Platelets enhance endotoxin-induced monocyte tissue factor (TF) activity in the horse

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.

Effect of recombinant canine interleukin-6 and interleukin-8 on tissue factor procoagulant activity in canine peripheral blood mononuclear cells and purified canine monocytes

BACKGROUND

Inflammation is a major cause of disseminated intravascular coagulation (DIC) in dogs, but underlying mechanisms for its initiation are unknown. We hypothesized that pro-inflammatory cytokines, interleukin (IL)-6 and IL-8, induce tissue factor (TF) expression on canine monocyte surfaces, which may contribute to DIC initiation.

OBJECTIVES

The objectives of this study were to determine if (1) IL-6 and IL-8 would induce TF activity on canine monocytes, (2) fetal bovine serum or autologous plasma was required for IL-6- or IL-8-induced TF responses in canine monocytes, and (3) these pro-inflammatory cytokines would enhance TF activity on canine monocytes in response to low concentrations of lipopolysaccharide (LPS).

METHODS

Canine monocytes were isolated from EDTA-anticoagulated blood as peripheral blood mononuclear cells (PBMC) by double-density gradient centrifugation and adhesion to plastic. Adherent cells were stimulated for 4 hours with recombinant canine (rc)-IL-6 or rc-IL-8 (10-5000 pg/mL) with or without 10% heat-inactivated (HI) fetal bovine serum, untreated autologous canine plasma (ACP), or HI-ACP. Lipopolysaccharide (100 ng/mL) served as a positive control. Cells were also costimulated with either cytokine (100 pg/mL) or low concentrations of LPS (0.1 and 1 ng/mL). Monocytes immunopurified from PBMC with anti-CD14 antibodies were also stimulated with both cytokines (100 and 5000 pg/mL). TF activity on cell surfaces was measured by a 2-stage amidolytic assay, based on activated factor X generation.

RESULTS

Neither rc-IL-6 nor rc-IL-8 consistently stimulated TF procoagulant activity in canine PBMC or purified monocytes after 4 hours. Serum, plasma, or low concentrations of LPS did not enhance the TF response to these cytokines.

CONCLUSIONS

IL-6 or IL-8 at evaluated concentrations may not play major roles in coagulation activation by induction of TF expression on monocytes in dogs with inflammation.

The immunopathology of sepsis: pathogen recognition, systemic inflammation, the compensatory anti-inflammatory response, and regulatory T cells

Sepsis, the systemic inflammatory response to infection, represents the major cause of death in critically ill veterinary patients. Whereas important advances in our understanding of the pathophysiology of this syndrome have been made, much remains to be elucidated. There is general agreement on the key interaction between pathogen‐associated molecular patterns and cells of the innate immune system, and the amplification of the host response generated by pro‐inflammatory cytokines. More recently, the concept of immunoparalysis in sepsis has also been advanced, together with an increasing recognition of the interplay between regulatory T cells and the innate immune response. However, the heterogeneous nature of this syndrome and the difficulty of modeling it in vitro or in vivo has both frustrated the advancement of new therapies and emphasized the continuing importance of patient‐based clinical research in this area of human and veterinary medicine.

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.

Cloning and functional characterization of recombinant equine P-selectin

The recent molecular characterization and sequencing of equine P-selectin (ePsel), and its glycoprotein ligand, P-selectin glycoprotein ligand-1 (PSGL-1), have provided the tools for further investigation into their role in leukocyte trafficking. Here, we report the generation of a genetically engineered chimeric protein (ePsel-IgG) in which the equine P-selectin lectin and epithelial growth factor (EGF) domains were covalently linked to the equine IgG1 heavy chain constant region. The soluble ePsel-IgG was observed to bind to equine monocytes by confocal microscopy and flow cytometry. Furthermore, equine monocytes bound to immobilized ePsel-IgG in a time course and dose dependent manner. Not only did ePsel-IgG act as an adhesion molecule, it was also found to activate ERK1/2 kinase and induce IL-8 mRNA expression in equine monocytes. That all of the aforementioned ePsel-IgG-induced cell binding and cell signaling were abolished by the addition of EDTA, suggested that ePsel-IgG chimera mediated events occurred via the P-selectin ligand, PSGL-1. We were able to demonstrate that 78% of equine monocytes cross-reacted with anti-human HECA-452 antibody, which recognizes the sialy-Lewis X (sLex) epitope, a well-known carbohydrate binding site on human PSGL-1. Pre-incubation of equine PBMC with neuraminidase or O-sialoglycoprotein endopeptidase (OSGP) reduced ePsel-IgG monocyte binding to 36% or 60%, respectively. Taken together, these data suggest that there might be two ligand recognition sites on P-selectin, one of which recognizes sLex and another which recognizes P-selectin ligand core protein. The ePsel-IgG chimera can be a useful as a reagent for further studies on the role of equine P-selectin and signal transduction in inflammatory events in horse.

Effect of fetal bovine serum and heat-inactivated fetal bovine serum on microbial cell wall-induced expression of procoagulant activity by equine and canine mononuclear cells in vitro

OBJECTIVE

To determine the effect of fetal bovine serum (FBS) and heat-inactivated FBS (HI-FBS) on lipopolysaccharide (LPS)- and zymosan-induced procoagulant activity of equine and canine mononuclear cells.

SAMPLE POPULATION

Mononuclear cells from 18 horses and 3 dogs.

PROCEDURES

Cells were incubated with various concentrations of FBS, HI-FBS, LPS, zymosan, polymyxin B, and anti-LPS-binding protein monoclonal antibody or combinations of these constituents. A 1 stage recalcification assay was used to determine procoagulant activity.

RESULTS

Addition of FBS to media significantly increased procoagulant activity; equine and canine cells were stimulated by 1% and 10% FBS, respectively. Coincubation of cells with FBS and polymyxin B did not reduce this effect, suggesting that the response was not attributable to LPS contamination. Addition of HI-FBS to media did not stimulate procoagulant activity of equine or canine cells, and the sensitivity of the equine cells to LPS was significantly increased by HI-FBS. This increased LPS sensitivity was reduced 40% with monoclonal antibody directed against human recombinant LPS-binding protein. Increasing concentrations of HIFBS significantly increased LPS- and zymosan-induced procoagulant activity of canine cells.

CONCLUSION AND CLINICAL RELEVANCE

Procoagulant activity production in equine and canine mononuclear cells was significantly increased by addition of FBS, whereas heat inactivation of FBS eliminated this effect. Heat inactivation did not eliminate the function of serum proteins involved in enhancement of LPS and zymosan-induced procoagulant activity. Results suggest that HI-FBS can be used as a source of serum proteins that increase the sensitivity of mononuclear cells to bacterial and yeast cell wall components.