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Infection and Immunity, July 2004, p. 4322-4326, Vol. 72, No. 7
0019-9567/04/$08.00+0 DOI: 10.1128/IAI.72.7.4322-4326.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Department of Cell and Molecular Biology, Lund University, Lund,1 Department of Medical Microbiology, Lund University, Malmö University Hospital, Malmö, Sweden,3 Biochemical Pharmacology, University of Konstanz, Konstanz, Germany2
Received 8 February 2004/ Returned for modification 15 March 2004/ Accepted 5 April 2004
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Peripheral blood mononuclear cells (PBMC) isolated over Ficoll-Paque (Pharmacia Biotech, Uppsala, Sweden) as described previously (12) were diluted in RPMI 1640 (Gibco, Life Technologies, Paisley, Scotland). The cells (final concentration, 2 x 106 cells/ml) were stimulated with cLTA (Sigma, St. Louis, Mo.), LTA (18), or PG (12), all derived from S. aureus. In brief, the isolation procedure for highly purified LTA was as follows. A defrosted aliquot of bacteria was mixed with an equal volume of n-butanol (Merck, Darmstadt, Germany), and the mixture was stirred for 30 min at room temperature. After centrifugation (13,000 x g) for 20 min, the aqueous phase was lyophilized, resuspended with chromatography start buffer (15% n-propanol in 0.1 M ammonium acetate, pH 4.7), and centrifuged (45,000 x g) for 15 min. The supernatant was subjected to hydrophobic interaction chromatography on octyl-Sepharose. The purity of LTA was >99% according to results of nuclear magnetic resonance and mass spectrometry analysis. Incubations were performed on a rotator at 37°C for 4 h. LPS from Escherichia coli O111:B4 (Sigma) served as a positive control. Subsequently, 100 µl of the cell incubation mixture was mixed with 200 µl of human plasma (obtained from healthy volunteers) that 1 min earlier had been recalcified with 30 mM CaCl2 at a ratio of 1:1. Clotting time was determined in duplicate by using a coagulometer (Amelung, Lemgo, Germany).
LTA, but not cLTA, induced PCA in PBMC in a concentration-dependent fashion (Fig. 1). LTA was about 10-fold more potent than PG on a weight basis in inducing significant PCA. However, LPS was an even stronger inducer of PCA than LTA. Analysis of time kinetics revealed that LTA induced PCA in a pattern similar to that previously shown for LPS and PG (13). Thus, stimulation by LTA resulted in a rapid process with increased PCA after 1 h, and a maximum PCA was reached after 4 h (data not shown).
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FIG. 1. Highly purified LTA from S. aureus induces PCA in PBMC. PBMC were incubated in medium alone or stimulated with various concentrations (given in micrograms per milliliter) of cLTA, highly purified LTA (hLTA), PG, or LPS for 4 h. Clotting time was determined for recalcified human plasma incubated with the cells. Values are means ± standard deviations (n = 3). Statistical analysis was performed by means of an analysis of variance (ANOVA). The overall P value was <0.0001. In addition, P values for differences between the mean for each stimulus and the mean for the control were calculated using the standard deviation from the ANOVA model and compared to the time distribution. No adjustments for multiple tests have been used. The level of statistical significance was set at P of <0.005 (***).
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10 µg/ml, upregulation of TF on the cell surface and PCA showed a tendency to decrease compared to those at concentrations of 1 µg/ml (Fig. 1 and 2). This paradox with regard to dose response effects from LTA has been shown by others and may be explained by micelle formation that renders LTA less active at higher concentrations (3, 6). |
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TABLE 1. LTA-induced PCA in human PBMC in the presence or absence of anti-TF IgG or IL-1 Ra
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FIG. 2. LTA induces expression of TF on the surfaces of monocytes as detected by flow cytometry. (A) Monocytes (M ) were gated using their characteristics in forward scatter and expression of CD14. PE, phycoerythrin. (B) Histogram showing mean fluorescence intensities of monocytes after labeling with FITC-conjugated antibody against TF. Cells incubated in the presence of highly purified LTA (1 µg/ml) showed increased expression of TF, demonstrated by a shift to the right, compared with cells incubated in medium alone. (C) Comparison of levels of TF expression on the surfaces of monocytes after incubation in medium alone and labeling with FITC-conjugated control IgG or after incubation in medium alone, in the presence of highly purified LTA at the indicated concentrations, or in the presence of LPS for 4 h and labeling with FITC-conjugated TF IgG. The results from one representative of four experiments are shown.
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), could theoretically be produced during stimulation of cells. However, IL-1ß is considered to be the key cytokine in the activation of TF expression (4, 20).
To study a more in vivo-like situation, LTA or PG was incubated in undiluted or diluted (40% blood in RPMI 1640) blood at 37°C for 4 h. Subsequently, 300 µl of blood was recalcified with 40 µl of CaCl2 (100 mM) and clotting time was determined. LTA was a weak inducer of PCA in whole blood compared with LTA incubated with PBMC. The threshold dose of LTA required to induce PCA was around 100 µg/ml, compared with 0.01 µg/ml in PBMC incubations (Fig. 1 and 3). Therefore, it seems likely that a neutralizing factor is present in whole blood, making LTA less active. Ellingsen et al. showed that LTA is a poor inducer of TNF-
release in whole blood but that diluting the blood lowers the threshold dose of LTA needed to induce TNF-
(8). This finding suggests the presence of such a factor. However, neither dilution of the blood (40% blood in RPMI 1640) nor prolonged incubation time changed the LTA-induced PCA in our hands. In a study of LTA-induced TNF-
release from monocytes, it was recently demonstrated that chylomicron-associated LPS-binding protein mediates the detoxification of LTA (23). This mechanism may represent such a neutralizing factor. The focus for coming studies will be to determine whether the neutralizing factor is also present in a local extravascular infection in which LTA could play a proinflammatory role by inducing TF in monocytes (7). An interesting finding in the present study is that our observations of PG-induced PCA in whole blood are identical to earlier observations of PG-induced PCA in PBMC, showing that PG is not affected by neutralizing factors in whole blood (13). This suggests that PG is a more important virulence factor than LTA with regard to TF induction in monocytes, at least during systemic S. aureus infection.
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FIG. 3. PG is a stronger inducer of PCA in whole blood than LTA. Undiluted human whole blood was incubated with LTA or PG at the indicated concentrations (given in micrograms per milliliter) for 4 h. Subsequently, the blood was recalcified and the clotting time was determined. Values are means ± standard deviations (n = 3). Statistical analysis was performed by means of ANOVA. The overall P value was <0.0001, and P values for differences between the mean for each stimulus and the mean for the control were calculated using the standard deviation from the ANOVA model and compared to the time distribution. No adjustments for multiple tests have been used. In addition, means with the same concentrations of LTA and PG on a weight basis were compared (indicated by a line). The level of statistical significance was set at P of <0.005 (***). n.s., not significant.
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in patients with severe sepsis and septic shock. J. Infect. Dis. 180:116-121.[CrossRef][Medline]
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