An unusual inhibitory receptor--the mast cell function-associated antigen (MAFA)

The role of KLRG1: a novel biomarker and new therapeutic target

Killer cell lectin-like receptor G1 (KLRG1) is an immune checkpoint receptor expressed predominantly in NK and T-cell subsets that downregulates the activation and proliferation of immune cells and participates in cell-mediated immune responses. Accumulating evidence has demonstrated the importance of KLRG1 as a noteworthy disease marker and therapeutic target that can influence disease onset, progression, and prognosis. Blocking KLRG1 has been shown to effectively mitigate the effects of downregulation in various mouse tumor models, including solid tumors and hematologic malignancies. However, KLRG1 inhibitors have not yet been approved for human use, and the understanding of KLRG1 expression and its mechanism of action in various diseases remains incomplete. In this review, we explore alterations in the distribution, structure, and signaling pathways of KLRG1 in immune cells and summarize its expression patterns and roles in the development and progression of autoimmune diseases, infectious diseases, and cancers. Additionally, we discuss the potential applications of KLRG1 as a tool for tumor immunotherapy.

New Targets for Antiviral Therapy: Inhibitory Receptors and Immune Checkpoints on Myeloid Cells

Immune homeostasis is achieved by balancing the activating and inhibitory signal transduction pathways mediated via cell surface receptors. Activation allows the host to mount an immune response to endogenous and exogenous antigens; suppressive modulation via inhibitory signaling protects the host from excessive inflammatory damage. The checkpoint regulation of myeloid cells during immune homeostasis raised their profile as important cellular targets for treating allergy, cancer and infectious disease. This review focuses on the structure and signaling of inhibitory receptors on myeloid cells, with particular attention placed on how the interplay between viruses and these receptors regulates antiviral immunity. The status of targeting inhibitory receptors on myeloid cells as a new therapeutic approach for antiviral treatment will be analyzed.

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

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

An atypical KLRG1 in Nile tilapia involves in adaptive immunity as a potential marker for activated T lymphocytes

Killer cell lectin-like receptor G subfamily 1 (KLRG1) is a receptor generally expressed on effector CD8⁺ T cells or NK cells at terminal differentiation stage, and it will be highly induced for lymphocyte cytotoxicity upon pathogen infection or lymphocyte activation. However, little is known about the character or function of KLRG1 in lower vertebrates. In present study, we reappraised a molecule that previously defined as KLRG1 in the genomic sequence of Nile tilapia Oreochromis niloticus, and identified it as an atypical KLRG1-like molecule (defined as On-KLRG1-L), and illustrated its potential function serving as a marker representing effector T lymphocytes of fish species. On-KLRG1-L consists of two C-type lectin-like domains (CTLDs) without transmembrane region, and the tertiary structure of the CTLD is highly alike to that in mouse KLRG1. As a CTLD-containing protein, the recombinant On-KLRG1-L could bind PGN and several microbes in vitro. On-KLRG1-L was widely expressed in immune-associated tissues, with the highest expression level in the gill. Once Nile tilapia is infected by Aeromonas hydrophila, mRNA level of On-KLRG1-L in spleen lymphocytes were significantly up-regulated on 5 days after infection. Meanwhile, On-KLRG1-L protein was also induced on 5 or 8 days after A. hydrophila infection. Furthermore, we found both mRNA and protein levels of On-KLRG1-L were dramatically enhanced within several hours after spleen lymphocytes were activated by T cell-specific mitogen PHA in vitro. More importantly, the ratio of On-KLRG1-L⁺ T cells was also augmented after PHA stimulation. The observations suggested that the KLRG1-like molecule from Nile tilapia participated in lymphocyte activation and anti-bacterial adaptive immune response, and could serve as an activation marker of T lymphocytes. Our study thus provided new evidences to understand lymphocyte-mediated adaptive immunity of teleost.

Strategies for Mast Cell Inhibition in Food Allergy

Mast cells are tissue resident allergic effector cells that drive IgE-mediated food allergies. There are several steps leading to mast cell activation in the context of allergic disease that can be targeted to prevent mast cell activation and degranulation. These include blocking IgE-FcεRI crosslinking and type 2 cytokine receptor activation; modulating cell-surface neural chemical receptors; stabilizing mast cell membranes to prevent co-localization of activating receptors; impeding intracellular signaling; and engaging cell surface inhibitory receptors. This review highlights several ITIM-containing inhibitory mast cell surface receptors that could serve as pharmaceutical targets to prevent mast cell activation and degranulation in the context of food allergy. When activated, these ITIM-containing inhibitory receptors recruit the phosphatases SHP-1, SHP-2, and/or SHIP to dephosphorylate the tyrosine kinases responsible for activation signals downstream of the IgE-FcεRI complex. We describe several members of the Ig and Ig-like inhibitory receptor and C-type lectin inhibitory receptor superfamilies. Fundamental studies exploring the behavior of these receptors within the context of experimental food allergy models are needed. A deeper understanding of how these receptors modulate mast cell-driven food allergic responses will shape future strategies to harness these inhibitory receptors to treat food allergy.

Effects of extracts and compounds from Tricholoma populinum Lange on degranulation and IL-2/IL-8 secretion of immune cells

Tricholoma populinum Lange is an edible basidiomycete from the family Tricholomataceae. Extracts, fractions, and different metabolites isolated from the fruiting bodies of this mushroom were tested for degranulation-inhibiting activities on RBL-2H3 cells (rat basophils). Dichloromethane extracts decreased degranulation significantly, as did a fraction after column chromatography. In addition, the extract decreased the IL-2 release from Jurkat T cells and the release of IL-8 from HMC-1 human mast cells. The results show the significant effects of extracts of T. populinum on cells of the innate (basophils and mast cells) and adaptive (T cells) immune system and indicate the influence of the mushroom on different immunological processes. As one fraction showed activity, it seems to be possible that it includes an active principle. The compounds responsible for this effect, however, could not be identified as the contents oleic acid (1), ergosterol peroxide (2), and 9,11-dehydroergosterol peroxide (3) showed no effects. Nevertheless, the mushroom could be used for supporting allergy treatment in future studies.

Mast cell function: a new vision of an old cell

Since first described by Paul Ehrlich in 1878, mast cells have been mostly viewed as effectors of allergy. It has been only in the past two decades that mast cells have gained recognition for their involvement in other physiological and pathological processes. Mast cells have a widespread distribution and are found predominantly at the interface between the host and the external environment. Mast cell maturation, phenotype and function are a direct consequence of the local microenvironment and have a marked influence on their ability to specifically recognize and respond to various stimuli through the release of an array of biologically active mediators. These features enable mast cells to act as both first responders in harmful situations as well as to respond to changes in their environment by communicating with a variety of other cells implicated in physiological and immunological responses. Therefore, the critical role of mast cells in both innate and adaptive immunity, including immune tolerance, has gained increased prominence. Conversely, mast cell dysfunction has pointed to these cells as the main offenders in several chronic allergic/inflammatory disorders, cancer and autoimmune diseases. This review summarizes the current knowledge of mast cell function in both normal and pathological conditions with regards to their regulation, phenotype and role.

Immunopharmacological modulation of mast cells

Mast cells produce a wide spectrum of mediators and they have been implicated in several physiopathological conditions (e.g. allergic reactions and certain tumors). Pharmacologic agents that modulate the release of mediators from mast cells has helped to elucidate the biochemical mechanisms by which immunological and non-immunological stimuli activate these cells. Furthermore, the study of surface receptors and signaling pathways associated with mast cell activation revealed novel pharmacologic targets. Thus, the development of pharmacologic agents based on this new wave of knowledge holds promise for the treatment of several mast cell-mediated disorders.

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

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

Mannose receptor and macrophage galactose-type lectin are involved in Bordetella pertussis mast cell interaction

Mast cells are crucial in the development of immunity against Bordetella pertussis, and the function of TLRs in this process has been investigated. Here, the interaction between mast cells and B. pertussis with an emphasis on the role of CLRs is examined. In this study, two CLRs, MGL and MR, were detected for the first time on the surface of mast cells. The involvement of MR and MGL in the stimulation of mast cells by heat-inactivated BP was investigated by the use of blocking antibodies and specific carbohydrate ligands. The cell wall LOS of BP was also isolated to explore its role in this interaction. Mast cells stimulated with heat-inactivated BP or BP LOS induced TNF-α, IL-6, and IFN-γ secretion, which was suppressed by blocking MR or MGL. Inhibition of CLRs signaling during BP stimulation affected the ability of mast cells to promote cytokine secretion in T cells but had no effect on the cell-surface expression of ICAM1. Blocking MR or MGL suppressed BP-induced NF-κB expression but not ERK phosphorylation. Syk was involved in the CLR-mediated activation of mast cells by BP. Bacterial recognition by immune cells has been predominantly attributed to TLRs; in this study, the novel role of CLRs in the BP-mast cell interaction is highlighted.

Immunoglobulin E receptor signaling and asthma

Elevated IgE levels and increased IgE sensitization to allergens are central features of allergic asthma. IgE binds to the high-affinity Fcε receptor I (FcεRI) on mast cells, basophils, and dendritic cells and mediates the activation of these cells upon antigen-induced cross-linking of IgE-bound FcεRI. FcεRI activation proceeds through a network of signaling molecules and adaptor proteins and is negatively regulated by a number of cell surface and intracellular proteins. Therapeutic neutralization of serum IgE in moderate-to-severe allergic asthmatics reduces the frequency of asthma exacerbations through a reduction in cell surface FcεRI expression that results in decreased FcεRI activation, leading to improved asthma control. Our increasing understanding of IgE receptor signaling may lead to the development of novel therapeutics for the treatment of asthma.

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

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

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).

Inhibitor and activator: dual functions for SHIP in immunity and cancer

SHIP1 is at the nexus of intracellular signaling pathways in immune cells that mediate bone marrow (BM) graft rejection, production of inflammatory and immunosuppressive cytokines, immunoregulatory cell formation, the BM niche that supports development of the immune system, and immune cancers. This review summarizes how SHIP participates in normal immune physiology or the pathologies that result when SHIP is mutated. This review also proposes that SHIP can have either inhibitory or activating roles in cell signaling that are determined by whether signaling pathways distal to PI3K are promoted by SHIP's substrate (PI(3,4,5)P(3) ) or its product (PI(3,4)P(2) ). This review also proposes the "two PIP hypothesis" that postulates that both SHIP's product and its substrate are necessary for a cancer cell to achieve and sustain a malignant state. Finally, due to the recent discovery of small molecule antagonists and agonists for SHIP, this review discusses potential therapeutic settings where chemical modulation of SHIP might be of benefit.

Ubiquitination and endocytosis of the high affinity receptor for IgE

The high affinity receptor for IgE (FcvarepsilonRI) is constitutivelly expressed on the surface of mast cells and basophils as a multimeric complex. Upon antigen ligation to FcvarepsilonRI-bound IgE molecules, the receptor complex transduces intracellular signals leading to the release of preformed and newly synthesised pro-inflammatory mediators. FcvarepsilonRI engagement also generates negative intracellular signals involving the coordinated action of adapters, phosphatases and ubiquitin ligases that limits the intensity and duration of positive signals. Relevant to this, antigen-induced FcvarepsilonRI ubiquitination has become recognized as an important signal for the internalization and delivery of engaged receptor complexes to lysosomes for degradation. In this article, we review recent advances in our understanding of molecular mechanisms that guarantee the clearance of antigen-stimulated FcvarepsilonRI complexes from the cell surface. A particular emphasis will be given on how lipid rafts and the ubiquitin pathway cooperate to ensure receptor internalization and sorting along the endocytic compartments. A brief discussion regarding how ubiquitination regulates the endocytosis of Fc receptors other than FcvarepsilonRI will be included.

KLRG1--more than a marker for T cell senescence

The co-inhibitory receptor killer-cell lectin like receptor G1 (KLRG1) is expressed on NK cells and antigen-experienced T cells and has been postulated to be a marker of senescence. Whilst KLRG1 has frequently been used as a marker of cellular differentiation, data are emerging indicating that KLRG1 plays an inhibitory role. In this review we examine evidence highlighting this view of KLRG1 with emphasis on the functional defects that arise during T cell differentiation with age that may, in part, be actively maintained by inhibitory receptor signalling.

Role of mast cells in allergic and non-allergic immune responses: comparison of human and murine data

The versatile role of mast cells in allergy, in innate immune responses and in the regulation of tissue homeostasis is well recognized. However, it is often not made clear that most mast-cell data derive solely from experiments in mice or rats, species that obviously never suffer from allergic and most other mast-cell-associated human diseases. Data on human mast cells are limited, and the mast-cell source and species from which findings derive are frequently not indicated in the titles and summaries of research publications. This Review summarizes recent data on human mast cells, discusses differences with murine mast cells, and describes new tools to study this increasingly meaningful cell type in humans.

Adaptive and innate immune reactions regulating mast cell activation: from receptor-mediated signaling to responses

In this article, we have described studies that have demonstrated that mast cells can be activated as a consequence of adaptive and innate immune reactions and that these responses can be modified by ligands for other receptors expressed on the surface of mast cells. These various stimuli differentially activate multiple signaling pathways within the mast cells required for the generation and/or release of inflammatory mediators. Thus, the composition of the suite of mediators released and the physiologic ramifications of these responses are dependent on the stimuli and the microenvironment in which the mast cells are activated. Knowledge of the different signaling molecules used by cell surface receptors may allow selective pharmacologic targeting such that inhibiting the adverse effects of mast cell activation can be achieved without influencing the beneficial effects of mast cell activation. The exact interconnections between the signaling pathways initiated by the surface receptors described in this article remain to be completely worked out; thus, this remains a topic for future investigation.

Killer cell lectin-like receptor G1 binds three members of the classical cadherin family to inhibit NK cell cytotoxicity

Killer cell lectin-like receptor G1 (KLRG1) is an inhibitory receptor expressed on subsets of natural killer (NK) cells and T cells, for which no endogenous ligands are known. Here, we show that KLRG1 binds three of the classical cadherins (E-, N-, and R-), which are ubiquitously expressed in vertebrates and mediate cell–cell adhesion by homotypic or heterotypic interactions. By expression cloning using the mouse KLRG1 tetramer as a probe, we identified human E-cadherin as a xenogeneic ligand. We also identified a syngeneic interaction between mouse KLRG1 and mouse E-cadherin. Furthermore, we show that KLRG1 binds N- and R-cadherins. Finally, we demonstrate that E-cadherin binding of KLRG1 prevents the lysis of E-cadherin–expressing targets by KLRG1⁺ NK cells. These results suggest that KLRG1 ligation by E-, N-, or R-cadherins may regulate the cytotoxicity of killer cells to prevent damage to tissues expressing the cadherins.

Selective inhibition of the Fc epsilon RI-induced de novo synthesis of mediators by an inhibitory receptor

Aggregation of the type 1 Fc-epsilon receptors (Fc-epsilon-RI) on mast cells initiates a network of biochemical processes culminating in secretion of both granule-stored and de novo-synthesized inflammatory mediators. A strict control of this response is obviously a necessity; nevertheless, this regulation is hardly characterized. Here we report that a prototype inhibitory receptor, the mast cell function-associated antigen (MAFA), selectively regulates the Fc-epsilon-RI stimulus-response coupling network and the subsequent de novo production and secretion of inflammatory mediators. Specifically, MAFA suppresses the PLC-gamma2-[Ca2+]i, Raf-1-Erk1/2, and PKC-p38 coupling pathways, while the Fyn-Gab2-mediated activation of PKB and Jnk is essentially unaffected. Hence, the activities of several transcription/nuclear factors for inflammatory mediators (NF-kappaB, NFAT) are markedly reduced, while those of others (Jun, Fos, Fra, p90rsk) are unaltered. This results in a selective inhibition of gene transcription of cytokines including IL-1beta, IL-4, IL-8, and IL-10, while that of TNF-alpha, MCP-1, IL-3, IL-5, or IL-13 remains unaffected. Taken together, these results illustrate the capacity of an immunoreceptor tyrosine-based inhibitory motif-containing receptor to cause tight and specific control of the production and secretion of inflammatory mediators by mast cells.

Co-clustering activating and inhibitory receptors: impact at varying expression levels of the latter

Clustering the mast cell function-associated antigen (MAFA) has earlier been shown to inhibit mast cells' secretory response to the type 1 Fcepsilon receptor (FcepsilonRI) stimulus. MAFA is a type II membrane glycoprotein first identified on rat mast cells and contains an immunoreceptor tyrosine-based inhibitory motif (ITIM) in its cytosolic domain. This inhibition is induced already upon clustering MAFA alone. Still, the inhibitory capacity of MAFA-FcepsilonRI co-clustering has recently been characterized and co-clustered MAFA molecules were found to exhibit a markedly higher inhibition capacity than MAFA-clusters alone. We have now compared the inhibitory capacity of FcepsilonRI co-clustered MAFA on the secretory response of rat mucosal-type mast cells (RBL-2H3 line) expressing different levels of this inhibitory protein. Reacting these cells carrying an IgE class, 2,4 dinitrophenyl (DNP)-specific monoclonal antibody with DNP-conjugated F(ab')2 fragments of non-specific polyclonal mouse IgG causes clustering of the FcepsilonRI-IgE. Reaction of these cells with DNP-conjugated F(ab')2 fragments of the MAFA-specific, monoclonal antibody G63 co-aggregates MAFA together with the FcepsilonRI-IgE thereby producing FcepsilonRI-IgE-MAFA co-clusters. Results of measurements of the secretory responses of RBL-2H3 cells expressing higher or lower MAFA levels than those of unmodified cells provided further support to the notion that co-clustered MAFA molecules exhibit a markedly higher inhibition capacity than MAFA-clusters alone. The molecular basis for this enhanced inhibition is most probably the increased concentration of the inhibitory cell components in the immediate proximity of the co-clustered FcepsilonRI-MAFA.

The Inhibitory Receptor IRp60 (CD300a) Is Expressed and Functional on Human Mast Cells

Mast cell-mediated responses are likely to be regulated by the cross talk between activatory and inhibitory signals. We have screened human cord blood mast cells for recently characterized inhibitory receptors expressed on NK cells. We found that IRp60, an Ig superfamily member, is expressed on human mast cells. On NK cells, IRp60 cross-linking leads to the inhibition of cytotoxic activity vs target cells in vitro. IRp60 is constitutively expressed on mast cells but is down-regulated in vitro by the eosinophil proteins major basic protein and eosinophil-derived neurotoxin. An immune complex-mediated cross-linking of IRp60 led to inhibition of IgE-induced degranulation and stem cell factor-mediated survival via a mechanism involving tyrosine phosphorylation, phosphatase recruitment, and termination of cellular calcium influx. To evaluate the role of IRp60 in regulation of allergic responses in vivo, a murine model of allergic peritonitis was used in which the murine homolog of IRp60, LMIR1, was neutralized in BALB/c mice by mAbs. This neutralization led to a significantly augmented release of inflammatory mediators and eosinophilic infiltration. These data demonstrate a novel pathway for the regulation of human mast cell function and allergic responses, indicating IRp60 as a candidate target for future treatment of allergic and mast cell-associated diseases.

Identification of tyrosine residues crucial for CD200R-mediated inhibition of mast cell activation

CD200 and its receptor CD200R are type-1 membrane glycoproteins, which contain two immunoglobulin-like domains. Engagement of CD200R by CD200 inhibits activation of myeloid cells. Unlike the majority of immune inhibitory receptors, CD200R does not contain an immunoreceptor tyrosine-based inhibitory motif but contains three tyrosine residues (Y286, Y289, and Y297) in the cytoplasmic domain. Y297 is located in an NPxY motif. Previously, we have shown that engagement of CD200R in mouse mast cells induces its tyrosine phosphorylation and recruitment of inhibitory adaptor proteins Dok1 and Dok2, leading to the inhibition of Ras/mitogen-activated protein kinase activation. In the present study, we examined the roles of these three tyrosines in CD200R-mediated inhibition by site-directed mutagenesis in mouse mast cells. Our data show that Y286 and Y297 are the major phosphorylation sites and are critical for CD200R-mediated inhibition of mast cell activation, and Y289 is dispensable. Our data also suggest that the Src family kinase may mediate the phosphorylation of CD200R and Dok.

Regulation of mast cells? secretory response by co-clustering the Type 1 Fc? receptor with the mast cell function-associated antigen

The mast cell function-associated antigen (MAFA) is a type II membrane glycoprotein first identified on rat mast cells and basophils. Clustering MAFA inhibits these cells' secretory response to the type 1 Fcepsilon receptor (FcepsilonRI) stimulus. To quantitatively characterize this inhibition and its dependence on MAFA-FcepsilonRI co-clustering, we investigated the secretory response of rat mucosal-type mast cells of the RBL 2H3 line carrying an IgE class, 2,4 dinitrophenyl (DNP) specific monoclonal antibody to DNP-conjugated Fab and F(ab')(2) fragments of (1) mouse IgG, and (2) of the MAFA-specific, monoclonal antibody G63. The first reagent clusters FcepsilonRI-IgE complexes into oligomers by reacting with the DNP residues. The DNP conjugated G63 Fab and F(ab')(2) fragments, additionally aggregate MAFA and form FcepsilonRI-IgE-MAFA co-clusters. All experiments using these ligands were performed in the absence or presence of an excess of intact mAb G63, which clusters MAFA molecules. Empirical Hill functions were used to relate the secretory response of mast cells to the equilibrium concentrations of FcepsilonRI-IgE or MAFA clusters and co-clusters calculated as function of the employed ligands concentrations. This analysis of the experimental results indicates that co-clustered MAFA molecules have a markedly higher inhibitory capacity than MAFA-clusters alone. The molecular basis of the enhanced inhibition observed upon co-clustering MAFA with the FcepsilonRI is most probably the increased concentration of the inhibitory cell components in the immediate proximity of the activation coupling elements.

A new self: MHC-class-I-independent Natural-killer-cell self-tolerance

A fundamental tenet of the immune system is the requirement for lymphocytes to respond to transformed or infected cells while remaining tolerant of normal cells. Natural killer (NK) cells discriminate between self and non-self by monitoring the expression of MHC class I molecules. According to the 'missing-self' hypothesis, cells that express self-MHC class I molecules are protected from NK cells, but those that lack this self-marker are eliminated by NK cells. Recent work has revealed that there is another system of NK-cell inhibition, which is independent of MHC class I molecules. Newly discovered NK-cell inhibitory receptors that have non-MHC-molecule ligands broaden the definition of self as seen by NK cells.

Molecular Mechanisms of CD200 Inhibition of Mast Cell Activation

CD200 and its receptor CD200R are both type I membrane glycoproteins that contain two Ig-like domains. Engagement of CD200R by CD200 inhibits activation of myeloid cells. Unlike the majority of immune inhibitory receptors, CD200R lacks an ITIM in the cytoplasmic domain. The molecular mechanism of CD200R inhibition of myeloid cell activation is unknown. In this study, we examined the CD200R signaling pathways that control degranulation of mouse bone marrow-derived mast cells. We found that upon ligand binding, CD200R is phosphorylated on tyrosine and subsequently binds to adapter proteins Dok1 and Dok2. Upon phosphorylation, Dok1 binds to SHIP and both Dok1 and Dok2 recruit RasGAP, which mediates the inhibition of the Ras/MAPK pathways. Activation of ERK, JNK, and p38 MAPK are all inhibited by CD200R engagement. The reduced activation of these MAPKs is responsible for the observed inhibition of mast cell degranulation and cytokine production. Similar signaling events were also observed upon CD200R engagement in mouse peritoneal cells. These data define a novel inhibitory pathway used by CD200R in modulating mast cell function and help to explain how engagement of this receptor in vivo regulates myeloid cell function.

The mouse homologue of the leukocyte-associated Ig-like receptor-1 is an inhibitory receptor that recruits Src homology region 2-containing protein tyrosine phosphatase (SHP)-2, but not SHP-1

We report the molecular cloning and characterization of the first leukocyte-associated Ig-like receptor 1 (LAIR-1) homologue in mice that we have named mouse LAIR-1 (mLAIR-1). The mLAIR-1 gene maps to the proximal end of mouse chromosome 7 in a region syntenic with human chromosome 19q13.4 where the leukocyte receptor cluster is located. The protein shares 40% sequence identity with human LAIR-1, has a single Ig-like domain, and contains two immunoreceptor tyrosine-based inhibitory motif-like structures in its cytoplasmic tail. Mouse LAIR-1 is broadly expressed on various immune cells, and cross-linking of the molecule on stably transfected RBL-2H3 and YT.2C2 cells results in strong inhibition of their degranulation and cytotoxic activities, respectively. Upon pervanadate stimulation, the mLAIR-1 cytoplasmic tail becomes phosphorylated, thereby recruiting Src homology region 2-containing tyrosine phosphatase-2. Interestingly, unlike human LAIR-1, Src homology region 2-containing tyrosine phosphatase-1 is not recruited to the mLAIR-1 cytoplasmic tail. Screening human and mouse cell lines for mLAIR-1 and human LAIR-1 binding partners identified several lines expressing putative ligand(s) for both receptors.

Stable knockdown of MAFA expression in RBL-2H3 cells by siRNA retrovirus-delivery system

The mast cell function-associated antigen (MAFA) is a type II membranal glycoprotein first discovered on the surface of the rat mucosal-type mast cells of the RBL-2H3 line. A C-type lectin domain and an immunoreceptor tyrosine-based inhibitory motif (ITIM) are located in the extracellular and intracellular domains of MAFA, respectively. MAFA clustering by its specific monoclonal antibody G63, has been previously shown to cause a dose-dependent inhibition of the secretory response of these cells to the FcRI stimulus. More recently, human and mouse homologues of MAFA have also been discovered. However, they are expressed also or only by NK- and T cells, suggesting that MAFA may play additional functional role(s). To further pursue MAFA's functional significance in mast cells, we have employed the recently developed siRNA (small inhibitory RNA)-dependent gene-specific silencing protocol. We have first incorporated the key elements of the well-defined and commercially available siRNA vector (pSilencer1.0-U6) together with anti-MAFA hairpin into a retroviral vector (pLXSNneo). This enabled generating retroviruses that induced a stable and efficient downregulation (knockdown) of MAFA's expression. RBL-2H3 cells with either normal or knocked-down MAFA expression levels are currently examined for their biological responses e.g. FcRI mediated secretion, cell proliferation and cell adhesion. So far however, no significant changes were resolved in the above biological functions.

Identification and characterization of a novel human myeloid inhibitory C-type lectin-like receptor (MICL) that is predominantly expressed on granulocytes and monocytes

Inhibitory and activatory C-type lectin-like receptors play an important role in immunity through the regulation of leukocytes. Here, we report the identification and characterization of a novel myeloid inhibitory C-type lectin-like receptor (MICL) whose expression is primarily restricted to granulocytes and monocytes. This receptor, which contains a single C-type lectin-like domain and a cytoplasmic immunoreceptor tyrosine-based inhibitory motif, is related to LOX-1 (lectin-like receptor for oxidized low density lipoprotein-1) and the beta-glucan receptor (Dectin-1) and is variably spliced and highly N-glycosylated. We demonstrate that it preferentially associates with the signaling phosphatases SHP-1 and SHP-2, but not with SHIP. Novel chimeric analyses with a construct combining MICL and the beta-glucan receptor show that MICL can inhibit cellular activation through its cytoplasmic immunoreceptor tyrosine-based inhibitory motif. These data suggest that MICL is a negative regulator of granulocyte and monocyte function.