PURIFICATION PROCESS FOR HEPATITIS SURFACE ANTIGEN AND PRODUCT THEREOF

Abstract: Methods for the purification of hepatitis surface antigen are provided which employ protein precipitating agents or a immunoaffinity adsorbent column, followed by fractionating on an anion exchange bed.

Description: Background The present invention relates to the purification of a protein produced by recombinant cells. Specifically, it provides a process for the efficient purification of hepatitis surface antigen (HBsAg) from cultures of recombinant cells producing this protein, particularly mammalian recombinant cells. Viral hepatitis B is transmitted by carriers of hepatitis B through blood and body secretions. HBsAg has been purified from blood serum and made into a vaccine. Purcell, R.H. and Gerin, J.L. in Viral Hepatitis , The Franklin Institute Press (1978) p. 491 and Hilleman, M. R. et al ibid. p. 525. This publication describes a procedure for isolation of HBsAg in which the plasma from hepatitis B carriers is defribinated, and the HBsAg in the serum is 1) concentrated by ammonium sulfate precipitation, 2) subjected to isopycnic banding in a centrifuge using sodium bromide as the gradient medium, 3) subjected to rate-zonal sedimentation using a sucrose gradient, 4) inactivated with a variety of steps, 5) subjected to gel filtration, and 6) produced into a vaccine. Use of serum as a source or HBsAg is dangerous because of the presence of hepatitis B virus as well as the presence of as yet undefined adventitious agents. In addtion, the techniques employed for purification of the HBsAg are time consuming and complicated. Formulation of hepatitis surface antigen for vaccine purposes has been performed by compounding with aluminum adjuvants such as aluminum hydroxide (op cit. Viral Hepatitis) and aluminum phosphate as described by Reerink-Brongers, E.E., et al , Developments in Biological Standardization , S. Karger (Basel, Switzerland) (1983) 54, 197. Purification of a protein such as HBsAg produced in a recombinant cell may require different techniques from those which are used to purify HBsAg from "natural" sources such as blood serum due to the significantly different compositions of the two sources. Thus, one cannot predict that a technique used for purification of HBsAg from natural sources would be useful for purification of HBsAg from recombinant cell sources. Predictability is diminished even further where the final product must be exceptionally pure, e.g. for use as a human vaccine. Wampler et al.: "Hepatitis B vaccine purification by immunoaffinity chromatography" Modern Approaches Vaccines: Mol. Chem. Basis Virus Virulence Immunogenecity, (Pap. Conf.) 1983 (pub. 1984), 251-256 discloses the preparation of compositions containing hepatitis virus antigen from hepatoma cell lines or yeast by use of affinity chromatography followed by dialysis or ultrafiltration. McAleer et al. Nature 307, 12 January 1984, p 178-180 describes purification of hepatitis virus antigen from recombinant yeast by a single application of immune affinity chromatography. US-A-4,162,192 teaches a method for purifying the antigen in which a preparation undergoes precipitation followed by two ion exchange steps; firstly anion exchange and secondly cation exchange. The present invention is directed to a process based upon chromatography which is successful in isolating HBsAg from recombinant cell cultures. A unique sequence of fractionation techniques is performed to produce HBsAg of high purity for use in a human vaccine. This product is free of active adventitious agents. Summary of the Invention In accordance with the present invention, a method of purifying HBsAg produced by culture of a recombinant cell provides a product of sufficient purity for incorporation into a vaccine. Cell debris is first separated from the cell culture medium which contains HBsAg. Clarified culture fluid containing HBsAg is then concentrated, preferably by ultrafiltration, and subjected to a two-stage fractionation by precipitation, preferably with ammonium sulfate, in which contaminant protein is removed in the first stage and HBsAg is precipitated in the second stage. The precipitated product is redissolved and may be dialyzed to remove salts. The dialyzed HBsAg solution is then further purified by immunoaffinity adsorption followed by anion exchange chromatography. Brief Description of the Drawings Figure 1 is a flow diagram of a method for purification of HBsAg from recombinant cell culture according to the present invention. Detailed Description A. Definitions As used herein "hepatitis surface antigen" or "HBsAg" refers to the protein with the amino acid sequence of the surface antigen in any of its forms as encoded by the hepatitis B virus genome. Such form variations are included in the definition. In addition, as is well known, protein sequences are often modified by glycosylation, acetylation, or other chemical derivatization and may be associated with lipids. Finally, those proteins can exist in various states of aggregation, such as dimers, trimers, or even large aggregates with or without the aforementioned modification. These aggregates may include lipids. All of these are intended to be included within the definition of HBsAg. As stated by Rutter, W. J. et al., EPO Patent Appln. Publ. No. 0072318, published February 16, 1983, hepatitis B virus encodes the protein of hepatitis B surface antigen, which is 226 amino acids in length with a molecular weight of 22,000 Daltons. Hepatitis B surface antigen as isolated from the serum of chronic carriers of hepatitis B virus is a spherical particle with a diameter of 22 nm and a molecular weight of 2-4 million Daltons. This particle is thought to be composed of lipid and dimers of the hepatitis B surface antigen protein. However, the precise structure of the 22 nm particle is not well defined. HBsAg isolated from cultures of recombinant mammalian cells is in the form of 22 nm particles which are indistinguishable from those derived from serum as judged by standard laboratory tests. The surface antigen isolated from recombinant yeast also appears to be in the form of 22 nm particles (Valenzuela, P., et al ., Nature , 298 : 347 (1982)). The designations of the techniques employed herein are used in the sense of their standard meanings. Thus, ultrafiltration refers to the technique whereby materials of low molecular weight, water, salts and some proteins, can be separated from those of higher molecular weight by providing a molecular sieve or filter with a pore size sufficiently small to retain larder molecular weight materials. Anion exchange chromatography refers to a technique whereby anionic materials can be adsorbed onto a support and subsequently eluted by the modification of solution conditions. Gel permeation chromatography refers to a molecular sieve process which separates a mixture of molecules according to size. Each of these individual techniques is known in the art as are the range of conditions under which they may be conducted. B. General Method B.1 Suitable Cell Cultures HBsAg has been produced using a variety of microorganisms and tissue culture cells as recombinant hosts. Bacterial systems (European Publication No. 0020251, published Dec. 10, 1980), yeast systems (EPO Publ. No. 073657, published March 9, 1983, and No. 0072318) and cell culture systems (EPO Publ. No. 0073656, published March 9, 1983) have been used. HBsAg so produced has been shown to be immunogenic. See, e.g. EPO Publ. No. 0073656 (supra). It is particularly preferred to use a cell source which secretes the protein into the medium as this allows one to begin with a more pure starting material. However, if the desired protein is not secreted, lysis of the cells by standard means such as sonication, homogenization, or osmotic shock will succeed in freeing the HBsAg into the medium, and such purification method is applicable. In one embodiment of the invention, the medium for cell culture is a defined medium composed of specified vitamins, trace elements, and other nutrients to replace serum, thus eliminating a major source of contaminating protein. In another embodiment, serum is used in the medium. B.2 Description of the Process Referring to the flow diagram in Figure 1, a procedure is illustrated which is applicable to purification of HBsAg whether of not the culture media contains serum. The description first will refer to the purification from cell culture medium without added serum. Referring to step 9, the host cell debris, preferably in the form of non-disrupted host cells, is separated from supernatant fluid by a conventional solid-liquid separation technique such as filtration or centrifugation. Referring to step 10, the soluble fraction is concentrated to a significant extent to permit an effective multi-stage precipitation fractionation as described in steps 11-13, infra . For that purpose, it is preferable to concentrate the HBsAg at least 25 to 30 fold, and to as high as 50-fold or more, in this step. Ultrafiltration is highly efficient for this purpose. Commercially available membranes fabricated from cellulose esters or polysulfone, for example, Amicon™YM-30™ and PM-10™ and Dorr-Oliver™ C-30™, may be used. A suitable molecular weight cut-off for the membrane is less than 100,000. Suitable operating pressures are on the order of 20 to 60 psi. The use of ultrafiltration membranes in a hollow fiber configuration such as a Romicon™ PM-100™ and PM-10™ have been used successfully for cell culture media which have serum added. The retentate from ultrafiltration, step 10, is then fractionated by a two-step process as illustrated in Figure 1, steps 11, 12 and 13. The ability to concentrate to the degree set forth above by ultrafiltration is a significant factor for obtaining good yields. Referring to step 11, the first stage of the fractionation by precipitation is particularly effective for HBsAg produced from tissue culture, specifically CHO cells. A suitable first or preliminary precipitation step is performed by adding a protein-precipitating agent to the concentrate from step 10 to precipitate a first fraction of contaminant protein which is removed from the remainder of the solution by a suitable clarification technique such as centrifugation, filtration or the like. A particularly effective first stage fractionation is accomplished by adding sufficient solid ammonium sulfate to precipitate the desired amount of contaminant protein without precipitating the HBsAg. For this purpose, sufficient ammonium sulfate may be added to the concentrated cell culture at, for example, 4-8° C to produce a concentration of ammonium sulfate in the range of 20-30% of saturation, preferably 25%. Referring to step 12, the contaminant protein precipitate is removed in a clarification step, suitably performed by centrifugation or by settling followed by filtration. Referring to step 13, the supernatant fraction from clarification is subjected to a second stage of precipitation fractionation by increasing the ammonium sulfate level to an amount effective to precipitate substantially all of the HBsAg from the solution. The preferred technique to accomplish this is to increase the ammonium sulfate concentration to about 60% of saturation by adding additional ammonium sulfate. The product is precipitated and is collected by centrifugation for subsequent processing. Other protein-precipitating agents may be utilized in place of the ammonium sulfate so long as they accomplish the objectives set forth above. In that regard, it has been found that polyethylene glycol, specifically, PEG 8000, may be utilized at a concentration of 2-4% w/v in step 3 and 8-10% w/v in step 12. Referring to step 14, the precipitate is redissolved and dialyzed to remove salts in preparation for fractionation by the ensuing chromatography steps 15 and 17. Dialysis is performed to remove salts which would interfere with chromatography. This step may be performed by dissolving the precipitate in dilute buffer, preferably at a pH of about 6-8, and dialyzing at a low temperature against dilute buffer primarily to remove the ammonium sulfate. An appropriate buffer comprises 1-50 mM Tris-Cl or sodium phosphate, preferably about 10mM Tris-Cl. Other methods for salt removal may also be employed such as gel filtration or diafiltration. In step 15, the solution from step 14 is purified by passage through an immunoaffinity absorbent column containing monoclonal antibodies specific for HBsAg. The HBsAg is adsorbed on the column and the contaminants either flow through the column or are washed off. Thereafter the adsorped HBsAg is desorbed by passing an appropriate eluent through the column. An appropriate immunoaffinity adsorbent column for use in step 15 is formed by covalently coupling a monoclonal antibody specific for HBsAg to a cross-linked agarose such as Sepharose™ CL-2B™ (Pharmacia, Inc.) with carbonyldiimidazole using the method of Bethell, G.S., et.al., J. Biol. Chem. 254 , 2572 (1979). HBsAg can be obtained in a nearly homogenous form by a single passage through this immunoaffinity adsorbent column from crude cell culture fluid obtained from the culture of mammalian cells in the presence of animal serum. After passage of the crude culture fluid through the column, an appropriate buffered eluent such as 10 mM Tris-Cl at a pH of 7.5 containing 1.0 M sodium chloride is used to wash the column. The bound HBsAg is released from the adsorbent by an appropriate eluent (e.g. 0.1 M sodium acetate/acetic acid pH 4.0 containing 1.0 M NaCl, or 10 mM Tris-Cl pH 7.5, containing 1.0 M NaCl, 3M KSCN). In step 16, the eluent from the immunoaffinity column preferably is dialyzed or diafiltered against a suitable medium such as 10 mM Tris-Cl pH 7.5, containing 0.18 M NaCl. Dialysis may be performed in a manner similar to that used in step 6. Diafiltration may be performed with an Amicon™ PM-10™ or YM-30™ membrane. In step 17, the product from step 16 is subjected to anion exchange chromatography. A suitable technique is by passage over a column of DE-52™ which is washed with 10 mM Tris-Cl at pH 7.5 containing 0.18 M NaCl. The eluent and wash are combined to form the purified product. In another embodiment, steps 11-14 may be eliminated for some cell cultures. B.3 Pharmaceutical Composition HBsAg purified according to the present invention can be formulated according to known methods to prepare pharmaceutically useful compositions, whereby the HBsAg is combined in admixture with a pharmaceutically acceptable carrier vehicle. Suitable vehicles and their formulation are described in Remington's Pharmaceutical Science , E. W. Martin, ed; incorporated herein by reference. B.4 Vaccine Preparation HBsAg purified according to the present invention, can be formulated according to known methods to, prepare vaccines whereby the HBsAg is combined in admixture with a suitable vehicle. Suitable vehicles include various known adjuvants. Such vaccines contain an effective amount of HBsAg and a suitable amount of vehicle in order to prepare a vaccine for effective administration to the host. A particularly effective vaccine is produced by compounding HBsAg with aluminum salts in the form of a gel as an adjuvant. The gel may be formed in situ by mixing a sufficient amount of HBsAg with a sufficient amount of a soluble aluminum salt such as potassium aluminum sulfate (e.g. at pH 3.5 ) and then coprecipitating the aluminum salt with HBsAg by titration of the solution with a basic titrant to a hither pH (e.g. 5 to 7) to form the gel. Such basic titrants may be an alkali hydroxide, such as sodium hydroxide, or alkali salts such as sodium phosphate. As defined herein, the "aluminum hydroxide adjuvant" means an adjuvant for the vaccine containing HBsAg purified according to the present invention containing aluminum primarily as the hydroxide in a formulation suitable for injection; while the term "aluminum phosphate adjuvant" means an adjuvant for the HBsAg vaccine containing aluminum primarily as the phosphate in a formulation suitable for injection. It has been found that the aluminum phosphate adjuvant vaccine has superior antigenic properties compared to those of the aluminum hydroxide adjuvant vaccine and that both of these have superior properties to that of a commercial serum-based vaccine, Heptavax B™, based upon intraperitoneal injection of mice. Because of the unique methods by which the active component of these vaccines are produced, they are likely to be essentially free of extraneous protein(s), viral and cellular components, and DNA. Therefore, they are less likely to produce the complications of virus vaccine preparations made from serum. It has further been found that vaccines containing HBsAg purified according to the present invention has vastly superior antigenic properties in chimpanzees than Heptavax B™ as reported in Hillman, M.R., et al , Viral Hepatitis , International Symposium, The Franklin Institute Press (1978), 525. These results are believed to be due, in major part, to the method of purification used in the present invention compared to the relatively harsher purification methods reported for production of HBsAg used in Heptavax B™. Specifically, the present methods avoid harsh proteases and protein denaturants of those reported methods. The following process and results are intended to exemplify, but not limit, the invention. C Example 1 This example illustrates the procedure of Figure 1 for the purification of HBsAg from a cell culture medium containing serum. The concentration of serum added to the cell culture fluid may range from 0-7.5% v/v. The culture fluid is filtered through a 0.22 micron Sartorius™ filter fabricated from cellulose acetate and cellulose to remove cells and cell debris which arise during cell culture. Approximately 100L of cell culture fluid is rapidly filtered through 2 square feet of membrane. Clarified cell culture fluid containing serum is concentrated by ultrafiltration using a Romicon™ hollow fiber system with a PM-100™ or PM-10™ type membrane. The ultrafiltration process is performed at ambient temperature at a pressure up to 25 psi. The cell culture fluid is concentrated and the retentate is drained from the system. The system is flushed with purified water which is combined with the retentate to effect a quantitative recovery of HBsAg. A permeate rate of 6-10 L/hr/sq.ft. is achieved with a PM-100™ type membrane with a cell culture medium which contains 7.5% serum. A concentration factor equal to or greater than 20 for cell culture fluids containing equal to or greater than 1% v/v added serum is required to achieve the optimum yield in the subsequent purification step. Concentrated cell culture fluid is fractionated in two steps. Sufficient solid ammonium sulfate is added to the concentrated cell culture fluid at 4-8°C to produce a concentration of 25% saturation (706 g/L is used as 100% saturation at 4°C). The solution is clarified by centrifugation at 6000g for 20 minutes. The pellet is discarded and the supernatant is brought to 60% saturation in ammonium sulfate. The second-stage pellet is collected by centrifugation at 6000g for 20 minutes. The yield on ammonium sulfate fractionation is dependent on the concentration factor achieved by ultrafiltration. A concentration factor equal to or greater than 35 typically achieves a maximal yield for the step. Above a concentration factor of 35, there is no substantial increase in the yield on ammonium sulfate fractionation. The second-stage pellet from the ammonium sulfate fractionation is dissolved in 10mM Tris-Cl pH 7.5 (pH measured at room temperature) in a volume in mL approximately equal to 0.25 times the original culture medium volume in liters. The solution is dialyzed at 2-8°C against 10mM Tris-Cl, pH 7.5 containing 1.0M NaCl. to remove the ammonium sulfate. The combination of ultrafiltration, ammonium sulfate fractionation and redissolution of the pellet results in a concentration factor of approximately 2000 fold based on the volume of starting cell culture medium. The dialyzate is applied to an immunoaffinity column and the column is washed with 10mM Tris-Cl pH 7.5 containing 1.0M NaCl. The HBsAg bound to the column is eluted with a solution of 0.1M sodium acetate pH 4.0 containing 1.0M NaCl. The procedure is performed at refrigerated temperature (2-8°C) or at room temperature without any significant difference. The eluent is then dialyzed against 10mM Tris-Cl containing 0.18M NaCl. Alternatively, the eluent is diafiltered using an Amicon YM-30™ or PM-10™ membrane against the same buffer. The dialyzed eluent is then passed through a DE-52™ column. HBsAg is not bound by DE-52™ in this medium. The column is washed with 10mM Tris-Cl pH 7.5 containing 0.18M NaCl to effect a quantitative recovery of the HBsAg. Approximately 1mL of DE-52™ is used per 10-20mg of HBsAg. The product from anion exchange chromatography using DE-52™ is adjusted to a concentration of approximately 0.2mg/mL by either dilution or concentration by ultrafiltration using for example an Amicon™ stirred cell with a PM-10™ or YM-30™ membrane. The resulting solution is filtered through a 0.22 micron filter and is heated at 60°C for a period of from 3 to 10 hours. The HBsAg was formulated into a vaccine by compounding with aluminum salts in the form of a gel as adjuvant as follows. 20µg of HBsAg is mixed with 0.1M NaCl and 0.88% w/v alum and 0.05% w/v Thimerosal™ as a preservative. The solution is then titrated to a pH of between 5-7 (5.5) with 0.1 M sodium phosphate at pH 12. Each ml of adjuvant-vaccine contains 0.5mg equivalent Al⁺³ ion, 20µg HBsAg and 0.05% w/v Thimerosal™. The overall yield of HBsAg according to this procedure is approximately 80%. C Product Characterization It is estimated that there is less than 20pg of DNA per mg of HBsAg produced by the procedure described in this report. This value was determined using the clearance factor by the process for DNA of one million, a concentration of HBsAg in the cell culture fluid of 1 mg/L and an amount of DNA per liter of cell culture fluid of 20µg as estimated by dot-blot hybridization. Direct measurement of DNA in the product by dot-blot hybridization shows that there is less than 10pg of DNA per mg of HBsAg. No significant decrease in stability of the HBsAg product at 4°C was observed over a period of six months. A solution composed of 0.025 mg/mL HBsAg purified by the procedure outlined, 10mM Tris-Cl; pH 7.5, and 0.15M NaCl can be heated at 60°C for a period of 10 hours with negligible loss as measured by both ELISA and Abbott AUSRIA II™ assays. The HBsAg is further characterized by a purity of greater than 98% as indicated by SDS-PAGE. Example 2 The aluminum phosphate adjuvant vaccine of Example 1 was compared with (a) an aluminum hydroxide adjuvant vaccine formed by substituting sodium hydroxide for sodium phosphate in the procedure, and (b) commercial Heptavax B™ supplied by Merck, Sharp and Dohme in a mouse potency assay. The results shown in the following Table 1 demonstrate the increased potency of the claimed vaccine relative to a commercial preparation.

Claims: A process for purifying HBsAg having 226 amino acid residues, comprising the steps of: (a) separating the HBsAg from host cell debris in a recombinant cell culture medium producing the HBsAg; (b) concentrating the separated HBsAg from step (a) by ultrafiltration; (c) adding ammonium sulphate to the concentrate from step (b) to an ammonium sulphate concentration of about 60% of saturation. (d) redissolving the precipitated HBsAg from step (c); (e) passing the separated HBsAg through an immunoaffinity adsorbent column containing monoclonal antibody specific for HBsAg to adsorb the HBsAg and remove contaminants; (f) releasing the adsorbed HBsAg by passing an eluent through the column; and (g) fractionating the HBsAg in said eluent by passing the same through an anion exchange bed to yield at least one purified HBsAg-containing fraction. A process according to claim 1 wherein the eluent of step (f) contains sodium acetate/acetic acid.

Inventors: HERSHBERG, ROBERT DAVID
Applicants: GENENTECH, INC.
Application year: 1985
Main classification: C07K3
IPC codes: A61K39, C12N15, C07K14, C12R1, C12P21, C12N5