Humoral immune response to the entire human immunodeficiency virus envelope glycoprotein made in insect cells

Fc receptors and the diversity of antibody responses to HIV infection and vaccination

The development of an effective vaccine against HIV is desperately needed. The successive failures of HIV vaccine efficacy trials in recent decades have shown the difficulty of inducing an appropriate protective immune response to fight HIV. Different correlates of antibody parameters associated with a decreased risk of HIV-1 acquisition have been identified. However, these parameters are difficult to reproduce and improve, possibly because they have an intricate and combined action. Here, we describe the numerous antibody (Ab) functions associated with HIV-1 protection and report the interrelated parameters regulating their complex functions. Indeed, besides neutralizing and Fc-mediated activity, additional factors such as Ab type, concentration and kinetics of induction, and Fc-receptor expression and binding capacity also influence the protective effect conferred by Abs. As these parameters were described to be associated with ethnicity, age and sex, these additional factors must be considered for the development of an effective immune response. Therefore, future vaccine designs need to consider these multifaceted Ab functions together with the demographic attributes of the patient populations.

High level stable expression of recombinant HIV gp120 in glutamine synthetase gene deficient HEK293T cells

Due to the important pathological roles of the HIV-1 gp120, the protein has been intensively used in the research of HIV. However, recombinant gp120 preparation has proven to be difficult because of extremely low expression levels. In order to facilitate gp120 expression, previous methods predominantly involved the replacement of native signal peptide with a heterologous one, resulting in very limited improvement. Currently, preparation of recombinant gp120 with native glycans relies solely on transient expression systems, which are not amendable for large scale production. In this work, we employed a different approach for gp120 expression. Besides replacing the native gp120 signal peptide with that of rat serum albumin and optimizing its codon usage, we generated a stable gp120-expressing cell line in a glutamine synthetase knockout HEK293T cell line that we established for the purpose of amplification of recombinant gene expressions. The combined usage of these techniques dramatically increased gp120 expression levels and yielded a functional product with human cell derived glycan. This method may be applicable to large scale preparation of other viral envelope proteins, such as that of the emerging SARS-CoV-2, or other glycoproteins which require the presence of authentic human glycans.

Sequential immunizations with a panel of HIV-1 Env virus-like particles coach immune system to make broadly neutralizing antibodies

Broadly neutralizing antibodies (bnAbs) are correlated with passive HIV/SHIV protection and are desirable components of a HIV protective immunity. In the current study, we have designed a sequential-immunization strategy with a panel of envelope glycoprotein (Env)-enriched virus-like particles (VLPs) from various HIV-1 clades (A-E) to elicit bnAbs with high breadth and potency of neutralization in rabbits. We have compared this regimen with repetitive immunizations of individual Env (subtype B) VLPs or a mixture of various Env VLPs. Our results demonstrate that the sequential immunization group of animals induced significantly higher IgG endpoint titers against respective HIV Env (autologous) antigen than other control groups. Animals vaccinated sequentially showed an increase in the antibody endpoint titers and IgG antibody secreting cells (ASCs) against Con-S Env protein. Sequential immunizations with various Env VLPs promoted antibody avidity indices and enhanced bnAb responses against a panel of HIV pseudotyped virions including some of the tier 3 pseudostrains. Sequential immunizations with various VLPs displaying "native-like" HIV-1 Envs elicited bnAb responses with increased breadth and potency of neutralization.

Glycosyl-Phosphatidylinositol-Anchored Anti-HIV Env Single-Chain Variable Fragments Interfere with HIV-1 Env Processing and Viral Infectivity

In previous studies, we demonstrated that single-chain variable fragments (scFvs) from anti-human immunodeficiency virus (HIV) Env monoclonal antibodies act as entry inhibitors when tethered to the surface of target cells by a glycosyl-phosphatidylinositol (GPI) anchor. Interestingly, even if a virus escapes inhibition at entry, its replication is ultimately controlled. We hypothesized that in addition to functioning as entry inhibitors, anti-HIV GPI-scFvs may also interact with Env in an infected cell, thereby interfering with the infectivity of newly produced virions. Here, we show that expression of the anti-HIV Env GPI-scFvs in virus-producing cells reduced the release of HIV from cells 5- to 22-fold, and infectivity of the virions that were released was inhibited by 74% to 99%. Additionally, anti-HIV Env GPI-scFv X5 inhibited virion production and infectivity after latency reactivation and blocked transmitter/founder virus production and infectivity in primary CD4⁺ T cells. In contrast, simian immunodeficiency virus (SIV) production and infectivity were not affected by the anti-HIV Env GPI-scFvs. Loss of infectivity of HIV was associated with a reduction in the amount of virion-associated Env gp120. Interestingly, an analysis of Env expression in cell lysates demonstrated that the anti-Env GPI-scFvs interfered with processing of Env gp160 precursors in cells. These data indicate that GPI-scFvs can inhibit Env processing and function, thereby restricting production and infectivity of newly synthesized HIV. Anti-Env GPI-scFvs therefore appear to be unique anti-HIV molecules as they derive their potent inhibitory activity by interfering with both early (receptor binding/entry) and late (Env processing and incorporation into virions) stages of the HIV life cycle.IMPORTANCE The restoration of immune function and persistence of CD4⁺ T cells in HIV-1-infected individuals without antiretroviral therapy requires a way to increase resistance of CD4⁺ T cells to infection by both R5- and X4-tropic HIV-1. Previously, we reported that anchoring anti-HIV-1 single-chain variable fragments (scFvs) via glycosyl-phosphatidylinositol (GPI) to the surface of permissive cells conferred a high level of resistance to HIV-1 variants at the level of entry. Here, we report that anti-HIV GPI-scFvs also derive their potent antiviral activity in part by blocking HIV production and Env processing, which consequently inhibits viral infectivity even in primary infection models. Thus, we conclude that GPI-anchored anti-HIV scFvs derive their potent blocking activity of HIV replication by interfering with successive stages of the viral life cycle. They may be effectively used in genetic intervention of HIV-1 infection.

HIV-1 protein Nef inhibits activity of ATP-binding cassette transporter A1 by targeting endoplasmic reticulum chaperone calnexin

HIV-infected patients are at increased risk of developing atherosclerosis, in part due to an altered high density lipoprotein profile exacerbated by down-modulation and impairment of ATP-binding cassette transporter A1 (ABCA1) activity by the HIV-1 protein Nef. However, the mechanisms of this Nef effect remain unknown. Here, we show that Nef interacts with an endoplasmic reticulum chaperone calnexin, which regulates folding and maturation of glycosylated proteins. Nef disrupted interaction between calnexin and ABCA1 but increased affinity and enhanced interaction of calnexin with HIV-1 gp160. The Nef mutant that did not bind to calnexin did not affect the calnexin-ABCA1 interaction. Interaction with calnexin was essential for functionality of ABCA1, as knockdown of calnexin blocked the ABCA1 exit from the endoplasmic reticulum, reduced ABCA1 abundance, and inhibited cholesterol efflux; the same effects were observed after Nef overexpression. However, the effects of calnexin knockdown and Nef on cholesterol efflux were not additive; in fact, the combined effect of these two factors together did not differ significantly from the effect of calnexin knockdown alone. Interestingly, gp160 and ABCA1 interacted with calnexin differently; although gp160 binding to calnexin was dependent on glycosylation, glycosylation was of little importance for the interaction between ABCA1 and calnexin. Thus, Nef regulates the activity of calnexin to stimulate its interaction with gp160 at the expense of ABCA1. This study identifies a mechanism for Nef-dependent inactivation of ABCA1 and dysregulation of cholesterol metabolism.

Role of the OB-fold of RNA helicase A in the synthesis of HIV-1 RNA

RNA helicase A (RHA), a DExD/H protein, contains a stretch of repeated arginine and glycine-glycine (RGG) residues and an oligonucleotide/oligosaccharide-binding fold (OB-fold) at the C-terminus. RHA has been reported to function as a transcriptional cofactor. This study shows the role of RGG and OB-fold domains of RHA in the activation of transcription and splicing of HIV-1 RNA. RHA stimulates HIV-1 transcription by enhancing the occupancy of RNA polymerase II on the proviral DNA. Deletion of RGG or both RGG and OB-fold does not change the transcriptional activity of RHA, nor does the stability of viral RNA. However, deletion of both RGG and OB-fold rather than deletion of RGG only results in less production of multiply spliced 6D RNAs. The results suggest that the OB-fold is involved in modulating HIV-1 RNA splicing in the context of some HIV-1 strains while it is dispensable for the activation of HIV-1 transcription.

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.

Quantitative proteomic analysis of exosomes from HIV-1-infected lymphocytic cells

HIV-1 infection causes profound effects both inside and outside of cells through multiple mechanisms, including those mediated by exosomes. Using the technique of stable isotope labeling by amino acids in cell culture, we compared protein expression patterns in the exosomal compartment of HIV-1-infected and -uninfected lymphocytic H9 cells. Of 770 proteins identified in two independent sets of exosomal samples, 14 proteins were found to be differentially expressed in the exosomal fraction of HIV-1-infected cells versus -uninfected controls. Gene Ontology survey and DAVID analysis revealed that identified proteins were enriched for functional categories such as binding. Of these 14 proteins, three immunomodulatory molecules were reproducibly identified in both replicates and included ADP-ribosyl cyclase 1 (CD38), L-lactate dehydrogenase B chain (LDHB), and Annexin A5 (ANXA5). In addition to previously reported HIV-1 associations with CD38 and LDHB, new interactions were identified and validated for ANXA5, CD38, and LDHB, which were found to bind to HIV-1 p24 and Tat. In summary, our studies reveal that exosomes released from HIV-1-infected cells are composed of a unique and quantitatively different protein signature and harbor regulatory molecules that impact the processes of cellular apoptosis (ANXA5 and LDHB) and proliferation (CD38).

Use of silver nanoparticles increased inhibition of cell-associated HIV-1 infection by neutralizing antibodies developed against HIV-1 envelope proteins

Background

HIV/AIDS pandemic is a worldwide public health issue. There is a need for new approaches to develop new antiviral compounds or other therapeutic strategies to limit viral transmission. The envelope glycoproteins gp120 and gp41 of HIV are the main targets for both silver nanoparticles (AgNPs) and neutralizing antibodies. There is an urgency to optimize the efficiency of the neutralizing antibodies (NABs). In this study, we demonstrated that there is an additive effect between the four NABs and AgNPs when combined against cell-associated HIV-1 infection in vitro

Results

Four NABs (Monoclonal antibody to HIV-1 gp41 126-7, HIV-1 gp120 Antiserum PB1 Sub 2, HIV-1 gp120 Antiserum PB1, HIV-1 gp120 Monoclonal Antibody F425 B4e8) with or without AgNPs of 30-50 nm in size were tested against cell free and cell-associated HIV_IIIB virus. All NABs inhibited HIV-1 cell free infection at a dose response manner, but with AgNPs an antiviral additive effect was not achieved Although there was no inhibition of infection with cell-associated virus by the NABs itself, AgNPs alone were able to inhibit cell associated virus infection and more importantly, when mixed together with NABs they inhibited the HIV-1 cell associated infection in an additive manner.

Discussion

The most attractive strategies to deal with the HIV problem are the development of a prophylactic vaccine and the development of effective topical vaginal microbicide. For two decades a potent vaccine that inhibits transmission of infection of HIV has been searched. There are vaccines that elicit NABs but none of them has the efficacy to stop transmission of HIV-1 infection. We propose that with the addition of AgNPs, NABs will have an additive effect and become more potent to inhibit cell-associated HIV-1 transmission/infection.

Conclusions

The addition of AgNPs to NABs has significantly increased the neutralizing potency of NABs in prevention of cell-associated HIV-1 transmission/infection. Further exploration is required to standardize potentiation of NABs by AgNPs. It is also required to evaluate in vivo toxicity of AgNPs before AgNPs could be incorporated in any antiviral vaginal creams.

Expression-system-dependent modulation of HIV-1 envelope glycoprotein antigenicity and immunogenicity

Recombinant expression systems differ in the type of glycosylation they impart on expressed antigens such as the human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins, potentially affecting their biological properties. We performed head-to-head antigenic, immunogenic and molecular profiling of two distantly related Env surface (gp120) antigens produced in different systems: (a) mammalian (293 FreeStyle cells; 293F) cells in the presence of kifunensine, which impart only high-mannose glycans; (b) insect cells (Spodoptera frugiperda, Sf9), which confer mainly paucimannosidic glycans; (c) Sf9 cells recombinant for mammalian glycosylation enzymes (Sf9 Mimic), which impart high-mannose, hybrid and complex glycans without sialic acid; and (d) 293F cells, which impart high-mannose, hybrid and complex glycans with sialic acid. Molecular models revealed a significant difference in gp120 glycan coverage between the Sf9-derived and wild-type mammalian-cell-derived material that is predicted to affect ligand binding sites proximal to glycans. Modeling of solvent-exposed surface electrostatic potentials showed that sialic acid imparts a significant negative surface charge that may influence gp120 antigenicity and immunogenicity. Gp120 expressed in systems that do not incorporate sialic acid displayed increased ligand binding to the CD4 binding and CD4-induced sites compared to those expressed in the system that do, and imparted other more subtle differences in antigenicity in a gp120 subtype-specific manner. Non-sialic-acid-containing gp120 was significantly more immunogenic than the sialylated version when administered in two different adjuvants, and induced higher titers of antibodies competing for CD4 binding site ligand-gp120 interaction. These findings suggest that non-sialic-acid-imparting systems yield gp120 immunogens with modified antigenic and immunogenic properties, considerations that should be considered when selecting expression systems for glycosylated antigens to be used for structure-function studies and for vaccine use.

Gammadelta T cell immune manipulation during chronic phase of simian-human immunodeficiency virus infection [corrected] confers immunological benefits

Vgamma2Vdelta2 T cells, a major human gammadelta T cell subset, recognize the phosphoantigen (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMBPP) produced by mycobacteria and some opportunistic pathogens, and they contribute to innate/adaptive/homeostatic and anticancer immunity. As initial efforts to explore Vgamma2Vdelta2 T cell-based therapeutics against HIV/AIDS-associated bacterial/protozoal infections and neoplasms, we investigated whether a well-defined HMBPP/IL-2 therapeutic regimen could overcome HIV-mediated immune suppression to massively expand polyfunctional Vgamma2Vdelta2 T cells, and whether such activation/expansion could impact AIDS pathogenesis in simian HIV (SHIV)-infected Chinese rhesus macaques. While HMBPP/IL-2 coadministration during acute or chronic phase of SHIV infection induced massive activation/expansion of Vgamma2Vdelta2 T cells, the consequences of such activation/expansions were different between these two treatment settings. HMBPP/IL-2 cotreatment during acute SHIV infection did not prevent the increases in peak and set-point viral loads or the accelerated disease progression seen with IL-2 treatment alone. In contrast, HMBPP/IL-2 cotreatment during chronic infection did not exacerbate disease, and more importantly it could confer immunological benefits. Surprisingly, although viral antigenic loads were not increased upon HMBPP/IL-2 cotreatment during chronic SHIV infection, HMBPP activation of Vgamma2Vdelta2 T cells boosted HIV Env-specific Ab titers. Such increases in Abs were sustained for >170 days and were immediately preceded by increased production of IFN-gamma, TNF-alpha, IL-4, and IL-10 during peak expansion of Vgamma2Vdelta2 T cells displaying memory phenotypes, as well as the short-term increased effector function of Vgamma2Vdelta2 T cells and CD4(+) and CD8(+) alphabeta T cells producing antimicrobial cytokines. Thus, HMBPP/Vgamma2Vdelta2 T cell-based intervention may potentially be useful for combating neoplasms and HMBPP-producing opportunistic pathogens in chronically HIV-infected individuals.

Inter-subunit disulfide bonds in soluble HIV-1 envelope glycoprotein trimers

Soluble forms of the trimeric human immunodeficiency virus (HIV-1) envelope glycoproteins are important tools for structural studies and in the construction of improved immunogens. We found that a substantial fraction of soluble envelope glycoprotein trimers contain inter-subunit disulfide bonds (inter-S-S bonds) that render the trimers resistant to heat and denaturing agents. These inter-S-S bonds can be reduced without disrupting the trimers by treatment with a low concentration of beta-mercaptoethanol or DTT. Antibody mapping studies suggest that the soluble HIV-1 envelope glycoprotein trimers lacking the inter-S-S bonds exhibit a conformation closer to that of the native HIV-1 envelope glycoprotein complex. However, reducing these inter-S-S bonds had only modest effects on the inefficient elicitation of neutralizing antibodies by the soluble trimers. These studies provide guidance in improving the resemblance of tractable, soluble forms of the HIV-1 envelope glycoproteins to the native virion spikes.

Recombinant HIV-1 Pr55gag virus-like particles: potent stimulators of innate and acquired immune responses

Several previous reports have clearly demonstrated the strong effectiveness of human immunodeficiency virus (HIV) Gag polyprotein-based virus-like particles (VLP) to stimulate humoral and cellular immune responses in complete absence of additional adjuvants. Yet, the mechanisms underlying the strong immunogenicity of these particulate antigens are still not very clear. However, current reports strongly indicate that these VLP act as "danger signals" to trigger the innate immune system and possess potent adjuvant activity to enhance the immunogenicity of per se only weakly immunogenic peptides and proteins. Here, we review the current understanding of how various particle-associated substances and other impurities may contribute to the observed immune-activating properties of these complex immunogens.

Characterization of the outer domain of the gp120 glycoprotein from human immunodeficiency virus type 1

The core of the gp120 glycoprotein from human immunodeficiency virus type 1 (HIV-1) is comprised of three major structural domains: the outer domain, the inner domain, and the bridging sheet. The outer domain is exposed on the HIV-1 envelope glycoprotein trimer and contains binding surfaces for neutralizing antibodies such as 2G12, immunoglobulin G1b12, and anti-V3 antibodies. We expressed the outer domain of HIV-1(YU2) gp120 as an independent protein, termed OD1. OD1 efficiently bound 2G12 and a large number of anti-V3 antibodies, indicating its structural integrity. Immunochemical studies with OD1 indicated that antibody responses against the outer domain of the HIV-1 gp120 envelope glycoprotein are rare in HIV-1-infected human sera that potently neutralize the virus. Surprisingly, such outer-domain-directed antibody responses are commonly elicited by immunization with recombinant monomeric gp120. Immunization with soluble, stabilized HIV-1 envelope glycoprotein trimers elicited antibody responses that more closely resembled those in the sera of HIV-1-infected individuals. These results underscore the qualitatively different humoral immune responses elicited during natural infection and after gp120 vaccination and help to explain the failure of gp120 as an effective vaccine.

Oral epithelial cells are susceptible to cell-free and cell-associated HIV-1 infection in vitro

Epithelial cells lining the oral cavity are exposed to HIV-1 through breast-feeding and oral-genital contact. Genital secretions and breast milk of HIV-1-infected subjects contain both cell-free and cell-associated virus. To determine if oral epithelial cells can be infected with HIV-1 we exposed gingival keratinocytes and adenoid epithelial cells to cell-free virus and HIV-1-infected peripheral blood mononuclear cells and monocytes. Using primary isolates we determined that gingival keratinocytes are susceptible to HIV-1 infection via cell-free CD4-independent infection only. R5 but not X4 viral strains were capable of infecting the keratinocytes. Further, infected cells were able to release infectious virus. In addition, primary epithelial cells isolated from adenoids were also susceptible to infection; both cell-free and cell-associated virus infected these cells. These data have potential implications in the transmission of HIV-1 in the oral cavity.

Neutralization epitopes of the HIV-1 primary isolate DH012

To study HIV-1 primary isolate neutralization, we have used DH012 as a model to study the immunogenicity of several DH012 immunogens and determine the potential neutralization epitopes in the virus envelope glycoprotein. Previously, we identified that DH012 infected animals mount potent neutralizing activity against a conformational epitope (CEV) that involves multiple variable regions. In this study, we show that the conformational epitope can be reconstituted with one gp120 recombinant fragment containing sequences from the V1/V2 loop and the bridging sheet of gp120 and a V3 peptide. In contrast to DH012 infection, we previously demonstrated that animals immunized with DH012 gp120 induced potent neutralizing antibodies directed at the V3 domain of gp120. In this study, a second neutralizing activity against the V1/V2 region of gp120 was identified from the same guinea pig sera. In summary, several neutralization epitopes are identified on DH012, including the CEV, V1/V2, V3, 17b, IgG1b12, and 2G12 epitopes. Infectious DH012 virus carrying oligomeric envelope appears to raise primarily neutralizing antibodies that recognize a discontinuous conformationally dependent epitope whereas the monomeric gp120 induces antibodies that are primarily directed at epitopes in the V3 and V1/V2 domains. The DH012 neutralizing epitopes, such as V1/V2 and V3, are either cryptic or poorly immunogenic in chimpanzees. However, immunogens, such as gp120, could be designed to induce neutralizing activity against epitopes that are poorly immunogenic, such as V1/V2 of DH012, in the native envelope glycoproteins.

HIV-1 replication cycle

The HIV-1 is a formidable pathogen with establishment of a persistent infection based on the ability to integrate the proviral genome into chronically infected cells, and by the rapid evolution made possible by a high mutation rate and frequent recombination during the viral replication. HIV-1 has a variety of novel genes that facilitate viral persistence and regulation of HIV replication, but this virus also usurps cellular machinery for HIV replication, particularly during gene expression and virion assembly and budding. Recent success with antiretroviral therapy may be limited by the emergence HIV drug resistance and by toxicities and other requirements for successful long-term therapy. Further investigation of HIV-1 replication may allow identification of novel targets of antiretroviral therapy that may allow continued virus suppression in patients of failing current regiments, particularly drugs that target HIV-1 entry and HIV-1 integration.

Induction and characterization of neutralizing antibodies against a human immunodeficiency virus type 1 primary isolate

Chimpanzees infected with the primary isolate DH012 mount potent neutralizing antibodies. This DH012 neutralizing activity is highly strain specific. Immune sera from guinea pigs immunized with recombinant DH012 gp120 could also neutralize this primary isolate. The neutralizing activity in chimpanzee and guinea pig sera against wild-type DH012 appears to be independent of a linear epitope in the V3 region of gp120. Interestingly, the neutralization escape mutant derived from growing DH012 in the presence of the potent neutralizing chimpanzee serum is at least 50-fold more sensitive than wild-type DH012 to neutralization by guinea pig immune sera. The unusually potent neutralizing activity against the DH012 neutralization-resistant virus is due to the presence of anti-V3 antibodies in guinea pig sera. These results suggested that recombinant gp120 could induce neutralizing antibodies against primary isolate DH012. The V3 of wild-type DH012 is poorly immunogenic in infected chimpanzees and is not accessible to neutralizing V3 antibodies. It is likely that this cryptic V3 region became exposed when the virus escaped the neutralizing activity of the chimpanzee serum.

Improved elicitation of neutralizing antibodies against primary human immunodeficiency viruses by soluble stabilized envelope glycoprotein trimers

Human immunodeficiency virus (HIV-1) envelope glycoprotein subunits, such as the gp120 exterior glycoprotein, typically elicit antibodies that neutralize T-cell-line-adapted (TCLA), but not primary, clinical isolates of HIV-1. Here we compare the immunogenicity of gp120 and soluble stabilized trimers, which were designed to resemble the functional envelope glycoprotein oligomers of primary and TCLA HIV-1 strains. For both primary and TCLA virus proteins, soluble stabilized trimers generated neutralizing antibody responses more efficiently than gp120 did. Trimers derived from a primary isolate elicited antibodies that neutralized primary and TCLA HIV-1 strains. By contrast, trimers derived from a TCLA isolate generated antibodies that neutralized only the homologous TCLA virus. Thus, soluble stabilized envelope glycoprotein trimers derived from primary HIV-1 isolates represent defined immunogens capable of eliciting neutralizing antibodies that are active against clinically relevant HIV-1 strains.

Mapping of the interaction between the immunodominant loop of the ectodomain of HIV-1 gp41 and human complement protein C1q

The human immunodeficiency virus type 1 transmembrane envelope glycoprotein gp41 has been previously shown to activate the C1 complex of human complement through direct interaction with its C1q subunit. The major interaction site has been located within the gp41 immunodominant region (residues 590-620), and a synthetic peptide overlapping residues 601-613 of gp41 (sequence GIWGCSGKLICTT) was shown to inhibit binding of gp41 to C1q in vitro (Thielens, N.M., Bally, I.M., Ebenbichler, C.F., Dierich, M.P. & Arlaud, G.J. (1993) J. Immunol. 151, 6583-6592). The ectodomain of gp41 (s-gp41) was secreted from the methylotrophic yeast Pichia pastoris and purified by immunoaffinity chromatography. Enzymatic deglycosylation of the recombinant s-gp41 was necessary to allow its in vitro interaction with C1q. A solid-phase competition assay was used to monitor the effect of mutant peptides derived from segment 601-613 of gp41 on the binding of deglycosylated s-gp41 to C1q. Whereas mutation of Ser606 had no effect, replacement of Ile602, Trp603, Lys608, Leu609 and Ile610 by Ala abolished the ability of the resulting peptides to inhibit binding of s-gp41 to C1q, suggesting that these residues participate in the interaction between gp41 and C1q. These findings are discussed in the light of a structural model of the immunodominant loop of gp41. It is proposed that the recognition of gp41 by C1q is driven by hydrophobic interactions, and that the sites of gp41 responsible for interaction with gp120 and C1q partly overlap.

gag, vif, and nef genes contribute to the homologous viral interference induced by a nonproducer human immunodeficiency virus type 1 (HIV-1) variant: identification of novel HIV-1-inhibiting viral protein mutants

We previously demonstrated that expression of the nonproducer F12-human immunodeficiency virus type 1 (HIV-1) variant induces a block in the replication of superinfecting HIV that does not depend on the down-regulation of CD4 HIV receptors. In order to individuate the gene(s) involved in F12-HIV-induced interference, vectors expressing each of the nine F12-HIV proteins were transfected in HIV-susceptible HeLa CD4 cells. Pools of cell clones stably producing each viral protein were infected with HIV-1, and virus release was measured in terms of reverse transcriptase activity in supernatants. We hereby demonstrate that HeLa CD4 cells expressing the F12-HIV gag, vif, or nef gene were resistant, to different degrees, to infection with T-cell-line-adapted HIV-1 strains. Conversely, expression of either the tat, rev, or vpu F12-HIV gene increased the rate of HIV release, and no apparent effects on HIV replication were observed in cells expressing either the F12-HIV vpr, pol, or env gene. No variation of CD4 exposure was detected in any of the uninfected HeLa CD4 pools. These data indicate that F12-HIV homologous viral interference is the consequence of the synergistic anti-HIV effects of Gag, Vif, and Nef proteins. Retrovirus vectors expressing F12-HIV vif or nef allowed us to further establish that the expression of each mutated protein (i) inhibits the replication of clinical HIV-1 isolates as well, (ii) impairs the infectivity of the virus released by cells chronically infected with HIV-1, and (iii) limitedly to F12-HIV Vif protein, induces HIV resistance in both vif-permissive and vif-nonpermissive cells. The levels of action of F12-HIV vif and nef anti-HIV effects were also determined. We observed that HIV virions emerging from the first viral cycle on F12-HIV vif-expressing cells, although released in unaltered amounts, had a strongly reduced ability to initiate the retrotranscription process when they reinfected parental HeLa CD4 cells. Differently, we observed that expression of F12-HIV Nef protein affects the HIV life cycle at the level of viral assembling and/or release. For the first time, an inhibitory effect on the HIV life cycle in both acutely and chronically infected cells induced by mutated Vif and Nef HIV-1 proteins is described. These genes could thus be proposed as new useful reagents for anti-HIV gene therapy.

Immune response to recombinant visna virus Gag and Env precursor proteins synthesized in insect cells

Two different recombinant visna virus (VV) gag-baculoviruses were constructed for the expression of precursor VV Gag in insect cells. Both recombinant Gag viruses expressed proteins migrating on SDS PAGE at the predicted rate for VV Gag precursor, Pr50gag. However, differences were seen in the morphology of the virus-like particles produced. Monoclonal antibody directed against the VV Gag capsid protein (p25) and sera from sheep infected with ovine lentiviruses reacted to both 50-kDa proteins. A recombinant VV env-baculovirus was constructed, substituting sequences encoding the signal peptide of VV Env with the murine IFN-gamma analogue. Sera from ovine lentivirus infected sheep reacted in immunoblots with two proteins of approximately 100 and 200 kDa found in the plasma membrane of insect cells infected with env-recombinant virus. Sheep immunized with either the recombinant Gag or the Env proteins developed high antibody titers to VV in ELISA. The serum of sheep and ascitic fluid of mice immunized with the recombinant Gag reacted with native Pr50gag and the processed Gag proteins in immunoblots, whereas serum of the recombinant Env immunized sheep reacted with VV gp135 and a putative oligomer of gp135. The immunized sheep responded specifically to visna virus by lymphocyte proliferation in vitro.

Increased incorporation of chimeric human immunodeficiency virus type 1 gp120 proteins into Pr55gag virus-like particles by an Epstein-Barr virus gp220/350-derived transmembrane domain

Noninfectious Pr55gag virus-like particles containing high quantities of oligomeric human immunodeficiency virus type 1 (HIV-1) envelope (Env) proteins represent potential candidate immunogens for a vaccine against HIV-1 infection. Thus, chimeric env genes were constructed encoding the HIV-1 exterior glycoprotein gp120 which was covalently linked at different C-terminal positions to a transmembrane domain (TM) from the Epstein-Barr virus (EBV) major Env glycoprotein gp220/ 350. All chimeric Env-TM polypeptides as well as the wild-type HIV Env proteins were equally produced and incorporated at the outer surface of insect cells using the baculovirus expression system. In the presence of coexpressed HIV Pr55gag polyproteins significantly decreased amounts of wild-type Env proteins were presented at the cell surface, whereas the membrane incorporation of the Env-TM chimeras was not affected. Biochemical and immunoelectron microscopical analysis of particles that were efficiently released from these cells displayed the incorporation of both wild-type Env and chimeric Env-TM proteins on the surface of VLPs. However, the quantities of particle-associated chimeric Env-TM proteins exceeded those of incorporated wild-type Env proteins by a factor of 5-10. Chemical cross-linking and subsequent polyacrylamide gel electrophoresis of VLP-entrapped Env proteins revealed that the chimeric Env-TM proteins form homodimers and a higher-order oligomer, similar to that observed for wild-type Env proteins. Thus, the results of this study clearly demonstrate that the replacement of the gp41 transmembrane protein of gp160 by a heterologous, EBV gp220/350-derived membrane anchor provides an effective strategy to incorporate high quantities of oligomeric HIV gp120 proteins on the surface of Pr55gag virus-like particles.

Expression of human endogenous retrovirus type K envelope glycoprotein in insect and mammalian cells

The human endogenous retrovirus type K (HERV-K) family codes for the human teratocarcinoma-derived retrovirus (HTDV) particles. The existence of the envelope protein (ENV) of HERV-K encoded by the subgenomic env mRNA has not yet been demonstrated. To study the genetic requirements for successful expression of ENV, we have constructed a series of recombinant HERV-K env expression vectors for infection and transfection experiments in insect cells and mammalian cells, respectively. Six baculovirus constructs bearing full-length or truncated HERV-K env with or without homologous or heterologous signal peptides were used for infections of insect cells. All recombinant baculoviruses yielded ENV proteins with the expected molecular masses. The full-length 80- to 90-kDa HERV-K ENV protein including the cORF leader sequence was glycosylated in insect cells. In addition, the 14-kDa cORF protein was expressed due to splicing of the full-length env mRNA. The ENV precursor protein is not cleaved to the surface (SU) and transmembrane (TM) glycoproteins; it does not appear on the surface of infected insect cells and is not secreted into the medium. For ENV expression in COS cells, plasmid vectors harboring the cytomegalovirus immediate-early promoter/intron A element and the tissue plasminogen activator (t-PA) signal peptide or the homologous HERV-K signal peptide upstream of the env gene were employed. Glycosylated and uncleaved ENV was expressed as in GH teratocarcinoma cells but at higher levels. The heterologous t-PA signal sequence was instrumental for expression of HERV-K ENV on the cell surface. Hence, we have shown for the first time that the HERV-K env gene has the potential to be expressed as a full-length envelope protein with appropriate glycosylation. In addition, our data provide explanations for the lack of infectivity of HERV-K/HTDV particles.

Effects of inefficient cleavage of the signal sequence of HIV-1 gp 120 on its association with calnexin, folding, and intracellular transport

The HIV-1 envelope glycoprotein gp120 displays inefficient intracellular transport, which is caused by its retention in the endoplasmic reticulum. Coexpression in insect cells (Sf9) of HIV-1 gp120 with calnexin has shown that their interaction was modulated by the signal sequence of HIV-1 gp120. gp120, with its natural signal sequence, showed a prolonged association with calnexin with a t1/2 of greater than 20 min. Replacement of the natural signal sequence with the signal sequence from mellitin led to a decreased time of association of gp120 with calnexin (t1/2 < 10 min). These different times of calnexin association coincided both with the folding of gp120 as measured by the ability of bind CD4 and with endoplasmic reticulum to Golgi transport as analyzed by the acquisition of partial endoglycosidase H resistance. Using a monospecific antibody to the HIV-1 gp120 natural signal peptide, we showed that calnexin associated with N-glycosylated but uncleaved gp120. Only after dissociation from calnexin was gp120 cleaved, but very inefficiently. Only the small proportion of signal-cleaved gp120 molecules acquired transport competence and were secreted. This is the first report demonstrating the effect of the signal sequence on calnexin association.

Anti-human immunodeficiency virus type 1 activity of an oligocationic compound mediated via gp120 V3 interactions

An oligocationic peptide compound (ALX40-4C) was developed for consideration in the treatment of human immunodeficiency virus type 1 (HIV-1) infection. This compound was designed to mimic the basic domain of the HIV-1 transactivation protein, Tat, and will competitively inhibit Tat binding to its specific RNA hairpin target (TAR [transactivation region]), found at the 5' end of all HIV-1 transcripts. Blocking Tat-TAR interactions can abrogate HIV-1 replication. ALX40-4C was shown to inhibit replication of HIV-1NL4-3 in a range of cell types, including primary cells and transformed cell lines, by as much as 10(4)-fold. In some experiments, virus rescue was not possible even after removal of ALX40-4C from the cultures. Strain-dependent resistance has been demonstrated for all antiretroviral agents tested; therefore, we tested for variable sensitivity to ALX40-4C. The cloned primary strains, HIV-JR-CSF and HIV-JR-FL, were less sensitive to ALX40-4C inhibition. Unexpectedly, determinants for efficient ALX40-4C inhibition were mapped by using recombinant virus strains to the V3 region of gpl20 and were shown to act at early events in viral replication, which include viral entry. If entry and reverse transcription are bypassed by transfection, a more modest, virus strain-independent inhibition is shown; this inhibition is likely due to blocking of Tat-TAR interaction. Thus, the highly basic oligocationic Tat inhibitor ALX40-4C appears to interfere with initial virus-target cell interactions which involve HIV-1 gp120 V3 determinants, most efficiently for T-cell line-adapted strains.

Induction of HIV-1 envelope (gp120)-specific cytotoxic T lymphocyte responses in mice by recombinant CHO cell-derived gp120 is enhanced by enzymatic removal of N-linked glycans

Priming of CD8+ cytotoxic T lymphocyte (CTL) responses with recombinant proteins has been facilitated by the development of novel adjuvants that deliver antigens into the class I major histocompatibility complex (MHC) pathway. However, the extent to which secondary structure or glycosylation of these proteins prevents priming of class I MHC-restricted CTL responses is not clear. To address this issue, recombinant HIV-1 gp120 envelope proteins produced in yeast insect, or mammalian cells were compared for the ability to elicit CD8+ CTL activity in mice. Envelope-specific CD8+ T lymphocytes were detected in BALB/c mice immunized with env 2-3, a 55-kDa yeast-derived envelope protein that is not glycosylated and lacks a native conformation. This response was directed against a previously described epitope in the V3 region of gp120, as well as a newly identified epitope located near the carboxy-terminus of the molecule. Similar levels of V3-directed CTL activity were observed in mice immunized with recombinant gp120 produced in insect (Spodoptera fugiperda) cells using a baculovirus expression system (gp120BAC). In contrast, induction of CTL responses was considerably less efficient when mice were immunized with gp120CHO, a native, fully glycosylated envelope protein produced in mammalian CHO cells. Denaturation of gp120CHO prior to immunization was not sufficient to prime CTL responses. However, envelope-specific CD8+ CTL activity was elicited when N-linked glycans were removed by treatment with an endoglycosidase. Possible mechanisms by which N-linked glycans influence delivery or processing of recombinant proteins for class I MHC presentation, and the implications of these findings for the design of subunit vaccines, are discussed.

Combinatorially selected guanosine-quartet structure is a potent inhibitor of human immunodeficiency virus envelope-mediated cell fusion

The phosphorothioate oligonucleotide T2G4T2 was identified as an inhibitor of HIV infection in vitro by combinatorial screening of a library of phosphorothioate oligonucleotides that contained all possible octanucleotide sequences. The oligonucleotide forms a parallel-stranded tetrameric guanosine-quartet structure. Tetramer formation and the phosphorothioate backbone are essential for antiviral activity. The tetramer binds to the human immunodeficiency virus envelope protein gp120 at the V3 loop and inhibits both cell-to-cell and virus-to-cell infection.

Envelope sequence variation, neutralizing antibodies, and primate lentivirus persistence

Studies in ungulate lentivirus systems clearly indicate that neutralization escape variants emerge over time in chronically infected animals. Studies in the EIAV system, in particular, have provided strong evidence that the humoral branch of the immune system is at least one selective force acting on an array of viral variants. In previous studies with the ungulate lentiviruses, molecularly cloned virus was never used, and plaque-purified virus was only sometimes used; the genetic determinants responsible for antigenic variation and immune selection were not determined. While molecular clones are available for HIV-1, immune selection studies have been hampered in this system by the fact that HIV-1 is infectious only for chimpanzees, which do not develop disease and are available in only limited numbers. Experiments on immune selection in humans are generally complicated by lack of knowledge on the time of infection and the genetic make-up of the infecting virus. Our studies on SIV immune selection summarized in this review provide definitive evidence that neutralization-resistant variants emerge in an individual during persistent infection by primate lentiviruses. By cloning viral envelope genes from rhesus monkeys over time and obtaining sequential serum samples from them, we have been able to study not only the evolution of envelope sequences but also the emergence of neutralization-resistant variants. Reciprocal neutralization studies were performed using parental and variant specific sera, and immune selection was demonstrated using molecularly cloned virus of defined sequence. During the course of persistent infection with SIV and HIV, there is clear selective pressure for change in discrete variable regions of envelope. The host neutralizing antibody response appears to be at least one of the selective forces driving sequence change in envelope since one result of the sequence variation is the emergence of neutralization escape mutants. This indicates that neutralizing antibodies do serve to limit HIV and SIV replication during the lengthy asymptomatic stage of infection. The coincidence of neutralization domains of HIV and/or SIV with variable regions V1, V2, V3, V4, V5, and V6 suggests a direct relationship between neutralization domains and the emergence of sequence variants. However, different selective forces may be responsible all or in part for driving sequence changes in some variable domains (summarized in Table 2). For example, alterations in cell and/or tissue tropism may be responsible at least in part for driving change in V3 and the cytotoxic T-lymphocyte response may be responsible for driving change in the signal peptide (V0; Henderson et al. 1992; Wei and Cresswell 1992).(ABSTRACT TRUNCATED AT 400 WORDS)

High antigenic cross-reactivity of the V3 consensus sequences of HIV-1 gp120

The principal neutralization determinant (PND) of the human immunodeficiency virus type 1 (HIV-1) is located within the variable V3 region of the external envelope protein gp120. Although it is recognized that V3 sequences induce antibody response with restricted neutralization activity in vitro, we observed that the V3 consensus sequences representing North American/European and African isolates were highly cross-reactive, binding 94 and 77%, respectively, of sera collected from HIV-1 individuals originating from various parts of the world. Even HIV-1-positive sera from some East African residents, infected by strains whose V3 loop sequences are undoubtedly distinct from the North American/European consensus V3 loop sequence, reacted better to the V3 North American/European consensus peptide than to African-specific V3 sequences. Results indicate that the V3 consensus sequences represent the best candidates for optimal cross-reactivity with a wide variety of strains. Furthermore, using immunoassays for antibodies to prototype-specific V3 sequences, it is shown that HIV-1 strains related to the MN group are prevalent in West Africa, indicating either a West African origin of the MN-related viruses or more probably an introduction of this group of viruses through European/North American contacts.

Site-specific N-glycosylation and oligosaccharide structures of recombinant HIV-1 gp120 derived from a baculovirus expression system

We report the complete structures of the N-linked oligosaccharides and the site-specificity of the N-glycosylation of recombinant gp120 (rgp120) of the HIV-1 BH8 isolate produce by a baculovirus expression system. Glycopeptides derived from the tryptic digests of intact rgp120 or of cyanogen bromide-generated fragments of rgp120 were isolated by their binding to concanavalin A-Sepharose and were purified by reversed-phase HPLC. The isolated glycopeptides were treated with PNGase F, releasing the carbohydrate moiety while converting Asn to Asp, and identified by amino acid analysis and/or peptide sequencing. Our results indicate that all 22 potential N-glycosylation sites in the rgp120 sequence are utilized. We did not detect N-acetylgalactosamine in rgp120, indicating that the glycoprotein lacks typical O-linked oligosaccharides. To investigate the oligosaccharide structures at the sites of glycosylation, we determined the carbohydrate composition for each site and characterized the oligosaccharides by 1H-NMR spectroscopy and by oligosaccharide mapping using high pH anion-exchange chromatography. Mannose and N-acetylglucosamine were the only sugars observed in the intact rgp120 and likewise in individual glycopeptides. All glycopeptides derived from rgp120 contained high mannose-type N-linked oligosaccharides, ranging from GlcNAc2Man5 to GlcNAc2Man9. However, different glycosylation sites showed varied degrees of processing of the high mannose-type oligosaccharides, as characterized by the ratio of GlcNAc2Man8-9 to GlcNAc2Man5-7. These results demonstrate that N-glycosylation of rgp120 in the baculovirus expression system occurs at all potential sites and is site specific in terms of oligosaccharide structures.

The use of baculoviruses as expression vectors

The use of recombinant baculoviruses as high level expression systems is becoming more and more popular. This review aims to provide a summary of the impact of this expression system in biochemistry and biotechnology, highlighting important advances that have been made utilizing the system. The potential of newly developed multiple baculovirus expression systems to enable the reconstruction of complex biological molecules and processes is also reviewed.

Antibodies to discontinuous or conformationally sensitive epitopes on the gp120 glycoprotein of human immunodeficiency virus type 1 are highly prevalent in sera of infected humans

We have used an indirect-capture enzyme-linked immunosorbent assay to quantitate the reactivity of sera from human immunodeficiency virus type 1 (HIV-1)-infected humans with native recombinant gp120 (HIV-1 IIIB or SF-2) or with the gp120 molecule (IIIB or SF-2) denatured by being boiled in the presence of dithiothreitol with or without sodium dodecyl sulfate. Denaturation of IIIB gp120 reduced the titers of sera from randomly selected donors by at least 100-fold, suggesting that the majority of cross-reactive anti-gp120 antibodies present are directed against discontinuous or otherwise conformationally sensitive epitopes. When SF-2 gp120 was used, four of eight serum samples reacted significantly with the denatured protein, albeit with ca. 3- to 50-fold reductions in titer. Only those sera reacting with denatured SF-2 gp120 bound significantly to solid-phase-adsorbed SF-2 V3 loop peptide, and none bound to IIIB V3 loop peptide. Almost all antibody binding to reduced SF-2 gp120 was blocked by preincubation with the SF-2 V3 loop peptide, as was about 50% of the binding to native SF-2 gp120. When sera from a laboratory worker or a chimpanzee infected with IIIB were tested, the pattern of reactivity was reversed, i.e., there was significant binding to reduced IIIB gp120, but not to reduced SF-2 gp120. Binding of these sera to reduced IIIB gp120 was 1 to 10% that to native IIIB gp120 and was substantially decreased by preincubation with IIIB (but not SF-2) V3 loop peptide. To analyze which discontinuous or conformational epitopes were predominant in HIV-1-positive sera, we prebound monoclonal antibodies (MAbs) to IIIB gp120 and then added alkaline phosphatase-labelled HIV-1-positive sera. MAbs (such as 15e) that recognize discontinuous epitopes and compete directly with CD4 reduced HIV-1-positive sera binding by about 50%, whereas neutralizing MAbs to the C4, V2, and V3 domains of gp120 were either not inhibitory or only weakly so. Thus, antibodies to the discontinuous CD4-binding site on gp120 are prevalent in HIV-1-positive sera, antibodies to linear epitopes are less common, most of the antibodies to linear epitopes are directed against the V3 region, and most cross-reactive antibodies are directed against discontinuous epitopes, including regions involved in CD4 binding.

Inhibition of human immunodeficiency virus infection by the lectin jacalin and by a derived peptide showing a sequence similarity with gp120

Jacalin is a plant lectin known to specifically induce the proliferation of CD4+ T lymphocytes in human. We demonstrate here that jacalin completely blocks human immunodeficiency virus type 1 (HIV-1) in vitro infection of lymphoid cells. Jacalin does not bind the viral envelope glycoprotein gp120. Besides other T cell surface molecules, it interacts with CD4, the high-affinity receptor to HIV. Binding of jacalin to CD4 does not prevent gp120-CD4 interaction and does not inhibit virus binding and syncytia formation. The anti-HIV effect of the native lectin can be reproduced by its separated alpha-subunits. More importantly, we have defined in the alpha-chain of jacalin a 14-amino acid sequence which shows high similarities with a peptide of the second conserved domain of gp120. A synthetic peptide corresponding to this similar stretch also exerts a potent anti-HIV effect. This peptide is not mitogenic for peripheral blood mononuclear cells and does not inhibit anti-CD3-induced lymphocyte proliferation. These results make jacalin alpha chain-derived peptide a potentially valuable therapeutic agent for acquired immunodeficiency syndrome.

Insect-transmitted vertebrate viruses: flaviviridae

The Flaviviridae include almost 70 viruses, nearly half of which have been associated with human disease. These viruses are among the most important arthropod-borne viruses worldwide and include dengue, yellow fever, and Japanese encephalitis viruses. Morbidity and mortality caused by these viruses vary, but collectively they account for millions of encephalitis, hemorrhagic fever, arthralgia, rash, and fever cases per year. Most of the members of this family are transmitted between vertebrate hosts by arthropod vectors, most commonly mosquitoes or ticks. Transmission cycles can be simple or complex depending on the hosts, vectors, the virus, and the environmental factors affecting both hosts and viruses. Replication of virus in invertebrate hosts does not seem to result in any significant pathology, which suggests a close evolutionary relationship between virus and vector. Another example of this relationship is the ability of these viruses to grow in invertebrate cell culture, where replication usually results in a steady state, persistent infection, often without cytopathic effect. Yields of virus from insect cell culture vary but are generally similar to yields in vertebrate cells. Replication kinetics are comparable between insect and vertebrate cell lines, despite differences in incubation temperature. Both vertebrate and insect cell culture systems continue to play a significant role in flavivirus isolation and the diagnosis of disease caused by these agents. Additionally, these culture systems permit the study of flavivirus attachment, penetration, replication, and release from cells and have been instrumental in the production and characterization of live-attenuated vaccines. Both vertebrate and insect cell culture systems will continue to play a significant role in basic and applied flavivirus research in the future.

Enhanced proliferative cellular responses to HIV-1 V3 peptide and gp120 following immunization with V3:Ty virus-like particles

The induction of CD4+ T-helper (Th) cell responses is likely to be an important requirement of vaccine candidates designed to prevent or moderate human immunodeficiency virus-1 (HIV-1) infection. We have investigated the ability of hybrid Ty virus-like particles carrying the V3 loop region of the HIV-1 IIIB envelope gp120 (V3:Ty-VLP) to elicit V3-specific proliferative responses. Significant proliferation in response to stimulation in vitro with homologous IIIB V3 peptide was observed following immunization of mice with V3:Ty-VLP either as an aluminium hydroxide precipitate or without adjuvant. Responses to MN V3 peptide were also observed in certain mouse haplotypes. To assess the effect of presenting the V3 loop in this particulate form, we compared the responses induced by V3:Ty-VLP with those obtained with two non-particulate immunogens, recombinant gp120 (rgp120) and V3 peptide conjugated to albumin. V3-specific responses to V3 peptide in vitro were reproducibly higher following immunization with V3:Ty-VLP than with either rgp120 or V3-albumin coagulate (V3-alb). The data indicate that immunization with the V3 loop as a hybrid Ty-VLP results in enhanced proliferative responses to V3 peptide and recognition of rgp120 in vitro. Some cross-reactivity of Th cells for V3 sequences from different isolates was also observed.

Immunogenicity and toxicity testing of an experimental HIV-1 vaccine in nonhuman primates

A highly purified saponin from Q. saponaria (QS-21) was tested in juvenile rhesus macaques for adjuvant activity and toxicity. The QS-21 was tested alone or as part of an experimental subunit HIV-1 vaccine containing a truncated recombinant HIV-1 envelope protein (gp160D) adsorbed to alum. Antibody responses were measured using ELISA and cell-mediated immunity was measured using cellular proliferation assays. Potential toxicity was monitored by standard clinical pathology testing using peripheral blood and urine samples. No toxic effects were observed, even after the administration of the experimental vaccines three times at monthly intervals. The QS-21 saponin adjuvant enhanced total antibody production levels by greater than 100-fold and broadened the specificity of the response so that additional epitopes were recognized, when compared with alum-adsorbed HIV-1 gp160D formulation. Low-level, antigen-specific proliferative responses to HIV-1 recombinant gp160 were induced by either vaccine formulation. Proliferative responses were induced by a sham challenge with soluble recombinant HIV-1 gp160 for all of the animals that had been vaccinated. However, those that received the HIV-complete vaccine formulation containing QS-21 responded significantly better. These data demonstrated that the QS-21 adjuvant augmented both antibody responses and cell-mediated immunity and established immunological memory. The potent adjuvant activity and lack of toxicity suggest that this adjuvant should be safe and effective for use in HIV-1 vaccines.

Transcellular activation of the human immunodeficiency virus type 1 long terminal repeat in T lymphocytes requires CD4-gp120 binding

Cells expressing human immunodeficiency virus type 1 (HIV-1) tat can transactivate the HIV-1 long terminal repeat (LTR) in cocultured T lymphocytes. In this report, we describe the molecular requirements for transcellular activation of the LTR in Jurkat cells. An analysis with deletion mutants and blocking antibodies demonstrated a requirement for env expression in addition to tat expression for transcellular activation to occur. The results suggest that the transient association of CD4 and gp120 in cocultured cells is required for tat-mediated transcellular activation. The events that follow CD4-gp120 binding in transactivation, however, do not require the gp120-neutralizing domain, in contrast to HIV-mediated fusion and infection. The consequences of this interaction on cellular function are currently under investigation.

Efficient production of human immunodeficiency virus proteins in transgenic mice

Transgenic mice containing the complete human immunodeficiency virus (HIV) coding sequences fused to the mouse mammary tumor virus long terminal repeat were generated. They were found to produce high levels of authentic gag and env HIV proteins in several tissues known to support mouse mammary tumor virus-driven transcription. HIV proteins were also detected in serum and in body fluids (milk and epididymal secretions) known to be natural sites of retrovirus, and specifically of HIV, production. These results indicate that primary mouse cells from different tissues have the capacity to produce HIV proteins. These mice represent a novel animal model for HIV infection.

Induction of interleukin-1 and tumor necrosis factor alpha in brain cultures by human immunodeficiency virus type 1

Cytokines such as interleukin-1 (IL-1) and tumor necrosis factor alpha (TNF alpha) are produced by leukocytes and play a role in immune responses. They also function in normal brain physiology as well as in pathological conditions within the central nervous system, where they are produced by brain macrophages (microglia) and brain astrocytes. In this study, we document the ability of human immunodeficiency virus type 1 (HIV-1) to induce TNF alpha and IL-1 in primary rat brain cultures. While productive infection did not occur in these cells, it was not required for cytokine induction. Using monocyte/macrophage-tropic (JRFL) and T-cell-tropic (IIIB) strains of HIV-1, we were able to induce cytokines in both microglia and astrocytes. In addition to whole virus, recombinant envelope proteins also induced these cytokines. The induction of IL-1 and TNF alpha could be blocked by a panel of antibodies recognizing epitopes in the gp120 and gp41 areas of the envelope. Soluble recombinant CD4 did not block TNF alpha and IL-1 production. If TNF alpha and IL-1 can be induced in brain tissue by HIV-1, they may contribute to some of the neurologic disorders associated with AIDS.

Human immunodeficiency virus type 1 major neutralizing determinant exposed on hepatitis B surface antigen particles is highly immunogenic in primates

Hepatitis B surface antigen (HBsAg) produced by recombinant DNA technology is now widely and safely used worldwide for hepatitis B vaccination. We used the HBsAg particle as a carrier molecule for presentation of selected human immunodeficiency virus type 1 (HIV-1) determinants to the immune system. Immunization of rhesus monkeys with an HBsAg chimera carrying the HIV-1 envelope major neutralizing determinant allowed us to generate proliferative T-cell responses and, in some cases, neutralizing antibodies and antibody-dependent cellular cytotoxicity. Since there is an overlap between populations at risk for hepatitis B virus and HIV, HBsAg recombinant particles may be relevant carriers for HIV-1 epitopes and could offer a new approach to the development of an AIDS vaccine.

The principal neutralization determinant of simian immunodeficiency virus differs from that of human immunodeficiency virus type 1

To identify the principal neutralization determinant (PND) of simian immunodeficiency virus (SIV), antisera were generated using recombinant gp110 [the SIV analog of the human immunodeficiency virus type 1 (HIV-1) external envelope glycoprotein, gp120], gp140, several large recombinant and proteolytic envelope fragments, and synthetic peptides of the SIVmac251 isolate. When purified under conditions that retain its native structure, gp110 bound CD4 and elicited antisera that neutralized SIVmac251 with high titer. Native gp110 also completely inhibited neutralizing antibody in sera from SIVmac251-infected macaques. In contrast, denatured gp110 and gp140, large envelope fragments, and synthetic peptides (including peptides analogous to the HIV-1 PND) elicited very low or undetectable neutralizing antibody titers and did not inhibit neutralizing antibody in infected macaque sera. Enzymatically deglycosylated gp110 efficiently absorbed neutralizing antibodies from macaque sera, showing that neutralizing antibodies primarily bind the protein backbone. A 45-kDa protease digest product, mapping to the carboxyl-terminal third of gp110, also completely absorbed neutralizing antibodies from infected macaque sera. These results show that the PND(s) of this SIV isolate depends on the native conformation and that linear peptides corresponding to the V3 loop of SIV envelope, in contrast to that of HIV-1, do not elicit neutralizing antibody. This may affect the usefulness of SIVmac for evaluating HIV-1 envelope vaccine approaches that rely on eliciting neutralizing antibody.

Comparison and fine mapping of both high and low neutralizing monoclonal antibodies against the principal neutralization domain of HIV-1

Monoclonal antibodies raised against viral lysate of HIV-1 (strain LAV-1) and against recombinant gp 160 of HIV-1 (strain HTLV IIIB) which neutralized HIV-1 in a type specific manner were mapped with the aid of peptides (Pepscan analysis). Each of these monoclonal antibodies bound to peptides located on the principal neutralizing domain (PND) of HIV-1. We found that the antigenic sites of the MAbs described in this paper are represented by linear peptides of at least 10 amino acids long. The affinity of the MAbs is high for these peptides and in the same order of magnitude as for native gp 160. The fine mapping of the epitopes may reflect structural features of the PND, for instance which amino acid side chains are exposed and which are buried in the protein. Furthermore the fine mapping of the epitopes explained the HIV type-specific neutralizing activity of the MAbs. Antibodies that bound to the tip of the loop (amino acids QRGPGRAF) have a higher neutralizing activity than antibodies that bound to amino acids towards the N-terminal side of the loop (amino acids KSIRI). Furthermore, MAbs that bound to virtually the same amino acids on the tip of the loop (amino acids IQRGPGRAF and RGPGRAFV) had different neutralizing activities due to different affinities for native gp 160. These data reveal that neutralizing activity not only is determined by the affinity of an antibody to the neutralizing site but also by its fine binding specificities to the V 3 loop of gp 120.

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.

Human cytomegalovirus inhibits human immunodeficiency virus replication in cells productively infected by both viruses

We have been studying the role of human cytomegalovirus (HCMV) as a potential cofactor in human immunodeficiency virus (HIV)-related disease. The clinical relevance of HCMV is highlighted by the fact that it is a principal viral pathogen in patients with AIDS and is known to infect the same cells as HIV. In this study, we focused on the molecular interactions between HIV and HCMV in human fibroblasts and in the human glioblastoma/astrocytoma-derived cell line U373 MG, cells which can be productively infected by both viruses. Because these cells are CD4-, we used HIV pseudotyped with a murine amphotropic retrovirus as described previously (D. H. Spector, E. Wade, D. A. Wright, V. Koval, C. Clark, D. Jaquish, and S. A. Spector, J. Virol. 64:2298-2308, 1990). Initial studies showed that when cells were preinfected with HIV (Ampho-1B) for 5 days and then superinfected with HCMV, HIV antigen production dropped significantly in the coinfected cells but continued to rise in cells infected with HIV (Ampho-1B) alone. HCMV production, however, was unaffected by the presence of HIV. Further analysis showed that HIV steady-state RNA levels and gag and env protein production were also inhibited in the presence of HCMV. The transcriptional inhibition of HIV was particularly surprising in view of the previous results of several other laboratories as well as our own that HCMV infection stimulates HIV long terminal repeat-chloramphenicol acetyltransferase (LTR-CAT) expression in transient expression assays. To investigate this further, we transfected the HIV LTR-CAT construct into either uninfected cells or cells which had been preinfected with HIV. The cells were infected with HCMV 24 h posttransfection and assayed for CAT gene expression at 48 h after HCMV infection. Although there was some stimulation of the LTR-CAT in cells that were dually infected by HIV and HCMV, it was 16-fold less than that in the cells infected only with HCMV. This suggests that in the presence of the HIV infection, the stimulation of the HIV LTR-CAT gene by HCMV is significantly reduced. Experiments with UV-irradiated HCMV and the HCMV DNA polymerase inhibitor ganciclovir showed that HCMV transcription is necessary for the reduction in HIV production to occur; however, replication of the HCMV genome or any events which take place after DNA replication are not necessary. These results, coupled with the observation that inhibition is usually first seen between 8 and 24 h after HCMV infection, suggest that an HCMV early protein is involved in repression of HIV.

Cross-neutralizing antibodies in rabbits immunized with HIV-1 gp160 purified from simian cells infected with a recombinant vaccinia virus

A recombinant vaccinia virus in which the transcription of the human immunodeficiency virus type 1 (BRU isolate) env gene is driven by the 11K late vaccinia promoter yields about 10-fold higher amounts of gp160 env protein upon infection of monkey cells than does a recombinant in which gp160 is expressed using the 7.5K early-late promoter. The gp160 was purified from detergent lysates of infected cells by lentil lectin affinity chromatography followed by immunoaffinity chromatography, and was obtained in yields of 1-2 mg/10(9) cells of material estimated to be about 70% pure. Pairs of rabbits were immunized with purified gp160 using either one of five different adjuvants or an immunostimulating complex. In all cases a substantial humoral immune response was obtained after boosting, including an activity that neutralized the homologous (BRU) isolate of HIV-1. In some cases, this activity also neutralized two distantly related isolates, SF2 and MN.

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.