Function-modulating human monoclonal antibodies against platelet-membrane receptors isolated from a phage-display library

Methodologies for the isolation of alternative binders with improved clinical potentiality over conventional antibodies

The availability of binders to different functional domains of the same protein or to physiologically co-operating proteins allows for the simultaneous inhibition of independent downstream signaling pathways. This multi-target approach represents a promising therapeutic strategy, as demonstrated in the case of the synergistic effect of anti-Her2 treatment based on the combined use of the trastuzumab and pertuzumab monoclonal antibodies that induce cellular cytotoxicity and impair the receptor dimerization, respectively. Therefore, a reliable selection method for the recovery of epitope-specific antibodies is highly needed. Animal immunization with short peptides resembling the epitope sequence for raising conventional antibodies represents an alternative. Panning phage displayed libraries of recombinant antibodies such as scFvs and nanobodies or of other peptide collections is another option. Although recombinant antibodies can provide the same specificity as conventional antibodies, they offer at least two further advantages: i) the protocols for the selection of epitope-specific antibodies can be rationally designed, and ii) their expression as multivalent, bispecific and biparatopic molecules is feasible. This review will analyze the recent literature concerning technical aspects related to the isolation, the expression as multivalent molecules, and the therapeutic applications of binders able to interfere with antigen functional domains. The term binder will be preferred when possible to include those molecules, such as peptides or affibodies, with at least some proven practical uses.

Molecular characterization of an human monoclonal antibody that interacts with a sulfated tyrosine-containing epitope of the GPIb receptor and inhibits platelet functions

Modification of tyrosine residues in extracellular proteins by a sulfate moity plays an important role in many ligand/receptors interactions. In the present work, we describe a unique human monoclonal antibody, termed Y1-scFv, that is specific for a sulfated epitope in the platelat receptor GPIb. The Y1-scFv single chain antibody (scFv) competes with von Willebrand factor (vWF) for binding to human platelets and thus effectively inhibits platelet aggregation. Limited proteolysis of GPIb molecule, using the endoproteases, mocarhagin and cathepsin G, revealed that a seven amino-acid epitope, Tyr-276 to Glu-282, contains the recognition site for Y1-scFv. This GPIb region contains three sulfated tyrosine residues. Binding studies of Y1-scFv to cells and to synthetic peptides in vitro indicated that of the seven residues comprising the epitope only sulfo-Tyr-276 and adjacent Asp-277 are critical for the interaction. To identify the reciprocal sequences in the antibody that recognize the sulfated epitope, we introduced mutations within the complementary-determining region of the heavy chain (CDR3H) of Y1-scFv (MRAPVI). Arginine residue in the second position was critical for the binding. Moreover, a mutant, containing two sequential arginine residues, in the second and third positions of the CDR3H (MRRPVI), showed a nine-fold increased binding to GPIb. This antibody mutant also demonstrated a significant increase in inhibition of vWF-dependent platelet aggregation and adhesion under flow. In conclusion, this unique antibody and mutants, that recognize a sulfated epitope in GP1b receptor, efficiently inhibited platelet adhesion and aggregation, making it a candidate for a new anti-thrombotic agent.

Generation and characterization of a recombinant single chain Fv antibody to von Willebrand factor A1 domain from phage display library

In this study, cDNA of von Willebrand factor domain A1 (VWF-A1) was cloned from human umbilical vein endothelial cells, and expressed in E. coli. The expressed VWF-A1 was used as antigen to immunize Balb/c mice. Reverse transcribed-polymerase chain reaction (RT-PCR) was used to amplify VH and VL gene segment from the RNA isolated from the immunized mouse spleens. Single chain Fv (ScFv) genes were generated by splicing overlap extension and subcloned into a phagemid vector pHEN-1. Phage display library of repertoire single chain antibodies of anti-VWF-A1 was then constructed and screened for ScFvs that interact with VWF-A1. The ScFv gene of phage clone 28 that has best binding activity to VWF-A1 was cloned into pET20b vector for higher expression in E. coli strain BL-21(DE3) pLysS. The recombinant ScFv specifically reacted with VWF, rVWF-A1 and rVWF-A1/A3, but not with rVWF-A3, P-selectin and BSA. It inhibited ristocetin-induced platelet aggregation with an IC50 of 0.75 micromol/l, and its maximal extent of inhibition is 62.5%. The ScFv had no effect on thrombin-induced platelet aggregation. These data show that the ScFv is specific against VWF-A1 and could have a potential to be used as an antithrombotic agent.