PE38 is a 38-kDa derivative of the 66-kDa Pseudomonas
exotoxin (PE) in which the cell binding domain of PE (domain Ia,
amino acids 1-252) and a portion of domain Ib (
amino acids 365-380) are deleted. The
immunotoxins LMB-1 and
LMB-7 contain PE38 and kill
cancer cells by exploiting the cytotoxic action of PE38. The major human
B cell epitopes of PE38 were mapped by measuring the reactivity of 45 serum samples from patients treated with the PE38-containing
immunotoxins LMB-1 or
LMB-7 to two panels of overlapping synthetic
peptides representing the sequence of PE38. One panel of
peptides is ten
amino acids long and overlap by seven
amino acids, and the second panel of
peptides is twenty
amino acids long and overlap by ten. Five major
epitopes were identified:
amino acids 274-283, 470-492, 531-540, 555-564, and the C-terminal
amino acids 596-609. Two minor
epitopes were identified as well:
amino acids 501-510 and 582-589. These
epitopes are predominantly located on the surface of the
protein. The
amino acids believed to be critical for binding are highly
solvent-accessible residues. The results of the human antibody response to
peptides are compared to the pattern of reactivity previously identified with serum samples obtained from monkeys administered LMB-1 and
LMB-7. The
epitopes between monkey and human are almost identical, demonstrating similarity in the response of antibody repertoires between the two species and providing further support that these are the
immunodominant epitopes. This information is critical for genetically engineering less immunogenic
immunotoxins and provides a foundation for the development of a
vaccine against pseudomonal
infections which
plague immunocompromised individuals and individuals with
cystic fibrosis.