ITIM-dependent internalization of antibody-bound CD33 in transduced cell lines. (A) 32D sublines, (B) OCI-AML3 sublines, and (C) KG-1a sublines, transduced with either wild-type CD33 or CD33 mutants, were labeled with unconjugated hP67.6 on ice water before the cells were incubated at 37°C in antibody-free medium to allow internalization of antibody-bound CD33 for 2 (shaded bars) or 4 hours (filled bars). Subsequently, remaining cell surface–associated hP67.6 was detected with biotin-conjugated mouse anti–human IgG₄ monoclonal antibody and a streptavidin-PE conjugate. Results are shown as mean ± SEM from 3 to 4 (for 32D and KG-1a sublines) or 2 (for OCI-AML3 sublines) independent experiments. n.d. indicates no internalization detected.