Seven sets of recombinant expression vectors were constructed for the expression of
the human anti-bovine serum albumin (BSA) single-chain Fv fragment (scFv) 13CG2 fused
to the C terminus of GFP(uv) or bombyxin (bx) signal peptide in the hemolymph and
fat body of silkworm larvae and pupae, using cysteine protease- (BmNPV-CP(-)), and
cysteine protease- and chitinase-deficient (BmNPV-CP(-)Chi(-)) bacmids. When BmNPV-CP(-)
or BmNPV-CP(-)Chi(-) bacmids were used, 16.9-18.9 mg/l (11.6-15.0 microg/larva) of
scFv was expressed at 6 d.p.i., whereas wild-type BmNPV bacmid expressed only 4.4
mg/l, probably because of proteolytic degradation of the protein. The scFv yield in
silkworm pupae was only 0.67-1.0 microg/pupa, which was 5.4% of that in the hemolymph
of silkworm larvae. The bx signal peptide enabled the secretion of scFv into the hemolymph.
Without the signal sequence, the fusion protein accumulated in the fat body and lost
its biological function. The removal of GFP(uv) significantly increased the scFv yield
in the hemolymph of silkworms to 188.4 mg/l (132.4 mg/larva), which was ten times
higher than that of the fusion protein.