Roles of a macrophage receptor with collagenous structure (MARCO) in host defense and heterogeneity of splenic marginal zone macrophages

Disruption of monocyte-macrophage differentiation and trafficking by a heme analog during active inflammation

Heme metabolism is a key regulator of inflammatory responses. Cobalt protoporphyrin IX (CoPP) is a heme analog and mimic that potently activates the NRF2/heme oxygenase-1 (HO-1) pathway, especially in monocytes and macrophages. We investigated the influence of CoPP on inflammatory responses using a murine model of colitis. Surprisingly, conditional deletion of myeloid HO-1 did not impact the colonic inflammatory response or the protective influence of CoPP in the setting of dextran sodium sulfate-induced colitis. Rather, we reveal that CoPP elicits a contradictory shift in blood myeloid populations relative to the colon during active intestinal inflammation. Major population changes include markedly diminished trafficking of CCR2⁺Ly6C^hi monocytes to the inflamed colon, despite significant mobilization of this population into circulation. This resulted in significantly diminished colonic expansion of monocyte-derived macrophages and inflammatory cytokine expression. These findings were linked with significant induction of systemic CCL2 leading to a disrupted CCL2 chemoattractant gradient toward the colon and concentration-dependent suppression of circulating monocyte CCR2 expression. Administration of CoPP also induced macrophage differentiation toward a Marco^hiHmox1^hi anti-inflammatory erythrophagocytic phenotype, contributing to an overall decreased inflammatory profile. Such findings redefine protective influences of heme metabolism during inflammation, and highlight previously unreported immunosuppressive mechanisms of endogenous CCL2 induction.

Stabilin-1 plays a protective role against Listeria monocytogenes infection through the regulation of cytokine and chemokine production and immune cell recruitment

Scavenger receptors are part of a complex surveillance system expressed by host cells to efficiently orchestrate innate immune response against bacterial infections. Stabilin-1 (STAB-1) is a scavenger receptor involved in cell trafficking, inflammation, and cancer; however, its role in infection remains to be elucidated. Listeria monocytogenes (Lm) is a major intracellular human food-borne pathogen causing severe infections in susceptible hosts. Using a mouse model of infection, we demonstrate here that STAB-1 controls Lm-induced cytokine and chemokine production and immune cell accumulation in Lm-infected organs. We show that STAB-1 also regulates the recruitment of myeloid cells in response to Lm infection and contributes to clear circulating bacteria. In addition, whereas STAB-1 appears to promote bacterial uptake by macrophages, infection by pathogenic Listeria induces the down regulation of STAB-1 expression and its delocalization from the host cell membrane.

We propose STAB-1 as a new SR involved in the control of Lm infection through the regulation of host defense mechanisms, a process that would be targeted by bacterial virulence factors to promote infection.

Characterization of Conventional Dendritic Cells and Macrophages in the Spleen Using the CSF1R-Reporter Transgenic Chickens

The spleen is a major site for the immunological responses to blood-borne antigens that is coordinated by cells of the mononuclear phagocyte system (MPS). The chicken spleen is populated with a number of different macrophages while the presence of conventional dendritic cells (cDC) has been described. However, a detailed characterization of the phenotype and function of different macrophage subsets and cDC in the chicken spleen is limited. Using the CSF1R-reporter transgenic chickens (CSF1R-tg), in which cells of the MPS express a transgene under the control elements of the chicken CSF1R, we carried out an in-depth characterization of these cells in the spleen. Immunohistological analysis demonstrated differential expression of MRC1L-B by periarteriolar lymphoid sheaths (PALS)-associated CSF1R-tg+ cells. In the chicken's equivalent of the mammalian marginal zone, the peri-ellipsoid white-pulp (PWP), we identified high expression of putative CD11c by ellipsoid-associated cells compared to ellipsoid-associated macrophages. In addition, we identified a novel ellipsoid macrophage subset that expressed MHCII, CD11c, MRC1L-B, and CSF1R but not the CSF1R-tg. In flow cytometric analysis, diverse expression of the CSF1R-tg and MHCII was observed leading to the categorization of CSF1R-tg cells into CSF1R-tgdim MHCIIinter−hi, CSF1R-tghi MHCIIhi, and CSF1R-tghi MHCIIinter subpopulations. Low levels of CD80, CD40, MHCI, CD44, and Ch74.2 were expressed by the CSF1R-tghi MHCIIinter cells. Functionally, in vivo fluorescent bead uptake was significantly higher in the CSF1R-tghi MHCIIhi MRC1L-B+ cells compared to the CSF1R-tgdim and CSF1R-tghi MHCIIinter MRC1L-B+ subpopulations while LPS enhanced phagocytosis by the CSF1R-tghi MHCIIinter subpopulation. The analysis of bead localization in the spleen suggests the presence of ellipsoid-associated macrophage subsets. In addition, we demonstrated the functionality of ex vivo derived CSF1R-tg+ MRC1L-Bneg cDC. Finally, RNA-seq analysis of the CSF1R-tg subpopulations demonstrated that separating the CSF1R-tghi subpopulation into CD11chi and CD11cdim cells enriched for cDC and macrophage lineages, respectively, while the CSF1R-tghi MHCIIinter subpopulation was enriched for red pulp macrophages. However, our analysis could not define the cell lineage of the heterogeneous CSF1R-tgdim subpopulation. This detailed overview of the MPS in the chicken spleen will contribute to future research on their role in antigen uptake and presentation.

Marginal zone SIGN-R1+ macrophages are essential for the maturation of germinal center B cells in the spleen

The mechanisms that regulate germinal center (GC) B cell responses in the spleen are not fully understood. Here we use a combination of pharmacologic and genetic approaches to delete SIGN-R1+ marginal zone (MZ) macrophages and reveal their specific contribution to the regulation of humoral immunity in the spleen. We find that while SIGN-R1+ macrophages were not essential for initial activation of B cells, they were required for maturation of the response and development of GC B cells. These defects could be corrected when follicular helper T (Tfh) cells were induced before macrophage ablation or when Tfh responses were enhanced. Moreover, we show that in the absence of SIGN-R1+ macrophages, DCIR2+ dendritic cells (DCs), which play a key role in priming Tfh responses, were unable to cluster to the interfollicular regions of the spleen and were instead displaced to the MZ. Restoring SIGN-R1+ macrophages to the spleen corrected positioning of DCIR2+ DCs and rescued the GC B cell response. Our study reveals a previously unappreciated role for SIGN-R1+ macrophages in regulation of the GC reaction and highlights the functional specification of macrophage subsets in the MZ compartment.

Limited Effect of Indolamine 2,3-Dioxygenase Expression and Enzymatic Activity on Lupus-Like Disease in B6.Nba2 Mice

B6.Nba2 mice spontaneously develop a lupus-like disease characterized by elevated levels of serum anti-nuclear autoantibody (ANA) immune complexes and constitutive type I interferon (IFNα) production. During disease progression, both plasmacytoid dendritic cells (pDCs) and antibody secreting plasma cells accumulate in spleens of B6.Nba2 mice. Indoleamine 2,3-dioxygenase (IDO) has been suggested to play a role in several autoimmune diseases including in the MRL/lpr model of mouse lupus-like disease; however, it remains unknown if IDO is involved in disease development and/or progression in other spontaneous models. We show here that IDO1 protein and total IDO enzymatic activity are significantly elevated in lupus-prone B6.Nba2 mice relative to B6 controls. IDO1 expression was restricted to PCs and SignR1⁺ macrophages in both strains, while significantly increased in B6.Nba2-derived SiglecH⁺ (SigH⁺) pDCs. Despite this unique expression pattern, neither pharmacologic inhibition of total IDO nor IDO1 gene ablation altered serum autoantibody levels, splenic immune cell activation pattern, or renal inflammation in B6.Nba2 mice. Interestingly, IDO pharmacologic inhibition, but not IDO1 deficiency, resulted in diminished complement factor C'3 fixation to kidney glomeruli, suggesting a possible therapeutic benefit of IDO inhibition in SLE patients with renal involvement.

Scavenger Receptors: Promiscuous Players during Microbial Pathogenesis

Innate immunity is the most broadly effective host defense, being essential to clear the majority of microbial infections. Scavenger Receptors comprise a family of sensors expressed in a multitude of host cells, whose dual role during microbial pathogenesis gained importance over recent years. SRs regulate the recruitment of immune cells and control both host inflammatory response and bacterial load. In turn, pathogens have evolved different strategies to overcome immune response, avoid recognition by SRs and exploit them to favor infection. Here, we discuss the most relevant findings regarding the interplay between SRs and pathogens, discussing how these multifunctional proteins recognize a panoply of ligands and act as bacterial phagocytic receptors.

Cutting Edge: Marginal Zone Macrophages Regulate Antigen Transport by B Cells to the Follicle in the Spleen via CD21

Marginal zone macrophages (MZM) are strategically located in the spleen, lining the marginal sinus where they sense inflammation and capture Ag from the circulation. One of the receptors expressed by MZM is scavenger receptor macrophage receptor with collagenous structure (MARCO), which has affinity for modified self-antigens. In this article, we show that engagement of MARCO on murine macrophages induces extracellular ATP and loss of CD21 and CD62L on marginal zone B cells. Engagement of MARCO also leads to reduction of Ag transport by marginal zone B cells and affects the subsequent immune response. This study highlights a novel function for MZM in regulating Ag transport and activation, and we suggest that MARCO-dependent ATP release regulates this through shedding of CD21 and CD62L. Because systemic lupus erythematosus patients were shown to acquire autoantibodies against MARCO, this highlights a mechanism that could affect a patient's ability to combat infections.

Galectin-3 deficiency exacerbates hyperglycemia and the endothelial response to diabetes

Background

Diabetes promotes maladaptive changes in the endothelium that lead to its dysfunction and contribute to the vascular pathology of diabetes. We have previously reported the up-regulation of galectin-3, a β-galactoside-binding lectin, in the endothelium and sera of diabetic mice, implicating this molecule in diabetic vasculopathy and suggesting its potential as a biomarker of the disease. Therefore, we sought to assess the role of galectin-3 in the vascular pathology of diabetes.

Methods

Galectin-3 knockout mice (KO) and wild-type mice (WT) were fed either a high-fat diet (HFD) (60 % fat calories) to produce insulin resistant diabetes, or standard chow (12 % fat calories), and their metabolic and endothelial responses were measured. After 8 weeks, the aortic and skeletal muscle endothelia were isolated by fluorescence sorting of CD105⁺/CD45⁻ cells and comprehensive transcriptional analyses were performed. Transcripts differentially dysregulated by HFD in KO endothelium compared to WT were confirmed by semi-quantitative RT-PCR, and protein expression was determined by immunofluorescence of aortic and muscle tissue. Ingenuity® Pathway Analysis was used to identify pathways up-regulated by HFD in the KO, such as the coagulation cascade, and measurements of blood clotting activity were performed to confirm these results.

Results

KO mice demonstrate greater hyperglycemia and impaired glucose tolerance but lower insulin levels on HFD compared to WT. KO mice demonstrate a more robust transcriptional response to HFD in the vascular endothelium compared to WT. Transcripts dysregulated in the KO endothelium after HFD are involved in glucose uptake and insulin signaling, vasoregulation, coagulation, and atherogenesis. One of the most down-regulated transcripts in the endothelium of the KO after HFD was the glucose transporter, Glut4/Slc2a4. GLUT4 immunofluorescence confirmed lower protein abundance in the endothelium and muscle of the HFD-fed KO. Prothrombin time was decreased in the diabetic KO indicating increased coagulation activity.

Conclusions

Galectin-3 deficiency leads to exacerbated metabolic derangement and endothelial dysfunction. The impaired tissue uptake of glucose in KO mice can be attributed to the reduced expression of GLUT4. Enhanced coagulation activity in the diabetic KO suggests a protective role for galectin-3 against thrombosis. These studies demonstrate that galectin-3 deficiency contributes both to the pathogenesis of diabetes and the associated vasculopathy.

Electronic supplementary material

The online version of this article (doi:10.1186/s12933-015-0230-3) contains supplementary material, which is available to authorized users.

Combination of gene expression patterns in whole blood discriminate between tuberculosis infection states

Background

Genetic factors are involved in susceptibility or protection to tuberculosis (TB). Apart from gene polymorphisms and mutations, changes in levels of gene expression, induced by non-genetic factors, may also determine whether individuals progress to active TB.

Methods

We analysed the expression level of 45 genes in a total of 47 individuals (23 healthy household contacts and 24 new smear-positive pulmonary TB patients) in Addis Ababa using a dual colour multiplex ligation-dependent probe amplification (dcRT-MLPA) technique to assess gene expression profiles that may be used to distinguish TB cases and their contacts and also latently infected (LTBI) and uninfected household contacts.

Results

The gene expression level of BLR1, Bcl2, IL4d2, IL7R, FCGR1A, MARCO, MMP9, CCL19, and LTF had significant discriminatory power between sputum smear-positive TB cases and household contacts, with AUCs of 0.84, 0.81, 0.79, 0.79, 0.78, 0.76, 0.75, 0.75 and 0.68 respectively. The combination of Bcl2, BLR1, FCGR1A, IL4d2 and MARCO identified 91.66% of active TB cases and 95.65% of household contacts without active TB. The expression of CCL19, TGFB1, and Foxp3 showed significant difference between LTBI and uninfected contacts, with AUCs of 0.85, 0.82, and 0.75, respectively, whereas the combination of BPI, CCL19, FoxP3, FPR1 and TGFB1 identified 90.9% of QFT^- and 91.6% of QFT⁺ household contacts.

Conclusions

Expression of single and especially combinations of host genes can accurately differentiate between active TB cases and healthy individuals as well as between LTBI and uninfected contacts.

Cell depletion in mice that express diphtheria toxin receptor under the control of SiglecH encompasses more than plasmacytoid dendritic cells

Plasmacytoid dendritic cells (pDC) produce IFN-I in response to viruses and are routinely identified in mice by SiglecH expression. SiglecH is a sialic acid-binding Ig-like lectin that has an immunomodulatory role during viral infections. In this study, we evaluated the impact of SiglecH deficiency on cytokine responses in the presence and absence of pDC. We found that lack of SiglecH enhanced IFN-I responses to viral infection, regardless of whether pDC were depleted. We also examined the expression pattern of SiglecH and observed that it was expressed by specialized macrophages and progenitors of classical dendritic cells and pDC. Accordingly, marginal zone macrophages and pDC precursors were eliminated in newly generated SiglecH-diphtheria toxin receptor (DTR)-transgenic (Tg) mice but not in CLEC4C-DTR-Tg mice after diphtheria toxin (DT) treatment. Using two bacterial models, we found that SiglecH-DTR-Tg mice injected with DT had altered bacterial uptake and were more susceptible to lethal Listeria monocytogenes infection than were DT-treated CLEC4C-DTR-Tg mice. Taken together, our findings suggest that lack of SiglecH may affect cytokine responses by cell types other than pDC during viral infections, perhaps by altering viral distribution or burden, and that cell depletion in SiglecH-DTR-Tg mice encompasses more than pDC.

Marginal Zone Macrophage Receptor MARCO Is Trapped in Conduits Formed by Follicular Dendritic Cells in the Spleen

The marginal zone (MZ) region of the spleen plays an important role in leukocyte traffic and the removal of blood-borne pathogens by resident macrophages. Macrophage receptor with a collagenous structure (MARCO), expressed by MZ macrophages, recognizes several microbial ligands and is also involved in the retention of MZ B cells. Here, we report that MARCO is also associated with follicular dendritic cells (FDCs) in the spleen. In its FDC-associated form MARCO is arranged in 0.3-0.5-μm diameter granular-fibrillar structures with an appearance similar to the white pulp conduit system formed by fibroblastic reticular cells (FRCs), but with different compartment preference. The follicular display of MARCO resists irradiation and requires the presence of both MZ macrophages and differentiated FDCs. The follicular delivery of MARCO is independent from the shuffling of marginal zone B cells, and it persists after clodronate liposome-mediated depletion of MZ macrophages. Our findings thus indicate that MARCO is distributed to both MZ and follicles within the spleen into conduit-like structures, where FDC-bound MARCO may mediate communication between the stromal microenvironments of MZ and follicles.

Autophagy regulates phagocytosis by modulating the expression of scavenger receptors

Autophagy and phagocytosis are conserved cellular functions involved in innate immunity. However, the nature of their interactions remains unclear. We evaluated the role of autophagy in regulating phagocytosis in macrophages from myeloid-specific autophagy-related gene 7-deficient (Atg7⁻/⁻) mice. Atg7⁻/⁻ macrophages exhibited higher bacterial uptake when infected with Mycobacterium tuberculosis (Mtb) or with M. tuberculosis var. bovis BCG (BCG). In addition, BCG-infected Atg7⁻/⁻ mice showed increased bacterial loads and exacerbated lung inflammatory responses. Atg7⁻/⁻ macrophages had increased expression of two class A scavenger receptors: macrophage receptor with collagenous structure (MARCO) and macrophage scavenger receptor 1 (MSR1). The increase in scavenger receptors was caused by increased activity of the nuclear factor (erythroid-derived 2)-like 2 (NFE2L2) transcription factor resulting from accumulated sequestosome 1 (SQSTM1 or p62) in Atg7⁻/⁻ macrophages. These insights increase our understanding of the host-pathogen relationship and suggest that therapeutic strategies should be designed to include modulation of both phagocytosis and autophagy.

An evolving new paradigm: endothelial cells--conditional innate immune cells

Endothelial cells (ECs) are a heterogeneous population that fulfills many physiological processes. ECs also actively participate in both innate and adaptive immune responses. ECs are one of the first cell types to detect foreign pathogens and endogenous metabolite-related danger signals in the bloodstream, in which ECs function as danger signal sensors. Treatment with lipopolysaccharide activates ECs, causing the production of pro-inflammatory cytokines and chemokines, which amplify the immune response by recruiting immune cells. Thus, ECs function as immune/inflammation effectors and immune cell mobilizers. ECs also induce cytokine production by immune cells, in which ECs function as immune regulators either by activating or suppressing immune cell function. In addition, under certain conditions, ECs can serve as antigen presenting cells (antigen presenters) by expressing both MHC I and II molecules and presenting endothelial antigens to T cells. These facts along with the new concept of endothelial plasticity suggest that ECs are dynamic cells that respond to extracellular environmental changes and play a meaningful role in immune system function. Based on these novel EC functions, we propose a new paradigm that ECs are conditional innate immune cells. This paradigm provides a novel insight into the functions of ECs in inflammatory/immune pathologies.

Genetic variants of MARCO are associated with susceptibility to pulmonary tuberculosis in a Gambian population

BACKGROUND

The two major class A scavenger receptors are scavenger receptor A (SRA), which is constitutively expressed on most macrophage populations, and macrophage receptor with collagenous structure (MARCO), which is constitutively expressed on a more restricted subset of macrophages, (e.g. alveolar macrophages) but whose expression increases on most macrophages during the course of infection. Although the primary role of SRA appears to be clearance of modified host proteins and lipids, mice defective in expression of either MARCO or SRA are immunocompromised in multiple models of infection and in vitro assays, the scavenger receptors have been demonstrated to bind bacteria and to enhance pro-inflammatory signalling to many bacterial lung pathogens; however their importance in Mycobacterium tuberculosis infection, is less clear.

METHODS

To determine whether polymorphisms in either SRA or MARCO were associated with tuberculosis, a case-control study of was performed. DNA samples from newly-detected, smear-positive, pulmonary tuberculosis cases were collected from The Gambia. Controls for this study consisted of DNA from cord bloods obtained from routine births at local Gambian health clinics. Informed written consent was obtained from patients or their parents or guardians. Ethical approval was provided by the joint The Gambian Government/MRC Joint Ethics Committee.

RESULTS

We studied the frequencies of 25 polymorphisms of MSR1 (SRA) and 22 in MARCO in individuals with tuberculosis (n=1284) and matched controls (n=1349). No SNPs within the gene encoding or within 1 kb of the promoter sequence of MSR1 were associated with either susceptibility or resistance to tuberculosis. Three SNPs in MARCO (rs4491733, Mantel-Haenszel 2x2 χ2 = 6.5, p = 0.001, rs12998782, Mantel-Haenszel 2x2 χ2 = 6.59, p = 0.001, rs13389814 Mantel-Haenszel 2x2 χ2 = 6.9, p = 0.0009) were associated with susceptibility to tuberculosis and one (rs7559955, Mantel-Haenszel 2x2 χ2 = 6.9, p = 0.0009) was associated with resistance to tuberculosis.

CONCLUSIONS

These findings identify MARCO as a potentially important receptor in the host response to tuberculosis.

Macrophage heterogeneity in lymphoid tissues

Macrophages in lymphoid organs exhibit a wide variety of phenotypes and functions. These cells excel in the removal of apoptotic cells that arise during the generation of immune cells and are thereby essential for the prevention of auto-immune responses. In addition to this macrophages in the secondary lymphoid organs form an important barrier for spreading of infections by phagocytosis of pathogens and the activation of both innate and adaptive immune responses. Thus, the remarkable ability of macrophages to phagocytose and handle a wide range of self and non-self material and to produce immunomediators is effectively exploited within lymphoid organs to regulate immune activation.

Effect of Lactobacillus brevis KB290 on the cell-mediated cytotoxic activity of mouse splenocytes: a DNA microarray analysis

Lactic acid bacteria confer a variety of health benefits. Here, we investigate the mechanisms by which Lactobacillus brevis KB290 (KB290) enhances cell-mediated cytotoxic activity. Female BALB/c mice aged 9 weeks were fed a diet containing KB290 (3 × 10(9) colony-forming units/g) or starch for 1 d. The resulting cytotoxic activity of splenocytes against YAC-1 cells was measured using flow cytometry and analysed for gene expression using DNA microarray technology. KB290 enhanced the cell-mediated cytotoxic activity of splenocytes. DNA microarray analysis identified 327 up-regulated and 347 down-regulated genes that characterised the KB290 diet group. The up-regulated genes were significantly enriched in Gene Ontology terms related to immunity, and, especially, a positive regulation of T-cell-mediated cytotoxicity existed among these terms. Almost all the genes included in the term encoded major histocompatibility complex (MHC) class I molecules involved in the presentation of antigen to CD8(+) cytotoxic T cells. Marco and Signr1 specific to marginal zone macrophages (MZM), antigen-presenting cells, were also up-regulated. Flow cytometric analysis confirmed that the proportion of MZM was significantly increased by KB290 ingestion. Additionally, the over-represented Kyoto Encyclopedia of Genes and Genomes pathways among the up-regulated genes were those for natural killer (NK) cell-mediated cytotoxicity and antigen processing and presentation. The results for the selected genes associated with NK cells and CD8(+) cytotoxic T cells were confirmed by quantitative RT-PCR. These results suggest that enhanced cytotoxic activity could be caused by the activation of NK cells and/or of CD8(+) cytotoxic T cells stimulated via MHC class I presentation.

Identification and characterization of a cell surface scavenger receptor cysteine-rich protein of Sciaenops ocellatus: bacterial interaction and its dependence on the conserved structural features of the SRCR domain

The scavenger receptor cysteine-rich (SRCR) proteins are secreted or membrane-bound receptors with one or multiple SRCR domains. Members of the SRCR superfamily are known to have diverse functions that include pathogen recognition and immunoregulation. In teleost, although protein sequences with SRCR structure have been identified in some species, very little functional investigation has been carried out. In this study, we identified and characterized a teleost SRCR protein from red drum Sciaenops ocellatus. The protein was named S. ocellatus SRCR1 (SoSRCRP1). SoSRCRP1 is 410-residue in length and was predicted to be a transmembrane protein, with the extracellular region containing a collagen triple helix repeat and a SRCR domain. The SRCR domain has six conserved cysteines, of which, C338 and C399, C351 and C409, and C379 and C389 were predicted to form three disulfide bonds. SoSRCRP1 expression was detected mainly in immune-relevant tissues and upregulated by bacterial and viral infection. In head kidney leukocytes, bacterial infection stimulated the expression of SoSRCRP1, and the expressed SoSRCRP1 was localized on cell surface. Recombinant SoSRCRP1 (rSoSRCRP1) corresponding to the SRCR domain was purified from Escherichia coli and found to be able to bind Gram-negative and Gram-positive bacteria. To examine the structure-function relationship of SoSRCRP1, the mutant proteins SoSRCRP1M1, SoSRCRP1M2, SoSRCRP1M3, and SoSRCRP1M4 were created, which bear C351S and C409S, C338S, C379S, and R325A mutations respectively. Compared to rSoSRCRP1, all mutants were significantly reduced in the ability of bacterial interaction, with the highest reduction observed with SoSRCRP1M4. Taken together, these results indicate that SoSRCRP1 is a cell surface-localized SRCR protein that binds bacterial ligands in a manner that depends on the conserved structural features of the SRCR domain.

Genetic variation in the scavenger receptor MARCO and its association with chronic obstructive pulmonary disease and lung infection in 10,604 individuals

BACKGROUND

MARCO (macrophage receptor with collagenous structure) is a dominant receptor for unopsonized particles and bacteria in the lungs. Reduced function of this receptor due to genetic variation may be associated with susceptibility to chronic obstructive pulmonary disease (COPD) and lung infection.

OBJECTIVES

To identify novel genetic variants in MARCO that are associated with reduced lung function, or increased risk of COPD or lung infection.

METHODS

We first screened 760 individuals with extreme lung phenotypes in a large general population study to identify novel variants in the MARCO gene. We next genotyped the entire cohort consisting of 10,604 individuals to assess the clinical relevance of these variants.

RESULTS

We identified 4 novel (R124H, K201N, P303L and G340W) and 5 previously described (H101Q, F282S, G319V, K387Q and E511D) non-synonymous variants. When screening the entire cohort for these variants, we found low minor allele frequencies ranging from 0.005 to 5%. None of the individual MARCO genotypes were associated with reduced lung function, or risk of COPD or lung infection. H101Q heterozygotes had an increased odds ratio for sepsis of 2.2 (95% CI: 1.1-4.4) compared to non-carriers, but none of the other MARCO genotypes were associated with the risk of sepsis.

CONCLUSIONS

We identified 9 non-synonymous variants in the MARCO gene and showed that these variants are not major risk factors for COPD or lung infection in the Danish population. H101Q heterozygotes had increased sepsis risk, but further research is required to confirm this finding. This study is the first to examine genetic variants in MARCO and the risk of COPD and infections in humans.

Genetic variants in MARCO are associated with the susceptibility to pulmonary tuberculosis in Chinese Han population

Background

Susceptibility to tuberculosis is not only determined by Mycobacterium tuberculosis infection, but also by the genetic component of the host. Macrophage receptor with a collagenous structure (MARCO) is essential components required for toll like receptor-signaling in macrophage response to Mycobacterium tuberculosis, which may contribute to tuberculosis risk.

Principal Findings

To specifically investigated whether single nucleotide polymorphisms (SNPs) in MARCO gene are associated with pulmonary tuberculosis in Chinese Han population. By selecting tagging SNPs in MARCO gene, 17 tag SNPs were identified and genotyped in 923 pulmonary tuberculosis patients and 1033 healthy control subjects using a hospital based case-control association study. Single-point and haplotype analysis revealed an association in intron and exon region of MARCO gene. One SNP (rs17009726) was associated with susceptibility to pulmonary tuberculosis, where the carriers of the G allele had a 1.65 fold (95% CI = 1.32–2.05, p_corrected = 9.27E–5) increased risk of pulmonary tuberculosis. Haplotype analysis revealed that haplotype GC containing G allele of 17009726 and haplotype TGCC (rs17795618T/A, rs1371562G/T, rs6761637T/C, rs2011839C/T) were also associated with susceptibility to pulmonary tuberculosis (p_corrected = 0.0001 and 0.029, respectively).

Conclusions

Our study suggested that genetic variants in MARCO gene were associated with pulmonary tuberculosis susceptibility in Chinese Han population, and the findings emphasize the importance of MARCO mediated immune responses in the pathogenesis of tuberculosis.

Marginal zone B cells regulate antigen capture by marginal zone macrophages

The marginal zone (MZ) of the mouse spleen contains macrophages that express receptors that trap pathogens, including the scavenger receptor macrophage receptor with a collagenous structure and the C-type lectin specific intracellular adhesion molecule-grabbing nonintegrin receptor 1 (SIGN-R1). We previously reported that expression of SIGN-R1 was decreased in CD19-deficient mice. In this study, we demonstrate that SIGN-R1 is expressed on a subset of macrophage receptor with a collagenous structure (MARCO)(+) macrophages. This subset is diminished when MZ B cells are absent due to either genetic developmental defects or following transient migration of B cells out of the MZ. When B cells return to the MZ, there is a delay in recovery of SIGN-R1-expressing macrophages. During this period, capture of Ficoll, which for the macrophages requires SIGN-R1, remains defective not only by the macrophages, but also by the B cells. Thus, MZ B cells regulate expression of molecules on macrophages that are important for trapping Ag, which, in turn, is required for Ag capture by the B cells.

Sustained release nitric oxide from long-lived circulating nanoparticles

The current limitations of nitric oxide (NO) delivery systems have stimulated an extraordinary interest in the development of compounds that generate NO in a controlled and sustained manner with a heavy emphasis on the treatment of cardiovascular disease states. This work describes the positive physiological response to the infusion of NO-releasing nanoparticles prepared using a new platform based on hydrogel/glass hybrid nanoparticles. When exposed to moisture, these nanoparticles slowly release therapeutic levels of NO, previously generated through thermal reduction of nitrite to NO trapped within the dry particles. The controlled and sustained release of NO observed from these nanoparticles (NO-np) is regulated by its hydration over extended periods of time. In a dose-dependent manner, circulating NO-np both decreased mean arterial blood pressure and increased exhaled concentrations of NO over a period of several hours. Circulating NO-np induced vasodilatation and increased microvascular perfusion during their several hour circulation lifetime. Control nanoparticles (control-np; without nitrite) did not induce changes in arterial pressure, although a decrease in the number of capillaries perfused and an increase in leukocyte rolling and immobilization in the microcirculation were observed. The NO released by the NO-np prevents the inflammatory response observed after infusion of control-np. These data suggest that NO release from NO-np is advantageous relative to other NO-releasing compounds, because it does not depend on chemical decomposition or enzymatic catalysis; it is only determined by the rate of hydration. Based on the observed physiological properties, NO-np has clear potential as a therapeutic agent and as a research tool to increase our understanding of NO signaling mechanisms within the vasculature.

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.

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.

Disparate regulation and function of the class A scavenger receptors SR-AI/II and MARCO

The macrophage class A scavenger receptors, macrophage receptor with a collagenous structure (MARCO) and type I/II class A scavenger receptor (SR-AI/II), share structural features and roles in host defense, but little is known about their regulation and signaling properties. Ligation of MARCO on mouse thioglycollate-elicited peritoneal macrophages (PEMs) with immobilized mAb costimulated IL-12 production, in contrast to previously reported inhibition by SR-AI/II. PEMs from MARCO-deficient mice exhibited 2.7 times lower IL-12 production in responses to stimulation with LPS and IFN-gamma and lack of significant IL-12 production on stimulation with LPS alone. Conversely, SR-AI/II-deficient PEMs produced 2.4 and 1.8 times more IL-12 than wild-type PEMs in response to LPS or LPS and IFN-gamma, respectively. Corresponding differences in regulation of SR-A and MARCO expression were also observed. Th1 adjuvants (LPS, a CpG motif-containing oligodeoxynucleotide (CpG-ODN), IL-12, and GM-CSF) increased, whereas Th2-polarizing factors (IL-4, M-CSF, and non-CpG ODN) decreased expression of MARCO on J774 macrophage-like cells. Expression of SR-A was regulated in the opposite manner to MARCO or not affected. Whereas MARCO was involved in opsonin-independent phagocytosis in CpG-ODN-pretreated but not in IL-4-pretreated J774 cells, anti-SR-A Abs inhibited particle uptake in untreated and IL-4-pretreated but not in CpG-ODN-pretreated cells. SR-A and MARCO are regulated differently and mediate distinct negative and positive effects on IL-12 production in macrophages. These differences may contribute to sustained Th1 or Th2 polarization of ongoing immune responses.

Defective microarchitecture of the spleen marginal zone and impaired response to a thymus-independent type 2 antigen in mice lacking scavenger receptors MARCO and SR-A

The macrophage scavenger receptor macrophage receptor with a collagenous structure (MARCO) is expressed in mice by the marginal zone macrophages of the spleen and by macrophages of the medullary cords of lymph nodes, as well as the peritoneal macrophages. MARCO is a relative of scavenger receptor A (SR-A), the more widely expressed prototypic member of the scavenger receptor family. In the present study, we found that genetic ablation of MARCO leads to changes in the organization of the splenic marginal zone, and causes a significant reduction in the size of the resident peritoneal macrophage population, possibly due to changes in adhesion and migration capacity. In mice lacking both MARCO and SR-A these effects are even more apparent. During ontogeny, the appearance and organization of the MARCO-expressing cells in the spleen precedes the appearance of other receptors on macrophages in the marginal zone, such as SIGNR1 and Siglec-1. In the absence of MARCO, a clear delay in the organization of the marginal zone was observed. Similar findings were seen when the reappearance of the various subsets from precursors was studied after depleting macrophages from the adult spleen by a liposome treatment. When challenged with a pneumococcal polysaccharide vaccine, a T-independent type 2 Ag for which an intact marginal zone is crucial, the knockout mice exhibited a clearly impaired response. These findings suggest that both MARCO and SR-A, in addition to being important scavenger receptors, could be involved in the positioning and differentiation of macrophages, possibly through interaction with endogenous ligands.

A phage display screen and binding studies with acetylated low density lipoprotein provide evidence for the importance of the scavenger receptor cysteine-rich (SRCR) domain in the ligand-binding function of MARCO

MARCO is a class A scavenger receptor capable of binding both gram-negative and -positive bacteria. Using the surface plasmon resonance technique, we show here that a recombinant, soluble form of MARCO, sMARCO, binds the major gram-negative and -positive bacterial surface components, lipopolysaccharide and lipoteichoic acid. Yet, the interaction of these two polyanions with sMARCO is of much lower affinity than that of polyinosinic acid, a polyanionic inhibitor of bacterial binding to MARCO. To further elucidate the ligand-binding functions of MARCO, we performed a phage display screen with sMARCO. The screening resulted in the enrichment of only a handful of phage clones. Contrary to expectations, no polyanionic peptides, but only those with a predominantly hydrophobic nature, were enriched. One peptide, VRWGSFAAWL, was displayed on two-thirds of the phages recovered after four rounds of screening. The VRWGSFAAWL phage-sMARCO interaction had significantly slower dissociation kinetics than that between sMARCO and lipopolysaccharide or lipoteichoic acid. Further work with this phage, and the second most enriched phage, displaying the peptide RLNWAWWLSY, demonstrated that both peptides bind to the SRCR domain of MARCO, and that they probably bind to the same site. Data base searches suggested that the VRWGSFAAWL peptide represents complement component C4, but we could not convincingly confirm this suggestion. A study with chimeric scavenger receptors indicated that even minor sequence changes in the MARCO scavenger receptor cysteine-rich (SRCR) domain can have profound effects on the binding of the prototypic scavenger receptor ligand, acetylated low density lipoprotein. As shown by differential binding of glutathione S-transferase-VR-WGSFAAWL, these differences were very likely due to conformational changes. These findings led to experiments that demonstrated a crucial role of the SRCR domain for acetylated low density lipoprotein binding in MARCO. Thus, our results strengthen the notion that the SRCR domain is the major ligand-binding domain in MARCO. Furthermore, they suggest that the domain may contain multiple ligand-binding sites.

Species-specific restriction of cell surface expression of mouse MARCO glycoprotein in murine cell lines

The MARCO (macrophage receptor with collagenous structure) glycoprotein belongs to the scavenger receptor type family of pattern-recognition molecules produced by a subset of marginal zone macrophages in the spleen. Stimulation with LPS leads to its appearance on macrophages located at other tissue compartments. In the present work, we report its in vitro expression by various cell lines using transient and stable (lentiviral) gene delivery aimed at investigating the signaling properties of this receptor and its analysis using a novel rat monoclonal antibody against the SRCR-domain of mouse MARCO. When trying to establish stable mouse MARCO-transfectants using lentiviral transduction and other methods, we consistently found that MARCO accumulated intracellularly in various murine host cells. In contrast, such a phenomenon was not observed in non-murine cell lines. Our observations indicate the presence of an unexpected limitation of the in vitro expression of mouse MARCO glycoprotein in murine cell lines. We believe that the failure to express MARCO on the cell surface of the many murine cell lines is likely due to the absence of endoplasmic reticulum molecular chaperones needed for the correct folding and assembly of the trimeric MARCO molecule.

Induction of macrophage scavenger receptor MARCO in nonalcoholic steatohepatitis indicates possible involvement of endotoxin in its pathogenic process

Nonalcoholic steatohepatitis (NASH) is one of the life-threatening hepatic diseases; however, its pathogenesis is still unknown. To evaluate the causative role of hyperlipidaemia and high-fat diet, we compared C57BL/6 mice with inherited hyperlipidaemic model mice (LDLR(-/-)mice and ApoE(-/-) mice) fed a normal or a high-fat diet. LDLR(-/-) and ApoE(-/-) mice fed the normal diet showed significantly higher serum cholesterol level than that of C57BL/6 mice fed the high-fat diet. These mice, however, have shown neither significant elevation of serum alanine transaminase (ALT) level nor histopathologic features of steatohepatitis. High-fat diet groups of all three strains showed histopathological characteristics of steatohepatitis with elevated serum ALT levels and high expression of macrophage scavenger receptor MARCO mRNA in the liver. Semiquantitative endotoxin analysis showed an elevated serum endotoxin level in the portal vein but not in the vena cava in ApoE(-/-) mice fed the high-fat diet. These results indicate that long-term feeding of a high-fat diet induces NASH, whereas hyperlipidaemia alone is not enough to induce NASH. Liver-restricted induction of MARCO in mice with high-fat diet and portal endotoxaemia in ApoE(-/-) mice fed the high-fat diet suggest the possible involvement of endotoxin in the pathogenesis of NASH.

The scavenger receptor MARCO is required for lung defense against pneumococcal pneumonia and inhaled particles

Alveolar macrophages (AMs) express the class A scavenger receptor macrophage receptor with collagenous structure (MARCO), but its role in vivo in lung defense against bacteria and environmental particles has not been studied. We used MARCO-deficient mice to directly test the in vivo role of AM MARCO in innate defense against pneumococcal infection and environmental particles. In a murine model of pneumococcal pneumonia, MARCO^−/− mice displayed an impaired ability to clear bacteria from the lungs, increased pulmonary inflammation and cytokine release, and diminished survival. In vitro binding of Streptococcus pneumoniae and in vivo uptake of unopsonized particles by MARCO^−/− AMs were dramatically impaired. MARCO^−/− mice treated with the “inert” environmental particle TiO₂ showed enhanced inflammation and chemokine expression, indicating that MARCO-mediated clearance of inert particles by AMs prevents inflammatory responses otherwise initiated by other lung cells. Our findings point to an important role of MARCO in mounting an efficient and appropriately regulated innate immune response against inhaled particles and airborne pathogens.

The distribution of antigen in lymphoid tissues following its injection into the anterior chamber of the rat eye

Injection of Ag into the anterior chamber (AC) of the eye induces deviant immune responses. It has been proposed that Ag internalized by ocular APCs is presented in a tolerogenic fashion in the spleen. However, the nature and distribution of the Ag-bearing cells in the lymphoid organs remain unclear. Fluorescent-labeled Ag (dextran, BSA) injected into the AC of Lewis rats was detected in the subcapsular sinus of the right submandibular lymph nodes (LNs) and cervical LNs, the marginal zone of the spleen, and the medulla of the mesenteric LNs. In the spleen, Ag-bearing cells were CD1(+), CD11b(+), ED1(+), ED2(low), ED3(+), CD86(low), OX6(+), CD11c(-), ED5(-) and in the LNs were CD4(+), CD8(+), CD80(+), and OX41(+) suggesting these were lymphoid organ resident macrophages. These Ag-bearing macrophages were located adjacent to CD4(+) cells, CD8(+) cells, and NK cells in the LNs and spleen and to marginal zone B cells in the spleen. No interaction with gamma delta T cells was observed. The data demonstrates that Ag derived from the AC of the eye is mainly internalized by resident macrophages in the LNs and spleen which are ideally placed to interact with cells involved in the induction of deviant ocular immune responses. The extensive distribution of Ag in LNs draining the upper airway and gastrointestinal tracts, together with the phenotype of Ag-bearing cells in the lymphoid organs, suggests that Ag leaves the eye predominantly in a soluble form and implies other mechanisms of tolerance may contribute to ocular-specific immune responses.

Macrophages of the splenic marginal zone are essential for trapping of blood-borne particulate antigen but dispensable for induction of specific T cell responses

Rapid removal of pathogens from the circulation by secondary lymphoid organs is prerequisite for successful control of infection. Blood-borne Ags are trapped mainly in the splenic marginal zone. To identify the cell populations responsible for Ag trapping in the marginal zone, mice were selectively depleted of marginal zone macrophages and marginal metallophilic macrophages. In the absence of these cells, trapping of microspheres and Listeria monocytogenes organisms was lost, and early control of infection was impaired. Depletion of marginal zone macrophages and marginal metallophilic macrophages, however, did not limit Ag presentation because Listeria-specific protective T cell immunity was induced. Therefore, marginal zone macrophages and marginal metallophilic macrophages are crucial for trapping of particulate Ag but dispensable for Ag presentation.

Macrophages control the retention and trafficking of B lymphocytes in the splenic marginal zone

The marginal zone of the spleen is a precisely ordered region that contains specialized subsets of B lymphocytes and macrophages. Disruption of the negative signaling inositol phosphatase, SH2-containing inositol-5-phosphatase 1 (SHIP), results in the loss of marginal zone B cells (MZBs) with reorganization of marginal zone macrophages (MZMOs) to the red pulp of the spleen. This primary macrophage defect, as revealed by selectively depleting SHIP in myeloid cells shows that MZMOs are specifically required for the retention of MZBs. The MZMO phenotype was reverted in SHIP/Bruton's tyrosine kinase (Btk) double knockout mice, thus identifying the Btk activating pathway as an essential component being regulated by SHIP. Furthermore, we identified a direct interaction between the MARCO scavenger receptor on MZMOs and MZBs. Activation or disruption of this interaction results in MZB migration to the follicle. The migration of the MZMOs was further studied after the response to Staphylococcus aureus, which induced MZMOs to move into the red pulp while MZBs migrated into the follicular zone. The marginal zone is therefore a dynamic structure in which retention and trafficking of B cells requires specific macrophage-B cell interactions.

Characterization of recombinant soluble macrophage scavenger receptor MARCO

MARCO is a type II transmembrane protein of the class A scavenger receptor family. It has a short N-terminal cytoplasmic domain, a transmembrane domain, and a large extracellular part composed of a 75-residue long spacer domain, a 270-residue collagenous domain, and a 99-residue long scavenger receptor cysteine-rich (SRCR) domain. Previous studies have indicated a role for this receptor in anti-microbial host defense functions. In this work we have produced the extracellular part of MARCO as a recombinant protein, and analyzed its binding properties. The production of this protein, soluble MARCO (sMARCO), has made it possible for the first time to study MARCO and its binding properties in a cell-free system. Using circular dichroism analyses, a protease-sensitive assay, and rotary shadowing electron microscopy, sMARCO was shown to have a triple-helical collagenous structure. Rotary shadowing also demonstrated that the molecules often associate with each other via the globes. sMARCO was found to bind avidly both heat-killed and living bacteria. Lipopolysaccharide, an important component of the outer membrane of Gram-negative bacteria, was shown to be a ligand of MARCO. Studies with different bacterial strains indicated that the O-side chain of lipopolysaccharide is not needed for the bacterial recognition. Finally, the C-terminal SRCR domain was also produced as a recombinant protein, and its bacteria-binding capability was studied. Although the transfection experiments with transmembrane MARCO variants have indicated a crucial role for this domain in bacterial binding, the monomeric domain exhibited low, barely detectable bacteria-binding activity. Thus, it is possible that cooperation between the SRCR domain and the collagenous domain is needed for high-affinity bacterial binding, or that the SRCR domain has to be in a trimeric form to effectively bind to bacteria.

Regulation of antibacterial and antifungal innate immunity in fruitflies and humans

Insects have been very successful in adapting to their environment, and the ability of the insect immune system to detect and elicit the appropriate response against various invading pathogens has helped in this success. Unlike the vertebrate immune system, which consists of both innate and adaptive components, insect immunity probably consists entirely of an innate immune response, as no evidence of an adaptive response has been found. The innate immune response is described as either a reaction against "lack of self," or the interaction between host germline-encoded receptors and molecules unique to a particular class of invading organisms. Once the invading organism is recognized, the host immune response can be activated via signaling pathways that lead to the appropriate reaction. This review endeavors to put forth how through genetic, molecular, and biochemical studies of the fruit fly Drosophila melanogaster, as well as other insects, it is now understood that aspects of the insect and vertebrate innate immune system are very similar.

Arginine residues in domain V have a central role for bacteria-binding activity of macrophage scavenger receptor MARCO

MARCO is a bacteria-binding macrophage-specific scavenger receptor that plays a role in innate immune response. MARCO has short intracellular and transmembrane domains, as well as a large extracellular domain composed of a spacer domain, a long collagenous domain, and a C-terminal scavenger receptor cysteine-rich domain (SRCR), domain V. As yet, no specific function has been assigned to the SRCR domain of scavenger receptors. In the present study, we generated several human and mouse MARCO variants with deletions or single amino acid substitutions and localized the primary bacteria-binding region to domain V. Furthermore, analysis of the MARCO variants containing only portions of domain V demonstrated a crucial role for an arginine-rich segment for this function. More precisely, the motif RXR was identified as an essential element for high-affinity bacterial binding. The results indicate that the binding properties of MARCO differ from those of the other class A scavenger receptors, SR-A and SRCL, whose ligand-binding function has been localized to the collagenous domain.

Aberrant macrophage and neutrophil population dynamics and impaired Th1 response to Listeria monocytogenes in colony-stimulating factor 1-deficient mice

Listeria monocytogenes, a facultative intracellular bacterium, has been used extensively to study innate immune responses. Macrophages act as hosts for this bacterium as well as a major defense against it. Using mice homozygous for a null mutation (Csf1(op)) in the gene for the mononuclear phagocytic growth factor colony-stimulating factor 1 (CSF-1), we have demonstrated that CSF-1-regulated macrophages were essential to defend against a listerial infection. In the absence of CSF-1, monocytes were not recruited to the sites of infection due to the lack of synthesis of the macrophage chemoattractant chemokine MCP-1. In addition, there was no burst of interleukin-10 (IL-10) synthesis that has been shown to result in the egress of neutrophils from sites of infection. Consequently, neutrophils were not replaced by macrophages, and numerous neutrophil-filled microabscesses developed, followed by tissue destruction and death of the mice. In the CSF-1 nullizygous mice compared to wild-type mice, there was also a very low synthesis of gamma interferon (IFN-gamma), resulting in reduced macrophage activation. However, the concentrations of the IFN-gamma-inducing cytokines IL-12 and IL-18 at this bacterial load were similar in these mutant mice. In contrast, IL-6 concentrations were dramatically reduced. Administration of IL-6 to Csf1(op)/Csf1(op) mice significantly increased the synthesis of IFN-gamma and reduced the bacterial burden to a greater extent than treatment with IFN-gamma alone. These data indicate that IL-6 occupies a central role in the CSF-1-regulated macrophage response to L. monocytogenes.

Expression profiling in knockout mice: lymphotoxin versus tumor necrosis factor in the maintenance of splenic microarchitecture

Expression profiling provides a powerful approach to define the underlying molecular mechanisms in disease. Several techniques referred collectively to as gene profiling may be also helpful in the analysis of the phenotype of mice with targeted mutations, especially if applied to distinct histological compartments, to specific cell types or to evaluate the effect of specific challenges, such as infection. Here we review several of the existing techniques applicable to genetic knockout studies, and share our experience from the study of mice with tumor necrosis factor (TNF) and lymphotoxin (LT) deficiencies, with specific emphasis on the distinction between TNF- and LT-mediated signalling pathways in vivo. Gene expression profiling analysis of TNF/LT-deficient mice supports the notion that TNF and LT, originally discovered as distinct biological activities, manifest both distinct and redundant functions in vivo.

Role of macrophage scavenger receptor in endotoxin shock

Lipopolysaccharide (LPS) is known to bind to several receptors on macrophages, including CD14 and macrophage scavenger receptor class A types I and II (MSR-A), and stimulates macrophages to release various inflammatory mediators. MSR-A recognizes a broad range of polyanionic ligands such as chemically modified lipoproteins, LPS of Gram-negative bacteria, and lipoteichoic acid of Gram-positive bacteria, suggesting a role in host defence. In this study, mice lacking MSR-A were used to elucidate the role of MSR-A in endotoxin shock. Peritoneal macrophages from MSR-A-deficient (MSR-A(-/-)) mice bound less remarkably to LPS than those from wild-type (MSR-A(+/+)) mice and the binding activity of MSR-A(+/+) macrophages to LPS was reduced by the addition of an anti-MSR-A antibody. Clearance of LPS in serum was retarded in MSR-A(-/-) mice after intraperitoneal administration of LPS. LPS-induced expression of cytokines in the liver was similar in MSR-A(+/+) and MSR-A(-/-) mice, but levels of interleukin (IL)-1beta expression and serum IL-1beta were lower in MSR-A(-/-) mice. Administration of large doses of LPS resulted in a higher mortality of MSR-A(+/+) mice and pretreatment with an IL-1 receptor antagonist reduced the mortality. Thus, MSR-A-mediated macrophage activation plays a negative role in protecting mice from endotoxin shock by enhancing IL-1beta production by macrophages.

Molecular characterization of a human scavenger receptor, human MARCO

Murine MARCO has been identified recently in subsets of macrophages located in the peritoneum, marginal zone of the spleen, and the medullary cord of lymph nodes, where it has been proposed that it serves as a bacteria-binding receptor. A scavenger receptor family member with an extended collagenous domain, murine MARCO has also been demonstrated in atherosclerotic lesions of susceptible mice. We report here the identification, tissue and chromosomal localization, and pharmacological characterization of human (h)MARCO. hMARCO was identified from a macrophage cDNA library by electronic screening with the murine MARCO sequence. Nucleotide sequence analysis confirmed that the full-length hMARCO clone encoded a 519-amino acid protein sharing 68.5% identity with murine MARCO. RNA blot analysis indicated that the hMARCO transcript is 2.0 kb in length and is predominantly expressed in human lung, liver, and lymph nodes. Radiation hybrid mapping localized hMARCO to chromosome 2q14. Ligand-binding studies of COS cells expressing hMARCO demonstrated significant specific binding of both Escherichia coli and Staphylococcus aureus. In contrast, the hMARCO receptor expressed in COS cells did not specifically bind the scavenger receptor ligand acetylated low-density lipoprotein (LDL), despite its similarity to the elongated collagen-like binding domain of the macrophage scavenger receptor. In addition, acetylated (Ac)LDL and oxidized (Ox)LDL did not inhibit E. coli binding to hMARCO. These data suggest that hMARCO may play an important role in host defense, but it has no obvious role in the accumulation of modified lipoproteins during atherogenesis.