Molecular cloning of a human immunoglobulin G Fc receptor

DNA methylation patterns and gene expression from amygdala tissue of mature Brahman cows exposed to prenatal stress

Prenatal stress can alter postnatal performance and temperament of cattle. These phenotypic effects may result from changes in gene expression caused by stress-induced epigenetic alterations. Specifically, shifts in gene expression caused by DNA methylation within the brain’s amygdala can result in altered behavior because it regulates fear, stress response and aggression in mammals Thus, the objective of this experiment was to identify DNA methylation and gene expression differences in the amygdala tissue of 5-year-old prenatally stressed (PNS) Brahman cows compared to control cows. Pregnant Brahman cows (n = 48) were transported for 2-h periods at 60 ± 5, 80 ± 5, 100 ± 5, 120 ± 5, and 140 ± 5 days of gestation. A non-transported group (n = 48) were controls (Control). Amygdala tissue was harvested from 6 PNS and 8 Control cows at 5 years of age. Overall methylation of gene body regions, promoter regions, and cytosine-phosphate-guanine (CpG) islands were compared between the two groups. In total, 202 genes, 134 promoter regions, and 133 CpG islands exhibited differential methylation (FDR ≤ 0.15). Following comparison of gene expression in the amygdala between the PNS and Control cows, 2 differentially expressed genes were identified (FDR ≤ 0.15). The minimal differences observed could be the result of natural changes of DNA methylation and gene expression as an animal ages, or because this degree of transportation stress was not severe enough to cause lasting effects on the offspring. A younger age may be a more appropriate time to assess methylation and gene expression differences produced by prenatal stress.

Fc Binding by FcγRIIa Is Essential for Cellular Activation by the Anti-FcγRIIa mAbs 8.26 and 8.2

FcγR activity underpins the role of antibodies in both protective immunity and auto-immunity and importantly, the therapeutic activity of many monoclonal antibody therapies. Some monoclonal anti-FcγR antibodies activate their receptors, but the properties required for cell activation are not well defined. Here we examined activation of the most widely expressed human FcγR; FcγRIIa, by two non-blocking, mAbs, 8.26 and 8.2. Crosslinking of FcγRIIa by the mAb F(ab’)₂ regions alone was insufficient for activation, indicating activation also required receptor engagement by the Fc region. Similarly, when mutant receptors were inactivated in the Fc binding site, so that intact mAb was only able to engage receptors via its two Fab regions, again activation did not occur. Mutation of FcγRIIa in the epitope recognized by the agonist mAbs, completely abrogated the activity of mAb 8.26, but mAb 8.2 activity was only partially inhibited indicating differences in receptor recognition by these mAbs. FcγRIIa inactivated in the Fc binding site was next co-expressed with the FcγRIIa mutated in the epitope recognized by the Fab so that each mAb 8.26 molecule can contribute only three interactions, each with separate receptors, one via the Fc and two via the Fab regions. When the Fab and Fc binding were thus segregated onto different receptor molecules receptor activation by intact mAb did not occur. Thus, receptor activation requires mAb 8.26 Fab and Fc interaction simultaneously with the same receptor molecules. Establishing the molecular nature of FcγR engagement required for cell activation may inform the optimal design of therapeutic mAbs.

Therapeutic Value of Single Nucleotide Polymorphisms on the Efficacy of New Therapies in Patients with Multiple Sclerosis

The introduction of new therapies for the treatment of multiple sclerosis (MS) is a very recent phenomenon and little is known of their mechanism of action. Moreover, the response is subject to interindividual variability and may be affected by genetic factors, such as polymorphisms in the genes implicated in the pathologic environment, pharmacodynamics, and metabolism of the disease or in the mechanism of action of the medications, influencing the effectiveness of these therapies. This review evaluates the impact of pharmacogenetics on the response to treatment with new therapies in patients diagnosed with MS. The results suggest that polymorphisms detected in the GSTP1, ITGA4, NQO1, AKT1, and GP6 genes, for treatment with natalizumab, ZMIZ1, for fingolimod and dimethyl fumarate, ADA, for cladribine, and NOX3, for dimethyl fumarate, may be used in the future as predictive markers of treatment response to new therapies in MS patients. However, there are few existing studies and their samples are small, making it difficult to generalize the role of these genes in treatment with new therapies. Studies with larger sample sizes and longer follow-up are therefore needed to confirm the results of these studies.

Role of platelets and megakaryocytes in adaptive immunity

The immune system is comprised of two principal interconnected components called innate and adaptive immunity. While the innate immune system mounts a nonspecific response that provides protection against the spread of foreign pathogens, the adaptive immune system has developed to specifically recognize a given pathogen and lead to immunological memory. Platelets are small fragments produced from megakaryocytes in bone marrow and lungs. They circulate throughout the blood to monitor the integrity of the vasculature and to prevent bleeding. Given their large repertoire of immune receptors and inflammatory molecules, platelets and megakaryocytes can contribute to both innate and adaptive immunity. In adaptive immunity, platelets and megakaryocytes can process and present antigens to lymphocytes. Moreover, platelets, via FcγRIIA, rapidly respond to pathogens in an immune host when antibodies are present. This manuscript reviews the reported contributions of platelets and megakaryocytes with emphasis on antigen presentation and antibody response in adaptive immunity.

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.

The Rare Anaphylaxis-Associated FcγRIIa3 Exhibits Distinct Characteristics From the Canonical FcγRIIa1

FcγRIIa is an activating FcγR, unique to humans and non-human primates. It induces antibody-dependent proinflammatory responses and exists predominantly as FcγRIIa1. A unique splice variant, we designated FcγRIIa3, has been reported to be associated with anaphylactic reactions to intravenous immunoglobulins (IVIg) therapy. We aim to define the functional consequences of this FcγRIIa variant associated with adverse responses to IVIg therapy and evaluate the frequency of associated SNPs. FcγRIIa forms from macaque and human PBMCs were investigated for IgG-subclass specificity, biochemistry, membrane localization, and functional activity. Disease-associated SNPs were analyzed by sequencing genomic DNA from 224 individuals with immunodeficiency or autoimmune disease. FcγRIIa3 was identified in macaque and human PBMC. The FcγRIIa3 is distinguished from the canonical FcγRIIa1 by a unique 19-amino acid cytoplasmic insertion and these two FcγRIIa forms responded distinctly to antibody ligation. Whereas FcγRIIa1 was rapidly internalized, FcγRIIa3 was retained longer at the membrane, inducing greater calcium mobilization and cell degranulation. Four FCGR2A SNPs were identified including the previously reported intronic SNP associated with anaphylaxis, but in only 1 of 224 individuals. The unique cytoplasmic element of FcγRIIa3 delays internalization and is associated with enhanced cellular activation. The frequency of the immunodeficiency-associated SNP varies between disease populations but interestingly occurred at a lower frequency than previously reported. None-the-less enhanced FcγRIIa3 function may promote a proinflammatory environment and predispose to pathological inflammatory responses.

The Role and Function of Fcγ Receptors on Myeloid Cells

A key determinant for the survival of organisms is their capacity to recognize and respond efficiently to foreign antigens. This is largely accomplished by the orchestrated activity of the innate and adaptive branches of the immune system. Antibodies are specifically generated in response to foreign antigens, facilitating thereby the specific recognition of antigens of almost infinite diversity. Receptors specific for the Fc domain of antibodies, Fc receptors, are expressed on the surface of the various myeloid leukocyte populations and mediate the binding and recognition of antibodies by innate leukocytes. By directly linking the innate and the adaptive components of immunity, Fc receptors play a central role in host defense and the maintenance of tissue homeostasis through the induction of diverse proinflammatory, anti-inflammatory, and immunomodulatory processes that are initiated upon engagement by the Fc domain. In this chapter, we discuss the mechanisms that regulate Fc domain binding to the various types of Fc receptors and provide an overview of the astonishing diversity of effector functions that are mediated through Fc-FcR interactions on myeloid cells. Lastly, we discuss the impact of FcR-mediated interactions in the context of IgG-mediated inflammation, autoimmunity, susceptibility to infection, and responsiveness to antibody-based therapeutics.

The platelet Fc receptor, FcγRIIa

Human platelets express FcγRIIa, the low-affinity receptor for the constant fragment (Fc) of immunoglobulin (Ig) G that is also found on neutrophils, monocytes, and macrophages. Engagement of this receptor on platelets by immune complexes triggers intracellular signaling events that lead to platelet activation and aggregation. Importantly these events occur in vivo, particularly in response to pathological immune complexes, and engagement of this receptor on platelets has been causally linked to disease pathology. In this review, we will highlight some of the key features of this receptor in the context of the platelet surface, and examine the functions of platelet FcγRIIa in normal hemostasis and in response to injury and infection. This review will also highlight pathological consequences of engagement of this receptor in platelet-based autoimmune disorders. Finally, we present some new data investigating whether levels of the extracellular ligand-binding region of platelet glycoprotein VI which is rapidly shed upon engagement of platelet FcγRIIa by autoantibodies, can report on the presence of pathological anti-heparin/platelet factor 4 immune complexes and thus identify patients with pathological autoantibodies who are at the greatest risk of developing life-threatening thrombosis in the setting of heparin-induced thrombocytopenia.

Dimeric FcγR Ectodomains as Probes of the Fc Receptor Function of Anti-Influenza Virus IgG

Ab-dependent cellular cytotoxicity, phagocytosis, and Ag presentation are key mechanisms of action of Abs arising in vaccine or naturally acquired immunity, as well of therapeutic mAbs. Cells expressing the low-affinity FcγRs (FcγRII or CD32 and FcγRIII or CD16) are activated for these functions when receptors are aggregated following the binding of IgG-opsonized targets. Despite the diversity of the Fc receptor proteins, IgG ligands, and potential responding cell types, the induction of all FcγR-mediated responses by opsonized targets requires the presentation of multiple Fc regions in close proximity to each other. We demonstrated that such "near-neighbor" Fc regions can be detected using defined recombinant soluble (rs) dimeric low-affinity ectodomains (rsFcγR) that have an absolute binding requirement for the simultaneous engagement of two IgG Fc regions. Like cell surface-expressed FcγRs, the binding of dimeric rsFcγR ectodomains to Ab immune complexes was affected by Ab subclass, presentation, opsonization density, Fc fucosylation, or mutation. The activation of an NK cell line and primary NK cells by human IgG-opsonized influenza A hemagglutinin correlated with dimeric rsFcγRIIIa binding activity but not with Ab titer. Furthermore, the dimeric rsFcγR binding assay sensitively detected greater Fc receptor activity to pandemic H1N1 hemagglutinin after the swine influenza pandemic of 2009 in pooled human polyclonal IgG. Thus these dimeric rsFcγR ectodomains are validated, defined probes that should prove valuable in measuring the immune-activating capacity of IgG Abs elicited by infection or vaccination or experimentally derived IgG and its variants.

The FcγR of humans and non-human primates and their interaction with IgG: implications for induction of inflammation, resistance to infection and the use of therapeutic monoclonal antibodies

Considerable effort has focused on the roles of the individual members of the FcγR receptor (FcγR) family in inflammatory diseases and humoral immunity. Recent work has revealed major roles in infection and in particular HIV pathogenesis and immunity. In addition, FcγR functions underpin the action of many of the successful therapeutic monoclonal antibodies. This emphasises the need for a greater understanding of FcγR function in humans and in the NHP which provides a key model for human immunity and preclinical testing of antibodies. We discuss recent key aspects of the human FcγR receptor biology and structure to define differences and similarities in activity between the human and macaque Fc receptors. These differences and similarities nuance the interpretation of infection and vaccine studies in the macaque. Indeed passive IgG antibody protection in lentivirus infection models in the macaque provided early evidence for the role of Fc receptors in anti-HIV immunity that have subsequently gained support from human vaccine trials. None-the-less the diverse functions and cellular contexts of FcγR receptor expression ensure there is much still to understand of the protective and deleterious effects of FcγRs in HIV infection. Careful comparative studies of human and non-human primate FcγRs will facilitate our appreciation of what attributes of HIV specific IgG antibodies, either acquired naturally or via vaccination, are most important for protection.

Polymorphisms and interspecies differences of the activating and inhibitory FcγRII of Macaca nemestrina influence the binding of human IgG subclasses

Little is known of the impact of Fc receptor (FcR) polymorphism in macaques on the binding of human (hu)IgG, and nothing is known of this interaction in the pig-tailed macaque (Macaca nemestrina), which is used in preclinical evaluation of vaccines and therapeutic Abs. We defined the sequence and huIgG binding characteristics of the M. nemestrina activating FcγRIIa (mnFcγRIIa) and inhibitory FcγRIIb (mnFcγRIIb) and predicted their structures using the huIgGFc/huFcγRIIa crystal structure. Large differences were observed in the binding of huIgG by mnFcγRIIa and mnFcγRIIb compared with their human FcR counterparts. MnFcγRIIa has markedly impaired binding of huIgG1 and huIgG2 immune complexes compared with huFcγRIIa (His(131)). In contrast, mnFcγRIIb has enhanced binding of huIgG1 and broader specificity, as, unlike huFcγRIIb, it avidly binds IgG2. Mutagenesis and molecular modeling of mnFcγRIIa showed that Pro(159) and Tyr(160) impair the critical FG loop interaction with huIgG. The enhanced binding of huIgG1 and huIgG2 by mnFcγRIIb was shown to be dependent on His(131) and Met(132). Significantly, both His(131) and Met(132) are conserved across FcγRIIb of rhesus and cynomolgus macaques. We identified functionally significant polymorphism of mnFcγRIIa wherein proline at position 131, also an important polymorphic site in huFcγRIIa, almost abolished binding of huIgG2 and huIgG1 and reduced binding of huIgG3 compared with mnFcγRIIa His(131). These marked interspecies differences in IgG binding between human and macaque FcRs and polymorphisms within species have implications for preclinical evaluation of Abs and vaccines in macaques.

ITAM receptor-mediated generation of reactive oxygen species in human platelets occurs via Syk-dependent and Syk-independent pathways

BACKGROUND

Ligation of the platelet-specific collagen receptor, GPVI/FcRγ, causes rapid, transient disulfide-dependent homodimerization, and the production of intracellular reactive oxygen species (ROS) generated by the NADPH oxidase, linked to GPVI via TRAF4.

OBJECTIVES

The aim of this study was to evaluate the role of early signaling events in ROS generation following engagement of either GPVI/FcRγ or a second immunoreceptor tyrosine-based activation motif (ITAM)-containing receptor on platelets, FcγRIIa.

METHODS AND RESULTS

Using an H(2) DCF-DA-based flow cytometric assay to measure intracellular ROS, we show that treatment of platelets with either the GPVI agonists, collagen-related peptide (CRP) or convulxin (Cvx), or the FcγRIIa agonist 14A2, increased intraplatelet ROS; other platelet agonists such as ADP and TRAP did not. Basal ROS in platelet-rich plasma from 14 healthy donors displayed little inter-individual variability. CRP, Cvx or 14A2 induced an initial burst of ROS within 2 min followed by additional ROS reaching a plateau after 15-20 min. The Syk inhibitor BAY61-3606, which blocks ITAM-dependent signaling, had no effect on the initial ROS burst, but completely inhibited the second phase.

CONCLUSIONS

Together, these results show for the first time that ROS generation downstream of GPVI or FcγRIIa consists of two distinct phases: an initial Syk-independent burst followed by additional Syk-dependent generation.

Cloning and characterization of ovine immunoglobulin G Fc receptor II (FcgammaRII)

Immunoglobulin G (IgG) Fc receptors (FcgammaRs) bind to immune complexes through interactions with the Fc region of IgG to initiate or inhibit the defense mechanism of the leukocytes on which they are expressed. In this study, we describe the cloning, sequencing and characterization of ovine FcgammaRII. By screening a translated expression sequence tag (EST) database with the protein sequence of bovine IgG Fc receptor II, we identified a putative ovine homologue. Using rapid amplification of cDNA ends (RACE), we isolated the cDNA encoding ovine FcgammaRII from peripheral blood leucocyte RNA. The ovine FcgammaRII cDNA contains an 894bp open-reading frame, encoding a 297 amino acid transmembrane glycoprotein composed of two immunoglobulin-like extracellular domains, a transmembrane region and a cytoplasmic tail with an immunoreceptor tyrosine-based inhibitory motif (ITIM). The glycoprotein encoded by the cloned cDNA was then expressed on the surface of COS-7 cells and immunoglobulin-binding assays show that it binds ovine IgG1, but not IgG2. Identification of the ovine FcgammaRII will aid in the understanding of the molecular basis of IgG-FcgammaR interaction.

Genetic variation in Fc gamma receptor IIa and risk of coronary heart disease: negative results from two large independent populations

BACKGROUND

The role of the Fc gamma receptor IIa (Fc gamma RIIa), a receptor for C-reactive protein (CRP), the classical acute phase protein, in atherosclerosis is not yet clear. We sought to investigate the association of Fc gamma RIIa genotype with risk of coronary heart disease (CHD) in two large population-based samples.

METHODS

Fc gamma RIIa-R/H131 polymorphisms were determined in a population of 527 patients with a history of myocardial infarction and 527 age and gender matched controls drawn from a population-based MONICA- Augsburg survey. In the LURIC population, 2227 patients with angiographically proven CHD, defined as having at least one stenosis >or= 50%, were compared with 1032 individuals with stenosis <50%.

RESULTS

In both populations genotype frequencies of the Fc gamma RIIa gene did not show a significant departure from the Hardy-Weinberg equilibrium. Fc gamma RIIa R(-131) --> H genotype was not independently associated with lower risk of CHD after multivariable adjustments, neither in the MONICA population (odds ratio (OR) 1.08; 95% confidence interval (CI) 0.81 to 1.44), nor in LURIC (OR 0.96; 95% CI 0.81 to 1.14).

CONCLUSION

Our results do not confirm an independent relationship between Fc gamma RIIa genotypes and risk of CHD in these populations.

Eptifibatide-induced thrombocytopenia and thrombosis in humans require FcgammaRIIa and the integrin beta3 cytoplasmic domain

Thrombocytopenia and thrombosis following treatment with the integrin alphaIIbbeta3 antagonist eptifibatide are rare complications caused by patient antibodies specific for ligand-occupied alphaIIbbeta3. Whether such antibodies induce platelet clearance by simple opsonization, by inducing mild platelet activation, or both is poorly understood. To gain insight into the mechanism by which eptifibatide-dependent antibodies initiate platelet clearance, we incubated normal human platelets with patient serum containing an alphaIIbbeta3-specific, eptifibatide-dependent antibody. We observed that in the presence of eptifibatide, patient IgG induced platelet secretion and aggregation as well as tyrosine phosphorylation of the integrin beta3 cytoplasmic domain, the platelet FcgammaRIIa Fc receptor, the protein-tyrosine kinase Syk, and phospholipase Cgamma2. Each activation event was inhibited by preincubation of the platelets with Fab fragments of the FcgammaRIIa-specific mAb IV.3 or with the Src family kinase inhibitor PP2. Patient serum plus eptifibatide did not, however, activate platelets from a patient with a variant form of Glanzmann thrombasthenia that expressed normal levels of FcgammaRIIa and the alphaIIbbeta3 complex but lacked most of the beta3 cytoplasmic domain. Taken together, these data suggest a novel mechanism whereby eptifibatide-dependent antibodies engage the integrin beta3 subunit such that FcgammaRIIa and its downstream signaling components become activated, resulting in thrombocytopenia and a predisposition to thrombosis.

Identification of FcgammaRIIa as the ITAM-bearing receptor mediating alphaIIbbeta3 outside-in integrin signaling in human platelets

Immunoreceptor tyrosine-based activation motif (ITAM)-containing proteins have recently been demonstrated in macrophages and neutrophils to be required for cell surface integrins to transmit activation signals into the cell. To identify ITAM-bearing proteins that mediate signaling via the platelet-specific integrin alphaIIbbeta3, fibrinogen binding was induced by (1) allowing platelets to spread directly on immobilized fibrinogen, or (2) activating the PAR1 thrombin receptor on platelets in suspension. Both initiated strong, ligand binding-dependent tyrosine phosphorylation of the ITAM-bearing platelet Fc receptor, FcgammaRIIa, as well as downstream phosphorylation of the protein tyrosine kinase Syk and activation of phospholipase Cgamma2 (PLCgamma2). Addition of Fab fragments of an FcgammaRIIa-specific monoclonal antibody strongly inhibited platelet spreading on immobilized fibrinogen, as well as downstream tyrosine phosphorylation of FcgammaRIIa, Syk, and PLCgamma2, and platelets from a patient whose platelets express reduced levels of FcgammaRIIa exhibited markedly reduced spreading on immobilized fibrinogen. Finally, fibrinogen binding-induced FcgammaRIIa phosphorylation did not occur in human platelets expressing a truncated beta3 cytoplasmic domain. Taken together, these data suggest that ligand binding to platelet alphaIIbbeta3 induces integrin cytoplasmic domain-dependent phosphorylation of FcgammaRIIa, which then enlists selected components of the immunoreceptor signaling cascade to transmit amplification signals into the cell.

FcgammaRIIB as a modulator of autoimmune disease susceptibility

Antibodies are secreted to recognize and in some cases directly neutralize pathogens. Another important means by which they are essential components of the immune system is through binding to Fc receptors. Effector responses triggered by antibody binding of Fc receptors affect a host of important cellular responses such as phagocytosis, inflammatory cytokine release, antigen presentation, and regulation of humoral responses. A crucial check on this antibody-mediated signal is through the inhibitory receptor, FcgammaRIIB. In this review we discuss how dysregulation of FcgammaRIIB can result in a lowered threshold for autoimmunity in mice and humans. We close with a discussion of the potential for applying these findings to immunotherapy.

Dual ITAM-mediated proteolytic pathways for irreversible inactivation of platelet receptors: de-ITAM-izing FcgammaRIIa

Collagen binding to glycoprotein VI (GPVI) induces signals critical for platelet activation in thrombosis. Both ligand-induced GPVI signaling through its coassociated Fc-receptor gamma-chain (FcRgamma) immunoreceptor tyrosine-activation motif (ITAM) and the calmodulin inhibitor, W7, dissociate calmodulin from GPVI and induce metalloproteinase-mediated GPVI ectodomain shedding. We investigated whether signaling by another ITAM-bearing receptor on platelets, FcgammaRIIa, also down-regulates GPVI expression. Agonists that signal through FcgammaRIIa, the mAbs VM58 or 14A2, potently induced GPVI shedding, inhibitable by the metalloproteinase inhibitor, GM6001. Unexpectedly, FcgammaRIIa also underwent rapid proteolysis in platelets treated with agonists for FcgammaRIIa (VM58/14A2) or GPVI/FcRgamma (the snake toxin, convulxin), generating an approximate 30-kDa fragment. Immunoprecipitation/pull-down experiments showed that FcgammaRIIa also bound calmodulin and W7 induced FcgammaRIIa cleavage. However, unlike GPVI, the approximate 30-kDa FcgammaRIIa fragment remained platelet associated, and proteolysis was unaffected by GM6001 but was inhibited by a membrane-permeable calpain inhibitor, E64d; consistent with this, micro-calpain cleaved an FcgammaRIIa tail-fusion protein at (222)Lys/(223)Ala and (230)Gly/(231)Arg, upstream of the ITAM domain. These findings suggest simultaneous activation of distinct extracellular (metalloproteinase-mediated) and intracellular (calpain-mediated) proteolytic pathways irreversibly inactivating platelet GPVI/FcRgamma and FcgammaRIIa, respectively. Activation of both pathways was observed with immunoglobulin from patients with heparin-induced thrombocytopenia (HIT), suggesting novel mechanisms for platelet dysfunction by FcgammaRIIa after immunologic insult.

Alteration of the Fc gamma RIIa dimer interface affects receptor signaling but not ligand binding

The aggregation of cell surface FcRs by immune complexes induces a number of important Ab-dependent effector functions. However, despite numerous studies that examine receptor function, very little is known about the molecular organization of these receptors within the cell. In this study, protein complementation, mutagenesis, and ligand binding analyses demonstrate that human FcgammaRIIa is present as a noncovalent dimer form. Protein complementation studies found that FcgammaRIIa molecules are closely associated. Mutagenesis of the dimer interface, as identified by crystallographic analyses, did not affect ligand binding yet caused significant alteration to the magnitude and kinetics of receptor phosphorylation. The data suggest that the ligand binding and the dimer interface are distinct regions within the receptor, and noncovalent dimerization of FcgammaRIIa may be an essential feature of the FcgammaRIIa signaling cascade.

Raft localisation of FcγRIIa and efficient signaling are dependent on palmitoylation of cysteine 208

Ligand-dependent aggregation of FcgammaRIIa initiates multiple biochemical processes including the translocation to detergent resistant membrane domains (DRMs) and receptor tyrosine phosphorylation. Palmitoylation of cysteine residues is considered to be one process that assists in the localisation of proteins to DRMs. Within the juxtamembrane region of FcgammaRIIa there is cysteine residue (C208) that we show to be palmitoylated. Mutation of this cysteine residue results in the disruption of FcgammaRIIa translocation to DRMs as empirically defined by insolubility at high Triton X-100 concentrations. This study also demonstrates that the lack of lipid raft association diminishes FcgammaRIIa signaling as measured by receptor phosphorylation and calcium mobilisation functions suggesting that FcgammaRIIa signaling is partially dependent on lipid rafts.

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.

Inhibition of immunoglobulin E synthesis through Fc gammaRII (CD32) by a mechanism independent of B-cell receptor co-cross-linking

The inhibitory effect on antibody production by immune complexes has been shown to depend on co-ligation of the B-cell antigen receptor (BCR) with the low-affinity receptor for immunoglobulin G (IgG) (Fc gammaRIIb, CD32). Here we report that immunoglobulin E (IgE) synthesis, induced in a BCR-independent manner by interleukin-4 (IL-4) and anti-CD40 antibody, was inhibited by CD32 ligation. The observed effect was specific for CD32 as, first, antibodies directed against other B-cell surface structures had no inhibitory effect, and, second, treatment with anti-CD32 of cells that had been in culture for 2 days was ineffective owing to the down-regulation of CD32 expression. IgE inhibition was also observed in cells stimulated by IL-4/CD40 F(ab')(2) or IL-4 plus soluble CD40 ligand, demonstrating that co-cross-linking of CD32 and CD40 was not necessary to induce inhibition. Mechanistic studies into the IgE class switch process demonstrated that IL-4/anti-CD40-induced IgE germline gene transcription and B-cell proliferation were not affected by CD32 ligation. The data demonstrate that the negative regulatory role of the CD32 molecule is not restricted to BCR-induced B-cell activation, but is also functional on other B-cell activation pathways mediated by CD40 and IL-4.

The role of FcgammaRIIa as an inflammatory mediator in rheumatoid arthritis and systemic lupus erythematosus

Despite their essential role in host protection, immunoglobulins are also involved in autoimmune processes where antibodies recognize the host's own tissue, triggering inflammatory responses that result in extensive tissue damage. A complex interaction of genetic predisposition, together with environment factors, is thought to trigger immune dysfunction. Although recent studies have dissected the essential role of Fc receptors in autoimmune antibody mediated processes, the uniquely human FcgammaRIIa has not been studied in detail. This Fc receptor is of particular interest, as it is the most abundantly expressed Fc receptor in humans and is implicated in immune complex disease. Investigation of its role has been hampered to date due to lack of suitable animal models. This review examines the evidence for the direct role of this receptor in diseases such as systemic lupus erythematosus and rheumatoid arthritis.

Demonstration of a tissue factor pathway inhibitor 2 messenger RNA synthesis by pure villous cytotrophoblast cells isolated from term human placentas

Tissue factor pathway inhibitor 2 (TFPI-2), a Kunitz-type proteinase inhibitor, might play an important role during placenta growth by regulating trophoblast invasion and differentiation. Many TFPI-2 transcripts have been detected in syncytiotrophoblast cells, but conflicting results have been reported concerning TFPI-2 synthesis by the cytotrophoblast. To address this issue, we developed a method to isolate pure preparations of human villous cytotrophoblast cells from normal term placentas, and the synthesis of tissue factor, TFPI-1, and TFPI-2 mRNAs was then evaluated. Cells were isolated by trypsin-DNase-EDTA digestion, followed by Percoll gradient separation and immunodepletion of human leukocyte antigen-positive cells. The quality of villous cytotrophoblast cells was verified by electron microscopy. Purity of cell preparations was assessed by labeling cells with GB25, a monoclonal antibody specific to villous trophoblast cells, and by checking the absence of contaminating cells using anti-CD9 antibody. The lack of hCG, CD32 mRNA, and tissue factor mRNA also indicated the absence of contaminating cells. Using competitive reverse transcription polymerase chain reaction, we showed that freshly isolated villous cytotrophoblast cells synthesized significant levels of TFPI-1 mRNA and larger amounts of TFPI-2 mRNA. TFPI-1 and TFPI-2 mRNA synthesis remained unchanged when cytotrophoblast cells were cultured in complete medium and evolved as a multinucleated syncytiotrophoblast. These results indicate that the villous cytotrophoblast and syncytiotrophoblast are both important sites of TFPI-2 synthesis in the human placenta. This study also indicates that tissue factor detection should be used systematically to check the purity of cytotrophoblast cell preparations because it allows detection of contamination by monocytes/macrophages and by syncytial fragments.

IgG Fc receptors

Since the description of the first mouse knockout for an IgG Fc receptor seven years ago, considerable progress has been made in defining the in vivo functions of these receptors in diverse biological systems. The role of activating Fc gamma Rs in providing a critical link between ligands and effector cells in type II and type III inflammation is now well established and has led to a fundamental revision of the significance of these receptors in initiating cellular responses in host defense, in determining the efficacy of therapeutic antibodies, and in pathological autoimmune conditions. Considerable progress has been made in the last two years on the in vivo regulation of these responses, through the appreciation of the importance of balancing activation responses with inhibitory signaling. The inhibitory FcR functions in the maintenance of peripheral tolerance, in regulating the threshold of activation responses, and ultimately in terminating IgG mediated effector stimulation. The consequences of deleting the inhibitory arm of this system are thus manifested in both the afferent and efferent immune responses. The hyperresponsive state that results leads to greatly magnified effector responses by cytotoxic antibodies and immune complexes and can culminate in autoimmunity and autoimmune disease when modified by environmental or genetic factors. Fc gamma Rs offer a paradigm for the biological significance of balancing activation and inhibitory signaling in the expanding family of activation/inhibitory receptor pairs found in the immune system.

Expression of low-affinity Fc gamma receptor by a human metastatic melanoma line

The class IIa of low-affinity receptors for the Fc region of IgG, Fc gamma RIIa, are expressed on immune cells. The cross-linking of Fc gamma RIIa by complexed IgG triggers activation of protein tyrosine kinase and internalization of immune complexes. In this report, we demonstrate the expression of Fc gamma RIIa by a human melanoma cell line (VIO) derived from a metastasis of a patient with regressive melanoma. The analysis of Fc gamma RIIa functions was performed in VIO cells and Fc gamma RlIa- or Fc gamma RIlb-transfected human melanoma cells (A375). The Fc gamma RIIa cross-linking induced protein tyrosine phosphorylation, including Fc gamma RIIa phosphorylation, and led to its internalization in a clathrin-independent way in human melanoma cells. Moreover, we showed that a part of internalized Fc gamma RIIa migrates in late endosomes, lysosomes and class II-containing compartments. These results suggest that melanoma cells can express functional Fc gamma RII, which might play a role in tumor-host relationships.

Fine structure analysis of interaction of FcepsilonRI with IgE

The high affinity receptor for IgE (FcepsilonRI) plays an integral role in triggering IgE-mediated hypersensitivity reactions. The IgE-interactive site of human FcepsilonRI has previously been broadly mapped to several large regions in the second extracellular domain (D2) of the alpha-subunit (FcepsilonRIalpha). In this study, the IgE binding site of human FcepsilonRIalpha has been further localized to subregions of D2, and key residues putatively involved in the interaction with IgE have been identified. Chimeric receptors generated between FcepsilonRIalpha and the functionally distinct but structurally homologous low affinity receptor for IgG (FcgammaRIIa) have been used to localize two IgE binding regions of FcepsilonRIalpha to amino acid segments Tyr129-His134 and Lys154-Glu161. Both regions were capable of independently binding IgE upon placement into FcgammaRIIa. Molecular modeling of the three-dimensional structure of FcepsilonRIalpha-D2 has suggested that these binding regions correspond to the "exposed" C'-E and F-G loop regions at the membrane distal portion of the domain. A systematic site-directed mutagenesis strategy, whereby each residue in the Tyr129-His134 and Lys154-Glu161 regions of FcepsilonRIalpha was replaced with alanine, has identified key residues putatively involved in the interaction with IgE. Substitution of Tyr131, Glu132, Val155, and Asp159 decreased the binding of IgE, whereas substitution of Trp130, Trp156, Tyr160, and Glu161 increased binding. In addition, mutagenesis of residues Trp113, Val115, and Tyr116 in the B-C loop region, which lies adjacent to the C'-E and F-G loops, has suggested Trp113 also contributes to IgE binding, since the substitution of this residue with alanine dramatically reduces binding. This information should prove valuable in the design of strategies to intervene in the FcepsilonRIalpha-IgE interaction for the possible treatment of IgE-mediated allergic disease.

Biochemical analysis and crystallisation of Fc gamma RIIa, the low affinity receptor for IgG

Fc gamma RIIa is one of a family of specific cell surface receptors for immunoglobulin. Fc gamma RIIa, which binds immune complexes of certain IgG isotypes, plays important roles in immune homeostasis. However, the precise characteristics of IgG binding and three-dimensional structure of Fc gamma RIIa have not been reported. This study describes the affinity of the Fc gamma RIIa:IgG interaction as well as biochemical characterisation of recombinant Fc gamma RIIa that has been used to generate high quality crystals. Equilibrium binding analysis of the Fc gamma RII:IgG interaction found, IgG3 binds with an affinity of K(D) = 0.6 microM, as expected. Unlike other Fc gamma R, IgG4 also bound to Fc gamma RIIa, K(D) = 3 microM, clearly establishing Fc gamma RIIa as an IgG4 receptor. Biochemical analysis of mammalian and insect cell derived Fc gamma RIIa established the genuine N-terminus with Q being the first amino acid in the sequence Q, A, A, A, P... extending the N-terminus further than previously thought. Furthermore, both potential N-linked glycosylation sites are occupied. Electrospray ionisation mass spectrometry (ESMS) indicate that the N-glycans of baculovirus derived Fc gamma RIIa are core mannose oligosaccharide side chains. Finally, we describe the first crystallisation of diffraction quality crystals of soluble Fc gamma RIIa. Orthorhombic crystals diffract X-rays beyond 2.1 A resolution in the space group P2(1)2(1)2 with cell dimensions a = 78.8 A, b = 100.5 A, c = 27.8 A. This marks a significant advance towards understanding the three-dimensional structure of Fc gamma RIIa and related FcR proteins that share high amino acid identity with Fc gamma RIIa.

Inhibition of B-cell receptor-antigen complex internalization by FcgammaRIIB1 signals

Membrane-expressed immunoglobulins are B-cell receptors (BCR) for specific antigens (Ag). Upon Ag engagement of the BCR, B-cells are activated to internalize Ag-BCR complexes, process Ag and subsequently present Ag-peptides loaded in class II MHC. Due to the specific nature of the BCR, the cognate interaction between T-cells expressing Ag-specific T-cell receptor and these Ag-presenting B-cells occur in a highly regulated and precise manner. Accordingly, efficient control of T-cell activation may be achieved through regulation of Ag presenting B-cells. A potent form of regulation of lymphocyte responses is mediated by Ig end-product and anti-idiotypic antibodies via Fc-dependent mechanisms. In this communication, the authors present data that an anti-idiotype (anti-Id) Ab inhibits BCR-mediated internalization of specific Ag. Coupling of BCR to the cytoskeleton was also abortive in anti-Id Ab-treated B-cells. Inhibition by anti-Id Ab was dependent upon the presence of FcgammaRIIB1 on B-cells. As a result of anti-Id Ab suppression, B-cells were unable to initiate Ca2+ responses in Ag-specific T-cells. The results suggest that co-crosslinking of FcgammaRIIB1 and BCR inhibits cytoskeletal coupling and internalization of the Ag-BCR complex thereby preventing specific Ag presentation by B-cells. Anti-Id Ab may mediate a negative regulatory mechanism that suppresses B-cell-mediated Ag-specific T-cell activation.

Fc receptor biology

This review deals with membrane Fc receptors (FcR) of the immunoglobulin superfamily. It is focused on the mechanisms by which FcR trigger and regulate biological responses of cells on which they are expressed. FcR deliver signals when they are aggregated at the cell surface. The aggregation of FcR having immunoreceptor tyrosine-based activation motifs (ITAMs) activates sequentially src family tyrosine kinases and syk family tyrosine kinases that connect transduced signals to common activation pathways shared with other receptors. FcR with ITAMs elicit cell activation, endocytosis, and phagocytosis. The nature of responses depends primarily on the cell type. The aggregation of FcR without ITAM does not trigger cell activation. Most of these FcR internalize their ligands, which can be endocytosed, phagocytosed, or transcytosed. The fate of internalized receptor-ligand complexes depends on defined sequences in the intracytoplasmic domain of the receptors. The coaggregation of different FcR results in positive or negative cooperation. Some FcR without ITAM use FcR with ITAM as signal transduction subunits. The coaggregation of antigen receptors or of FcR having ITAMs with FcR having immunoreceptor tyrosine-based inhibition motifs (ITIMs) negatively regulates cell activation. FcR therefore appear as the subunits of multichain receptors whose constitution is not predetermined and which deliver adaptative messages as a function of the environment.

Redirected cytotoxic effector function. Requirements for expression of chimeric single chain high affinity immunoglobulin e receptors

The aim of this study was to construct a single chain chimeric FcepsilonRIalpha receptor capable of effector function in leukocytes, including cytotoxic lymphocytes. To determine the most effective single chain FcepsilonRIalpha receptor with respect to IgE binding and signaling function, a variety of chimeric gene constructs were transiently transfected into COS-7 cells. The most effective chimera consisted of four parts including: wild-type or mutated extracellular domains (Trp130 --> Ala130, W130A) of FcepsilonRIalpha, membrane proximal and transmembrane regions of FcgammaRIIa, and intracellular CD3zeta (epsilonIIaIIazeta). Scatchard analysis indicated that these FcepsilonRIalpha chimeric receptor bound ligand with an affinity of 0.9 to 2.2 x 10(9) -1. Ligand binding capacity was dramatically reduced with the deletion of 11 membrane proximal amino acids of FcepsilonRIalpha; however, function was restored by substitution with the equivalent region of FcgammaRIIa, suggesting a crucial requirement for a "spacer" segment between the transmembrane and extracellular ligand binding domain. Chimeras that bound IgE effectively also mediated phagocytosis. Chimeric receptors that contained transmembrane zeta were expressed as multimers and consequently did not bind IgE effectively; however, cotransfection of these chimeras with gamma-chain largely reconstituted IgE-mediated phagocytosis. The mouse cytotoxic T lymphocyte cell line, CTLLR8 was stably transfected with epsilonIIaIIazeta, and cloned transfectants were demonstrated to lyse target cells in an anti-FcepsilonRIalpha or IgE antibody-dependent manner. Therefore, functional single chain chimeric FcepsilonRIalpha receptors were expressed in the absence or presence of associated zeta or gamma molecules and were used to redirect killer lymphocytes to target cells.

Abrogation of the Fc gamma receptor IIA-mediated phagocytic signal by stem-loop Syk antisense oligonucleotides

The role of Syk kinase in Fc gamma receptor (Fc gamma R) IIA-mediated phagocytosis was examined with two forms of antisense oligodeoxynucleotides (ODNs) designed to hybridize to human Syk mRNA. Monocytes were incubated with linear and stem-loop antisense ODNs targeted to Syk mRNA. When complexed with cationic liposomes, stem-loop Syk antisense ODN with phosphorothioate modification exhibited stability in fetal bovine and human serum. The stem-loop Syk antisense ODN at a concentration of 0.2 microM inhibited Fc gamma RIIA-mediated phagocytosis by 90% and completely eliminated Syk mRNA and protein in monocytes, whereas scrambled-control ODNs had no effect. The Syk antisense ODNs did not change beta-actin mRNA levels and Fc gamma RII cell-surface expression. In addition, stem-loop Syk antisense ODN inhibited Fc gamma RI and Fc gamma RIIIA-mediated phagocytosis. These data indicate the efficacy of stem-loop Syk antisense ODN for targeting and degrading Syk mRNA and protein and the importance of Syk kinase in Fc gamma receptor-mediated phagocytosis. Immunoblotting assay demonstrated that Fc gamma RII tyrosine phosphorylation after Fc gamma RII cross-linking did not change in the absence of Syk protein. These results indicate that Syk kinase is required for Fc gamma RIIA-mediated phagocytic signaling and that Fc gamma RII cross-linking leads to tyrosine phosphorylation of Fc gamma RII independent of Syk kinase.

Defining a similarity threshold for a functional protein sequence pattern: the signal peptide cleavage site

When preparing data sets of amino acid or nucleotide sequences it is necessary to exclude redundant or homologous sequences in order to avoid overestimating the predictive performance of an algorithm. For some time methods for doing this have been available in the area of protein structure prediction. We have developed a similar procedure based on pair-wise alignments for sequences with functional sites. We show how a correlation coefficient between sequence similarity and functional homology can be used to compare the efficiency of different similarity measures and choose a nonarbitrary threshold value for excluding redundant sequences. The impact of the choice of scoring matrix used in the alignments is examined. We demonstrate that the parameter determining the quality of the correlation is the relative entropy of the matrix, rather than the assumed (PAM or identity) substitution mode. Results are presented for the case of prediction of cleavage sites in signal peptides. By inspection of the false positives, several errors in the database were found. The procedure presented may be used as a general outline for finding a problem-specific similarity measure and threshold value for analysis of other functional amino acid or nucleotide sequence patterns.

Characterization of IgG Fc receptors on CD34 antigen-expressing cell lines (KG-1 and KG-1a)

Although Fc gamma receptors (Fc gamma R) on mature blood cells have been extensively studied, there are only limited data on Fc gamma R expression in the early haematopoietic progenitor cells. In this study, we used the stem cell antigen (CD34)-expressing cell line (KG-1) and its less differentiated subline (KG-1a) as a model for the study of Fc gamma R in the early haematopoietic progenitors. Flow cytometry and immunoprecipitation studies on KG-1 and KG-1a cells with anti-Fc gamma R mAb showed that Fc gamma RII is the only Fc gamma R expressed on the cell surface. Analysis of the steady-state levels of Fc gamma R mRNA in KG-1 and KG-1a cells using a quantitative in situ hybridization assay revealed the presence of only Fc gamma RII mRNA. On further analysis Fc gamma RIIA mRNA but no Fc gamma RIIB or Fc gamma RIIC transcripts were found in these cells; Fc gamma RIIA transcripts with and without the transmembrane exon were present in approximately equal amounts. These findings are surprisingly similar to those observed previously with Fc gamma R in platelets and megakaryocytic cells but different from those found with Fc gamma R in cells of other lineages. These data suggest that the Fc gamma R transcript distribution pattern observed in the early haematopoietic progenitors (KG-1 cells) is retained in later stages of haematopoietic differentiation only in cells of megakaryocytic lineage.

Single amino acid mutation of Fc gamma receptor is associated with the development of heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is mediated by a heparin-dependent antibody/platelet factor 4/heparin complex binding to platelets via the Fc gamma receptor (type IIA). A single base polymorphism at position 131 of Fc gamma RIIA changes the native arginine to histidine. In the presence of murine monoclonal IgG1 the former phenotype (Fc gamma RIIAArg131) is functionally characterized by strong platelet aggregation (high responder) and the latter (Fc gamma RIIAHis131) by poor aggregation (low responder). In the presence of human IgG2 the opposite response is observed. It has recently been shown that the heparin-dependent antibody is predominantly of this subclass. We hypothesize that a relationship exists between Fc gamma RIIAHis131 and the development of HIT. We studied 24 normal individuals and 20 HIT patients using VM58, a murine monoclonal IgG1, to characterize the phenotype by platelet aggregrometry, and PCR products, amplified with primers bordering the Fc gamma RIIA polymorphism and hybridized with oligonucleotide probes specific for the single base mutation, to determine the genotype. The distribution of phenotypes and genotypes in the two populations differed, with a greater prevalence of the Fc gamma RIIAHis131 allele in the HIT patient population. Homozygous Fc gamma RIIAArg131 individuals were absent from this group. We conclude that the presence of the Fc gamma RIIAHis131 allele is associated with a predisposition to HIT.

Multiple regions of human Fc gamma RII (CD32) contribute to the binding of IgG

The low affinity receptor for IgG, Fc gamma RII (CD32), has a wide distribution on hematopoietic cells where it is responsible for a diverse range of cellular responses crucial for immune regulation and resistance to infection. Fc gamma RII is a member of the immunoglobulin superfamily, containing an extracellular region of two Ig-like domains. The IgG binding site of human Fc gamma RII has been localized to an 8-amino acid segment of the second extracellular domain, Asn154-Ser161. In this study, evidence is presented to suggest that domain 1 and two additional regions of domain 2 also contribute to the binding of IgG by Fc gamma RII. Chimeric receptors generated by exchanging the extracellular domains and segments of domain 2 between Fc gamma RII and the structurally related Fc epsilon RI alpha chain were used to demonstrate that substitution of domain 1 in its entirety or the domain 2 regions encompassing residues Ser109-Val116 and Ser130-Thr135 resulted in a loss of the ability of these receptors to bind hIgG1 in dimeric form. Site-directed mutagenesis performed on individual residues within and flanking the Ser109-Val116 and Ser130-Thr135 domain 2 segments indicated that substitution of Lys113, Pro114, Leu115, Val116, Phe129, and His131 profoundly decreased the binding of hIgG1, whereas substitution of Asp133 and Pro134 increased binding. These findings suggest that not only is domain 1 contributing to the affinity of IgG binding by Fc gamma RII but, importantly, that the domain 2 regions Ser109-Val116 and Phe129-Thr135 also play key roles in the binding of hIgG1. The location of these binding regions on a molecular model of the entire extracellular region of Fc gamma RII indicates that they comprise loops that are juxtaposed in domain 2 at the interface with domain 1, with the putative crucial binding residues forming a hydrophobic pocket surrounded by a wall of predominantly aromatic and basic residues.

Quantitation of soluble and membrane-bound Fc gamma RIIA (CD32A) mRNA in platelets and megakaryoblastic cell line (Meg-01)

Fc gamma receptors (Fc gamma Rs) are glycoproteins on platelet surface that bind IgG-containing immune complexes. However, excessive binding of immune complexes leads to platelet activation and thrombosis or increased platelet clearance and thrombocytopenia. In this study, Fc gamma R transcripts in platelets and megakaryoblastic cell line (Meg-01) were investigated using specifically designed oligonucleotides and a new quantitative in situ hybridization assay. Platelets and Meg-01 cells were found to express only Fc gamma RII transcripts. Of Fc gamma RIIA mRNA isoforms (Fc gamma RIIA, B and C), Fc gamma RIIA mRNA predominates in these cells. Platelets and Meg-01 cells contain both alternative spliced forms of Fc gamma RIIA mRNA, those with and without the transmembrane (TM) exon and both forms were present in near equal amounts. In contrast, Fc gamma RIIA transcript with the TM exon predominates in neutrophils and monocytes, suggesting that the splicing of the TM exon is under lineage-specific control.

Human neutrophil Fc receptor-mediated adhesion under flow: a hollow fibre model of intravascular arrest

Human polymorphonuclear cells (PMN) were found to adhere to a novel model of blood vessel wall-associated IgG. The internal surfaces of cellulose acetate hollow fibres, of comparable internal diameter to small blood vessels, were coated with normal serum human IgG, heat-aggregated IgG (HAIgG), laminin or fibrinogen. Under conditions of flow mimicking those in a small vessel, PMN were found to adhere markedly only to immunoglobulin-coated fibres. Arrest on HAIgG was inhibited by excess soluble IgG but not by bovine serum albumin (BSA), demonstrating that the adhesion was IgG-specific and presumably mediated by Fc gamma R on the PMN surface. Pre-adsorption of serum components onto HAIgG-coated fibres enhanced PMN arrest, due most probably to fixation of complement components by immobilized HAIgG, resulting in additional potential to entrap PMN via complement receptors such as CR3. Treatment of PMN with the regulatory neuropeptide substance P also enhanced adhesion to HAIgG-coated fibres and caused increased surface expression of Fc gamma RI, Fc gamma RII and Fc gamma RIII. A mouse cell line derived from L cells, hR4C6, stably transfected with human Fc gamma RII, was found to adhere under flow to HAIgG-coated fibres, whilst untransfected parent L cells did not. This adhesion was similarly inhibited by excess soluble IgG, confirming the capability of Fc gamma R to mediate cell arrest. The study strongly suggests that Fc gamma R may play an important role in intravascular PMN arrest and we speculate that in inflammatory diseases PMN may adhere via Fc gamma R to immobilized immunoglobulin on the vascular endothelium, with subsequent degranulation and tissue damage.

Transfection of an Fc gamma receptor cDNA induces T cells to become phagocytic

The human receptor Fc gamma RIIA for the Fc portion of IgG (Fc gamma) was expressed in a human T-cell line and conferred on these cells the ability to perform IgG antibody-stimulated phagocytosis. Crosslinking Fc gamma RIIA with anti-Fc gamma RII monoclonal antibody also induced tyrosine phosphorylation of multiple proteins including Fc gamma RIIA, ZAP-70, p72SYK, and phospholipase C gamma 1 subunit and an increase in intracellular Ca2+ concentration. The T cell receptor-associated zeta-chain was not tyrosine-phosphorylated after crosslinking of Fc gamma RIIA, suggesting that the Fc gamma RIIA-mediated signals were independent of CD3. Fc gamma RIIA-mediated signal transduction was defective in a transfected mutant T-cell line exhibiting reduced expression of the tyrosine kinases LCK and FYN. These studies indicate that certain T cells can assume phagocytic properties after transfection of cDNA encoding an Fc gamma receptor with the capability of inducing a phagocytic signal.

Putative nuclear localization signals (NLS) in protein transcription factors

We have recognized about ten distinct forms of strongly basic hexapeptides, containing at least four arginines and lysines, characteristic of nuclear proteins among all eukaryotic species, including yeast, plants, flies and mammals. These basic hexapeptides are considered to be different versions of a core nuclear localization signal, NLS. Core NLSs are present in nearly all nuclear proteins and absent from nearly all "nonassociated" cytoplasmic proteins that have been investigated. We suggest that the few (approximately 10%) protein factors lacking a typical NLS core peptide may enter the nucleus via their strong crosscomplexation with their protein factor partners that possess a core NLS. Those cytoplasmic proteins found to possess a NLS-like peptide are either tightly associated with cell membrane proteins or are integral components of large cytoplasmic protein complexes. On the other hand, some versions of core NLSs are found in many cell membrane proteins and secreted proteins. It is hypothesized that in these cases the N-terminal hydrophobic signal peptide of extracellular proteins and the internal hydrophobic domains of transmembrane proteins are stronger determinants for their subcellular localization. The position of core NLSs among homologous nuclear proteins may or may not be conserved; however, if lost from an homologous site it appears elsewhere in the protein. This search provides a set of rules to our understanding of the nature of core nuclear localization signals: (1) Core NLS are proposed to consist most frequently of an hexapeptide with 4 arginines and lysines; (2) aspartic and glutamic acid residues as well as bulky amino acids (F, Y, W) need not to be present in this hexapeptide; (3) acidic residues and proline or glycine that break the alpha-helix are frequently in the flanking region of this hexapeptide stretch; (4) hydrophobic residues ought not to be present in the core NLS flanking region allowing for the NLS to be exposed on the protein. In this study we attempt to classify putative core NLS from a wealth of nuclear protein transcription factors from diverse species into several categories, and we propose additional core NLS structures yet to be experimentally verified.

Human high-affinity Fc gamma RI (CD64) gene mapped to chromosome 1q21.2-q21.3 by fluorescence in situ hybridization

The human Fc gamma RI gene encodes for a high-affinity Fc gamma receptor that plays pivotal roles in the immune response. We have used fluorescence in situ hybridization analysis to localize the Fc gamma RI gene to human chromosome 1. The human Fc gamma RI (CD64) gene has been assigned to human chromosome 1q21.2-q21.3 using R-banded human (pro)metaphase chromosomes.

Recombinant soluble human Fc gamma RII: production, characterization, and inhibition of the Arthus reaction

A recombinant soluble form of human Fc gamma RII (rsFc gamma RII) was genetically engineered by the insertion of a termination codon 5' of sequences encoding the transmembrane domain of a human Fc gamma RII cDNA. Chinese hamster ovary cells were transfected with the modified cDNA and the secreted rsFc gamma RII purified from the tissue culture supernatant (to > 95%, assessed by SDS-PAGE) using heat aggregated human immunoglobulin G (IgG) immunoaffinity chromatography. The IgG-purified rsFc gamma RII was relatively homogeneous (approximately 31,000 M(r)) whereas the total unpurified rsFc gamma RII secreted into the tissue culture supernatant was heterogeneous relating to N-linked glycosylation differences. Functional in vitro activity of the rsFc gamma RII was demonstrated by: (a) ability to bind via the Fc portion of human IgG and mouse IgG (IgG2a > IgG1 > > IgG2b); (b) complete inhibition of binding of erythrocytes sensitized with rabbit IgG to membrane-bound Fc gamma RII on K562 cells; and (c) inhibition of the anti-Leu4-induced T cell proliferation assay. Blood clearance and biodistribution studies show the rsFc gamma RII was excreted predominantly through the kidney in a biphasic manner, with an alpha-phase (t1/2 approximately 25 min) and a beta-phase (t1/2 approximately 4.6 h); the kidneys were the only organs noted with tissue-specific accumulation. In vivo, the administration of rsFc gamma RII significantly inhibited the immune complex-mediated inflammatory response induced by the reversed passive Arthus reaction model in rats. There was a specific and dose-dependent relationship between the amount of rsFc gamma RII administered, and the reduction in the size and severity of the macroscopic inflammatory lesion. Histological analysis of the skin showed a diffuse neutrophil infiltrate in both control and rsFc gamma RII-treated rats, however the perivascular infiltrate and the red cell extravasation was less intense in the rsFc gamma RII-treated group. It is likely that complement activation leads to neutrophil chemotaxis, but neutrophil activation via Fc gamma RII, which results in inflammatory mediator release, is inhibited. The data indicate that rsFc gamma RII is a potential therapeutic agent for the treatment of antibody or immune complex-mediated tissue damage.

Association of immunoglobulin G Fc receptor II with Src-like protein-tyrosine kinase Fgr in neutrophils

The interaction of Fc receptors with antibody-antigen complexes activates multiple biological functions in hematopoietic cells. Recently, protein-tyrosine phosphorylation has been suggested to be involved in Fc receptor-mediated cell signaling. Here we show that the Src-like protein-tyrosine kinase Fgr, which is specifically expressed in mature myelomonocytic cells, coimmunoprecipitates with IgG Fc receptor II (Fc gamma RII), but not with Fc gamma RIII from detergent lysates of human peripheral neutrophils. Crosslinking of Fc gamma RII induced a rapid increase in the tyrosine kinase activity and comodulation of Fgr. These results suggest that Fgr is physically and functionally associated with Fc gamma RII and involved in Fc gamma RII-mediated signal transduction pathways.

Chimeric Fc receptors identify immunoglobulin-binding regions in human Fc gamma RII and Fc epsilon RI

Fc gamma RII and Fc epsilon RI are functionally distinct cell surface receptors for immunoglobulin (Ig); Fc gamma RII binds IgG with low affinity, whereas Fc epsilon RI binds IgE with high affinity, yet they are homologous in structure and sequence having extracellular regions containing two Ig-like domains with 38% amino acid identity. Chimeric receptors derived from human Fc gamma RII and Fc epsilon RI were produced by exchanging homologous regions of the two receptors to define binding region(s) for IgG in Fc gamma RII and IgE in Fc epsilon RI. Firstly, a chimeric form of the Fc epsilon RI alpha chain was produced by replacing the transmembrane region and cytoplasmic tail with that of Fc gamma RII. This mutant alpha chain could be expressed on the cell surface independently of associated beta and gamma subunits, and retained high-affinity IgE binding, indicating that the extracellular region of the Fc epsilon RI alpha chain is sufficient for high-affinity IgE binding. Secondly, to identify the role of the individual domains in Fc binding of both Fc gamma RII and Fc epsilon RI, chimeric receptors were generated by exchanging the first extracellular domains between Fc gamma RII and the alpha chain mutant and used to demonstrate that the second extracellular domain of both receptors contains region(s) directly involved in Ig binding. Additional chimeric receptors were constructed to localize the Ig interactive regions in domain two of Fc gamma RII and Fc epsilon RI; these identified a single region of IgG binding in Fc gamma RII located between residues Ser136 to Val169, and at least three independent IgE binding regions in the Fc epsilon RI alpha chain, between residues Trp87 to Lys128, Tyr129 to Asp145, and Ser146 to Val169.