Divergent intracellular sorting of Fc{gamma}RIIA and Fc{gamma}RIIB2

Activating FcγR function depends on endosomal-signaling platforms

Cell surface receptor internalization can either terminate signaling or activate alternative endosomal signaling pathways. We investigated here whether endosomal signaling is involved in the function of the human receptors for Fc immunoglobulin fragments (FcRs): FcαRI, FcγRIIA, and FcγRI. All these receptors were internalized after their cross-linking with receptor-specific antibodies, but their intracellular trafficking was different. FcαRI was targeted directly to lysosomes, while FcγRIIA and FcγRI were internalized in particular endosomal compartments described by the insulin esponsive minoeptidase (IRAP), where they recruited signaling molecules, such as the active form of the kinase Syk, PLCγ and the adaptor LAT. Destabilization of FcγR endosomal signaling in the absence of IRAP compromised cytokine secretion downstream FcγR activation and macrophage ability to kill tumor cells by antibody-dependent cell-mediated cytotoxicity (ADCC). Our results indicate that FcγR endosomal signaling is required for the FcγR-driven inflammatory reaction and possibly for the therapeutic action of monoclonal antibodies.

Human immunoglobulins are transported to HCMV viral envelope by viral Fc gamma receptors-dependent and independent mechanisms

Human cytomegaloviruses (HCMVs) employ many different mechanisms to escape and subvert the host immune system, including expression of the viral IgG Fcγ receptors (vFcγRs) RL11 (gp34), RL12 (gp95), RL13 (gpRL13), and UL119 (gp68) gene products. The role of vFcγRs in HCMV pathogenesis has been reported to operate in infected cells by interfering with IgG-mediated effector functions. We found that gp34 and gp68 are envelope proteins that bind and internalize human IgGs on the surface of infected cells. Internalized IgGs are then transported on the envelope of viral particles in a vFcR-dependent mechanism. This mechanism is also responsible for the incorporation on the virions of the anti-gH neutralizing antibody MSL-109. Intriguingly, we show that gp68 is responsible for MSL-109 incorporation, but it is dispensable for other anti-HCMV antibodies that do not need this function to be transported on mature virions. HCMV-infected cells grown in presence of anti-HCMV monoclonal antibodies generate a viral progeny still infective and possible to be neutralized. This is the first example of a virus carrying neutralizing IgGs on its surface and their possible role is discussed.

Agonist antibody discovery: Experimental, computational, and rational engineering approaches

Agonist antibodies that activate cellular signaling have emerged as promising therapeutics for treating myriad pathologies. Unfortunately, the discovery of rare antibodies with the desired agonist functions is a major bottleneck during drug development. Nevertheless, there has been important recent progress in discovering and optimizing agonist antibodies against a variety of therapeutic targets that are activated by diverse signaling mechanisms. Herein, we review emerging high-throughput experimental and computational methods for agonist antibody discovery as well as rational molecular engineering methods for optimizing their agonist activity.

Human Platelets Take up Anti-VEGF Agents

Purpose

Growing evidence suggests different systemic exposure of anti-vascular endothelial growth factor (anti-VEGF) agents with repeated intravitreal application. Since the penetration of anti-VEGF agents through vascular barrier was reported, the interaction of anti-VEGF with nonresident platelets has become a topic of interest. The purpose of this study was to evaluate, with the help of visualization techniques, whether platelets take up the anti-VEGF agents ranibizumab, aflibercept, and bevacizumab.

Methods

The uptake of anti-VEGF agents with or without VEGF treatment was investigated using immunofluorescence and immunogold staining in human platelets. The role of actin filaments and clathrin-coated vesicles in the transport of ranibizumab, aflibercept, and bevacizumab was evaluated by two pharmacologic inhibitors: staurosporine (protein kinase C inhibitor) and cytochalasin D.

Results

All three anti-VEGF agents were taken up by platelets and colocalized with VEGF. Ranibizumab and aflibercept were mainly detected in alpha-granules; however, bevacizumab was equally localized in alpha-granules and in platelet vesicles. Both staurosporine and cytochalasin D completely inhibited the uptake of aflibercept into platelets. Both pharmacological inhibitors also decreased the transport of ranibizumab and bevacizumab into platelets. Bevacizumab was significantly more frequently colocalized within clathrin-coated vesicles than ranibizumab and aflibercept.

Conclusion

All three anti-VEGF agents are taken up by platelets and internalized in alpha-granules, which may result in a higher local exposure of anti-VEGF after the activation of platelets, potentially contributing to arterial thromboembolic events. Clathrin-coated vesicles seem to be more prominent in the transport of bevacizumab than ranibizumab and aflibercept. Nevertheless, whether the different localization and transport of bevacizumab are truly related to specific differences of receptor-mediated endocytosis has to be revealed by further research.

Macrophage hypophagia as a mechanism of innate immune exhaustion in mAb-induced cell clearance

Macrophage antibody (Ab)-dependent cellular phagocytosis (ADCP) is a major cytotoxic mechanism for both therapeutic unconjugated monoclonal Abs (mAbs) such as rituximab and Ab-induced hemolytic anemia and immune thrombocytopenia. Here, we studied the mechanisms controlling the rate and capacity of macrophages to carry out ADCP in settings of high target/effector cell ratios, such as those seen in patients with circulating tumor burden in leukemic phase disease. Using quantitative live-cell imaging of primary human and mouse macrophages, we found that, upon initial challenge with mAb-opsonized lymphocytes, macrophages underwent a brief burst (<1 hour) of rapid phagocytosis, which was then invariably followed by a sharp reduction in phagocytic activity that could persist for days. This previously unknown refractory period of ADCP, or hypophagia, was observed in all macrophage, mAb, and target cell conditions tested in vitro and was also seen in vivo in Kupffer cells from mice induced to undergo successive rounds of αCD20 mAb-dependent clearance of circulating B cells. Importantly, hypophagia had no effect on Ab-independent phagocytosis and did not alter macrophage viability. In mechanistic studies, we found that the rapid loss of activating Fc receptors from the surface and their subsequent proteolytic degradation were the primary mechanisms responsible for the loss of ADCP activity in hypophagia. These data suggest hypophagia is a critical limiting step in macrophage-mediated clearance of cells via ADCP, and understanding such limitations to innate immune system cytotoxic capacity will aid in the development of mAb regimens that could optimize ADCP and improve patient outcome.

Bone Marrow Endothelial Cells Take Up Blood-Borne Immune Complexes via Fcγ Receptor IIb2 in an Erythropoietin-Dependent Manner

Immune complexes (ICs) in blood are efficiently removed mainly by liver reticuloendothelial systems consisting of sinusoidal endothelial cells and Kupffer cells expressing FcγR. The bone marrow (BM) also has sinusoidal vasculatures, and sinusoidal BM endothelial cells (BMECs) bear unique function, including hematopoietic niches and traffic regulation of hematopoietic cells. In this study, we found that sinusoidal BMECs express FcγRIIb2, which is markedly increased in anemic conditions or by the administration of erythropoietin (Epo) in healthy mice. BMECs expressed Epo receptor (EpoR), and the Epo-induced increase in FcγRIIb2 expression was abolished in Epor^-/- ::HG1-Epor transgenic mice, which lack EpoR in BMECs except for BM erythroblasts, suggesting the effect was directly mediated via EpoR on BMECs. Further, although BMECs hardly captured i.v.-injected soluble ICs in healthy mice, Epo administration induced a remarkable increase in the uptake of ICs in a FcγRIIb-dependent manner. Enhancement of the IC incorporation capacity by Epo was also observed in cultured BMECs in vitro, suggesting the direct effect of Epo on BMECs. Moreover, we found that i.v.-injected ICs in Epo-treated mice were more rapidly removed from the circulation than in PBS-treated mice. These results reveal a novel function of BMECs to efficiently remove circulating blood-borne ICs in an FcγRIIb2-mediated manner.

Aberrant FcγRIIb and FcγRIII expression on monocytes from patients with Behçet's disease

Behçet's disease (BD) patients have abnormal FcγR polymorphisms, the implication of which remains elusive. We examined FcγRIIb expression on neutrophils, monocytes, B cells, natural killer cells, dendritic cells and T cells, and FcγRI and FcγRIII expression on monocytes in BD patients and healthy controls using flow cytometry. We further stimulated monocytes with IgG and (or) lipopolysaccharide (LPS) and measured IL-6 and TNF-α production by enzyme-linked immunosorbent assay. We found that BD monocytes expressed a lower level of FcγRIIb and a higher level of FcγRIII, which were correlated with erythrocyte sedimentation rate and C-reactive protein and were rescued after treatment. Furthermore, LPS- and IgG-stimulated BD monocytes produced higher levels of IL-6 and TNF-α than HC monocytes. In summary, we found that BD monocytes downregulated FcγRIIb expression and upregulated FcγRIII expression, which were correlated with disease activity and potentially contributed to monocyte hyperactivation in BD.

T-DM1-induced thrombocytopenia in breast cancer patients: New perspectives

PURPOSE

Human epidermal growth factor receptor 2 (HER2) is overexpressed in 15-20% of patients with breast cancer. HER2 overexpression is the result of a genetic alteration and this marker is associated with poor clinical outcomes. HER2-targeted therapy can significantly improve the prognosis of patients with either early or advanced HER2-positive breast cancer. One such therapy is the antibody drug conjugate (ADC) trastuzumab emtansine (T-DM1), a combination of trastuzumab and the cytotoxic antimicrotubule agent DM1. After T-DM1 binds HER2, DM1 is subsequently released into the cell. T-DM1 is generally well tolerated and has a relatively low incidence of adverse events. However, there are clinical concerns regarding T-DM1-induced high-grade thrombocytopenia.

METHODS

Here, we summarize the incidence of thrombocytopenia from several clinical trials and review experimental studies to explore the causes for T-DM1-induced thrombocytopenia. Progress in several other ADCs targeting HER2-positive breast cancer was also reviewed.

CONCLUSIONS

We conclude that T-DM1 uptake by megakaryocytes occurs through either Fcγ receptor binding or through pinocytosis, and we suggest several methods through which these processes could be interrupted to potentially improve the clinical safety of T-DM1. More generally, we recommend that toxicity should be carefully addressed during the development of ADCs.

Alterations in Platelet Alpha-Granule Secretion and Adhesion on Collagen under Flow in Mice Lacking the Atypical Rho GTPase RhoBTB3

Typical Rho GTPases, such as Rac1, Cdc42, and RhoA, act as molecular switches regulating various aspects of platelet cytoskeleton reorganization. The loss of these enzymes results in reduced platelet functionality. Atypical Rho GTPases of the RhoBTB subfamily are characterized by divergent domain architecture. One family member, RhoBTB3, is expressed in platelets, but its function is unclear. In the present study we examined the role of RhoBTB3 in platelet function using a knockout mouse model. We found the platelet count, size, numbers of both alpha and dense granules, and surface receptor profile in these mice were comparable to wild-type mice. Deletion of Rhobtb3 had no effect on aggregation and dense granule secretion in response to a range of agonists including thrombin, collagen, and adenosine diphosphate (ADP). By contrast, alpha-granule secretion increased in mice lacking RhoBTB3 in response to thrombin, collagen related peptide (CRP) and U46619/ADP. Integrin activation and spreading on fibrinogen and collagen under static conditions were also unimpaired; however, we observed reduced platelet accrual on collagen under flow conditions. These defects did not translate into alterations in tail bleeding time. We conclude that genetic deletion of Rhobtb3 leads to subtle alterations in alpha-granule secretion and adhesion to collagen without significant effects on hemostasis in vivo.

Multivalent Fcγ-receptor engagement by a hexameric Fc-fusion protein triggers Fcγ-receptor internalisation and modulation of Fcγ-receptor functions

Engagement of Fcγ-receptors triggers a range of downstream signalling events resulting in a diverse array of immune functions. As a result, blockade of Fc-mediated function is an important strategy for the control of several autoimmune and inflammatory conditions. We have generated a hexameric-Fc fusion protein (hexameric-Fc) and tested the consequences of multi-valent Fcγ-receptor engagement in in vitro and in vivo systems. In vitro engagement of hexameric-Fc with FcγRs showed complex binding interactions that altered with receptor density and triggered the internalisation and degradation of Fcγ-receptors. This caused a disruption of Fc-binding and phagocytosis. In vivo, in a mouse ITP model we observed a short half-life of hexameric-Fc but were nevertheless able to observe inhibition of platelet phagocytosis several days after hexameric-Fc dosing. In cynomolgus monkeys, we again observed a short half-life, but were able to demonstrate effective FcγR blockade. These findings demonstrate the ability of multi-valent Fc-based therapeutics to interfere with FcγR function and a potential mechanism through which they could have a sustained effect; the internalisation and degradation of FcγRs.

Fc receptors in antibody-dependent enhancement of viral infections

Sensitization of the humoral immune response to invading viruses and production of antiviral antibodies forms part of the host antiviral repertoire. Paradoxically, for a number of viral pathogens, under certain conditions, antibodies provide an attractive means of enhanced virus entry and replication in a number of cell types. Known as antibody‐dependent enhancement (ADE) of infection, the phenomenon occurs when virus‐antibody immunocomplexes interact with cells bearing complement or Fc receptors, promoting internalization of the virus and increasing infection. Frequently associated with exacerbation of viral disease, ADE of infection presents a major obstacle to the prevention of viral disease by vaccination and is thought to be partly responsible for the adverse effects of novel antiviral therapeutics such as intravenous immunoglobulins. There is a growing body of work examining the intracellular signaling pathways and epitopes responsible for mediating ADE, with a view to aiding rational design of antiviral strategies. With in vitro studies also confirming ADE as a feature of infection for a growing number of viruses, challenges remain in understanding the multilayered molecular mechanisms of ADE and its effect on viral pathogenesis.

Antimicrobial Mechanisms of Macrophages and the Immune Evasion Strategies of Staphylococcus aureus

Habitually professional phagocytes, including macrophages, eradicate microbial invaders from the human body without overt signs of infection. Despite this, there exist select bacteria that are professional pathogens, causing significant morbidity and mortality across the globe and Staphylococcus aureus is no exception. S. aureus is a highly successful pathogen that can infect virtually every tissue that comprises the human body causing a broad spectrum of diseases. The profound pathogenic capacity of S. aureus can be attributed, in part, to its ability to elaborate a profusion of bacterial effectors that circumvent host immunity. Macrophages are important professional phagocytes that contribute to both the innate and adaptive immune response, however from in vitro and in vivo studies, it is evident that they fail to eradicate S. aureus. This review provides an overview of the antimicrobial mechanisms employed by macrophages to combat bacteria and describes the immune evasion strategies and some representative effectors that enable S. aureus to evade macrophage-mediated killing.

Inhibitory FcγRIIb-Mediated Soluble Antigen Clearance from Plasma by a pH-Dependent Antigen-Binding Antibody and Its Enhancement by Fc Engineering

Fc engineering can modulate the Fc-FcγR interaction and thus enhance the potency of Abs that target membrane-bound Ags, but it has not been applied to Abs that target soluble Ags. In this study, we revealed a previously unknown function of inhibitory FcγRII in vivo and, using an Ab that binds to Ag pH dependently, demonstrated that the function can be exploited to target soluble Ag. Because pH-dependent Ab dissociates Ag in acidic endosome, its Ag clearance from circulation reflects the cellular uptake rate of Ag/Ab complexes. In vivo studies showed that FcγR but not neonatal FcR contributes to Ag clearance by the pH-dependent Ab, and when Fc binding to mouse FcγRII and III was increased, Ag clearance was markedly accelerated in wild-type mice and FcR γ-chain knockout mice, but the effect was diminished in FcγRII knockout mice. This demonstrates that mouse FcγRII efficiently promotes Ab uptake into the cell and its subsequent recycling back to the cell surface. Furthermore, when a human IgG1 Fc variant with selectively increased binding to human FcγRIIb was tested in human FcγRIIb transgenic mice, Ag clearance was accelerated without compromising the Ab half-life. Taken together, inhibitory FcγRIIb was found to play a prominent role in the cellular uptake of monomeric Ag/Ab immune complexes in vivo, and when the Fc of a pH-dependent Ab was engineered to selectively enhance human FcγRIIb binding, the Ab could accelerate soluble Ag clearance from circulation. We assume such a function would enhance the therapeutic potency of Abs that target soluble Ags.

Three major uncertainties in the antibody therapy of cancer

Antibodies against surface molecules of human tumors are now frequently administered in combination with strong chemotherapy, increasing therapeutic efficacy but making the task of elucidating immunological events more difficult. Experiments on genetically manipulated mice indicate that antibody efficacy is greatest when IgG antibody coating tumor cells is engaged by the Fcγ-receptors of effector cells, chiefly the monocyte/macrophage lineage. Evidence suggests lesser roles for NK cells, neutrophils, receptor-mediated cytotoxicity and complement-mediated cytotoxicity. The classical mode of killing employed by macrophages is phagocytosis, but much has to be learned about optimally activating macrophages for this task, and about any other modes of cytotoxicity used. There is renewed interest in antigenic modulation, which implies removal of therapeutic antibody linked with antigen from target-cell surfaces. It is now apparent that this removal of immune complexes can be achieved either by internalization by the target cell, or by transfer of the complexes to another cell by trogocytosis. In trials, anti-idiotype antibodies surprisingly proved therapeutically more effective than anti-CD20, despite anti-idiotype being more effectively removed from target-cell surfaces by antigenic modulation. This anomalous result might reflect the fact that persistence of anti-CD20 immune complexes in large amounts induces serious effector modulation, which paralyzes macrophage attacks on antibody-coated cells. The case for effector modulation is argued by analogy with the therapeutic suppression of autoimmune inflammation by effector modulation, achieved by infusion either of normal IgG in large amounts, or of anti-red cell IgG in relatively small amounts.

Activatory and inhibitory Fcγ receptors augment rituximab-mediated internalization of CD20 independent of signaling via the cytoplasmic domain

Type I anti-CD20 mAb such as rituximab and ofatumumab engage with the inhibitory FcγR, FcγRIIb on the surface of B cells, resulting in immunoreceptor tyrosine-based inhibitory motif (ITIM) phosphorylation. Internalization of the CD20·mAb·FcγRIIb complex follows, the rate of which correlates with FcγRIIb expression. In contrast, although type II anti-CD20 mAb such as tositumomab and obinutuzumab also interact with and activate FcγRIIb, this interaction fails to augment the rate of CD20·mAb internalization, raising the question of whether ITIM phosphorylation plays any role in this process. We have assessed the molecular requirements for the internalization process and demonstrate that in contrast to internalization of IgG immune complexes, FcγRIIb-augmented internalization of rituximab-ligated CD20 occurs independently of the FcγRIIb ITIM, indicating that signaling downstream of FcγRIIb is not required. In transfected cells, activatory FcγRI, FcγRIIa, and FcγRIIIa augmented internalization of rituximab-ligated CD20 in a similar manner. However, FcγRIIa mediated a slower rate of internalization than cells expressing equivalent levels of the highly homologous FcγRIIb. The difference was maintained in cells expressing FcγRIIa and FcγRIIb lacking cytoplasmic domains and in which the transmembrane domains had been exchanged. This difference may be due to increased degradation of FcγRIIa, which traffics to lysosomes independently of rituximab. We conclude that the cytoplasmic domain of FcγR is not required for promoting internalization of rituximab-ligated CD20. Instead, we propose that FcγR provides a structural role in augmenting endocytosis that differs from that employed during the endocytosis of immune complexes.

Regulation of fc receptor endocytic trafficking by ubiquitination

Most immune cells, particularly phagocytes, express various receptors for the Fc portion of the different immunoglobulin isotypes (Fc receptors, FcRs). By binding to the antibody, they provide a link between the adaptive immune system and the powerful effector functions triggered by innate immune cells such as mast cells, neutrophils, macrophages, and NK cells. Upon ligation of the immune complexes, the downstream signaling pathways initiated by the different receptors are quite similar for different FcR classes leading to the secretion of preformed and de novo synthesized pro-inflammatory mediators. FcR engagement also promotes negative signals through the combined action of several molecules that limit the extent and duration of positive signaling. To this regard, ligand-induced ubiquitination of FcRs for IgE (FcεR) and IgG (FcγR) has become recognized as a key modification that generates signals for the internalization and/or delivery of engaged receptor complexes to lysosomes or cytoplasmic proteasomes for degradation, providing negative-feedback regulation of Fc receptor activity. In this review, we discuss recent advances in our understanding of the molecular mechanisms that ensure the clearance of engaged Fcε and Fcγ receptor complexes from the cell surface with an emphasis given to the cooperation between the ubiquitin pathway and endosomal adaptors including the endosomal sorting complex required for transport (ESCRT) in controlling receptor internalization and sorting along the endocytic compartments.

Engineered antibody Fc variant with selectively enhanced FcγRIIb binding over both FcγRIIa(R131) and FcγRIIa(H131)

Engaging inhibitory FcγRIIb by Fc region has been recently reported to be an attractive approach for improving the efficacy of antibody therapeutics. However, the previously reported S267E/L328F variant with enhanced binding affinity to FcγRIIb, also enhances binding affinity to FcγRIIa(R131) allotype to a similar degree because FcγRIIb and FcγRIIa(R131) are structurally similar. In this study, we applied comprehensive mutagenesis and structure-guided design based on the crystal structure of the Fc/FcγRIIb complex to identify a novel Fc variant with selectively enhanced FcγRIIb binding over both FcγRIIa(R131) and FcγRIIa(H131). This novel variant has more than 200-fold stronger binding affinity to FcγRIIb than wild-type IgG1, while binding affinity to FcγRIIa(R131) and FcγRIIa(H131) is comparable with or lower than wild-type IgG1. This selectivity was achieved by conformational change of the C(H)2 domain by mutating Pro to Asp at position 238. Fc variant with increased binding to both FcγRIIb and FcγRIIa induced platelet aggregation and activation in an immune complex form in vitro while our novel variant did not. When applied to agonistic anti-CD137 IgG1 antibody, our variant greatly enhanced the agonistic activity. Thus, the selective enhancement of FcγRIIb binding achieved by our Fc variant provides a novel tool for improving the efficacy of antibody therapeutics.

C-reactive protein inhibits plasmacytoid dendritic cell interferon responses to autoantibody immune complexes

OBJECTIVE

C-reactive protein (CRP) is a serum pattern recognition molecule that binds to apoptotic cells and nucleoprotein autoantigens and Fcγ receptors (FcγR). In systemic lupus erythematosus (SLE), immune complexes (ICs) containing nucleoprotein autoantigens activate plasmacytoid dendritic cells (PDCs) to produce type I interferon (IFN), which contributes to disease pathogenesis. Autoantibody ICs are taken up by PDCs through FcγR type IIa into endosomes, where the nucleic acid components activate Toll-like receptor 7 (TLR-7) or TLR-9. The objective of this study was to investigate the effect of CRP on PDC and monocyte responses to nucleoprotein autoantigens and ICs.

METHODS

Peripheral blood mononuclear cells (PBMCs), purified monocytes, and PDCs were isolated from healthy volunteers and stimulated with autoantibody ICs containing apoptotic cells, small nuclear RNPs (snRNPs), or DNA, or directly with TLR-7 and TLR-9 agonists. Supernatants were analyzed for IFNα and cytokine levels by enzyme-linked immunosorbent assay and multiplex assay. Small nuclear RNPs were fluorescence-labeled, and the effect of CRP on binding, uptake, and intracellular localization of autoantibody snRNP complexes was measured by flow cytometry and confocal microscopy.

RESULTS

CRP bound to autoantigen did not induce IFNα in PBMCs or PDCs, whereas complexes formed with autoantibody did. Significantly, CRP inhibited the IFNα response to both anti-U1 RNP-snRNP complexes and anti-DNA-DNA complexes, but not to other TLR-7 and TLR-9 agonists. CRP directly inhibited PDC IFNα release, promoted PDC differentiation, and increased late endosome localization of autoantigen in PDCs and monocytes.

CONCLUSION

CRP is a regulator of the type I IFN response to SLE ICs. CRP increased the intracellular processing of ICs in late endosomes, which is associated with decreased synthesis of type I IFN after intracellular TLR activation.

A novel hypothesis for an alkaline phosphatase 'rescue' mechanism in the hepatic acute phase immune response

The liver isoform of the enzyme alkaline phosphatase (AP) has been used classically as a serum biomarker for hepatic disease states such as hepatitis, steatosis, cirrhosis, drug-induced liver injury, and hepatocellular carcinoma. Recent studies have demonstrated a more general anti-inflammatory role for AP, as it is capable of dephosphorylating potentially deleterious molecules such as nucleotide phosphates, the pathogenic endotoxin lipopolysaccharide (LPS), and the contact clotting pathway activator polyphosphate (polyP), thereby reducing inflammation and coagulopathy systemically. Yet the mechanism underlying the observed increase in liver AP levels in circulation during inflammatory insults is largely unknown. This paper hypothesizes an immunological role for AP in the liver and the potential of this system for damping generalized inflammation along with a wide range of ancillary pathologies. Based on the provided framework, a mechanism is proposed in which AP undergoes transcytosis in hepatocytes from the canalicular membrane to the sinusoidal membrane during inflammation and the enzyme's expression is upregulated as a result. Through a tightly controlled, nucleotide-stimulated negative feedback process, AP is transported in this model as an immune complex with immunoglobulin G by the asialoglycoprotein receptor through the cell and secreted into the serum, likely using the receptor's State 1 pathway. The subsequent dephosphorylation of inflammatory stimuli by AP and uptake of the circulating immune complex by endothelial cells and macrophages may lead to decreased inflammation and coagulopathy while providing an early upstream signal for the induction of a number of anti-inflammatory gene products, including AP itself.

Human FcγRII cytoplasmic domains differentially influence antibody-mediated dengue virus infection

Ab-dependent enhancement (ADE) of dengue virus (DENV) infection is mediated through the interaction of viral immune complexes with FcγRs, with notable efficiency of FcγRII. Most human dengue target cells coexpress activating (FcγRIIa) and inhibitory (FcγRIIb) isoforms, but their relative roles in ADE are not well understood. We studied the effects of FcγRIIa and FcγRIIb by transfecting cells to express each individual receptor isoform or through coexpression of both isoforms. We showed that although both isoforms similarly bind dengue-immune complexes, FcγRIIa efficiently internalized virus leading to productive cellular infection, unlike FcγRIIb. We next focused on the main discriminating feature of these isoforms: their distinct intracytoplasmic tails (FcγRIIa with an immunoreceptor tyrosine-based activation motif [ITAM] and FcγRIIb with an immunoreceptor tyrosine-based inhibitory motif [ITIM]). We engineered cells to express "swapped" versions of their FcγRII by switching the cytoplasmic tails containing the ITAM/ITIM motifs, leaving the remainder of the receptor intact. Our data show that both FcγRIIa and FcγRIIb comparably bind dengue immune complexes. However, wild type FcγRIIa facilitates DENV entry by virtue of the ITAM motif, whereas the swapped version FcγRIIa-ITIM significantly inhibited ADE. Similarly, replacing the inhibitory motif in FcγRIIb with an ITAM (FcγRIIb-ITAM) reconstituted ADE capacity to levels of the wild type activating counterpart, FcγRIIa. Our data suggest that FcγRIIa and FcγRIIb isoforms, as the most abundantly distributed class II Fcγ receptors, differentially influence Ab-mediated DENV infection under ADE conditions both at the level of cellular infection and viral production.

The gamma chain subunit of Fc receptors is required for alpha-synuclein-induced pro-inflammatory signaling in microglia

Background

The protein alpha-synuclein (α-SYN), which is found in the Lewy bodies of dopamine-producing (DA) neurons in the substantia nigra (SN), has an important role in the pathogenesis of Parkinson’s disease (PD). Previous studies have shown that neuroinflammation plays a key role in PD pathogenesis. In an AAV-synuclein mouse model of PD, we have found that over-abundance of α-SYN triggers the expression of NF-κB p65, and leads to microglial activation and DA neurodegeneration. We also have observed that Fcγ receptors (FcγR), proteins present on the surface of microglia that bind immunoglobulin G (IgG) and other ligands, are key modulators of α-SYN-induced neurodegeneration.

Methods

In order to study the role of FcγRs in the interactions of α-SYN and microglia, we treated the primary microglial cultures from wild-type (WT) and FcγR^−/− mice with aggregated human α-SYN in vitro.

Results

Using immunocytochemistry, we found that α-SYN was taken up by both WT and FcγR^−/− microglia, however, their patterns of internalization were different, with aggregation in autophagosomes in WT cells and more diffuse localization in FcγR^−/− microglia. In WT microglia, α-SYN induced the nuclear accumulation of NF-κB p65 protein and downstream chemokine expression while in FcγR^−/− mouse microglia, α-SYN failed to trigger the enhancement of nuclear NF-κB p65, and the pro-inflammatory signaling was reduced.

Conclusions

Our results suggest that α-SYN can interact directly with microglia and can be internalized and trafficked to autophagosomes. FcγRs mediate this interaction, and in the absence of the gamma chain, there is altered intracellular trafficking and attenuation of pro-inflammatory NF-κB signaling. Therefore, blocking either FcγR signaling or downstream NF-κB activation may be viable therapeutic strategies in PD.

High-content live-cell imaging assay used to establish mechanism of trastuzumab emtansine (T-DM1)--mediated inhibition of platelet production

Proplatelet production represents a terminal stage of megakaryocyte development during which long, branching processes composed of platelet-sized swellings are extended and released into the surrounding culture. Whereas the cytoskeletal mechanics driving these transformations have been the focus of many studies, significant limitations in our ability to quantify the rate and extent of proplatelet production have restricted the field to qualitative analyses of a limited number of cells over short intervals. A novel high-content, quantitative, live-cell imaging assay using the IncuCyte system (Essen BioScience) was therefore developed to measure the rate and extent of megakaryocyte maturation and proplatelet production under live culture conditions for extended periods of time. As proof of concept, we used this system in the present study to establish a mechanism by which trastuzumab emtansine (T-DM1), an Ab-drug conjugate currently in clinical development for cancer, affects platelet production. High-content analysis of primary cell cultures revealed that T-DM1 is taken up by mouse megakaryocytes, inhibits megakaryocyte differentiation, and disrupts proplatelet formation by inducing abnormal tubulin organization and suppressing microtubule dynamic instability. Defining the pathways by which therapeutics such as T-DM1 affect megakaryocyte differentiation and proplatelet production may yield strategies to manage drug-induced thrombocytopenias.

Differences in endocytosis mediated by FcγRIIA and FcγRIIB2

An important function of Fcγ receptors is the removal of IgG-containing immune complexes from the circulation. The activating receptor FcγRIIA and inhibitory receptor FcγRIIB2 are both expressed on human myeloid cells, and are both capable of mediating endocytosis of immune complexes. We studied endocytosis of these two receptors expressed by transfection in ts20 Chinese hamster fibroblasts. We find that while FcγRIIA-mediated endocytosis requires the participation of the ubiquitin-conjugating system, the endocytosis of FcγRIIB2 does not. Little if any ubiquitylation of FcγRIIB2 was observed in response to immune complex binding. FcγRIIB2 mediates internalization of immune complexes at a faster rate than FcγRIIA, and facilitates the endocytosis of FcγRIIA upon co-engagement of both receptors. This may represent a novel mechanism by which the inhibitory receptor can reduce signalling from the activating Fcγ receptor.