Endotoxin-induced activation of equine platelets: evidence for direct activation of p38 MAPK pathways and vasoactive mediator production

Intravenous lipopolysaccharide challenge in early- versus mid-lactation dairy cattle. I: The immune and inflammatory responses

Cows in early lactation (EL) are purportedly immune suppressed, which renders them more susceptible to disease. Thus, the study objective was to compare key biomarkers of immune activation from i.v. LPS between EL and mid-lactation (ML) cows. Multiparous EL (20 ± 2 DIM; n = 11) and ML (131 ± 31 DIM; n = 12) cows were enrolled in a 2 × 2 factorial design and assigned to 1 of 2 treatments by lactation stage (LS): (1) EL (EL-LPS; n = 6) or ML (ML-LPS; n = 6) cows administered a single LPS bolus from Escherichia coli O55:B5 (0.09 µg/kg of BW), or (2) pair-fed (PF) EL (EL-PF; n = 5) or ML (ML-PF; n = 6) cows administered i.v. saline. After LPS administration, cows were intensely evaluated for 3 d to analyze their response and recovery to LPS. Rectal temperature increased in LPS relative to PF cows (1.1°C in the first 9 h), and the response was more severe in EL-LPS relative to ML-LPS cows (2.3 vs. 1.3°C increase at 4 h post-LPS; respectively). Respiration rate increased only in EL-LPS cows (47% relative to ML-LPS in the first hour post-LPS). Circulating tumor necrosis factor-α, IL-6, monocyte chemoattractant protein-1, macrophage inflammatory protein (MIP)-1α, MIP-1β, and IFN-γ-inducible protein-10 increased within the first 6 h after LPS and these changes were exacerbated in EL-LPS relative to ML-LPS cows (6.3-fold, 4.8-fold, 57%, 93%, 10%, and 61%, respectively). All cows administered LPS had decreased circulating iCa relative to PF cows (34% at the 6 h nadir), but the hypocalcemia was more severe in EL-LPS than ML-LPS cows (14% at 6 h nadir). In response to LPS, neutrophils decreased regardless of LS, then increased into neutrophilia by 24 h in all LPS relative to PF cows (2-fold); however, the neutrophilic phase was augmented in EL- compared with ML-LPS cows (63% from 24 to 72 h). Lymphocytes and monocytes rapidly decreased then gradually returned to baseline in LPS cows regardless of LS; however, monocytes were increased (57%) at 72 h in EL-LPS relative to ML-LPS cows. Platelets were reduced (46%) in LPS relative to PF cows throughout the 3-d following LPS, and from 24 to 48 h, platelets were further decreased (41%) in EL-LPS compared with ML-LPS. During the 3-d following LPS, serum amyloid A (SAA), LPS-binding protein (LBP), and haptoglobin (Hp) increased in LPS compared with PF groups (9-fold, 72%, and 153-fold, respectively), and the LBP and Hp responses were more exaggerated in EL-LPS than ML-LPS cows (85 and 79%, respectively) whereas the SAA response did not differ by LS. Thus, our data indicates that EL immune function does not appear "suppressed," and in fact many aspects of the immune response are seemingly functionally robust.

Activated platelets and platelet-leukocyte aggregates in the equine systemic inflammatory response syndrome

In humans, activated platelets contribute to sepsis complications and to multiple organ failure. In our prospective analytical study of cases of the equine systemic inflammatory response syndrome (SIRS), we adapted a standard human protocol for the measurement of activated platelets and platelet-leukocyte aggregates (PLAs) in equine platelet-leukocyte-rich plasma (PLRP) by flow cytometry, and we investigated the hypothesis that activated platelets and PLAs are increased in clinical cases of SIRS. We included 17 adult horses and ponies fulfilling at least 2 SIRS criteria, and 10 healthy equids as controls. Activation of platelets was determined by increased expression of CD62P on platelets. Activated platelets and PLAs were measured before and after in vitro activation of platelets with collagen. Median expression of CD62P on platelets was significantly increased after activation in the control group: 1.45% (interquartile range [IQR]: 1.08-1.99%) initially versus 8.78% (IQR: 6.79-14.78%, p = 0.002) after activation. The equids with SIRS had significantly more activated platelets and PLAs in native PLRP than controls: CD62P 4.92% (median, IQR: 2.21-12.41%) versus 1.45% in controls (median, IQR: 1.08-1.99%, p = 0.0007), and PLAs 4.16% (median, IQR: 2.50-8.58%) versus 2.95% in controls (median, IQR: 1.57-3.22%, p = 0.048). To our knowledge, increased platelet activation and PLAs have not been demonstrated previously with flow cytometry in clinical cases of equine SIRS.

Effect of the p38 MAPK inhibitor doramapimod on the systemic inflammatory response to intravenous lipopolysaccharide in horses

Background

Doramapimod, a p38 MAPK inhibitor, is a potent anti‐inflammatory drug that decreases inflammatory cytokine production in equine whole blood in vitro. It may have benefits for treating systemic inflammation in horses.

Objective

To determine whether doramapimod is well tolerated when administered IV to horses, and whether it has anti‐inflammatory effects in horses in a low‐dose endotoxemia model.

Animals

Six Standardbred horses.

Methods

Tolerability study, followed by a blinded, randomized, placebo‐controlled cross‐over study. Horses were given doramapimod, and clinical and clinicopathological variables were monitored for 24 hours. Horses then were treated with doramapimod or placebo, followed by a low dose infusion of lipopolysaccharide (LPS). Clinical variables (heart rate, rectal temperature, noninvasive blood pressure), leukocyte count and tumor necrosis factor alpha (TNF‐α) and interleukin‐1 beta (IL‐1β) concentrations were measured at multiple time points until 6 hours post‐LPS infusion.

Results

No adverse effects or clinicopathological changes were seen in the safety study. When treated with doramapimod as compared to placebo, horses had significantly lower heart rates (P = .03), rectal temperatures (P = .03), and cytokine concentrations (P = .03 for TNF‐α and IL‐1β), and a significantly higher white blood cell count (P = .03) after LPS infusion.

Conclusions and Clinical Importance

Doramapimod has clinically relevant anti‐inflammatory effects in horses, likely mediated by a decrease in leukocyte activation and decrease in the release of pro‐inflammatory cytokines. To evaluate its potential as a novel treatment for systemic inflammatory response syndrome in horses, clinical trials will be necessary to determine its efficacy in naturally occurring disease.

Bacterial Endotoxins and Their Role in Periparturient Diseases of Dairy Cows: Mucosal Vaccine Perspectives

During the periparturient period there is a significant increase in the incidence of multiple metabolic and infectious diseases in dairy cows. Dairy cows are fed high-grain diets immediately after calving to support production of large amounts of milk. Mounting evidence indicates these types of diets are associated with the release of high amounts of endotoxins in the rumen fluid. If infected, the udder and uterus additionally become important sources of endotoxins during the postpartum period. There is increasing evidence that endotoxins translocate from rumen, uterus, or udder into the systemic circulation and trigger chronic low-grade inflammatory conditions associated with multiple diseases including fatty liver, mastitis, retained placenta, metritis, laminitis, displaced abomasum, milk fever, and downer cow syndrome. Interestingly, endotoxin-related diseases are triggered by a bacterial component and not by a specific bacterium. This makes prevention of these type of diseases different from classical infectious diseases. Prevention of translocation of endotoxins into the host systemic circulation needs to take priority and this could be achieved with a new approach: mucosal vaccination. In this review article, we discuss all the aforementioned issues in detail and also report some of our trials with regards to mucosal vaccination of periparturient dairy cows.

Anti-inflammatory effects of a p38 MAP kinase inhibitor, doramapimod, against bacterial cell wall toxins in equine whole blood

Doramapimod (BIRB-796-BS), is an anti-inflammatory compound, acting through p38 MAPK inhibition, but its anti-inflammatory effects have not previously been studied in the horse. Whole blood aliquots from healthy horses diluted 1:1 with cell culture medium were incubated for 21 h with 1 μg/ml of lipopolysaccharide (LPS), lipoteichoic acid (LTA) or peptidoglycan (PGN) in the presence of increasing concentrations of doramapimod (3 × 10^-8 M to 10^-5 M). Cell bioassays were used to measure TNF-α and IL-1β activity. Doramapimod significantly and potently inhibited TNF-α and IL-1β activity induced by all three bacterial toxins. There was no significant difference in IC₅₀ or maximum inhibition of TNF-α or IL-1β production between any of the toxins. Maximum inhibition of IL-1β was higher than that of TNF-α for all toxins, and this difference was significant for LPS (P = 0.04). Doramapimod was a potent inhibitor of TNF-α and IL-1β for inflammation induced by LPS, LTA and PGN, with potency much greater than that of other drugs previously tested using similar methods.

Immunological considerations in the development of Pseudomonas aeruginosa vaccines

Pseudomonas aeruginosa is an opportunistic human pathogen capable of causing a wide range of potentially life-threatening infections. With multidrug-resistant P. aeruginosa infections on the rise, the need for a rationally-designed vaccine against this pathogen is critical. A number of vaccine platforms have shown promising results in pre-clinical studies, but no vaccine has successfully advanced to licensure. Growing evidence suggests that an effective P. aeruginosa vaccine may require Th17-type CD4⁺ T cells to prevent infection. In this review, we summarize recent pre-clinical studies of P. aeruginosa vaccines, specifically focusing on those that induce Th17-type cellular immunity. We also highlight the importance of adjuvant selection and immunization route in vaccine design in order to target vaccine-induced immunity to infected tissues. Advances in cellular immunology and adjuvant biology may ultimately influence better P. aeruginosa vaccine platforms that can protect targeted human populations.

Effect of antigen challenge on dynamics of CD62P and CD41/61 expression on platelets in horses with recurrent airway obstruction (RAO)

Recurrent airway obstruction (RAO), also known as heaves, is an allergic respiratory condition that develops in horses following an exposure to aeroallergens in hay and straw. This is manifested by airway hyperreactivity, inflammation, bronchoconstriction, as well as a leukocyte and platelet infiltration into the airways. Platelet activation and an increase in circulating platelet-leukocyte aggregates may lead to airway remodeling. The aim of this study was to explore the effect of seven-day antigen challenge on dynamics of platelet indices and CD41/61 and CD62 P expression on platelets in horses with RAO. Ten RAO-affected horses and ten healthy horses were included in this study. All horses were exposed to 7 days hay and straw challenge. Blood samples were collected prior to the challenge (Pre-challenge) and 1, 2, 3, 7 and 14 days after the initiating the antigen challenge. Blood samples were obtained to determine the platelet count (PLT), mean platelet volume (MPV) and platelet large cell ratio (P-LCR). Expression of CD62 P and CD41/61 was detected by flow cytometry on activated platelets. Antigen challenge resulted in a significant gradual decrease of PLT in RAO horses, but not in controls. MPV and P-LCR in control and RAO-affected horses remained unchanged after antigen challenge. The expression of CD62 P and CD41/61 in RAO horses was significantly higher compared to control horses. The antigen challenge resulted in an increase expression of CD62 P and CD41/61 on the platelets of RAO-affected horses, while did not lead to significant changes in the control group. An increased expression of CD62 P and CD41/61 indicates platelet activation what may contribute to the formation of platelet aggregates in their respiratory system.

Lipopolysaccharide potentiates platelet responses via toll-like receptor 4-stimulated Akt-Erk-PLA2 signalling

Lipopolysaccharide (LPS) from the cell envelope of Gram-negative bacteria is a principal cause of the symptoms of sepsis. LPS has been reported to modulate the function of platelets although the underlying mechanisms of LPS action in these cells remain unclear. Platelets express the Toll-like receptor 4 (TLR4) which serves as a receptor for LPS, although the potential role of TLR4 and associated cell signalling in controlling platelet responses to LPS has not been extensively explored. In this study, we therefore investigated the actions of LPS prepared from different strains of Escherichia coli on platelet function, the underlying signalling mechanisms, and the potential role of TLR4 in orchestrating these. We report that LPS increased the aggregation of washed platelets stimulated by thromboxane (U46619) or GPVI collagen receptor agonists, effects that were prevented by a TLR4 antagonist. Associated with this, LPS enhanced fibrinogen binding, P-selectin exposure and reactive oxygen species (ROS) release. Increase of ROS was found to be important for the actions of LPS on platelets, since these were inhibited in the presence of superoxide dismutase or catalase. The effects of LPS were associated with phosphorylation of Akt, ERK1/2 and PLA2 in stimulated platelets, and inhibitors of PI3-kinase, Akt and ERK1/2 reduced significantly LPS enhanced platelet function and associated ROS production. Furthermore, inhibition of platelet cyclooxygenase or the thromboxane receptor, revealed an important role for thromboxane A2. We therefore conclude that LPS increases human platelet activation through a TLR4-PI3K-Akt-ERK1/2-PLA2 -dependent pathway that is dependent on ROS and TXA2 formation.

Treatment of endotoxaemia and septicaemia in the equine patient

Endotoxins, constituents of the cell wall of gram-positive and gram-negative bacteria, regularly result in severe illness and death in horses. In endotoxaemia, these constituents are present in the systemic circulation; in septicaemia, whole microbes invade normally sterile parts of the body. Interaction of these endotoxins with pathogen recognition receptors leads to an inflammatory response that cannot always be sufficiently contained and hence needs direct treatment. Over the last decennia, our understanding of the pathophysiology of endotoxaemia and septicaemia has significantly increased. Based on improved understanding of the interaction between receptors and endotoxins as well as the subsequent downstream signalling pathways, new therapeutic targets have been identified in laboratory animal species and humans. Important species differences in the recognition of endotoxins and pathogens by their receptors as well as the inflammatory response to receptor activation hamper extrapolation of this information to the horse (and other species). Historically, horses with endotoxaemia and septicaemia have been treated mainly symptomatically and supportively. Based on the identified therapeutic targets, this review describes the current knowledge of the treatment for endotoxaemia and septicaemia in the horse with reference to the findings in other animal species and humans.

p38 MAPK in cardioprotection - are we there yet?

PKs transfer a phosphate from ATP to the side-chain hydroxyl group of a serine, threonine or tyrosine residue of a substrate protein. This in turn can alter that protein's function; modulating fundamental cellular processes including, metabolism, transcription, growth, division, differentiation, motility and survival. PKs are subdivided into families based on homology. One such group are the stress-activated kinases, which as the name suggests, are activated in response to cellular stresses such as toxins, cytokines, mechanical deformation and osmotic stress. Members include the p38 MAPK family, which is composed of α, β, γ and δ, isoforms which are encoded by separate genes. These kinases transduce extracellular signals and coordinate the cellular responses needed for adaptation and survival. However, in cardiovascular and other disease states, these same systems can trigger maladaptive responses that aggravate, rather than alleviate, the disease. This situation is analogous to adrenergic, angiotensin and aldosterone signalling in heart failure, where inhibition is beneficial despite the importance of these hormones to homeostasis. The question is whether similar benefits could accrue from p38 inhibition? In this review, we will discuss the structure and function of p38, the history of p38 inhibitors and their use in preclinical studies. Finally, we will summarize the results of recent cardiovascular clinical trials with p38 inhibitors.

Expression of surface platelet receptors (CD62P and CD41/61) in horses with recurrent airway obstruction (RAO)

Recurrent airway obstruction (RAO) is an allergic disease of horses similar to human asthma, which is characterized by airway inflammation and activation of neutrophils, lymphocytes and platelets. Platelet activation and an increase in circulating platelet-leukocyte aggregates may lead to airway remodeling. The aim of this study was to investigate platelet status in RAO-affected horses based on the platelet morphology and platelet surface expression of CD41/61 and CD62P. Ten RAO-affected horses and ten healthy horses were included in this study. Blood samples were obtained to determine the platelet count (PLT), mean platelet volume (MPV) and platelet large cell ratio (P-LCR). Expression of CD62P and CD41/61 was detected by flow cytometry on activated platelets. The median PLT was significantly reduced in horses with RAO compared to the controls. The MPV and the P-LCR values were significantly higher in RAO horses than controls. Expression of CD41/61 on platelets was increased in RAO horses, while CD62P expression was reduced. This study demonstrated the morphological changes in platelets and expression of platelet surface receptors. Despite the decrease of CD62P expression, the observed increased surface expression of CD41/61 on platelets in horses with RAO may contribute to the formation of platelet aggregates in their respiratory system.

Inhibition of platelet function with clopidogrel, as measured with a novel whole blood impedance aggregometer in horses

This study aimed to validate a loading and maintenance clopidogrel dosing scheme for the inhibition of platelet function, measured by whole blood impedance aggregometry in healthy adult horses. Ten Warmblood horses received oral clopidogrel once daily. Doses were based on 50 kg weight categories and resulted in one loading dose of 6-6.5 mg/kg bodyweight and maintenance doses of 1.2-1.4 mg/kg over the next 4 days. Platelet function was measured via whole blood multiple electrode impedance aggregometry prior to (T0) and at 6, 12, 24, 48, 72, 96, 144, 192 and 240 h following the loading dose. Aggregometries for collagen (COLtest), arachidonic acid (ASPItest), adenosine diphosphate (ADPtest) and ADP with prostaglandin E1 (ADPtestHS) were performed. Statistical analyses included one way repeated measures ANOVAs and subsequent Dunnett's tests. Platelet aggregation induced by collagen remained unchanged. There were significant inhibitions in the ASPItest (P <0.01 at 192 h, and P <0.05 at 240 h) and the ADPtest and ADPtestHS (P < 0.01, with the exception of 240 h). The loading dose of clopidogrel induced rapid inhibition of platelet function within hours, and the low dose was suitable for maintaining the inhibition over the 4 days of therapy. Recovery of platelet function was restored 6 days after the cessation of medication, determined with the ADPtest and ADPtestHS, but remained inhibited with the ASPItest. The prolonged effect of clopidogrel may indicate differences in the activation of platelets between horses and humans that were previously unknown.

Effects of clopidogrel on horses with experimentally induced endotoxemia

OBJECTIVE

To evaluate the effects of clopidogrel on clinical and clinicopathologic variables in healthy horses with experimentally induced endotoxemia.

ANIMALS

12 adult mares. Procedures-Horses were assigned with a randomization procedure to receive clopidogrel (4 mg/kg, once, then 2 mg/kg, q 24 h; n = 6) or a placebo (6) through a nasogastric tube. After 72 hours of treatment, horses received lipopolysaccharide (LPS; 30 ng/kg, IV). Heart rate, respiratory rate, rectal temperature, CBC variables, plasma fibrinogen concentration, serum tumor necrosis factor-α concentration, plasma von Willebrand factor concentration, and measures of platelet activation (including ADP- and collagen-induced platelet aggregation and closure times, thrombelastography variables, and results of flow cytometric detection of platelet membrane P-selectin, phosphatidylserine, and microparticles) were determined at various times before and after LPS administration by investigators unaware of the treatment groups. Statistical analyses were performed with repeated-measures ANOVA.

RESULTS

4 of 6 clopidogrel-treated horses had significant decreases in ADP-induced platelet aggregation before and after LPS administration. Heart rate increased significantly after LPS administration only for the placebo group. No significant differences were detected between groups for CBC variables, closure time, and plasma concentration of fibrinogen or serum concentration of tumor necrosis factor-α, and no clinically relevant differences were detected for other hemostatic variables.

CONCLUSIONS AND CLINICAL RELEVANCE

In this study, administration of LPS did not induce platelet hyperreactivity in horses on the basis of measures of platelet adhesion, aggregation, degranulation, and procoagulant activity. Administration of clopidogrel was associated with variable platelet antiaggregatory activity and attenuated some clinical signs of endotoxemia.

Arachidonic acid depletion extends survival of cold-stored platelets by interfering with the [glycoprotein Ibα--14-3-3ζ] association

BACKGROUND

Cold storage of platelets reduces bacterial growth and preserves their hemostatic properties better than current procedures do. However, storage at 0°C induces [14-3-3ζ-glycoprotein Ibα] association, 14-3-3ζ release from phospho-Bad, Bad activation and apoptosis.

DESIGN AND METHODS

We investigated whether arachidonic acid, which also binds 14-3-3ζ, contributes to coldinduced apoptosis.

RESULTS

Cold storage activated P38-mitogen-activated protein kinase and released arachidonic acid, which accumulated due to cold inactivation of cyclooxygenase-1/thromboxane synthase. Accumulated arachidonic acid released 14-3-3ζ from phospho-Bad and decreased the mitochondrial membrane potential, which are steps in the induction of apoptosis. Addition of arachidonic acid did the same and its depletion made platelets resistant to cold-induced apoptosis. Incubation with biotin-arachidonic acid revealed formation of an [arachidonic acid-14-3-3ζ-glycoprotein Ibα] complex. Indomethacin promoted complex formation by accumulating arachidonic acid and released 14-3-3ζ from cyclo-oxygenase-1. Arachidonic acid depletion prevented the cold-induced reduction of platelet survival in mice.

CONCLUSIONS

We conclude that cold storage induced apoptosis through an [arachidonic acid-14-3-3ζ-glycoprotein Ibα] complex, which released 14-3-3ζ from Bad in an arachidonic acid-dependent manner. Although arachidonic acid depletion reduced agonist-induced thromboxane A(2) formation and aggregation, arachidonic acid repletion restored these functions, opening ways to reduce apoptosis during storage without compromising hemostatic functions post-transfusion.

Cocaine up-regulation of the norepinephrine transporter requires threonine 30 phosphorylation by p38 mitogen-activated protein kinase

The norepinephrine (NE) transporter (NET) regulates NE signaling by rapidly clearing synaptic NE. Cocaine binds NET and modulates NE transport. These actions contribute to rewarding effects and abuse liability of cocaine. Activation of mitogen-activated protein kinase (MAPK) cascades is implicated in cocaine-induced neuroadaptations. However, the role of MAPK and the mechanisms involved in cocaine modulation of NET are not clear. Acute intra-peritoneal injections of cocaine (20 mg/kg body weight) to rats resulted in increased NE uptake by prefrontal cortex (PFC) synaptosomes with a parallel increase in the surface expression of endogenous NET. Cocaine also enhanced the immunoreactivity of phospho-p38 MAPK in the PFC synaptosomes without affecting the total p38 MAPK. In vitro cocaine (30-50 μM) treatment of rat PFC synaptosomes increased native NET function, surface expression, and phosphorylation in a manner sensitive to p38 MAPK inhibition by PD169316. We next examined cocaine-elicited effects on wild-type human NET (hNET) expressed heterologously in human placental trophoblast cells to gain more insights into the mechanisms involved. Cocaine treatment of hNET expressing human placental trophoblast cells up-regulated the function, surface expression, and phosphorylation of hNET in a PD169316-sensitive manner. In addition, cocaine inhibited constitutive endocytosis of hNET. Mutational analysis of serine and threonine residues revealed that substitution of threonine 30, located at the amino terminus of hNET with alanine (T30A-hNET), abolished cocaine-induced up-regulation of NET function, surface expression, and phosphorylation. Furthermore, cocaine did not alter T30A-hNET endocytosis. These studies identify a novel molecular mechanism that cocaine-activated p38 MAPK-mediated phosphorylation of NET-T30 dictates surface NET availability, and hence, NE transport.

Aggregation and microparticle production through toll-like receptor 4 activation in platelets from recently menopausal women

Bacterial infection may increase risk for thrombosis and atherosclerosis. Human platelets express toll-like receptor 4 (TLR4), the receptor for gram-negative bacterial lipopolysaccharide (LPS). Experiments were designed to evaluate direct, acute effects of TLR4 activation on aggregation, secretion, and generation of prothrombogenic microparticles in vitro on platelets derived from healthy women at risk for development of cardiovascular disease because of their hormonal status. Platelet-rich plasma from recently menopausal women was incubated with ultrapure Escherichia coli LPS in the absence or presence of antibodies that neutralize the human TLR4. Incubating platelets with LPS (100 ng/mL) for 5 minutes decreased aggregation and dense granule adenosine triphosphate secretion induced by thrombin receptor agonist peptide (TRAP) but not by adenosine diphosphate or collagen. The antibody to TLR4 blocked this effect of LPS. TLR4 activation increased phosphorylation of p38 mitogen-activated protein kinase and decreased production of prothrombotic phosphatidylserine and P-selectin-positive microparticles in response to TRAP. Therefore, acute, direct activation of TLR4 reduces platelet reactivity to TRAP stimulation in vitro. Increased thrombotic and cardiovascular risk with bacterial infection most likely reflects the sum of TLR4 activation on other blood and vascular cells to release proinflammatory cytokines/chemokines, which indirectly affect platelet reactivity.

Regulation of platelet activating factor-induced equine platelet activation by intracellular kinases

Lipopolysaccharide (LPS) can activate equine platelets directly or indirectly, via leukocyte-derived platelet activating factor (PAF). Thromboxane (Tx) production by LPS-stimulated equine platelets requires p38 MAPK and this kinase has been suggested as a therapeutic target in endotoxaemia. The present study has utilised selective inhibitors to investigate the role of p38 MAPK and two other kinases, phosphatidylinositol-3 kinase (PI3K) and protein kinase C (PKC), in regulating PAF-induced Tx production, aggregation and 5-HT release in equine platelets, and the modification of these responses by LPS. LPS enhanced PAF-induced 5-HT release, an effect that was reduced by the p38 MAPK inhibitor, SB203580 (60 +/- 8% reduction; n = 6). SB203580 did not affect responses to PAF alone; whereas inhibition of PKC reduced PAF-induced 5-HT release, Tx production and aggregation (maximal inhibition by the PKCdelta inhibitor, rottlerin: 69 +/- 13%, 63 +/- 14% and 97 +/- 1%, respectively; n = 6). Wortmannin and LY249002, which inhibit PI3K, also caused significant inhibition of PAF-induced aggregation (maximal inhibition 78 +/- 3% and 88 +/- 2%, respectively; n = 6). These data suggest that inhibition of platelet p38 MAPK may be of benefit in equine endotoxaemia by counteracting some of the effects of LPS. However, detrimental effects of platelet activation mediated by PAF and not enhanced by LPS are unlikely to be markedly affected.

Plasma concentrations of endotoxin and platelet activation in the developmental stage of oligofructose-induced laminitis

The link between the fermentation of carbohydrate in the equine large intestine and the development of acute laminitis is poorly understood. Absorption of endotoxin (lipopolysaccharide; LPS) into the plasma has been observed in one experimental model of laminitis, but does not cause laminitis when administered alone. Thus, the potential role of endotoxin is unclear. Platelet activation has previously been demonstrated in the developmental stage of laminitis. Equine platelets are more sensitive than leukocytes to activation by endotoxin, and can be activated directly by LPS in the low pg/ml range, activating p38 MAP kinase and releasing serotonin (5-HT) and thromboxane. The objectives of this study were firstly to determine whether endotoxin and platelet activation could be measured in the plasma of horses in the developmental phase of laminitis induced with oligofructose. Secondly, the time course of events involving platelet activation and platelet-derived vasoactive mediator production was investigated. Laminitis was induced in six Standardbred horses by the administration of 10 g/kg bwt of oligofructose. Plasma samples were obtained every 4h, and platelet pellets were obtained by centrifugation. LPS was measured using a kinetic limulus amebocyte lysate assay, and platelet activation was assessed by Western blotting for the phosphorylated form of p38 MAP kinase. Plasma 5-HT was assayed by HPLC with electrochemical detection and thromboxane B(2) was measured by radioimmunoassay. Clinical signs of laminitis and histopathologic changes were observed in lamellar sections from five of the six horses. Onset of lameness was between 20 and 30 h after the administration of oligofructose. LPS increased above the limit of detection (0.6 pg/ml) to reach a peak of 2.4+/-1.0 pg/ml at 8 h. TNFalpha was also detectable in the plasma from 12 to 24 h. There was a time-dependent increase in platelet p38 MAPK phosphorylation, which peaked at approximately 12 h (3.8+/-1.3 fold increase); plasma 5-HT and thromboxane increased steadily after this time (2.9+/-0.6 and 11.3+/-5.0 fold increases, respectively). These data indicate that small quantities of endotoxin may move into the circulation from the large intestine after the sharp decrease in pH that occurs as a result of carbohydrate fermentation. Correlating these findings with in vitro studies suggests that LPS may primarily activate platelets, leading indirectly to the activation of leukocytes. Therefore, endotoxin may contribute in the initiation of the early inflammatory changes observed in experimental models of acute laminitis.

Role of p38 MAPK in LPS induced pro-inflammatory cytokine and chemokine gene expression in equine leukocytes

Endotoxemia occurs when bacterial lipopolysaccharide (LPS) in the blood induces a dysregulated inflammatory response, resulting in circulatory shock and multi-organ failure. Laminitis is a common complication in endotoxemic horses and is frequently the reason for humane euthanasia of these cases. Blood leukocytes are a principal target of LPS in endotoxemia leading to activation of multiple signal transduction pathways involved in the induction of a number of pro-inflammatory genes. In other animal models, the p38 mitogen activated protein kinase (MAPK) pathway has been associated with induced expression of tumor necrosis factor-alpha (TNFalpha), interleukin (IL)-1beta, IL-6 and IL-8. The goal of this study was to determine the role of the p38 MAPK pathway in the induction of these pro-inflammatory cytokine and chemokine genes in LPS-stimulated equine leukocytes. Stimulation of equine peripheral blood leukocytes resulted in an increase in TNFalpha, IL-1beta, IL-6 and IL-8 mRNA levels. Pharmacological inhibition of p38 MAPK activity with SB203580 or SB202190 reduced the ability of LPS stimulation to increase mRNA concentrations for all four genes. However, only SB203580 pretreatment significantly reduced LPS-stimulated IL-1beta and IL-8 mRNA expression and only pretreatment with SB202190 significantly reduced LPS-stimulated TNFalpha and IL-6 mRNA expression. From this study we conclude TNFalpha, IL-1beta, IL-6 and IL-8 are induced upon LPS stimulation of equine leukocytes and that this induction of gene expression is dependent on the p38 MAPK pathway. However, there are differences in the efficacy of the p38 inhibitors tested here that may be explained by differences in specificity or potency. This study provides evidence for the use of selective p38 MAPK inhibitors as potential therapeutics for the treatment of equine endotoxemia.

Equine recurrent airway obstruction and insect bite hypersensitivity: understanding the diseases and uncovering possible new therapeutic approaches

Recurrent airway obstruction (RAO) and insect bite hypersensitivity (IBH) are allergic conditions that are commonly encountered in the horse. Whilst complete allergen avoidance is an effective management strategy for both diseases, this may not be achievable in all cases and treatment options are therefore required. The inflammatory response is the main therapeutic target for glucocorticoids given to horses with RAO and severe cases of IBH, whilst the bronchodilators used in RAO primarily target airway smooth muscle. Such drugs are effective in most but not all individuals and there may be unwanted adverse effects. This article will review how knowledge of drug action and the pathogenesis of RAO and IBH can be utilised to identify potential targets for novel therapeutic agents that, in the longer term, may be safer and/or more effective in managing the allergic horse.