The Effect of pH and Ionic Strength on the Filtration of Bacteriophage φX-174 through a Commercial Virus Removal Filter

Many mammalian cell lines used to produce recombinant protein products naturally produce viruses that can pose a serious threat to human health. Effective virus clearance is a crucial part of bioprocessing, and virus filtration membranes are a major component of the overall virus removal strategy. These membranes require high levels of virus retention and high transmission of the desired protein product. Virus filtration complements other virus clearance steps and is thought to provide high virus removal based on a size-exclusion principle. The pores of virus filtration membranes are designed to allow antibodies (~8 nm) to pass through the membrane, while the viruses (≥20 nm) are retained. However, there is some evidence that virus retention may also be a function of solution conditions, suggesting that adsorptive phenomena may also play a role in virus retention. This presentation examines the effects of solution pH and ionic strength on the virus retention characteristics of a commercial virus removal filter (Millipore’s Viresolve NFP membrane). The data show that virus retention increases as pH decreases. The increase in retention at pH below the isoelectric point of the virus may be due to electrostatic attraction between the charged virus and the charged pores of the membrane. At higher pH, the membrane becomes more negatively charged, which may lead to a repulsive electrostatic interaction and a corresponding reduction in virus retention. Virus retention at low pH was also greater at lower ionic strength, consistent with the effects of electrostatic interactions on virus retention. These results provide important insights into the factors governing virus filtration membrane performance in bioprocessing applications.