α-Ketoglutarate plays an inflammatory inhibitory role by regulating scavenger receptor class a expression through N6-methyladenine methylation during sepsis

Sepsis arises from an uncontrolled inflammatory response triggered by infection or stress, accompanied by alteration in cellular energy metabolism, and a strong correlation exists between these factors. Alpha-ketoglutarate (α-KG), an intermediate product of the TCA cycle, has the potential to modulate the inflammatory response and is considered a crucial link between energy metabolism and inflammation. The scavenger receptor (SR-A5), a significant pattern recognition receptor, assumes a vital function in anti-inflammatory reactions. In the current investigation, we have successfully illustrated the ability of α-KG to mitigate inflammatory factors in the serum of septic mice and ameliorate tissue damage. Additionally, α-KG has been shown to modulate metabolic reprogramming and macrophage polarization. Moreover, our findings indicate that the regulatory influence of α-KG on sepsis is mediated through SR-A5. We also elucidated the mechanism by which α-KG regulates SR-A5 expression and found that α-KG reduced the N6-methyladenosine level of macrophages by up-regulating the m6A demethylase ALKBH5. α-KG plays a crucial role in inhibiting inflammation by regulating SR-A5 expression through m6A demethylation during sepsis. The outcomes of this research provide valuable insights into the relationship between energy metabolism and inflammation regulation, as well as the underlying molecular regulatory mechanism.

Identification of large yellow croakers (Larimichthys crocea) scavenger receptor genes: Involvement in immune response to Pseudomonas plecoglossicida infection and hypoxia-exposure experiments

Scavenger receptors (SRs) are pattern recognition receptors involved in the innate immune defense against pathogen infection in fish. However, there has not been much research done on teleosts. In this study, 18 members of the SR gene family were found in large yellow croaker. The identification of the SR gene family showed that the protein length of SR members in large yellow croaker were quite different, and most SR genes were distributed in nuclear and endoplasmic. The evolutionary relationship, exon/intron structure and motif analysis revealed that members of the SR gene family were highly conserved. The results of the expression profiles after Pseudomonas plecoglossicida infection and hypoxia-exposure demonstrated that SR members were involved in inflammatory reactions. Especially, COLEC12 and SCARF1 exhibited substantial changes in response to both P. plecoglossicida and hypoxia stress, indicating their possible immunological functions. The result of this study revealed that SR genes played a vital part in the innate immune response of large yellow croaker, and would give important details for a deeper comprehension of the SR gene family's regulation mechanism under various conditions in large yellow croaker.

Promoting AMPK/SR-A1-mediated clearance of HMGB1 attenuates chemotherapy-induced peripheral neuropathy

BACKGROUND

Chemotherapy-induced peripheral neuropathy (CIPN) is a serious side effect of chemotherapy with poorly understood mechanisms and few treatments. High-mobility group box 1 (HMGB1)-induced neuroinflammation is the main cause of CIPN. Here, we aimed to illustrate the role of the macrophage scavenger receptor A1 (SR-A1) in HMGB1 clearance and CIPN resolution.

METHODS

Oxaliplatin (L-OHP) was used to establish a CIPN model. Recombinant HMGB1 (rHMGB1) (his tag) was used to evaluate the phagocytosis of HMGB1 by macrophages.

RESULTS

In the clinic, HMGB1 expression and MMP-9 activity were increased in the plasma of patients with CIPN. Plasma HMGB1 expression was positively correlated with the cumulative dose of L-OHP and the visual analog scale. In vitro, engulfment and degradation of rHMGB1 increased and inflammatory factor expression decreased after AMP-activated protein kinase (AMPK) activation. Neutralizing antibodies, inhibitors, or knockout of SR-A1 abolished the effects of AMPK activation on rHMGB1 engulfment. In vivo, AMPK activation increased SR-A1 expression in the dorsal root ganglion, decreased plasma HMGB1 expression and MMP-9 activity, and attenuated CIPN, which was abolished by AMPK inhibition or SR-A1 knockout in the CIPN mice model.

CONCLUSION

Activation of the AMPK/SR-A1 axis alleviated CIPN by increasing macrophage-mediated HMGB1 engulfment and degradation. Therefore, promoting HMGB1 clearance may be a potential treatment strategy for CIPN. Video abstract.

SCARA5 inhibits oral squamous cell carcinoma via inactivating the STAT3 and PI3K/AKT signaling pathways

Oral squamous cell carcinoma (OSCC) is a common tumor in the world. Despite the rapid development of medical care, OSCC is also accompanied by high incidence and mortality every year. Therefore, it is still necessary to continuously develop new methods or find new targets to treat OSCC. Previous research showed that scavenger receptor class A member 5 (SCARA5) was one of the potential biomarkers of OSCC, and its expression is significantly low in OSCC. This study aimed to explore the role and related molecular mechanisms of SCARA5 in OSCC. In this study, we found that the SCARA5 expression was lower in CAL-27 and SCC-9 cells than that in human normal oral epithelial keratinocytes. SCARA5 overexpression significantly inhibited cell proliferation and induced apoptosis of CAL-27 and SCC-9 cells. In addition, SCARA5 repressed OSCC cell epithelial-mesenchymal transformation (EMT), evidenced by increased E-cadherin expression and reduced N-cadherin expression. Finally, we found that SCARA5 could suppress STAT3, PI3K, and AKT phosphorylation. Therefore, SCARA5 was related to STAT3 and PI3K/AKT signaling pathways in OSCC. In conclusion, SCARA5 inhibited the proliferation and EMT and induced the apoptosis of OSCC cells through the inhibition of STAT3 and PI3K/AKT signaling pathways, thereby exerting a tumor suppressor effect.

Extensive involvement of CD40 and CD154 costimulators in multiple T cell-mediated responses in a perciform fish Larimichthys crocea

CD40 and CD154 are well-characterized costimulatory molecules involved in adaptive humoral immunity in humans and other mammals. These two costimulatory molecules were found to be originated from teleost fish during vertebrate evolution. However, the functionality of fish CD40 and CD154 remains to be explored. In this study, we identified the CD40 and CD154 homologs (LcCD40 and LcCD154) from large yellow croaker (Larimichthys crocea), a marine species of the perciform fish family. The LcCD40 and LcCD154 share conserved structural features to their mammalian counterparts, and are widely expressed in immune-relevant tissues and leukocytes at different transcriptional levels. Immunofluorescence staining and FCM analysis showed that LcCD40 and LcCD154 proteins are distributed on MHC-II⁺ APCs and CD4-2⁺ T cells, and are significantly upregulated in response to antigen stimulation. Co-IP assay exhibited strong association between LcCD40 and LcCD154 proteins. Blockade of LcCD154 with anti-LcCD154 antibody (Ab) or recombinant soluble LcCD40-Ig fusion protein remarkably decreased the MHC-II⁺ APC-initiated CD4⁺ T cell response upon Aeromonas hydrophila stimulation, and alloreactive T cell activation as examined by mixed lymphocyte reaction (MLR). These findings highlight the costimulatory role of LcCD40 and LcCD154 in T cell activities in Larimichthys crocea. Thus, the CD40 and CD154 costimulators may extensively participate in the regulation of multiple T cell-mediated immune responses in teleost fish. It is anticipated that this study would provide a cross-species understanding of the evolutionary history of CD40 and CD154 costimulatory signals from fish to mammals.

Negative Regulatory Role of the Spring Viremia of Carp Virus Matrix Protein in the Host Interferon Response by Targeting the MAVS/TRAF3 Signaling Axis

Spring viremia of carp virus (SVCV) is a severe infectious pathogen that causes high rates of mortality in cyprinids and other fish species. Despite numerous investigations of SVCV infection, the underlying molecular mechanisms remain poorly understood. In this study, we found that the SVCV matrix protein (SVCV-M) played an inhibitory role in the host interferon (IFN) response by targeting the MAVS/TRAF3 signaling axis, thereby uncovering a previously unrecognized mechanism of SVCV escape from host innate antiviral immunity. Mechanistically, SVCV-M was located at the mitochondria independent of MAVS, which allowed SVCV-M to build an arena for competition with the MAVS platform. A microscale thermophoresis assay showed that SVCV-M had a high affinity for TRAF3, as indicated by a lower equilibrium dissociation constant (K_D) value than that of MAVS with TRAF3. Therefore, the association of MAVS with TRAF3 was competitively impaired by SVCV-M in a dose-dependent manner. Accordingly, SVCV-M showed a potent ability to inhibit the K63-linked polyubiquitination of TRAF3. This inhibition was accompanied by the impairment of the IFN response, as shown by the marked decline in IFN-φ1-promoter (pro) luciferase reporter activity. By constructing truncated TRAF3 and SVCV-M proteins, the RING finger, zinc finger, and coiled-coil domains of TRAF3 and the hydrophobic-pocket-like structure formed by the α2-, α3-, and α4-helices of SVCV-M may be the major target and antagonistic modules responsible for the protein-protein interaction between the TRAF3 and SVCV-M proteins. These findings highlighted the intervention of SVCV-M in host innate immunity, thereby providing new insights into the extensive participation of viral matrix proteins in multiple biological activities. IMPORTANCE The matrix protein of SVCV (SVCV-M) is an indispensable structural element for nucleocapsid condensation and virion formation during viral morphogenesis, and it connects the core nucleocapsid particle to the outer membrane within the mature virus. Previous studies have emphasized the architectural role of SVCV-M in viral construction; however, the potential nonstructural functions of SVCV-M in viral replication and virus-host interactions remain poorly understood. In this study, we identified the inhibitory role of the SVCV-M protein in host IFN production by competitively recruiting TRAF3 from the MAVS signaling complex and impairing TRAF3 activation via inhibition of K63-linked polyubiquitination. This finding provided new insights into the regulatory role of SVCV-M in host innate immunity, which highlighted the broader functionality of rhabdovirus matrix protein apart from being a structural protein. This study also revealed a previously unrecognized mechanism underlying SVCV immune evasion by inhibiting the IFN response by targeting the MAVS/TRAF3 signaling axis.

Japanese Flounder HMGB1: A DAMP Molecule That Promotes Antimicrobial Immunity by Interacting with Immune Cells and Bacterial Pathogen

High mobility group box (HMGB) proteins are DNA-associated proteins that bind and modulate chromosome structures. In mammals, HMGB proteins can be released from the cell nucleus and serve as a damage-associated molecular pattern (DAMP) under stress conditions. In fish, the DAMP function of HMGB proteins in association with bacterial infection remains to be investigated. In this study, we examined the immunological functions of two HMGB members, HMGB1 and HMG20A, of Japanese flounder. HMGB1 and HMG20A were expressed in multiple tissues of the flounder. HMGB1 was released from peripheral blood leukocytes (PBLs) upon bacterial challenge in a temporal manner similar to that of lactate dehydrogenase release. Recombinant HMGB1 bound to PBLs and induced ROS production and the expression of inflammatory genes. HMGB1 as well as HMG20A also bound to various bacterial pathogens and caused bacterial agglutination. The bacteria-binding patterns of HMGB1 and HMG20A were similar, and the binding of HMGB1 competed with the binding of HMG20A but not vice versa. During bacterial infection, HMGB1 enhanced the immune response of PBLs and repressed bacterial invasion. Collectively, our results indicate that flounder HMGB1 plays an important role in antimicrobial immunity by acting both as a modulator of immune cells and as a pathogen-interacting DAMP.

Genome-wide identification and analysis of scavenger receptors and their expression profiling in response to Edwardsiella tarda infection in Japanese flounder (Paralichthys olivaceus)

The scavenger receptors (SRs) gene family, as one of pattern recognition receptors, participates in the innate immune response in diverse lineages. However, the systematic identification, characteristics and functions of SRs family are lacking in teleost. Here, we identified all 19 SRs family members in Japanese flounder (Paralichthys olivaceus) based on the genome and transcriptome data. Phylogenetic and Ka/Ks analysis demonstrated that these SRs genes were divided into five classes and all exhibited pronounced purified selection pressures. Whole genome duplication event was found in colec12, scarb2, and lamp1. Gene structure, functional domain and motif distribution analyses indicated that SRs within the different subfamilies are severely conservative. SRs genes showed diverse expression patterns in the embryogenesis and unchanged tissues. The regulations of 14 SRs genes in blood, gill and kidney after E. tarda infection suggested their roles in innate immune response. Meanwhile, ten SRs genes were differentially expressed after E. tarda stimulation in macrophages in vitro. Then we proved that PoSCARA3 could suppress the activity of NF-κB and AP-1 in HEK 293T cells by dual-luciferase assays. In summary, this study provided valuable basis for further functional characterization and immune functions of SRs genes in P. olivaceus.

Highly Expressing SCARA5 Promotes Proliferation and Migration of Esophageal Squamous Cell Carcinoma

Background

Thrombospondin type 1 domain-containing 7A (THSD7A) was reported to play a procancer role in esophageal squamous cell carcinoma (ESCC). The aim of the study was to screen the downstream functional genes of THSD7A and explore their functions in ESCC, based on the reported research into THSD7A function and on gene microarrays.

Methods

We adopted quantitative reverse-transcription polymerase chain reaction (qRT-PCR) and Celigo high-content screening (HCS) technology to screen the downstream genes of THSD7A. The expression level of target genes was examined by PCR, western blot, and immunohistochemistry (IHC). The effects of these target genes on ESCC malignant biological behavior were performed in vivo and in vitro. The Kaplan-Meier (K-M) survival analysis and Cox regression were used to analyze the prognostic significance of target genes in ESCC patients. Experiments in the literature on liver cancer (LC) were repeated to verify the functions of these genes in different tumors. We further explored the cancer-promoting mechanism of target genes in ESCC by sequencing of the genes' exons.

Results

Scavenger receptor class A member 5 (SCARA5) was proved to be the downstream driving gene of THSD7A. SCARA5 promoted cell proliferation and migration but inhibited apoptosis in ESCC. IHC results confirmed that SCARA5 expression in ESCC exceeded that in normal tissues. The K-M survival analysis indicated that SCARA5 expression quantity was not related to prognosis, but tumor volume and T classification were both the independent prognostic factors. Repetition of experiments in LC in the literature confirmed that SCARA5 had exactly opposite functions in EC and LC.

Conclusion

SCARA5 was related to the development and occurrence of ESCC. Our findings suggested that it was a potentially diagnostic individualized therapeutic target for ESCC in the future and that its application could possibly be combined with that of upstream THSD7A gene.

IL-33 Enhances the Lipopolysaccharide-Induced Secretion of Inflammatory Cytokines and Ameliorates Lipopolysaccharide Desensitization in Macrophages

Background: Lipopolysaccharide (LPS) desensitization, which is characterized by hyporesponsiveness and a form of immunosuppression, is important in the negative regulation of responses to LPS and inflammatory disease such as sepsis. However, effect of IL-33 in the desensitization to LPS remains unclear. Methods: We used RNA-sequencing technology to analyze changes in mRNA in bone-marrow-derived macrophages (BMDMs) stimulated with LPS. Changes in expression and secretion of inflammatory cytokines were detected by qPCR and ELISA, respectively. Mechanisms were further studied through p65 phosphorylation detection. Results: IL-33 expression was significantly increased in LPS-treated macrophages, indicating its involvement in LPS-induced inflammation. Exogenous IL-33 increased the inflammatory response and ameliorated LPS desensitization by increasing the secretion of proinflammatory cytokines. It also activated p65 phosphorylation in resistant cells. Conclusion: IL-33 can enhance the inflammatory response induced by LPS and ameliorate LPS desensitization possibly by activating the NF-κB pathway in mouse macrophages.

New Proteins Contributing to Immune Cell Infiltration and Pannus Formation of Synovial Membrane from Arthritis Diseases

An inflamed synovial membrane plays a major role in joint destruction and is characterized by immune cells infiltration and fibroblast proliferation. This proteomic study considers the inflammatory process at the molecular level by analyzing synovial biopsies presenting a histological inflammatory continuum throughout different arthritis joint diseases. Knee synovial biopsies were obtained from osteoarthritis (OA; n = 9), chronic pyrophosphate arthropathy (CPPA; n = 7) or rheumatoid arthritis (RA; n = 8) patients. The histological inflammatory score was determined using a semi-quantitative scale based on synovial hyperplasia, lymphocytes, plasmocytes, neutrophils and macrophages infiltration. Proteomic analysis was performed by liquid chromatography-mass spectrometry (LC-MS/MS). Differentially expressed proteins were confirmed by immunohistochemistry. Out of the 1871 proteins identified and quantified by LC-MS/MS, 10 proteins (LAP3, MANF, LCP1, CTSZ, PTPRC, DNAJB11, EML4, SCARA5, EIF3K, C1orf123) were differentially expressed in the synovial membrane of at least one of the three disease groups (RA, OA and CPPA). Significant increased expression of the seven first proteins was detected in RA and correlated to the histological inflammatory score. Proteomics is therefore a powerful tool that provides a molecular pattern to the classical histology usually applied for synovitis characterization. Except for LCP1, CTSZ and PTPRC, all proteins have never been described in human synovitis.

U3-1402 sensitizes HER3-expressing tumors to PD-1 blockade by immune activation

Immunotherapy targeting programmed cell death-1 (PD-1) induces durable antitumor efficacy in many types of cancer. However, such clinical benefit is limited because of the insufficient reinvigoration of antitumor immunity with the drug alone; therefore, rational therapeutic combinations are required to improve its efficacy. In our preclinical study, we evaluated the antitumor effect of U3-1402, a human epidermal growth factor receptor 3-targeting (HER3-targeting) antibody-drug conjugate, and its potential synergism with PD-1 inhibition. Using a syngeneic mouse tumor model that is refractory to anti-PD-1 therapy, we found that treatment with U3-1402 exhibited an obvious antitumor effect via direct lysis of tumor cells. Disruption of tumor cells by U3-1402 enhanced the infiltration of innate and adaptive immune cells. Chemotherapy with exatecan derivative (Dxd, the drug payload of U3-1402) revealed that the enhanced antitumor immunity produced by U3-1402 was associated with the induction of alarmins, including high-mobility group box-1 (HMGB-1), via tumor-specific cytotoxicity. Notably, U3-1402 significantly sensitized the tumor to PD-1 blockade, as a combination of U3-1402 and the PD-1 inhibitor significantly enhanced antitumor immunity. Further, clinical analyses indicated that tumor-specific HER3 expression was frequently observed in patients with PD-1 inhibitor-resistant solid tumors. Overall, U3-1402 is a promising candidate as a partner of immunotherapy for such patients.

The characterization and initial immune functional analysis of SCARA5 in turbot (Scophthalmus maximus L.)

Scavenger receptors (SRs) are a group of membrane-bound receptors that could bind to a variety of ligands including endogenous proteins and pathogens. SRs have been recognized to play vital roles in innate immune response against pathogen infection in both vertebrates and invertebrates. In this regard, one SmSCARA5 gene was captured in turbot (Scophthalmus maximus). The full-length SmSCARA5 transcript contains an open reading frame (ORF) of 1494 bp. SmSCARA55 showed both the highest identity and similarity to half-smooth tongue sole (Cynoglossus semilaevis), and a high degree of conservation of genomic structure to the teleost species. In addition, the phylogenetic tree analysis showed SmSCARA5 had the closest relationship to half-smooth tongue sole, the syntenic analysis revealed a relatively conserved synteny pattern of SmSCARA5 to other species. Moreover, SmSCARA5 was ubiquitously expressed in all the examined tissues, with the highest expression level in brain and the lowest expression level in blood. And it was significantly down-regulated in intestine following Gram-negative bacteria Vibrio anguillarum, and Gram-positive bacteria Streptococcus iniae challenge. Finally, the recombinant SmSCARA5 showed the highest affinity to lipopolysaccharide (LPS), followed by peptidoglycan (PGN) and lipoteichoic acid (LTA), as well as the strong inhibition effect on the growth of V. anguillarum. Taken together, our results suggested SmSCARA5 plays vital roles in innate immune response in teleost, further studies should be carried out to better understand its regulatory mechanism for innate inflammation response in teleost.

SPAG5 promotes hepatocellular carcinoma progression by downregulating SCARA5 through modifying β-catenin degradation

BACKGROUND

The sperm-associated antigen 5 (SPAG5) plays a key role in controlling various cellular phenomena, including cell cycle progression and proliferation. However, the role of SPAG5 in hepatocellular carcinoma (HCC) remains unknown.

METHODS

This study investigated the function and clinical significance of SPAG5 protein expression in hepatocellular carcinoma. We analyzed SPAG5 expression in surgical specimens from 136 HCC patients. The correlation between the clinical characteristics and prognosis was also determined. Furthermore, the SPAG5 was overexpressed in HCC cell and silenced with shRNA in HCC cells. Moreover, cell proliferation and apoptosis were measured using Edu assay and flow cytometry and a molecular mechanism of SPAG5 promotes HCC progression was explored.

RESULTS

Herein, our study showed that upregulation of SPAG5 was detected frequently in primary HCC tissues, and was associated with significantly worse survival among the HCC patients. Multivariate analyses revealed that high SPAG5 expression was an independent predictive marker for the poor prognosis of HCC. SPAG5 silence effectively abolished the proliferation abilities of SPAG5 in vivo and in vitro, while induced apoptosis in HCC cells. Furthermore, our results indicate that SPAG5 promoted cell progression by decreasing SCARA5 expression, which has been reported to control the progression of HCC, and our data demonstrated that SCARA5 is crucial for SPAG5-mediated HCC cell progression in vitro and in vivo. Moreover, we found that the expression of SPAG5 and SCARA5 are inversely correlated in HCC tissues. In addition, we demonstrated that SPAG5 promoted progression in HCC via downregulating SCARA5 depended on the β-catenin/TCF4 signaling pathway. Interestingly, the underlying mechanism is which SPAG5 regulates SCARA5 expression by modulating β-catenin degradation.

CONCLUSIONS

Taken together, our data provide a novel evidence for the biological and clinical significance of SPAG5 as a potential biomarker, and we demonstrate that SPAG5-β-catenin-SCARA5 might be a novel pathway involved in HCC progression.

Characterization of an NLRP1 Inflammasome from Zebrafish Reveals a Unique Sequential Activation Mechanism Underlying Inflammatory Caspases in Ancient Vertebrates

NLRP1 inflammasome is one of the best-characterized inflammasomes in humans and other mammals. However, the existence of this inflammasome in nonmammalian species remains poorly understood. In this study, we report the molecular and functional identification of an NLRP1 homolog, Danio rerio NLRP1 (DrNLRP1) from a zebrafish (D. rerio) model. This DrNLRP1 possesses similar structural architecture to mammalian NLRP1s. It can trigger the formation of a classical inflammasome for the activation of zebrafish inflammatory caspases (D. rerio Caspase [DrCaspase]-A and DrCaspase-B) and maturation of D. rerio IL-1β in a D. rerio ASC (DrASC)-dependent manner. In this process, DrNLRP1 promotes the aggregation of DrASC into a filament with DrASC^CARD core and DrASC^PYD cluster. The assembly of DrNLRP1 inflammasome depends on the CARD-CARD homotypic interaction between DrNLRP1 and DrASC^CARD core, and PYD-PYD interaction between DrCaspase-A/B and DrASC^PYD cluster. The FIIND domain in DrNLRP1 is necessary for inflammasome assembly. To understand the mechanism of how the two DrCaspases are coordinated in DrNLRP1 inflammasome, we propose a two-step sequential activation model. In this model, the recruitment and activation of DrCaspase-A/B in the inflammasome is shown in an alternate manner, with a preference for DrCaspase-A followed by a subsequent selection for DrCaspase-B. By using morpholino oligonucleotide-based knockdown assays, the DrNLRP1 inflammasome was verified to play important functional roles in antibacterial innate immunity in vivo. These observations demonstrate that the NLRP1 inflammasome originated as early as in teleost fish. This finding not only gives insights into the evolutionary history of inflammasomes but also provides a favorable animal model for the study of NLRP1 inflammasome-mediated immunology and diseases.

Identification of three class A scavenger receptors from rainbow trout (Oncorhynchus mykiss): SCARA3, SCARA4, and SCARA5

Class A scavenger receptors (SR-As) are a family of five surface receptors whose functions in mammals are associated with innate immunity; however, their role in fish immunity requires further elucidation. The present study identifies, performs sequence analysis, and constitutive transcript expression analysis for three SR-A family members, SCARA3, SCARA4 and SCARA5, from rainbow trout. This work will provide a basis for future studies on SR-A function and their role in innate immunity in this economically important fish.

Role of a macrophage receptor with collagenous structure (MARCO) in regulating monocyte/macrophage functions in ayu, Plecoglossus altivelis

Macrophage receptor with collagenous structure (MARCO) plays essential roles in phagocytic cell-mediated innate immune responses. However, studies regarding MARCO, especially its functions, are limited in teleost species. In this study, we identified a MARCO molecule (PaMARCO) from ayu (Plecoglossus altivelis). PaMARCO shared conserved functional domains with its mammalian counterparts. Sequence analysis showed that PaMARCO was most closely related to its rainbow trout (Oncorhynchus mykiss) counterpart. PaMARCO expression was upregulated in all tested immune tissues and monocytes/macrophages (MO/MΦ) upon Vibrio anguillarum infection, and blocking its function significantly decreased the immune responses of MO/MΦ during infection. PaMARCO could bind to the tested gram-positive and -negative bacteria in a Ca²⁺-dependent manner in vitro. Furthermore, the phagocytosis and bacterial killing activities of MO/MΦ were significantly decreased upon PaMARCO blockade using anti-PaMARCO IgG. PaMARCO was also involved in the polarization processes of ayu MO/MΦ. The upregulated expression of representative cytokines in LPS-induced M1 type (TNF-α, IL-1β) or cAMP-induced M2 type (TGF-β, IL-10) were inhibited in the anti-PaMARCO IgG-treated group, indicating that PaMARCO may be involved in the regulation of both inflammation priming and inflammation resolution of MO/MΦ. In conclusion, our results implicate that PaMARCO has essential regulatory roles for bacterial binding, clearance, and the polarization processes of ayu MO/MΦ.

Role of scavenger receptors as damage-associated molecular pattern receptors in Toll-like receptor activation

Damage-associated molecular patterns (DAMPs) have been implicated in sterile inflammation in various tissue injuries. High-mobility group box 1 (HMGB1) is a representative DAMP, and has been shown to transmit signals through receptors for advanced glycation end products (RAGEs) and TLRs, including TLR2 and TLR4. HMGB1 does not, however, bind to TLRs with high affinity; therefore, the mechanism of HMGB1-mediated TLR activation remains unclear. In this study, we found that fluorescently labeled HMGB1 was efficiently internalized into macrophages through class A scavenger receptors. Although both M1- and M2-type macrophages internalized HMGB1, only M1-type macrophages secreted cytokines in response to HMGB1. The pan-class A scavenger receptor competitive inhibitor, maleylated bovine serum albumin (M-BSA), inhibited HMGB1 internalization and reduced cytokine production from macrophages in response to HMGB1 but not to LPS. The C-terminal acidic domain of HMGB1 is responsible for scavenger receptor-mediated internalization and cytokine production. HMGB1 and TLR4 co-localized in macrophages, and this interaction was disrupted by M-BSA, suggesting that class A scavenger receptors function as co-receptors of HMGB1 for TLR activation. M-BSA ameliorated LPS-induced sepsis and dextran sulfate sodium (DSS)-induced colitis models in which HMGB1 has been shown to play progressive roles. These data suggest that scavenger receptors function as co-receptors along with TLRs for HMGB1 in M1-type inflammatory macrophages.