In the absence of other Fc receptors, Fc gamma RIIIA transmits a phagocytic signal that requires the cytoplasmic domain of its gamma subunit

Serum immunoglobulin and the threshold of Fc receptor-mediated immune activation

Antibodies can mediate immune recruitment or clearance of immune complexes through the interaction of their Fc domain with cellular Fc receptors. Clustering of antibodies is a key step in generating sufficient avidity for efficacious receptor recognition. However, Fc receptors may be saturated with prevailing, endogenous serum immunoglobulin and this raises the threshold by which cellular receptors can be productively engaged. Here, we review the factors controlling serum IgG levels in both healthy and disease states, and discuss how the presence of endogenous IgG is encoded into the functional activation thresholds for low- and high-affinity Fc receptors. We discuss the circumstances where antibody engineering can help overcome these physiological limitations of therapeutic antibodies. Finally, we discuss how the pharmacological control of Fc receptor saturation by endogenous IgG is emerging as a feasible mechanism for the enhancement of antibody therapeutics.

Genetic variations in low-to-medium-affinity Fcγ receptors and autoimmune neutropenia in early childhood in a Danish cohort

Autoimmune neutropenia (AIN) in early childhood is caused by autoantibodies directed against antigens on the neutrophil membrane and is a frequent cause of neutropenia in children. Association of AIN with Fcγ receptor (FCGR) 3B variants is well described. In this study, we investigate genetic variations in the FCGR locus and copy number variation of FCGR3B. A total of 130 antibody-positive AIN patients, 64 with specific anti-HNA-1a antibodies and 66 with broad-reacting anti-FcγRIIIb antibodies, were genotyped with a multiplex ligation probe assay and compared with healthy controls. Positive findings were confirmed with real-time q-PCR. We determined copy numbers of the FCGR2 and FCGR3 genes and the following SNPs: FCGR2A Q62W (rs201218628), FCGR2A H166R (rs1801274), FCGR2B I232T (rs1050501), FCGR3A V176F (rs396991), haplotypes for FCGR2B/C promoters (rs3219018/rs780467580), FCGR2C STOP/ORF and HNA-1 genotypes in FCGR3B (rs447536, rs448740, rs52820103, rs428888 and rs2290834). Generally, associations were antibody specific, with all associations being representative of the anti-HNA-1a-positive group, while the only association found in the anti-FcγRIIIb group was with the HNA-1 genotype. An increased risk of AIN was observed for patients with one copy of FCGR3B; the HNA genotypes HNA-1a, HNA-1aa or HNA-1aac; the FCGR2A 166H and FCGR2B 232I variations; and no copies of FCGR2B 2B.4. A decreased risk was observed for HNA genotype HNA-1bb; FCGR2A 166R; FCGR2B 232T; and one copy of FCGR2B promoter 2B.4. We conclude that in our Danish cohort, there was a strong association between variation in the FCGR locus and AIN. The findings of different genetic associations between autoantibody groups could indicate the presence of two different disease entities and disease heterogeneity.

Fc-Mediated Functions of Porcine IgG Subclasses

The pig is an important agricultural species and powerful biomedical model. We have established the pig, a large natural host animal for influenza with many physiological similarities to humans, as a robust model for testing the therapeutic potential of monoclonal antibodies. Antibodies provide protection through neutralization and recruitment of innate effector functions through the Fc domain. However very little is known about the Fc-mediated functions of porcine IgG subclasses. We have generated 8 subclasses of two porcine monoclonal anti influenza hemagglutinin antibodies. We characterized their ability to activate complement, trigger cytotoxicity and phagocytosis by immune cells and assayed their binding to monocytes, macrophages, and natural killer cells. We show that IgG1, IgG2a, IgG2b, IgG2c and IgG4 bind well to targeted cell types and mediate complement mediated cellular cytotoxicity (CDCC), antibody dependent cellular cytotoxicity (ADCC) and antibody mediated cell phagocytosis (ADCP). IgG5b and IgG5c exhibited weak binding and variable and poor functional activity. Immune complexes of porcine IgG3 did not show any Fc-mediated functions except for binding to monocytes and macrophages and weak binding to NK cells. Interestingly, functionally similar porcine IgG subclasses clustered together in the genome. These novel findings will enhance the utility of the pig model for investigation of therapeutic antibodies.

Antibody-Dependent Enhancement of Ebola Virus Infection by Human Antibodies Isolated from Survivors

Some monoclonal antibodies (mAbs) recovered from survivors of filovirus infections can protect against infection. It is currently unknown whether natural infection also induces some antibodies with the capacity for antibody-dependent enhancement (ADE). A panel of mAbs obtained from human survivors of filovirus infection caused by Ebola, Bundibugyo, or Marburg viruses was evaluated for their ability to facilitate ADE. ADE was observed readily with all mAbs examined at sub-neutralizing concentrations, and this effect was not restricted to mAbs with a particular epitope specificity, neutralizing capacity, or subclass. Blocking of specific Fcγ receptors reduced but did not abolish ADE that was associated with high-affinity binding antibodies, suggesting that lower-affinity interactions still cause ADE. Mutations of Fc fragments of an mAb that altered its interaction with Fc receptors rendered the antibody partially protective in vivo at a low dose, suggesting that ADE counteracts antibody-mediated protection and facilitates dissemination of filovirus infections.

Genetic Variation in Low-To-Medium-Affinity Fcγ Receptors: Functional Consequences, Disease Associations, and Opportunities for Personalized Medicine

Fc-gamma receptors (FcγR) are the cellular receptors for Immunoglobulin G (IgG). Upon binding of complexed IgG, FcγRs can trigger various cellular immune effector functions, thereby linking the adaptive and innate immune systems. In humans, six classic FcγRs are known: one high-affinity receptor (FcγRI) and five low-to-medium-affinity FcγRs (FcγRIIA, -B and -C, FcγRIIIA and -B). In this review we describe the five genes encoding the low-to-medium -affinity FcγRs (FCGR2A, FCGR2B, FCGR2C, FCGR3A, and FCGR3B), including well-characterized functionally relevant single nucleotide polymorphisms (SNPs), haplotypes as well as copy number variants (CNVs), which occur in distinct copy number regions across the locus. The evolution of the locus is also discussed. Importantly, we recommend a consistent nomenclature of genetic variants in the FCGR2/3 locus. Next, we focus on the relevance of genetic variation in the FCGR2/3 locus in auto-immune and auto-inflammatory diseases, highlighting pathophysiological insights that are informed by genetic association studies. Finally, we illustrate how specific FcγR variants relate to variation in treatment responses and prognosis amongst autoimmune diseases, cancer and transplant immunology, suggesting novel opportunities for personalized medicine.

Potential Mechanisms for IgG4 Inhibition of Immediate Hypersensitivity Reactions

IgG4 is the least abundant IgG subclass in human serum, representing less than 5 % of all IgG. Increases in IgG4 occur following chronic exposure to antigen and are generally associated with states of immune tolerance. In line with this, IgG4 is regarded as an anti-inflammatory antibody with a limited ability to elicit effective immune responses. Furthermore, IgG4 attenuates allergic responses by inhibiting the activity of IgE. The mechanism by which IgG4 inhibits IgE-mediated hypersensitivity has been investigated using a variety of model systems leading to two proposed mechanisms. First by sequestering antigen, IgG4 can function as a blocking antibody, preventing cross-linking of receptor bound IgE. Second IgG4 has been proposed to co-stimulate the inhibitory IgG receptor FcγRIIb, which can negatively regulate FcεRI signaling and in turn inhibit effector cell activation. Recent advances in our understanding of the structural features of human IgG4 have shed light on the unique functional and immunologic properties of IgG4. The aim of this review is to evaluate our current understanding of IgG4 biology and reassess the mechanisms by which IgG4 functions to inhibit IgE-mediated allergic responses.

Immunomodulation by IVIg and the Role of Fc-Gamma Receptors: Classic Mechanisms of Action after all?

Intravenous IgG (IVIg) contains polyclonal immunoglobulin G (IgG) from thousands of donors. It is administered at a low dose at regular intervals as antibody replacement therapy and at a higher dose as immunomodulatory treatment in various auto-immune or auto-inflammatory diseases. The working mechanism of immunomodulation is not well understood. Many different explanations have been given. During the last decade, we have focused on classical antibody binding via the Fc-domain of the IgG molecules to the common IgG receptors, i.e. the Fcγ receptors (FcγRs). Variation in the genes encoding human FcγRs determines function as well as expression among immune cells. As described here, NK cells and myeloid cells, including macrophages, can express different FcγR variants, depending on the individual's genotype, copy number variation (CNV), and promoter polymorphisms. B-cells seem to only express the single inhibitory receptor. Although these inhibitory FcγRIIb receptors are also expressed by monocytes, macrophages, and only rarely by NK cells or neutrophils, their presence is unlikely to explain the immunomodulatory capacity of IVIg, nor does the sialylation of IgG. Direct IVIg effects at the level of the activating FcγRs, including the more recently described FcγRIIc, deserve renewed attention to describe IVIg-related immunomodulation.

Role of novel rat-specific Fc receptor in macrophage activation associated with crescentic glomerulonephritis

Crescentic glomerulonephritis (Crgn) is a complex disease where the initial insult is often the glomerular deposition of antibodies against intrinsic or deposited antigens in the glomerulus. The role of Fc receptors in the induction and progression of Crgn is increasingly recognized, and our previous studies have shown that copy number variation in Fcgr3 partially explains the genetic susceptibility of the Wistar-Kyoto (WKY) rat to nephrotoxic nephritis, a rat model of Crgn. The Fcgr3-related sequence (Fcgr3-rs) is a novel rat-specific Fc receptor with a cytoplasmic domain 6 amino acids longer than its paralogue, Fcgr3. The Fcgr3-rs gene is deleted from the WKY rat genome, and this deletion is associated with enhanced macrophage activity in this strain. Here, we investigated the mechanism by which the deletion of Fcgr3-rs in the WKY strain leads to increased macrophage activation. By lentivirus-mediated gene delivery, we generated stably transduced U937 cells expressing either Fcgr3-rs or Fcgr3. In these cells, which lack endogenous Fcgr3 receptors, we show that Fcgr3-rs interacts with the common Fc-γ chain but that Fc receptor-mediated phagocytosis and signaling are defective. Furthermore, in primary macrophages, expression of Fcgr3-rs inhibits Fc receptor-mediated functions, because WKY bone marrow-derived macrophages transduced with Fcgr3-rs had significantly reduced phagocytic activity. This inhibitory effect on phagocytosis was mediated by the novel cytoplasmic domain of Fcgr3-rs. These results suggest that Fcgr3-rs may act to inhibit Fcgr3-mediated signaling and phagocytosis and could be considered as a novel mechanism in the modulation of Fc receptor-mediated cell activation in autoimmune diseases.

The role of calcium signaling in phagocytosis

Immune cells kill microbes by engulfing them in a membrane-enclosed compartment, the phagosome. Phagocytosis is initiated when foreign particles bind to receptors on the membrane of phagocytes. The best-studied phagocytic receptors, those for Igs (FcgammaR) and for complement proteins (CR), activate PLC and PLD, resulting in the intracellular production of the Ca(2+)-mobilizing second messengers InsP3 and S1P, respectively. The ensuing release of Ca(2+) from the ER activates SOCE channels in the plasma and/or phagosomal membrane, leading to sustained or oscillatory elevations in cytosolic Ca(2+) concentration. Cytosolic Ca(2+) elevations are required for efficient ingestion of foreign particles by some, but not all, phagocytic receptors and stringently control the subsequent steps involved in the maturation of phagosomes. Ca(2+) is required for the solubilization of the actin meshwork that surrounds nascent phagosomes, for the fusion of phagosomes with granules containing lytic enzymes, and for the assembly and activation of the superoxide-generating NADPH oxidase complex. Furthermore, Ca(2+) entry only occurs at physiological voltages and therefore, requires the activity of proton channels that counteract the depolarizing action of the phagocytic oxidase. The molecules that mediate Ca(2+) ion flux across the phagosomal membrane are still unknown but likely include the ubiquitous SOCE channels and possibly other types of Ca(2+) channels such as LGCC and VGCC. Understanding the molecular basis of the Ca(2+) signals that control phagocytosis might provide new, therapeutic tools against pathogens that subvert phagocytic killing.

Phagocytosis: a repertoire of receptors and Ca(2+) as a key second messenger

Receptor-mediated phagocytosis is a complex process that mediates the internalization, by a cell, of other cells and large particles; this is an important physiological event not only in mammals, but in a wide diversity of organisms. Of simple unicellular organisms that use phagocytosis to extract nutrients, to complex metazoans in which phagocytosis is essential for the innate defence system, as a first line of defence against invading pathogens, as well as for the clearance of damaged, dying or dead cells. Evolution has armed multicellular organisms with a range of receptors expressed on many cells that serve as the molecular basis to bring about phagocytosis, regardless of the organism or the specific physiological event concerned. Key to all phagocytic processes is the finely controlled rearrangement of the actin cytoskeleton, in which Ca(2+) signals play a major role. Ca(2+) is involved in cytoskeletal changes by affecting the actions of a number of contractile proteins, as well as being a cofactor for the activation of a number of intracellular signalling molecules, which are known to play important roles during the initiation, progression and resolution of the phagocytic process. In mammals, the requirement of Ca(2+) for the initial steps in phagocytosis, and the subsequent phagosome maturation, can be quite different depending on the type of cell and on the type of receptor that is driving phagocytosis. In this review we discuss the different receptors that mediate professional and non-professional phagocytosis, and discuss the role of Ca(2+) in the different steps of this complex process.

Roles of the transmembrane domain and the cytoplasmic domain of Fc?RI? in immunoglobulin E-mediated up-regulation of surface Fc?RI expression

BACKGROUND

Human mast cells express both Fc epsilon RI alpha and Fc gamma RI alpha. IgE up-regulates Fc epsilon RI alpha expression, but IgG1 does not up-regulate Fc gamma RI alpha expression. The transmembrane domain (TM) of Fc gamma RI alpha determines the stability of cell surface expression of this receptor.

OBJECTIVE

The aim of this study was to clarify the roles of the TM and cytoplasmic domain (CY) of Fc epsilon RI alpha in IgE-mediated Fc epsilon RI up-regulation.

METHODS

Chimeric receptors created by domain shuffling between Fc epsilon RI alpha and Fc gamma RI alpha were transduced into human mast cell line HMC-1. Cell surface expression of the chimeric receptors and the effect of IgE or IgG1 on chimeric receptor expression were examined by FACS. The association of the chimeric receptors with FcR gamma was investigated by immunoprecipitation.

RESULTS

The results showed that the TM and CY of Fc epsilon RI alpha are not essential for IgE-mediated up-regulation of surface Fc epsilon RI.

CONCLUSION

The extracellular domain of each Fc receptor determines the diversity of Ig-regulated Fc receptor expression.

The monocyte Fcgamma receptors FcgammaRI/gamma and FcgammaRIIA differ in their interaction with Syk and with Src-related tyrosine kinases

There are important differences in signaling between the Fc receptor for immunoglobulin G (IgG) FcgammaRIIA, which uses the Ig tyrosine-activating motif (ITAM) within its own cytoplasmic domain, and FcgammaRI, which transmits signals by means of an ITAM located within the cytoplasmic domain of its associated gamma-chain. For example, in transfected epithelial cells and COS-1 cells, FcgammaRIIA mediates phagocytosis of IgG-coated red blood cells more efficiently than does FcgammaRI/gamma, and enhancement of phagocytosis by Syk kinase is more pronounced for FcgammaRI/gamma than for FcgammaRIIA. In addition, structure/function studies indicate that the gamma-chain ITAM and the FcgammaRIIA ITAM have different requirements for mediating the phagocytic signal. To study the differences between FcgammaRIIA and FcgammaRI/gamma, we examined the interaction of FcgammaRIIA and the FcgammaRI/gamma chimera FcgammaRI-gamma-gamma (extracellular domain-transmembrane domain-cytoplasmic domain) with Syk kinase and with the Src-related tyrosine kinases (SRTKs) Hck and Lyn in transfected COS-1 cells. Our data indicate that FcgammaRIIA interacts more readily with Syk than does FcgammaRI-gamma-gamma and suggest that one consequence may be the greater phagocytic efficiency of FcgammaRIIA compared with FcgammaRI/gamma. Furthermore, individual SRTKs affect the efficiency of phagocytosis differently for FcgammaRI-gamma-gamma and FcgammaRIIA and also influence the ability of these receptors to interact with Syk kinase. Taken together, the data suggest that differences in signaling by FcgammaRIIA and FcgammaRI-gamma-gamma are related in part to interaction with Syk and Src kinases and that individual SRTKs play different roles in FcgammaR-mediated phagocytosis.

Fcgamma receptor transmembrane domains: role in cell surface expression, gamma chain interaction, and phagocytosis

We constructed chimeric receptors to dissect the role of the transmembrane (TM) domain in cell surface expression of and phagocytosis by the gamma chain-dependent Fcgamma receptors FcgammaRIIIA and FcgammaRI. FcgammaR chimeras containing the TM and cytoplasmic (CY) domains of the gamma chain were expressed on the cell surface and mediated an efficient phagocytic signal. In contrast, chimeras containing the FcgammaRIIIA TM were poorly expressed. Receptors containing the FcgammaRI TM and the gamma chain CY but lacking the gamma chain TM also were expressed efficiently and mediated phagocytosis, suggesting that a gamma chain dimer induced by the gamma chain TM is not required for efficient phagocytosis. Cotransfection of FcgammaRI or FcgammaRIIIA with the chimera CD8-gamma-gamma (EC-TM-CY) resulted in FcgammaR cell surface expression and phagocytosis, whereas CD8-CD8-gamma, whose TM does not associate with FcgammaR, allowed cell surface expression of (but not phagocytosis by) FcgammaRI. CD8-CD8-gamma also did not allow surface expression of FcgammaRIIIA. Exchanging FcgammaRI and CD8 TMs indicated that the C-terminal 11 amino acids of the FcgammaRI TM are essential for association of FcgammaRI with the gamma chain and phagocytosis. The data indicate that specific sequences in the FcgammaRIIIA and FcgammaRI TMs govern their different interactions with the gamma chain in cell surface expression and phagocytosis and that gamma chain TM sequences are not required for gamma chain-mediated phagocytosis. The data identify a specific region of the FcgammaRI TM and its asparagine as important for FcgammaRI cell surface expression in the absence of the gamma chain and for distinguishing the FcgammaRI and FcgammaRIIIA phenotypes.

The Fc gamma receptor genotype as a severity factor for chronic periodontitis in Japanese patients

BACKGROUND

Functional polymorphisms of immunoglobulin G (IgG) Fc receptors (Fc gamma R) have been shown to be associated with generalized aggressive periodontitis (GAgP) or recurrence of chronic periodontitis (CP) in Japanese patients. The purpose of this study was to evaluate whether Fc gamma R polymorphisms are also associated with severity of CP.

METHODS

Fifty Japanese non-smoking patients with severe CP and 39 Japanese non-smoking patients with moderate CP were identified according to established clinical criteria, including measurements of probing depth (PD), clinical attachment level (CAL), and alveolar bone loss (BL). Fc gamma R genotypes for 3 bi-allelic polymorphisms (Fc gamma RIIa-R/H131, Fc gamma RIIIa-158V/F, Fc gamma RIIIb-NA1/NA2) were determined in these CP patients and 64 race-matched, non-smoking healthy controls by means of allele-specific polymerase chain reactions.

RESULTS

There was a significant over-representation of Fc gamma RIIIa-158V allele in severe CP patients compared to moderate CP patients (odds ratio 2.03, 95% confidence interval [CI] 1.03-4.01, chi 2 = 4.86, P = 0.028). In addition, we found a strong association between CP severity and Fc gamma R composite genotype comprising Fc gamma RIIIa-158V plus Fc gamma RIIIb-NA2 (severe CP versus moderate CP: odds ratio 4.69, 95% CI 1.52-15.10, chi 2 = 9.35, P = 0.002; severe CP versus healthy controls: odds ratio 4.10, 95% CI 1.62-10.59, chi 2 = 11.13, P = 0.0009). Moreover, CP patients positive for the composite genotype exhibited more severe signs of periodontitis than composite genotype-negative individuals (positive versus negative; mean PD: 3.8 mm versus 3.2 mm, P = 0.005; mean CAL: 4.5 mm versus 3.7 mm, P = 0.005; mean % BL: 37.6% versus 29.9%, P = 0.008).

CONCLUSION

Our results document the Fc gamma RIIIa-158V allele and possibly Fc gamma RIIIb-NA2 to be associated with severity of CP in Japanese patients.

Ig-binding receptors on human NK cells as effector and regulatory surface molecules

The receptors on human natural killer 9NK cells which can specifically bind the Fc portion of immunoglobulin molecules (Fc receptors) have been extensively studied. The best known and studied Fc receptor on human NK cells is FcgammaRIIIa. Interactions of NK cells with IgG antibodies via this receptor are well known to induce a signal transduction cascade and lead to antibody-dependent cell-mediated cytotoxicity (ADCC) as well as release of various cytokines. In addition, interactions with monomeric IgG and FcgammaRIIIa have been demonstrated, which result in negative regulation of NK activity and other immunomodulatory effects. Over the past several years, it has also become increasingly appreciated that human NK cells express a variety of other Fc receptors, including FcmuR, which also can mediate effector and immunoregulatory functions. Also, a novel form of FcgammaR has been demonstrated on human NK cells, termed FcgammaRIIc. Recent molecular studies have shown considerable polymorphism in the genes for FcgammaIIc and the functional consequences are being dissected. This appears to include cross-talk between FcgammaRIIIa and at least some forms of FcgammaRIIc, which may have important functional consequences.

Indirect role for COPI in the completion of FCgamma receptor-mediated phagocytosis

Recent evidence suggests that extension of pseudopods during phagocytosis requires localized insertion of endomembrane vesicles. The nature of these vesicles and the processes mediating their release and insertion are unknown. COPI plays an essential role in the budding and traffic of membrane vesicles in intracellular compartments. We therefore assessed whether COPI is also involved in phagosome formation. We used ldlF cells, a mutant line derived from Chinese hamster ovary cells that express a temperature-sensitive form of epsilonCOP. To confer phagocytic ability to ldlF cells, they were stably transfected with Fc receptors type IIA (FcgammaRIIA). In the presence of functional COPI, FcgammaRIIA-transfected ldlF cells effectively internalized opsonized particles. In contrast, phagocytosis was virtually eliminated after incubation at the restrictive temperature. Similar results were obtained impairing COPI function in macrophages using brefeldin A. Notably, loss of COPI function preceded complete inhibition of phagocytosis, suggesting that COPI is indirectly required for phagocytosis. Despite their inability to internalize particles, COPI-deficient cells nevertheless expressed normal levels of FcgammaRIIA, and signal transduction appeared unimpeded. The opsonized particles adhered normally to COPI-deficient cells and were often found on actin-rich pedestals, but they were not internalized due to the inability of the cells to extend pseudopods. The failure to extend pseudopods was attributed to the inability of COPI-deficient cells to mobilize endomembrane vesicles, including a VAMP3-containing compartment, in response to the phagocytic stimulus.

Phagocytosis and the actin cytoskeleton

The process of engulfing a foreign particle - phagocytosis - is of fundamental importance for a wide diversity of organisms. From simple unicellular organisms that use phagocytosis to obtain their next meal, to complex metazoans in which phagocytic cells represent an essential branch of the immune system, evolution has armed cells with a fantastic repertoire of molecules that serve to bring about this complex event. Regardless of the organism or specific molecules concerned, however, all phagocytic processes are driven by a finely controlled rearrangement of the actin cytoskeleton. A variety of signals can converge to locally reorganise the actin cytoskeleton at a phagosome, and there are significant similarities and differences between different organisms and between different engulfment processes within the same organism. Recent advances have demonstrated the complexity of phagocytic signalling, such as the involvement of phosphoinostide lipids and multicomponent signalling complexes in transducing signals from phagocytic receptors to the cytoskeleton. Similarly, a wide diversity of ‘effector molecules’ are now implicated in actin-remodelling downstream of these receptors.

Fc gamma receptors differ in their structural requirements for interaction with the tyrosine kinase Syk in the initial steps of signaling for phagocytosis

Receptors for the constant region of IgG, Fc gamma receptors, are expressed on the surface of hematopoietic cells, where they mediate signaling events, such as phagocytosis, essential for host defense. Fc gamma receptors also play a role in the pathophysiology of autoimmune diseases. We have demonstrated that members of each of the three classes of human Fc gamma receptors, Fc gamma RI, Fc gamma RII, and Fc gamma RIII, mediate phagocytosis, but that important differences exist in their requirements for phagocytic signaling. For example, the Fc gamma receptors Fc gamma RI and Fc gamma RIIIA induce signaling largely by association with a gamma subunit containing a conserved cytoplasmic motif (ITAM) whose tyrosines are phosphorylated following receptor stimulation. Fc gamma RIIA contains a similar motif in its own cytoplasmic domain and does not require the gamma chain for phagocytic signaling. The tyrosine kinase Syk associates with the cytoplasmic domain of both the Fc gamma receptor gamma chain and Fc gamma RIIA and is required for phagocytosis by both Fc gamma receptor systems. To elucidate the differences in phagocytic signaling by the gamma chain and Fc gamma RIIA, we investigated the requirements for Fc gamma receptor/Syk co-immunoprecipitation, tyrosine phosphorylation, and phagocytosis. Both Fc gamma RIIA and the human gamma chain contain a tyrosine seven amino acids upstream of the ITAM motif. We observed that the upstream tyrosine plays a role in Fc gamma RIIA phagocytic signaling but is not involved in phagocytic signaling by the human gamma chain. Our data also indicate that the two ITAM tyrosines of the human gamma chain and Fc gamma RIIA do not contribute equally to Fc gamma receptor association with Syk kinase and phagocytic signaling. The data indicate that the carboxy-terminal tyrosine of the receptor cytoplasmic domain is especially important both for the interaction with Syk kinase and for phagocytosis. Elucidating such differences in gamma chain and Fc gamma RIIA signaling may be valuable in designing strategies for therapeutic intervention in hematopoietic and immunological disorders.

The Fcgamma receptor genotype as a risk factor for generalized early-onset periodontitis in Japanese patients

BACKGROUND

Genetic polymorphisms of immunoglobulin G (IgG) Fc receptors (FcgammaR) were recently shown to be associated with recurrence rates of adult periodontitis (AP). The purpose of this study was to evaluate whether FcgammaR polymorphisms are also associated with generalized early-onset periodontitis (G-EOP) in Japanese patients.

METHODS

Thirty-eight Japanese patients with G-EOP and 83 Japanese patients with AP were identified according to established clinical criteria, including measurements of probing depth, clinical attachment level, and alveolar bone level. FcgammaR genotypes for 3 bi-allelic polymorphisms were determined in these G-EOP and AP patients and 104 race-matched healthy controls by means of allele-specific polymerase chain reactions.

RESULTS

There was a significant difference in the distribution of FcgammaRIIIb genotypes between G-EOP patients and healthy controls (P = 0.02). Additionally, a significant over-representation of FcgammaRIIIb-NA2 allele was observed in G-EOP patients as compared to AP patients and controls (P= 0.02, P= 0.009, respectively). Moreover, we found a strong association between G-EOP and the composite genotype comprising FcgammaRIIIb-NA2 and FcgammaRIIIa-158F (G-EOP versus controls: odds ratio 2.4, 95% CI 1.0-6.0, chi2 = 4.13, P= 0.04).

CONCLUSIONS

This study indicates that the FcgammaRIIIb-NA2 allele and possibly FcgammaRIIIa-158F could be associated with susceptibility to G-EOP in Japanese patients.

Differential involvement of Src family kinases in Fc gamma receptor-mediated phagocytosis

The tyrosine phosphorylation cascade originated from Fc gamma receptors (Fc gamma Rs) is essential for macrophage functions including phagocytosis. Although the initial step is ascribed to Src family tyrosine kinases, the role of individual kinases in phagocytosis signaling is still to be determined. In reconstitution experiments, we first showed that expression in the RAW 264.7 cell line of C-terminal Src kinase (Csk) inhibited and that of a membrane-anchored, gain-of-function Csk abolished the Fc gamma R-mediated signaling that leads to phagocytosis in a kinase-dependent manner. We next tested reconstruction of the signaling in the membrane-anchored, gain-of-function Csk-expressing cells by introducing Src family kinases the C-terminal negative regulatory sequence of which was replaced with a c-myc epitope. Those constructs derived from Lyn and Hck (a-Lyn and a-Hck) that associated with detergent-resistant membranes successfully reconstructed Fc gamma R-mediated Syk activation, filamentous actin rearrangement, and phagocytosis. In contrast, c-Src-derived construct (a-Src), that was excluded from detergent-resistant membranes, could not restore the series of phagocytosis signaling. Tyrosine phosphorylation of Vav and c-Cbl was restored in common by a-Lyn, a-Hck, and a-Src, but Fc gamma RIIB tyrosine phosphorylation, which is implicated in negative signaling, was reconstituted solely by a-Lyn and a-Hck. These findings suggest that Src family kinases are differentially involved in Fc gamma R-signaling and that selective kinases including Lyn and Hck are able to fully transduce phagocytotic signaling.

Enhancement of Fcγ Receptor-Mediated Phagocytosis by Transforming Mutants of Cbl

Phagocytosis mediated by FcγR plays an important role in host defense. The molecular events involved in this process have not been completely defined. The adapter protein Cbl has been implicated in FcγR signaling, but the function of Cbl in phagocytosis is unknown. Here we show that overexpression of the transforming mutants of Cbl, Cbl-70Z, and v-Cbl, but not wild-type (wt) Cbl, enhance phagocytosis mediated by FcγR in COS cells. Cbl-70Z, but not Cbl-wt, also enhanced FcγR-mediated phagocytosis in P388D1 murine macrophage cells. Cbl-70Z did not affect tyrosine phosphorylation or in vitro kinase activity of Syk, indicating that Syk may not be the direct target of Cbl-70Z in the enhancement of phagocytosis. A point mutation (G306E) in the phosphotyrosine domain of Cbl-70Z, as well as a C-terminal 67-aa deletion, partially abolished the enhancing effect on FcγR-mediated phagocytosis. A double mutant of Cbl-70Z containing both the G306E mutation and the C-terminal deletion completely lacked the ability to enhance phagocytosis. Thus, both the phosphotyrosine binding domain and the carboxyl-terminal tail were required for optimal enhancement of phagocytosis by Cbl-70Z. Functional phosphatidylinositol 3-kinase was required for Cbl-70Z to enhance phagocytosis, since wortmannin, a phosphatidylinositol 3-kinase inhibitor, inhibited FcγR-mediated phagocytosis in the presence of Cbl-70Z. These studies demonstrate that mutants of Cbl can modulate the phagocytic pathway mediated by FcγR and imply a functional involvement of c-Cbl in Fcγ receptor-mediated phagocytosis.

Relevance of Fc gamma RIIIa-158V-F polymorphism to recurrence of adult periodontitis in Japanese patients

The immunoglobulin receptor Fc gamma RIIIa (CD16) is distributed on natural killer (NK) cells, macrophages, and gamma delta T cells, and is polymorphic. Fc gamma RIIIa-158V has a higher affinity for both monomeric and immune complexed IgG1, IgG3, and IgG4 than IIIa-158F. We determined Fc gamma RIIIa-158V/F genotypes of Japanese patients with adult periodontitis. A significant over-representation of Fc gamma RIIIa-158F was found in patients with recurrence, compared with patients without recurrence, making Fc gamma RIIIA a candidate gene for recurrence risk of adult periodontitis.

Structural requirements of Syk kinase for Fcgamma receptor-mediated phagocytosis

The tyrosine kinase Syk plays a critical role in the phagocytic pathway mediated by Fcgamma receptors (FcgammaR). In transfected COS1 cells co-expression of Syk enhances FcgammaR mediated phagocytosis. The other member of the Syk kinase family, the highly homologous tyrosine kinase Zap70, also plays a role in signaling by immunoglobulin gene family receptors, but does not increase the phagocytic efficiency of FcgammaRs. The homologous tandem SH2 and kinase domains of Syk and Zap70 are separated by a nonhomologous region referred to as the unique domain. Zap70's inability to enhance phagocytosis was not due to unique domain tyrosine 292, previously implicated in negative regulation of Zap70 function. We determined the regions of Syk important for its interaction with the phagocytic pathway. An intact kinase domain was required for Syk's effect on phagocytosis. Furthermore, the Syk variant SykB, lacking 23 amino acids in the unique region, signaled for phagocytosis as efficiently as did Syk. We then constructed exchange chimeras between Syk and Zap70 and determined the contributions of the SH2, unique and kinase domains to phagocytic signaling. Our data suggest that the Syk kinase domain, which has high intrinsic kinase activity, is important for facilitating phagocytic signaling by FcgammaRI and FcgammaRIIIA.

Fcγ Receptor-Mediated Activation of Phospholipase D Regulates Macrophage Phagocytosis of IgG-Opsonized Particles

Receptors for the Fc portion of IgG (FcγRs) integrate the innate and acquired components of immunity by coupling the specific recognition of IgG Abs to the activation of phagocytic leukocytes. Knowledge of the molecular mechanisms that regulate phagocyte stimulation by FcγRs may permit therapeutic modulation to augment immunoprotective aspects and minimize damage to host tissues in diverse inflammatory diseases. Since phospholipase D (PLD) has been linked to the stimulation of cytotoxic leukocyte responses, we characterized FcγR-dependent activation of PLD in human macrophages. IgG-coated SRBCs (EIgG) stimulated a 9.4-fold increase in PLD activity compared with SRBCs treated with control Ab (p < 0.001), determined by formation of the PLD-specific product phosphatidylethanol in the presence of 0.5% ethanol. Levels of phosphatidic acid, the physiologic product of PLD-mediated catalyzis, were significantly increased in the absence of ethanol (6.4-fold, p < 0.001). PLD activity was also stimulated by immune complex-coated latex beads or cross-linking of Abs specific for FcγRI, FcγRII, or FcγRIII. Phagocytosis of EIgG was reduced by two inhibitors of PLD-mediated signaling, 2,3-diphosphoglycerate or 1-butanol. Addition of purified PLD restored control levels of phagocytosis in cells in which endogenous PLD was inhibited. The tyrosine kinase inhibitors genistein and herbimycin A caused concordant reductions in FcγR-stimulated PLD activity and phagocytosis. These studies demonstrate that FcγR-mediated phagocytosis is accompanied by tyrosine kinase-dependent activation of PLD and support the hypothesis that stimulation of PLD functions to regulate the ingestion of IgG-opsonized particles.

Activation of three classes of nonreceptor tyrosine kinases following Fc gamma receptor crosslinking in human monocytes

Fc gamma receptors on monocytes/macrophages play an important role in both host defense and autoimmune disorders. Fc gamma receptor signaling can lead to such downstream events as phagocytosis and the release of intracellular cytokines and reactive oxygen species. Freshly isolated human monocytes express two major classes of Fc gamma receptor proteins, Fc gamma RI (CD64) and Fc gamma RII (CD32). Crosslinking of Fc gamma RI and Fc gamma RII gives rise to rapid and transient phosphorylation of multiple monocyte intracellular proteins including proteins of 40, 68-72, 75-85, 95, and 115-165 kDa. A 72-kDa protein was earlier identified as the tyrosine kinase Syk. Here we identify one of the proteins in the 115- to 165-kDa cluster as FAK, a protein tyrosine kinase localized to focal adhesions. A 68-kDa phosphoprotein was identified as paxillin, a cytoskeleton associated substrate for tyrosine kinases, and a 95-kDa protein was found to be the proto-oncogene product Vav. The Src family protein tyrosine kinase Fgr (p58) also displayed enhanced tyrosine phosphorylation after Fc gamma RI and Fc gamma RII crosslinking. Although Fc gamma RIIA utilizes tyrosines within its own cytoplasmic domain for signaling while Fc gamma RI utilizes the cytoplasmic tyrosines of its associated gamma subunit, our results indicate sharing of several proteins for signaling in monocytes by these Fc receptors. These molecules include three distinct classes of tyrosine kinases, Syk, FAK, and Fgr, and the functionally diverse proteins Vav and paxillin.

Structural bases of Fc gamma R functions

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

Inhibition of Fcγ Receptor-Mediated Phagocytosis by a Nonphagocytic Fcγ Receptor

There are three major classes of human Fcγ receptors (FcγRI, FcγRII, and FcγRIII) and various isoforms of each class are capable of mediating phagocytosis. FcγRIIA is an unusual Fcγ receptor in that it transmits a phagocytic signal in the absence of an additional receptor subunit. The cytoplasmic domain of FcγRIIA contains a conserved motif containing two copies of the sequence YXXL. The tyrosines (Y) within the motif are phosphorylated after receptor crosslinking and the integrity of these conserved sequences is required for efficient phagocytosis. The FcγRIIB receptors, FcγRIIB1 and FcγRIIB2, contain one copy of the cytoplasmic YXXL sequence and do not transmit a phagocytic signal. In B cells, FcγRIIB negatively regulates B-cell activation by the B-cell antigen receptor. Human macrophages express both FcγRIIA and FcγRIIB and while FcγRIIA mediates phagocytosis, the function of FcγRIIB in these cells is unknown. Using the epithelial/fibroblast-like cell line COS-1 as a model to examine the molecular events that regulate the phagocytosis of IgG-coated cells (EA), we investigated the effect of FcγRIIB on FcγRIIA signaling. FcγRIIB inhibited phagocytosis mediated both by FcγRIIA and by a chimeric FcγRIIA receptor containing the extracellular domain of FcγRI and the transmembrane and cytoplasmic domains of FcγRIIA. This inhibition occurred at an early signaling stage because tyrosine phosphorylation of the FcγRIIA cytoplasmic domain was inhibited after concurrent stimulation of these receptors with EA. FcγRIIB mutations showed the importance of the FcγRIIB YXXL for inhibition of FcγRIIA-mediated phagocytosis. Deletion of the FcγRIIB YXXL or conservative replacement of the YXXL tyrosine substantially reduced the inhibitory signal. FcγRIIB had a lesser inhibitory effect on phagocytosis by the Fcγ receptor FcγRIIIA, which requires a γ subunit to mediate a phagocytic signal. These results show that FcγRIIB negatively regulates phagocytic signaling by FcγRIIA and suggests that FcγRIIB plays a role in modulating FcγRIIA function in vivo.

Inhibition of Fcγ Receptor-Mediated Phagocytosis by a Nonphagocytic Fcγ Receptor

There are three major classes of human Fcγ receptors (FcγRI, FcγRII, and FcγRIII) and various isoforms of each class are capable of mediating phagocytosis. FcγRIIA is an unusual Fcγ receptor in that it transmits a phagocytic signal in the absence of an additional receptor subunit. The cytoplasmic domain of FcγRIIA contains a conserved motif containing two copies of the sequence YXXL. The tyrosines (Y) within the motif are phosphorylated after receptor crosslinking and the integrity of these conserved sequences is required for efficient phagocytosis. The FcγRIIB receptors, FcγRIIB1 and FcγRIIB2, contain one copy of the cytoplasmic YXXL sequence and do not transmit a phagocytic signal. In B cells, FcγRIIB negatively regulates B-cell activation by the B-cell antigen receptor. Human macrophages express both FcγRIIA and FcγRIIB and while FcγRIIA mediates phagocytosis, the function of FcγRIIB in these cells is unknown. Using the epithelial/fibroblast-like cell line COS-1 as a model to examine the molecular events that regulate the phagocytosis of IgG-coated cells (EA), we investigated the effect of FcγRIIB on FcγRIIA signaling. FcγRIIB inhibited phagocytosis mediated both by FcγRIIA and by a chimeric FcγRIIA receptor containing the extracellular domain of FcγRI and the transmembrane and cytoplasmic domains of FcγRIIA. This inhibition occurred at an early signaling stage because tyrosine phosphorylation of the FcγRIIA cytoplasmic domain was inhibited after concurrent stimulation of these receptors with EA. FcγRIIB mutations showed the importance of the FcγRIIB YXXL for inhibition of FcγRIIA-mediated phagocytosis. Deletion of the FcγRIIB YXXL or conservative replacement of the YXXL tyrosine substantially reduced the inhibitory signal. FcγRIIB had a lesser inhibitory effect on phagocytosis by the Fcγ receptor FcγRIIIA, which requires a γ subunit to mediate a phagocytic signal. These results show that FcγRIIB negatively regulates phagocytic signaling by FcγRIIA and suggests that FcγRIIB plays a role in modulating FcγRIIA function in vivo.

Functional separation of pseudopod extension and particle internalization during Fc gamma receptor-mediated phagocytosis

Receptors for the Fc portion of immunoglobulin (Ig)G (Fc gamma R) mediate phagocytosis of IgG-opsonized particles by a process that can be divided into four major steps: receptor-ligand binding, pseudopod extension, internalization, and lysosomal fusion. We have expressed single classes of Fc gamma R in COS fibroblasts to examine the structural determinants necessary to complete the four steps of phagocytosis. Using phase contrast, fluorescence, confocal, and electron microscopy we have demonstrated that Fc gamma R-expressing COS cells can phagocytose in a manner similar to that of professional phagocytes. We have further analyzed the capacity of the three classes of Fc gamma R to phagocytose, placing special emphasis on the Fc gamma RIA-gamma chain complex, which allowed us to examine independently the roles of the ligand-binding unit (Fc gamma RIA) and the signaling unit (gamma chain). We found that receptor complexes containing a conserved tyrosine activation motif (ITAM), as found in the cytoplasmic domain of Fc gamma RIIA and in the gamma chain associated with Fc gamma RIA and Fc gamma RIIIA, readily internalized target particles. In contrast, Fc gamma RIA alone, having no ITAM, was unable to internalize target particles efficiently, but did mediate pseudopod extension. Cotransfection of gamma chain with Fc gamma RIA restored the ability of the receptor to internalize target particles. A mutant Fc gamma RIA in which the cytoplasmic domain had been deleted was also capable of mediating pseudopod extension, showing that neither the gamma chain nor the cytoplasmic domain of Fc gamma RIA were required for this step. Cytochalasin D, an inhibitor of actin polymerization, blocked particle internalization by all Fc gamma R, but did not block pseudopod extension. Staining the Fc gamma RIA COS cells for F-actin and for tyrosine phosphoproteins, we found that actin did not polymerize during Fc gamma RIA-mediated pseudopod extension, nor were tyrosine kinases activated. Our data suggest that pseudopod extension and internalization are functionally distinct steps mediated through different pathways.

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

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

Ligand binding and phagocytosis by CD16 (Fc gamma receptor III) isoforms. Phagocytic signaling by associated zeta and gamma subunits in Chinese hamster ovary cells

CD16, the low affinity Fc gamma receptor III for IgG (Fc gamma RIII), exists as a polypeptide-anchored form (Fc gamma RIIIA or CD16A) in human natural killer cells and macrophages and as a glycosylphosphatidylinositol-anchored form (Fc gamma RIIIB or CD16B) in neutrophils. CD16A requires association of the gamma subunit of Fc epsilon RI or the zeta subunit of the TCR-CD3 complex for cell surface expression. The CD16B is polymorphic and the two alleles are termed NA1 and NA2. In this study, CD16A and the two alleles of CD16B have been expressed in Chinese hamster ovary (CHO) cells and their ligand binding and phagocytic properties analyzed. The two allelic forms of CD16B showed a similar affinity toward human IgG1. However, the NA1 allele showed approximately 2-fold higher affinity for the IgG3 than the NA2 allele. Although all three forms of CD16 efficiently bound rabbit IgG-coated erythrocytes (EA), only CD16A coexpressed with the gamma subunit phagocytosed EA. The phagocytosis mediated by CD16A expressed on CHO cells was independent of divalent cations but dependent on intact microfilaments. CHO cells expressing CD16A-gamma and CD16A-zeta chimeras also phagocytosed EA. The phagocytosis was specifically inhibited by tyrphostin-23, a tyrosine kinase inhibitor. In summary, our results demonstrate that glycosylphosphatidylinositol-anchored CD16B alleles differ from CD16A in their ability to mediate phagocytosis. Furthermore, since studies with other Fc gamma Rs have shown that CHO cells lack the phagocytic pathway mediated by the cytoplasmic domain of Fc gamma Rs, the phagocytosis of EA by CHO cells stably transfected with CD16A and CD16A-subunit chimera provides an ideal system to dissect the phagocytic signaling pathways mediated by these Fc gamma R-associated subunits.

Determinants of the phagocytic signal mediated by the type IIIA Fc gamma receptor, Fc gamma RIIIA: sequence requirements and interaction with protein-tyrosine kinases

The Fc gamma receptor-associated gamma and zeta subunits contain a conserved cytoplasmic motif, termed the immunoglobulin gene tyrosine activation motif, which contains a pair of YXXL sequences. The tyrosine residues within these YXXL sequences have been shown to be required for transduction of a phagocytic signal. We have previously reported that the gamma subunit of the type IIIA Fc gamma receptor (Fc gamma RIIIA) is approximately 6 times more efficient in mediating phagocytosis than the zeta subunit of Fc gamma RIIIA. By exchanging regions of the cytoplasmic domains of the homologous gamma and zeta chains, we observed that the cytoplasmic area of the gamma chain bearing a pair of the conserved YXXL sequences is important in phagocytic signaling. Further specificity of phagocytic signaling is largely determined by the two internal XX amino acids in the YXXL sequences. In contrast, the flanking amino acids of the YXXL sequences including the seven intervening amino acids between the two YXXL sequences do not significantly affect the phagocytic signal. Furthermore, the protein-tyrosine kinase Syk, but not the related kinase ZAP-70, stimulated Fc gamma RIIIA-mediated phagocytosis. ZAP-70, however, increased phagocytosis when coexpressed with the Src family kinase Fyn. These data demonstrate the importance of the two specific amino acids within the gamma subunit YXXL cytoplasmic sequences in phagocytic signaling and explain the difference in phagocytic efficiency of the gamma and zeta chains. These results indicate the importance of Syk in Fc gamma RIIIA-mediated phagocytosis and demonstrate that ZAP-70 and syk differ in their requirement for a Src-related kinase in signal transduction.

Structure/function relationships of Fc gamma receptors in phagocytosis

An important function of Fc gamma receptors is the ingestion or phagocytosis of IgG sensitized cells. It has been difficult to clearly define the individual function of each receptor in phagocytosis because hematopoietic cells express multiple Fc gamma receptor isoforms. To examine this issue, an in-vitro model system in COS-1 cells has been developed. When transfected with an appropriate Fc gamma receptor, COS-1 cells which lack endogeneous Fc receptors, ingest IgG-sensitized cells. Using this model, a single class of human Fc gamma receptor in the absence of other Fc receptors was observed to mediate phagocytosis. Furthermore, isoforms from each Fc gamma receptor class can mediate phagocytosis although the requirements for phagocytosis differ. Investigation of the relationship between structure and function for Fc receptor-mediated phagocytosis established the importance of the cytoplasmic tyrosines of the receptor or its associated gamma chains. For example, two cytoplasmic YXXL sequences, in a configuration similar to the conserved tyrosine containing motif found in immunoglobulin gene family receptors, are important for phagocytosis by the human Fc gamma receptor, Fc gamma RIIA. Fc gamma RI and Fc gamma RIIIA do not possess cytoplasmic tyrosines, but transmit a phagocytic signal through interaction with an associated gamma-subunit which contains two YXXL sequences in a conserved motif required for phagocytosis. The human Fc gamma RII isoforms, Fc gamma RIIB2, do not induce phagocytosis and have only a single YXXL sequence. Crosslinking of the phagocytic Fc gamma receptors induces tyrosine phosphorylation of either Fc gamma RIIA or the gamma chain and treatment with tyrosine kinase inhibitors reduces both phagocytosis and phosphorylation of the receptor tyrosine residues. The protein tyrosine kinase Syk, which is associated with the gamma chain in monocytes/macrophages, dramatically enhances phagocytosis mediated by Fc gamma RI and Fc gamma RIIIA and also induces non-phagocytic Fc gamma RI or Fc gamma RIIIA expressing cells to acquire phagocytic capability.

Molecular dissection of Fc gamma receptor-mediated phagocytosis

Using an experimental model in COS-1 cells, we have examined the structural requirements for phagocytosis of IgG-sensitized cells by Fc gamma receptors. We have established that isoforms of each of the 3 classes of the Fc gamma receptors, Fc gamma RI, Fc gamma RII and Fc gamma RIII, are able to transmit a phagocytic signal in the absence of the other receptor class. Fc gamma I and Rc gamma RIIIA require a gamma-subunit for this signaling event, but Fc gamma RIIA does not. Fc gamma RIIA and the gamma-subunit associated with Fc gamma RI and Fc gamma RIIIA contain 2 copies of a conserved tyrosine-containing cytoplasmic sequence, YXXL. This sequence is important for phagocytosis and is phosphorylated on tyrosine after receptor ligation. The Fc gamma receptors Fc gamma RIIB1 and Fc gamma RIIB2 which contain only 1 copy of the YXXL cytoplasmic sequence do not include the phagocytosis of IgG-coated cells. Thus, the Fc gamma receptor isoforms differ in their ability to transmit a phagocytic signal. Structure/function studies also indicate that the Fc gamma receptors which induce phagocytosis differ in their requirements for phagocytic signaling.

Conserved cytoplasmic tyrosine residues of the gamma subunit are required for a phagocytic signal mediated by Fc gamma RIIIA

Fc receptors for immunoglobulins are found on many cells and are important in host defense. We transfected Fc gamma RIIIA, present on macrophages and natural killer (NK) cells, into COS-1 cells to study its role in phagocytosis and calcium mobilization in the absence of other Fc gamma receptors. Human Fc gamma RIIIA-alpha (CD16) was cotransfected with its associated chains, either Fc gamma RIIIA gamma or zeta. Both gamma and zeta were observed to induce a phagocytic signal, but gamma was at least sixfold more effective than zeta. Conservative substitution by phenylalanine of either one of the two cytoplasmic tyrosine residues in the gamma chain resulted in markedly diminished phagocytosis and calcium mobilization. Tyrphostin 23, an inhibitor of tyrosine kinases, reversibly inhibited phagocytosis. Further, in vitro kinase assays with the wild type and mutant gamma chains demonstrated that the wild type gamma chain, but not the mutant gamma chains, is phosphorylated. These results suggest that the cytoplasmic tyrosine residues and tyrosine phosphorylation are required for Fc gamma RIIIA to mediate two signal transduction events: phagocytosis and calcium mobilization.