Structural investigations into the interaction of hemoglobin and part structures with bacterial endotoxins

Quantitative assessment of LPS-HBsAg interaction by introducing a novel application of immunoaffinity chromatography

Lipopolysaccharide (LPS), as a stubborn contamination, should be monitored and kept in an acceptable level during the pharmaceutical production process. Recombinant hepatitis B surface antigen (r-HBsAg) is one of the recombinant biological products, which is probable to suffer from extrinsic endotoxin due to its long and complex production process. This research aims to assess the potential interaction between LPS and r-HBsAg by recruiting immunoaffinity chromatography (IAC) as a novel tool to quantify the interaction. Molecular modeling was performed on the HBsAg molecule to theoretically predict its potential binding and interaction sites. Then dynamic light scattering (DLS) analysis was implemented on HBsAg, LPS, and mixtures of them to reveal the interaction. The virus-like particle (VLP) structure of HBsAg and the ribbon-like structure of LPS were visualized by transmission electron microscopy (TEM). Finally, the interaction was quantified by applying various LPS/HBsAg ratios ranging from 1.67 to 120 EU/dose in the IAC. Consequently, the LPS/HBsAg ratios in the eluate were measured from 1.67 to a maximum of 92.5 EU/dose. The results indicated that 77 to 100% of total LPS interacted with HBsAg by an inverse relationship to the incubated LPS concentration. The findings implied that the introduced procedure is remarkably practical in the quantification of LPS interaction with a target recombinant protein.

Biophysical Analysis of Lipopolysaccharide Formulations for an Understanding of the Low Endotoxin Recovery (LER) Phenomenon

Lipopolysaccharides (LPS, endotoxin) are complex and indispensable components of the outer membrane of most Gram-negative bacteria. They represent stimuli for many biological effects with pathophysiological character. Recombinant therapeutic proteins that are manufactured using biotechnological processes are prone to LPS contaminations due to their ubiquitous occurrence. The maximum endotoxin load of recombinant therapeutic proteins must be below the pyrogenic threshold. Certain matrices that are commonly used for recombinant therapeutic proteins show a phenomenon called “Low Endotoxin Recovery (LER)”. LER is defined as the loss of detectable endotoxin activity over time using compendial Limulus amebocyte lysate (LAL) assays when undiluted products are spiked with known amount of endotoxin standards. Because LER poses potential risks that endotoxin contaminations in products may be underestimated or undetected by the LAL assay, the United States (U.S.) Food and Drug Administration’s (FDA’s) Center for Drug Evaluation and Research (CDER) has recently started requesting that companies conduct endotoxin spike/hold recovery studies to determine whether a given biological product causes LER. Here, we have performed an analysis of different LPS preparations with relevant detergents studying their acyl chain phase transition, their aggregate structures, their size distributions, and binding affinity with a particular anti-endotoxin peptide, and correlating it with the respective data in the macrophage activation test. In this way, we have worked out biophysical parameters that are important for an understanding of LER.

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.

Mechanism of Hbγ-35-induced an increase in the activation of the human immune system by endotoxins

Endotoxins (LPS) are highly potent immune stimulatory molecules and are mainly known for triggering Gram-negative sepsis. However, besides their toxic effects, this stimulatory function may be advantageous, for example when used as an adjuvant during vaccination. Thus, there is always a narrow range between the useful wake-up of the immune system and its overwhelming reaction, which can lead to diseases like sepsis. This raises the question of which conformational properties are responsible for making the LPS aggregates more or less potent. As described previously, the size, type and form of LPS aggregates play a major role in their immune stimulatory activity. In this study we investigate the role of these parameters. On the one hand, we use a peptide (Pep19-2.5; Aspidasept) that causes a change of the LPS aggregate structure into a less toxic state; on the other hand, we use a potent immune stimulating peptide (Hbγ-35), leading to higher toxicity. We have found opposing effects on LPS aggregate conformations allowing a better understanding of the processes of immune stimulation.

Pulmonary surfactant protein A-induced changes in the molecular conformation of bacterial deep-rough LPS lead to reduced activity on human macrophages

The lung is constantly exposed to immune stimulation by LPS from inhaled microorganisms. A primary mechanism to maintain immune homeostasis is based on anti-inflammatory regulation by surfactant protein A (SP-A), a secreted component of lung innate immunity. The architecture of LPS aggregates is strongly associated with biological activity. We therefore investigated whether SP-A affects the physico-chemical properties of LPS. Determination of the three-dimensional aggregate structure of LPS by small-angle X-ray scattering demonstrated that SP-A induced the formation of multi-lamellar aggregate structures. Determination of the acyl-chain-fluidity of LPS aggregates by Fourier transform infrared (FTIR) spectroscopy showed that the phase transition temperature of LPS was reduced in the presence of SP-A. The phosphate groups at the diglucosamine backbone of LPS represent important functional groups for the bioactivity of LPS. FTIR analysis revealed changes in the vibrational bands νas PO-(2), indicating an interaction of SP-A with the 1-phosphate, but not with the 4'-phosphate. The physico-chemical changes induced by SP-A were associated with up to 90% reduction in LPS-induced TNF-α-production by human macrophages. In conclusion, our data demonstrate that the SP-A/LPS interaction induces conformational changes in LPS aggregates leading to biologically less active structures, thereby providing a new molecular mechanism of immune modulation by SP-A.

Pathways involved in the synergistic activation of macrophages by lipoteichoic acid and hemoglobin

Lipoteichoic acid (LTA) is a Gram-positive cell surface molecule that is found in both a cell-bound form and cell-free form in the host during an infection. Hemoglobin (Hb) can synergize with LTA, a TLR2 ligand, to potently activate macrophage innate immune responses in a TLR2- and TLR4-dependent way. At low levels of LTA, the presence of Hb can result in a 200-fold increase in the secretion of IL-6 following macrophage activation. Six hours after activation, the macrophage genes that are most highly up-regulated by LTA plus Hb activation compared to LTA alone are cytokines, chemokines, receptors and interferon-regulated genes. Several of these genes exhibit a unique TLR4-dependent increase in mRNA levels that continued to rise more than eight hours after stimulation. This prolonged increase in mRNA levels could be the result of an extended period of NF-κB nuclear localization and the concurrent absence of the NF-κB inhibitor, IκBα, after stimulation with LTA plus Hb. Dynasore inhibition experiments indicate that an endocytosis-dependent pathway is required for the TLR4-dependent up-regulation of IL-6 secretion following activation with LTA plus Hb. In addition, interferon-β mRNA is present after activation with LTA plus Hb, suggesting that the TRIF/TRAM-dependent pathway may be involved. Hb alone can elicit the TLR4-dependent secretion of TNF-α from macrophages, so it may be the TLR4 ligand. Hb also led to secretion of high mobility group box 1 protein (HMGB1), which synergized with LTA to increase secretion of IL-6. The activation of both the TLR2 and TLR4 pathways by LTA plus Hb leads to an enhanced innate immune response.

Identification of hemopexin as an anti-inflammatory factor that inhibits synergy of hemoglobin with HMGB1 in sterile and infectious inflammation

Hemoglobin is released from lysed RBCs in numerous clinical settings. High mobility group box 1 (HMGB1) is a nuclear and cytosolic DNA-binding protein released from injured cells that has been shown to play an important role in inducing inflammation. Because both of these endogenous molecules are frequently present in sites of necrosis and inflammation, we studied their interaction on the activation of macrophages. We report in this article that hemoglobin and HMGB1 synergize to activate mouse macrophages to release significantly increased proinflammatory cytokines. Addition of microbial ligands that activate through TLR2 or TLR4 resulted in further significant increases, in a "three-way" synergy between endogenous and microbial ligands. The synergy was strongly suppressed by hemopexin (Hx), an endogenous heme-binding plasma protein. The findings suggest that hemoglobin may play an important role in sterile and infectious inflammation, and that endogenous Hx can modulate this response. Administration of Hx may be beneficial in clinical settings characterized by elevated extracellular hemoglobin and HMGB1.

Biophysical investigations into the interactions of endotoxins with bile acids

The interaction of selected endotoxin preparations (lipid A from Erwinia carotovora and LPS Re and Ra from Salmonella enterica sv. Minnesota strains R595 and R60, respectively) with selected bile acids was investigated biophysically. Endotoxin aggregates were analyzed for their gel-to-liquid crystalline phase behavior, the type of their aggregates, the conformation of particular functional groups, and their Zeta potential in the absence and presence of the bile acids by applying Fourier-transform infrared spectroscopy, differential scanning calorimetry, measurements of the electrophoretic mobility, and synchrotron radiation X-ray scattering. In addition, the ability of the endotoxins to induce cytokines in human mononuclear cells was tested in the absence and presence of varying concentrations of bile acids. The data show that the endotoxin:bile acid interaction is not governed by Coulomb forces, rather a hydrophobic interaction takes place. This leads to an enhanced formation of the inherent cubic aggregate structures of the endotoxins, concomitant with a slight disaggregation, as evidenced by freeze-fracture electron microscopy. Parallel to this, the addition of bile acids increased the bioactivity of lipid A and, to a lower degree, also that of the tested rough mutant LPS at lower concentrations of the endotoxin preparation, a finding similar as reported for the interaction of other agents such as hemoglobin. These data imply that there are general mechanisms that govern the expression of biological activities of endotoxins.

Delineation of lipopolysaccharide (LPS)-binding sites on hemoglobin: from in silico predictions to biophysical characterization

Hemoglobin (Hb) functions as a frontline defense molecule during infection by hemolytic microbes. Binding to LPS induces structural changes in cell-free Hb, which activates the redox activity of the protein for the generation of microbicidal free radicals. Although the interaction between Hb and LPS has implications for innate immune defense, the precise LPS-interaction sites on Hb remain unknown. Using surface plasmon resonance, we found that both the Hb α and β subunits possess high affinity LPS-binding sites, with K(D) in the nanomolar range. In silico analysis of Hb including phospho-group binding site prediction, structure-based sequence comparison, and docking to model the protein-ligand interactions showed that Hb possesses evolutionarily conserved surface cationic patches that could function as potential LPS-binding sites. Synthetic Hb peptides harboring predicted LPS-binding sites served to validate the computational predictions. Surface plasmon resonance analysis differentiated LPS-binding peptides from non-binders. Binding of the peptides to lipid A was further substantiated by a fluorescent probe displacement assay. The LPS-binding peptides effectively neutralized the endotoxicity of LPS in vitro. Additionally, peptide B59 spanning residues 59-95 of Hbβ attached to the surface of Gram-negative bacteria as shown by flow cytometry and visualized by immunogold-labeled scanning electron microscopy. Site-directed mutagenesis of the Hb subunits further confirmed the function of the predicted residues in binding to LPS. In summary, the integration of computational predictions and biophysical characterization has enabled delineation of multiple LPS-binding hot spots on the Hb molecule.

A rabbit vaginal cell-derived antimicrobial peptide, RVFHbαP, blocks lipopolysaccharide-mediated inflammation in human vaginal cells in vitro

Antimicrobial peptides (AMPs) constitute a phylogenetically ancient form of innate immunity that provides host defense at various mucosal surfaces, including the vagina. Recently, we have identified one such AMP, rabbit vaginal fluid hemoglobin alpha peptide (RVFHbαP), from the vaginal lavage of rabbits (Oryctolagus cuniculus). The recent demonstration of a protective role of this peptide in erythrocytes and vaginal cells led us to investigate (i) the lipopolysaccharide (LPS) interactive domain in RVFHbαP and (ii) whether RVFHbαP of rabbit origin modulates the cellular immune responses of another species (humans) in vitro. HeLa-S3, a human vaginal epithelial cell line (hVEC), was exposed to LPS alone (10 μg/ml for 6 h), or LPS-induced cells were treated with RVFHbαP (70.45 μM for 1 h) and cultured for 24 h, and the results obtained were compared with the medium control. We show here that RVFHbαP exerts an anti-inflammatory activity in hVECs, as suggested by the prevention of LPS-induced production of extracellular (supernatant) and intracellular (lysate) levels of cytokines (interleukin 6 [IL-6] and IL-1α) and chemokines (IL-8 and monocyte chemoattractant protein 1 [MCP-1]). The demonstration of Toll-like receptor 4 (TLR4) and NF-κB expression in hVECs and the observations of RVFHbαP suppression of human β-defensin-1 (hBD1) mRNA expression further support the hypothesis of a genomic activity of RVFHbαP. Confocal microscopy and flow cytometry results demonstrate that RVFHbαP inhibits LPS-induced phagocytosis of Escherichia coli by macrophages. The chemotaxis studies performed using the Boyden chamber Transwell method showed the increased migration of U937 cells when supernatants of LPS-induced hVECs were used, and this effect was inhibited by RVFHbαP. In conclusion, our study proposes a novel explanation for the protective role of RVFHbαP in inflammation-associated infections, which not only may provide the new cellular targets for the screening of RVFHbαP ligands acting in the vaginal tissue but also has the potential to develop RVFHbαP as a therapeutic agent for reproductive tract infections.

Morphology, size distribution, and aggregate structure of lipopolysaccharide and lipid A dispersions from enterobacterial origin

Lipopolysaccharides (LPSs) from Gram-negative bacteria are strong elicitors of the human immune systems. There is strong evidence that aggregates and not monomers of LPS play a decisive role at least in the initial stages of cell activation of immune cells such as mononuclear cells. In previous reports, it was shown that the biologically most active part of enterobacterial LPS, hexa-acyl bisphosphorylated lipid A, adopts a particular supramolecular conformation, a cubic aggregate structure. However, little is known about the size and morphology of these aggregates, regarding the fact that LPS may have strong variations in the length of the saccharide chains (various rough mutant and smooth-form LPS). Thus, in the present paper, several techniques for the determination of details of the aggregate morphology such as freeze-fracture and cryo-electron microscopy, analytical ultracentrifugation, laser backscattering analysis, and small-angle X-ray scattering were applied for various endotoxin (lipid A and different LPS) preparations. The data show a variety of different morphologies not only for different endotoxins but also when comparing different applied techniques. The data are interpreted with respect to the suitability of the single techniques, in particular on the basis of available literature data.

Toll-like receptor-2 mediates diet and/or pathogen associated atherosclerosis: proteomic findings

Background

Accumulating evidence implicates a fundamental link between the immune system and atherosclerosis. Toll-like receptors are principal sensors of the innate immune system. Here we report an assessment of the role of the TLR2 pathway in atherosclerosis associated with a high-fat diet and/or bacteria in ApoE^+/− mice.

Methods and Results

To explore the role of TLR2 in inflammation- and infection-associated atherosclerosis, 10 week-old ApoE^+/−-TLR2^+/+, ApoE^+/−-TLR2^+/− and ApoE^+/−-TLR2^−/− mice were fed either a high fat diet or a regular chow diet. All mice were inoculated intravenously, once per week for 24 consecutive weeks, with 50 µl live Porphyromonas gingivalis (P.g) (10⁷ CFU) or vehicle (normal saline). Animals were euthanized 24 weeks after the first inoculation. ApoE^+/−-TLR2^+/+ mice showed a significant increase in atheromatous lesions in proximal aorta and aortic tree compared to ApoE^+/−-TLR2^+/− and ApoE^+/−-TLR2^−/− mice for all diet conditions. They also displayed profound changes in plaque composition, as evidenced by increased macrophage infiltration and apoptosis, increased lipid content, and decreased smooth muscle cell mass, all reflecting an unstable plaque phenotype. SAA levels from ApoE^+/−-TLR2^+/+ mice were significantly higher than from ApoE^+/−-TLR2^+/− and ApoE^+/−-TLR2^−/− mice. Serum cytokine analysis revealed increased levels of pro-inflammatory cytokines in ApoE^+/−-TLR2^+/+ mice compared to ApoE^+/−-TLR2^+/− and TLR2^−/− mice, irrespective of diet or bacterial challenge. ApoE^+/−-TLR2^+/+ mice injected weekly for 24 weeks with FSL-1 (a TLR2 agonist) also demonstrated significant increases in atherosclerotic lesions, SAA and serum cytokine levels compared to ApoE^+/−-TLR2^−/− mice under same treatment condition. Finally, mass-spectrometry (MALDI-TOF-MS) of aortic samples analyzed by 2-dimentional gel electrophoresis differential display, identified 6 proteins upregulated greater than 2-fold in ApoE^+/−-TLR2^+/+ mice fed the high fat diet and inoculated with P.g compared to any other group.

Conclusion

Genetic deficiency of TLR2 reduces diet- and/or pathogen-associated atherosclerosis in ApoE^+/− mice, along with differences in plaque composition suggesting greater structural stability while TLR-2 ligand-specific activation triggers atherosclerosis. The present data offers new insights into the pathophysiological pathways involved in atherosclerosis and paves the way for new pharmacological interventions aimed at reducing atherosclerosis.