UNIQUE PROPERTIES OF HFBRS

There are four fundamental characteristics that differentiate hollow fiber cell culture from classic suspension culture:

• Cells are bound to a porous matrix much as they are in vivo.
• Cells are cultured in a gently perfused, non-shear environment.
• The molecular weight cut off of the support matrix can be controlled.
• The system possesses an extremely high surface area to volume ratio (150 cm² or more per mL).

Cells are bound to a porous support much as they are in vivo .

Cells are cultured in a gently perfused, non-shear environment.

Unlike suspension culture, the cells are constrained within a non-shear compartment with gas, nutrition and waste transport moving across the fibers. Lack of shear simplifies the medium composition, and perfusion permits a more homeostatic, in vivo-like environment for the cells. This can result in improved protein translational fidelity. These growth conditions and lack of shear also result in significantly reduced apoptosis (3). The majority of cells that become necrotic will not release host cell proteins or DNA into the culture medium, resulting in a product that is cleaner and easier to purify from the bulk harvest

The molecular weight cut off of the fiber can be controlled.

Desired products can be retained to 10–100 times higher concentrations. Toxic or unstable products can be selectively removed, and the effects of cytokines controlled. An example of this is hybridoma culture, in which the inhibitory cytokine transforming growth factor beta can be selectively removed from the culture, while the secreted antibody is retained.

The system possesses an extremely high surface area to volume ratio.

HFBRs have an extremely high surface area to volume ratio (in the range of 100–200 cm² /mL of volume). Coupled with the high gross filtration rate of FiberCell Systems' polysulfone fibers, the exchange of nutrients and waste products is rapid. Cell densities of 1–2 × 10⁸ or more are achieved, close to in vivo tissue-like densities. A module with a volume of 1 L would support as many cells as a 100 L stirred tank, as well as providing continuous production. High cell densities support higher volumetric productivities, and facilitate adaptation to lower serum concentrations or serum free culture. In fact, a simplified, protein-free, chemically defined, animal component-free serum replacement, CDM–HD has been developed by FiberCell Systems and is optimized for use in hollow fiber systems. The use of a protein-free medium such as CDM–HD provides cleaner harvests of product, easier regulatory compliance and simplified downstream purification.

The more in vivo-like cell culture conditions can also result in improved protein folding and more uniform glycosylation patterns over time. Since it is a continuous perfusion system, the amount of protein produced is determined as much by the length of time the culture is maintained as by such parameters as the clone's specific productivity or the size of the cartridge.

The combination of an unlimited nutrient supply and the ability to de-bulk the culture through the cartridge ports allows the system to be maintained at relative equilibrium for several months or longer. This continuous production over long periods of time, rather than the severely batch-style results from other systems, provides several benefits, including: consistency in culture condition, dramatically increased production per unit footprint and culture volume, continuous or daily product harvest allowing timely and convenient stabilizing treatment or storage conditions, and products that might be toxic or inhibitory to cells can be selectively removed from the culture.