A novel role for the Fc receptor gamma subunit: enhancement of Fc gamma R ligand affinity

A leukocyte immune-type receptor specific polyclonal antibody recognizes goldfish kidney leukocytes and activates the MAPK pathway in isolated goldfish kidney neutrophil-like cells

Leukocyte immune-type receptors (LITRs) belong to a large family of teleost immunoregulatory receptors that share phylogenetic and syntenic relationships with mammalian Fc receptor-like molecules (FCRLs). Recently, several putative stimulatory Carassius auratus (Ca)-LITR transcripts, including CaLITR3, have been identified in goldfish. CaLITR3 has four extracellular immunoglobulin-like (Ig-like) domains, a transmembrane domain containing a positively charged histidine residue, and a short cytoplasmic tail region. Additionally, the calitr3 transcript is highly expressed by goldfish primary kidney neutrophils (PKNs) and macrophages (PKMs). To further investigate the immunoregulatory potential of CaLITR3 in goldfish myeloid cells, we developed and characterized a CaLITR3-epitope-specific polyclonal antibody (anti-CaL3.D1 pAb). We show that the anti-CaL3.D1 pAb stains various hematopoietic cell types within the goldfish kidney, as well as in PKNs and PKMs. Moreover, cross-linking of the anti-CaL3.D1-pAb on PKN membranes induces phosphorylation of p38 and ERK1/2, critical components of the MAPK pathway involved in controlling a wide variety of innate immune effector responses such as NETosis, respiratory burst, and cytokine release. These findings support the stimulatory potential of CaLITR3 proteins as activators of fish granulocytes and pave the way for a more in-depth examination of the immunoregulatory functions of CaLITRs in goldfish myeloid cells.

Characterization of a membrane Fcγ receptor in largemouth bass (Micropterus saloumoides) and its response to bacterial challenge

Fc receptors (FcRs), specific to the Fc portion of immunoglobulin (Ig), are required to regulate immune responses against pathogenic infections. However, FcγR is a member of FcRs family, whose structure and function remains to be elucidated in teleost fish. In this study, the FcγRII, from largemouth bass (Micropterus saloumoides), named membrane MsFcγRII (mMsFcγRII), was cloned and identified. The opening reading frame (ORF) of mMsFcγRII was 750 bp, encoding 249 amino acids with a predicted molecular mass of 27 kDa. The mMsFcγRII contained a signal peptide, two Ig domains, a transmembrane domain, and an intracellular region, which was highly homology with FcγR from other teleost fish. The mRNA expression analysis showed that mMsFcγRII was widely distributed in all tested tissues and with the highest expression level in spleen. After bacterial challenge, the expression of mMsFcγRII was significantly upregulated in vivo (spleen and head kidney), as well as in vitro (leukocytes from head kidney). The subcellular localization assay revealed that mMsFcγRII was mostly observed on the membrane of HEK293T cells which were transfected with mMsFcγRII overexpression plasmid. Flow cytometric analysis showed that natural mMsFcγRII protein was highly expressed in head kidney lymphocytes. Moreover, indirect immunofluorescence assay and pull-down assay indicated that mMsFcγRII could bind to IgM purified from largemouth bass serum. These results suggested that mMsFcγRII was likely to play an influential role in the immune response against pathogens and provided valuable insights for studying the function of FcRs in teleost.

Cross-reactivity of mouse IgG subclasses to human Fc gamma receptors: Antibody deglycosylation only eliminates IgG2b binding

Immunoglobulin G (IgG) antibodies are important for protection against pathogens and exert effector functions through binding to IgG-Fc receptors (FcγRs) on myeloid and natural killer cells, resulting in destruction of opsonized target cells. Despite interspecies differences, IgG subclasses and FcγRs show substantial similarities and functional conservation between mammals. Accordingly, binding of human IgG (hIgG) to mouse FcγRs (mFcγRs) has been utilized to study effector functions of hIgG in mice. In other applications, such as immunostaining with mouse IgG monoclonal antibodies (mAbs), these cross-reactivities are undesired and prone to misinterpretation. Despite this drawback, the binding of mouse IgG (mIgG) subclasses to human FcγR (hFcγR) classes has never been fully documented. Here, we report detailed and quantifiable characterization of binding affinities for all mIgG subclasses to hFcγRs, including functional polymorphic variants. mIgG subclasses show the strongest binding to hFcγRIa, with relative affinities mIgG2a = mIgG2c > mIgG3 >> mIgG2b, and no binding by mIgG1. hFcγRIIa/b showed general low reactivities to all mIgG (mIgG1> mIgG2a/c > mIgG2b), with no reactivity to mIgG3. A particularly high affinity was observed for mIgG1 to the hFcγRIIa-R131 polymorphic variant. hFcγRIIIa showed lower binding (mIgG2a/c > mIgG3), slightly favouring binding to the hFcγRIIIa-V158 over the F158 polymorphic variant. No binding was observed of mIgG to hFcγRIIIb. Deglycosylation of mIgG1 did not abrogate binding to hFcγRIIa-R131, nor did deglycosylation of mIgG2a/c and mIgG3 prevent hFcγRIa binding. Importantly, deglycosylation of the least cross-reactive mIgG subclass, mIgG2b, abrogated reactivity to all hFcγRs. Together, these data document for the first time the full spectrum of cross-reactivities of mouse IgG to human FcγRs.

Impact of Plasma Membrane Domains on IgG Fc Receptor Function

Lipid cell membranes not only represent the physical boundaries of cells. They also actively participate in many cellular processes. This contribution is facilitated by highly complex mixtures of different lipids and incorporation of various membrane proteins. One group of membrane-associated receptors are Fc receptors (FcRs). These cell-surface receptors are crucial for the activity of most immune cells as they bind immunoglobulins such as immunoglobulin G (IgG). Based on distinct mechanisms of IgG binding, two classes of Fc receptors are now recognized: the canonical type I FcγRs and select C-type lectin receptors newly referred to as type II FcRs. Upon IgG immune complex induced cross-linking, these receptors are known to induce a multitude of cellular effector responses in a cell-type dependent manner, including internalization, antigen processing, and presentation as well as production of cytokines. The response is also determined by specific intracellular signaling domains, allowing FcRs to either positively or negatively modulate immune cell activity. Expression of cell-type specific combinations and numbers of receptors therefore ultimately sets a threshold for induction of effector responses. Mechanistically, receptor cross-linking and localization to lipid rafts, i.e., organized membrane microdomains enriched in intracellular signaling proteins, were proposed as major determinants of initial FcR activation. Given that immune cell membranes might also vary in their lipid compositions, it is reasonable to speculate, that the cell membrane and especially lipid rafts serve as an additional regulator of FcR activity. In this article, we aim to summarize the current knowledge on the interplay of lipid rafts and IgG binding FcRs with a focus on the plasma membrane composition and receptor localization in immune cells, the proposed mechanisms underlying this localization and consequences for FcR function with respect to their immunoregulatory capacity.

Regulation of antibody effector functions through IgG Fc N-glycosylation

Immunoglobulin gamma (IgG) antibodies are key effector proteins of the immune system. They recognize antigens with high specificity and are indispensable for immunological memory following pathogen exposure or vaccination. The constant, crystallizable fragment (Fc) of IgG molecules mediates antibody effector functions such as complement-dependent cytotoxicity, antibody-mediated cellular cytotoxicity, and antibody-dependent cell-mediated phagocytosis. These functions are regulated by a single N-linked, biantennary glycan of the heavy chain, which resides just below the hinge region, and the presence of specific sugar moieties on the glycan has profound implications on IgG effector functions. Emerging knowledge of how Fc glycans contribute to IgG structure and functions has opened new avenues for the therapeutic exploitation of defined antibody glycoforms in the treatment of cancer and autoimmune diseases. Here, we review recent advances in understanding proinflammatory IgG effector functions and their regulation by Fc glycans.

Monomeric IgG1 Fc molecules displaying unique Fc receptor interactions that are exploitable to treat inflammation-mediated diseases

The IgG1 Fc is a dimeric protein that mediates important antibody effector functions by interacting with Fcγ receptors (FcγRs) and the neonatal Fc receptor (FcRn). Here, we report the discovery of a monomeric IgG1 Fc (mFc) that bound to FcγRI with very high affinity, but not to FcγRIIIa, in contrast to wild-type (dimeric) Fc. The binding of mFc to FcRn was the same as that of dimeric Fc. To test whether the high-affinity binding to FcγRI can be used for targeting of toxins, a fusion protein of mFc with a 38 kDa Pseudomonas exotoxin A fragment (PE38), was generated. This fusion protein killed FcγRI-positive macrophage-like U937 cells but not FcγRI-negative cells, and mFc or PE38 alone had no killing activity. The lack of binding to FcγRIIIa resulted in the absence of Fc-mediated cytotoxicity of a scFv-mFc fusion protein targeting mesothelin. The pharmacokinetics of mFc in mice was very similar to that of dimeric Fc. The mFc's unique FcγRs binding pattern and related functionality, combined with its small size, monovalency and the preservation of FcRn binding which results in relatively long half-life in vivo, suggests that mFc has great potential as a component of therapeutics targeting inflammation mediated by activated macrophages overexpressing FcγRI and related diseases, including cancer.

The binding of soluble recombinant human Fcγ receptor I for human immunoglobulin G is conferred by its first and second extracellular domains

Human FcγRI is a high affinity receptor for the Fc portion of human immunoglobulin G (IgG), and has extracellular, transmembrane and cytoplasmic regions. The extracellular region of human FcγRI, which is the part that interacts with human IgG, is comprised of three immunoglobulin-like domains. Unlike low affinity Fcγ receptors (FcγRII and FcγRIII), FcγRI has a unique third extracellular domain (D3). This study investigated the contribution of D3 to the binding between recombinant human FcγRI (rhFcγRI) and human IgG. The three extracellular domains and the first and second extracellular domains of human FcγRI were expressed by Escherichia coli as rhFcγRI and rhFcγRI-D1D2, respectively. The binding specificity of rhFcγRI-D1D2 to human IgG subclasses was the same as that of rhFcγRI. From surface plasmon resonance analysis, the binding affinity of rhFcγRI-D1D2 for human IgG1/κ was high (the equilibrium dissociation constant: KD=8.04 × 10(-10)M), but slightly lower than that of rhFcγRI (KD=2.59 × 10(-10)M). While the association of rhFcγRI-D1D2 with human IgG1/κ was same as that of rhFcγRI, the dissociation of rhFcγRI-D1D2 was faster than that of rhFcγRI. From these results, D3 of rhFcγRI would not contribute directly to the binding specificity and association of rhFcγRI, but to the holding bound human IgG.

FcγRIIb on liver sinusoidal endothelium clears small immune complexes

It has long been known that the ITIM-bearing IgG Fc receptor (FcγRIIb, RIIb) is expressed on liver sinusoidal endothelial cells (LSEC) and that the liver is the major site of small immune complex (SIC) clearance. Thus, we proposed that RIIb of LSEC eliminates blood-borne SIC, thereby controlling immune complex-mediated autoimmune disease. Testing this hypothesis, we found most RIIb of the mouse, fully three-quarters, to be expressed in liver. Moreover, most (90%) liver RIIb was expressed in LSEC, the remainder in Kupffer cells. An absent FcRγ in LSEC implied that RIIb is the sole FcγR expressed. Testing the capacity of liver RIIb to clear blood-borne SIC, we infused mice intravenously with radio-iodinated SIC made of OVA and rabbit IgG anti-OVA. Tracking decay of SIC from the blood, we found the RIIb knockout strain to be severely deficient in eliminating SIC compared with the wild-type strain, terminal half-lives being 6 and 1.5 h, respectively. RIIb on LSEC, a major scavenger, keeps SIC blood concentrations low and minimizes pathologic deposition of inflammatory immune complex.

Serine phosphorylation of FcγRI cytoplasmic domain directs lipid raft localization and interaction with protein 4.1G

The high-affinity IgG receptor (CD64, FcγRI) has several special capacities, including the receptor-stimulated cleavage of the cell surface B cell-activating factor of the TNF superfamily (TNFSF13B). With the use of the yeast two-hybrid system, we and others have shown that FcγRI interacts with protein 4.1G (EPB41L2). Our mutational analyses identified two required 4.1G-interacting regions in the FcγRI CY and one FcγRI-interacting site in the C-terminus of protein 4.1G. Herein, we explore mechanism(s) that may regulate the interaction between protein 4.1G and FcγRI CY and influence FcγRI membrane mobility and function. We show that FcγRI CY interacts with protein 4.1G in vitro and that FcγRI coimmunoprecipitates protein 4.1G in freshly isolated human PBMC. With the use of immunostaining, we show that FcγRI colocalizes with protein 4.1G in unstimulated U937 cells, in which the FcγRI CY is constitutively serine-phosphorylated, but significant uncoupling occurs following FcγRI cross-linking, suggesting phosphoserine-regulated interaction. In vitro, protein 4.1G interacted preferentially with CK2-phosphorylated FcγRI CY, and compared with WT FcγRI, a nonphosphorylatable FcγRI mutant receptor was excluded from lipid rafts, suggesting a key role for protein 4.1G in targeting phosphorylated FcγRI to rafts. These data are consistent with a phosphoserine-dependent tethering role for protein 4.1G in maintaining FcγRI in lipid rafts and provide insight into the unique phosphoserine-based regulation of receptor signaling by FcγRI CY.

Anti-severe acute respiratory syndrome coronavirus spike antibodies trigger infection of human immune cells via a pH- and cysteine protease-independent FcγR pathway

Public health measures successfully contained outbreaks of the severe acute respiratory syndrome coronavirus (SARS-CoV) infection. However, the precursor of the SARS-CoV remains in its natural bat reservoir, and reemergence of a human-adapted SARS-like coronavirus remains a plausible public health concern. Vaccination is a major strategy for containing resurgence of SARS in humans, and a number of vaccine candidates have been tested in experimental animal models. We previously reported that antibody elicited by a SARS-CoV vaccine candidate based on recombinant full-length Spike-protein trimers potentiated infection of human B cell lines despite eliciting in vivo a neutralizing and protective immune response in rodents. These observations prompted us to investigate the mechanisms underlying antibody-dependent enhancement (ADE) of SARS-CoV infection in vitro. We demonstrate here that anti-Spike immune serum, while inhibiting viral entry in a permissive cell line, potentiated infection of immune cells by SARS-CoV Spike-pseudotyped lentiviral particles, as well as replication-competent SARS coronavirus. Antibody-mediated infection was dependent on Fcγ receptor II but did not use the endosomal/lysosomal pathway utilized by angiotensin I converting enzyme 2 (ACE2), the accepted receptor for SARS-CoV. This suggests that ADE of SARS-CoV utilizes a novel cell entry mechanism into immune cells. Different SARS vaccine candidates elicit sera that differ in their capacity to induce ADE in immune cells despite their comparable potency to neutralize infection in ACE2-bearing cells. Our results suggest a novel mechanism by which SARS-CoV can enter target cells and illustrate the potential pitfalls associated with immunization against it. These findings should prompt further investigations into SARS pathogenesis.

Cytokine-induced immune complex binding to the high-affinity IgG receptor, FcγRI, in the presence of monomeric IgG

FcγRI is the sole high-affinity immunoglobulin G (IgG) receptor on leukocytes. Its role in immunity and the clearance of opsonized particles has been challenged, as the receptor function may well be hindered by serum IgG. Here, we document immune complex binding by FcγRI to be readily enhanced by cytokine stimulation, whereas binding of monomeric IgG only modestly increased. Enhanced immune complex binding was independent of FcγRI surface expression levels. FcγRI, saturated with prebound IgG, was found capable of effective immune complex binding upon cytokine stimulation. Cytokine-enhanced binding was observed across a variety of immune complexes, including huIgG3- or mIgG2a-opsonized red blood cells, rituximab- or ofatumumab-opsonized B-cell lymphoma, and cetuximab-opsonized glioblastoma cells. This study contributes to our understanding of how FcγRI can participate in the clearance of opsonized particles despite saturation by monomeric IgG.

Specificity and affinity of human Fcγ receptors and their polymorphic variants for human IgG subclasses

Distinct genes encode 6 human receptors for IgG (hFcγRs), 3 of which have 2 or 3 polymorphic variants. The specificity and affinity of individual hFcγRs for the 4 human IgG subclasses is unknown. This information is critical for antibody-based immunotherapy which has been increasingly used in the clinics. We investigated the binding of polyclonal and monoclonal IgG1, IgG2, IgG3, and IgG4 to FcγRI; FcγRIIA, IIB, and IIC; FcγRIIIA and IIIB; and all known polymorphic variants. Wild-type and low-fucosylated IgG1 anti-CD20 and anti-RhD mAbs were also examined. We found that (1) IgG1 and IgG3 bind to all hFcγRs; (2) IgG2 bind not only to FcγRIIAH131, but also, with a lower affinity, to FcγRIIAR131 and FcγRIIIAV158; (3) IgG4 bind to FcγRI, FcγRIIA, IIB and IIC and FcγRIIIAV158; and (4) the inhibitory receptor FcγRIIB has a lower affinity for IgG1, IgG2, and IgG3 than all other hFcγRs. We also identified parameters that determine the specificity and affinity of hFcγRs for IgG subclasses. These results document how hFcγR specificity and affinity may account for the biological activities of antibodies. They therefore highlight the role of specific hFcγRs in the therapeutic and pathogenic effects of antibodies in disease.

FcgammaRIV is a mouse IgE receptor that resembles macrophage FcepsilonRI in humans and promotes IgE-induced lung inflammation

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

Signaling through Fc gamma RIII is required for optimal T helper type (Th)2 responses and Th2-mediated airway inflammation

Although inhibitory Fc gamma receptors have been demonstrated to promote mucosal tolerance, the role of activating Fc gamma receptors in modulating T helper type (Th)2-dependent inflammatory responses characteristic of asthma and allergies remains unclear. Here, we demonstrate that signaling via activating Fc gamma receptors in conjunction with Toll-like receptor 4 stimulation modulated cytokine production from bone marrow-derived dendritic cells (DCs) and augmented their ability to promote Th2 responses. Ligation of the low affinity receptor Fc gamma RIII was specifically required for the enhanced Th2 responses, as Fc gamma RIII(-/-) DCs failed to augment Th2-mediated airway inflammation in vivo or induce Th2 differentiation in vitro. Further, Fc gamma RIII(-/-) mice had impaired Th2 cytokine production and exhibited reduced airway inflammation, whereas no defect was found in Fc gamma RI(-/-) mice. The augmentation of Th2 immunity was regulated by interleukin 10 production from the DCs but was distinct and independent of the well-established role of Fc gamma RIII in augmenting antigen presentation. Thus, our studies reveal a novel and specific role for Fc gamma RIII signaling in the regulation of Th cell responses and suggest that in addition to immunoglobulin (Ig)E, antigen-specific IgG also contributes to the pathogenesis of Th2-mediated diseases such as asthma and allergies.

Affinity of C-reactive protein toward FcgammaRI is strongly enhanced by the gamma-chain

C-reactive protein (CRP), the prototype human acute phase protein, is widely regarded as a key player in cardiovascular disease, but the identity of its cellular receptor is still under debate. By using ultrasensitive confocal imaging analysis, we have studied CRP binding to transfected COS-7 cells expressing the high-affinity IgG receptor FcgammaRI. Here we show that CRP binds to FcgammaRI on intact cells, with a kd of 10+/-3 micromol/L. Transfection of COS-7 cells with a plasmid coding for both FcgammaRI and its functional counterpart, the gamma-chain, markedly increases CRP affinity to FcgammaRI, resulting in a kd of 0.35+/-0.10 micromol/L. The affinity increase results from an approximately 30-fold enhanced association rate coefficient. The pronounced enhancement of affinity by the gamma-chain suggests its crucial involvement in the CRP receptor interaction, possibly by mediating interactions between the transmembrane moieties of the receptors. Dissociation of CRP from the cell surfaces cannot be detected throughout the time course of several hours and is thus extremely slow. Considering the pentameric structure of CRP, this result indicates that multivalent binding and receptor clustering are crucially involved in the interaction of CRP with nucleated cells.

Affinity and kinetic analysis of Fcgamma receptor IIIa (CD16a) binding to IgG ligands

Binding of pathogen-bound immunoglobulin G (IgG) to cell surface Fc gamma receptors (FcgammaRs) triggers a wide variety of effector functions. The binding kinetics and affinities of IgG-FcgammaR interactions are hence important parameters for understanding FcgammaR-mediated immune functions. We have measured the kinetic rates and equilibrium dissociation constants of IgG binding to a soluble FcgammaRIIIa fused with Ig Fc (sCD16a) using the surface plasmon resonance technique. sCD16a interacted with monomeric human IgG and its subtypes IgG1 and IgG3 as well as rabbit IgG with on-rates of 6.5 x 10(3), 8.2 x 10(3), 1.1 x 10(4) and 1.8 x 10(4) m(-1) s(-1), off-rates of 4.7 x 10(-3), 5.7 x 10(-3), 5.9 x 10(-3), and 1.9 x 10(-2) s(-1), and equilibrium dissociation constants of 0.72, 0.71, 0.56, and 1.1 mum, respectively. The kinetics and affinities measured by surface plasmon resonance agreed with those obtained from real time flow cytometry and competition inhibition binding experiments using cell surface CD16a. These data add to our understanding of IgG-FcgammaR interactions.

Differential enhancement of dengue virus immune complex infectivity mediated by signaling-competent and signaling-incompetent human Fcgamma RIA (CD64) or FcgammaRIIA (CD32)

Fcgamma receptor (FcgammaR)-mediated entry of infectious dengue virus immune complexes into monocytes/macrophages is hypothesized to be a key event in the pathogenesis of complicated dengue fever. FcgammaRIA (CD64) and FcgammaRIIA (CD32), which predominate on the surface of such dengue virus-permissive cells, were compared for their influence on the infectivity of dengue 2 virus immune complexes formed with human dengue virus antibodies. A signaling immunoreceptor tyrosine-based activation motif (ITAM) incorporated into the accessory gamma-chain subunit that associates with FcgammaRIA and constitutively in FcgammaRIIA is required for phagocytosis mediated by these receptors. To determine whether FcgammaRIA and FcgammaRIIA activation functions are also required for internalization of infectious dengue virus immune complexes, we generated native and signaling-incompetent versions of each receptor by site-directed mutagenesis of ITAM tyrosine residues. Plasmids designed to express these receptors were transfected into COS-7 cells, and dengue virus replication was measured by plaque assay and flow cytometry. We found that both receptors mediated enhanced dengue virus immune complex infectivity but that FcgammaRIIA appeared to do so far more effectively. Abrogation of FcgammaRIA signaling competency, either by expression without gamma-chain or by coexpression with gamma-chain mutants, was associated with significant impairment of phagocytosis and of dengue virus immune complex infectivity. Abrogation of FcgammaRIIA signaling competency was also associated with equally impaired phagocytosis but had no discernible effect on dengue virus immune complex infectivity. These findings point to fundamental differences between FcgammaRIA and FcgammaRIIA with respect to their immune-enhancing capabilities and suggest that different mechanisms of dengue virus immune complex internalization may operate between these FcgammaRs.

IgG Fc receptor III homologues in nonhuman primate species: genetic characterization and ligand interactions

Ig Fc receptors bind to immune complexes through interactions with the Fc regions of specific Ab subclasses to initiate or inhibit the defense mechanisms of the leukocytes on which they are expressed. The mechanism of action of IgG-based therapeutic molecules, which are routinely evaluated in nonhuman primate models, involves binding to the low-affinity FcRIII (CD16). The premise that IgG/CD16 interactions in nonhuman primates mimic those present in humans has not been evaluated. Therefore, we have identified and characterized CD16 and associated TCR zeta-chain homologues in rhesus macaques, cynomolgus macaques, baboons, and sooty mangabeys. Similar to humans, CD16 expression was detected on a lymphocyte subpopulation, on monocytes, and on neutrophils of sooty mangabeys. However, CD16 was detected only on a lymphocyte subpopulation and on monocytes in macaques and baboons. A nonhuman primate rCD16 generated in HeLa cells interacted with human IgG1 and IgG2. By contrast, human CD16 binds to IgG1 and IgG3. As shown for humans, the mAb 3G8 was able to block IgG binding to nonhuman primate CD16 and inhibition of nonhuman primate CD16 N-glycosylation enhanced IgG binding. Clearly, differences in interaction with IgG subclasses and in cell-type expression should be considered when using these models for in vivo evaluation of therapeutic Abs.

FcγRIV: A Novel FcR with Distinct IgG Subclass Specificity

Mouse IgG subclasses display a hierarchy of in vivo activities, with IgG2a and IgG2b showing the greatest protective and pathogenic properties. These enhanced activities result, in part, from their ability to bind to a novel, gamma chain-dependent, activating IgG Fc receptor, FcgammaRIV. FcgammaRIV maps in the 75 kb genomic interval between FcgammaRII and FcgammaRIII; its expression is restricted to myeloid lineage cells, and it binds to IgG2a and IgG2b with intermediate affinity. No binding to IgG1 or IgG3 was observed. Blocking FcgammaRIV binding to pathogenic anti-platelet antibodies is sufficient to protect mice from antibody-induced thrombocytopenia. Thus, the FcgammaR system has evolved distinct activation receptors displaying selectivity for IgG subclasses, with IgG1 antibodies exclusively dependent on FcgammaRIII, whereas IgG2a and IgG2b show preferential dependence on FcgammaRIV activation. These distinct binding affinities for the IgG subclasses to FcgammaRs account for their differential protective and pathogenic activities in vivo.

Functional regulation of human neutrophil Fc gamma receptors

Interaction between Fc receptors expressed on phagocytic cells and antibodies play a critical role in innate immune response. Interestingly, immune cells such as neutrophils, monocytes, and dendritic cells (DCs) express multiple Fc receptors for IgG (FcgammaR) with overlapping ligand specificity. These receptors compete for the same ligand on the target and are known to transduce positive and negative signals to the same cell, depending on presence of type of signaling motif in their cytoplasmic domain. Neutrophils, the first line of defense against bacterial infection and the major phagocytic cell in the blood, express two types of FcgammaRs depending on the species. In humans, the neutrophils co-express immunoreceptor tyrosine-based activation motif (ITAM) containing CD32A and glycosyl-phosphatidyl inositol (GPI)-anchored CD16B, which is in contrast to co-expression of ITAM containing CD16A and ITIM containing CD32B in mouse neutrophils. Recent studies in gene knockout mice have demonstrated that the negative signaling by CD32B plays a critical role in preventing immune complex (IC)-mediated autoimmune diseases by regulating the activation signal delivered by CD16A. However, it is not known how the function of ITAM signaling CD32A is regulated in human neutrophils. Recent observations from our laboratory suggest that in human neutrophils, the CD32A receptor is regulated at the ligand-binding stage. Using a CD16B-deficient donor, we found that the CD32A expressed on resting neutrophils is unable to bind ligand; however, once neutrophils are activated with fMLP, a bacterial chemotactic peptide, the CD32A is functionally active in binding ligand. We also observed that this regulation is neutrophil-specific phenomenon. These observations suggest that FcgammaR can be regulated by distinct mechanisms and factors such as membrane-anchoring, cell-specific signaling, and avidity modulation that may be coordinately involved in regulating the function of human FcgammaR. Because neutrophils may be activated during infectious and inflammatory diseases, the knowledge of functional regulation of FcgammaR will be useful in designing therapies for many autoimmune diseases.

Modulation of FcγRI (CD64) Ligand Binding by Blocking Peptides of Periplakin

FcgammaRI requires both the intracellular domain of the alpha-chain and associated leukocyte Fc receptor (FcR) gamma-chains for its biological function. We recently found the C terminus of periplakin to selectively interact with the cytoplasmic domain of the FcgammaRI alpha-chain. It thereby enhances the capacity of FcgammaRI to bind, internalize, and present antigens on MHC class II. Here, we characterized the domains involved in FcgammaRI-periplakin interaction using truncated and alanine-substituted FcgammaRI mutants and randomly mutagenized periplakin. This allowed us to design TAT peptides that selectively interfered with endogenous FcgammaRI-periplakin interactions. The addition of these peptides to FcgammaRI-expressing cells modulated FcgammaRI ligand binding, as assessed by erythrocyte-antibody-rosetting. These data support a dominant-negative role of C-terminal periplakin for FcgammaRI biological activity and implicate periplakin as a novel regulator of FcgammaRI in immune cells.

Fc receptors and the common gamma-chain in experimental autoimmune encephalomyelitis

Fcgamma receptors (FcgammaRs), composed of a ligand-binding alpha-chain (FcRalpha) sometimes associated with the homodimeric, cell-signaling common gamma-chain (FcRgamma), comprise an important family of effector molecules linking humoral and cell-mediated adaptive immunity and regulating innate immunity. In peripheral autoimmune diseases, FcgammaRs contribute to inflammation and tissue damage through inappropriate activation of macrophages and neutrophils, release of cytokines and oxidants, and destruction of autoantibody-opsonized cells. In the central nervous system (CNS), the role of FcgammaRs in autoimmune disease such as multiple sclerosis (MS) remains largely unexplored despite extensive documentation of CNS-specific antibodies in cerebrospinal fluid and plaques. Several studies have now examined the role of FcgammaRs in experimental autoimmune encephalomyelitis (EAE), the animal model for MS, using mice genetically deficient in one or more FcgammaRs or in FcRgamma. These studies indicate that none of the FcgammaR alpha-chains are critical for EAE development and progression. In contrast, it is unequivocal that FcRgamma contributes to EAE, and surprisingly it seems that this effect is independent of FcgammaRs. Recent studies now indicate that FcRgamma expression in gammadelta T cells, most likely as a component of the TCR/CD3 signaling complex, is a critical requirement for EAE development. These studies support previous evidence implicating a pathogenic role for gammadelta T cells in EAE.

Direct interaction between FcgammaRI (CD64) and periplakin controls receptor endocytosis and ligand binding capacity

FcgammaRI depends for its biological function on both the intracellular domain of the alpha-chain and associated Fc receptor (FcR) gamma-chains. However, functional protein effectors of FcgammaRI's intracellular domain have not been identified. In this study, we identified periplakin (PPL) as a selective interacting protein for the intracellular tail of FcgammaRI but no other activatory FcRs. The interaction was confirmed by coimmunoprecipitation and blot-overlay assays. PPL and FcgammaRI colocalized at the plasma membrane in monocytes and cell transfectants, and both were up-regulated by IFN-gamma. By expressing C-terminal PPL in transfectants, we established a pivotal role for this protein in FcgammaRI ligand binding, endocytosis, and antigen presentation. These data illustrate that intracellular protein interactions with a multisubunit FcR alpha-chain can confer unique properties to the receptor.

Transmembrane domains in the functions of Fc receptors

In the present study, we use a novel method, PHDhtm, to predict the exact locations and extents of the transmembrane (TM) domains of multisubunit immunoglobulin Fc-receptors. Whereas most previous studies have used single residue hydrophobicity plots for characterizing of these domains, PHDhtm utilizes a system of neural networks and the evolutionary information contained in multiple alignments of related sequences to predict the above. Present PHDhtm application predicts TM domains of immunoglobulin Fc-receptors that in many cases differ significantly from those derived by using earlier methods. Comparisons of helical wheel projections of the presently derived TM domains from PHDhtm with those produced earlier reveal different hydrophobic moments as well as hydrophobic and hydrophilic surfaces. These differences probably alter the character of subunit association within the receptor complexes. This new algorithm can also be used for other membrane protein complexes and may advance both understanding the principles underlying such complexes formation and design of peptides that can interfere with such TM domain association so as to modulate specific cellular responses.

FcgammaR polymorphisms: Implications for function, disease susceptibility and immunotherapy

Leukocyte Fcgamma receptors (FcgammaR) confer potent cellular effector functions to the specificity of IgG. FcgammaR-induced leukocyte functions, including antibody-dependent cellular cytotoxicity, phagocytosis, superoxide generation, degranulation, cytokine production and regulation of antibody production, are essential for host defense and immune regulation. The efficacy of IgG-induced FcgammaR function displays inter-individual heterogeneity due to genetic polymorphisms of three FcgammaR subclasses, FcgammaRIIa (CD32a), FcgammaRIIIa (CD16a), and FcgammaRIIIb (CD16b). FcgammaR polymorphisms have been associated with infectious and autoimmune disease, or with disease severity. FcgammaR polymorphisms may furthermore serve as markers for therapeutic efficacy and side-effects of treatment with monoclonal antibodies. In this review, FcgammaR function and the relevance of FcgammaR polymorphisms as prognostic markers for inflammatory disease and antibody-based immunotherapy are discussed.

The CY domain of the Fcgamma RIa alpha-chain (CD64) alters gamma-chain tyrosine-based signaling and phagocytosis

Although the cytoplasmic domain of the human FcgammaRIa alpha-chain lacks tyrosine-based phosphorylation motifs, it modulates receptor cycling and receptor-specific cytokine production. The cytoplasmic domain of FcgammaRIa is constitutively phosphorylated, and the inhibition of dephosphorylation with okadaic acid, an inhibitor of type 1 and type 2A protein serine/threonine phosphatase, inhibits both receptor-induced activation of the early tyrosine phosphorylation cascade and receptor-specific phagocytosis. To explore the basis for these effects of the cytoplasmic domain of FcgammaRIa, we developed a series of human FcgammaRIa molecular variants, expressed in the murine macrophage cell line P388D1, and demonstrate that serine phosphorylation of the cytoplasmic domain is an important regulatory mechanism. Truncation of the cytoplasmic domain and mutation of the cytoplasmic domain serine residues to alanine abolish the okadaic acid inhibition of phagocytic function. In contrast, the serine mutants did not recapitulate the selective effects of cytoplasmic domain truncation on cytokine production. These results demonstrate for the first time a direct functional role for serine phosphorylation in the alpha-chain of FcgammaRIa and suggest that the cytoplasmic domain of FcgammaRI regulates the different functional capacities of the FcgammaRIa-receptor complex.

Mac-1 (CD11b/CD18) as Accessory Molecule for FcαR (CD89) Binding of IgA

IgA, the principal ligand for FcalphaRI, exists in serum as monomeric IgA and at mucosal sites as secretory IgA (SIgA). SIgA consists of dimeric IgA linked by joining chain and secretory components. Human polymorphonuclear leukocytes (PMN) and mouse PMN transgenic for human FcalphaRI exhibited spreading and elicited respiratory burst activity upon interaction with either serum or SIgA. However, PMN devoid of the beta(2) integrin Mac-1 (Mac-1(-/-)) were unable to bind SIgA, despite expression of FcalphaRI. Consistent with this, serum IgA stimulated Mac-1(-/-) PMN oxygen radical production, in contrast to SIgA. Binding studies showed the secretory component, by itself, to interact with Mac-1-expressing PMN, but not with Mac-1(-/-) PMN. These data demonstrate an essential role for Mac-1 in establishing SIgA-FcalphaRI interactions.

Mutagenesis within human FcepsilonRIalpha differentially affects human and murine IgE binding

Soluble fragments of the alpha-chain of FcepsilonRI, the high-affinity receptor for IgE, compete with membrane-bound receptors for IgE and may thus provide a means to combat allergic responses. Mutagenesis within FcepsilonRIalpha is used in this study, in conjunction with the crystal structure of the FcepsilonRIalpha/IgE complex, to define the relative importance of specific residues within human FcepsilonRIalpha for IgE binding. We have also compared the effects of these mutants on binding to both human and mouse IgE, with a view to evaluating the mouse as an appropriate model for the analysis of future agents designed to mimic the human FcepsilonRIalpha and attenuate allergic disease. Three residues within the C-C' region of the FcepsilonRIalpha2 domain and two residues within the alpha2 proximal loops of the alpha1 domain were selected for mutagenesis and tested in binding assays with human and mouse IgE. All three alpha2 mutations (K117D, W130A, and Y131A) reduced the affinity of human IgE binding to different extents, but K117D had a far more pronounced effect on mouse IgE binding, and although Y131A had little effect, W130A modestly enhanced binding to mouse IgE. The mutations in alpha1 (R15A and F17A) diminished binding to both human and mouse IgE, with these effects most likely caused by disruption of the alpha1/alpha2 interface. Our results demonstrate that the effects of mutations in human FcepsilonRIalpha on mouse IgE binding, and hence the inhibitory properties of human receptor-based peptides assayed in rodent models of allergy, may not necessarily reflect their activity in a human IgE-based system.

Cloning and sequencing of a cDNA encoding the bovine FcR gamma chain

Phagocytic cells of the immune system express specific receptors for the Fc region of immunoglobulins (FcRs). In humans, most FcRs for IgG (FcgammaR), IgA (FcalphaR) and IgE (FcvarepsilonR) consist of an immunoglobulin (Ig) -binding subunit associated with a specialized signaling molecule, the FcR gamma chain. The FcR gamma chain is crucial for the transmission of intracellular signals following receptor ligation. In cattle, however, although four distinct complimentary DNAs (cDNAs) encoding IgG-binding subunits have been described (corresponding to bovine FcgammaRI, FcgammaRII, FcgammaRIII, and Fcgamma2R), virtually, nothing is known about signal transduction via bovine FcRs. Therefore, in this study, a cDNA encoding the bovine FcR gamma chain was cloned. The cDNA is 258 base pairs long and encodes a protein of 85 amino-acids. The mature protein shows high homology with the FcR gamma chains from several other species. Interestingly, the cytoplasmic domain of the bovine FcR gamma chain is one amino-acid shorter than those previously described. Cloning of a cDNA encoding, the bovine FcR gamma chain will allow for a better understanding of signal transduction processes triggered by bovine FcRs.

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.

High resolution mapping of the binding site on human IgG1 for Fc gamma RI, Fc gamma RII, Fc gamma RIII, and FcRn and design of IgG1 variants with improved binding to the Fc gamma R

Immunoglobulin G (IgG) Fc receptors play a critical role in linking IgG antibody-mediated immune responses with cellular effector functions. A high resolution map of the binding site on human IgG1 for human Fc gamma RI, Fc gamma RIIA, Fc gamma RIIB, Fc gamma RIIIA, and FcRn receptors has been determined. A common set of IgG1 residues is involved in binding to all Fc gamma R; Fc gamma RII and Fc gamma RIII also utilize residues outside this common set. In addition to residues which, when altered, abrogated binding to one or more of the receptors, several residues were found that improved binding only to specific receptors or simultaneously improved binding to one type of receptor and reduced binding to another type. Select IgG1 variants with improved binding to Fc gamma RIIIA exhibited up to 100% enhancement in antibody-dependent cell cytotoxicity using human effector cells; these variants included changes at residues not found at the binding interface in the IgG/Fc gamma RIIIA co-crystal structure (Sondermann, P., Huber, R., Oosthuizen, V., and Jacob, U. (2000) Nature 406, 267-273). These engineered antibodies may have important implications for improving antibody therapeutic efficacy.

DAP12 and KAP10 (DAP10)-novel transmembrane adapter proteins of the CD3zeta family

Transmembrane adapter proteins are molecules that associate with receptors and mediate intracellular signals following interaction of the receptor with its ligand. Many such molecules have been characterized in detail, particularly the small TM adapters of the CD3zeta class at the core of the T cell receptor. Recently, two new genetically linked members of this class of transmembrane adapters have been identified called DAP12 (KARAP) and KAP10 (DAP10), respectively. In this review, we discuss this new class of TM adapters using the wealth of knowledge concerning CD3zeta and FcRgamma to highlight similarities and differences with DAP12 and KAP10. In addition, novel receptor families which interact with these TM adapters have also been identified. The role of these receptors and their inhibitory isoforms are discussed.

Stimulatory and inhibitory signals originating from the macrophage Fcgamma receptors

The macrophage receptors for the Fc portion of immunoglobulin G (FcgammaR) have long been known to mediate a variety of effector functions that are vital to the adaptive immune response. Recent studies, however, have begun to stress potential regulatory roles that these receptors can play in modulating immune and inflammatory responses. In this article we discuss the activating and inhibitory properties of the individual macrophage FcgammaR and the conditions under which these heterologous responses can occur.

Conformation of the isolated cepsilon3 domain of IgE and its complex with the high-affinity receptor, FcepsilonRI

Immunoglobulin E (IgE) exhibits a uniquely high affinity for its receptor, FcepsilonRI, on the surface of mast cells and basophils. Previous work has implicated the third domain of the constant region of the epsilon-heavy chain (Cepsilon3) in binding to FcepsilonRI, but the smallest fragment of IgE that is known to bind with full affinity is a covalent dimer of the Cepsilon3 and Cepsilon4 domains. We have expressed the isolated Cepsilon3 in Escherichia coli, measured its affinity for FcepsilonRI, and examined its conformation alone and in the complex with FcepsilonRI. Sedimentation equilibrium in the analytical centrifuge reveals that this product is a monomer. The kinetics of binding to an immobilized fragment of the FcepsilonRI alpha-chain, measured by surface plasmon resonance, yields an affinity constant K(a) = 5 x 10(6) M(-)(1), as compared with 4 x 10(9) M(-)(1) for IgE. The circular dichroism spectrum and measurements of fluorescence as a function of the concentration of a denaturant do not reveal any recognizable secondary structure or hydrophobic core. On binding to the FcepsilonRI alpha-chain fragment, there is no change in the circular dichroism spectrum, indicating that the conformation of Cepsilon3 is unchanged in the complex. Thus the isolated Cepsilon3 domain is sufficient for binding to FcepsilonRI, but with lower affinity than IgE. This may be due to the loss of its native immunoglobulin domain structure or to the requirement for two Cepsilon3 domains to constitute the complete binding site for FcepsilonRI or to a combination of these factors.

Markedly different pathogenicity of four immunoglobulin G isotype-switch variants of an antierythrocyte autoantibody is based on their capacity to interact in vivo with the low-affinity Fcgamma receptor III

Using three different Fcgamma receptor (FcgammaR)-deficient mouse strains, we examined the induction of autoimmune hemolytic anemia by each of the four immunoglobulin (Ig)G isotype-switch variants of a 4C8 IgM antierythrocyte autoantibody and its relation to the contributions of the two FcgammaR, FcgammaRI, and FcgammaRIII, operative in the phagocytosis of opsonized particles. We found that the four IgG isotypes of this antibody displayed striking differences in pathogenicity, which were related to their respective capacity to interact in vivo with the two phagocytic FcgammaRs, defined as follows: IgG2a > IgG2b > IgG3/IgG1 for FcgammaRI, and IgG2a > IgG1 > IgG2b > IgG3 for FcgammaRIII. Accordingly, the IgG2a autoantibody exhibited the highest pathogenicity, approximately 20-100-fold more potent than its IgG1 and IgG2b variants, respectively, while the IgG3 variant, which displays little interaction with these FcgammaRs, was not pathogenic at all. An unexpected critical role of the low-affinity FcgammaRIII was revealed by the use of two different IgG2a anti-red blood cell autoantibodies, which displayed a striking preferential utilization of FcgammaRIII, compared with the high-affinity FcgammaRI. This demonstration of the respective roles in vivo of four different IgG isotypes, and of two phagocytic FcgammaRs, in autoimmune hemolytic anemia highlights the major importance of the regulation of IgG isotype responses in autoantibody-mediated pathology and humoral immunity.

The membrane anchor influences ligand binding two-dimensional kinetic rates and three-dimensional affinity of FcgammaRIII (CD16)

Kinetic rates and affinity are essential determinants for biological processes that involve receptor-ligand binding. By using a micropipette method, we measured the kinetics of human Fcgamma receptor III (CD16) interacting with IgG when the two molecules were bound to apposing cellular membranes. CD16 is one of only four eukaryotic receptors known to exist natively in both the transmembrane (TM, CD16a) and glycosylphosphatidylinositol (GPI, CD16b) isoforms. The biological significance of this anchor isoform coexistence is not clear. Here we showed that the anchor influenced kinetic rates; compared with CD16a-TM, CD16a-GPI bound faster and with higher affinities to human and rabbit IgGs but slower and with lower affinity to murine IgG2a. The same differential affinity patterns were observed using soluble IgG ligands. A monoclonal antibody bound CD16a-GPI with higher affinity than CD16a-TM, whereas another monoclonal antibody reacted strongly with CD16a-TM but weakly with CD16a-GPI. No major differential glycosylation between the two CD16a isoforms was detected by SDS-polyacrylamide gel electrophoresis analysis. We suggest a conformational difference as the mechanism underlying the observed anchor effect, as it cannot be explained by the differing diffusivity, flexibility, orientation, height, distribution, or clustering of the two molecules on the cell membrane. These data demonstrate that a covalent modification of an Ig superfamily receptor at the carboxyl terminus of the ectodomain can have an impact on ligand binding kinetics.

Crosslinking of the Human Fc Receptor for IgA (FcαRI/CD89) Triggers FcR γ-Chain-Dependent Shedding of Soluble CD89

CD89/FcαRI is a 55- to 75-kDa type I receptor glycoprotein, expressed on myeloid cells, with important immune effector functions. At present, no information is available on the existence of soluble forms of this receptor. We developed an ELISA for the detection of soluble CD89 (sCD89) forms and investigated the regulation of sCD89 production. PMA/ionomycin stimulation of monocytic cell lines (U937, THP-1, and MM6), but not of neutrophils, resulted in release of sCD89. Crosslinking of CD89 either via its ligand IgA or with anti-CD89 mAbs similarly resulted in sCD89 release. Using CD89-transfected cells, we showed ligand-induced shedding to be dependent on coexpression of the FcR γ-chain subunit. Shedding of sCD89 was dependent on signaling via the γ-chain and prevented by addition of inhibitors of protein kinase C (staurosporine) or protein tyrosine kinases (genistein). Western blotting revealed sCD89 to have an apparent molecular mass of 30 kDa and to bind IgA in a dose-dependent fashion. In conclusion, the present data document a ligand-binding soluble form of CD89 that is released upon activation of CD89-expressing cells. Shedding of CD89 may play a role in fine-tuning CD89 immune effector functions.

The cytoplasmic domain of human FcgammaRIa alters the functional properties of the FcgammaRI.gamma-chain receptor complex

The gamma/zeta-chain family of proteins mediate cell activation for multiple immunoglobulin receptors. However, the recognition that these receptors may have distinct biologic functions suggests that additional signaling elements may contribute to functional diversity. We hypothesized that the cytoplasmic domain (CY) of the ligand binding alpha-chain alters the biological properties of the receptor complex. Using macrophage FcgammaRIa as a model system, we created stable transfectants expressing a full-length or a CY deletion mutant of human FcgammaRIa. Both receptors functionally associate with the endogenous murine gamma-chain. However, we have established that the CY of FcgammaRIa directly contributes to the functional properties of the receptor complex. Deletion of the FcgammaRIa CY leads to slower kinetics of receptor-specific phagocytosis and endocytosis as well as lower total phagocytosis despite identical levels of receptor expression. Deletion of the CY also converts the phenotype of calcium independent FcgammaRIa-specific phagocytosis to a calcium-dependent phenotype. Finally, deletion of the CY abrogates FcgammaRIa-specific secretion of interleukin-6 but does not affect production of interleukin-1beta. These results demonstrate a functional role for the CY of FcgammaRIa and provide a general model for understanding how multiple receptors that utilize the gamma-chain can generate diversity in function.

Influence of the human high-affinity IgG receptor FcgammaRI (CD64) on residual infectivity of neutralized dengue virus

We examined dengue virus immune complex-phagocyte interaction with respect to a single Fc receptor class using a transient expression system involving the high-affinity human macrophage receptor, FcgammaRI. We found that New Guinea C strain dengue 2 virus formed well-defined plaques in normal and transfected COS cells and we analyzed the structural determinants of FcgammaRI-mediated binding and internalization of dengue 2 virus immune complexes by expressing native or truncated forms of the receptor in COS cells, alone or with its accessory gamma chain signaling unit, which bears an immunoreceptor tyrosine-based activation motif (ITAM). The residual infectivity of dengue 2 virus treated with neutralizing human antiserum was strikingly higher in FcgammaRI-bearing COS cells than in controls. Compatible with the IgG subclass specificity of FcgammaRI, this difference was abrogated quantitatively by treatment of FcgammaRI-transfected cells with human IgG1 but not IgG2 myeloma protein. The magnitude of receptor-mediated plaque formation after cotransfection with gamma chain was also significantly higher than in controls but was less than that observed with FcgammaRI transfection only, a difference probably explained by reduced levels of FcgammaRI expression in gamma chain cotransfectants. Deletion of the FcgammaRI cytoplasmic domain had no effect on receptor-mediated immune complex infectivity. We conclude that the FcgammaRI extracellular domain is sufficient for internalization of infectious dengue virus immune complexes through a mechanism that does not involve classical ITAM-dependent signaling.

The FcγRIa (CD64) Ligand Binding Chain Triggers Major Histocompatibility Complex Class II Antigen Presentation Independently of Its Associated FcR γ-Chain

Within multi-subunit Ig receptors, the FcR γ-chain immunoreceptor tyrosine-based activation motif (ITAM) plays a crucial role in enabling antigen presentation. This process involves antigen-capture and targeting to specific degradation and major histocompatibility complex (MHC) class II loading compartments. Antigenic epitopes are then presented by MHC class II molecules to specific T cells. The high-affinity receptor for IgG, hFcγRIa, is exclusively expressed on myeloid lineage cells and depends on the FcR γ-chain for surface expression, efficient ligand binding, and most phagocytic effector functions. However, we show in this report, using the IIA1.6 cell model, that hFcγRIa can potentiate MHC class II antigen presentation, independently of a functional FcR γ-chain ITAM. Immunoelectron microscopic analyses documented hFcγRIa -chain/rabbit IgG-Ovalbumin complexes to be internalized and to migrate via sorting endosomes to MHC class II-containing late endosomes. Radical deletion of the hFcγRIa -chain cytoplasmic tail did not affect internalization of rabbit IgG-Ovalbumin complexes. Importantly, however, this resulted in diversion of receptor-ligand complexes to the recycling pathway and decreased antigen presentation. These results show the hFcγRIa cytoplasmic tail to contain autonomous targeting information for intracellular trafficking of receptor-antigen complexes, although deficient in canonical tyrosine- or dileucine-targeting motifs. This is the first documentation of autonomous targeting by a member of the multichain FcR family that may critically impact the immunoregulatory role proposed for hFcγRIa (CD64).

The FcγRIa (CD64) Ligand Binding Chain Triggers Major Histocompatibility Complex Class II Antigen Presentation Independently of Its Associated FcR γ-Chain

Within multi-subunit Ig receptors, the FcR γ-chain immunoreceptor tyrosine-based activation motif (ITAM) plays a crucial role in enabling antigen presentation. This process involves antigen-capture and targeting to specific degradation and major histocompatibility complex (MHC) class II loading compartments. Antigenic epitopes are then presented by MHC class II molecules to specific T cells. The high-affinity receptor for IgG, hFcγRIa, is exclusively expressed on myeloid lineage cells and depends on the FcR γ-chain for surface expression, efficient ligand binding, and most phagocytic effector functions. However, we show in this report, using the IIA1.6 cell model, that hFcγRIa can potentiate MHC class II antigen presentation, independently of a functional FcR γ-chain ITAM. Immunoelectron microscopic analyses documented hFcγRIa -chain/rabbit IgG-Ovalbumin complexes to be internalized and to migrate via sorting endosomes to MHC class II-containing late endosomes. Radical deletion of the hFcγRIa -chain cytoplasmic tail did not affect internalization of rabbit IgG-Ovalbumin complexes. Importantly, however, this resulted in diversion of receptor-ligand complexes to the recycling pathway and decreased antigen presentation. These results show the hFcγRIa cytoplasmic tail to contain autonomous targeting information for intracellular trafficking of receptor-antigen complexes, although deficient in canonical tyrosine- or dileucine-targeting motifs. This is the first documentation of autonomous targeting by a member of the multichain FcR family that may critically impact the immunoregulatory role proposed for hFcγRIa (CD64).

Alternative endocytic pathway for immunoglobulin A Fc receptors (CD89) depends on the lack of FcRgamma association and protects against degradation of bound ligand

IgA is the most abundant immunoglobulin in mucosal areas but is only the second most common antibody isotype in serum because it is catabolized faster than IgG. IgA exists in monomeric and polymeric forms that function through receptors expressed on effector cells. Here, we show that IgA Fc receptor(s) (FcalphaR) are expressed with or without the gamma chain on monocytes and neutrophils. gamma-less FcalphaR represent a significant fraction of surface FcalphaR molecules even on cells overexpressing the gamma chain. The FcalphaR-gamma2 association is up-regulated by phorbol esters and interferon-gamma. To characterize gamma-less FcalphaR functionally, we generated mast cell transfectants expressing wild-type human FcalphaR or a receptor with a point mutation (Arg --> Leu at position 209) which was unable to associate with the gamma chain. Mutant gamma-less FcalphaR bound monomeric and polymeric human IgA1 or IgA2 but failed to induce exocytosis after receptor clustering. The two types of transfectant showed similar kinetics of FcalphaR-mediated endocytosis; however, the endocytosis pathways of the two types of receptor differed. Whereas mutant FcalphaR were localized mainly in early endosomes, those containing FcalphaR-gamma2 were found in endo-lysosomal compartments. Mutant gamma-less FcalphaR recycled the internalized IgA toward the cell surface and protected against IgA degradation. Cells expressing the two forms of FcalphaR, associated or unassociated with gamma chains, may thus have differential functions either by degrading IgA antibody complexes or by recycling serum IgA.

Interaction of poliovirus with its receptor affords a high level of infectivity to the virion in poliovirus infections mediated by the Fc receptor

Poliovirus infects susceptible cells through the poliovirus receptor (PVR), which functions to bind virus and to change its conformation. These two activities are thought to be necessary for efficient poliovirus infection. How binding and conformation conversion activities contribute to the establishment of poliovirus infection was investigated. Mouse L cells expressing mouse high-affinity Fcgamma receptor molecules were established and used to study poliovirus infection mediated by mouse antipoliovirus monoclonal antibodies (MAbs) (immunoglobulin G2a [IgG2a] subtypes) or PVR-IgG2a, a chimeric molecule consisting of the extracellular moiety of PVR and the hinge and Fc portion of mouse IgG2a. The antibodies and PVR-IgG2a showed the same degree of affinity for poliovirus, but the infectivities mediated by these molecules were different. Among the molecules tested, PVR-IgG2a mediated the infection most efficiently, showing 50- to 100-fold-higher efficiency than that attained with the different MAbs. A conformational change of poliovirus was induced only by PVR-IgG2a. These results strongly suggested that some specific interaction(s) between poliovirus and the PVR is required for high-level infectivity of poliovirus in this system.

Crystal structure of the human high-affinity IgE receptor

Allergic responses result from the activation of mast cells by the human high-affinity IgE receptor. IgE-mediated allergic reactions may develop to a variety of environmental compounds, but the initiation of a response requires the binding of IgE to its high-affinity receptor. We have solved the X-ray crystal structure of the antibody-binding domains of the human IgE receptor at 2.4 A resolution. The structure reveals a highly bent arrangement of immunoglobulin domains that form an extended convex surface of interaction with IgE. A prominent loop that confers specificity for IgE molecules extends from the receptor surface near an unusual arrangement of four exposed tryptophans. The crystal structure of the IgE receptor provides a foundation for the development of new therapeutic approaches to allergy treatment.