Neonatal Fc receptor expression in macrophages is indispensable for IgG homeostasis

ABSTRACT

The maintenance of the homeostasis of immunoglobulin G (IgG) represents a fundamental aspect of humoral immunity that has direct relevance to the successful delivery of antibody-based therapeutics. The ubiquitously expressed neonatal Fc receptor (FcRn) salvages IgG from cellular degradation following pinocytic uptake into cells, conferring prolonged in vivo persistence on IgG. However, the cellular sites of FcRn function are poorly defined. Pinocytic uptake is a prerequisite for FcRn-mediated IgG salvage, prompting us to investigate the consequences of IgG uptake and catabolism by macrophages, which represent both abundant and highly pinocytic cells in the body. Site-specific deletion of FcRn to generate mice harboring FcRn-deficient macrophages results in IgG hypercatabolism and ~threefold reductions in serum IgG levels, whereas these effects were not observed in mice that lack functional FcRn in B cells and dendritic cells. Consistent with the degradative activity of FcRn-deficient macrophages, depletion of these cells in FcRn-deficient mice leads to increased persistence and serum levels of IgG. These studies demonstrate a pivotal role for FcRn-mediated salvage in compensating for the high pinocytic and degradative activities of macrophages to maintain IgG homeostasis.     

Ovarian cancer cells polarize macrophages toward a tumor-associated phenotype

Tumor-associated macrophages (TAM) may have tumor-promoting activity, but it is not clear how their phenotype is achieved. In this study, we demonstrate that ovarian cancer cells switch cocultured macrophages to a phenotype similar to that found in ovarian tumors. Tumor cells caused dynamic changes in macrophage cytokine, chemokine, and matrix metalloprotease mRNA, and protein-inducing mediators that are found in human cancer. Macrophage mannose, mannose receptor, and scavenger receptors (SR-As) were also up-regulated by coculture, but not by conditioned medium. To further validate the model, we studied SR-A regulation on TAM in vitro and in vivo. Coculture of murine macrophages from mice deficient in TNF-alpha or its receptors revealed that TNF-alpha was key to SR-A induction via its p75 receptor. SR-A expression was also reduced in TAM from ovarian cancers treated with anti-TNF-alpha Abs or grown in TNF-alpha(-/-) mice. Chemical communication between tumor cells and macrophages may be important in regulating the cancer cytokine microenvironment.     

Estradiol treatment redirects the isotype of the autoantibody response and prevents the development of autoimmune arthritis

A number of clinical and experimental observations have been made relating elevated estrogen levels with the amelioration of autoimmune diseases, yet questions remain about the levels required for efficacy as well as the mechanism of disease inhibition. Using the collagen-induced arthritis (CIA) model, we have studied the effects of physiological, sustained levels of 17beta-estradiol in preventing the development of autoimmune arthritis and analyzed the changes in the autoimmune response. Using time-release pellets of 17beta-estradiol, arthritis development was significantly inhibited in three different strains of CIA-susceptible mice compared with the effect of placebo treatment, and serum estradiol levels similar to those of mice in estrus were found to be equally effective as higher estradiol concentrations. Analysis of the autoimmune response in the estradiol-treated mice indicated that T cell production of IFN-gamma was markedly decreased, and significant decreases were also observed in levels of IL-10 and GM-CSF produced by lymph nodes cells from estradiol-treated mice. Although the total IgG anti-CII response was only minimally affected by estrogen treatment, a significant reduction in the levels of IgG2a anti-CII Abs and an increase in the levels of IgG1 anti-CII Abs were observed in estradiol-treated mice. These data indicate that estradiol treatment altered the Th profile of the autoimmune T cell response, which, in turn, altered the production of IgG Abs to an isotype that is poor at fixing complement, an important component in the immunopathogenesis of CIA.     

FcRn overexpression in transgenic mice results in augmented APC activity and robust immune response with increased diversity of induced antibodies

Our previous studies have shown that overexpression of bovine FcRn (bFcRn) in transgenic (Tg) mice leads to an increase in the humoral immune response, characterized by larger numbers of Ag-specific B cells and other immune cells in secondary lymphoid organs and higher levels of circulating Ag-specific antibodies (Abs). To gain additional insights into the mechanisms underlying this increase in humoral immune response, we further characterized the bFcRn Tg mice. Our Western blot analysis showed strong expression of the bFcRn transgene in peritoneal macrophages and bone marrow derived dendritic cells; and a quantitative PCR analysis demonstrated that the expression ratios of the bFcRn to mFcRn were 2.6- and 10-fold in these cells, respectively. We also found that overexpression of bFcRn enhances the phagocytosis of Ag-IgG immune complexes (ICs) by both macrophages and dendritic cells and significantly improves Ag presentation by dendritic cells. Finally, we determined that immunized bFcRn mice produce a much greater diversity of Ag-specific IgM, whereas only the levels, but not the diversity, of IgG is increased by overexpression of bFcRn. We suggest that the increase in diversity of IgG in Tg mice is prevented by a selective bias towards immunodominant epitopes of ovalbumin, which was used in this study as a model antigen. These results are also in line with our previous reports describing a substantial increase in the levels of Ag-specific IgG in FcRn Tg mice immunized with Ags that are weakly immunogenic and, therefore, not affected by immunodominance.     

The mannose receptor fails to enhance processing and presentation of a glycoprotein antigen in transfected fibroblasts

One function proposed for the mannose receptor found on dendritic cells as well as on macrophages and hepatic endothelial cells is in enhancing uptake and processing of glycoprotein antigens for presentation by major histocompatibility complex (MHC) class II molecules. In this study, a direct assessment of the possible role of the mannose receptor in this process was made in the absence of other endocytic receptors that can internalize glycoproteins. Presentation of RNase A and B peptides was compared in transfected fibroblasts coexpressing the mannose receptor and MHC class II molecules. RNase B bears a high-mannose oligosaccharide and is a ligand for the mannose receptor, whereas RNase A is not glycosylated and is taken up by pinocytosis. Incubation of RNase A or B with the transfected cells resulted in identical stimulation of ribonuclease-specific T cells, indicating that endocytosis of the glycosylated protein by the mannose receptor does not enhance presentation of this antigen. The postulated role of the mannose receptor in presentation of glycoprotein-derived antigen is reevaluated in light of these results.     

Human immature dendritic cells efficiently bind and take up secretory IgA without the induction of maturation.

Immature dendritic cells (DC) reside in peripheral tissues, where they pick up and process incoming pathogens via scavenger receptors or FcR such as FcgammaR and FcepsilonR. At mucosal surfaces, IgA is the main Ig to protect the body from incoming pathogens. In addition, DC are present in high numbers at these sites. We detected expression of FcalphaR (CD89) on the CD14+ population of CD34+ progenitor-derived DC and on monocyte-derived DC (MoDC). However, CD89 expression was strongly decreased upon differentiation from monocyte to DC. We found only minimal binding of serum IgA to MoDC but strong binding of secretory IgA (SIgA). The SIgA binding to MoDC could not be blocked by anti-CD89 blocking Abs. DC efficiently internalized SIgA, but not serum IgA, and uptake of SIgA could be blocked by specific sugars or partially by Ab reactive with mannose receptor. Importantly, binding and uptake of SIgA was not accompanied by signs of DC maturation, such as increased expression of CD86 and CD83 or induction of cytokine secretion. These data indicate that SIgA can interact with DC not via CD89, but via carbohydrate-recognizing receptors like mannose receptor and suggest that uptake of SIgA-containing immune complexes by immature DC may be a mechanism to modulate mucosal immune responses.     

Characterization of peripheral blood acetylcholine receptor-binding B cells in experimental myasthenia gravis

In myasthenia gravis (MG), the neuromuscular transmission is impaired by antibodies (Abs) specific for muscle acetylcholine receptor (AChR). Anti-AChR Abs can be detected in the serum of MG patients, although their levels do not correlate with disease severity. In this study, we developed a flow cytometric assay for the detection of peripheral blood AChR-specific B cells to characterize B cell phenotypes associated with experimental autoimmune myasthenia gravis (EAMG). Alexa-conjugated AChR was used as a probe for AChR-specific B cells (B220+Ig+). Mice with EAMG had significantly elevated frequencies of AChR-specific IgG2+ and IgM+ B cells. While the frequencies of IgG2+ B cells and plasma anti-AChR IgG2 levels significantly correlated with the clinical grades of EAMG, the frequencies of IgM+ B cells and plasma anti-AChR IgM levels did not. These results indicate that the frequency of AChR-specific and IgG1+ (mouse IgG2 equivalent) peripheral blood B cells and anti-AChR IgG1 levels could be potential biomarkers for MG disease severity.     

Mannose receptor (MR) engagement by mesothelin GPI anchor polarizes tumor-associated macrophages and is blocked by anti-MR human recombinant antibody

Tumor-infiltrating macrophages respond to microenvironmental signals by developing a tumor-associated phenotype characterized by high expression of mannose receptor (MR, CD206). Antibody cross-linking of CD206 triggers anergy in dendritic cells and CD206 engagement by tumoral mucins activates an immune suppressive phenotype in tumor-associated macrophages (TAMs). Many tumor antigens are heavily glycosylated, such as tumoral mucins, and/or attached to tumor cells by mannose residue-containing glycolipids (GPI anchors), as for example mesothelin and the family of carcinoembryonic antigen (CEA). However, the binding to mannose receptor of soluble tumor antigen GPI anchors via mannose residues has not been systematically studied. To address this question, we analyzed the binding of tumor-released mesothelin to ascites-infiltrating macrophages from ovarian cancer patients. We also modeled functional interactions between macrophages and soluble mesothelin using an in vitro system of co-culture in transwells of healthy donor macrophages with human ovarian cancer cell lines. We found that soluble mesothelin bound to human macrophages and that the binding depended on the presence of GPI anchor and of mannose receptor. We next challenged the system with antibodies directed against the mannose receptor domain 4 (CDR4-MR). We isolated three novel anti-CDR4-MR human recombinant antibodies (scFv) using a yeast-display library of human scFv. Anti-CDR4-MR scFv #G11 could block mesothelin binding to macrophages and prevent tumor-induced phenotype polarization of CD206(low) macrophages towards TAMs. Our findings indicate that tumor-released mesothelin is linked to GPI anchor, engages macrophage mannose receptor, and contributes to macrophage polarization towards TAMs. We propose that compounds able to block tumor antigen GPI anchor/CD206 interactions, such as our novel anti-CRD4-MR scFv, could prevent tumor-induced TAM polarization and have therapeutic potential against ovarian cancer, through polarization control of tumor-infiltrating innate immune cells.     

Glycosylation and functionality of recombinant β-glucocerebrosidase from various production systems

The glycosylation of recombinant β-glucocerebrosidase, and in particular the exposure of mannose residues, has been shown to be a key factor in the success of ERT (enzyme replacement therapy) for the treatment of GD (Gaucher disease). Macrophages, the target cells in GD, internalize β-glucocerebrosidase through MRs (mannose receptors). Three enzymes are commercially available for the treatment of GD by ERT. Taliglucerase alfa, imiglucerase and velaglucerase alfa are each produced in different cell systems and undergo various post-translational or post-production glycosylation modifications to expose their mannose residues. This is the first study in which the glycosylation profiles of the three enzymes are compared, using the same methodology and the effect on functionality and cellular uptake is evaluated. While the major differences in glycosylation profiles reside in the variation of terminal residues and mannose chain length, the enzymatic activity and stability are not affected by these differences. Furthermore, the cellular uptake and in-cell stability in rat and human macrophages are similar. Finally, in vivo studies to evaluate the uptake into target organs also show similar results for all three enzymes. These results indicate that the variations of glycosylation between the three regulatory-approved β-glucocerebrosidase enzymes have no effect on their function or distribution.     

Natural antibodies to CCR5 from breast milk block infection of macrophages and dendritic cells with primary R5-tropic HIV-1

In the present study, we demonstrate that breast milk of 66% and 83% of HIV-seronegative and seropositive women, respectively, contains natural Abs of the secretory IgA and IgG isotypes directed against the CCR5 coreceptor for R5-tropic strains of HIV-1. Abs to CCR5 were affinity purified on a matrix to which a synthetic peptide corresponding to the second extracellular loop of CCR5 had been coupled. The purified Abs bound to the CCR5 peptide in a dose-dependent fashion and to both native CCR5 expressed by Chinese hamster ovary cells transfected with CCR5 gene, macrophages, and immature dendritic cells. Although the avidity differed, the amount of anti-CCR5 Abs did not significantly differ between breast milk of HIV-seropositive and -seronegative women. Purified anti-CCR5 Abs inhibited up to 75% infection of macrophages and dendritic cells with HIV(BaL) and HIV(JR-CSF). Our observations provide evidence for a role of natural Abs to CCR5 in breast milk in controlling transmissibility of HIV through breastfeeding.     

Mannosylation of Virus-Like Particles Enhances Internalization by Antigen Presenting Cells

Internalization of peptides by antigen presenting cells is crucial for the initiation of the adaptive immune response. Mannosylation has been demonstrated to enhance antigen uptake through mannose receptors, leading to improved immune responses. In this study we test the effect of surface mannosylation of protein-based virus-like particles (VLP) derived from Rabbit hemorrhagic disease virus (RHDV) on uptake by murine and human antigen presenting cells. A monomannoside and a novel dimannoside were synthesized and successfully conjugated to RHDV VLP capsid protein, providing approximately 270 mannose groups on the surface of each virus particle. VLP conjugated to the mannoside or dimannoside exhibited significantly enhanced binding and internalization by murine dendritic cells, macrophages and B cells as well as human dendritic cells and macrophages. This uptake was inhibited by the inclusion of mannan as a specific inhibitor of mannose specific uptake, demonstrating that mannosylation of VLP targets mannose receptor-based uptake. Consistent with mannose receptor-based uptake, partial retargeting of the intracellular processing of RHDV VLP was observed, confirming that mannosylation of VLP provides both enhanced uptake and modified processing of associated antigens.     

IL-4 modulates transcriptional control of the mannose receptor in mouse FSDC dendritic cells

The mannose receptor is a 175 kDa protein found on the surface of macrophages and dendritic cells whose functions include clearance of extracellular hydrolases, internalization of pathogens, and antigen capture. Receptor expression is closely linked to the functional state of these cells and is regulated by cytokines. Previous work has shown that treatment of macrophages and dendritic cells with interleukin-4 leads to increased mannose receptor expression. We have examined the mechanism of this IL-4-mediated up-regulation in the murine dendritic cell line FSDC. IL-4 increased mannose receptor activity, protein, and mRNA. The mannose receptor promoter was functional in FSDCs using transient transfection assays, and IL-4 treatment increased promoter activity 2.6-fold. The responsive region was localized to the proximal 228 bp. Electrophoretic mobility shift assays detected an IL-4-inducible protein that bound to the mannose receptor promoter at a site spanning the region between -147 and -108 bp. The sequence TTAC(N)4CACC (-135 and -124 bp) is similar to the IL-4 response region in the Fc receptor II. Mutation of the flanking TT and CC in this motif blocked IL-4 responsiveness and binding of the IL-4-induced mannose receptor binding protein. This protein does not appear to be STAT6 since neither an anti-STAT6 antibody nor a STAT6 consensus oligonucleotide altered factor binding.     

Aggregated bovine IgG inhibits mannose receptor expression of murine bone marrow-derived macrophages via activation

We previously described the presence of an inhibitory protein contained in the 20 to 40% (NH4)2SO4 precipitable fraction of FCS that down-regulates expression of mannose receptors on bone marrow-derived macrophages. We now identify aggregated bovine IgG as the main inhibitory component. Heat-aggregated bovine IgG was capable of down-regulating expression of the macrophage mannose receptor in a dose-dependent manner without inducing changes in ligand affinity whereas neither F(ab')2 fragments nor nonaggregated IgG displayed any inhibitory effect. Depleting of IgG from heat inactivated FCS by protein G affinity chromatography completely removes the inhibitory activity. Moreover, readdition of the Ig eluate from the protein G chromatography column restored inhibition in a dose-dependent manner. Macrophages were able to clear exogenously added aggregated bovine IgG, thus leading to loss of inhibitory activity in macrophage-conditioned media as compared to sham-conditioned media containing aggregated IgG. These results indicate that aggregated IgG down-regulates mannose receptor expression by macrophage activation via interaction with Fc-gamma R.     

Auto-oxidation and oligomerization of protein S on the apoptotic cell surface is required for Mer tyrosine kinase-mediated phagocytosis of apoptotic cells

Prompt phagocytosis of apoptotic cells prevents inflammatory and autoimmune responses to dying cells. We have previously shown that the blood anticoagulant factor protein S stimulates phagocytosis of apoptotic human B lymphoma cells by human monocyte-derived macrophages. In this study, we show that protein S must first undergo oxidative activation to stimulate phagocytosis. Binding of human protein S to apoptotic cells or to phosphatidylserine multilamellar vesicles promotes auto-oxidation of Cys residues in protein S, resulting in covalent, disulfide-linked dimers and oligomers that preferentially bind to and activate the human Mer tyrosine kinase (MerTK) receptor on the macrophages. The prophagocytic activity of protein S is eliminated when disulfide-mediated oligomerization is prevented, or when MerTK is blocked with neutralizing Abs. Protein S oligomerization is independent of phospholipid oxidation. The data suggest that membranes containing phosphatidylserine serve as a scaffold for protein S-protein S interactions and that the resulting auto-oxidation and oligomerization is required for the prophagocytic activity of protein S. In this way, apoptotic cells facilitate their own uptake by macrophages. The requirement for oxidative modification of protein S can explain why this abundant blood protein does not constitutively activate MerTK in circulating monocytes and tissue macrophages.     

Receptor-mediated endocytosis of fucosylated neoglycoprotein by macrophages

The characteristics of the recognition system involved in the receptor mediated endocytosis of the neoglycoprotein, fucose human serum albumin (HSA) were studied. It was found that (i) fucose-HSA showed strong affinity binding and uptake by various macrophages; (ii) binding was specific for L-fucose and D-mannose; (iii) binding was found to be inhibited by oxidant like H₂O₂ and swainsonine whereas it was elevated by dexamethasone; (iv) clearance of¹²⁵I-fucose-HSA was rapid and strongly inhibited by unlabelled fucose-HSA. Greater than 70% of fucose-HSA was found in liver and more than 60% of this was found in liver lysosomes; (v) uptake of fucose-HSA was thirty-fold more efficient in liver macrophages (Kupffer cells) than in hepatocytes; (vi) moreover, mannose-HSA and ovalbumin which are potent inhibitors of mannose/N-acetylglucosamine receptors inhibited clearance and uptake of fucose-HSA by liver as well as by isolated Kupffer cells suggesting the involvement of both fucose and mannose receptors or a single type of receptor having greater affinity for fucose-HSA than for mannose-HSA. These results emphasize the important role of fucose-terminated glycoproteins in site-specific drug targeting.     

Unique monoclonal antibodies define expression of Fc gamma RI on macrophages and mast cell lines and demonstrate heterogeneity among subcutaneous and other dendritic cells

The mouse Fc gamma RI is one of the most fundamentally important FcRs. It participates in different stages of immunity, being a low affinity receptor for T-independent IgG3 and yet a high affinity receptor for IgG2a, the product of a Th1 immune response. However, analysis of this receptor has been difficult due largely to the failure to generate specific Abs to this FcR. We have made use of the polymorphic differences between BALB/c and NOD/Lt mice to generate mAb specific for the Fc gamma RI of BALB/c and the majority of in-bred mouse strains. Three different mAb were obtained that detected Fc gamma RI encoded by the more common Fcgr1(a) and Fcgr1(b) alleles, and although they identified different epitopes, none inhibited the binding of IgG to Fc gamma RI. When bound to Fc gamma RI, these mAb induced calcium mobilization upon cross-linking. Several novel observations were made of the cellular distribution of Fc gamma RI. Resting and IFN-gamma-induced macrophages expressed Fc gamma RI as well as mast cell lines. Both bone marrow-derived and freshly isolated dendritic cells from spleen and lymph nodes expressed Fc gamma RI. A class of DC, uniquely found in s.c. lymph nodes, expressed the highest level of Fc gamma RI and also high levels of MHC class II, DEC205, CD40, and CD86, with a low level of CD8 alpha, corresponding to the phenotype for Langerhans-derived DC, which are highly active in Ag processing. Thus, in addition to any role in effector functions, Fc gamma RI on APC may act as a link between innate and adaptive immunities by binding and mediating the uptake of T-independent immune complexes for presentation, thereby assisting in the development of T-dependent immune responses.     

Archaeosomes varying in lipid composition differ in receptor-mediated endocytosis and differentially adjuvant immune responses to entrapped antigen

Archaeosomes prepared from total polar lipids extracted from six archaeal species with divergent lipid compositions had the capacity to deliver antigen for presentation via both MHC class I and class II pathways. Lipid extracts from Halobacterium halobium and from Halococcus morrhuae strains 14039 and 16008 contained archaetidylglycerol methylphosphate and sulfated glycolipids rich in mannose residues, and lacked archaetidylserine, whereas the opposite was found in Methanobrevibacter smithii, Methanosarcina mazei and Methanococcus jannaschii. Annexin V labeling revealed a surface orientation of phosphoserine head groups in M. smithii, M. mazei and M. jannaschii archaeosomes. Uptake of rhodamine-labeled M. smithii or M. jannaschii archaeosomes by murine peritoneal macrophages was inhibited by unlabeled liposomes containing phosphatidylserine, by the sulfhydryl inhibitor N-ethylmaleimide, and by ATP depletion using azide plus fluoride, but not by H. halobium archaeosomes. In contrast, N-ethylmaleimide failed to inhibit uptake of the four other rhodamine-labeled archaeosome types, and azide plus fluoride did not inhibit uptake of H. halobium or H. morrhuae archaeosomes. These results suggest endocytosis of archaeosomes rich in surface-exposed phosphoserine head groups via a phosphatidylserine receptor, and energy-independent surface adsorption of certain other archaeosome composition classes. Lipid composition affected not only the endocytic mechanism, but also served to differentially modulate the activation of dendritic cells. The induction of IL-12 secretion from dendritic cells exposed to H. morrhuae 14039 archaeosomes was striking compared with cells exposed to archaeosomes from 16008. Thus, archaeosome types uniquely modulate antigen delivery and dendritic cell activation.     

Mannose receptor-mediated uptake of ricin toxin and ricin A chain by macrophages. Multiple intracellular pathways for a chain translocation

The role of the high mannose carbohydrate chains in the mechanism of action of ricin toxin was investigated. Ricin is taken up by two routes in macrophages, by binding to cell surface mannose receptors, or by binding of the ricin galactose receptor to cell surface glycoproteins. Removal of carbohydrate from ricin by periodate oxidation led to a large loss in toxicity via both routes of uptake by an effect on the B chain not due to a loss of galactose binding affinity. These data suggest that the carbohydrate chains of ricin B chain may be required for full toxicity. The pathway of uptake of ricin by the macrophage mannose receptor was found to differ in several respects from uptake via the galactose-specific pathway. Analysis of intoxication of macrophages by ricin in the presence of ammonium chloride suggested that mannose receptor bound ligand passes through acidic vesicles prior to translocation, unlike galactose bound ligand. Intoxication by ricin via galactose-specific uptake was potentiated by swainsonine but not by castanospermine, suggesting that ricin may be attacked by an endogenous mannosidase within the cell, and that ricin passes through either a lysosomal or a Golgi compartment prior to translocation.     

MHC class I-related neonatal Fc receptor for IgG is functionally expressed in monocytes, intestinal macrophages, and dendritic cells

The neonatal Fc receptor (FcRn) for IgG, an MHC class I-related molecule, functions to transport IgG across polarized epithelial cells and protect IgG from degradation. However, little is known about whether FcRn is functionally expressed in immune cells. We show here that FcRn mRNA was identifiable in human monocytes, macrophages, and dendritic cells. FcRn heavy chain was detectable as a 45-kDa protein in monocytic U937 and THP-1 cells and in purified human intestinal macrophages, peripheral blood monocytes, and dendritic cells by Western blot analysis. FcRn colocalized in vivo with macrosialin (CD68) and Ncl-Macro, two macrophage markers, in the lamina propria of human small intestine. The heavy chain of FcRn was associated with the beta(2)-microglobulin (beta(2)m) light chain in U937 and THP-1 cells. FcRn bound human IgG at pH 6.0, but not at pH 7.5. This binding could be inhibited by human IgG Fc, but not Fab. FcRn could be detected on the cell surface of activated, but not resting, THP-1 cells. Furthermore, FcRn was uniformly present intracellularly in all blood monocytes and intestinal macrophages. FcRn was detectable on the cell surface of a significant fraction of monocytes at lower levels and on a small subset of tissue macrophages that expressed high levels of FcRn on the cell surface. These data show that FcRn is functionally expressed and its cellular distribution is regulated in monocytes, macrophages, and dendritic cells, suggesting that it may confer novel IgG binding functions upon these cell types relative to typical Fc gamma Rs: Fc gamma RI, Fc gamma RII, and Fc gamma RIII.