FIGURE 4.

D132F and D132Y, but not R206E, triggered TACI internalization on endogenously expressed receptors. Cells were stained with a PE-coupled anti-TACI antibody and incubated with the indicated ligands for 1 h at 37 °C to allow receptor internalization. Subsequently, cells were placed on ice to halt membrane movements and then treated with either PBS (control) or acid solution (pH 2) to strip off labeled receptors that were not internalized. A, example of FACS profile of the TACI internalization for A20 cells. The high PE signal that remained after acid treatment (marked boxes) reflects TACI being internalized and protected inside cells. B, quantification of A expressed as percentage of APRIL-induced TACI internalization. C, quantification of TACI internalization for human Raji cells.