Immobilization of Biologically Active Material in Capsules Prepared From a Water-Soluble Polymer and Chitosan Acetate

The present invention is concerned with the immobilization of biologically-active materials including biological cells, particularly hybridomacells, for use in bioreactors for the production of biological products.

Various attempts have been made to encapsulate and otherwise immobilize biological cells so that they remain viable and in a protected state within a semi-permeable membrane which permits the passage into the capsule of nutrients, oxygen and other species required for sustaining cell viability and metabolic functions and which also permits cell metabolites to pass out of the capsules.

In one prior art process, described in U.S. Pat. No. 4,251,387, capsules are manufactured by interfacial polymerization effected by reacting cationic monomers containing multifunctional amine groups with complementary anionic species. Polyelectric complex formation is accomplished by the formation of salt-bridges between anionic and cationic species.

Prior art immobilization procedures generally have suffered from one or more of the drawbacks of high material costs, high potential for contamination during the immobilization process, oxygen and nutrient limitations in the immobilized cell matrices, mechanical and/or chemical instability of the immobilized cell matrices, a narrow range of biocompatibility of matrix materials with cells and the use of expensive weighting materials.

In accordance with the present invention, there is provided a novel procedure for immobilizing biological cells and other biologically-active materials to enable them to be used in bioreactors for the production of a wide variety of biological products, including monoclonal antibodies, vaccines and pharmaceuticals.

By immobilizing the cells using the present invention, the cells are able to proliferate in concentrations significantly higher than for non-immobilized conditions and to remain viable within the immobilization matrices. The immobilized cells are particularly applicable to large-scale operations as required for industrial manufacturing. The immobilization procedures promote the essential mass transfer of nutrients, including oxygen, to the cells and the desirable passage of waste metabolites from the immobilized cells.

In the present invention, the process of immobilization is effected using chitosan as the key immobilizing agent. A solid immobilizing medium is formed for the biologically-active materials and cells by ionic interaction of polycationic groups on the chitosan with polyanionic groups on a polyanionic polymer. The procedure of immobilization may be effected in any convenient manner, including encapsulation, entrapment and occlusion.

In the present invention, the immobilization may be effected using one of three different, but related, procedures, namely encapsulation, entrapment and occlusion. Essential to the present invention is the use of chitosan, a partially deacetylated chitin. Chitosan is a polycationic polymeric material having free amino acid groups, which are protonated for use in the present invention. The molecular weight of the chitosan may be varied to vary the properties of the immobilized product. Generally, the chitosan has an average molecular weight of about 5.times.10.sup.5 to about 20.times.10.sup.5, preferably about 5.times.10.sup.5 to about 15.times.10.sup.5. Chitosan is biocompatible, mechanically-stable, inexpensive and provides good mechanical protection to the cells. The chitosan may be employed in a variety of mild polymerization processes involving ionic interactions between polycationic and polyanionic polymers to immobilize the cells.

In this embodiment of the invention, the cells are encapsulated within an outer semi-permeable membrane which is an ionically-interacted combination of chitosan and alginate or other polyanionic polymer. The semipermeable membrane is permeable to nutrients, ions and other low molecular weight materials required for sustaining cell viability and metabolic functions. Depending on permeability, the membrane may or may not be permeable to the product secreted from the cells.

Once encapsulated, the immobilized cells in the microcapsules then are cultivated in aqueous culture medium.

The cells are suspended in an aqueous solution of a polyanionic polymer, such as a polysaccharide gum, which can be reversibly gelled, generally an alkali metal alginate, preferably sodium alginate. The suspension then is formed into droplets by passing the suspension through any convenient droplet-forming apparatus and the droplets are gelled in an aqueous solution of calcium chloride to form temporary capsules. The droplet-forming apparatus generally is capable of forming droplets of such a size that the final microcapsules have a diameter from about 500 to about 3000 microns, preferably about 500 to about 1000 microns. The temporary capsules then are contacted with an aqueous solution of chitosan, which usually is in the form of a quaternary salt, such as the acetate. The aqueous solution of chitosan generally has a concentration of about 0.15 to about 3 wt.%, preferably about 1 to about 2 wt.%. The whole procedure is effected at a pH of about 5.6 to 6.0 to ensure that the free amino groups on the chitosan are protonated.

A sequestering agent for calcium, preferably sodium phosphate, then is slowly added to the mixture to cause precipitation of calcium phosphate. Ionotropic gelation then takes place between the protonated amino groups on the chitosan and the freed carboxylic groups on the alginate, to form a coating on the surface of each of the beads, so as to encapsulate the cells.

The coating or semipermeable membrane, which is formed in a period generally of about 1 to about 20 minutes, preferably about 4 to about 6 minutes, generally has a thickness of about 2 to about 10 microns, preferably about 5 to about 10 microns.

The majority of the calcium alginate remains in that form during the encapsulation step. The calcium alginate may be reliquified by sequestering the calcium ions, such as by exposure to sodium citrate solution.

The permeability and structural integrity of the chitosan-alginate microcapsules may be altered during membrane formation by varying the concentration and molecular weight of the polycationic polymer (i.e., the chitosan) within the ranges noted above, as well as the duration of the ionotropic gelation.

In this embodiment of the invention, the cells are impregnated within the pores of chitosan-stabilized calcium alginate beads. The metabolic product of the cells is excreted into the growth medium from the pores, permitting batch or continuous operation of a bioreactor vessel in which the beads are retained and may be reused.

The cells are suspended in an aqueous solution of a polyanionic polymer which can be gelled, such as a polysaccharide gum, generally an alkali metal alginate, preferably sodium alginate. The suspension then is formed into droplets by passing the suspension through any convenient droplet-forming apparatus. The droplet-forming apparatus generally is capable of forming droplets of such a size that the final beads have a diameter from about 500 to about 3000 microns, preferably about 500 to about 1000 microns.

The droplets then are passed into an aqueous solution containing the chitosan, which usually is in the form of a quaternary salt, such as the acetate. The aqueous solution of chitosan generally has a concentration of about 0.1 to about 1 wt.%, preferably about 0.5 to about 1 wt.%. The procedure is effected at a pH of 5.6 to 6.0 to ensure that the free amino groups on the polycationic chitosan are protonated. The solution into which the droplets are extruded also contains additional counterions, usually calcium, present as calcium chloride. Competition between the calcium and the cationic moieties on the chitosan results in the formation of highly cross-linked and hardened porous beads. The gelation proceeds for about 5 to about 30 minutes, preferably about 10 to about 20 minutes.

In this embodiment of the invention, the cells are immobilized within the pores of a fibrous chitosan-alginate matrix. The metabolic product of the cells is excreted into the growth medium from the pores in the fibrous matrix, permitting continuous operation of a bioreactor vessel in which the fibrous matrix is used.

The cells are suspended in an aqueous solution of a polyanionic polymer, generally an alkali metal alginate, preferably sodium alginate. The chitosan, which usually is in the form of a quaternary ammonium salt, such as the acetate, then is added to the suspension, resulting in the formation of a soluble polyelectric complex with the polyanionic polymer. The chitosan solution generally has a concentration of about 0.1 to about 2 wt.%, preferably about 0.5 to about 1 wt.%. The suspension is maintained at a pH of about 5.6 to 6.0 to ensure that the free amino groups on the chitosan are protonated.

The suspension then is extruded into an aqueous cross-linking and hardening solution containing multivalent anions, such as a pyrophosphate, preferably sodium pyrophosphate, and having a mildly alkaline pH, preferably about 7.3 to 7.5, resulting in fibre formation in which the cells are occluded within pores. The cross-linking proceeds for about 1 to about 10 minutes, preferably about 4 to about 6 minutes. Control of fiber porosity is accomplished by altering the duration of the crosslinking of the polyelectrolytes.

This Example illustrates the immobilization of hybridoma cells by encapsulation and growth of the immobilized cells in culture.

HP-6 murine hybridoma cells which produce monoclonal antibodies (MAb) directed against horse radish peroxidase were cultivated to a density of approximately 10.sup.6 cells/mL in Dulbecco's Modified Eagle's Medium (DMEM) supplemented with 10% fetal calf serum at 37.degree. C. in a 5% CO.sub.2 air-based environment.

M4-1 murine hybridoma cells producing MAbs against bovine light chain IgG.sub.1 also were grown under the same environmental conditions to a density of about 10.sup.6 cells/mL in Modified Eagle's Medium (MEM) supplemented with 10% horse serum.

5 mL of each hybridoma cell suspension was mixed with 5 mL of a 3% (w/v) sodium alginate solution (pH=5.6 to 6). The cell-alginate solution then was formed into droplets by passing the suspension through a droplet forming apparatus.

The droplet-forming apparatus consisted of a 20 cc syringe equipped with a 0.01 inch I.D. Teflon (trademark for polytetrafluoroethylene) coated needle. The syringe and needle assembly were positioned within a plexiglass housing such that the tip of the needle was subjected to a constant laminar air flow which acted as an air knife. A constant force applied to the top of the housing forced droplets of the cell-alginate suspension contained within the syringe from the tip of the needle. Each drop was disengaged from the needle tip by the laminar air stream and allowed to fall approximately 5 cm into a 50 mM CaCl.sub.2 solution (pH 5.6 to 6) wherein it was cross-linked to form a shape-retaining protective temporary bead. The bead size was controlled to be within the range of 900 to 1200 microns by adjusting the flow rate of the air stream.

After bead hardening for 5 to 20 minutes in the calcium chloride solution and washing with 125 mM NaCl (pH 5.6 to 6), a semi-permeable capsule membrane was formed by thoroughly coating the gelled beads with an aqueous 2% (w/v) chitosan-acetate solution (pH 5.6 to 6), followed by the slow addition of enough sodium phosphate (24.2 g/L monobasic and 3.5 g/L dibasic, pH 5.6 to 6), to cause the precipitation of calcium phosphate. Ionotropic gelation between the protonated amino groups on the chitosan and the carboxylic groups on the alginate proceeded for 5 minutes.

The calcium alginate within the resulting microcapsules was reliquified by exposing the capsules to 110 mM sodium citrate (pH 7) for 2 to 4 minutes. After a thorough washing in 125 mM NaCl, the resultant microcapsules (approximately 1,100 microns in diameter) were resuspended in tissue culture medium and were transferred to 75 cm.sup.3 culture flasks for incubation at 37.degree. C. and 5% CO.sub.2.

For comparative purposes, HP-6 hybridoma cells from the same stock culture also were grown in conventional tissue culture flasks according to two known methods, namely (1) conventional batch suspension and (2) immobilized in polylysine microcapsules of the same size prepared according to the procedure described in U.S. Pat. No. 4,251,387 referred to above. The maximum viable cell densities for the three HP-6 hybridoma cell cultures were determined at a suitable period, usually 5 to 20 days of culture, and the results obtained are tabulated in the following Table I:

As may be seen from the results set forth in Table I, the microcapsules prepared according to the present invention produce significantly higher viable cell densities than simple suspension culturing and microencapsulation by prior art procedures.

This Example illustrates the immobilization of hybridoma cells by entrapment in beads and growth of the immobilized cells in culture.

A 5 mL suspension of hybridoma cells (10.sup.6 cells/mL) was mixed with 5 mL of a 3% (w/v) sodium alginate solution (pH 5.6 to 6). The cell-alginate suspension then was passed through the droplet forming apparatus as described in Example 1 above, into a gently stirred solution containing one part 2% (w/v) chitosan-acetate solution (pH 5.6 to 6) and two parts 50 mM CaCl.sub.2 (pH 5.6 to 6). The ionotropic gelation of chitosan and calcium with alginate proceeded for a maximum of 20 minutes. The hardened beads were washed twice with 125 mM NaCl followed by one wash with DMEM or MEM. The procedure was effected for both the HP-6 and M4-1 hybridoma cells described in Example 1.

The resultant chitosan-alginate beads, (approximately 1,000 microns in diameter), then were transferred to conventional tissue culture flasks and were cultivated at 37.degree. C. and 5% CO.sub.2. The immobilized cells were cultivated in a batch-mode for the first 5 days of the culture period. After this initial time, the cell culture was operated semi-continuously by exchanging approximately 70% of the medium daily for the duration of the culture.

For comparative purposes, HP-6 hybridoma cells from the same stock culture also were grown in tissue culture flasks by conventional batch suspension. The maximum viable cell densities and monoclonal antibody production for the two HP-6 hybridoma cell cultures were determined at a suitable material, usually from 5 to 20 days of culture, and the results are tabulated in the following Table II:

As may be seen from the results set forth in Table II, the cells immobilized according to the invention produced a higher cell growth and MAb production than conventional batch suspension cultures.

HP-6 cells also were immobilized in agarose beads of diameter 1000 microns following the procedure of Nilsson et al referred to above and the cell growth in culture compared with the HP-6 hybridoma cells immobilized as described above. The results are set forth in the following Table III:

As may be seen from these results, a higher overall cell growth was experienced for the cells immobilized according to the present invention.

This Example illustrates the immobilization of hybridoma cells by occlusion in fibers and growth of the immobilized cells in culture.

A 5 mL cell culture suspension containing approximately 10.sup.6 cells/mL was centrifuged at 2000 RPM for 5 minutes. The resultant cell pellet was resuspended in 10 mL of a 1.5% (w/v) sodium alginate solution (pH 5.6 to 6). Ten mL of a 0.5% (w/v) chitosan-acetate solution (pH 5.6 to 6) was then added to the cell-alginate suspension and mixed well. The cell-alginate-chitosan suspension was immediately extruded into a gently stirred 1.5% (w/v) sodium pyrophosphate solution (pH 7.3 to 7.5) using the needle and syringe assembly described above in Example 1 (but omitting the air knife). After crosslinking for 4 minutes, the fibers were washed extensively with 125 mM NaCl and then once with tissue culture medium. The procedure was effected for both HP-6 and M4-1 hybridoma cells described in Example 1.

Cells occluded within the pores (40 to 60 microns in diameter) of the fibrous chitosan-alginate matrix were cultivated in stationary flasks and in 1 L airlift (internal-loop) and 0.5 L bubble columns. After 5 days of batch culture, between 75 to 80% of the medium was exchanged daily for the duration of the culture period. Immobilized cultures were operated semi-continuously for over 1 month.

The viable cell densities and monoclonal antibody production for the HP-6 hybridoma cells immobilized by the occlusion procedure described above were compared with those produced by the same cells grown in conventional batch suspension. The results obtained are set forth in the following Table IV:

As may be seen from these results, a significantly higher cell density and MAb productivity was obtained by occlusion of the fibers, according to the invention.