Bioreactor Considerations

Transcription

1 Bioreactor Considerations for Animal Cell Culture Animal cells are difficult to cultivate in large-scale because: They are larger (10-30 µm) and more complex than most microorganisms; Their growth rate is very slow (D t ~10 to 50 h) compared to the microorganisms, therefore, the productivity is low and the maintenance of sterility is difficult; Their nutritional requirements are not fully defined yet, sometimes requiring expensive serum for medium or serum-free medium; They are enclosed with a delicate plasma membrane without the tough cell wall normally found in microorganisms, as a result they are fragile and very shear sensitive; They are part of an organized tissue rather than an individual cellular organism; Most animal cells only grow when attached to a surface (mostly on treated plastic); Toxic metabolites (ammonium, lactate) are produced during growth and product concentration (titre) is usually very low (µg/ml).

2 Bioreactor Considerations for Animal Cell Culture Animal cell features set certain constraints on the bioreactor design: The reactor should be gently aerated and agitated. Some mechanically agitated reactors operating at agitation speeds over 20 rpm and air-lift reactors operating at high aeration rates may cause shear damage to cells; shear sensitivity is cell line dependent. Stirred-tank Bioreactor Air-lift Bioreactor

3 Bioreactor Considerations for Animal Cell Culture Animal cell features set certain constraints on the bioreactor design: A large support material surface-volume ratio needs to be provided for adherent cells. Development of microcarrier culture, i.e. growth of cells on small particles, typically µm in size (based on dextran, gelatin, glass, silica,...), of various shapes, suspended in stirred culture media Source: Microcarrier Cell Culture Handbook, GE Healthcare Well-controlled homogeneous environmental conditions (T, ph, dissolved oxygen, redox potential) and a supply of CO 2 -enriched air need to be provided; The removal of toxic products of metabolism (lactate, ammonium) and the concentration of high values products (MAb s, vaccines) should be accomplished during cell cultivation.

4 Bioreactor Systems for Animal Cell Culture Multiplate culture Stirred bioreactor Roller Bottle culture Air-lift bioreactor Spinner Flask culture Fixed-bed bioreactor Hollow fiber and ceramic matrix modules Fluidized-bed bioreactor

5 Multiplate Culture Static system consisting of multiplates, made of polystyrene; Typically used for the establishment and subcultivation cell cultures from human diploid working cell banks for vaccine production; Variable volume and surface area; Used to inoculate similar systems or microcarrier culture systems. Multiplate System Source:

6 Roller Bottle Culture Basis of the roller culture system: to place multiple cylindrical bottles ( ml) into an apparatus that will rotate the bottles evenly at set rotational speeds (5-60 rph) all the internal surface is used to cell growth. Advantages: smaller volume of medium and thus a higher product titre can be achieved; the cells are more efficiently oxygenated due to alternate exposure to medium and the gas phase; dynamic systems usually generate higher unit cell densities than stationary systems. Apparatus are available to accommodate four to hundreds of bottles. (Production of vaccines uses bottles per batch); To prevent oxygen limitation a medium/air volume ratio between 1:5 and 1:10 is used.

7 Roller Bottle Culture Source: Source: Small bench top rack Large bench top or free standing extendable rack

8 Spinner Flask Culture The spinner flask is a glass vessel, usually intended to be used for replicate cultures. Sizes range from a few milliliters to 20 L; The stirring speed is between 10 and 300 rpm; The first scale-up step for cells growing in suspension or adherent cells on microcarriers is the spinner flask. Spinner Flask, from Bellco Source:

9 Spinner Flask Culture Spinner flask culture with gelatin microcarriers Can potentially be used with other support materials (scaffolds). Source: Bueno, EM, et al J. Biotechnol, 2007

10 Hollow Fiber Bioreactor Basis of the hollow fiber system: cells grow in the extracapillary space. The medium is pumped through capillaries and the ph and oxygen saturation are adjusted after each passage. The fibers are made of semi-permeable membranes, allowing diffusion of oxygen, low molecular weight nutrients and waste products, but not proteins; The product is harvested in a concentrated form from the extracapillary space. Source: Diagram of a hollow fiber perfusion system

11 Ceramic Matrix Module Basis of the ceramic matrix bioreactor: is the same as for hollow fiber systems, with the exception that cells are not separated from the medium circulation by a membrane. Cells are held in the highly porous matrix and are supplied with nutrients by circulating medium through small squared channels in the matrix. 37ºC Al 2 O 3 -TiO 2 coated with ZrO 2 membrane Diagram of a Ceramic Matrix Module Bioreactor System

12 Stirred Culture Different agitation systems, e.g. turbine-type or marine-type impeller, vibromixer and cell lift, were developed in order to prevent exposure of animal cells, cultured in stirred bioreactors, to high shear forces; In large-scale or high cell density perfusion cultures, air sparging or oxygenpermeable silicone/polypropylene tubing will provide adequate aeration. Source: Stirred-tank Bioreactor Type of impeller: (1) spin filter; (2) double screen cell lift; (3) draft-tube; (4) pitched blade; and (5) marine blade

13 Air-lift Bioreactor Basis of the air-lift bioreactor: Air or oxygen sparged into the bioreactor at the bottom drives circulation to keep cells in suspension, instead of a mechanical stirring device To improve circulation and oxygen input and minimize cell damage, the height to diameter ratio is normally high. Source: Air-lift bioreactor: 2 concentric cylinders create an outer and inner chambers. 5% CO 2 in air is bubbled in the bottom of the inner chamber. The bubbles rise, carrying the cell suspension. CO 2 is vented from the top and displacement ensures the return of the cell suspension down the outer chamber.

14 Microcarrier Cell Culture Fixed and Fluidized Bed Bioreactors Basis of the microcarrier cell culture: immobilization of cells (both adherent and suspension) in micro/macroporous microspheres. Microcarrier technology is currently the most successful scale-up method for high density perfused cultures; Compatible with stirred, fixed-bed /packed bed and fluidized-bed bioreactor. Stirred-tank Bioreactor Fixed-Bed/Fluidized-Bed Bioreactor

15 Microcarriers and Fluidized-Bed Technology Basis of the fluidized-bed: microcarriers are weighted (specific gravity ) so that they can be used in fluidized beds at high recycle flow rates (typically 75 cm min -1 ). Microcarriers have a sponge-like structure with a pore volume of 85%, allowing the immobilization of cells to high density (1-4x10 8 cells ml -1 ); Source: GE Healthcare

16 Pictures from GE Healthcare Fluidized-Bed Bioreactor Cytopilot by GE Healthcare

17 Other Bioreactor Types Rotary Cell Culture System TM Source: Synthecon, Inc.

18 Other Bioreactor Types Rotary Cell Culture System TM HARV Vessels (high aspect ratio vessel) Reusable Vessel. Oxygenation by a flat silicone rubber gas transfer membrane. Used to culture both suspension and anchorage dependent cells. Available in 50 ml and 10 ml sizes. STLV Vessels (slow turning lateral vessel) Reusable Vessel. Oxygenated by a central core silicone rubber gas transfer membrane. The STLV is intended for microcarrier cell culture and explant tissue cultures. Available in 55, 110, 250, or 500 ml sizes. Source: Synthecon, Inc.

19 Other Bioreactor Types Rotary Perfusion Culture System Source: System allow a for continuous feeding of the cell chamber from external media bottle; Cells are retained in the cell chamber by molecular weight cutoff membrane.

20 Large-scale Animal Cell Culture A typical animal cell culture bioreactor is conceptually similar to the bacterial fermentor apart from modifications such as: - a marine (not turbine) impeller; - curved or convex base for better mixing at low speeds; - water-jacket (not immersion heater) temperature control. Large-scale mammalian cell culture systems have been widely used for multiple applications such as production of recombinant proteins and monoclonal antibodies. Example: hybridoma cells - engineered to produce a desired antibody in large amounts; to produce monoclonal antibodies, B-cells are removed from the spleen of an animal that has been exposed to the relevant antigen; B-cells are then fused with myeloma tumor cells that can grow indefinitely in culture. CULTURE MODES: 1. Batch culture; 2. Fed-batch culture; 3. Continuous-flow (chemostat) culture; 4. Continuous perfusion culture

21 Animal Cell Culture Modes Source: Ratledge, C, Kristiansen, B, Basic Biotechnology, Cambridge University Press, 2001

22 BATCH Rationale: cell inoculum is added to the total final volume of medium. The inoculum size is relatively high ( 2x10 5 cell ml -1, or more are often used) Growth stops when a substrate is depleted or a by-product has reached inhibitory levels; Mammalian cells are routinely maintained in the laboratory by successive sub-cultures in stationary T-flasks (containing ml medium) with a large surface-to-volume ratio; Adherent cells attach to the bottom of the flask and further passages require that cells are detached by using trypsin; suspension cells attach more loosely and can often be removed by shaking the flask; Large-scale cultures: 200 L or less is often sufficient to satisfy the demand for high value therapeutic proteins.

23 BATCH (cont.) The scale-up factor from stationary cultures is not more than 5 (inoculum volume of at least 20%). In bioreactors, where higher cell densities are obtained, the scale-up factor can be up to 10; A typical batch culture of hybridoma cells lasts 3 to 5 days and reaches a cell density of 2-6x10 6 cells ml -1. The maximum specific growth rate of hybridoma cells is about 0.05 h -1. The amount of monoclonal antibodies produced in a batch culture of hybridoma reaches 1-4 g L -1. Early commercial production with adhrent cells was often performed in roller bottles. Typically a surface of cm 2 with ml medium will yield 1-2x10 8 cells.

24 FED-BATCH Objectives: maintenance of a high viable cell concentration; maintenance of nutrient concentrations, often prolonging culture time and thus increasing final product concentration; achievement of energetically more efficient metabolic rates in the presence of an overflow metabolism; accumulation of both cell and product concentrations at high levels and more flexibility in industrial processes. Although a glucose- and glutamine-limited fed-batch culture solves the problem of overflow metabolism in animal cells, it is not enough to obtain a substantial increase in cell density; By feeding a balanced mixture of nutrients, both the cell density and the product titre can be improved more than 10-fold as compared to batch cultivation; Processes up to 15 m 3 have been described. Cell densities of around 1 to 1.4x10 7 viable cell ml -1 have been reported for fed-batch processes.

25 CONTINUOUS-FLOW Rationale: continuous addition of fresh medium and withdraw of spent medium, keeping the culture volume constant (production processes with up to 2 m 3 reactor volume have been described). Animal cell cultures contain multiple carbon and nitrogen sources, so it is difficult to establish steady-state growth limited to a single nutrient, contrarily to microbial systems. Therefore growth of animal cells is likely to result in multiple nutrient limitation. Disadvantages: long duration of a culture (at least 5 weeks) which the contamination risk; increase validation has to include proof that the cell line is stable over the cultivation period.

26 PERFUSION Rationale: cells are retained within the reactor via a retention device, while fresh medium is introduced and spent medium removed Cell densities up to 3x10 7 cells ml -1 and product titres an order of magnitude higher than in batch cultures can be achieved; Devices to separate cells from the culture fluid can be placed inside or outside the reactor. Spinner-filter devices make use of a rotating cage of wire mesh with pores of 5 to 75 µm; Membrane filters (hollow fibers) can be used for separation of cells from the culture fluid; Settling devices, using slightly higher density of cells to separate the cells have been developed; Source: Centrifugation as a means of cell retention has been applied to large-scale processes. Fed-batch and perfusion cultures are the two dominant modes of operation for animal cell culture processes

27 Cell Growth and Death Rates Free cells in suspension Balance on total cells: rate of accumulation of total cells = = rate at which cells are added in the feed - - rate at which cells are removed in the outlet + + rate of cell growth - - rate of cell lysis Balance on total cells for a sterile feed: d(nv)/dt = 0 - nf o + µ app nv, (eq. 1) Schematic diagram of a continuous-flow stirred suspension vessel. For fed-batch culture F o = 0 and for batch culture F o = F i = 0. n = total cell concentration, V = vessel volume µ app = apparent specific growth rate (includes cell growth and lysis) F i = volumetric flow rate of fresh medium F o = volumetric flow rate of spent medium (including suspended cells)

28 Cell Growth and Death Rates Free cells in suspension (cont.) Only viable cells can divide and viable cells are not directly lysed (assumption): µ app n = µn v - k l n d (eq. 2) µ= true specific growth rate n v = viable cell concentration n d = dead cell concentration k l = dead cell specific lysis rate Note: Cell death converts a viable cell into a dead cell, not changing the total cell concentration. Viable cell balance: (from eq. 1) d(n v V)/dt = 0 - n V F o + µ app n v V (eq. 3) µ app = viable cell-derived apparent specific growth rate: µ app n v = µn v - k d n v (eq. 4) k d = specific death rate

29 Cell Growth and Death Rates Free cells in suspension continuous culture If V = constant: F i = F o = F, and eq. 1 (d(nv)/dt = 0 - nf o + µ app nv) becomes: dn/dt = - nd + µ app n (eq. 5) D = dilution rate = F/ V General expression for µ app µ app = d ln(n)/dt + D (eq. 6) If steady state: µ app = D If lysis is negligible (low k l ): (from eq.2) µ = µ app (n/n v ) = µ app /f v (eq. 7) µ diverges from µ app as the viability decreases Cell death rate, k d : k d = µ - D - d ln(n v )/dt (eq. 8) High dilution rates Low k d

30 Cell Growth and Death Rates Free cells in suspension batch and fed-batch Batch Culture: F i = F o = F = 0 If lysis is negligible and setting D = 0: µ app = d ln(n)/dt µ= µ app /f v k d = µ - d ln(n v )/dt (eq. 9) Fed-batch Culture: F i 0; F o = 0 (V not constant) µ app = d ln(nv)/dt µ app = d ln(n v V)/dt (eq. 10) V(t) = F i dt Low k l : µ = µ app /f v From equations 4 and 10: k d = µ d ln(n v V)/dt (eq. 11) (valid for periodic and continuous medium additions)

31 Cell Growth and Death Rates Free cells in suspension continuous with retention Continuous culture limits cell concentration by nutrient supply: Higher cell concentrations achieved with retention of cells and increase of medium perfusion rate (in general a purge stream is used to allow stable operation at an elevated cell concentration). Cell retention systems: spin filters, settling columns and recirculation loops containing ultrafiltration membranes. Schematic diagram of a continuousflow stirred suspension vessel with cell retention. The distribution of outlet flow between F 1 and F 2 is varied to maintain the desired cell concentration in the reactor.

32 Cell Growth and Death Rates Free cells in suspension continuous with retention Total cell concentration in the outlet stream: n o = α s n Assumption: α s (viable cells) = α s (non-viable cells) Retention fraction (fraction of cells retained in the bioreactor): 1 - α s. Note: For the system shown in Figure 6 α s = F 2 /F o If V = constant: F i = F o = F, and the total cell balance becomes: If steady state: µ app = α s D < D dn/dt = - α s nd + µ app n (eq. 12) Replacing D with α s D in equations 6-8: µ app = d ln(n)/dt + α s D µ= µ app /f v k d = µ - α s D - d ln(n v )/dt (eq. 13) If α s (viable cells) α s (non-viable cells): dn/dt = - (α sv n v + α sd n d )D + µ app n 1 - α sv = retention ratio for viable cells 1-α sd = retention ratio for non-viable cells

33 Cell Growth and Death Rates Microcarrier Cell Culture Microcarrier culture: perfusion or batch systems PERFUSION SYSTEMS: Microcarriers are retained; free cells exit in outlet stream. Balance on total cells (V=constant): dn/dt = -n F D + µ app n (eq. 14) n A = concentration of attached cells n F = concentration of free cells n = n A + n F If attached cells are all viable (present predominantly as a monolayer on the bead), comes n v = n A + f Fv n F (f Fv = viable fraction of free cells): or : µ app n = µ A n A + µ F (f Fv n F ) (eq. 15) µ A = µ app (n/n A ) - µ F f Fv (n F /n A ) n A (eq.16) µ app is given by eq. 14 as: µ app = d ln(n)/dt + D (n F /n) (eq. 17)

34 Cell Growth and Death Rates Microcarrier Cell Culture (cont.) Perfusion systems: Free cells are removed with the outlet medium: n F << n Balance on adherent cells: dn A /dt = µ appa n A = µ A n A - k R n A (eq. 18) µ appa = apparent specific growth rate of adherent cells k R = rate of cell release from the beads: k R = µ A - d ln(n A )/dt (eq. 19) Batch systems: Equations valid with D = 0 Contribution from free cells in equations 14 and 15 is greater because free cells are not removed from the system, leading potentially to aggregate formation, which further complicates analysis.

35 Cell Growth and Death Rates Cell aggregates and spheroids The situation is much more complicated for multilayers of cells on microcarriers or for cells that grow in clumps or spheroids. The specific growth rate in such systems is normally function of cell location. For example, cells at the outside of tumor cell spheroids generally proliferate rapidly, while those further in are quiescent (i.e. not dividing); In these systems it is only possible to determine apparent growth rates. These can be expressed in terms of total cell numbers, viable cell numbers or spheroid volume.

36 TIME COURSE of animal cell growth, nutrients consumption and product formation Source: Ratledge, C, Kristiansen, B, Basic Biotechnology, Cambridge University Press, 2001

37 Kinetics of Metabolism and Product Formation Mass balance on the concentration of a dissolved metabolite M: d(mv)/dt = F i M F F o M + q M n v V (eq. 20) M F = concentration in the feed stream M = concentration in the vessel q M = metabolic quotient, i.e. the net specific formation rate of M (mol M time -1 viable cell -1 ) Note: Equation 20 applies for reactors with cell retention because small metabolites are not retained along with the cells. However, the relation must be modified if product proteins are (partially) retained in the vessel. If V = constant: q M = [D(M M F ) + (dm/dt)] / n v (eq. 21) Nutrients: q M < 0 Products: q M > 0

38 Metabolic quotients Dissolved substrates and products Specific substrate consumption rate (q S ) for substrates: q S = [D(S F - S) - (ds/dt)] / n v (eq. 22) Specific product formation rate (q P ) for products: q P = [D(P P F ) + (dp/dt)] / n v (eq. 23) For batch systems, comes: For fed-batch systems, comes: q S = - (ds/dt) / n v (eq. 24) and q P = (dp/dt) / n v (eq. 25) q S = F i S F - [d(sv) /dt)] / n v V (eq. 26) and q P = - F i P F + [d(pv) /dt)] / n v V (eq. 27)

39 Metabolic quotients Oxygen Oxygen requirements for animal cell range from molo 2 /(h.cell) (less than oxygen requirements for plant and microbial cells); Typical values of k L a range from 5-25 h -1 for mammalian suspension cultures. Table 2. Overall transfer coefficients for oxygenation using different aeration systems Overall transfer coefficients for oxygenation using different aeration systems. Aeration system kla(s -1 ) Static liquid surface 10-6 Stirred liquid surface x 10-3 Silastic membrane aeration Sparged with agitation 7 x Gas exchange impeller 4 x x 10-2 Source: Doyle, A, Griffiths, J.B, Cell and Tissue Culture: Laboratory Procedures in Biotechnology, Griffiths, John Wiley & Sons Ltd, 1998

40 Metabolic quotients Oxygen (cont.) Specific oxygen consumption rate (q O2 ): q O 2 = {D (C O2F - C O2 ) + k L a[(p O2 /H) - C O2 ] - [d (C O2 )/dt]}/ n v (eq. 28) C = dissolved O O 2F 2 concentration (mm) in the vessel C = dissolved O O 2 2 concentration (mm) in the feed stream = oxygen partial pressure (mm Hg) in the gas phase P O 2 H = Henry s law constant (~760 mm Hg) for oxygen in culture medium at 37ºC k L a = volumetric oxygen mass transfer coefficient In most agitated vessels, the contributions from oxygen in the feed stream is negligible, yielding: q O 2 = {k L a[(p O2 /H) - C O2 ] - [d (C O2 )/dt]}/ n v (eq. 29)

41 Metabolic quotients Oxygen (cont.) Table Reported 3. Reported oxygen oxygen consumption consumption rates rates of cells of cells in culture in culture. Cell Type Specific Oxygen consumption rate (µm h cells) CHO 0.15 BHK Human foreskin (FS-4) 0.05 Human granulocytes Murine hybridoma Murine/human hybridoma Murine myeloma Murine macrophages Vero 0.24 Source: Doyle, A, Griffiths, J.B, Cell and Tissue Culture: Laboratory Procedures in Biotechnology, Griffiths, John Wiley & Sons Ltd, 1998

42 Metabolic Yield Ratios Apparent yield, Y P,S : Y P,S = q P /q S (eq. 30) Apparent yield of lactate from glucose, Y lac,glu (estimative of the fraction of glucose converted to lactate via glycolysis): Y lac,glu = q lac / q glu (eq. 31) Note 1: This is an apparent yield because lactate can be produced via metabolism of other substrates such as glutamine, and because pyruvate derived from glycolysis can be converted into other compounds such as alanine. Note 2: The theoretical maximum yield is 2 because no more than two molecules of lactate can be obtained from a single molecule of glucose. However, production of lactate from glutamine may result in yields greater than 2. Cell yield on substrate, Y n, S : Y n, S = µ/q S (eq. 32) µ = specific growth rate q S = specific consumption rate for substrate S

43 Metabolic Yield Ratios Product Formation Specific product formation rate, q P,, is the characteristic parameter for product formation During batch growth, frequently q P constant, e.g. monoclonal antibody production by hybridoma cells. Assumption: q P does not vary with time, then eq. 23 can be integrated to yield: P = q P 0 t n v dt (eq. 33) If V = constant and product is retained: (1-αP) = retention fraction of P q P = [D(α P P - P F ) + (dp/dt)]/ n v (eq. 34)

44 Metabolic Yield Ratios Immobilized Cell Reactors Cell density estimated using metabolic quotients Volumetric production rate determined from the metabolite concentration versus time data: Q M = D (M F - M o ) (eq. 35) M o = metabolite concentration in the outlet stream M F = concentration in the feed stream to the reactor (M is often oxygen or glucose). Q M and q M are related by: q M = Q M / n v (eq. 36)