Phagotopes derived by antibody screening of phage-displayed random peptide libraries vary in immunoreactivity: studies using an exemplary monoclonal antibody, CII-C1, to type II collagen

Mimotope peptides selected from phage display combinatorial library by serum antibodies of pigs experimentally infected with Taenia solium as leads to developing diagnostic antigens for human neurocysticercosis

Neurocysticercosis is caused by penetration of the tapeworm Taenia solium larvae into the central nervous system resulting in a diverse range of neurologic complications including epilepsy in endemic areas that globalization spreads worldwide. Sensitive and specific immunodiagnosis is needed for the early detection and elimination of the parasite, but the lack of standardized, readily obtainable antigens is a challenge. Here, we used the phage display for resolving the problem. The rationale of the strategy rests on the concept that the screening of combinatorial libraries with polyclonal serum to pathogens reveals families of peptides mimicking the pathogen most immunodominant epitopes indispensable for the successful diagnosis. The screening of a 7mer library with serum IgG of four pigs experimentally infected with parasite followed by computer aided segregation of the selected sequences resulted in the discovery of four clusters of homologous sequences of which one presented a family of ten mimotopes selected by three infected pig serum IgGs; the common motif sequence LSPF carried by the family was considered to be the core of an immunodominant epitope of the parasite critical for the binding with the antibody that selected the mimotopes. The immunoassay testing permitted to select a mimotope whose synthetic peptide free of the phage with the amino acid sequence Leu-Ser-Fen-Pro-Ser-Val-Val that distinguished well a panel of 21 cerebrospinal fluids of neurocysticercosis patients from the fluids of individuals with neurological complications of other etiology. This peptide is proposed as a lead for developing a novel molecularly defined diagnostic antigen(s) for the neurocysticercosis.

Antibody reactivity of conformational peptide mimics of a conserved H5N1 neutralization site in different fusion proteins

Several peptide mimics of a conserved H5N1 avian influenza virus neutralization site recognized by 8H5 mAb have been reported previously. In this study, the secondary and possibly higher structural orders of the peptide mimics 122 and 125 were investigated and found to be closely related to the specific binding with 8H5 mAb. These two peptide mimics were fused to three different carrier proteins, and the antibody binding activities were recovered in 4 of the 11 fusion proteins. HEV structural protein p239 and HBc were more suitable than the outer membrane protein T47 of the Treponema pallidum particle for the recovery of reactivity. The increase in the copy number of peptide mimics was important for the recovery of antibody-binding activity and the interaction between peptide and carrier protein may affect the spatial structure of both the peptide and the carrier protein. These results are likely to be of relevance for conformational peptide mimics in diagnostic tests, vaccine and inhibitors.

Autoimmune epitopes: autoepitopes

The identity of reactants for autoantibodies has been successively refined from whole cellular organelles (immunofluorescence), identified molecules (immunoblot; gene expression libraries), epitope regions (truncated cDNAs; peptide scanning) to contact residues, as described here. Most autoantibodies react with conformational epitopes, in which amino acids distant in the linear sequence come into contiguity by protein folding. Identification of contact sites with the antibody paratope requires particular technologies, crystallography, or antibody screening of phage-displayed random peptide libraries. The latter is illustrated by our studies on the autoepitope for anti-PDC-E2 (AMA) in primary biliary cirrhosis (PBC), anti-GAD65 in type 1 diabetes, and anti-C1 of type II collagen in collagen-induced arthritis. More precise definition of the structure of conformational autoepitopes could (a) clarify controversial aspects of autoimmunity including epitope mimicry, epitope spreading, and molecular spatial relationships between B and T cell autoepitopes, and (b) impact on novel diagnostic and therapeutic (vaccine) molecules.

Requirement of multiple phage displayed peptide libraries for optimal mapping of a conformational antibody epitope on CCR5

In the absence of information from crystallography, conformational epitopes can often be discerned by antibody screening of phage displayed random peptide libraries. However the context in which the peptide is displayed, and the number of copies displayed in the library, can influence results and interpretations. Here, the monoclonal antibodies 3A9 specific for the transmembrane chemokine receptor CCR5, and CII-C1 specific for type II collagen, were used to screen multiple phage-displayed peptide libraries in which peptides were displayed in either the pIII or pVIII coat proteins. ELISA was used to test for reactivity and cross-inhibitory activity of isolated phage clones. Based on sequences of reactive phage inserts, epitope motifs were initially inferred from a molecular model of CCR5 and subsequently confirmed experimentally using mutagenesis to alanine. For each mAb, phage sequences from pIII biopannings were more diverse than from pVIII biopannings. Notably, sequences from either biopanning were cross-inhibitory despite a lack of linear sequence homology. For CCR5, residues 88H and 94W in the first loop of CCR5 were identified by pIII biopannings, and 7S9IYD11 at the N-terminus by pVIII biopannings. Thus conformational epitopes can be identified using phage display, but optimal mapping of complex epitopes can require the use of multiple peptide libraries.

Arthritogenic anti-type II collagen antibodies are pathogenic for cartilage-derived chondrocytes independent of inflammatory cells

OBJECTIVE

Some monoclonal antibodies (mAb) to type II collagen (CII) are arthritogenic upon passive transfer to mice. We undertook this study to investigate whether such mAb are pathogenic in the absence of mediators of inflammation.

METHODS

The arthritogenic mAb CIIC1 and M2139, and the nonarthritogenic mAb CIIF4, each reactive with a distinct and well-defined conformational epitope on CII, were compared with control mAb GAD6. Bovine chondrocytes were cultured with one of the mAb, and on days 3, 6, and 9, antibody binding by chondrocytes and newly synthesized extracellular matrix (ECM) was examined by immunofluorescence, morphologic effects were studied by electron microscopy, and synthesis of matrix components was determined by metabolic labeling with (3)H-proline for collagen and (35)S-sulfate for proteoglycans.

RESULTS

All 3 mAb to CII bound to the matrix. CIIC1 and M2139 adversely affected the cultures, whereas CIIF4 did not. CIIC1 caused disorganization of CII fibrils in the ECM without affecting chondrocyte morphology, and increased matrix synthesis. M2139 caused thickening and aggregation of CII fibrils in the ECM and abnormal chondrocyte morphology but matrix synthesis was unaffected.

CONCLUSION

The unique arthritogenic capacity of particular anti-CII mAb upon passive transfer could be explained by their adverse, albeit differing, effects in primary cultures of chondrocytes. Such effects occur independent of inflammation mediators and are related to the epitope specificity of the mAb. Interference with the structural integrity of CII could precede, and even initiate, the inflammatory expression of disease.

An arthritogenic monoclonal antibody to type II collagen, CII‐C1, impairs cartilage formation by cultured chondrocytes

Antibodies to type II collagen (CII) cause articular damage in collagen-induced arthritis (CIA) in mice as judged by passive transfer to naive animals of mAb to CII. We tested the hypothesis that mAb degrade cartilage structure by reacting with functionally important regions of the collagen molecule by examining the effects of an arthritogenic mAb to CII, CII-C1, on cultured bovine chondrocytes at high density, at days 7 and 14. The effects were compared of CII-C1, an isotype-matched control mAb, or medium alone, on chondrocyte proliferation and viability, cell morphology, matrix structure by light and electron microscopy, and matrix synthesis by metabolic labelling with 3H-proline for collagen or 35SO4 for proteoglycans. Chondrocytes in culture remained viable, proliferated, and produced an extracellular matrix in which CII was the major collagen. The addition of CII-C1, but not a control mAb, increased the synthesis of CII and proteoglycan, and caused disorganization of the extracellular matrix and thin collagen fibrils ultrastructurally. Moreover, using a cell-free assay, CII-C1 inhibited the normal self-assembly of collagen fibrils from CII in solution. The finding that the mAb to CII, CII-C1 has striking degradative effects in vitro on cartilage synthesis suggests that antibodies to collagen perpetuate the chronic phase of CIA and that, in mice at least, such antibodies are an important component of pathogenesis.

Phage display for epitope determination: a paradigm for identifying receptor-ligand interactions

Antibodies that react with many different molecular species of protein and non-protein nature are widely studied in biology and have particular utilities, but the precise epitopes recognized are seldom well defined. The definition of epitopes by X-ray crystallography of the antigen-antibody complex, the gold standard procedure, has shown that most antibody epitopes are conformational and specified by interactions with topographic determinants on the surface of the antigenic molecule. Techniques available for the definition of such epitopes are limited. Phage display using either gene-specific libraries, or random peptide libraries, provides a powerful technique for an approach to epitope identification. The technique can identify amino acids on protein antigens that are critical for antibody binding and, further, the isolation of peptide motifs that are both structural and functional mimotopes of both protein and non-protein antigens. This review discusses techniques used to isolate such mimotopes, to confirm their specificity, and to characterize peptide epitopes. Moreover there are direct practical applications to deriving epitopes or mimotopes by sequence, notably the development of new diagnostic reagents, or therapeutic agonist or antagonist molecules. The techniques developed for mapping of antibody epitopes are applicable to probing the origins of autoimmune diseases and certain cancers by identifying "immunofootprints" of unknown initiating agents, as we discuss herein, and are directly applicable to examination of a wider range of receptor-ligand interactions.

Characterization of epitopes for virus-neutralizing monoclonal antibodies to Ross River virus E2 using phage-displayed random peptide libraries

Ross River virus (RRV) is the predominant cause of epidemic polyarthritis in Australia, yet the antigenic determinants are not well defined. We aimed to characterize epitope(s) on RRV-E2 for a panel of monoclonal antibodies (MAbs) that recognize overlapping conformational epitopes on the E2 envelope protein of RRV and that neutralize virus infection of cells in vitro. Phage-displayed random peptide libraries were probed with the MAbs T1E7, NB3C4, and T10C9 using solution-phase and solid-phase biopanning methods. The peptides VSIFPPA and KTAISPT were selected 15 and 6 times, respectively, by all three of the MAbs using solution-phase biopanning. The peptide LRLPPAP was selected 8 times by NB3C4 using solid-phase biopanning; this peptide shares a trio of amino acids with the peptide VSIFPPA. Phage that expressed the peptides VSIFPPA and LRLPPAP were reactive with T1E7 and/or NB3C4, and phage that expressed the peptides VSIFPPA, LRLPPAP, and KTAISPT partially inhibited the reactivity of T1E7 with RRV. The selected peptides resemble regions of RRV-E2 adjacent to sites mutated in neutralization escape variants of RRV derived by culture in the presence of these MAbs (E2 210-219 and 238-245) and an additional region of E2 172-182. Together these sites represent a conformational epitope of E2 that is informative of cellular contact sites on RRV.

Use of phage display technology to investigate allergen-antibody interactions

Phage display is an advanced technology that can be used to characterize the interactions of antibody with antigen at the molecular level. It provides valuable data when applied to the investigation of IgE interaction with allergens. The aim of this rostrum article is to provide an explanation of the potential of phage display for increasing the understanding of allergen-IgE interaction, the discovery of diagnostic reagents, and the development of novel therapeutics for the treatment of allergic disease. The significance of initial studies that have applied phage display technology in allergy research will be highlighted. Phage display has been used to clone human IgE to timothy grass pollen allergen Phl p 5, to characterize the epitopes for murine and human antibodies to a birch pollen allergen Bet v 1, and to elucidate the epitopes of a murine mAb to the house dust mite allergen Der p 1. The technology has identified peptides that functionally mimic sites of human IgE constant domains and that were used to raise antiserum for blocking binding of IgE to the FcepsilonRI on basophils and subsequent release of histamine. Phage display has also been used to characterize novel peanut and fungal allergens. The method has been used to increase our understanding of the molecular basis of allergen-IgE interactions and to develop clinically relevant reagents with the pharmacologic potential to block the effector phase of allergic reactions. Many advances from these early studies are likely as phage display technology evolves and allergists gain expertise in its research applications.