Change in sensitivity to lipopolysaccharide during the differentiation of human monocytes to macrophages in vitro

The dangerous liaisons in innate immunity involving recombinant proteins and endotoxins: Examples from the literature and the Leptospira field

Endotoxins, also known as lipopolysaccharides (LPS), are essential components of cell walls of diderm bacteria such as Escherichia coli. LPS are microbe-associated molecular patterns that can activate pattern recognition receptors. While trying to investigate the interactions between proteins and host innate immunity, some studies using recombinant proteins expressed in E. coli reported interaction and activation of immune cells. Here, we set out to provide information on endotoxins that are highly toxic to humans and bind to numerous molecules, including recombinant proteins. We begin by outlining the history of the discovery of endotoxins, their receptors and the associated signaling pathways that confer extreme sensitivity to immune cells, acting alone or in synergy with other microbe-associated molecular patterns. We list the various places where endotoxins have been found. Additionally, we warn against the risk of data misinterpretation due to endotoxin contamination in recombinant proteins, which is difficult to estimate with the Limulus amebocyte lysate assay, and cannot be completely neutralized (e.g., treatment with polymyxin B or heating). We further illustrate our point with examples of recombinant heat-shock proteins and viral proteins from severe acute respiratory syndrome coronavirus 2, dengue and HIV, for which endotoxin contamination has eventually been shown to be responsible for the inflammatory roles previously ascribed. We also critically appraised studies on recombinant Leptospira proteins regarding their putative inflammatory roles. Finally, to avoid these issues, we propose alternatives to express recombinant proteins in nonmicrobial systems. Microbiologists wishing to undertake innate immunity studies with their favorite pathogens should be aware of these difficulties.

Differential susceptibility to lipopolysaccharide affects the activation of toll-like-receptor 4 signaling in THP-1 cells and PMA-differentiated THP-1 cells

Monocytes and macrophages that originate from common myeloid progenitors perform various crucial roles in the innate immune system. Stimulation with LPS combined with TLR4 drives the production of pro-inflammatory cytokines through MAPKs and NF-κB pathway in different cells. However, the difference in LPS susceptibility between monocytes and macrophages is poorly understood. In this study, we found that pro-inflammatory cytokines-IL-1β, IL-6 and TNFα showed greater induction in phorbol-12-myristate-13-acetate (PMA)-differentiated THP-1 cells than in THP-1 cells. To determine the difference in cytokine expression, the surface proteins such as TLR4-related proteins and intracellular adaptor proteins were more preserved in PMA-differentiated THP-1 cells than in THP-1 cells. MyD88 is a key molecule responsible for the difference in LPS susceptibility. Moreover, MAPKs and NF-κB pathway-related molecules showed higher levels of phosphorylation in PMA-differentiated THP-1 cells than in THP-1 cells. Upon MyD88 depletion, there was no difference in the phosphorylation of MAPK pathway-related molecules. Therefore, these results demonstrate that the difference in LPS susceptibility between THP-1 cells and PMA-differentiated THP-1 cells occur as a result of gap between the activated MAPKs and NF-κB pathways via changes in the expression of LPS-related receptors and MyD88.

Metformin inhibits monocyte adhesion to endothelial cells and foam cell formation

The United Kingdom Prospective Diabetes Study (UKPDS) found that metformin reduces macrovascular complications in type 2 diabetic patients. To investigate the mechanisms involved we examined the effect of metformin on monocyte adhesion to human endothelial cells (ECs) induced by advanced glycation end-products (AGE), and on monocyte differentiation into macrophages and foam cell formation. Treatment of human ECs with AGEs (100 µg/ml) for up to 12 hours significantly increased human monocyte adhesion. Pre-treatment of the cells with metformin (0.1—2.5 µg/ml) inhibited AGE-induced monocyte adhesion and expression of endothelial cell adhesion molecules. In culture, human monocytes spontaneously differentiated into macrophages, as indicated by phenotypic changes, and increased expression of lectin-like oxidised low-density lipoprotein (LDL) receptor and scavenger receptor type A. Incubation of these cells in the presence of metformin decreased expression of all of these parameters. Metformin also inhibited foam cell formation induced by minimally modified LDL. Overall, these results suggest new mechanisms by which metformin may reduce the risk of vascular complications in patients with type 2 diabetes.

Movement-evoked hyperalgesia induced by lipopolysaccharides is not suppressed by glucocorticoids

Systemic exposure to lipopolysaccharides (LPS) produces a variety of effects, including movement-evoked hyperalgesia that can be measured using the grip force assay in mice. Because both lethality and enhanced sensitivity to cutaneous pain following exposure to endotoxins have each been attributed to inflammatory mediators, we explored the possibility that LPS-induced movement-evoked hyperalgesia is also sensitive to manipulations of glucocorticoids that regulate these other LPS responses. We found that the hyperalgesic effect of LPS (5mg/kg s.c.) in mice that were adrenalectomized did not differ from that in control mice that were sham operated, even though mortality after LPS was potentiated by adrenalectomy. The development of tolerance to the movement-evoked hyperalgesic effect of LPS also did not differ between adrenalectomized and sham-operated control mice. In addition, mifepristone (25mg/kg s.c.), a glucocorticoid antagonist, did not attenuate the hyperalgesic effect of LPS (2mg/kg s.c.), yet this dose of mifepristone was sufficient to enhance the incidence of lethality induced by LPS. Enhancement of glucocorticoid activity by two injections of dexamethasone (1mg/kg s.c.) had no effect on the degree of hyperalgesia in mice injected with LPS (5mg/kg s.c.), yet this dose of dexamethasone was sufficient to attenuate the incidence of mortality induced by LPS in adrenalectomized mice. Finally, morphine (10mg/kg i.p.) reversed the decrease in grip force caused by LPS (5mg/kg i.p.), supporting the interpretation that decreases in grip force produced by LPS reflect muscle hyperalgesia that is not sensitive to glucocorticoids.

Gliclazide inhibits differentiation-associated biologic events in human monocyte-derived macrophages

We investigated the in vitro effect of gliclazide on human monocyte-derived macrophage scavenger receptor expression and activity, foam cell formation, and lipopolysaccharide-induced cytokine production. Differentiation of human monocytes into macrophages in the presence of gliclazide (1-10 microg/mL) decreased CD36 expression by 20% to 50%, with maximal effect occurring at 2.5 microg/mL (P<.05). This effect was mimicked by vitamin E (50 micromol/L) and N-acetyl-L-cysteine (10 mmol/L). Incubation of the cells with gliclazide and N-acetyl-L-cysteine also reduced CD36 activity by 30% (P<.02). Despite these effects, neither gliclazide nor vitamin E did affect foam cell formation. In contrast, gliclazide significantly reduced lipopolysaccharide-stimulated macrophage tumor necrosis factor alpha and interleukin 6 secretion (P<.05). Overall, these data indicate that gliclazide, at concentrations in the therapeutic range, may regulate some key biologic events associated with the process of monocyte differentiation into macrophages.

Characterization of calves exhibiting a novel inheritable TNF-α hyperresponsiveness to endotoxin: associations with increased pathophysiological complications

A subpopulation of calves, herein termed “hyperresponders” (HPR), was identified and defined by the patterns of plasma TNF-α concentrations that developed following two challenges with endotoxin (LPS, 0.8 μg Escherichia coli 055:B5 LPS/kg0.75live body wt) separated by 5 days. The principle characteristic of HPR calves was a failure to develop tolerance to repeated LPS challenge that was evident in the magnitude of the TNF-α concentrations and prolonged severity of pathological sequellae. Whereas calves failing to develop LPS tolerance were identified on the basis of their excessive in vivo plasma TNF-α concentration responses, in vitro TNF-α responses of peripheral blood mononuclear cells isolated from each calf and challenged with LPS or PMA did not correlate or predict the magnitude of in vivo plasma TNF response of the calf. Intentional breeding to obtain calves from bulls and/or cows documented as HPR resulted in offspring displaying the HPR character when similar progeny calves were tested with LPS in vivo, with extensive controls in place to account for sources of variability in the general TNF-α response to LPS that might compromise interpretation of the data. Feed intake, clinical serology and hematology profiles, and acute-phase protein responses of HPR calves following LPS were significantly different from those of calves displaying tolerance. These results suggest that the pattern of plasma TNF-α changes that evolve from a low-level double LPS challenge effectively reveal the presence of a genetic potential for animals to display excessive or prolonged pathological response to LPS-related stress and compromised prognosis for recovery.

Tolerance develops to the effect of lipopolysaccharides on movement-evoked hyperalgesia when administered chronically by a systemic but not an intrathecal route

Single exposures to lipopolysaccharides (LPS) produce deep tissue pain in humans and cutaneous hyperalgesia in rodents. While tolerance develops to many effects of LPS, sensitization to hyperalgesia is documented after a single injection. To determine the effect of long-term exposure to LPS, we explored the chronic effect of LPS on movement-evoked pain using a new assay based on grip force in mice. We found that a single systemic injection of LPS (i.p. or s.c.) induced a dose-related decrease in forelimb grip force responses beginning 6-8 h after injection and peaking between 9 and 24 h. The consequence of LPS is likely hyperalgesia rather than weakness as these decreases were rapidly attenuated by either 10 mg/kg of morphine i.p. or 10 microg of morphine injected intrathecally (i.t.). Complete tolerance to this hyperalgesia developed after repeated injections of LPS at doses of 0.9 mg/kg i.p. or 5 mg/kg s.c. Tolerance began after a single injection and was fully developed after as few as four injections of 5 mg/kg of LPS delivered s.c. The concentration of circulating LPS 5 h after a single parenteral injection was less in LPS-tolerant mice than naïve controls, suggesting that tolerance may result from a more efficient clearance of LPS from the circulation. Injected i.t., LPS also induced hyperalgesia, however, tolerance did not develop to multiple injections by this route. There was no cross-tolerance between s.c. and i.t. injections of LPS. These data indicate that decreases in grip force are a sensitive measure of LPS-induced movement-evoked hyperalgesia and that tolerance develops to parenteral but not central hyperalgesic effects of LPS.

C-Reactive Protein Does Not Directly Induce Tissue Factor in Human Monocytes

OBJECTIVE

It is generally assumed that C-reactive protein (CRP) induces synthesis of tissue factor (TF) in monocytic cells, thereby potentially initiating intravascular blood coagulation. We aimed to elucidate the mechanism of CRP-induced TF expression in monocytes and monocyte-derived macrophages (MDMs) in vitro.

METHODS AND RESULTS

Monocytes were isolated from the blood of healthy donors and cultured with or without CRP or lipopolysaccharide (LPS) to study the time course of TF antigen and TF mRNA expression. Addition of 100 microg/mL CRP did not result in a significant increase in TF antigen (range: 9 to 163 pg/10(6) cells, n=11) and TF mRNA (relative number of TF transcripts; N(TF)=0.01 to 0.33), when compared with nonstimulated cells (TF antigen 7 to 46 pg/10(6) cells, N(TF)=0.01 to 0.13). Variation of CRP concentration and exposure time did not affect the TF response. Similar results were obtained in monocytes cultured in suspension and in MDMs. In contrast, TF was strongly induced by 10 microg/mL LPS (TF antigen 1125 to 6120 pg/10(6) cells, N(TF)=5.94 to 23.43). Cultured monocytes did express FcRgammaII, a putative CRP receptor, and addition of CRP induced a 7-fold increase in the production of monocyte chemoattractant protein-1 (MCP-1). Interestingly, CRP addition to peripheral blood mononuclear cells (PBMCs) did result in TF expression on monocytic cells.

CONCLUSIONS

The absence of TF induction after incubation of purified monocytes with CRP indicates that CRP is unable to induce TF expression in monocytes and MDMs directly. The presence of CRP-induced TF expression in PBMCs suggests that CRP can induce TF indirectly, probably through cross-talk between cells.

Lipopolysaccharide (LPS) and zymosan-resistant mutant isolated from a macrophage-like cell line, WEHI-3, with a defective response to LPS under serum-free conditions

A LPS-resistant mutant, W3SF-1, was isolated from a murine macrophage-like cell line, WEHI-3. The W3SF-1 mutant did not produce a significant amount of nitric oxide (NO) or TNF-alpha even with high concentrations of LPS in the presence or absence of FCS, whereas the parental WEHI-3 cells produced them in response to LPS. The parental cells expressed a significant level of TNF-alpha mRNA after LPS stimulation, whereas the mutant cells did not. This defective response of the mutant cells to LPS was neither dependent on the concentration or chemical structure of LPS, nor on the time of LPS treatment. The mutant cells also showed a defective response to zymosan, suggesting that the defect in the mutant cells is common to LPS and zymosan in the signal transduction pathways. The parental and mutant cells showed similar levels of Mac1, F4/80 and CD14, suggesting that these surface markers of macrophages are not linked directly to the defective responses of the mutant to LPS. The treatment of mutant cells with IFN-gamma did not restore the defect of NO or TNF-alpha production on LPS treatment. Binding experiments with 125I-labelled LPS showed a similar binding affinity for LPS in the parental and the mutant cells. These results suggest that the defect in the W3SF-1 mutant cells may not reside in the LPS binding but rather in the early step of signal transduction pathways in the cells after LPS binding.

The biology and tumour-related properties of monocyte tissue factor

Tissue factor (TF, coagulation factor III, CD142) is not only the main physiological initiator of normal blood coagulation, but is also important in the natural history of solid malignancies in that it potentiates metastasis and angiogenesis and mediates outside-in signalling. TF is expressed constitutively by many tissues which are not in contact with blood and by other cells upon injury or activation; the latter include endothelial cells, tissue macrophages, and peripheral blood monocytes. It can exist encrypted and unavailable functionally in the plasma membrane and the appearance of functional TF may be due to synthesis and/or de-encryption. Inflammatory cells often express TF and act to induce its production or de-encryption by other cells locally and, apparently, at remote sites. Inappropriate expression of TF by endothelial cells, macrophages or monocytes is thought to be an important trigger of coagulation in various pathological conditions. Several studies have shown that measurements of monocyte TF (mTF) may provide clinically significant information, particularly in patients with malignant and inflammatory diseases.

Suppressive effects of serum on the LPS-induced production of nitric oxide and TNF-alpha by a macrophage-like cell line, WEHI-3, are dependent on the structure of polysaccharide chains in LPS

The effect of serum on LPS-induced activation of a murine macrophage-like cell line, WEHI-3, was examined. Foetal calf serum strongly inhibited the production of nitric oxide (NO) and TNF-alpha by LPS-stimulated WEHI-3 cells, while it enhanced the production of both by other macrophage-like cell lines, J774.1 and BAM3, on treatment with LPS. This suppressive effect of serum on WEHI-3 cells was most remarkable when the cells were stimulated with rough-chemotype LPS, Ra LPS, Rc LPS and Rd2 LPS. Foetal calf serum also inhibited TNF-alpha production by the same cells stimulated with high concentrations of smooth-form LPS (S LPS; > 1000 ng/mL). Serum-mediated suppression was also observed for expression of the TNF-alpha gene in Rc LPS-stimulated WEHI-3 cells. This suppressive effect of FCS was most remarkable during the 1-2 h before the addition of LPS, but it was not observed when FCS was added at 1 h after the addition of LPS, suggesting dependence on the time of FCS addition to LPS-stimulated cells. No significant difference was observed in the expression of CD14 on WEHI-3 cells cultured in the presence and absence of serum, suggesting that CD14 is not involved in the serum-mediated suppression of these LPS-responses. On the contrary, FCS showed enhancing effects on the production of NO and TNF-alpha by WEHI-3 cells stimulated with low concentrations (< 100 ng/mL) of S LPS and rough mutant Salmonella minnesota Re LPS. These results suggest that the ability of FCS to suppress LPS-induced activation of WEHI-3 cells in mainly dependent on the structure of polysaccharide chains and also on the concentration of LPS employed.

Tissue factor expression in human monocytic cell lines

Tissue factor (TF) is a main initiator of the coagulation protease cascade. Control of the expression of this protein in monocytes is essential, since these cells are the only circulating blood cells responsible for TF expression. In this report we have used two human cell lines, arrested at different stages of monocytic differentiation, to study TF expression. The monoblastic cell line U-937 had a constitutive expression of TF surface protein and low TF mRNA levels detected by immunofluorescence or quantitative reverse transcriptase polymerase chain reaction respectively. The phorbol-12-myristate-13-acetate (PMA) was a potent enhancer of TF expression in U-937. Lipopolysaccharide (LPS) and tumor necrosis factor (TNF) had no effect on TF expression in U-937. The Mono Mac 6 cell line, with phenotypic features similar to that of mature monocytes, expressed lower basal levels of TF mRNA and surface TF antigen. However, in Mono Mac 6 cells TF expression was induced in response to LPS and TNF. These results indicate differences in basal and induced TF expression between U-937 and Mono Mac 6 cell lines.

Serum factors, cell membrane CD14, and beta2 integrins are not required for activation of bovine macrophages by lipopolysaccharide

The role of serum factors such as lipopolysaccharide (LPS)-binding protein (LBP) and of macrophage-expressed CD14 and beta2 integrins in the activation of bovine macrophages by LPS was investigated. Macrophage activation was determined by measuring tumor necrosis factor production, NO generation, and upregulation of procoagulant activity by LPS (Escherichia coli O55:B5) at concentrations of 100 pg/ml to 100 ng/ml. The 50% effective dose for LPS was 1 order of magnitude higher than that for activating human macrophages. Macrophages were activated by LPS in the presence of serum or in the presence of albumin demonstrated to be free of LBP. The capacity to react to LPS in the absence of LBP was not due to the acquisition of LBP during a previous culture in serum. It was then established which CD14-specific antibodies block LPS binding to monocytes. Among the CD14-specific antibodies recognizing bovine mononuclear phagocytes (60bca, 3C10, My4, CAM36, VPM65, CMRF31, and TUK4), the first four blocked the binding of LPS-fluorescein isothiocyanate to bovine monocytes at low concentrations. Anti-CD14 antibodies did not block LPS-mediated activation of bovine bone marrow-derived macrophages, monocyte-derived macrophages, and alveolar macrophages. This was observed in experiments in which anti-CD14 concentrations exceeded the 50% inhibitory dose by >30-fold (3C10 and My4) or >300-fold (60bca), as defined in the binding assay described above. Monocyte-derived macrophages from an animal deficient in beta2 integrins and control macrophages were activated by similar concentrations of LPS, suggesting that beta2 integrins are not important bovine LPS receptors. Thus, in bovine macrophages, LPS recognition pathways which are independent of exogenous LBP, of membrane-expressed CD14, and of beta2 integrins may exist.

Human macrophages respond to LPS in a serum-independent, CD14-dependent manner

Two crucial mediators of monocyte activation by lipopolysaccharide (LPS) are the acute phase plasma factor, lipopolysaccharide binding protein (LBP) and cell-surface-expressed CD14. Whether macrophage (M phi) recognized and respond to LPS in a similar manner is unknown. Here we show that human monocyte-derived M phi respond to LPS by tumor necrosis factor-alpha release and procoagulant activity upregulation by a similar dose response curve in the presence or absence of serum, suggesting that humoral factors such as LBP are relatively unimportant in the activation of M phi. Both serum-dependent and serum-independent activation of M phi by LPS require cellular CD14, as evidence by blocking studies with CD14-specific antibodies. Clones from the monocytoid cell line Mono Mac-6 selected for high LPS sensitivity displayed similar properties. When washed free of serum and cultured in the presence of calcitriol, they responded to LPS in a similar manner, regardless of the presence or absence of serum, and this response was inhibited by anti-CD14. It is hypothesized that during their differentiation. M phi acquire a functional substitute for the serum factor LBP, thereby being able to recognize low LPS concentrations in a milieu low in LBP concentration. It will be of interest to determine whether this is a high-affinity LBP receptor, LBP itself, or another cell surface constituent.

The use of human monocytoid lines as indicators of endotoxin

We wish to develop an in vitro test system for pyrogenic substances. A major source of pyrogen activity is endotoxin. Here we describe a highly sensitive endotoxin-monitoring system based on cytokine measurement in human cell lines of myelomonocytoid origin. The following measures were taken to develop an endotoxin monitoring system of high sensitivity. (i) Mono Mac 6 (MM6) and THP-1 cells, which both represent advanced stages of myelomonocytic development, were better suited as endotoxin indicators than the more immature U-937 line. (ii) In order to enhance cell surface expression of CD14, a major lipopolysaccharide (LPS) receptor, cells were pretreated for 2 days with calcitriol. (iii) The use of fetal calf serum (FCS) without detectable endotoxin traces was essential for maintaining a high LPS sensitivity. (iv) Selected subclones of either THP-1 or MM6 were significantly more sensitive LPS indicators than bulk cultures from which the clones originated. (v) Based on stimulation indices, a commercial tumor necrosis factor-alpha (TNF-alpha) immunoassay proved to be a more sensitive LPS indicator than other cytokine assays or the expression of procoagulant activity/tissue factor. Thus we were able to eliminate the disadvantage of previous cell line-based systems (i.e., low sensitivity) without loss of reproducibility which is seen when using fresh blood, monocytes or monocyte-derived macrophages. High endotoxin sensitivity is a prerequisite for a test system specifically indicating pyrogen activity, because it permits the testing of substances at higher dilutions, thereby minimizing nonspecific interference.

Induction of nitric oxide synthase in bovine mononuclear phagocytes is differentiation stage-dependent

Bovine monocytes and monocyte-derived macrophages (MDM) activated by various means were assessed for induction of inducible nitric oxide synthase (iNOS), using the Griess assay, Northern blotting and reverse transcription/polymerase chain reaction (RT-PCR). Interferon-gamma (IFN-gamma) induced little, if any, iNOS expression and NO production in MDM, although these cells responded to IFN-gamma in other regards. In contrasts, MDM produced copious amounts of NO when stimulated with LPS or Salmonella dublin, and this was paralleled by high steady state levels of iNOS mRNA. Heat-killed Listeria monocytogenes induced more iNOS mRNA and nitrite than IFN-gamma, but much less than L. mono-cytogenes and IFN-gamma combined. Monocytes differed from M phi with respect to iNOS induction and nitrite production in several regards: (i) LPS and S. dublin induced only low levels of iNOS mRNA and nitrite in monocytes, although cells responded to these stimuli in various other ways: (ii) IFN-gamma alone induced in monocytes iNOS mRNA generation and NO formation, although to a low and variable degree; (iii) upon maximal stimulation (e.g. by L. monocytogenes and IFN-gamma combined), monocytes produced much less nitrite than MDM, and mRNA levels were lower. Regulation of macrophage iNOS varies considerably between species. We provide the first evidence in any species that the steady state levels of iNOS mRNA and NO generation in monocytes and macrophages activated by various means depend on the stage of mononuclear phagocyte differentiation.

Transforming growth factor-beta and interleukin-10, but not interleukin-4, down-regulate procoagulant activity and tissue factor expression in human monocyte-derived macrophages

The effect of IL-4, IL-10, and TGF-beta on expression of procoagulant activity (PCA) and of surface-associated tissue factor (TF) by human monocyte-derived macrophages was determined. Monocytes were allowed to mature to macrophages in teflon bags, and were primed either in suspension cultures, or after subculturing in microtiter plates. PCA was determined in PBS-stimulated cells (constitutive PCA) or after stimulation with LPS for 6 hr. TGF-beta significantly reduced constitutive and LPS-induced PCA. This effect was associated with a reduction in surface-expressed TF, but was not correlated with TNF-alpha production in LPS-stimulated cells. The TGF-beta effect was seen both in suspension cultures and in adherent cultures. IL-10 strongly down-regulated LPS-induced PCA, an effect closely correlated with TNF production. It had a weaker, albeit significant effect on constitutive PCA, when tested on suspended cells, and PCA down-regulation was associated with reduction in TF surface expression. IL-4 reduced neither constitutive nor induced PCA in macrophages, and had little effect on TF surface expression, although it strongly down-regulated CD14 expression. Also in monocytes, IL-4 influenced TF expression to a lesser degree than IL-10 and TGF-beta. In the monocytoid cell line, THP-1, PCA/TF was down-regulated preferentially by TGF-beta. Our findings point to a complex cytokine-mediated regulation of PCA at the level of TF expression and possibly at additional levels.