Parameters that determine virus adsorption kinetics: toward the design of better infection strategies for the insect cell - baculovirus expression system

High productivities of bioprocesses involving viruses can be attained through infection strategies based on adequate understanding of parameters ruling cell-virus interactions. Two factors that affect virus binding and infection efficiency were studied: the utilization of an adsorption step, where infection volume at constant cell/virus ratio was varied; and the concentration of fetal bovine serum (FBS). The insect cell-baculovirus expression system and recombinant protein VP4 of rotavirus were used as models. Virus binding kinetics were adequately described by a sigmoidal response curve. The adsorption step, with or without FBS, increased virus attachment rate, whereas it increased bound virus at equilibrium only in FBS-free infections. A first-order dependance of virus attachment on cell concentration was found above 5 × 106 cell/mL in infections with 10% FBS. Addition of 10% FBS decreased maximum bound baculovirus and binding rate by as much as 3 times and VP4 concentration up to 4 times. In contrast, heat inactivation of FBS increased bound virus from 20% to over 90%, an increase of 1.4 times compared to FBS-free infections. A direct linear relation was found between attached virus and maximum VP4 concentration for the different FBS concentrations tested, indicating that baculovirus-cell attachment was the limiting step for recombinant protein production. Interestingly, virus progeny accumulation was not affected by differences in virus binding. In conclusion, infection strategies aimed at increasing productivity should be performed at high cell concentrations and without FBS, or with heat-inactivated FBS.