basics of cell culture part 2: choosing a host cell
TRANSCRIPT
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Basics of Cell Culture
Part 2:
Choosing a Host Cell
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Overview
• Kinds of cells used in industry– Advantages– Disadvantages
• Large scale cell growth issues– Sterility– Cleanliness– Consistency
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Cells Used in Bioprocessing
• Different options
• Choice based on many variables
• No perfect single choice– Bacterial cells– Yeast cells– Insect cells– Mammalian cells
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Industrial Bacteria
• Gram negative bacteria– E. coli– Produce lipopolysaccharide (an endotoxin)
• Gram positive bacteria– B. subtillus– No endotoxins
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Industrial Yeast
• Eukaryotic• S. cerevisiae
– Brewer’s and baker’s yeast– Alcohol dehydrogenase system
• P. pastoris– Methanol producer– High level of protein synthesis– High cell density
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Microbial Advantages
• Simple media
• Fast growth rates
• Sturdy cells– Cell walls outside plasma membrane– Faster mixing possible– Faster sparging rate
• Long history of large-scale use
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Microbial Disadvantages
• Potentially infectious
• Endotoxin (with some cells)
• Sturdy cells– Harder to break open– Harder to purify internal protein
• Simpler post-translational modifications
• Bacterial inclusion bodies
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Protein Modifications
• Functional groups added to proteins
• After translation is done
• Increase functionality of protein– Membrane association– Extra-cellular protection– Activity regulation
• Limited in bacteria, extensive in mammals
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Inclusion Bodies
• Bacterial defense– Normally have balanced protein profile– Too much of one protein is “bad”
• Protection by isolation– Excess protein stored in aggregation– Too much for cell to process
• Active protein may be recoverable
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Renaturing Inclusion Bodies
• Lyse cells
• Separate soluble from particulate
• Add strong protein denaturant– Aggregate held by hydrophobic interactions– Denaturant breaks hydrophobic bonds– Renature protein into proper conformation– Optimization can be difficult
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Mammalian Cells
• Various species used– Monkey– Rat– Mouse– Hamster
• Various cell types used
• Each originally from a single animal
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Primary Cell Culture• Obtain source of cells (organ, tissue, etc.)• Dissociate cells
– Scissors– Digestive enzymes
• Grow on treated plates• Isolate cell of interest
– Fastest growing cells?– Selective conditions for cell type?– Divide cells as they fill up surface of plates
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Mammalian Cells- Live or Die?
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Transformation
• Different from bacterial “transformation”
• De-differentiation– Anchorage dependence may be lost– Lower serum requirement for growth– Loss of normal behaviors or functions– Immortality
• Induced by some chemicals or viruses
• Spontaneous in some cell types
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Cell Characteristics
• Normal mammalian– Surface attachment*– Contact inhibition– Require growth stimuli– Mortal– Non-tumorigenic– “Typical behaviors”
• Transformed cells– May not require
surface– Overgrow monolayer– Independent of stimuli– Immortal– May be tumorigenic – Loss of “typical
behaviors”
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Mammalian Cell Culture
• Mammalian cells used industrially are transformed– Continuous cell lines– Immortal– Usually suspension culture
• Chinese Hamster Ovary (CHO)– Surface or suspension growth– Versatile synthetic machinery– Intracellular or extracellular proteins
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Mammalian Disadvantages
• Viruses
• Mycoplasma
• Slow growth rate
• Serum costs
• Risk of genetic instability
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Mammalian Advantages
• Good at making mammalian proteins– Secretory signals– Post-translational modifications
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“Fermentor” vs. “Bioreactor”
• Microbial cells grown in “fermentor”– Original stirred tank reactor (STR)– Generally use rapid stirring
• Mammalian cells grown in “bioreactor”– Same general structure– Gentler mixing since cells more fragile– Size range more limited
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Review
• Many cell options are available
• Choose on cost and usability• Media/growth expense• Ability to make protein properly
• Balance advantages vs. disadvantages– Bottom line is useful product– Minimize cost but make product