Cross-talk between TLR4 and FcgammaReceptorIII (CD16) pathways

Nanocluster-mediated signaling crosstalk between FcγR and TLR4 in macrophage inflammatory responses

Receptor crosstalk, the interaction between different receptors to modulate signaling, is crucial for fine-tuning the inflammatory responses of innate immune cells. Although the synergistic crosstalk between Toll-like receptor (TLR)4 and Fc gamma receptor (FcγR) is well documented, the detailed mechanism underlying this synergy remains unclear. In this study, we addressed this knowledge gap by imaging the molecular organization of TLR4 and FcγR on the macrophage cell surface and correlating it with their synergistic co-activation using ligands functionalized on lipid bilayers. We confirmed that co-activation of TLR4 and FcγR enhances whole-cell pro-inflammatory responses and tyrosine phosphorylation at the receptor level. Super-resolution microscopy revealed that TLR4 and FcγR each form discrete nanoclusters after ligand stimulation, and their synergistic co-activation increases both the size and spatial overlap of these nanoclusters. Contrary to previous assumptions that TLR4 and FcγR form heterodimers during their crosstalk, our results emphasize the critical role of nanoscale spatial organization between distinct receptor clusters in modulating innate immune responses. Additionally, these findings align with similar receptor interaction mechanisms that we previously reported in other receptor pairs, such as Dectin-1/TLR2 and FcγR/TLR2, suggesting that nanocluster interactions may represent a predominant mechanism governing crosstalk between TLRs and ITAM-containing receptors.

Optimization of IgG1 immune complexes to stimulate cytokine production in innate cells

Fcγ receptors are key immunoreceptors, that when bound by IgG immune complexes, trigger activation of downstream signaling pathways. However, there are limited in vitro assays that stimulate innate cells via Fcγ receptors that allow for evaluation of drugs or chemicals on antibody-triggered signaling. Our study investigated the activation of Fcγ receptors in innate cells using immune complexes. Our findings revealed that immobilized IgG antibodies did not elicit a significant immune response, so we designed two IgG1 immune complexes: trinitrophenyl-bovine serum albumin (TNP-BSA)/TNP-IgG1 and streptavidin-biotinylated IgG1 (Strept-Biotin IgG1). Strept-Biotin IgG1 immune complex was particularly effective, significantly enhancing IL-6, TNFα, and C3a levels, whereas TNP-BSA/TNP-IgG1 immune complex showed a modest IL-6 increase. Both TNP-BSA/TNP-IgG1 and Strept-Biotin IgG1 stimulated CD86 marker expression on F4/80+ macrophages. We also confirmed the binding of Strept-Biotin IgG1 to innate cells with fluorochrome-conjugated streptavidin. To further understand the Fcγ receptor-mediated activation of innate cells, we blocked the downstream phosphatidylinositol 3-kinase (PI3K) pathway. We found out that the PI3K inhibitor successfully suppressed IL-6 cytokine release and C3a production. However, specific Fcγ receptor-blocking antibodies failed to block IL-6 cytokine production and only modestly suppressed TNFα cytokine release, suggesting either that the antibodies were not effective blockers or that these immune complexes use other receptors. Regardless, the use of the Strept-Biotin IgG1 immune complex to stimulate cytokine production and other immune signaling was consistent with Fcγ receptor activation on innate cells which might be useful in assessing the effects of drugs or chemicals in innate cells.

Acetylation of TIR domains in the TLR4-Mal-MyD88 complex regulates immune responses in sepsis

Activation of the Toll-like receptor 4 (TLR4) by bacterial endotoxins in macrophages plays a crucial role in the pathogenesis of sepsis. However, the mechanism underlying TLR4 activation in macrophages is still not fully understood. Here, we reveal that upon lipopolysaccharide (LPS) stimulation, lysine acetyltransferase CBP is recruited to the TLR4 signalosome complex leading to increased acetylation of the TIR domains of the TLR4 signalosome. Acetylation of the TLR4 signalosome TIR domains significantly enhances signaling activation via NF-κB rather than IRF3 pathways. Induction of NF-κB signaling is responsible for gene expression changes leading to M1 macrophage polarization. In sepsis patients, significantly elevated TLR4-TIR acetylation is observed in CD16+ monocytes combined with elevated expression of M1 macrophage markers. Pharmacological inhibition of HDAC1, which deacetylates the TIR domains, or CBP play opposite roles in sepsis. Our findings highlight the important role of TLR4-TIR domain acetylation in the regulation of the immune responses in sepsis, and we propose this reversible acetylation of TLR4 signalosomes as a potential therapeutic target for M1 macrophages during the progression of sepsis.

Direct Binding of Bovine IgG-Containing Immune Complexes to Human Monocytes and Their Putative Role in Innate Immune Training

Bovine milk IgG (bIgG) was shown to bind to and neutralize the human respiratory synovial virus (RSV). In animal models, adding bIgG prevented experimental RSV infection and increased the number of activated T cells. This enhanced activation of RSV-specific T cells may be explained by receptor-mediated uptake and antigen presentation after binding of bIgG-RSV immune complexes (ICs) with FcγRs (primarily CD32) on human immune cells. This indirect effect of bIgG ICs on activation of RSV-specific T cells was confirmed previously in human T cell cultures. However, the direct binding of ICs to antigen-presenting cells has not been addressed. As bovine IgG can induce innate immune training, we hypothesized that this effect could be caused more efficiently by ICs. Therefore, we characterized the expression of CD16, CD32, and CD64 on (peripheral blood mononuclear cells (PBMCs), determined the optimal conditions to form ICs of bIgG with the RSV preF protein, and demonstrated the direct binding of these ICs to human CD14⁺ monocytes. Similarly, bIgG complexed with a murine anti-bIgG mAb also bound efficiently to the monocytes. To evaluate whether the ICs could induce innate immune training more efficiently than bIgG itself, the resulted ICs, as well as bIgG, were used in an in vitro innate immune training model. Training with the ICs containing bIgG and RSV preF protein-but not the bIgG alone-induced significantly higher TNF-α production upon LPS and R848 stimulation. However, the preF protein itself nonsignificantly increased cytokine production as well. This may be explained by its tropism to the insulin-like growth factor receptor 1 (IGFR1), as IGF has been reported to induce innate immune training. Even so, these data suggest a role for IgG-containing ICs in inducing innate immune training after re-exposure to pathogens. However, as ICs of bIgG with a mouse anti-bIgG mAb did not induce this effect, further research is needed to confirm the putative role of bIgG ICs in enhancing innate immune responses in vivo.

Obesity Exacerbates Lupus Activity in Fc Gamma Receptor IIb Deficient Lupus Mice Partly through Saturated Fatty Acid-Induced Gut Barrier Defect and Systemic Inflammation

The prevalence of obesity is increasing, and the coexistence of obesity and systemic lupus erythematosus (lupus) is possible. A high-fat diet (HFD) was orally administered for 6 months in female 8-week-old Fc gamma receptor IIb deficient (FcgRIIb-/-) lupus or age and gender-matched wild-type (WT) mice. Lupus nephritis (anti-dsDNA, proteinuria, and increased creatinine), gut barrier defect (fluorescein isothiocyanate dextran), serum lipopolysaccharide (LPS), serum interleukin (IL)-6, liver injury (alanine transaminase), organ fibrosis (liver and kidney pathology), spleen apoptosis (activated caspase 3), and aorta thickness (but not weight gain and lipid profiles) were more prominent in HFD-administered FcgRIIb-/- mice than the obese WT, without injury in regular diet-administered mice (both FcgRIIb-/- and WT). In parallel, combined palmitic acid (PA; a saturated fatty acid) with LPS (PA + LPS) induced higher tumor necrotic factor-α, IL-6, and IL-10 in the supernatant, inflammatory genes (inducible nitric oxide synthase and IL-1β), reactive oxygen species (dihydroethidium), and glycolysis with reduced mitochondrial activity (extracellular flux analysis) when compared with the activation by each molecule alone in both FcgRIIb-/- and WT macrophages. However, the alterations of these parameters were more prominent in PA + LPS-administered FcgRIIb-/- than in the WT cells. In conclusion, obesity accelerated inflammation in FcgRIIb-/- mice, partly due to the more potent responses from the loss of inhibitory FcgRIIb against PA + LPS with obesity-induced gut barrier defect.

Toll-Like Receptor 4: A Promising Therapeutic Target for Alzheimer's Disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease that primarily manifests as memory deficits and cognitive impairment and has created health challenges for patients and society. In AD, amyloid β-protein (Aβ) induces Toll-like receptor 4 (TLR4) activation in microglia. Activation of TLR4 induces downstream signaling pathways and promotes the generation of proinflammatory cytokines, such as tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), and interleukin-1β (IL-1β), which also trigger the activation of astrocytes and influence amyloid-dependent neuronal death. Therefore, TLR4 may be an important molecular target for treating AD by regulating neuroinflammation. Moreover, TLR4 regulates apoptosis, autophagy, and gut microbiota and is closely related to AD. This article reviews the role of TLR4 in the pathogenesis of AD and a range of potential therapies targeting TLR4 for AD. Elucidating the regulatory mechanism of TLR4 in AD may provide valuable clues for developing new therapeutic strategies for AD.

Immobile ligands enhance FcγR-TLR2/1 crosstalk by promoting interface overlap of receptor clusters

Innate immune cells detect pathogens through simultaneous stimulation of multiple receptors, but how cells use the receptor crosstalk to elicit context-appropriate responses is unclear. Here, we reveal that the inflammatory response of macrophages from FcγR-TLR2/1 crosstalk inversely depends on the ligand mobility within a model pathogen membrane. The mechanism is that FcγR and TLR2/1 form separate nanoclusters that interact at their interfaces during crosstalk. Less mobile ligands induce stronger interactions and more overlap between the receptor nanoclusters, leading to enhanced signaling. Different from the prevailing view that immune receptors colocalize to synergize their signaling, our results show that FcγR-TLR2/1 crosstalk occurs through interface interactions between non-colocalizing receptor nanoclusters, which are modulated by ligand mobility. This suggests a mechanism by which innate immune cells could use physical properties of ligands to fine-tune host responses.

Detection and functional resolution of soluble immune complexes by an FcγR reporter cell panel

Fc-gamma receptor (FcγR) activation by soluble IgG immune complexes (sICs) represents a major mechanism of inflammation in certain autoimmune diseases such as systemic lupus erythematosus (SLE). A robust and scalable test system allowing for the detection and quantification of sIC bioactivity is missing. We developed a comprehensive reporter cell panel detecting activation of FcγRs. The reporter cell lines were integrated into an assay that enables the quantification of sIC reactivity via ELISA or a faster detection using flow cytometry. This identified FcγRIIA(H) and FcγRIIIA as the most sIC-sensitive FcγRs in our test system. Reaching a detection limit in the very low nanomolar range, the assay proved also to be sensitive to sIC stoichiometry and size reproducing for the first time a complete Heidelberger-Kendall curve in terms of immune receptor activation. Analyzing sera from SLE patients and mouse models of lupus and arthritis proved that sIC-dependent FcγR activation has predictive capabilities regarding severity of SLE disease. The assay provides a sensitive and scalable tool to evaluate the size, amount, and bioactivity of sICs in all settings.

The Functional Role of IgA in the IgM/IgA-Enriched Immunoglobulin Preparation Trimodulin

In comparison to human immunoglobulin (Ig) G, antibodies of IgA class are not well investigated. In line with this, the functional role of the IgA component in IgM/IgA-enriched immunoglobulin preparations is also largely unknown. In recent years, powerful anti-pathogenic and immunomodulatory properties of human serum IgA especially on neutrophil function were unraveled. Therefore, the aim of our work is to investigate functional aspects of the trimodulin IgA component, a new plasma-derived polyvalent immunoglobulin preparation containing ~56% IgG, ~23% IgM and ~21% IgA. The functional role of IgA was investigated by analyzing the interaction of IgA with FcαRI, comparing trimodulin with standard intravenous IgG (IVIG) preparation and investigating Fc receptor (FcR)-dependent functions by excluding IgM-mediated effects. Trimodulin demonstrated potent immunomodulatory, as well as anti-pathogenic effects in our neutrophil model (neutrophil-like HL-60 cells). The IgA component of trimodulin was shown to induce a strong FcαRI-dependent inhibitory immunoreceptor tyrosine-based activation motif (ITAMi) signaling, counteract lipopolysaccharide-induced inflammation and mediate phagocytosis of Staphylococcus aureus. The fine-tuned balance between immunomodulatory and anti-pathogenic effects of trimodulin were shown to be dose-dependent. Summarized, our data demonstrate the functional role of IgA in trimodulin, highlighting the importance of this immunoglobulin class in immunoglobulin therapy.

Cholesterol and sphingomyelin are critical for Fcγ receptor-mediated phagocytosis of Cryptococcus neoformans by macrophages

Cryptococcus neoformans is a fungal pathogen that causes life-threatening meningoencephalitis in lymphopenic patients. Pulmonary macrophages comprise the first line of host defense upon inhalation of fungal spores by aiding in clearance but can also potentially serve as a niche for their dissemination. Given that macrophages play a key role in the outcome of a cryptococcal infection, it is crucial to understand factors that mediate phagocytosis of C. neoformans. Since lipid rafts (high-order plasma membrane domains enriched in cholesterol and sphingomyelin [SM]) have been implicated in facilitating phagocytosis, we evaluated whether these ordered domains govern macrophages' ability to phagocytose C. neoformans. We found that cholesterol or SM depletion resulted in significantly deficient immunoglobulin G (IgG)-mediated phagocytosis of fungus. Moreover, repletion of macrophage cells with a raft-promoting sterol (7-dehydrocholesterol) rescued this phagocytic deficiency, whereas a raft-inhibiting sterol (coprostanol) significantly decreased IgG-mediated phagocytosis of C. neoformans. Using a photoswitchable SM (AzoSM), we observed that the raft-promoting conformation (trans-AzoSM) resulted in efficient phagocytosis, whereas the raft-inhibiting conformation (cis-AzoSM) significantly but reversibly blunted phagocytosis. We observed that the effect on phagocytosis may be facilitated by Fcγ receptor (FcγR) function, whereby IgG immune complexes crosslink to FcγRIII, resulting in tyrosine phosphorylation of FcR γ-subunit (FcRγ), an important accessory protein in the FcγR signaling cascade. Correspondingly, cholesterol or SM depletion resulted in decreased FcRγ phosphorylation. Repletion with 7-dehydrocholesterol restored phosphorylation, whereas repletion with coprostanol showed FcRγ phosphorylation comparable to unstimulated cells. Together, these data suggest that lipid rafts are critical for facilitating FcγRIII-mediated phagocytosis of C. neoformans.

Protocatechuic acid attenuates lipopolysaccharide-induced septic lung injury in mice: The possible role through suppressing oxidative stress, inflammation and apoptosis

Here, we investigated the protective efficacy of protocatechuic acid (PCA) against lipopolysaccharide (LPS)-induced septic lung injury. Eighty-two male Balb/c mice were divided into six groups: control, PCA30 (30 mg/kg), LPS (10 mg/kg), PCA10-LPS, PCA20-LPS, and PCA30-LPS treated with 10, 20 and 30 mg/kg PCA, respectively, for seven days before intraperitoneal LPS injection. PCA pre-treatment, especially at higher dose, significantly reduced LPS-induced lung tissue injury as indicated by increased heat shock protein 70 and antioxidant molecules (reduced glutathione, superoxide dismutase, catalase, glutathione peroxidase, and glutathione reductase) accompanied by lower oxidative stress indices (malondialdehyde and nitric oxide). PCA administration decreased inflammatory mediators including myeloperoxidase, nuclear factor kappa B (NF-κB p65), and pro-inflammatory cytokines, and prevented the development of apoptotic events in the lung tissue. At the molecular level, PCA downregulated mRNA expression of nitric oxide synthase 2, C/EBP homologous protein, and high mobility group box1 in the lungs of all PCA-LPS treated mice. Thus, PCA-pre-treatment effectively counteracted sepsis-induced acute lung injury in vivo by promoting and antioxidant status, while inhibiting inflammation and apoptosis. PRACTICAL IMPLICATIONS: Sepsis-mediated organ dysfunction and high mortality is aggravated by acute lung injury (ALI). Therefore, new therapeutic approaches are needed to encounter sepsis-mediated ALI. Protocatechuic acid (PCA) is a naturally occurring phenolic acid with various biological and pharmacological activities. PCA is abundant in edible plants including Allium cepa L., Oryza sativa L., Hibiscus sabdariffa, Prunus domestica L., and Eucommia ulmoides. In this investigation we studied the potential protective role of pure PCA (10, 20 and 30 mg/kg) on LPS-mediated septic lung injury in mice through examining oxidative challenge, inflammatory response, apoptotic events and histopathological changes in addition to evaluating the levels and mRNA expression of heat shock protein 70, C/EBP homologous protein and high mobility group box1 in the lung tissue. The recorded results showed that PCA pre-administration was able to significantly abrogate the damages in the lung tissue associated septic response. This protective effect comes from its strong antioxidant, anti-inflammatory, and anti-apoptotic activities, suggesting that PCA may be applied to alleviate ALI associated with the development of sepsis.

Spatial organization of FcγR and TLR2/1 on phagosome membranes differentially regulates their synergistic and inhibitory receptor crosstalk

Many innate immune receptors function collaboratively to detect and elicit immune responses to pathogens, but the physical mechanisms that govern the interaction and signaling crosstalk between the receptors are unclear. In this study, we report that the signaling crosstalk between Fc gamma receptor (FcγR) and Toll-like receptor (TLR)2/1 can be overall synergistic or inhibitory depending on the spatial proximity between the receptor pair on phagosome membranes. Using a geometric manipulation strategy, we physically altered the spatial distribution of FcγR and TLR2 on single phagosomes. We demonstrate that the signaling synergy between FcγR and TLR2/1 depends on the proximity of the receptors and decreases as spatial separation between them increases. However, the inhibitory effect from FcγRIIb on TLR2-dependent signaling is always present and independent of receptor proximity. The overall cell responses are an integration from these two mechanisms. This study presents quantitative evidence that the nanoscale proximity between FcγR and TLR2 functions as a key regulatory mechanism in their signaling crosstalk.

Acute Kidney Injury Induced Lupus Exacerbation Through the Enhanced Neutrophil Extracellular Traps (and Apoptosis) in Fcgr2b Deficient Lupus Mice With Renal Ischemia Reperfusion Injury

Renal ischemia is the most common cause of acute kidney injury (AKI) that might be exacerbate lupus activity through neutrophil extracellular traps (NETs) and apoptosis. Here, the renal ischemia reperfusion injury (I/R) was performed in Fc gamma receptor 2b deficient (Fcgr2b-/-) lupus mice and the in vitro experiments. At 24 h post-renal I/R injury, NETs in peripheral blood neutrophils and in kidneys were detected using myeloperoxidase (MPO), neutrophil elastase (NE) and citrullinated histone H3 (CitH3), as well as kidney apoptosis (activating caspase-3), which were prominent in Fcgr2b-/- mice more compared to wild-type (WT). After 120 h renal-I/R injury, renal NETs (using MPO and NE) were non-detectable, whereas glomerular immunoglobulin (Ig) deposition and serum anti-dsDNA were increased in Fcgr2b-/- mice. These results imply that renal NETs at 24 h post-renal I/R exacerbated the lupus nephritis at 120 h post-renal I/R injury in Fcgr2b-/- lupus mice. Furthermore, a Syk inhibitor attenuated NETs, that activated by phorbol myristate acetate (PMA; a NETs activator) or lipopolysaccharide (LPS; a potent inflammatory stimulator), more prominently in Fcgr2b-/- neutrophils than the WT cells as determined by dsDNA, PAD4 and MPO. In addition, the inhibitors against Syk and PAD4 attenuated lupus characteristics (serum creatinine, proteinuria, and anti-dsDNA) in Fcgr2b-/- mice at 120 h post-renal I/R injury. In conclusion, renal I/R in Fcgr2b-/- mice induced lupus exacerbation at 120 h post-I/R injury partly because Syk-enhanced renal NETs led to apoptosis-induced anti-dsDNA, which was attenuated by a Syk inhibitor.

Lipopolysaccharide-Enhanced Responses against Aryl Hydrocarbon Receptor in FcgRIIb-Deficient Macrophages, a Profound Impact of an Environmental Toxin on a Lupus-Like Mouse Model

Fc gamma receptor IIb (FcgRIIb) is the only inhibitory-FcgR in the FcgR family, and FcgRIIb-deficient (FcgRIIb^−/−) mice develop a lupus-like condition with hyper-responsiveness against several stimulations. The activation of aryl hydrocarbon receptor (Ahr), a cellular environmental sensor, might aggravate activity of the lupus-like condition. As such, 1,4-chrysenequinone (1,4-CQ), an Ahr-activator, alone did not induce supernatant cytokines from macrophages, while the 24 h pre-treatment by lipopolysaccharide (LPS), a representative inflammatory activator, prior to 1,4-CQ activation (LPS/1,4-CQ) predominantly induced macrophage pro-inflammatory responses. Additionally, the responses from FcgRIIb^−/− macrophages were more prominent than wild-type (WT) cells as determined by (i) supernatant cytokines (TNF-α, IL-6, and IL-10), (ii) expression of the inflammation associated genes (NF-κB, aryl hydrocarbon receptor, iNOS, IL-1β and activating-FcgRIV) and cell-surface CD-86 (a biomarker of M1 macrophage polarization), and (iii) cell apoptosis (Annexin V), with the lower inhibitory-FcgRIIb expression. Moreover, 8-week-administration of 1,4-CQ in 8 week old FcgRIIb^−/− mice, a genetic-prone lupus-like model, enhanced lupus characteristics as indicated by anti-dsDNA, serum creatinine, proteinuria, endotoxemia, gut-leakage (FITC-dextran), and glomerular immunoglobulin deposition. In conclusion, an Ahr activation worsened the disease severity in FcgRIIb^−/− mice possibly through the enhanced inflammatory responses. The deficiency of inhibitory-FcgRIIb in these mice, at least in part, prominently enhanced the pro-inflammatory responses. Our data suggest that patients with lupus might be more vulnerable to environmental pollutants.

Sexually dimorphic placental responses to maternal SARS-CoV-2 infection

There is a persistent male bias in the prevalence and severity of COVID-19 disease. Underlying mechanisms accounting for this sex difference remain incompletely understood. Interferon responses have been implicated as a modulator of disease in adults, and play a key role in the placental anti-viral response. Moreover, the interferon response has been shown to alter Fc-receptor expression, and therefore may impact placental antibody transfer. Here we examined the intersection of viral-induced placental interferon responses, maternal-fetal antibody transfer, and fetal sex. Placental interferon stimulated genes (ISGs), Fc-receptor expression, and SARS-CoV-2 antibody transfer were interrogated in 68 pregnancies. Sexually dimorphic placental expression of ISGs, interleukin-10, and Fc receptors was observed following maternal SARS-CoV-2 infection, with upregulation in males. Reduced maternal SARS-CoV-2-specific antibody titers and impaired placental antibody transfer were noted in pregnancies with a male fetus. These results demonstrate fetal sex-specific maternal and placental adaptive and innate immune responses to SARS-CoV-2.

Prominent Indomethacin-Induced Enteropathy in Fcgriib Defi-cient lupus Mice: An Impact of Macrophage Responses and Immune Deposition in Gut

A high dose of NSAIDs, a common analgesic, might induce lupus activity through several NSAIDs adverse effects including gastrointestinal permeability defect (gut leakage) and endotoxemia. Indomethacin (25 mg/day) was orally administered for 7 days in 24-wk-old Fc gamma receptor IIb deficient (FcgRIIb-/-) mice, an asymptomatic lupus model (increased anti-dsDNA without lupus nephritis), and age-matched wild-type (WT) mice. Severity of indomethacin-induced enteropathy in FcgRIIb-/- mice was higher than WT mice as demonstrated by survival analysis, intestinal injury (histology, immune-deposition, and intestinal cytokines), gut leakage (FITC-dextran assay and endotoxemia), serum cytokines, and lupus characteristics (anti-dsDNA, renal injury, and proteinuria). Prominent responses of FcgRIIb-/- macrophages toward lipopolysaccharide (LPS) compared to WT cells due to the expression of only activating-FcgRs without inhibitory-FcgRIIb were demonstrated. Extracellular flux analysis indicated the greater mitochondria activity (increased respiratory capacity and respiratory reserve) in FcgRIIb-/- macrophages with a concordant decrease in glycolysis activity when compared to WT cells. In conclusion, gut leakage-induced endotoxemia is more severe in indomethacin-administered FcgRIIb-/- mice than WT, possibly due to the enhanced indomethacin toxicity from lupus-induced intestinal immune-deposition. Due to a lack of inhibitory-FcgRIIb expression, mitochondrial function, and cytokine production of FcgRIIb-/- macrophages were more prominent than WT cells. Hence, lupus disease-activation from NSAIDs-enteropathy-induced gut leakage is possible.

Polymorphisms in Fc Gamma Receptors and Susceptibility to Malaria in an Endemic Population

Repeated infections by Plasmodium falciparum result in a humoral response that could reduce disease symptoms and prevent the development of clinical malaria. The principal mechanism underlying this humoral response is that immunoglobulin G (IgG) binds directly to the parasites, thus causing their neutralization. However, the action of antibodies alone is not always sufficient to eliminate pathogens from an organism. One key element involved in the recognition of IgG that plays a crucial role in the destruction of the parasites responsible for spreading malaria is the family of Fc gamma receptors. These receptors are expressed on the surface of immune cells. Several polymorphisms have been detected in the genes encoding these receptors, associated with susceptibility or resistance to malaria in different populations. In this review, we describe identified polymorphisms within the family of Fc gamma receptors and the impact of these variations on the response of a host to infection as well as provide new perspectives for the design of an effective vaccine for malaria.

Tumor necrosis factor α inhibition overcomes immunosuppressive M2b macrophage-induced bevacizumab resistance in triple-negative breast cancer

Bevacizumab in neoadjuvant therapy provides a new hope of improved survival for patients with triple-negative breast cancer (TNBC) by targeting vascular endothelial growth factor in combination with chemotherapy, but curative effect is limited by bevacizumab’s continuous use while mechanisms remain incompletely understood. More and more researches reported that tumor-associated macrophages mediate resistance to chemotherapy and radiotherapy in various tumors. Here we developed a TNBC model resistant to bevacizumab under bevacizumab continuous administration. It was found that proportion of a specific subset of tumor-associated macrophages characterized as M2b (CD11b⁺ CD86^high IL10^high) increased and responsible for acquired resistance to bevacizumab. Then, we showed that RAW264.7 macrophages could be polarized to M2b subtype on simultaneous exposure to bevacizumab and TLR4 ligands as occurs in the context of continuous bevacizumab treatment. Concordantly, in TLR4-deleted C57BL/10ScNJNju (TLR4^lps–del) ^mut/mut mice with bevacizumab treatment model, it was verified that the M2b macrophage could be induced by Fc gamma receptor-TLR4 cross-talk. In MDA-MB-231-resistant tumor-bearing mice, the content of TNFα in serum kept going up consistent with CCL1, a chemokine of M2b macrophage. In vitro neutralizing tumor necrosis factor α (TNFα) could inhibit the tumor progression caused by M2b culture medium and tumor IDO1 expression. Therefore, we thought that TNFα is a key tumor-promoting effector molecule secreted by M2b macrophage. Accordingly, the curative effect of bevacizumab was proved to be significantly improved by neutralizing TNFα with anti-TNFα nanobody. This study is expected to provide theoretical and clinical evidence elucidating the drug resistance in patients receiving bevacizumab.

Syk inhibitor attenuates inflammation in lupus mice from FcgRIIb deficiency but not in pristane induction: the influence of lupus pathogenesis on the therapeutic effect

Macrophages are responsible for the recognition of pathogen molecules. The downstream signalling of the innate immune responses against pathogen molecules, lipopolysaccharide (LPS) and (1→3)-β-D-glucan (BG), and the adaptive immune response to antibodies, Fc gamma receptor (FcgR), is spleen tyrosine kinase (Syk). Because pathogen molecules and antibodies could be presented in lupus, impact of Syk and macrophages in lupus is explored. FcgR-IIb deficient (FcgRIIb^-/-) mice, a model of inhibitory signalling loss, at 40 weeks old, but not pristane mice (a chemical induction lupus model) demonstrated spontaneous elevation of LPS and BG in serum from gut translocation despite the similarity in faecal microbiome analysis. Syk abundance in FcgRIIb^-/- mice was higher than in pristane mice, possibly due to several Syk activators (anti-dsDNA, LPS and BG), and Syk inhibitor-attenuated proteinuria and serum cytokines only in FcgRIIb^-/- mice. In addition, LPS + BG enhanced the expression of activating FcgRs, NF-κB and Syk, together with supernatant TNF-α predominantly in FcgRIIb^-/- compared to wild-type macrophages. The inhibitors against Dectin-1, Syk and nuclear factor kappa B, but not anti-Raf-1, reduced supernatant TNF-α in LPS+BG-activated macrophages, implying Syk-dependent signalling. The pathogen molecules enhanced activating-FcgRs, without inhibition, through Syk, a shared downstream innate and adaptive signalling, is responsible for the hyper-responsiveness in FcgRIIb^-/- macrophages. In conclusion, Syk inhibitor attenuated inflammation in FcgRIIb^-/- but not in pristane mice, implying the influence of a lupus genetic background in treatment modalities.

[Chromato-mass spectrometric analysis of urine proteins associated with the functions of Toll-receptors in a healthy person under conditions of 17-day isolation.]

Under controlled conditions of 17-day isolation (Sirius-17 experiment), the protein composition of urine was studied in 6 healthy test volunteers-3 women and 3 men. Collection of samples in the form of a second freely separated morning urine fraction was carried out in the background (seven days before the experiment), as well as 1 day after the end of exposure. Chromatographic-mass-spectrometric semi-quantitative analysis of the protein composition of samples was performed on a system consisting of an Agilent 1100 chromatograph and an LTQ-FT Ultra hybrid mass spectrometer using bioinformatics resources UniProtKB, GeneOntology. An asymptomatic change in the immune defense system of kidney tissue after isolation in a closed hermetic object is associated with a change in the content of 7 proteins that provide functional activity of the TLR tubules of the kidneys - FcRIII, MUC1, Galectin-3, Ficolin-2, APOA1, FLNA, FCGR3A and Clusterin. These proteins are found to be useful biomarkers in the study of physiology and kidney diseases. They can be attributed to candidates for protein markers of the initial stages of impaired recognition by the epithelium of renal tubules of bacteria with known pathogenic potential.

Prolyl Oligopeptidase From Leishmania infantum: Biochemical Characterization and Involvement in Macrophage Infection

Leishmania infantum is a flagellated protozoan and one of the main causative agents of visceral leishmaniasis. This disease usually affects the human reticuloendothelial system, can cause death and available therapies may lead to serious side effects. Since it is a neglected tropical disease, the incentives for the development of new drugs are insufficient. It is important to know Leishmania virulence factors that contribute most to the disease in order to develop drugs. In the present work, we have produced L. infantum prolyl oligopeptidase (rPOPLi) in Escherichia coli, and investigated its biochemical properties as well as the effect of POP inhibitors on its enzymatic activity and on the inhibition of the macrophage infection by L. infantum. The optimal activity occurred at pH 7.5 and 37°C in the presence of DTT, the latter increased rPOPLi catalytic efficiency 5-fold on the substrate N-Suc-Gly-Pro-Leu-Gly-Pro-AMC. The enzyme was inhibited by TPCK, TLCK and by two POP specific inhibitors, Z-Pro-prolinal (ZPP, IC₅₀ 4.2 nM) and S17092 (IC₅₀ 3.5 nM). Besides being a cytoplasmic enzyme, POPLi is also found in punctuate structures within the parasite cytoplasm or associated with the parasite plasma membrane in amastigotes and promastigotes, respectively. Interestingly, S17092 and ZPP prevented parasite invasion in murine macrophages, supporting the involvement of POPLi in the invasive process of L. infantum. These data suggest POPLi as a virulence factor that offers potential as a target for designing new antileishmanial drugs.

THP-1 cells increase TNF-α production upon LPS + soluble human IgG co-stimulation supporting evidence for TLR4 and Fcγ receptors crosstalk

The lipopolysaccharide (LPS) of Gram-negative bacteria is recognized on human monocytes and macrophages by TLR4 and MD2 and induces the production of inflammatory cytokines; the LPS + IgG complexes co-stimulation increases the cytokine production, mediated by the Fc-γRIIa (CD32a). We stimulated human CD14 + monocytes or THP-1 cells with LPS or LPS + soluble human IgG (sIgG) and TNF-α transcription and production, assessed RT-qPCR, ELISA, or flow cytometry, was enhanced by 30% upon LPS + sIgG compared to LPS stimulation. LPS + sIgG co-stimulation affected the NF-κB pathway (p65 phosphorylation and nucleus translocation, and IkB- α degradation). The biochemical inhibition of IRAK 1/4 and Syk kinases suppressed the enhancer effect of LPS + sIgG on TNF- α production, suggesting the involvement of both MyD88 dependent and independent pathways. Our results suggest that during LPS activation, sIgG may participate in a TLR4 - Fc-γR crosstalk.

Fc-gamma receptors and S100A8/A9 cause bone erosion during rheumatoid arthritis. Do they act as partners in crime?

Bone erosion is one of the central hallmarks of RA and is caused by excessive differentiation and activation of osteoclasts. Presence of autoantibodies in seropositive arthritis is associated with radiographic disease progression. ICs, formed by autoantibodies and their antigens, activate Fcγ-receptor signalling in immune cells, and as such stimulate inflammation-mediated bone erosion. Interestingly, ICs can also directly activate osteoclasts by binding to FcγRs on their surface. Next to autoantibodies, high levels of alarmins, among which is S100A8/A9, are typical for RA and they can further activate the immune system but also directly promote osteoclast function. Therefore, IC-activated FcγRs and S100A8/A9 might act as partners in crime to stimulate inflammation and osteoclasts differentiation and function, thereby stimulating bone erosion. This review discusses the separate roles of ICs, FcγRs and alarmins in bone erosion and sheds new light on the possible interplay between them, which could fuel bone erosion.

FcRγ deficiency improves survival in experimental sepsis by down-regulating TLR4 signaling pathway

Fc receptor common γ signaling chain (FcRγ), a common subunit shared by Fc receptors (FcγRI, III, IV, FcαRI, and FcεRI), is an important immune regulator both in innate and adaptive immunity. Previous studies have shown that FcRγ was a potential target of inflammatory diseases, whereas the role of FcRγ in sepsis has been poorly understood. In this study, we found that deficiency of FcRγ resulted in increased survival in lipopolysaccharide (LPS)/D-galactosamine and E. coli-induced sepsis in mice. This protective effect was characterized by decreased TNF-α, IL-6, and IL-10. Further experiments in bone marrow-derived macrophages (BMDMs) in vitro also showed that FcRγ deficiency resulted in decreased production of TNF-α, IL-6, and IL-10 upon LPS stimulation. The mechanism study showed that FcRγ was physiologically associated with toll-like receptor 4 (TLR4), and tyrosine phosphorylation of FcRγ mediated TLR4 signaling pathway, followed by increased ERK phosphorylation upon LPS stimulation. Our results suggest that FcRγ might be a potential therapeutic target of sepsis.

Geometrical reorganization of Dectin-1 and TLR2 on single phagosomes alters their synergistic immune signaling

Receptors of innate immune cells function synergistically to detect pathogens and elicit appropriate immune responses. Many receptor pairs also appear "colocalized" on the membranes of phagosomes, the intracellular compartments for pathogen ingestion. However, the nature of the seemingly receptor colocalization and the role it plays in immune regulation are unclear, due to the inaccessibility of intracellular phagocytic receptors. Here, we report a geometric manipulation technique to directly probe the role of phagocytic receptor "colocalization" in innate immune regulation. Using particles with spatially patterned ligands as phagocytic targets, we can decouple the receptor pair, Dectin-1 and Toll-like receptor (TLR)2, to opposite sides on a single phagosome or bring them into nanoscale proximity without changing the overall membrane composition. We show that Dectin-1 enhances immune responses triggered predominantly by TLR2 when their centroid-to-centroid proximity is <500 nm, but this signaling synergy diminishes upon receptor segregation beyond this threshold distance. Our results demonstrate that nanoscale proximity, not necessarily colocalization, between Dectin-1 and TLR2 is required for their synergistic regulation of macrophage immune responses. This study elucidates the relationship between the spatial organization of phagocytic receptors and innate immune responses. It showcases a technique that allows spatial manipulation of receptors and their signal cross-talk on phagosomes inside living cells.

Toll-like receptor 4 and breast cancer: an updated systematic review

Toll-like receptors (TLRs) may play dual roles in human cancers. TLR4 is a key molecule which may participate in both friend and foe roles against breast cancer. This review article collected recent data regarding the mechanisms used by TLR4 in the eradication of breast cancer cells and induction of the tumor cells, and discussed the mechanisms involved in the various functions of TLR4. The literature searches revealed that TLR4 is a key molecule that participates in breast cancer cell eradication or induction of breast cancer development and also transformation of the normal cells. TLR4 eradicates breast cancer cells via recognition of their DAMPs and then induces immune responses. Over-expression of TLR4 and also alterations in its signaling, including association of some intrinsic pathways such as TGF-β signaling and TP53, are the crucial factors to alter TLR4 functions against breast cancer.

Inside-Out Control of Fc-Receptors

Receptors recognizing the Fc-part of immunoglobulins (FcR) are important in the engagement of phagocytes with opsonized micro-organisms, but they also play a major role in the pathogenesis of chronic inflammatory diseases. Different FcRs are specifically recognizing and binding the different classes of immunoglobulins, transmitting different signals into the cell. The function of IgG (FcγR's) and IgA (FcαR) recognizing receptors is controlled by cellular signals evoked by activation of heterologous receptors in a process generally referred to as inside-out control. This concept is clearly described for the regulation of integrin receptors. Inside-out control can be achieved at different levels by modulation of: (i) receptor affinity, (ii) receptor avidity/valency, (iii) interaction with signaling chains, (iv) interaction with other receptors and (v) localization in functionally different membrane domains. The inside-out control of FcRs is an interesting target for novel therapy by therapeutical antibodies as it can potentiate or decrease the functionality of the response to the antibodies depending on the mechanisms of the diseases they are applied for.

Impact of CD14 on Reactive Oxygen Species Production from Human Leukocytes Primed by Escherichia coli Lipopolysaccharides

Lipopolysaccharides (LPS) from Gram-negative bacteria prime human polymorphonuclear neutrophils (PMNs) via multicomponent receptor cluster including CD14 and MD-2·TLR4 for the enhanced release of reactive oxygen species (ROS) were triggered by bacterial derived peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP). In this study, we investigated the impact of CD14 on LPS-induced priming of human PMNs for fMLP-triggered ROS generation (respiratory or oxidative) burst. Monoclonal antibodies against human CD14 (mAbs) as well as isotype-matched IgG2a did not influence significantly fMLP-triggered ROS production from LPS-unprimed PMNs. Anti-CD14 mAbs (clone UCHM-1) attenuated LPS-induced priming of PMNs as it had been mirrored by fMLP-triggered decrease of ROS production. Similar priming activity of S-LPS or Re-LPS from Escherichia coli for fMLP-triggered ROS release from PMNs was found. Obtained results suggest that glycosylphosphatidylinositol-anchored CD14 is the key player in LPS-induced PMN priming for fMLP-triggered ROS production. We believe that blockade of CD14 on the cell surface and clinical use of anti-CD14 mAbs or their Fab fragments may diminish the production of ROS and improve outcomes during cardiovascular diseases manifested by LPS-induced inflammation.

Beyond binding: antibody effector functions in infectious diseases

Antibodies play an essential role in host defence against pathogens by recognizing microorganisms or infected cells. Although preventing pathogen entry is one potential mechanism of protection, antibodies can control and eradicate infections through a variety of other mechanisms. In addition to binding and directly neutralizing pathogens, antibodies drive the clearance of bacteria, viruses, fungi and parasites via their interaction with the innate and adaptive immune systems, leveraging a remarkable diversity of antimicrobial processes locked within our immune system. Specifically, antibodies collaboratively form immune complexes that drive sequestration and uptake of pathogens, clear toxins, eliminate infected cells, increase antigen presentation and regulate inflammation. The diverse effector functions that are deployed by antibodies are dynamically regulated via differential modification of the antibody constant domain, which provides specific instructions to the immune system. Here, we review mechanisms by which antibody effector functions contribute to the balance between microbial clearance and pathology and discuss tractable lessons that may guide rational vaccine and therapeutic design to target gaps in our infectious disease armamentarium.

Granulocyte-augmented chemokine production induced by type II collagen containing immune complexes is mediated via TLR4 in rheumatoid arthritis patients

Rheumatoid arthritis (RA) patients with early elevations of antibodies against collagen type II (CII) have a distinct acute onset phenotype, associated with cytokine induction by surface‐bound anti‐CII‐containing immune complexes (ICs) and high C‐reactive protein (CRP) and erythrocyte sedimentation rate (ESR). Polymorphonuclear granulocytes (PMNs) and peripheral blood mononuclear cells (PBMCs) are abundant in the vicinity of CII in RA joints, and both PMN and PBMC reactivity against anti‐CII IC individually relate to early joint destruction and early elevation of CRP and ESR in RA. We searched for CII‐dependent mechanisms that might attract PMNs and PBMCs to RA joints. Human PBMCs and PMNs were stimulated with anti‐CII ICs and control ICs, either individually or in cocultures. Cocultured PMNs and PBMCs stimulated with anti‐CII ICs synergistically augmented production of the chemokines CXCL8, RANTES and MCP‐1, whereas downregulation was seen with control IC. This upregulation was unique to chemokines, as TNF‐α, IL‐1β, and GM‐CSF were downregulated in anti‐CII IC‐stimulated cocultures. The coculture‐associated chemokine upregulation depended on endogenous TLR4 ligand(s) and functionally active PMN enzymes, and was partially mediated by GM‐CSF. As anti‐CII levels peak around the time of RA diagnosis, this mechanism can attract inflammatory cells to joints in early RA and intensify the anti‐CII‐associated acute onset RA phenotype.

FcγRIIIa-Syk Co-signal Modulates CD4+ T-cell Response and Up-regulates Toll-like Receptor (TLR) Expression

CD4(+) T-cells in systemic lupus erythematosus (SLE) patients show altered T-cell receptor signaling, which utilizes Fc-receptor γ-chain FcRγ-Syk. A role for FcγRIIIa activation from immune complex (IC) ligation and sublytic terminal complement complex (C5b-9) in CD4(+) T-cell responses is not investigated. In this study, we show that the ICs present in SLE patients by ligating to FcγRIIIa on CD4(+) T-cells phosphorylate Syk and provide a co-stimulatory signal to CD4(+) T-cells in the absence of CD28 signal. This led to the development of pathogenic IL-17A(+) and IFN-γ(high) CD4(+) T-cells in vitro. Cytokines IL-1β, IL-6, TGF-β1, and IL-23 were the only requirement for the development of both populations. SLE patients CD4(+) T-cells that expressed CD25, CD69, and CD98 bound to ICs showed pSyk and produced IFN-γ and IL-17A. This FcγRIIIa-mediated co-signal differentially up-regulated the expression of IFN pathway genes compared with CD28 co-signal. FcγRIIIa-pSyk up-regulated several toll-like receptor genes as well as the HMGB1 and MyD88 gene transcripts. ICs co-localized with these toll-like receptor pathway proteins. These results suggest a role for the FcγRIIIa-pSyk signal in modulating adaptive immune responses.

Protective Role of Rho Guanosine Diphosphate Dissociation Inhibitor, Ly-GDI, in Pulmonary Alveolitis

Growing evidences indicate that Ly-GDI, an inhibitory protein of Rho GTPases, plays an essential role in regulating actin cytoskeletal alteration which is indispensible for the process such as phagocytosis. However, the role of Ly-GDI in inflammation remains largely unknown. In the current study, we found that Ly-GDI expression was significantly decreased in the IgG immune complex-injured lungs. To determine if Ly-GDI might regulate the lung inflammatory response, we constructed adenovirus vectors that could mediate ectopic expression of Ly-GDI (Adeno-Ly-GDI). In vivo mouse lung expression of Ly-GDI resulted in a significant attenuation of IgG immune complex-induced lung injury, which was due to the decreased pulmonary permeability and lung inflammatory cells, especially neutrophil accumulation. Upon IgG immune complex deposition, mice with Ly-GDI over-expression in the lungs produced significant less inflammatory mediators (TNF-α, IL-6, MCP-1, and MIP-1α) in bronchoalveolar lavage fluid when compared control mice receiving airway injection of Adeno-GFP. Mechanically, IgG immune complex-induced NF-κB activity was markedly suppressed by Ly-GDI in both alveolar macrophages and lungs as measured by luciferase assay and electrophoretic mobility shift assay. These findings suggest that Ly-GDI is a critical regulator of inflammatory injury after deposition of IgG immune complexes and that it negatively regulates the lung NF-κB activity.

Genomic regulation of senescence and innate immunity signaling in the retinal pigment epithelium

The tumor suppressor p53 is a major regulator of genes important for cell cycle arrest, senescence, apoptosis, and innate immunity, and has recently been implicated in retinal aging. In this study we sought to identify the genetic networks that regulate p53 function in the retina using quantitative trait locus (QTL) analysis. First we examined age-associated changes in the activation and expression levels of p53; known p53 target proteins and markers of innate immune system activation in primary retinal pigment epithelial (RPE) cells that were harvested from young and aged human donors. We observed increased expression of p53, activated caspase-1, CDKN1A, CDKN2A (p16INK4a), TLR4, and IFNα in aged primary RPE cell lines. We used the Hamilton Eye Institute (HEI) retinal dataset ( www.genenetwork.org ) to identify genomic loci that modulate expression of genes in the p53 pathway in recombinant inbred BXD mouse strains using a QTL systems biology-based approach. We identified a significant trans-QTL on chromosome 1 (region 172-177 Mb) that regulates the expression of Cdkn1a. Many of the genes in this QTL locus are involved in innate immune responses, including Fc receptors, interferon-inducible family genes, and formin 2. Importantly, we found an age-related increase in FCGR3A and FMN2 and a decrease in IFI16 levels in RPE cultures. There is a complex multigenic innate immunity locus that controls expression of genes in the p53 pathway in the RPE, which may play an important role in modulating age-related changes in the retina.

Cooperative and alternate functions for STIM1 and STIM2 in macrophage activation and in the context of inflammation

Calcium (Ca(2+)) signaling in immune cells, including macrophages, controls a wide range of effector functions that are critical for host defense and contribute to inflammation and autoimmune diseases. However, receptor-mediated Ca(2+) responses consist of complex mechanisms that make it difficult to identify the pathogenesis and develop therapy. Previous studies have revealed the importance of the Ca(2+) sensor STIM1 and store-operated Ca(2+)-entry (SOCE) for Fcγ-receptor activation and IgG-induced inflammation. Here, we identify the closely related STIM2 as mediator of cell migration and cytokine production downstream of GPCR and TLR4 activation in macrophages and show that mice lacking STIM2 are partially resistant to inflammatory responses in peritonitis and LPS-induced inflammation. Interestingly, STIM2 modulates the migratory behavior of macrophages independent from STIM1 and without a strict requirement for Ca(2+) influx. While STIM2 also contributes in part to FcγR activation, the C5a-induced amplification of IgG-mediated phagocytosis is mainly dependent on STIM1. Blockade of STIM-related functions limits mortality in experimental models of AIHA and LPS-sepsis in normal mice. These results suggest benefits of Ca(2+)-inhibition for suppression of exacerbated immune reactions and illustrate the significance of alternate functions of STIM proteins in macrophage activation and in the context of innate immune inflammation.

Co-operation of TLR4 and raft proteins in LPS-induced pro-inflammatory signaling

Toll-like receptor 4 (TLR4) is activated by lipopolysaccharide (LPS), a component of Gram-negative bacteria to induce production of pro-inflammatory mediators aiming at eradication of the bacteria. Dysregulation of the host responses to LPS can lead to a systemic inflammatory condition named sepsis. In a typical scenario, activation of TLR4 is preceded by binding of LPS to CD14 protein anchored in cholesterol- and sphingolipid-rich microdomains of the plasma membrane called rafts. CD14 then transfers the LPS to the TLR4/MD-2 complex which dimerizes and triggers MyD88- and TRIF-dependent production of pro-inflammatory cytokines and type I interferons. The TRIF-dependent signaling is linked with endocytosis of the activated TLR4, which is controlled by CD14. In addition to CD14, other raft proteins like Lyn tyrosine kinase of the Src family, acid sphingomyelinase, CD44, Hsp70, and CD36 participate in the TLR4 signaling triggered by LPS and non-microbial endogenous ligands. In this review, we summarize the current state of the knowledge on the involvement of rafts in TLR4 signaling, with an emphasis on how the raft proteins regulate the TLR4 signaling pathways. CD14-bearing rafts, and possibly CD36-rich rafts, are believed to be preferred sites of the assembly of a multimolecular complex which mediates the endocytosis of activated TLR4.

Targeting the Fc receptor in autoimmune disease

INTRODUCTION

The Fc receptors (FcRs) and their interactions with immunoglobulin and innate immune opsonins, such as C-reactive protein, are key players in humoral and cellular immune responses. As the effector mechanism for some therapeutic monoclonal antibodies, and often a contributor to the pathogenesis and progression of autoimmunity, FcRs are promising targets for treating autoimmune diseases.

AREAS COVERED

This review discusses the nature of different FcRs and the various mechanisms of their involvement in initiating and modulating immunocyte functions and their biological consequences. It describes a range of current strategies in targeting FcRs and manipulating their interaction with specific ligands, while presenting the pros and cons of these approaches. This review also discusses potential new strategies including regulation of FcR expression and receptor crosstalk.

EXPERT OPINION

FcRs are appealing targets in the treatment of inflammatory autoimmune diseases. However, there are still knowledge limitations and technical challenges, the most important being a better understanding of the individual roles of each of the FcRs and enhancement of the specificity in targeting particular cell types and specific FcRs.

Signaling through FcRγ-associated receptors on dendritic cells drives IL-33-dependent TH2-type responses

BACKGROUND

Although allergic sensitization can be generated against various allergens, it is unknown how such a diversity of antigens is able to promote TH2-mediated inflammation leading to atopy. Our previous studies demonstrated that allergen-specific IgG immune complexes (ICs) and house dust mite (HDM) extract both induced dendritic cells (DCs) to drive TH2-mediated inflammation, but the mechanism by which these diverse stimuli produce similar responses is unknown.

OBJECTIVE

We sought to identify the DC signaling pathways used by TH2 stimuli to promote TH2-mediated inflammation.

METHODS

C57BL/6, FcγRIII(-/-), FcRγ(-/-), and ST2(-/-) mice were sensitized and challenged with HDM, and inflammation was assessed based on results of flow cytometry and histology and cytokine production. Bone marrow-derived DCs from these strains were used in signaling and adoptive transfer experiments.

RESULTS

Our findings indicate that 2 distinct TH2 stimuli, ICs and HDM, use the FcRγ-associated receptors FcγRIII and Dectin-2, respectively, to promote TH2-mediated lung inflammation. In this study we demonstrate that both ICs and HDM induce expression of IL-33, a critical mediator in asthma pathogenesis and the differentiation of TH2 cells, in DCs. Upregulation of IL-33 in DCs is dependent on FcRγ, Toll-like receptor 4, and phosphoinositide 3-kinase. Exogenous IL-33 is sufficient to restore the development of TH2 responses in FcRγ-deficient mice. Finally, adoptive transfer of allergen-pulsed FcRγ(+/-) bone-marrow derived DCs restores the development of TH2-type inflammation in FcRγ-deficient mice, demonstrating the necessity of this signaling pathway in DCs for allergen-induced inflammation.

CONCLUSION

These data identify a mechanism whereby TH2 stimuli signal through FcRγ-associated receptors on DCs to upregulate IL-33 production and induce TH2-mediated allergic airway inflammation.

Toll-like receptor 4 and MAIR-II/CLM-4/LMIR2 immunoreceptor regulate VLA-4-mediated inflammatory monocyte migration

Inflammatory monocytes play an important role in host defense against infections. However, the regulatory mechanisms of transmigration into infected tissue are not yet completely understood. Here we show that mice deficient in MAIR-II (also called CLM-4 or LMIR2) are more susceptible to caecal ligation and puncture (CLP)-induced peritonitis than wild-type (WT) mice. Adoptive transfer of inflammatory monocytes from WT mice, but not from MAIR-II, TLR4 or MyD88-deficient mice, significantly improves survival of MAIR-II-deficient mice after CLP. Migration of inflammatory monocytes into the peritoneal cavity after CLP, which is dependent on VLA-4, is impaired in above mutant and FcRγ chain-deficient mice. Lipopolysaccharide stimulation induces association of MAIR-II with FcRγ chain and Syk, leading to enhancement of VLA-4-mediated adhesion to VCAM-1. These results indicate that activation of MAIR-II/FcRγ chain by TLR4/MyD88-mediated signalling is essential for the transmigration of inflammatory monocytes from the blood to sites of infection mediated by VLA-4.

Inflammatory monocytes play an important role in host defense against infections. Here the authors provide insights into the mechanism behind the recruitment of inflammatory monocytes to sites of infection by demonstrating the involvement of Toll-like receptor 4 and MAIR-II immunoreceptors in this process.

Selective antibody intervention of Toll-like receptor 4 activation through Fc γ receptor tethering

Inflammation is mediated mainly by leukocytes that express both Toll-like receptor 4 (TLR4) and Fc γ receptors (FcγR). Dysregulated activation of leukocytes via exogenous and endogenous ligands of TLR4 results in a large number of inflammatory disorders that underlie a variety of human diseases. Thus, differentially blocking inflammatory cells while sparing structural cells, which are FcγR-negative, represents an elegant strategy when targeting the underlying causes of human diseases. Here, we report a novel tethering mechanism of the Fv and Fc portions of anti-TLR4 blocking antibodies that achieves increased potency on inflammatory cells. In the presence of ligand (e.g. lipopolysaccharide (LPS)), TLR4 traffics into glycolipoprotein microdomains, forming concentrated protein platforms that include FcγRs. This clustering produces a microenvironment allowing anti-TLR4 antibodies to co-engage TLR4 and FcγRs, increasing their avidity and thus substantially increasing their inhibitory potency. Tethering of antibodies to both TLR4 and FcγRs proves valuable in ameliorating inflammation in vivo. This novel mechanism of action therefore has the potential to enable selective intervention of relevant cell types in TLR4-driven diseases.

Human resistin promotes neutrophil proinflammatory activation and neutrophil extracellular trap formation and increases severity of acute lung injury

Although resistin was recently found to modulate insulin resistance in preclinical models of type II diabetes and obesity, recent studies also suggested that resistin has proinflammatory properties. We examined whether the human-specific variant of resistin affects neutrophil activation and the severity of LPS-induced acute lung injury. Because human and mouse resistin have distinct patterns of tissue distribution, experiments were performed using humanized resistin mice that exclusively express human resistin (hRTN(+/-)(/-)) but are deficient in mouse resistin. Enhanced production of TNF-α or MIP-2 was found in LPS-treated hRtn(+/-/-) neutrophils compared with control Rtn(-/-/-) neutrophils. Expression of human resistin inhibited the activation of AMP-activated protein kinase, a major sensor and regulator of cellular bioenergetics that also is implicated in inhibiting inflammatory activity of neutrophils and macrophages. In addition to the ability of resistin to sensitize neutrophils to LPS stimulation, human resistin enhanced neutrophil extracellular trap formation. In LPS-induced acute lung injury, humanized resistin mice demonstrated enhanced production of proinflammatory cytokines, more severe pulmonary edema, increased neutrophil extracellular trap formation, and elevated concentration of the alarmins HMGB1 and histone 3 in the lungs. Our results suggest that human resistin may play an important contributory role in enhancing TLR4-induced inflammatory responses, and it may be a target for future therapies aimed at reducing the severity of acute lung injury and other inflammatory situations in which neutrophils play a major role.

Fc gamma receptors in respiratory syncytial virus infections: implications for innate immunity

RSV infections are a major burden in infants less than 3 months of age. Newborns and infants express a distinct immune system that is largely dependent on innate immunity and passive immunity from maternal antibodies. Antibodies can regulate immune responses against viruses through interaction with Fc gamma receptors leading to enhancement or neutralization of viral infections. The mechanisms underlying the immunomodulatory effect of Fc gamma receptors on viral infections have yet to be elucidated in infants. Herein, we will discuss current knowledge of the effects of antibodies and Fc gamma receptors on infant innate immunity to RSV. A better understanding of the pathogenesis of RSV infections in young infants may provide insight into novel therapeutic strategies such as vaccination.

Amelioration of murine passive immune thrombocytopenia by IVIg and a therapeutic monoclonal CD44 antibody does not require the Myd88 signaling pathway

Immune thrombocytopenia (ITP) is an autoimmune bleeding disorder characterized by a low platelet count and the production of anti-platelet antibodies. The majority of ITP patients have antibodies to platelet integrin α_IIbβ₃ (GPIIbIIIa) which can direct platelet phagocytosis by macrophages. One effective treatment for patients with ITP is intravenous immunoglobulin (IVIg) which rapidly reverses thrombocytopenia. The exact mechanism of IVIg action in human patients is unclear, although in mouse models of passive ITP, IVIg can rapidly increase platelet counts in the absence of adaptive immunity. Another antibody therapeutic that can similarly increase platelet counts independent of adaptive immunity are CD44 antibodies. Toll-like receptors (TLRs) are pattern recognition receptors which play a central role in helping direct the innate immune system. Dendritic cells, which are notable for their expression of TLRs, have been directly implicated in IVIg function as an initiator cell, while CD44 can associate with TLR2 and TLR4. We therefore questioned whether IVIg, or the therapeutic CD44 antibody KM114, mediate their ameliorative effects in a manner dependent upon normal TLR function. Here, we demonstrate that the TLR4 agonist LPS does not inhibit IVIg or KM114 amelioration of antibody-induced thrombocytopenia, and that these therapeutics do not ameliorate LPS-induced thrombocytopenia. IVIg was able to significantly ameliorate murine ITP in C3H/HeJ mice which have defective TLR4. All known murine TLRs except TLR3 utilize the Myd88 adapter protein to drive TLR signaling. Employing Myd88 deficient mice, we found that both IVIg and KM114 ameliorate murine ITP in Myd88 deficient mice to the same extent as normal mice. Thus both IVIg and anti-CD44 antibody can mediate their ameliorative effects in murine passive ITP independent of the Myd88 signaling pathway. These data help shed light on the mechanism of action of IVIg and KM114 in the amelioration of murine ITP.

Neuron-derived IgG protects dopaminergic neurons from insult by 6-OHDA and activates microglia through the FcγR I and TLR4 pathways

Oxidative and immune attacks from the environment or microglia have been implicated in the loss of dopaminergic neurons of Parkinson's disease. The role of IgG which is an important immunologic molecule in the process of Parkinson's disease has been unclear. Evidence suggests that IgG can be produced by neurons in addition to its traditionally recognized source B lymphocytes, but its function in neurons is poorly understood. In this study, extensive expression of neuron-derived IgG was demonstrated in dopaminergic neurons of human and rat mesencephalon. With an in vitro Parkinson's disease model, we found that neuron-derived IgG can improve the survival and reduce apoptosis of dopaminergic neurons induced by 6-hydroxydopamine toxicity, and also depress the release of NO from microglia triggered by 6-hydroxydopamine. Expression of TNF-α and IL-10 in microglia was elevated to protective levels by neuron-derived IgG at a physiologic level via the FcγR I and TLR4 pathways and microglial activation could be attenuated by IgG blocking. All these data suggested that neuron-derived IgG may exert a self-protective function by activating microglia properly, and IgG may be involved in maintaining immunity homeostasis in the central nervous system and serve as an active factor under pathological conditions such as Parkinson's disease.

Effects of C-reactive protein on the neutrophil respiratory burst in vitro

This study determined the influence of physiologically relevant concentrations of C-reactive protein (CRP) on reactive oxygen species (ROS) production by neutrophils. Neutrophils from healthy individuals were incubated with soluble pentameric CRP prior to TLR stimulation with Fusobacterium nucleatum, or FcγR stimulation with IgG-opsonised Staphylococcus aureus or heat-aggregated IgG. ROS generation by unstimulated cells and those after stimulation were determined using luminol, isoluminol and lucigenin chemiluminescence, detecting predominantly intracellular hypochlorous acid (HOCl), extracellular hydrogen peroxide (detected as HOCl) and extracellular superoxide respectively. Baseline (unstimulated) neutrophil ROS generation and release was reduced compared with vehicle control by 10 µg/ml CRP. There was no consistent effect of CRP on FcγR-stimulated HOCl production, but the extracellular superoxide response was reduced by 10 µg/ml CRP. By contrast, CRP reduced intracellular (10 µg/ml) and extracellular (3 and 10 µg/ml) HOCl generation, but increased superoxide release (1-10 µg/ml) in response to TLR stimulation. Physiologically relevant concentrations of CRP inhibited baseline ROS generation and reduced FcγR-stimulated extracellular superoxide and TLR-stimulated HOCl release, suggesting that CRP may offer some degree of host protection from neutrophil-associated, low-level oxidative stress. However, CRP enhanced TLR-mediated superoxide release from neutrophils, potentially increasing oxidative stress but aiding host protection from infection.

FcγRIII (CD16) equips immature 6-sulfo LacNAc–expressing dendritic cells (slanDCs) with a unique capacity to handle IgG-complexed antigens

The expression of CD16 by immature slanDCs equips these cells with a unique capacity to handle immune complexes. CD16 expression on slanDCs is rapidly downregulated during maturation by activation of ADAM10 and ADAM17.

Lipopolysaccharide-induced phosphorylation of c-Met tyrosine residue 1003 regulates c-Met intracellular trafficking and lung epithelial barrier function

c-Met, the receptor tyrosine kinase whose natural ligand is hepatocyte growth factor, is known to have a key role in cell motility. We have previously shown that lysophosphatidic acid (LPA) induced a decrease in c-Met activation via serine phosphorylation of c-Met at cell-cell contacts. Here, we demonstrate that lipopolysaccharide (LPS) treatment of human bronchial epithelial cells induced internalization of c-Met via phosphorylation at its tyrosine residue 1003. In addition, it induced epithelial barrier dysfunction as evidenced by a decrease in transepithelial resistance (TER) in a time-dependent manner. Pretreatment with a c-Met inhibitor (PHA-665752) or inhibition of protein kinase C (PKC)-α attenuated the LPS-mediated phosphorylation of c-Met and its internalization. LPS-induced c-Met tyrosine 1003 phosphorylation, activation of PKCα, and c-Met internalization were, however, reversed by pretreatment of cells with LPA, which increased c-Met accumulation at cell-cell contacts. Inhibition of LPS-mediated c-Met tyrosine (Y1003) phosphorylation and internalization by prior treatment with PHA-665752, inhibition of PKCα, or overexpression of c-MetY1003A mutant attenuated LPS-induced reduction of TER. Furthermore, we found that c-Met accumulation at cell-cell contacts contributed to LPA-enhanced epithelial barrier integrity, since downregulation of c-Met by specific small-interfering RNA attenuated LPA-increased TER. The data reveal a novel biological function of c-Met in the regulation of lung epithelial barrier integrity.

IL-33-dependent induction of allergic lung inflammation by FcγRIII signaling

Atopic asthma is a chronic inflammatory disease of the lungs generally marked by excessive Th2 inflammation. The role of allergen-specific IgG in asthma is still controversial; however, a receptor of IgG-immune complexes (IgG-ICs), FcγRIII, has been shown to promote Th2 responses through an unknown mechanism. Herein, we demonstrate that allergen-specific IgG-ICs, formed upon reexposure to allergen, promoted Th2 responses in two different models of IC-mediated inflammation that were independent of a preformed T cell memory response. Development of Th2-type airway inflammation was shown to be both FcγRIII and TLR4 dependent, and T cells were necessary and sufficient for this process to occur, even in the absence of type 2 innate lymphoid cells. We sought to identify downstream targets of FcγRIII signaling that could contribute to this process and demonstrated that bone marrow-derived DCs, alveolar macrophages, and respiratory DCs significantly upregulated IL-33 when activated through FcγRIII and TLR4. Importantly, IC-induced Th2 inflammation was dependent on the ST2/IL-33 pathway. Our results suggest that allergen-specific IgG can enhance secondary responses by ligating FcγRIII on antigen-presenting cells to augment development of Th2-mediated responses in the lungs via an IL-33-dependent mechanism.

Zonulin as prehaptoglobin2 regulates lung permeability and activates the complement system

Zonulin is a protein involved in the regulation of tight junctions (TJ) in epithelial or endothelial cells. Zonulin is known to affect TJ in gut epithelial cells, but little is known about its influences in other organs. Prehaptoglobin2 has been identified as zonulin and is related to serine proteases (MASPs, C1qrs) that activate the complement system. The current study focused on the role of zonulin in development of acute lung injury (ALI) in C57BL/6 male mice following intrapulmonary deposition of IgG immune complexes. A zonulin antagonist (AT-1001) and a related peptide with permeability agonist activities (AT-1002) were employed and given intratracheally or intravenously. Also, zonulin was blocked in lung with a neutralizing antibody. In a dose-dependent manner, AT-1001 or zonulin neutralizing antibody attenuated the intensity of ALI (as quantitated by albumin leak, neutrophil accumulation, and proinflammatory cytokines). A similar pattern was found using the bacterial lipopolysaccharide model of ALI. Using confocal microscopy on sections of injured lungs, staining patterns for TJ proteins were discontinuous, reduced, and fragmented. As expected, the leak of blood products into the alveolar space confirmed the passage of 3 and 20 kDa dextran, and albumin. In contrast to AT-1001, application of the zonulin agonist AT-1002 intensified ALI. Zonulin both in vitro and in vivo induced generation of complement C3a and C5a. Collectively, these data suggest that zonulin facilitates development of ALI both by enhancing albumin leak and complement activation as well as increased buildup of neutrophils and cytokines during development of ALI.