ON THE INTERACTION BETWEEN SINGLE-CHAIN FV ANTIBODIES AND BACTERIAL IMMUNOGLOBULIN-BINDING PROTEINS

Using four bacterial immunoglobulin-binding proteins, we have analyzed the binding characteristics of a panel of 34 human single chain Fv antibodies, expressed in E. coli and with known specificity and sequence. Several of the single chain Fv antibodies showed affinity for staphylococcal protein A and peptostreptococcal protein L, but not for the streptococcal proteins G or H. The affinity of the binding was higher for protein L (4.5 and 1.4 x 10(9) M(-1)) than for protein A (7.7 and 6.7 x 10(8) M(-1)), using the two single chain Fv antibodies displaying the strongest binding activity to these ligands. The binding was shown to be specific by Western blotting, and the single chain Fv antibodies could be purified from crude bacterial culture media by affinity chromatography on protein L- or A-Sepharose. Protein A, which has affinity for the V-H domain of the scFv antibodies, was tested against scFv antibodies containing V(H)1, V(H)3, V(H)4 and V(H)5 domains, and its binding was restricted to approximately half of the scFv antibodies with a V(H)3 domain. Protein L, which has affinity for the V-L domain, was tested against kappa 1, kappa 4, lambda 1, lambda 2 and lambda 3 domains, and it bound all kappa 1 domains, one lambda 2 and one lambda 3 domain. Comparison of the amino acid sequences of binding and non-binding V-L domains demonstrated that amino acid residues crucial to the binding of protein L were distributed over a large area outside the hypervariable antigen-binding regions.