The cytotoxicity of anti-CD22 immunotoxin is enhanced by bryostatin 1 in B-cell lymphomas through CD22 upregulation and PKC-ßII depletion.

ABSTRACT

BACKGROUND: In spite of potent first-line therapies for chronic lymphocytic leukemia, treatment remains palliative and all patients frequently relapse. Treatment options for these patients are more limited. BL22 is a recombinant protein composed of the variable region of a monoclonal antibody that binds to CD22 and of PE38, a truncated Pseudomonas exotoxin. BL22 is a very potent drug already used in patients with hairy cell leukemia, whereas in chronic lymphocytic leukemia its cytotoxicity is limited by a lower expression of CD22. Here we demonstrate that this limitation can be overcome by pre-activation of chronic lymphocytic leukemia cells with bryostatin 1. DESIGN AND METHODS: Primary malignant B cells from chronic lymphocytic leukemia and mantle cell lymphoma patients were used in vitro to assess the therapeutic impact of drug combinations using BL22 and bryostatin 1. RESULTS: We demonstrate that bryostatin 1 sensitizes chronic lymphocytic leukemia cells for the cytotoxic effects of BL22 through activation of protein kinase C and subsequently increased CD22 surface expression. Dose and time response analysis reveals that activation of protein kinase C further activates an autocrine feedback loop degrading protein kinase C-ßII protein. Depletion of protein kinase C-ßII and upregulation of CD22 persist for several days following pre-stimulation with bryostatin 1. Therefore, our data provide a rationale for the sequential administration of BL22 following bryostatin 1 treatment. In addition to primary chronic lymphocytic leukemia cells, bryostatin 1 also sensitizes diffuse large B-cell lymphoma and mantle cell lymphoma cells to BL22 induced apoptosis. CONCLUSIONS: Our data suggest that the combination of bryostatin 1 with antibodies directed against CD22 is a potent drug combination for the treatment of low- and high-grade B-cell lymphoma.