The macrophage beta-glucan receptor mediates arachidonate release induced by zymosan: essential role for Src family kinases

β-glucans: a potential source for maintaining gut microbiota and the immune system

The human gastrointestinal (GI) tract holds a complex and dynamic population of microbial communities, which exerts a marked influence on the host physiology during homeostasis and disease conditions. Diet is considered one of the main factors in structuring the gut microbiota across a lifespan. Intestinal microbial communities play a vital role in sustaining immune and metabolic homeostasis as well as protecting against pathogens. The negatively altered gut bacterial composition has related to many inflammatory diseases and infections. β-glucans are a heterogeneous assemblage of glucose polymers with a typical structure comprising a leading chain of β-(1,4) and/or β-(1,3)-glucopyranosyl units with various branches and lengths as a side chain. β-glucans bind to specific receptors on immune cells and initiate immune responses. However, β-glucans from different sources differ in their structures, conformation, physical properties, and binding affinity to receptors. How these properties modulate biological functions in terms of molecular mechanisms is not known in many examples. This review provides a critical understanding of the structures of β-glucans and their functions for modulating the gut microbiota and immune system.

Differential Mobilization of the Phospholipid and Triacylglycerol Pools of Arachidonic Acid in Murine Macrophages

Innate immune cells such as monocytes and macrophages contain high levels of arachidonic acid (AA), part of which can be mobilized during cellular activation for the formation of a vast array of bioactive oxygenated metabolites. Monocytes and macrophages present in inflammatory foci typically incorporate large amounts of AA, not only in membrane phospholipids, but also in neutral lipids such as triacylglycerol. Thus, it was of interest to investigate the metabolic fate of these two AA pools in macrophages. Utilizing a variety of radiolabeling techniques to distinguish the phospholipid and triacylglycerol pools, we show in this paper that during an acute stimulation of the macrophages with yeast-derived zymosan, the membrane phospholipid AA pool acts as the major, if not the only, source of releasable AA. On the contrary, the AA pool in triacylglycerol appears to be used at a later stage, when the zymosan-stimulated response has declined, as a source to replenish the phospholipid pools that were consumed during the activation process. Thus, phospholipids and triacylglycerol play different in roles AA metabolism and dynamics during macrophage activation.

Whole grain cereals: the potential roles of functional components in human health

Whole grain cereals have been the basis of human diet since ancient times. Due to rich in a variety of unique bioactive ingredients, they play an important role in human health. This review highlights the contents and distribution of primary functional components and their health effects in commonly consumed whole grain cereals, especially dietary fiber, protein, polyphenols, and alkaloids. In general, cereals exert positive effects in the following ways: 1) Restoring intestinal flora diversity and increasing intestinal short-chain fatty acids. 2) Regulating plasma glucose and lipid metabolism, thereby the improvement of obesity, cardiovascular and cerebrovascular diseases, diabetes, and other chronic metabolic diseases. 3) Exhibiting antioxidant activity by scavenging free radicals. 4) Preventing gastrointestinal cancer via the regulation of classical signaling pathways. In summary, this review provides a scientific basis for the formulation of whole-grain cereals-related dietary guidelines, and guides people to form scientific dietary habits, so as to promote the development and utilization of whole-grain cereals.

β-glucans and eicosapolyenoic acids as MAMPs in plant-oomycete interactions: past and present

Branched β-1,3-glucans and the eicosapolyenoic acids (EP) are among the best characterized oomycete elicitors that trigger innate immune responses in plants. These elicitors were identified over three decades ago, and they were useful in the study of the sequence of physiological, biochemical and molecular events that induce resistance in plants. However, in spite of the cross-kingdom parallels where these molecules are well-characterized as immune system modulators in animals, their perception and modes of action in plants remains obscure. Oomycetes are among the most important plant pathogens, responsible for diseases that devastate crops, ornamentals, and tree species worldwide. With the recent interest and advances in our understanding of innate immunity in plants, and the redefining of many of the classical elicitors as microbe-associated molecular patterns (MAMPs), it seems timely and important to reexamine β-glucans and EP using contemporary approaches. In this review, we highlight early studies of β-glucans and EP, discuss their roles as evolutionarily conserved signals, and consider their action in relation to current models of MAMP-triggered immunity.

Dectin-1-mediated signaling leads to characteristic gene expressions and cytokine secretion via spleen tyrosine kinase (Syk) in rat mast cells

Dectin-1 recognizes β-glucan and plays important roles for the antifungal immunity through the activation of spleen tyrosine kinase (Syk) in dendritic cells or macrophages. Recently, expression of Dectin-1 was also identified in human and mouse mast cells, although its physiological roles were largely unknown. In this report, rat mast cell line RBL-2H3 was analyzed to investigate the molecular mechanism of Dectin-1-mediated activation and responses of mast cells. Treatment of cells with Dectin-1-specific agonist curdlan induced tyrosine phosphorylation of cellular proteins and the interaction of Dectin-1 with the Src homology 2 domain of Syk. These responses depended on tyrosine phosphorylation of the hemi-immunoreceptor tyrosine-based activation motif in the cytoplasmic tail of Dectin-1, whereas they were independent of the γ-subunit of high-affinity IgE receptor. DNA microarray and real-time PCR analyses showed that Dectin-1-mediated signaling stimulated gene expression of transcription factor Nfkbiz and inflammatory cytokines, such as monocyte chemoattractant protein-1, IL-3, IL-4, IL-13, and tumor necrosis factor (TNF)-α. The response was abrogated by pretreatment with Syk inhibitor R406. These results suggest that Syk is critical for Dectin-1-mediated activation of mast cells, although the signaling differs from that triggered by FcϵRI activation. In addition, these gene expressions induced by curdlan stimulation were specifically observed in mast cells, suggesting that Dectin-1-mediated signaling of mast cells offers new insight into the antifungal immunity.

Interstitial lung disease with multiple microgranulomas in chronic granulomatous disease

BACKGROUND

Chronic granulomatous disease (CGD) is a primary immunodeficiency disease that is characterized by susceptibility to bacterial and fungal infections. CGD patients also suffer from immune regulatory disorders, such as CGD-associated bowel inflammation with granuloma, which could be caused by excessive inflammation without demonstrable infection.

PURPOSE

We investigated the clinical manifestation of interstitial lung disease (ILD) resulting from excessive inflammation in X-linked CGD patients.

METHODS

Pulmonary CT images and testing of serum KL-6 levels were performed to assess ILD in the patients. For this study, patients with pulmonary lesions due to demonstrable infections were excluded from among ILD patients.

RESULTS

Among 33 CGD patients, four developed ILD; they had increased reticulo-nodular opacities on CT images and elevated serum KL-6 levels. Histopathological examinations revealed multiple homogeneous microgranulomas in the lesions of inflammatory cell infiltration. Mononuclear cells obtained from their pulmonary lesions produced higher amounts of inflammatory cytokines than the peripheral blood mononuclear cells of CGD patients, suggesting that the only infiltrating cells in the pulmonary lesions were activated and produced large amounts of inflammatory cytokines in ILD patients. Interestingly, an anti-inflammatory drug, such as a corticosteroid or thalidomide, but not anti-bacterial or anti-fungal drugs, improved CT image findings and reduced their KL-6 levels.

CONCLUSIONS

CGD patients' daily exposures to inhaled antigens may induce excessive reactions with the production of inflammatory cytokines leading to the development of ILD with multiple microgranulomas, which could be due to an inadequate production of reactive oxygen species in CGD.

Bruton's Tyrosine Kinase (BTK) and Vav1 contribute to Dectin1-dependent phagocytosis of Candida albicans in macrophages

Phagocytosis of the opportunistic fungal pathogen Candida albicans by cells of the innate immune system is vital to prevent infection. Dectin-1 is the major phagocytic receptor involved in anti-fungal immunity. We identify two new interacting proteins of Dectin-1 in macrophages, Bruton's Tyrosine Kinase (BTK) and Vav1. BTK and Vav1 are recruited to phagocytic cups containing C. albicans yeasts or hyphae but are absent from mature phagosomes. BTK and Vav1 localize to cuff regions surrounding the hyphae, while Dectin-1 lines the full length of the phagosome. BTK and Vav1 colocalize with the lipid PI(3,4,5)P3 and F-actin at the phagocytic cup, but not with diacylglycerol (DAG) which marks more mature phagosomal membranes. Using a selective BTK inhibitor, we show that BTK contributes to DAG synthesis at the phagocytic cup and the subsequent recruitment of PKCε. BTK- or Vav1-deficient peritoneal macrophages display a defect in both zymosan and C. albicans phagocytosis. Bone marrow-derived macrophages that lack BTK or Vav1 show reduced uptake of C. albicans, comparable to Dectin1-deficient cells. BTK- or Vav1-deficient mice are more susceptible to systemic C. albicans infection than wild type mice. This work identifies an important role for BTK and Vav1 in immune responses against C. albicans.

Fungal recognition enhances mannose receptor shedding through dectin-1 engagement

The mannose receptor (MR) is an endocytic type I membrane molecule with a broad ligand specificity that is involved in both hemostasis and pathogen recognition. Membrane-anchored MR is cleaved by a metalloproteinase into functional soluble MR (sMR) composed of the extracellular domains of intact MR. Although sMR production was initially considered a constitutive process, enhanced MR shedding has been observed in response to the fungal pathogen Pneumocystis carinii. In this work, we have investigated the mechanism mediating enhanced MR shedding in response to fungi. We show that other fungal species, including Candida albicans and Aspergillus fumigatus, together with zymosan, a preparation of the cell wall of Saccharomyces cerevisiae, mimic the effect of P. carinii on sMR production and that this effect takes place mainly through β-glucan recognition. Additionally, we demonstrate that MR cleavage in response to C. albicans and bioactive particulate β-glucan requires expression of dectin-1. Our data, obtained using specific inhibitors, are consistent with the canonical Syk-mediated pathway triggered by dectin-1 being mainly responsible for inducing MR shedding, with Raf-1 being partially involved. As in the case of steady-state conditions, MR shedding in response to C. albicans and β-glucan particles requires metalloprotease activity. The induction of MR shedding by dectin-1 has clear implications for the role of MR in fungal recognition, as sMR was previously shown to retain the ability to bind fungal pathogens and can interact with numerous host molecules, including lysosomal hydrolases. Thus, MR cleavage could also impact on the magnitude of inflammation during fungal infection.

Beta-glucan recognition by the innate immune system

Beta-glucans are recognized by the innate immune system. This recognition plays important roles in host defense and presents specific opportunities for clinical modulation of the host immune response. Neutrophils, macrophages, and dendritic cells among others express several receptors capable of recognizing beta-glucan in its various forms. This review explores what is currently known about beta-glucan recognition and how this recognition stimulates immune responses. Special emphasis is placed on Dectin-1, as we know the most about how this key beta-glucan receptor translates recognition into intracellular signaling, stimulates cellular responses, and participates in orchestrating the adaptive immune response.

Dectin-1 is inducible and plays an essential role for mycobacteria-induced innate immune responses in airway epithelial cells

INTRODUCTION

Airway epithelial cells are the first cells to be challenged upon contact with mycobacteria. In response, they express pattern-recognition receptors that play fundamental roles as sentinels and mediators of pulmonary innate immunity. The c-type lectin Dectin-1 is expressed predominantly on the surface of myeloid lineage cells. In this study, we examined the induction, regulation, and functions of Dectin-1 in pulmonary epithelial cells.

RESULTS

Mycobacterium tuberculosis (Mtb) actively induced the expression of Dectin-1 mRNA and protein in A549 cells in a toll-like receptor (TLR) 2-dependent manner. In addition, Mtb-mediated generation of reactive oxygen species and Dectin-1 induction were mutually dependent. Moreover, Mtb actively induced the phosphorylation of Src family kinases at Tyr416 via TLR2. Selective inhibition of Src markedly attenuated the induction of Mtb-dependent Dectin-1 expression, indicating that Src kinases are crucial regulators of Dectin-1-dependent signaling. Mtb internalization was partially blocked by silencing Dectin-1 expression, inhibiting Src kinases, or pretreating with antioxidants. Finally, Dectin-1 was required for pro-inflammatory cytokine release and antimicrobial effects on intracellular mycobacterial growth in A549 cells.

CONCLUSION

Collectively, our findings demonstrate the novel induction of Dectin-1 in type II airway epithelial cells and its critical role in the innate immune response against Mtb in non-phagocytic cells.

Beta-glucans in the treatment of diabetes and associated cardiovascular risks

Diabetes mellitus is characterized by high blood glucose level with typical manifestations of thirst, polyuria, polydipsia, and weight loss. It is caused by defects in insulin-mediated signal pathways, resulting in decreased glucose transportation from blood into muscle and fat cells. The major risk is vascular injury leading to heart disease, which is accelerated by increased lipid levels and hypertension. Management of diabetes includes: control of blood glucose level and lipids; and reduction of hypertension. Dietary intake of beta-glucans has been shown to reduce all these risk factors to benefit the treatment of diabetes and associated complications. In addition, beta-glucans also promote wound healing and alleviate ischemic heart injury. However, the mechanisms behind the effect of beta-glucans on diabetes and associated complications need to be further studied using pure beta-glucan.

Oligomannose-coated liposomes activate ERK via Src kinases and PI3K/Akt in J774A.1 cells

We have previously shown that liposomes coated with a neoglycolipid constructed from mannotriose and dipalmitoylphosphatidylethanolamine (Man3-DPPE) activate peritoneal macrophages to induce enhanced expression of co-stimulatory molecules and MHC class II. In this study, we investigated the signaling pathways activated by the Man3-DPPE-coated liposomes (OMLs) in a murine macrophage cell line, J774A.1. In response to OML stimulation, ERK among MAPKs was clearly and transiently phosphorylated in J774 cells. ERK phosphorylation was also induced by treatment of the cells with Man3-DPPE and Man3-BSA, but not by uncoated liposomes. In addition, rapid and transient phosphorylation of Akt and Src family kinases (SFKs) was observed in response to OMLs. OML-induced ERK phosphorylation was inhibited by specific inhibitors of PI3K and SFKs, and OML-induced Akt phosphorylation was inhibited by a inhibitor of SFKs. Therefore, OMLs may activate the PI3K/Akt pathway through phosphorylation of Src family kinases to induce ERK activation.

The biological role of dectin-1 in immune response

The identification of antigen-recognizing receptors in the innate immune system significantly helped us to understand the initiation and regulation of the innate immune response. Dectin-1 was recently identified as the most important receptor for beta-glucan. Recognition of beta-glucan by dectin-1 triggers effective immune response, including phagocytosis and proinflammatory factor production, to eliminate infecting fungi, which especially benefits immunocompromised patients against opportunistic fungal infection. In addition, dectin-1 is involved in the adaptive immune response as well as autoimmune diseases and immune tolerance. In this paper, we will mainly focus on the characteristics of dectin-1 and its biological role in immune response.

Possible role for early-life immune insult including developmental immunotoxicity in chronic fatigue syndrome (CFS) or myalgic encephalomyelitis (ME)

Chronic fatigue syndrome (CFS), also known as myalgic encephalomyelitis (ME) in some countries, is a debilitating disease with a constellation of multi-system dysfunctions primarily involving the neurological, endocrine and immune systems. While substantial information is available concerning the complex dysfunction-associated symptoms of CFS, environmental origins of the disease have yet to be determined. Part of the dilemma in identifying the cause(s) has been the focus on biomarkers (hormones, neurotransmitters, cytokines, infectious agents) that are contemporary with later-life CFS episodes. Yet, recent investigations on the origins of environmental diseases of the neurological, endocrine, reproductive, respiratory and immune systems suggest that early life toxicologic and other insults are pivotal in producing later-life onset of symptoms. As with autism and childhood asthma, CFS can also occur in children where the causes are certainly early-life events. Immune dysfunction is recognized as part of the CFS phenotype but has received comparatively less attention than aberrant neurological or endocrine function. However, recent research results suggest that early life immune insults (ELII) including developmental immunotoxicity (DIT), which is induced by xenobiotics, may offer an important clue to the origin(s) of CFS. The developing immune system is a sensitive and novel target for environmental insult (xenobiotic, infectious agents, stress) with major ramifications for postnatal health risks. Additionally, many prenatal and early postnatal neurological lesions associated with postnatal neurobehavioral diseases are now recognized as linked to prenatal immune insult and inflammatory dysregulation. This review considers the potential role of ELII including DIT as an early-life component of later-life CFS.