Identification of Residues That Stabilize the Single-chain Fv of Monoclonal Antibodies B3

Abstract

B3(Fv)-PE38 is a recombinant single-chain immunotoxin in which the Fv portion of the B3 antibody in a single-chain form, which serves as the targeting moiety, is fused to PE38, a truncated form of Pseudomonas exotoxin A, which serves as the cytotoxic moiety. B3(Fv)-PE38 is specifically cytotoxic to many human cancer cell lines and is currently evaluated in a clinical trial. Monoclonal antibodies B3 (IgG1k) and B5 (IgMk) recognize related carbohydrate epitopes on human carcinoma cells. The Fv regions of these antibodies were previously cloned and expressed as the single-chain Fv-immunotoxins B3(Fv)-PE38 and B5(Fv)-PE38, respectively. The B3(Fv)-PE38 immunotoxin binds to antigen-positive cancer cells with a higher affinity than B5(Fv)-PE38 and is a more potent cytotoxic agent than B5(Fv)-PE38. However, it is less stable and rapidly aggregates upon incubation at 37°C. The V L domains of the two Fvs are very similar, differing by only three residues, the fourth and seventh Fr1 residues and the fifth CDR1 residue. The V H domains of the two Fvs vary considerably. To investigate whether any of the different V L residues may influence the stability of the B3(Fv), we constructed a chimeric immunotoxin containing the B3V H and the B5V L . This chimera had an improved stability and a higher apparent antigen binding affinity and cytotoxic activity when compared with B3(Fv)-PE38. Site-specific mutagenesis was used to show that the V L M4L mutation has an important role in stabilizing B3(Fv), although residues V L Ser-7 and V L Ile-28 also play a role in the increased stability. When tested in an in vivo model system, the chimera containing the B3V H and the B5V L had an improved antitumor activity in a human xenograft mouse model. These studies indicate that the common use of degenerate ("family-specific") primers to clone Fv fragments may introduce destabilizing mutations.Monoclonal antibodies B3 and B5 are murine antibodies directed against Lewis Y -related carbohydrate antigens, which are abundant on the surface of many carcinomas (1). The B3 IgG or its fragments are currently used as the targeting moiety of immunotoxins that are being developed as anticancer agents. Both conventional whole IgG conjugates and singlechain recombinant immunotoxins have been prepared (1-4). The single-chain Fv 1 immunotoxin of B3 is unstable at 37°C; it undergoes inactivation mainly by aggregation, especially upon incubation in PBS or in cell culture medium. In contrast, the B5(Fv)-PE38 immunotoxin is less susceptible to inactivation under those conditions, but it has lower apparent antigen binding affinity and cytotoxicity (5). We reasoned that we might be able to combine the advantages of each Fv by chimerization of their variable domains, since the Fvs of monoclonal antibody B3 and B5 bind the same carbohydrate antigen and are homologous in sequence (particularly in the V L domain, where 109 of 112 residues are identical). Therefore, we sought to gain insight on the possible involvement of individual residues of the light chain on the st...

Comprehensive optimization of a single-chain variable domain antibody fragment as a targeting ligand for a cytotoxic nanoparticle

“…29,30 We also changed M4 to L and V58 to I to improve packing of the VL domain core. 31,32 Finally, we ran the protein stability prediction algorithm Eris, 33 and changed R66 to K or to A. We introduced 3 mutations (VH-A49S, VL-R66K and VL-F83E) into the 116v4 framework and 7 mutations (VH-A49S, VL-M4L, VL-S12E, VL-V58I, VL-S60D, VL-R66A and VL-F83E) in the framework of 116v5.…”

Site-specific chemical modification with polyethylene glycol of recombinant immunotoxin anti-Tac(Fv)-PE38 (LMB-2) improves antitumor activity and reduces animal toxicity and immunogenicity

“…This effect is attributed to the increased molecular size and steric hindrance that result from attaching PEG to proteins. In most cases, the application of PEGylation has been limited to enzymes whose substrates are very small molecules, such as adenosine deaminase and superoxide disumutase, because the attached PEG chain can sterically inhibit ligand-receptor and antigenantibody binding, which are based on macromolecular interactions (19,20). Additionally, PEGylation usually occurs at lysine residues, some of which may be in or near the active site of the protein, and this lysine modification by PEG is random and difficult to control (25).…”

“…PEGylation of proteins increases their plasma half-lives by reducing proteolysis and restricting tissue-distribution such as glomerular filtration by the kidney and hepatic uptake (12)(13)(14)(15)(16)(17)(18)(19)(20). This effect is attributed to the increased molecular size and steric hindrance that result from attaching PEG to proteins.…”

“…In most cases, PEGylation of proteins is nonspecific and targeted at all of the lysine residues in the protein, some of which may be in or near the active-site. To overcome this drawback, we previously attempted to do site-specific PEGylation of mutant PE molecules that were engineered to contain one or two cysteine residues on the surface of PE (19,20). Free thiol chemistry was used for the attachment of PEG to these residues.…”

Restoration of Ig Secretion: Mutation of Germline-Encoded Residues in T15L Chains Leads to Secretion of Free Light Chains and Assembled Antibody Complexes Bearing Secretion-Impaired Heavy Chains

“…V L FW1 residues have frequently been implicated in the stability of Fv proteins (23,35,36). Wall and Pluckthun (23) showed that mutation of four FW1 residues in T15L, including S 16 Ͼ G, increased folding and secretion of the T15Fv.…”