Different effects of a single amino acid substitution on three adjacent epitopes in the gp41 C-terminal tail of a neutralizing antibody escape mutant of human immunodeficiency virus type 1

Ability of antibodies specific to the HIV-1 envelope glycoprotein to block the fusion inhibitor T20 in a cell-cell fusion assay

The anti-HIV peptide T20 is able to inhibit the syncytia formation between CHO-WT and HeLa CD4(+)cells. We found that several sera of HIV-infected patients have the capacity to block the inhibition of fusion by T20. Suggesting that these sera may contain antibody which can block T20 access and prevent membrane fusion, we studied the ability of a panel of antibodies directed to different regions of HIV-1 envelope glycoprotein to block the inhibition of fusion by T20. We found that the C1 and V3 loop regions of gp120 and the heptad repeat 1, the immunodominant C-C region and the Kennedy epitope of gp41 located in the intracytoplasmic tail were the target for antibodies capable to block the inhibition of syncytia formation by T20. We suggest that these antibodies have the capacity to counteract the anti-fusion effect of T20 by preventing its binding to the interaction sites. Further studies are needed to determine if some of them recognize new T20 interaction sites.

The complex antigenicity of a small external region of the C-terminal tail of the HIV-1 gp41 envelope protein: a lesson in epitope analysis

The newly discovered external tail loop within the C-terminal tail of the gp41 transmembrane subunit of the HIV-1 envelope protein comprises approximately 40 residues, and within this are 18-residues ((734)PDRPEGIEEEGGERDRDR(751)) that include three antibody-reactive regions. The antigenicity is complex, and changes according to the biological context of the gp41. It is thus of interest both to the HIV specialist and protein immunologists. The antibody-reactive region, centred on the sequence ERDRD, encompasses three distinct epitopes which are expressed in different combinations on infected cells, wt virions, prefusion virion-cell complexes, and a neutralising antibody escape mutant virion. In addition ERDRD-specific antibodies have one or more antiviral activities, and variously neutralise the infectivity of free virions, neutralise virions already attached to the target cell, reduce the production of infectious progeny, and inhibit the ability of infected cells to fuse with non-infected cells. Antibodies to PDRPEG and IEEE have no apparent antiviral activity even though the footprints of the IEEE- and ERDRD-specific antibodies overlap. This review marshals the available experimental data with the aim of understanding the significance of the gp41 tail loop to the HIV-1 life cycle, and its relevance to potential anti-viral measures. There are lessons here, too, that are relevant to the comprehension of the antigenicity of short protein segments in general.

The C-terminal tail of the gp41 transmembrane envelope glycoprotein of HIV-1 clades A, B, C, and D may exist in two conformations: an analysis of sequence, structure, and function

In addition to the major ectodomain, the gp41 transmembrane glycoprotein of HIV-1 is now known to have a minor ectodomain that is part of the long C-terminal tail. Both ectodomains are highly antigenic, carry neutralizing and non-neutralizing epitopes, and are involved in virus-mediated fusion activity. However, data have so far been biologically based, and derived solely from T cell line-adapted (TCLA), B clade viruses. Here we have carried out sequence and theoretically based structural analyses of 357 gp41 C-terminal sequences of mainly primary isolates of HIV-1 clades A, B, C, and D. Data show that all these viruses have the potential to form a tail loop structure (the minor ectodomain) supported by three, β-sheet, membrane-spanning domains (MSDs). This means that the first (N-terminal) tyrosine-based sorting signal of the gp41 tail is situated outside the cell membrane and is non-functional, and that gp41 that reaches the cell surface may be recycled back into the cytoplasm through the activity of the second tyrosine-sorting signal. However, we suggest that only a minority of cell-associated gp41 molecules – those destined for incorporation into virions – has 3 MSDs and the minor ectodomain. Most intracellular gp41 has the conventional single MSD, no minor ectodomain, a functional first tyrosine-based sorting signal, and in line with current thinking is degraded intracellularly. The gp41 structural diversity suggested here can be viewed as an evolutionary strategy to minimize HIV-1 envelope glycoprotein expression on the cell surface, and hence possible cytotoxicity and immune attack on the infected cell.

An antibody specific for the C-terminal tail of the gp41 transmembrane protein of human immunodeficiency virus type 1 mediates post-attachment neutralization, probably through inhibition of virus-cell fusion

Evidence has been presented which shows that part of the C-terminal tail of the gp41 transmembrane protein of human immunodeficiency virus type 1 (HIV-1) contains a neutralization epitope and is thus exposed on the external surface of the virion. Here, SAR1, a monoclonal antibody, which was stimulated by immunization with a plant virus expressing 60 copies of the GERDRDR sequence from the exposed gp41 tail, and has an unusual pattern of neutralization activity, giving little or no neutralization of free virions, but effecting modest post-attachment neutralization (PAN) of virus bound to target cells was investigated. Here, the properties of PAN were investigated. It was found that PAN could be mediated at 4 or 20 degrees C, but that at 20 degrees C maximum PAN required virus-cell complexes to be incubated for 3 h before addition of antibody. Further PAN appeared stable at 20 degrees C and could be mediated for at least 5 h at this temperature. In contrast, when virus-cell complexes formed at 20 degrees C but then shifted to 37 degrees C for various times before addition of SAR1, PAN was maximal after just 10 min, and was lost after 30 min incubation. Thus, PAN at 37 degrees C is transient and temperature-dependent. Since this scenario recalled the temperature requirements of virus-cell fusion, fusion of HIV-1-infected and non-infected cells was investigated, and it was found that SAR1 inhibited this process by up to 75 %, in a dose-dependent manner. However, antibodies to adjacent epitopes did not inhibit fusion. These data confirm the external location of the SAR1 epitope, implicate the gp41 C-terminal tail in the HIV-1 fusion process for the first time, and suggest that SAR1 mediates PAN by inhibiting virus-mediated fusion.

Part of the C-terminal tail of the envelope gp41 transmembrane glycoprotein of human immunodeficiency virus type 1 is exposed on the surface of infected cells and is involved in virus-mediated cell fusion

The C-terminal tail of the gp41 transmembrane glycoprotein of the human immunodeficiency virus type 1 (HIV-1) virion is usually thought to be inside the virion, but it has been shown recently that part of the tail is exposed on the virion exterior. Here, using a panel of antibodies, it was demonstrated that the same part of the tail is exposed on the surface of HIV-1-infected C8166 lymphoblastoid cells and HeLa cells infected with a gp41-expressing vaccinia virus recombinant. Both types of infected cell failed to react with p17 matrix protein-specific IgGs until permeabilized with saponin, confirming the integrity of the plasma membrane. Cell-surface exposure of the gp41 tail was independently demonstrated by inhibition of HIV-1-mediated cell–cell fusion by one of the gp41 tail-specific antibodies. These data also implicate the exposed region of the gp41 C-terminal tail either directly or indirectly in the viral fusion process. Its surface exposure suggests that the gp41 C-terminal tail may be a candidate for immune intervention or chemotherapy of infection.

Attenuating mutations in coxsackievirus B3 map to a conformational epitope that comprises the puff region of VP2 and the knob of VP3

Ten antibody escape mutants of coxsackievirus B3 (CVB3) were used to identify nucleotide substitutions that determine viral virulence for the heart and pancreas. The P1 region, encoding the structural genes of each mutant, was sequenced to identify mutations associated with the lack of neutralization. Eight mutants were found to have a lysine-to arginine mutation in the puff region of VP2, while two had a glutamate-to-glycine substitution in the knob of VP3. Two mutants, EM1 and EM10, representing each of these mutations, were further analyzed, initially by determining their entire sequence. In addition to the mutations in P1, EM1 was found to have two mutations in the 3D polymerase, while EM10 had a mutation in stem-loop II of the 5' nontranslated region (5'NTR). The pathogenesis of the mutants relative to that of CVB3 strain RK [CVB3(RK)] then was examined in A/J mice. Both mutants were found to be less cardiotropic than the parental strain, with a 40-fold (EM1) or a 100- to 1,000-fold (EM10) reduction in viral titers in the heart relative to the titers of CVB3(RK). The mutations in VP2, VP3, and the 5'NTR were introduced independently into the RK infectious clone, and the phenotypes of the progeny viruses were determined. The results substantiated that the VP2 and VP3 mutations reduced cardiovirulence, while the 5'NTR mutation in EM10 was associated with a more virulent phenotype when expressed on its own. Stereographic imaging of the two mutations in the capsomer showed that they lie in close proximity on either side of a narrow cleft between the puff and the knob, forming a conformational epitope that is part of the putative binding site for coreceptor DAF.

Widespread adaptive evolution in the human immunodeficiency virus type 1 genome

We investigated variable selective pressures among amino acid sites in HIV-1 genes. Selective pressure at the amino acid level was measured by using the nonsynonymous/synonymous substitution rate ratio (omega = dN/dS). To identify amino acid sites under positive selection with omega > 1, we applied maximum likelihood models that allow variable omega ratios among sites to analyze genomic sequences of 26 HIV-1 lineages including subtypes A, B, and C. Likelihood ratio tests detected sites under positive selection in each of the major genes in the genome: env, gag, pol, vif, and vpr. Positive selection was also detected in nef, tat, and vpu, although those genes are very small. The majority of positive selection sites is located in gp160. Positive selection was not detected if omega was estimated as an average across all sites, indicating the lack of power of the averaging approach. Candidate positive selection sites were mapped onto the available protein tertiary structures and immunogenic epitopes. We measured the physiochemical properties of amino acids and found that those at positive selection sites were more diverse than those at variable sites. Furthermore, amino acid residues at exposed positive selection sites were more physiochemically diverse than at buried positive selection sites. Our results demonstrate genomewide diversifying selection acting on the HIV-1.

Detection of IgA inhibiting the interaction between gp120 and soluble CD4 receptor in serum and saliva of HIV-1-infected patients

OBJECTIVE

To evaluate the presence of IgA directed to the CD4-binding domain of gp120 and to a conserved region of gp41 (the Kennedy epitope) in serum and parotid saliva of HIV-1-seropositive patients.

METHODS

IgA were separated from IgG by anion-exchange chromatography and protein G treatment. The reactivity of IgA was tested against peptides and fusion proteins of the maltose-binding protein (MBP) and the CD4-binding site (MBP24) and MBP and the Kennedy epitope (MBP42). The capacity of serum and saliva IgA to interfere with the gp120-soluble CD4 (sCD4) interaction was examined. IgA were also purified by affinity chromatography using the MBP proteins adsorbed to a resin.

RESULTS

Peptides representing the CD4-binding domain and the Kennedy epitope were recognized by serum and saliva IgA of HIV-1-seropositive patients. Of the sera and saliva samples tested, 6/26 serum IgA and 5/25 saliva IgA inhibited the gp120-sCD4 interaction by approximately 50%. The gp120-sCD4 interaction was inhibited by MBP24 affinity-purified IgA but not by MBP42 affinity-purified IgA.

CONCLUSION

Immunogens capable of eliciting IgA antibodies that inhibit gp120-CD4 binding might be efficiently used in vaccine to prevent mucosal transmission of HIV-1.

A novel monoclonal antibody specific to the C-terminal tail of the gp41 envelope transmembrane protein of human immunodeficiency virus type 1 that preferentially neutralizes virus after it has attached to the target cell and inhibits the production of infectious progeny

SAR1 is a new IgG2a murine monoclonal antibody derived by immunization with a plant virus expressing the sequence GERDRDR from the C-terminal tail of the gp41 transmembrane glycoprotein of human immunodeficiency virus type 1 (HIV-1). SAR1 binds to peptides and proteins carrying the GERDRDR sequence, to some but not all preparations of purified virus, and to cells infected with all viruses tested. In a standard neutralization assay, SAR1 failed to neutralize, or neutralized poorly, a number of T cell line-adapted viruses. However, it was more effective at postattachment neutralization. This was measured by two assays, the inhibition of the syncytium production by input virus, and the inhibition of the production of infectious progeny virus. In general SAR1 was more effective at neutralizing progeny virus than inoculum virus. Fifty percent inhibition of progeny virus production by different HIV-1 strains was obtained with 2-26 microg/ml of SAR1. The SAR1 neutralizing epitope was mapped specifically to the gp41 C-terminal tail. SAR1 is an unusual, if not unique, antibody whose activity supports the view that part of the gp41 C-terminal tail is exposed on the outside of the virion.

A region of the C-terminal tail of the gp41 envelope glycoprotein of human immunodeficiency virus type 1 contains a neutralizing epitope: evidence for its exposure on the surface of the virion

The approximately 150 amino acid C-terminal tail of the gp41 transmembrane glycoprotein of human immunodeficiency virus type 1 (HIV-1) is generally thought to be located inside the virion. However, we show here that both monoclonal IgG and polyclonal epitope-purified IgG specific for the (746)ERDRD(750) epitope that lies within the C-terminal tail neutralized infectious virus. IgG was mapped to the C-terminal tail by its failure to neutralize tail-deleted virus, and by sequencing of antibody-escape mutants. The fact that antibody does not cross lipid membranes, and infectious virus is by definition intact, suggested that ERDRD was exposed on the surface of the virion. This was confirmed by reacting virus and IgG, separating virus and unbound IgG by centrifugation, and showing that virus was neutralized to essentially the same extent as virus that had been in constant contact with antibody. Epitope exposure on virions was independent of temperature and therefore constitutive. Monoclonal antibodies specific to epitopes PDRPEG and IEEE, upstream of ERDRD, also bound to virions, suggesting that they too were located externally. Protease digestion destroyed the ERDRD and PDRPEG epitopes, consistent with their proposed external location. Altogether these data are consistent with part of the C-terminal tail of gp41 being exposed on the outside of the virion. Possible models of the structure of the gp41 tail, taking these observations into account, are discussed.

Monoclonal antibodies to elongation factor-1alpha inhibit in vitro translation in lysates of Sf21 cells

Elongation factor-1alpha (EF-1alpha) is an enzyme that is essential for protein synthesis. Although EF-1alpha offers an excellent target for the disruption of insect metabolism, agents known to interfere with EF-1alpha activity are toxic to humans. In this article, we describe the development of monoclonal antibodies (MAbs) that can disrupt the activity of insect EF-1alpha without cross-reacting with the human enzyme. MAbs were generated to EF-1alpha from Sf21 cells derived from the fall armyworm, Spodoptera frugiperda, by immunizing mice with EF-1alpha eluted from SDS-PAGE gels. The MAbs reacted with EF-1alpha in eggs and first through fifth instars of the fall armyworm in immunoblots of SDS-PAGE gels, but did not recognize EF-1alpha in human carcinoma cells and normal tissues. MAbs with the ability to recognize EF-1alpha in its native conformation, identified through immunoprecipitation experiments, were added to Sf21 cell lysates to determine whether the antibodies could inhibit incorporation of [(35)S]methionine into newly synthesized in vitro translation products. Of the four EF-1alpha-specific MAbs tested, three significantly inhibited protein synthesis when compared to the negative control antibody (P < 0.001, one-way ANOVA; followed by Dunnett's test, P < 0.05).

Rational development of prophylactic HIV vaccines based on structural and regulatory proteins

The severity of the AIDS epidemic clearly emphasises the urgent need to expedite HIV vaccine candidates into clinical trials. Prophylactic HIV vaccine candidates have been evaluated in non-human primates. Based on specific proof of principle studies the first phase III clinical studies have recently begun in humans. However, a truly effective HIV vaccine is not yet at hand and many problems related to specific properties of the virus remain to be overcome. Previously proven empirical approaches have largely failed and now rational thinking based on an understanding of immunity to lentiviral infections is needed. This review addresses the scientific problems and complications facing the development of an HIV vaccine as well as the possible strategies currently available to overcome these problems. Recent attention has focussed on identifying the immune correlates and mechanisms of protection from either HIV infection or protection from disease progression. Based on these observations, the logic and rational behind the development of multiple component vaccine strategies are highlighted.