HIV-1 neutralization by chimeric CD4-CG10 polypeptides fused to human IgG1

MHC-I presentation of peptides derived from intact protein products of the pioneer round of translation

Among the earliest protein products of most cellular genes are those synthesized during the pioneer round of translation (PRT), a key step in nonsense-mediated mRNA decay (NMD) that allows scanning of new transcripts for the presence of a premature termination codon (PTC). It has been demonstrated that at least some PRT degradation products can be targeted to major histocompatibility (MHC)-I presentation. To gain new insight into this putative PRT-to-MHC-I route, we have assembled 2 pairs of reporter genes so that the 2 genes in each pair encode an identical fusion protein between a model antigenic peptide and enhanced green fluorescent protein (EGFP), one of which harbors a PTC. We expressed these genes in different mouse and human cell lines and confirmed enhanced NMD activity for the PTC(+) gene in each pair by monitoring the effect of cycloheximide on the level of the respective mRNA. We then exploited several strategies for establishing the ratio between level of peptide presentation and total amount of protein product. We consistently observed significantly higher ratios for the PTC(+) mRNAs compared with the PTC(-) ones, pointing to correlation between the turnover of otherwise identical proteins and the fate of their template mRNA. Using confocal microscopy, we showed a clear link between NMD, the presence of misfolded EGFP polypeptides, and enhanced MHC-I peptide presentation. Altogether, these findings imply that identical full-length gene products differing only in 3' noncoding sequences can be differentially degraded and targeted to the MHC-I presentation pathway, suggesting a more general role for the PRT in establishing the MHC-I peptidome.-Weinstein-Marom, H., Hendel, L., Laron, E. A., Sharabi-Nov, A., Margalit, A., Gross, G. MHC-I presentation of peptides derived from intact protein products of the pioneer round of translation.

Conformational and structural features of HIV-1 gp120 underlying the dual receptor antagonism by cross-reactive neutralizing antibody m18

We investigated the interaction between cross-reactive HIV-1 neutralizing human monoclonal antibody m18 and HIV-1YU-2 gp120 in an effort to understand how this antibody inhibits the entry of virus into cells. m18 binds to gp120 with high affinity (KD≈5 nM) as measured by surface plasmon resonance (SPR) and isothermal titration calorimetry (ITC). SPR analysis further showed that m18 inhibits interactions of gp120 with both soluble CD4 and CD4-induced antibodies that have epitopes overlapping the coreceptor binding site. This dual receptor site antagonism, which occurs with equal potency for both inhibition effects, argues that m18 is not functioning as a mimic of CD4, in spite of the presence of a putative CD4-like loop formed by HCDR3 in the antibody. Consistent with this view, m18 was found to interact with gp120 in the presence of saturating concentrations of a CD4-mimicking small molecule gp120 inhibitor, suggesting that m18 does not require unoccupied CD4 Phe43 binding cavity residues of gp120. Thermodynamic analysis of the m18-gp120 interaction suggests that m18 stabilizes a conformation of gp120 that is unique from and less structured than the CD4-stabilized conformation. Conformational mutants of gp120 were studied for their impact on m18 interaction. Mutations known to disrupt the coreceptor binding region and to lead to complete suppression of 17b binding had minimal effects on m18 binding. This argues that energetically important epitopes for m18 binding lie outside the disrupted bridging sheet region used for 17b and coreceptor binding. In contrast, mutations in the CD4 region strongly affected m18 binding. Overall, the results obtained in this work argue that m18, rather than mimicking CD4 directly, suppresses both receptor binding site functions of HIV-1 gp120 by stabilizing a nonproductive conformation of the envelope protein. These results can be related to prior findings about the importance of conformational entrapment as a common mode of action for neutralizing CD4bs antibodies, with differences mainly in epitope utilization and the extent of gp120 structuring.

Enhanced HIV-1 neutralization by a CD4-VH3-IgG1 fusion protein

HIV-1 gp120 is an alleged B cell superantigen, binding certain VH3+ human antibodies. We reasoned that a CD4-VH3 fusion protein could possess higher affinity for gp120 and improved HIV-1 inhibitory capacity. To test this we produced several human IgG1 immunoligands harboring VH3. Unlike VH3-IgG1 or VH3-CD4-IgG1, CD4-VH3-IgG1 bound gp120 considerably stronger than CD4-IgG1. CD4-VH3-IgG1 exhibited approximately 1.5-2.5-fold increase in neutralization of two T-cell laboratory-adapted strains when compared to CD4-IgG1. CD4-VH3-IgG1 improved neutralization of 7/10 clade B primary isolates or pseudoviruses, exceeding 20-fold for JR-FL and 13-fold for Ba-L. It enhanced neutralization of 4/8 clade C viruses, and had negligible effect on 1/4 clade A pseudoviruses. We attribute this improvement to possible pairing of VH3 with CD4 D1 and stabilization of an Ig Fv-like structure, rather than to superantigen interactions. These novel findings support the current notion that CD4 fusion proteins can act as better HIV-1 entry inhibitors with potential clinical implications.

Molecular decoys: antidotes, therapeutics and immunomodulators

Receptor–ligand interactions are fundamental to the regulation of cell physiology, enabling the communication between cells and their environment via signal transduction. Receptors are also exploited by toxins and infectious agents to mediate pathogenesis. Over the past 20 years, however, this bi-partite paradigm for cellular regulation, that is, receptors and their ligands, has been revised to include an unforeseen participant namely, soluble receptors or molecular decoys. Decoys function as nature's modifiers of potent responses such as inflammation, stimulation of cell proliferation and triggering apoptosis. Decoys not only provide the means to fine tune the regulation of these phenomena; they also serve as potential leads for the development of recombinant anti-toxins, anti-viral agents and novel therapeutics for combating cancer and inflammatory disease.

Delivery of DNA HIV-1 vaccine to the liver induces high and long-lasting humoral immune responses

The quality of immune responses induced by DNA vaccination depends on the site of DNA administration, the expression, and the properties of the encoded antigen. In the present study, we demonstrate that intravenous hydrodynamic HIV-1 envelope DNA injection resulted in high levels of expression of HIV-1 envelope antigen in the liver. When compared to the administration of DNA by i.n., i.d., i.m., and i.splenic routes, hydrodynamic vaccination induced, upon DNA boosting, levels of HIV-1 envelope-specific antibodies 40-fold higher than those elicited by the other routes tested. Hydrodynamic vaccination with 1 microg DNA induced higher humoral responses than 100 microg DNA given intramuscularly in the prime-boost regimen. High levels of envelope-specific IgG and IgA antibodies were induced in genital tract secretions after two doses of DNA followed by intranasal boosting with recombinant HIV-1 gp120 protein. Furthermore, two doses of 100 microg DNA generated interferon-gamma production in approximately 4.3+/-1.7% of CD8(+) splenocytes after in vitro stimulation with HIV-1 envelope peptides. These results demonstrate that DNA vaccines targeted to tissues with high proteosynthetic activity, such as the liver, results in enhanced immune responses.

Antibody-based inhibitors of HIV infection

The demand for new treatment options against HIV is becoming increasingly desperate as the side effects and the expansion and spread of drug-resistant virus within the infected population limit the clinical benefits provided by available anti-HIV drugs. Preparations of polyclonal antibodies have a long history of proven clinical utility against some viruses; however, they have enjoyed very limited success against HIV. Recent clinical trials and in vitro experiments suggest that monoclonal antibodies against HIV may have promise clinically. These antibodies and antibody-based reagents target either the viral envelope glycoprotein, the receptor (CD4) or coreceptor (CCR5) molecules, or transition-state structures that appear during viral entry. The challenge is whether an antibody-based therapy can be identified (with or without their small molecule brethren) that presents long-term clinical efficacy, low toxicity and minimal risk of clinical failure from viral resistance.

A recombinant allosteric lectin antagonist of HIV-1 envelope gp120 interactions

The first, critical stage of HIV-1 infection is fusion of viral and host cellular membranes initiated by a viral envelope glycoprotein gp120. We evaluated the potential to form a chimeric protein entry inhibitor that combines the action of two gp120-targeting molecules, an allosteric peptide inhibitor 12p1 and a higher affinity carbohydrate-binding protein cyanovirin (CVN). In initial mixing experiments, we demonstrated that the inhibitors do not interfere with each other and instead show functional synergy in inhibiting viral cell infection. Based on this, we created a chimera, termed L5, with 12p1 fused to the C-terminal domain of CVN through a linker of five penta-peptide repeats. L5 revealed the same broad specificity as CVN for gp120 from a variety of clades and tropisms. By comparison to CVN, the L5 chimera exhibited substantially increased inhibition of gp120 binding to receptor CD4, coreceptor surrogate mAb 17b and gp120 antibody F105. These binding inhibition effects by the chimera reflected both the high affinity of the CVN domain and the allosteric action of the 12p1 domain. The results open up the possibility to form high potency chimeras, as well as noncovalent mixtures, as leads for HIV-1 envelope antagonism that can overcome potency limits and potential virus mutational resistance for either 12p1 or CVN alone.