Development of prophylactic vaccines against HIV-1

Allosteric induction of the CD4-bound conformation of HIV-1 Gp120

BACKGROUND

HIV-1 infection of target cells is mediated via the binding of the viral envelope protein, gp120, to the cell surface receptor CD4. This interaction leads to conformational rearrangements in gp120 forming or revealing CD4 induced (CD4i) epitopes which are critical for the subsequent recognition of the co-receptor required for viral entry. The CD4-bound state of gp120 has been considered a potential immunogen for HIV-1 vaccine development. Here we report on an alternative means to induce gp120 into the CD4i conformation.

RESULTS

Combinatorial phage display peptide libraries were screened against HIV-1 gp120 and short (14aa) peptides were selected that bind the viral envelope and allosterically induce the CD4i conformation. The lead peptide was subsequently systematically optimized for higher affinity as well as more efficient inductive activity. The peptide:gp120 complex was scrutinized with a panel of neutralizing anti-gp120 monoclonal antibodies and CD4 itself, illustrating that peptide binding does not interfere with or obscure the CD4 binding site.

CONCLUSIONS

Two surfaces of gp120 are considered targets for the development of cross neutralizing antibodies against HIV-1; the CD4 binding site and CD4i epitopes. By implementing novel peptides that allosterically induce the CD4i epitopes we have generated a viral envelope that presents both of these surfaces simultaneously.

Functional conservation of HIV-1 Gag: implications for rational drug design

Background

HIV-1 replication can be successfully blocked by targeting gag gene products, offering a promising strategy for new drug classes that complement current HIV-1 treatment options. However, naturally occurring polymorphisms at drug binding sites can severely compromise HIV-1 susceptibility to gag inhibitors in clinical and experimental studies. Therefore, a comprehensive understanding of gag natural diversity is needed.

Findings

We analyzed the degree of functional conservation in 10862 full-length gag sequences across 8 major HIV-1 subtypes and identified the impact of natural variation on known drug binding positions targeted by more than 20 gag inhibitors published to date. Complete conservation across all subtypes was detected in 147 (29%) out of 500 gag positions, with the highest level of conservation observed in capsid protein. Almost half (41%) of the 136 known drug binding positions were completely conserved, but all inhibitors were confronted with naturally occurring polymorphisms in their binding sites, some of which correlated with HIV-1 subtype. Integration of sequence and structural information revealed one drug binding pocket with minimal genetic variability, which is situated at the N-terminal domain of the capsid protein.

Conclusions

This first large-scale analysis of full-length HIV-1 gag provided a detailed mapping of natural diversity across major subtypes and highlighted the considerable variation in current drug binding sites. Our results contribute to the optimization of gag inhibitors in rational drug design, given that drug binding sites should ideally be conserved across all HIV-1 subtypes.

F(ab')2 fragment of a gp41 NHR-trimer-induced IgM monoclonal antibody neutralizes HIV-1 infection and blocks viral fusion by targeting the conserved gp41 pocket

Using a recombinant protein N46FdFc that mimics the HIV-1 gp41 N-helix trimer to immunize mice, we identified the first IgM monoclonal antibody 18D3 that specifically bound to the conserved gp41 pocket. Its F(ab')2 fragment potently inhibited HIV-1 Env-mediated cell-cell fusion and neutralized infection by laboratory-adapted and primary HIV-1 isolates with different subtypes and tropism, including the T20-resistant variants. This F(ab')2 fragment can be used to develop a bispecific broad neutralizing monoclonal antibody or HIV-1 inactivator as a novel immunotherapeutic for treatment and prevention of HIV-1 infection.

Cell-to-cell spread of HIV-1 and evasion of neutralizing antibodies

Cell-to-cell spread of human immunodeficiency virus (HIV-1) between immune cells was first observed over 20 years ago. During this time, the question of whether this infection route favours viral evasion of neutralizing antibodies (NAbs) targeting the virus envelope glycoprotein (Env) has been repeatedly investigated, but with conflicting results. A clearer picture has formed in the last few years as more broadly neutralizing antibodies have been isolated and we gain further insight into the mechanisms of HIV-1 transmission at virological and infectious synapses. Nevertheless consensus is still lacking, a situation which may be at least partly explained by variability in the experimental approaches used to study the activity of NAbs in the cell-to-cell context. In this review we focus on the most critical question concerning the activity of NAbs against cell-to-cell transmission: is NAb inhibition of cell-to-cell HIV-1 quantitatively or qualitatively different from cell-free infection? Overall, data consistently show that NAbs are capable of blocking HIV-1 infection at synapses, supporting the concept that cell-to-cell infection occurs through directed transfer of virions accessible to the external environment. However, more recent findings suggest that higher concentrations of certain NAbs might be needed to inhibit synaptic infection, with important potential implications for prophylactic vaccine development. We discuss several mechanistic explanations for this relative and selective loss of activity, and highlight gaps in knowledge that are still to be explored.

Thirty years on: HIV receptor gymnastics and the prevention of infection

During 30 years of research on human immunodeficiency virus (HIV), our knowledge of its cellular receptors - CD4, CCR5 and CXCR4 - has illuminated aspects of the pathogenesis of the acquired immune deficiency syndrome (AIDS). Studying how the HIV envelope glycoproteins interact with the receptors led to anti-retroviral drugs based on blocking the docking or fusion of virus to the host cell. Genetic polymorphisms of CCR5 determine resistance to HIV infection and the rate of progression to AIDS. Eliciting neutralizing antibodies to the sites of receptor interaction on HIV glycoproteins is a promising approach to HIV vaccine development.