IE47101B1 - Purifying hepatitis b surface antigen - Google Patents

Description

This invention is concerned with the purification of hepatitis Ts surface antigen. Attention is directed to British Patent Specification No. 1 554 811 which describes and claims related subject matter.

In accordance with the present invention there is provided a process for obtaining HBsAg from human biological fluid obtained from human hepatitis B donors comprising treating the fluid with sufficient ammonium sulfate to precipitate an antigen fraction comprising HBsAg, subjecting the precipitated fraction to an isopycnic banding and to a rate zonal banding, and recovering HBgAg in the density range of from 1.13 to 1.16 g/ce.

The starting material for the process of the present invention may be any human biological fluid containing HBgAg such as, for example, plasma, saliva, fecal extracts, nasal pharyngeal secretions, bile, spinal fluid, sweat, urine, semen, vaginal secretions, or menstrual blood. The plasma is obtained in conventional manner, e.g., by plasmaphoresis. The level of HBgAg in the human biological fluid may be measured in known manner by any suitable means, e.g., reversed passive hemagglutination or complement fixation. When the biological fluid is plasma, it may be treated directly or cooled and the cryoprecipitate which forms may be removed by light centrifugation, or - 3 CaClz may be added to remove fibrinogen and clarified. The resulting fluid is treated with ammonium sulfate and the HBgAg in the resulting fluid is isolated by an isopycnic banding step followed by a rate zonal banding step.

Xn isopycnic banding the partially purified concentrate is contacted with a liquid medium having a density gradient therein which includes the density of the specific antigen being isolated. The liquid medium is then subjected to ultracentrifugation to attain an equilibrium distribution of the serum components through the density gradient according to their individual densities. Successive fractions of the medium are displaced and those containing the desired antigen, i.e. the fractions having a density of from 1.21 to 1.24 g/cc, are separated. The application of this technique to the purification of HBsAg is described in German Specification 2,049,515 and U.S.

Patent 3,636,191. The concentrations of the solutions forming the gradient are selected so as to encompass the density range of from 1.0 to 1.41 g/cc. The liquid medium may be in the form of a linear gradient or a step gradient, preferably a step gradient due to its inherent higher capacity for fractionation.

In rate zonal banding the partially purified cencentrate is subjected to ultracentrifugation in contact with a liquid medium having a density gradient therein, but this time using the rate zonal technique i.e., at a rate and for a period such that equilibrium is not attained, the HBgAg and other residual serum components being distributed through the medium according to their sedimentation coefficients in the medium. The concentrations of the solutions forming the step gradient are selected so as to encompass the density range of from 1.0 to 1.28 g/cc. The rate zonal step is carried out until the HBsAg resides in the 1.13 to 1.16 density region. At this point the HBgAg is separated from the bulk of the crude plasma proteins and, most significantly, is also separated from the 471°1 macroglobulin complement of the plasma. If the rate zonal step is carried out such that the desired HBgAg antigen reaches its equilibrium position, i.e., 1.18 to 1.20 g/cc, it has been found that a plasma macroglobulin fraction will appear as a contaminant in the desired HBgAg antigen fraction.

The liquid media used in the isopycnic banding step may be any density gradient in the appropriate ranges. Piror-art solutes for such solutions include, e.g. sucrose, potassium bromide, cesium chloride and potassium tartrate.

The isopycnic banding step is conveniently carried out in a centrifuge, for example, Electronucleonics-K, by filling the stationary rotor with saline solution, then successively displacing the saline solution upwards with aliquots of a liquid medium solution of increasing density until a step gradient is formed. The plasma is introduced at the top of the rotor displacing some of the highest density solution from the bottom. Typically, the volume of plasma is from about 15¾ to about 40% that of the step gradient. The centrifuge is brought up to speed through a programmed speed control system which prevents mixing during the initial reorientation phase. When equilibrium is attained and the product is in its proper density position, the rotor is slowed down through the same system to prevent mixing upon reorientation to the original configuration.

Then the gradient is drained from below and the proper density cut collected. A similar technique is used in the rate zonal banding. The proper density cut from the rate zonal banding is the desired concentrate of hepatitis B antigen.

Due to the small size of HBgAg, approximately 20 nm, the isopycnic banding step is quite time-consuming, requiring about 18 hours of centrifuging. As a result, even operating 24 hours a day, 7 days a week, it is possible to process only about 4 batches per centrifuge per week. Productivity can be increased, of course, by utilizing - 5 additional centrifuges. This involves a tremendous capital investment, however, as each centrifuge is very expensive.

It has now been found that substantial increases in productivity and substantially reduced operating costs are obtained by treating the human biological fluid with ammonium sulfate before subjecting the fluid to isopycnic banding conditions. The amount of ammonium sulfate used should be at least about that amount sufficient to precipitate substantially all of the HBsAg in the fluid while avoiding precipitation of additional undesired proteinaceous matter. As a result of this precipitation, the HBgAg from several liters of fluid can be subjected to isopycnic banding in one batch whereas without ammonium sulfate precipitation, only a much smaller quantity of fluid can be subjected to ispoycnic banding in a single batch. Generally, when the biological fluid is plasma from 200 to 250 g of ammonium sulfate is used per liter of plasma, and preferably about 225 g per liter of plasma. Lesser amounts do not precipitate all of the HBsAg while greater amounts percipitate additional undesired proteinaceous matter. When the fluid is plasma, with ammonium sulfate precipitation, the HBgAg in about 20 liters can be subjected to isopycnic banding in one batch; without ammonium sulfate precipitation only about 1.5 liters of plasma can be subjected to isopycnic banding.

After the ammonium sulfate is added, the fluid is agitated to help dissolve the ammonium sulfate. Preferably the fluid is agitated for at least about 3 hours at lowered temperature of from 0 to 10°C, preferably at about 5°C, and preferably at least about 4 hours. Additional agitation beyond about 4 hours is not harmful. The precipitate which forms is collected by centrifugation and the pellets resuspended in saline and dialysed against saline to remove the ammonium sulfate. The resulting fluid concentrate is then subjected to an isopycnic banding using a gradient material having a permissable density - 6 range of from 1.1 to 1.4 g/cc.

The product from the isopycnic banding is dialysed against PBS to remove the gradient material if it is dialysable.

The product is then subjected to a rate zonal banding until the HBsAg is in the density range of from 1.13 to 1.16 g/cc. Any gradient material that yields this density range and is dialysable and physiologically acceptable may be used. Typically this banding takes from 16 to 20 hours, preferably from 17 to 18 hours at about 30,000 rpm. At higher speeds less time is required, and at lower speeds more time is required.

According to a preferred aspect of the present invention the gradient is formed of sodium bromide. In contrast to heretofore used materials sodium bromide has definite advantages. The solubility of sodium bromide allows the use of high density solutions in the formation of gradients at refrigerator temperatures (2-6°C). There are definite economic advantages to using sodium bromide over a salt such as caesium chloride as well as not having to contend with the problem of human toxicity from residual and HBsAg bound caesium ions. In sodium bromide gradient any ions bound to the HBgAg due to biophysical characteristics, will be sodium ions which are very compatible with the human biological system and do not present any toxicity problems.

The superior solubility of NaBr at lowered temperatures with respect to KBr permits the use of lowered temperatures more conducive to stability of biological materials. The use of a step gradient rather than a linear gradient is preferred as it accumulates impurities at the step boundaries and permits processing a larger volume of plasma in a single gradient.

The following example illustrates the present invention without, however, limiting it. The word Millipore is a trade mark. - 7 EXAMPLE Liters of clarified plasma from hepatitis B donors is filtered through a 293 mm fiter containing an AP 20 filter membrane (Millipore). Ammonium sulfate, 4.53 kg, is added to the filtrate which is then agitated gently overnight at 5°C. The precipitate which forms is collected by batch centrifugation at 7000 X g for 30 minutes using the JA-10 rotor (3 liters' capacity per batch). The pellets post centrifugation are suspended in about 2.25 liters of saline. The concentrated suspension is then dialysed against 40 liters of saline to remove the ammonium sulfate.

The rotor of a centrifuge, Electronucleonics K, is filled with 8,400 ml of phosphate buffer. After running the rotor up to 10,000 rpm to degas the system, the following step gradient is pumped into the bottom of the stationary rotor: 1. 2,000 ml of 10% NaBr, p = 1.08 2. 1,000 ml of 20% NaBr, p = 1.17 3. 1,500 ml of 30% NaBr, p = 1.28 4. 3,900 ml of 40% NaBr, p = 1.41 The dialysed suspension containing HBsAg, 2,250 ml, is pumped into the top of the stationary rotor displacing 2,250 ml of 40% NaBr from the bottom of the rotor. The rotor is accelerated to 30,000 rpm and run at this speed for 18 hours. After stopping the rotor 2,000 ml of HBsAg rich material in the 1.21 - 1.24 density region is collected and dialysed against phosphate buffer.

The rotor is then filled with phosphate buffer, degassed as above, and the following step gradient pumped into the bottom of the stationary rotor: 1. 2,000 ml of 5% sucrose, P = 1.02 2. 1,650 ml of 15% sucrose, P = 1.06 3. 1,750 ml of 25% sucrose, P = 1.10 4. 3,000 ml of 50% sucrose, P = 1.23 The HBgAg rich material from the NaBr isopycnic - 8 banding step 2,000 ml is pumped into the rotor top displacing 2,000 ml of 50% sucrose out the rotor bottom.

The rotor is then run at 28,000 rpm for 18 hours. After stopping the rotor, 1,000 ml of HBgAg rich material in the 1.135 - 1.165 density region is collected.

Claims (13)

1. A process for obtaining HB g Ag from human biological fluid obtained from human hepatitis B donors comprising treating the fluid with sufficient ammonium sulfate to precipitate an antigen fraction comprising HBgAg, subjecting the precipitated fraction to an isopycnic banding and to a rate zonal banding, and recovering HB Ag in the density range of from 1.13 to 1.16 g/cc. s

2. A process according to claim 1 wherein the amount of ammonium sulfate is sufficient to precipitate substantially all of the HB g Ag in the fluid but insufficient to precipitate a substantial amount of additional proteinaceous matter.

3. A process according to claim 1 or 2 wherein the fluid is plasma.

4. A process according to claim 3 wherein the amount of ammonium sulfate is up to about 250 g per liter of plasma.

5. A process according to claim 4 wherein the amount of ammonium sulfate is from 200 to 250 g per liter of plasma.

6. A process according to claim 5 wherein the amount of ammonium sulfate is about 225 g per liter of plasma.

7. A process according to Claim 1 wherein after addition of ammonium sulfate, the fluid is agitated at from 0 to 10°C for at least about 3 hours.

8. A process according to Claim 1 wherein the fluid is agitated at from 0 to 10°C for at least 4 hours.

9. A process according to Claim 7 or 8 5 wherein the agitation is carried out at about 5°C.

10. A process according to any one of the preceding claims wherein the isopycnic banding gradient is NaBr.

11. A process according to any one of the 10 preceding claims wherein the rate zonal banding gradient is sucrose.

12. A process according to Claim 1 substantially as hereinbefore described in the foregoing Example.

13. HBgAg when obtained by a process according 15 to any one of the preceding claims.