Poster Sessions

CANCER-10

Johanna Hansen

J. Hansen, J. Weldon, L. Xiang, R. Beers, M . Onda, I. Pastan

A Recombinant Immunotoxin Targeting CD22 with Low Immunogenicity, Low Nonspecific Toxicity, and High Anti-tumor Activity

Recombinant immunotoxins (RITs) are genetically engineered proteins designed to kill cancer cells. Immunotoxin HA22 contains the Fv portion of a cancer-specific antibody (anti-CD22) fused to a 38 kDa fragment of Pseudomonas exotoxin A (PE38). Because PE38 is a bacterial protein patients treated with it frequently produce antibodies that neutralize its activity, preventing re-treatment. PE38 contains seven major B-cell epitopes located in domains II and III. We describe here a new mutant RIT, HA22-LR-6X, in which we removed most B-cell epitopes by deleting domain II and mutating six residues in domain III. HA22-LR-6X is cytotoxic to lymphoma cell lines and has low non-specific toxicity in mice. To assess immunogenicity three MHC-divergent strains of mice were immunized with 5μg doses of HA22-LR-6X. We found that HA22-LR-6X elicited significantly lower antibody responses than HA22, or other mutant RITs with fewer epitopes removed. Furthermore, large (50μg) doses of HA22-LR-6X induced lower antibody responses than 5μg of HA22, indicating that high doses can be administered with low immunogenicity. Thus, we have removed B-cell epitopes from PE38, producing a highly active immunotoxin with low immunogenicity and low animal toxicity. It will be important to determine if these properties carry over to cancer patients.