Type I mAbs internalize in human cell lines and traffic to the lysosome. (A) A selection of NHL cell lines (Raji, Daudi, SU-DHL-4, DOHH2, Ramos, Granta-519, RL, and HBL-1) was treated with rituximab–Alexa 488 (Ritux-488) or Tosit-488 (5 μg/mL) for 2, 6, or 24 hours and then assessed for internalization as before. *At each time point, the level of surface-accessible CD20 with Ritux-488 was significantly lower than with Tosit-488: P < .001 at each time point. (B) Appearance of Raji B cells treated with Ritux-488 or Tosit-488 (5 μg/mL) for 24 hours in vitro and assessed by confocal microscopy using an HCX PL APO CS 100×/1.4 lens with 1× optical zoom (Rit-488) and 1.5× optical zoom (Tosit-488) with a Z section through the center of the cell and overlaid versus the bright field (Bf). (C) Colocalization of Ritux-488 with transferrin-647 or Lysostracker on Raji cells.