The murine Fc receptor for immunoglobulin: purification, partial amino acid sequence, and isolation of cDNA clones

Harnessing the immune system via FcγR function in immune therapy: a pathway to next-gen mAbs

The human fragment crystallizable (Fc)γ receptor (R) interacts with antigen‐complexed immunoglobulin (Ig)G ligands to both activate and modulate a powerful network of inflammatory host‐protective effector functions that are key to the normal physiology of immune resistance to pathogens. More than 100 therapeutic monoclonal antibodies (mAbs) are approved or in late stage clinical trials, many of which harness the potent FcγR‐mediated effector systems to varying degrees. This is most evident for antibodies targeting cancer cells inducing antibody‐dependent killing or phagocytosis but is also true to some degree for the mAbs that neutralize or remove small macromolecules such as cytokines or other Igs. The use of mAb therapeutics has also revealed a “scaffolding” role for FcγR which, in different contexts, may either underpin the therapeutic mAb action such as immune agonism or trigger catastrophic adverse effects. The still unmet therapeutic need in many cancers, inflammatory diseases or emerging infections such as severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) requires increased effort on the development of improved and novel mAbs. A more mature appreciation of the immunobiology of individual FcγR function and the complexity of the relationships between FcγRs and antibodies is fueling efforts to develop more potent “next‐gen” therapeutic antibodies. Such development strategies now include focused glycan or protein engineering of the Fc to increase affinity and/or tailor specificity for selective engagement of individual activating FcγRs or the inhibitory FcγRIIb or alternatively, for the ablation of FcγR interaction altogether. This review touches on recent aspects of FcγR and IgG immunobiology and its relationship with the present and future actions of therapeutic mAbs.

The Neonatal Fc Receptor (FcRn): A Misnomer?

Antibodies are essential components of an adaptive immune response. Immunoglobulin G (IgG) is the most common type of antibody found in circulation and extracellular fluids. Although IgG alone can directly protect the body from infection through the activities of its antigen binding region, the majority of IgG immune functions are mediated via proteins and receptors expressed by specialized cell subsets that bind to the fragment crystallizable (Fc) region of IgG. Fc gamma (γ) receptors (FcγR) belong to a broad family of proteins that presently include classical membrane-bound surface receptors as well as atypical intracellular receptors and cytoplasmic glycoproteins. Among the atypical FcγRs, the neonatal Fc receptor (FcRn) has increasingly gained notoriety given its intimate influence on IgG biology and its ability to also bind to albumin. FcRn functions as a recycling or transcytosis receptor that is responsible for maintaining IgG and albumin in the circulation, and bidirectionally transporting these two ligands across polarized cellular barriers. More recently, it has been appreciated that FcRn acts as an immune receptor by interacting with and facilitating antigen presentation of peptides derived from IgG immune complexes (IC). Here we review FcRn biology and focus on newer advances including how emerging FcRn-targeted therapies may affect the immune responses to IgG and IgG IC.

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 multiple facets of FcRn in immunity

The neonatal Fc receptor, FcRn, is best known for its role in transporting IgG in various tissues, providing newborns with humoral immunity, and for prolonging the half-life of IgG. Recent findings implicate the involvement of FcRn in a far wider range of biological and immunological processes, as FcRn has been found to bind and extend the half-life of albumin; to be involved in IgG transport and antigen sampling at mucosal surfaces; and to be crucial for efficient IgG-mediated phagocytosis. Herein, the function of FcRn will be reviewed, with emphasis on its recently documented significance for IgG polymorphisms affecting the half-life and biodistribution of IgG3, on its role in phagocyte biology, and the subsequent role for the presentation of antigens to lymphocytes.

Fc Receptors in Immune Responses

Antibodies are major molecular effectors of adaptive immune responses. Most, if not all, biological activities of antibodies, however, depend on the functional properties of cells that express receptors for the Fc portion of antibodies (FcR). Most FcR are activating receptors; some are inhibitory. When engaged by antibodies and antigen, the various FcR expressed by a given cell trigger a mixture of positive and negative signals whose integration determines cellular responses. Responses of cell populations can be either protective or pathogenic. As a consequence, FcR are potential target/tools in a variety of diseases including infection, allergy, autoimmune diseases, and cancer.

Transgenic mice expressing human FcgammaRIIa have enhanced sensitivity to induced autoimmune arthritis as well as elevated Th17 cells

The major human Fc receptor, huFcgammaRIIa, is implicated in the development of autoimmune arthritis in humans but until recently has not been studied in mouse models. We evaluated potential roles of FcgammaRIIa by using transgenic mice expressing the receptor. We examined two models of induced autoimmune arthritis pristane-induced arthritis (PIA) and collagen-induced arthritis (CIA) as well as the anti-collagen-II antibody-induced arthritis (CAIA) model. In the induced arthritis models PIA and CIA, the transgenic mice developed a more severe arthritis than the other arthritis-prone SJL or DBA1 mice. Interestingly, anti-collagen-II antibodies were elevated in PIA in the susceptible mice. In the CIA model, the highly susceptible transgenic mouse had IgG subclass levels equivalent to the unaffected and disease resistant C57BL/6 mouse strain implying that the FcgammaRIIa lowers the threshold of IgG dependent leukocyte activation. This is consistent with the greatly enhanced sensitivity of the FcgammaRIIa transgenic mice to CAIA which clearly indicates a role for the receptor at least at the inflammatory effector cell level. Other roles for huFcgammaRIIa or other gene products in the development of autoimmunity cannot be ruled out however, especially as the mice exhibited elevated Th1 or Th17 CD4 T cells in the draining lymph nodes.

Fc receptor-targeted mucosal vaccination as a novel strategy for the generation of enhanced immunity against mucosal and non-mucosal pathogens

Numerous studies have demonstrated that targeting immunogens to Fcgamma receptors (FcgammaR) on antigen (Ag)-presenting cells (APC) can enhance humoral and cellular immunity in vitro and in vivo. FcgammaR are classified based on their molecular weight, IgG-Fc binding affinities, IgG subclass binding specificity, and cellular distribution and they consist of activating and inhibitory receptors. However, despite the potential advantages of targeting Ag to FcR at mucosal sites, very little is known regarding the role of FcR in mucosal immunity or the efficacy of FcR-targeted mucosal vaccines. In addition, recent work has suggested that FcRn is present in the lungs of adult mice and humans and can transport FcRn-targeted Ag to FcgammaR-bearing APC within mucosal lymphoid tissue. In this review we will discuss the need for new vaccine strategies, the potential for FcR-targeted vaccines to fill this need, the impact of activating versus inhibitory FcgammaR on FcR-targeted vaccination, the significance of focusing on mucosal immunity, as well as caveats that could impact the use of FcR targeting as a mucosal vaccine strategy.

Fc gamma receptor IIB on dendritic cells enforces peripheral tolerance by inhibiting effector T cell responses

The uptake of immune complexes by FcRs on APCs augments humoral and cellular responses to exogenous Ag. In this study, CD11c+ dendritic cells are shown to be responsible in vivo for immune complex-triggered priming of T cells. We examine the consequence of Ab-mediated uptake of self Ag by dendritic cells in the rat insulin promoter-membrane OVA model and identify a role for the inhibitory FcgammaRIIB in the maintenance of peripheral CD8 T cell tolerance. Effector differentiation of diabetogenic OT-I CD8+ T cells is enhanced in rat insulin promoter-membrane OVA mice lacking FcgammaRIIB, resulting in a high incidence of diabetes. FcgammaRIIB-mediated inhibition of CD8 T cell priming results from suppression of both DC activation and cross-presentation through activating FcgammaRs. Further FcgammaRIIB on DCs inhibited the induction of OVA-specific Th1 effectors, limiting Th1-type differentiation and memory T cell accumulation. In these MHC II-restricted responses, the presence of FcgammaRIIB only modestly affected initial CD4 T cell proliferative responses, suggesting that FcgammaRIIB limited effector cell differentiation primarily by inhibiting DC activation. Thus, FcgammaRIIB can contribute to peripheral tolerance maintenance by inhibiting DC activation alone or by also limiting processing of exogenously acquired Ag.

The mast cell IgG receptors and their roles in tissue inflammation

Mast cells are effector cells of the innate immune system, but because they express Fc receptors (FcRs), they can be engaged in adaptive immunity by antibodies. Mast cell FcRs include immunoglobulin E (IgE) and IgG receptors and, among these, activating and inhibitory receptors. The engagement of mast cell IgG receptors by immune complexes may or may not trigger cell activation, depending on the type of mast cell. The coengagement of IgG and IgE receptors results in inhibition of mast cell activation. The Src homology-2 domain-containing inositol 5-phosphatase-1 is a major effector of negative regulation. Biological responses of mast cells depend on the balance between positive and negative signals that are generated in FcR complexes. The contribution of human mast cell IgG receptors in allergies remains to be clarified. Increasing evidence indicates that mast cells play critical roles in IgG-dependent tissue-specific autoimmune diseases. Convincing evidence was obtained in murine models of multiple sclerosis, rheumatoid arthritis, bullous pemphigoid, and glomerulonephritis. In these models, the intensity of lesions depended on the relative engagement of activating and inhibitory IgG receptors. In vitro models of mature tissue-specific murine mast cells are needed to investigate the roles of mast cells in these diseases. One such model unraveled unique differentiation/maturation-dependent biological responses of serosal-type mast cells.

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.

Molecular analysis of expression and function of hFcgammaRIIbl and b2 isoforms in myeloid cells

The inhibitory receptor FcgammaRIIb becomes tyrosine phosphorylated and associates with the inositol phosphatase SHIP to downregulate phagocytosis. The two splice variants of FcgammaRIIb, b1 and b2, are differentially expressed in hematopoetic cells. Both isoforms of FcgammaRIIb are expressed in human myeloid cells although FcgammaRIIb2 predominates. In murine B cells FcgammaRIIb2 associates with clathrin-coated pits and undergoes endocytosis, whereas FcgammaRIIbl is excluded from the coated pits, indicating that the two isoforms serve partially differing functions. In humans, there are conflicting reports with regard to the ability of FcgammaRIIb2 to become tyrosine phosphorylated, and the functional capacities of the two isoforms are poorly understood. We, and others, have previously reported that the expression of FcgammaRIIb is upregulated in human monocytes by the anti-inflammatory cytokine IL-4. Here, we extend these findings to demonstrate that the IL-4-induced upregulation of FcgammaRIIb is synergistically enhanced by the addition of IL-10, both at the protein and the mRNA level. The upregulated receptors are functional as assessed by their ability to become tyrosine phosphorylated and to downregulate phagocytosis. Interestingly, both b1 and b2 isoforms are upregulated by anti-inflammatory cytokines. Transfection experiments expressing human FcgammaRIIbl or b2 in Raw 264.7 murine macrophage cells revealed that both isoforms are tyrosine phosphorylated and promote SHIP phosphorylation. Finally, both b1 and b2 isoforms of FcgammaRIIb downregulate phagocytosis to a similar extent. Thus we conclude that FcgammaRIIbl and b2 are both functional inhibitory receptors in the phagocytic process.

The production and characterisation of a chimaeric human IgE antibody, recognising the major mite allergen Der p 1, and its chimaeric human IgG1 anti-idiotype

BACKGROUND

Two mouse monoclonal antibodies have been described, namely: mAb 2C7 (IgG2bkappa), which is directed against the major house dust mite allergen Der p 1, and mAb 2G10 (IgG1kappa), which is an anti-idiotypic antibody raised against mAb 2C7. Given its broad IgE specificity, anti-idiotype mAb 2G10 could potentially have immunomodulatory applications. For example, a chimaeric human IgG version of mAb 2G10 could prove to be a useful molecule for binding to mast cell and basophil FcepsilonRI bound IgE, and in doing so co-ligating FcepsilonRI with FcgammaRIIB, which has been reported to have downregulatory effects.

AIMS

To produce a chimaeric human IgE version of mAb 2C7 (mAb 2C7huE) and a chimaeric human IgG1 version of its anti-idiotype mAb 2G10 (mAb 2G10huG1).

METHODS

The Vkappa and VH regions of mAb 2C7 and its anti-idiotype mAb 2G10 were engineered into human constant regions of the IgE and IgG1 isotypes, respectively.

RESULTS

The production of chimaeric mAb 2C7huE and its anti-idiotype mAb 2G10huG1 confirmed that the respective mouse antibody V regions were successfully engineered into human constant regions and still retained the specificity of the original murine V regions.

CONCLUSION

The newly constructed chimaeric antibodies will be useful to investigate the downregulation of IgE mediated hypersensitivity by the crosslinking of FcepsilonRI with FcgammaRIIB.

Structural bases of Fc gamma R functions

This review describes structures which determine the biological activities triggered by Fc gamma R and account for the cell-mediated functions of IgG antibodies in physiology and pathology. The binding specificity and affinity of Fc gamma R depend primarily on IgG-binding structures, in their immunoglobulin-like extracellular domains. Binding is however also influenced by subunits that associate to multichain Fc gamma R. Effector and regulatory intracytoplasmic sequences that are unique to molecules of the Fc gamma RIIB family determine the internalization properties of these receptors. Immunoreceptor Tyrosine-based Activation Motifs (ITAMs) are intracytoplasmic effector sequences shared by Fc gamma R and other receptors involved in the recognition of antigen, which trigger cell activation and internalization. Immunoreceptor Tyrosine-based Inhibition Motifs (ITIMs) are intracytoplasmic sequences, shared by Fc gamma RIIB and a growing number of negative coreceptors which negatively regulate cell activation via ITAM-bearing receptors. Altogether, these structures enable IgG antibodies to exert a variety of finely tuned biological effects during the immune response.

Fcε Receptor I-Associated lyn-Dependent Phosphorylation of Fcγ Receptor IIB During Negative Regulation of Mast Cell Activation

FcγRIIB are low-affinity receptors for IgG whose intracytoplasmic domain contains an immunoreceptor tyrosine-based inhibition motif (ITIM). FcγRIIB inhibit cell activation triggered by receptors that signal via immunoreceptor tyrosine-based activation motifs. This inhibition requires ITIM tyrosyl phosphorylation and is correlated with the binding of SH2 domain-containing phosphatases that may mediate the inhibitory signal. In the present work, we investigated the mechanism of FcγRIIB phosphorylation and its consequences in mast cells. We demonstrate that the phosphorylation of FcγRIIB requires coaggregation with FcεRI and that, once phosphorylated, FcγRIIB selectively recruit the inositol polyphosphate 5 phosphatase SHIP, in vivo. In vitro, however, the phosphorylated FcγRIIB ITIM binds not only SHIP, but also the two protein tyrosine phosphatases, SHP-1 and SHP-2. We show that the coaggregation of FcγRIIB with FcεRI does not prevent FcεRI-mediated activation of lyn and syk. Both kinases can phosphorylate FcγRIIB in vitro. However, when coaggregated with FcεRI, FcγRIIB was in vivo phosphorylated in syk-deficient mast cells, but not in lyn-deficient mast cells. When FcεRI are coaggregated with FcγRIIB by immune complexes, FcεRI-associated lyn may thus phosphorylate FcγRIIB. By this mechanism, FcεRI initiate ITIM-dependent inhibition of intracellular propagation of their own signals.

FcγRII (CD32) Is Linked to Apoptotic Pathways in Murine Granulocyte Precursors and Mature Eosinophils

Murine granulocytes and precursors express low-affinity IgG Fc receptors (FcγR). We investigated the effects of FcγR ligation on the development of eosinophils in cultures of normal murine bone marrow. Eosinophilopoiesis was induced by culture of bone marrow cells in the presence of cytokines (granulocyte-macrophage colony-stimulating factor [GM-CSF], interleukin-3 [IL-3], and IL-5). Addition to the cultures of 2.4G2, a rat monoclonal antibody (mAb) that reacts with FcγRII (CD32) and FcγRIII (CD16), induced granulocyte apoptosis within 24 hours. Granulocytes in cultures that contained 2.4G2 showed chromatin condensation, binding of Annexin-V, and fas induction, and by electron microscopy, apoptosis was most commonly observed in cells of the eosinophil lineage. Since murine granulocytes can express both FcγRII (CD32) and FcγRIII (CD16), we investigated the effect of 2.4G2 on cultures of bone marrow obtained from FcγRIII (CD16) gene–disrupted mice and found that the apoptosis induced with 2.4G2 was CD16-independent. Studies with bone marrow cultures from B6MLR-lpr/lpr and C3H/HEJ-gld/gld mice established that the FcγRII (CD32)-triggered apoptosis was fas-fasL–dependent. When mature eosinophils isolated from hepatic granulomas of Schistosoma mansoni–infected mice were cultured in cytokines in the presence of 2.4G2, the eosinophils underwent apoptosis within 24 hours. These findings identify a previously unknown linkage between FcγR on eosinophils and fas-mediated apoptosis, a connection that could be relevant to mechanisms by which eosinophils mediate tissue injury and antibody-dependent cellular cytotoxicity reactions.

FcγRII (CD32) Is Linked to Apoptotic Pathways in Murine Granulocyte Precursors and Mature Eosinophils

Murine granulocytes and precursors express low-affinity IgG Fc receptors (FcγR). We investigated the effects of FcγR ligation on the development of eosinophils in cultures of normal murine bone marrow. Eosinophilopoiesis was induced by culture of bone marrow cells in the presence of cytokines (granulocyte-macrophage colony-stimulating factor [GM-CSF], interleukin-3 [IL-3], and IL-5). Addition to the cultures of 2.4G2, a rat monoclonal antibody (mAb) that reacts with FcγRII (CD32) and FcγRIII (CD16), induced granulocyte apoptosis within 24 hours. Granulocytes in cultures that contained 2.4G2 showed chromatin condensation, binding of Annexin-V, and fas induction, and by electron microscopy, apoptosis was most commonly observed in cells of the eosinophil lineage. Since murine granulocytes can express both FcγRII (CD32) and FcγRIII (CD16), we investigated the effect of 2.4G2 on cultures of bone marrow obtained from FcγRIII (CD16) gene–disrupted mice and found that the apoptosis induced with 2.4G2 was CD16-independent. Studies with bone marrow cultures from B6MLR-lpr/lpr and C3H/HEJ-gld/gld mice established that the FcγRII (CD32)-triggered apoptosis was fas-fasL–dependent. When mature eosinophils isolated from hepatic granulomas of Schistosoma mansoni–infected mice were cultured in cytokines in the presence of 2.4G2, the eosinophils underwent apoptosis within 24 hours. These findings identify a previously unknown linkage between FcγR on eosinophils and fas-mediated apoptosis, a connection that could be relevant to mechanisms by which eosinophils mediate tissue injury and antibody-dependent cellular cytotoxicity reactions.

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.

Regulation of high-affinity IgE receptor-mediated mast cell activation by murine low-affinity IgG receptors

Allergic symptoms result from the release of granular and lipidic mediators and of cytokines by inflammatory cells. The whole process is initiated by the aggregation of mast cell and basophil high-affinity IgE receptors (Fc epsilon RI) by IgE and antigen. We report here that IgE-induced release of mediator and cytokine can be inhibited by cross-linking Fc epsilon RI to low-affinity IgG receptors (Fc gamma RII) which are constitutively expressed on mast cells and basophils. Using a model of stable transfectants in RBL-2H3 cells expressing endogeneous rat Fc epsilon RI and recombinant murine Fc gamma RII, we showed that inhibition requires that Fc epsilon RI be crosslinked to Fc gamma RII by the same multivalent ligand. Inhibition of cross-linked receptors left non-cross-linked Fc epsilon RI capable of triggering mediator release and was reversible upon disengagement. Both isoforms of wild-type Fc gamma RII were equally capable of inhibiting Fc epsilon RI-mediated mast cell activation provided they had an intact intracytoplasmic domain. Our results demonstrate that mast cell secretory responses triggered by high-affinity receptors for IgE may be controlled by low-affinity receptors for IgG. This regulation of Fc epsilon RI-mediated mast cell activation is of potential interest in mast cell physiology and in allergic pathology.

Fc gamma receptors: gene structure and receptor function

Molecular studies of murine Fc gamma R have revealed much exciting new information about the structure and regulation of Fc gamma RI and Fc gamma RII genes and of the Fc gamma RI protein. The Fc gamma RI gene is composed of six exons, whereas the Fc gamma RII gene is composed of ten. The extracellular domains are encoded by individual exons in both genes (three in Fc gamma RI and two in Fc gamma RII); however, the Fc gamma RII gene shows greatest complexity in the region encoding the cytoplasmic tail and membrane spanning region, which is encoded by four exons compared to only one in the Fc gamma RI gene. Expression of Fc gamma RII is controlled by elements within the first 641 bases upstream of the transcription initiation site. The function of the domains of Fc gamma RI has been defined with the surprising finding that in the absence of the third domain the first two extracellular domains function as a broadly specific low affinity Fc gamma RII-like receptor.

Regulation of the expression of murine alpha- and beta-Fc gamma R genes

Murine low-affinity receptors for the Fc portion of IgG are of two types: Fc gamma RII and Fc gamma RIII. Murine Fc gamma RII and III have 95% homologous extracellular (EC) domains and bind the same ligands, but different transmembrane (TM) and intracytoplasmic (IC) domains. They, however, have unrelated TM and IC domains. Murine Fc gamma RII are single-chain receptors, encoded by the beta-Fc gamma R gene. Murine Fc gamma RIII are composed of two subunits: the ligand-binding alpha-subunit, encoded by the alpha-Fc gamma R gene and the gamma-subunit, encoded by another gene which belongs to a family of genes encoding dimeric subunits of multichain receptors. The expression of murine Fc gamma RII and Fc gamma RIII depends on a number of mechanisms which do the following: (1) determine the tissue-specific expression of the alpha- and beta-Fc gamma R genes by selectively unmethylating DNA in specific 5' sequences in different cell types; (2) regulate the initiation of the transcription of the alpha- and beta-Fc gamma R genes via several transcription factors; (3) up- and downregulate the amount of alpha- and beta-Fc gamma R transcripts in response to cytokines; (4) decide the alternative splicing of IC exons of the beta-Fc gamma R gene and generate the different Fc gamma RII isoforms; (5) possibly regulate the translation of alpha- and beta-Fc gamma R transcripts in different cells; (6) control the assembly of the Fc gamma RIII subunits and their membrane insertion, and (7) determine the turnover of Fc gamma RII and III in the presence and absence of ligands by affecting the internalization, shedding and proteolytic cleavage of the receptors. These mechanisms altogether contribute to make a variety of cells capable of responding differently to antigen-antibody complexes, depending on environmental stimuli.

Cytoplasmic domain heterogeneity and functions of IgG Fc receptors in B lymphocytes

B lymphocytes and macrophages express closely related immunoglobulin G (IgG) Fc receptors (Fc gamma RII) that differ only in the structures of their cytoplasmic domains. Because of cell type-specific alternative messenger RNA splicing, B-cell Fc gamma RII contains an insertion of 47 amino acids that participates in determining receptor function in these cells. Transfection of an Fc gamma RII-negative B-cell line with complementary DNA's encoding the two splice products and various receptor mutants indicated that the insertion was responsible for preventing both Fc gamma RII-mediated endocytosis and Fc gamma RII-mediated antigen presentation. The insertion was not required for Fc gamma RII to modulate surface immunoglobulin-triggered B-cell activation. Instead, regulation of activation involved a region of the cytoplasmic domain common to both the lymphocyte and macrophage receptor isoforms. In contrast, the insertion did contribute to the formation of caps in response to receptor cross-linking, consistent with suggestions that the lymphocyte but not macrophage form of the receptor can associate with the detergent-insoluble cytoskeleton.

Fc receptor endocytosis is controlled by a cytoplasmic domain determinant that actively prevents coated pit localization

Macrophages and B-lymphocytes express two major isoforms of Fc receptor (FcRII-B2 and FcRII-B1) that exhibit distinct capacities for endocytosis. This difference in function reflects the presence of an in-frame insertion of 47 amino acids in the cytoplasmic domain of the lymphocyte isoform (FcRII-B1) due to alternative mRNA splicing. By expressing wild type and mutant FcRII cDNAs in fibroblasts, we have now examined the mechanism by which the insertion acts to prevent coated pit localization and endocytosis. We first identified the region of the FcRII-B2 cytoplasmic domain that is required for rapid internalization. Using a biochemical assay for endocytosis and an immuno-EM assay to determine coated pit localization directly, we found that the distal half of the cytoplasmic domain, particularly a region including residues 18-31, as needed for coated pit-mediated endocytosis. Elimination of the tyrosine residues at position 26 and 43, separately or together, had little effect on coated pit localization and a partial effect on endocytosis of ligand. Since the FcRII-B1 insertion occurs in the membrane-proximal region of the cytoplasmic domain (residue 6) not required for internalization, it is unlikely to act by physically disrupting the coated pit localization determinant. In fact, the insertion was found to prevent endocytosis irrespective of its position in the cytoplasmic tail and appeared to selectively exclude the receptor from coated regions. Moreover, receptors bearing the insertion exhibited a temperature- and ligand-dependent association with a detergent-insoluble fraction and with actin filaments, perhaps in part explaining the inability of FcRII-B1 to enter coated pits.

Soluble Fc gamma receptors II (Fc gamma RII) are generated by cleavage of membrane Fc gamma RII

This study describes the production of soluble Fc gamma RII by a cell line, D1B1, obtained by transfection of mouse L cells with a murine beta 1 Fc gamma RII cDNA. Upon incubation at 37 degrees C, radioiodinated D1B1 cells release a 39-kDa soluble Fc gamma RII, reacting with the rat anti-mouse Fc gamma RII monoclonal antibody 2.4G2, and binding to mouse IgG2a, IgG2b and IgG1 but not IgG3. In contrast to the transmembrane 50- to 70-kDa receptor, this soluble Fc gamma RII does not react with antibodies directed against a peptide corresponding to the 15 carboxy-terminal intracytoplasmic amino acids of beta Fc gamma RIII. N-Glycosidase F treatment generates a 18-kDa polypeptide. A 32- to 40-kDa soluble Fc gamma RII, which resolves into 18.5- and 20-kDa polypeptides after deglycosylation, was also isolated from the culture medium of unlabeled D1B1 cells. Therefore, this study indicates that soluble Fc gamma RII corresponding to the two extracellular domains of Fc gamma RII are generated by cleavage of membrane Fc gamma RII. Proteolysis occurs most probably at the vicinity of the transmembrane region of the receptor, around amino acids 165 to 180.

Toxoplasma gondii: fusion competence of parasitophorous vacuoles in Fc receptor-transfected fibroblasts

After actively entering its host cells, the protozoan parasite Toxoplasma gondii resides in an intracellular vacuole that is completely unable to fuse with other endocytic or biosynthetic organelles. The fusion blocking requires entry of viable organisms but is irreversible: fusion competence of the vacuole is not restored if the parasite is killed after entry. The fusion block can be overcome, however, by altering the parasite's route of entry. Thus, phagocytosis of viable antibody-coated T. gondii by Chinese hamster ovary cells transfected with macrophage-lymphocyte Fc receptors results in the formation of vacuoles that are capable of both fusion and acidification. Phagocytosis and fusion appear to involve a domain of the Fc receptor cytoplasmic tail distinct from that required for localization at clathrin-coated pits. These results suggest that the mechanism of fusion inhibition is likely to reflect a modification of the vacuole membrane at the time of its formation, as opposed to the secretion of a soluble inhibitor by the parasite.

Organization of the human and mouse low-affinity Fc gamma R genes: duplication and recombination

Receptors for immunoglobulin G immune complexes (Fc gamma RII and Fc gamma RIII) are expressed on most hematopoietic cells and show much structural and functional diversity. In order to determine the genetic basis for this diversity, a family of genes encoding the human and mouse receptors was isolated and characterized. Humans have five distinct genes for low-affinity Fc gamma Rs, in contrast to two in the mouse. With the use of yeast artificial chromosomes, the genes encoding the human receptors were oriented and linked, which established the structure of this complex locus. Comparison of the human and mouse genes generated a model for the evolutionary amplification of this locus.

Immunoglobulin Fc binding activity is associated with the mouse hepatitis virus E2 peplomer protein

Antigenic variation among murine coronaviruses is associated primarily with the surface peplomer protein E2 (180,000 Da). E2 is responsible for attachment of the virus to the host cell, MHV-induced cell fusion, and eliciting neutralizing antibody. We report here the molecular mimicry between E2 and Fc gamma receptor (Fc gamma R). Molecular mimicry between E2 and Fc gamma R may allow the escape of virus-infected cells from destruction by immunological mechanisms. Rabbit IgG, monoclonal rat IgG1 and IgG2b, monoclonal mouse IgG2a and IgG2b, and the rat anti-mouse Fc gamma R monoclonal antibody 2.4G2 immunoprecipitated from MHV-JHM-infected cells a polypeptide with a molecular mass identical to that immunoprecipitated by anti-E2 antibodies. F(ab')2 fragments of rabbit IgG did not immunoprecipitate any proteins from MHV-infected cells. All of these antibodies did not immunoprecipitate any proteins from uninfected cells. The anti-mouse Fc gamma R monoclonal antibody 2.4G2 immunoprecipitated from MHV-JHM-, MHV-3-, or MHV-A59-infected L-2 cells and 17CL-1 cells, or MHV-JHM-infected cultures of neonatal BALB/c brain cells, a protein with a molecular weight identical to that of MHV-JHM E2. The anti-Fc gamma R monoclonal antibody did not immunoprecipitate any proteins from uninfected cells. Furthermore, the 2.4G2 monoclonal antibody (mab), unrelated rat and mouse monoclonal antibodies, and a goat antiserum against E2, but not normal goat serum, immunoprecipitated a 75,000- to 77,000-Da molecule from uninfected WEHI-3 cells, a Fc gamma R bearing cell line. Several lines of evidence demonstrated that the protein immunoprecipitated by the anti-Fc gamma R mab from MHV-JHM-infected cells is the E2 glycoprotein: (1) Partial proteolytic maps obtained by Staphylococcus aureus V-8 protease treatment of the 180,000-Da proteins immunoprecipitated by the anti Fc gamma R mab and the anti-E2 mab were identical. (2) Sequential immunoprecipitation experiments from MHV-JHM-infected cells revealed that the same polypeptide chain was recognized by the anti-E2 mab and by the anti-Fc gamma R mab 2.4G2, (3) Actinomycin D did not influence the induction and expression of the 180,000-Da polypeptide chain that was immunoprecipitated by the anti-Fc gamma R mab, demonstrating that this protein is of viral origin.

Identification of Fc gamma RIIa, a product of the murine alpha Fc gamma R gene

Two genes, alpha and beta, encode murine low-affinity receptors for the Fc portion of IgG (Fc gamma RII). The amino acid sequences deduced from the nucleotidic sequences of alpha and beta cDNA are highly homologous in extracellular domains. As a consequence, the protein product of the alpha Fc gamma R gene has not yet been distinguished from that of the beta Fc gamma R gene. alpha and beta cDNA, however, show no homology in sequences coding for intracellular portions. We therefore raised antibodies against a synthetic peptide corresponding to the 26 intracytoplasmic amino acids of the expected product of the murine alpha Fc gamma R gene. F(ab')2 fragments of alpha-specific antibodies thus obtained stained specifically membrane proteins which were present in cells containing alpha transcripts but not in cells containing beta transcripts only; they bound molecules carrying the 2.4G2 epitope, characteristic of the extracellular domains of murine Fc gamma RII; they immunoprecipitated material which migrated as heterogeneous glycosylated proteins of 45-55 kDa when native and, when deglycosylated, as a single polypeptide with an apparent molecular mass of 29 kDa, which is compatible with the calculated molecular mass of the protein expected to be translated from alpha Fc gamma R transcripts. These criteria identify a product of the murine alpha Fc gamma R gene as a subtype of Fc gamma RII which can be designated Fc gamma RIIa.

Mouse macrophage beta subunit (CD11b) cDNA for the CR3 complement receptor/Mac-1 antigen

The cDNA for the common beta Mac-1 subunit (CD11b) of the mouse LFA-1/Mac-1/p150,95 group of leukocyte cell adhesion receptors, formally designated integrin beta 2, has been cloned and sequenced. Clones were isolated from cDNA libraries made from J774 macrophage and WEHI-3B myelomonocytic tumor cells which express this subunit as a component of the macrophage activation antigen 1 (Mac-1), also known as complement receptor type 3 (CR3). This subunit is expressed as a single, abundant mRNA species approximately 2.7 kilobase (kb) in size. The 2422 base pair (bp) cDNA sequence obtained codes for a 771 amino acid protein organized with leader, extracellular, transmembrane, and cytoplamic domains of 23, 680, 23, and 46 amino acids, respectively, yielding an 82,700 mature protein of 747 amino acids. The mouse beta Mac-1 subunit is highly similar to its human counterpart with an overall sequence identity of 81% and identical positioning of 5 out of 6 potential N-linked glycosylation sites, as well as 56 Cys residues that are organized in repeating motifs characteristic of integrin beta subunits. The most highly conserved regions are the transmembrane and cytoplasmic domains where only 4 out of 69 amino acids differ, indicating that the functions associated with this domain in Mac-1-mediated processes, such as iC3b-triggered phagocytosis, have been evolutionarily conserved.

Interaction of antibodies with Fc receptors in substrate-supported planar membranes measured by total internal reflection fluorescence microscopy

A procedure for constructing substrate-supported planar membranes using membrane fragments isolated from the macrophage-related cell line J774A.1 is described. Total internal reflection (TIR) fluorescence microscopy is employed to demonstrate that fluorescently labeled Fab fragments of a monoclonal antibody (2.4G2) with specificity for a murine macrophage cell-surface receptor for IgG (moFc gamma RII) bind to the planar model membranes. These measurements show that the planar membranes contain moFc gamma RII and yield a value for the association constant of 2.4G2 Fab fragments with moFc gamma RII equal to (9.6 +/- 0.4) x 10(8) M-1 and indicate that the surface density of reconstituted moFc gamma RII is approximately 50 molecules/microns 2. In addition, TIR fluorescence microscopy is used to investigate the Fc-mediated competition of unlabeled, polyclonal murine IgG with labeled 2.4G2 Fab fragments for moFc gamma RII in the planar membranes. These measurements indicate that the reconstituted moFc gamma RII recognized by 2.4G2 Fab fragments also retains the ability to bind murine IgG Fc regions and yield a value for the association constant of polyclonal murine IgG with moFc gamma RII equal to (1-5) x 10(5) M-1. This work represents one of the first applications of TIR fluorescence microscopy to specific ligand-receptor interactions.

Regulation of IgG production by suppressor Fc gamma RII+ T hybridomas

In this work, we analyzed the immunoglobulin heavy (H) and light (L) chain production by two variant B hybridomas, UN2.C3 and UN2.C17.K1 co-cultured with cells from a Fc gamma RII+, IgG-binding factor (IgG-BF)-producer T hybridoma (T2D4.C1) or with cells of a Fc gamma RII-, IgG-BF-nonproducer variant (D10C5). We showed that only the Fc gamma RII+ hybridoma directly inhibits the IgG secretion by UN2.C3 through a soluble mediator. This inhibition affects the H and L chain synthesis as well as the H and L chain-encoding mRNA steady state. No apparent cytotoxic effect could be detected. In contrast, the production of kappa chain by an H chain-negative variant (UN2.C17.K1) was unaffected. This indicates that a complete IgG molecule is required to observe the inhibitory effect induced by T2D4.C1. The pattern of effector/target cell interactions observed in our work suggests that the soluble factor involved in the suppression of IgG production is IgG-BF, able to transiently modify the IgG gene expression in target cells.

Recombinant soluble receptors for the Fc gamma portion inhibit antibody production in vitro

The problem of the structural relationship between suppressive IgG-binding factor and low-affinity receptors for the Fc portion of IgG (Fc gamma RII) has not yet been solved. In the present work we have isolated a recombinant soluble Fc gamma RII containing only the two external domains of Fc gamma RII, and analyzed its biochemical characteristics and biological activity. A cDNA encoding Fc gamma RII was mutated by the creation of a stop codon at the Lys175 codon. L cells have been transfected with this cDNA inserted into an expression vector. A cell line was obtained that secretes recombinant soluble Fc gamma RII which reacts with a monoclonal anti-Fc gamma RII antibody and binds to IgG1, IgG2a and IgG2b murine isotypes but not to IgG3 or F(ab')2 fragments of IgG2a. The secreted molecule contains two molecular species of relative molecular mass (Mr) 44,000 and 34,000-38,000 and of pI 4.5 and 6.3. They correspond to different glycosylations of a single polypeptide of Mr 19,000. After purification to homogeneity, soluble Fc gamma RII has found to suppress secondary and primary in vitro antibody responses in a dose-dependent way. The present work shows that recombinant soluble Fc gamma RII has biochemical characteristics, immunoreactivity and biological activity similar to those of suppressive IgG-binding factor.

Expression of macrophage-lymphocyte Fc receptors in Madin-Darby canine kidney cells: polarity and transcytosis differ for isoforms with or without coated pit localization domains

Many cells of the immune system and certain epithelia express receptors for the Fc domain of IgG (FcR). On mouse macrophages and lymphocytes, two distinct receptor isoforms have been identified, designated FcRII-B1 and FcRII-B2. The isoforms are identical except for an in-frame insertion of 47 amino acids in the cytoplasmic tail of FcRII-B1 that blocks its ability to be internalized by clathrin-coated pits. We have recently found that at least one IgG-transporting epithelium, namely placental syncytial trophoblasts, expresses transcripts encoding a receptor similar or identical to macrophage-lymphocyte FcRII. To determine whether FcRII of hematopoietic cells might also function as a transcytotic receptor if expressed in epithelial cells, FcRII-B1 and -B2 were transfected into Madin-Darby canine kidney (MDCK) cells and grown on permeable filter units. The two FcRII isoforms exhibited different patterns of polarized expression: FcRII-B1 was localized mainly to the apical plasma membrane domain, whereas FcRII-B2 was found predominantly on the basolateral surface. As expected for FcR in placenta, FcRII-B2 and to a lesser extent FcRII-B1 mediated transcellular transport of IgG-complexes from the apical to the basolateral plasma membrane. Neither receptor mediated transcytosis in the opposite direction, although FcRII-B2 also delivered ligand to lysosomes when internalized from either the basolateral or apical domains. Furthermore, FcRII-B2 was capable of transporting monovalent antireceptor antibody Fab fragments across the cell, suggesting that transcytosis was not dependent on receptor cross-linking. These findings suggest the possibility that FcRII can mediate transepithelial IgG transport when expressed in placental syncytial trophoblasts in addition to its "classical" endocytic and signaling activities when expressed in macrophages. Because FcRII-B1 and -B2 are expressed with distinct polarities, the results also suggest that interactions with clathrin-coated pits may play a role in generating the polarized distribution of at least some plasma membrane proteins in MDCK cells.

Fc gamma RII expression in resting and activated B lymphocytes

In this report, we analyze the expression of the type II receptor for the Fc region of IgG (Fc gamma RII) in resting and lipopolysaccharide (LPS)-activated murine B lymphocytes. Fc gamma RII is encoded by two genes, alpha and beta. The beta gene encodes two mRNA, beta 1 and beta 2, which are generated by alternative splicing. Using an S1 nuclease protection assay, we found that resting and activated B lymphocytes express predominantly the beta 1 transcript. Very low levels of the beta 2 mRNA were detected in this assay, while no expression of the alpha transcript could be detected. Quantitative Northern blot analysis showed that the amount of Fc gamma RII beta mRNA was increased 9-fold in LPS-activated B lymphocytes. The expression of Fc gamma RII during the various phases of B cell activation was then studied by immunofluorescence using the monoclonal antibody 2.4G2. LPS stimulation induced an increase of the Fc gamma RII cellular pool as well as of its expression at the surface of B lymphocytes. The rise in Fc gamma RII surface expression occurred after the induction of class II antigens (Ia) and before transferrin receptor induction. Fc gamma RII expression was found to be enhanced during the G1 phase of the cell cycle since (a) only large cells (i.e. those that had entered the G1 phase) expressed an increased amount of Fc gamma RII and (b) blocking the entry of activated cells into the S phase (with the ion channel blocker quinine) did not affect the Fc gamma RII induction by LPS. Furthermore, only B cell activators that induced cells to enter into G1 [LPS and F(ab')2 anti-IgM antibodies, but not interleukin 4] caused an increase in the expression of Fc gamma RII. These results show that the increase in the membrane expression of Fc gamma RII occurs during the early G1 phase, establishing it as a marker for the entry of B lymphocytes into the cell cycle.

Murine natural killer cells express functional Fc gamma receptor II encoded by the Fc gamma R alpha gene

We report evidence that murine NK cells express a functional Fc gamma RII encoded by the Fc gamma RII alpha gene. Several lines of indirect evidence indicate that freshly obtained NK cells from mice of several strains bear a functional Fc gamma RII: (a) anti-Fc gamma RII antibody 2.4G2 detects a small but significant proportion of sIg- cells and a small proportion of the 2.4G2+ cells are included in the Thy-1+ population; (b) sIg- lymphocytes contain 2.4G2+ and Fc gamma R-bearing cells in similar proportions; (c) binding of particulate immune complexes by sIg- lymphocytes is completely inhibited by 2.4G2; (d) 2.4G2+ cells mediate greater than 50% of the spontaneous cytotoxicity in sIg- splenic lymphocytes. Direct evidence for the presence of Fc gamma RII on murine NK cells is provided by the results of two-color immunofluorescence studies performed on splenic lymphocytes from C57BL/6 mice showing coexpression of NK-1.1 and 2.4G2. Studies of in vitro propagated homogeneous NK cell populations confirm that murine NK cells express only Fc gamma RII and that this Fc gamma R is functional, as shown in experiments of inhibition of ADCC by the anti-Fc gamma RII antibody 2.4G2. The results of studies at the molecular level show that an Fc gamma RII alpha transcript identical to that expressed in macrophages is the only molecule encoding Fc gamma RII in murine NK cells.

Isolation and expression of functional high-affinity Fc receptor complementary DNAs

Human and murine mononuclear phagocytes express a high-affinity receptor for immunoglobulin G that plays a central role in macrophage antibody-dependent cellular cytotoxicity and clearance of immune complexes. The receptor (FcRI) may also be involved in CD4-independent infection of human macrophages by human immunodeficiency virus. This report describes the isolation of cDNA clones encoding the human FcRI by a ligand-mediated selection technique. Expression of the cDNAs in COS cells gave rise to immunoglobulin G binding of the expected affinity and subtype specificity. RNA blot analysis revealed expression of a 1.7-kilobase transcript in macrophages and in cells of the promonocytic cell line U937 induced with interferon-gamma. The extracellular region of FcRI consists of three immunoglobulin-like domains, two of which share homology with low-affinity receptor domains.

cDNA heterogeneity suggests structural variants related to the high-affinity IgE receptor

The high-affinity IgE receptor present on mast cells and basophils is responsible for the IgE-mediated activation of these cells. The current model for this receptor depicts a four-subunit structure, alpha beta gamma 2. A cDNA for the alpha subunit was recently cloned and predicts a structure consisting of two homologous extracellular domains, a transmembrane segment, and a cytoplasmic tail. Using a synthetic oligonucleotide corresponding to the amino-terminal sequence of the alpha subunit, we identified a number of cDNA clones from a rat basophilic leukemia cell cDNA library. Nucleotide sequencing established four different forms of cDNA: one is nearly identical to the published cDNA; the second differs from the first in the 5' untranslated sequence; the other two forms use either one or the other of the 5'-end sequences as above and lack 163 base pairs in the region coding for the second extracellular domain. RNase protection analysis with radioactive RNA probes established the heterogeneity of rat basophilic leukemia cell mRNA with regard to both the 5' and the internal sequences. Our results suggest the existence of at least four different protein forms related to the alpha subunit of the high-affinity IgE receptor.

Murine Fc gamma receptor proteins: identification of a previously unrecognized molecule with a monoclonal antibody (12-15)

Previously we studied differential expression of cell surface molecules between the metastatic murine lymphoma ESb and an adhesion variant ESb-MP. Here we describe the specificity of a monoclonal antibody (12-15) that showed strong binding to the adhesion variant and weak reactivity against ESb cells. The antibody also reacted to lymphoid but not to macrophage-derived cell lines and immunoprecipitated a molecule of approx. 60-69 kDa from ESb-MP cells. N-terminal sequencing of the antigen revealed identity to the beta protein of mouse Fc gamma receptors. Using monoclonal antibodies against Fc gamma receptors (2.4G2 and K9.361) in immunofluorescence assays and cDNA probes specific for alpha, beta 1 and beta 2 Fc receptor transcripts in Northern blot experiments the differential expression of Fc receptors in ESb and ESb-MP cells was confirmed. Biochemical analysis of endoglycosidase F-treated precipitates revealed that antibody 12-15 reacted to products of all three transcripts with molecular masses for the protein core of 38.5 kDa (beta 1), 34 kDa (beta 2) and 31 kDa alpha). In addition, an unknown protein of 37 kDa (termed beta 3) was identified by antibody 12-15 which could also be detected in ESb cells and EL4 cells. Antibodies 2.4G2 and K9.361 did not react to the beta 3 chain but reacted to varying extents to the other Fc proteins in macrophage and lymphoid cells. Comparison by peptide mapping of the novel beta 3 chain to beta 1, beta 2 and alpha proteins revealed similar, but also distinct peptides. The tissue-specific reactivity of monoclonal antibody 12-15 is likely to be due to a carbohydrate epitope associated with all Fc gamma receptors in lymphoid but not macrophage cell lines.

Molecular cloning of a human immunoglobulin G Fc receptor

Human IgG Fc receptor (Fc gamma R) cDNA clones were isolated by cross-species hybridization by probing cDNA libraries with the low-affinity Fc gamma R beta 1 cDNA clone from mouse as well as a pool of oligonucleotides constructed from the nucleotide sequence of this Fc gamma R. Three cDNA clones were isolated and analysis of the predicted amino acid sequence indicated that the human Fc gamma R protein is synthesized with a 34-amino acid leader and the mature protein is composed of 281 amino acids. The extracellular region of this Fc gamma R was divided into two domains, which were very similar to each other and to the corresponding regions of both mouse alpha and beta Fc gamma Rs and showed a clear relationship to immunoglobulin variable regions. One possible N-linked glycosylation site was found in each of the extracellular domains. The human Fc gamma R leader sequence was shown to be similar to the mouse alpha Fc gamma R leader sequence, but the transmembrane region was most similar to the mouse beta 1 Fc gamma R. The intracellular domain of the human Fc gamma R was surprisingly different from both mouse Fc gamma Rs. RNA blot analysis of human cells demonstrated two transcripts (2.5 and 1.5 kilobases) that arise by use of different adenylylation signals. The cellular expression of these transcripts suggest that they encode the low-affinity p40 Fc gamma R protein.

Isolation of cDNAs for two distinct human Fc receptors by ligand affinity cloning

Two cDNA clones encoding different but related receptors for immunoglobulin G constant domains were isolated from cDNA expression libraries by a ligand-mediated selection procedure ('affinity cloning'). Because both of the receptors encoded by the cDNAs react with CDw32 monoclonal antibodies, and both show the appropriate IgG binding affinity, both appear to be forms of the receptor formerly thought to be a single species called FcRII. The extracellular domains encoded by the isolated clones are closely related to the murine IgG2b/1 beta receptor extracellular domains, but the intracellular domains are unrelated. The receptors expressed in COS cells show a preference for IgG1 among IgG subtypes and no affinity for IgM, IgA or IgE. Abundant expression of the RNAs was detected in myeloid cell lines and placenta.

Expression and characterization of a truncated murine Fc gamma receptor

We have isolated a recombinant secreted Fc gamma R beta molecule by deletion of the transmembrane and cytoplasmic domains encoding sequence from a Fc gamma R beta 1 cDNA clone, and insertion of the truncated cDNA into a eukaryotic expression vector, pcEXV-3. To express and amplify the production of the truncated Fc gamma R beta molecule, we transfected the truncated cDNA plasmid into a dihydrofolate reductase-minus CHO cell line along with a dhfr minigene, and amplified the gene products with methotrexate. The resulting cell line secretes 2-3 micrograms/ml/24 h of truncated Fc gamma R beta, which can be readily purified by affinity chromatography on IgG-Sepharose. The truncated Fc gamma R beta has a Mr of 31-33,000 on SDS-PAGE and is glycosylated. N-glycosidase F cleavage reduces the Mr to 19,000, consistent with the size of the truncated product, 176 amino acid residues. There are two disulfide bonds in the protein. Binding of immune complexes formed between DNP20BSA and anti-DNP mAbs reveals better binding of IgG1 aggregates than that of IgG2b and IgG2a aggregates. The binding of the immune complexes was somewhat better at more acidic pH, in contrast to previous experiments with binding of purified Fc gamma R to immune complex-coated beads. We were surprised to observe that the truncated Fc gamma R beta did not react with the anti-Fc gamma R mAb 6B7C. Previous work had shown that 6B7C reacts with Fc gamma R on immunoblots, fails to bind to the surface of resting B cells and peritoneal macrophages, but does bind to macrophage cell lines and LPS-stimulated B cells. We show, by binding of mAb 6B7C to a peptide conjugate, that the 6B7C epitope lies within residues 169-183 of the intact Fc gamma R beta, which is just outside the plasma membrane. The availability of the truncated Fc gamma R beta in microgram quantities should facilitate further analysis of structure and function of these receptors.

Mlsa determinants: relationship to Fc gamma receptor and tissue distribution

We have analysed the genetic relationship between Mlsa and Fc gamma R in mice. Using the Fc gamma R-specific DNA probes, we were unable to detect a restriction fragment length polymorphism (RFLP) which is consistent in DNA derived from Mlsa strains and which differed from that of Mlsb strains, while we could see a polymorphism that distinguishes Ly17.1 from Ly17.2, alleles of the Fc gamma R. These results strongly suggest that Mlsa is neither a product of the alpha Fc gamma R nor of the beta Fc gamma R gene. Furthermore, we have re-examined the tissue distribution of Mlsa determinants using a major histocompatibility complex (MHC) class II antigen-positive T-cell tumour as well as a pure population of bone marrow derived macrophages of Mlsa genotype. Both these cell types were recognized to varying degrees by alloreactive cells; however, none of them expressed functionally detectable Mlsa determinants. We conclude from our studies that Mlsa is a highly stimulatory self peptide that is exclusively expressed in B cells.