Tubular immunostimulating complex based on cucumarioside A2-2 and monogalactosyldiacylglycerol from marine macrophytes
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There is an urgent need to develop safe and effective adjuvants for the new generation of subunit vaccines. We developed the tubular immunostimulating complex (TI-complex) as a new nanoparticulate antigen delivery system. The morphology and composition of TI-complexes principally differ from the known vesicular immunostimulating complexes (ISCOMs). However, methodology for the preparation of TI-complexes has suffered a number of shortcomings. The aim of the present work was to obtain an antigen carrier consisting of triterpene glycosides from Cucumaria japonica, cholesterol, and monogalactosyldiacylglycerol from marine macrophytes with reproducible properties and high adjuvant activity.
The cucumarioside A2-2 - cholesterol - MGalDG ratio of 6:2:4 (by weight) was found to provide the most effective formation of TI-complexes and the minimum hemolytic activity in vitro. Tubules of TI-complexes have an outer diameter of about 16 nm, an inner diameter of 6 nm, and a length of 500 nm. A significant dilution by the buffer gradually destroyed the tubular nanoparticles. The TI-complex was able to increase the immunogenicity of the protein antigens from Yersinia pseudotuberculosis by three to four times.
We propose an optimized methodology for the preparation of homogeneous TI-complexes containing only tubular particles, which would achieve reproducible immunization results. We suggest that the elaborated TI-complexes apply as a universal delivery system for different subunit antigens within anti-infectious vaccines and enhance their economic efficacy and safety.
KeywordsSaponin Hemolytic Activity Triterpene Glycoside Adjuvant Activity Marine Macrophyte
Immunostimulating complexes (ISCOMs) are a commonly known adjuvant that represents a supramolecular combination of saponins from Quillaja saponaria, cholesterol, and phosphatidylcholine. It has been shown that ISCOMs display high adjuvant activity against a broad range of bacterial and viral antigens [1, 2, 3, 4, 5]. However, the side effects of ISCOMs are their toxicity, caused by the presence of the hemolytic saponins of Q. saponaria[2, 6], a consistently insufficient adjuvant activity, and the absence of a satisfactory method of preparing them for industrial applications [6, 7, 8, 9]. The development of the ISCOMATRIX™ adjuvant, based on purified saponin fractions from Quil A substantially overcomes these shortcomings [1, 10]. However, we proposed principally new biologically active components, glycoglycerolipids and triterpene glycosides isolated from marine macrophytes and invertebrates, respectively, to modify and optimize ISCOM vehicles for microbial antigens. The immunologically inert phospholipid phosphatidylcholine (PC) of ISCOMs was replaced by glycolipid monogalactosyldiacylglycerol (MGalDG) from marine macrophytes (algae and seagrasses) . Subsequent substitution of the sum of saponins from Q. saponaria by triterpene glycosides from sea cucumbers, possessing immunostimulating activity, resulted in a tubular superstructure of the complexes (TI-complexes) instead of the vesicular one of classical ISCOMs . Based on these results, we developed a prototype of TI-complex consisting of the sum of monosulfated triterpene glycosides from Cucumaria japonica, cholesterol, and MGalDG from marine macrophytes with a component weight ratio of 3:2:6, respectively. To obtain an antigen carrier, we further developed a method  based on that proposed earlier by Copland et al.. Biological tests of the TI-complex prototype revealed that it had adjuvant activity [14, 15, 16].
However, these pilot preparations of TI-complexes appeared to be heterogeneous with a large amount of source material (amorphous phase of MGalDG + cholesterol), and intermediate structures apart from the complex particles. Perhaps this was a cause of non-reproducible and often contradictory immunization results. The presence of intermediate structures was also characteristic of classical ISCOM preparations obtained by methods of dialysis , the hydration of lipid films , and the injection of diethyl ether . The aim of the present work was to obtain an antigen carrier based on triterpene glycosides from C. japonica, cholesterol, and MGalDG from marine macrophytes with reproducible properties and high adjuvant activity. The practical part of the work is protected by Application № 2010122159/10(031466) for Patents of the Russian Federation .
Results and Discussion
Immunological properties of the prototype TI-complex
These results showed the bioavailability of TI-complexes as a new type of promising adjuvant carrier for antigens, although adjuvant activity of TI-complexes with a component weight ratio of 3:2:6 was not sufficiently high.
The Choice of optimum composition for the TI-complex formation
We obtained chromatographically pure cucumarioside A2-2 from the sum of the triterpene glycosides of C. japonica and identified its structure by 13C NMR . It is advantageous to use pure cucumarioside A2-2 rather than the sum of monosulfated triterpene glycosides, because it exhibits the highest immunostimulatory activity. Thus, the possible inclusion of immunosuppressive di- and tri-sulfated triterpene glycosides of C. japonica in TI-complexes is avoided.
Energy-filtered transmission electron microscopy (EFTEM) was applied to unstained samples to investigate the homogeneity of the preparations of TI-complexes prepared with pure cucumarioside A2-2. The structure of the particles of the TI-complex was studied in samples that were negatively stained by phosphotungstic acid.
Thus, a cucumarioside A2-2 - cholesterol - MGalDG ratio of 6:2:4 (Figures 4G, H) prevented the formation of any particle other than the tubular type. This weight ratio of the components of the TI-complex was equimolar.
Hemolytic activity of TI-complex
Hemolytic activity (concentration causing 50% hemolysis of red blood cells - HD50) of TI-complexes and their components
Individual components of TI-complex
TI-complexes with different ratios of cucumarioside A2-2 - cholesterol - MGalDG
Saponins of Quillaja saponaria
MGalDG from Zostera marina
MGalDG from Sargassum pallidum
Optimal dilution of the TI-complex preparations
The TI-complex with a cucumarioside A2-2 - cholesterol - MGalDG component weight ratio of 6:2:4 diluted 10-folds with phosphate buffer was selected for the biological tests. The volume of the preparation for immunization was 10 μl per mouse. A further decrease in the dilution of the TI-complex in order to reduce its destruction was impossible, since the injection of less than 10 μl is technically difficult.
Immunostimulating activity of TI-complex
To confirm the adjuvant activity of the TI-complex, thermostable toxin (TST) from the same bacterium was also used . Experiments addressing the immunization of mice with TST revealed that the injection of TST at a dose of 0.1 μg per mouse in the 6:2:4 TI-complex resulted in a three-fold increase in optical density in ELISA in comparison with the free antigen, indicating a significant adjuvant effect of the TI-complex with respect to TST (Figure 6B).
In this work, we have determined the optimum ratio of components (cucumarioside A2-2, cholesterol and MGalDG) to form maximally homogeneous TI-complexes with minimal toxicity. The evidence of the complex destruction at the high dilution is also important to achieve reproducible immunization results. The data from biological tests revealed that the TI-complex can serve as a powerful adjuvant for subunit bacterial antigens.
Cucumarioside A2-2 isolation
Cucumarioside A2-2 was isolated by a modified method [21, 22]. The body wall of the holothurian Cucumaria japonica (3 kg) was extracted twice for 3 h with an equal volume of 50% ethanol under conditions of boiling. The extracts thus obtained were combined and evaporated to dryness by the addition of 20% n-butanol.
80 g of the resulting extract was dissolved in 1.5 L of distilled water and the sum of triterpene glycosides and sterols was obtained by reverse phase chromatography using a column with 0.5 kg of Polichrom-1 (fraction of 0.5-1 mm) purchased from "Biolar", Latvia. The column length to diameter ratio was 5:1. The column was washed with distilled water up to a negative reaction for the chloride ion. The sum of triterpene glycosides and sterols was eluted with 50% ethanol. The yield of the substances was monitored by TLC on silica gel with chloroform-ethanol-water at 100:100:17 (v/v). The plates were sprayed with a solution of 10% sulfuric acid in methanol, followed by heating at 150°C until dark spots appeared.
For further removal of macromolecular impurities, the fraction was evaporated to dryness (3 g), and then repeatedly extracted with systems of chloroform-ethanol-water 100:75:10 and 100:100:17 (v/v). The extracts were combined, evaporated to dryness (1.5 g), and re-dissolved in a minimum volume of chloroform-ethanol-water at 100:100:17 (v/v).
Sterols and sterol sulfates were separated on a silica gel (100/160 μm fraction, Chemapol, Czech Republic) column (the ratio of length to diameter was 10:1) equilibrated with chloroform-ethanol 3:1 (v/v). The sorbent-substance ratio was 100:1. The column was eluted with chloroform-ethanol 3:1 (v/v). After separation of the sterols (750 mg), the triterpene glycosides were eluted with chloroform-ethanol-water at 100:100:17. The yield of substances was monitored by TLC as described above. Fractions corresponding to the sum of monosulfated triterpene glycosides were pooled and evaporated to dryness (450 mg). Then, the sum of monosulfated triterpene glycosides was separated on a silica gel (40-63 μm, Merck, Germany) column (length to diameter ratio 10:1), equilibrated with chloroform-ethanol-water 100:100:17 (v/v). The sorbent - triterpene glycoside ratio was 150:1. The substances were eluted with the same system. Fractions corresponding to cucumarioside A2-2 (control by TLC using a standard of cucumarioside A2-2) were combined and evaporated to dryness (200 mg).
Cucumarioside A2-2 was isolated by preparative reverse phase HPLC-MS followed by HPLC-MS on silica gel. This was performed with a SHIMADZU LC-8A chromatograph. Substance output was detected in negative ion mode at 3 kV using an HPLC LCMS - 2010EV mass spectrometer. Reverse phase chromatography was performed on a Sim-pack PREP-ODS (20 × 250 mm) column. 1.5 ml of an aqueous solution of the sum of triterpene glycosides at a concentration of 10 mg/ml was injected into the column and eluted with a methanol-water gradient of 70% - 20 min; 75% - 20 min, and 90% - 40 min at a rate of 10 ml/min. The fraction corresponding to ion m/z of 1299 was selected. HPLC was performed on a PREP-sil silica gel Sim-pack column (20 × 250 mm). 2 ml of the triterpene glycosides at a concentration of 5 mg/ml in a 73.2:22.95:3.8 (v/v) chloroform-methanol-water system was injected into column and eluted with the same system at a rate of 10 ml/min. The selected fraction, corresponding to ion m/z of 1299, was a pure cucumarioside A2-2 (80 mg).
The structure of purified cucumarioside A2-2 in deuteropyridine was identified by 13C NMR spectroscopy on a Brucker-500 spectrometer. The spectrum was identical to that previously published for cucumarioside A2-2 [22, 25].
Isolation of monogalactosyldiacylglycerol
MGalDGs from marine macrophytes were isolated and their fatty acid analysis was performed as described previously . Chromatographically pure MGalDG was dissolved in chloroform and stored at -20°C in a light-proof container.
Preparation of TI-complexes
To obtain the TI-complex with a cucumarioside A2-2-cholesterol-MGalDG component ratio of 6:2:4, 5 mg of MGalDG was dissolved in 1 ml of chloroform; 5 mg of cholesterol was dissolved in 1 ml of chloroform; and 4 mg of cucumarioside A2-2 was dissolved in 1 ml of distilled water. Then, a 66-μl solution of MGalDG and a 33-μl solution of cholesterol were evaporated to dryness under a stream of air at a temperature of 60°C. A 125-μl solution of cucumarioside A2-2 was added to this dry residue. Then, 375 μl of PBS, pH 7.2, was added to this mixture, adjusting the concentration of MGalDG and cholesterol to 1 mg/ml. The suspension thus obtained was sonicated for 5 min on a SONOPULS Ultrashall-Homogenizatoren HD 2070 (Germany) ultrasonic disintegrator at 10% maximum power (0.7 sec - work; 0.3 sec - interval). After sonication, the preparation was left to stand at room temperature for 2 h.
To obtain the TI-complexes with other component ratios, the volume of the aliquots of the components was increased or decreased. However, the final volume of the preparation was always 0.5 ml and the total concentration of MGalDG and cholesterol was 1 mg/ml.
To obtain the YompF-containing TI-complex with 0.1 μg of protein per 1 μg of cucumarioside A2-2, 100 μl of the lipid-saponin complex was collected immediately after sonication and combined with a 10-μl YompF solution in PBS, pH 7.2, at a concentration of 1 mg/ml. This mixture was vortexed for 1 min. Then, the preparation was left to stand at room temperature for 2 h.
The complex containing thermostable toxin (TST) was prepared in a similar way to the complex containing YompF. To obtain the complex with 0.1 μg TST per 1 μg of cucumarioside A2-2, 75 μl of lipid-saponin complex was collected and added to 25 μl of the TST solution in PBS, pH 7.2, at a concentration of 0.3 mg/ml.
To study immunological properties of the TI-complex prototype, the complex of monosulfated triterpene glycosides from C. japonica, cholesterol and MGalDG from Ulva fenestrata (or PC from egg yolk) were mixed at the weight ratio of 3:2:6. Then denaturated monomeric porin from Y. pseudotuberculosis at a respective dose (0.1, 1 or 10 μg of protein per 1 μg of glycosides) was added to TI-complexes. The resulted mixture was sonicated by ultrasonic disintegrator SONOPULS Ultrashall-Homogenizatoren HD 2070 (Germany) at 10% maximum power (0.7 sec - work; 0.3 sec - interval) [13, 14, 15]. Porin-containing ISCOMs were prepared as described .
Trimer and denatured monomer forms of YompF were isolated from Y. pseudotuberculosis (strain 598, I serovar) by researchers at the Laboratory of the Molecular Bases of Antibacterial Immunity, PIBOC, FEB RAS following a method . Thermostable toxin  was provided by collaborators at Laboratory of the Molecular Bases of Bacterial Pathogenicity at the Research Institute of Epidemiology and Microbiology, SB RAMS.
Transmission electron microscopy
Transmission electron microscopy (TEM) and energy-filtered transmission electron microscopy (EFTEM) were performed with Libra-120 microscope (Zeiss, Germany) at an accelerating voltage of 120 kV. Each suspension of the TI-complexes was investigated by TEM on the negatively stained samples and by EFTEM on the unstained samples. Copper grids of 300-400 mesh with a formvar substrate were used to prepare the samples .
To prepare the samples, 1-2 μl of a TI-complex suspension was placed on a grid and then dried in a stream of warm air. The dried grid was washed three times with 3 μl of distilled water, removing the excess with filter paper. To prepare the unstained samples, the washed grids were dried in a stream of warm air. To obtain the negative stained samples, 1 μl of a 0.5% solution of phosphotungstic acid, pH 7.2, was placed on the washed grid and then dried in a stream of warm air.
The hemolytic activity of the TI-complexes was determined by a direct hemolysis reaction. A 0.5% suspension of human erythrocytes in 0.01 M PBS, pH 7.2, was used as a test system. 0.1 ml of the erythrocyte suspension was added per well onto a 96-well plate. Then, 0.1 ml of TI-complexes at several dilutions was added in 3 repeats. The mixtures were incubated at 37°C for 1 h and centrifuged at 2000 rpm for 10 min. The supernatant was transferred to the wells of a 96-well flat-bottomed plate. Photometry was performed at 540 nm using an ELX808IU photometer (BioTek Instr., USA) to determine the minimum effective concentration of the substance that caused hemolysis in 50% of red blood cells (HD50).
Laboratory mice (Balb/c strain) with weight of 18-21 g were used. Animals were keeping in vivarium at standard conditions with unlimited access to food and water according with the rules accepted by European Convention for the Protection of Vertebrate Animals used for Experimental and Other Scientific Purposes (Strasbourg, 1986). Research was carried out according with the rules of the proper laboratory practice (GLP), Order of Ministry of Health of Russian Federation №267, 19.06.2003 "About the approval of rules for laboratory practice", Guide on experimental (pre-clinical) study of new pharmacological substances" (2005).
The TI-complex was injected at a dose of 1 μg of cucumarioside A2-2 and 0.1 μg of YompF or thermostable toxin per mouse. Immediately before injection, the original preparations of antigen-containing TI-complexes were diluted with PBS, and 10 μl of this solution was injected subcutaneously into the thighs of the animals.
To study the immunological properties of the TI-complex prototype, pure porin and porin with adjuvants (TI-complex prototype, ISCOMs or Freund's complete adjuvant (Sigma)) were injected intraperitoneally into mice at doses of 0.1, 1, and 10 μg/mouse in 100 μl of PBS.
The preparations were administered on the 1st and 7th day of the experiment. Sera were collected on day 21. In all experiments each experimental group consisted of 10 animals.
Specific antibodies in blood serum were determined by the ELISA method. The relative concentration of antibodies was expressed in optical density units at a wavelength of 450 nm (dilution of serum - 1/100).
Cytokines (INF-γ and IL-1β) were determined in the blood serum of mice in the respective groups by non-competitive ELISA using BD OptEIA™ Mouse ELISA kits (BD, USA). Absorption was recorded with an Elx808IU microplate photometer, Biotek Instr., USA, at a wavelength of 450 nm.
The authors thank Drs. Novikova O. D. and Portniagina O.Y. for YompF preparations as well as Dr. Timchenko N. F. for preparation of the thermostable toxin from Yersinia pseudotuberculosis.
This work was supported by Ministry of Education and Science of the Russian Federation (Projects № 22.214.171.124/10113 of ADTP, Grant opportunities for Russian scientists living abroad № 02.740.11.5088), the Government of the Russian Federation (Grant for state support of research projects implemented by leading scientists at Russian institutions of higher learning, contract № 11.G34.31.0010).
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