Permissive residues within the minimal epitopes of neutralizing monoclonal antibodies to the V3 loop of HIV-1

Phage display: an important tool in the discovery of peptides with anti-HIV activity

Human immunodeficiency virus (HIV) remains a worldwide health problem despite huge investments and research breakthroughs, and no single drug is effective in killing the virus yet. Among new strategies to control HIV infection, the phage display (PD) technology has become a promising tool in the discovery of peptides that can be used as new drugs, or also as possible vaccine candidates. This review discusses basic aspects of PD and its use to advance two main objectives related to combating HIV-1 infection: the identification of peptides that inhibit virus replication and the identification of peptides that induce the production of neutralizing antibodies. We will cover the different approaches used for mapping and selection of mimotopes, and discuss the promising results of these biologicals as antiviral agents.

Phages and HIV-1: from display to interplay

The complex hide-and-seek game between HIV-1 and the host immune system has impaired the development of an efficient vaccine. In addition, the high variability of the virus impedes the long-term control of viral replication by small antiviral drugs. For more than 20 years, phage display technology has been intensively used in the field of HIV-1 to explore the epitope landscape recognized by monoclonal and polyclonal HIV-1-specific antibodies, thereby providing precious data about immunodominant and neutralizing epitopes. In parallel, biopanning experiments with various combinatorial or antibody fragment libraries were conducted on viral targets as well as host receptors to identify HIV-1 inhibitors. Besides these applications, phage display technology has been applied to characterize the enzymatic specificity of the HIV-1 protease. Phage particles also represent valuable alternative carriers displaying various HIV-1 antigens to the immune system and eliciting antiviral responses. This review presents and summarizes the different studies conducted with regard to the nature of phage libraries, target display mode and biopanning procedures.

Dual specificity of a human neutralizing monoclonal antibody, specific for the V3 loop of GP120 (HIV-1)

Neutralizing antibodies specific for the third variable (V3) domain of gp120, the HIV-1 surface envelope protein, appear early in infection. However, they are usually highly specific for the priming isolate. To identify potential mimotopes of the V3 domain, we have screened a hexapeptide phage library with a human neutralizing mAb, mAb 268, specific for the V3 loop of the viral MN isolate. We have identified two groups of sequences. Within the first group, sequence 268-1 reproduces the linear epitope identified using a conventional epitope mapping approach. The sequence 268-1, H L G P G R, corresponds to amino acids 315-320, localized in the highly conserved tip of the V3 loop. A second group of sequences was identified, including sequence 268-2, K A I H R I. Partial homology with a more variable region of the V3 loop can be found. Using synthetic peptides, we demonstrated that peptides, 268-1 and 268-2, both interact with the same binding site as the V3 region on the 268 mAb. Moreover, both peptides can inhibit the interaction of the 268 mAb with the original immunogen, gp120MN. Peptide 268-1 can compete with peptide 268-2, albeit poorly, for binding of the 268 mAb. When injected into rabbits, KLH conjugated peptide 268-2 elicited antibodies that interact specifically with the initial immunogen gp120MN. These data suggest that peptide 268-2 is both an antigenic and immunogenic mimic of the natural antigen, gp120MN.

The maltose-binding protein as a scaffold for monovalent display of peptides derived from phage libraries

Random peptide libraries are displayed on filamentous bacteriophage as fusions to either the minor coat protein, pIII, or the major coat protein, pVIII. We have devised a means of isolating the peptide displayed on a phage clone by transferring it to the N-terminus of the maltose-binding protein (MBP) of Escherichia coli encoded by malE. Transfer of a peptide sequence to monomeric MBP eliminates phage-encoded amino acids downstream of the insert peptide as well as avidity effects caused by multivalent display on phage. Peptide:MBP fusions are also easily affinity purified on amylose columns. The pMal-p2 vector was engineered to accept phage DNA encoding pIII- and pVIII-displayed peptides fused to their respective leader sequences. Both types of leader sequence were shown to target the peptide:MBP fusions to the periplasm of E. coli. A streamlined procedure for transferring peptides to MBP was applied to clones that had been isolated from a panel of pVIII-displayed peptide libraries by screening with an HIV-1-specific monoclonal antibody (Ab). By enzyme-linked immunosorbent assay, the Ab bound each of the peptide:MBP fusions and required the presence of a disulfide bridge within each peptide. Some of the peptide:MBP fusions were also analyzed using surface plasmon resonance. Thus, our study shows the value of malE fusion vectors in characterizing phage-displayed peptides.

Peptide mimics of a conformationally constrained protective epitopes of respiratory syncytial virus fusion protein

AIMS

To identify peptides that mimic (mimotopesi conformational and protective epitopes of RSV fusion protein and to assess their efficacy as immunogens and potential vaccines.

MATERIAL AND METHODS

An 8-mer solid-phase (TG resin) library was screened with a neutralising and protective RSV fusion protein specific monoclonal antibodies (Mab-19). After selection of positive beads, reactive sequences were identified by microsequencing and 8-mer peptides were synthesised. Improvement of binding was analysed by amino acid replacement using the SPOTs method.

RESULTS

Mabs were not able to bind to the free and soluble peptides, nor did these peptides induce anti-RSV specific antibodies. However, several peptides re-synthesised on a TG resin (to produce de-protected 8-mer peptides linked to the resin) or as SPOTs reacted specifically. Therefore it was critical to be able to reproduce this conformation in order to use these mimotopes as immunogens and potential vaccines. Using C-terminal constrained versions of the mimotopes, strong binding of one of the Mabs to the peptides was demonstrated by surface-plasmon resonance. Immunisation of Balb/c mice with these peptide-mimics produced anti-sera that: (1) reacted specifically with RSV; (2) inhibited the binding of the Mab to the virus; (3) neutralised RSV in vitro with high titres (range: 80-640); and (4) reduce significantly the viral load in the lungs of mice challenged with RSV (P < 0.01).

CONCLUSIONS

This report demonstrates for the first time that: (1) a protective epitope of the conserved RSV fusion protein can be mimicked by synthetic peptides; and (2) immunisations with these mimotopes induced specific anti-RSV neutralising antibodies and reduced viral load in vivo. These results represent a novel concept for the development of a vaccine against RSV.