Background: CD33, the target for the anti-leukemia immunoconjugate, gemtuzumab ozogamicin (GO; Mylotarg), is a transmembrane glycoprotein that contains two cytoplasmic immunoreceptor tyrosine-based inhibitory motifs (ITIMs). Although we have previously shown that disruption of the ITIMs prevents effective uptake of antibody-bound CD33 and significantly reduces GO-induced cytotoxicity, the mechanisms underlying this uptake and intracellular trafficking of the antibody-CD33 complex are not known. In this study, we tested whether CD33 is a target for monoubiquitination, a posttranslational modification that marks proteins for lysosomal degradation, and how this modification relates to tyrosine phosphorylation and endocytosis.

Methods: CD33 Jurkat and 32D cells transduced with a lentiviral vector expressing either wild-type or mutant CD33 were analyzed for CD33 expression as well as internalization of anti-CD33 antibody by flow cytometry. Pull-down experiments were performed with glutathione S-transferase (GST) proteins fused to phosphorylated cytoplasmic tails of CD33 or TKB domain of Cbl/Cbl-b, using human myeloid cell lysates. For co-immunoprecipitation experiments, constructs encoding wild-type and mutant CD33, ubiquitin, Cbl/Cbl-b, and wild-type Fyn were transfected into HEK293T cells.

Results: In engineered Jurkat cells, treatment with either anti-CD33 antibody or the tyrosine phosphatase inhibitor pervandate increased tyrosine phosphorylation of CD33. Pervanadate enhanced uptake of antibody-bound CD33; this effect was dependent upon the integrity of the ITIMs and was prevented by co-treatment with the Src tyrosine kinase inhibitor PP2. CD33 interacted with the TKB domains of Cbl and Cbl-b in GST fusion protein pulldown assays. Similarly, Cbl and Cbl-b were co-immunoprecipitated with CD33 in transfected 293T cells. Experiments in 293T cells further showed that CD33 is monoubiquitinated in an ITIM-dependent manner. Cbl or Cbl-b significantly increased the amount of CD33-associated ubiquitination; Cbl/Cbl-b-dependent ubiquitination could further be enhanced by co-transfected Fyn. Finally, 32D cells expressing lysine-to-arginine mutants of CD33 displayed much higher levels of surface CD33 but had reduced internalization of antibody-bound CD33 compared to cells expressing wild-type CD33, consistent with a reduced internalization/degradation of the lysine-to-arginine mutants.

Conclusion: These studies indicate that Src-family kinase dependent phosphorylation favors internalization of antibody-bound CD33 and identify Cbl family proteins as potential binding partners of CD33. Importantly, CD33 is a target for ITIM-dependent monoubiquitination, and Cbl family proteins can act as an E3 ligase in this reaction. Our data therefore suggest a model where lysosomal routing and degradation of antibody-bound CD33 is secondary to Src-family kinase-induced phosphorylation of CD33 with subsequent phosphotyrosine-dependent binding of Cbl to and monoubiquitination of CD33.

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