Deletion of Slam locus in mice reveals inhibitory role of SLAM family in NK cell responses regulated by cytokines and LFA-1

Suppression of adaptive NK cell expansion by macrophage-mediated phagocytosis inhibited by 2B4-CD48

Infection of mice by mouse cytomegalovirus (MCMV) triggers activation and expansion of Ly49H+ natural killer (NK) cells, which are virus specific and considered to be "adaptive" or "memory" NK cells. Here, we find that signaling lymphocytic activation molecule family receptors (SFRs), a group of hematopoietic cell-restricted receptors, are essential for the expansion of Ly49H+ NK cells after MCMV infection. This activity is largely mediated by CD48, an SFR broadly expressed on NK cells and displaying augmented expression after MCMV infection. It is also dependent on the CD48 counter-receptor, 2B4, expressed on host macrophages. The 2B4-CD48 axis promotes expansion of Ly49H+ NK cells by repressing their phagocytosis by virus-activated macrophages through inhibition of the pro-phagocytic integrin lymphocyte function-associated antigen-1 (LFA-1) on macrophages. These data identify key roles of macrophages and the 2B4-CD48 pathway in controlling the expansion of adaptive NK cells following MCMV infection. Stimulation of the 2B4-CD48 axis may be helpful in enhancing adaptive NK cell responses for therapeutic purposes.

CD47 masks pro-phagocytic ligands in cis on tumor cells to suppress antitumor immunity

Cancer cells often overexpress CD47, which triggers the inhibitory receptor SIRPα expressed on macrophages, to elude phagocytosis and antitumor immunity. Pharmacological blockade of CD47 or SIRPα is showing promise as anticancer therapy, although CD47 blockade has been associated with hematological toxicities that may reflect its broad expression pattern on normal cells. Here we found that, in addition to triggering SIRPα, CD47 suppressed phagocytosis by a SIRPα-independent mechanism. This mechanism prevented phagocytosis initiated by the pro-phagocytic ligand, SLAMF7, on tumor cells, due to a cis interaction between CD47 and SLAMF7. The CD47-SLAMF7 interaction was disrupted by CD47 blockade and by a first-in-class agonist SLAMF7 antibody, but not by SIRPα blockade, thereby promoting antitumor immunity. Hence, CD47 suppresses phagocytosis not only by engaging SIRPα, but also by masking cell-intrinsic pro-phagocytic ligands on tumor cells and knowledge of this mechanism may influence the decision between CD47 blockade or SIRPα blockade for therapeutic purposes.

Cis interactions between CD2 and its ligands on T cells are required for T cell activation

CD2 is largely described to promote T cell activation when engaged by its ligands, CD48 in mice and CD58 in humans, that are present on antigen-presenting cells (APCs). However, both CD48 and CD58 are also expressed on T cells. By generating new knockout mouse strains lacking CD2 or CD48 in the C57BL/6 background, we determined that whereas CD2 was necessary on T cells for T cell activation, its ligand CD48 was not required on APCs. Rather, CD48 was also needed on T cells. One exception was during cytotoxicity, which required CD48 on T cells and APCs. Fluorescence resonance energy transfer (FRET) studies in nonimmune cells provided evidence that cis interactions between CD2 and CD48 existed within individual cells. CD2-CD48 interactions on T cells enabled more robust T cell receptor (TCR) signals, including protein tyrosine phosphorylation. Using T cells from a CD2 knock-in mouse in which a tag was inserted at the carboxyl terminus of CD2, mass spectrometry analyses revealed that the role of CD2 in T cell activation correlated with its ability to interact with components of the TCR complex and the protein tyrosine kinase Lck. CD2-CD58 provided a similar function in human T cells. Thus, our data imply that T cell-intrinsic cis interactions of CD2 with its ligands are required for TCR signaling and T cell activation. Interactions with ligands on APCs contribute during cytotoxicity.

When killers become thieves: Trogocytosed PD-1 inhibits NK cells in cancer

Trogocytosis modulates immune responses, with still unclear underlying molecular mechanisms. Using leukemia mouse models, we found that lymphocytes perform trogocytosis at high rates with tumor cells. While performing trogocytosis, both Natural Killer (NK) and CD8+ T cells acquire the checkpoint receptor PD-1 from leukemia cells. In vitro and in vivo investigation revealed that PD-1 on the surface of NK cells, rather than being endogenously expressed, was derived entirely from leukemia cells in a SLAM receptor-dependent fashion. PD-1 acquired via trogocytosis actively suppressed NK cell antitumor immunity. PD-1 trogocytosis was corroborated in patients with clonal plasma cell disorders, where NK cells that stained for PD-1 also stained for tumor cell markers. Our results, in addition to shedding light on a previously unappreciated mechanism underlying the presence of PD-1 on NK and cytotoxic T cells, reveal the immunoregulatory effect of membrane transfer occurring when immune cells contact tumor cells.

Inflammatory macrophages exploit unconventional pro-phagocytic integrins for phagocytosis and anti-tumor immunity

Blockade of the inhibitory checkpoint SIRPα-CD47 promotes phagocytosis of cancer cells by macrophages and is a promising avenue in anti-cancer therapy. Productive phagocytosis is strictly predicated on co-engagement of pro-phagocytic receptors-namely, Fc receptors (FcRs), integrin CD11b, or SLAMF7-by their ligands on cancer cells. Here, we examine whether additional pro-phagocytic receptors could be harnessed to broaden the scope of phagocytosis. Inflammatory stimuli, including multiple cytokines and Toll-like receptor (TLR) ligands, augment phagocytosis efficiency and fully alleviate the requirement of FcRs, CD11b, and SLAMF7 for phagocytosis. These effects are mediated by the unconventional pro-phagocytic integrins CD11a and CD11c, which act with CD18 to initiate actin polarization, leading to phagocytosis. Some inflammatory stimuli enable phagocytosis even in the absence of SIRPα-CD47 blockade. Higher CD11c expression in macrophage-enriched tumors correlates with improved survival in clinical studies. Thus, inflammatory macrophages exploit unconventional pro-phagocytic integrins for improved phagocytosis and anti-tumor immunity.

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.

Targeting natural killer cells to enhance vaccine responses

Vaccination serves as a cornerstone of global health. Successful prevention of infection or disease by vaccines is achieved through elicitation of pathogen-specific antibodies and long-lived memory T cells. However, several microbial threats to human health have proven refractory to past vaccine efforts. These shortcomings have been attributed to either inefficient triggering of memory T and B cell responses or to the unfulfilled need to stimulate non-conventional forms of immunological memory. Natural killer (NK) cells have recently emerged as both key regulators of vaccine-elicited T and B cell responses and as memory cells that contribute to pathogen control. We discuss potential methods to modulate these functions of NK cells to enhance vaccine success.

SLAM family receptors control pro-survival effectors in germinal center B cells to promote humoral immunity

Expression of the signaling lymphocytic activation molecule (SLAM)-associated protein (SAP) is critical for the germinal center (GC) reaction and T cell-dependent antibody production. However, when SAP is expressed normally, the role of the associated SLAM family receptors (SFRs) in these processes is nebulous. Herein, we established that in the presence of SAP, SFRs suppressed the expansion of the GC reaction but facilitated the generation of antigen-specific B cells and antibodies. SFRs favored the generation of antigen-reactive B cells and antibodies by boosting expression of pro-survival effectors, such as the B cell antigen receptor (BCR) and Bcl-2, in activated GC B cells. The effects of SFRs on the GC reaction and T cell-dependent antibody production necessitated expression of multiple SFRs, both in T cells and in B cells. Hence, while in the presence of SAP, SFRs inhibit the GC reaction, they are critical for the induction of T cell-mediated humoral immunity by enhancing expression of pro-survival effectors in GC B cells.

Plasma membrane lipid scrambling causing phosphatidylserine exposure negatively regulates NK cell activation

One of the hallmarks of live cells is the asymmetric distribution of lipids across their plasma membrane. Changes in this asymmetry due to lipid "scrambling" result in phosphatidylserine exposure at the cell surface that is detected by annexin V staining. This alteration is observed during cell death processes such as apoptosis, and during physiological responses such as platelet degranulation and membrane repair. Previous studies have shown that activation of NK cells is accompanied by exposure of phosphatidylserine at the cell surface. While this response was thought to be indicative of ongoing NK cell death, it may also  reflect the regulation of NK cell activation in the absence of cell death. Herein, we found that NK cell activation was accompanied by rapid phosphatidylserine exposure to an extent proportional to the degree of NK cell activation. Through enforced expression of a lipid scramblase, we provided evidence that activation-induced lipid scrambling in NK cells is reversible and does not lead to cell death. In contrast, lipid scrambling attenuates NK cell activation. This response was accompanied by reduced cell surface expression of activating receptors such as 2B4, and by loss of binding of Src family protein tyrosine kinases Fyn and Lck to the inner leaflet of the plasma membrane. Hence, lipid scrambling during NK cell activation is, at least in part, a physiological response that reduces the NK cell activation level. This effect is due to the ability of lipid scrambling to alter the distribution of membrane-associated receptors and kinases required for NK cell activation.

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.

Concomitant deletion of SLAM-family receptors, NKG2D and DNAM-1 reveals gene redundancy of NK cell activating receptors in NK cell development and education

NK cells recognize "unwanted" cells using a variety of germline-encoded activating receptors, such as the seven members of signaling lymphocyte activating molecule (SLAM)-family receptors (SFRs), natural killer cell group 2D (NKG2D), and DNAX accessory molecule-1(DNAM-1). Whether these receptors redundantly or synergistically regulate NK cell development and effector function remains poorly understood. By generating mice lacking SFRs, NKG2D, and DNAM-1, separately or in combination, we found that SLAMF6, one of the SFR members, was associated with NK cell differentiation, but its absence had no severe effect on NK cell differentiation and function, likely due to SFR redundancy. Moreover, we revealed that SFRs might work with other NK cell activating receptors in regulating NK cell development and function. We found that SFR deficiency caused an increase in immature NK cell subsets (CD27⁺ CD11b^- ), and this effect was further augmented by the additional deficiency of NKG2D but not DNAM-1. However, SFR-deficient NK cells exhibited elevated responsiveness against "missing-self" hematopoietic targets, whereas the deletion of either NKG2D or DNAM-1 could partially abrogate the elevated effect of SFR deficiency on NK cell activation. Therefore, our results reveal the complexity of activating receptors in regulating NK cell differentiation and activation, extending our insights into the gene redundancy and compensatory effect of NK cell activating receptors.

Aging boosts antiviral CD8+T cell memory through improved engagement of diversified recall response determinants

The determinants of protective CD8+ memory T cell (CD8+TM) immunity remain incompletely defined and may in fact constitute an evolving agency as aging CD8+TM progressively acquire enhanced rather than impaired recall capacities. Here, we show that old as compared to young antiviral CD8+TM more effectively harness disparate molecular processes (cytokine signaling, trafficking, effector functions, and co-stimulation/inhibition) that in concert confer greater secondary reactivity. The relative reliance on these pathways is contingent on the nature of the secondary challenge (greater for chronic than acute viral infections) and over time, aging CD8+TM re-establish a dependence on the same accessory signals required for effective priming of naïve CD8+T cells in the first place. Thus, our findings reveal a temporal regulation of complementary recall response determinants that is consistent with the recently proposed "rebound model" according to which aging CD8+TM properties are gradually aligned with those of naïve CD8+T cells; our identification of a broadly diversified collection of immunomodulatory targets may further provide a foundation for the potential therapeutic "tuning" of CD8+TM immunity.

The Good and the Bad of Natural Killer Cells in Virus Control: Perspective for Anti-HBV Therapy

Immune modulatory therapies are widely believed to represent potential therapeutic strategies for chronic hepatitis B infection (CHB). Among the cellular targets for immune interventions, Natural Killer (NK) cells represent possible candidates because they have a key role in anti-viral control by producing cytokines and by exerting cytotoxic functions against virus-infected cells. However, in patients with chronic hepatitis B, NK cells have been described to be more pathogenic than protective with preserved cytolytic activity but with a poor capacity to produce anti-viral cytokines. In addition, NK cells can exert a regulatory activity and possibly suppress adaptive immune responses in the setting of persistent viral infections. Consequently, a potential drawback of NK-cell targeted modulatory interventions is that they can potentiate the suppressive NK cell effect on virus-specific T cells, which further causes impairment of exhausted anti-viral T cell functions. Thus, clinically useful NK-cell modulatory strategies should be not only suited to improve positive anti-viral NK cell functions but also to abrogate T cell suppression by NK cell-mediated T cell killing. This review outlines the main NK cell features with a particular focus on CHB infection. It describes different mechanisms involved in NK-T cell interplay as well as how NK cells can have positive anti-viral effector functions and negative suppressive effects on T cells activity. This review discusses how modulation of their balance can have potential therapeutic implications.

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.

From the "missing self" hypothesis to adaptive NK cells: Insights of NK cell-mediated effector functions in immune surveillance

The original discovery of NK cells approximately 40 yr ago was based on their unique capability to kill tumor cells without prior sensitization or priming, a process named natural cytotoxicity. Since then, several studies have documented that NK cells can kill hematopoietic and nonhematopoietic cancer cells. NK cells also recognize and kill cells that have undergone viral infections. Besides natural cytotoxicity, NK cells are also major effectors of antibody-dependent cell cytotoxicity (ADCC). Therefore, NK cells are well "armed" to recognize and mount immune responses against "insults" that result from cell transformation and viral infections. Because of these attributes, an essential role of NK cells in tumor surveillance was noted. Indeed, several studies have shown a correlation between impaired NK cell cytotoxicity and a higher risk of developing cancer. This evidence led to the idea that cancer initiation and progress is intimately related to an abnormal or misdirected immune response. Whereas all these ideas remain current, it is also true that NK cells represent a heterogeneous population with different abilities to secrete cytokines and to mediate cytotoxic functions. In addition, recent data has shown that NK cells are prone to suffer epigenetic modifications resulting in the acquisition of previously unrecognized attributes such as memory and long-term survival. Such NK cells, referred as "adaptive" or "memory-like," also display effector functions that are not necessarily equal to those observed in conventional NK cells. Given the new evidence available, it is essential to discuss the conceptual reasoning and misconceptions regarding the role of NK cells in immune surveillance and immunotherapy.

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.

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.

Weak vaccinia virus-induced NK cell regulation of CD4 T cells is associated with reduced NK cell differentiation and cytolytic activity

Natural killer (NK) cells control antiviral adaptive immune responses in mice during some virus infections, but the universality of this phenomenon remains unknown. Lymphocytic choriomeningitis virus (LCMV) infection of mice triggered potent cytotoxic activity of NK cells (NK^LCMV) against activated CD4 T cells, tumor cells, and allogeneic lymphocytes. In contrast, NK cells activated by vaccinia virus (VACV) infection (NK^VACV) exhibited weaker cytolytic activity against each of these target cells. Relative to NK^LCMV cells, NK^VACV cells exhibited a more immature (CD11b^-CD27⁺) phenotype, and lower expression levels of the activation marker CD69, cytotoxic effector molecules (perforin, granzyme B), and the transcription factor IRF4. NK^VACV cells expressed higher levels of the inhibitory molecule NKG2A than NK^LCMV cells. Consistent with this apparent lethargy, NK^VACV cells only weakly constrained VACV-specific CD4 T-cell responses. This suggests that NK cell regulation of adaptive immunity, while universal, may be limited with viruses that poorly activate NK cells.

Inhibition of IL-6-JAK/Stat3 signaling in castration-resistant prostate cancer cells enhances the NK cell-mediated cytotoxicity via alteration of PD-L1/NKG2D ligand levels

To investigate whether IL‐6 signaling affects the susceptibility of castration‐resistant prostate cancer (CRPC) cells to cytotoxic action of natural killer (NK) cells, CRPC cell lines (having different IL‐6 levels) were developed by lentiviral transduction. While observing no secreted IL‐6 level in parental C4‐2 and CWR22Rv1 cells, we found the IL‐6 expression/secretion in these cells was induced after the transduction process and the IL‐6 level difference in C4‐2siIL‐6/sc and CWR22siIL‐6/sc cell CRPC cell sets could be detected. We then found that IL‐6‐knockdown cells were more susceptible to NK cell cytotoxicity than control cells due to lowered programmed death receptor ligand 1 (PD‐L1) and increased NK group 2D (NKG2D) ligand levels. In animal studies, to concur with the in vitro results, we found that IL‐6‐expressing cell‐derived tumors were more resistant to NK cell action than the tumors of IL‐6‐knockdown cells. Further, we discovered that JAK‐Stat3 is the most critical IL‐6 downstream signaling that modulates PD‐L1/NKG2D ligand levels in CRPC cells. Furthermore, inhibition of the JAK or Stat3 signaling effectively increased the susceptibility of C4‐2sc and CWRsc cells to NK cell cytotoxicity. We observed the most effective cytotoxicity when the PD‐L1 Ab and JAK inhibitor (or Stat 3 inhibitor) were used together. These results suggest that the strategy of targeting IL‐6 signaling (or its downstream signaling) may enhance the NK cell‐mediated immune action to CRPC tumors, thus yielding clinical implications in developing future immunotherapeutics of exploiting this strategy to treat patients with CRPC.

SLAMF7 is critical for phagocytosis of haematopoietic tumour cells via Mac-1 integrin

Cancer cells elude anti-tumour immunity through multiple mechanisms, including upregulated expression of ligands for inhibitory immune checkpoint receptors. Phagocytosis by macrophages plays a critical role in cancer control. Therapeutic blockade of signal regulatory protein (SIRP)-α, an inhibitory receptor on macrophages, or of its ligand CD47 expressed on tumour cells, improves tumour cell elimination in vitro and in vivo, suggesting that blockade of the SIRPα-CD47 checkpoint could be useful in treating human cancer. However, the pro-phagocytic receptor(s) responsible for tumour cell phagocytosis is(are) largely unknown. Here we find that macrophages are much more efficient at phagocytosis of haematopoietic tumour cells, compared with non-haematopoietic tumour cells, in response to SIRPα-CD47 blockade. Using a mouse lacking the signalling lymphocytic activation molecule (SLAM) family of homotypic haematopoietic cell-specific receptors, we determined that phagocytosis of haematopoietic tumour cells during SIRPα-CD47 blockade was strictly dependent on SLAM family receptors in vitro and in vivo. In both mouse and human cells, this function required a single SLAM family member, SLAMF7 (also known as CRACC, CS1, CD319), expressed on macrophages and tumour cell targets. In contrast to most SLAM receptor functions, SLAMF7-mediated phagocytosis was independent of signalling lymphocyte activation molecule-associated protein (SAP) adaptors. Instead, it depended on the ability of SLAMF7 to interact with integrin Mac-1 (refs 18, 19, 20) and utilize signals involving immunoreceptor tyrosine-based activation motifs. These findings elucidate the mechanism by which macrophages engulf and destroy haematopoietic tumour cells. They also reveal a novel SAP adaptor-independent function for a SLAM receptor. Lastly, they suggest that patients with tumours expressing SLAMF7 are more likely to respond to SIRPα-CD47 blockade therapy.