Expression of the class A macrophage scavenger receptor on specific subpopulations of murine dendritic cells limits their endotoxin response

Plasmodium Infection-Cure Cycles Increase the Capacity of Phagocytosis in Conventional Dendritic Cells

Malaria stands as one of the most pervasive human infectious diseases globally and represents a prominent cause of mortality. Immunity against clinical malaria disease is achieved through multiple infection and treatment cycles, culminating in a substantial reduction in parasite burden. To investigate this phenomenon, we established a murine model involving repeated infection-cure cycles, whereby mice were infected with the lethal rodent malarial parasite Plasmodium berghei ANKA and subsequently treated with the anti-malarial drug pyrimethamine. Our earlier study revealed a significant decrease in the capacity of conventional dendritic cells (cDCs) to produce cytokines upon stimulation in infection-cured mice. In the present study, we aimed to further elucidate the modulation of cDC functionality during repeated infection-cure cycles by examining their phagocytic capacity. Administering fluorescent beads to mice resulted in no significant difference in the total number of bead-positive cells within the spleens of both uninfected and 3-cure (three cycles of infection-cure) mice. However, the proportion of the CD11c+F4/80- population within bead-positive cells was notably higher in 3-cure mice compared to uninfected mice. Subsequent in vitro analysis of bead phagocytosis by purified CD11c+cDCs revealed that the cDC2 subset from 3-cure mice exhibited significantly enhanced phagocytic capacity in comparison to their uninfected counterparts. These findings underscore the substantial impact of repeated infection-cure cycles on various facets of cDC function, potentially influencing the trajectory of immune responses against subsequent malaria infections.

Poria cocos polysaccharide-functionalized graphene oxide nanosheet induces efficient cancer immunotherapy in mice

Introduction: Tumor vaccines that induce robust humoral and cellular immune responses have attracted tremendous interest for cancer immunotherapy. Despite the tremendous potential of tumor vaccines as an effective approach for cancer treatment and prevention, a major challenge in achieving sustained antitumor immunity is inefficient antigen delivery to secondary lymphoid organs, even with adjuvant aid. Methods: Herein, we present antigen/adjuvant integrated nanocomplexes termed nsGO/PCP/OVA by employing graphene oxide nanosheet (nsGO) as antigen nanocarriers loaded with model antigen ovalbumin (OVA) and adjuvant, Poria cocos polysaccharides (PCP). We evaluated the efficacy of nsGO/PCP/OVA in activating antigen-specific humoral as well as cellular immune responses and consequent tumor prevention and rejection in vivo. Results: The optimally formed nsGO/PCP/OVA was approximately 120-150 nm in diameter with a uniform size distribution. Nanoparticles can be effectively engulfed by dendritic cells (DCs) through receptor-mediated endocytosis, induced the maturation of DCs and improved the delivery efficiency both in vitro and in vivo. The nsGO/PCP/OVA nanoparticles also induced a significant enhancement of OVA antigen-specific Th1 and Th2 immune responses in vivo. In addition, vaccination with nsGO/PCP/OVA not only significantly suppressed tumor growth in prophylactic treatments, but also achieved a therapeutic effect in inhibiting the growth of already-established tumors. Conclusion: Therefore, this potent nanovaccine platform with nanocarrier nsGO and PCP as adjuvants provides a promising strategy for boosting anti-tumor immunity for cancer immunotherapy.

Metabolic regulation of dendritic cell activation and immune function during inflammation

Dendritic cells (DCs) are antigen-presenting cells that bridge innate and adaptive immune responses. Multiple cell types, including DCs, rely on cellular metabolism to determine their fate. DCs substantially alter cellular metabolic pathways during activation, such as oxidative phosphorylation, glycolysis, fatty acid and amino acid metabolism, which have crucial implications for their functionality. In this review, we summarize and discuss recent progress in DC metabolic studies, focusing on how metabolic reprogramming influences DC activation and functionality and the potential metabolic differences among DC subsets. Improving the understanding of the relationship between DC biology and metabolic regulation may provide promising therapeutic targets for immune-mediated inflammatory diseases.

Characterization of microglia/macrophage phenotypes in the spinal cord following intervertebral disc herniation

Dogs frequently suffer from traumatic spinal cord injury (SCI). Most cases of SCI have a favorable prognosis but 40-50% of dogs with paraplegia and absence of nociception do not regain ambulatory abilities, eventually leading to euthanasia. Microglia and infiltrating macrophages play a crucial role in inflammatory process after SCI. However, little is known about microglia/macrophage phenotypes representing a potential target for future therapeutic strategies. In the present study, the microglia/macrophage phenotype was characterized by immunohistochemistry in the morphologically unaltered canine spinal cord (10 control dogs) and during acute and subacute SCI (1-4 and 5-10 days post injury, 9 and 8 dogs, respectively) using antibodies directed against IBA1, MAC387, MHC-II, lysozyme, EGR2, myeloperoxidase, CD18, CD204 and lectin from Griffonia simplicifolia (BS-1). The expression of these markers was also analyzed in the spleen as reference for the phenotype of histiocytic cells. Histological lesions were absent in controls. In acute SCI, 4 dogs showed mild to moderate hemorrhages, 2 dogs bilateral gray matter necrosis and 6 dogs mild multifocal axonal swellings and myelin sheath dilation. One dog with acute SCI did not show histological alterations except for few dilated myelin sheaths. In subacute SCI, variable numbers of gitter cells, axonal changes and dilated myelin sheaths were present in all dogs and large areas of tissue necrosis in 2 dogs. Neuronal chromatolysis was found in 3 dogs with acute and subacute SCI, respectively. In control dogs, microglia/macrophage constitutively expressed IBA1 and rarely other markers. In acute SCI, a similar marker expression was found except for an increase in MAC387-positive cells in the spinal cord white matter due to an infiltration of few blood-borne macrophages. In subacute SCI, increased numbers of microglia/macrophages expressed CD18, CD204 and MHC-II in the gray matter SCI indicating enhanced antigen recognition, processing and presentation as well as cell migration and phagocytosis during this stage. Interestingly, only CD204-positive cells were upregulated in the white matter, which might be related to gray-white matter heterogeneity of microglia as previously described in humans. The present findings contribute to the understanding of the immunological processes during SCI in a large animal model for human SCI.

Breaking Entry-and Species Barriers: LentiBOOST® Plus Polybrene Enhances Transduction Efficacy of Dendritic Cells and Monocytes by Adenovirus 5

Due to their ability to trigger strong immune responses, adenoviruses (HAdVs) in general and the serotype5 (HAdV-5) in particular are amongst the most popular viral vectors in research and clinical application. However, efficient transduction using HAdV-5 is predominantly achieved in coxsackie and adenovirus receptor (CAR)-positive cells. In the present study, we used the transduction enhancer LentiBOOST^® comprising the polycationic Polybrene to overcome these limitations. Using LentiBOOST^®/Polybrene, we yielded transduction rates higher than 50% in murine bone marrow-derived dendritic cells (BMDCs), while maintaining their cytokine expression profile and their capability to induce T-cell proliferation. In human dendritic cells (DCs), we increased the transduction rate from 22% in immature (i)DCs or 43% in mature (m)DCs to more than 80%, without inducing cytotoxicity. While expression of specific maturation markers was slightly upregulated using LentiBOOST^®/Polybrene on iDCs, no effect on mDC phenotype or function was observed. Moreover, we achieved efficient HAdV5 transduction also in human monocytes and were able to subsequently differentiate them into proper iDCs and functional mDCs. In summary, we introduce LentiBOOST^® comprising Polybrene as a highly potent adenoviral transduction agent for new in-vitro applications in a set of different immune cells in both mice and humans.

Scavenger Receptor A1 Signaling Pathways Affecting Macrophage Functions in Innate and Adaptive Immunity

First discovered on macrophages by Goldstein and Brown in 1979, Scavenger Receptors have since been shown to participate in a diverse number of cell functions; equally diverse are their structures and the ligands they bind. Macrophage activation is crucial in the outcome of an immune response. SR-A1 is highly abundant on macrophages and recognizes both host- and microorganism-derived molecules that impact processes that are initiated, perpetuated, or modified. This review summarizes the involvement of SR-A1 in both inflammatory and anti-inflammatory responses, the multiple-ligand internalization mechanisms and the diversity of signaling pathways that impact macrophage function and activation. Engagement of SR-A1 results in the stimulation of differential signaling pathways and patterns of cytokine expression, kinetics, magnitude of response and activation status. SR-A1 plays essential roles in phagocytosis and efferocytosis, interacting with other receptors and promoting tolerance in response to apoptotic cell uptake. In cell adhesion, tissue remodeling, and cell migration, SR-A1 signals through different pathways engaging different cytoplasmic motifs. We describe the role of SR-A1 during innate and adaptive immune responses, such as participation in macrophage polarization and interaction with other innate receptors, as well as in antigen uptake, processing, and presentation, regulating T and B cell activation. The dichotomous contribution of SR-A1 on macrophage functions is discussed. A better understanding of the role SR-A1 plays through molecular mechanisms and crosstalk with other receptors may provide insights into developing novel therapeutic strategies to modulate immune responses and immunopathologies.

Remodeling the Tumor Myeloid Landscape to Enhance Antitumor Antibody Immunotherapies

Among the diverse tumor resident immune cell types, tumor-associated macrophages (TAMs) are often the most abundant, possess an anti-inflammatory phenotype, orchestrate tumor immune evasion and are frequently associated with poor prognosis. However, TAMs can also be harnessed to destroy antibody-opsonized tumor cells through the process of antibody-dependent cellular phagocytosis (ADCP). Clinically important tumor-targeting monoclonal antibodies (mAb) such as Rituximab, Herceptin and Cetuximab, function, at least in part, by inducing macrophages to eliminate tumor cells via ADCP. For IgG mAb, this is mediated by antibody-binding activating Fc gamma receptors (FcγR), with resultant phagocytic activity impacted by the level of co-engagement with the single inhibitory FcγRIIb. Approaches to enhance ADCP in the tumor microenvironment include the repolarization of TAMs to proinflammatory phenotypes or the direct augmentation of ADCP by targeting so-called 'phagocytosis checkpoints'. Here we review the most promising new strategies targeting the cell surface molecules present on TAMs, which include the inhibition of 'don't eat me signals' or targeting immunostimulatory pathways with agonistic mAb and small molecules to augment tumor-targeting mAb immunotherapies and overcome therapeutic resistance.

Sirt6 deletion in bone marrow-derived cells increases atherosclerosis - Central role of macrophage scavenger receptor 1

AIMS

Sirtuin 6 (Sirt6) is a NAD⁺-dependent deacetylase that plays a key role in DNA repair, inflammation and lipid regulation. Sirt6-null mice show severe metabolic defects and accelerated aging. Macrophage-foam cell formation via scavenger receptors is a key step in atherogenesis. We determined the effects of bone marrow-restricted Sirt6 deletion on foam cell formation and atherogenesis using a mouse model.

METHODS AND RESULTS

Sirt6 deletion in bone marrow-derived cells increased aortic plaques, lipid content and macrophage numbers in recipient Apoe^-/- mice fed a high-cholesterol diet for 12 weeks (n = 12-14, p < .001). In RAW macrophages, Sirt6 overexpression reduced oxidized low-density lipoprotein (oxLDL) uptake, Sirt6 knockdown enhanced it and increased mRNA and protein levels of macrophage scavenger receptor 1 (Msr1), whereas levels of other oxLDL uptake and efflux transporters remained unchanged. Similarly, in human primary macrophages, Sirt6 knockdown increased MSR1 protein levels and oxLDL uptake. Double knockdown of Sirt6 and Msr1 abolished the increase in oxLDL uptake observed upon Sirt6 single knockdown. FACS analyses of macrophages from aortic plaques of Sirt6-deficient bone marrow-transplanted mice showed increased MSR1 protein expression. Double knockdown of Sirt6 and the transcription factor c-Myc in RAW cells abolished the increase in Msr1 mRNA and protein levels; c-Myc overexpression increased Msr1 mRNA and protein levels.

CONCLUSIONS

Loss of Sirt6 in bone marrow-derived cells is proatherogenic; hereby macrophages play an important role given a c-Myc-dependent increase in MSR1 protein expression and an enhanced oxLDL uptake in human and murine macrophages. These findings assign endogenous SIRT6 in macrophages an important atheroprotective role.

Characterization of respiratory dendritic cells from equine lung tissues

BACKGROUND

Dendritic cells (DCs) are professional antigen-presenting cells that have multiple subpopulations with different phenotypes and immune functions. Previous research demonstrated that DCs have strong potential for anti-viral defense in the host. However, viruses including alphaherpesvirinae have developed strategies to interfere with the function or maturation of DCs, causing immune dysfunction and avoidance of pathogen elimination. The goal of the present study was to isolate and characterize equine lung-derived DCs (L-DCs) for use in studies of respiratory viruses and compare their features with equine blood-derived DCs (B-DCs), which are currently used for these types of studies.

RESULTS

We found that L-DCs were morphologically similar to B-DCs. Overall, B-DCs demonstrated higher expression of CD86 and CD172α than L-DCs, but both cell types expressed high levels of MHC class II and CD44, as well as moderate amounts of CD163, CD204, and Bla36. In contrast, the endocytic activity of L-DCs was elevated compared to that of B-DCs. Finally, mononuclear cells isolated from lung (L-MCs), which are used as precursors for L-DCs, expressed more antigen-presenting cell-associated markers such as MHC class II and CD172α compared to their counterparts from blood.

CONCLUSIONS

Our results indicate that L-DCs may be in an earlier differentiation stage compared to B-DCs. Concurrent with this observation, L-MCs possessed significantly more antigen-uptake capacity compared to their counterparts from blood. It is likely that L-DCs play an important role in antigen uptake and processing of respiratory pathogens and are major contributors to respiratory tract immunity and may be ideal tools for future in vitro or ex vivo studies.

A Consensus Definitive Classification of Scavenger Receptors and Their Roles in Health and Disease

Scavenger receptors constitute a large family of proteins that are structurally diverse and participate in a wide range of biological functions. These receptors are expressed predominantly by myeloid cells and recognize a diverse variety of ligands including endogenous and modified host-derived molecules and microbial pathogens. There are currently eight classes of scavenger receptors, many of which have multiple names, leading to inconsistencies and confusion in the literature. To address this problem, a workshop was organized by the United States National Institute of Allergy and Infectious Diseases, National Institutes of Health, to help develop a clear definition of scavenger receptors and a standardized nomenclature based on that definition. Fifteen experts in the scavenger receptor field attended the workshop and, after extensive discussion, reached a consensus regarding the definition of scavenger receptors and a proposed scavenger receptor nomenclature. Scavenger receptors were defined as cell surface receptors that typically bind multiple ligands and promote the removal of nonself or altered-self targets. They often function by mechanisms that include endocytosis, phagocytosis, adhesion, and signaling that ultimately lead to the elimination of degraded or harmful substances. Based on this definition, nomenclature and classification of these receptors into 10 classes were proposed. This classification was discussed at three national meetings and input from participants at these meetings was requested. The following manuscript is a consensus statement that combines the recommendations of the initial workshop and incorporates the input received from the participants at the three national meetings.

CD163+CD204+ tumor-associated macrophages contribute to T cell regulation via interleukin-10 and PD-L1 production in oral squamous cell carcinoma

Tumor-associated macrophages (TAMs) promote cancer cell proliferation, invasion, and metastasis by producing various mediators. Although preclinical studies demonstrated that TAMs preferentially express CD163 and CD204, the TAM subsets in oral squamous cell carcinoma (OSCC) remain unknown. In this study, we examined the expression and role of TAM subsets in OSCC. Forty-six patients with OSCC were analyzed for expression of TAMs in biopsy samples by immunohistochemistry. We examined TAM subsets and their production of immune suppressive molecules (IL-10 and PD-L1) in peripheral blood mononuclear cells from three OSCC patients by flow cytometry. CD163 was detected around the tumor or connective tissue, while CD204 was detected in/around the tumors. Flow cytometric analysis revealed that CD163+CD204+ TAMs strongly produced IL-10 and PD-L1 in comparison with CD163+CD204- and CD163-CD204+ TAMs. Furthermore, the number of activated CD3+ T cells after co-culture with CD163+CD204+ TAMs was significantly lower than that after co-culture with other TAM subsets. In clinical findings, the number of CD163+CD204+ TAMs was negatively correlated with that of CD25+ cells and 5-year progression-free survival. These results suggest that CD163+CD204+ TAMs possibly play a key role in the invasion and metastasis of OSCC by T-cell regulation via IL-10 and PD-L1 production.

Ultradeformable Archaeosomes for Needle Free Nanovaccination with Leishmania braziliensis Antigens

Total antigens from Leishmania braziliensis promastigotes, solubilized with sodium cholate (dsLp), were formulated within ultradeformable nanovesicles (dsLp-ultradeformable archaeosomes, (dsLp-UDA), and dsLp-ultradeformable liposomes (dsLp-UDL)) and topically administered to Balb/c mice. Ultradeformable nanovesicles can penetrate the intact stratum corneum up to the viable epidermis, with no aid of classical permeation enhancers that can damage the barrier function of the skin. Briefly, 100 nm unilamellar dsLp-UDA (soybean phosphatidylcholine: Halorubrum tebenquichense total polar lipids (TPL): sodium cholate, 3:3:1 w:w) of -31.45 mV Z potential, containing 4.84 ± 0.53% w/w protein/lipid dsLp, 235 KPa Young modulus were prepared. In vitro, dsLp-UDA was extensively taken up by J774A1 and bone marrow derive cells, and the only that induced an immediate secretion of IL-6, IL-12p40 and TNF-α, followed by IL-1β, by J774A1 cells. Such extensive uptake is a key feature of UDA ascribed to the highly negatively charged archaeolipids of the TPL, which are recognized by a receptor specialized in uptake and not involved in downstream signaling. Despite dsLp alone was also immunostimulatory on J774A1 cells, applied twice a week on consecutive days along 7 weeks on Balb/c mice, it raised no measurable response unless associated to UDL or UDA. The highest systemic response, IgGa2 mediated, 1 log lower than im dsLp Al₂O₃, was elicited by dsLp-UDA. Such findings suggest that in vivo, UDL and UDA acted as penetration enhancers for dsLp, but only dsLp-UDA, owed to its pronounced uptake by APC, succeeded as topical adjuvants. The actual TPL composition, fully made of sn2,3 ether linked saturated archaeolipids, gives the UDA bilayer resistance against chemical, physical and enzymatic attacks that destroy ordinary phospholipids bilayers. Together, these properties make UDA a promising platform for topical drug targeted delivery and vaccination, that may be of help for countries with a deficient healthcare system.

Scavenger Receptors: Emerging Roles in Cancer Biology and Immunology

Scavenger receptors constitute a large family of evolutionally conserved protein molecules that are structurally and functionally diverse. Although scavenger receptors were originally identified based on their capacity to scavenge modified lipoproteins, these molecules have been shown to recognize and bind to a broad spectrum of ligands, including modified and unmodified host-derived molecules or microbial components. As a major subset of innate pattern recognition receptors, scavenger receptors are mainly expressed on myeloid cells and function in a wide range of biological processes, such as endocytosis, adhesion, lipid transport, antigen presentation, and pathogen clearance. In addition to playing a crucial role in maintenance of host homeostasis, scavenger receptors have been implicated in the pathogenesis of a number of diseases, e.g., atherosclerosis, neurodegeneration, or metabolic disorders. Emerging evidence has begun to reveal these receptor molecules as important regulators of tumor behavior and host immune responses to cancer. This review summarizes our current understanding on the newly identified, distinct functions of scavenger receptors in cancer biology and immunology. The potential of scavenger receptors as diagnostic biomarkers and novel targets for therapeutic interventions to treat malignancies is also highlighted.

Oxidation by neutrophils-derived HOCl increases immunogenicity of proteins by converting them into ligands of several endocytic receptors involved in antigen uptake by dendritic cells and macrophages

The initiation of adaptive immune responses to protein antigens has to be preceded by their uptake by antigen presenting cells and intracellular proteolytic processing. Paradoxically, endocytic receptors involved in antigen uptake do not bind the majority of proteins, which may be the main reason why purified proteins stimulate at most weak immune responses. A shared feature of different types of adjuvants, capable of boosting immunogenicity of protein vaccines, is their ability to induce acute inflammation, characterized by early influx of activated neutrophils. Neutrophils are also rapidly recruited to sites of tissue injury or infection. These cells are the source of potent oxidants, including hypochlorous acid (HOCl), causing oxidation of proteins present in inflammatory foci. We demonstrate that oxidation of proteins by endogenous, neutrophils-derived HOCl increases their immunogenicity. Upon oxidation, different, randomly chosen simple proteins (yeast alcohol dehydrogenase, human and bovine serum albumin) and glycoproteins (human apo-transferrin, ovalbumin) gain the ability to bind with high affinity to several endocytic receptors on antigen presenting cells, which seems to be the major mechanism of their increased immunogenicity. The mannose receptor (CD206), scavenger receptors A (CD204) and CD36 were responsible for the uptake and presentation of HOCl-modified proteins by murine dendritic cells and macrophages. Other scavenger receptors, SREC-I and LOX-1, as well as RAGE were also able to bind HOCl-modified proteins, but they did not contribute significantly to these ligands uptake by dendritic cells because they were either not expressed or exhibited preference for more heavily oxidised proteins. Our results indicate that oxidation by neutrophils-derived HOCl may be a physiological mechanism of conferring immunogenicity on proteins which in their native forms do not bind to endocytic receptors. This mechanism might enable the immune system to detect infections caused by pathogens not recognized by pattern recognition receptors.

The dual role of scavenger receptor class A in development of diabetes in autoimmune NOD mice

Human type 1 diabetes is an autoimmune disease that results from the autoreactive destruction of pancreatic β cells by T cells. Antigen presenting cells including dendritic cells and macrophages are required to activate and suppress antigen-specific T cells. It has been suggested that antigen uptake from live cells by dendritic cells via scavenger receptor class A (SR-A) may be important. However, the role of SR-A in autoimmune disease is unknown. In this study, SR-A-/- nonobese diabetic (NOD) mice showed significant attenuation of insulitis, lower levels of insulin autoantibodies, and suppression of diabetes development compared with NOD mice. We also found that diabetes progression in SR-A-/- NOD mice treated with low-dose polyinosinic-polycytidylic acid (poly(I:C)) was significantly accelerated compared with that in disease-resistant NOD mice treated with low-dose poly(I:C). In addition, injection of high-dose poly(I: C) to mimic an acute RNA virus infection significantly accelerated diabetes development in young SR-A-/- NOD mice compared with untreated SR-A-/- NOD mice. Pathogenic cells including CD4+CD25+ activated T cells were increased more in SR-A-/- NOD mice treated with poly(I:C) than in untreated SR-A-/- NOD mice. These results suggested that viral infection might accelerate diabetes development even in diabetes-resistant subjects. In conclusion, our studies demonstrated that diabetes progression was suppressed in SR-A-/- NOD mice and that acceleration of diabetes development could be induced in young mice by poly(I:C) treatment even in SR-A-/- NOD mice. These results suggest that SR-A on antigen presenting cells such as dendritic cells may play an unfavorable role in the steady state and a protective role in a mild infection. Our findings imply that SR-A may be an important target for improving therapeutic strategies for type 1 diabetes.

An investigation of scavenger receptor A mediated leukocyte binding to polyanionic and uncharged polymer hydrogels

Cell adhesion to biomaterials can be mediated in part by mechanisms aside from the traditionally recognized opsinization and integrin binding mechanisms. In this study, we investigated the role of scavenger receptor A (SR-A) in leukocyte binding to a series of well-controlled polyanionic and uncharged hydrogels based on a poly(N-isopropylacrylamide) backbone. The hydrogels were injected in the peritoneal cavity of SR-A knockout (KO) and wild-type mice using a minimally invasive procedure and allowed to set in situ. After 24 h, the hydrogels were recovered and analyzed, the peritoneal cavity was lavaged, and cytokine concentrations were assessed by ELISA. The polyanionic hydrogels retrieved from the KO animals were found to be completely devoid of adherent leukocytes, which were present in other materials regardless of the mouse strain in which they were injected. Results from a subsequent in vitro cellular adhesion study with a RAW264.7 cell line failed to yield a similarly definitive role for SR-A in the cellular binding of a polyanionic hydrogel. Taken together, the results of this study show that SR-A mediates leukocyte adhesion to a polyanionic hydrogel in the peritoneal cavity, but other adhesion mechanisms contribute to cellular binding in vitro.

Standardizing scavenger receptor nomenclature

Scavenger receptors constitute a large family of proteins that are structurally diverse and participate in a wide range of biological functions. These receptors are expressed predominantly by myeloid cells and recognize a variety of ligands, including endogenous and modified host-derived molecules and microbial pathogens. There are currently eight classes of scavenger receptors, many of which have multiple names, leading to inconsistencies and confusion in the literature. To address this problem, a workshop was organized by the U.S. National Institute of Allergy and Infectious Diseases, National Institutes of Health to help develop a clear definition of scavenger receptors and a standardized nomenclature based on that definition. Fifteen experts in the scavenger receptor field attended the workshop and, after extensive discussion, reached a consensus regarding the definition of scavenger receptors and a proposed scavenger receptor nomenclature. Scavenger receptors were defined as cell surface receptors that typically bind multiple ligands and promote the removal of non-self or altered-self targets. They often function by mechanisms that include endocytosis, phagocytosis, adhesion, and signaling that ultimately lead to the elimination of degraded or harmful substances. Based on this definition, nomenclature and classification of these receptors into 10 classes were proposed. The discussion and nomenclature recommendations described in this report only refer to mammalian scavenger receptors. The purpose of this article is to describe the proposed mammalian nomenclature and classification developed at the workshop and to solicit additional feedback from the broader research community.

Fucoidan as a marine anticancer agent in preclinical development

Fucoidan is a fucose-containing sulfated polysaccharide derived from brown seaweeds, crude extracts of which are commercially available as nutritional supplements. Recent studies have demonstrated antiproliferative, antiangiogenic, and anticancer properties of fucoidan in vitro. Accordingly, the anticancer effects of fucoidan have been shown to vary depending on its structure, while it can target multiple receptors or signaling molecules in various cell types, including tumor cells and immune cells. Low toxicity and the in vitro effects of fucoidan mentioned above make it a suitable agent for cancer prevention or treatment. However, preclinical development of natural marine products requires in vivo examination of purified compounds in animal tumor models. This review discusses the effects of systemic and local administration of fucoidan on tumor growth, angiogenesis, and immune reaction and whether in vivo and in vitro results are likely applicable to the development of fucoidan as a marine anticancer drug.

The class A scavenger receptor SR-A/CD204 and the class B scavenger receptor CD36 regulate immune functions of macrophages differently

SR-A/CD204 and CD36 are major receptors responsible for oxidized lipoproteins uptake by macrophages in atherosclerotic plaques. Both receptors also share the role as receptors for different pathogens, but studies on their signaling have been hampered by the lack of selective ligands. We report that, upon specific ligation by Ab, SR-A does not induce cytokine production, but mediates inhibition of LPS-stimulated production of IL-6 and IL-12/23p40, enhancement of IL-10 release, and has no effect on TNF-α and RANTES production in murine macrophages. In contrast, anti-CD36 Ab alone stimulated production of all these cytokines, with IL-10 production being exceptionally high. Effects of anti-CD36 Ab, except of IL-10 production, were mediated by CD14 and TLR2, whereas those of SR-A ligation by heterotrimeric Gi/o proteins and by phosphatidylinositol 3-kinases. Surprisingly, we found that LPS uptake by macrophages was mediated in part by CD36 cooperating with CD14, whereas SR-A was not involved in this process. Finely, during in vitro Ag presentation to naïve CD4(+) lymphocytes, pre-incubation of macrophages with anti-CD36 Ab enhanced IFN-γ production in the co-culture, but exerted the opposite effect under conditions enabling IL-10 accumulation. In contrast, anti-SR-A Ab was ineffective alone, but reversed the Th1-polarizing effect of LPS.

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.

An interplay between scavenger receptor A and CD14 during activation of J774 cells by high concentrations of LPS

Lipopolysaccharide (LPS) activates macrophages by binding to the TLR4/MD-2 complex and triggers two pro-inflammatory signaling pathways: one relies on MyD88 at the plasma membrane, and the other one depends on TRIF in endosomes. When present in high doses, LPS is internalized and undergoes detoxification. We found that the uptake of a high concentration of LPS (1000ng/ml) in macrophage-like J774 cells was upregulated upon inhibition of clathrin- and dynamin-mediated endocytosis which, on the other hand, strongly reduced the production of pro-inflammatory mediators TNF-α and RANTES. The binding and internalization of high amounts of LPS was mediated by scavenger receptor A (SR-A) with participation of CD14 without an engagement of TLR4. Occupation of SR-A by dextran sulfate or anti-SR-A antibodies enhanced LPS-induced production of TNF-α and RANTES by about 70%, with CD14 as a limiting factor. Dextran sulfate also elevated the cell surface levels of TLR4 and CD14, which could have contributed to the upregulation of the pro-inflammatory responses. Silencing of SR-A expression inhibited the LPS-triggered TNF-α production whereas RANTES release was unchanged. These data indicate that SR-A is required for maximal production of TNF-α in cells stimulated with LPS, possibly by modulating the cell surface levels of TLR4 and CD14.

The class A macrophage scavenger receptor CD204 is a useful immunohistochemical marker of canine histiocytic sarcoma

The immunohistochemical expression of the class A macrophage scavenger receptor CD204, was investigated in 50 canine histiocytic sarcomas (HSs) and compared with that of CD18, CD163, CD11d and class II molecules of the major histocompatibility complex (MHC). Expression of CD204 was also determined in 81 canine round cell tumours and pleomorphic sarcomas including T- and B-cell lymphomas, mast cell tumours, extramedullary plasmacytomas, cutaneous histiocytomas, transmissible venereal tumours, pigmented or amelanotic melanomas, poorly differentiated haemangiosarcomas and rhabdomyosarcomas. All of the 50 HSs expressed CD204, CD18 and MHC class II; 27 were positive for CD163 and seven expressed CD11d. All of the round cell tumours, except for one grade III mast cell tumour, were negative for CD204; however, they showed varying immunoreactivity patterns for CD18 and MHC class II. None of the pleomorphic sarcomas were immunoreactive for CD204. CD204 would appear to be a useful marker for canine HS.

Signaling in innate immunity and inflammation

Inflammation is triggered when innate immune cells detect infection or tissue injury. Surveillance mechanisms involve pattern recognition receptors (PRRs) on the cell surface and in the cytoplasm. Most PRRs respond to pathogen-associated molecular patterns (PAMPs) or host-derived damage-associated molecular patterns (DAMPs) by triggering activation of NF-κB, AP1, CREB, c/EBP, and IRF transcription factors. Induction of genes encoding enzymes, chemokines, cytokines, adhesion molecules, and regulators of the extracellular matrix promotes the recruitment and activation of leukocytes, which are critical for eliminating foreign particles and host debris. A subset of PRRs activates the protease caspase-1, which causes maturation of the cytokines IL1β and IL18. Cell adhesion molecules and chemokines facilitate leukocyte extravasation from the circulation to the affected site, the chemokines stimulating G-protein-coupled receptors (GPCRs). Binding initiates signals that regulate leukocyte motility and effector functions. Other triggers of inflammation include allergens, which form antibody complexes that stimulate Fc receptors on mast cells. Although the role of inflammation is to resolve infection and injury, increasing evidence indicates that chronic inflammation is a risk factor for cancer.

Suppression of antigen-specific CD4+ T cell activation by SRA/CD204 through reducing the immunostimulatory capability of antigen-presenting cell

Pattern recognition scavenger receptor SRA/CD204, primarily expressed on specialized antigen-presenting cells (APCs), including dendritic cells (DCs) and macrophages, has been implicated in multiple physiological and pathological processes, including atherosclerosis, Alzheimer's disease, endotoxic shock, host defense, and cancer development. SRA/CD204 was also recently shown to function as an attenuator of vaccine response and antitumor immunity. Here, we, for the first time, report that SRA/CD204 knockout (SRA(-/-)) mice developed a more robust CD4(+) T cell response than wild-type mice after ovalbumin immunization. Splenic DCs from the immunized SRA(-/-) mice were much more efficient than those from WT mice in stimulating naïve OT-II cells, indicating that the suppressive activity of SRA/CD204 is mediated by DCs. Strikingly, antigen-exposed SRA(-/-) DCs with or without lipopolysaccharide treatment exhibited increased T-cell-stimulating activity in vitro, which was independent of the classical endocytic property of the SRA/CD204. Additionally, absence of SRA/CD204 resulted in significantly elevated IL12p35 expression in DCs upon CD40 ligation plus interferon gamma (IFN-γ) stimulation. Molecular studies reveal that SRA/CD204 inhibited the activation of STAT1, mitogen activated protein kinase p38, and nuclear factor-kappa B signaling activation in DCs treated with anti-CD40 antibodies and IFN-γ. Furthermore, splenocytes from the generated SRA(-/-) OT-II mice showed heightened proliferation upon stimulation with OVA protein or MHC-II-restricted OVA(323-339) peptide compared with cells from the SRA(+/+) OT-II mice. These results not only establish a new role of SRA/CD204 in limiting the intrinsic immunogenicity of APCs and CD4(+) T cell activation but also provide additional insights into the molecular mechanisms involved in the immune suppression by this molecule.

Targeting the immunoregulator SRA/CD204 potentiates specific dendritic cell vaccine-induced T-cell response and antitumor immunity

Although dendritic cell (DC) vaccines offer promise as cancer immunotherapy, further improvements are needed to amplify their clinical therapeutic efficacy. The pattern recognition scavenger receptor SRA/CD204 attenuates the ability of DCs to activate CD8(+) T-cell responses. Therefore, we examined the impact of SRA/CD204 on antitumor responses generated by DC vaccines and we also evaluated the feasibility of enhancing DC vaccine potency by SRA/CD204 blockade. DCs from SRA/CD204-deficient mice were more immunogenic in generating antitumor responses to B16 melanoma, compared with DCs from wild-type mice. Similarly, siRNA-mediated knockdown of SRA/CD204 by lentiviral vectors improved the ability of wild-type DCs to stimulate the expansion and activation of CD8(+) T cells specific for idealized or established melanoma antigens in mice. Using SRA/CD204-silenced DCs to generate antigen-targeted vaccines, we documented a marked increase in the level of antitumor immunity achieved against established B16 tumors and metastases. This increase was associated with enhanced activation of antigen specific CTLs, greater tumor infiltration by CD8(+) T cells and NK cells, and increased intratumoral ratios of both CD4(+) and CD8(+) T-effector cells to CD4(+)CD25(+) T-regulatory cells. Our studies establish that downregulating SRA/CD204 strongly enhances DC-mediated antitumor immunity. In addition, they provide a rationale to enhance DC vaccine potency through SRA/CD204-targeting approaches that can improve clinical outcomes in cancer treatment.

CD204 suppresses large heat shock protein-facilitated priming of tumor antigen gp100-specific T cells and chaperone vaccine activity against mouse melanoma

We previously reported that scavenger receptor A (SRA/CD204), a binding structure on dendritic cells (DCs) for large stress/heat shock proteins (HSPs; e.g., hsp110 and grp170), attenuated an antitumor response elicited by large HSP-based vaccines. In this study, we show that SRA/CD204 interacts directly with exogenous hsp110, and lack of SRA/CD204 results in a reduction in the hsp110 binding and internalization by DCs. However, SRA(-/-) DCs pulsed with hsp110 or grp170-reconstituted gp100 chaperone complexes exhibit a profoundly increased capability of stimulating melanoma Ag gp100-specific naive T cells compared with wild-type (WT) DCs. Similar results were obtained when SRA/CD204 was silenced in DCs using short hairpin RNA-encoding lentiviruses. In addition, hsp110-stimulated SRA(-/-) DCs produced more inflammatory cytokines associated with increased NF-κB activation, implicating an immunosuppressive role for SRA/CD204. Immunization with the hsp110-gp100 vaccine resulted in a more robust gp100-specific CD8(+) T cell response in SRA(-/-) mice than in WT mice. Lastly, SRA/CD204 absence markedly improved the therapeutic efficacy of the hsp110-gp100 vaccine in mice established with B16 melanoma, which was accompanied by enhanced activation and tumor infiltration of CD8(+) T cells. Given the presence of multiple HSP-binding scavenger receptors on APCs, we propose that selective scavenger receptor interactions with HSPs may lead to highly distinct immunological consequences. Our findings provide new insights into the immune regulatory functions of SRA/CD204 and have important implications in the rational design of protein Ag-targeted recombinant chaperone vaccines for the treatment of cancer.

Suppression of TLR4-mediated inflammatory response by macrophage class A scavenger receptor (CD204)

The class A scavenger receptor (SR-A, CD204), one of the principal receptors expressed on macrophages, has been found to regulate inflammatory response and attenuate septic endotoxemia. However, the detailed mechanism of this process has not yet been well characterized. To clarify the regulative mechanisms of lipopolysaccharide (LPS)-induced macrophage activation by SR-A, we evaluated the activation of Toll-like receptor 4 (TLR4)-mediated signaling molecules in SR-A-deficient (SR-A(-/-)) macrophages. In a septic shock model, the blood levels of tumor necrosis factor (TNF)-α, interleukin (IL)-6 and interferon (IFN)-β were significantly increased in SR-A(-/-) mice compared to wild-type mice, and elevated nuclear factor kappa B (NFκB) activation was detected in SR-A(-/-) macrophages. SR-A deletion increased the production of pro-inflammatory cytokines, and the phosphorylation of mitogen-activated protein kinase (MAPK) and NFκB in vitro. SR-A deletion also promoted the nuclear translocation of NFκB and IFN regulatory factor (IRF)-3. In addition, a competitive binding assay with acetylated low-density lipoprotein, an SR-A-specific ligand, and anti-SR-A antibody induced significant activation of TLR4-mediated signaling molecules in wild-type macrophages but not in SR-A(-/-) macrophages. These results suggest that SR-A suppresses the macrophage activation by inhibiting the binding of LPS to TLR4 in a competitive manner and it plays a pivotal role in the regulation of the LPS-induced inflammatory response.

Absence of scavenger receptor A promotes dendritic cell-mediated cross-presentation of cell-associated antigen and antitumor immune response

Given the primary expression of scavenger receptor A (SRA) or CD204 on antigen-presenting cells, we investigate the immunoregulatory activities of SRA/CD204 in the context of cross-presentation of cell-associated antigen and the immunogenicity of dying tumor cells. Immunization with dying prostate cancer cells results in profoundly increased control of subsequently inoculated tumors in SRA/CD204 knockout mice. Using OVA-expressing RM1 prostate tumor line (RM1-OVA), we show for the first time that SRA absence greatly enhances dendritic cells (DCs)-mediated cross-presentation of OVA antigen derived from dying RM1 cells. While the phagocytic ability of DCs is not significantly impacted by the lack of SRA/CD204, DCs deficient in SRA/CD204 display increased expression of inflammatory cytokines and chemokines, as well as co-stimulatory molecules upon interaction with dying RM1 cells, implicating a suppressive regulation of the functional activation of DCs by SRA/CD204. Further, SRA/CD204-deficient DCs pulsed with dying RM1-OVA cells are more effective than wild-type counterparts in priming antigen-specific T-cell responses, resulting in improved control of RM1 tumor growth in both prophylactic and therapeutic settings. Our findings suggest that the increased immunogenicity of dying tumor cells in SRA/CD204 knockout mice is attributed to the altered functions of DCs in the absence of SRA/CD204, which underscores the important role of SRA/CD204 in host immune homeostasis. Selective downregulation or blockade of this immunoregulatory molecule may lead to enhanced potency of DC-based vaccines capable of breaking immune tolerance against cancer.

Scavenger receptors and their potential as therapeutic targets in the treatment of cardiovascular disease

Scavenger receptors act as membrane-bound and soluble proteins that bind to macromolecular complexes and pathogens. This diverse supergroup of proteins mediates binding to modified lipoprotein particles which regulate the initiation and progression of atherosclerotic plaques. In vascular tissues, scavenger receptors are implicated in regulating intracellular signaling, lipid accumulation, foam cell development, and cellular apoptosis or necrosis linked to the pathophysiology of atherosclerosis. One approach is using gene therapy to modulate scavenger receptor function in atherosclerosis. Ectopic expression of membrane-bound scavenger receptors using viral vectors can modify lipid profiles and reduce the incidence of atherosclerosis. Alternatively, expression of soluble scavenger receptors can also block plaque initiation and progression. Inhibition of scavenger receptor expression using a combined gene therapy and RNA interference strategy also holds promise for long-term therapy. Here we review our current understanding of the gene delivery by viral vectors to cells and tissues in gene therapy strategies and its application to the modulation of scavenger receptor function in atherosclerosis.

Lipid accumulation and dendritic cell dysfunction in cancer

Professional antigen presenting cells, dendritic cells (DC) are responsible for initiation and maintenance of immune responses. Here, we report that a substantial proportion of DCs in tumor-bearing mice and cancer patients have increased levels of triglycerides. Lipid accumulation in DCs was caused by increased uptake of extracellular lipids due to up-regulation of scavenger receptor A. DCs with high lipid content were not able to effectively stimulate allogeneic T cells or present tumor-associated antigens. DCs with high and normal lipid levels did not differ in expression of MHC and co-stimulatory molecules. However, lipid-laden DCs had reduced capacity to process antigens. Pharmacological normalization of lipid levels in DCs with an inhibitor of acetyl-CoA carboxylase restored the functional activity of DCs and substantially enhanced the effects of a cancer vaccine. These findings support the regulation of immune responses in cancer by manipulation of lipid levels in DCs.

Functional and phenotypic effects of AhR activation in inflammatory dendritic cells

Aryl hydrocarbon receptor (AhR) activation by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces immune suppression. Dendritic cells (DCs) are key antigen presenting cells governing T cell activation and differentiation. However, the consequences of AhR activation in DCs are not fully defined. We hypothesized that AhR activation alters DC differentiation and generates dysfunctional DCs. To test this hypothesis, inflammatory bone marrow-derived DCs (BMDCs) from C57Bl/6 mice were generated in the presence of vehicle or TCDD. TCDD decreased CD11c expression but increased MHC class II, CD86 and CD25 expression on the BMDCs. The effects of TCDD were strictly AhR-dependent but not exclusively DRE-mediated. Similar effects were observed with two natural AhR ligands, 6-formylindolo[3,2-b]carbazole (FICZ) and 2-(1H-Indol-3-ylcarbonyl)-4-thiazolecarboxylic acid (ITE). TCDD increased LPS- and CpG-induced IL-6 and TNF-alpha production by BMDCs but decreased their NO production. TCDD decreased CpG-induced IL-12p70 production by BMDCs but did not affect their secretion of IL-10. TCDD downregulated LPS- and CpG-induced NF-kB p65 levels and induced a trend towards upregulation of RelB levels in the BMDCs. AhR activation by TCDD modulated BMDC uptake of both soluble and particulate antigens. Induction of indoleamine-2,3-dioxygenase (IDO) and TGF-beta3 has been implicated in the generation of regulatory T cells following AhR activation. TCDD increased IDO1, IDO2 and TGF-beta3 mRNA levels in BMDCs as compared to vehicle. Despite the induction of regulatory mediators, TCDD-treated BMDCs failed to suppress antigen-specific T cell activation. Thus, AhR activation can directly alter the differentiation and innate functions of inflammatory DCs without affecting their ability to successfully interact with T cells.

Scavenger receptor A1 is required for sensing HCMV by endosomal TLR-3/-9 in monocytic THP-1 cells

Monocytes provide initial surveillance for pathogenic glycopeptides via scavenger receptors (SRs) and for viruses via Toll-like receptors (TLRs) which trigger pro-inflammatory response. However, specific interactions between SR-A1 and TLRs have not yet been assessed in human cytomegalovirus (HCMV)-exposed monocytes. Our results showed two patterns of gene expression upon HCMV exposure: genes that were induced within 10 min include SR-A1, Lyn, TLR-2, and IL-12p35, whereas those induced at 1h are TLR-3, TLR-9, TRIF, IRF-3, and IFN-beta. NF-kappaB p65 and TNF-alpha were elevated at both 10 min and 1h post exposure. Further, inhibitory studies using neutralizing antibodies and morpholino antisense oligonucleotides suggested that within 10 min of HCMV exposure, transcription of TNF-alpha and IL-12 genes is TLR-2-dependent fashion. However, induction of both TLR-3-mediated IFN-beta and TLR-9-mediated TNF-alpha at 1h was dependent on SR-A1. These findings reveal a novel mechanistic insight into an interrelationship between SR-A1 and TLR-3/-9 signaling in HCMV-exposed monocytes.

Uncoupling scavenger receptor A-mediated phagocytosis of bacteria from endotoxic shock resistance

Unresolved infection by gram-negative bacteria can result in the potentially lethal condition known as endotoxic shock, whereby uncontrolled inflammation can lead to multiple organ failure and death of the infected host. Previous results have demonstrated that animals deficient in class A scavenger receptor (SRA), a trafficking receptor for bacteria and bacterium-derived molecules, are more susceptible to endotoxic shock. This has been proposed to be a result of impaired SRA-dependent phagocytic clearance of bacteria resulting in stronger proinflammatory stimuli. In this report, we test the hypothesis that there is an obligate reciprocal relationship between SRA-mediated phagocytosis of bacteria and susceptibility to endotoxic shock. Here, we demonstrate that both SRA-dependent and -independent gram-negative bacterial strains elicit SRA-dependent increased cytokine production in vitro and in vivo and increased susceptibility to endotoxic shock in SRA-deficient mice. This is the first evidence showing that SRA-mediated clearance of LPS is functionally distinct from the role of SRA in bacterial phagocytosis and is a formal demonstration that the SRA-dependent cytokine responses and the resultant endotoxic shock are not coupled to SRA-mediated clearance of bacteria.

Conserved domains of the class A scavenger receptors: evolution and function

The class A scavenger receptors are phagocytic pattern recognition receptors that are well represented in vertebrate genomes. The high level of conservation among vertebrates implies that this is an evolutionarily conserved family of receptors and indicates the presence of a common ancestral gene. The identity of this ancestral gene is not clear, as it appears that many of the domains of the scavenger receptors (e.g. collagenous, scavenger receptor cysteine rich) originated early in evolutionary history and are found in many combinations, often in genes of unknown function. These early receptors may function in cell-cell recognition, aggregation, or lipid recognition, and their involvement in pattern recognition, phagocytosis, and homeostasis may have been adaptations of such conserved patterns. Herein, we reclassify the class A scavenger receptors based on recent discoveries of new members of this family, describe the evolution of the various domains of the class A scavenger receptors, and discuss the appearance and function of these domains through evolutionary history.

PolyI:C-induced reduction in uptake of soluble antigen is independent of dendritic cell activation

Dendritic cells (DC) are key players in the initiation and modulation of adaptive immune responses due to their ability to acquire and present antigen and stimulate T cells. For the induction of effector T cell functions, antigen must be presented by activated DC. In this study, we have compared uptake of antigen by mouse DC in the presence of different Toll-like receptor (TLR) agonists, which are potent inducers of DC activation. Here we show that the reduction in uptake of soluble antigen in the presence of the viral double-stranded RNA (dsRNA) analogues polyinosinic-polycytidylic acid and Ampligen is independent of TLR-mediated DC activation. A reduction in antigen uptake by bone marrow-derived and splenic DC was also observed in response to other RNA homopolymers such as polyinosinic and polyguanylic acids, which are known inhibitors of scavenger receptor-mediated endocytosis. Pinocytosis and mannose receptor-mediated uptake of soluble antigen were not affected by any of the tested nucleic acids. The reduction in antigen uptake by dsRNA did not negatively influence the T cell stimulating properties of the DC. In summary, we conclude that the decrease in antigen endocytosis observed in the presence of a variety of TLR agonists is independent of TLR signalling and is caused by competition for specific surface receptors that are involved in the uptake of these TLR agonists and the antigen.

Ligand of scavenger receptor class A indirectly induces maturation of human blood dendritic cells via production of tumor necrosis factor-alpha

Dendritic cells (DCs) are the most potent antigen-presenting cells for naive T cells. In this study, scavenger receptor class A type I and type II (SR-A) were shown to be expressed by peripheral blood DCs (PBDCs) and monocyte-derived DCs (MDDCs). In addition, the binding of anti-SR-A antibody to these cells was lower in the presence of fucoidan, an SR-A agonist. Treatment of these DCs with fucoidan or anti-SR-A antibody markedly increased the surface expression of costimulatory molecules CD83 and major histocompatibility complex class II on the CD11c(high)CD123(low) myeloid subset of PBDCs. Furthermore, fucoidan-treated PBDCs produced tumor necrosis factor-alpha (TNF-alpha) but not IL-12p70. In addition, fucoidan-induced maturation was eliminated by pretreatment with TNF-alpha-neutralizing antibody. Finally, interferon-gamma secretion and T-cell proliferation were enhanced by coculture of T cells with fucoidan-matured PBDCs. Specific inhibitors of p38 MAPK and glycogen synthase kinase 3 suppressed TNF-alpha production and maturation of fucoidan-treated PBDCs. Moreover, MDDCs lacking SR-A failed to up-regulate CD83 expression, TNF-alpha production, and phosphorylation of p38 MAPK and glycogen synthase kinase 3-beta in the presence of fucoidan. Taken together, these results suggest that ligation of SR-A leads to induction of TNF-alpha, which subsequently induces PBDC maturation, thereby leading to enhanced T-cell stimulatory capacity.

Pattern recognition scavenger receptor SRA/CD204 down-regulates Toll-like receptor 4 signaling-dependent CD8 T-cell activation

Class A scavenger receptor (SRA), also known as CD204, has been shown to participate in the pathogenesis of atherosclerosis and the pattern recognition of pathogen infection. However, its role in adaptive immune responses has not been well defined. In this study, we report that the lack of SRA/CD204 promotes Toll-like receptor (TLR)4 agonist-augmented tumor-protective immunity, which is associated with enhanced activation of CD8(+) effector T cell and improved inhibition of tumor growth. Dendritic cells (DCs) deficient in SRA/CD204 display more effective immunostimulatory activities upon TLR4 engagement than those from wild-type counterparts. Silencing of SRA/CD204 by RNA interference improves the ability of DCs to prime antigen-specific CD8(+) T cells, suggesting that antigen-presenting cells, for example, DCs, play a major role in SRA/CD204-mediated immune modulation. Our findings reveal a previously unrecognized role for SRA/CD204, a non-TLR pattern recognition receptor, as a physiologic negative regulator of TLR4-mediated immune consequences, which has important clinical implications for development of TLR-targeted immunotherapeutic intervention.

The macrophage scavenger receptor A is host-protective in experimental meningococcal septicaemia

Macrophage Scavenger Receptor A (SR-A) is a major non-opsonic receptor for Neisseria meningitidis on mononuclear phagocytes in vitro, and the surface proteins NMB0278, NMB0667, and NMB1220 have been identified as ligands for SR-A. In this study we ascertain the in vivo role of SR-A in the recognition of N. meningitidis MC58 (serogroup B) in a murine model of meningococcal septicaemia. We infected wild-type and SR-A(-/-) animals intraperitoneally with N. meningitidis MC58 and monitored their health over a period of 50 hours. We also determined the levels of bacteraemia in the blood and spleen, and measured levels of the pro-inflammatory cytokine interleukin-6 (IL-6). The health of SR-A(-/-) animals deteriorated more rapidly, and they showed a 33% reduction in survival compared to wild-type animals. SR-A(-/-) animals consistently exhibited higher levels of bacteraemia and increased levels of IL-6, compared to wild-type animals. Subsequently, we constructed a bacterial mutant (MC58-278-1220) lacking two of the SR-A ligands, NMB0278 and NMB1220. Mutation of NMB0667 proved to be lethal. When mice were infected with the mutant bacteria MC58-278-1220, no significant differences could be observed in the health, survival, bacteraemia, and cytokine production between wild-type and SR-A(-/-) animals. Overall, mutant bacteria appeared to cause less severe symptoms of septicaemia, and a competitive index assay showed that higher levels of wild-type bacteria were recovered when animals were infected with a 1ratio1 ratio of wild-type MC58 and mutant MC58-278-1220 bacteria. These data represent the first report of the protective role of SR-A, a macrophage-restricted, non-opsonic receptor, in meningococcal septicaemia in vivo, and the importance of the recognition of bacterial protein ligands, rather than lipopolysaccharide.

Pivotal Advance: Toll-like receptor regulation of scavenger receptor-A-mediated phagocytosis

Class-A scavenger receptors (SR-A) and TLR mediate early immune responses against pathogenic bacteria. SR-A and TLR molecules are expressed on phagocytes and interact with common ligands from Gram-negative and Gram-positive bacteria; however, the contribution of TLR activity to SR-A-mediated phagocytosis has not been assessed directly. Herein, we provide genetic and functional evidence that ligand- and TLR-specific stimuli synergize with SR-A to mediate bacterial phagocytosis. Although complete loss of SR-A (SR-A(-/-)) is known to impair bacterial clearance, here we identify the first deficiency attributable to SR-A heterozygosity: SR-A(+/-)TLR4(+/-) cells and mice are impaired significantly in the clearance of Gram-negative Escherichia coli. This phenotype is specific to the TLR signaling event, as SR-A(+/-)TLR4(+/-) cells are not deficient for the clearance of Gram-positive Staphylococcus aureus bacteria, which contain cell-surface TLR2 ligands but lack TLR4 ligands. We demonstrate that this is a global, phagocytic mechanism, regulated independently by multiple TLRs, as analogous to the SR-A(+/-)TLR4(+/-) deficit, SR-A(+/-)TLR2(+/-) cells are impaired for S. aureus uptake. In support of this, we show that SR-A(+/-)MyD88(+/-) cells recapitulate the phagocytosis defect observed in SR-A(+/-)TLR4(+/-) cells. These data identify for the first time that TLR-driven innate immune responses, via a MyD88 signaling mechanism, regulate SR-A-dependent phagocytosis of bacteria. These findings provide novel insights into how innate immune cells control SR-A-mediated trafficking and are the first demonstration that subtle changes in the expression of SR-A and TLRs can substantially affect host bacterial clearance.

SR-A, MARCO and TLRs differentially recognise selected surface proteins from Neisseria meningitidis: an example of fine specificity in microbial ligand recognition by innate immune receptors

Macrophages express various classes of pattern recognition receptors involved in innate immune recognition of artificial, microbial and host-derived ligands. These include the scavenger receptors (SRs), which are important for phagocytosis, and the Toll-like receptors (TLRs) involved in microbe sensing. The class A macrophage scavenger receptor (SR-A) and macrophage receptor with a collagenous structure (MARCO) display similar domain structures and ligand-binding specificity, which has led to the assumption that these two receptors may be functionally redundant. In this study we show that SR-A and MARCO differentially recognise artificial polyanionic ligands as well as surface proteins from the pathogenic bacterium Neisseria meningitidis. We show that, while acetylated low-density lipoprotein (AcLDL) is a strong ligand for SR-A, it is not a ligand for MARCO. Of the neisserial proteins that were SR ligands, some were ligands for both receptors, while other proteins were only recognised by either SR-A or MARCO. We also analysed the potential of these ligands to act as TLR agonists and assessed the requirement for SR-A and MARCO in pro-inflammatory cytokine induction. SR ligation alone did not induce cytokine production; however, for proteins that were both SR and TLR ligands, the SRs were required for full activation of TLR pathways.

The scavenger receptor SR-A I/II (CD204) signals via the receptor tyrosine kinase Mertk during apoptotic cell uptake by murine macrophages

Apoptotic cells (AC) must be cleared by macrophages (Mø) to resolve inflammation effectively. Mertk and scavenger receptor A (SR-A) are two of many receptors involved in AC clearance. As SR-A lacks enzymatic activity or evident intracellular signaling motifs, yet seems to signal in some cell types, we hypothesized that SR-A signals via Mer receptor tyrosine kinase (Mertk), which contains a multisubstrate docking site. We induced apoptosis in murine thymocytes by dexamethasone and used Western blotting and immunoprecipitation to analyze the interaction of Mertk and SR-A in the J774A.1 (J774) murine Mø cell line and in peritoneal Mø of wild-type mice and SR-A-/- mice. Phagocytosis (but not adhesion) of AC by J774 was inhibited by anti-SR-A or function-blocking SR-A ligands. In resting J774, SR-A was associated minimally with unphosphorylated (monomeric) Mertk; exposure to AC induced a time-dependent increase in association of SR-A with Mertk in a direct or indirect manner. Anti-SR-A inhibited AC-induced phosphorylation of Mertk and of phospholipase Cgamma2, essential steps in AC ingestion. Relative to tissue Mø of wild-type mice, AC-induced Mertk phosphorylation was reduced and delayed in tissue Mø of SR-A-/- mice, as was in vitro AC ingestion at early time-points. Thus, during AC uptake by murine Mø, SR-A is essential for optimal phosphorylation of Mertk and subsequent signaling required for AC ingestion. These data support the Mertk/SR-A complex as a potential target to manipulate AC clearance and hence, resolution of inflammation and infections.

Redundancy renders the glycoprotein 96 receptor scavenger receptor A dispensable for cross priming in vivo

CD8(+) T cells (T(CD8+)) differentiate into effector cells following recognition of specific peptide-major histocompatibility complex (MHC) class I complexes (pMHC-I) on the surface of professional APCs (pAPCs), such as dendritic cells. Antigenic pMHC-I can be generated from two spatially distinct sources. The direct presentation pathway involves generation of peptide from protein substrate synthesized within the cell that is presenting the pMHC-I. Alternatively, the cross presentation pathway involves presentation of antigen that is not synthesized within the presenting cell, but is derived from exogenous proteins synthesized within other donor cells. The mechanisms by which cross presentation of exogenous antigens occur in vivo remain controversial. The C-type lectin scavenger receptor A (SR-A) has been implicated in a number of potential cross presentation pathways, including the presentation of peptide bound to heat shock proteins, such as glycoprotein 96 (gp96), and the transfer of pMHC-I from a donor cell to the pAPC. We demonstrate here that initiation of T(CD8+) responses is normal in mice lacking SR-A, and that the redundancy of ligand binding exhibited by the SR family is likely to be an important mechanism that ensures cross presentation in vivo. These observations emphasize the requirement to target multiple receptors and antigen-processing pathways during the rational design of vaccines aimed at eliciting protective T(CD8+).

Macrophage scavenger receptors and host-derived ligands

The scavenger receptors are a large family of molecules that are structurally diverse and have been implicated in a range of functions. They are expressed by myeloid cells, selected endothelial cells and some epithelial cells and recognise many different ligands, including microbial pathogens as well as endogenous and modified host-derived molecules. This review will focus on the eight classes of scavenger receptors (class A-H) in terms of their structure, expression and recognition of host-derived ligands. Scavenger receptors have been implicated in a range of physiological and pathological processes, such as atherosclerosis and Alzheimer's disease, and function in adhesion and tissue maintenance. More recently, some of the scavenger receptors have been shown to mediate binding and endocytosis of chaperone proteins, such as the heat shock proteins, thereby playing an important role in antigen cross-presentation.