. In addition, CD244 activation inhibited production of proinflammatory cytokines in human DC in vitro. Importantly, CD244-/- mice showed significantly impaired tumor growth of HNSCC and interventional treatment of WT mice with anti-CD244 monoclonal antibody significantly impaired the growth of established HNSCC tumors and increased tumor-infiltrating CD8+ T cells.

CONCLUSIONS

Together these data suggest that CD244 contributes to the overall immune-suppressive environment and therefore has potential as a new immunotherapy target in the treatment of malignancies.

Signaling pathways involved in the T-cell-mediated immunity against Epstein-Barr virus: Lessons from genetic diseases

Primary immunodeficiencies (PIDs) provide researchers with unique models to understand in vivo immune responses in general and immunity to infections in particular. In humans, impaired immune control of Epstein-Barr virus (EBV) infection is associated with the occurrence of several different immunopathologic conditions; these include non-malignant and malignant B-cell lymphoproliferative disorders, hemophagocytic lymphohistiocytosis (HLH), a severe inflammatory condition, and a chronic acute EBV infection of T cells. Studies of PIDs associated with a predisposition to develop severe, chronic EBV infections have led to the identification of key components of immunity to EBV - notably the central role of T-cell expansion and its regulation in the pathophysiology of EBV-associated diseases. On one hand, the defective expansion of EBV-specific CD8 T cells results from mutations in genes involved in T-cell activation (such as RASGRP1, MAGT1, and ITK), DNA metabolism (CTPS1) or co-stimulatory pathways (CD70, CD27, and TNFSFR9 (also known as CD137/4-1BB)) leads to impaired elimination of proliferating EBV-infected B cells and the occurrence of lymphoma. On the other hand, protracted T-cell expansion and activation after the defective killing of EBV-infected B cells is caused by genetic defects in the components of the lytic granule exocytosis pathway or in the small adapter protein SH2D1A (also known as SAP), a key activator of T- and NK cell-cytotoxicity. In this setting, the persistence of EBV-infected cells results in HLH, a condition characterized by unleashed T-cell and macrophage activation. Moreover, genetic defects causing selective vulnerability to EBV infection have highlighted the role of co-receptor molecules (CD27, CD137, and SLAM-R) selectively involved in immune responses against infected B cells via specific T-B cell interactions.

Regulation of MHC class I-independent NK cell education by SLAM family receptors

Seven members of signaling lymphocytic activation molecule (SLAM) family receptors (SFRs) are ubiquitously expressed on hematopoietic cells and they play critical roles in immune cell differentiation and activation. The engagement of these receptors transmits intracellular signaling mainly by recruiting SLAM-associated protein (SAP) and its related adaptors, EWS-FLI1-activated transcript-2 (EAT-2) and EAT-2-related transducer (ERT). The critical roles of SFRs and SAP-family adaptors are highlighted by the discovery that SAP is mutated in human X-linked lymphoproliferative (XLP1) disease in which the contact between T and B cells in germinal center and cytotoxic lymphocytes (NK cells and CD8⁺ T cells) function are severely compromised. These immune defects are closely associated with the defective antibody production and the high incidence of lymphoma in the patients with XLP1. In addition to these well-known functions, SLAM-SAP family is involved in NK cell education, a process describing NK cell functional competence. In this chapter, we will mainly discuss these unappreciated roles of SAP-dependent and SAP-independent SFR signaling in regulating MHC-I-independent NK cell education.

SLAM-SAP-Fyn: Old Players with New Roles in iNKT Cell Development and Function

Invariant natural killer T (iNKT) cells are a unique T cell lineage that develop in the thymus and emerge with a memory-like phenotype. Accordingly, following antigenic stimulation, they can rapidly produce copious amounts of Th1 and Th2 cytokines and mediate activation of several immune cells. Thus, it is not surprising that iNKT cells play diverse roles in a broad range of diseases. Given their pivotal roles in host immunity, it is crucial that we understand the mechanisms that govern iNKT cell development and effector functions. Over the last two decades, several studies have contributed to the current knowledge of iNKT cell biology and activity. Collectively, these studies reveal that the thymic development of iNKT cells, their lineage expansion, and functional properties are tightly regulated by a complex network of transcription factors and signaling molecules. While prior studies have clearly established the importance of the SLAM-SAP-Fyn signaling axis in iNKT cell ontogenesis, recent studies provide exciting mechanistic insights into the role of this signaling cascade in iNKT cell development, lineage fate decisions, and functions. Here we summarize the previous literature and discuss the more recent studies that guide our understanding of iNKT cell development and functional responses.

SLAM family member 8 is involved in oncogenic KIT-mediated signalling in human mastocytosis

The signalling lymphocytic activation molecule family member 8 (SLAMF8)/CD353 is a member of the CD2 family of proteins. Its ligand has not been identified. SLAMF8 is expressed by macrophages and suppresses cellular functions. No study has yet explored SLAMF8 expression or function in human mastocytosis, which features oncogenic KIT-mediated proliferation of human mast cells. SLAMF8 protein was expressed in human mastocytosis cells, immunohistochemically. SLAMF8 expression was also evident in the human mast cell lines, HMC1.2 (expressing oncogenic KIT) and LAD2 (expressing wild-type KIT) cells. SLAMF8 knock-down significantly reduced the KIT-mediated growth of HMC1.2 cells but not that of LAD2 cells. SLAMF8 knock-down HMC1.2 cells exhibited significant attenuation of SHP-2 activation and oncogenic KIT-mediated RAS-RAF-ERK signalling. An interaction between SLAMF8 and SHP-2 was confirmed in HMC1.2 cells and all pathological mastocytosis specimens examined (19 of 19 cases, 100%). Thus, SLAMF8 is involved in oncogenic KIT-mediated RAS-RAF-ERK signalling and the subsequent growth of human neoplastic mast cells mediated by SHP-2. SLAMF8 is a possible therapeutic target in human mastocytosis patients.

NK cell recognition of hematopoietic cells by SLAM-SAP families

The signaling lymphocyte activation molecule (SLAM) family of receptors (SFRs) are ubiquitously expressed on immune cells, and they regulate multiple immune events by recruiting SH2 (Src homology 2) domain-containing SAP family adapters, including SAP and its homologs, Ewing's sarcoma-associated transcript 2 (EAT-2) and EAT-2 related transducer (ERT). In human patients with X-linked lymphoproliferative (XLP) disease, which is caused by SAP mutations, SFRs alternatively bind other inhibitory SH2 domain-containing molecules to suppress immune cell activation and development. NK cells express multiple SFRs and all SAP family adapters. In recent decades, SFRs have been found to be critical for enhancing NK cell activation in response to abnormal hematopoietic cells in SAP-family-intact NK cells; however, SFRs might suppress NK cell activation in SAP-family-deficient mice or patients with XLP1. In this paper, we review how these two distinct SFR signaling pathways orchestrate NK cell activation and inhibition and highlight the importance of SFR regulation of NK cell biology and their physiological status and pathological relevance in patients with XLP1.

The Emerging Role of CD244 Signaling in Immune Cells of the Tumor Microenvironment

In cancer, immune exhaustion contributes to the immunosuppressive tumor microenvironment. Exhausted immune cells demonstrate poor effector function and sustained expression of certain immunomodulatory receptors, which can be therapeutically targeted. CD244 is a Signaling Lymphocyte Activation Molecule (SLAM) family immunoregulatory receptor found on many immune cell types—including NK cells, a subset of T cells, DCs, and MDSCs—that represents a potential therapeutic target. Here, we discuss the role of CD244 in tumor-mediated immune cell regulation.

Crosstalks between mTORC1 and mTORC2 variagate cytokine signaling to control NK maturation and effector function

The metabolic checkpoint kinase mechanistic/mammalian target of rapamycin (mTOR) regulates natural killer (NK) cell development and function, but the exact underlying mechanisms remain unclear. Here, we show, via conditional deletion of Raptor (mTORC1) or Rictor (mTORC2), that mTORC1 and mTORC2 promote NK cell maturation in a cooperative and non-redundant manner, mainly by controlling the expression of Tbx21 and Eomes. Intriguingly, mTORC1 and mTORC2 regulate cytolytic function in an opposing way, exhibiting promoting and inhibitory effects on the anti-tumor ability and metabolism, respectively. mTORC1 sustains mTORC2 activity by maintaining CD122-mediated IL-15 signaling, whereas mTORC2 represses mTORC1-modulated NK cell effector functions by restraining STAT5-mediated SLC7A5 expression. These positive and negative crosstalks between mTORC1 and mTORC2 signaling thus variegate the magnitudes and kinetics of NK cell activation, and help define a paradigm for the modulation of NK maturation and effector functions.

The metabolic regulator protein family, mTOR, regulate natural killer (NK) cell development and function, but the underlying mechanism is unclear. Here, the authors show that Raptor/mTORC1 and Rictor/mTORC2 form a feedback crosstalk network to variegate cytokine and cellular signaling and modulate NK maturation and effector functions.

SLAM family receptors in natural killer cells - Mediators of adhesion, activation and inhibition via cis and trans interactions

SLAM family receptors are important for the fine-tuning of immune reactions. Their expression is restricted to cells of hematopoietic origin and most SLAM family receptors are their own ligand. Here we review how these receptors are involved in regulating the functions of Natural Killer (NK) cells. We discuss that promoting cellular adhesion may be a main function of SLAM family receptors in NK cells. The homophilic interactions of SLAM family receptors can not only occur in trans between different cells, but also in cis on the surface of the same cell. This cis interaction additionally modulates the function of the receptors and subsequently affects the activities of NK cells. Finally, SLAM-family receptors can also mediate inhibitory signals under certain conditions. These inhibitory signals can contribute to the functional maturation of NK cells during NK cell education. Therefore, SLAM family receptors are critically involved in many aspects of NK cell functionality.

PD-L1 Mediates Dysfunction in Activated PD-1+ NK Cells in Head and Neck Cancer Patients

Inhibitory immune-checkpoint receptors (ICRs), including programmed death 1 (PD-1), have been characterized as exhaustion markers on T cells that infiltrate the tumor microenvironment (TME) of many cancer types, including head and neck cancer (HNC). However, expression and function of ICRs, including PD-1, on natural killer (NK) cells remains less defined. NK cells are innate immune effector cells that lyse epidermal growth factor receptor-overexpressing HNC cells via cetuximab-mediated antibody-dependent cytotoxicity. Cetuximab is clinically effective but only in 10% to 15% of patients. Therefore, it is necessary to investigate how immunomodulation with cetuximab or PD-1 blockade might enhance NK cell responses in the TME and improve monoclonal antibody therapeutic efficacy. We observed that expression of PD-1 on NK cells marks an activated phenotype, which was suppressed only after binding programmed death ligand-1 (PD-L1). HNC patients who exhibit higher circulating PD-1⁺ NK cells associate with better clinical outcome, and these cells are enriched in the TME. Cetuximab-mediated NK cell activation increased PD-1 expression on NK cells in vitro, which was confirmed in vivo in a prospective neoadjuvant cetuximab trial. In contrast, PD-L1 ligation of PD-1⁺ NK cells diminished their activation status, whereas PD-1 blockade increased cetuximab-mediated NK cell activation and cytotoxicity, but only against HNC targets with high PD-L1 expression. Therefore, blocking the PD-1-PD-L1 axis may be a useful strategy to reverse immune evasion of HNC tumors with high PD-L1 expression during cetuximab therapy by reversing NK cell dysfunction.

Inherited Immunodeficiencies With High Predisposition to Epstein-Barr Virus-Driven Lymphoproliferative Diseases

Epstein–Barr Virus (EBV) is a gamma-herpes virus that infects 90% of humans without any symptoms in most cases, but has an oncogenic potential, especially in immunocompromised individuals. In the past 30 years, several primary immunodeficiencies (PIDs) associated with a high risk to develop EBV-associated lymphoproliferative disorders (LPDs), essentially consisting of virus-associated hemophagocytic syndrome, non-malignant and malignant B-cell LPDs including non-Hodgkin and Hodgkin’s types of B lymphomas have been characterized. Among them are SH2D1A (SAP), XIAP, ITK, MAGT1, CD27, CD70, CTPS1, RASGRP1, and CORO1A deficiencies. Penetrance of EBV infection ranges from 50 to 100% in those PIDs. Description of large cohorts and case reports has refined the specific phenotypes associated with these PIDs helping to the diagnosis. Specific pathways required for protective immunity to EBV have emerged from studies of these PIDs. SLAM-associated protein-dependent SLAM receptors and MAGT1-dependent NKG2D pathways are important for T and NK-cell cytotoxicity toward EBV-infected B-cells, while CD27–CD70 interactions are critical to drive the expansion of EBV-specific T-cells. CTPS1 and RASGRP1 deficiencies further strengthen that T-lymphocyte expansion is a key step in the immune response to EBV. These pathways appear to be also important for the anti-tumoral immune surveillance of abnormal B cells. Monogenic PIDs should be thus considered in case of any EBV-associated LPDs.

X-Linked Lymphoproliferative Disease Type 1: A Clinical and Molecular Perspective

X-linked lymphoproliferative disease (XLP) was first described in the 1970s as a fatal lymphoproliferative syndrome associated with infection with Epstein–Barr virus (EBV). Features include hemophagocytic lymphohistiocytosis (HLH), lymphomas, and dysgammaglobulinemias. Molecular cloning of the causative gene, SH2D1A, has provided insight into the nature of disease, as well as helped characterize multiple features of normal immune cell function. Although XLP type 1 (XLP1) provides an example of a primary immunodeficiency in which patients have problems clearing primarily one infectious agent, it is clear that XLP1 is also a disease of severe immune dysregulation, even independent of EBV infection. Here, we describe clinical features of XLP1, how molecular and biological studies of the gene product, SAP, and the associated signaling lymphocyte activation molecule family receptors have provided insight into disease pathogenesis including specific immune cell defects, and current therapeutic approaches including the potential use of gene therapy. Together, these studies have helped change the outcome of this once almost uniformly fatal disease.

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.

Molecular checkpoints controlling natural killer cell activation and their modulation for cancer immunotherapy

Natural killer (NK) cells have gained considerable attention as promising therapeutic tools for cancer therapy due to their innate selectivity against cancer cells over normal healthy cells. With an array of receptors evolved to sense cellular alterations, NK cells provide early protection against cancer cells by producing cytokines and chemokines and exerting direct cytolytic activity. These effector functions are governed by signals transmitted through multiple receptor–ligand interactions but are not achieved by engaging a single activating receptor on resting NK cells. Rather, they require the co-engagement of different activating receptors that use distinct signaling modules, due to a cell-intrinsic inhibition mechanism. The redundancy of synergizing receptors and the inhibition of NK cell function by a single class of inhibitory receptor suggest the presence of common checkpoints to control NK cell activation through different receptors. These molecular checkpoints would be therapeutically targeted to harness the power of NK cells against diverse cancer cells that express heterogeneous ligands for NK cell receptors. Recent advances in understanding the activation of NK cells have revealed promising candidates in this category. Targeting such molecular checkpoints will facilitate NK cell activation by lowering activation thresholds, thereby providing therapeutic strategies that optimize NK cell reactivity against cancer.

T cell exhaustion: from pathophysiological basics to tumor immunotherapy

The immune system is capable of distinguishing between danger- and non-danger signals, thus inducing either an appropriate immune response against pathogens and cancer or inducing self-tolerance to avoid autoimmunity and immunopathology. One of the mechanisms that have evolved to prevent destruction by the immune system, is to functionally silence effector T cells, termed T cell exhaustion, which is also exploited by viruses and cancers for immune escape In this review, we discuss some of the phenotypic markers associated with T cell exhaustion and we summarize current strategies to reinvigorate exhausted T cells by blocking these surface marker using monoclonal antibodies.

Dissection of SAP-dependent and SAP-independent SLAM family signaling in NKT cell development and humoral immunity

Chen et al. dissect SAP-dependent and SAP-independent SLAM family signaling in the regulation of NKT cell development and follicular T helper cell differentiation using a novel mouse model lacking all seven SLAM family receptors.

Signaling lymphocytic activation molecule (SLAM)–associated protein (SAP) mutations in X-linked lymphoproliferative disease (XLP) lead to defective NKT cell development and impaired humoral immunity. Because of the redundancy of SLAM family receptors (SFRs) and the complexity of SAP actions, how SFRs and SAP mediate these processes remains elusive. Here, we examined NKT cell development and humoral immunity in mice completely deficient in SFR. We found that SFR deficiency severely impaired NKT cell development. In contrast to SAP deficiency, SFR deficiency caused no apparent defect in follicular helper T (T_FH) cell differentiation. Intriguingly, the deletion of SFRs completely rescued the severe defect in T_FH cell generation caused by SAP deficiency, whereas SFR deletion had a minimal effect on the defective NKT cell development in SAP-deficient mice. These findings suggest that SAP-dependent activating SFR signaling is essential for NKT cell selection; however, SFR signaling is inhibitory in SAP-deficient T_FH cells. Thus, our current study revises our understanding of the mechanisms underlying T cell defects in patients with XLP.

Primary immunodeficiencies associated with EBV-Induced lymphoproliferative disorders

Primary immunodeficiency diseases (PIDs) are a subgroup of inherited immunological disorders that increase susceptibility to viral infections. Among the range of viral pathogens involved, EBV remains a major threat because of its high prevalence of infection among the adult population and its tendency to progress to life-threatening lymphoproliferative disorders (LPDs) and/or malignancy. The high mortality in immunodeficient patients with EBV-driven LPDs, despite institution of diverse and often intensive treatments, prompts the need to better study these PIDs to identify and understand the affected molecular pathways that increase susceptibility to EBV infection and progression. In this article, we have provided a detailed literature review of the reported cases of EBV-driven LPDs in patients with PID. We discuss the PIDs associated with development of EBV-LPDs. Then, we review the nature and the therapeutic outcome of common EBV- driven LPDs in the PID patients and review the mechanisms common to the major PIDs. Deep study of these common pathways and gaining a better insight into the disease nature and outcomes, may lead to earlier diagnosis of the disease, choosing the best treatment modalities available and development of novel therapeutic strategies to decrease morbidity and mortality brought about by EBV infection.

Stepwise phosphorylation of p65 promotes NF-κB activation and NK cell responses during target cell recognition

NF-κB is a key transcription factor that dictates the outcome of diverse immune responses. How NF-κB is regulated by multiple activating receptors that are engaged during natural killer (NK)-target cell contact remains undefined. Here we show that sole engagement of NKG2D, 2B4 or DNAM-1 is insufficient for NF-κB activation. Rather, cooperation between these receptors is required at the level of Vav1 for synergistic NF-κB activation. Vav1-dependent synergistic signalling requires a separate PI3K-Akt signal, primarily mediated by NKG2D or DNAM-1, for optimal p65 phosphorylation and NF-κB activation. Vav1 controls downstream p65 phosphorylation and NF-κB activation. Synergistic signalling is defective in X-linked lymphoproliferative disease (XLP1) NK cells entailing 2B4 dysfunction and required for p65 phosphorylation by PI3K-Akt signal, suggesting stepwise signalling checkpoint for NF-κB activation. Thus, our study provides a framework explaining how signals from different activating receptors are coordinated to determine specificity and magnitude of NF-κB activation and NK cell responses.

NK cell activation requires multiple signals. Here the authors show that while NKG2D, 2B4, or DNAM-1 receptor activation is insufficient to induce cytokine production, these signals synergize by Vav-1-mediated NF-κB multiphosphorylation, and this signaling checkpoint is defective in X-linked lymphoproliferative disease.

A hematopoietic cell-driven mechanism involving SLAMF6 receptor, SAP adaptors and SHP-1 phosphatase regulates NK cell education

Activation of natural killer (NK) cells by hematopoietic target cells is controlled by the SLAM family of receptors and by the associated SAP family of adaptors. Here we found that SLAM receptors also enhanced NK cell activation by nonhematopoietic target cells, which lack ligands for SLAM receptors. This function was mediated by SLAMF6, a homotypic SLAM receptor found on NK cells and other hematopoietic cells, and was regulated by SAP adaptors, which uncoupled SLAM receptors from phosphatase SHP-1 and diminished the effect of SLAMF6 on NK cell responsiveness toward nonhematopoietic cells. Thus, in addition to their role in NK cell activation by hematopoietic cells, the SLAM-SAP pathways influence responsiveness toward nonhematopoietic targets by a process akin to NK cell 'education'.

Fine-tuning of CD8(+) T-cell effector functions by targeting the 2B4-CD48 interaction

Polyfunctionality and cytotoxic activity dictate CD8(+) T-cell efficacy in the eradication of infected and malignant cells. The induction of these effector functions depends on the specific interaction between the T-cell receptor (TCR) and its cognate peptide-MHC class I complex, in addition to signals provided by co-stimulatory or co-inhibitory receptors, which can further regulate these functions. Among these receptors, the role of 2B4 is contested, as it has been described as either co-stimulatory or co-inhibitory in modulating T-cell functions. We therefore combined functional, transcriptional and epigenetic approaches to further characterize the impact of disrupting the interaction of 2B4 with its ligand CD48, on the activity of human effector CD8(+) T-cell clones. In this setting, we show that the 2B4-CD48 axis is involved in the fine-tuning of CD8(+) T-cell effector function upon antigenic stimulation. Blocking this interaction resulted in reduced CD8(+) T-cell clone-mediated cytolytic activity, together with a subtle drop in the expression of genes involved in effector function regulation. Our results also imply a variable contribution of the 2B4-CD48 interaction to the modulation of CD8(+) T-cell functional properties, potentially linked to intrinsic levels of T-bet expression and TCR avidity. The present study thus provides further insights into the role of the 2B4-CD48 interaction in the fine regulation of CD8(+) T-cell effector function upon antigenic stimulation.

Roles of CD48 in regulating immunity and tolerance

CD48, a member of the signaling lymphocyte activation molecule family, participates in adhesion and activation of immune cells. Although constitutively expressed on most hematopoietic cells, CD48 is upregulated on subsets of activated cells. CD48 can have activating roles on T cells, antigen presenting cells and granulocytes, by binding to CD2 or bacterial FimH, and through cell intrinsic effects. Interactions between CD48 and its high affinity ligand CD244 are more complex, with both stimulatory and inhibitory outcomes. CD244:CD48 interactions regulate target cell lysis by NK cells and CTLs, which are important for viral clearance and regulation of effector/memory T cell generation and survival. Here we review roles of CD48 in infection, tolerance, autoimmunity, and allergy, as well as the tools used to investigate this receptor. We discuss stimulatory and regulatory roles for CD48, its potential as a therapeutic target in human disease, and current challenges to investigation of this immunoregulatory receptor.

DNAM-1 controls NK cell activation via an ITT-like motif

DNAM-1­–mediated active biochemical signals initiated by a conserved ITT-like motif are essential for its capacity to enhance NK cell cytotoxicity and cytokine production. Enhancement of these signals may underlie the therapeutic impact of blocking anti-TIGIT antibodies for treatment of cancer and viral infections.

DNAM-1 (CD226) is an activating receptor expressed on natural killer (NK) cells, CD8⁺ T cells, and other immune cells. Upon recognition of its ligands, CD155 and CD112, DNAM-1 promotes NK cell–mediated elimination of transformed and virus-infected cells. It also has a key role in expansion and maintenance of virus-specific memory NK cells. Herein, the mechanism by which DNAM-1 controls NK cell–mediated cytotoxicity and cytokine production was elucidated. Cytotoxicity and cytokine production triggered by DNAM-1 were mediated via a conserved tyrosine- and asparagine-based motif in the cytoplasmic domain of DNAM-1. Upon phosphorylation by Src kinases, this motif enabled binding of DNAM-1 to adaptor Grb2, leading to activation of enzymes Vav-1, phosphatidylinositol 3′ kinase, and phospholipase C-γ1. It also promoted activation of kinases Erk and Akt, and calcium fluxes. Although, as reported, DNAM-1 promoted adhesion, this function was signal-independent and insufficient to promote cytotoxicity. DNAM-1 signaling was also required to enhance cytotoxicity, by increasing actin polymerization and granule polarization. We propose that DNAM-1 promotes NK cell activation via an immunoreceptor tyrosine tail (ITT)–like motif coupling DNAM-1 to Grb2 and other downstream effectors.

A Comprehensive Immunoreceptor Phosphotyrosine-based Signaling Network Revealed by Reciprocal Protein-Peptide Array Screening

Cells of the immune system communicate with their environment through immunoreceptors. These receptors often harbor intracellular tyrosine residues, which, when phosphorylated upon receptor activation, serve as docking sites to recruit downstream signaling proteins containing the Src Homology 2 (SH2) domain. A systematic investigation of interactions between the SH2 domain and the immunoreceptor tyrosine-based regulatory motifs (ITRM), including inhibitory (ITIM), activating (ITAM), or switching (ITSM) motifs, is critical for understanding cellular signal transduction and immune function. Using the B cell inhibitory receptor CD22 as an example, we developed an approach that combines reciprocal or bidirectional phosphopeptide and SH2 domain array screens with in-solution binding assays to identify a comprehensive SH2-CD22 interaction network. Extending this approach to 194 human ITRM sequences and 78 SH2 domains led to the identification of a high-confidence immunoreceptor interactome containing 1137 binary interactions. Besides recapitulating many previously reported interactions, our study uncovered numerous novel interactions. The resulting ITRM-SH2 interactome not only helped to fill many gaps in the immune signaling network, it also allowed us to associate different SH2 domains to distinct immune functions. Detailed analysis of the NK cell ITRM-mediated interactions led to the identification of a network nucleated by the Vav3 and Fyn SH2 domains. We showed further that these SH2 domains have distinct functions in cytotoxicity. The bidirectional protein-peptide array approach described herein may be applied to the numerous other peptide-binding modules to identify potential protein-protein interactions in a systematic and reliable manner.

Homotypic NK cell-to-cell communication controls cytokine responsiveness of innate immune NK cells

While stationary organ cells are in continuous contact with neighboring cells, immune cells circulate throughout the body without an apparent requirement for cell-cell contact to persist in vivo. This study challenges current convention by demonstrating, both in vitro and in vivo, that innate immune NK cells can engage in homotypic NK-to-NK cell interactions for optimal survival, activation, and proliferation. Using a specialized cell-laden microwell approach, we discover that NK cells experiencing constant NK-to-NK contact exhibit a synergistic increase in activation status, cell proliferation, and anti-tumor function in response to IL-2 or IL-15. This effect is dependent on 2B4/CD48 ligation and an active cytoskeleton, resulting in amplification of IL-2 receptor signaling, enhanced CD122/CD132 colocalization, CD25 upregulation, and Stat3 activation. Conversely, 'orphan' NK cells demonstrate no such synergy and fail to persist. Therefore, our data uncover the existence of homotypic cell-to-cell communication among mobile innate lymphocytes, which promotes functional synergy within the cytokine-rich microenvironment.

SAP facilitates recruitment and activation of LCK at NTB-A receptors during restimulation-induced cell death

Upon TCR restimulation, activated, cycling T cells can undergo a self-regulatory form of apoptosis known as restimulation-induced cell death (RICD). We previously demonstrated that RICD is impaired in T cells from patients with X-linked lymphoproliferative disease, which lack SLAM-associated protein (SAP) expression. Both SAP and the specific SLAM receptor NK, T, and B cell Ag (NTB-A) are required for RICD, but the mechanism by which these molecules promote a strong, proapoptotic signal through the TCR remains unclear. In this article, we show that the Src-family kinase LCK, but not FYN, associates with NTB-A in activated human T cells. This association increased after TCR restimulation in a SAP-dependent manner, requiring both immunoreceptor tyrosine-based switch motifs in the NTB-A cytoplasmic tail. Both NTB-A-associated LCK phosphorylation and kinase activity were enhanced in restimulated T cells, amplifying proximal TCR signaling. In contrast, TCR-induced LCK association with NTB-A, as well as phosphorylation and kinase activity, was reduced in T cells from patients with X-linked lymphoproliferative disease or normal T cells transfected with SAP-specific small interfering RNA, consistent with RICD resistance. Collectively, our data reveal how SAP nucleates a previously unknown signaling complex involving NTB-A and LCK to potentiate RICD of activated human T cells.

Recognition of tumors by the innate immune system and natural killer cells

In recent years, roles of the immune system in immune surveillance of cancer have been explored using a variety of approaches. The roles of the adaptive immune system have been a major emphasis, but increasing evidence supports a role for innate immune effector cells such as natural killer (NK) cells in tumor surveillance. Here, we discuss some of the evidence for roles in tumor surveillance of innate immune cells. In particular, we focus on NK cells and other immune cells that express germline-encoded receptors, often labeled NK receptors. The impact of these receptors and the cells that express them on tumor suppression is summarized. We discuss in detail some of the pathways and events in tumor cells that induce or upregulate cell-surface expression of the ligands for these receptors, and the logic of how those pathways serve to identify malignant, or potentially malignant cells. How tumors often evade tumor suppression mediated by innate killer cells is another major subject of the review. We end with a discussion on some of the implications of the various findings with respect to possible therapeutic approaches.

Large granular lymphocytosis during dasatinib therapy

Dasatinib is a second generation tyrosine kinase inhibitor (TKI) approved for clinical use in patients with imatinib-resistant chronic myeloid leukemia (CML) and Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph(+) ALL). Large granular lymphocytes (LGLs) are medium to large cells with eccentric nuclei and abundant cytoplasm with coarse azurophilic granules. LGL lymphocytosis is caused by a proliferation of cytotoxic (CD8+) T cells and/or NK cells. In a proportion of CML and Ph(+) ALL patients, there is a significant expansion of LGLs during dasatinib therapy. LGL lymphocytosis is seen in some cases with fevers, colitis, and pleural effusions (PE), suggesting an aberrant immune response mediated by these LGLs. LGLs may participate in the elimination of the residual leukemic cells, and LGL clonal expansion is associated with excellent, long-lasting therapy responses in dasatinib-treated patients. For a more comprehensive analysis, we analyzed the morphologic, phenotypic, clinical, and functional features of the LGL subsets amplified in vivo during dasatinib therapy.

Dasatinib, large granular lymphocytosis, and pleural effusion: useful or adverse effect?

Dasatinib is a second generation tyrosine kinase inhibitor approved for clinical use in first line and imatinib-resistant chronic myeloid leukemia and Philadelphia positive (Ph+) acute lymphoblastic leukemia. In addition to BCR-ABL1, dasatinib inhibits TEC kinases and SRC family kinases and is more potent than imatinib in the treatment of Ph+ leukemias. In the last 3 years, increases in cytotoxic T and natural-killer cells in peripheral blood samples have been reported in cases treated by dasatinib. The awareness of the clonal expansion of large granular lymphocytes and beneficial effect of these clonal cells increased the interest to dasatinib in cases receiving this drug. Clonal expansion of large granular lymphocytes is an important effect of dasatinib therapy, shown to be an off-target phenomenon associated with pleural effusion and better clinical response. The benefit of dasatinib-induced lymphocytosis and its underlying mechanism of this are important points for clinicians working in hematology and oncology.

Requirement and redundancy of the Src family kinases Fyn and Lyn in perforin-dependent killing of Cryptococcus neoformans by NK cells

Natural killer (NK) cells directly recognize and kill fungi, such as the pathogenic fungus Cryptococcus neoformans, via cytolytic mechanisms. However, the precise signaling pathways governing this NK cell microbicidal activity and the implications for fungal recognition are still unknown. Previously, it was reported that NK cell anticryptococcal activity is mediated through a conserved phosphatidylinositol 3-kinase-extracellular signal-regulated kinase 1/2 (PI3K-ERK1/2) pathway. Using YT (a human NK-like cell line) and primary human NK cells, we sought to identify the upstream, receptor-proximal signaling elements that led to fungal cytolysis. We demonstrate that Src family kinases were activated in response to C. neoformans. Furthermore, pharmacologic inhibition with an Src kinase inhibitor blocked C. neoformans-induced downstream activation of PI3K and ERK1/2 and abrogated cryptococcal killing. At the same time, the inhibitor disrupted the polarization of perforin-containing granules toward the NK cell-cryptococcal synapse but had no effect on conjugate formation between the organism and the NK cell. Finally, small interfering RNA (siRNA) double (but not single) knockdown of two Src family kinases, Fyn and Lyn, blocked cryptococcal killing. Together these data demonstrate a mechanism whereby the Src family kinases, Fyn and Lyn, redundantly mediate anticryptococcal activity through the activation of PI3K and ERK1/2, which in turn facilitates killing by inducing the polarization of perforin-containing granules to the NK cell-cryptococcal synapse.

The adaptor molecule SAP plays essential roles during invariant NKT cell cytotoxicity and lytic synapse formation

The adaptor molecule signaling lymphocytic activation molecule-associated protein (SAP) plays critical roles during invariant natural killer T (iNKT) cell ontogeny. As a result, SAP-deficient humans and mice lack iNKT cells. The strict developmental requirement for SAP has made it difficult to discern its possible involvement in mature iNKT cell functions. By using temporal Cre recombinase-mediated gene deletion to ablate SAP expression after completion of iNKT cell development, we demonstrate that SAP is essential for T-cell receptor (TCR)-induced iNKT cell cytotoxicity against T-cell and B-cell leukemia targets in vitro and iNKT-cell-mediated control of T-cell leukemia growth in vivo. These findings are not restricted to the murine system: silencing RNA-mediated suppression of SAP expression in human iNKT cells also significantly impairs TCR-induced cytolysis. Mechanistic studies reveal that iNKT cell killing requires the tyrosine kinase Fyn, a known SAP-binding protein. Furthermore, SAP expression is required within iNKT cells to facilitate their interaction with T-cell targets and induce reorientation of the microtubule-organizing center to the immunologic synapse (IS). Collectively, these studies highlight a novel and essential role for SAP during iNKT cell cytotoxicity and formation of a functional IS.

Molecular mechanisms of T cell co-stimulation and co-inhibition

Co-stimulatory and co-inhibitory receptors have a pivotal role in T cell biology, as they determine the functional outcome of T cell receptor (TCR) signalling. The classic definition of T cell co-stimulation continues to evolve through the identification of new co-stimulatory and co-inhibitory receptors, the biochemical characterization of their downstream signalling events and the delineation of their immunological functions. Notably, it has been recently appreciated that co-stimulatory and co-inhibitory receptors display great diversity in expression, structure and function, and that their functions are largely context dependent. Here, we focus on some of these emerging concepts and review the mechanisms through which T cell activation, differentiation and function is controlled by co-stimulatory and co-inhibitory receptors.

NK cell cytotoxicity mediated by 2B4 and NTB-A is dependent on SAP acting downstream of receptor phosphorylation

2B4 (CD244) and NK-T-B-antigen (NTB-A, CD352) are activating receptors on human natural killer (NK) cells and belong to the family of signaling lymphocyte activation molecule (SLAM)-related receptors (SRR). Engagement of these receptors leads to phosphorylation of their cytoplasmic tails and recruitment of the adapter proteins SLAM-associated protein (SAP) and Ewing's sarcoma-activated transcript-2 (EAT-2). X-linked lymphoproliferative syndrome (XLP) is a severe immunodeficiency that results from mutations in the SAP gene. 2B4 and NTB-A-mediated cytotoxicity are abrogated in XLP NK cells. To elucidate the molecular basis for this defect we analyzed early signaling events in SAP knockdown cells. Similar to XLP NK cells, knockdown of SAP in primary human NK cells leads to a reduction of 2B4 and NTB-A-mediated cytotoxicity. We found that early signaling events such as raft recruitment and receptor phosphorylation are not affected by the absence of SAP, indicating the defect in the absence of SAP is downstream of these events. In addition, knockdown of EAT-2 does not impair 2B4 or NTB-A-mediated cytotoxicity. Surprisingly, EAT-2 recruitment to both receptors is abrogated in the absence of SAP, revealing a novel cooperativity between these adapters.

Intronic SH2D1A mutation with impaired SAP expression and agammaglobulinemia

X-linked lymphoproliferative (XLP) disease is a primary immunodeficiency syndrome associated with the inability to control Epstein-Barr virus (EBV), lymphoma, and hypogammaglobulinemia. XLP is caused by mutations in the SH2D1A gene, which encodes the SLAM-associated protein (SAP), or in the BIRC4 gene, which encodes the X-linked inhibitor of apoptosis protein (XIAP). Here we report a patient with recurrent respiratory tract infections and early onset agammaglobulinemia who carried a unique disease-causing intronic loss-of-function mutation in SH2D1A. The intronic mutation affected SH2D1A gene transcription but not mRNA splicing, and led to markedly reduced level of SAP protein. Despite undetectable serum immunoglobulins, the patient's B cells replicated and differentiated into antibody producing cells normally in vitro.

The adaptor SAP controls NK cell activation by regulating the enzymes Vav-1 and SHIP-1 and by enhancing conjugates with target cells

The adaptor SAP, mutated in X-linked lymphoproliferative disease, has critical roles in multiple immune cell types. Among these, SAP is essential for the ability of natural killer (NK) cells to eliminate abnormal hematopoietic cells. Herein, we elucidated the molecular and cellular bases of this activity. SAP enhanced NK cell responsiveness by a dual molecular mechanism. It coupled SLAM family receptors to the kinase Fyn, which triggered the exchange factor Vav-1 and augmented NK cell activation. SAP also prevented the inhibitory function of SLAM family receptors. This effect was Fyn independent and correlated with uncoupling of SLAM family receptors from the lipid phosphatase SHIP-1. Both mechanisms cooperated to enable conjugate formation with target cells and to stimulate cytotoxicity and cytokine secretion by NK cells. These data showed that SAP secures NK cell activation by a dichotomous molecular mechanism, which is required for conjugate formation. These findings may have implications for the role of SAP in other immune cell types.

Inherited defects causing hemophagocytic lymphohistiocytic syndrome

Hemophagocytic lymphohistiocytosis (HLH) manifests as the uncontrolled activation of T lymphocytes and macrophages infiltrating multiple organs. Molecular studies of individuals with HLH have demonstrated in most of these conditions a critical role of granule-dependent cytotoxic activity in the regulation of lymphocyte homeostasis, and have allowed the characterization of key effectors regulating cytotoxic granule release. The cytolytic process may now be considered a multistep process, including cell activation; the polarization of cytotoxic granules toward the conjugated target cell; the tethering, priming, and fusion of the cytotoxic granules with the plasma membrane; and the release of their contents (perforin and granzymes) into the intercellular cleft, leading to target cell death. Cytolytic cells have a second effector function involving the production of cytokines, principally γ-interferon, which is secreted independently of the exocytosis cytotoxic granule pathway. An analysis of the mechanisms underlying HLH has identified γ-interferon as a key cytokine inducing uncontrolled macrophage activation, and thus represents a potential therapeutic target.

Tim-3 is an inducible human natural killer cell receptor that enhances interferon gamma production in response to galectin-9

NK-cell function is regulated by the integration of signals received from activating and inhibitory receptors. Here we show that a novel immune receptor, T-cell Ig and mucin-containing domain-3 (Tim-3), is expressed on resting human NK cells and is up-regulated on activation. The NK92 NK-cell line engineered to overexpress Tim-3 showed a marked increase in IFN-γ production in the presence of soluble rhGal-9 or Raji tumor cells engineered to express Gal-9. The Tim-3(+) population of low-dose IL-12/IL-18-activated primary NK cells significantly increased IFN-γ production in response to soluble rhGal-9, Gal-9 presented by cell lines, and primary acute myelogenous leukemia (AML) targets that endogenously express Gal-9. This effect is highly specific as Tim-3 Ab blockade significantly decreased IFN-γ production, and Tim-3 cross-linking induced ERK activation and degradation of IκBα. Exposure to Gal-9-expressing target cells had little effect on CD107a degranulation. Reconstituted NK cells obtained from patients after hematopoietic cell transplantation had diminished expression of Tim-3 compared with paired donors. This observation correlates with the known IFN-γ defect seen early posttransplantation. In conclusion, we show that Tim-3 functions as a human NK-cell coreceptor to enhance IFN-γ production, which has important implications for control of infectious disease and cancer.

Kinome analysis of receptor-induced phosphorylation in human natural killer cells

BACKGROUND

Natural killer (NK) cells contribute to the defense against infected and transformed cells through the engagement of multiple germline-encoded activation receptors. Stimulation of the Fc receptor CD16 alone is sufficient for NK cell activation, whereas other receptors, such as 2B4 (CD244) and DNAM-1 (CD226), act synergistically. After receptor engagement, protein kinases play a major role in signaling networks controlling NK cell effector functions. However, it has not been characterized systematically which of all kinases encoded by the human genome (kinome) are involved in NK cell activation.

RESULTS

A kinase-selective phosphoproteome approach enabled the determination of 188 kinases expressed in human NK cells. Crosslinking of CD16 as well as 2B4 and DNAM-1 revealed a total of 313 distinct kinase phosphorylation sites on 109 different kinases. Phosphorylation sites on 21 kinases were similarly regulated after engagement of either CD16 or co-engagement of 2B4 and DNAM-1. Among those, increased phosphorylation of FYN, KCC2G (CAMK2), FES, and AAK1, as well as the reduced phosphorylation of MARK2, were reproducibly observed both after engagement of CD16 and co-engagement of 2B4 and DNAM-1. Notably, only one phosphorylation on PAK4 was differentally regulated.

CONCLUSIONS

The present study has identified a significant portion of the NK cell kinome and defined novel phosphorylation sites in primary lymphocytes. Regulated phosphorylations observed in the early phase of NK cell activation imply these kinases are involved in NK cell signaling. Taken together, this study suggests a largely shared signaling pathway downstream of distinct activation receptors and constitutes a valuable resource for further elucidating the regulation of NK cell effector responses.

SLAM family receptors and SAP adaptors in immunity

The signaling lymphocyte activation molecule (SLAM)-associated protein, SAP, was first identified as the protein affected in most cases of X-linked lymphoproliferative (XLP) syndrome, a rare genetic disorder characterized by abnormal responses to Epstein-Barr virus infection, lymphoproliferative syndromes, and dysgammaglobulinemia. SAP consists almost entirely of a single SH2 protein domain that interacts with the cytoplasmic tail of SLAM and related receptors, including 2B4, Ly108, CD84, Ly9, and potentially CRACC. SLAM family members are now recognized as important immunomodulatory receptors with roles in cytotoxicity, humoral immunity, autoimmunity, cell survival, lymphocyte development, and cell adhesion. In this review, we cover recent findings on the roles of SLAM family receptors and the SAP family of adaptors, with a focus on their regulation of the pathways involved in the pathogenesis of XLP and other immune disorders.