Phages and HIV-1: from display to interplay

Abstract

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.

Keywords: CCR5; CXCR4; HIV-1; HIV-1 inhibitor; HIV-1 vaccine; epitope mapping; gp120; gp41; mimotopes; phage display.

Figure 1

Model for HIV-1 entry. (A and B) Binding of Cluster of Differentiation (CD)4 to glycoprotein (gp)120 exposes a coreceptor binding site in gp120; (C and D) Coreceptor binding causes the exposure of the gp41 fusion peptide and its insertion into the membrane of the target cell in a triple-stranded coiled-coil; (E) Formation of a helical hairpin structure in which gp41 folds back on itself is coincident with membrane fusion.

Figure 2

Phage display applications in the field of HIV-1. (A) Epitope mapping (section 1): Randomized Peptide Library (RPL) or antigen-fragment libraries are used to map the residues forming the epitope(s) recognized by either monoclonal or polyclonal antibodies immobilized on a solid support; (B) Inhibitor discovery (section 2): RPL, antibody-fragments or ligand variants libraries are used to select for viral or host-cell protein inhibitors; (C) Phage substrate (section 3): phage particles immobilized on a solid support are specifically eluted by the proteolytic activity of the HIV-1 protease; (D) Phage as carrier (section 4): phage particles or derived Virus-Like Particles (VLPs) are used to display immunogenic peptides or whole antigens to the immune system.

Figure 3

Epitope landscape of HIV-1 Env proteins. Monoclonal antibodies were mapped to different sites on the surface of the gp120 and gp41 proteins.

Figure 4

Antibodies and antibody fragments used in phage display. (A) Whole human IgG; (B) Fab (Fragment antigen binding); (C) ScFv (Single-chain variable Fragment); (D) Camelid HcAb (Heavy-chain only Antibody); (E) V_HH/Nanobodies (Variable domain of HcAb).

Figure 5

HIV-1 entry inhibitors identified by phage display and their target. Potent HIV-1 inhibitors blocking key steps in the entry process were identified using the phage display technology. These inhibitors target: the CD4 binding site (Fab b12 and Z13), the coreceptors CCR5 (CCL5 variants) or CXCR4 (V_HH 238D2 and 238D4), the CD4-induced epitope of gp120 (Fab X5) or the heptad repeat region of gp41 (peptide PIE12₃).