Viral culture.

Viruses were serially passaged in MDCK cells (2.5 × 10⁵ cells/well). The multiplicity of infection (MOI) for passages was 0.01 except for late passages in the first experiment, for which output virus was low and the MOI was adjusted to accommodate. Trajectories were prepared both in the presence and absence of escalating concentrations of F10 antibody or equivalent concentrations of the control monoclonal antibody 80R. In passage 4, the antibody concentration was 1× the EC₅₀. For the next passage, the concentration was increased to 2× the EC₅₀ and then doubled for each subsequent passage as long as >50% cytopathic effect (CPE) was present. If <50% CPE was present, the concentration of antibody was escalated at a lower rate.