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

Novel Human FCGR1A Variants Affect CD64 Functions and Are Risk Factors for Sarcoidosis

CD64 (or FcγRIA) is the sole functional high affinity IgG Fc receptor coded by FCGR1A gene in humans. The FCGR1A genetics has not been comprehensively investigated and effects of human FCGR1A variants on immune functions remain unknown. In the current study, we identified three novel FCGR1A variants including the single nucleotide variant (SNV) rs1848781 (c.-131) in the proximal FCGR1A gene promoter region, the rs587598788 indel variant within the FCGR1A intron 5, and the non-synonymous SNV rs1050204 (c.970G>A or FcγRIA-p.D324N) in the FCGR1A coding region. Genotype-phenotype analyses revealed that SNV rs1848781 genotypes were significantly associated with CD64 expression levels. Promoter reporter assays show that rs1848781G allele had significantly higher promoter activity than the rs1848781C, confirming that the rs1848781 is a functional FCGR1A SNV affecting promoter activity and gene expression. The rs587598788 indel genotypes were also significantly associated with levels of CD64 expression. Moreover, the non-synonymous SNV rs1050204 (FcγRIA-p.D324N) alleles significantly affected CD64-mediated phagocytosis, degranulation, and pro-inflammatory cytokine productions. Genetic analyses revealed that FCGR1A genotypes were significantly associated with sarcoidosis susceptibility and severity. Our data suggest that FCGR1A genetic variants may affect immune responses and play a role in sarcoidosis.

Identification of periplakin as a major regulator of lung injury and repair in mice

Periplakin is a component of the desmosomes that acts as a cytolinker between intermediate filament scaffolding and the desmosomal plaque. Periplakin is strongly expressed by epithelial cells in the lung and is a target antigen for autoimmunity in idiopathic pulmonary fibrosis. The aim of this study was to determine the role of periplakin during lung injury and remodeling in a mouse model of lung fibrosis induced by bleomycin. We found that periplakin expression was downregulated in the whole lung and in alveolar epithelial cells following bleomycin-induced injury. Deletion of the Ppl gene in mice improved survival and reduced lung fibrosis development after bleomycin-induced injury. Notably, Ppl deletion promoted an antiinflammatory alveolar environment linked to profound changes in type 2 alveolar epithelial cells, including overexpression of antiinflammatory cytokines, decreased expression of profibrotic mediators, and altered cell signaling with a reduced response to TGF-β1. These results identify periplakin as a previously unidentified regulator of the response to injury in the lung.

The many faces of FcγRI: implications for therapeutic antibody function

Fcγ receptor I (FcγRI or CD64) is the sole human Fc receptor with high affinity for monovalent IgG. While it contains an immunoreceptor tyrosine-based activation motif in its cytoplasmic domain, binding of FcγRI can result in a complex array of activating and inhibitory outcomes. For instance, binding of monomeric IgG provides a low-intensity tonic signal through FcγRI that is necessary for full interferon γ receptor signaling in the same cell. Interaction of FcγRI with larger high-avidity complexes can result in phagocytosis, the generation of reactive oxygen species, as well as the synthesis and release of inflammatory cytokines. However, numerous reports also document potent anti-inflammatory effects brought about by FcγRI engagement with immune complexes such as the inhibition of IFNγ and TLR4 signaling, and secretion of interleukin-10. This has led to conflicting hypotheses regarding the function of FcγRI, especially with regard to its role in the efficacy of several therapeutic monoclonal antibodies. While many of these issues are still unclear, continued characterization of the regulation and context dependence of FcγRI function, as well as the molecular mechanisms responsible for these various outcomes, will improve our understanding of FcγRI biology as well as the therapeutic strategies designed to harness or constrain its actions.

Optimal complement-mediated phagocytosis of Pseudomonas aeruginosa by monocytes is cystic fibrosis transmembrane conductance regulator-dependent

Cystic fibrosis (CF) is caused by mutations of the cystic fibrosis transmembrane conductance regulator (CFTR) gene, and is characterized by chronic pulmonary infections. The mechanisms underlying chronic infection and inflammation remain incompletely understood. Mutant CFTR in nonepithelial tissues such as immune cells has been suggested to contribute to infection, inflammation, and the resultant lung disease. However, much controversy still exists regarding the intrinsic role of CFTR in immune cells, especially phagocytes. Therefore, we investigated CFTR expression and function in neutrophils and monocytes isolated from human peripheral blood. CFTR function was assessed by comparing non-CF and CF cells, before and after the chemical inhibition of CFTR. We found CFTR protein expression in monocytes, but this expression was limited or undetectable in neutrophils. Furthermore, the phagocytosis and intracellular killing of Pseudomonas aeruginosa was reduced in CF monocytes, and impaired phagocyte effector mechanisms were phenocopied in non-CF monocytes upon the pharmacological inhibition of CFTR. Reduced phagocytosis in CF monocytes relied on the complement-dependent opsonization of Pseudomonas aeruginosa, and was also observed in the context of latex particles labeled with purified C3b. In mechanistic terms, we observed that CFTR function in monocytes is required for the optimal expression of CD11b. We observed no role for CFTR in neutrophil-mediated phagocytosis. These data support an intrinsic role for CFTR in monocytes, and suggest that CFTR-dependent alterations in complement-mediated interactions between Pseudomonas aeruginosa and monocytes may contribute to enhanced susceptibility to infection in patients with CF.

CFTR expression analysis in human nasal epithelial cells by flow cytometry

Rationale

Unbiased approaches that study aberrant protein expression in primary airway epithelial cells at single cell level may profoundly improve diagnosis and understanding of airway diseases. We here present a flow cytometric procedure to study CFTR expression in human primary nasal epithelial cells from patients with Cystic Fibrosis (CF). Our novel approach may be important in monitoring of therapeutic responses, and better understanding of CF disease at the molecular level.

Objectives

Validation of a panel of CFTR-directed monoclonal antibodies for flow cytometry and CFTR expression analysis in nasal epithelial cells from healthy controls and CF patients.

Methods

We analyzed CFTR expression in primary nasal epithelial cells at single cell level using flow cytometry. Nasal cells were stained for pan-Cytokeratin, E cadherin, and CD45 (to discriminate epithelial cells and leukocytes) in combination with intracellular staining of CFTR. Healthy individuals and CF patients were compared.

Measurements and Main Results

We observed various cellular populations present in nasal brushings that expressed CFTR protein at different levels. Our data indicated that CF patients homozygous for F508del express varying levels of CFTR protein in nasal epithelial cells, although at a lower level than healthy controls.

Conclusion

CFTR protein is expressed in CF patients harboring F508del mutations but at lower levels than in healthy controls. Multicolor flow cytometry of nasal cells is a relatively simple procedure to analyze the composition of cellular subpopulations and protein expression at single cell level.

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.

Fcγ receptor I alpha chain (CD64) expression in macrophages is critical for the onset of meningitis by Escherichia coli K1

Neonatal meningitis due to Escherichia coli K1 is a serious illness with unchanged morbidity and mortality rates for the last few decades. The lack of a comprehensive understanding of the mechanisms involved in the development of meningitis contributes to this poor outcome. Here, we demonstrate that depletion of macrophages in newborn mice renders the animals resistant to E. coli K1 induced meningitis. The entry of E. coli K1 into macrophages requires the interaction of outer membrane protein A (OmpA) of E. coli K1 with the alpha chain of Fcγ receptor I (FcγRIa, CD64) for which IgG opsonization is not necessary. Overexpression of full-length but not C-terminal truncated FcγRIa in COS-1 cells permits E. coli K1 to enter the cells. Moreover, OmpA binding to FcγRIa prevents the recruitment of the γ-chain and induces a different pattern of tyrosine phosphorylation of macrophage proteins compared to IgG2a induced phosphorylation. Of note, FcγRIa^−/− mice are resistant to E. coli infection due to accelerated clearance of bacteria from circulation, which in turn was the result of increased expression of CR3 on macrophages. Reintroduction of human FcγRIa in mouse FcγRIa^−/− macrophages in vitro increased bacterial survival by suppressing the expression of CR3. Adoptive transfer of wild type macrophages into FcγRIa^−/− mice restored susceptibility to E. coli infection. Together, these results show that the interaction of FcγRI alpha chain with OmpA plays a key role in the development of neonatal meningitis by E. coli K1.

Escherichia coli K1 is the most common cause of meningitis in premature infants; the mortality rate of this disease ranges from 5% to 30%. A better understanding of the pathogenesis of E. coli K1 meningitis is needed to develop new preventative strategies. We have shown that outer membrane protein A (OmpA) of E. coli K1, independent of antibody opsonization, is critical for bacterial entrance and survival within macrophages. Using a newborn mouse model, we found that depletion of macrophages renders the animals resistant to E. coli K1 induced meningitis. OmpA binds to α-chain of Fcγ-receptor I (FcγRIa) in macrophages, but does not induce expected gamma chain association and signaling. FcγRIa knockout mice are resistant to E. coli K1 infection because their macrophages express more CR3 and are thus able to kill bacteria with greater efficiency, preventing the development of high-grade bacteremia, a pre-requisite for the onset of meningitis. These novel observations demonstrate that inhibiting OmpA binding to FcγRIa is a promising therapeutic target for treatment or prevention of neonatal meningitis.

Genetic characterization and ligand specificity of the ovine Fc gamma receptor I (ovFc gamma RI)

Receptors for the Fc region of IgG (Fc gamma Rs) play a critical role in the immune system and host protection against infection. In this study, we describe the cloning, mRNA expression and IgG subclass specificity of ovine Fc gamma receptor I (ovFc gamma RI). The ovFc gamma RI cDNA contains a 1047bp open-reading frame, and is predicted to encode a 349 amino acid trans-membrane glycoprotein composed of three immunoglobulin-like domains, a trans-membrane region and a short cytoplasmic tail. The overall identity of the ovine Fc gamma RI to its cattle, human and mouse counterparts at the level of the amino acid sequence was 92%, 61% and 54%, respectively. Rosetting analysis shows that COS-7 cells were transfected with an expressional vector carrying the cDNA open-reading frame of ovFc gamma RI and expressed this receptor on the surface. Identification of ovine Fc gamma RI will aid in the understanding of molecular basis of the ovine immune system and further studies of the receptor function.

Regulation of Treg functionality by acetylation-mediated Foxp3 protein stabilization

Regulatory T cells (Tregs) are a specific subset of lymphocytes that are critical for the maintenance of self-tolerance. Expression levels of the transcription factor Foxp3 have been causally associated with Treg differentiation and function. Recent studies show that Foxp3 can also be transiently expressed in effector T cells; however, stable Foxp3 expression is required for development of a functional Treg suppressor phenotype. Here, we demonstrate that Foxp3 is acetylated, and this can be reciprocally regulated by the histone acetyltransferase p300 and the histone deacetylase SIRT1. Hyperacetylation of Foxp3 prevented polyubiquitination and proteasomal degradation, therefore dramatically increasing stable Foxp3 protein levels. Moreover, using mouse splenocytes, human peripheral blood mononuclear cells, T cell clones, and skin-derived T cells, we demonstrate that treatment with histone deacetylase inhibitors resulted in significantly increased numbers of functional Treg cells. Taken together, our data demonstrate that modulation of the acetylation state of Foxp3 provides a novel molecular mechanism for assuring rapid temporal control of Foxp3 levels in T cells, thereby regulating Treg numbers and functionality. Manipulating Foxp3 acetylation levels could therefore provide a new therapeutic strategy to control inappropriate (auto)immune responses.

Regulation of myelopoiesis through syntenin-mediated modulation of IL-5 receptor output

The granulocyte-macrophage colony-stimulating factor (GM-CSF)/interleukin (IL)–3/IL-5 receptor family regulates the production and function of myeloid cells. These cytokines signal through receptor complexes that consist of unique ligand-binding α-chains and common signaling β-chains. IL-5 is distinct from IL-3 and GM-CSF in its capacity to induce eosinophil development, however, the molecular mechanisms that generate functional diversity within this receptor family are mostly unknown. Here, we characterized the selective IL-5Rα–binding adapter protein syntenin in IL-5R function. Syntenin and IL-5Rα colocalize at the plasma membrane and in early endosomal compartments. Manipulation of syntenin expression by ectopic expression or knockdown selectively modulated IL-5R but not GM-CSF receptor signaling, and severely affected IL-5–induced eosinophil differentiation from primary human CD34+ hematopoietic progenitor cells. We found syntenin up-regulated during eosinophilopoiesis but down-regulated during neutropoiesis. Syntenin forms complexes with multiple IL-5Rα chains, suggesting that syntenin-enhanced IL-5R output may result from stabilization of an IL-5–induced oligomeric receptor complex. These data demonstrate that cytokine-specific functions can be transduced by unique receptor α-chain–associating adapter proteins.

Phagocytosis in macrophages lacking Cbl reveals an unsuspected role for Fc gamma receptor signaling and actin assembly in target binding

Fc gamma receptor (Fc gammaR)-mediated phagocytosis is known to require tyrosine kinases (TKs). We identified c-Cbl and Cbl-b as proteins that undergo tyrosine phosphorylation during phagocytosis. Cbl-deficient macrophages displayed enhanced Fc gammaR-mediated signaling and phagocytosis. Surprisingly, binding of IgG-coated targets (EIgG) was also enhanced. c-Cbl-deficient macrophages expressed less Fc gammaRIIb, the inhibitory Fc gamma receptor; however, this did not account for enhanced target binding. We isolated the function of one Fc receptor isoform, Fc gammaRI, using IgG2a-coated targets (EIgG2a). Cbl-deficient macrophages demonstrated a disproportionate increase in binding EIgG2a, suggesting that signal strength regulates binding efficiency toward opsonized targets. In resting cells, Fc gammaRI colocalized with the Src family TK Hck in F-actin-rich structures, which was enhanced in Cbl-deficient macrophages. Target binding was sensitive to TK inhibitors, profoundly inhibited following depletion of cholesterol, and ablated at 4 degrees C or in the presence of inhibitors of actin polymerization. Sensitivity of EIgG binding to cytoskeletal disruption was inversely proportional to opsonin density. These findings challenge the view that Fc gammaR-mediated binding is a passive event. They suggest that dynamic engagement of TKs and the cytoskeleton enables macrophages to serve as cellular "Venus fly traps", with the capacity to capture phagocytic targets under conditions of limiting opsonin density.

OS9 interacts with DC-STAMP and modulates its intracellular localization in response to TLR ligation

Dendritic cell-specific transmembrane protein (DC-STAMP) has been first identified as an EST in a cDNA library of human monocyte-derived dendritic cells (DC). DC-STAMP is a multimembrane spanning protein that has been implicated in skewing haematopoietic differentiation of bone marrow cells towards the myeloid lineage, and in cell fusion during osteoclastogenesis and giant cell formation. To gain molecular insight in how DC-STAMP exerts its function, DC-STAMP interacting proteins were identified in a yeast-2-hybrid analysis. Herein, we report that amplified in osteosarcoma 9 (OS9) physically interacts with DC-STAMP, and that both proteins colocalize in the endoplasmic reticulum in various cell lines, including immature DC. OS9 has previously been implicated in ER-to-Golgi transport and transcription factor turnover. Interestingly, we now demonstrate that toll-like receptor (TLR)-induced maturation of DC leads to the translocation of DC-STAMP from the ER to the Golgi while OS9 localization is unaffected. Applying TLR-expressing CHO cells we could confirm ER-to-Golgi translocation of DC-STAMP following TLR stimulation and demonstrated that the DC-STAMP/OS9 interaction is involved in this process. Collectively, the data indicate that OS9 is critically involved in the modulation of ER-to-Golgi transport of DC-STAMP in response to TLR triggering, suggesting a novel role for OS9 in myeloid differentiation and cell fusion.

FcgammaRI (CD64) resides constitutively in lipid rafts

Cellular membranes contain microdomains known as 'lipid rafts' or detergent-insoluble microdomains (DRM), enriched in cholesterol and sphingolipids. DRM can play an important role in many cellular processes, including signal transduction, cytoskeletal organization, and pathogen entry. Many receptors like T cell receptors, B cell receptors and IgE receptors have been shown to reside in DRM. The majority of these receptors depend on multivalent ligand interaction to associate with these microdomains. We, here, study association between the high affinity IgG receptor, FcgammaRI (CD64), and membrane microdomains. FcgammaRI is a 72kDa type I glycoprotein that can mediate phagocytosis of opsonized pathogens, but can also effectively capture small immune complexes, and facilitates antigen presentation. We found FcgammaRI to predominantly reside within detergent-insoluble buoyant membranes, together with FcRgamma-chain, but independent of cross-linking ligand. With the use of confocal imaging, FcgammaRI was found to co-patch with GM1, a microdomain-enriched glycolipid. Depletion of cellular cholesterol, furthermore, modulated FcgammaRI-ligand interactions. These data indicated FcgammaRI to reside within lipid rafts without prior triggering of the receptor.

Molecular cloning and expression of the porcine high-affinity immunoglobulin G Fc receptor (FcgammaRI)

Receptors for the Fc region (FcgammaRs) of immunoglobulin G (IgG) play a crucial role in the immune system and host protection against infection. In this study, we describe the cloning, sequencing, and expression of the high-affinity IgG receptor from pig. By screening a translated Expressed Sequence Tags database with the human FcgammaRI (CD64) protein sequence, we identified a putative porcine homologue. Subsequent polymerase chain reaction amplification confirmed that the identified full-length cDNA was expressed in porcine cells. Rosetting analysis shows that COS-7 cells transfected with a plasmid containing the cloned cDNA were able to bind chicken erythrocytes sensitized with porcine IgG. Scatchard analysis indicated that monomeric IgG bound to transiently transfected cells with an affinity of approximately 4x10(7) M(-1). The porcine FcgammaRI cDNA is 1,038 nucleotides long and is predicted to encode a 346-amino-acid transmembrane glycoprotein composed of three Ig-like domains, a transmembrane region, and a short cytoplasmic tail. The overall identity of the porcine FcgammaRI to its human and mouse counterparts at the level of the amino acid sequence was 75% and 57%, respectively. Identification of porcine FcgammaRI will aid in the understanding of the molecular basis of the porcine immune system and further studies of the receptor function.

Identification and characterization of DC-SCRIPT, a novel dendritic cell-expressed member of the zinc finger family of transcriptional regulators

Dendritic cells (DC) compose a heterogeneous population of cells that hold a leading role in initiating and directing immune responses. Although their function in recognizing, capturing, and presenting Ags is well defined, the molecular mechanisms that control their differentiation and immune functions are still largely unknown. In this study, we report the isolation and characterization of DC-SCRIPT, a novel protein encoded by an 8-kb mRNA that is preferentially expressed in DC. DC-SCRIPT is expressed in multiple DC subsets in vivo, including myeloid DC, plasmacytoid DC, and Langerhans cells. At the protein level, DC-SCRIPT consists of a proline-rich region, 11 C2H2-type zinc fingers, and an acidic region. Localization studies reveal that DC-SCRIPT resides in the nucleus and that nuclear localization is critically dependent on the zinc fingers. The protein displays no transcriptional activation properties according to assorted transactivation assays, but interacts with the corepressor C-terminal binding protein 1. Taken together, our results show that we have isolated a novel DC marker that could be involved in transcriptional repression. In contrast to other DC molecules, DC-SCRIPT identifies all DC subsets tested to date.

Breaking the Connection: Caspase 6 Disconnects Intermediate Filament-Binding Domain of Periplakin from its Actin-Binding N-Terminal Region

Periplakin is a member of the plakin family of cytolinkers that connect cytoskeletal networks to each other as well as to the cell junctional complexes. Here, we demonstrate a direct molecular interaction between actin and periplakin. Furthermore, the oligomerization state of periplakin was shown to determine specificity of its binding to intermediate filaments (IF) in vitro. Both the filament association and the cell membrane localization of periplakin were confirmed in the cells overexpressing human periplakin. Double labeling of the N- and C-terminally tagged periplakin revealed unexpected lack of co-localization of periplakin ends in a confluent culture, and separation of the periplakin ends was even more pronounced in apoptotic cells. Western analysis revealed that after induction of apoptosis, periplakin becomes cleaved close to its C-terminal tail. Only the distinct cleavage products, but not the full-length periplakin, were present in the cells detached from the solid support during the apoptotic process. We show that caspase 6 cleaves periplakin at an unconventional recognition site, amino acid sequence TVAD. Thus, the separation of periplakin ends disconnects the actin-binding head-rod domain from the IF-binding C-terminal domain. We show that specific cleavage products co-exist with the full-length periplakin in cells, suggesting physiological consequences due to their altered binding specificities.

Periplakin interferes with G protein activation by the melanin-concentrating hormone receptor-1 by binding to the proximal segment of the receptor C-terminal tail

In mice genetic ablation of expression of either melanin-concentrating hormone or the melanin-concentrating hormone-1 receptor results in alterations in energy metabolism and a lean phenotype. There is thus great interest in the function and regulation of this receptor. Using the yeast two-hybrid system we identified an interaction of the actin- and intermediate filament-binding protein periplakin with the intracellular C-terminal tail of the melanin-concentrating hormone-1 receptor. Direct association of these proteins was verified in pull-down and coimmunoprecipitation experiments. Truncations and internal deletions delineated the site of interaction to a group of 11 amino acids proximal to transmembrane helix VII, which was distinct from the binding site for the melanin-concentrating hormone-1 receptor-interacting zinc finger protein. Immunohistochemistry demonstrated coexpression of periplakin with melanin-concentrating hormone-1 receptor in specific cells of the piriform cortex, amygdala, and other structures of the adult mouse brain. Coexpression of the melanin-concentrating hormone-1 receptor with periplakin in human embryonic kidney 293 cells did not prevent agonist-mediated internalization of the receptor but did interfere with binding of (35)S-labeled guanosine 5'-3-O-(thio)triphosphate ([(35)S]GTPgammaS) to the G protein Galpha(o1) and the elevation of [Ca(2+)](i). Coexpression of the receptor with the interacting zinc finger protein did not modulate receptor internalization or G protein activation. The interaction of periplakin with receptors was selective. Coexpression of periplakin with the IP prostanoid receptor did not result in coimmunoprecipitation nor interfere with agonist-mediated binding of [(35)S]GTPgammaS to the G protein Galpha(s). Periplakin is the first protein described to modify the capacity of the melanin-concentrating hormone-1 receptor to initiate signal transduction.