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From the Departments of Vascular Medicine (R.J.B., J.J.P.K., J.H.M.L., B.v.d.B., J.C.M.M., M.L., E.S.G.S.), Hematology (J.J.Z.), and Experimental Medicine (P.H.R.), Academic Medical Center, Amsterdam, the Netherlands; and Pfizer Global Research and Development (D.H.), Ann Arbor, Mich.
Correspondence to Erik S. Stroes, MD, PhD, Department of Vascular Medicine, Rm F4.275, Academic Medical Center, Meibergdreef 9, 1105 AZ Amsterdam, The Netherlands. E-mail e.s.stroes{at}amc.uva.nl
| Abstract |
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Key Words: inflammation endothelium atherosclerosis thrombosis
| Introduction |
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| Materials and Methods |
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The rhCRP (BiosPacific), derived from Escherichia coli (K12, substrain NM522), was supplied in 20 mmol/L Tris, 140 mmol/L NaCL, 2 mmol/L CaCl2, pH 7.5, and 0.05% (wt/vol) sodium azide and revealed a single 23-kDa band (>99%) after CBBR-staining (1 µg; SDS-polyacrylamide gel). Before purification, the host cell protein concentration was 85 ppm, as determined by a high-sensitive ELISA in accordance with manufacturers instructions (Cygnus Technologies Inc). Subsequently, the rhCRP was purified using size exclusion chromatography to remove contaminants including endotoxin and sodium azide (Univalid bv). Purity as well as stability were evaluated using sequential high-performance liquid chromatography and time-of-flight mass spectrometry, showing no other protein fractions, including the monomeric variant of CRP, besides the CRP pentamer. Endotoxin levels were below 1.5 endotoxin units (EU)/mL as evaluated by Limulus assay (turbidimetric kinetic method; ACC Inc). Purified rhCRP, added to whole blood (final concentration 88 µg/mL) obtained from 3 volunteers for 24 hours at 37°C and 5% CO2, significantly stimulated interleukin (IL)-8 production, whereas boiled or trypsinized rhCRP did not elicit a cytokine response. Moreover, in vitro assays, including cell culture experiments with human umbilical vein endothelial cells revealed no toxicity of the purified rhCRP solution.
In separate, single-dose toxicity studies in mice (n=6) at CRP concentrations more than four times higher than peak concentrations obtained in humans, we observed no direct effects of the purified rhCRP solution on temperature, blood pressure, or heart rate. In analogy to the findings in humans, a minor cytokine response was observed on purified rhCRP infusion (data not shown). The rhCRP was stored in a CaCl2-containing buffer (pH 8.5) at 0 to 4°C, and all experiments were performed within 4 weeks after rhCRP preparation.
CRP concentrations were measured with high-sensitivity and immunonephelometric assays (Roche Diagnostics Corporation). Tumor necrosis factor-
(TNF-
), IL-6, and IL-8 were assayed by cytometric bead array analysis (BD Biosciences). We measured concentrations of soluble E-selectin, (R&D Systems), von Willebrand factor (vWFAg; Dako), prothrombin F1+2 (Dade-Behring), plasminogen activator inhibitor type-1 (Monozyme), and D-dimer (Asserachrom D-dimer, Roche) using ELISAs. Additionally, serum amyloid A protein (SAA; Anogen) and type II secretory phospholipase A2 (sPLA2; CLB) were measured with this technique. Monocytic expression of CD11b and CD18 was quantified using a fluorescence-activated cell sorter Vantage flowcytometer (Becton Dickinson) at baseline, and at 4 and 24 hours after infusion. Monocytes were gated by their specific forward and side-scatter pattern and further identified by high CD14 expression.
Data are medians and ranges. Differences between treatment groups were tested by analysis of variance for repeated measures. Comparisons within groups were done with the Wilcoxon signed rank test. A probability value less than 0.05 was regarded significant.
| Results |
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In accord with previous in vitro studies, vWFAg and E-selectin rose from 82% (60 to 127) to 127% (84 to 208) (P<0.01) and 44.1 ng/mL (19.1 to 69.0) to 67.7 ng/mL (25.8 to 115.7) (P<0.05), respectively. After 4 hours, IL-6 increased significantly from less than 1.6 pg/mL (<1.6 to 14.7) to 99.6 pg/mL (5.0 to 709.5, P<0.05 versus baseline) and so did IL-8 from 14.1 pg/mL (6.4 to 29.2) to 106.0 pg/mL (37.7 to 243.0; P<0.05) (Figure 1). TNF-
concentrations remained unaltered; furthermore, a trend toward monocytic CD11b and CD18 upregulation was recorded. After 8 hours, both serum amyloid A protein [4.7 mg/L (0.3 to 27.9) to 206.2 mg/L (27.8 to 2099.2), P<0.05 versus baseline] and sPLA2 [2.0 ng/mL (2.0 to 6.0) to 22.0 ng/mL (9.0 to 60.0), P<0.05] concentrations rose significantly (Figure 1).
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Coagulation activation was associated with a 3-fold increase in prothrombin F1 +2 ng/mL concentrations 4 hours after rhCRP-infusion (P<0.05), and a 3.5-fold increase of D-dimer concentrations (P<0.05) (Figure 2). Additionally, plasminogen activator inhibitor type-1 was significantly enhanced [35.0 ng/mL (14.0 to 109.0) to 71.0 ng/mL (18.0 to 83.0); P<0.05].
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| Discussion |
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Although CRP concentrations greater than 3 mg/L already denote heightened cardiovascular risk,1 patients are usually exposed to these raised concentrations for many years before potential onset of cardiovascular events. Of note, mechanisms behind this association between modestly elevated CRP levels and cardiovascular events may not necessarily reflect the effects observed in this study. In view of the acute nature of the present study, CRP concentrations targeted at 25 mg/L, in accordance with previous in vitro studies,4 seemed appropriate to assess potential direct effects of CRP in vivo. The induction of a proinflammatory state in response to CRP is illustrated by IL-6 and IL-8 increases at 4 hours, followed by significant rises in the acute phase reactants SAA and sPLA2 from 8 hours onwards. Accordingly, a second peak of endogenous CRP release was noted after 24 hours.
A previously reported drawback of inflammatory activation by CRP is potential lipopolysaccharide-contamination in commercially available rhCRP solutions. Using extensively purified rhCRP, several details preclude that possibility in our study. First, we did not note an increase in TNF-
, which is an established hallmark of lipopolysaccharide-induced inflammatory activation. Second, the calculated quantity of lipopolysaccharide given during rhCRP infusion was at most 1.75 EU/kg, whereas the critical amount to induce activation of coagulation and/or TNF-
release exceeds is 20 EU/kg.9 Last, there was lack of a febrile response in any of the subjects, whereas the kinetics of the observed cytokine responses subsequent to rhCRP infusion differ profoundly from that evoked by lipopolysaccharide infusion.
Activation of coagulation induced by CRP might result from enhanced monocytic tissue-factor activity, because in vitro CRP has been shown to induce monocytic tissue-factor expression.10 Alternative mechanisms might include downregulation of the anticoagulant protein C pathway secondary to the inflammatory response. The precise mechanisms underlying the CRP-mediated activation of the inflammatory and coagulation pathways need further investigation. Although we evaluated only seven individuals, the consistency of the observations clearly lend further support to the development of compounds that specifically block CRP bioactivity in vivo.
| Acknowledgments |
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| Footnotes |
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| References |
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3. Ridker PM, Rifai N, Rose L, Buring JE, Cook NR. Comparison of C-reactive protein and low-density lipoprotein cholesterol levels in the prediction of first cardiovascular events. N Engl J Med. 2002; 347: 15571565.
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5. Venugopal SK, Devaraj S, Yuhanna I, Shaul P, Jialal I. Demonstration that C-reactive protein decreases eNOS expression and bioactivity in human aortic endothelial cells. Circulation. 2002; 106: 14391441.
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8. Paul A, Ko KW, Li L, Yechoor V, McCrory MA, Szalai AJ, Chan L. C-reactive protein accelerates the progression of atherosclerosis in apolipoprotein Edeficient mice. Circulation. 2004; 109: 647655.
9. Levi M, Ten Cate H, Bauer KA, van der PT, Edgington TS, Buller HR, Van Deventer SJ, Hack CE, ten Cate JW, Rosenberg RD. Inhibition of endotoxin-induced activation of coagulation and fibrinolysis by pentoxifylline or by a monoclonal anti-tissue factor antibody in chimpanzees. J Clin Invest. 1994; 93: 114120.[Medline] [Order article via Infotrieve]
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