MDP and LPS act synergistically to induce arginine-dependent cytostatic activity in rat alveolar macrophages

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.

Anti-inflammatory effect of preserved egg with simulated gastrointestinal digestion on LPS-stimulated RAW264.7 cells

Preserved egg, a traditional Chinese egg product, is regarded as an anti-inflammatory food in traditional Chinese medicine. This study was aimed to examine anti-inflammatory effect of the simulated gastrointestinal digestive products of whole preserved egg (DWPE), preserved egg white (DPEW), and preserved egg yolk (DPEY) on lipopolysaccharide (LPS)-stimulated RAW264.7 cells. The results demonstrated that DWPE, DPEW, or DPEY inhibited the LPS-induced secretion of Nitric oxide (NO), without marked cytotoxicity. The DWPE, DPEW, and DPEY significantly suppressed the secretion of pro-inflammatory cytokines, such as tumor necrosis factor (TNF)-α and interleukin (IL)-6. Particularly, the inhibition rate of DPEW on NO, IL-6, and TNF-α could reach 25 to 27%, 31 to 42%, and 26 to 38%, respectively (P < 0.05). Furthermore, the DPEW and DPEY downregulated the gene expression of Cyclooxygenase-2 and inducible nitric oxide synthase in a dose-dependent manner. Taken together, this study indicated that DWPE, DPEW, and DPEY, especially DPEW, might serve as functional food for anti-inflammatory therapy.

A nanoparticle-based approach to improve the outcome of cancer active immunotherapy with lipopolysaccharides

This study sought to develop a simple nanoparticle-based approach to enhance the efficiency and tolerability of lipopolysaccharide (LPS), a potent ligand of Toll-like Receptor 4 (TLR4), for immunotherapy in cancer. Despite holding promise within this context, the strong pro-inflammatory properties of LPS also account for its low tolerability given localized and systemic side effects, which restrict the administrable dosage. Herein, we investigated the effect of LPS decoration as a surface-active molecule on a polymeric matrix upon its efficiency and tolerability. The LPS-decorated nanoparticles (LPS-NP) were about 150 nm in size, with slightly negative zeta potential (about -15 mV) and acceptable LPS incorporation (about 70%). In vitro, the particles accounted for a higher induction of apoptosis in tumor cells cultured with murine splenocytes compared to LPS solution. When used for the treatment of a murine syngeneic colorectal tumor model, higher intratumoral deposition of the particle-bound LPS was observed. Furthermore, unlike LPS solution, which accounted for localized necrosis at high concentrations, treatment of tumor-bearing animals with equivalent doses of LPS-NP was well tolerated. We propose that the observed localized necrosis can be Shwartzman phenomenon, which, due to modulated 24-h post-injection systemic TNF-α and LPS concentrations, have been avoided in case of LPS-NP. This has in turn enhanced the therapeutic efficiency and enabled complete tumor regression at concentrations at which LPS solution was intolerable. The findings indicate that nanoparticles can serve as beyond carriers for the delivery of superficially decorated LPS molecules, but impact their overall efficiency and tolerability in cancer therapy.

Modulation of nitric oxide synthase activity in macrophages

L-Arginine is converted to the highly reactive and unstable nitric oxide (NO) and L-citrulline by an enzyme named nitric oxide synthase (NOS). NO decomposes into other nitrogen oxides such as nitrite (NO₂^-) and nitrate (NO₂^-), and in the presence of superoxide anion to the potent oxidizing agent peroxynitrite (ONOO⁻). Activated rodent macrophages are capable of expressing an inducible form of this enzyme (iNOS) in response to appropriate stimuli, i.e., lipopolysaccharide (LPS) and interferon-γ (IFNγ). Other cytokines can modulate the induction of NO biosynthesis in macrophages. NO is a major effector molecule of the anti-microbial and cytotoxic activity of rodent macrophages against certain micro-organisms and tumour cells, respectively. The NO synthesizing pathway has been demonstrated in human monocytes and other cells, but its role in host defence seems to be accessory. A delicate functional balance between microbial stimuli, host-derived cytokines and hormones in the microenvironment regulates iNOS expression. This review will focus mainly on the known and proposed mechanisms of the regulation of iNOS induction, and on agents that can modulate NO release once the active enzyme has been expressed in the macrophage.

Anti-inflammatory effects of hydroxycinnamic acid derivatives

NF-kappaB family of transcription factors are involved in numerous cellular processes, including differentiation, proliferation, and inflammation. It was reported that hydroxycinnamic acid derivatives (HADs) are inhibitors of NF-kappaB activation. Rice bran oil contains a lot of phytosteryl ferulates, one of HADs. We have investigated effects of phytosteryl ferulates on NF-kappaB activation in macrophage. Cycloartenyl ferulate (CAF), one of phytosteryl ferulates, significantly reduced lipopolysaccharide (LPS)-induced NO production and mRNA expression of inducible NO synthase and cyclooxygenese-2 but upregulated SOD activity. Electrophoresis mobility shift assay revealed that CAF inhibited DNA-binding of NF-kappaB. CAF and phytosteryl ferulates probably have potentially anti-inflammatory properties.

Prevention of experimental septic shock by pretreatment of mice with muramyl peptides

Muramyl peptides, immunostimulators with macrophage as a main target cell, are used for protecting mice from LPS-lethality (the experimental model of septic shock). Different protocols of pretreatment mice by muramyl peptides lead to opposite results. LPS and glycopeptides act synergistically in the induction of lethal shock, when mice receive peptides 1 day prior to lethal dose of LPS. However, extension of the period between the peptide and LPS injections to 6 days cancels the effect of synergism. Moreover, a 14-day interval between the same injections leads to protection of 70-90% animals from the toxic effect of LPS. Lipophilic analogs require 10-100 lower concentrations to protect the animals than the parent highly hydrophilic glycopeptides. Production of TNF, IL-1 and phagocytosis by macrophages was studied within the periods corresponding to "synergism" and LPS-resistance. High level of macrophage activity was observed during the "synergism" period. Low TNF production and reduced macrophage phagocyte activity corresponded to LPS-resistant state. These results partly explain the LPS-unresponsiveness in mice after their pretreatment by muramyl peptides.

Upregulation of iNOS by COX-2 in muscularis resident macrophage of rat intestine stimulated with LPS

We investigated the effect of lipopolysaccharide (LPS) on the induction of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) in muscularis resident macrophages of rat intestine in situ. When the tissue was incubated with LPS for 4 h, mRNA levels of iNOS and COX-2 were increased. The majority of iNOS and COX-2 proteins appeared to be localized to the dense network of muscularis resident macrophages immunoreactive to ED2. LPS treatment also increased the production of nitric oxide (NO), PGE(2), and PGI(2). The increased expression of iNOS mRNA by LPS was suppressed by indomethacin but not by N(G)-monomethyl-L-arginine (L-NMMA). The increased expression of COX-2 mRNA by LPS was affected neither by indomethacin nor by L-NMMA. Muscle contractility stimulated by 3 microM carbachol was significantly inhibited in the LPS-treated muscle, which was restored by treatment of the tissue with L-NMMA, aminoguanidine, indomethacin, or NS-398. Together, these findings show that LPS increases iNOS expression and stimulates NO production in muscularis resident macrophages to inhibit smooth muscle contraction. LPS-induced iNOS gene expression may be mediated by autocrine regulation of PGs through the induction of COX-2 gene expression.

Synergistic epithelial responses to endotoxin and a naturally occurring muramyl peptide

We have investigated the synergistic interactions of a naturally occurring peptidoglycan fragment (muramyl peptide) and bacterial endotoxin in the induction of inflammatory processes within respiratory epithelial cells, at the levels of both signal transduction events and ultimate cellular metabolic effects. The source of the muramyl peptide is Bordetella pertussis, the causative agent of the respiratory disease pertussis. During log-phase growth, B. pertussis releases the muramyl peptide tracheal cytotoxin (TCT), which has the structure N - acetylglucosaminyl - 1,6 - anhydro - N - acetylmuramyl - (L) - alanyl - gamma - (D) - glutamyl - meso - diaminopimelyl - (D) - alanine, equivalent to a monomeric subunit of gram-negative bacterial peptidoglycan. When applied to hamster trachea epithelial (HTE) cells, TCT and endotoxin were found to be highly synergistic in the induction of interleukin-1alpha (IL-1alpha), type II (inducible) nitric oxide synthase (iNOS), nitric oxide production, and inhibition of DNA synthesis. Neither molecule alone significantly triggered these responses. The serine/threonine protein kinase inhibitor H7 blocked induction of both IL-1alpha and iNOS. More selective inhibitors of protein kinase C, cyclic AMP-dependent protein kinase, and cyclic GMP-dependent protein kinase were not capable of blocking the effects of TCT and endotoxin, suggesting that the H7-inhibited component in this pathway is not among the commonly described kinase targets of H7. Treatment of HTE cells with exogenous IL-1 reproduced the induction of iNOS and DNA synthesis inhibition caused by TCT and endotoxin. H7 was not capable of interfering with effects caused by exogenous IL-1, implying that the H7-sensitive step in the pathway is upstream of IL-1 protein production. Similar assays with the phorbol ester phorbol myristate acetate indicate that it could effectively synergize with endotoxin but not with TCT, suggesting that TCT and endotoxin induce different signal transduction events that combine synergistically. The synergy observed with TCT and endotoxin in epithelial cells is significantly different from their interaction with other cell types, revealing a unique inflammatory response by epithelial cells to these natural bacterial products.

cAMP and purinergic P2y receptors upregulate and enhance inducible NO synthase mRNA and protein in vivo

Adenosine 3',5'-cyclic monophosphate (cAMP) and purinergic P2y receptor agonists upregulate inducible nitric oxide (NO) synthase (iNOS) but inhibit Escherichia coli endotoxin lipopolysaccharide (LPS)- and cytokine-mediated upregulation of iNOS in cultured cells. We examined the effects of cAMP and P2y receptor agonists on the iNOS system in vivo. Intratracheal administration of dibutyryl-cAMP (DBcAMP, 0.1 and 1 mg/kg), a P2y receptor agonist [2-methylthioadenosine 5'-triphosphate (MeS-ATP), 5 mg/kg], or LPS (0.6 mg/kg) to rats 2 h before bronchoalveolar lavage (BAL) increased iNOS mRNA (competitor-equalized reverse transcription-polymerase chain reaction) and iNOS protein (Western blot) in rat alveolar macrophages compared with the effects of sterile phosphate-buffered saline (0.5 ml it). At equal levels of upregulation of iNOS mRNA, 1) LPS, but not DBcAMP or MeS-ATP, upregulated nuclear transcription factor-kappa B (NF-kappa B) and 2) iNOS protein and formation of NO were greater in alveolar macrophages from LPS- and MeS-ATP-treated rats than from DBcAMP-treated rats. Administration of DBcAMP or MeS-AMP 15 min before LPS did not inhibit LPS-induced alveolar macrophage-derived iNOS mRNA, iNOS protein, and NO. Diethyldithiocarbamate (DETC, 5 mg/kg it) inhibited LPS-induced iNOS mRNA but did not affect upregulation of iNOS mRNA produced by the other agonists. We conclude that an LPS-dependent and -independent pathway of iNOS mRNA induction exists in vivo. The former is activated by IPS and most cytokines, is associated with upregulation of NF-kappa B and inhibited by DETC, and elicits an inflammatory response. The latter, activated by DBcAMP and MeS-ATP, is not associated with upregulation of NF-kappa B, inhibition by DETC, or activation of inflammation. The two systems are additive in vivo rather than antagonistic. Speculatively, if the LPS-independent iNOS pathway exists in humans, the iNOS in tissues from patients taking drugs affecting cAMP or P2y receptors may be iatrogenic rather than pathogenetic in origin.

Interferon-gamma/tumour necrosis factor-alpha synergism as a mechanism for enhanced nitric oxide production following in vivo administration of muramyl dipeptide (MDP) to mice

The aim of the present study was to investigate the role of muramyl dipeptide (MDP) in triggering host non-specific defence mechanisms, namely its influence upon the L-arginine-dependent biotransformation pathways leading to formation of nitric oxide (NO). MDP was applied subcutaneously to mice as one or three daily injections, 300 micrograms MDP each. Production of NO by peritoneal macrophages from these animals or the capability of their splenocytes to render virgin macrophages for enhanced NO production was assayed. It has been found that macrophages from MDP-treated animals produce considerably enhanced amounts of NO and that the splenocytes secrete soluble factors providing all the signals required for induction of NO biosynthesis. Introduction of anti-IFN-gamma or anti-TNF-alpha antibodies into these splenocyte supernatants resulted in complete suppression of inducible NO production by macrophages.

Modulation of murine macrophage nitric oxide synthesis by liposomal phospholipids: correlation with liposome immune adjuvant activity

The influence of alum and liposomal phospholipids on interferon-gamma-(IFN-gamma), IFN-gamma/N-acetylmuramyl-L-alanyl-D-isoglutamine- (MDP) or IFN-gamma/tumor necrosis factor-alpha- (IFN-gamma/TNF-alpha) induced macrophage nitric oxide (NO) synthesis has been investigated. IFN-gamma induced NO synthesis in a dose-dependent manner. TNF-alpha and MDP did not induce NO synthesis, but interacted synergistically with sub-optimal doses of IFN-gamma. Alum strongly inhibited IFN-gamma-induced NO synthesis (ID50 25 microgram/ml). Liposomes composed of dipalmitoylphosphatidylcholine (DPPC) had no effect on IFN-gamma-induced NO synthesis. IFN-gamma-induced NO synthesis was stimulated by DPPC/dimyristoylphosphatidylglycerol (DMPG) liposomes (9:1 mol ratio, ED50 45 nmol phospholipid/ml), and inhibited by DPPC/dipalmitoylphosphatidylethanolamine (DPPE) liposomes (9:1 mol ratio, ID50 > 500 nmol phospholipid/ml), and DPPC/phosphatidylserine (PS) liposomes (7:3 mol ratio, ID50 150 nmol phospholipid/ml). Alum, DPPC/PE and DPPC/PS liposomes also inhibited IFN-gamma/MDP- and IFN-gamma/TNF-alpha-induced NO synthesis. Neither alum or the liposome preparations had significant toxicity towards macrophages in vitro at concentrations that induced maximal inhibition or stimulation of IFN-gamma-induced NO synthesis. Immunization of mice with alum-adsorbed and liposome-incorporated bovine serum albumin (BSA) demonstrated that enhancement or reduction of both IgG antibody and the proportion of IgG2a/IgG2b was correlated with stimulation or inhibition of IFN-gamma-induced NO synthesis.

Factors influencing macrophage activation by muramyl peptides: inhibition of NO synthase activity by high levels of NO

Treatment with muramyldipeptide (MDP) or a lipophilic derivative (MTP-Chol) included in nanocapsules renders macrophages cytostatic towards tumor cells. At the same time, nitric oxide (NO) synthase (EC 1.14.23) activity is induced, as determined by measurement of the two end products of the reaction (nitrite and L-citrulline). The objective of this study was to investigate some factors which might influence this activation and explain the decreased response observed at high nanocapsule concentrations. The glucose content of the medium did not seem to be limiting. Addition of indomethacin decreased nitrite production in the effector phase, suggesting a role for prostaglandins in the maintenance of the activated state. We also tested the hypothesis that NO itself might regulate inducible nitric oxide synthase activity. The addition of NO donors (SIN-1 and nitrosoglutathione) or superoxide dismutase to cultures of activated macrophages inhibited the NO synthase activity. Since these NO donors were non toxic towards macrophages, these observations indicate clearly that the addition of exogenous NO to that formed by the enzymatic reaction can cause inhibition of the inducible NO synthase.

Anti-metastatic activity of MDP-L-alanyl-cholesterol incorporated into various types of nanocapsules

A lipophilic immunomodulator (MTP-Chol) was included in nanocapsules prepared from different polymers and the anti-metastatic effects of the resulting drug delivery systems were evaluated in a murine model of liver metastases. Loaded nanocapsules were effective if they were given 2 days before tumor inoculation. Neither the nature of the polymer nor the total dose of immunomodulator affected the antimetastatic capacity. However, enhanced anti-metastatic activity was obtained when indomethacin nanocapsules were associated with MTP-Chol nanocapsules. These results show that nanocapsules containing an immunomodulator possess anti-metastatic activity, but only when given as a prophylactic treatment; this would correspond, in the clinic, to patients undergoing surgery for a primary tumor and at risk of developing liver metastases.

Improved intracellular delivery of a muramyl dipeptide analog by means of nanocapsules

A lipophilic derivative of muramyl dipeptide, muramyl tripeptide cholesterol, was incorporated into poly(D,L-lactide) nanocapsules and its immunomodulating properties were assessed in vitro. The nanocapsule form was more effective than the free drug in activating rat alveolar macrophages for a cytostatic effect toward syngeneic tumor cells. Induction of NO synthase correlated with anti-proliferative activity. The time course of activation and the effect of inhibitors of endocytosis suggested that this increased efficiency was due to improved intracellular delivery by phagocytosis of nanocapsules. Such nanocapsules might be useful for immunotherapy of metastases resistant to conventional treatment, since they could overcome two problems associated with soluble muramyl peptides: rapid elimination and poor uptake by macrophages.

Muramyldipeptide and granulocyte-macrophage colony-stimulating factor enhance interferon-gamma-induced nitric oxide production by rat alveolar macrophages

Rat alveolar macrophages incubated with recombinant rat interferon-gamma produce L-arginine-dependent nitric oxide, which is rapidly decomposed into nitrite: this production by interferon-gamma was markedly enhanced by granulocyte-macrophage colony-stimulating factor and muramyldipeptide, but not by other cytokines. The enhancement was dependent on the presence of L-arginine in the incubation medium. It was based on a simple synergism between interferon-gamma and muramyldipeptide and a priming effect of granulocyte-macrophage colony-stimulating factor for interferon-gamma-induced nitrite production. These data suggest that cytokine networks are important in the induction of nitric oxide in rat alveolar macrophages.

Differential stimulation of macrophages for tumor cytostasis and monokine production

We have investigated whether antitumor activity could be expressed independently of cytokine production. Resident macrophages treated with interferon-gamma (IFN-gamma), lipopolysaccharide (LPS) plus muramyldipeptide (MDP) expressed a cytostatic activity against P815 tumor cells and released interleukin 6 (IL-6) and nitrite but produced neither IL-1 nor tumor necrosis factor (TNF). Thioglycollate-elicited macrophages required only LPS plus IFN-gamma for cytostatic activity which was expressed concomitantly with the release of high levels of TNF, IL-1 and IL-6, whereas C3H/HeJ macrophages produced low levels of monokines and were not cytostatic. LPS, alone, was sufficient for triggering Concanavalin A-primed macrophages leading to a full cytostatic activity, even in C3H/HeJ macrophages that was expressed, for these latter, in the absence of monokine production. TNF did not appear to play a role either in autocrine stimulation of macrophages or in the cytostatic process because anti-TNF antiserum affected neither the cytostatic activity nor the nitrite production.