Chinese Hamster Ovary (CHO) cells are a primary tool in biotechnology, serving the pharmaceutical industry for producing complex proteins. Isolated in the 1950s from a Chinese hamster ovary, these cells have become indispensable for generating therapeutic proteins such as monoclonal antibodies. Their widespread adoption stems from their adaptability and capacity to mimic human cellular processes, making them ideal for developing medicines.
Why CHO Cells Are Crucial in Biotechnology
CHO cells are the preferred mammalian cell line for biopharmaceutical manufacturing. They possess cellular machinery for complex post-translational modifications, particularly glycosylation. Glycosylation, the addition of sugar molecules to proteins, is essential for the stability, activity, and function of many therapeutic proteins, ensuring they are well-tolerated and effective.
The ability of CHO cells to perform human-like glycosylation reduces the risk of an immune response in patients. Unlike bacterial or yeast systems, CHO cells process proteins in a way that closely resembles human cells. This, coupled with their robustness in culture and regulatory acceptance, makes them the gold standard; approximately 70% of FDA-approved therapeutic proteins are produced using CHO cells.
How Cell Size Impacts CHO Cell Performance
The size of CHO cells directly influences their performance in large-scale bioreactors, affecting various bioprocess parameters. Larger cells may exhibit higher protein productivity per cell, but this can also lead to increased nutrient consumption and greater susceptibility to stress. Studies have shown that specific monoclonal antibody productivity can be directly proportional to cell volume, with larger cells (e.g., 17 µm in diameter) demonstrating higher productivity compared to smaller ones (e.g., 12-14 µm).
Robustness, including viability and susceptibility to shear stress, is linked to cell size. Larger cells, lacking a rigid cell wall, are more fragile and prone to damage from bioreactor agitation and mixing forces. This stress can lead to cell lysis, impacting cell density and viability.
Cell size plays a role in downstream processing efficiency, involving protein purification. Filtration and centrifugation, common purification steps, are affected by cell size, as larger cells can alter separation efficiency. The surface area-to-volume ratio, influenced by cell size, affects nutrient uptake and waste removal efficiency. Larger cells may struggle to efficiently exchange nutrients and expel metabolic byproducts like lactate and ammonia, which can accumulate and negatively impact cell growth and protein quality.
What Influences CHO Cell Size
CHO cell size varies due to several factors during biopharmaceutical production. The cell cycle phase is a natural determinant; cells typically grow larger before dividing. For example, G1-arrested cells are often metabolically more active and larger, linked to increased recombinant protein production.
Culture conditions impact cell growth and size. Nutrient availability, including glucose and amino acids, affects cell metabolism and biomass accumulation. Environmental parameters such as oxygen levels, pH, temperature, and osmolality influence cell volume. Hyperosmolar conditions, often from nutrient additions and metabolite accumulation in fed-batch cultures, tend to increase cell size.
Genetic factors play a role; different CHO cell clones or engineered cell lines may exhibit distinct size profiles. The accumulation of intracellular metabolites or the recombinant protein product itself can affect cell volume. These internal and external influences drive changes in CHO cell size throughout a culture.
Techniques for Measuring and Managing CHO Cell Size
Scientists employ methods to quantify and manage CHO cell size. Flow cytometry is used for size determination, providing rapid, accurate measurements of individual cells. Automated cell counters, like the LUNA-FX7™, offer precise cell density and viability tracking, often including size analysis. Microscopy, particularly image analysis, determines the mean diameter of cells and aggregates, especially for suspension cultures.
Understanding factors influencing cell size allows for process adjustments to maintain desired ranges. Optimizing media formulations, by controlling nutrient concentrations and osmolality, can modulate cell size. Adjusting bioreactor agitation rates is another strategy, as excessive shear stress can damage cells and alter their size distribution. Modifying feeding strategies to prevent nutrient depletion or excessive metabolite accumulation can maintain optimal cell size and performance.