The immune response to HIV: implications for vaccine development

HIV infection is accompanied by a vigorous immune response to the virus consisting of humoral and cellular elements that effectively neutralize virus infectivity and lyse infected cells when analyzed in cell culture models. However, this immune response shows no evidence of being able to eliminate the infection. The inability to clear this infection places HIV in a category of viruses for which vaccines have yet to be successfully developed. Indeed, effective vaccines have emerged only for viruses where natural immunity is part of the pathogenic process; it is such immunity that became the guiding principle for the development of the respective vaccines. It follows that the unique features of HIV infection and pathogenesis are issues to consider carefully in formulating vaccine strategies, particularly those that relate to its susceptibility to immune attack on the one hand, and its mechanisms of immune escape on the other.

Phylogenetic analysis of gag genes from 70 international HIV-1 isolates provides evidence for multiple genotypes

OBJECTIVE

To determine the extent of genetic variation among internationally collected HIV-1 isolates, to analyse phylogenetic relationships and the geographic distribution of different variants.

DESIGN

Phylogenetic comparison of 70 HIV-1 isolates collected in 15 countries on four continents.

METHODS

To sequence the complete gag genome of HIV-1 isolates, build multiple sequence alignments and construct phylogenetic trees using distance matrix methods and maximum parsimony algorithms.

RESULTS

Phylogenetic tree analysis identified seven distinct genotypes. The seven genotypes were evident by both distance matrix methods and maximum parsimony analysis, and were strongly supported by bootstrap resampling of the data. The intra-genotypic gag distances averaged 7%, whereas the inter-genotypic distances averaged 14%. The geographic distribution of variants was complex. Some genotypes have apparently migrated to several continents and many areas harbor a mixture of genotypes. Related variants may cluster in certain areas, particularly isolates from a single city collected over a short time.

CONCLUSIONS

The genetic variation among HIV-1 isolates is more extensive than previously appreciated. At least seven distinct HIV-1 genotypes can be identified. Diversification, migration and establishment of local, temporal 'blooms' of particular variants may all occur concomitantly.

[Production of recombinant proteins and their use in immunoblotting]

Chemico-enzymatic synthesis and cloning of genes coding viral proteins--precursors of p50 and gp120 in the eukaryotic vector system were carried out. The recombinant proteins obtained were shown to be used in immunochemical evaluation of AIDS virus antibodies in blood serum.

[Preparation of a specific immunogen based on bacteriophage M13]

Earlier we developed an expression vector on the basis of bacteriophage M13 allowing the exposure of short peptides on the virion surface. It was used to obtain a recombinant phage carrying the antigenic determinant of HIVI gag protein p17. This phage was tested as immunogen in rabbits. It was shown by ELISA that Ig against the fusion phage reacted with the 17-kDa core protein of the virus and with its polyprotein precursor p55 on strips activated by the transfer of HIVI viral proteins. These data may be used in vaccine development.

The HIV-1 Nef protein shares an antigenic determinant with a T-cell surface protein

OBJECTIVE

To characterize antigens on uninfected T lymphocytes reactive with monoclonal antibodies (MAb) directed against the HIV-1 Nef protein, and to search for antibodies directed against this epitope in HIV-1-infected individuals.

DESIGN

Murine MAb directed against an epitope of Nef defined by amino acids 60-73 reacted with cell surface antigens of normal peripheral blood lymphocytes and permanent human T-cell lines.

METHODS

The specificity of the MAb reaction was investigated by flow cytometry and immunofluorescence. The antigen was precipitated from lysates or uninfected cells using MAb or sera from HIV-1-infected individuals and analysed by Western blot and isoelectrofocusing.

RESULTS

An antigen with an apparent relative molecular mass of 137,000 and an isoelectric point of 8.45 was immunoprecipitated with the cross-reactive MAb from uninfected human T cells. Sera from HIV-positive individuals recognizing a Nef epitope partially overlapping with the binding site of the cross-reactive MAb stained the 137 kD protein precipitated with the MAb in Western blot analysis, while HIV-positive sera without antibodies to this Nef region and sera from uninfected individuals were negative.

CONCLUSION

The induction of autoantibodies cross-reactive with cellular surface proteins may play a role in the pathogenesis of AIDS.

A method to quantify binding of unlabeled peptides to class I MHC molecules and detect their allele specificity

A general method has been developed for measuring the stabilization of class I MHC molecules in extracts of the mutant cell lines .174/T2 and RMA-S. 35S-Met-labeled class I molecules which have been stabilized by peptides in vitro are immunoprecipitated with conformation dependent monoclonal antibodies and electrophoresed on polyacrylamide gels. The heavy and light chains are excised from the dried gel and quantified on a flat bed scintillation counter. The stabilizing effect of peptides on class I molecules in vitro correlates well with peptide binding measured by direct methods and can be therefore used to assess peptide binding affinity. We show that a peptide from HIV-1 gag (which has a high affinity for Db) is a CTL epitope restricted through Db, and also use the assay to analyse the effects of amino acid substitution on peptide affinity. In addition, the effect of a given peptide on a class I molecule within a mixture of human class I molecules can be distinguished by immunoprecipitation with the monomorphic antibody W6/32 and separation by 1-D isoelectric focussing. The technique therefore requires neither labeled peptide ligands nor allele-specific antibodies. It can be used to identify the peptide ligand of any human class I molecule, and gives a measure of peptide binding affinity. The technique should be of value in identifying epitopes recognized by CTL since we have found that these tend to bind with the highest affinities.

Correlation between seroreactivity to HIV-1 V3 loop peptides and male-to-female heterosexual transmission

OBJECTIVE

To evaluate the correlation between seroreactivity to peptides corresponding to the V3 loop of the major envelope glycoprotein from different HIV-1 strains and the risk of heterosexual HIV-1 transmission.

METHODS

Sera from 39 infected individuals (16 transmitters and 23 non-transmitters) were tested for reactivity against synthetic peptides representing sequences of the V3 loop apex from HIV-1 strains MN, SC, WMJ2, RF and IIIB.

RESULTS

A skewed distribution in seroreactivity to RF and IIIB peptides was observed between the two groups: reactivity was more prevalent in sera from non-transmitting than from transmitting individuals. Reactivity to the MN, SC and WMJ2 peptides was very frequent and there were no differences between the two groups.

CONCLUSION

These data suggest that antibodies reactive with a larger set of V3 apex peptides (i.e., cross-reactive antibodies) could play a role in the prevention of heterosexual transmission.

Principal neutralizing domain of HIV-1 is highly immunogenic when expressed on the surface of hepatitis B core particles

The development of subunit vaccines against HIV requires the identification of immunologically relevant antigens and a suitable method of antigen delivery. Ideally, defined epitopes with neutralizing activity should be included in a vaccine preparation. The carrier for such peptide sequences should enhance the immunogenicity of the selected epitopes. In this study hepatitis B virus core antigen (HBcAg) was used as a carrier moiety for the principal neutralizing domain (PND, V3-loop) of HIV-1. A 25 amino acid V3-loop sequence was fused to HBcAg at various positions by genetic engineering. The resulting hybrid HBcAg/HIV polypeptides were analysed for particle formation and immunogenicity. Fusion of the PND to an internal position replacing an immunodominant antibody-binding region of HBcAg or to a C-terminally truncated HBcAg resulted in the formation of hybrid particles with biochemical and biophysical properties similar to those of wild-type HBcAg particles. Both types of hybrids are recognized by monoclonal and polyclonal antisera raised against PND peptides of various HIV-1 isolates. Hybrid particles with a C-terminal fusion but not an internal fusion are also recognized by a polyvalent anti-HBcAg serum. In both cases the V3 domain is surface accessible. Immunization of mice with hybrid particles induces an enhanced antibody response against the V3 sequence. The internal fusion is more immunogenic than the C-terminal fusion.

Subtyping of human immunodeficiency virus isolates with a panel of monoclonal antibodies: identification of conserved and divergent epitopes on p17 and p25 core proteins

We have investigated the feasibility and significance of subtyping of human immunodeficiency virus (HIV) isolates with monoclonal antibodies (mAb) raised against the core proteins of HIV. A panel of 37 mAb tested for reactivity with HIV1 oligopeptides was used to analyse the antigenic relatedness among 14 HIV isolates which included 12 isolates of HIV1 from different geographical origins and 2 isolates of HIV2. Three out of these 37 mAb reacted with conserved epitopes expressed by all 14 HIV isolates tested. These reagents which included 2 mAb reacting with the 285-310 amino acid sequence of p25 and 1 mAb reacting with an epitope of p25 not mapped by the peptides' approach, also reacted with a non-human primate lentivirus. Five mAb reacting either with the 11-25 or 121-132 amino acid sequences of p17 or the 302-320 amino acid sequence of p25 reacted with strain-specific epitopes. The other 29 mAb reacted with polymorphic epitopes and thereby define subfamily and subtype-specific markers.

Bacterial expression, purification, and in vitro N-myristoylation of HIV-1 p17gag

The coding region of the N-terminal 17-kDa portion of HIV-1 Pr55gag (p17gag) was cloned into the pET-3c expression vector and was used to overexpress HIV-1 p17gag in Escherichia coli. Induction of the transformed bacteria caused the accumulation of a 17-kDa polypeptide in the soluble cell fraction which was released by sonication in hypotonic nondetergent buffer. The 17-kDa polypeptide was purified by ammonium sulfate precipitation and successive chromatography on G-75 Sephadex, DEAE-Sephacel, and S-Sephadex. The final product was purified 12-fold with about a 16% recovery from the original soluble cell lysate and was judged to be 97+% pure by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Western blotting with two different antibodies confirmed the identify of the purified 17-kDa polypeptide as authentic p17gag. In the presence of myristoyl-CoA and bovine brain N-myristoyl-transferase, p17gag was quantitatively N-myristoylated in vitro with a pseudo-first-order rate constant of 4.7 +/- 1.0 x 10(-3) min-1, but with only about 3% of the catalytic efficiency of N-myristoylation of a 16-residue peptide homologous to the N-terminus of p17gag. The myristate group in the N-myristoylated p17gag was stable to treatment with detergent and hydroxylamine consistent with a covalent N-acyl-amide linkage. The N-myristoylglycyl linkage was confirmed by partial acid hydrolysis and identification of the p-nitrobenzylazlactone derivative of the resulting N-myristoylglycine by high-performance liquid chromatography.

Immunodominant epitopes of HIV-1 p17 and p24

Immunodominant antibody-binding sites were mapped using overlapping synthetic peptides of the structural proteins p17 and p24 of human immunodeficiency virus type 1 (HIV-1). Using sera from HIV-1-infected individuals at a variety of disease states, three major epitopes were identified within p17 and one within p24. Antibodies which recognized these epitopes were present in all risk groups throughout all stages of HIV infection, regardless of the presence of high levels of serum p24 antigen.

Identification and characterization of fusion and processing domains of the human immunodeficiency virus type 2 envelope glycoprotein

The envelope glycoprotein of the human immunodeficiency virus type 2 (HIV-2) is synthesized as a polyprotein precursor which is proteolytically processed to produce the mature surface and transmembrane envelope glycoproteins. The processed envelope glycoprotein species are responsible for the fusion between the viral envelope and the host cell membrane during the infection process. The envelope glycoprotein also induces syncytium formation between envelope-expressing cells and receptor-bearing cells. To characterize domains of the HIV-2 envelope glycoprotein involved in membrane fusion and in proteolytic processing, we introduced single amino acid mutations into the region of the HIV-2 surface glycoprotein corresponding to the principal neutralizing determinant (the V3 loop) of HIV-1, the putative HIV-2 envelope precursor-processing sequence, and the hydrophobic amino terminus of the HIV-2 transmembrane envelope glycoprotein. The effects of these mutations on syncytium formation, virus infectivity, envelope expression, envelope processing, and CD4 binding were analyzed. Our results suggest that the V3-like region of the HIV-2 surface glycoprotein and the hydrophobic amino terminus of the transmembrane glycoprotein are HIV-2 fusion domains and characterize the effects of mutations in the HIV-2 envelope glycoprotein precursor-processing sequence.

HIV-1 genomic RNA diversification following sexual and parenteral virus transmission

Human immunodeficiency virus type 1 (HIV-1) genomic RNA variation was studied in seven presumed donor-recipient pairs directly following sexual (6/7) or parenteral (1/7) transmission. The first RNA-positive serum sample of each recipient and the serum sample of the virus transmitter, identified by epidemiological history and taken within a time bracket of three months of the recipient seroconversion, were analyzed by polymerase chain reaction amplification followed by sequencing of eight cDNA clones of 276 bp, including the V3 coding region. The sequence populations of the recipients were without exception homogeneous, while the sequence populations of the transmitters showed varying degrees of heterogeneity. Nucleotide distance between consensus sequences of unrelated individuals from the Amsterdam population (interpatient variation) averaged 11% (range 7-15%). The largest distance between two clonal sequences of one individual (intrapatient variation) was also 11%. Consensus sequences of five recipients differed by only 0-1% from the consensus sequence of the presumed transmitter, including two pairs of which the transmission was either proven or highly probable. This contrasted with a difference of 10-12% in two pairs, casting doubt on the epidemiological relatedness. Antibody reactivity to a panel of V3 peptides with varying degrees of similarity to the V3 sequences obtained did not augment the discriminatory power of sequence analysis. Results of the sequential sequencing of samples of one transmitter suggest that this was due to an anamnestic antibody response of the transmitter to early variants. From the loss of sequence heterogeneity following transmission and the consensus sequence similarities observed within five transmitter-recipient pairs, we conclude that HIV-1 transmission results in the selection of a limited number of genomes carrying on the infection in the new host, but does not generally lead to a shift in the sequence population as defined by the consensus sequence.

[Molecular analysis of the principal neutralization epitope (V3 loop) of human immunodeficiency virus type 1 in Argentina]

The main goal of the present paper was to analyze the molecular diversity of the principal neutralizing domain (V3 loop) of the HIV 1 gp120 in samples from patients of Argentina. The study was carried out on a total of 30 HIV 1 positive blood samples, obtained during 1991-1992, belonging to 15 intravenous drug users (group A), 5 homosexual men (group B), 8 children born to HIV 1 positive mothers (group C) and 2 AIDS patients (group D). By using extracted DNA from peripheral blood lymphocytes and from infected cells of the viral isolates in the case of the 2 AIDS patients, the V3 loop region was amplified by means of polymerase chain reaction. Direct sequencing by Sanger methodology was then performed on DNA fragments and nucleotide sequences obtained were translated into the correspondent amino acids. Consensus sequences for each group and a general consensus sequence were established (Table 1). Its alignment with V3 loop amino acid sequences of the major HIV 1 strains isolated worldwide is showed in table 2. Homology analysis between each sequence of the study population and sequence of different HIV 1 isolates showed that most of these samples share high homology with SF2 and BH10 strains. In contrast a low homology was found with JH3 and MN isolated (table 3). The presence of highly conserved amino acid residues as substitutions and insertions was determined in the Argentinian V3 loop sequences giving them a local pattern. The present paper is of great importance for our country, considering that the V3 loop is the main neutralizing domain becoming a major target in the development of HIV 1 vaccine. To our knowledge, this is the first report on the sequencing of the principal neutralizing domain of the Human Immunodeficiency Virus type 1 in Latin America.

Gag proteins of the highly replicative MN strain of human immunodeficiency virus type 1: posttranslational modifications, proteolytic processings, and complete amino acid sequences

The MN strain of human immunodeficiency virus type 1 was grown in H9 cells, concentrated by centrifugation, and disrupted, and proteins were purified by reversed-phase high-pressure liquid chromatography. Complete amino acid sequences were determined for the mature Gag proteins, showing natural proteolytic cleavage sites and the order of proteins (p17-p24-p2-p7-p1-p6) in the Gag precursors. At least two sequence variants of p24 and eight sequence variants of p17 were detected. The two most abundant variants of p24 and p17 represented at least 50% +/- 5% and 20% +/- 5% of their totals, respectively. These data suggest heterogeneity in the virus population, with 50% of the total virus containing the most abundant forms of p17 and p24 and 20% of the virus containing the second most abundant forms. The Gag precursors of these suggested viruses differ from each other by only 3 amino acid residues but differ from the precursors predicted by the published MN proviral DNA sequence by 10 residues. Electrospray ionization mass spectrometry analysis of the purified p24 forms showed that the measured molecular weight of the protein was 200 +/- 50 atomic mass units greater than the calculated molecular weight. The source of additional mass for the p24 forms was not determined, but the observation is consistent with previous suggestions that the protein is phosphorylated. Greater than 98% of the total recovered p17 was myristylated at the N-terminal glycine residue, and the measured molecular weights (as determined by electrospray ionization mass spectrometry) of the most abundant forms were within 3 atomic mass units of the calculated molecular weights (15,266).

Viral culture and p24 antigenemia of human immunodeficiency virus (HIV)-infected individuals correlated with antibody profiles determined with recombinant polypeptides of all HIV-1 open-reading frames

The association between viral activity and antibody profiles was investigated in 202 individuals infected by the human immunodeficiency virus (HIV) grouped according to their Walter Reed clinical stage. Each study group was subdivided into subjects positive or negative for markers of active viral replication: presence of serum p24 antigen and viral culture. In Western blots using recombinant antigens, sera of HIV-positive individuals with positive viral markers had a significantly lower antibody reactivity to several viral proteins than did individuals without viral markers. Noticeably, proteins of the gag (p24, p17) and env (gp120, COOH-terminal part of gp41) open-reading frames revealed a decreased reactivity. The antibody response to the regulatory proteins revealed no or poor association with viral activity in the host. The results suggest that seroreactivity is mainly influenced by factors reflecting the viral activity of an HIV-infected individual, while the clinical stage of the patient is less important. Especially, reductions in antibodies against gp120 and p17 were useful markers associated with increased viral activity.

Selective transmission of human immunodeficiency virus type-1 variants from mothers to infants

Multiple human immunodeficiency virus type-1 sequences from the V3 and V4-V5 regions of the envelope gene were analyzed from three mother-infant pairs. The infants' viral sequences were less diverse than those of their mothers. In two pairs, a proviral form infrequently found in the mother predominated in her infant. A conserved N-linked glycosylation site within the V3 region, present in each mother's sequence set, was absent in all of the infants' sequence sets. These findings demonstrate that a minor subset of maternal virus is transmitted to the infant.

Development of a vaccine for the prevention of AIDS, a critical appraisal

The pathogenesis and clinical expression of HIV-1 infection in humans is considered in terms of classical pathogenetic studies of viral infections for which successful vaccines have been produced. The unique features of HIV pathogenesis are defined, and gaps in knowledge identified as a framework for considering designs for immune intervention. Envelope-derived candidate vaccines have been used in immunization and challenge experiments in SIV/macaque or HIV/chimpanzee models, presented either as vaccinia recombinant vectors or as subunits, singly or in sequence. These studies have been paralleled by clinical trials for safety and immunogenicity in seronegative individuals. Data generated will permit comparison of immune responses to specific antigens and delivery systems in animal models and in humans. In limited studies conducted under optimized conditions, non-human primates have been protected against virus challenge when immunized with some candidate vaccines or following passive transfer of high-titred antibody. Consideration of current information suggests that in order to prevent HIV infection it may be necessary to devise new strategies capable of inducing and maintaining high threshold titres of biologically relevant antibody as well as persistence of active cytotoxic T cells recognizing multiple epitopes.

A mathematical model of vaccination against HIV to prevent the development of AIDS

Vaccination and post-exposure immunization against the human immunodeficiency viruses (HIV-1 and HIV-2) faces the problem of the extensive genetic and antigenic variability of these viruses. This raises the question of what fraction of all possible antigen strains of the virus must be recognized by the immune response to a vaccine to prevent development of acquired immunodeficiency disease (AIDS). The success of a vaccine can depend on the variability of the target epitopes. The different HIV variants must be suppressed faster than new escape mutants can be produced. In this paper the antigenic variation of HIV during an individual infection is described by a stochastic process. The central assumption is that antigenic drift is important for the virus to survive immunological attack and to establish a persistent infection that leads to the development of AIDS after a long incubation period. The mathematical analysis reveals that the fraction of antigenic variants recognized by the immune response, that is induced by a successful immunogen, must exceed 1-1/R, where R is the diversification rate of the virus population. This means that if each HIV strain can produce, on average, five new escape mutants, then more than 80% of the possible variants must be covered by the immunogen. A generic result of the model is that, no matter how immunogenic a vaccine is, it will fail if it does not enhance immune attack against a sufficiently large fraction of strains. Furthermore, it is shown that the timing of the application of post-exposure immunization is important.

Antigenic diversity thresholds and the development of AIDS

Longitudinal studies of patients infected with HIV-1 reveal a long and variable incubation period between infection and the development of AIDS. Data from a small number of infected patients show temporal changes in the number of genetically distinct strains of the virus throughout the incubation period, with a slow but steady rise in diversity during the progression to disease. A mathematical model of the dynamic interaction between viral diversity and the human immune system suggests the existence of an antigen diversity threshold, below which the immune system is able to regulate viral population growth but above which the virus population induces the collapse of the CD4+ lymphocyte population. The model suggests that antigenic diversity is the cause, not a consequence, of immunodeficiency disease. The model is compared with available data, and is used to assess how the timing of the application of chemotherapy or immunotherapy influences the rate of progress to disease.