Selection of Mammalian Cell Lines for Expression

    Mammalian cells serve as a system for expressing human genes. The advantages of mammalian cells include promoting proper protein folding and providing accurate post-translational modifications, such as glycosylation and acetylation. They can secrete mature proteins into the extracellular space by incorporating signal peptides, and high-activity proteins can be easily obtained through affinity purification. Recombinant proteins produced using mammalian cell expression systems are widely used in the production of therapeutic recombinant proteins because their tertiary structure, post-translational modifications, physicochemical properties, and biochemical activity more closely resemble those of natural higher-order biological protein molecules. Depending on the temporal and spatial differences in target protein expression, mammalian cell expression systems can be classified into transient, stable, and inducible expression systems.

    Since mammalian cell expression requires stable, continuous cell growth and the expression of target genes can be achieved by introducing foreign genes, suitable cells must be obtained from animal tissues through cell-based panning and culture optimization. Commonly used cell lines include HEK293 (human embryonic kidney cells) and CHO (Chinese hamster ovary cells), both of which utilize suspension cell culture and can produce several grams of protein within a month. However, mammalian expression systems also have some drawbacks, such as high experimental costs, long expression cycles, and technical challenges. The main issues with expressing recombinant proteins in mammalian cells are low transfection efficiency and low protein levels. Nevertheless, with advances in scientific research, HEK293 cells and CHO cells have been developed into effective transient and stable expression systems, respectively.

    For transient transfection, liposomes, calcium phosphate, or PEI are used as transfection agents. HEK293 cells are relatively simple to work with and offer the advantages of higher cell density and faster growth rates. They are easy to culture and transfect and are widely used in the production of proteins, vaccines, and anticancer agents. However, scaling up their cultivation is difficult. CHO cells, on the other hand, are fibroblasts—a type of non-secretory cell—that rarely secrete endogenous proteins, which facilitates the isolation and purification of the target protein. They can be used for the stable production of large quantities of recombinant proteins and are currently among the most widely used and successful cell lines for expressing exogenous proteins.