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Infection and Immunity, August 2002, p. 4092-4098, Vol. 70, No. 8
0019-9567/02/$04.00+0 DOI: 10.1128/IAI.70.8.4092-4098.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Department of Bacteriology, The Kitasato Institute, Tokyo 108-8642,1 Department of Bacteriology, National Institute of Infectious Diseases, Tokyo 162-8640,2 Department of Microbiology, Jichi Medical School, Tochigi 329-0498, Japan,4 Division of Biophysics, Research Center Borstel, D-23845 Borstel, Germany3
Received 11 February 2002/ Returned for modification 9 April 2002/ Accepted 2 May 2002
| ABSTRACT |
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). In both cell lines the LPS and the lipid A from bacteria grown at 27°C were stronger inducers of TNF-
than those from bacteria grown at 37°C. However, the difference in activity was more prominent in human macrophage cells. These results suggest that in order to reduce the activation of human macrophages, it may be more advantageous for Y. pestis to produce less-acylated lipid A at 37°C. | INTRODUCTION |
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Darveau et al. (8) reported that the molecular size of the LPS from Y. pestis strain EV76 changed slightly when the growth temperature was shifted from 26 to 37°C. Modification of the antigenicity by growth temperature was also reported by another group (13). It was previously reported that the amount of C16:1 in the lipid A of Escherichia coli increased when it was grown at lower temperatures (38). The palmitoleoyl transferase that functions at low temperatures was identified, and its character was studied (6). Since C16:1 was also present in the lipid A of Y. pestis (2, 4, 39), we assumed that a similar phenomenon could occur in this bacterium. Furthermore, it was proved by several groups (15, 32, 36) using synthetic lipid A compounds that the amounts, kinds, and linkages of fatty acids in lipid A influenced its biological activity. Therefore, if the composition and the structure of fatty acids in lipid A are modified, the biological activity may also be changed. This possibility attracted our interest in relation to pathogenicity.
In this study modification of the structure and activity of Y. pestis lipid A by growth temperature was investigated, and the possible association of such modification with the pathogenesis of Y. pestis is discussed.
| MATERIALS AND METHODS |
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Extraction of LPS and preparation of lipid A. Killed bacterial cells were washed with distilled water, ethanol, acetone, and diethyl ether to prepare dry cells. LPS was extracted from the dry cells by a conventional phenol-water method (42), and the aqueous phase was dialyzed extensively to remove phenol and then lyophilized. The crude LPS preparation was treated with nucleases and proteases as described previously (34) and was purified by repeated ultracentrifugation. The LPS in the precipitate from centrifugation was further purified by extraction with 45% phenol plus triethylamine and sodium deoxycholate (23) and was used for the macrophage stimulation assay. In order to release lipid A, the purified LPS was hydrolyzed in 0.1 M sodium acetate buffer (pH 4.4) at 100°C for 2 h. The hydrolysate was dialyzed, and the retentate was ultracentrifuged to obtain lipid A in the precipitate.
SDS-PAGE. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) was performed by using a 4 to 20% gradient gel (Tefco, Tokyo, Japan) with the buffer system of Laemmli (20). The gel was oxidized with periodate and silver stained as described by Tsai and Frasch (37). LPS samples from Salmonella enterica were used as references. Smooth type LPS of Salmonella enterica serovar Abortus-equi, kindly donated by C. Galanos (Max-Planck-Institut für Immunbiologie, Freiburg, Germany), was purified as described elsewhere (11). Ra type LPS of Salmonella enterica serovar Minnesota R60 was prepared by phenol-chloroform-petroleum ether extraction (10). Re type LPS of Salmonella serovar Minnesota R595 was purchased from Sigma Chemical Co. (St. Louis, Mo.).
Chemical analysis. Neutral sugars were determined by gas-liquid chromatography (GLC) in the form of alditol acetate after they were released from LPS by hydrolysis in 0.1 M HCl (at 100°C for 48 h). D-Glucosamine (GlcN) was determined by the same method used for neutral sugars after it was released by hydrolysis in 4 M HCl (at 100°C for 16 h). Fatty acids were determined by GLC in the methyl ester derivatives after hydrolysis in 4 M HCl (at 100°C for 5 h). For GLC analysis, a GC-14A chromatograph equipped with a 25-m CBP1 capillary column (both from Shimadzu, Kyoto, Japan) was used. Sugar derivatives were analyzed by a temperature program of 170°C for 2 min raised to 270°C by 5°C/min, and fatty acid methyl esters were analyzed by a program of 150°C for 2 min raised to 230°C by 5°C/min. 3-Deoxy-D-manno-oct-2-ulosonic acid (Kdo) and phosphorus were determined colorimetrically as described previously (17).
Mass spectrometry. Lipid A preparations were analyzed by matrix-assisted laser desorption ionization-time-of-flight (MALDI-TOF) mass spectrometry with a Bruker-Reflex III (Bruker-Franzen Analytik, Bremen, Germany) in a linear TOF configuration at an acceleration voltage of 20 kV. Details of the applied methods have been described elsewhere (21). In general, the compounds were dissolved in distilled water at a concentration of 10 µg/µl and treated with an ion exchanger (Amberlite IR-120; Merck, Darmstadt, Germany) to remove disturbing cations. A 1-µl aliquot of the sample was then mixed with 0.5 M 2,5-dihydroxybenzoic acid (Aldrich, Milwaukee, Wis.) in methanol, and 0.5-µl aliquots were deposited on a metallic sample holder. Mass scale calibration was performed externally with similar compounds of known chemical structure.
Cell culture.
The murine macrophage cell line RAW 264.7 (American Type Culture Collection, Manassas, Va.) and the human macrophage cell line U937 (Japanese Cancer Research Resources Bank, Tokyo, Japan) were used for induction of tumor necrosis factor alpha (TNF-
) upon stimulation with LPS or lipid A as reported previously (9, 24). For culture of cells, RPMI 1640 medium (Flow Laboratories, Inc., Rockville, Md.) supplemented with 10 mM HEPES, 2 mM L-glutamine, 100 U of penicillin per ml, 100 µg of streptomycin per ml, and 0.2% NaHCO3 was used as the basic medium, and heat-inactivated fetal calf serum (FCS; Flow Laboratories) was added at a concentration of 5 or 10% (5 or 10% FCS-RPMI medium). RAW 264.7 cells were suspended in 5% FCS-RPMI medium at 106 cells per ml. These cell suspensions were dispensed (0.5 ml) to each well of a 48-well culture plate and cultured for 2 h. The cells in each well were washed three times with 0.5 ml of Hanks' balanced salt solution (Flow Laboratories), and adherent cells were cultured with 5% FCS-RPMI medium in the presence of test samples (0.5 ml/well). The culture supernatant obtained at 24 h after stimulation was assayed for TNF-
activity. U937 cells were prepared for experiments by adding phorbol myristate acetate at a final concentration of 30 ng per ml in 10% FCS-RPMI medium (2 x 105 cells/ml) and culturing cells for 3 days on a 48-well culture plate (0.5 ml/well) to induce differentiation into macrophage-like cells. Cells were washed once with 10% FCS-RPMI medium (0.5 ml/well), and adherent cells were cultured with 10% FCS-RPMI medium in the presence of test samples (0.5 ml/well). The culture supernatant obtained at 4 h after stimulation was assayed for TNF-
activity.
TNF-
assay.
TNF-
was determined by a cytotoxic assay with L-929 cells (33). Briefly, L929 cells were cultured with 5% FCS-RPMI medium in a 96-well flat-bottom culture plate for 3 h, and actinomycin D (Sigma Chemical Co.) was added to a final concentration of 1 µg/ml with serial dilution of test samples. Viable cells in the overnight culture were stained with crystal violet, and absorbance of the blue color extracted with 30% acetic acid was measured at 540 nm. TNF-
activity was calculated from the dilution factor of test samples necessary for 50% cell lysis, with correction by an internal standard of recombinant human TNF-
in each assay, and was expressed in units per milliliter.
| RESULTS |
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/ml was about 100 times greater than that of LPS-27°C (Fig. 6).
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| DISCUSSION |
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For E. coli, it was found that acyltransferase encoded on the lpxP gene transfers C16:1 to the intermediate of lipid A biosynthesis at a low temperature (6). A similar thermoregulation mechanism may also be present in acyltransferases in the biosynthesis of Y. pestis lipid A. This should be investigated in the future in comparison with genes encoding E. coli acyltransferases by using the genomic sequence data of Y. pestis (27).
The proposed structure of lipid A-37°C shown in Fig. 4 is similar to that of the lipid A biosynthesis intermediate lipid IVA (precursor Ia) (31) or synthetic compound 406 (16). Lipid IVA has been reported to act as an LPS agonist to murine cells but to behave as an antagonist to human cells (12, 18, 22). From this structure and the reported data, we expected, and proved in this study, that lipid A-37°C or LPS-37°C, in comparison with lipid A-27°C or LPS-27°C, would exhibit weaker TNF-inducing activity in murine macrophages, and much weaker activity in human macrophages.
It is extremely interesting that Y. pestis produces lipid A with reduced biological activity at 37°C, i.e., the body temperature of humans and other host animals. This phenomenon may be related to the infectious ability of Y. pestis. When this pathogen is injected into mammalian tissue through a flea bite, the environmental temperature for the pathogen shifts up to 37°C, giving a signal to modify the lipid A structure. The modified structure of lipid A will then stimulate the host immune system less actively. This reduction in activity may be more prominent in the human immune system than in the mouse system and may be of great advantage to Y. pestis, enabling the organism to succeed in infection by escaping from the host defense systems. In order to elucidate the complete mechanism of lipid A modification and its effects on the host immune system during infection by and proliferation of Y. pestis, factors other than growth temperature, such as culture medium, pH, or oxygen supply, should also be considered in future studies.
| FOOTNOTES |
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