Serological responses in chimpanzees inoculated with human immunodeficiency virus glycoprotein (gp120) subunit vaccine

A brief history of the global effort to develop a preventive HIV vaccine

Soon after HIV was discovered as the cause of AIDS in 1983-1984, there was an expectation that a preventive vaccine would be rapidly developed. In trying to achieve that goal, three successive scientific paradigms have been explored: induction of neutralizing antibodies, induction of cell mediated immunity, and exploration of combination approaches and novel concepts. Although major progress has been made in understanding the scientific basis for HIV vaccine development, efficacy trials have been critical in moving the field forward. In 2009, the field was reinvigorated with the modest results obtained from the RV144 trial conducted in Thailand. Here, we review those vaccine development efforts, with an emphasis on events that occurred during the earlier years. The goal is to provide younger generations of scientists with information and inspiration to continue the search for an HIV vaccine.

Lessons in nonhuman primate models for AIDS vaccine research: from minefields to milestones

Nonhuman primate (NHP) disease models for AIDS have made important contributions to the search for effective vaccines for AIDS. Viral diversity, persistence, capacity for immune evasion, and safety considerations have limited development of conventional approaches using killed or attenuated vaccines, necessitating the development of novel approaches. Here we highlight the knowledge gained and lessons learned in testing vaccine concepts in different virus/NHP host combinations.

Epitope mapping: the first step in developing epitope-based vaccines

Antibodies are an effective line of defense in preventing infectious diseases. Highly potent neutralizing antibodies can intercept a virus before it attaches to its target cell and, thus, inactivate it. This ability is based on the antibodies’ specific recognition of epitopes, the sites of the antigen to which antibodies bind. Thus, understanding the antibody/epitope interaction provides a basis for the rational design of preventive vaccines. It is assumed that immunization with the precise epitope, corresponding to an effective neutralizing antibody, would elicit the generation of similarly potent antibodies in the vaccinee. Such a vaccine would be a ‘B-cell epitope-based vaccine’, the implementation of which requires the ability to backtrack from a desired antibody to its corresponding epitope.

In this article we discuss a range of methods that enable epitope discovery based on a specific antibody. Such a reversed immunological approach is the first step in the rational design of an epitope-based vaccine. Undoubtedly, the gold standard for epitope definition is x-ray analyses of crystals of antigen: antibody complexes. This method provides atomic resolution of the epitope; however, it is not readily applicable to many antigens and antibodies, and requires a very high degree of sophistication and expertise. Most other methods rely on the ability to monitor the binding of the antibody to antigen fragments or mutated variations. In mutagenesis of the antigen, loss of binding due to point modification of an amino acid residue is often considered an indication of an epitope component. In addition, computational combinatorial methods for epitope mapping are also useful. These methods rely on the ability of the antibody of interest to affinity isolate specific short peptides from combinatorial phage display peptide libraries. The peptides are then regarded as leads for the definition of the epitope corresponding to the antibody used to screen the peptide library. For epitope mapping, computational algorithms have been developed, such as Mapitope, which has recently been found to be effective in mapping conformational discontinuous epitopes. The pros and cons of various approaches towards epitope mapping are also discussed.

Differences in early virus loads with different phenotypic variants of HIV-1 and SIV(cpz) in chimpanzees

OBJECTIVE

A comparative study of the replication kinetics of different HIV-1 variants (including SIV(cpz)) was undertaken to determine which viral characteristics were associated with sustained plasma viraemia in chimpanzees.

DESIGN

Plasma samples from chimpanzees infected with six different HIV-1 clade B isolates were compared with plasma samples from SIV(cpz-ant)-infected chimpanzees.

METHODS

A pan-clade quantitative competitive reverse transcriptase-polymerase chain reaction assay was developed based on conserved primer sequences recognizing M, N and O human lentiviruses as well as different SIV(cpz) isolates.

RESULTS

Important differences between early kinetics in the human lentivirus isolates as well as compared with the chimpanzee isolate SIV(cpz-ant) were observed. R5-dependent non-syncytium-inducing (NSI) isolates (5016, Ba-L, SIV(cpz)) were found to have relatively higher viral loads than the syncytium-inducing (SI), X4-dependent primary (SF2), T cell-adapted (IIIB) or X4/R5 (Han2, DH12) SI primary isolates.

CONCLUSION

Infection of chimpanzees with NSI R5-utilizing isolates correlated with persistent viraemia (approximately 10(4) RNA equivalents/ml) in contrast to transient viraemia observed after infection with SI X4-utilizing isolates.

Induction of neutralizing antibodies to T-cell line-adapted and primary human immunodeficiency virus type 1 isolates with a prime-boost vaccine regimen in chimpanzees

Five chimpanzees were immunized by administration of one or more intranasal priming doses of one to three recombinant adenoviruses containing a gp160 insert from human immunodeficiency virus type 1 (HIV-1) MN (HIV-1MN) followed by one or more boosts of recombinant HIV-1SF2 gp120 delivered intramuscularly with MF59 adjuvant. This regimen resulted in humoral immune responses in three of five animals. Humoral responses included immunochemically active anti-H1V-1 antibodies (Abs) directed to recombinant gp120 and neutralizing Abs reactive with T-cell-line-adapted HIV-1MN and HIV-1SF2. In addition, neutralizing activity was detected to the two homologous primary isolates and to two of three heterologous primary isolates which, like the immunizing strains, can use CXCR4 as a coreceptor for infection. The three animals with detectable neutralizing Abs and a fourth exhibiting the best cytotoxic T-lymphocyte response were protected from a low-dose intravenous challenge with a cell-free HIV-1SF2 primary isolate administered 4 weeks after the last boost. Animals were rested for 46 weeks and then rechallenged, without a boost, with an eightfold-higher challenge dose of HIV-1SF2. The three animals with persistent neutralizing Abs were again protected. These data show that a strong, long-lived protective Ab response can be induced with a prime-boost regimen in chimpanzees. The data suggest that in chimpanzees, the presence of neutralizing Abs correlates with protection for animals challenged intravenously with a high dose of a homologous strain of HIV-1, and they demonstrate for the first time the induction of neutralizing Abs to homologous and heterologous primary isolates.

Correlation of titer of antibody to principal neutralizing domain of HIVMN strain with disease progression in Japanese hemophiliacs seropositive for HIV type 1

With the use of the principal neutralizing determinant (PND) peptide-based ELISA to measure anti-PND antibodies that specifically bound synthetic peptides derived from HIVIIIB, HIVMN, HIVRF, HIVSC, HIVWJM-2, HIVAf1l.con, or HIVAf2.con, type-specific antibodies to the HIVMN peptide were studied in 350 serum specimens from Japanese with hemophilia A who had been injected with known unheated factor VIII concentrates until 1985 and had been infected with HIV-1 subtype B. These antibodies were not found in any of the seronegative sera of hemophiliacs, patients with autoimmune diseases, or normal healthy controls. Further, all hemophiliacs rapidly progressing to AIDS and death among the 95 hemophiliacs in a restricted Nara area had antibody titers of less than 20 and their low levels preceded the rapid progression to the disease state. In contrast, slowly progressing hemophiliacs maintained an antibody titer of more than 100 from the initial stages of viral infection and remained asymptomatic. Sequence analysis of the V3 regions of HIV-1 indicated that the hemophiliacs who maintained a high anti-PNDMN antibody level showed a conserved MN sequence. In contrast, the HIV-infected hemophiliacs with nonreactivity in the ELISA showed sequence changes in the neutralizing epitopes of HIVMN. The dynamic of the serum anti-PNDMN antibody titer appear to be a characteristic indicator of the progression of the HIV-infected status in Japanese hemophiliacs seropositive for HIV-1.

Envelope glycoproteins of HIV-1, HIV-2, and SIV purified with Galanthus nivalis agglutinin induce strong immune responses

Lectin affinity chromatography was used to purify in a single step the envelope glycoproteins of HIV-1, HIV-2, and SIV. Envelope glycoproteins carry the major determinants essential for protection by the humoral immune response. The purification of these proteins has previously been a laborious procedure. The glycoproteins were purified by a one-step procedure to a high level of purity by using Galanthus nivalis agglutinin (GNA). The purified glycoprotein had CD4-binding and antigenic reactivities. Strong immune responses to envelope proteins and peptides were seen in mice and primates after immunization with these preparations.

Statistical analysis of sparse infection data and its implications for retroviral treatment trials in primates

Reports on retroviral primate trials rarely publish any statistical analysis. Present statistical methodology lacks appropriate tests for these trials and effectively discourages quantitative assessment. This paper describes the theory behind VACMAN, a user-friendly computer program that calculates statistics for in vitro and in vivo infectivity data. VACMAN's analysis applies to many retroviral trials using i.v. challenges and is valid whenever the viral dose-response curve has a particular shape. Statistics from actual i.v. retroviral trials illustrate some unappreciated principles of effective animal use: dilutions other than 1:10 can improve titration accuracy; infecting titration animals at the lowest doses possible can lower challenge doses; and finally, challenging test animals in small trials with more virus than controls safeguards against false successes, "reuses" animals, and strengthens experimental conclusions. The theory presented also explains the important concept of viral saturation, a phenomenon that may cause in vitro and in vivo titrations to agree for some retroviral strains and disagree for others.

Expression and characterization of genetically engineered human immunodeficiency virus-like particles containing modified envelope glycoproteins: implications for development of a cross-protective AIDS vaccine

Noninfectious human immunodeficiency virus type 1 (HIV-1) viruslike particles containing chimeric envelope glycoproteins were expressed in mammalian cells by using inducible promoters. We engineered four expression vectors in which a synthetic oligomer encoding gp120 residues 306 to 328 (amino acids YNKRKRIHIGP GRAFYTTKNIIG) from the V3 loop of the MN viral isolate was inserted at various positions within the endogenous HIV-1LAI env gene. Expression studies revealed that insertion of the heterologous V3(MN) loop segment at two different locations within the conserved region 2 (C2) of gp120, either 173 or 242 residues away from the N terminus of the mature subunit, resulted in the secretion of fully assembled HIV-like particles containing chimeric LAI/MN envelope glycoproteins. Both V3 loop epitopes were recognized by loop-specific neutralizing antibodies. However, insertion of the V3(MN) loop segment into other regions of gp120 led to the production of envelope-deficient viruslike particles. Immunization with HIV-like particles containing chimeric envelope proteins induced specific antibody responses against both the autologous and heterologous V3 loop epitopes, including cross-neutralizing antibodies against the HIV-1LAI and HIV-1MN isolates. This study, therefore, demonstrates the feasibility of genetically engineering optimized HIV-like particles capable of eliciting cross-neutralizing antibodies.

High-titer HIV-1 neutralizing antibody response of rhesus macaques to gp160 and env peptides

Three groups of four rhesus macaques were immunized twice, one month apart with purified recombinant HIV-1LAI gp160 in the presence of either alum, incomplete Freund's adjuvant (IFA), or SAF-1. Two months later, the animals were injected twice again with a synthetic peptide with the sequence of the principal neutralization determinant (PND) of the HIV-1LAI isolate mixed with the same adjuvants. All animals received a booster injection of gp160 and PND peptide at 6 months. This regimen of immunization induced in the SAF-1 and IFA groups a high-titer neutralizing antibody response that declined progressively over the course of the following 6 months. In contrast, only a weak response was observed in the alum group. Neutralizing antibody titers varied as anti-PND titers, suggesting that they were principally targeted to the PND. A shortened immunization protocol comprising two injections of gp160 at 0 and 1 month followed by one injection of PND peptide at 3 months is suggested as optimal for the induction of high titers of HIV-1 neutralizing antibodies in primates.

The auto-regulation model: a unified concept of how HIV regulates its infectivity, pathogenesis and persistence

The life cycle of HIV can be divided into two distinct stages: intracellular and extracellular. The prevailing view is that the intracellular stage provides the only locus for regulating the virus in response to physiologic stimuli. Such regulation is accomplished by modulating the rates of transcription, translation and viral assembly. The extracellular stage consists of physical processes such as diffusion, adhesion and penetration of cells by viral particles. These latter processes are commonly thought to be "automatic" and not subject to regulation. For the past several years, we have developed means of more carefully measuring and characterizing the extracellular stage of HIV infection, and we have obtained evidence indicating that novel regulatory processes do, in fact, take place during this extracellular stage. We believe that this extracellular regulation permits HIV to adapt to a wide range of physiologic cell densities, to maintain persistent but slow growing infection, and to defeat the protective activity of humoral blockers. The overall purpose of this review is to consider our evidence for this hypothesis.

Human immunodeficiency virus

A successful AIDS vaccine must elicit an immune state that will prevent the establishment of an HIV-1 persistent infection. This is a unique and difficult goal for a vaccine. Most vaccines elicit or prime for immune responses that prevent or attenuate the expression of clinical disease following infection with the pathogen. However, current evidence suggest that, following persistent infection with HIV-1, antiviral immune responses do not prevent the long-term progression to disease. Hence, it seems that the development of the persistent infection must be prevented. The ability of the immune response to accomplish this goal depends upon the efficiency with which the virus establishes persistence in the host. This is unknown for HIV-1. As a result, early efforts at vaccine development have focused on humoral immune responses directed against the virus particle in the attempt to prevent any infection of the host's cells. Studies with chimpanzees, as a model for HIV-1 infection, suggest that virus-neutralizing antibodies directed against the third hypervariable (V3) domain of the viral gp120 envelope glycoprotein may be particularly effective in preventing this infection. Studies also are in progress, both in chimpanzees and humans, to define the immunogenicity and effectiveness of various immunogens derived from the viral envelope and core structural proteins. Efforts that have concentrated on the gp120 V3 domain (or PND) have defined the extent of this region's variability and have established elements of generally conserved structure and sequence. The construction of these elements into practical and effective immunogens is an important goal. Finally, it is essential that basic studies be performed to determine if humoral or cellular immune responses directed against virus-infected cells would aid in preventing the establishment of an HIV-1 persistent infection. Such immune responses, if effective and in conjunction with specific virus-neutralizing antibody responses, would enhance the probability that an effective HIV-1 vaccine could be developed.

Native but not denatured recombinant human immunodeficiency virus type 1 gp120 generates broad-spectrum neutralizing antibodies in baboons

The protection of individuals from human immunodeficiency virus type 1 (HIV-1) infection with an envelope subunit derived from a single isolate will require the presentation of conserved epitopes in gp120. The objective of the studies presented here was to test whether a native recombinant gp120 (rgp120) immunogen would elicit responses to conserved neutralization epitopes that are not present in a denatured recombinant gp120 antigen from the same virus isolate. In a large study of 51 baboons, we have generated heterologous neutralizing activity with native, glycosylated rgp120SF2 but not with denatured, nonglycosylated env 2-3SF2. After repeated exposure to rgp120SF2 formulated with one of several adjuvants, virus isolates from the United States, the Caribbean, and Africa were neutralized. The timing of the immunization regimen and the choice of adjuvant affected the virus neutralization titers both quantitatively and qualitatively. These results suggest that vaccination with native, glycosylated rgp120 from a single virus isolate, HIV-SF2, may elicit a protective immune response effective against geographically and sequentially distinct HIV-1 isolates.

Neutralization of divergent HIV-1 isolates by conformation-dependent human antibodies to Gp120

The spectrum of human immunodeficiency virus type 1 (HIV-1) isolates neutralized by antibodies from HIV-1-infected humans is broader than the spectrum of isolates neutralized by sera from animals immunized with purified gp120 subunits. This broader neutralization was due, in part, to the presence of antibodies to conserved gp120 conformational epitopes. Purified conformation-dependent gp120-specific human antibodies neutralized a wider range of virus isolates than human antibodies directed to linear determinants in gp120 and were also responsible for the majority of the gp120-specific CD4-blocking activity of HIV-1-infected human sera. A gp120 subunit vaccine that effectively presents these conformation-dependent neutralization epitopes should protect against a broader range of HIV-1 variants than a vaccine that presents exclusively linear determinants.

Biological Agents in Infectious Diseases

This article examines the use of classical as well as novel immunological agents to augment the immune system. Strategies to prevent infectious disease using vaccines to prime the immune system are discussed. A prospective overview of acquired immunodeficiency syndrome (AIDS) vaccine development provides insights into the possible agents that may be developed in the near future. The rationale for the use of intravenous immunoglobulin and colony stimulating factors in selected clinical situations is also discussed.

High prevalence of antibodies to the gp120 V3 region principal neutralizing determinant of HIV-1MN in sera from Africa and the Americas

Neutralizing antibodies (NA) against HIV-1MN and HIV-1IIIB, and antibodies binding to synthetic peptides (BA) derived from the gp120 envelope V3 region principal neutralizing determinants (PND) of the HIV-1MN, HIV-1IIIB, and HIV-1Z3 virus strains were assayed in HIV-1 antibody-positive sera from the United States, Haiti, Brazil, Zaire, and Zimbabwe. The ability of soluble PND peptide to block neutralization of the corresponding virus by representative sera was also tested. In each country, NA and BA titers were highest against the HIV-1MN strain, and compared with other countries, NA and BA titers against HIV-1MN were higher in sera from the United States and Haiti. When NA titers were compared with BA titers against either HIV-1MN or HIV-1IIIB, no correlation was found for the HIV-1IIIB strain, but there was a significant correlation for HIV-1MN. Addition of the HIV-1MN strain peptide to a neutralization assay for HIV-1MN resulted in a four- to tenfold reduction in NA titers in sera from the United States, Zaire, and Brazil. The results suggest that HIV-1MN and closely related variants are prevalent in many parts of the world, and that antibodies directed against the PND account for most of the neutralizing activity in sera of infected individuals.

Antibody responses of chimpanzees immunized with synthetic peptides corresponding to full-length V3 hypervariable loops of HIV-1 envelope glycoproteins

Immunization of primates or humans with human immunodeficiency virus type 1 (HIV-1) glycoproteins usually elicited moderate immune responses to the principal neutralizing determinant (PND) located within the V3 hypervariable loop of gp120. Since an antibody response to the PND appears to be protective, experiments were carried out to determine the responsiveness of chimpanzees to immunization with synthetic peptides corresponding to the full-length V3 loop. Seven chimpanzees (4 preimmunized with gp160, 2 preimmunized with HIV-1 antigens unrelated to gp160, and 1 unimmunized) were vaccinated with a mixture of full-length V3 loop peptides from 21 distinct HIV-1 isolates (clones) either in unconjugated form or linked to carrier proteins from HIV-1 nef and gag P18, respectively. Six chimpanzees developed high levels of antibodies to the peptides (dilution endpoints 1: greater than 25,000), and 5 had high levels of antibodies to gp120 from HIV-1IIIB (endpoint titers 1: greater than 500,000). Chimpanzees immunized with peptide-carrier conjugates (4) had antibodies to the carrier proteins nef and gag P18, respectively (endpoint titers 1: greater than or equal to 35,000). Virus-neutralizing (VN) antibodies were detected in sera of 5 of 7 chimpanzees, but were present at titers of 1: greater than or equal to 400 only in sera of 2 chimpanzees. One of these was challenged with HIV-1 and was protected against infection, as reported elsewhere. The antibodies were primarily specific for the HIV-1 isolate used for primary immunization before boosting with peptides. The relatively low dilution endpoints of VN antibodies as compared with endpoints determined by site-specific immunoassays probably can be ascribed to imperfect mimicry of conformational epitopes by synthetic peptides. Nevertheless, sequential or simultaneous immunization with recombinant envelope glycoproteins of HIV-1 and selected synthetic peptides offers an approach for eliciting protective immunity against HIV-1.

The inability of human immunodeficiency virus to infect chimpanzee monocytes can be overcome by serial viral passage in vivo

Studies of lentivirus infection in ruminants, nonhuman primates, and humans suggest that virus infection of macrophages plays a central role in the disease process. To investigate whether human immunodeficiency virus type 1 (HIV-1) can infect chimpanzee macrophages, we recovered monocytes from peripheral blood mononuclear cells of HIV-1-negative animals and inoculated these and control human monocytes with a panel of four human-passaged monocytotropic virus strains and one chimpanzee-passaged isolate. HIV-1 infected human monocytes synthesized proviral DNA, viral mRNA, p24 antigen, and progeny virions. In contrast, except for the chimpanzee-passaged HIV-1 isolate, chimpanzee monocytes failed to support HIV-1 replication when cultured under both identical and a variety of other conditions. Proviral DNA was demonstrated only at background levels in these cell cultures by polymerase chain reaction for gag- and env-related sequences. Interestingly, the chimpanzee-passaged HIV-1 isolate did not replicate in human monocytes; viral p24 antigens and progeny virions were not detected. The same monocytotropic panel of HIV-1 strains replicated in both human and chimpanzee CD4+ T lymphoblasts treated with phytohemagglutinin and interleukin-2. The failure of HIV-1 to infect chimpanzee monocytes, which can be overcome by serial in vivo viral passage, occurs through a block early in the viral life cycle.

Immunization of chimpanzees with the HIV-1 glycoprotein gp160 induces long-lasting T-cell memory

The goal of the present study was to investigate the antigen-specific T-cell response to the recombinant HIV envelope glycoprotein (gp160) and to test the effect of various adjuvant formulations on the efficiency of T-cell priming as well as on magnitude and longevity of the gp160-specific T-cell response. Our studies revealed that, in combination with an appropriate adjuvant (lipid-based adjuvant or mineral carrier complex), immunization with recombinant gp160 led to the appearance of gp160-primed T cells. The T-cell response obtained was substantial (proliferative response of greater than 100,000 delta dpm after one primary and two booster immunizations), gp160-specific (proliferation only in response to gp160, no proliferation after addition of a mock gp160 preparation), and long-lasting (T cell responses of greater than 50,000 delta dpm were observed more than one year after the last booster). The results presented here differ from those of previous studies in that they show the presence of substantial and long-lasting T-cell memory toward the immunogen gp160. Therefore further investigations on the use of these preparations as HIV candidate vaccines appear to be justified.

Sequential immunizations with rgp120s from independent isolates of human immunodeficiency virus type 1 induce the preferential expansion of broadly crossreactive B cells

The gp120 envelope glycoprotein of human immunodeficiency virus type 1 (HIV-1) is a dominant target against which the host's humoral immune response is directed. Unfortunately, gp120 proteins from different isolates of HIV are antigenically distinct, complicating the use of the envelope glycoprotein in vaccines designed to prevent acquired immunodeficiency syndrome. Using an enzyme-linked immunosorbent spot assay (ELISA), BALB/c mice immunized and boosted with recombinant purified gp120 were studied at the single cell level for their humoral immune response to HIV-1 envelope proteins. Approximately 90% of responding B cells produced antibodies reactive with the immunizing form of gp120 but not with gp120s from other strains of HIV. A novel sandwich ELISA was then used to analyze the frequency with which individual in vivo activated B cells produced antibodies that crossreacted with heterologous gp120s. Repeated immunizations with a single gp120 or with a mixture of different gp120s resulted in the activation of primarily mono-specific (noncrossreactive) B cells. In contrast, the sequential immunization of mice with recombinant purified envelope proteins from different strains of HIV (IIIB, SF2, and Zr6) induced the selective expansion of B cells producing highly crossreactive antibodies.

Production of immunogenic HIV-1 viruslike particles in stably engineered monkey cell lines

A proviral fragment from human immunodeficiency virus type 1 (HIV-1) (LAV-1BRU) containing only protein-coding information, was expressed in COS cells using constitutive promoters in transient and stable transfection experiments. The presence of viruslike particles in cell supernatants was verified by Western blot analysis, density gradient centrifugation, and electron microscopy. Transfection of Vero cells with a similar construct employing the human metallothionein promoter led to the isolation of stable cell lines exhibiting inducible viruslike particle expression in response to cadmium chloride treatment. Induction ratios for viruslike particle expression were in excess of 1000-fold with production levels of p24 core antigen as high as 0.6 mg/L per 24 h. HIV-1 viruslike particles were immunogenic in mice, leading to strong envelope and core-specific humoral responses after two immunizations. The development of stable cell lines expressing significant quantities of HIV-1 viruslike particles offers an alternative to the use of live virus vectors for the production and evaluation of particle-based AIDS vaccines.

Induction of high-titer neutralizing antibodies, using hybrid human immunodeficiency virus V3-Ty viruslike particles in a clinically relevant adjuvant

The localization of neutralization determinants within the envelope glycoproteins of human immunodeficiency virus (HIV) has been largely achieved by immunizing small animals in conjunction with Freund's adjuvant. However, for eventual use in humans, candidate HIV vaccine components must also be efficacious in a nontoxic formulation. We describe here the production of hybrid Ty viruslike particles carrying the major neutralizing domain of HIV and demonstrate the induction of high-titer virus-neutralizing antibodies and an HIV-specific T-cell proliferative response after immunization in conjunction with aluminum hydroxide. As aluminum hydroxide and aluminum phosphate are the only adjuvants currently licensed for use in humans, these observations have implications for the development of an effective vaccine against HIV.

Emergence of viruses resistant to neutralization by V3-specific antibodies in experimental human immunodeficiency virus type 1 IIIB infection of chimpanzees

Emergence in two chimpanzees of human immunodeficiency virus type 1 (HIV-1) IIIB variants resistant to neutralization by the preexisting antibody is described. Viruses isolated from the HIV-1 IIIB gp120-vaccinated and -challenged animal were more resistant to neutralization by the chimpanzee's own serum than viruses isolated from the naive infected animal, indicating immune pressure as the selective mechanism. However, all reisolated viruses were 16- to 256-fold more neutralization resistant than the inoculum virus to antibodies binding to the third variable domain (V3) of the HIV-1 external envelope. Early chimpanzee serum samples that neutralized the inoculum strain but not the reisolated viruses were found to bind an HIV-1 IIIB common nonapeptide (IQRGPGRAF) derived from the gp120 isolate-specific V3 domain shown to induce isolate-specific neutralization in other animals. Amplification of the V3 coding sequence by polymerase chain reaction and subsequent sequence analysis of the neutralization-resistant variants obtained from in vivo-infected animals indicated that early resistance to neutralization by an HIV-1 IIIB monoclonal antibody (0.5 beta) was conferred by changes outside the direct binding site for the selective neutralizing antibody. The reisolated neutralization-resistant isolates consisted of the lower-replication-competent virus subpopulations of the HIV-1 IIIB stock, as confirmed by biological and sequence analyses. In vitro passage of the HIV-1 IIIB stock through chimpanzee and human peripheral blood mononuclear cell cultures void of HIV-specific antibody resulted in homogenic amplification of the more-replication-competent subpopulation preexisting in the original viral stock, suggesting a role for the immune system in suppressing the more-replication-competent viruses.

Antibody-mediated in vitro neutralization of human immunodeficiency virus type 1 abolishes infectivity for chimpanzees

This study was undertaken to establish whether antibody directed against the human immunodeficiency virus type 1 (HIV-1) principal gp120 type-specific neutralization determinant can abolish the infectivity of HIV-1 in chimpanzees. Challenge inocula of the IIIb virus isolate were mixed in vitro with either immunoglobulin G (IgG) from an uninfected chimpanzee, nonneutralizing IgG from an HIV-seropositive human, a virus-neutralizing murine monoclonal antibody directed against the HIV-1 IIIb isolate, or virus-neutralizing IgG from a chimpanzee infected with the IIIb isolate. Both neutralizing antibodies were directed against the principal neutralization determinant of the challenge isolate. Establishment of infection following inoculation of each virus-antibody mixture into chimpanzees was assessed by virus-specific antibody development and by virus isolation. No protective effect was noted either with the control IgG or with the nonneutralizing anti-HIV IgG. By contrast, the polyclonal chimpanzee virus-neutralizing IgG prevented HIV-1 in vivo infection, while the neutralizing monoclonal antibody notably decreased the infectivity of the challenge virus. Hence, antibody to the gp120 principal neutralization determinant is able both to prevent HIV-1 infection in vitro and to inhibit infection in vivo.

Importance of hypervariable regions of HIV-1 gp120 in the generation of virus neutralizing antibodies

Variants of the envelope gene of the HIV-SF2 isolate of HIV-1 with deletions of one or more of the hypervariable domains of gp120 were produced in genetically engineered yeast as nonglycosylated denatured polypeptide analogs of gp120. Purified antigens were used to immunize experimental animals to determine whether the removal of hypervariable regions from this type of gp120 immunogen had any effect on (1) the ability of the antigen to elicit virus neutralizing antibodies; and (2) the isolate specificity of the neutralizing antibodies that were elicited. The results of these studies demonstrate that, in addition to the previously identified V3 domain, at least two other hypervariable regions in gp120 are capable of eliciting neutralizing antibodies in experimental animals. However, when all five of the hypervariable regions were deleted, the resulting antigen was no longer capable of eliciting neutralizing antibodies. Finally, the neutralizing antibodies elicited by all of these nonglycosylated antigens were effective against HIV-SF2, the isolate from which the antigens were derived, but were not able to neutralize two divergent isolates, HIV-BRU or HIV-Zr6.

Characterization of the secreted, native gp120 and gp160 of the human immunodeficiency virus type 1

We have previously shown that the cell line 6D5(451) chronically infected with the HIV-1 isolate HTLV-III(451), secretes the HIV-1 envelope glycoproteins gp120 and gp160 in the extracellular medium. The HTLV-III(451) gp120 and gp160 were purified by sequential affinity chromatographic steps using a monoclonal antibody to HIV-1 gp41 and an anti-HIV-1-positive human serum. Amino acid sequence analysis of gp120 and gp160 showed the loss of the signal peptide. Digestion of the purified gp120 and gp160 with endoglycosidases revealed that both proteins are heavily glycosylated and contain complex carbohydrates, in contrast to the intracellular form of gp160 which has been shown to contain mannose-rich immature sugars. Competitive binding analysis showed that while both gp120 and gp160 bind CD4, the affinity of gp160 was five times lower than that of gp120. Both gp120 and gp160 inhibited syncytia formation by HIV-1-infected cells when mixed with CD4+ cells. Furthermore, both gp120 and gp160 had strong mitogenic effects on the T cells from HIV-1-infected gibbons but not on cells from uninfected gibbons.

Adjuvants

In summary, HIV vaccine studies described have generally not been designed to measure the effect of the adjuvant or to make comparisons between adjuvants. In only one study was a head-to-head comparison made between HIV antigen alone and antigen formulated with different adjuvants. We hope that future experiments with HIV/SIV vaccine candidates will be designed to determine the relative potency and safety of different adjuvants. Unfortunately, such experiments tend to be tedious and expensive. The design of these studies will need to address a number of variables which influence the response to the vaccine, including route and schedule of immunization, genotype and species of the vaccinated subject, and intrinsic characteristics of the antigen. In addition, the immunologic endpoints should include measurement of both B and T cell function. The carrier/adjuvant/antigen formulation should be hand-tailored and then standardized so that it is manufactured reproducibly without producing different biological effects between lots, and the vaccine formulation should be stable on storage and shipping. Finally, we obviously need to identify and test the protective antigen or antigens. The best adjuvant will never correct the choice of the wrong epitope.

Challenge of chimpanzees (Pan troglodytes) immunized with human immunodeficiency virus envelope glycoprotein gp120

The human immunodeficiency virus type 1 (HIV-1), the causative agent of acquired immunodeficiency syndrome, infects humans and chimpanzees. To determine the efficacy of immunization for preventing infection, chimpanzees were immunized with gp120 purified from human T-cell lymphotrophic virus type IIIB (HTLV-IIIB)-infected cell membranes and challenged with the homologous virus, HTLV-IIIB. A challenge stock of HTLV-IIIB was prepared by using unconcentrated HTLV-IIIB produced in H9 cells. The titer of the virus from this stock on human and chimpanzee peripheral blood mononuclear cells and in human lymphoid cell lines was determined; a cell culture infectivity of 10(4) was assigned. All chimpanzees inoculated intravenously with 40 cell culture infectious units or more became infected, as demonstrated by virus isolation and seroconversion. One of two chimpanzees inoculated with 4 cell culture infectious units became infected. Chimpanzees immunized with gp120 formulated in alum developed antibodies which precipitated gp120 and neutralized HTLV-IIIB. Peripheral blood mononuclear cells from gp120-vaccinated and HIV-infected animals showed a significantly greater response in proliferation assays with HIV proteins than did peripheral blood mononuclear cells from nonvaccinated and non-HIV-infected chimpanzees. Two of the gp120-alum-immunized chimpanzees were challenged with virus from the HTLV-IIIB stock. One animal received 400 cell culture infectious units, and one received 40 infectious units. Both animals became infected with HIV, indicating that the immune response elicited by immunization with gp120 formulated in alum was not effective in preventing infection with HIV-1.

Oligomeric structure of gp41, the transmembrane protein of human immunodeficiency virus type 1

We characterized the structural forms of the human immunodeficiency virus env-encoded proteins with a panel of monoclonal and polyclonal antibodies. Western blot (immunoblot) assays with antibodies specific for gp41 invariably recognized a major component of 160 kilodaltons and a less intense component of 120 kilodaltons in viral lysates. We demonstrated that these species are noncovalently associated tetramers and trimers of gp41 which represent the native form of this protein in virions. These complexes were stable when boiled in the presence of low concentrations of sodium dodecyl sulfate but were dissociated to gp41 monomers at high sodium dodecyl sulfate concentrations. Moreover, two human monoclonal antibodies preferentially recognized the oligomeric complexes over monomeric gp41 in Western blots, indicating the presence of epitopes recognized by the human immune system on the gp41 multimers which are not efficiently expressed by the dissociated monomers. The demonstration of the existence of multimeric env complexes and the enhanced and altered antigenicity of such multimers may be relevant to the design of subunit and recombinant human immunodeficiency virus env vaccines.

Isolate- and group-specific immune responses to the envelope protein of human immunodeficiency virus induced by a live recombinant vaccinia virus in macaques

The immune responses produced by four macaques inoculated intradermally with a recombinant vaccinia virus that expresses the envelope gene of human immunodeficiency virus (HIV) were analyzed. Antibody capable of immunoprecipitating the glycosylated envelope protein precursor gp160 was detected in all animals within 3 weeks after the primary intradermal vaccination. The level of antibody was increased following a booster inoculation, and the sera from three of the four animals were then capable of immunoprecipitating gp120. Sera from two of the macaques with anti-gp120 antibody were able to prevent syncytium formation mediated by the parent HIV but not that induced by an unrelated HIV isolate. The fusion-inhibiting antibody was directed toward the highly variable central portion of the gp120 molecule since the effect was abrogated by incubation with PB1. The latter is a recombinant protein produced in Escherichia coli containing amino acids 295-474, a major neutralizing epitope. Sera from the three macaques with anti-gp120 neutralized HIV infectivity in an isolate-specific manner. The serum with highest neutralizing activity was also best at inhibiting syncytium formation. In contrast to the isolate-specific nature of the neutralizing antibody, T lymphocytes from immunized animals proliferated in response to divergent HIV isolates as well as to purified gp120. These studies provide baseline immunologic information for further development of recombinant vaccinia virus vectors.

Purified envelope glycoproteins from human immunodeficiency virus type 1 variants induce individual, type-specific neutralizing antibodies

Repeated immunizations of goats, horses, or chimpanzees with envelope glycoprotein gp120 isolated from human immunodeficiency virus type 1 (HIV-1) resulted in type-specific neutralizing-antibody responses, which began to decay approximately 20 days following the administration of antigen. This was true repeatedly for serum samples from animals hyperimmunized with gp120s from either the HTLV-IIIB (IIIB) or the envelope-divergent HTLV-IIIRF (RF) HIV-1 isolates. Animals previously immunized with the IIIB gp120 were then inoculated with purified RF gp120. The first response in these animals was an anamnestic resurgence of neutralizing antibody to IIIB without detectable neutralizing antibody for RF. However, with later RF gp120 boosts, the IIIB neutralizing-antibody titers fell and an RF type-specific neutralizing-antibody response developed. When assessed with other HIV-1 variants, no group-specific neutralizing antibody was seen in any of the vaccination protocols evaluated. These results will pose real obstacles in the development of an effective vaccine for HIV.

Human immunodeficiency virus type 1 challenge of chimpanzees immunized with recombinant envelope glycoprotein gp120

The major envelope glycoprotein, gp120, of human immunodeficiency virus type 1 (HIV-1) was purified from a Chinese hamster ovary cell line transfected with a truncated form of the HIV-1 env gene. The recombinant glycoprotein (rgp120) was formulated with aluminum hydroxide adjuvant and was used to immunize chimpanzees. The recombinant preparation was effective in eliciting cellular and humoral immunity as well as immunologic memory. Anti-rgp 120 antibodies reacted with authentic viral gp120 in immunological blot assays and were able to neutralize HIV-1 infectivity in vitro. Sera from the rgp120-immunized animals were able to neutralize HIV-1 pseudotypes of vesicular stomatitis virus prepared from the IIIB isolate, from which the gene encoding rgp120 was derived, as well as two heterologous isolates, ARV-2 and RF. The immune response elicited against the rgp120 was not effective in preventing viral infection after intravenous challenge with HIV-1. The implications of these results on HIV-1 vaccine development are discussed.

Microinjection and expression of an infectious proviral clone and subgenomic envelope construct of a human immunodeficiency virus

An infectious proviral clone of the human immunodeficiency virus (HIV) was microinjected into the cell nucleus in six cell lines derived from caprine, ovine, bovine, or human solid tissue to study the utility of this method in effecting viral gene expression in nonlymphoid cells. Immunofluorescence assays for HIV demonstrated viral gene expression in only 5% of cells (100-200 cells per line) 24-48 h after microinjection; however, no reverse transcriptase activity was detectable, presumably due to a low level of virus release in this limited number of cells. Therefore, to indirectly assess infectious virus release, microinjected cells were cocultured with human T4 antigen-positive lymphocytes (H9) sensitive to HIV infection. Syncytia formation, electron microscopy, reverse transcriptase activity, and radioimmunoassay for HIV p24 were used to monitor viral gene expression in cocultures. HIV was efficiently recovered by cocultivating H9 with microinjected cells 48 h after microinjection, regardless of the tissue type or species of origin. H9 syncytia were visualized in some cocultures as early as day 5 but were readily apparent in all experiments on days 7-10. Syncytia induction in H9 was the earliest and most reliable indicator of infectious virus release. A recombinant construct containing a subgenomic envelope gene derived from the proviral clone of HIV was microinjected into human glioblastoma cells. Twenty-four to 48 h after manipulation, 5-20% of microinjected cells were found by immunofluorescence assay to express low levels of a putative gp120. These results suggest a possible approach to producing virus-free HIV envelope antigens in mammalian cells and may be relevant to subunit vaccine development.