Sorting polyclonal antibodies into functionally distinct fractions using peptide phage display: 'a library on top of a library'.

A general approach for sorting antibodies (Abs) to a restricted protein domain was developed using phage-displayed peptide libraries. The method is demonstrated by fractionating polyclonal antibodies (pAbs), raised against a short peptide derived from the extracellular, juxtamembrane region of fibroblast growth factor receptor 1 (FGFR1) into fractions with distinct chemical and biological characteristics. Screening two combinatorial peptide libraries, with the pAb, several sequences, homologous to different regions within the original peptide, were identified. Four of the corresponding peptides were synthesized and used as peptide-conjugated affinity columns for the fractionation of the pAbs. The fractions obtained were unique in their recognition patterns and in their capacity to immunoprecipitate and immunoblot, as well as to modulate the activity of FGFR1. This technique is, therefore, highly sufficient in separating pAbs to monospecific fractions and may also be used for fine mapping of different, even overlapping, sequences within a restricted peptide or protein domain.

Prevention of experimental antiphospholipid syndrome and endothelial cell activation by synthetic peptides.

Antiphospholipid syndrome (APS) is characterized by recurrent fetal loss, repeated thromboembolic phenomena, and thrombocytopenia. The syndrome is believed to be caused by antiphospholipid beta-2-glycoprotein-I (beta2GPI)-dependent Abs or anti-beta2GPI Abs by themselves. Using a hexapeptide phage display library, we identified three hexapeptides that react specifically with the anti-beta2GPI mAbs ILA-1, ILA-3, and H-3, which cause endothelial cell activation and induce experimental APS. To enhance the binding of the peptides to the corresponding mAbs, the peptides were lengthened to correspond with the site of the beta2GPI epitope being recognized by these mAbs. As a result, the following three peptides were prepared: A, NTLKTPRVGGC, which binds to ILA-1 mAb; B, KDKATFGCHDGC, which binds to ILA-3 mAb; and C, CATLRVYKGG, which binds to H-3 mAb. Peptides A, B, and C specifically inhibit both in vitro and in vivo the biological functions of the corresponding anti-beta2GPI mAbs. Exposure of endothelial cells to anti-beta2GPI mAbs and their corresponding peptides led to the inhibition of endothelial cell activation, as shown by decreased expression of adhesion molecules (E-selectin, ICAM-1, VCAM-1) and monocyte adhesion. In vivo infusion of each of the anti-beta2GPI mAbs into BALB/c mice, followed by administration of the corresponding specific peptides, prevented the peptide-treated mice from developing experimental APS. The use of synthetic peptides that focus on neutralization of pathogenic anti-beta2GPI Abs represents a possible new therapeutic approach to APS.

Identification of a hexapeptide that mimics a conformation-dependent binding site of acetylcholine receptor by use of a phage-epitope library.

Monoclonal antibody (mAb) 5.5 is directed against the ligand-binding site of the nicotinic acetylcholine receptor. The epitope for this antibody is conformation-dependent, and the antibody does not react with synthetic peptides derived from the receptor sequence. We have identified a ligand peptide that mimics this conformation-dependent epitope from a phage-epitope library composed of filamentous phage displaying random hexapeptides. Among 38 positive phage clones, individually selected from the library, 34 positive clones carried the sequence Asp-Leu-Val-Trp-Leu-Leu (DLVWLL), 1 positive clone had the sequence Asp-Ile-Val-Trp-Leu-Leu (DIVWLL), and 3 positive clones expressed the sequence Leu-Ile-Glu-Trp-Leu-Leu (LIEWLL), none of which are significantly homologous with the nicotinic acetylcholine receptor alpha subunit sequence. All of these phages bind specifically to mAb 5.5. The synthetic peptide DLVWLL inhibits binding of mAb 5.5 to the related peptide-presenting phage and to the nicotinic acetylcholine receptor in a concentration-dependent manner; the IC50 value is of the order of 10(-4) M. Bioactivity of the peptide "mimotope" DLVWLL was demonstrated in vivo in hatched chickens by inhibition of the mAb 5.5 effect by the peptide. The neuromuscular block and myasthenia gravis-like symptoms that are induced in chicken by passive transfer of mAb 5.5 were specifically abolished by DLVWLL. This study shows the potential of a random peptide phage-epitope library for selecting a mimotope for an antibody that recognizes a folded form of the protein, where peptides from the linear amino acid sequence of the protein are not applicable.

Isolation of peptides that inhibit binding of basic fibroblast growth factor to its receptor from a random phage-epitope library.

Basic fibroblast growth factor (bFGF) is known to bind to its cell-surface receptors with high affinity and in a heparin-dependent manner. In an attempt to predict the receptor recognition site on bFGF we screened phage-epitope libraries with monoclonal antibodies DG2 and DE6, which inhibit bFGF binding to its receptor. On the affinity-isolated phages, we identified several peptide sequences as the putative antibody-binding epitopes on bFGF. The identified library epitopes shared the consensus sequence Pro-(Pro/Ser)-Gly-His-(Tyr/Phe)-Lys, corresponding to two continuous protein sequences of bFGF: Pro-Pro-Gly-His-Phe-Lys and Arg-Thr-Gly-Gln-Tyr-Lys at amino acids 13-18 and 120-125 of bFGF, respectively. Synthetic peptides of the corresponding phage epitopes or of the above bFGF sequences specifically inhibited binding of the antibodies to bFGF, blocked binding of bFGF to its high-affinity receptor, and inhibited basal and bFGF-induced proliferation of vascular endothelial cells at submicromolar peptide concentrations. The potent inhibition of bFGF binding and biological activity by peptides recognized by the antibodies suggests that these sequences are functionally involved in receptor binding and may constitute part of the receptor-binding determinants on bFGF.