Cell binding of purified HA.hIgG proteins. (A) Diagram of HA.hIgG constructs. SP, signal peptide. (B) Coomassie staining of purified HA.hIgG proteins resolved by SDS-PAGE. (C) Binding of the purified HA.hIgG to 293T cells. Different amounts of proteins were used to bind 293T cells and the binding was measured by flow cytometry as described in Materials and Methods. Relative mean fluorescent intensity (MFI) was calculated by deducting MFI of negative control (no purified protein added) from that of cells incubated with purified proteins. (D) Binding of 2 μg purified proteins to three different types of cells.