Induction of proinflammatory responses in human monocytes by particulate and soluble forms of lipopolysaccharide

Sensitive and rapid detection of bacterial endotoxin with a functional carbon nanotube field-effect transistor biosensor

Endotoxin in the blood can cause unexplained fever, and even death due to endotoxemia and bacteremia. Rapid and sensitive detection of endotoxin has become a priority event to intervene in the occurrence of dangerous diseases in time. In this context, a carbon nanotubes-based field-effect transistor (CNTs FET) nanosensor is developed to realize the rapid, label-free and sensitive detection of endotoxin. The CNTs FET was fabricated by assembling the polymer-sorted high-purity semiconductor CNT films onto the sensing channel. In order to improve the detection sensitivity, carboxylated graphene quantum dots (cGQDs) were coupled to the CNT surface via the poly-l-lysine (PLL). After that, polymyxin B (PMB), which is highly specific for endotoxin, was covalently combined with cGQDs, thus enabling the capture and detection of endotoxin. The method not only displayed an extremely low level of limitation of detection in PBS (4.6 fg/mL) and serum (30.3 fg/mL), respectively, with superior resistance to interference, but also enabled the analysis of Gram-negative bacterial infections in blood samples in a short duration of time. Meanwhile, the receiver operating characteristic curve (ROC) proved the superior diagnostic accuracy of this method (AUC = 0.990). Considering its excellent performance, the constructed CNTs FET biosensor is a promising tool to provide early warning of disease for clinical Gram-negative bacteremia and endotoxemia.

The Anti-Microbial Peptide (Lin-SB056-1)2-K Reduces Pro-Inflammatory Cytokine Release through Interaction with Pseudomonas aeruginosa Lipopolysaccharide

The ability of many anti-microbial peptides (AMPs) to modulate the host immune response has highlighted their possible therapeutic use to reduce uncontrolled inflammation during chronic infections. In the present study, we examined the anti-inflammatory potential of the semi-synthetic peptide lin-SB056-1 and its dendrimeric derivative (lin-SB056-1)₂-K, which were previously found to have anti-microbial activity against Pseudomonas aeruginosa in in vivo-like models mimicking the challenging environment of chronically infected lungs (i.e., artificial sputum medium and 3-D lung mucosa model). The dendrimeric derivative exerted a stronger anti-inflammatory activity than its monomeric counterpart towards lung epithelial- and macrophage-cell lines stimulated with P. aeruginosa lipopolysaccharide (LPS), based on a marked decrease (up to 80%) in the LPS-induced production of different pro-inflammatory cytokines (i.e., IL-1β, IL-6 and IL-8). Accordingly, (lin-SB056-1)₂-K exhibited a stronger LPS-binding affinity than its monomeric counterpart, thereby suggesting a role of peptide/LPS neutralizing interactions in the observed anti-inflammatory effect. Along with the anti-bacterial and anti-biofilm properties, the anti-inflammatory activity of (lin-SB056-1)₂-K broadens its therapeutic potential in the context of chronic (biofilm-associated) infections.

Inflammatory Response to Different Toxins in Experimental Sepsis Models

Sepsis is defined as life-threatening organ dysfunction caused by the dysregulated host response to infection. Despite serious mortality and morbidity, no sepsis-specific drugs exist. Endotoxemia is often used to model the hyperinflammation associated with early sepsis. This model classically uses lipopolysaccharide (LPS) from Gram-negative pathogens to activate the immune system, leading to hyperinflammation, microcirculatory disturbances and death. Other toxins may also be used to activate the immune system including Gram-positive peptidoglycan (PG) and lipoteichoic acid (LTA). In addition to these standard toxins, other bacterial components can induce inflammation. These molecules activate different signaling pathways and produce different physiological responses which can be taken advantage of for sepsis modeling. Endotoxemia modeling can provide information on pathways to inflammation in sepsis and contribute to preclinical drug development.

Clinical utility of C-reactive protein to predict treatment response during cystic fibrosis pulmonary exacerbations

Rationale

C-reactive protein (CRP) is a systemic marker of inflammation that correlates with disease status in cystic fibrosis (CF). The clinical utility of CRP measurement to guide pulmonary exacerbation (PEx) treatment decisions remains uncertain.

Objectives

To determine whether monitoring CRP during PEx treatment can be used to predict treatment response. We hypothesized that early changes in CRP can be used to predict treatment response.

Methods

We reviewed all PEx events requiring hospitalization for intravenous (IV) antibiotics over 2 years at our institution. 83 PEx events met our eligibility criteria. CRP levels from admission to day 5 were evaluated to predict treatment non-response, using a modified version of a prior published composite definition. CRP was also evaluated to predict time until next exacerbation (TUNE).

Measurements and main results

53% of 83 PEx events were classified as treatment non-response. Paradoxically, 24% of PEx events were characterized by a ≥ 50% increase in CRP levels within the first five days of treatment. Absolute change in CRP from admission to day 5 was not associated with treatment non-response (p = 0.58). Adjusted for FEV₁% predicted, admission log₁₀ CRP was associated with treatment non-response (OR: 2.39; 95% CI: 1.14 to 5.91; p = 0.03) and shorter TUNE (HR: 1.60; 95% CI: 1.13 to 2.27; p = 0.008). The area under the receiver operating characteristics (ROC) curve of admission CRP to predict treatment non-response was 0.72 (95% CI 0.61–0.83; p<0.001). 23% of PEx events were characterized by an admission CRP of > 75 mg/L with a specificity of 90% for treatment non-response.

Conclusions

Admission CRP predicts treatment non-response and time until next exacerbation. A very elevated admission CRP (>75mg/L) is highly specific for treatment non-response and might be used to target high-risk patients for future interventional studies aimed at improving exacerbation outcomes.

Contribution of soluble endotoxin released from Gram-negative bacteria by antibiotics to the pathogenesis of experimental sepsis in mice

The results from our laboratory summarized in this review support the importance of endotoxin release from the Gram-negative microbe as a requirement for the full manifestation of the biological activity of the lipopolysaccharide (LPS) macromolecule. They further suggest that LPS is the most important bacterial component released from Gram-negative microbes treated in vitro with cell-wall active antibiotics in terms of biological activity. The use of several experimental mouse models of infection confirm that mice treated with the reversible hepatotoxin, D-galactosamine, are sensitive to both endotoxin lethality and to infection with Gram-negative microbes. Infected LPS-sensitive animals can be protected if they are treated with antibiotics, but the degree of protection (e.g. 5-10-fold) is significantly less than that observed when infection-sensitive but LPS-insensitive mice are similarly treated (500-fold protection). Additional levels of protection in infected, antibiotic-treated mice can be observed with antiendotoxin agents (e.g. polymyxin B-dextran, monoclonal antibody). Finally, antibiotics with equivalent minimal inhibitory concentrations for Escherichia coli, but differing in their mode of action and endotoxin releasing potential in vitro, also differ in their protective efficacy in vivo to reduce mortality in infected animals. Collectively, these data provide evidence to support a role for microbe-derived endotoxin, released from the microbial surface following treatment with cell-wall active antibiotics, in the pathogenesis of experimental Gram-negative sepsis.

Biosynthetically engineered lipopolysaccharide as vaccine adjuvant

Lipopolysaccharide (LPS), a dominant component of the Gram-negative bacterial outer membrane, is a strong activator of the innate immune system, and thereby an important determinant in the adaptive immune response following bacterial infection. This adjuvant activity can be harnessed following immunization with bacteria-derived vaccines that naturally contain LPS, and when LPS or molecules derived from it are added to purified vaccine antigens. However, the downside of the strong biological activity of LPS is its ability to contribute to vaccine reactogenicity. Modification of the LPS structure allows triggering of a proper immune response needed in a vaccine against a particular pathogen while at the same time lowering its toxicity. Extensive modifications to the basic structure are possible by using our current knowledge of bacterial genes involved in LPS biosynthesis and modification. This review focuses on biosynthetic engineering of the structure of LPS and implications of these modifications for generation of safe adjuvants.

Effect of empiric antibiotic treatment on plasma endotoxin activity in septic patients

OBJECTIVES

Efficient empiric antibiotic therapy remains the cornerstone of sepsis treatment. However, antibiotics could be responsible for the transient clinical deterioration provoked by the release of bacterial cell-wall constituents, such as endotoxin, into the blood stream. The aim of this study was to evaluate if a transient elevation of endotoxin level occurred in septic patients following antibiotic administration.

METHODS

Thirty-three septic intensive care unit (ICU) patients were enrolled in this prospective trial. Four blood samples were collected from each of these patients during a 24-h period, and endotoxin activity was measured in these samples by the chemiluminescence technique. Fifteen ICU non-septic patients and 15 healthy volunteers were also observed for possible daily fluctuations in endotoxin activity.

RESULTS

There was no significant increase in endotoxin levels following the initiation of empiric antibiotic therapy in septic patients. A clinical deterioration in the 4 h following antibiotic administration was observed in 14 septic patients (42 %). These patients had significantly higher endotoxin levels than stable septic patients.

CONCLUSIONS

Although endotoxin levels failed to increase after the administration of antibiotic(s) to critically ill patients, they were higher in the septic patients presenting a transient deterioration than in the other patients. This observation suggests that a possible release of endotoxin due to bacteria lysis by antibiotics could be responsible for the observed clinical deterioration.

Modulation of bacterial ghosts--induced nitric oxide production in macrophages by bacterial ghost-delivered resveratrol

The present study aimed to investigate the capacity of resveratrol (RV) delivered into macrophages by bacterial ghosts (BGs), representing intact empty nonliving envelopes of Gram-negative bacteria, to modulate nitric oxide (NO) production related to the presence of the pathogen-associated molecular patterns on the surface of BGs. Incubation of the murine macrophage cell line RAW 264.7 with BGs leads to a dose-dependent activation of inducible NO synthase. To modify BG-induced NO formation in RAW 264.7 cells by RV; BGs were loaded with RV (RV-BGs) and incubated with murine macrophages in a dose-dependent manner. RV-BGs delivering RV to the target macrophages significantly reduced BG-induced NO production with concentration of RV more than one order of magnitude lower than the amount of RV capable of reducing NO formation when applied directly. Moreover, no cytotoxic impact of BGs on the viability of RAW 264.7 cells added to macrophages alone or loaded with RV was detected after a mutual 24 h incubation, whereas cell viability slightly decreased (~ 10%) when RV concentrations of 30 μm alone were applied. The results obtained in the present study clearly indicate that the intracellular delivery of RV by BGs significantly enhances the total RV effect.

Antibiotics modulate the stimulated cytokine response to endotoxin in a human ex vivo, in vitro model

BACKGROUND

Sepsis may lead to the suppression of stimulated cytokine release after Gram-negative stimuli, correlating with a fatal outcome. Treatment of sepsis includes adequate therapy with antibiotics. The aim of this study was to investigate the role of antibiotics in the modulation of the lipopolysaccharide (LPS)-stimulated cytokine response of human monocytes.

METHODS

In this ex vivo, in vitro study, whole blood samples were taken from 10 healthy volunteers, stimulated with LPS in the presence or absence of various antibiotics (penicillin, amoxicillin, cefuroxime, ceftazidime, cefotaxime, piperacillin/tazobactam, imipenem/cilastatin, gentamicin, netilmicin, ciprofloxacin, vancomycin) and cultured for 24 h. Thereafter, tumor necrosis factor-alpha (TNF-alpha) and interleukin-10 (IL-10) were measured in the supernatants by enzyme-linked immunosorbent assay (ELISA). Furthermore, CD14 and HLA-DR expression on monocytes was assessed using flow cytometry.

RESULTS

All cephalosporins decreased LPS-stimulated IL-10 release. Cefuroxime and cefotaxime also decreased the expression density of the LPS recognition molecule CD14 on monocytes. An increase in LPS-stimulated IL-10 release was observed with vancomycin. A suppression of LPS-stimulated TNF-alpha and IL-10 release was observed in the presence of ciprofloxacin.

CONCLUSION

These results indicate a modulation of the expression density of CD14 on monocytes, together with a shift from a balanced to an inflammatory cytokine release pattern, by cefuroxime and cefotaxime. Vancomycin changes the response to an anti-inflammatory release pattern. After ciprofloxacin, a profound unresponsiveness of immune-competent cells to LPS stimulation is observed. Because of the critical role of a balanced innate immune response, these data may be of importance for the selection of antibiotics in septic patients.

Antimicrobial-induced endotoxin and cytokine activity in an in vitro model of septicemia in foals

OBJECTIVE

To determine which antimicrobials that are used to treat neonatal foals with septicemia attributable to Escherichia coli will minimize endotoxin release from bacteria and subsequent activity of inflammatory mediators while maintaining bactericidal efficacy.

SAMPLE POPULATION

Blood samples from 10 healthy foals.

PROCEDURE

Escherichia coli isolates A and B were isolated from 2 septicemic foals, and minimal inhibitory concentrations (MIC) were determined for 9 antimicrobials. Five of these antimicrobials were tested in vitro at 2 and 20 times their respective MIC. Whole blood or mononuclear cells grown in tissue-culture media were incubated with 105 colony-forming units of E. coli and each antimicrobial or saline (0.9% NaCl) solution. After 6 hours, number of viable bacteria remaining was determined, and supernatant was tested for endotoxin and tumor necrosis activity.

RESULTS

Testing in whole blood was compromised by bactericidal effects of the blood itself. In mononuclear cell suspensions, each antimicrobial significantly reduced the number of viable bacteria to low or undetectable amounts. Antimicrobials did not differ significantly in efficacy of bacterial killing. Amikacin used alone or in combination with ampicillin resulted in significantly less endotoxin activity than did ampicillin, imipenem, or ceftiofur alone. There was a correlation between TNF-alpha and endotoxin activity.

CONCLUSIONS AND CLINICAL RELEVANCE

Aminoglycosides appear less likely to induce endotoxemia and TNF-alpha synthesis during bactericidal treatment of E. coli septicemia, compared with beta-lactam antimicrobials. Use of ampicillin, imipenem, or ceftiofur in the treatment of septicemic neonatal foals should be accompanied by appropriate treatment for endotoxemia.

Diphosphoryl lipid A from Rhodobacter sphaeroides blocks the binding and internalization of lipopolysaccharide in RAW 264.7 cells

Diphosphoryl lipid A derived from the nontoxic LPS of Rhodobacter sphaeroides (RsDPLA) has been shown to be a powerful LPS antagonist in both human and murine cell lines. In addition, RsDPLA also can protect mice against the lethal effects of toxic LPS. In this study, we complexed both the deep rough LPS from Escherichia coli D31 m4 (ReLPS) and RsDPLA with 5- and 30-nm colloidal gold and compared their binding to the RAW 264.7 cell line by electron microscopy. Both ReLPS and RsDPLA bound to the cells with the following observations. First, binding studies revealed that pretreatment with RsDPLA completely blocked the binding and thus internalization of ReLPS-gold conjugates to these cells at both 37 degrees C and 4 degrees C. Second, ReLPS was internalized via micropinocytosis (noncoated plasma membrane invaginations) involving formation of caveolae-like structures and leading to the formation of micropinocytotic vesicles, macropinocytosis (or phagocytosis), formation of clathrin-coated pits (receptor mediated), and penetration through plasma membrane into cytoplasm. Third, in contrast, RsDPLA was internalized predominantly via macropinocytosis. These studies show for the first time that RsDPLA blocks the binding and thus internalization of LPS as observed by scanning and transmission electron microscopy.

Protective role of Raf-1 in Salmonella-induced macrophage apoptosis

Invasive Salmonella induces macrophage apoptosis via the activation of caspase-1 by the bacterial protein SipB. Here we show that infection of macrophages with Salmonella causes the activation and degradation of Raf-1, an important intermediate in macrophage proliferation and activation. Raf-1 degradation is SipB- and caspase-1-dependent, and is prevented by proteasome inhibitors. To study the functional significance of Raf-1 in this process, the c-raf-1 gene was inactivated by Cre-loxP-mediated recombination in vivo. Macrophages lacking c-raf-1 are hypersensitive towards pathogen-induced apoptosis. Surprisingly, activation of the antiapoptotic mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) and nuclear factor kappaB pathways is normal in Raf-1-deficient macrophages, and mitochondrial fragility is not increased. Instead, pathogen-mediated activation of caspase-1 is enhanced selectively, implying that Raf-1 antagonizes stimulus-induced caspase-1 activation and apoptosis.

Impaired mitogen-activated protein kinase activation and altered cytokine secretion in endotoxin-tolerant human monocytes

BACKGROUND

Dysregulation of monocyte/macrophage cytokine production after exposure to multiple inflammatory stimuli may contribute to multiple organ failure and sepsis. Endotoxin (lipopolysaccharide [LPS]) activation of murine macrophage results in the phosphorylation of kinases in the mitogen-activated protein kinase cascade. Pretreatment of murine macrophages with LPS induces LPS-tolerance, with inhibition of LPS-stimulated activation of kinases (ERK1/2 and p38) and diminished release of tumor necrosis factor (TNF). We sought to determine whether similar alterations in LPS-dependent signal transduction are present in LPS-tolerant human peripheral blood monocytes.

METHODS

Human peripheral blood monocytes from healthy volunteer donors (n = 12) were incubated in RPMI 1640 culture medium +/- 10 ng/mL of LPS for 18 hours, then stimulated with 0 to 1,000 ng/mL of LPS. Supernatant TNF and interleukin-1 (IL-1) levels were measured after 5 hours by enzyme-linked immunosorbent assay. Activation of the p42/p44 kinases (ERK1/2) was measured 15 minutes after LPS with monoclonal antibodies to diphosphorylated (active) ERK1/2 using novel flow cytometric methods.

RESULTS

LPS-tolerant (10 ng/mL LPS pretreatment) human monocytes had significant inhibition of LPS-stimulated TNF secretion but augmented IL-1 release (p < 0.05). Nontolerant human monocytes had a dramatic increase in the percentage of ERK1/2-positive cells in response to an initial stimulation with LPS. This did not occur in the LPS-tolerant cells. Phorbol-12-myristate-13 acetate restored ERK1/2 activation in LPS-tolerant human monocytes.

CONCLUSION

LPS-tolerance in human monocytes is associated with inhibition of LPS-stimulated TNF secretion, augmented release of IL-1, and defective activation of mitogen-activated protein kinase cascade (ERK1/2). These results suggest a method of identifying LPS-tolerance and monocyte dysfunction in patients with sepsis.

Endotoxin release due to ciprofloxacin measured by three different methods

Antibiotics are known to induce the release of bioactive endotoxin (LPS) from gram-negative bacterial cells. Because varying data have been published on the influence of quinolone antibiotics on LPS liberation, we studied the effect of ciprofloxacin on a culture of Escherichia coli by determining bacterial killing and free LPS concentrations in comparison with imipenem and ceftazidime. LPS levels were measured by three different methods, namely (1) the Limulus amebocyte lysate test, (2) an ELISA method based on capture of LPS by monoclonal antibodies, and (3) indirect determination by measuring the ability of antibiotic-induced LPS to trigger TNFalpha release from a monocytic cell line. With both the Limulus and ELISA tests, a low endotoxin-releasing activity of ciprofloxacin was confirmed. In contrast to previous studies, this LPS also had low bioactivity in terms of TNFalpha induction. Limulus LPS determinations correlated more precisely with LPS bioactivity than did ELISA values, an observation which underlines the crucial role of LPS determination methods in studies of antibiotic-induced LPS release.

Structure-function relationships of bacterial endotoxins. Contribution to microbial sepsis

A substantial body of knowledge has emerged over the past several decades concerning the primary and tertiary, and quaternary structure of endotoxic LPS and their contribution to the pathogenesis of gram-negative sepsis; however, important questions remain. Among them are the precise three-dimensional configuration of the LPS macromolecule and the contribution of the quaternary structure to the ability of these potent microbial factors to interact with host humoral and cellular inflammatory mediator systems. Also remaining to be sufficiently addressed is the relative contribution of endotoxin interactions with the host to the overall manifestation of disease and conditions under which such contributions serve as the pivotal event in determining outcome. The answers to these questions can be expected to provide valuable insights into potential novel therapeutic intervention strategies and approaches that will ultimately reduce both morbidity and mortality in infection from gram-negative microbes.

Lipoprotein release by bacteria: potential factor in bacterial pathogenesis

Lipoprotein (LP) is a major component of the outer membrane of bacteria in the family Enterobacteriaceae. LP induces proinflammatory cytokine production in macrophages and lethal shock in LPS-responsive and -nonresponsive mice. In this study, the release of LP from growing bacteria was investigated by immuno-dot blot analysis. An immuno-dot blot assay that could detect LP at levels as low as 100 ng/ml was developed. By using this assay, significant levels of LP were detected in culture supernatants of growing Escherichia coli cells. During mid-logarithmic growth, approximately 1 to 1.5 microgram of LP per ml was detected in culture supernatants from E. coli. In contrast, these culture supernatants contained 5 to 6 microgram/ml of lipopolysaccharide (LPS). LP release was not unique to E. coli. Salmonella typhimurium, Yersinia enterocolitica, and two pathogenic E. coli strains also released LP during in vitro growth. Treatment of bacteria with the antibiotic ceftazidime significantly enhanced LP release. Culture supernatants from 5-h cultures of E. coli were shown to induce in vitro production of interleukin-6 (IL-6) by macrophages obtained from LPS-nonresponsive C3H/HeJ mice. In contrast, culture supernatants from an E. coli LP-deletion mutant were significantly less efficient at inducing IL-6 production in C3H/HeJ macrophages. These results suggest, for the first time, that LP is released from growing bacteria and that this released LP may play an important role in the induction of cytokine production and pathologic changes associated with gram-negative bacterial infections.

Effect of O-antigenic polysaccharide of Escherichia coli on endotoxin neutralizing activity of lysozyme

Endotoxemia is considered to be associated with the high mortality of gram-negative septic patients. Increasing evidence shows that beta-lactam antibiotics have a propensity to induce endotoxin release from the bacterial outer membrane while killing bacteria. We have recently found that egg white lysozyme (EW-LZM) shows strong inhibition of beta-lactam induced bacteriolysis and lipopolysaccharide (LPS) release from Escherichia coli O111, resulting in reduction of the LPS-initiated inflammatory response. In this study, we compared the effect of EW-LZM on E. coli J5, which possesses rough-type LPS (RaLPS), in order to demonstrate the effect of O-antigenic polysaccharide on endotoxin neutralizing activity of EW-LZM and on inhibition of beta-lactam induced lysis by LZM. Both of the beta-lactam induced bacterial lysis and subsequent LPS release were almost completely inhibited by EW-LZM. The effect was more potent than that of wild-type LPS as assessed by released LPS concentration and LPS induced cytokine syntheses. In addition, EW-LZM was effective against lethal infection of E. coli J5 in cyclophosphamide induced leukopenic mice. These facts strongly suggested that O-antigenic polysaccharide negatively modulates LPS neutralizing activity of EW-LZM.

Pre-exposure to heat shock inhibits peroxynitrite-induced activation of poly(ADP) ribosyltransferase and protects against peroxynitrite cytotoxicity in J774 macrophages

The reaction of nitric oxide (NO) with superoxide yields the cytotoxic oxidant peroxynitrite, produced during inflammation and shock. A novel pathway of peroxynitrite cytotoxicity involves activation of the nuclear enzyme poly(ADP) ribosyltransferase, and concomitant ADP-ribosylation. NAD+ consumption and exhaustion of intracellular energy stores. In the present report we provide evidence that pre-exposure of J774 macrophages to heat shock reduces peroxynitrite-induced activation of poly(ADP) ribosyltransferase and protects against the peroxynitrite-induced suppression of mitochondrial respiration. The protection was significant at 8 h after heat shock, but was absent at 24 h after heat shock. Thus, the protection showed a temporal correlation with the expression of heat shock protein 70, the expression of which was maximal at 8 h. Exposure to heat shock did not alter the expression of poly(ADP) ribosyltransferase over 24 h. In summary, the heat shock phenotype or heat shock proteins may protect against peroxynitrite induced cytotoxicity.

Inhibition of terminal calcium overload protects against peroxynitrite-induced cellular injury in macrophages

The inducible isoform of nitric oxide synthase (iNOS) produces large quantities of nitric oxide (NO) during inflammation and shock. Recent studies show that the reaction of NO with superoxide yields the cytotoxic oxidant peroxynitrite (ONOO-). An important pathway of ONOO- cytotoxicity involves DNA strand breakage, activation of the nuclear repair enzyme poly(ADP) ribosyltransferase (PARS), and concomitant ADP-ribosylation, NAD+ consumption, and exhaustion of intracellular energy stores. Using quin-2, a calcium chelator, we have investigated the role of calcium in the cytotoxicity elicited by ONOO-. Quin-2 (10-100 microM) ameliorated the suppression of mitochondrial respiration in response to ONOO- (1 mM) in J774 macrophages. Quin-2 at 100 microM, but not at 10 microM, caused a small (20%) inhibition of PARS activity, and did not significantly affect NAD+ depletion. Quin-2 exhibited a slight protective effect against the decrease in mitochondrial respiration in immunostimulated macrophages which endogenously produce ONOO-. These results suggest that the protective effect of quin-2 against the ONOO(-)-induced cellular injury is not due to interference with PARS activation or NAD+ depletion, but rather due to interference with a delayed intracellular event, possibly terminal calcium overload due to inhibition of mitochondrial enzymes and membrane pumps. Inhibition of calcium overload may be a viable experimental strategy to limit ONOO- cytotoxicity.