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The Journal of Immunology, 2008, 180, 618 -629
Copyright © 2008 by The American Association of Immunologists, Inc.
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Critical but Overlapping Role of Fc{gamma}RIII and Fc{gamma}RIV in Activation of Murine Neutrophils by Immobilized Immune Complexes1,2

Zoltán Jakus*, Tamás Németh*, J. Sjef Verbeek{dagger} and Attila Mócsai3,*

* Department of Physiology, School of Medicine, Semmelweis University, Budapest, Hungary; and {dagger} Department of Human Genetics, Leiden University Medical Center, Leiden, The Netherlands


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 Disclosures
 References
 
Immune complex-induced activation of neutrophils through cell surface FcRs plays a central role in the pathogenesis of autoimmune inflammatory diseases. These diseases are often modeled using genetically modified mice. However, in contrast to the number of studies on human cells, the identity of FcRs involved in immune complex activation of murine neutrophils is at present unknown. Furthermore, little is known about the cellular functions mediated by the recently identified murine Fc{gamma}RIV. In this study, we tested the identity of FcRs involved in the activation of neutrophils by plate-bound immune complexes, using various knockout mouse strains, function-blocking mAbs, or the combination of both approaches. Activation of murine neutrophils by immobilized IgG immune complexes was abrogated in FcR {gamma}-chain-deficient cells, but not by the single or combined deficiency of the {gamma}-chain-associated Fc{gamma}RI and Fc{gamma}RIII, or by blocking Abs against either Fc{gamma}RIII or Fc{gamma}RIV alone. However, treatment of Fc{gamma}RIII-deficient neutrophils with Fc{gamma}RIV-blocking Abs or simultaneous blocking of Fc{gamma}RIII and Fc{gamma}RIV in wild-type cells completely inhibited the immune complex-induced cellular responses. In parallel studies, activation of human neutrophils by immobilized immune complexes was abrogated by blocking Abs against either Fc{gamma}RIIA or Fc{gamma}RIIIB alone. Taken together, neutrophil activation by immobilized immune complexes requires the murine Fc{gamma}RIII/Fc{gamma}RIV or the human Fc{gamma}RIIA/Fc{gamma}RIIIB molecules. Although both of the two human receptors are required for this response, the two murine receptors play overlapping, redundant roles. These results promote our understanding of autoimmune diseases and identify an IgG-dependent cellular function of Fc{gamma}RIV.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 Disclosures
 References
 
Engagement of FcRs by IgG-opsonized microorganisms is one of the major routes of pathogen recognition by neutrophils, triggering a series of antimicrobial elimination mechanisms such as generation of reactive oxygen species or exocytosis of intracellular granules. During a normal immune response, this activation only occurs at the site of microbial invasion. However, during autoimmune diseases, generation of autoantibodies against host Ags leads to immune complex deposition and concomitant FcR-mediated neutrophil activation, which is targeted against the host tissues. Examples of such immune complex-induced, neutrophil-mediated autoimmune inflammatory diseases include rheumatoid arthritis (1, 2) and autoantibody-induced acute glomerulonephritis (3). In both these cases, pathogenic immune complexes are formed on a solid surface: the cartilaginous lining of the articular cavity (4) or the glomerular basement membrane. Surface-bound immune complexes thus appear to provoke strong activation of neutrophils and concomitant tissue damage.

Human neutrophils express a number of receptors for the Fc portion of IgG which may be involved in their activation by immobilized immune complexes. These include the ITAM-bearing single-chain Fc{gamma}RIIA and the GPI-linked Fc{gamma}RIIIB (human neutrophils do not express Fc{gamma}RIIIA, a transmembrane receptor associated with the ITAM-bearing FcR {gamma}-chain). Besides these low-affinity Fc{gamma}Rs, human neutrophils also express low levels of the high-affinity Fc{gamma}RI under resting conditions, and this expression increases upon activation of the cells by inflammatory stimuli (5, 6, 7).

The contribution of the above-mentioned Fc{gamma}Rs (in particular, Fc{gamma}RIIA and Fc{gamma}RIIIB) to activation of human neutrophils by immune complexes has been tested by a number of groups (8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19). The overall conclusion from those studies is that both Fc{gamma}RIIA and Fc{gamma}RIIIB participate in the activation of human neutrophils by immune complexes (8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19). However, the relative contribution of the two receptors appears to vary between the various experimental conditions because some studies reported a predominant role of Fc{gamma}RIIA (8, 9), while Fc{gamma}RIIIB was suggested to be the most important receptor involved under other conditions (10, 11, 12, 13), and both receptors appeared to be involved, supposedly in a cooperative manner, in yet other assay systems (11, 12, 13, 14, 15, 16, 17, 18, 19). Despite these differences, the receptors involved in immune complex activation of human neutrophils are relatively well-characterized.

During the last several years, genetically modified mice have become major tools of dissecting immunological and inflammatory processes at the molecular level. Studies on such animals strongly contributed to our understanding of basic biological mechanisms and disease pathogeneses, and they pointed to novel targets of therapeutic intervention. Genetically engineered animals provide two major advantages over the human system: the introduction of germline genomic mutations allows the functional analysis of any chosen protein and they can also be subjected to in vivo disease models, enabling detailed molecular analysis of disease pathogenesis in the context of the entire organism.

A number of studies have shown that neutrophils play a critical role in mouse models of immune complex-mediated diseases such as autoimmune arthritis (1, 20, 21) or autoantibody-induced glomerulonephritis (22, 23, 24). FcRs also likely participate in the development of these diseases because the genetic deficiency of the FcR {gamma}-chain completely protects mice from autoimmune arthritis (25, 26, 27, 28) or autoantibody-induced glomerulonephritis (29, 30, 31, 32, 33, 34, 35, 36), and these diseases are also attenuated (though not completely abolished) in Fc{gamma}RIII-deficient mice (26, 27, 37, 38, 39, 40). Though it is difficult to directly prove, it is likely that the FcRs on the surface of neutrophils participate in these autoimmune diseases.

Given the likely role of neutrophil FcRs in autoimmune disease models, it would be important to know what receptors are involved in immune complex-induced activation of mouse neutrophils. In contrast to human cells, murine neutrophils appear to primarily express FcR {gamma}-chain-associated Fc{gamma}Rs. Traditionally, the most prominent member of this group was thought to be Fc{gamma}RIII, a low-affinity Fc{gamma}R highly expressed on murine neutrophils. In contrast, the expression of the high-affinity activating Fc{gamma}RI (which is also an FcR {gamma}-chain-associated molecule) is questionable: while resting murine neutrophils (similar to the human cells) fail to express high levels of this molecule (40), there have been no studies on the expression of Fc{gamma}RI in activated murine neutrophils. In addition to these conventional Fc{gamma}Rs, recent reports have also described a novel low-affinity Fc{gamma}R (termed Fc{gamma}RIV) in mice (41, 42, 43). The expression of Fc{gamma}RIV is restricted to the myeloid lineage with neutrophils being one of the most highly expressing cell types (42). Murine neutrophils also likely express a number of other FcR {gamma}-chain-associated molecules such as PIR-A (44, 45, 46), OSCAR (47), and LILRC1 (48). Though no Ig binding of these receptors have been reported, their direct or indirect contribution to immune complex activation of murine neutrophils cannot be excluded.

Despite the extensive characterization of cell surface expression of FcRs and related molecules on murine neutrophils, and in sharp contrast to the large number of papers on the role of individual Fc{gamma}Rs in immune complex activation of human neutrophils (8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19), there is practically no information available on the identity of FcR(s) involved in immune complex-induced activation of murine neutrophils. The only related reports studied the role of the FcR {gamma}-chain in the activation of murine neutrophils by immobilized immune complexes (3) or in the initial tethering of these cells under physiological shear rates (49). Although these studies indicate a role for FcR {gamma}-chain-associated receptors in immune complex-induced activation of neutrophils, they do not allow the identification of the receptor(s) involved.

There are very few reports on the functional role of the recently identified Fc{gamma}RIV molecule and most of those studies deal with the contribution of Fc{gamma}RIV to autoantibody-induced in vivo processes such as autoimmune thrombocytopenia (42), nephrotoxic nephritis (36), or B cell depletion triggered by monoclonal anti-CD20 Abs (50). Unfortunately, no experiments aimed at the identification of the Fc{gamma}RIV-bearing cell types responsible for the reported findings have been performed in these studies. Furthermore, though a recent study has suggested a functional role of Fc{gamma}RIV in certain IgE immune complex-induced macrophage functions (43), there are no published reports on the role of Fc{gamma}RIV in any cellular responses triggered by its principal ligand, IgG. Despite the very high expression of Fc{gamma}RIV on neutrophils, the function of this receptor on these cells is also entirely unclear.

The above issues strongly hinder our understanding of the cellular mechanisms behind immune complex-induced autoimmune inflammatory diseases. This prompted us to test the role of various Fc{gamma}Rs in neutrophil activation by immobilized IgG immune complexes. Our results indicate that Fc{gamma}RIII and the recently identified Fc{gamma}RIV molecule play critical but overlapping roles in immune complex activation of murine neutrophils. Immune complex activation of human neutrophils, in contrast, requires both Fc{gamma}RIIA and Fc{gamma}RIIIB.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 Disclosures
 References
 
Animals

Fc{gamma}RI-deficient (Fcgr1tm1Sjv/tm1Sjv, referred to as Fc{gamma}R1–/–) (40) and Fc{gamma}RIII-deficient (Fcgr3tm1Sjv/tm1Sjv, referred to as Fc{gamma}R3–/–) (51) single mutant mice have been described and were used to generate Fc{gamma}R1–/–Fc{gamma}R3–/– double mutant animals. FcR {gamma}-chain-deficient (Fcer1gtm1Rav/tm1Rav, referred to as FcR{gamma}–/–) (52) mice were purchased from Taconic Farms. DNAX-activating protein of 12 kDa (DAP12)-deficient (Tyrobptm1Lll/tm1Lll, referred to as DAP12–/–) (53) mice were provided by L. Lanier (University of California, San Francisco, CA) and the DNAX Research Institute. All mice were backcrossed to the C57BL/6 genetic background for six or more generations. Animals were housed in individually sterile ventilated cages in a conventional facility. All animal experiments were approved by the Semmelweis University Animal Experimentation Review Board.

Neutrophil isolation

Mouse neutrophils were isolated from the bone marrow by hypotonic lysis followed by Percoll (GE Healthcare) gradient centrifugation as described (54). Human neutrophils were isolated from venous blood of healthy volunteers by Ficoll (GE Healthcare) gradient centrifugation followed by hypotonic lysis of RBC as described (55). Neutrophil isolation was performed at room temperature using sterile and endotoxin-free reagents. Cells were kept at room temperature in Ca2+- and Mg2+-free medium until use (usually <30 min) and prewarmed to 37°C before activation. Neutrophil assays were performed at 37°C in HBSS (Invitrogen Life Technologies) supplemented with 20 mM HEPES (pH 7.4).

FcR-blocking Abs

The monoclonal FcR blocking Abs and their isotype controls used in this study are described in Table I. The Abs were purchased from BD Biosciences except for the anti-mouse Fc{gamma}RIV mAb (42) and the anti-human Fc{gamma}RIIA mAb (56, 57) provided by J. Ravetch (Rockefeller University, New York, NY) and J. Leusen (University Medical Center, Utrecht, The Netherlands), respectively. Wherever possible, isotype controls binding to other neutrophil cell surface receptors were used, or the obtained results were confirmed by Fab preparation of the FcR-blocking mAbs (see Results).


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  		Table I. FcR-blocking mAbs used in this study

 
In experiments using FcR-blocking Abs, neutrophils were preincubated with the indicated blocking Abs or their isotype controls at 1 or 4 µg Ab/106 murine or human cells, respectively. After a 50-min incubation at room temperature, neutrophils were washed and stimulated in functional assays.

Preparation of plate-bound immune complexes

Unless otherwise stated, immobilized immune complexes were formed using human serum albumin (HSA)4 Ag and rabbit polyclonal anti-HSA IgG Abs (both reagents were obtained from Sigma-Aldrich). Immune complex-covered surfaces were prepared by incubating 96-well Maxisorp F96 (Nalge Nunc International) ELISA plates with 20 µg/ml HSA in 50 mM carbonate/bicarbonate buffer (pH 9.6) for 1 h, followed by blocking with 10% FCS (Invitrogen Life Technologies) in PBS for 1 h and a further 1-h incubation with anti-HSA Abs at a 1/400 dilution (~10 µg/ml HSA-specific IgG) in 10% FCS. Parallel wells prepared without the HSA Ag-incubation step served as controls. In some experiments, immune complexes were generated using 20 µg/ml OVA or human lactoferrin (Lfr; both obtained from Sigma-Aldrich) as Ags and 1/400 rabbit polyclonal anti-OVA or anti-Lfr (both obtained from Sigma-Aldrich) as Abs.

Because of the poor optical features of 96-well ELISA plates, an alternative protocol was used to immobilize immune complexes for microscopic observation. Tissue-culture dishes (BD Biosciences) were incubated with 0.1 mg/ml poly-L-lysine (Sigma-Aldrich) for 30 min, washed, then incubated with 2.5% glutaraldehyde (Sigma-Aldrich) for another 15 min and washed again. The dishes were then incubated with 20 µg/ml HSA for 1 h, then blocked and incubated with anti-HSA Abs as described above.

F(ab')2 of the anti-HSA Abs were prepared using the ImmunoPure F(ab')2 preparation kit (Pierce Biotechnology) according to the manufacturer’s instructions. Optimal conditions for Ab digestion were determined by SDS-PAGE under both reducing and nonreducing conditions. F(ab')2 were then used instead of full IgG to prepare the immune complex-coated surface. The binding of the anti-HSA full IgG or F(ab')2 to the capturing HSA Ag was determined by a direct ELISA using peroxidase-labeled Abs directed against either the Fc or the Fab portion of rabbit IgG (Jackson ImmunoResearch Laboratories).

Functional assays

Neutrophil activation by immobilized immune complexes was achieved by plating the cells on the immune complex-coated surfaces without any additional stimulus. Other routes of cell activation (cytochalasin B (CB) plus fMLP, TNF on fibrinogen, immobilized anti-CD18 Abs, PMA) were performed as described (54, 55, 58, 59). For respiratory burst and degranulation assays, 1 x 105 human or 4 x 105 murine neutrophils/well were used. Release of superoxide was determined by a cytochrome c reduction test as described (59). Unless otherwise stated, unstimulated control values were subtracted from the stimulated superoxide release. Degranulation of gelatinase (a marker of the specific and gelatinase granules) was determined by an in-gel gelatinase zymography assay. Neutrophil supernatants collected after a 30-min stimulation were centrifuged to remove any remaining cells, supplemented with 4x concentrated nonreducing Laemmli sample buffer and run on an 8% SDS-polyacrylamide gel containing 0.1% gelatin (Sigma-Aldrich). Gels were renatured in 2.5% Triton X-100 and incubated overnight at 37°C in 200 mM NaCl, 5 mM CaCl2, 50 mM Tris (pH 7.4). Digestion of gelatin prepolymerized into the gel was visualized by Coomassie Blue staining.

For microscopic observations, 2 x 106 human or 5 x 106 murine neutrophils in 1 ml of assay medium were plated on immune complex-covered 3.5-cm tissue-culture dishes. After 20 min of incubation, the cells were cooled and fixed by the addition of 100 µl of formalin (Sigma-Aldrich) to the assay medium. Nonadherent cells were allowed to settle and images were taken on a Leica DMI 6000B inverted microscope with a x20 phase contrast objective, connected to a Leica DFC480 CCD camera.

Presentation of the data

Kinetic curves of superoxide release assays represent three or more independent experiments with similar results. Error bars represent SD of triplicate or quadruplicate readings. Bar graphs of immune complex-induced superoxide release data summarize three or more independent experiments, where stimulus-induced responses have been expressed in percent of that in wild-type or isotype control mAb-treated samples. Error bars represent SD from the indicated number of experiments. Statistical analyses of these data have been performed using the Student paired two-population t test. A difference was considered statistically significant at p < 0.05. Gelatinase release data and microscopic pictures are representative of three or more independent experiments.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 Disclosures
 References
 
Neutrophil activation by immobilized immune complexes

We first set up an assay system (similar to those used in Refs. 3 and 60) for neutrophil activation by immobilized immune complexes and performed a general characterization of this assay. To this end, 96-well ELISA plates were coated with HSA, blocked with FCS, and then incubated with rabbit polyclonal anti-HSA IgG Abs. As shown in Fig. 1A, human neutrophils plated on such HSA-anti-HSA immune complex surfaces responded with a robust respiratory burst that could not be observed if either HSA or anti-HSA was omitted from the surface preparation protocol. Neutrophil activation could also be induced by the combination of OVA with anti-OVA or Lfr with anti-Lfr, but not by noncorresponding Ag-Ab combinations (Fig. 1B), confirming the specificity of the assay. As seen in Fig. 1, A and B, control samples did not induce considerable superoxide release. Hence, and to simplify the presentation of our results, control values (obtained in the absence of the capturing Ag) will be subtracted from the values of immune complex-stimulated samples and only the thus obtained (stimulus-induced) responses will be presented in the following figures.


Figure 1
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  		FIGURE 1. Neutrophil activation by immobilized immune complexes. A and B, Respiratory burst of human neutrophils plated on immobilized immune complexes formed by various Ag-Ab pairs. C and D, The effect of removal of the Fc portion of the immune complex-forming anti-HSA Abs on superoxide production of human neutrophils (C) and on Ab binding to the HSA Ag surface detected by a direct ELISA using peroxidase-labeled anti-Fc or anti-Fab Abs (D). E, Comparison of the responses of human neutrophils to immobilized immune complexes and various other routes of cell activation (10 µM CB followed by 1 µM fMLP; 20 ng/ml human TNF on a fibrinogen-coated (Fbg) surface; 20 µg/ml anti-human CD18 Ab-coated surface; 100 nM PMA). F and G, Nonoxidative responses (gelatinase release (F) and cell spreading (G)) of human neutrophils plated on immobilized immune complexes. H–J, Comparison of the responses of wild-type murine neutrophils to immobilized immune complexes and various other routes of cell activation (10 µM CB followed by 3 µM fMLP; 50 ng/ml murine TNF on a Fbg surface; 20 µg/ml anti-murine CD18 Ab-coated surface; 100 nM PMA). Unstimulated control values were subtracted in C, E, and H. PMN, Neutrophils (polymorphonuclear cells); PO, peroxidase.

 
Immune complex activation likely proceeds through cell surface receptors recognizing the Fc portion of the complex-forming Abs used. To confirm this, we prepared F(ab')2 of the anti-HSA Abs and used them to generate plate-bound immune complexes. As expected, immune complexes with F(ab')2 failed to induce functional responses of neutrophils (Fig. 1C). This defect was not due to the lack of F(ab')2 binding to HSA because similar levels of F(ab')2 and full IgG were detected on the immune complex surfaces with peroxidase-labeled Abs against the Fab portion of rabbit IgG (Fig. 1D). In contrast, peroxidase-labeled Abs against the Fc portion labeled full IgG but not F(ab')2 immune complexes (Fig. 1D), confirming efficient digestion of the full Abs. These results indicate that the respiratory burst triggered by immobilized full IgG immune complexes indeed proceeds through recognition of the Fc portion of the IgG molecules used.

Comparison of the responses of neutrophils to various routes of cell activation (Fig. 1E) revealed that the amount of superoxide production triggered by immobilized immune complexes exceeded that induced by the bacterial tripeptide fMLP in the presence of CB (a Gi protein-mediated response (54, 55)), by TNF stimulation on a fibrinogen surface (an integrin-dependent response (58, 61)), or by plate-bound anti-CD18 mAbs (which requires both integrins and low-affinity Fc{gamma}Rs (59, 62)), and approached that induced by PMA, the most robust nonphysiological activator of these cells.

We next tested whether neutrophil activation by plate-bound immune complexes activates cellular functions other than the respiratory burst. Immobilized immune complexes were able to trigger the release of gelatinase, a constituent of secondary (specific) and tertiary (gelatinase) granules (63), though this response was slightly lower than that induced by other routes of cell activation tested (Fig. 1F). Neutrophils plated on immobilized immune complexes also spread over the activation surface (Fig. 1G). On average, ~60–80% of human neutrophils spread on immune complex-coated surfaces (data not shown), compared with the spreading of 30–50% of TNF-stimulated and 80–95% of PMA-stimulated human neutrophils on the fibrinogen surface (64).

All the above experiments were performed on human neutrophils. Because our further studies involved the analysis of genetically modified mice, we also tested the effect of immobilized immune complexes on murine neutrophils. Similar to human cells (Fig. 1E), plating murine neutrophils on immobilized immune complexes induced a respiratory burst which exceeded that induced by other physiological stimuli and approached the response triggered by PMA (Fig. 1H). Immune complex activation of murine neutrophils also induced gelatinase release, which was comparable to that of TNF-stimulated cells on the fibrinogen surface but less than that induced by fMLP stimulation of CB-treated cells or by the nonphysiological activator PMA (Fig. 1H). Murine neutrophils also spread over the immune complex surface (Fig. 1J). The average percentage of spreading under these conditions (60–80%) was significantly higher than the 10–20% spreading of TNF-stimulated murine neutrophils on the fibrinogen surface and approached the 80–95% spreading response triggered by PMA (58). Taken together, plating human or murine neutrophils on immobilized immune complexes induces a robust activation through engagement of cell surface FcRs.

Immune complex activation requires the FcR {gamma}-chain

A number of FcRs and related molecules associate with an ITAM-bearing transmembrane adapter molecule, the FcR {gamma}-chain (52, 65, 66). This adapter may be required for the signal transduction of these receptors because the FcR {gamma}-chain is necessary for the expression of the associated receptors on the cell surface, and/or because the phosphorylation of its ITAM may be involved in triggering downstream signal transduction events. As shown in Fig. 2A, immune complex stimulation failed to trigger a respiratory burst response from FcR {gamma}-chain-deficient (FcR{gamma}–/–) neutrophils. In four independent experiments (Fig. 2B), the FcR{gamma}–/– mutation caused an average inhibition of 99.6 ± 3.7% of the response of wild-type cells (p = 1.4 x 10–5). The FcR {gamma}-chain was also required for immune complex-induced gelatinase release (Fig. 2C) and spreading of neutrophils over the immobilized immune complex surface (Fig. 2D). In contrast, FcR{gamma}–/– neutrophils responded normally to the nonphysiological-activating agent PMA (Fig. 2E).


Figure 2
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  		FIGURE 2. The FcR {gamma}-chain is required for immune complex activation of murine neutrophils. A and B, Superoxide production of wild-type (WT) and FcR{gamma}–/– murine neutrophils plated on immune complex (IC)-covered surfaces. A shows a representative experiment while B summarizes results from four independent experiments expressed in percent of WT. C and D, Gelatinase release (C) and spreading (D) of FcR{gamma}–/– neutrophils plated on IC-covered surfaces. E, Respiratory burst of FcR{gamma}–/– neutrophils triggered by 100 nM PMA. Unstimulated control values were subtracted in A, B, and E. *, p < 0.05 compared with WT.

 
Neutrophils also express DAP12, an FcR {gamma}-chain-related ITAM-bearing transmembrane adapter molecule (67). However, the respiratory burst of DAP12–/– neutrophils plated on immobilized immune complexes was indistinguishable (4.5 ± 9.2% inhibition; p = 0.49; n = 3) from that of wild-type cells (Fig. 3, A and B). Similarly, the immune complex-induced degranulation and spreading response (Fig. 3, C and D), as well as the PMA-induced respiratory burst (Fig. 3E) also proceeded normally in the DAP12–/– cells. Hence, unlike integrin-mediated neutrophil functions (67), FcR-induced responses do not require DAP12.


Figure 3
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  		FIGURE 3. DAP12 is dispensable for immune complex activation. A and B, Superoxide production of wild-type (WT) and DAP12–/– murine neutrophils plated on immune complex (IC)-covered surfaces. A shows a representative experiment whereas B summarizes results from three independent experiments expressed in percent of WT. C and D, Gelatinase release (C) and spreading (D) of DAP12–/– neutrophils plated on IC-covered surfaces. Unstimulated control values were subtracted in A, B, and E.

 
Normal immune complex-induced responses in the absence of Fc{gamma}RI and Fc{gamma}RIII

Of the best known FcR {gamma}-chain-associated murine FcRs, Fc{gamma}RIII would be expected to be primarily involved in immune complex activation of murine neutrophils, given its high expression on the surface of these cells and its low IgG-binding affinity, suggesting that it is only activated by high-valency immune complexes. In contrast to our expectations, the Fc{gamma}RIII-deficient (Fc{gamma}R3–/–) murine neutrophils showed nearly normal respiratory burst when plated on immobilized immune complexes (Fig. 4A), though statistical analysis of nine independent experiments (Fig. 4B) revealed a minor but statistically significant inhibition (14.5 ± 18.1% inhibition; p = 0.032).


Figure 4
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  		FIGURE 4. Fc{gamma}RI and Fc{gamma}RIII are dispensable for immune complex activation of murine neutrophils. A and B, Superoxide production of wild-type (WT), Fc{gamma}R1–/–, and Fc{gamma}R3–/– single mutant and Fc{gamma}R1–/–Fc{gamma}R3–/– double mutant murine neutrophils plated on immune complex (IC)-covered surfaces. A shows a representative experiment while B summarizes results from four to nine independent experiments expressed in percent of WT. C and D, Gelatinase release (C) and spreading (D) of the various mutant neutrophils plated on IC-covered surfaces. E, Respiratory burst of the various mutant neutrophils triggered by 100 nM PMA. Unstimulated control values were subtracted in A, B, and E. *, p < 0.05 compared with WT.

 
Murine neutrophils fail to express the high-affinity Fc{gamma}RI under resting conditions (40) but, similar to the human receptor (5, 6, 7), Fc{gamma}RI may become expressed upon stimulation of the cells. Hence, we tested whether the genetic deficiency of Fc{gamma}RI would affect immune complex-induced neutrophil responses. As shown in Fig. 4, A and B, the respiratory burst of Fc{gamma}RI-deficient (Fc{gamma}R1–/–) neutrophils was indistinguishable from that of wild-type cells (1.3 ± 12.3% inhibition; p = 0.84; n = 4). We next tested the effect of the combined deficiency of both Fc{gamma}RI and Fc{gamma}RIII. The Fc{gamma}R1–/–Fc{gamma}R3–/– double mutant neutrophils also released nearly normal amount of superoxide when plated on immune complex surfaces (Fig. 4A), though a moderate but statistically significant decrease (21.1 ± 10.0% inhibition; p = 0.0036; n = 6) could be observed (Fig. 4B).

We also tested other functional responses in the same mutants. Genetic deficiency of Fc{gamma}RIII or Fc{gamma}RI, either alone or in combination, did not abrogate the immune complex-induced gelatinase release (Fig. 4C) or spreading (Fig. 4D) response, nor did it affect the responses of the cells to the PMA stimulus (Fig. 4E). Taken together, neither Fc{gamma}RIII nor Fc{gamma}RI, alone or in combination, play a critical role in neutrophil functions triggered by immobilized IgG immune complexes, though Fc{gamma}RIII likely plays a partial role in this response.

Blocking Fc{gamma}RIV abrogates immune complex responses of Fc{gamma}R3–/– neutrophils

A novel FcR {gamma}-chain-associated murine Fc{gamma}R, named Fc{gamma}RIV, has recently been identified by several groups (41, 42, 43). The fact that this receptor is highly expressed on neutrophils raised the possibility that it may be responsible for the Fc{gamma}RIII-independent component of neutrophil activation by plate-bound immune complexes. This was tested using wild-type or Fc{gamma}R3–/– neutrophils incubated with function-blocking mAbs against Fc{gamma}RIV (Fig. 5, A and B). As expected from prior experiments, Fc{gamma}R3–/– neutrophils pretreated with an isotype-matched control mAb showed a moderate but statistically significant decrease of immune complex-induced superoxide generation (18.1 ± 7.8% inhibition, p = 0.0024; n = 6). Preincubation of wild-type neutrophils with Fc{gamma}RIV-blocking Abs (clone 9E9) also led to a moderate but statistically significant decrease (22.9 ± 15.5% inhibition; p = 0.0080; n = 7). Importantly, pretreatment of Fc{gamma}R3–/– neutrophils with the Fc{gamma}RIV-blocking Abs completely abolished the immune complex-induced respiratory burst of the cells (99.3 ± 4.3% inhibition; p = 3.4 x 10–8; n = 6). Similarly, preincubation of Fc{gamma}R3–/– neutrophils with Fc{gamma}RIV-blocking Abs also abrogated the degranulation (Fig. 5C) and spreading (Fig. 5D) responses. Again, the Fc{gamma}R3–/– mutation or the Fc{gamma}RIV blockade alone had no effect except for a significant but incomplete decrease of gelatinase release in Fc{gamma}RIV Ab-treated wild-type cells (Fig. 5C). The responses to PMA were normal in all samples tested (Fig. 5E).


Figure 5
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  		FIGURE 5. Blocking Fc{gamma}RIV inhibits immune complex activation of Fc{gamma}RIII-deficient neutrophils. A and B, Superoxide production of wild-type (WT) or Fc{gamma}R3–/– murine neutrophils pretreated with an Fc{gamma}RIV-blocking mAb or its isotype control (Iso Co) and plated on immune complex (IC)-covered surfaces. A shows a representative experiment while B summarizes results from six to seven independent experiments expressed in percent of WT Iso Co. C and D, Gelatinase release (C) and spreading (D) of neutrophils from the indicated sources plated on IC-covered surfaces. E, Respiratory burst of neutrophils triggered by 100 nM PMA. Unstimulated control values were subtracted in A, B, and E. *, p < 0.05 compared with WT Iso Co.

 
Blocking both Fc{gamma}RIII and Fc{gamma}RIV abrogates immune complex-induced responses of wild-type murine neutrophils

The above experiments showed complete inhibition of neutrophil responses by Fc{gamma}RIV-blocking Abs when used on the Fc{gamma}R3–/– genetic background. We next tested whether a similar inhibition can also be attained by Ab-mediated blocking of both Fc{gamma}RIII and Fc{gamma}RIV. As shown in Fig. 6, A and B, an Fc{gamma}RII/III-blocking Ab (clone 2.4G2) did not affect immune complex-induced respiratory burst of wild-type murine neutrophils (3.0 ± 12.5% increase; p = 0.58; n = 6) while the Fc{gamma}RIV blockade led to a marginal inhibition that did not prove to be statistically significant in this series of experiments (13.2 ± 14.5% inhibition; p = 0.17; n = 4). Combination of both Fc{gamma}RII/III- and Fc{gamma}RIV-blocking Abs, however, drastically reduced the respiratory burst triggered by immobilized immune complexes (88.8 ± 3.3% inhibition; p = 1.4 x 10–5; n = 4). Again, combination of the two Abs led to complete inhibition of immune complex-induced degranulation (Fig. 6C) or spreading (Fig. 6D) response while neither of the Abs alone were effective except for an incomplete inhibition of the gelatinase release in the Fc{gamma}RIV Ab-treated cells (Fig. 6C). The PMA-induced respiratory burst was not affected by any of these interventions (Fig. 6E).


Figure 6
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  		FIGURE 6. Inhibition of immune complex activation of neutrophils by simultaneously blocking Fc{gamma}RIII and Fc{gamma}RIV. A and B, Superoxide production of wild-type murine neutrophils pretreated with Fc{gamma}RII/III-blocking and/or Fc{gamma}RIV-blocking mAbs or their respective isotype controls (Iso Co) and plated on immune complex (IC)-covered surfaces. A shows a representative experiment while B summarizes results from four to six independent experiments expressed in percent of 2x Iso Co. C and D, Gelatinase release (C) and spreading (D) of neutrophils from the indicated sources plated on IC-covered surfaces. E, Respiratory burst of neutrophils triggered by 100 nM PMA. Unstimulated control values were subtracted in A, B, and E. *, p < 0.05 compared with 2x Iso Co.

 
Taken together, the experiments presented in Figs. 2–6 indicate that the responses of murine neutrophils to immobilized IgG immune complexes (respiratory burst, degranulation, and spreading) are mediated by two FcR {gamma}-chain-coupled low-affinity Fc{gamma}Rs, Fc{gamma}RIII and Fc{gamma}RIV. These two proteins play a mostly redundant, overlapping role during the activation of the cells.

Fc{gamma}RIIA-blocking Abs abrogate immune complex activation of human neutrophils

In contrast to murine neutrophils that rely on FcR {gamma}-chain-associated low-affinity Fc{gamma}Rs for recognition of IgG immune complexes, human neutrophils do not possess any such receptors but instead express two unique low-affinity Fc{gamma}Rs, Fc{gamma}RIIA and Fc{gamma}RIIIB. Of these, Fc{gamma}RIIA has an ITAM in its cytoplasmic domain and thus is able to be expressed and signal without the FcR {gamma}-chain. Fc{gamma}RIIIB, in contrast, is a GPI-linked protein that is anchored to the outer leaflet of the plasma membrane through a GPI moiety. Although there have been a number of studies trying to identify the role of these molecules in immune complex-induced activation of human neutrophils (8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19), variable results have been obtained in the different experimental systems (see Introduction). For this reason and to facilitate the comparison of experimental results on human and mouse neutrophils, we next attempted to identify the human FcRs involved in the same immobilized immune complex activation assays that was used in our previous mouse studies.

As shown in Fig. 7, A and B, preincubation of human neutrophils with an Fc{gamma}RIIA-blocking Ab (clone IV.3) completely abolished the respiratory burst of cells plated on immobilized IgG immune complexes (96.1 ± 4.7% inhibition; p = 6.1 x 10–8; n = 6) relative to cells treated with an isotype-matched control Ab. Similar results were obtained using an Fab (59) of the Fc{gamma}RIIA-blocking Ab (not shown). Blocking of Fc{gamma}RIIA also inhibited the degranulation (Fig. 7C) and spreading (Fig. 7D) response of immune complex-bound human neutrophils. However, the PMA-induced respiratory burst was not affected by the Fc{gamma}RIIA-blocking Abs (Fig. 7E).


Figure 7
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  		FIGURE 7. Fc{gamma}RIIA is required for immune complex activation of human neutrophils. A and B, Superoxide production of human neutrophils pretreated with an Fc{gamma}RIIA-blocking mAb or its isotype control (Iso Co) and plated on immune complex (IC)-covered surfaces. A shows a representative experiment while B summarizes results from six independent experiments expressed in percent of Iso Co. C and D, Gelatinase release (C) and spreading (D) of Ab-treated neutrophils plated on IC-covered surfaces. E, Respiratory burst of neutrophils triggered by 100 nM PMA. Unstimulated control values were subtracted in A, B, and E. *, p < 0.05 compared with Iso Co.

 
Blockade of Fc{gamma}RIIIB inhibits immune complex activation of human neutrophils

Next, we tested the effect of blocking Fc{gamma}RIIIB on immune complex-induced responses of human neutrophils. As shown in Fig. 8, A and B, an Fc{gamma}RIIIB-blocking mAb (clone 3G8) strongly inhibited the respiratory burst of human neutrophils plated on immobilized immune complex surfaces (84.3 ± 19.4% inhibition; p = 5.3 x 10–6; n = 8). The Fc{gamma}RIIIB-blocking Ab also prevented gelatinase release (Fig. 8C) and spreading of the cells (Fig. 8D) in response to activation by immobilized immune complexes. However, the PMA-induced respiratory burst was not affected by the Ab treatment (Fig. 8E).


Figure 8
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  		FIGURE 8. Fc{gamma}RIIIB is required for immune complex activation of human neutrophils. A and B, Superoxide production of human neutrophils pretreated with an Fc{gamma}RIII-blocking mAb or its isotype control (Iso Co) and plated on immune complex (IC)-covered surfaces. A shows a representative experiment while B summarizes results from eight independent experiments expressed in percent of Iso Co. C and D, Gelatinase release (C) and spreading (D) of Ab-treated neutrophils plated on IC-covered surfaces. E, Respiratory burst of neutrophils triggered by 100 nM PMA. Unstimulated control values were subtracted in A, B, and E. *, p < 0.05 compared with Iso Co.

 
Taken together, activation of human neutrophils by immobilized IgG immune complexes is mediated by Fc{gamma}RIIA and Fc{gamma}RIIIB. In contrast to the results with murine cells, however, both receptors are required for activation of human neutrophils.


   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
Disclosures
 References
 
Ligation of neutrophil FcRs by immobilized immune complexes is thought to be one of the major triggers of tissue destruction during autoimmune inflammatory diseases. Although such diseases are often studied in animal models using genetically modified mice, very little is known about what cell surface receptors are involved in activation of murine neutrophils by immobilized immune complexes. This may be due to the fact that an apparently very important murine Fc{gamma}R, Fc{gamma}RIV, has only recently been identified (41, 42, 43). Our functional studies focusing on the role of this receptor, as well as murine Fc{gamma}RIII and Fc{gamma}RI, revealed that neutrophil activation by immobilized IgG immune complexes is mediated by Fc{gamma}RIII and Fc{gamma}RIV (Figs. 5 and 6) but not by Fc{gamma}RI (Fig. 4). The relative contribution of Fc{gamma}RIII and Fc{gamma}RIV appears to be rather balanced, because genetic deficiency or Ab-mediated blocking of either one of them leads to a moderate (10–20%) inhibition, whereas deleting or blocking both receptors completely abrogates the responses of the cells (Figs. 4–6). These experiments identify the FcRs involved in activation of murine neutrophils by immobilized immune complexes and provide the first evidence for an IgG-mediated cellular function of the recently described Fc{gamma}RIV molecule.

Despite the similar role of murine Fc{gamma}RIII and Fc{gamma}RIV in our neutrophil assays, other studies indicate that there are significant differences between these molecules. Although both receptors are highly expressed on neutrophils and macrophages (42, 43), NK cells and mast cells appear to more specifically express Fc{gamma}RIII (42, 43) while Fc{gamma}RIV expression is somewhat higher on dendritic cells (43). There also appears to be a moderate difference between the level of induction of the two proteins upon stimulation of macrophages with proinflammatory agents (43). Sequence analysis of Fc{gamma}RIV also revealed a YEEP motif in the cytoplasmic tail of Fc{gamma}RIV but not Fc{gamma}RIII in all rodent species tested (43). Phosphorylation of the tyrosine residue in this YEEP motif may be involved in initiation of Fc{gamma}RIV-specific intracellular signals through binding of Src homology 2 domain-containing signaling molecules, as suggested by the cross-linking-induced phosphorylation of the Fc{gamma}RIV YEEP motif and its association with Crk-L (43).

Besides the differences in their expression and signaling motifs, Fc{gamma}RIII and Fc{gamma}RIV also differ in their specificity and affinity toward various Ig classes and subclasses (42, 43). Although both receptors are able to bind IgG2a and IgG2b molecules, the affinity of Fc{gamma}RIV toward these IgG subclasses is 30- to 50-fold higher than that of Fc{gamma}RIII (42, 43). In contrast, murine IgG1 binds to Fc{gamma}RIII but not Fc{gamma}RIV (42, 43). Hence, Fc{gamma}RIV appears to be specific for IgG2 subclasses whereas Fc{gamma}RIII is skewed toward recognition of IgG1. Interestingly, Fc{gamma}RIV (but not Fc{gamma}RIII) is able to bind IgE molecules of the "b" but not the "a" allotype, and IgEb, but not IgEa immune complexes are able to activate macrophages in an Fc{gamma}RIV-dependent manner (43). Hence, Fc{gamma}RIV, but not Fc{gamma}RIII, may also be involved in IgE-mediated inflammatory processes in certain mouse strains.

In light of these differences, the apparently similar functions of Fc{gamma}RIII and Fc{gamma}RIV in our experiments may be somewhat surprising. There are, however, indications in the literature that these molecules do not perform uniquely specific functions in vivo either. Genetic deficiency of Fc{gamma}RIII attenuates the development of autoantibody-mediated arthritis, though the published effects of the Fc{gamma}R3–/– mutation range from nearly complete (37) through considerable (26, 38, 39) to moderate inhibition (27). Interestingly, genetic deficiency of the FcR {gamma}-chain leads to complete inhibition of arthritis development in all reported studies (25, 26, 27, 28). Fc{gamma}R1–/– animals are protected from arthritis-induced cartilage destruction (39, 40) but not from the inflammatory reaction itself (26, 39). The overall impression from these studies is that an FcR {gamma}-chain-associated receptor other than Fc{gamma}RI is able to compensate for the lack of Fc{gamma}RIII during the development of autoimmune arthritis. Genetic deficiency of Fc{gamma}RIII also attenuates autoantibody-induced glomerulonephritis, but again the different studies report variable phenotypes ranging from nearly complete (31) through intermediate (30) to negligible (36) levels of protection from disease. However, deletion of the FcR {gamma}-chain completely eliminated autoantibody-induced glomerulonephritis in all reported studies (29, 30, 31, 32, 33, 34, 35, 36). Hence, also in the case of autoantibody-induced glomerulonephritis, an FcR {gamma}-chain-associated receptor other than Fc{gamma}RIII appears to play an important pathogenetic role, though currently available studies provide conflicting data about whether this compensating receptor is Fc{gamma}RI (30) or Fc{gamma}RIV (36). Taken together, various autoimmune diseases are likely mediated by both Fc{gamma}RIII and another FcR {gamma}-chain-associated receptor, and it is reasonable to assume that this latter receptor is, at least under certain conditions, the Fc{gamma}RIV molecule.

Though studies on the in vivo role of Fc{gamma}RIV are scarce at the moment, these reports also support an overlapping role between Fc{gamma}RIII and Fc{gamma}RIV. Although Fc{gamma}RIV-blocking mAbs inhibited IgG2-mediated experimental immune thrombocytopenia in vivo, the inhibition was only partial and did not reach the level of protection caused by the genetic deficiency of the FcR {gamma}-chain (42), and a similar disease induced by anti-platelet mAb of the IgG1 subclass was entirely dependent on Fc{gamma}RIII rather than Fc{gamma}RIV (42). Hence, in a "real world" scenario when both IgG1 and IgG2 autoantibodies are present, both Fc{gamma}RIII and Fc{gamma}RIV would be expected to contribute to disease pathogenesis. Furthermore, while Kaneko et al. (36) suggested Fc{gamma}RIV rather than Fc{gamma}RIII to be primarily involved in autoimmune glomerulonephritis, other groups showed considerable protection of Fc{gamma}R3–/– mice from the disease under very similar conditions (30, 36). Again, the overall impression from these glomerulonephritis studies would be that both Fc{gamma}RIII and Fc{gamma}RIV play an important role in this disease.

A possible or even likely explanation for the functional overlap between Fc{gamma}RIII and Fc{gamma}RIV in the above in vivo experiments and our in vitro studies would be the different isotype specificity of the two receptors. A spontaneous polyclonal autoimmune reaction likely generates both IgG1 and IgG2 autoantibodies, and the polyclonal Ab preparation used in our in vitro studies also likely includes both IgG subclasses. It is then possible that IgG1 molecules primarily activate Fc{gamma}RIII whereas IgG2 molecules preferentially activate Fc{gamma}RIV. Although this could be tested in vitro using immune complexes prepared with mAbs of defined IgG subclasses, we believe that our assay system using polyclonal Abs much better reflect the situation in a real autoimmune reaction.

Although the involvement of various FcRs in in vivo autoimmune models has been quite extensively tested, it is mostly unclear what cell types use these receptors to mediate the autoimmune inflammatory reaction. Neutrophils may be one such cell type. This possibility is supported by our findings that immobilized immune complexes strongly activate these cells (Fig. 1) and that the role of Fc{gamma}RIII and Fc{gamma}RIV in our in vitro assays is in agreement with their supposed role in in vivo disease pathogenesis.

In the experiments presented in Fig. 6, an Fc{gamma}RII/III-blocking mAb (clone 2.4G2) was used to block Fc{gamma}RIII function. It should be mentioned that this Ab blocks both murine Fc{gamma}RII and Fc{gamma}RIII. However, because mice only express the inhibitory (Fc{gamma}RIIB) but not the activating (Fc{gamma}RIIA) isoform of Fc{gamma}RII and the effect of this mAb was very similar to that of the Fc{gamma}R3–/– mutation (compare Figs. 5 and 6), it is unlikely that the inhibitory effect of the 2.4G2 mAb was due to inhibition of an Fc{gamma}RII isoform. The 2.4G2 mAb has also been shown to bind to Fc{gamma}RIV in heterologous expression systems. However, the effect of this mAb in our experiments closely mirrored that of the genetic deficiency of Fc{gamma}RIII, and an additional Fc{gamma}RIV-blocking mAb was clearly required for a considerable inhibition of the immune complex-induced neutrophil responses. Furthermore, even a 4-fold higher concentration (4 µg/106 cells) of 2.4G2 failed to affect the immune complex-induced respiratory burst of wild-type neutrophils despite a strong inhibition of the Fc{gamma}RIII-mediated response triggered by plate-bound anti-CD18 mAbs (59) (data not shown). Hence, it is unlikely that the 2.4G2 mAb caused a significant inhibition of Fc{gamma}RIV function in our experiments. It is interesting to note, however, that the 2.4G2 mAb blocked IgEb-mediated phagocytosis of macrophages (36) raising the possibility that this Ab interferes with IgEb but not IgG binding to Fc{gamma}RIV.

Though neither Fc{gamma}RIII nor Fc{gamma}RIV appeared to have any major nonredundant role in the experiments presented in this article, it should be noted that the immune complex-induced gelatinase release was consistently lower in samples treated with Fc{gamma}RIV-blocking Abs alone (Figs. 5C and 6C). Though this inhibition ranged from significant to moderate across the number of experiments performed, these results suggest that Fc{gamma}RIV may be more directly involved in initiating the degranulation response upon neutrophil activation by plate-bound immune complexes. Further studies, such as detailed kinetic analyses and the determination of the release of the various other exocytic compartments (63), will be required to further clarify this issue. Nevertheless, Fc{gamma}RIII and Fc{gamma}RIV do appear to play strongly overlapping functions in most assay systems tested.

Taken together, the experiments presented in this article indicate that Fc{gamma}RIII and Fc{gamma}RIV play a redundant, overlapping role in activation of murine neutrophils by IgG immune complexes. This is in contrast with the human situation where both Fc{gamma}RIIA and Fc{gamma}RIIIB are required. These results will promote our understanding of the mechanism of autoimmune inflammation in mice and its extrapolation to the pathogenesis of human disease. They also raise novel questions, such as whether human and murine neutrophils use similar or different signal transduction mechanism during activation by immobilized immune complexes. Furthermore, this study provides the first evidence for an IgG-mediated cellular function of the recently identified Fc{gamma}RIV molecule.


   Acknowledgments
 
We thank Lewis Lanier and the DNAX Research Institute for the DAP12–/– mice; Jeffrey Ravetch and Jeanette Leusen for FcR-blocking mAbs; Krisztina Makara and Edina Simon for colony management and expert technical assistance; Árpád Mikesy for mouse colony maintenance; and Eric Brown for initial suggestions related to the immune complex activation assay.


   Disclosures
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Disclosures
References
 
The authors have no financial conflict of interest.


   Footnotes
 
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

1 Z.J. and A.M. designed the work, interpreted the results, and wrote the paper. Z.J., T.N., and A.M. performed the experiments and analyzed the data. J.S.V. provided experimental tools (Fc{gamma}RI and Fc{gamma}RIII-deficient mice). A.M. supervised the project. Back

2 This work was supported by the Hungarian Scientific Research Fund (OTKA T046409, to A.M.), the Hungarian Office for Research and Technology (NKFP-A1-0069/2006, to A.M.), and the U.S. National Institutes of Health (R03 TW006831, to A.M.). A.M. is an International Senior Research Fellow of the Wellcome Trust and a European Molecular Biology Organization/Howard Hughes Medical Institute Scientist. Z.J. and A.M. are recipients of Bolyai Research Fellowships from the Hungarian Academy of Sciences. Back

3 Address correspondence and reprint requests to Dr. Attila Mócsai, Department of Physiology, School of Medicine, Semmelweis University, P. O. Box 259, 1444 Budapest, Hungary. E-mail address: mocsai{at}puskin.sote.hu Back

4 Abbreviations used in this paper: HSA, human serum albumin; Lfr, lactoferrin; DAP12, DNAX-activating protein of 12 kDa; CB, cytochalasin B. Back

Received for publication July 23, 2007. Accepted for publication October 22, 2007.


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 Introduction
 Materials and Methods
 Results
 Discussion
 Disclosures
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