WO1993003134A1

WO1993003134A1 - Process for isolating and purifying recombinant interleukin-7

Info

Publication number: WO1993003134A1
Application number: PCT/US1992/006232
Authority: WO
Inventors: Per Wikstrom
Original assignee: Immunex Corporation
Priority date: 1991-07-29
Filing date: 1992-07-27
Publication date: 1993-02-18
Also published as: AU3350893A

Abstract

There is disclosed a method for isolating, renaturing and purifying recombinant interleukin-7 (IL-7) from prokaryotic transformed host cells that produce IL-7. The method for isolating purified recombinant IL-7 comprises entrapping the host cells in a porous matrix, wherein the porous matrix comprises a complex polysaccharide, extracting IL-7 from the porous matrix with an extraction buffer comprising a chaotropic agent and a reducing agent, renaturing extracted IL-7 by diluting IL-7 in a refolding buffer, and purifying renatured IL-7 from the refolding buffer.

Description

TITLE

PROCESS FOR ISOLATING AND PURIFYING RECOMBINANT INTERLEUKIN-7

TECHNICAL FIELD OF THE TNVENTTON

This invention relates generally to methods for isolating, purifying, and renaturing recombinant interleukin-7 (IL-7) polypeptides. More particularly, this invention relates to a method for isolating purified recombinant IL-7 from prokaryotic transformed host cells utilizing a porous matrix which entraps said host cells.

BACKGROUND OF THE INVENTION Interleukin-7 had been previously designated "Lymphopoietin-1." The cloning and expression of human and murine IL-7 is described in United States Patent 4, 965,195, issued on October 23, 1990, the disclosure of which is incorporated by reference herein. IL-7 is a lymphopoietic growth factor that was first isolated and cloned by virtue of its ability to stimulate the growth of B- and T-cell progenitors in bone marrow. Published PCT Application WO 89/03884 (May 5, 1989) and EP-A- 0314415 (May 3, 1989) disclosed DNA vectors and related processes for producing IL-7 by recombinant DNA technology. The relevant disclosures of these published patent applications are incorporated by reference herein. The cloning of murine IL-7 was first reported in the scientific literature by Namen et al., Nature 333:571 (1988) and human IL-7 by Goodwin et al., Proc. Natl. Acad. Sci. USA 86:302 (1989).

Purification of murine IL-7 from supernatants of transformed bone marrow stromal cell lines indicated an apparent molecular weight of approximately 25,000 daltons (see, e.g., Namen et al., /. Exp. Med. 26:7988 (1988)). The cDNA sequences reported by Namen et al. and Goodwin et al. suggest minimum molecular weights for murine and human IL-7 polypeptides of 14,897 and 17,387 daltons, respectively, exclusive of any glycosylation. Cloning, characterization and expression of IL-7 has enabled the characterization of its spectrum of biological activities.

In the production of recombinant proteins, such as IL-7, separation of the expressed polypeptide from the transformed host cells or their cultured supernants is often a major problem (Dwyer, Bio/Technology 2: 1957 (1984)). Often, mammalian polypeptides expressed in prokaryotic or bacterial cells form precipitates (which are also sometimes referred to as "aggregates," "refractile bodies," or "inclusion bodies"). See, e.g., United States Patent 4, 569,790; Langley et al; Eur. J. Biochem.163:313 (1987); Winkler et al., Bio/Technology 3:990 (1985); and DeLamarter et al., EMBO J.4:2515 (1985). Such aggregates are often associated with improper formation of inteπnolecular bonds or hydrogen bonds and disulfide bonds between amino acids within a polypeptide chain. Inclusion bodies probably afford protection to recombinant polypeptides against endogenous proteases. However, inclusion bodies present problems of extraction and purification from endogenous host cell proteins because they are often insoluble in aqueous buffers. Another problem in the recovery of the desired recombinant polypeptide, expressed in the form of refractile or inclusion bodies, is that it is often difficult to separate the recombinant polypeptide from other host cell materials. Another problem is due to strong attraction that inclusion body polypeptides have for one another, due perhaps to ionic attractions or hydrophobic bonding. This attraction can crystallize inclusion bodies in particular pH and ionic strength environments.

The general approach that has been used for obtaining biologically active polypeptides from such inclusion bodies often involves first treating prokaryotic transformed host cells with chaotropic agents and reducing agents to solubilize the inclusion bodies. One next removes the chaotropic agent and reducing agent to permit the polypeptides to refold into their biologically active conformation while minimizing the reformation of aggregates. A procedure to separate a recombinant polypeptide (in soluble or insoluble form) from other host cell materials is often difficult, and usually involves centrifugation. Centrifugation is a difficult process for large-scale purification of recombinant polypeptides because centrifugation is generally conducted batchwise and not continuously. Moreover, centrifugation to separate soluble from insoluble materials in the presence of a high concentration of a chaotropic agent, such as 7M guanidine HC1, is often unsuccessful. Therefore, it is desirable to avoid centrifugation in large- scale purification processes and allow continuous processing of recombinant polypeptides.

Therefore, it would be useful to provide a method that solves problems in purifying, renaturing and isolating recombinant IL-7 polypeptides using procedures that allow for continuous large-scale processing to isolate and purify biologically active IL-7 in a large-scale process.

SUMMARY OF THE INVENTION

The present invention relates to a process for isolating recombinant IL-7 from prokaryotic transformed host cells that produce IL-7. The process for isolating recombinant IL-7 first involves entrapping the prokaryotic transformed host cells in a porous matrix, extracting IL-7 from the porous matrix with an extraction buffer comprising a chaotropic agent and a reducing agent, renaturing the extracted IL-7 by diluting the extract in a refolding buffer, and purifying renatured IL-7 from the refolding buffer. Preferably, the porous matrix comprises an alginate salt. The present invention further describes a method for renaturing extracted IL-7 and a method for purifying renatured IL-7.

The transformed prokaryotic host cells are preferably E. coli cells that have been transformed with a cDNA coding for an IL-7 polypeptide, such as the cDNAs described in United States Patent 4,965,195. Such transformed E. coli cells will produce recombinant human IL-7 when grown in proper fermentation conditions. The method of entrapping the prokaryotic host involves forming a porous matrix to entrap the host cells. Preferably, one forms an alginate porous matrix by mixing from about 2.0% to about 4.0% (w/v) solution of sodium alginate with suspended host cells. One then adds the alginate-host cell mixture to a solution comprising from about 20mM to about lOOmM of a divalent cation salt solution (preferably calcium or magnesium as the divalent cation) and allowing the porous matrix (alginate beads) to form after at least about 5 minutes, and preferably after 30 minutes of mixing. Other complex polysaccharides can be substituted for alginate. These include, for example, carrageenan, and other complex polysaccharide constituents of marine algae. Divalent cations include calcium, magnesium, aluminum, barium, and others.

The porous matrix form that entraps the host cells allows for diffusion of various external chemical agents and solvents to the entrapped host cells but retains insoluble and large molecular weight materials (300 kDa or larger) within the porous matrix. One then extracts IL-7 from the porous matrix with an extraction buffer. IL-7 polypeptides, when soluble, are small enough to diffuse out of the porous matrix. The extraction buffer disrupts the cell membranes of the entrapped cells, and allows soluble materials of molecular weight less than 100 kDa to diffuse from the porous matrix while retaining insoluble materials within the porous matrix. IL-7 is a soluble material within the extraction buffer. Therefore, after washing the porous matrix with extraction buffer, IL-7 is removed from the porous matrix in the extraction buffer.

The extraction buffer comprises a chaotropic agent and a reducing agent. Preferably, the chaotropic agent is selected from the group consisting of guanidine, urea, anionic surfactants, ammonia, and combinations thereof. Sodium dodecyl sulfate (SDS) can also be used as a chaotropic agent if calcium is not used as the divalent cation. The extraction buffer further comprises a reducing agent. A reducing agent is selected from the group consisting of dithiothreitol (DTT), glutathione, mercaptoethanol, lipoic acid, thioglycolic acid, thioredoxin, and combinations thereof. Preferably, guanidine HCl is the chaotropic agent and DTT is the reducing agent.

Extracted IL-7 needs to be renatured to provide biological activity. A preferred renaturing step involves diluting the extraction buffer in a refolding buffer, wherein the protein concentration is diluted to a concentration of from about 0.05 mg ml to about 1.0 mg/ml. Preferably, the protein concentration can be determined by optical density measurement at 550 nm to have an OD reading of from about 5 to about 9, when the original cell density OD is from 4.0 to 100 at 550 nm. The refolding buffer comprises a chelating agent, such as EDTA, and a slightly basic pH of from about 7.0 to about 9.0. The IL-7 solution is incubated in the refolding buffer at room temperature for at least 10 hours, and preferably one day, in a mildly oxidizing atmosphere. One can create a mildly oxidizing atmosphere, for example, by allowing the tank of refolding buffer to be exposed to air. Preferably, the refolding buffer consists essentially of an oxicktion/reduction buffer, such as glutathione.

Purification of refolded IL-7 from the refolding buffer follows conventional protein purification techniques. Preferably, a reverse phase high performance liquid chromatography (RP-HPLC) technique is used. One method of purifying recombinant human IL-7 involves precipitating protein in a purification buffer by cooling the buffer, filtering the buffered solution to remove precipitated proteins, fractionating the filtrate in a cation exchange column with a salt gradient, fractionating the pooled IL-7 fractions in a RP-HPLC procedure with a bonded alkyl column in an acetonitrile gradient, and refractionating the pooled RP-HPLC IL-7 fractions in a cation exchange column with a salt gradient.

BRIEF PESCRimQN QF THE PfiAWINfi

The Figure shows a schematic diagram of a procedure for isolating recombinant IL-7 from a fermentation broth of transformed prokaryotic host cells that produce IL-7.

PETAILEP PES (. JPTIPN QF THE INVENTION

The present invention provides a process for isolating, renaturing, and urifying recombinant IL-7 from transformed prokaryotic host cells that express IL-7 as internal inclusion bodies. The isolation process utilizes a porous matrix, preferably an alginate porous matrix, to entrap the host cells prior to removing recombinant IL-7 from the host cells. The use of a porous matrix allows large-scale isolation of recombinant IL-7 in a continuous manner, without having to resort to a more cumbersome batch process that utilizes centrifugation to separate soluble from insoluble materials. The use of a porous matrix further reduces endotoxin content of isolated _L- 7 to have less reliance on the purification process to reduce endotoxin levels. The method for isolating recombinant IL-7 from prokaryotic transformed host cells comprises two steps. First, the cells are entrapped in a porous matrix. Second, IL-7 is extracted from the entrapped cells.

Preferably, prior to entrapping the cells, one concentrates the host cells within their fermentation broth. Concentration may be performed, for example, by means of diafiltration, filtration, centrifugation, or other conventional means. Concentration to an optical density of from about 40 to about 700, preferably from about 280 to about 600, at 550 nm, is preferred.

The entrapping step forms a porous matrix to entrap the prokaryotic host cells. Preferably, when forming an alginate porous matrix, one mixes a sodium alginate solution (or another alginate salt) with a suspension of the host cells. Other complex polysaccharides that can form a porous matrix can be substituted in place of alginate. Alginic acid is a co-polymer of β-D-mannuronic acid and α-L-glycuronic acid linked by the αl -► 4 glycolytic linkages. Alginic acid, like carrageenan, is a constituent of marine algae. Alginic acid is produced by the brown algae of the phaeophyceae, which occur in intertidal zones and are free living. Phaeophyceae commonly are found in the Sargasso Sea. A primary commercial source of alginic acid is Macrocystis pyrifera which is a giant kelp harvested off the Pacific Coast of the United States. Alginic acid can also be obtained from Laminaria species which grow off the coast of Europe, Japan and Northeast America, anάAscophylum species which grow around the English coast. In nature, alginic acid occurs as its mixed salt with sodium, calcium and magnesium. Commercial preparation of alginic acid extracts alginic acid by digestion of the seaweed with sodium hydroxide to produce a dilute solution of sodium alginate. The dilute solution of sodium alginate can be filtered to remove paniculate matter. Sodium alginate is then further purified. Alginate is commonly sold as a sodium salt. However, alginate is also commercially available as an ammonium or propylene glycol salt and as a propylene glycol ester of alginic acid. Sodium alginate is useful for forming a porous matrix because it has a high affinity for divalent cations, such as calcium, magnesium, barium, and strontium.

Various methods exist for entrapping host cells within a porous matrix. For example, an alginate (e.g., sodium alginate) solution is mixed with the host cells to form a mixture. This mixture is slowly added to a salt solution comprising a divalent cation, preferably a calcium salt such as calcium chloride. The salt solution is gently stirred to allow newly formed porous matrix, in the form of beads, to move away from the entry point of the mixture of the two solutions.

In case of error the porous matrix is removed by adding a divalent cation chelating agent, such as EDTA or citrate, to remove the divalent cation from the porous matrix and thereby restore the alginate solution as a soluble suspension. The porous matrix is maintained by the presence of divalent cations.

The porosity of a calcium alginate porous matrix can be controlled by altering the starting concentration of alginate. Preferred concentrations of alginate in a host cell- alginate suspension is from about 1% to about 8% (w/v) alginate. A preferred concentration of a divalent cation salt (such as a calcium chloride) is from about 20 mM to about 200 mM salt. The porous matrix entraps host cells upon formation. Preferably, the porous matrix is formed in about 50 mM calcium. After the porous matrix is formed, it can be maintained in a lower concentration of divalent cation; for example, ten times lower than the concentration of divalent cation used in forming the porous matrix. If a monovalent cation is present, for example sodium, then the maintenance concentration of divalent cation should be increased.

The second step involves extracting IL-7 from the entrapped host cells within the porous matrix. This is accomplished by changing the buffer solution that bathes the entrapped host cells to an extraction buffer. The extraction buffer comprises a chaotropic agent and a reducing agent. Chaotropic agents include, for example, guanidine, urea, anionic surfactants, ammonia, sodium dodecyl sulfate (SDS), and combinations thereof. A preferred chaotropic agent is guanidine and a preferred concentration is from about 5 M to about 10 M. Reducing agents are selected from the group consisting of dithϊothreital (DTT), glutathione, mercaptoethanol, lipoic acid, thioglycolic acid, thioredoxin, and combinations thereof. A preferred reducing agent is DTT at a concentration of from about 5 mM to about 20 mM. The extraction buffer may ftrrther comprise from about 1 mM to about 20 mM of a salt solution comprising a divalent cation. The presence of a divalent cation helps keep the porous matrix stable. Extraction is usually performed at room temperature for at least about four hours. Preferably, the extraction step is performed twice, in an effort to reduce the volume of extract, and the extract, containing soluble IL-7, is pooled for subsequent renaturation and purification.

The renaturing process begins by diluting the IL-7 concentration to a protein concentration of from about 0.05 mg/ml to about 1.0 mg ml, and preferably from about 0.1 mg ml to about 0.4 mg ml. One can monitor the protein concentration for this dilution step by measuring optical density of the solution at 550 nm. Preferred optical density readings are from about 5 to about 8. IL-7 is renatured by dilution in a refolding buffer. The refolding buffer consists essentially of an oxidation reduction buffer, a cation chelating agent, a hydrophobicity agent, and a pH buffer. An example of an oxidation/reduction buffer is a gluthathione buffer. An example of a chelating agent is EDTA. An example of a hydrophobicity agent is ammonium sulfate, and an example of a pH buffer is TRIS . A preferred refolding buffer is, for example, 2M guanidine hydrochloride, 2mM EDTA, 0.1 M ammonium sulfate, 1 mM glutathione (reduced), 0.2 mM glutathione (oxidized), and 0.1 M TRIS pH 8.

After diluting IL-7 in the refolding buffer, this mixture is stirred and allowed to oxidize in a mildly oxidizing atmosphere (e.g., air) at room temperature for at least ten hours, but no longer than one week. This incubation in a mildly oxidizing atmosphere allows IL-7 to renature into its active, tertiary conformation.

After renaturing, IL-7 requires purification to obtain substantially homogenous recombinant IL-7. The purification process can begin by diafiltering IL-7 in refolding buffer against a purification buffer. A preferred ratio for diafiltering IL-7 in refolding buffer against a purification buffer is approximately 1 :5. A purification buffer comprises, for example, a weakly ionic buffer with a pH of from about 6.0 to about 8.0. Preferably, the purification buffer comprises TRIS and ammonium sulfate. Most preferably, approximately 50 mM TRIS and approximately 50 mM ammonium sulfate at pH 7.4 comprise the purification buffer. Purification is accomplished by first precipitating denatured proteins. This can be accomplished by refrigerating the E -7 solution in purification buffer at least 8 hours, followed by filtering the refrigerated solution through a 0.2 μm filter, or by centrifuging the refrigerated IL-7 solution in purification buffer at approximately 9000 rpm at 20° C for approximately 1 hour. Preferably, precipitated proteins are removed by filtration.

Purification is further accomplished by a series of column fractionation steps. Preferably, the fractionation process involves three steps: Two cation exchange column chromatography steps, and a reverse phase high performance liquid chromatography (RP-HPLC) procedure. In a preferred embodiment, one applies the IL-7 solution in purification buffer to a cation exchange column and then elutes EL-7 with a linear salt gradient. Preferably, the cation exchange column is a Mono S column (Pharmacia) comprising monobeads made of polystyrene crosslinked with divinylbenzene and derivatized with sulfonyl groups.

An elution buffer can comprise, for example, 1 M NaCl in purification buffer over a gradient of 0.07 M to about 0.2 M NaCl on the column. Each fraction is analyzed for IL-7 activity (such as by a pre-B cell assay). Fractions containing IL-7 activity are pooled for the next chromatography step. A RP-HPLC step further purifies the IL-7 solution. Preferably, the IL-7 solution is passed over a bonded alkyl column in trifluoroacetic acid (TEA) with an acetonitrile gradient. Preferably, the buffer is approximately 0.1% TEA and the acetonitrile gradient is from 0% to about 80%. IL-7 elutes at an acetonitrile concentration of approximately 40% to about 55%. Fractions containing IL-7 biological activity are pooled for the final purification step.

The final purification step employs another cation exchange column, preferably a Mono S column, and an acetate buffer. Preferably, the acetate buffer is about 50 mM sodium acetate at an acid pH of from about 3.0 to about 5.5. Preferably, the pH of the sodium acetate buffer is from about 4.3 to about 5.0. The IL-7 solution is applied to the cation exchange column and eluted in a salt gradient up to about 1 M NaCl. IL-7 elutes at approximately 0.54 M to about 0.58 M NaCl. The fractions containing IL-7 activity are substantially homogenous and free from other contaminating proteins.

The following example illustrates a preferred, large scale purification process for recombinant IL-7 obtained from transformed E . coli host cells.

EXAMPLE

This example illustrates the isolation, renaturing and purification of human IL-7 from transformed E. coli host cells. IL-7 was produced as inclusion bodies by incubating transformed E. coli host cells in a fermentation broth. A 14 liter fermentation broth was concentrated to 2 liters, prior to extraction, by a diafiltration method. Diafiltration was accomplished by pumping the fermentation broth through a 0.45 μm filter. Buffer exchange was performed 10 times by adding 2 liters of 20 mM tris pH 8.0 and concentrated to 2 liters. The concentrated cells were entrapped in an alginate porous matrix. The alginate porous matrix was formed with a 5% (w/w) solution of alginic acid (Sigma, St. Louis, MO) dissolved in deionized water the day before it was used. Approximately 1.4 liters of 5% alginic acid was mixed with 2 liters of concentrated fermentation solution to form a homogenous slurry. The slurry was pumped through a multi-nozzle immobilization device, such that droplets formed (approximately 203 mm diameter) when entering into a tank. The tank contained 8 liters of 50 mM calcium chloride in 50 mM TRIS HC1 buffer pH 8.0. No stirring of this solution was necessary. The flow rate was chosen such that droplets were formed without sticking to each other when entering into the calcium chloride solution. We placed the nozzle approximately 15-25 cm above the calcium solution. The alginate porous matrix formed in the tank and entrapped host cells. The alginate porous matrix was incubated for approximately one hour at room temperature. The alginate porous matrix was then washed at least two times with about 6 liters of 5 mM calcium chloride in 0.1 M TRIS HC1 buffer to remove cells and other fermentation debris that did not become entrapped in the porous matrix. The alginate porous matrix was removed from this solution with a strainer. The extraction step was performed twice with 8 liters of 7 M guanidine hydrochloride in 0.1 M TRIS HC1, 5 mM calcium chloride and 10 mM of the reducing agent, DTT for each extraction of IL-7 from the alginate porous matrix. Both extraction steps required at least about 6 hours of incubation at room temperature.

Renaturing was accomplished by diluting the extract containing IL-7 to an optical density of approximately 7 at 550 nm. This corresponds to a final protein concentration of approximately 0.2 mg/ml in approximately 2 M guanidine. The extract was diluted with a refolding buffer comprising 0.2 mM glutathione (oxidized), 1 mM glutathione (reduced), 2 mM of a chelating agent (EDTA), 0.1 M ammonium sulfate and 0.1 M TRIS at a pH of 8.0. The volume of extracted IL-7 in refolding buffer was approximately 120 to 180 hters. The refolding buffer was stirred in a large tank with an open lid at room temperature to allow slow air oxidation. The tank lid was open to allow for a mildly oxidizing atmosphere.

Purification was a multi-step process that began by diafiltering IL-7. Approximately 20 liters of refolding buffer was diafiltered against approximately 100 liters of purification buffer. Purification buffer comprised 50 mM ammonium sulfate and 50 mM TRIS at a pH of 7.4. Diafiltration was performed continuously with a slow decrease of refolding buffer concentration.

After diafiltration the solution was refrigerated at approximately 4° C overnight. This allowed for precipitation of denatured proteins. The precipitated proteins were removed by filtering the solution through a 0.2 μm filter.

Next, the IL-7 solution and purification buffer was purified by 3 column chromatography steps. First, the solution was applied to a cation exchange (Mono S 60/100) column. Approximately 40 liters of the IL-7 solution was applied to the column at a rate of approximately 25 ml/min. Application of this solution to the column was followed by 10 column volumes of wash buffer (purification buffer) also applied at a flow rate of 25 ml/min. The column was eluted with 15 column volumes of a linear salt gradient at a flow rate of 50 ml/min. The salt gradient ranged from 0 M NaCl to 1.0 M NaCl. IL-7 eluted between 5.6% and 24.4% of the second buffer, which corresponds to 0.07 M NaCl to 0.24 M NaCl. Each fraction within the range contained IL-7 and was pooled for further purification.

After each use of the Mono S column, the column was washed. We followed a washing procedure adding 1/2 column volume of filtered 2 M NaCl solution at a flow rate of 50 cm/hour (24 ml/min), followed by 1 column volume of water and 4 column volumes of 1 M sodium hydroxide at 40 cm/hour (19 ml/min). This was followed by 1 column volume of water, 2 column volumes of 75% acetic acid, 2 column volumes of low ionic strength buffer and 1 column volume of 2 M NaCl at a flow rate of 100 cm hour (48 ml/min). Lastly, 10 column volumes of 40% ethanol was added at a flow rate of 40 cm/hour (19 ml min). This procedure was sufficient to regenerate the column for further purification uses.

The pooled IL-7 solution fractions were added to a C18 (Vydac) column, whose dimensions were 2.5 cm x 10 cm in a 0.1% trifluoracetic acid (TFA) buffer with acetonitrile elution. The column was first equilibrated in a 0.1% TFA solution. The flow rate was 20 ml/min. Elution was with a linear gradient of acetonitrile in 0.1 % TFA over 20 column volumes. IL-7 eluted at 60-65% acetonitrile.

RP-HPLC fractions containing IL-7 were pooled and applied to a Mono-S cation exchange column (60/100) in buffer A (50 mM sodium acetate pH 4.7) and eluted with a linear gradient of buffer B (1 M NaCl in 50 mM sodium acetate pH 4.7). After application of the IL-7 pooled fraction, the column was washed with 10 column volumes of 50 mM sodium acetate (pH 4.7), and eluted with 15 column volumes of with buffer B. Substantially homogenous IL-7 eluted at 0.54 M to 0.58 M NaCl. The procedure employing an alginate porous matrix produced purified hIL-7 with an endotoxin level less than 0.012 ng/mg IL-7, which is the limit of detection. Thus, no endotoxin could be detected.

Claims

CLAIMSPROCESS FOR ISOLATING AND PURIFYING RECOMBINANT INTERLEUKIN-7What is claimed is:

1. A method for isolating purified recombinant Interleukin-7 (IL-7) from prokaryotic transformed host cells that produce IL-7, comprising:

(a) entrapping said host cells in porous matrix, wherein said porous matrix comprises a complex polysaccharide;

(b) extracting IL-7 from the porous matrix with an extraction buffer comprising a chaotropic agent and a reducing agent; (c) renaturing extracted IL-7 by diluting in a refolding buffer, and

(d) purifying refolded IL-7 from the refolding buffer.

2. The method of claim 1 wherein the method of entrapping prokaryotic host cells in a porous matrix comprises: (a) mixing a from about 1.0% to about 10.0% (w/v) solution of an alginate salt with suspended host cells to form an alginate-cell mixture; and

(b) adding the alginate-cell mixture to a solution comprising from about 20 mM to about 100 mM salt solution comprising a divalent cation to form the porous matrix.

3. The method of claim 1 wherein the chaotropic agent is selected from the group consisting of guanidine, urea, anionic surfactants, ammonia, sodium dodecyl sulfate and combinations thereof.

4. The method of claim 1 wherein the reducing agent is selected from the group consisting of dithiothreitol (DTT), glutathione, mercaptoethanol, lipoic acid, thioglycolic acid, thioredoxin, and combinations thereof.

5. The method of claim 1 wherein the renaturing process comprises: (a) diluting IL-7 with a refolding buffer to a protein concentration of from about 0.05 mg/ml to about 1.0 mg ml, wherein the refolding buffer has a pH of from about 7.0 to about 9.0, and corhprises a chelating agent; and (b) incubating IL-7 in the refolding buffer for at least 10 hours at room temperature in a mildly oxidizing atmosphere.

6. The method of claim 1 wherein the purification process comprises: (a) precipitating denatured proteins by cooling IL-7 in a purification buffer, wherein the purification buffer pH is adjusted to about pH 6.0 to about pH 8.0;

(b) filtering the IL-7 through a filter no larger than 0.5 μm to remove precipitated proteins and collecting a filtrate;

(c) fractionating the filtrate in a cation exchange column with a salt gradient buffer, and collecting and pooling IL-7 fractions;

(d) fractionating pooled IL-7 fractions in a reverse phase high performance liquid chromatography (RP-HPLC) procedure with a bonded alkyl column and an acetonitrile gradient in trifluoracetic acid (TFA) and collecting and pooling RP- HPLC IL-7 fractions; and (e) refractionating the pooled RP-HPLC IL-7 fractions in a cation exchange column buffered with a pH from about 4.0 to about 5.0 with salt gradient elution, whereby substantially homogenous IL-7 will elute at approximately 0.4 M salt.

7. A method for refolding a solution of denatured IL-7 comprising: (a) diluting IL-7 with a refolding buffer to a protein concentration of from about 0.05 mg/ml to about 1.0 mg ml, wherein the refolding buffer has a pH of from about 7.0 to about 9.0, and comprises a chelating agent; and

(b) incubating IL-7 in the refolding buffer for at least 24 hours at room temperature in a mildly oxidizing atmosphere.

8. A method for purifying a solution of IL-7 comprising:

(a) precipitating denatured proteins by cooling IL-7 in a purification buffer, wherein the purification buffer has a pH range from about pH 6.0 to about pH 8.0; (b) filtering IL-7 in purification buffer through a filter no larger than 0.5 μm to remove precipitated proteins and collecting a filtrate;

(c) fractionating the filtrate in a cation exchange column with a salt gradient buffer and collecting and pooling IL-7 fractions;

(d) fractionating pooled IL-7 fractions in a reverse phase high performance liquid chromatography (RP-HPLC) procedure with a bonded alkyl column and an acetonitrile gradient in trifluoracetic acid (TEA) and collecting and pooling RP- HPLC IL-7 fractions; and (e) refractionating the pooled RP-HPLC IL-7 fractions in a cation exchange column buffered with a pH of from about 4.0 to about 5.0 with a salt gradient elution, whereby substantially homogenous IL-7 will elute at approximately 0.4 M salt.