Distinct genes encode 6 human receptors for IgG (hFc␥Rs), 3 of which have 2 or 3 polymorphic variants. The specificity and affinity of individual hFc␥Rs for the 4 human IgG subclasses is unknown. This information is critical for antibodybased immunotherapy which has been increasingly used in the clinics. We investigated the binding of polyclonal and monoclonal IgG1, IgG2, IgG3, and IgG4 to Fc␥RI; Fc␥RIIA, IIB, and IIC; Fc␥RIIIA and IIIB; and all known polymorphic variants. Wild-type and low-fucosylated IgG1 anti-CD20 and anti-RhD mAbs were also examined. We found that (1) IgG1 and IgG3 bind to all hFc␥Rs; (2) IgG2 bind not only to Fc␥RIIA H131 , but also, with a lower affinity, to Fc␥RIIA R131 4 Other FcRs are inserted in the outer layer of the plasma membrane by a glycosylphosphatidylinositol (GPI) anchor and contain no signaling motif. 5 FcRs have been associated with many antibodydependent diseases 6 and are key molecules in antibody-based immunotherapy. These include the treatment, for instance, of non-Hodgkin lymphomas by mouse/human chimeric IgG1 anti-CD20 antibodies 7 and the prevention of hemolytic disease of the newborn by a mixture of polyclonal IgG1 and IgG3 anti-RhD antibodies (eg, Rophylac). Therapeutic antibodies are, however, potentially harmful, as exemplified by a recent clinical trial using IgG4 anti-CD28 antibodies.Four human subclasses of IgG are produced in different amounts in response to various antigens. T-dependent protein antigens elicit primarily IgG1 and IgG3 antibodies, whereas T-independent carbohydrate antigens elicit primarily IgG2 antibodies. Chronic antigen stimulation, as in allergic desensitization, elicits IgG4 antibodies. The biological activities of each subclass of IgG are poorly known. IgG receptors (Fc␥Rs) are strikingly numerous in humans. They comprise high-affinity and low-affinity receptors. 8 Both high-affinity and low-affinity Fc␥Rs bind IgGimmune complexes with a high avidity, but only high-affinity Fc␥Rs bind monomeric IgG. There is one high-affinity IgG receptor in humans, hFc␥RI (CD64), and 2 families of low-affinity IgG receptors, hFc␥RIIA, IIB, and IIC (CD32), and hFc␥RIIIA and IIIB (CD16). hFc␥RI and hFc␥RIIIA are FcR␥-associated activating receptors, hFc␥RIIA and hFc␥RIIC are single-chain activating receptors, hFc␥RIIB are single-chain inhibitory receptors, and hFc␥RIIIB are GPI-anchored receptors whose function is uncertain. 1 The multiplicity of hFc␥Rs is further increased by a series of polymorphisms in their extracellular domains (reviewed in van Sorge et al 9 ). Two alleles of the gene encoding hFc␥RIIA generate 2 variants differing at position 131, named low-responder (H 131 ) and high-responder (R 131 ). 10 The H 131 and R 131 alleles are differentially distributed in whites, Japanese, and Chinese. 11 Two alleles of the gene-encoding hFc␥RIIIA generate 2 variants differing at 23 and hFc␥RIIIB NA2 to SLE in Japanese people. 24 The subclass specificity of hFc␥Rs has been investigated since the 1980s, that is, at a time when the complexity of hFc␥R...

Anaphylaxis is a life-threatening hyperacute immediate hypersensitivity reaction. Classically, it depends on IgE, FcεRI, mast cells, and histamine. However, anaphylaxis can also be induced by IgG antibodies, and an IgG1-induced passive type of systemic anaphylaxis has been reported to depend on basophils. In addition, it was found that neither mast cells nor basophils were required in mouse models of active systemic anaphylaxis. Therefore, we investigated what antibodies, receptors, and cells are involved in active systemic anaphylaxis in mice. We found that IgG antibodies, FcγRIIIA and FcγRIV, platelet-activating factor, neutrophils, and, to a lesser extent, basophils were involved. Neutrophil activation could be monitored in vivo during anaphylaxis. Neutrophil depletion inhibited active, and also passive, systemic anaphylaxis. Importantly, mouse and human neutrophils each restored anaphylaxis in anaphylaxis-resistant mice, demonstrating that neutrophils are sufficient to induce anaphylaxis in mice and suggesting that neutrophils can contribute to anaphylaxis in humans. Our results therefore reveal an unexpected role for IgG, IgG receptors, and neutrophils in anaphylaxis in mice. These molecules and cells could be potential new targets for the development of anaphylaxis therapeutics if the same mechanism is responsible for anaphylaxis in humans.

FcγRIV is a recently identified mouse activating receptor for IgG2a and IgG2b that is expressed on monocytes, macrophages, and neutrophils; herein it is referred to as mFcγRIV. Although little is known about mFcγRIV, it has been proposed to be the mouse homolog of human FcγRIIIA (hFcγRIIIA) because of high sequence homology. Our work, however, has revealed what we believe to be new properties of mFcγRIV that endow this receptor with a previously unsuspected biological significance; we have shown that it is a low-affinity IgE receptor for all IgE allotypes. Although mFcγRIV functioned as a high-affinity IgG receptor, mFcγRIV-bound monomeric IgGs were readily displaced by IgE immune complexes. Engagement of mFcγRIV by IgE immune complexes induced bronchoalveolar and peritoneal macrophages to secrete cytokines, suggesting that mFcγRIV may be an equivalent of human FcεRI(αγ), which is expressed by macrophages and neutrophils and especially in atopic individuals, rather than an equivalent of hFcγRIIIA, which has no affinity for IgE. Using mice lacking 3 FcγRs and 2 FcεRs and expressing mFcγRIV only, we further demonstrated that mFcγRIV promotes IgE-induced lung inflammation. These data lead us to propose a mouse model of IgE-induced lung inflammation in which cooperation exists between mast cells and mFcγRIV-expressing lung cells. We therefore suggest that a similar cooperation may occur between mast cells and hFcεRI-expressing lung cells in human allergic asthma.

IgE and IgE receptors (FcϵRI) are well-known inducers of allergy. We recently found in mice that active systemic anaphylaxis depends on IgG and IgG receptors (FcγRIIIA and FcγRIV) expressed by neutrophils, rather than on IgE and FcϵRI expressed by mast cells and basophils. In humans, neutrophils, mast cells, basophils, and eosinophils do not express FcγRIIIA or FcγRIV, but FcγRIIA. We therefore investigated the possible role of FcγRIIA in allergy by generating novel FcγRIIA-transgenic mice, in which various models of allergic reactions induced by IgG could be studied. In mice, FcγRIIA was sufficient to trigger active and passive anaphylaxis, and airway inflammation in vivo. Blocking FcγRIIA in vivo abolished these reactions. We identified mast cells to be responsible for FcγRIIA-dependent passive cutaneous anaphylaxis, and monocytes/macrophages and neutrophils to be responsible for FcγRIIA-dependent passive systemic anaphylaxis. Supporting these findings, human mast cells, monocytes and neutrophils produced anaphylactogenic mediators after FcγRIIA engagement. IgG and FcγRIIA may therefore contribute to allergic and anaphylactic reactions in humans.

Key Points• Human Fc␥RI can trigger antibody-induced inflammatory arthritis, thrombocytopenia, airway inflammation, and systemic anaphylaxis.• Human Fc␥RI can trigger antibody-mediated immunotherapy of mouse metastatic melanoma.Receptors for the Fc portion of IgG (Fc␥Rs) are mandatory for the induction of various IgG-dependent models of autoimmunity, inflammation, anaphylaxis, and cancer immunotherapy. A few Fc␥Rs have the ability to bind monomeric IgG: high-affinity mouse mFc␥RI, mFc␥RIV, and human hFc␥RI. All others bind IgG only when aggregated in complexes or bound to cells or surfaces: low-affinity mouse mFc␥RIIB and mFc␥RIII and human hFc␥RIIA/B/C and hFc␥RIIIA/B. Although it has been proposed that high-affinity Fc␥Rs are occupied by circulating IgG, multiple roles for mFc␥RI and mFc␥RIV have been reported in vivo. However, the potential roles of hFc␥RI that is expressed on monocytes, macrophages, and neutrophils have not been reported. In the present study, we therefore investigated the role of hFc␥RI in antibody-mediated models of disease and therapy by generating hFc␥RI-transgenic mice deficient for multiple endogenous FcRs. hFc␥RI was sufficient to trigger autoimmune arthritis and thrombocytopenia, immune complex-induced airway inflammation, and active and passive systemic anaphylaxis. We found monocyte/macrophages to be responsible for thrombocytopenia, neutrophils to be responsible for systemic anaphylaxis, and both cell types to be responsible for arthritis induction.

K/BxN serum-induced passive arthritis was reported to depend on the activation of mast cells, triggered by the activating IgG receptor FcγRIIIA, when engaged by IgG1 autoantibodies present in K/BxN serum. This view is challenged by the fact that FcγRIIIA-deficient mice still develop K/BxN arthritis and because FcγRIIIA is the only activating IgG receptor expressed by mast cells. We investigated the contribution of IgG receptors, IgG subclasses, and cells in K/BxN arthritis. We found that the activating IgG2 receptor FcγRIV, expressed only by monocytes/macrophages and neutrophils, was sufficient to induce disease. K/BxN arthritis occurred not only in mast cell-deficient Wsh mice, but also in mice whose mast cells express no activating IgG receptors. We propose that at least two autoantibody isotypes, IgG1 and IgG2, and two activating IgG receptors, FcγRIIIA and FcγRIV, contribute to K/BxN arthritis, which requires at least two cell types other than mast cells, monocytes/macrophages, and neutrophils.

• Neutrophils are necessary and sufficient for mAbinduced therapy of subcutaneous syngeneic or xenograft tumors in mice.• Antitumor immunoglobulin G mAb therapy requires a Syk-dependent FcgRinduced killing of tumors by neutrophils.

Background Animal models have demonstrated that allergen-specific IgG confers sensitivity to systemic anaphylaxis that relies on IgG receptors (FcγRs). Mouse IgG2a and IgG2b bind activating FcγRI, FcγRIII and FcγRIV, and inhibitory FcγRIIB; mouse IgG1 binds only FcγRIII and FcγRIIB. Although these interactions are of strikingly different affinities, these three IgG subclasses have been shown to enable induction of systemic anaphylaxis. Objective Determine which pathways control the induction of IgG1-, IgG2a- and IgG2b-passive systemic anaphylaxis. Methods Mice were sensitized with IgG1, IgG2a or IgG2b anti-TNP mAbs and challenged with TNP-BSA intravenously to induce systemic anaphylaxis that was monitored using rectal temperature. Anaphylaxis was evaluated in mice deficient for FcγRs, injected with mediator antagonists or in which basophils, monocyte/macrophages or neutrophils had been depleted. The expression of FcγRs was evaluated on these cells before and after anaphylaxis. Results Activating FcγRIII is the receptor primarily responsible for all three models of anaphylaxis, and subsequent down regulation of this receptor was observed. These models differentially relied on histamine release and on the contribution of mast cells, basophils, macrophages and neutrophils. Strikingly, basophil contribution and histamine predominance in IgG1- and IgG2b-mediated anaphylaxis correlated with the ability of inhibitory FcγRIIB to negatively regulate these models of anaphylaxis. Conclusion We propose that the differential expression of inhibitory FcγRIIB on myeloid cells and its differential binding of IgG subclasses controls the contributions of mast cells, basophils, neutrophils and macrophages to IgG subclass-dependent anaphylaxis. Collectively, our results unravel novel complexities in the involvement and regulation of cell populations in IgG-mediated reactions in vivo.