Insufficient phosphorylation prevents fc gamma RIIB from recruiting the SH2 domain-containing protein-tyrosine phosphatase SHP-1

Inhibitory Receptor Trap: A Platform for Discovery of Inhibitory Receptors That Utilize Inositol Lipid and Phosphotyrosine Phosphatase Effectors

Among the areas of most impactful recent progress in immunology is the discovery of inhibitory receptors and the subsequent translation of this knowledge to the clinic. Although the original and canonical member of this family is FcγRIIB, more recent studies defined PD1 as an inhibitory receptor that constrains T cell immunity to tumors. These studies led to development of “checkpoint blockade” immunotherapies (CBT) for cancers in which PD1 interactions with its ligand are blocked. Unfortunately, although very effective in some patients, only a small proportion respond to this therapy. This suggests that additional as yet undescribed inhibitory receptors exist, which could be exploited. Here, we describe a new platform, termed inhibitory receptor trap (IRT), for discovery of members of this family. The approach takes advantage of the fact that many of the known inhibitory receptors mediate signaling by phospho-immunoreceptor tyrosine-based inhibition motif (ITIM) mediated recruitment of Src Homology 2 (SH2) domain-containing phosphatases including the SH2 domain-containing inositol phosphatase SHIP1 encoded by the INPP5D gene and the SH2 domain-containing phosphotyrosine phosphatases SHP1 and SHP2 encoded by the PTPN6 and PTPN11 genes respectively. Here, we describe the IRT discovery platform in which the SH2 domains of inhibitory phosphatases are used for affinity-based isolation and subsequent identification of candidate effectors via immunoblotting and high sensitivity liquid chromatography–mass spectrometry. These receptors may represent alternative targets that can be exploited for improved CBT. Salient observations from these studies include the following: SH2 domains derived from the respective phosphatases bind distinct sets of candidates from different cell types. Thus, cells of different identity and different activation states express partially distinct repertoires of up and downstream phosphatase effectors. Phosphorylated PD1 binds not only SHP2 but also SHIP1, thus the latter may be important in its inhibitory function. B cell antigen receptor signaling leads predominantly to CD79 mono-phosphorylation as indicated by much greater binding to LynSH2 than Syk(SH2)₂. This balance of ITAM mono- versus bi-phosphorylation likely tunes signaling by varying activation of inhibitory (Lyn) and stimulatory (Syk) pathways.

Overexpression of FcγRIIB regulates downstream protein phosphorylation and suppresses B cell activation to ameliorate systemic lupus erythematosus

The present study aimed to examine the effects of FcγRIIB on systemic lupus erythematosus (SLE) and to investigate the underlying mechanisms. For this purpose, lentiviral vector carrying the membrane-bound type FcγRIIB gene (mFcγRIIB lentivirus) and soluble FcγRIIB (sFcγRIIB) protein were used to treat B cells from patients with SLE. The B cells were treated with calf thymus DNA (ctDNA) and anti-calf thymus DNA-immune complexes (anti-ctDNA-IC). mFcγRIIB lentivirus and sFcγRIIB protein were also injected into MRL/lpr SLE mice. The results revealed that anti-ctDNA-IC treatment significantly downregulated the IgG antibody secretion of B cells treated with mFcγRIIB lentivirus. mFcγRIIB and sFcγRIIB decreased the phosphorylation level of Bruton's tyrosine kinase (BTK) in B cells, and increased the phosphorylation level of Lyn proto-oncogene (Lyn), docking protein 1 (DOK1) and inositol polyphosphate-5-phospha-tase D (SHIP). mFcγRIIB promoted the apoptosis of B cells. Following the treatment of MRL/lpr SLE mice with mFcγRIIB lentivirus, the levels of urinary protein, serum anti-nuclear and anti-dsDNA antibodies were decreased, while the levels of mFcγRIIB in B cells were increased. mFcγRIIB ameliorated the pathologies of the kidneys, liver and lymph node tissues of the MRL/lpr SLE mice. Following treatment of the MRL/lpr SLE mice with sFcγRIIB, the levels of urinary protein, serum anti-dsDNA antibody and BTK and SHIP phosphorylation levels in B cells were decreased, while the serum sFcγRIIB and sFcγRIIB-IgG levels were increased. On the whole, the findings of the present study demonstrate that recombinant FcγRIIB inhibits the secretion of IgG antibody by B cells from patients with SLE, ameliorates the symptoms of SLE in mice, and alters the phosphorylation levels of downstream proteins of the FcγRIIB signaling pathway in B cells. These results suggest that FcγRIIB may play preventive and therapeutic roles in SLE by inhibiting B cell activation via the FcγRIIB signaling pathway, which provides a novel theory and strategy for the prevention and treatment of SLE.

Therapeutic inhibition of FcγRIIb signaling targets leukemic stem cells in chronic myeloid leukemia

Despite the successes achieved with molecular targeted inhibition of the oncogenic driver Bcr-Abl in chronic myeloid leukemia (CML), the majority of patients still require lifelong tyrosine kinase inhibitor (TKI) therapy. This is primarily caused by resisting leukemic stem cells (LSCs), which prevent achievement of treatment-free remission in all patients. Here we describe the ITIM (immunoreceptor tyrosine-based inhibition motif)-containing Fc gamma receptor IIb (FcγRIIb, CD32b) for being critical in LSC resistance and show that targeting FcγRIIb downstream signaling, by using a Food and Drug Administration-approved BTK inhibitor, provides a successful therapeutic approach. First, we identified FcγRIIb upregulation in primary CML stem cells. FcγRIIb depletion caused reduced serial re-plaiting efficiency and cell proliferation in malignant cells. FcγRIIb targeting in both a transgenic and retroviral CML mouse model provided in vivo evidence for successful LSC reduction. Subsequently, we identified BTK as a main downstream mediator and targeting the Bcr-Abl-FcγRIIb-BTK axis in primary CML CD34⁺ cells using ibrutinib, in combination with standard TKI therapy, significantly increased apoptosis in quiescent CML stem cells thereby contributing to the eradication of LSCs.. As a potential curative therapeutic approach, we therefore suggest combining Bcr-Abl TKI therapy along with BTK inhibition.

The Human FcγRII (CD32) Family of Leukocyte FcR in Health and Disease

FcγRs have been the focus of extensive research due to their key role linking innate and humoral immunity and their implication in both inflammatory and infectious disease. Within the human FcγR family FcγRII (activatory FcγRIIa and FcγRIIc, and inhibitory FcγRIIb) are unique in their ability to signal independent of the common γ chain. Through improved understanding of the structure of these receptors and how this affects their function we may be able to better understand how to target FcγR specific immune activation or inhibition, which will facilitate in the development of therapeutic monoclonal antibodies in patients where FcγRII activity may be desirable for efficacy. This review is focused on roles of the human FcγRII family members and their link to immunoregulation in healthy individuals and infection, autoimmunity and cancer.

IgG-Independent Co-aggregation of FcεRI and FcγRIIB Results in LYN- and SHIP1-Dependent Tyrosine Phosphorylation of FcγRIIB in Murine Bone Marrow-Derived Mast Cells

Activation of the high-affinity receptor for IgE (FcεRI) follows a bell-shaped dose-response curve. Upon supra-optimal stimulation, mast cell effector responses are down-regulated by inhibitory molecules like the SH2-containing inositol-5'-phosphatase SHIP1 and the SRC-family-kinase LYN. To identify further molecules involved in a negative regulatory signalosome, we screened for proteins showing the same pattern of tyrosine phosphorylation as SHIP1, which is tyrosine-phosphorylated strongest upon supra-optimal antigen (Ag) stimulation. The low-affinity IgG receptor, FcγRIIB, was found to be most strongly phosphorylated under supra-optimal conditions. This phosphorylation is the consequence of passive, Ag/IgE-dependent and progressive co-localization of FcεRI and FcγRIIB, which is not dependent on IgG. Upon supra-optimal FcεRI cross-linking, FcγRIIB phosphorylation is executed by LYN and protected from dephosphorylation by SHIP1. Analysis of FcγRIIB-deficient bone marrow-derived mast cells revealed an ambiguous phenotype upon FcεRI cross-linking. Absence of FcγRIIB significantly diminished the level of SHIP1 phosphorylation and resulted in augmented Ca²⁺ mobilization. Though, degranulation and IL-6 production were only weakly altered. Altogether our data establish the LYN/FcγRIIB/SHIP1 signalosome in the context of FcεRI activation, particularly at supra-optimal Ag concentrations. The fact that SHIP1 tyrosine phosphorylation/activation not only depends on FcγRIIB, highlights the necessity for its tight backup control.

Lyn and Fyn function as molecular switches that control immunoreceptors to direct homeostasis or inflammation

Immunoreceptors can transduce either inhibitory or activatory signals depending on ligand avidity and phosphorylation status, which is modulated by the protein kinases Lyn and Fyn. Here we show that Lyn and Fyn control immune receptor signaling status. SHP-1 tyrosine 536 phosphorylation by Lyn activates the phosphatase promoting inhibitory signaling through the immunoreceptor. By contrast, Fyn-dependent phosphorylation of SHP-1 serine 591 inactivates the phosphatase, enabling activatory immunoreceptor signaling. These SHP-1 signatures are relevant in vivo, as Lyn deficiency exacerbates nephritis and arthritis in mice, whereas Fyn deficiency is protective. Similarly, Fyn-activating signature is detected in patients with lupus nephritis, underlining the importance of this Lyn-Fyn balance. These data show how receptors discriminate negative from positive signals that respectively result in homeostatic or inflammatory conditions.Src-family kinases Fyn and Lyn are signaling components downstream of ITAM-bearing antigen receptors. Here the authors show that by phosphorylating SHP-1 at different residues, Lyn and Fyn can have opposing regulatory effects on ITAM receptors.

Of ITIMs, ITAMs, and ITAMis: revisiting immunoglobulin Fc receptor signaling

Receptors for immunoglobulin Fc regions play multiple critical roles in the immune system, mediating functions as diverse as phagocytosis, triggering degranulation of basophils and mast cells, promoting immunoglobulin class switching, and preventing excessive activation. Transmembrane signaling associated with these functions is mediated primarily by two amino acid sequence motifs, ITAMs (immunoreceptor tyrosine-based activation motifs) and ITIMs (immunoreceptor tyrosine-based inhibition motifs) that act as the receptors' interface with activating and inhibitory signaling pathways, respectively. While ITAMs mobilize activating tyrosine kinases and their consorts, ITIMs mobilize opposing tyrosine and inositol-lipid phosphatases. In this review, we will discuss our current understanding of signaling by these receptors/motifs and their sometimes blurred lines of function.

Fc receptors as adaptive immunoreceptors

Most biological activities of antibodies depend on their ability to engage Receptors for the Fc portion of immunoglobulins (FcRs) on a variety of cell types. As FcRs can trigger positive and negative signals, as these signals control several biological activities in individual cells, as FcRs are expressed by many cells of hematopoietic origin, mostly of the myeloid lineage, as these cells express various combinations of FcRs, and as FcR-expressing cells have different functional repertoires, antibodies can exert a wide spectrum of biological activities. Like B and T Cell Receptors (BCRs and TCRs), FcRs are bona fide immunoreceptors. Unlike BCRs and TCRs, however, FcRs are immunoreceptors with an adaptive specificity for antigen, with an adaptive affinity for antibodies, with an adaptive structure and with an adaptive signaling. They induce adaptive biological responses that depend on their tissue distribution and on FcR-expressing cells that are selected locally by antibodies. They critically determine health and disease. They are thus exquisitely adaptive therapeutic tools.

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).

Osteoclast inhibitory peptide-1 binding to the Fc gammaRIIB inhibits osteoclast differentiation

Osteoclast inhibitory peptide-1 (OIP) is an autocrine/paracrine inhibitor of osteoclast differentiation, and mice that overexpress OIP-1 in osteoclast lineage cells develop an osteopetrosis bone phenotype. In this study, we show that OIP-1 binding to the Fc gamma receptor IIB (Fc gammaRIIB) inhibits osteoclast differentiation. Confocal microscopy revealed colocalization of OIP-1 with Fc gammaRIIB in osteoclasts, and we observed that OIP-1 carboxy-terminal GPI-linked peptide forms a 1:1 complex with recombinant Fc gammaRIIB protein with an affinity binding of a dissociation constant of approximately 4 microm. Immunoreceptor tyrosine-based activation motif (ITAM)-bearing adapter proteins (FcR gamma and DNAX-activating protein of molecular mass 12 kDa) are critical for osteoclast development, and OIP-1 transgenic mouse-derived preosteoclast cells demonstrated suppression (6-fold) of ITAM phosphorylation of FcR gamma but not DNAX-activating protein of molecular mass 12 kDa. Interestingly, these preosteoclast cells demonstrated increased levels (4-fold) of immunoreceptor tyrosine-based inhibitory motif phosphorylation of Fc gammaRIIB and Src homology 2-domain-containing proteins tyrosine phosphatase 1 activation. Further, OIP-1 mouse-derived preosteoclasts cells demonstrated inhibition of spleen tyrosine kinase activation (4.5-fold), compared with wild-type mice. These results suggest that cross-regulation of immunoreceptor tyrosine-based inhibitory motif and ITAM bearing Fc receptors may play a role in OIP-1 suppression of spleen tyrosine kinase activation and inhibition of osteoclast differentiation. Thus, OIP-1 is an important physiologic regulator of osteoclast development and may have therapeutic utility for bone diseases with high bone turnover.

Regulation of hematopoietic cell function by inhibitory immunoglobulin G receptors and their inositol lipid phosphatase effectors

Numerous autoimmune and inflammatory disorders stem from the dysregulation of hematopoietic cell activation. The activity of inositol lipid and protein tyrosine phosphatases, and the receptors that recruit them, is critical for prevention of these disorders. Balanced signaling by inhibitory and activating receptors is now recognized to be an important factor in tuning cell function and inflammatory potential. In this review, we provide an overview of current knowledge of membrane proximal events in signaling by inhibitory/regulatory receptors focusing on structural and functional characteristics of receptors and their effectors Src homology 2 (SH2) domain-containing tyrosine phosphatase 1 and SH2 domain-containing inositol 5-phosphatase-1. We review use of new strategies to identify novel regulatory receptors and effectors. Finally, we discuss complementary actions of paired inhibitory and activating receptors, using Fc gammaRIIA and Fc gammaRIIB regulation human basophil activation as a prototype.

A mouse Fcgamma-Fcepsilon protein that inhibits mast cells through activation of FcgammaRIIB, SH2 domain-containing inositol phosphatase 1, and SH2 domain-containing protein tyrosine phosphatases

BACKGROUND

A human Fcgamma-Fcepsilon fusion protein (GE2) designed to inhibit FcepsilonRI signaling by coaggregating FcepsilonRI with the inhibitory receptor FcgammaRIIB has been shown to inhibit mast cell activation and block cutaneous anaphylaxis. A critical issue remained as to whether the mechanism of GE2 inhibition is competition for IgE binding or inhibitory signaling through FcgammaRIIB.

OBJECTIVE

Our aim was to define the in vitro and in vivo mechanism of action of a mouse homolog of GE2 (mGE) and to assess the potential of human GE2 (hGE2) for therapeutic administration.

METHODS

The in vitro activity of mGE on mediator release and signaling pathways was characterized in IgE-sensitized bone marrow-derived mast cells (BMMCs). The in vivo activity of mGE was examined in mouse passive cutaneous and passive systemic anaphylaxis models, and the therapeutic activity of hGE2 was evaluated in Ascaris suum-sensitized cynomolgus monkeys.

RESULTS

mGE inhibited release of histamine and cytokines by BMMCs from wild-type mice but not by BMMCs from FcgammaRIIB-deficient mice. In mice mGE blocked IgE-dependent anaphylaxis mediated by mast cells with sustained efficacy. In BMMCs mGE decreased spleen tyrosine kinase and extracellular signal-regulated kinases 1/2 phosphorylation and induced FcgammaRIIB phosphorylation and the subsequent recruitment of SH2 domain-containing inositol polyphosphate 5' phosphatase (SHIP) 1 and SH2 domain-containing protein tyrosine phosphatase (SHP) 1/2 phosphatases. When administered therapeutically, hGE2 protected sensitized monkeys from local anaphylaxis for 3 weeks.

CONCLUSION

mGE-mediated inhibition of mast cell activation is associated with inhibitory signaling through FcgammaRIIB that results from activation of SHIP-1 and SHP-1/2 phosphatases.

The SH2 domain-containing inositol 5-phosphatase SHIP1 is recruited to the intracytoplasmic domain of human FcγRIIB and is mandatory for negative regulation of B cell activation

Murine FcgammaRIIB were demonstrated to recruit SH2 domain-containing inositol 5-phosphatases (SHIP1/2), when their ITIM is tyrosyl-phosphorylated upon co-aggregation with BCR, and SHIP1 to account for FcgammaRIIB-dependent negative regulation of murine B cell activation. Although human FcgammaRIIB share the same ITIM as murine FcgammaRIIB and similarly inhibit human B cell activation, which among the four known SH2 domain-containing (tyrosine or inositol) phosphatases is/are recruited by human FcgammaRIIB is unclear. Our recent finding that, besides the ITIM, a second tyrosine-based motif is mandatory for murine FcgammaRIIB to recruit SHIP1 challenged the possibility that human FcgammaRIIB recruit this phosphatase. Human FcgammaRIIB indeed lack this motif. Using an experimental model which enabled us to compare human FcgammaRIIB and murine FcgammaRIIB under strictly controlled conditions, we show that SHIP1 is recruited to the intracytoplasmic domain of human FcgammaRIIB and inhibits the same biological responses and intracellular signals as when recruited by murine FcgammaRIIB. Identical results were observed in murine and in human B cells. We demonstrate that SHIP is necessary for human FcgammaRIIB to inhibit BCR signaling, and cannot be replaced by SHP-1 or SHP-2. Although it contains no tyrosine, the C-terminal segment of human FcgammaRIIB was as mandatory as the tyrosine-containing C-terminal segment of murine FcgammaRIIB for SHIP1 to be recruited to the ITIM. This segment, however, did not recruit the adapters Grb2/Grap which were demonstrated to stabilize the recruitment of SHIP1 to the ITIM in murine FcgammaRIIB.

Fc receptors as determinants of allergic reactions

Activation of the high-affinity receptor for IgE (FcepsilonRI) on allergic effector cells induces a multitude of positive signals via immunoreceptor tyrosine-based activation motifs, which leads to the rapid manifestation of allergic inflammatory reactions. As a counterbalance, the coaggregation of the IgG receptor FcgammaRIIB mediates inhibitory signals via immunoreceptor tyrosine-based inhibition motifs. Advances in the positive and negative regulation of Fc receptor expression and signaling have shed light on the role of Fc receptors in our immune system, indicating them to be bifunctional, inhibitory and activating structures. Based on these findings, exciting new therapeutic strategies have been developed, such as the use of chimeric fusion proteins, which concomitantly activate FcepsilonRI and FcgammaRIIB. These new approaches successfully take advantage of the bivalent character of Fc receptors and pave the way for innovative strategies to modulate allergic immune reactions.

Inhibition of inflammatory responses by leukocyte Ig-like receptors

The immune system must effectively regulate the balance between beneficial and detrimental inflammation. This process is achieved in part through cell surface receptors that rapidly integrate activating and inhibitory signals. The inhibitory members of the leukocyte Ig-like receptor (LILR) family, termed LILRBs, are broadly distributed among cell populations in the immune system and potently counterregulate cell activation induced by stimuli of innate and adaptive immune responses. Studies in mice and humans indicate that LILRBs appreciably downregulate harmful inflammatory responses induced by microbial, allergic, and cytotoxic mechanisms. Hence, the LILRBs likely play significant roles in regulating the incidence and severity of many inflammatory diseases, making them potential targets for therapeutic interventions.

Negative Signaling in Fc Receptor Complexes

Cell activation results from the transient displacement of an active balance between positive and negative signaling. This displacement depends in part on the engagement of cell surface receptors by extracellular ligands. Among these are receptors for the Fc portion of immunoglobulins (FcRs). FcRs are widely expressed by cells of hematopoietic origin. When binding antibodies, FcRs provide these cells with immunoreceptors capable of triggering numerous biological responses in response to a specific antigen. FcR-dependent cell activation is regulated by negative signals which are generated together with positive signals within signalosomes that form upon FcR engagement. Many molecules involved in positive signaling, including the FcRbeta subunit, the src kinase lyn, the cytosolic adapter Grb2, and the transmembrane adapters LAT and NTAL, are indeed also involved in negative signaling. A major player in negative regulation of FcR signaling is the inositol 5-phosphatase SHIP1. Several layers of negative regulation operate sequentially as FcRs are engaged by extracellular ligands with an increasing valency. A background protein tyrosine phosphatase-dependent negative regulation maintains cells in a "resting" state. SHIP1-dependent negative regulation can be detected as soon as high-affinity FcRs are occupied by antibodies in the absence of antigen. It increases when activating FcRs are engaged by multivalent ligands and, further, when FcR aggregation increases, accounting for the bell-shaped dose-response curve observed in excess of ligand. Finally, F-actin skeleton-associated high-molecular weight SHIP1, recruited to phosphorylated ITIMs, concentrates in signaling complexes when activating FcRs are coengaged with inhibitory FcRs by immune complexes. Based on these data, activating and inhibitory FcRs could be used for new therapeutic approaches to immune disorders.

Two Distinct Tyrosine-based Motifs Enable the Inhibitory Receptor FcγRIIB to Cooperatively Recruit the Inositol Phosphatases SHIP1/2 and the Adapters Grb2/Grap

FcgammaRIIB are low-affinity receptors for IgG that contain an immunoreceptor tyrosine-based inhibition motif (ITIM) and inhibit immunoreceptor tyrosine-based activation motif (ITAM)-dependent cell activation. When coaggregated with ITAM-bearing receptors, FcgammaRIIB become tyrosyl-phosphorylated and recruit the Src homology 2 (SH2) domain-containing inositol 5'-phosphatases SHIP1 and SHIP2, which mediate inhibition. The FcgammaRIIB ITIM was proposed to be necessary and sufficient for recruiting SHIP1/2. We show here that a second tyrosine-containing motif in the intracytoplasmic domain of FcgammaRIIB is required for SHIP1/2 to be coprecipitated with the receptor. This motif functions as a docking site for the SH2 domain-containing adapters Grb2 and Grap. These adapters interact via their C-terminal SH3 domain with SHIP1/2 to form a stable receptor-phosphatase-adapter trimolecular complex. Both Grb2 and Grap are required for an optimal coprecipitation of SHIP with FcgammaRIIB, but one adapter is sufficient for the phosphatase to coprecipitate in a detectable manner with the receptors. In addition to facilitating the recruitment of SHIPs, the second tyrosine-based motif may confer upon FcgammaRIIB the properties of scaffold proteins capable of altering the composition and stability of the signaling complexes generated following receptor engagement.

SHIP2 Is Recruited to the Cell Membrane upon Macrophage Colony-Stimulating Factor (M-CSF) Stimulation and Regulates M-CSF-Induced Signaling

The Src homology 2-containing inositol phosphatase SHIP1 functions in hemopoietic cells to limit activation events mediated by PI3K products, including Akt activation and cell survival. In contrast to the limited cellular expression of SHIP1, the related isoform SHIP2, is widely expressed in both parenchymal and hemopoietic cells. The goal of this study was to determine how SHIP2 functions to regulate M-CSF signaling. We report that 1) SHIP2 was tyrosine-phosphorylated in M-CSF-stimulated human alveolar macrophages, human THP-1 cells, murine macrophages, and the murine macrophage cell line RAW264; 2) SHIP2 associated with the M-CSF receptor after M-CSF stimulation; and 3) SHIP2 associated with the actin-binding protein filamin and localization to the cell membrane, requiring the proline-rich domain, but not on the Src homology 2 domain of SHIP2. Analyzing the function of SHIP2 in M-CSF-stimulated cells by expressing either wild-type SHIP2 or an Src homology 2 domain mutant of SHIP2 reduced Akt activation in response to M-CSF stimulation. In contrast, the expression of a catalytically deficient mutant of SHIP2 or the proline-rich domain of SHIP2 enhanced Akt activation. Similarly, the expression of wild-type SHIP2 inhibited NF-kappaB-mediated gene transcription. Finally, fetal liver-derived macrophages from SHIP2 gene knockout mice enhanced activation of Akt in response to M-CSF treatment. These data suggest a novel regulatory role for SHIP2 in M-CSF-stimulated myeloid cells.

A bispecific antibody against human IgE and human FcgammaRII that inhibits antigen-induced histamine release by human mast cells and basophils

BACKGROUND

FcgammaRIIB are low-affinity immunoglobulin (Ig)G receptors that we previously demonstrated to negatively regulate IgE-induced mast cell activation when coaggregated with FcepsilonRI. Here, we engineered and characterized a bispecific reagent capable of coaggregating FcgammaRIIB with FcepsilonRI on human mast cells and basophils.

METHODS

A bispecific antibody was constructed by chemically crosslinking one Fab' fragment against human IgE and one Fab' fragment against human FcgammaRII. This molecule was used to coaggregate FcepsilonRI with FcgammaRII on human mast cells and basophils sensitized with human IgE antibodies, and the effect of coaggregation was examined on mediator release upon challenge with specific antigen.

RESULTS

When used under these conditions, this bispecific antibody not only failed to trigger the release of histamine by IgE-sensitized cells, but it also prevented specific antigen from triggering histamine release. Comparable inhibitions were observed with mast cells and basophils derived in vitro from cord blood cells and with peripheral blood basophils.

CONCLUSIONS

The bispecific antibody described here is the prototype of similar molecules that could be used in new therapeutic approaches of allergic diseases based on the coaggregation of activating receptors, such as FcepsilonRI, with inhibitory receptors, such as FcgammaRIIB, that are constitutively expressed by mast cells and basophils.

Co-aggregation of FcgammaRII with FcepsilonRI on human mast cells inhibits antigen-induced secretion and involves SHIP-Grb2-Dok complexes

Signaling through the high affinity IgE receptor FcepsilonRI on human basophils and rodent mast cells is decreased by co-aggregating these receptors to the low affinity IgG receptor FcgammaRII. We used a recently described fusion protein, GE2, which is composed of key portions of the human gamma1 and the human epsilon heavy chains, to dissect the mechanisms that lead to human mast cell and basophil inhibition through co-aggregation of FcgammaRII and FcepsilonRI. Unstimulated human mast cells derived from umbilical cord blood express the immunoreceptor tyrosine-based inhibitory motif-containing receptor FcgammaRII but not FcgammaRI or FcgammaRIII. Interaction of the mast cells with GE2 alone did not cause degranulation. Co-aggregating FcepsilonRI and FcgammaRII with GE2 1) significantly inhibited IgE-mediated histamine release, cytokine production, and Ca(2+) mobilization, 2) reduced the antigen-induced morphological changes associated with mast cell degranulation, 3) reduced the tyrosine phosphorylation of several cellular substrates, and 4) increased the tyrosine phosphorylation of the adapter protein downstream of kinase 1 (p62(dok); Dok), growth factor receptor-bound protein 2 (Grb2), and SH2 domain containing inositol 5-phosphatase (SHIP). Tyrosine phosphorylation of Dok was associated with increased binding to Grb2. Surprisingly, in non-stimulated cells, there were complexes of phosphorylated SHIP-Grb2-Dok that were lost upon IgE receptor activation but retained under conditions of Fcepsilon-Fcgamma co-aggregation. Finally, studies using mast cells from Dok-1 knock-out mice showed that IgE alone triggers degranulation supporting an inhibitory role for Dok degranulation. Our results demonstrate how human FcepsilonRI-mediated responses can be inhibited by co-aggregation with FcgammaRIIB and implicate Dok, SHIP, and Grb2 as key intermediates in regulating antigen-induced mediator release.

Fcgamma receptors on mast cells: activatory and inhibitory regulation of mediator release

Mast cell activation and subsequent release of proinflammatory mediators are primarily a consequence of aggregation of the high affinity receptors for IgE (FcepsilonRI) on the mast cell surface following antigen-dependent ligation of FcepsilonRI-bound IgE. However, data obtained from rodent and human mast cells have revealed that IgG receptors (FcgammaR) can both promote and inhibit mast cell activation. These responses appear to be species and/or mast cell phenotype dependent. In CD34+-derived human mast cells exposed to interferon-gamma, FcgammaRI is upregulated, FcgammaRII is expressed but not upregulated, and FcgammaRIII is not expressed. In contrast, in mouse mast cells, FcgammaRII and FcgammaRIII receptors are expressed, whereas FcgammaRI is not. Aggregation of FcgammaRI on human mast cells promotes mediator release in a manner generally similar to that observed following FcepsilonRI aggregation. Aggregation of FcgammaRIIb in mouse mast cells fails to influence cellular processes; however, when coligated with FcepsilonRI, signaling events thus activated downregulate antigen-dependent mediator release. These divergent responses are a consequence of different motifs contained within the cytosolic tails of the signaling subunits of these receptors and the specific signaling molecules recruited by these receptors following ligation. The studies described imply that data obtained in rodent models regarding the influence of FcgammaRs on mast cells may not be directly translatable to the human. The exploitation of FcgammaRs for a potential therapy for the treatment of allergic disorders is discussed in this context.

Role of Fcgamma receptors in the pathogenesis and treatment of idiopathic thrombocytopenic purpura

Idiopathic thrombocytopenic purpura (ITP) is an autoimmune disease characterized by autoantibody-mediated platelet destruction. Platelets with associated IgG are targeted for destruction by phagocytic cells bearing Fc receptors in the reticuloendothelial system. While there are a variety of therapeutics available for this incompletely understood disease, one of the treatments of choice is infusion of intravenous immunoglobulin (IVIG). This review will discuss the pathophysiology of ITP with an emphasis on the role of Fcgamma receptors in both the pathogenesis and treatment of the disease by IVIG. Other prominent theories of the mechanisms of action of IVIG, including the role of anti-idiotype antibodies, will also be addressed.

1alpha, 25-dihydroxy-vitamin D3 alters Syk activation through FcgammaRII in monocytic THP-1 cells

In monocytes and macrophages, activation of the tyrosine kinase Syk is an essential step in the biochemical cascade linking aggregation of receptors for immunoglobulin G (FcgammaR) to initiation of effector functions. An increase in Syk activation during differentiation of myeloid cells by different agents has been reported. We studied the activation state of Syk in response to FcgammaRII crosslinking in monocytic cells before and after in vitro differentiation with 1alpha, 25-dihydroxy-vitamin D3. We show here that while in undifferentiated THP-1 cells clustering of FcgammaRII induces significant phosphorylation and activation of Syk, in THP-1 cells differentiated in vitro by 1alpha, 25-dihydroxy-vitamin D3, FcgammaRII crosslinking induced a decrease in Syk activity. In vitro differentiation did not induce changes in the expression of FcgammaRII isoforms. The observed effect on Syk activation though FcgammaRII could be mediated by differentiation-induced changes in the expression and basal activation level of Syk, as well as changes in the association of Syk with the tyrosine phosphatase SHP-1. These results suggest that the biochemical signaling pathways induced by FcgammaRII could be dependent on the differentiation state of the cell.

IVIg-mediated amelioration of murine ITP via FcgammaRIIB is independent of SHIP1, SHP-1, and Btk activity

It has been established that amelioration of murine immune thrombocytopenia purpura (ITP) by IVIg is dependent on the inhibitory receptor FcgammaRIIB. Co-cross-linking of the FcgammaRIIB with the B-cell receptor complex or with FcepsilonRI in mast cells results in cell inhibition, which is mediated by recruitment of the inositol phosphatase SHIP1 to the cytoplasmic tail of the FcgammaR. The FcgammaRIIB can also associate with protein tyrosine phosphatase SHP-1 as a potential secondary target of the receptor. Alternatively, homoaggregation of FcgammaRIIB can induce a proapoptotic state in B cells that is dependent on the presence of Bruton tyrosine kinase (Btk), a kinase also expressed in monocytes. We sought to determine if these signaling pathways may direct IVIg-mediated FcgammaRIIB-dependent regulation of in vivo monocyte function in a murine model of ITP in which IVIg functions in an FcgammaRIIB-dependent manner. We demonstrate that mice deficient in SHIP1, SHP-1, and Btk respond to the ameliorating effects of IVIg with the same kinetics as control mice. We conclude that IVIgmediated inhibitory pathways operating via monocyte FcgammaRIIB may involve a transmembrane signaling pathway different from that of B cells.

The effect of phosphatases SHP-1 and SHIP-1 on signaling by the ITIM- and ITAM-containing Fcgamma receptors FcgammaRIIB and FcgammaRIIA

Inositol and tyrosine phosphatases have been implicated in inhibitory signaling by an Fc receptor for immunoglobulin G, FcgammaRIIB, in B cells, mast cells, and monocytes. Here, we propose a role for the Src homology 2 (SH2)-containing tyrosine phosphatase-1 (SHP-1) in FcgammaRIIB-mediated inhibition of FcgammaR signaling. Coexpression of SHP-1 enhances FcgammaRIIB-mediated inhibition of FcgammaRIIA phagocytosis in COS-1 cells. SHP-1 also enhances the reduction in FcgammaRIIA tyrosine phosphorylation that accompanies this inhibition. Significantly, tyrosine phosphorylation of Syk kinase is substantially inhibited by SHP-1. Furthermore, the activation of SHP-1 tyrosine phosphorylation is observed following stimulation of FcgammaRII in COS-1 cells and in human monocytes. The SH2 domain containing inositol phosphatase (SHIP), SHIP-1 also enhances FcgammaRIIB-mediated inhibition of FcgammaRIIA, indicating that FcgammaRIIB can use more than one pathway for its inhibitory action. In addition, SHP-1 and SHIP-1 can inhibit FcgammaRIIA phagocytosis and signal transduction in the absence of FcgammaRIIB. The data support emerging evidence that SH2-containing phosphatases, such as SHP-1 and SHIP-1, can modulate signaling by "activating" receptors.

FcgammaR polymorphisms: Implications for function, disease susceptibility and immunotherapy

Leukocyte Fcgamma receptors (FcgammaR) confer potent cellular effector functions to the specificity of IgG. FcgammaR-induced leukocyte functions, including antibody-dependent cellular cytotoxicity, phagocytosis, superoxide generation, degranulation, cytokine production and regulation of antibody production, are essential for host defense and immune regulation. The efficacy of IgG-induced FcgammaR function displays inter-individual heterogeneity due to genetic polymorphisms of three FcgammaR subclasses, FcgammaRIIa (CD32a), FcgammaRIIIa (CD16a), and FcgammaRIIIb (CD16b). FcgammaR polymorphisms have been associated with infectious and autoimmune disease, or with disease severity. FcgammaR polymorphisms may furthermore serve as markers for therapeutic efficacy and side-effects of treatment with monoclonal antibodies. In this review, FcgammaR function and the relevance of FcgammaR polymorphisms as prognostic markers for inflammatory disease and antibody-based immunotherapy are discussed.

Regulated expression and inhibitory function of Fcgamma RIIb in human monocytic cells

Human monocytes/macrophages express three classes of receptors for IgG: FcgammaRI, FcgammaRII, and FcgammaRIII. The expression and function of these receptors has been extensively studied with the exception of one, FcgammaRIIb. While the mRNA for FcgammaRIIb has been detected in human monocytes, the protein has remained elusive. Studies in mouse models indicated that the macrophage FcgammaRIIb serves to down-regulate FcgammaR-mediated phagocytosis and immune complex-induced inflammation. FcgammaRIIb has also been shown to modulate the action of cytotoxic antibodies against tumors in mouse models. Hence, an understanding of how FcgammaRIIb expression is regulated is of great importance. Here we demonstrate for the first time FcgammaRIIb protein expression and function in human monocytes. We also report that the expression of FcgammaRIIb is highly up-regulated by interleukin-4, a Th2 cytokine, and that the up-regulation of FcgammaRIIb results in a decrease in the phagocytic efficiency of interleukin-4-treated THP-1 cells. Furthermore co-clustering FcgammaRIIb with FcgammaRIIa resulted in enhanced phosphorylation of the inositol phosphatase SHIP, association of SHIP with Shc, and phosphorylation of additional proteins around 120 and 60-65 kDa, with a concomitant attenuation of Akt activation. We, therefore, propose that FcgammaRIIb serves to inhibit FcgammaRI/IIa-mediated macrophage activation using SHIP as its effector.

Redox regulation of Fcgamma receptor-mediated phagocytosis: implications for host defense and tissue injury

Recent advances in our understanding of the mechanisms that regulate acute and chronic inflammatory responses have revealed a key role for reactive oxygen intermediates in modulating the activation of neutrophils. Opsonized microbes and immune complexes initiate the oxidative burst by the engagement of receptors for immunoglobulin G, termed Fcgamma receptors. The regulation of phagocytic cell function by oxidant-sensitive signaling pathways optimizes host defense capabilities, but it also amplifies tissue damage. This review will focus on the cross-talk between Fcgamma receptors and reactive oxygen intermediates at sites of inflammation and its role in microbial immunity.

Src Homology 2 Domain-containing Inositol 5-Phosphatase 1 Mediates Cell Cycle Arrest by FcγRIIB

We previously found that low affinity receptors for the Fc portion of IgG, FcgammaRIIB, which are widely expressed by hematopoietic cells, can negatively regulate receptor tyrosine kinase-dependent cell proliferation. We investigated here the mechanisms of this inhibition. We used as experimental models wild-type mast cells, which constitutively express the stem cell factor receptor Kit and FcgammaRIIB, FcgammaRIIB-deficient mast cells reconstituted with wild-type or mutated FcgammaRIIB, and Src homology 2 domain-containing inositol polyphosphate 5-phosphatase 1 (SHIP1)-deficient mast cells. We found that, upon coaggregation with Kit, FcgammaRIIB are tyrosyl-phosphorylated, recruit SHIP1, but not SHIP2, SH2 domain-containing protein tyrosine phosphatase-1 or -2, abrogate Akt phosphorylation, shorten the duration of the activation of mitogen-activated protein kinases of the Ras and Rac pathways, abrogate cyclin induction, prevent cells from entering the cell cycle, and block thymidine incorporation. FcgammaRIIB-mediated inhibition of Kit-dependent cell proliferation was reduced in SHIP1-deficient mast cells, whereas inhibition of IgE-induced responses was abrogated. Cell proliferation was, however, inhibited by coaggregating Kit with FcgammaRIIB whose intracytoplasmic domain was replaced with the catalytic domain of SHIP1. These results demonstrate that FcgammaRIIB use SHIP1 to inhibit pathways shared by receptor tyrosine kinases and immunoreceptors to trigger cell proliferation and cell activation, respectively, but that, in the absence of SHIP1, FcgammaRIIB can use other effectors that specifically inhibit cell proliferation.

Restricted accumulation of phosphatidylinositol 3-kinase products in a plasmalemmal subdomain during Fc gamma receptor-mediated phagocytosis

Phagocytosis is a highly localized and rapid event, requiring the generation of spatially and temporally restricted signals. Because phosphatidylinositol 3-kinase (PI3K) plays an important role in the innate immune response, we studied the generation and distribution of 3' phosphoinositides (3'PIs) in macrophages during the course of phagocytosis. The presence of 3'PI was monitored noninvasively in cells transfected with chimeras of green fluorescent protein and the pleckstrin homology domain of either Akt, Btk, or Gab1. Although virtually undetectable in unstimulated cells, 3'PI rapidly accumulated at sites of phagocytosis. This accumulation was sharply restricted to the phagosomal cup, with little 3'PI detectable in the immediately adjacent areas of the plasmalemma. Measurements of fluorescence recovery after photobleaching were made to estimate the mobility of lipids in the cytosolic monolayer of the phagosomal membrane. Stimulation of phagocytic receptors induced a marked reduction of lipid mobility that likely contributes to the restricted distribution of 3'PI at the cup. 3'PI accumulation during phagocytosis was transient, terminating shortly after sealing of the phagosomal vacuole. Two factors contribute to the rapid disappearance of 3'PI: the dissociation of the type I PI3K from the phagosomal membrane and the persistent accumulation of phosphoinositide phosphatases.