A Common Site of the Fc Receptor γ Subunit Interacts with the Unrelated Immunoreceptors FcαRI and FcϵRI

Antibody Fc-receptor FcεR1γ stabilizes cell surface receptors in group 3 innate lymphoid cells and promotes anti-infection immunity

Group 3 innate lymphoid cells (ILC3) are crucial for maintaining mucosal homeostasis and regulating inflammatory diseases, but the molecular mechanisms governing their phenotype and function are not fully understood. Here, we show that ILC3s highly express Fcer1g gene, which encodes the antibody Fc-receptor common gamma chain, FcεR1γ. Genetic perturbation of FcεR1γ leads to the absence of critical cell membrane receptors NKp46 and CD16 in ILC3s. Alanine scanning mutagenesis identifies two residues in FcεR1γ that stabilize its binding partners. FcεR1γ expression in ILC3s is essential for effective protective immunity against bacterial and fungal infections. Mechanistically, FcεR1γ influences the transcriptional state and proinflammatory cytokine production of ILC3s, relying on the CD16-FcεR1γ signaling pathway. In summary, our findings highlight the significance of FcεR1γ as an adapter protein that stabilizes cell membrane partners in ILC3s and promotes anti-infection immunity.

Fc Binding by FcγRIIa Is Essential for Cellular Activation by the Anti-FcγRIIa mAbs 8.26 and 8.2

FcγR activity underpins the role of antibodies in both protective immunity and auto-immunity and importantly, the therapeutic activity of many monoclonal antibody therapies. Some monoclonal anti-FcγR antibodies activate their receptors, but the properties required for cell activation are not well defined. Here we examined activation of the most widely expressed human FcγR; FcγRIIa, by two non-blocking, mAbs, 8.26 and 8.2. Crosslinking of FcγRIIa by the mAb F(ab’)₂ regions alone was insufficient for activation, indicating activation also required receptor engagement by the Fc region. Similarly, when mutant receptors were inactivated in the Fc binding site, so that intact mAb was only able to engage receptors via its two Fab regions, again activation did not occur. Mutation of FcγRIIa in the epitope recognized by the agonist mAbs, completely abrogated the activity of mAb 8.26, but mAb 8.2 activity was only partially inhibited indicating differences in receptor recognition by these mAbs. FcγRIIa inactivated in the Fc binding site was next co-expressed with the FcγRIIa mutated in the epitope recognized by the Fab so that each mAb 8.26 molecule can contribute only three interactions, each with separate receptors, one via the Fc and two via the Fab regions. When the Fab and Fc binding were thus segregated onto different receptor molecules receptor activation by intact mAb did not occur. Thus, receptor activation requires mAb 8.26 Fab and Fc interaction simultaneously with the same receptor molecules. Establishing the molecular nature of FcγR engagement required for cell activation may inform the optimal design of therapeutic mAbs.

C-type lectin MCL is an FcRγ-coupled receptor that mediates the adjuvanticity of mycobacterial cord factor

Cord factor, also called trehalose-6,6'-dimycolate (TDM), is a potent mycobacterial adjuvant. We herein report that the C-type lectin MCL (also called Clec4d) is a TDM receptor that is likely to arise from gene duplication of Mincle (also called Clec4e). Mincle is known to be an inducible receptor recognizing TDM, whereas MCL was constitutively expressed in myeloid cells. To examine the contribution of MCL in response to TDM adjuvant, we generated MCL-deficient mice. TDM promoted innate immune responses, such as granuloma formation, which was severely impaired in MCL-deficient mice. TDM-induced acquired immune responses, such as experimental autoimmune encephalomyelitis (EAE), was almost completely dependent on MCL, but not Mincle. Furthermore, by generating Clec4e(gfp) reporter mice, we found that MCL was also crucial for driving Mincle induction upon TDM stimulation. These results suggest that MCL is an FcRγ-coupled activating receptor that mediates the adjuvanticity of TDM.

Fc receptor-γ subunits with both polar or non-polar amino acids at position of T22 are capable of restoring surface expression of the high-affinity IgE receptor and degranulation in γ subunit-deficient rat basophilic leukemia cells

The high-affinity IgE receptor (FcɛRI) is formed by the IgE-binding α subunit, β subunit and γ subunits homodimer. All three subunits are required for proper expression of the receptor on the plasma membrane of mast cells and basophils. However, the exact molecular mechanism of inter-subunit interactions required for correct expression and function of the FcɛRI complex remains to be identified. A recent study suggested that polar aspartate at position 194 within the transmembrane domain of the α subunit could interact by hydrogen bonding with polar threonine at position 22 in the transmembrane domains of the γ subunits. To verify this, we used previously isolated rat basophilic leukemia (RBL)-2H3 variant cells deficient in the expression of the FcɛRI-γ subunit (FcR-γ), and transfected them with DNA vectors coding for FcR-γ of the wild-type or mutants in which T22 was substituted for nonpolar alanine (T22A mutant) or polar serine (T22S mutant). Analysis of the transfectants showed that both T22A and T22S mutants were capable to restore surface expression of the FcɛRI similar to wild-type FcR-γ. Furthermore, cells transfected with wild-type, T22A or T22S FcR-γ showed comparably enhanced FcɛRI-mediated degranulation. Our data indicate that substitution of FcR-γ T22 with non-polar amino acid does not interfere with surface expression of the FcɛRI and its signaling capacity.

A conserved host and pathogen recognition site on immunoglobulins: structural and functional aspects

A common site in the constant region (Fc) of immunoglobulins is recognized by host receptors and is a frequent target of proteins expressed by pathogens. This site is located at the junction of two constant domains in the antibody heavy chains and produces a large shallow cavity formed by loops of the CH2 and CH3 domains in IgG and IgA (CH3 and CH4 domains in IgM). Crystal structures have been determined for complexes of IgG-Fc and IgA-Fc with a structurally diverse set of host, pathogen and in vitro selected ligands. While pathogen proteins may directly block interactions with the immunoglobulins thereby evading host immunity, it is likely that the same pathogen molecules also interact with other host factors to carry out their primary biological function. Herein we review the structural and functional aspects of host and pathogen molecular recognition of the common site on the Fc of immunoglobulins. We also propose that some pathogen proteins may promote virulence by affecting the bridging between innate and adaptive immunity.

The human immunoglobulin A Fc receptor FcαRI: a multifaceted regulator of mucosal immunity

Immunoglobulin A (IgA) is commonly recognized as the most prevalent antibody (Ab) at mucosal sites with an important role in defense by shielding mucosal surfaces from invasion by pathogens. However, its potential to both actively dampen excessive immune responses or to initiate potent proinflammatory cellular processes is less well known. Interestingly, either functional outcome is mediated through interaction with the myeloid IgA Fc receptor FcαRI (CD89). Monomeric interaction of IgA with FcαRI triggers inhibitory signals that block activation via other receptors, whereas multimeric FcαRI crosslinking induces phagocytosis, reactive oxygen species production, antigen presentation, Ab-dependent cellular cytotoxicity, and cytokine release. Thus, FcαRI acts as a regulator between anti- and proinflammatory responses of IgA. As such, the biology of FcαRI, and its multifaceted role in immunity will be the focus of this review.

Structural basis for Fc gammaRIIa recognition of human IgG and formation of inflammatory signaling complexes

The interaction of Abs with their specific FcRs is of primary importance in host immune effector systems involved in infection and inflammation, and are the target for immune evasion by pathogens. FcγRIIa is a unique and the most widespread activating FcR in humans that through avid binding of immune complexes potently triggers inflammation. Polymorphisms of FcγRIIa (high responder/low responder [HR/LR]) are linked to susceptibility to infections, autoimmune diseases, and the efficacy of therapeutic Abs. In this article, we define the three-dimensional structure of the complex between the HR (arginine, R134) allele of FcγRIIa (FcγRIIa-HR) and the Fc region of a humanized IgG1 Ab, hu3S193. The structure suggests how the HR/LR polymorphism may influence FcγRIIa interactions with different IgG subclasses and glycoforms. In addition, mutagenesis defined the basis of the epitopes detected by FcR blocking mAbs specific for FcγRIIa (IV.3), FcγRIIb (X63-21), and a pan FcγRII Ab (8.7). The epitopes detected by these Abs are distinct, but all overlap with residues defined by crystallography to contact IgG. Finally, crystal structures of LR (histidine, H134) allele of FcγRIIa and FcγRIIa-HR reveal two distinct receptor dimers that may represent quaternary states on the cell surface. A model is presented whereby a dimer of FcγRIIa-HR binds Ag-Ab complexes in an arrangement that possibly occurs on the cell membrane as part of a larger signaling assembly.

Immunoglobulin E receptor signaling and asthma

Elevated IgE levels and increased IgE sensitization to allergens are central features of allergic asthma. IgE binds to the high-affinity Fcε receptor I (FcεRI) on mast cells, basophils, and dendritic cells and mediates the activation of these cells upon antigen-induced cross-linking of IgE-bound FcεRI. FcεRI activation proceeds through a network of signaling molecules and adaptor proteins and is negatively regulated by a number of cell surface and intracellular proteins. Therapeutic neutralization of serum IgE in moderate-to-severe allergic asthmatics reduces the frequency of asthma exacerbations through a reduction in cell surface FcεRI expression that results in decreased FcεRI activation, leading to improved asthma control. Our increasing understanding of IgE receptor signaling may lead to the development of novel therapeutics for the treatment of asthma.

Immunoglobulin A: A next generation of therapeutic antibodies?

Although immunoglobulin (Ig) A is commonly recognized as the most prevalent antibody subclass at mucosal sites with an important role in mucosal defense, its potential as a therapeutic monoclonal antibody is less well known. However, IgA has multifaceted anti-, non-, and pro-inflammatory functions that can be exploited for different immunotherapeutical strategies, which will be the focus of this review.

Characterization of leukocyte mono-immunoglobulin-like receptor 7 (LMIR7)/CLM-3 as an activating receptor: its similarities to and differences from LMIR4/CLM-5

Here we characterize leukocyte mono-Ig-like receptor 7 (LMIR7)/CLM-3 and compare it with an activating receptor, LMIR4/CLM-5, that is a counterpart of an inhibitory receptor LMIR3/CLM-1. LMIR7 shares high homology with LMIR4 in the amino acid sequences of its Ig-like and transmembrane domains. Flow cytometric analysis demonstrated that LMIR4 was predominantly expressed in neutrophils, whereas LMIR7 was highly expressed in mast cells and monocytes/macrophages. Importantly, LMIR7 engagement induced cytokine production in bone marrow-derived mast cells (BMMCs). Although FcRγ deficiency did not affect surface expression levels of LMIR7, it abolished LMIR7-mediated activation of BMMCs. Consistently we found significant interaction of LMIR7-FcRγ, albeit with lower affinity compared with that of LMIR4-FcRγ. Our results showed that LMIR7 transmits an activating signal through interaction with FcRγ. In addition, like LMIR4, LMIR7 synergizes with TLR4 in signaling. Analysis of several chimera receptors composed of LMIR4 and LMIR7 revealed these findings: 1) the transmembrane of LMIR7 with no charged residues maintained its surface expression at high levels in the absence of FcRγ; 2) the extracellular juxtamembrane region of LMIR7 had a negative effect on its surface expression levels; and 3) the strong interaction of LMIR4 with FcRγ depended on the extracellular juxtamembrane region as well as the transmembrane domain of LMIR4. Thus, LMIR7 shares similarities with LMIR4, although they are differentially regulated in their distribution, expression, and function.

Roles for the High Affinity IgE Receptor, FcεRI, of Human Basophils in the Pathogenesis and Therapy of Allergic Asthma: Disease Promotion, Protection or Both?

The role of basophils, the rarest of blood granulocytes, in the pathophysiology of allergic asthma is still incompletely understood. Indirect evidence generated over many decades is consistent with a role for basophils in disease promotion. Recent improvements in procedures to purify and analyze very small numbers of human cells have generally supported this view, but have also revealed new complexities. This chapter focuses on our analyses of Fcε R1 function in basophils in the context of understanding and treating human allergic asthma. In long-term studies, we demonstrated that asthmatic subjects have higher circulating numbers of basophils than non-atopic non-asthmatic subjects and that their basophils show higher rates of both basal and anti-IgE or antigen-stimulated histamine release. These results hint at a direct role for basophils in promoting asthma. Supporting this interpretation, the non-releaser phenotype that we linked to the excessive proteolysis of Syk via the ubiquitin/proteasomal pathway is less common in basophils from asthmatic than non-asthmatic donors. The discovery of a basophil-specific pathway regulating Syk levels presents a clear opportunity for therapy. Another route to therapy was revealed by evidence that basophil FcεRI signaling can be downregulated by co-crosslinking the ITAM-containing IgE receptor, FcγRI, to the ITIM-containing IgG receptor, FcγRIIB. Based on this discovery, hybrid co-crosslinking fusion proteins are being engineered as potential therapies targeting basophils. A third distinguishing property of human basophils is their high dependence on IgE binding to stabilize membrane FcεRI. The circulating IgE scavenging mAb, Omalizumab, reduces FcεRI expression in basophils from asthmatics by over 95% and produces a substantial impairment of IL-4, IL-8 and IL-13 production in response to the crosslinking of residual cell surface IgE-FcεRI. A search for small molecule inhibitors that similarly impair high affinity IgE binding to basophils may yield reagents that mimic Omalizumab's therapeutic benefits without the potential for immune side effects. Although studies on allergen and FcεRI-mediated basophil activation all point to a role in promoting disease, a case can also be made for an alternative or additional role of basophil FcεRI in protection against allergic asthma. Human basophils have high affinities for IgE, they upregulate receptor levels over a >100-fold range as circulating IgE levels increase and they have short half-lives in the circulation. Thus, when allergen is absent, basophil FcεRI could serve as scavengers of serum IgE and therefore protectors against mast cell IgE-mediated inflammatory responses. Further studies are clearly needed to determine if FcεR-expressing basophils play pathogenic or protective roles - or both - in human allergic asthma and other IgE-mediated inflammatory disorders.

The first transmembrane region of the beta-chain stabilizes the tetrameric Fc epsilon RI complex

The family of activating immune receptors stabilizes via the 3-helix assembly principle. A charged basic transmembrane residue interacts with two charged acidic transmembrane residues and forms a 3-helix interface to stabilize receptor complexes in the lipid bilayer. One family member, the high affinity receptor for IgE, Fc epsilon RI, is a key regulator of immediate allergic responses. Tetrameric Fc epsilon RI consists of the IgE-binding alpha-chain, the multimembrane-spanning beta-chain and a dimer of the gamma-subunit (Fc epsilon R gamma). Comparative analysis of these seven transmembrane regions indicates that Fc epsilon RI does not meet the charge requirements for the 3-helix assembly mechanism. We performed alanine mutagenesis to show that the only basic amino acid in the transmembrane regions, beta K97, is not involved in Fc epsilon RI stabilization or surface upregulation, a hallmark function of the beta-chain. Even a beta K97E mutant is functional despite four negatively charged acidic amino acids in the transmembrane regions. Using truncation mutants, we demonstrate that the first uncharged transmembrane domain of the beta-chain contains the interface for receptor stabilization. In vitro translation experiments depict the first transmembrane region as the internal signal peptide of the beta-chain. We also show that this beta-chain domain can function as a cleavable signal peptide when used as a leader peptide for a Type I protein. Our results provide evidence that tetrameric Fc epsilon RI does not assemble according to the 3-helix assembly principle. We conclude that receptors formed with multispanning proteins use different mechanisms of shielding transmembrane charged amino acids.

Fc receptor gamma-chain, a constitutive component of the IL-3 receptor, is required for IL-3-induced IL-4 production in basophils

The Fc receptor common gamma-chain (FcRgamma) is a widely expressed adaptor bearing an immunoreceptor tyrosine-based activation motif (ITAM) that transduces activation signals from various immunoreceptors. We show here that basophils lacking FcRgamma developed normally and proliferated efficiently in response to interleukin 3 (IL-3) but were very impaired in IL-3-induced production of IL-4 and in supporting T helper type 2 differentiation. Through its transmembrane portion, FcRgamma associated constitutively with the common beta-chain of the IL-3 receptor and signaled by recruiting the kinase Syk. Retrovirus-mediated complementation demonstrated the essential function of the ITAM of FcRgamma in IL-3 signal transduction. Our results identify a previously unknown mechanism whereby FcRgamma functions to 'route' selective cytokine-triggered signals into the ITAM-mediated IL-4 production pathway.

Fc receptors

The aggregation of cell surface Fc receptors by immune complexes induces a number of important antibody-dependent effector functions. It is becoming increasingly evident that the organization of key immune proteins has a significant impact on the function of these proteins. Comparatively little is known, however, about the nature of Fc receptor spatiotemporal organization. This review outlines the current literature concerning human Fc receptor spatial organization and physiological function.

The mechanism underlying defective Fcgamma receptor-mediated phagocytosis by HIV-1-infected human monocyte-derived macrophages

Clearance of IgG-opsonized erythrocytes is impaired in HIV-1-infected patients, suggesting defective FcgammaR-mediated phagocytosis in vivo. We have previously shown defective FcgammaR-mediated phagocytosis in HIV-1-infected human monocyte-derived macrophages (MDM), establishing an in vitro model for defective tissue macrophages. Inhibition was associated with decreased protein expression of FcR gamma-chain, which transduces immune receptor signals via ITAM motifs. FcgammaRI and FcgammaRIIIa signal via gamma-chain, whereas FcgammaRIIa does not. In this study, we showed that HIV-1 infection inhibited FcgammaRI-, but not FcgammaRIIa-dependent Syk activation in MDM, showing that inhibition was specific for gamma-chain-dependent signaling. HIV-1 infection did not impair gamma-chain mRNA levels measured by real-time PCR, suggesting a posttranscriptional mechanism of gamma-chain depletion. HIV-1 infection did not affect gamma-chain degradation (n = 7, p = 0.94) measured in metabolic labeling/chase experiments, whereas gamma-chain biosynthesis was inhibited (n = 12, p = 0.0068). Using an enhanced GFP-expressing HIV-1 strain, we showed that FcgammaR-mediated phagocytosis inhibition is predominantly due to a bystander effect. Experiments in which MDM were infected in the presence of the antiretroviral drug 3TC suggest that active viral replication is required for inhibition of phagocytosis in MDM. These data suggest that HIV-1 infection may affect only gamma-chain-dependent FcgammaR functions, but that this is not restricted to HIV-1-infected cells.

IgA and IgA-specific receptors in human disease: structural and functional insights into pathogenesis and therapeutic potential

IgA antibodies play an important role in humoral immunity. IgA is the predominant antibody in mucosal secretions and the second most prevalent in the serum. It occupies a unique position among human antibodies in that it can both trigger and suppress inflammatory responses, depending on the situation. Recent structural and functional studies have revealed details of the structure of IgA and its interaction with key cell-surface receptors. We look at the role IgA and IgA receptors (particularly FcalphaRI) play in the pathogenesis of diseases such as IgA nephropathy and other autoimmune conditions. Finally, we address the potential of IgA as a therapeutic tool to either trigger specific inflammatory responses to destroy target cells or suppress inflammatory responses in the case of autoimmune diseases, and the promise of mucosal vaccines for eliciting specific IgA responses to pathogens in mucosal environments.