Neutralizing cross-reactive and non-neutralizing monoclonal antibodies to HIV-1 gp120

Fluorescent protein pair emit intracellular FRET signal suitable for FACS screening

The fluorescent proteins ECFP and HcRed were shown to give an easily resolved FRET-signal when expressed as a fusion inside mammalian cells. HeLa-tat cells expressing ECFP, pHcRed, or the fusion protein pHcRed-ECFP were analyzed by flow cytometry after excitation of ECFP. Cells expressing HcRed-ECFP, or ECFP and HcRed, were mixed and FACS-sorted for FRET positive cells: HcRed-ECFP cells were greatly enriched (72 times). Next, cloned human antibodies were fused with ECFP and expressed anchored to the ER membrane. Their cognate antigens (HIV-1 gp120 or gp41) were fused to HcRed and co-expressed in the ER. An increase of 13.5+/-1.5% (mean+/-SEM) and 8.0+/-0.7% in ECFP fluorescence for the specific antibodies reacting with gp120 or gp41, respectively, was noted after photobleaching. A positive control (HcRed-ECFP) gave a 14.8+/-2.6% increase. Surprisingly, the unspecific antibody (anti-TT) showed 12.1+/-1.1% increase, possibly because overexpression in the limited ER compartment gave false FRET signals.

Rational monoclonal antibody development to emerging pathogens, biothreat agents and agents of foreign animal disease: The antigen scale

Many factors influence the choice of methods used to develop antibody to infectious agents. In this paper, we review the current status of the main technologies used to produce monoclonal antibodies (mAbs) from the B cells of antigen-sensitized animals. While companies are adopting advanced high-throughput methods, the major technologies used by veterinary and medical research laboratories are classical hybridoma fusion and recombinant library selection techniques. These methods have inherent advantages and limitations but have many common aspects when using immunized rodents. Laboratories with expertise in both methods of antibody development have a distinct advantage in their ability to advance mAb technology. New and re-emerging infectious threats in today's world emphasize the need for quality immunoreagents and the need to maintain expertise in mAb development. We provide examples of some common applications for mAb reagents used to identify pathogens such as the SARS-coronavirus (SARS-CoV), Bacillus anthracis, and foot-and-mouth disease (FMD) virus. We also outline a framework for investigators to make rational decisions concerning which method to use to develop mAbs based upon characteristics of the pathogen under study and the intended downstream application. Lastly, we provide parameters for the immunisation of mice and a classification system which describes the expected outcome for mAb development strategies when using classes of immunogens to generate mAbs with desired activities.

Intranasal HIV-1-gp160-DNA/gp41 peptide prime-boost immunization regimen in mice results in long-term HIV-1 neutralizing humoral mucosal and systemic immunity

An intranasal DNA vaccine prime followed by a gp41 peptide booster immunization was compared with gp41 peptide and control immunizations. Serum HIV-1-specific IgG and IgA as well as IgA in feces and vaginal and lung secretions were detected after immunizations. Long-term humoral immunity was studied for up to 12 mo after the booster immunization by testing the presence of HIV-1 gp41- and CCR5-specific Abs and IgG/IgA-secreting B lymphocytes in spleen and regional lymph nodes in immunized mice. A long-term IgA-specific response in the intestines, vagina, and lungs was obtained in addition to a systemic immune response. Mice immunized only with gp41 peptides and L3 adjuvant developed a long-term gp41-specific serum IgG response systemically, although over a shorter period (1-9 mo), and long-term mucosal gp41-specific IgA immunity. HIV-1-neutralizing serum Abs were induced that were still present 12 mo after booster immunization. HIV-1 SF2-neutralizing fecal and lung IgA was detectable only in the DNA-primed mouse groups. Intranasal DNA prime followed by one peptide/L3 adjuvant booster immunization, but not a peptide prime followed by a DNA booster, was able to induce B cell memory and HIV-1-neutralizing Abs for at least half of a mouse's life span.

Multi-subtype gp160 DNA immunization induces broadly neutralizing anti-HIV antibodies

A highly desirable feature for an human immunodeficiency virus type 1 (HIV-1) vaccine is the ability to induce broadly reactive anti-envelope antibodies that can neutralize primary HIV-1 isolates. Two immunizations with an HIV-1 envelope-encoding plasmid together with recombinant granulocyte-macrophage colony-stimulating factor (rGM-CSF) resulted in high antibody titers in mice. The antibody induction was further enhanced after immunization with genes encoding HIV-1 envelopes originating from subtypes A, B and C. The sera from these animals were able to neutralize A, B and C viral isolates, whereas the sera from animals immunized solely with subtype B DNA neutralized only subtype B virus. The combined DNA vaccine gave serum antibodies with broad recognition of HIV-1 envelope epitopes as determined by peptide mapping. Cell-mediated immunity was not compromised by the increased humoral immunity. This demonstrates the ability of multiple envelope genes to induce the desired antibody response against several subtypes. Moreover, it documents the ability of rGM-CSF to enhance the potency of such a vaccine when given simultaneously. The strategy may be useful for making an HIV vaccine more potent and broadly effective against strains of different clades.

Enhanced immune responses after DNA vaccination with combined envelope genes from different HIV-1 subtypes

In a multisubtype approach to HIV-1 vaccination, mice were immunized with HIV-1 envelope gp160 genes from subtypes A, B, and C. Subsequently the mice were challenged with syngeneic primary splenocytes infected with a HIV-1/MuLV pseudovirus carrying a subtype B genome. HIV-specific immune responses and protection were strongest in the group of animals immunized with a combination of subtype A, B, and C specific gp160 genes as compared to subtype B only. Immunization with the combination of the cross-reactive subtypes A and C envelope genes induced HIV-specific immune responses but did not result in significant protection to challenge with subtype B infected cells. From this we conclude that immunization with the envelope genes from several HIV-1 subtypes may indeed enhance immune responses. This study shows that by using a mix of subtype envelope genes, an enhanced protective immunity can be obtained experimentally, potentially also in humans.

Immunization procedures for anti-idiotypic antibody induction in mice and rats

Anti-idiotypic antibodies (Ab2) mimicking antigens (Ags)-defined by antibodies (Ab1) directed to tumors or pathogens have elicited Ag-specific humoral, cellular and/or protective immunity in experimental animals and in humans. In immunizations of rodents with Ab1, factors such as animal species, form of Ab1 and choice of adjuvant are crucial for the successful induction of Ab2 as candidate vaccines against tumors and pathogens. Here we survey the outcome of 362 fusion events (each event representing one animal), using nine immunization schedules in mice and seven schedules in rats and including 10 different Ab1 directed against human tumor- and immunodeficiency virus (HIV-1)-associated Ags. Ab1 IgG or F(ab')2 were administered uncoupled or coupled to keyhole limpet hemocyanin (KLH). As adjuvants, complete and incomplete Freund's adjuvant (CFA/IFA), lipid A, aluminum hydroxide, TiterMax or vaccinia virus were used. In syngeneic immunizations with murine Ab1 in mice, F(ab')2 coupled to KLH and emulsified in CFA/IFA preferentially induced Ab2 mimicking tumor or HIV-1 associated epitopes. In xenogeneic immunizations with mouse Ab1 in rats, various forms of Ab1 and adjuvants successfully induced Ab2 mimicking tumor Ags.

Impact of epitope permutations in the antibody response of mice to a multi-epitope polypeptide of the V3 loop of human immunodeficiency virus type 1

Our group have produced in Escherichia coli and evaluated the immunogenicity of different multi-epitope polypeptides (MEPs) bearing one copy of V3 loop sequential B cell epitopes from several isolates of human immunodeficiency virus type 1 (HIV-1) gp120. One of these MEPs called TAB9 comprises the 15 central amino acids of the V3 loop from isolates LR150, JY1, RF, MN, BRVA and IIIB in this order. Antibodies against all V3 regions were elicited after immunization of rabbits, macaques and humans with TAB9. In contrast, mice immunized with this protein only developed antibodies against epitopes JY1, LR150 and MN in that order (JY1>LR150>MN>>>RF, BRVA, IIIB) resembling an immunodominant gradient from the N-terminus to the C-terminal portion of this construction. To assess what role the location of the V3 epitopes in TAB9 could play, we constructed the protein TAB16, by altering the position of V3 epitopes in TAB9 primary structure and compared the pattern of antibodies elicited by both MEPs in H-2(d) Balb/c mice. The MEP TAB16 elicited antibody titers comparable to that of the sera from mice immunized with TAB9. There were no statistical differences in antibody titers between both groups (P>0.05). JY1, LR150 and MN V3 epitopes were again immunodominant in mice immunized with TAB16 fusion protein. The highest antibody titers detected in both groups among V3 epitopes corresponded to JY1, now located at the C-terminus of the permuted chimera. Antibodies against V3 epitopes RF, BRVA and IIIB were again not detected. Additionally, the MN V3 epitope showed to be significantly more immunogenic in its new orientation in TAB16, possibly as a result of a higher degree of accessibility in the surface of the protein. The results of the present investigation strongly suggest that the sequential order or the intramolecular position of V3 epitopes inside the primary structure of TAB9 and TAB16 MEPs does not interfere with the global immunogenicity or with the hierarchy of immunodominance of these regions.

Protection of neutralization epitopes in the V3 loop of oligomeric human immunodeficiency virus type 1 glycoprotein 120 by N-linked oligosaccharides in the V1 region

The V3 region of the human immunodeficiency virus type 1 envelope protein gp120 constitutes a potential neutralization target, but the oligosaccharide of one conserved N-glycosylation site in this region protects it from neutralizing antibodies. Here, we determined whether N-linked glycans of other gp120 domains were also involved in protection of V3 neutralization epitopes. Two molecular clones of HIV-1, one lacking three N-linked glycans of the V1 region (HIV-1(3N/V1)) and another lacking three N-linked glycans of the C2 region (HIV-1(3N/C2)), were created and characterized. gp120 from both mutated viral clones had higher electrophoretic mobilities than gp120 from wild-type virus, confirming loss of N-linked glycans. Wild-type virus and both mutant clones replicated equally well in established T cell lines and all three viruses were able to utilize CXCR4 but not CCR5 as a coreceptor. The induced mutations increased gp120 affinity for CXCR4 but caused no corresponding increase in viral ability to replicate in T cell lines. HIV-1(3N/V1) was neutralized at about 25 times lower concentrations of an antibody to the V3 region than were wild-type virus and HIV-1(3N/C2). Soluble, monomeric gp120 from HIV-1(3N/V1) and wild type virus had identical avidity for the V3 antibody, indicating that the V1 glycans were able to shield V3 only in oligomeric but not monomeric gp120. In conclusion, one or more N-linked glycans of gp120 V1 is engaged in protection of the V3 region from potential neutralizing antibodies, and this effect is dependent on the oligomeric organization of gp120/gp41.

Monoclonal antibodies to native HIV type 1 reverse transcriptase and their interaction with enzymes from different subtypes

Recombinant reverse transcriptase (RT) from HIV-1 subtype B was used to produce mouse anti-RT monoclonal antibodies (MAbs). Immunization was done by mixing RT with the ISCOM matrix-forming adjuvant saponin (Quil A). Two different assays, both based on the interaction of native RT and antibodies, were used to monitor the immune response in mice and for screening, selection, and characterization of the MAbs. The first assay measures the capacity of antibodies to inhibit the polymerase activity of the RT and the second assay measures the ability of antibodies to capture enzymatically active RT. Twelve clones with the capacity to inhibit at least 50% of the RT activity and 34 clones with high RT-capturing capacity were found. The MAb panel was utilized to evaluate the immunological properties of 18 different RTs representing 9 different HIV1 subtypes. The RT-inhibitory MAbs could be divided into two groups based on their pattern of cross-reactivity toward the different HIV-1 RTs. The degree of diversity recorded among MAbs with RT-capturing capacity was larger. At least seven groups of MAbs with distinct cross-reactivity patterns were identified. Thus, the degree of isoenzyme specificity varied greatly, from MAbs that were quite specific for subtype B RT to one MAb that was able to capture the RTs from all HIV-1 isolates tested except one of the two group O isolates. In conclusion, our study revealed that there exist surprisingly large immunological differences between RTs from different HIV-1 subtypes as well as from the same subtype.

A retro-inverso miniantibody with anti-HIV activity

An HIV-1-specific miniantibody, a peptide representing the third heavy chain complementarity-determining region (CDR) of an HIV-specific mouse antibody, was characterized and modified with unnatural D-isomeric amino acids. The CDR peptide and its parent antibody bound to a similar epitope, located in the V3 region of HIV-1 gp120. A shortened CDR sequence was modified with D-amino acids to create an all-D-amino acid retro-inverso (RI) peptide with a reversed sequence order. The RI CDR was less susceptible to proteolytic degradation than its L-counterpart and had a higher affinity for HIV-1 peptides. The miniantibody and its parent antibody showed neutralization of both primary and laboratory strains of HIV-1. In accordance with the binding studies, the RI CDR showed a stronger HIV-inhibiting capacity than its L-counterpart. We conclude that the anti-HIV retro-inverso CDR identified in this study has the potential to become a future anti-HIV drug. It has a virus-neutralizing capacity in vitro and appears to be stable. Future research should focus on characterizing its antiviral activity in vivo.

Nonproductive human immunodeficiency virus type 1 infection of human fetal astrocytes: independence from CD4 and major chemokine receptors

Human immunodeficiency virus type 1 (HIV-1) infection of the brain is associated with neurological manifestations both in adults and in children. The primary target for HIV-1 infection in the brain is the microglia, but astrocytes can also be infected. We tested 26 primary HIV-1 isolates for their capacity to infect human fetal astrocytes in culture. Eight of these isolates, independent of their biological phenotype and chemokine receptor usage, were able to infect astrocytes. Although no sustained viral replication could be demonstrated, the virus was recovered by coculture with receptive cells such as macrophages or on stimulation with interleukin-1beta. To gain knowledge into the molecular events that regulate attachment and penetration of HIV-1 in astrocytes, we investigated the expression of several chemokine receptors. Fluorocytometry and calcium-mobilization assay did not provide evidence of expression of any of the major HIV-1 coreceptors, including CXCR4, CCR5, CCR3, and CCR2b, as well as the CD4 molecule on the cell surface of human fetal astrocytes. However, mRNA transcripts for CXCR4, CCR5, Bonzo/STRL33/TYMSTR, and APJ were detected by RT-PCR. Furthermore, infection of astrocytes by HIV-1 isolates with different chemokine receptor usage was not inhibited by the chemokines SDF-1beta, RANTES, MIP-1beta, or MCP-1 or by antibodies directed against the third variable region or the CD4 binding site of gp120. These data show that astrocytes can be infected by primary HIV-1 isolates via a mechanism independent of CD4 or major chemokine receptors. Furthermore, astrocytes are potential carriers of latent HIV-1 and on activation may be implicated in spreading the infection to other neighbouring cells, such as microglia or macrophages.

Enhanced HIV infectivity and changes in GP120 conformation associated with viral incorporation of human leucocyte antigen class I molecules

BACKGROUND

Assembly of human immunodeficiency virus type 1 (HIV-1) occurs at the level of the plasma membrane of the host cell. During this process HIV incorporates significant quantities of cell surface-derived molecules into its lipid bilayer including human leucocyte antigen (HLA) class I and II, intercellular adhesion molecule-1 and lymphocyte function antigen-1. Several studies indicate that virion-bound host-cell-derived molecules are functional and affect the biological properties of HIV-1. Virion-associated HLA class II and intercellular adhesion molecule-1 enhance the infectivity of T-cell line-adapted (TCLA) viruses. No role for virion-associated HLA class I molecules has yet been identified.

OBJECTIVE

To investigate the role of HLA class I molecules in HIV replication and infectivity.

METHODS

HLA class I negative human cells lines transfected with the HLA Cw4 gene were infected with different TCLA viruses as well as primary X4 isolates. The infectivity of HLA Cw4 positive and negative viruses was determined on indicator cell lines and on phytohaemagglutinin-activated peripheral blood mononuclear cells. An entry polymerase chain reaction assay was used to determine differences in entry-competence of Cw4 positive and negative viruses. The expression of selected gp120 epitopes on native Env molecules derived from Cw4 positive and negative viruses was determined by a monoclonal antibody-based enzyme-linked immunosorbent assay. Immunoprecipitation experiments were performed to investigate the presence of gp120/HLA Cw4 complexes. Neutralization assays determined the differences in susceptibility to neutralization between HLA Cw4 negative and positive viruses.

RESULTS AND CONCLUSIONS

The infectivity of primary HIV-1 X4 isolates and of TCLA viruses is increased upon viral incorporation of HLA Cw4 molecules. This effect is associated with changes in viral envelope proteins conformation including an enhanced expression of the V3 loop of gp120, and of epitopes that are exposed upon CD4 binding. The gp120 conformational changes are consistent with the formation of a multimolecular complex between HLA class I and gp120/160. HLA Cw4 incorporation is also associated to a lower susceptibility to antibody neutralization. These findings have important implications for understanding the immune response to cryptic and conformational epitopes of the viral envelope.

Stoichiometry of monoclonal antibody neutralization of T-cell line-adapted human immunodeficiency virus type 1

In order to study the stoichiometry of monoclonal antibody (MAb) neutralization of T-cell line-adapted human immunodeficiency virus type 1 (HIV-1) in antibody excess and under equilibrium conditions, we exploited the ability of HIV-1 to generate mixed oligomers when different env genes are coexpressed. By the coexpression of Env glycoproteins that either can or cannot bind a neutralizing MAb in an env transcomplementation assay, virions were generated in which the proportion of MAb binding sites could be regulated. As the proportion of MAb binding sites in Env chimeric virus increased, MAb neutralization gradually increased. Virus neutralization by virion aggregation was minimal, as MAb binding to HIV-1 Env did not interfere with an AMLV Env-mediated infection by HIV-1(AMLV/HIV-1) pseudotypes of CD4(-) HEK293 cells. MAb neutralization of chimeric virions could be described as a third-order function of the proportion of Env antigen refractory to MAb binding. This scenario is consistent with the Env oligomer constituting the minimal functional unit and neutralization occurring incrementally as each Env oligomer binds MAb. Alternatively, the data could be fit to a sigmoid function. Thus, these data could not exclude the existence of a threshold for neutralization. However, results from MAb neutralization of chimeric virus containing wild-type Env and Env defective in CD4 binding was readily explained by a model of incremental MAb neutralization. In summary, the data indicate that MAb neutralization of T-cell line-adapted HIV-1 is incremental rather than all or none and that each MAb binding an Env oligomer reduces the likelihood of infection.

Co-expression of a trans-dominant negative mutant of the human immunodeficiency virus type 1 (HIV-1) Rev protein affects the Rev-dependent splicing pattern and expression of HIV-1 RNAs

Trans-dominant negative mutants of the human immunodeficiency virus type 1 (HIV-1) regulatory protein Rev inhibit the function of wild-type Rev in a dose-dependent manner. This was previously shown to be caused by nuclear retention of the wild-type protein. In the present work, further analysis of the trans-dominant negative effect was performed using cotransfection experiments with different constructs encoding HIV-1 Rev and viral structural proteins together with a plasmid encoding a trans-dominant negative Rev mutant. Thus, one species of pre-mRNA was transcribed from the reporter plasmids. This pre-mRNA was then either spliced or exported by Rev as unspliced RNA for translation of the HIV structural proteins. An immunofluorescence assay and Western blot analysis were used for analysis of protein expression. In situ hybridization was applied for labelling of unspliced mRNA in transfected cells, and RNase protection analysis was used to determine the relative amount of unspliced versus spliced mRNAs. The experiments confirmed that the transdominant negative mutant inhibited nuclear export of unspliced mRNA. It was, in addition, demonstrated for the first time that the trans-dominant negative mutant also affected a Rev-dependent regulatory step connected with viral pre-mRNA splicing. As a consequence, proteins expressed from unspliced and singly spliced HIV mRNAs decreased while there was an increase in protein products encoded by spliced and alternatively spliced mRNAs.

Long-term immunotherapy in HIV infection, combined with short-term antiretroviral treatment

Forty asymptomatic HIV-infected individuals with CD4+ lymphocyte levels above 400 × 106/1 were immunized over 5 years with recombinant envelope glycoprotein gp160 (rgp160). After 5 years there was a trend towards more nonprogressors in the immunized group as compared to the matched controls. Since immunizations could activate HIV, the first 6 immunizations were followed by 2 weeks of zidovudine or placebo, double-blind. The viral load did not change during the first 6 months and was not different from that of the matched controls after 5 years. The best effect on CD4+ lymphocyte development was seen in individuals with a high viral load randomized to rgp160+zidovudine and in individuals with a low viral load randomized to rgp160+placebo. We conclude that rgp160 is safe and results in temporarily improved CD4+ development. Concomitant antiviral treatment might be of benefit, especially in patients with a more advanced disease and can today be given with more effective combinations.

The V3 loop based multi-epitope polypeptide TAB9 adjuvated with montanide ISA720 is highly immunogenic in nonhuman primates and induces neutralizing antibodies against five HIV-1 isolates

In a previous work we selected montanide ISA720 (M-ISA720) among different adjuvants for the vaccination with a V3 loop based multi-epitope polypeptide TAB9. In this paper we present the evaluation of the toxicity and immunogenicity of this formulation in non-human primates. TAB9 in M-ISA720 was highly immunogenic in macaques (Macaca fascicularis) inducing antibodies against TAB9 in all animals after one inoculation and a strong anamnestic response after booster injections. Furthermore 97% of the V3 peptides included were recognized by TAB9 sera. No differences between doses of 200 microg and 1 mg of TAB9 in M-ISA720 were observed after four immunizations. Neutralizing antibodies against five HIV-1 isolates were detected in most animals. Animals remain healthy throughout the study and did not show lesions at the inoculation site.

Simplified procedure for fractionation and structural characterisation of complex mixtures of N-linked glycans, released from HIV-1 gp120 and other highly glycosylated viral proteins

HIV-1 gp120 is heavily glycosylated containing 24 N-glycosylation sites, and this makes elucidation of the significance of glycans at individual glycosylation sites a difficult task. A procedure is described where a complex mixture of biologically radiolabelled glycans of gp120, derived from a relatively small number of virus-infected cells may be characterized by a combination of N-glycanase release, single lectin separation, and normal phase HPLC (NP-HPLC). The method was applied in analysis of three N-linked glycosylation sites essential for the in vivo priming of T-cells, specific for an epitope in their vicinity (Sjölander, S., Bolmstedt, A., Akerblom, 1996. Virology 215, 124-133.). The carbohydrate compositions of wild type gp120 and of mutant variants gp120 lacking one, two, or all of these three active N-linked glycans were analysed. Cells were infected with r-vaccinia virus expressing wild-type gp120 or mutated gp120, or were infected with HIV-1BRU (wild type) or mutant virus variants. HIV-1 glycoproteins were purified by immunosorbent affinity chromatography and released glycans were separated on lectins, then analysed with NP-HPLC. Our data showed that the structural composition of glycans occupying two of the three glycosylation sites was heterogeneous but the site located adjacent to the T-cell epitope was equipped with one large, high mannose-type structure (> 11 units) with the capacity to cover a substantial part of the gp120 surface.

Inhibition of HIV type 1 infectivity by coexpression of a wild-type and a defective glycoprotein 120

An amino acid substitution (D --> K) in the C3 region of HIV-1 gp120 has previously been shown to inhibit binding of virions to CD4+ cells. We have introduced the same mutation into the HIV-1 isolate LAV-I(BRU), in which the mutation is denoted D373K. Here we show that the D373K envelope protein is processed and incorporated into virus particles, but that D373K virions have no detectable infectivity (below 0.1% relative to wild type). When D373K and the wild-type envelope gene were cotransfected in 293 cells at a 4:1 ratio, the resultant infectivity of the HIV-1 supernatant was reduced more than 100-fold. When the same ratio of plasmids was tested in COS-1 cells the inhibition of HIV-1 was an order of magnitude less than observed in 293 cells. COS-1 and 293 cells differed in that only 293 cells displayed saturation of virus production with respect to the envelope protein. Our data fit a simple model: when virion formation is saturated with envelope protein, expression and incorporation of a defective envelope protein imply a corresponding dilution of wild-type protein on the surface of virions. The cooperative function of wild-type envelope proteins is subsequently compromised, and a trans-dominant inhibition of virus infectivity is observed.

Induction of antibodies against epitopes inaccessible on the HIV type 1 envelope oligomer by immunization with recombinant monomeric glycoprotein 120

An N-glycan (N306) at the base of the V3 loop of HIV-BRU gp120 is shielding a linear neutralization epitope at the tip of the V3 loop on oligomeric Env. In contrast, this epitope is readily antigenic on monomeric gp120. Immunization with recombinant monomeric HIV-BRU gp120 may thus be expected to elicit antibodies preferentially neutralizing mutant variants of HIV-BRU lacking the N306 glycan. Therefore, two guinea pigs were immunized with monomeric wild-type HIV-BRU gp120 possessing the N306 glycan and immune sera were tested for neutralization against target viruses HIV-BRU, -A308, and -A308T321. HIV-A308 and HIV-A308T321 lack the N306 glycan; HIV-A308T321 contains an additional mutation at the tip of V3 rendering it resistant to MAb binding at this epitope. Both immune sera preferentially neutralized the two mutant virus variants lacking the N306 glycan, with a 10- to 20-fold increase in neutralization titer compared with the wild-type HIV-BRU. Thus, immunization with monomeric HIV-BRU gp120 elicited antibodies preferentially neutralizing HIV variants lacking the N306 glycan. In addition to antibodies directed against the tip of V3, other antibodies directed against epitopes shielded by the N306 glycan on the envelope oligomer were elicited by the immunization, as demonstrated by the ability of the immune sera to neutralize HIV-A308T321. One such epitope was overlapping the NEA-9284 epitope located at the amino-terminal flank of the V3 loop. Our results demonstrate that monomeric gp120 contains immunogenic structures inaccessible on the envelope oligomer. The limited ability of recombinant gp120 vaccines to induce neutralizing antibodies against primary isolates may thus not exclusively reflect genetic variation.

Outcome of immunization of cynomolgus monkeys with recombinant Semliki Forest virus encoding human immunodeficiency virus type 1 envelope protein and challenge with a high dose of SHIV-4 virus

Infection of macaques with chimeric simian-human immunodeficiency viruses (SHIVs) allows evaluation of HIV-1 envelope vaccines. SHIV-4 is based on SIVmac239 but carries the env, tat, and rev genes of HIV-1IIIB. In this study we used Semliki Forest virus (SFV) RNA vectors to express the envelope protein gp160 of HIV-1IIIB in cynomolgus macaques. Monkeys were immunized four times with recombinant suicide SFV. Whereas two of four monkeys showed T cell-proliferative responses, only one monkey had demonstrable levels of antibodies to HIV-1 gp41 and gp120 as shown by enzyme-linked immunosorbent assay (ELISA) and Western blot. The vaccinated monkeys and four control animals were challenged with 10,000 MID100 (100% minimum infectious doses) of cell-free monkey cell-grown SHIV-4 virus. As demonstrated by virus isolation, all macaques became infected after challenge. All vaccinated monkeys showed an HIV-1-specific anamnestic T cell-proliferative response. Three of four vaccines had developed HIV-1-Env-specific antibodies 2 weeks after challenge whereas none of the four controls showed any detectable immune response at this time point. Furthermore, three of four vaccinated monkeys had no demonstrable viral antigenemia and low viral load as opposed to one of the four naive control animals.

Neutralization sensitivity of cell culture-passaged simian immunodeficiency virus

CEMx174- and C8166-45-based cell lines which contain a secreted alkaline phosphatase (SEAP) reporter gene under the control of a tat-responsive promoter derived from either SIVmac239 or HIV-1(NL4-3) were constructed. Basal levels of SEAP activity from these cell lines were low but were greatly stimulated upon transfection of tat expression plasmids. Infection of these cell lines with simian immunodeficiency virus (SIV) or human immunodeficiency virus type 1 (HIV-1) resulted in a dramatic increase in SEAP production within 48 to 72 h that directly correlated with the amount of infecting virus. When combined with chemiluminescent measurement of SEAP activity in the cell-free supernatant, these cells formed the basis of a rapid, sensitive, and quantitative assay for SIV and HIV infectivity and neutralization. Eight of eight primary isolates of HIV-1 that were tested induced readily measurable SEAP activity in this system. While serum neutralization of cloned SIVmac239 was difficult to detect with other assays, neutralization of SIVmac239 was readily detected at low titers with this new assay system. The neutralization sensitivities of two stocks of SIVmac251 with different cell culture passage histories were tested by using sera from SIV-infected monkeys. The primary stock of SIVmac251 had been passaged only twice through primary cultures of rhesus monkey peripheral blood mononuclear cells, while the laboratory-adapted stock had been extensively passaged through the MT4 immortalized T-cell line. The primary stock of SIVmac251 was much more resistant to neutralization by a battery of polyclonal sera from SIV-infected monkeys than was the laboratory-adapted virus. Thus, SIVmac appears to be similar to HIV-1 in that extensive laboratory passage through T-cell lines resulted in a virus that is much more sensitive to serum neutralization.

Neutralization titres of HIV-1-specific monoclonal antibodies vary according to the batch of primary human peripheral blood lymphocytes, but do not vary coordinately

Human peripheral blood lymphocytes (PBL) were collected from five healthy adults under standard conditions and on a number of different occasions, and used in neutralization assays of human immunodeficiency virus type 1 (HIV-1) strain IIIB with three monoclonal antibodies (mAbs). Variations in neutralization titre were observed with different batches of PBLs with, for example, titres of ICR39.3b ranging from 1/10 to over 1/40000. However titres were as high, or higher, in PBLs than in C8166 cells (a human CD4+ T lymphoblastoid cell line) in 82% (28/34) of tests made. Most surprising was that neutralization by the three mAbs did not vary coordinately. In one batch of PBLs the neutralization titre of one of the mAbs might be increased while that of another mAb did not increase, or decrease. Thus PBLs could not be described as giving high or low levels of neutralization without reference to a specific mAb. This was not an assay problem as infectivity titres were relatively constant (varying by 1 to 1.4 fold with respect to C8166 cells), and neutralization titres were reproducible with the same batch of frozen PBLs over a three month period. Only one donor gave consistently low neutralization titres (defined here as 1/200; 2/2 batches tested) with all three mAbs, but all other donors gave similarly low titres with one of their batches of PBLs. The non-coordinate variation in neutralization titre indicates the advisability of using antibodies of several different specificities in any kind of preventive or therapeutic immunity.

Two PBMC-based neutralization assays depict low reactivity of both anti-V3 monoclonal antibodies and immune sera against HIV-1 primary isolates

The neutralizing activity of anti-V3 monoclonal antibodies (MAbs) and anti-HIV-1 immune sera was tested against HIV-1 laboratory strains and African primary isolates. Neutralization was investigated in Phytohaemagglutinin (PHA)-stimulated peripheral blood mononuclear cell (PBMC) cultures by means of two distinct viral titer reduction assays. In these assays, virus was detected by means of either p24 antigen measurement using ELISA or HIV provirus synthesis using PCR, respectively. Anti-V3 MAbs and anti-HIV-1 immune sera neutralized efficiently the homologous laboratory HIV-1 strains used for eliciting immune response but showed no neutralizing activity against most primary isolates. The two neutralization assays used provided similar results. However, a PCR-based assay circumvented the limitations due to low levels of virus replication. The mechanism of resistance of the primary isolates to neutralizing antibodies was complex and was not simply predicted by partial sequence determination of the epitopes. This points out the need for reliable neutralization assays of HIV-1 primary isolates in order to evaluate the role of humoral immunity during HIV-1 infection and for future vaccine strategies.

Human antibodies from phage libraries: neutralizing activity against human immunodeficiency virus type 1 equally improved after expression as Fab and IgG in mammalian cells

Human antibodies against HIV-1 have been sought to study neutralization events on the molecular level, and for possible use in passive immune intervention. The development of phage display techniques has opened the possibility of rapidly generating human monoclonal antibodies with desired specificities. We and others have isolated human HIV-1 neutralizing antibody fragments using this technique. Bacterial expression of isolated clones does, however, differ broadly both in expression levels and functional activity. In addition, intact IgG cannot be expressed in bacteria. By transferring the genes of isolated Fab clones to a mammalian expression system we could perform a comparison of functional activity between Fab expressed in bacterial and mammalian cells, as well as Fab and whole IgG. Fab fragments expressed in mammalian cells showed increased virus neutralizing activity compared to the same Fab clones expressed in Escherichia coli, underlining the inefficiency of procaryotic expression. No difference in HIV-1 neutralizing capacity was detected between monovalent (Fab) and divalent (whole antibody) reagents expressed in CHO cells. Thus, bivalency does not always confer improved neutralization efficacy.

Peptides isolated from random peptide libraries on phage elicit a neutralizing anti-HIV-1 response: analysis of immunological mimicry

Peptides binding to a murine, human immunodeficiency virus type 1 (HIV-1) neutralizing monoclonal antibody (F58/H3) were isolated from two random peptide libraries expressed on the surface of phage. The antibody was originally elicited by immunization with HIV-1 envelope protein gp120LAI, and has previously been shown to interact with the -I-GPGRA- motif of the V3 loop. The peptide libraries consisted of nine or 15 random amino acid residues flanked by two cysteines, and fused to the amino terminal end of the cpIII protein on the filamentous phage. Selection of specific peptides was carried out in three rounds, with decreasing antibody concentration. An expected peptide motif -GPGRA-, a similar segment, -GPAR-, and two unrelated motifs -FRLLG- and -WRM/ALG- were selected. Binding of antibody was tested both to synthetic peptides in solution, and the corresponding peptide on phage. The GPXR motifs bound in both formats, while the FRLLG bound antibody only when present on the phage The reactivity of peptides on phage was highly dependent on an intact disulphide bond between the cysteines flanking the peptide. The molecular mimicry of the found motifs was tested by immunizing mice and rabbits with conjugated synthetic peptides or peptide on phage. In mice, peptide-specific antisera were raised, but no reactivity to the whole protein (gp120) was detected. In rabbits, however, this was accomplished with the -GPGRA- containing peptide when present on phage. In addition, this antisera precipitated virus particles, and neutralized HIV-1SF2 virus in vitro.

Conserved N-linked oligosaccharides of the C-terminal portion of human immunodeficiency virus type 1 gp120 and viral susceptibility to neutralizing antibodies

We have constructed a mutated infectious HIV variant lacking the signals for addition of three N-linked glycans situated in the V4, C4 and V5 regions of HIV gp120. When comparing mutated virus with wildtype virus we found essentially no differences in the phenotypic characteristics of the two viruses except for the expected electrophoretic mobility shift of radioimmuno-precipitated mutated gp120, resulting from the missing N-glycans. Thus, the infectivity titer and the capacity to induce syncytia were similar for the two viruses. The sensitivity of mutant and wildtype virus to a number of neutralizing agents was determined. As expected, the mutant virus was significantly less sensitive to neutralization by Con A, with affinity for the N-glycans eliminated. We found, however, that antibodies to the V3 loop and sCD4 neutralized wild-type virus as efficiently as mutant virus, whereas 2G12, a monoclonal antibody, binding to a discontinuous neutralization epitope, and GP13, binding to the CD4-binding domain, neutralized wildtype virus better than mutant virus. Altogether the data suggest that the three conserved N-linked glycans, despite their location in immediate association with the CD4-binding domain, which is an important neutralization epitope, are not essential for virus replication in cell culture and they are not engaged in shielding neutralization epitopes of gp120 from neutralizing antibodies. However, the glycans evidently influence the three-dimensional conformation of gp120, since their presence increases the availability of the neutralization epitope of 2G12.

N-butyldeoxynojirimycin-mediated inhibition of human immunodeficiency virus entry correlates with changes in antibody recognition of the V1/V2 region of gp120

The alpha-glucosidase inhibitor N-butyldeoxynojirimycin (NB-DNJ) is an inhibitor of human immunodeficiency virus (HIV) replication and HIV-induced syncytium formation in vitro. Although NB-DNJ appears to inhibit HIV entry at the level of post-CD4 binding (P.B. Fischer, M. Collin, G.B. Karlsson, W. James, T.D. Butters, S.J. Davis, S. Gordon, R.A. Dwek, and F.M. Platt, J. Virol. 69:5791-5797, 1995), the exact mechanism of action remains to be established. In this study we have examined the effect of NB-DNJ on the structure of recombinant gp120 (rgpl20), expressed in CHO cells, by using a panel of 40 monoclonal antibodies. The levels of binding of antibodies to rgp120 produced in the presence [rgpl20(+)] and absence [rgpl20(-)] of NB-DNJ were compared by enzyme-linked immunosorbent assay and surface plasmon resonance (BIAcore; Pharmacia). The results showed an increase in the binding to rgp120(+) of antibodies directed against the C1 and C2 regions and a decrease in the binding of antibodies directed against the V1/V2 loops compared with antibody binding to rgpl20(-). A decrease in the binding to rgpl20(+) of antibodies directed against discontinuous epitopes was also observed. No differences were seen in the binding of antibodies directed against the crown of the V3 loop and the C4 region of gp120. Treatment of rgpl20 with alpha-glucosidases I and II had no effect on the differential binding observed, whereas treatment with sialidase abolished the differences seen in the binding of antibodies directed against the C1 and C2 regions of gp120. In addition to these findings, rgpl20(+) showed increased sensitivity to proteases released by CHO cells during expression, as well as to exogenous thrombin. Taken together, the data presented in this paper suggest that production of gp120 in the presence of NB-DNJ affects the conformation of the Vl/V2 loops of gpl20, as well as the overall charge of the C1 and C2 regions. These effects may play a role in the previously described NB-DNJ-mediated inhibition of HIV entry at the level of post-CD4 binding.

Influence of N-linked glycans in V4-V5 region of human immunodeficiency virus type 1 glycoprotein gp160 on induction of a virus-neutralizing humoral response

One of the functions of N-linked glycans of viral glycoproteins is protecting otherwise accessible neutralization epitopes of the viral envelope from neutralizing antibodies. The aim of the present study was to explore the possibility to obtain a more broadly neutralizing immune response by immunizing guinea pigs with gp160 depleted of three N-linked glycans in the CD4-binding domain by site-directed mutagenesis. Mutant and wild type gp160 were formulated into immunostimulating complexes and injected s.c. into guinea pigs. Both preparations induced high serum antibody response to native gp120 and V3 peptides. Both preparations also induced antibodies that bound equally well to the V3 loop or the CD4-binding region, as determined by a competitive enzyme-linked immunosorbent assay (ELISA). The sera from animals, immunized with mutated glycoprotein, did not neutralize nonrelated HIV strains better than did sera from animals, immunized with wild type glycoprotein. Instead, a pattern of preferred homologous neutralization was observed, i.e., sera from animals, immunized with mutant gp160, neutralized mutant virus better than wild type virus, and vice versa. These data indicated that elimination of the three N-linked glycans from gp160 resulted in an altered local antigenic conformation but did not uncover hidden neutralization epitopes, broadening the immune response.

Immunodominant glycoprotein 41 epitope identified by seroreactivity in HIV type 1-infected individuals

A highly conserved gp41 epitope (amino acid sequence ELDKWA) has been described to elicit antibodies neutralizing a broad variety of HIV-1 strains. We analyzed the antibody reactivity of HIV-1-infected individuals from Argentina and Sweden to overlapping synthetic peptides covering aa647-684 of HIV-1 MN gp41. An immunodominant antigenic determinant was discovered in the C-terminal region adjacent to the ELDKWA sequence. Most of the sera from both Argentina and Sweden reacted only with peptides representing this region. Two other patterns of reactivity were also observed: some sera reacted only with peptides spanning the central region of gp41, including the ELDKWA sequence; other sera reacted with both the central and C-terminal regions. Differences in reactivity were noted between Argentinian and Swedish sera in terms of peptides covering the central region. In addition, to determine the amino acids essential for antibody binding, seroreactivity against a set of substitution peptides covering the immunodominant epitope was studied. Results obtained indicated that carboxyl amino acids WNWFDI close to the ELDKWA sequence were the most important for antibody binding. Ability of sera, tested for peptide reactivity, to neutralize HIV-1 was also analyzed. High antibody reactivity to the central region was frequently found in sera with high neutralizing titers. This observation was not seen when seroreactivity to peptides spanning the C-terminal region was analyzed. These results suggest that the immunodominant epitope C terminal to the ELDKWA sequence is not involved in inducing neutralizing antibodies.

Sensitivity of HIV-1 to neutralization by antibodies against O-linked carbohydrate epitopes despite deletion of O-glycosylation signals in the V3 loop

It has been suggested that threonine or serine residues in the V3 loop of HIV-1 gp120 are glycosylated with the short-chain O-linked oligosaccharides Tn or sialosyl-Tn that function as epitopes for broadly neutralizing carbohydrate specific antibodies. In this study we examined whether mutation of such threonine or serine residues could decrease the sensitivity to infectivity inhibition by Tn or sialosyl-Tn specific antibodies. All potentially O-glycosylated threonine and serine residues in the V3 loop of cloned HIV-1 BRU were mutagenized to alanine thus abrogating any O-glycosylation at these sites. Additionally, one of these T-A mutants (T308A) also abrogated the signal for N-glycosylation at N306 inside the V3-loop. The mutant clones were compared with the wild type virus as to sensitivity to neutralization with monoclonal and polyclonal antibodies specific for the tip of the V3 loop of BRU or for the O-linked oligosaccharides Tn or sialosyl-Tn. Deletion of the N-linked oligosaccharide at N306 increased the neutralization sensitivity to antibodies specific for the tip of the loop, which indicates that N-linked glycosylation modulates the accessibility to this immunodominant epitope. However, none of the mutants with deletions of O-glycosylation signals in the V3 loop displayed any decrease in sensitivity to anti-Tn or anti-sialosyl-Tn antibody. This indicates that these broadly specific neutralization epitopes are located outside the V3 loop of gp 120.

Characterization of an internal permissive site in the cholera toxin B-subunit and insertion of epitopes from human immunodeficiency virus-1, hepatitis B virus and enterotoxigenic Escherichia coli

We previously described the construction of novel hybrid proteins based on the B-subunit of cholera toxin (CTB) [Bäckström et al., Gene 149 (1994) 211-217], in which a neutralizing B-cell epitope from the third variable (V3) loop in the envelope glycoprotein gp120 from human immunodeficiency virus type 1 (HIV-1) was inserted within a surface-exposed region between amino acids (aa) 55 and 64. The resulting protein retained properties of native CTB and could induce strong anti-CTB antibody (Ab) responses, but the inserted gp120 epitope was only modestly immunogenic. In this study, the potential use of this internal permissive site in CTB for the insertion of heterologous epitopes has been further investigated. Six additional plasmids were constructed encoding HIV::CTB hybrid proteins with ten to fourteen aa from the V3 loop of gp120 genetically inserted at different positions between aa 52 and 65, with deletions of different CTB aa. Plasmids encoding proteins with peptides inserted between aa 53 and 64 in CTB gave rise to stable proteins which reacted with CTB-specific monoclonal antibodies (mAb) and bound to GM1 gangliosides (GM1), indicating that insertions between these positions do not drastically alter the conformation or the receptor-binding properties of native CTB. Plasmids were also constructed encoding CTB hybrid proteins which had either an 11-aa peptide from hepatitis B virus (HBV) pre-S(2) or one of two peptides related to the heat-stable toxin (STa) of enterotoxigenic Escherichia coli inserted between aa 55 and 64 of CTB. This resulted in the production of HBV::CTB or ST::CTB hybrid proteins and illustrated that the internal permissive site can be used for insertion of peptides of varying aa composition. The reactivity of the inserted epitopes with epitope-specific mAb in GM1-ELISA and immunoblots varied greatly between hybrid proteins and depended on the position in CTB and the aa composition of the inserted peptides. Despite these differences, all the HIV::CTB, ST::CTB and HBV::CTB hybrid proteins could induce low, but significant, levels of serum Ab in mice against gp120, STa or pre-S(2), in addition to strong serum Ab responses against CTB. The Ab response against the internally inserted gp120 peptide was similar to that against the same peptide fused to the N-terminus of CTB, indicating that internally placed peptides had similar immunogenicity to the same peptides added terminally.

Serotyping of primary human immunodeficiency virus type 1 isolates from diverse geographic locations by flow cytometry

The immunologic relatedness of the various human immunodeficiency virus type 1 (HIV-1) clades was determined with 13 human anti-HIV-1 monoclonal antibodies (MAbs) to six immunogenic regions of the HIV-1 structural proteins. The immunoreactivity of the native, oligomeric viral envelope glycoproteins expressed on the surfaces of human peripheral blood mononuclear cells infected in vitro with primary isolates from clades A through E was determined by flow cytometry. Some epitopes in the immunodominant region of gp41 and the C terminus of gp120 appear to be HIV-1 group specific in that they are expressed on the surfaces of cells in cultures infected with the majority of viruses tested from clades A to E. Epitopes within the V3 region appear to be clade restricted. Surprisingly, one MAb to an epitope in the C terminus of gp120 was entirely clade B specific. Staining with anti-V2 and anti-CD4 binding domain (CD4bd) reagents was infrequently detected. Anti-CD4bd MAbs stained only CD4-negative T cells because the CD4bd of gp120 appeared to be complexed with membrane CD4. When present, the epitopes of V2 and the CD4bd appeared to be expressed on cells infected with various clades. Thus, the results suggest that MAbs to gp41, the C terminus, and the V3 loop of gp120 are most useful in serotyping primary isolates of HIV-1, providing group-specific, clade-restricted, and clade-specific reagents. The use of the immunofluorescent method with the reagents described herein distinguishes infection with clade B from that with all other HIV-1 clades. With additional MAbs, this technique will allow a broadly applicable, reproducible, and practical method for serotyping HIV-1.

Single peptide and anti-idiotype based immunizations can broaden the antibody response against the variable V3 domain of HIV-1 in mice

The third variable (V3) domain of the human immunodeficiency virus type 1 (HIV-1) external envelope glycoprotein gp120 is a major target of neutralizing antibodies in infected persons and in experimental immunized animals. Given the high degree of sequence variability of V3, the humoral response toward this region is very type-specific. In the present study, we evaluated the potential of a single peptide and an anti-idiotypic antibody to broaden the anti-V3 antibody specificity in BALB/c mice. We show that a synthetic peptide derived from the V3 determinant of HIV-1 MN isolate (V3MN), when used as an immunogen, was able to induce an antibody response to multiple (up to six) HIV-1 strains. The extent of this cross-reactivity, which tended to enlarge as the injections increased, appeared to be inversely correlated with the binding affinity to V3MN peptide. These data thus present evidence that, despite its great sequence heterogeneity, the V3 loop encompasses conserved amino-acid positions and/or stretches which may be less immunogenic than their variable counterparts. We additionally demonstrate that a rabbit anti-idiotype (Ab2), recognizing a binding site related idiotype on a V3-specific mouse monoclonal antibody (Ab1), could mount a broadened humoral response (Ab3) in mice. Unlike nominal antibody Ab1 which strictly reacted with the European HIV-1 LAI isolate, elicited Ab3 recognized the two divergent HIV-1 strains SF2 and 1286, originating respectively from North America and Central Africa, in addition to LAI. The reasons accounting for this Ab2-induced enlargement of the V3 antibody response are discussed. Our findings suggest that single peptide and anti-idiotype based immunizations may provide viable approaches to overcome, at least in part, HIV epitope variability.

Molecular characterization of a human anti-HIV 1 monoclonal antibody revealed a CD26-related motif in CDR2

Genes encoding the immunoglobulin variable regions of a human anti-HIV-1 IgG1 kappa monoclonal antibody were rescued from a hybridoma, derived from a sero-negative donor, using PCR cloning and expression in Escherichia coli. The ELISA binding results obtained from the expressed Fab fragment confirmed the anti-V3 loop specificity for HIV-1 (LAI) of the original antibody. In addition, an amino acid sequence derived from the second complementarity determining region (CDRH2) of the heavy chain was found to be very similar to the catalytic motif of CD26, a T-cell activation antigen. Furthermore, synthetic peptides containing both the catalytic domain of CD26 and CDRH2 of the antibody showed specific binding to an HIV peptide representing the V3 region in a dose-dependent manner. This suggests an involvement of CD26 as a possible coreceptor for HIV-1.

Infection of vaginal and colonic epithelial cells by the human immunodeficiency virus type 1 is neutralized by antibodies raised against conserved epitopes in the envelope glycoprotein gp120

The rectal and genital tract mucosae are considered to be major sites of entry for the human immunodeficiency virus (HIV) during sexual contact. We now demonstrate that vaginal epithelial cells can be infected by HIV type 1 (HIV-1) via a mechanism similar to that described for neuroglial cells and, more recently, for colorectal epithelial cells, involving initial interaction of the HIV-1 envelope glycoprotein gp120 with a cell-surface glycosphingolipid (sulfated lactosylceramide). A hyperimmune serum against gp120 was able to neutralize HIV-1 infection of vaginal epithelial cells. Site-directed immunization was employed to identify sites on gp120 recognized by antibodies neutralizing HIV-1 infection of vaginal and colonic epithelial cells. Hyperimmune sera were raised in monkeys against a series of 40 overlapping synthetic peptides covering the entire sequence of HIV-1 (HTLV-IIIB) gp120. Antisera raised against five synthetic peptides, corresponding to three relatively conserved regions and to the hypervariable region (V3 loop), efficiently neutralized HIV-1 infection of human vaginal epithelial cells in vitro. Similar results were obtained with the colonic cells. Hyperimmune sera to all five peptides have been shown earlier to neutralize HIV-1 infectivity in CD4+ T cells. These results have obvious implications for the design of mucosal subunit vaccines against sexually transmitted HIV-1 infections.

Cross-reactivity of monoclonal antibodies to a chimeric V3 peptide of HIV-1 with peptide analogues studied by biosensor technology and ELISA

The reactivity of monoclonal antibodies (Mabs) raised against a cyclic peptide representing a chimeric V3 loop of HIV-1 gp120 with different peptide analogues was studied with a biosensor system (BIAcore) and by ELISA. In both assays, the Mabs cross-reacted extensively with the V3 regions of different HIV-1 strains and recognized the cyclic form of the peptide immunogen better than its linear form. The highest degree of cross-reactivity was observed with peptides that shared a Lys312 with the chimeric sequence. Dissociation rate constants of ten Mabs measured with the BIAcore with respect to different peptides increased with increasing numbers of substitutions in the flanking regions of the V3 tip sequence Gly Pro Gly Arg. Immobilization of the cyclic peptide on the sensor chip via a thiol group added near the end of the loop structure preserved the conformation of the peptide. In view of the good correlation between the BIAcore and ELISA results, biosensor data should be useful for selecting peptides to be used in diagnostic solid phase assays.

Molecular characterization of variable heavy and light chain regions of five HIV type 1-specific human monoclonal antibodies

We have reported the generation and characterization of four HIV-1 neutralizing human monoclonal antibodies. Three antibodies recognize a conformational epitope within the CD4-binding site of HIV-1 gp120 and one recognizes a linear epitope located within the hypervariable V3 domain of gp120. In the present study we report the nucleotide sequences of the cDNAs encoding the variable regions of the heavy and light chains of these antibodies. Molecular characteristics, closet germline genes, and the putative extent of somatic mutation are presented. Two of the four heavy chain variable (VH) regions are derived from the VH1 gene family, one from the VH3 gene family, and one from the VH5 gene family. In addition, the VH chain of a previously described human monoclonal antibody, directed against HIV-1 gp41, is derived from the VH3 gene family. The degree of nucleotide variation between these five antibodies and their closest germline counterparts ranges from 4 to 12%, mainly located in the complementarity-determining regions. Significant nucleotide sequence homology with previously described germline diversity (D) genes could be found for only two of five antibody D segments. Joining (JH) gene segments utilized are JH4 or JH6. Two light chain variable (VL) regions are derived from a VK1 gene segment, one from a V kappa 4, one from a V lambda 2, and one from a lambda 6 gene segment.

A broadly neutralizing human monoclonal antibody against gp41 of human immunodeficiency virus type 1

We have established a hybridoma clone, designated 2F5, secreting a neutralizing human monoclonal antibody (MAb) specific for gp41 of human immunodeficiency virus type 1 (HIV-1). The epitope of MAb 2F5 was mapped to amino acid sequence Glu-Leu-Asp-Lys-Trp-Ala on the ectodomain of gp41. In this study different in vitro test systems were used to characterize the neutralizing properties of MAb 2F5. In syncytium inhibition assays, fusion inhibition experiments, and neutralization assays on different HIV-susceptible cells (H9, U937, and peripheral blood mononuclear cells) MAb 2F5 showed broad-spectrum neutralizing capacity against HIV-1 laboratory isolates IIIB, MN, RF, and SF2. In addition, primary isolates from AIDS patients were also neutralized.

Insertion of a HIV-1-neutralizing epitope in a surface-exposed internal region of the cholera toxin B-subunit

The non-toxic B-subunit of cholera toxin (CTB) is a powerful immunogen and has been investigated as a carrier for foreign peptide epitopes, with peptides genetically fused to either the N- or C terminus of CTB. In the present study, we have constructed a plasmid encoding a novel intrachain CTB fusion protein with a peptide epitope inserted into an internal region of CTB: eight amino acids (aa) in CTB (56-63) were substituted with a 10-aa peptide from the third variable (V3) loop of the HIV-1 envelope protein gp120. The resulting chimeric protein retained important functional characteristics of the native CTB including pentamerization and GM1 ganglioside receptor binding. The internal hybrid protein was also shown to be resistant to proteolytic degradation during production in Vibrio cholerae, whereas a terminal hybrid protein, where the same gp120-epitope was fused to the N terminus of CTB, was rapidly cleaved during culture. The inserted epitope, which is known to give rise to HIV-1 neutralizing Ab, could be detected with a V3 loop-specific monoclonal Ab when the chimeric protein was analyzed in ELISA and immunoblot, indicating that the epitope inserted at this site is presented on the surface of the protein. Consistent with these observations, immunization of mice with the CTB::HIV hybrid protein elicited a high titered serum Ab response to the CTB moiety and also, in some but not all animals, a detectable response to the inserted gp120 epitope.

Immunological evidence for interactions between the first, second, and fifth conserved domains of the gp120 surface glycoprotein of human immunodeficiency virus type 1

We have used a combination of genetic and immunological techniques to explore how amino acid substitutions in the second conserved (C2) domain of gp120 from human immunodeficiency virus type 1 (HIV-1) affect the conformation of the protein. It was reported previously (R. L. Willey, E. K. Ross, A. J. Buckler-White, T. S. Theodore, and M. A. Martin. J. Viol. 63:3595-3600, 1989) that an asparagine-glutamine (N/Q) substitution at C2 residue 267 of HIV-1 NL4/3 reduced virus infectivity, but that infectivity was restored by a compensatory amino acid change (serine-glutamine; S/N) at residue 128 in the C1 domain. Here we show that the 267 N/Q substitution causes the abnormal exposure of a segment of C1 spanning residues 80 to 120, which compromises the integrity of the CD4-binding site. The reversion substitution at residue 128 restores the normal conformation of the C1 domain and recreates a high-affinity CD4-binding site. The gp120 structural perturbation caused by changes in C2 extends also to the C5 domain, and we show by immunological analysis that there is a close association between areas of the C1 and C5 domains. This association might be important for forming a complex binding site for gp41 (E. Helseth, U. Olshevsky, C. Furman, and J. Sodroski. J. Virol. 65:2119-2123, 1991). Segments of the C1 and C2 domains are predicted to form amphipathic alpha helices. We suggest that these helices might be packed together in the core of the folded gp120 molecule, that the 267 N/Q substitution disrupts this interdomain association, and that the 128 S/N reversion substitution restores it.

Enzyme immunoassay (ELISA) for the evaluation of antibodies directed to the CD4 receptor-binding site of the HIV gp120 molecule

The interaction between the HIV envelope glycoprotein gp120 and the CD4 molecule is probably the most important primary event determining HIV infection. Reactivity with the native viral envelope has been difficult to measure due to the lack of gp120 ligand purified directly from primary virus cultures. We have developed an ELISA, utilizing Galanthus nivalis agglutinin (GNA) which selectively binds native HIV envelope gp120 in culture medium. The GNA-based ELISA eliminates the need for isotope-labelled reagents, live cells and recombinant non-natively glycosylated envelope proteins and offers an easy way of using gp120 directly from crude HIV culture medium. The reactivities of sera from several categories of HIV infected individuals were assayed for inhibition of the HIV-1 gp120-CD4 binding. 19/32 (59.3%) sera from asymptomatic individuals and 7/10 (70%) sera from ARC/AIDS patients blocked the CD4-gp120 binding. 20 serum samples from uninfected individuals showed a gp120-CD4 interaction blocking capacity of 0-15%. Two monoclonal antibodies, T4.2 directed to CD4 and 1171 directed to the CD4 binding site of gp120 were used as positive controls. Both Mabs inhibited CD4-gp120 binding by 66-90%.

Identification of an N-linked glycan in the V1-loop of HIV-1 gp120 influencing neutralization by anti-V3 antibodies and soluble CD4

Glycosylation is necessary for HIV-1 gp120 to attain a functional conformation, and individual N-linked glycans of gp120 are important, but not essential, for replication of HIV-1 in cell culture. We have constructed a mutant HIV-1 infectious clone lacking a signal for N-linked glycosylation in the V1-loop of HIV-1 gp120. Lack of an N-linked glycan was verified by a mobility enhancement of mutant gp120 in SDS-gel electrophoresis. The mutated virus showed no differences in either gp120 content per infectious unit or infectivity, indicating that the N-linked glycan was neither essential nor affecting viral infectivity in cell culture. We found that the mutated virus lacking an N-linked glycan in the V1-loop of gp120 was more resistant to neutralization by monoclonal antibodies to the V3-loop and neutralization by soluble recombinant CD4 (sCD4). Both viruses were equally well neutralized by ConA and a conformation dependent human antibody IAM-2G12. This suggests that the N-linked glycan in the V1-loop modulates the three-dimensional conformation of gp120, without changing the overall functional integrity of the molecule.

Cross-reactive potential of rabbit antibodies raised against a cyclic peptide representing a chimeric V3 loop of HIV-1 gp120 studied by biosensor technique and ELISA

Rabbit antibodies were induced against a free cyclic peptide representing the chimeric sequence of a consensus V3 loop of HIV-1 gp120. The reactivity of these antibodies was tested in a biosensor system (BIAcore, Pharmacia AB, Uppsala, Sweden) and in ELISA with the peptide immunogen in its cyclic and linear forms, as well as with peptides corresponding to the V3 region of different HIV-1 variants. The antibodies reacted with all the peptides tested both in ELISA and in biosensor assays and recognized the cyclic form of the chimeric peptide better than the linear form. Although antibodies raised against the V3 region of particular HIV-1 variants cross-react with other HIV-1 strains, it seems that the use of a chimeric peptide as immunogen improved the cross-reactivity spectrum of recognition of the antibodies. The anti-V3 antibodies were also tested for their ability to neutralize in vitro four HIV-1 laboratory strains. Only the HIVMN variant was found to be neutralized. Compared to conventional solid phase immunoassays, the BIAcore presents several advantages for measuring the differential reactivity of peptide analogues. In view of their broadly cross-reactive potential, antibodies raised against a consensus sequence should be useful in immunodiagnosis of viral antigenic variants.

Cross-reactivity between autoimmune anti-U1 snRNP antibodies and neutralizing epitopes of HIV-1 gp120/41

We report extensive amino acid sequence homology between HIV-1 gp120/41, and > 33% of a U1 RNA-associated splicing protein, 70K. The latter is a target of autoimmune anti-RNP antibodies in mixed connective tissue disease (MCTD). The homologies, involving dominant epitopes of 70K and neutralizing epitopes of gp120/41, are the basis for mutual antibody cross-reactivity. A key finding is that the epitope GRAFVTIG in the V3 loop of gp120 (strain IIIB) is homologous to the functionally essential U1 RNA-binding site of 70K. ELISA data reveal a mean reactivity of anti-RNP antibodies to V3 IIIB that is as high as that of HIV sera. V3 MN, containing the framework sequence G-AF-T, also cross-reacts with anti-RNP antibodies, as do hydrophilic epitopes in gp41 homologous to the COOH end of 70K. Further, there is strong cross-reactivity between HIV sera and 70K in Western blots. In contrast, antibodies from a related autoimmune disorder, Sjögren's syndrome (SS), are neither V3 nor gp41 selective. We conclude that the substantial cross-reactivities reported here are due to conserved, antigenically dominant B cell epitopes having homologous counterparts in 70K and gp120/41. Because antibody production in both MCTD and HIV-1 infection is T cell dependent, the results imply that common T cell clones are also activated in these two disease paradigms. Further exploration of the mechanisms that activate these clones, and that control their divergent fates in MCTD and AIDS, may provide new insights into immune dysregulation in HIV infection.(ABSTRACT TRUNCATED AT 250 WORDS)

Resistance to neutralization by broadly reactive antibodies to the human immunodeficiency virus type 1 gp120 glycoprotein conferred by a gp41 amino acid change

A neutralization-resistant variant of human immunodeficiency virus type 1 (HIV-1) that emerged during in vitro propagation of the virus in the presence of neutralizing serum from an infected individual has been described. A threonine-for-alanine substitution at position 582 in the gp41 transmembrane envelope glycoprotein of the variant virus was responsible for the neutralization-resistant phenotype (M.S. Reitz, Jr., C. Wilson, C. Naugle, R. C. Gallo, and M. Robert-Guroff, Cell 54:57-63, 1988). The mutant virus also exhibited reduced sensitivity to neutralization by 30% of HIV-1-positive sera that neutralized the parental virus, suggesting that a significant fraction of the neutralizing activity within these sera can be affected by the amino acid change in gp41 (C. Wilson, M. S. Reitz, Jr., K. Aldrich, P. J. Klasse, J. Blomberg, R. C. Gallo, and M. Robert-Guroff, J. Virol. 64:3240-3248, 1990). It is shown here that the change of alanine 582 to threonine specifically confers resistance to neutralizing by antibodies directed against both groups of discontinuous, conserved epitopes related to the CD4 binding site on the gp120 exterior envelope glycoprotein. Only minor differences in binding of these antibodies to wild-type and mutant envelope glycoproteins were observed. Thus, the antigenic structure of gp120 can be subtly affected by an amino acid change in gp41, with important consequences for sensitivity to neutralization.

Probing the structure of the human immunodeficiency virus surface glycoprotein gp120 with a panel of monoclonal antibodies

We have probed the structures of monomeric and oligomeric gp120 glycoproteins from the LAI isolate of human immunodeficiency virus type 1 (HIV-1) with a panel of monoclonal antibodies (MAbs); most of these MAbs are directed against continuous epitopes. On native monomeric gp120, most of the first conserved (C1) domain is accessible to MAbs, although some regions of C1 are relatively inaccessible. All of the MAbs directed against the C2, C3, and C5 domains bind preferentially to denatured monomeric gp120, indicating that these regions of gp120 are poorly accessible on the native monomer, although the extreme C terminus in C5 is well exposed. Segments of the V1, V2, and V3 loops are exposed on the surface of monomeric gp120, although the base of the V3 loop is inaccessible. A portion of C4 is also available for MAb binding on monomeric gp120, as is the extreme C terminus in C5. However, on oligomeric gp120-gp41 complexes, only the V2 and V3 loops (and perhaps V1) are well exposed and a segment of the C4 region is partially exposed; continuous epitopes in C1 and C5 that are accessible to antibodies on monomeric gp120 are occluded on the oligomer. Although deletion of the V1, V2, and V3 loops resulted in increased exposure of several discontinuous epitopes overlapping the CD4-binding site, the exposure of most continuous epitopes on the monomeric gp120 glycoprotein was not affected. These results imply a HIV-1 gp120 structure in which the conserved continuous determinants are inaccessible; in some cases, this inaccessibility is due to intramolecular interactions between conserved regions, and in other cases, it is due to intermolecular interactions with other components of the glycoprotein spike. These findings have implications for the design of subunit vaccines based on gp120.

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)

Polyclonal antiidiotypic antibodies mimicking gp120 of HIV-1

Rabbit antiidiotypic antibodies (Ab2) were produced against anti-HIV-1 antibody 0.5 beta (Ab1), which binds to gp120 of HIV-1 and shows virus-neutralizing activity. The Ab2 bound specifically to the Ab1 and their binding to Ab2 was inhibited by a recombinant fragment of gp120 (PB1) or a peptide (residues 301-324 of gp120), both expressing the Ab1-defined epitope. The Ab2 induced in rats antiantiidiotypic antibodies (Ab3) that were Ab1-like in their binding reactivities to PB1, native gp120 or peptide and shared idiotopes with the Ab1. However, the Ab2 did not induce virus-neutralizing Ab3, probably a reflection of the low avidity of the Ab3 as compared to the Ab1.