Inhibition of FcepsilonRI-dependent mediator release and calcium flux from human mast cells by sialic acid-binding immunoglobulin-like lectin 8 engagement

Siglec-8 and Siglec-F, the new therapeutic targets in asthma

The recruitment of eosinophils from the circulation into the airway is a prominent feature of allergic asthma. Persistent inflammatory responses may arise from inefficient mechanisms for resolution of inflammation, including delayed apoptosis. Several studies suggest that eosinophil apoptosis is delayed in asthma. Sialic acid-binding immunoglobulin-like lectins are characterized by their sequence similarities and abilities to bind sialic acids in glycoproteins and glycolipids. Siglec-8 is uniquely expressed on eosinophils, mast cells, and basophils. Engagement of Siglec-8 on blood eosinophils results in caspase- and mitochondria-dependent apoptosis. Eosinophil apoptosis is an important therapeutic target for the development of novel anti-asthma treatments that specifically target the eosinophil.

Granulocyte death regulation by naturally occurring autoantibodies

Programmed cell death (PCD) plays a central role in the regulation of granulocytes that are key effector cells of the innate immune system. Granulocytes are produced in high amounts in the bone marrow. A safe elimination of granulocytes by cell death (apoptosis) is essential to maintain the numbers of these cells balanced. In many acute and chronic inflammatory diseases, delayed apoptosis is one mechanism that contributes to accumulation of neutrophil and eosinophil granulocytes at the site of inflammation. On the other hand, a safe elimination of granulocytes by cell death is required to avoid unwanted tissue damage for instance by secretion of toxic products from these cells. Recent evidence shows that humans produce an array of naturally occurring autoantibodies (NAbs) with the capacity to regulate granulocyte death, including agonistic and antagonistic NAbs that bind to the receptors Fas, Siglec-8, and Siglec-9. Together with other factors, these various NAbs exhibit different properties in terms of the form of cell death they induce, the molecular signaling pathways they engage, as well as the efficacy or potency by which they induce cell death. Moreover, several regulatory mechanisms seem to exist that control their biological activity. Novel insights support the concept of granulocyte death regulation by NAbs, which might have important implications for our understanding of the pathogenesis and treatment of inflammatory diseases, including many autoimmune and allergic disorders.

Developmental, malignancy-related, and cross-species analysis of eosinophil, mast cell, and basophil siglec-8 expression

OBJECTIVE

The aim of this study is to determine when during hematopoiesis Siglec-8 gets expressed, whether it is expressed on hematologic malignancies, and if there are other non-human species that express Siglec-8.

METHODS

Siglec-8 mRNA and cell surface expression was monitored during in vitro maturation of human eosinophils and mast cells. Flow cytometry was performed on human blood and bone marrow samples, and on blood samples from dogs, baboons, and rhesus and cynomolgus monkeys.

RESULTS

Siglec-8 is a late maturation marker. It is detectable on eosinophils and basophils from subjects with chronic eosinophilic leukemia, chronic myelogenous leukemia, and on malignant and non-malignant bone marrow mast cells, as well as the HMC-1.2 cell line. None of the Siglec-8 monoclonal antibodies tested recognized leukocytes from dogs, baboons, and rhesus and cynomolgus monkeys.

CONCLUSIONS

Siglec-8-based therapies should not target immature human leukocytes but should recognize mature and malignant eosinophils, mast cells, and basophils. So far, there is no suitable species for preclinical testing of Siglec-8 monoclonal antibodies.

The two faces of mast cells in food allergy and allergic asthma: the possible concept of Yin Yang

The purpose of this review is to discuss the role of mast cells in allergic inflammation. We have focused on inflammation associated with allergic asthma and food allergy. Mast cells are 'first line of defense' innate/adaptive immune cells and are widely distributed in tissues in surfaces exposed to the environment. Especially in allergic settings mast cells are extensively studied, as they can be activated to release a wide range of mediators by allergen-IgE specific triggers. In addition, in allergic inflammation mast cells can also be activated non-allergic triggers. Recent studies revealed that mast cells, besides the classical role of pro-inflammatory effector cell, have also emerged as modulators of allergic sensitization and down-regulators of allergic inflammation. Therefore, mast cells can be regarded as 'Ying Yan' modulators in allergic responses in intestinal tract and airways. This article is part of a Special Issue entitled: Mast Cells in Inflammation.

Potential targeting of siglecs, mast cell inhibitory receptors, in interstitial cystitis

Mast cell increases and activation are detected in the chronic inflammatory bladder disease interstitial cystitis (IC), and their proinflammatory mediators are felt to contribute to regional pelvic pain and inflammatory pathophysiology. The immunoreceptor tyrosine-based inhibition motif-containing sialic acid-binding immunoglobulin-like lectins (Siglecs) expressed in mast cells could be evaluated as in vivo signaling regulators capable of inhibiting IC-related mast cell activation.

Targeting siglecs--a novel pharmacological strategy for immuno- and glycotherapy

The immune system must be tightly held in check to avoid bystander tissue damage as well as autoreactivity caused by overwhelming immune reactions. A novel family of immunoregulatory, carbohydrate-binding receptors, the Siglecs (sialic acid binding immunoglobulin-like lectins), has received particular attention in light of their capacity to mediate cell death, anti-proliferative effects and to regulate a variety of cellular activities. Siglec receptors are mainly expressed on leukocytes in a cell type-specific and differentiation-dependent manner. Siglecs might potentially be exploited as targets of novel immune- and glycotherapeutics for cell-directed therapies in autoimmune and allergic diseases, as well as in haematologic malignancies. Here we present novel insights on structural and functional characteristics, expression patterns and evolutionary aspects of Siglecs and their ligands. Pharmacological strategies using Siglec agonistic cross-linking therapeutics, such as monoclonal or engineered antibodies, intravenous immunoglobulin (IVIG), or glycomimetics are discussed. Modulation of immune responses by targeting Siglecs using agonistic or antagonistic therapeutics may have important clinical implications and may pave the way for novel pharmacological avenues for the treatment of autoimmune and allergic diseases or for tumor immunotherapy.

Down-regulation of mast cell responses through ITIM containing inhibitory receptors

The multiple cell types that comprise the immune system provide an efficient defense system against invading pathogens and micro-organisms. In general, immune cells are activated for disparate functions, such as proliferation, production and release of mediators and chemotaxis, as a result of interactions between ligands and their matching immunoreceptors. This in turn leads to the recruitment and activation of a cascade of second messengers, via their regulators/adaptors, that determine the net effect of the initial response. However, activation of cells of the immune system must be tightly regulated by a finely tuned interplay between activation and inhibition to avoid excessive or inappropriate responsiveness and to maintain homeostasis. Loss of inhibitory signals may disrupt this balance, leading to various pathological processes such as allergic and auto-immune diseases. In this chapter, we will discuss down-regulating mechanisms of mast cells focusing on immunoreceptor tyrosine-based inhibition motifs (ITIM)-containing inhibitory receptors (IR).

Regulation of mast cell responses in health and disease

Mast cells are multifunctional cells that initiate not only IgE-dependent allergic diseases but also play a fundamental role in innate and adaptive immune responses to microbial infection. They are also thought to play a role in angiogenesis, tissue remodeling, wound healing, and tumor repression or growth. The broad scope of these physiologic and pathologic roles illustrates the flexible nature of mast cells, which is enabled in part by their phenotypic adaptability to different tissue microenvironments and their ability to generate and release a diverse array of bioactive mediators in response to multiple types of cell-surface and cytosolic receptors. There is increasing evidence from studies in cell cultures that release of these mediators can be selectively modulated depending on the types or groups of receptors activated. The intent of this review is to foster interest in the interplay among mast cell receptors to help understand the underlying mechanisms for each of the immunological and non-immunological functions attributed to mast cells. The second intent of this review is to assess the pathophysiologic roles of mast cells and their products in health and disease. Although mast cells have a sufficient repertoire of bioactive mediators to mount effective innate and adaptive defense mechanisms against invading microorganisms, these same mediators can adversely affect surrounding tissues in the host, resulting in autoimmune disease as well as allergic disorders.

Human and mouse mast cells express and secrete the GPI-anchored isoform of CD160

CD160 is expressed by human and mouse natural killer (NK) cells and other cytotoxic lymphocyte subpopulations. CD160 is mostly expressed as a trimeric 83 kDa glycosylphosphatidylinositol (GPI)-anchored activating NK receptor, cleaved upon IL-15 stimulation in a secreted trimeric soluble form (sCD160) that binds to major histocompatibility complex (MHC) class I molecules, while a transmembrane isoform appears. sCD160 exhibits immunoregulatory function as it inhibits CD8(+) T-lymphocyte cytotoxic activity. We show that human mast cells (MCs) express CD160. In human and mouse skin, resident MCs expressed CD160, whereas in C57BL/6-Kit(W-sh/W-sh) mice, CD160(+) cells were only identified at the site of reconstitution with syngeneic cultured MCs. In the human mast cell line, HMC-1, we only identified the transcripts of the GPI-anchored CD160 isoform. Furthermore, CD160 was identified in HMC-1 and mouse MC supernatants, suggesting that MCs release sCD160. Supporting this hypothesis, HMC-1 express the GPI-specific phospholipase D variant 2 involved in the NK lymphocyte membrane cleavage of CD160, and morphological studies highlighted a relative loss of CD160 expression in inflammatory skin sites, where MC degranulation is expected to occur. We also demonstrated an inhibition of T-cell cytotoxicity by HMC-1 supernatant that was partially reversed by anti-CD160 mAb. In conclusion, sCD160, produced by MCs, may have a role in T-cell-MC interactions in vivo.

Cell death modulation by intravenous immunoglobulin

The induction of cell death in immune cells by naturally occurring antibodies specific for death receptors may present an important antiinflammatory mechanism of intravenous immunoglobulin (IVIG). Conversely, the protection of tissue cells from death receptor-mediated apoptosis by blocking antibodies is thought to contribute to the beneficial effects of IVIG in certain inflammatory disorders such as toxic epidermal necrolysis, also known as Lyell's syndrome. In this review, we focus on recent insights into the role of functional antibodies against Fas, sialic acid-binding immunoglobulin-like lectin (Siglec)-8, and Siglec-9 receptors in IVIG-mediated cell survival or death effects. In addition, we examine a variety of factors in inflammatory disease that may interplay with these cellular events and influence the therapeutic efficacy or potency of IVIG. These involve activation status of the target cell, cytokine microenvironment, pathogenesis and stage of disease, individual genetic determinants, species characteristics, and batch-to-batch variations of IVIG preparations.

Regulation of allergic inflammatory responses by inhibitory receptors

An intricate network of activation and inhibitory signals tightly regulates immune responses. To date, multiple activation receptors have been described. These include receptors that mediate cellular functions such as adhesion, chemotaxis, cytokine signalling, mediator release, survival and phagocytosis. In contrast to these activation pathways, an opposing and suppressive receptor system has evolved. These receptors can override the signals elicited by the activation pathways and are broadly termed inhibitory receptors. Inhibitory receptors share unique intracellular signalling motifs and have key roles in various cellular and pathological conditions. Therefore, such receptors are potential targets for future therapeutics. In this review, we will discuss the structure and function of inhibitory receptors. In particular, we will focus on the expression and function of inhibitory receptors on mast cells and eosinophils and illustrate strategies for their inhibition in the settings of allergic inflammation.

Mast cell-dependent contraction of human airway smooth muscle cell-containing collagen gels: influence of cytokines, matrix metalloproteases, and serine proteases

In asthma, mast cells infiltrate the airway smooth muscle cell layer and secrete proinflammatory and profibrotic agents that contribute to airway remodeling. To study the effects of mast cell activation on smooth muscle cell-dependent matrix contraction, we developed coculture systems of human airway smooth muscle cells (HASM) with primary human mast cells derived from circulating progenitors or with the HMC-1 human mast cell line. Activation of primary human mast cells by IgE receptor cross-linking or activation of HMC-1 cells with C5a stimulated contraction of HASM-embedded collagen gels. Contractile activity could be transferred with conditioned medium from activated mast cells, implicating involvement of soluble factors. Cytokines and proteases are among the agents released by activated mast cells that may promote a contractile response. Both IL-13 and IL-6 enhanced contraction in this model and the activity of IL-13 was ablated under conditions leading to expression of the inhibitory receptor IL-13Ralpha2 on HASM. In addition to cytokines, matrix metalloproteinases (MMPs), and serine proteases induced matrix contraction. Inhibitor studies suggested that, although IL-13 could contribute to contraction driven by mast cell activation, MMPs were critical mediators of the response. Both MMP-1 and MMP-2 were strongly expressed in this system. Serine proteases also contributed to contraction induced by mast cell-activating agents and IL-13, most likely by mediating the proteolytic activation of MMPs. Hypercontractility is a hallmark of smooth muscle cells in the asthmatic lung. Our findings define novel mechanisms whereby mast cells may modulate HASM-driven contractile responses.

Eosinophil-selective binding and proapoptotic effect in vitro of a synthetic Siglec-8 ligand, polymeric 6'-sulfated sialyl Lewis x

The lectin Siglec-8 (sialic acid-binding, immunoglobulin-like lectin), which is selectively expressed on eosinophil surfaces and regulates eosinophil survival, preferentially binds to the glycan 6'-sulfo-sialyl Lewis X (6'-sulfo-sLe(x)). Antibody engagement of Siglec-8 on eosinophils causes their apoptosis, suggesting that engagement of Siglec-8 with its natural glycan ligands in vivo may control allergic inflammation. We report that a soluble synthetic polymer displaying 6'-sulfo-sLe(x) glycan selectively binds to human eosinophils and human embryonic kidney 293 cells expressing Siglec-8. Binding was inhibited by anti-Siglec-8 antibody. In whole blood, eosinophils were the only leukocyte subtype to detectably bind polymeric 6'-sulfo-sLe(x). Interleukin-5-primed eosinophils underwent apoptosis when incubated with either anti-Siglec-8 monoclonal antibody or polymeric 6'-sulfo-sLe(x), although the glycan polymer was less effective. These data demonstrate that a soluble, multivalent glycan selectively binds to human eosinophils and induces their apoptosis in vitro and provide proof-of-concept that such a reagent could be used to selectively target eosinophils.

Siglecs as targets for therapy in immune-cell-mediated disease

The sialic-acid-binding immunoglobulin-like lectins (siglecs) comprise a family of receptors that are differentially expressed on leukocytes and other immune cells. The restricted expression of several siglecs to one or a few cell types makes them attractive targets for cell-directed therapies. The anti-CD33 (also known as Siglec-3) antibody gemtuzumab (Mylotarg) is approved for the treatment of acute myeloid leukemia, and antibodies targeting CD22 (Siglec-2) are currently in clinical trials for treatment of B cell non-Hodgkins lymphomas and autoimmune diseases. Because siglecs are endocytic receptors, they are well suited for a 'Trojan horse' strategy, whereby therapeutic agents conjugated to an antibody, or multimeric glycan ligand, bind to the siglec and are efficiently carried into the cell. Although the rapid internalization of unmodified siglec antibodies reduces their utility for induction of antibody-dependent cellular cytotoxicity or complement-mediated cytotoxicity, antibody binding of Siglec-8, Siglec-9 and CD22 has been demonstrated to induce apoptosis of eosinophils, neutrophils and depletion of B cells, respectively. Here, we review the properties of siglecs that make them attractive for cell-targeted therapies.

Advances in mechanisms of asthma, allergy, and immunology in 2008

This review summarizes selected articles appearing in 2008 in the Journal. Articles chosen include those improving our understanding of mechanisms of allergic diseases by focusing on human basophil, mast cell, and eosinophil biology; IgE and its high-affinity receptor on various cells; novel properties of omalizumab; airways remodeling; and genetics. Articles from other journals have been included to supplement the topics presented.

Siglec-8 on human eosinophils and mast cells, and Siglec-F on murine eosinophils, are functionally related inhibitory receptors

Siglecs (sialic acid-binding, Ig-like lectins) are a family of single-pass transmembrane cell surface proteins found predominantly on leucocytes. Their unique structural characteristics include an N-terminal carbohydrate-binding ('lectin') domain that binds sialic acid, followed by a variable number of Ig-like domains, hence these structures are a subset of the Ig gene superfamily. Another unique feature of Siglecs is that most, but not all, possess so-called immunoreceptor tyrosine-based inhibitory motifs in their cytoplasmic domains, suggesting that these molecules function in an inhibitory capacity. Siglec-8, the eighth member identified at the time, was discovered as part of an effort initiated almost a decade ago to identify novel human eosinophil and mast cell proteins. Since that time, its selective expression on human eosinophils and mast cells has been confirmed. On eosinophils, Siglec-8 engagement results in apoptosis, whereas on mast cells, inhibition of FcepsilonRI-dependent mediator release, without apoptosis, is seen. It has subsequently been determined that the closest functional paralog in the mouse is Siglec-F, selectively expressed by eosinophils but not expressed on mast cells. Despite only modest homology, both Siglec-8 and Siglec-F preferentially recognize a sulphated glycan ligand closely related to sialyl Lewis X, a common ligand for the selectin family of adhesion molecules. Murine experiments in normal, Siglec-F-deficient mice and hypereosinophilic mice have resulted in similar conclusions that Siglec-F, like Siglec-8, plays a distinctive and important role in regulating eosinophil accumulation and survival in vivo. Given the resurgent interest in eosinophil-directed therapies for a variety of disorders, plus its unique additional ability to also target the mast cell, therapies focusing on Siglec-8 could some day prove to be a useful adjunct to our current armamentarium for the treatment of asthma, allergies and related disorders where overproduction and overactivity of eosinophils and mast cells is occurring.

Basic and clinical immunology of Siglecs

Siglecs are cell-surface proteins found primarily on hematopoietic cells. By definition, they are members of the immunoglobulin gene super-family and bind sialic acid. Most contain cytoplasmic tyrosine motifs implicated in cell signaling. This review will first summarize characteristics common and unique to Siglecs, followed by a discussion of each human Siglec in numerical order, mentioning in turn its closest murine ortholog or paralog. Each section will describe its pattern of cellular expression, latest known immune functions, ligands, and signaling pathways, with the focus being predominantly on CD33-related Siglecs. Potential clinical and therapeutic implications of each Siglec will also be covered.

Molecular mimicry of host sialylated glycans allows a bacterial pathogen to engage neutrophil Siglec-9 and dampen the innate immune response

Human neutrophil Siglec-9 is a lectin that recognizes sialic acids (Sias) via an amino-terminal V-set Ig domain and possesses tyrosine-based inhibitory motifs in its cytoplasmic tail. We hypothesized that Siglec-9 recognizes host Sias as "self," including in cis interactions with Sias on the neutrophil's own surface, thereby dampening unwanted neutrophil reactivity. Here we show that neutrophils presented with immobilized multimerized Siaalpha2-3Galbeta1-4GlcNAc units engage them in trans via Siglec-9. The sialylated capsular polysaccharide of group B Streptococcus (GBS) also presents terminal Siaalpha2-3Galbeta1-4GlcNAc units, and similarly engages neutrophil Siglec-9, dampening neutrophil responses in a Sia- and Siglec-9-dependent manner. Reduction in the neutrophil oxidative burst, diminished formation of neutrophil extracellular DNA traps, and increased bacterial survival are also facilitated by GBS sialylated capsular polysaccharide interactions with Siglec-9. Thus, GBS can impair neutrophil defense functions by coopting a host inhibitory receptor via sialoglycan molecular mimicry, a novel mechanism of bacterial immune evasion.

Our perception of the mast cell from Paul Ehrlich to now

Just over a century ago Paul Ehrlich received the Nobel Prize for his studies of immunity. This review describes one of his legacies, the histochemical description of the mast cell, and the research that has ensued since then. After a long period of largely descriptive studies, which revealed little about the biological role of the mast cell, the field was galvanized in the 1950s by the recognition that the mast cell was the main repository of histamine and a key participant in anaphylactic reactions. Although the mast cell was long-viewed in these terms, recent research has now shown that the mast cell also plays a key role in innate and adaptive immune responses, autoimmune disease, and possibly tissue homeostasis by virtue of its expression of a diverse array of receptors and biologically active products. In addition, the responsiveness of mast cells to immunological and pathological stimulants is highly modulated by the tissue cytokine environment and by synergistic, or inhibitory, interactions among the various mast cell receptor systems. This once enigmatic cell of Paul Ehrlich has proved to be both adaptable and multifunctional.

Siglec-F antibody administration to mice selectively reduces blood and tissue eosinophils

BACKGROUND

Sialic acid-binding immunoglobulin-like lectins (Siglecs) are a family of receptors that bind sialic acid and mostly contain immunoreceptor tyrosine-based inhibitory motifs, suggesting that these molecules possess inhibitory functions. We have recently identified Siglec-8 as an eosinophil-prominent Siglec, and cross-linking of Siglec-8 on human eosinophils induces apoptosis. In this article, we address the in vivo consequences of Siglec engagement. We and others have identified mouse Siglec-F as the closest functional paralog of human Siglec-8, based on shared ligand-binding and expression pattern. We therefore hypothesized that Siglec-F engagement would affect levels and viability of eosinophils in vivo.

METHODS

Wild type and hypereosinophilic mice were administered Siglec-F antibody and levels of eosinophils in peripheral blood and tissue were measured. Eosinophil apoptosis (in vivo and in vitro) was determined by binding of Annexin-V.

RESULTS

Studies in IL-5 transgenic mice, displaying hypereosinophilia, show that administration of a single dose of Siglec-F antibody results in rapid reductions in quantum of eosinophils in the blood. This decrease was accompanied by reductions in tissue eosinophils. Quantum of eosinophils in blood was decreased using two separate antibodies, as well as in other mouse models (wild type mice and in a mouse model of chronic eosinophilic leukemia). Mechanistic studies demonstrated that Siglec-F antibody administration induced apoptosis of eosinophils in vivo and in vitro.

CONCLUSION

These data demonstrate that activation of innate immune receptors, like Siglec-F, can significantly reduce mouse eosinophil viability. As such, targeting Siglec-8/F may be a therapeutic approach for eosinophilic disorders.

Inducing an anergic state in mast cells and basophils without secretion

BACKGROUND

IgE-mediated secretion from mast cells or basophils depends on the activity of both spleen tyrosine kinase (syk) and phosphatidyl inositol 3' kinase (PI3K), but several specific downregulatory pathways (eg, loss of syk expression) do not.

OBJECTIVE

We tested whether stimulation with antigen in the presence of a syk inhibitor (NVP-QAB205) would ablate secretion while simultaneously allowing anergy.

METHODS

The anergic or desensitized state in human basophils, cultured-derived mast cells, and in situ stimulated airway mast cells (in organ baths) was assessed after stimulation with antigen in the presence of syk inhibitor.

RESULTS

Antigen caused 35 +/- 7% and 62 +/- 10% histamine release from basophils and mast cells, respectively, and it caused an 87 +/- 5% histamine/leukotriene D(4)-dependent contraction of human isolated bronchi. All of these responses were blocked >95% by the syk inhibitor. Rechallenging the preparations with antigen, after first washing out the syk inhibitor and antigen, revealed that near complete anergy (92% to 100%) occurred in each case. A similar result was found when using a PI3K inhibitor, LY294002, in studies of basophils.

CONCLUSION

Although the syk inhibitor nearly abolished the antigen-induced secretion from mast cells and basophils, it had little effect on the pathways involved in anergy. These results suggest that syk and PI3K are not involved in downregulation leading to anergy.