Macrophage scavenger receptor down-regulates mycobacterial cord factor-induced proinflammatory cytokine production by alveolar and hepatic macrophages

Scavenger receptor A in immunity and autoimmune diseases: Compelling evidence for targeted therapy

INTRODUCTION

Scavenger receptor A (SR-A) is reported to be involved in innate and adaptive immunity and in recent years, the soluble form of SR-A has also been identified. Intriguingly, SR-A displays double-edged sword features in different diseases. Moreover, targeted therapy on SR-A, including genetic modulation, small molecule inhibitor, inhibitory peptides, fucoidan, and blocking antibodies, provides potential strategies for treatment. Currently, therapeutics targeting SR-A are in preclinical studies and clinical trials, revealing great perspectives in future immunotherapy.

AREAS COVERED

Through searching PubMed (January 1979-March 2022) and clinicaltrials.gov, we review most of the research and clinical trials involving SR-A. This review briefly summarizes recent study advances on SR-A, with particular concern on its role in immunity and autoimmune diseases.

EXPERT OPINION

Given the emerging evidence of SR-A in immunity, its targeted therapy has been studied in various diseases, especially autoimmune diseases. However, many challenges still remain to be overcome, such as the double-sworded effects and the specific isoform targeting. For further clinical success of SR-A targeted therapy, the crystal structure illustration and the dual function discrimination of SR-A should be further investigated. Nevertheless, although challenging, targeting SR-A would be a potential effective strategy in the treatment of autoimmune diseases and other immune-related diseases.

The protective action of piperlongumine against mycobacterial pulmonary tuberculosis in its mitigation of inflammation and macrophage infiltration in male BALB/c mice

Introduction

Piperlongumine (PL) is a bioactive alkaloid and medicinal compound of piperamide isolated from the long pepper (Piper longum Linn). It has demonstrated bactericidal action against Mycobacterium tuberculosis (MTB), the cause of pulmonary tuberculosis; nevertheless, immunomodulatory activity had not been identified for it in MTB-triggered granulomatous inflammation. This study investigated if piperlongumine could inhibit such inflammation.

Material and Methods

Mycobacterium tuberculosis strain H37Rv was subjected to a broth microdilution assay. Piperlongumine at 5, 15, and 25 μg/mL, 0.2% dimethyl sulphoxide as control or 4 μM of dexamethasone were tested in vitro on MH-S murine alveolar macrophages. BALB/c mice were orally administered PL at 50, 100 and 150 mg/kg b.w. after trehalose-6,6-dimycolate (TDM) stimulation. Chemokine and cytokine concentrations were determined in lung supernatants. Flow cytometry and Western blot analysis were performed to determine phosphorylated spleen tyrosine kinase (Syk), c-Jun N-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK) pathways.

Results

Piperlongumine inhibited inflammatory mediators and adherence of lymphocyte function-associated antigen 1 to MH-S cells following TDM activation. It also improved macrophage clearance of MTB. In TDM-stimulated MH-S cells, PL significantly influenced the macrophage inducible Ca²⁺-dependent lectin receptor (Mincle)-Syk-ERK signalling pathway. Oral dosing of PL effectively suppressed the development of pulmonary granulomas and inflammatory reactions in the TDM-elicited mouse granuloma model.

Conclusion

PL as an inhibitor of MTB-triggered granulomatous inflammation may be an effective complementary treatment for mycobacterial infection.

Underwhelming or Misunderstood? Genetic Variability of Pattern Recognition Receptors in Immune Responses and Resistance to Mycobacterium tuberculosis

Human genetic control is thought to affect a considerable part of the outcome of infection with Mycobacterium tuberculosis (Mtb). Most of us deal with the pathogen by containment (associated with clinical "latency") or sterilization, but tragically millions each year do not. After decades of studies on host genetic susceptibility to Mtb infection, genetic variation has been discovered to play a role in tuberculous immunoreactivity and tuberculosis (TB) disease. Genes encoding pattern recognition receptors (PRRs) enable a consistent, molecularly direct interaction between humans and Mtb which suggests the potential for co-evolution. In this review, we explore the roles ascribed to PRRs during Mtb infection and ask whether such a longstanding and intimate interface between our immune system and this pathogen plays a critical role in determining the outcome of Mtb infection. The scientific evidence to date suggests that PRR variation is clearly implicated in altered immunity to Mtb but has a more subtle role in limiting the pathogen and pathogenesis. In contrast to 'effectors' like IFN-γ, IL-12, Nitric Oxide and TNF that are critical for Mtb control, 'sensors' like PRRs are less critical for the outcome of Mtb infection. This is potentially due to redundancy of the numerous PRRs in the innate arsenal, such that Mtb rarely goes unnoticed. Genetic association studies investigating PRRs during Mtb infection should therefore be designed to investigate endophenotypes of infection - such as immunological or clinical variation - rather than just TB disease, if we hope to understand the molecular interface between innate immunity and Mtb.

The biologic effect of hydrogen sulfide and its function in various diseases

INTRODUCTION

Hydrogen sulfide (H2S), a colorless, water soluble, flammable gas with a characteristic smell of rotten eggs, has been known as a highly toxic gas for several years. However, much like carbon monoxide (CO) and nitric oxide (NO), the initial negative perception of H2S has developed with the discovery that H2S is generated enzymatically in animals under normal conditions. With the result of this discovery, much more work is needed to elucidate the biologic effects of H2S. In recent years, its cytoprotective properties have been recognized in multiple organs and tissues. In particular, H2S plays important roles in combating oxidative species such as reactive oxygen species (ROS) and reactive nitrogen species (RNS) and protect the body from oxidative stress. Therefore, this review discusses the biologic effect of H2S and how it protects cells in various diseases by acting as an antioxidant that reduces excessive amounts of ROS and RNS.

ETHICS AND DISSEMINATION

Ethical approval and informed consent are not required, as the study will be a literature review and will not involve direct contact with patients or alterations to patient care.

CONCLUSION

H2S has been found to be cytoprotective in oxidative stress in a wide range of physiologic and pathologic conditions, an increasing number of therapeutic potentials of H2S also have been revealed. However, there is still much debate on the clear mechanism of action of H2S, so that the mechanisms of cell signaling that promote cellular survival and organ protection need to be further investigated to provide better H2S-based therapeutics.

Sophora flavescens protects against mycobacterial Trehalose Dimycolate-induced lung granuloma by inhibiting inflammation and infiltration of macrophages

The immune system responds to Mycobacterium tuberculosis (MTB) infection by forming granulomas to quarantine the bacteria from spreading. Granuloma-mediated inflammation is a cause of lung destruction and disease transmission. Sophora flavescens (SF) has been demonstrated to exhibit bactericidal activities against MTB. However, its immune modulatory activities on MTB-mediated granulomatous inflammation have not been reported. In the present study, we found that flavonoids from Sophora flavescens (FSF) significantly suppressed the pro-inflammatory mediators released from mouse lung alveolar macrophages (MH-S) upon stimulation by trehalose dimycolate (TDM), the most abundant lipoglycan on MTB surface. Moreover, FSF reduced adhesion molecule (LFA-1) expression on MH-S cells after TDM stimulation. Furthermore, FSF treatment on TDM-activated lung epithelial (MLE-12) cells significantly downregulated macrophage chemoattractant protein (MCP-1/CCL2) expression, which in turn reduced the in vitro migration of MH-S to MLE-12 cells. In addition, FSF increased the clearance of mycobacterium bacteria (Mycobacterium aurum) in macrophages. FSF mainly affected the Mincle-Syk-Erk signaling pathway in TDM-activated MH-S cells. In TDM-induced mouse granulomas model, oral administration with FSF significantly suppressed lung granulomas formation and inflammation. These findings collectively implicated an anti-inflammatory role of FSF on MTB-mediated granulomatous inflammation, thereby providing evidence of FSF as an efficacious adjunct treatment during mycobacterial infection.

Mesenchymal stem cells internalize Mycobacterium tuberculosis through scavenger receptors and restrict bacterial growth through autophagy

Human mesenchymal stem cells (MSCs) express scavenger receptors that internalize lipids, including oxidized low-density lipoprotein (oxLDL). We report that MSCs phagocytose Mycobacterium tuberculosis (Mtb) through two types of scavenger receptors (SRs; MARCO and SR-B1), as blockade of the receptors with antibodies or siRNA knockdown decreased the uptake of Mtb. MSCs also expressed mannose receptor (MR) that was found to endocytose rhodamine-labeled mannosylated BSA (rMBSA), though the receptor was not involved in the uptake of Mtb. Dil-oxLDL and rMBSA taken up into MSC endosomes colocalized with Mtb phagosomes, thus suggesting that the latter were fusion competent. Phagocytosed Mtb did not replicate within MSCs, thus suggesting an intrinsic control of bacterial growth. Indeed, MSCs exhibited intrinsic autophagy, which was up-regulated after activation with rapamycin. SiRNA knockdown of autophagy initiator beclin-1 enhanced Mtb survival, whereas rapamycin-induced autophagy increased intracellular killing of Mtb. In addition, MSCs secreted nitric oxide after Mtb infection, and inhibition of NO by N(G)-monomethyl-L-arginine enhanced intracellular survival of Mtb. MSCs can be grown in large numbers in vitro, and autologous MSCs transfused into tuberculosis patients have been found to be safe and improve lung immunity. Thus, MSCs are novel phagocytic cells with a potential for immunotherapy in treating multidrug-resistant tuberculosis.

Toll-Like Receptor 2 and Mincle Cooperatively Sense Corynebacterial Cell Wall Glycolipids

Nontoxigenic Corynebacterium diphtheriae and Corynebacterium ulcerans cause invasive disease in humans and animals. Host sensing of corynebacteria is largely uncharacterized, albeit the recognition of lipoglycans by Toll-like receptor 2 (TLR2) appears to be important for macrophage activation by corynebacteria. The members of the order Corynebacterineae (e.g., mycobacteria, nocardia, and rhodococci) share a glycolipid-rich cell wall dominated by mycolic acids (termed corynomycolic acids in corynebacteria). The mycolic acid-containing cord factor of mycobacteria, trehalose dimycolate, activates the C-type lectin receptor (CLR) Mincle. Here, we show that glycolipid extracts from the cell walls of several pathogenic and nonpathogenic Corynebacterium strains directly bound to recombinant Mincle in vitro Macrophages deficient in Mincle or its adapter protein Fc receptor gamma chain (FcRγ) produced severely reduced amounts of granulocyte colony-stimulating factor (G-CSF) and of nitric oxide (NO) upon challenge with corynebacterial glycolipids. Consistently, cell wall extracts of a particular C. diphtheriae strain (DSM43989) lacking mycolic acid esters neither bound Mincle nor activated macrophages. Furthermore, TLR2 but not TLR4 was critical for sensing of cell wall extracts and whole corynebacteria. The upregulation of Mincle expression upon encountering corynebacteria required TLR2. Thus, macrophage activation by the corynebacterial cell wall relies on TLR2-driven robust Mincle expression and the cooperative action of both receptors.

Stat6-Dependent Inhibition of Mincle Expression in Mouse and Human Antigen-Presenting Cells by the Th2 Cytokine IL-4

The C-type lectin receptors (CLRs) Mincle, Mcl, and Dectin-2 bind mycobacterial and fungal cell wall glycolipids and carbohydrates. Recently, we described that expression of these CLR is downregulated during differentiation of human monocytes to dendritic cells (DC) in the presence of GM-CSF and IL-4. Here, we demonstrate that the Th2 cytokine IL-4 specifically inhibits expression of Mincle, Mcl, and Dectin-2 in human antigen-presenting cells (APC). This inhibitory effect of IL-4 was observed across species, as murine macrophages and DC treated with IL-4 also downregulated these receptors. IL-4 blocked upregulation of Mincle and Mcl mRNA expression and cell surface protein by murine macrophages in response to the Mincle ligand Trehalose-6,6-dibehenate (TDB), whereas the TLR4 ligand LPS overcame inhibition by IL-4. Functionally, downregulation of Mincle expression by IL-4 was accompanied by reduced cytokine production upon stimulation with TDB. These inhibitory effects of IL-4 were dependent on the transcription factor Stat6. Together, our results show that the key Th2 cytokine IL-4 exerts a negative effect on the expression of Mincle and other Dectin-2 cluster CLR in mouse and human macrophages and DC, which may render these sentinel cells less vigilant for sensing mycobacterial and fungal ligands.

Microglia Function in the Normal Brain

The activation of microglia has been recognized for over a century by their morphological changes. Long slender microglia acquire a short sturdy ramified shape when activated. During the past 20 years, microglia have been accepted as an essential cellular component for understanding the pathogenic mechanism of many brain diseases, including neurodegenerative diseases. More recently, functional studies and imaging in mouse models indicate that microglia are active in the healthy central nervous system. It has become evident that microglia release several signal molecules that play key roles in the crosstalk among brain cells, i.e., astrocytes and oligodendrocytes with neurons, as well as with regulatory immune cells. Recent studies also reveal the heterogeneous nature of microglia diverse functions depending on development, previous exposure to stimulation events, brain region of residence, or pathological state. Subjects to approach by future research are still the unresolved questions regarding the conditions and mechanisms that render microglia protective, capable of preventing or reducing damage, or deleterious, capable of inducing or facilitating the progression of neuropathological diseases. This novel knowledge will certainly change our view on microglia as therapeutic target, shifting our goal from their general silencing to the generation of treatments able to change their activation pattern.

Sensing of Mycobacterium tuberculosis and consequences to both host and bacillus

Mycobacterium tuberculosis (Mtb), the primary causative agent of human tuberculosis, has killed more people than any other bacterial pathogen in human history and remains one of the most important transmissible diseases worldwide. Because of the long-standing interaction of Mtb with humans, it is no surprise that human mucosal and innate immune cells have evolved multiple mechanisms to detect Mtb during initial contact. To that end, the cell surface of human cells is decorated with numerous pattern recognition receptors for a variety of mycobacterial ligands. Furthermore, once Mtb is ingested into professional phagocytes, other host molecules are engaged to report on the presence of an intracellular pathogen. In this review, we discuss the role of specific mycobacterial products in modulating the host's ability to detect Mtb. In addition, we describe the specific host receptors that mediate the detection of mycobacterial infection and the role of individual receptors in mycobacterial pathogenesis in humans and model organisms.

Macrophage-inducible C-type lectin underlies obesity-induced adipose tissue fibrosis

In obesity, a paracrine loop between adipocytes and macrophages augments chronic inflammation of adipose tissue, thereby inducing systemic insulin resistance and ectopic lipid accumulation. Obese adipose tissue contains a unique histological structure termed crown-like structure (CLS), where adipocyte-macrophage crosstalk is known to occur in close proximity. Here we show that Macrophage-inducible C-type lectin (Mincle), a pathogen sensor for Mycobacterium tuberculosis, is localized to macrophages in CLS, the number of which correlates with the extent of interstitial fibrosis. Mincle induces obesity-induced adipose tissue fibrosis, thereby leading to steatosis and insulin resistance in liver. We further show that Mincle in macrophages is crucial for CLS formation, expression of fibrosis-related genes and myofibroblast activation. This study indicates that Mincle, when activated by an endogenous ligand released from dying adipocytes, is involved in adipose tissue remodelling, thereby suggesting that sustained interactions between adipocytes and macrophages within CLS could be a therapeutic target for obesity-induced ectopic lipid accumulation.

The recognition of biomaterials: pattern recognition of medical polymers and their adsorbed biomolecules

All biomedical materials are recognized as foreign entities by the host immune system despite the substantial range of different materials that have been developed by material scientists and engineers. Hydrophobic biomaterials, hydrogels, biomaterials with low protein binding surfaces, and those that readily adsorb a protein layer all seem to incite similar host responses in vivo that may differ in magnitude, but ultimately result in encapsulation by fibrotic tissue. The recognition of medical materials by the host is explained by the very intricate pattern recognition system made up of integrins, toll-like receptors, scavenger receptors, and other surface proteins that enable leukocytes to perceive almost any foreign body. In this review, we describe the various pattern recognition receptors and processes that occur on biomedical material surfaces that permit detection of a range of materials within the host.

Recognition of the mycobacterial cord factor by Mincle: relevance for granuloma formation and resistance to tuberculosis

The world's most successful intracellular bacterial pathogen, Mycobacterium tuberculosis (MTB), survives inside macrophages by blocking phagosome maturation and establishes chronic infection characterized by the formation of granulomas. Trehalose-6,6-dimycolate (TDM), the mycobacterial cord factor, is the most abundant cell wall lipid of virulent mycobacteria, is sufficient to cause granuloma formation, and has long been known to be a major virulence factor of MTB. Recently, TDM has been shown to activate the Syk-Card9 signaling pathway in macrophages through binding to the C-type lectin receptor Mincle. The Mincle-Card9 pathway is required for activation of macrophages by TDM in vitro and for granuloma formation in vivo following injection of TDM. Whether this pathway is also exploited by MTB to reprogram the macrophage into a comfortable niche has not been explored yet. Several recent studies have investigated the phenotype of Mincle-deficient mice in mycobacterial infection, yielding divergent results in terms of a role for Mincle in host resistance. Here, we review these studies, discuss possible reasons for discrepant results and highlight open questions in the role of Mincle and other C-type lectin receptors in the infection biology of MTB.

Scavenger receptor class A ligands induce secretion of IL1β and exert a modulatory effect on the inflammatory activation of astrocytes in culture

Class-A scavenger receptor (SR-A) is expressed by microglia, and we show here that it is also expressed by astrocytes, where it participates on their inflammatory activation. Astrocytes play a key role on the inflammatory response of the central nervous system, secreting several soluble mediators like cytokines and radical species. Exposure to SR ligands activated MAPKs and NF-κB signaling and increased production of IL1β and nitric oxide (NO). IL1β classically an inflammatory cytokine surprisingly did not increase but inhibited LPS+IFNγ-induced NO production by astrocytes. Our results suggest that SRs expressed by astrocytes participate in the modulation of inflammatory activation.

Mincle is not essential for controlling Mycobacterium tuberculosis infection

Individually and combined, Toll-like receptors (TLR)-2, -4, -9, nucleotide oligomerization domain (NOD) 2 and NALP3 contribute to the Mycobacterium tuberculosis (Mtb)-induced innate immune response only to a limited extent, particularly in terms of inducing antibacterial protection and granuloma formation in vivo. A singular essential sensory component of this initial response has not been discovered yet. Trehalose-6,6'-dimycolate (TDM), a well known mycobacterial cell wall glycolipid, is believed to be involved in these early inflammatory processes after Mtb infection. Only recently the macrophage inducible C-type lectin (Mincle) was demonstrated as an essential receptor for TDM. However, not much is known about the sensing capacity of Mincle during infection with live mycobacteria. To determine the significance of Mincle during tuberculosis (TB), we analyzed the outcome of Mtb infection in Mincle-deficient mice. Whereas in the absence of Mincle macrophages did not respond to TDM, Mincle-deficient mice were capable of mounting an efficient granulomatous and protective immune response after low and high dose infections with Mtb. Mutant mice generated a normal T helper (TH) 1 and TH17 immune response followed by the induction of efficient macrophage effector mechanisms and control of mycobacterial growth identical to wildtype mice. From our results we conclude that absence of the innate receptor Mincle can be fully compensated for in vivo in terms of sensing Mtb and mounting a protective inflammatory immune response.

Class A scavenger receptor promotes cerebral ischemic injury by pivoting microglia/macrophage polarization

Class A scavenger receptor (SR-A) is primarily expressed in microglia/macrophages and plays an important role in immune responses. However, whether SR-A can influence microglia/macrophage polarization in cerebral ischemic injury is not known. To this end we monitored the phenotypic alteration of microglia/macrophages in an animal model of cerebral ischemia injury. SR-A was up-regulated in mouse brains 24h after permanent occlusion of middle cerebral artery (MCAO). SR-A-deficient mice displayed reduced infarct size and improved neurological function compared with wild-type mice littermate controls. Furthermore, a decrease in inflammatory F4/80(+)CD11b(+)CD45(high)CD11c(+) microglia/macrophages and attenuated nuclear factor-kappaB (NF-κB) activation was found in ischemic brains in the SR-A null mice. This was accompanied by alleviation of classically activated M1 macrophage markers and preservation of alternatively activated M2 macrophage markers. These data suggest that SR-A contributes to cerebral ischemic injury by pivoting the phenotype of microglia/macrophages to a skewed M1 polarization.

In vitro cytokine stimulation assay for glycolipid biosurfactant from Rhodococcus ruber: role of monocyte adhesion

Glycolipid biosurfactant (GLB) from Rhodococcus ruber IEGM 231 was found to stimulate tumor necrosis factor-α (TNF-α), interleukin (IL) -1β and IL-6 production when applied as an ultrasonic emulsion to the adherent human peripheral blood monocyte culture. However, a lack of cytokine-stimulating activity was registered with the GLB applied as a hydrophobic film coating in 24-well culture plates, indicating that it may have been due to its inhibitory effect on monocyte adhesion. The mode of GLB application may therefore play an important role in in vitro assay of immunostimulatory activity of this compound as well as other bacterial glycolipids. Additionally, GLB from R. ruber displayed no cytotoxicity against human lymphocytes and therefore could be proposed as a potential immunomodulating and antitumor agent.

Prolonged survival of scavenger receptor class A-deficient mice from pulmonary Mycobacterium tuberculosis infection

The present study tested the hypothesis that the scavenger receptor SR-A modulates granuloma formation in response to pulmonary infection with Mycobacterium tuberculosis (MTB). To test this hypothesis, we monitored survival and histopathology in WT and SR-A-deficient mice following aerosol infection with MTB Rv. SR-A-deficient (SR-A-/-) mice infected with MTB survived significantly longer than WT mice; the mean survival of SR-A-/- mice exceeded 430 days compared to 230 days for WT mice. Early granuloma formation was not impaired in SR-A-/- mice. The extended survival of SR-A-/- mice was associated with 13- and 3-fold higher number of CD4+ lymphocytes and antigen presenting cells in SR-A-/- lungs compared to WT mice 280 after infection. The histopathology of chronically infected SR-A-/- lungs, however, was marked by abundant cholesterol clefts in parenchymal lesions containing infection in multinucleated giant cells. The present study indicates SR-A as a candidate gene of the innate immune system influencing the chronic phase of M. tuberculosis infection.

Elastase/LPS-exposed mice exhibit impaired innate immune responses to bacterial challenge: role of scavenger receptor A

Nontypeable Haemophilus influenzae (NTHi) is an important bacterial pathogen associated with lower respiratory tract colonization and with acute exacerbations and disease progression in chronic obstructive pulmonary disease (COPD). Why the immune system fails to eliminate NTHi and the exact contribution of the organism to COPD progression are not well understood, in part because we lack an animal model that mimics all aspects of COPD. For this study, we used an established murine model that exhibits typical features of COPD. Elastase/LPS-exposed mice infected with NTHi showed persistence of bacteria up to 5 days after infection, whereas mice exposed to elastase, LPS, or PBS cleared all bacteria by 3 days. Elastase/LPS-exposed mice also showed sustained lung neutrophilic inflammation, goblet cell metaplasia, airway hyperresponsiveness, and progression of emphysema at 15 days after infection. Alveolar macrophages isolated from elastase/LPS-exposed mice showed impaired bacterial phagocytosis, reduced expression of MARCO and of mannose receptor, and absent expression of scavenger receptor-A (SR-A). Neutralization of SR-A significantly decreased phagocytosis of NTHi by normal alveolar macrophages. Our results suggest that elastase/LPS-exposed mice show impaired bacterial clearance and sustained lung inflammation. Lack of SR-A expression may, in part, be responsible for impaired phagocytosis of bacteria by alveolar macrophages of elastase/LPS-exposed mice. These data validate the suitability of elastase/LPS model for investigating NTHi pathogenesis and progression of disease in COPD.

Structural studies of cord factors from Mycobacterium simiae related to the capacity for tumour necrosis factor alpha (α-TNF) induction

The structure of cord factor was studied in several strains of Mycobacterium simiae, including 'habana' TMC 5135, considered as highly immunogenic in experimental tuberculosis and leprosy. The mycolic acids liberated from cord factor were identified in all cases as α'-, α- and keto-mycolates. According to the general NMR and MS data, α'-mycolates were mono-unsaturated and contained from 64 to 68 carbon atoms, whereas α-mycolates mainly presented two 2,3-disubstituted cyclopropane rings and a chain length of 80-91 carbon atoms; keto-mycolates mostly contained one cyclopropane ring and 85-91 carbon atoms. Taking into account the (1)H-NMR results, strains varied in the ratio of the different mycolates, and the high levels of keto-mycolates found in the cord factors of TMC 5135 and ATCC 25275(T) stood out. Notably, MS revealed that the odd carbon number series of α-mycolates (C87-C89) predominated in the cord factor of TMC 5135, in contrast to the remaining studied strains, in which the even (C84-C86) and odd carbon number series appeared more equal. The fine structural differences detected among the cord factors studied did not seem to be relevant to the general capacity of these molecules to induce the secretion of tumour necrosis factor alpha, as the cord factors from several strains of M. simiae (TMC 5135, IPK-342 and ATCC 25275(T)) induced similar amounts of this cytokine in RAW 264.7 cells.

A regulatory role for macrophage class A scavenger receptors in TLR4-mediated LPS responses

Recognition of microbial components by TLR, key sensors of infection, leads to induction of inflammatory responses. We found that, in vivo, TLR4 engagement by LPS induces up-regulation of the class A scavenger receptors (SR) macrophage receptor with a collagenous structure (MARCO) and SR-A, which occurs, at least in the case of MARCO, via both MyD88-dependent and -independent pathways. When challenging mice with a low dose of LPS followed by a high dose, class A SR-deficient mice showed a higher survival rate than WT mice. This was paired with increased production of IL-10 and anti-LPS Ab, as well as increased activation status of marginal zone B cells. However, the receptors were not crucial for survival when challenging mice i.p. with Neisseria meningitidis or Listeria monocytogenes, but they were found to contribute to microbial capture and clearance. This indicates physiological significance for the up-regulation of class A SR during early stages of bacterial infection. Thus, we believe that we have revealed a mechanism where SR regulate the activation status of the immune system and are involved in balancing a proper immune response to infection. This regulation could also be important in maintaining tolerance since these receptors have been shown to be involved in regulation of self-reactivity.

Partial redundancy of the pattern recognition receptors, scavenger receptors, and C-type lectins for the long-term control of Mycobacterium tuberculosis infection

Mycobacterium tuberculosis is recognized by multiple pattern recognition receptors involved in innate immune defense, but their direct role in tuberculosis pathogenesis remains unknown. Beyond TLRs, scavenger receptors (SRs) and C-type lectins may play a crucial role in the sensing and signaling of pathogen motifs, as well as contribute to M. tuberculosis immune evasion. In this study, we addressed the relative role and potential redundancy of these receptors in the host response and resistance to M. tuberculosis infection using mice deficient for representative SR, C-type lectin receptor, or seven transmembrane receptor families. We show that a single deficiency in the class A SR, macrophage receptor with collagenous structure, CD36, mannose receptor, specific ICAM-3 grabbing nonintegrin-related, or F4/80 did not impair the host resistance to acute or chronic M. tuberculosis infection in terms of survival, control of bacterial clearance, lung inflammation, granuloma formation, and cytokine and chemokine expression. Double deficiency for the SRs class A SR types I and II plus CD36 or for the C-type lectins mannose receptor plus specific ICAM-3 grabbing nonintegrin-related had a limited effect on macrophage uptake of mycobacteria and TNF response and on the long-term control of M. tuberculosis infection. By contrast, mice deficient in the TNF, IL-1, or IFN-gamma pathway were unable to control acute M. tuberculosis infection. In conclusion, we document a functional redundancy in the pattern recognition receptors, which might cooperate in a coordinated response to sustain the full immune control of M. tuberculosis infection, in sharp contrast with the nonredundant, essential role of the TNF, IL-1, or IFN-gamma pathway for host resistance to M. tuberculosis.

Cutting edge: Mincle is essential for recognition and adjuvanticity of the mycobacterial cord factor and its synthetic analog trehalose-dibehenate

The mycobacterial cord factor trehalose-6,6-dimycolate (TDM) and its synthetic analog trehalose-6,6-dibehenate (TDB) are potent adjuvants for Th1/Th17 vaccination that activate Syk-Card9 signaling in APCs. In this study, we have further investigated the molecular mechanism of innate immune activation by TDM and TDB. The Syk-coupling adapter protein FcRgamma was essential for macrophage activation and Th17 adjuvanticity. The FcRgamma-associated C-type lectin receptor Mincle was expressed in macrophages and upregulated by TDM and TDB. Recombinant Mincle-Fc fusion protein specifically bound to the glycolipids. Genetic ablation of Mincle abolished TDM/TDB-induced macrophage activation and induction of T cell immune responses to a tuberculosis subunit vaccine. Macrophages lacking Mincle or FcRgamma were impaired in the inflammatory response to Mycobacterium bovis bacillus Calmette-Guérin. These results establish that Mincle is a key receptor for the mycobacterial cord factor and controls the Th1/Th17 adjuvanticity of TDM and TDB.

Direct recognition of the mycobacterial glycolipid, trehalose dimycolate, by C-type lectin Mincle

Tuberculosis remains a fatal disease caused by Mycobacterium tuberculosis, which contains various unique components that affect the host immune system. Trehalose-6,6′-dimycolate (TDM; also called cord factor) is a mycobacterial cell wall glycolipid that is the most studied immunostimulatory component of M. tuberculosis. Despite five decades of research on TDM, its host receptor has not been clearly identified. Here, we demonstrate that macrophage inducible C-type lectin (Mincle) is an essential receptor for TDM. Heat-killed mycobacteria activated Mincle-expressing cells, but the activity was lost upon delipidation of the bacteria; analysis of the lipid extracts identified TDM as a Mincle ligand. TDM activated macrophages to produce inflammatory cytokines and nitric oxide, which are completely suppressed in Mincle-deficient macrophages. In vivo TDM administration induced a robust elevation of inflammatory cytokines in sera and characteristic lung inflammation, such as granuloma formation. However, no TDM-induced lung granuloma was formed in Mincle-deficient mice. Whole mycobacteria were able to activate macrophages even in MyD88-deficient background, but the activation was significantly diminished in Mincle/MyD88 double-deficient macrophages. These results demonstrate that Mincle is an essential receptor for the mycobacterial glycolipid, TDM.

MARCO, TLR2, and CD14 are required for macrophage cytokine responses to mycobacterial trehalose dimycolate and Mycobacterium tuberculosis

Virtually all of the elements of Mycobacterium tuberculosis (Mtb) pathogenesis, including pro-inflammatory cytokine production, granuloma formation, cachexia, and mortality, can be induced by its predominant cell wall glycolipid, trehalose 6,6′-dimycolate (TDM/cord factor). TDM mediates these potent inflammatory responses via interactions with macrophages both in vitro and in vivo in a myeloid differentiation factor 88 (MyD88)-dependent manner via phosphorylation of the mitogen activated protein kinases (MAPKs), implying involvement of toll-like receptors (TLRs). However, specific TLRs or binding receptors for TDM have yet to be identified. Herein, we demonstrate that the macrophage receptor with collagenous structure (MARCO), a class A scavenger receptor, is utilized preferentially to “tether” TDM to the macrophage and to activate the TLR2 signaling pathway. TDM-induced signaling, as measured by a nuclear factor-kappa B (NF-κB)-luciferase reporter assay, required MARCO in addition to TLR2 and CD14. MARCO was used preferentially over the highly homologous scavenger receptor class A (SRA), which required TLR2 and TLR4, as well as their respective accessory molecules, in order for a slight increase in NF-κB signaling to occur. Consistent with these observations, macrophages from MARCO^−/− or MARCO^−/−SRA^−/− mice are defective in activation of extracellular signal-related kinase 1/2 (ERK1/2) and subsequent pro-inflammatory cytokine production in response to TDM. These results show that MARCO-expressing macrophages secrete pro-inflammatory cytokines in response to TDM by cooperation between MARCO and TLR2/CD14, whereas other macrophage subtypes (e.g. bone marrow–derived) may rely somewhat less effectively on SRA, TLR2/CD14, and TLR4/MD2. Macrophages from MARCO^−/− mice also produce markedly lower levels of pro-inflammatory cytokines in response to infection with virulent Mtb. These observations identify the scavenger receptors as essential binding receptors for TDM, explain the differential response to TDM of various macrophage populations, which differ in their expression of the scavenger receptors, and identify MARCO as a novel component required for TLR signaling.

The causative agent of tuberculosis, Mycobacterium tuberculosis, has a lipid-rich cell wall that contains a high percentage of mycolic acids. These mycolic acids contribute to both the impermeable nature of the cell wall and to the immunostimulatory properties of the bacterium. Indeed, it has been known for over 50 years that trehalose 6,6′-dimycolate (TDM/cord factor) is the major immunogenic lipid of M. tuberculosis, which induces potent pro-inflammatory responses from macrophages, although the receptor has not been identified. We have demonstrated that the toll-like receptor (TLR) pathway is required for pro-inflammatory cytokine production in response to TDM; however, the TLRs alone, or in conjunction with known co-receptors, are not sufficient to induce a response. We demonstrate that the macrophage receptor MARCO, a scavenger receptor, is utilized preferentially to “tether” TDM to the macrophage and activate the TLR2 signaling pathway, and is used preferentially over the related SRA. Macrophages from MARCO^−/− mice are defective in activation of TDM-induced signaling and subsequent pro-inflammatory cytokine production in response to both TDM-coated beads and virulent M. tuberculosis. By identifying the macrophage receptors involved in initial recognition we can now explain variable responses to TDM between different macrophage populations (which differ in scavenger receptor expression), and have identified a novel co-receptor that may be involved in lipid presentation to TLRs.

Adjuvanticity of a synthetic cord factor analogue for subunit Mycobacterium tuberculosis vaccination requires FcRgamma-Syk-Card9-dependent innate immune activation

Novel vaccination strategies against Mycobacterium tuberculosis (MTB) are urgently needed. The use of recombinant MTB antigens as subunit vaccines is a promising approach, but requires adjuvants that activate antigen-presenting cells (APCs) for elicitation of protective immunity. The mycobacterial cord factor Trehalose-6,6-dimycolate (TDM) and its synthetic analogue Trehalose-6,6-dibehenate (TDB) are effective adjuvants in combination with MTB subunit vaccine candidates in mice. However, it is unknown which signaling pathways they engage in APCs and how these pathways are coupled to the adaptive immune response. Here, we demonstrate that these glycolipids activate macrophages and dendritic cells (DCs) via Syk-Card9-Bcl10-Malt1 signaling to induce a specific innate activation program distinct from the response to Toll-like receptor (TLR) ligands. APC activation by TDB and TDM was independent of the C-type lectin receptor Dectin-1, but required the immunoreceptor tyrosine-based activation motif-bearing adaptor protein Fc receptor gamma chain (FcRgamma). In vivo, TDB and TDM adjuvant activity induced robust combined T helper (Th)-1 and Th-17 T cell responses to a MTB subunit vaccine and partial protection against MTB challenge in a Card9-dependent manner. These data provide a molecular basis for the immunostimulatory activity of TDB and TDM and identify the Syk-Card9 pathway as a rational target for vaccine development against tuberculosis.

Scavenger receptor A dampens induction of inflammation in response to the fungal pathogen Pneumocystis carinii

Alveolar macrophages are the effector cells largely responsible for clearance of Pneumocystis carinii from the lungs. Binding of organisms to beta-glucan and mannose receptors has been shown to stimulate phagocytosis of the organisms. To further define the mechanisms used by alveolar macrophages for clearance of P. carinii, mice deficient in the expression of scavenger receptor A (SRA) were infected with P. carinii, and clearance of organisms was monitored over time. SRA-deficient (SRAKO) mice consistently cleared P. carinii faster than did wild-type control mice. Expedited clearance corresponded to elevated numbers of activated CD4(+) T cells in the alveolar spaces of SRAKO mice compared to wild-type mice. Alveolar macrophages from SRAKO mice had increased expression of CD11b on their surfaces, consistent with an activated phenotype. However, they were not more phagocytic than macrophages expressing SRA, as measured by an in vivo phagocytosis assay. SRAKO alveolar macrophages produced significantly more tumor necrosis factor alpha (TNF-alpha) than wild-type macrophages when stimulated with lipopolysaccharide in vitro but less TNF-alpha in response to P. carinii in vitro. However, upon in vivo stimulation, SRAKO mice produced significantly more TNF-alpha, interleukin 12 (IL-12), and IL-18 in response to P. carinii infection than did wild-type mice. Together, these data indicate that SRA controls inflammatory cytokines produced by alveolar macrophages in the context of P. carinii infection.

Class A scavenger receptor (CD204) attenuates hyperoxia-induced lung injury by reducing oxidative stress

To clarify the role of macrophage class A scavenger receptors (SR-A, CD204) in oxidative lung injury, we examined lung tissue of SR-A deficient (SR-A(-/-)) and wild-type (SR-A(+/+)) mice in response to hyperoxic treatment. Protein levels of bronchoalveolar lavage fluid (BALF) and pulmonary oedema (wet : dry weight ratios) were higher in SR-A(-/-) mice than those in SR-A(+/+) mice. Cumulative survival was significantly decreased in SR-A(-/-) mice. However, there were no differences in BALF macrophage and neutrophil count between the two groups. Real-time reverse transcriptase-polymerase chain reaction (RT-PCR) revealed that messenger RNA (mRNA) levels of the inducible nitric oxide synthase (iNOS) were increased during hyperoxic injury, and this increase was more prominent in SR-A(-/-) mice. Expression levels of iNOS in alveolar macrophages after hyperoxia in vivo and in vitro were higher in SR-A(-/-) macrophages compared with SR-A(+/+) macrophages. Immunohistochemistry using anti-nitrotyrosine antibodies revealed distinctive oxidative stress in the injured lung in both groups, but it was more remarkable in the SR-A(-/-) mice. After hyperoxic treatment, pulmonary mRNA levels of tumour necrosis factor-alpha(TNF-alpha) were elevated more rapidly in SR-A(-/-) mice than in SR-A(+/+) mice. Together these results suggest that SR-A expression attenuates hyperoxia-induced lung injury by reducing macrophage activation.

Inflammation in prostate carcinogenesis

About 20% of all human cancers are caused by chronic infection or chronic inflammatory states. Recently, a new hypothesis has been proposed for prostate carcinogenesis. It proposes that exposure to environmental factors such as infectious agents and dietary carcinogens, and hormonal imbalances lead to injury of the prostate and to the development of chronic inflammation and regenerative 'risk factor' lesions, referred to as proliferative inflammatory atrophy (PIA). By developing new experimental animal models coupled with classical epidemiological studies, genetic epidemiological studies and molecular pathological approaches, we should be able to determine whether prostate cancer is driven by inflammation, and if so, to develop new strategies to prevent the disease.

Class A scavenger receptor-mediated cell adhesion requires the sequential activation of Lyn and PI3-kinase

Class A scavenger receptors (SR-A) participate in multiple macrophage functions including macrophage adhesion to modified proteins. SR-A-mediated adhesion may therefore contribute to chronic inflammation by promoting macrophage accumulation at sites of protein modification. The mechanisms that couple SR-A binding to modified proteins with increased cell adhesion have not been defined. In this study, SR-A expressing HEK cells and SR-A+/+ or SR-A-/- macrophages were used to delineate the signaling pathways required for SR-A-mediated adhesion to modified protein. Inhibiting G(i/o) activation, which decreases initial SR-A-mediated cell attachment, did not prevent the subsequent spreading of attached cells. In contrast, inhibition of Src kinases or PI3-kinase abolished SR-A-dependent cell spreading without affecting SR-A-mediated cell attachment. Consistent with these results, the Src kinase Lyn and PI3-kinase were sequentially activated during SR-A-mediated cell spreading. Furthermore, activation of both Lyn and PI3-kinase was required for enhancing paxillin phosphorylation. Activation of a Src kinase-PI3-kinase-Akt pathway was also observed in cells expressing a truncated SR-A protein that does not internalize indicating that SR-A-mediated activation of intracellular signaling cascades following adhesion to MDA-BSA is independent of receptor internalization. Thus SR-A binding to modified protein activates signaling cascades that have distinct roles in regulating initial cell attachment and subsequent cell spreading.