Efficient production of a functional mouse/human chimeric Fab′ against human urokinase-type plasminogen activator by Bacillus brevis

Expression/secretion vectors for the production of Fab′ and single-chain (sc) Fab′ by Bacillus brevis have been constructed. For the production of Fab′, the cDNAs encoding the L chain and Fd′ fragment (Fd with the hinge region) of a mouse-human chimeric Fab′ against human urokinase-type plasminogen activator were fused directly with the translation-start and signal-peptide-encoding regions of the mwp gene, the gene for one of the major cell-wall proteins of Bacillus brevis. The two fused genes were placed tandemly downstream from the promoter of the cell-wall protein gene operon (cwp) of B. brevis. For the production of scFab′, the two cDNAs were linked with a synthetic oligonucleotide encoding a flexible peptide linker of 17 or 24 amino acids, and fused with the translation start and signal-peptide-encoding regions of the mwp gene. Fab′ was efficiently produced by B. brevis, being accumulated at a level of 100 mg/l in the culture medium in a simple shake-flask culture, which is the highest level obtained so far for a gram-positive bacterium. On the other hand, the scFab′ remained at a level of a few milligrams per liter in the culture medium. The Fab′ produced by B. brevis showed comparable antigen-binding activity to that of the parental antibody. The L chain and Fd′ fragment, constituting the Fab′, had the correct N-terminal amino acid sequences. These results indicate that B. brevis is a very promising host for the production of native Ig fragments.