Endotoxin{middle dot}albumin complexes transfer endotoxin monomers to MD-2 resulting in activation of TLR4

Human milk affects TLR4 activation and LPS-induced inflammatory cytokine expression in Caco-2 intestinal epithelial cells

Human milk (HM) components affect immune cell toll-like receptor 4 (TLR4) signaling. However, studies examining the immunomodulatory impacts of HM on TLR4 signaling in intestinal epithelial cells (IECs) are limited. This study utilized both a TLR4 reporter cell line and a Caco-2 IEC model to examine the effects of HM on lipopolysaccharide (LPS)-induced TLR4 activation and cytokine responses, respectively. Additionally, we performed fast protein liquid chromatography and mass spectrometry to identify a HM component that contributes to the effect of HM on LPS/TLR4 signaling. HM enhances LPS-induced TLR4 signaling as well as LPS-induced IEC gene expression of pro-inflammatory cytokines and negative regulators of NF-κB. Human serum albumin (HSA) present in HM contributes to these effects. HSA within HM synergizes with LPS to induce IEC gene expression of pro-inflammatory cytokines and negative regulators of NF-κB. Altogether, this study provides mechanistic evidence behind the immunomodulatory function of HM on IECs, which may contribute to an enhanced immune response in breast-fed neonates.

Cordycepin enhances anti-tumor immunity in breast cancer by enhanceing ALB expression

Objective

The treatment of breast cancer still faces great challenges, and it is necessary to continuously explore effective drugs and targets to promote immune precision medicine. This study aims to investigate the immune-related regulatory mechanism of cordycepin in breast cancer.

Methods

Network pharmacology was employed to discovery the action of cordyceps on breast cancer targets, molecular docking was employed to analyze the interaction pattern between core components and targets, and biological information analysis was used to explore the target-related immune mechanism and verified in vitro experiments.

Results

The results of this study indicate that cordycepin can effectively inhibit breast cancer. The roles of cordycepin's active component and its target gene ALB were elucidated through the combined use of network pharmacology and molecular docking. Bioinformatics analysis revealed convincing associations between ALB and many immune pathway marker genes. ALB was inhibited in tumor expression, and cordycepin was found to enhance the expression of ALB in vitro to play an anti-tumor role.

Conclusion

Cordycepin regulates immune suppression of tumor, which is expected to open a new chapter of breast cancer immunotherapy.

Flotillins affect LPS-induced TLR4 signaling by modulating the trafficking and abundance of CD14

Lipopolysaccharide (LPS) induces a strong pro-inflammatory reaction of macrophages upon activation of Toll-like receptor 4 (TLR4) with the assistance of CD14 protein. Considering a key role of plasma membrane rafts in CD14 and TLR4 activity and the significant impact exerted on that activity by endocytosis and intracellular trafficking of the both LPS acceptors, it seemed likely that the pro-inflammatory reaction could be modulated by flotillins. Flotillin-1 and -2 are scaffolding proteins associated with the plasma membrane and also with endo-membranes, affecting both the plasma membrane dynamics and intracellular protein trafficking. To verify the above hypothesis, a set of shRNA was used to down-regulate flotillin-2 in Raw264 cells, which were found to also become deficient in flotillin-1. The flotillin deficiency inhibited strongly the TRIF-dependent endosomal signaling of LPS-activated TLR4, and to a lower extent also the MyD88-dependent one, without affecting the cellular level of TLR4. The flotillin depletion also inhibited the pro-inflammatory activity of TLR2/TLR1 and TLR2/TLR6 but not TLR3. In agreement with those effects, the depletion of flotillins down-regulated the CD14 mRNA level and the cellular content of CD14 protein, and also inhibited constitutive CD14 endocytosis thereby facilitating its shedding. Ultimately, the cell-surface level of CD14 was markedly diminished. Concomitantly, CD14 recycling was enhanced via EEA1-positive early endosomes and golgin-97-positive trans-Golgi network, likely to compensate for the depletion of the cell-surface CD14. We propose that the paucity of surface CD14 is the reason for the down-regulated signaling of TLR4 and the other TLRs depending on CD14 for ligand binding.

Recovering What Matters: High Protein Recovery after Endotoxin Removal from LPS-Contaminated Formulations Using Novel Anti-Lipid A Antibody Microparticle Conjugates

Endotoxins or lipopolysaccharides (LPS), found in the outer membrane of Gram-negative bacterial cell walls, can stimulate the human innate immune system, leading to life-threatening symptoms. Therefore, regulatory limits for endotoxin content apply to injectable pharmaceuticals, and excess LPS must be removed before commercialization. The majority of available endotoxin removal systems are based on the non-specific adsorption of LPS to charged and/or hydrophobic surfaces. Albeit effective to remove endotoxins, the lack of specificity can result in the unwanted loss of essential proteins from the pharmaceutical formulation. In this work, we developed microparticles conjugated to anti-Lipid A antibodies for selective endotoxin removal. Anti-Lipid A particles were characterized using flow cytometry and microscopy techniques. These particles exhibited a depletion capacity > 6 ×103 endotoxin units/mg particles from water, as determined with two independent methods (Limulus Amebocyte Lysate test and nanoparticle tracking analysis). Additionally, we compared these particles with a non-specific endotoxin removal system in a series of formulations of increasing complexity: bovine serum albumin in water < insulin in buffer < birch pollen extracts. We demonstrated that the specific anti-Lipid A particles show a higher protein recovery without compromising their endotoxin removal capacity. Consequently, we believe that the specificity layer integrated by the anti-Lipid A antibody could be advantageous to enhance product yield.

Heterogeneity of Lipopolysaccharide as Source of Variability in Bioassays and LPS-Binding Proteins as Remedy

Lipopolysaccharide (LPS), also referred to as endotoxin, is the major component of Gram-negative bacteria's outer cell wall. It is one of the main types of pathogen-associated molecular patterns (PAMPs) that are known to elicit severe immune reactions in the event of a pathogen trespassing the epithelial barrier and reaching the bloodstream. Associated symptoms include fever and septic shock, which in severe cases, might even lead to death. Thus, the detection of LPS in medical devices and injectable pharmaceuticals is of utmost importance. However, the term LPS does not describe one single molecule but a diverse class of molecules sharing one common feature: their characteristic chemical structure. Each bacterial species has its own pool of LPS molecules varying in their chemical composition and enabling the aggregation into different supramolecular structures upon release from the bacterial cell wall. As this heterogeneity has consequences for bioassays, we aim to examine the great variability of LPS molecules and their potential to form various supramolecular structures. Furthermore, we describe current LPS quantification methods and the LPS-dependent inflammatory pathway and show how LPS heterogeneity can affect them. With the intent of overcoming these challenges and moving towards a universal approach for targeting LPS, we review current studies concerning LPS-specific binders. Finally, we give perspectives for LPS research and the use of LPS-binding molecules.

The Role of Serum Albumin Level during Hospitalization as a Predictor of Complications and Mortality in Children with Burns

Background

Serum albumin can function as a potential biomarker to determine the severity of the injury and clinical staging of children with burns. Therefore, in this study, we investigated the association between serum albumin level and complications and mortality rate in children with burns.

Methods

In this descriptive-analytic cross-sectional study, 85 patients younger than 18 years with burns who were admitted to Shahid Motahari hospital between 2021 and 2022 were studied. Demographic information, including patients' age, sex, weight, underlying diseases, medical information, albumin level, and C-reactive protein (CRP), was obtained from patient records. Patients were observed until discharge. The independent t-test, chi-square, Pearson correlation, and logistic regression were used for analysis and to examine the predictive role of albumin.

Results

Out of 85 patients, 47 and 38 were boys and girls, respectively. The mean age of the participants was 3.69 ± 3.09 years. The mean length of hospital stay was 2.3 days, with a median of 1.5 days. The mean percentage of burns was 23.44 ± 16.50, and burn grade 2 was the most common. A total of 25 patients (29.41%) were admitted to the intensive care unit (ICU), and 13 deaths (15.29%) were observed among the patients. The mean albumin level was significantly lower than in other patients with outcomes of pulmonary infection, sepsis, renal failure, ICU admission, and death (P < 0.001).

Conclusion

Serum Albumin has a significant predictive value in death, pulmonary infection, sepsis, admission to the ICU, and renal failure. Serum albumin may be a good prognostic marker associated with morbidity and mortality.

Immune defenses of the mammary gland epithelium of dairy ruminants

The epithelium of the mammary gland (MG) fulfills three major functions: nutrition of progeny, transfer of immunity from mother to newborn, and its own defense against infection. The defense function of the epithelium requires the cooperation of mammary epithelial cells (MECs) with intraepithelial leucocytes, macrophages, DCs, and resident lymphocytes. The MG is characterized by the secretion of a large amount of a nutrient liquid in which certain bacteria can proliferate and reach a considerable bacterial load, which has conditioned how the udder reacts against bacterial invasions. This review presents how the mammary epithelium perceives bacteria, and how it responds to the main bacterial genera associated with mastitis. MECs are able to detect the presence of actively multiplying bacteria in the lumen of the gland: they express pattern recognition receptors (PRRs) that recognize microbe-associated molecular patterns (MAMPs) released by the growing bacteria. Interactions with intraepithelial leucocytes fine-tune MECs responses. Following the onset of inflammation, new interactions are established with lymphocytes and neutrophils recruited from the blood. The mammary epithelium also identifies and responds to antigens, which supposes an antigen-presenting capacity. Its responses can be manipulated with drugs, plant extracts, probiotics, and immune modifiers, in order to increase its defense capacities or reduce the damage related to inflammation. Numerous studies have established that the mammary epithelium is a genuine effector of both innate and adaptive immunity. However, knowledge gaps remain and newly available tools offer the prospect of exciting research to unravel and exploit the multiple capacities of this particular epithelium.

CD14 recycling modulates LPS-induced inflammatory responses of murine macrophages

TLR4 is activated by the bacterial endotoxin lipopolysaccharide (LPS) and triggers two pro-inflammatory signaling cascades: a MyD88-dependent one in the plasma membrane, and the following TRIF-dependent one in endosomes. An inadequate inflammatory reaction can be detrimental for the organism by leading to sepsis. Therefore, novel approaches to therapeutic modulation of TLR4 signaling are being sought after. The TLR4 activity is tightly connected with the presence of CD14, a GPI-anchored protein that transfers LPS monomers to the receptor and controls its endocytosis. In this study we focused on CD14 trafficking as a still poorly understood factor affecting TLR4 activity. Two independent assays were used to show that after endocytosis CD14 can recycle back to the plasma membrane in both unstimulated and stimulated cells. This route of CD14 trafficking can be controlled by sorting nexins (SNX) 1, 2, and 6, and is important for maintaining the surface level and the total level of CD14, but can also affect the amount of TLR4. Silencing of these SNXs attenuated especially the CD14-dependent endosomal signaling of TLR4, making them a new target for therapeutic regulation of the inflammatory response of macrophages to LPS.

Therapeutic solutions of human albumin - The possible effect of process-induced molecular alterations on clinical efficacy and safety

Human albumin solutions were developed as therapeutic during the Second World War to address blood loss due to battlefield injury. This indication was based on the recognition that albumin provided most of the oncotic capacity of human plasma. For the succeeding sixty years, this formed the basis for the use of albumin in traumatology and emergency medicine. In more recent times, the pharmacological properties arising from albumin's complex structure have become a focus of attention by clinical researchers. In particular, albumin, through anti-inflammatory and anti-oxidant properties, has been proposed as an agent for the treatment of sepsis, cirrhosis and other inflammatory states. Some evidence for these indications has accrued from a number of small clinical trials and observational studies. These studies have not been confirmed in other large trials. Together with other investigators, we have shown that the process of plasma fractionation results in alterations in the structure of albumin, including those parts of the molecule involved in anti-oxidant and anti-inflammatory effects. Albumin products from diverse manufacturers show heterogeneity in their ability to address these effects. In this article, we review the historical development of albumin solutions, pointing out the variations in fractionation chemistries which different manufacturers have adopted. We suggest ways by which the manufacturing processes have contributed to variations in the physico-chemical properties of molecule. We review the outcomes of clinical studies assessing the role of albumin in ameliorating conditions such as sepsis and cirrhosis, and we speculate as to the extent which heterogeneity in the products may have contributed to variable clinical outcomes. Finally, we argue for a change in the perception of the plasma product industry and its regulatory overseers. Historically, albumin has been viewed as a generic commodity, with different preparations being interchangeable in their clinical application. We suggest that this implied biosimilarity is not necessarily applicable for different albumin solutions. The use of albumin, in indications other than its historical role as a plasma expander, can only be validated by clinical investigation of each separate albumin product.

New Insights on End-Stage Renal Disease and Healthy Individual Gut Bacterial Translocation: Different Carbon Composition of Lipopolysaccharides and Different Impact on Monocyte Inflammatory Response

Chronic kidney disease induces disruption of the intestinal epithelial barrier, leading to gut bacterial translocation. Here, we appreciated bacterial translocation by analyzing circulating lipopolysaccharides (LPS) using two methods, one measuring only active free LPS, and the other quantifying total LPS as well as LPS lipid A carbon chain length. This was done in end-stage renal disease (ESRD) patients and healthy volunteers (HV). We observed both higher LPS concentration in healthy volunteers and significant differences in composition of translocated LPS based on lipid A carbon chain length. Lower LPS activity to mass ratio and higher concentration of high-density lipoproteins were found in HV, suggesting a better plasma capacity to neutralize LPS activity. Higher serum concentrations of soluble CD14 and pro-inflammatory cytokines in ESRD patients confirmed this hypothesis. To further explore whether chronic inflammation in ESRD patients could be more related to LPS composition rather than its quantity, we tested the effect of HV and patient sera on cytokine secretion in monocyte cultures. Sera with predominance of 14-carbon chain lipid A-LPS induced higher secretion of pro-inflammatory cytokines than those with predominance of 18-carbon chain lipid A-LPS. TLR4 or LPS antagonists decreased LPS-induced cytokine production by monocytes, demonstrating an LPS-specific effect. Thereby, septic inflammation observed in ESRD patients may be not related to higher bacterial translocation, but to reduced LPS neutralization capacity and differences in translocated LPS subtypes.

C-Reactive Protein-to-Albumin Ratio Predicts Sepsis and Prognosis in Patients with Severe Burn Injury

Background

Sepsis is a leading cause of mortality among severe burns. This study was conducted to investigate the predictive role of C-reactive protein-to-albumin ratio (CAR) for sepsis and prognosis in severe burns.

Methods

Patients with severe burn injuries from 2013 to 2017 were enrolled and divided into septic and nonseptic groups based on the presence of sepsis within 30 days postburn. Independent risk factors for sepsis were performed by the univariate and multivariate logistic regression analyses. The association between CAR level at admission and postburn 30-day mortality was designed via the Kaplan–Meier method.

Results

Of all the 196 enrolled patients, 83 patients developed sepsis within 30 days postburn injury, with an incidence of 42.3%. TBSA percentage (OR: 1.65, 95% CI: 1.17-2.32, P = 0.014) and CAR at admission (OR: 2.25, 95% CI: 1.33-3.56, P = 0.009) were the two independent risk factors for sepsis in severe burns by the multivariate logistic regression analysis. A higher CAR level (≥1.66) at admission was associated with a lower postburn 30-day survival rate (P = 0.005).

Conclusions

The CAR level at admission was an independent risk factor for sepsis and prognosis in severe burns.

Post-treatment with yiqifumai injection and its main ingredients attenuates lipopolysaccharide-induced microvascular disturbance in mesentery and ileum

OBJECTIVE

To investigate the effect of Yiqifumai injection (YQFM), a compound Chinese medicine, and its main active ingredients on lipopolysaccharide (LPS)-induced microvascular disturbance in mesentery and ileum.

METHODS

Rats were infused with LPS (5 mg/kg/h) for 90 min. Thirty minutes after initiation of LPS administration, YQFM (160 mg/kg/h), Rb1 (5 mg/kg/h), Sch (2.5 mg/kg/h), or Rb1+Sch (5 mg/kg/h + 2.5 mg/kg/h) was infused until 90 min. Human umbilical vein endothelial cells (HUVECs) were incubated with LPS (100 ng/ml) for 90 min. YQFM (1 mg/ml), Rb1 (100 µM), Sch (100 µM), or Rb1+Sch (200 µM) was added 30 min after initiation of LPS stimulation.

RESULTS

Yiqifumai injection and Rb1+Sch inhibited mesenteric venule hyperpermeability, suppressed microvillar erosion and submucosal edema, and protected claudin-5 from downregulation and interleukin-1β from upregulation in ileal tissues after LPS. Study in HUVECs confirmed the effect of YQFM and Rb1+Sch on JAM-1 after LPS and revealed a similar effect on other junction proteins. Moreover, YQFM and Rb1+Sch attenuated the dysfunctional energy metabolism and the activation of TLR-4/Src/NF-κB signaling with Rb1 and Sch being partially effective.

CONCLUSION

These results demonstrated the beneficial effect of post-treatment with YQFM, which is attributable to its main ingredient Rb1 and Sch, and likely mediated by targeting TLR-4/Src/NF-κB signaling pathway.

Biochemical transformation of bacterial lipopolysaccharides by acyloxyacyl hydrolase reduces host injury and promotes recovery

Animals can sense the presence of microbes in their tissues and mobilize their own defenses by recognizing and responding to conserved microbial structures (often called microbe-associated molecular patterns (MAMPs)). Successful host defenses may kill the invaders, yet the host animal may fail to restore homeostasis if the stimulatory microbial structures are not silenced. Although mice have many mechanisms for limiting their responses to lipopolysaccharide (LPS), a major Gram-negative bacterial MAMP, a highly conserved host lipase is required to extinguish LPS sensing in tissues and restore homeostasis. We review recent progress in understanding how this enzyme, acyloxyacyl hydrolase (AOAH), transforms LPS from stimulus to inhibitor, reduces tissue injury and death from infection, prevents prolonged post-infection immunosuppression, and keeps stimulatory LPS from entering the bloodstream. We also discuss how AOAH may increase sensitivity to pulmonary allergens. Better appreciation of how host enzymes modify LPS and other MAMPs may help prevent tissue injury and hasten recovery from infection.

Commercial albumin solution enhances endotoxin-induced vasoplegia and inflammation

BACKGROUND

The Gram-negative bacterium Escherichia coli, commonly involved in severe sepsis and septic shock, shed endotoxin that upon detection by the host triggers an inflammatory cascade. Efficiency of albumin solutions to restore hypovolemia during sepsis has been debated. To aid identification of subgroups of sepsis patients that may respond positively or negatively to treatment with albumin we investigated if preparations of albumin for medical use could affect endotoxin-induced inflammatory response.

METHODS

Isolated human omental arteries obtained during surgery were incubated with endotoxin in the presence or absence of albumin solution. Isolated human monocytes were incubated with endotoxin in the presence or absence of five different commercially available albumin solutions. Vascular contractile response to noradrenaline and release of interleukin (IL)-1β, IL-6, IL-8, IL-10, and tumor necrosis factor (TNF)-α were measured.

RESULTS

Incubation with albumin together with endotoxin decreased median maximum contraction and increased release of IL-6 and IL-8 from the arteries compared to incubation with endotoxin alone. All albumin solutions except one significantly increased endotoxin-induced TNF-α release from monocytes. IL-6 and IL-10 were also increased and no concentration dependency of TNF-α release was observed above 2 mg mL^-1 . Incubation with albumin alone did not affect contraction or release of cytokines while no potentially endotoxin-enhancing contaminant could be identified.

CONCLUSION

We have shown that albumin solution in combination with endotoxin cause vasoplegia in human omental arteries, paralleled by an inflammatory response. This finding could explain the variable efficiency of albumin solutions for sepsis treatment.

Recent Advances in Lipopolysaccharide Recognition Systems

Lipopolysaccharide (LPS), commonly known as endotoxin, is ubiquitous and the most-studied pathogen-associated molecular pattern. A component of Gram-negative bacteria, extracellular LPS is sensed by our immune system via the toll-like receptor (TLR)-4. Given that TLR4 is membrane bound, it recognizes LPS in the extracellular milieu or within endosomes. Whether additional sensors, if any, play a role in LPS recognition within the cytoplasm remained unknown until recently. The last decade has seen an unprecedented unfolding of TLR4-independent LPS sensing pathways. First, transient receptor potential (TRP) channels have been identified as non-TLR membrane-bound sensors of LPS and, second, caspase-4/5 (and caspase-11 in mice) have been established as the cytoplasmic sensors for LPS. Here in this review, we detail the brief history of LPS discovery, followed by the discovery of TLR4, TRP as the membrane-bound sensor, and our current understanding of caspase-4/5/11 as cytoplasmic sensors.

Substrate structure-activity relationship reveals a limited lipopolysaccharide chemotype range for intestinal alkaline phosphatase

Lipopolysaccharide (LPS) from the Gram-negative bacterial outer membrane potently activates the human innate immune system. LPS is recognized by the Toll-like receptor 4/myeloid differentiation factor-2 (TLR4/MD2) complex, leading to the release of pro-inflammatory cytokines. Alkaline phosphatase (AP) is currently being investigated as an anti-inflammatory agent for detoxifying LPS through dephosphorylating lipid A, thus providing a potential treatment for managing both acute (sepsis) and chronic (metabolic endotoxemia) pathologies wherein aberrant TLR4/MD2 activation has been implicated. Endogenous LPS preparations are chemically heterogeneous, and little is known regarding the LPS chemotype substrate range of AP. Here, we investigated the activity of AP on a panel of structurally defined LPS chemotypes isolated from Escherichia coli and demonstrate that calf intestinal AP (cIAP) has only minimal activity against unmodified enteric LPS chemotypes. P_i was only released from a subset of LPS chemotypes harboring spontaneously labile phosphoethanolamine (PEtN) modifications connected through phosphoanhydride bonds. We demonstrate that the spontaneously hydrolyzed O-phosphorylethanolamine is the actual substrate for AP. We found that the 1- and 4'-lipid A phosphate groups critical in TLR4/MD2 signaling become susceptible to hydrolysis only after de-O-acylation of ester linked primary acyl chains on lipid A. Furthermore, PEtN modifications on lipid A specifically enhanced hTLR4 agonist activity of underacylated LPS preparations. Computational binding models are proposed to explain the limitation of AP substrate specificity imposed by the acylation state of lipid A, and the mechanism of PEtN in enhancing hTLR4/MD2 signaling.

Serum Albumin Is Associated With Higher Inflammation and Carotid Atherosclerosis in Treated Human Immunodeficiency Virus Infection

Background

This study was conducted to explore the associations between serum albumin and markers of inflammation and cardiovascular disease in treated human immunodeficiency virus (HIV)-infected adults.

Methods

We conducted a nested study within in the SATURN-HIV trial in which 147 HIV⁺ adults on stable antiretroviral therapy were (1) virally suppressed, (2) had a low-density lipoprotein (LDL)-cholesterol level <130 mg/dL, and (3) were randomized to 10 mg daily rosuvastatin or placebo. Measures of serum albumin, carotid intima media thickness ([cIMT] surrogate marker of atherosclerosis), inflammation, T cells, monocyte activation, and gut integrity were assessed at baseline, 48 and 96 weeks later. Spearman correlations and linear mixed-effect models were used to assess associations with serum albumin.

Results

Mean age was 45 years, 80% of participants were male, and 69% were African American. Mean serum albumin was similar between the groups at all time points (4.01-4.09 g/dL in statin arm vs 4.02-4.11 g/dL in placebo arm; P = .08-0.35). Lower baseline serum albumin significantly predicted larger changes in cIMT, interleukin 6, D-dimer, tumor necrosis factor α receptor 1, fibrinogen, and high-sensitivity C-reactive protein (P ≤ .03) over 96 weeks independently of statin therapy. After adjusting for age, gender, smoking, body mass index, creatinine clearance, and LDL cholesterol, every 1 g/dL decrease in serum albumin at baseline remained associated with a 0.05-mm increase in cIMT over 96 weeks (P = .05).

Conclusions

Lower serum albumin in controlled HIV is associated with higher markers of chronic inflammation and hypercoagulation, which could explain the prior observation that serum albumin predicts nonacquired immune deficiency syndrome events in HIV. Serum albumin may predict progression of carotid atherosclerosis independent of traditional risk factors.

Diverse pro-inflammatory endotoxin recognition systems of mammalian innate immunity

In humans and other mammals, recognition of endotoxins—abundant surface lipopolysaccharides (LPS) of Gram-negative bacteria—provides a potent stimulus for induction of inflammation and mobilization of host defenses. The structurally unique lipid A region of LPS is the principal determinant of this pro-inflammatory activity. This region of LPS is normally buried within the bacterial outer membrane and aggregates of purified LPS, making even more remarkable its picomolar potency and the ability of discrete variations in lipid A structure to markedly alter the pro-inflammatory activity of LPS. Two recognition systems—MD-2/TLR4 and “LPS-sensing” cytosolic caspases—together confer LPS responsiveness at the host cell surface, within endosomes, and at sites physically accessible to the cytosol. Understanding how the lipid A of LPS is delivered and recognized at these diverse sites is crucial to understanding how the magnitude and character of the inflammatory responses are regulated.

Toll-Like Receptor 4 Signalling and Its Impact on Platelet Function, Thrombosis, and Haemostasis

Platelets are anucleated blood cells that participate in a wide range of physiological and pathological functions. Their major role is mediating haemostasis and thrombosis. In addition to these classic functions, platelets have emerged as important players in the innate immune system. In particular, they interact with leukocytes, secrete pro- and anti-inflammatory factors, and express a wide range of inflammatory receptors including Toll-like receptors (TLRs), for example, Toll-like receptor 4 (TLR4). TLR4, which is the most extensively studied TLR in nucleated cells, recognises lipopolysaccharides (LPS) that are compounds of the outer surface of Gram-negative bacteria. Unlike other TLRs, TLR4 is able to signal through both the MyD88-dependent and MyD88-independent signalling pathways. Notably, despite both pathways culminating in the activation of transcription factors, TLR4 has a prominent functional impact on platelet activity, haemostasis, and thrombosis. In this review, we summarise the current knowledge on TLR4 signalling in platelets, critically discuss its impact on platelet function, and highlight the open questions in this area.

Funiculosin variants and phosphorylated derivatives promote innate immune responses via the Toll-like receptor 4/myeloid differentiation factor-2 complex

The Toll-like receptor 4 (TLR4)/myeloid differentiation factor-2 (MD-2) complex is essential for LPS recognition and induces innate immune responses against Gram-negative bacteria. As activation of TLR4/MD-2 is also critical for the induction of adaptive immune responses, TLR4/MD-2 agonists have been developed as vaccine adjuvants, but their efficacy has not yet been ascertained. Here, we demonstrate that a funiculosin (FNC) variant, FNC-RED, and FNC-RED and FNC derivatives are agonists for both murine and human TLR4/MD-2. FNC-RED induced nuclear factor-κB (NF-κB) activation via murine TLR4/MD-2, whereas FNC had no TLR4/MD-2 stimulatory activity. Biacore analysis revealed that FNC-RED binds to murine TLR4/MD-2 but not murine radioprotective 105 (RP105)/myeloid differentiation factor-1 (MD-1), another LPS sensor. FNC-RED induced CD14-independent expressions of pro-inflammatory cytokines and co-stimulatory molecules in murine macrophages and dendritic cells. In contrast, FNC-RED stimulation was reduced in CD14-dependent LPS responses, including dimerization and internalization of TLR4/MD-2 and IFN-β expression. FNC-RED-induced IL-12p40 production from murine dendritic cells was dependent on NF-κB but not MAPK pathway. In addition, fetal bovine serum augmented lipid A-induced NF-κB activation but blocked FNC-RED-mediated responses. Two synthetic phosphate group-containing FNC-RED and FNC derivatives, FNC-RED-P01 and FNC-P01, respectively, activated human TLR4/MD-2, unlike FNC-RED. Finally, computational analysis revealed that this species-specific activation by FNC-RED and FNC-RED-P01 resulted from differences in electrostatic surface potentials between murine and human TLR4/MD-2. We conclude that FNC-RED and its synthetic derivative represent a novel category of murine and human TLR4/MD-2 agonist.

High-affinity caspase-4 binding to LPS presented as high molecular mass aggregates or in outer membrane vesicles

Caspases of the non-canonical inflammasome (caspases -4, -5, and -11) directly bind endotoxin (LOS/LPS) and can be activated in the absence of any co-factors. Models of LPS-induced caspase activation have postulated that 1:1 binding of endotoxin monomers to caspase trigger caspase oligomerization and activation, analogous to that established for endotoxin-induced activation of MD-2/TLR4. However, using metabolically radiolabeled LOS and LPS, we now show high affinity and selective binding of caspase-4 to high molecular mass aggregates of purified endotoxin and to endotoxin-rich outer membrane vesicles without formation of 1:1 endotoxin:caspase complexes. Thus, our findings demonstrate markedly different endotoxin recognition properties of caspase-4 from that of MD-2/TLR4 and strongly suggest that activation of caspase-4 (and presumably caspase-5 and caspase-11) are mediated by interactions with activating endotoxin-rich membrane interfaces rather than by endotoxin monomers.

Enhancing actions of peptides derived from the γ-chain of fetal human hemoglobin on the immunostimulant activities of monophosphoryl lipid A

Hemoglobin and its structures have been described since the 1990s to enhance a variety of biological activities of endotoxins (LPS) in a dose-dependent manner. To investigate the interaction processes in more detail, the system was extended by studying the interactions of newly designed peptides from the γ-chain of human hemoglobin with the adjuvant monophosphoryl lipid A (MPLA), a partial structure of lipid A lacking its 1-phosphate. It was found that some selected Hbg peptides, in particular two synthetic substructures designated Hbg32 and Hbg35, considerably increased the bioactivity of MPLA, which alone was only a weak activator of immune cells. These findings hold true for human mononuclar cells, monocytes and T lymphocytes. To understand the mechanisms of action in more detail, biophysical techniques were applied. These showed a peptide-induced change of the MPLA aggregate structure from multilamellar into a non-lamellar, probably inverted, cubic structure. Concomitantly, the peptides incorporated into the tightly packed MPLA aggregates into smaller units down to monomers. The fragmentation of the aggregates was an endothermic process, differing from a complex formation but rather typical for a catalytic reaction.

Role of albumin in diseases associated with severe systemic inflammation: Pathophysiologic and clinical evidence in sepsis and in decompensated cirrhosis

The metabolism of albumin in inflammatory states such as sepsis or major surgery is complex and still not well characterized. Nevertheless, in inflammatory states, albumin synthesis has been observed to increase. By contrast, in decompensated cirrhosis, a disease characterized by systemic inflammation, albumin synthesis by the liver may decrease to 30% to 50% of normal values. Furthermore, in these conditions, there are high capillary leakage and altered albumin kinetics. The discussion regarding the effect of exogenous albumin administration on intravascular volume in inflammatory states should therefore address albumin turnover. To add complexity to our understanding of the effects of albumin, there are many data indicating that the therapeutic action of albumin is mediated not only through the impact on plasma volume expansion but also through a modulatory effect on inflammation and oxidative stress. All these characteristics are relevant to diseases associated with systemic inflammation including sepsis and decompensated cirrhosis.

Dichotomous roles for externalized cardiolipin in extracellular signaling: Promotion of phagocytosis and attenuation of innate immunity

Among the distinct molecular signatures present in the mitochondrion is the tetra-acylated anionic phospholipid cardiolipin, a lipid also present in primordial, single-cell bacterial ancestors of mitochondria and multiple bacterial species today. Cardiolipin is normally localized to the inner mitochondrial membrane; however, when cardiolipin becomes externalized to the surface of dysregulated mitochondria, it promotes inflammasome activation and stimulates the elimination of damaged or nonfunctional mitochondria by mitophagy. Given the immunogenicity of mitochondrial and bacterial membranes that are released during sterile and pathogen-induced trauma, we hypothesized that cardiolipins might function as "eat me" signals for professional phagocytes. In experiments with macrophage cell lines and primary macrophages, we found that membranes with mitochondrial or bacterial cardiolipins on their surface were engulfed through phagocytosis, which depended on the scavenger receptor CD36. Distinct from this process, the copresentation of cardiolipin with the Toll-like receptor 4 (TLR4) agonist lipopolysaccharide dampened TLR4-stimulated production of cytokines. These data suggest that externalized, extracellular cardiolipins play a dual role in host-host and host-pathogen interactions by promoting phagocytosis and attenuating inflammatory immune responses.

Purified monomeric ligand.MD-2 complexes reveal molecular and structural requirements for activation and antagonism of TLR4 by Gram-negative bacterial endotoxins

A major focus of work in our laboratory concerns the molecular mechanisms and structural bases of Gram-negative bacterial endotoxin recognition by host (e.g., human) endotoxin-recognition proteins that mediate and/or regulate activation of Toll-like receptor (TLR) 4. Here, we review studies of wild-type and variant monomeric endotoxin.MD-2 complexes first produced and characterized in our laboratories. These purified complexes have provided unique experimental reagents, revealing both quantitative and qualitative determinants of TLR4 activation and antagonism. This review is dedicated to the memory of Dr. Theresa L. Gioannini (1949-2014) who played a central role in many of the studies and discoveries that are reviewed.

Emerging principles governing signal transduction by pattern-recognition receptors

The problem of recognizing and disposing of non-self-organisms, whether for nutrients or defense, predates the evolution of multicellularity. Accordingly, the function of the innate immune system is often intimately associated with fundamental aspects of cell biology. Here, we review our current understanding of the links between cell biology and pattern-recognition receptors of the innate immune system. We highlight the importance of receptor localization for the detection of microbes and for the initiation of antimicrobial signaling pathways. We discuss examples that illustrate how pattern-recognition receptors influence, and are influenced by, the general membrane trafficking machinery of mammalian cells. In the future, cell biological analysis likely will rival pure genetic analysis as a tool to uncover fundamental principles that govern host-microbe interactions.

Co-operation of TLR4 and raft proteins in LPS-induced pro-inflammatory signaling

Toll-like receptor 4 (TLR4) is activated by lipopolysaccharide (LPS), a component of Gram-negative bacteria to induce production of pro-inflammatory mediators aiming at eradication of the bacteria. Dysregulation of the host responses to LPS can lead to a systemic inflammatory condition named sepsis. In a typical scenario, activation of TLR4 is preceded by binding of LPS to CD14 protein anchored in cholesterol- and sphingolipid-rich microdomains of the plasma membrane called rafts. CD14 then transfers the LPS to the TLR4/MD-2 complex which dimerizes and triggers MyD88- and TRIF-dependent production of pro-inflammatory cytokines and type I interferons. The TRIF-dependent signaling is linked with endocytosis of the activated TLR4, which is controlled by CD14. In addition to CD14, other raft proteins like Lyn tyrosine kinase of the Src family, acid sphingomyelinase, CD44, Hsp70, and CD36 participate in the TLR4 signaling triggered by LPS and non-microbial endogenous ligands. In this review, we summarize the current state of the knowledge on the involvement of rafts in TLR4 signaling, with an emphasis on how the raft proteins regulate the TLR4 signaling pathways. CD14-bearing rafts, and possibly CD36-rich rafts, are believed to be preferred sites of the assembly of a multimolecular complex which mediates the endocytosis of activated TLR4.

Human serum albumin, systemic inflammation, and cirrhosis

Human serum albumin (HSA) is one of the most frequent treatments in patients with decompensated cirrhosis. Prevention of paracentesis-induced circulatory dysfunction, prevention of type-1 HRS associated with bacterial infections, and treatment of type-1 hepatorenal syndrome are the main indications. In these indications treatment with HSA is associated with improvement in survival. Albumin is a stable and very flexible molecule with a heart shape, 585 residues, and three domains of similar size, each one containing two sub-domains. Many of the physiological functions of HSA rely on its ability to bind an extremely wide range of endogenous and exogenous ligands, to increase their solubility in plasma, to transport them to specific tissues and organs, or to dispose of them when they are toxic. The chemical structure of albumin can be altered by some specific processes (oxidation, glycation) leading to rapid clearance and catabolism. An outstanding feature of HSA is its capacity to bind lipopolysaccharide and other bacterial products (lipoteichoic acid and peptidoglycan), reactive oxygen species, nitric oxide and other nitrogen reactive species, and prostaglandins. Binding to NO and prostaglandins are reversible, so they can be transferred to other molecules at different sites from their synthesis. Through these functions, HSA modulates the inflammatory reaction. Decompensated cirrhosis is a disease associated systemic inflammation, which plays an important role in the pathogenesis of organ or system dysfunction/failure. Although, the beneficial effects of HAS have been traditionally attributed to plasma volume expansion, they could also relate to its effects modulating systemic and organ inflammation.

Tetraacylated lipid A and paclitaxel-selective activation of TLR4/MD-2 conferred through hydrophobic interactions

LPS exerts potent immunostimulatory effects through activation of the TLR4/MD-2 receptor complex. The hexaacylated lipid A is an agonist of mouse (mTLR4) and human TLR4/MD-2, whereas the tetraacylated lipid IVa and paclitaxel activate only mTLR4/MD-2 and antagonize activation of the human receptor complex. Hydrophobic mutants of TLR4 or MD-2 were used to investigate activation of human embryonic kidney 293 cells by different TLR4 agonists. We show that each of the hydrophobic residues F438 and F461, which are located on the convex face of leucine-rich repeats 16 and 17 of the mTLR4 ectodomain, are essential for activation of with lipid IVa and paclitaxel, which, although not a structural analog of LPS, activates cells expressing mTLR4/MD-2. Both TLR4 mutants were inactive when stimulated with lipid IVa or paclitaxel, but retained significant activation when stimulated with LPS or hexaacylated lipid A. We show that the phenylalanine residue at position 126 of mouse MD-2 is indispensable only for activation with paclitaxel. Its replacement with leucine or valine completely abolished activation with paclitaxel while preserving the responsiveness to lipid IVa and lipid A. This suggests specific interaction of paclitaxel with F126 because its replacement with leucine even augmented activation by lipid A. These results provide an insight into the molecular mechanism of TLR4 activation by two structurally very different agonists.

Cell surface nucleolin is crucial in the activation of the CXCL12/CXCR4 signaling pathway

Recently, CXCL12-CXCR4 has been focused on therapeutic strategies for papillary thyroid carcinoma (PTC) and other cancers. At the same time, cell surface nucleolin is also over-expressed in PTC and others. Interestingly, a few reports suggest that either CXCR4 or cell surface nucleolin is a co-receptor for HIV-1 entry into CD4+ T cells, which indicates that there is a relationship between CXCR4 and nucleolin. In this study, antibody and siRNA were used to identify effects of cell surface nucleolin and CXCR4 on cell signaling; soft-agar colony formation assay and Transwell assay were used to determine roles of nucleolin and CXCR4 in cell proliferation and migration. Importantly, co-immunoprecipitation was used to demonstrate the relationship between CXCR4 and nucleolin. Results showed CXCR4 and nucleolin were co-expressed in PTC cell line K1, B-CPAP, and TPC-1. Either cell surface nucleolin or CXCR4 was necessary to prompt extracellular signal-regulated kinase phosphorylation. When blocked, CXCR4 or nucleolin can significantly affect TPC-1 proliferation and migration (p < 0.01). Co-immunoprecipitation analysis identified that nucleolin can bind and interact with CXCR4 to activate CXCR4 signaling. This study suggests that nucleolin is crucial in the activation of CXCR4 signaling, which affects cell growth, migration, and invasiveness. Further, nucleolin may interact with other receptors. Our study also offers new ideas for cancer therapy.

CD14 mediates binding of high doses of LPS but is dispensable for TNF-α production

Activation of macrophages with lipopolysaccharide (LPS) involves a sequential engagement of serum LPS-binding protein (LBP), plasma membrane CD14, and TLR4/MD-2 signaling complex. We analyzed participation of CD14 in TNF-α production stimulated with 1-1000 ng/mL of smooth or rough LPS (sLPS or rLPS) and in sLPS binding to RAW264 and J744 cells. CD14 was indispensable for TNF-α generation induced by a low concentration, 1 ng/mL, of sLPS and rLPS. At higher doses of both LPS forms (100-1000 ng/mL), TNF-α release required CD14 to much lower extent. Among the two forms of LPS, rLPS-induced TNF-α production was less CD14-dependent and could proceed in the absence of serum as an LBP source. On the other hand, the involvement of CD14 was crucial for the binding of 1000 ng/mL of sLPS judging from an inhibitory effect of the anti-CD14 antibody. The binding of sLPS was also strongly inhibited by dextran sulfate, a competitive ligand of scavenger receptors (SR). In the presence of dextran sulfate, sLPS-induced production of TNF-α was upregulated about 1.6-fold. The data indicate that CD14 together with SR participates in the binding of high doses of sLPS. However, CD14 contribution to TNF α production induced by high concentrations of sLPS and rLPS can be limited.

Direct proteomic quantification of the secretome of activated immune cells

Protein secretion allows communication of distant cells in an organism and controls a broad range of physiological functions. We describe a quantitative, high-resolution mass spectrometric workflow to detect and quantify proteins that are released from immune cells upon receptor ligation. We quantified the time-resolved release of 775 proteins, including 52 annotated cytokines from only 150,000 primary Toll-like receptor 4-activated macrophages per condition. Achieving low picogram sensitivity, we detected secreted proteins whose abundance increased by a factor of more than 10,000 upon stimulation. Secretome to transcriptome comparisons revealed the transcriptionally decoupled release of lysosomal proteins. From genetic models, we defined secretory profiles that depended on distinct intracellular signaling adaptors and showed that secretion of many proinflammatory proteins is safeguarded by redundant mechanisms, whereas signaling adaptor synergy promoted the release of anti-inflammatory proteins.

The TLR4 antagonist Eritoran protects mice from lethal influenza infection

There is a pressing need to develop alternatives to annual influenza vaccines and antiviral agents licensed for mitigating influenza infection. Previous studies reported that acute lung injury caused by chemical or microbial insults is secondary to the generation of host-derived, oxidized phospholipid that potently stimulates Toll-like receptor 4 (TLR4)-dependent inflammation. Subsequently, we reported that Tlr4(-/-) mice are highly refractory to influenza-induced lethality, and proposed that therapeutic antagonism of TLR4 signalling would protect against influenza-induced acute lung injury. Here we report that therapeutic administration of Eritoran (also known as E5564)-a potent, well-tolerated, synthetic TLR4 antagonist-blocks influenza-induced lethality in mice, as well as lung pathology, clinical symptoms, cytokine and oxidized phospholipid expression, and decreases viral titres. CD14 and TLR2 are also required for Eritoran-mediated protection, and CD14 directly binds Eritoran and inhibits ligand binding to MD2. Thus, Eritoran blockade of TLR signalling represents a novel therapeutic approach for inflammation associated with influenza, and possibly other infections.

Radioiodination of an endotoxin·MD-2 complex generates a novel sensitive, high-affinity ligand for TLR4

A purified complex of metabolically labeled [(3)H]lipooligosaccharide (LOS) and recombinant human myeloid differentiation factor 2 (MD-2), [(3)H]LOS·MD-2, has been used to demonstrate pM affinity binding interactions with soluble TLR4 ectodomain (TLR4ecd). For measurement of the binding parameters of membrane-bound TLR4, we took advantage of the stability of endotoxin·MD-2 and tyrosine(s) present on the surface of MD-2 to radioiodinate LOS·MD-2. Radioiodinated LOS·MD-2 generated a reagent with an estimated 1:1 molar ratio of [(125)I] to sMD-2 with 20-fold higher specific radioactivity and TLR4-activating properties comparable to metabolically-labeled LOS·MD-2. LOS·MD-2[(125)I] and [(3)H]LOS·MD-2 have similar affinities for soluble (FLAG) TLR4ecd and for membrane-bound TLR4 in HEK293T/TLR4 cells. In a similar dose-dependent manner, sMD-2 and LOS·MD-2 inhibit LOS·MD-2[(125)I] binding to TLR4 indicating the pM affinity binding of LOS·MD-2[(125)I] is agonist-independent. LOS·MD-2[(125)I] allowed measurement of low levels of cell-surface human or murine TLR4 expressed by stable cell lines (2000-3000 sites/cell) and quantitatively measures low levels of 'MD-2-free' TLR4 (est. 250 molecules/cell) in cells co-expressing TLR4 and MD-2. Occupation of 50-100 TLR4/cell by LOS·MD-2 is sufficient to trigger measurable TLR4-dependent cell activation. LOS·MD-2[(125)I] provides a powerful reagent to measure quantitatively functional human and murine cell-surface TLR4, including in cells where surface TLR4 is potentially functionally significant but not detectable by other methods.

NMR studies of hexaacylated endotoxin bound to wild-type and F126A mutant MD-2 and MD-2·TLR4 ectodomain complexes

Host response to invasion by many gram-negative bacteria depends upon activation of Toll-like receptor 4 (TLR4) by endotoxin presented as a monomer bound to myeloid differentiation factor 2 (MD-2). Metabolic labeling of hexaacylated endotoxin (LOS) from Neisseria meningitidis with [(13)C]acetate allowed the use of NMR to examine structural properties of the fatty acyl chains of LOS present in TLR4-agonistic and -antagonistic binary and ternary complexes with, respectively, wild-type or mutant (F126A) MD-2 ± TLR4 ectodomain. Chemical shift perturbation indicates that Phe(126) affects the environment and/or position of each of the bound fatty acyl chains both in the binary LOS·MD-2 complex and in the ternary LOS·MD-2·TLR4 ectodomain complex. In both wild-type and mutant LOS·MD-2 complexes, one of the six fatty acyl chains of LOS is more susceptible to paramagnetic attenuation, suggesting protrusion of that fatty acyl chain from the hydrophobic pocket of MD-2, independent of association with TLR4. These findings indicate that re-orientation of the aromatic side chain of Phe(126) is induced by binding of hexaacylated E, preceding interaction with TLR4. This re-arrangement of Phe(126) may act as a "hydrophobic switch," driving agonist-dependent contacts needed for TLR4 dimerization and activation.