ABSTRACT
Mammalian cells, like bacterial hosts, can be engineered to produce various types of recombinant proteins including secreted proteins, such as monoclonal antibodies or growth factors; intracellular proteins such as enzymes; and membrane proteins such as growth factor receptors. Mammalian expression systems offer numerous advantages over the use of bacteria, yeast, or insect cells for the production of complex recombinant proteins of human or mammalian origin. Mammalian cells can generate fully functional mammalian recombinant proteins including correct folding and proper posttranslational modifications such as disulfide bond formation, prenylation, carboxylation, and phosphorylation. Of special note is the capacity for certain mammalian cell lines to perform glycosylational modifications very similar to those obtained in humans. As a result, secreted and membrane glycoproteins generated by mammalian cells are not recognized as foreign by the human immune system and will remain in the circulatory system for extended periods. In addition, mammalian cells readily secrete some recombinant proteins, allowing for ease of isolation of the product from the surrounding culture medium. In contrast, many bacterial systems require intracellular expression and the attendant purification and refolding of proteins from cell lysates. Regardless of the type of recombinant protein being produced, expression is dependent upon multiple factors. These include factors at the level of the DNA, including gene copy number or insertion site in the genome, at the level of RNA , such as mRNA processing and stability, and polypeptide considerations, such as folding, transport, processing, and stability. In order to generate the product of interest, many facets of cell line development can be considered, including vector design, the use of
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inducible systems, type of mammalian cell lines, transfection methods, transient vs. stable transfection, selection, and single cell cloning. These facets are discussed in greater detail later.
