HIV Entry and Its Inhibition by Bifunctional Antiviral Proteins

Abstract

HIV entry is a highly specific and time-sensitive process that can be divided into receptor binding, coreceptor binding, and membrane fusion. Bifunctional antiviral proteins (bAVPs) exploit the multi-step nature of the HIV entry process by binding to two different extracellular targets. They are generated by expressing a fusion protein containing two entry inhibitors with a flexible linker. The resulting fusion proteins exhibit exceptional neutralization potency and broad cross-clade inhibition. In this review, we summarize the HIV entry process and provide an overview of the design, antiviral potency, and methods of delivery of bAVPs. Additionally, we discuss the advantages and limitations of bAVPs for HIV prevention and treatment.

Keywords: HIV; antiviral proteins; bifunctional; entry; entry inhibitors; gene therapy.

Figure 1

HIV Env Structure (A) Schematic representation of gp120 and gp41. (B) Crystal structure of native HIV Env. The positions of V1–V5 of gp120 as well as of HR1 and HR2 of gp41 are indicated. The image was generated with the software Chimera by using the coordinates from PDB: 5FUU.

Figure 2

The HIV Entry Cascade A schematic representation of the HIV entry process is shown. Receptor binding induces conformational changes in gp120 that result in the exposure of the coreceptor-binding site on gp120 and the HR1 (light green) and HR2 (dark green) of gp41. Coreceptor binding induces additional changes that result in the release of the FP of gp41 (yellow) and cause the HR1 and HR2 of gp41 to assume an extended conformation (pre-fusion intermediate). Insertion of the FP into the host cell membrane initiates the formation of the 6-helix bundle and lipid mixing between the viral and cellular membranes, leading to the formation of a fusion pore and content mixing.

Figure 3

Design of bAVPs (A) bAVPs targeting receptor binding. (B) bAVPs targeting receptor binding and coreceptor binding. (C) bAVP targeting coreceptor binding. (D) bAVPs targeting receptor and membrane fusion. (E) bAVPs targeting coreceptor binding and membrane fusion. C_H, antibody heavy chain constant domain; C_L, antibody light chain constant domain; D1 and D2, CD4 domain 1 and 2; L, linker sequence; L₁₆, linker sequence consisting of 16 G and S residues in random order; L_pIII, linker sequence derived from bacteriophage protein pIII; V_H, antibody heavy chain variable domain; V_L, antibody light chain variable domain.

Figure 4

Delivery of bAVPs bAVPs can be directly injected into patients. Alternatively, several genetic strategies may be used to deliver genes encoding bAVPs. Virus-derived vectors, such as AAV vectors, have been used to deliver genes to non-hematopoietic cells by direct injection into the target site. Probiotic bacteria of the genital tract can be engineered to secrete bAVPs for prevention. Autologous hematopoietic cells can be modified to secrete bAVPs with viral and non-viral vectors.