Effect of incubation or endocytosis/re-expression on sIgM signaling. Cells were either untreated (A-D) or treated (E-H) with polyclonal goat anti–L chain F(ab′)_2. The change in the ability of untreated cell populations to respond to ligation of sIgM was monitored over time in cases previously designated as (A,C,E,G) “responders” or (B,D,F,H) “nonresponders.” In parallel, (C-D) the level of expression of sIgM (MFI) was measured. Antibody-treated cell populations from “responders” or “nonresponders” underwent endocytosis of sIgM. (E-F) The ability of cell populations to reacquire signaling capacity and (G-H) sIgM expression was monitored. Unmutated cases are represented by open symbols; mutated cases, by filled symbols. Circled symbols in panels E-H indicate values at 72 hours for untreated cultures. Case numbers were as follows: “Responders” (A,C,E,G): 255 (□), 191 (▴), 269 (•), 273 (▪), 189 (♦), 231 (△), and 239 (▪). “Nonresponders” (B,D,F,H): 236 (•), 169 (large ♦), 159 (△), 153 (▪), 261 (▴), 201 (□), and 299 (small ♦).