CLONING, EXPRESSION AND CHARACTERIZATION OF THE FV FRAGMENTS OF THE ANTICARBOHYDRATE MABS B1 AND B5 AS SINGLE-CHAIN IMMUNOTOXINS

The mAbs B1 (IgG1 kappa) and B5 (IgM kappa) recognize carbohydrate epi topes on human carcinoma cells. The Fv regions of these antibodies wer e separately cloned from hybridoma RNA using reverse transcription and PCR with oligonucleotide primers designed according to the amino acid sequences of the N-termini. The Fv regions also provide sequences for translation initiation in Escherichia coli (Fr1 oligos) and sequences of the constant region of the heavy and light domains (CH1 or C-kappa oligos). Following the determination of the DNA sequence of the Fvs, primers were designed according to the 3' ends of the V-H and V-L doma ins. These also provided for a peptide linker at the C-terminus of the V-H and a short connector at the C-terminus of the V-L (Fr4 oligos). The V-H and V-L were then each PCR-amplified using their corresponding Fr1 and phosphorylated Fr4 oligos. The resulting PCR products were an nealed as 'mutagenic primers' to a uracil-containing single-stranded t emplate obtained from an expression plasmid encoding a single-chain im munotoxin in which the B3 single-chain Fv is fused to a truncated form of Pseudomonas exotoxin (PE). Thus, the B1 and B5 variable domains re placed their corresponding B3 domains in the expression plasmid by 'va riable domain shuffling' without subcloning. The resulting B1(Fv)-PE38 and B5(Fv)-PE38 were expressed in E. coli and purified to near homoge neity. Both show specific cytotoxicities to human carcinoma cell lines , but B1(Fv)-PE38 is much more active, reflecting its higher affinity to the target cells.