The reactivities of HIV-1+ human sera with solid-phase V3 loop peptides can be poor predictors of their reactivities with V3 loops on native gp120 molecules

What Do Chaotrope-Based Avidity Assays for Antibodies to HIV-1 Envelope Glycoproteins Measure?

When HIV-1 vaccine candidates that include soluble envelope glycoproteins (Env) are tested in humans and other species, the resulting antibody responses to Env are sifted for correlates of protection or risk. One frequently used assay measures the reduction in antibody binding to Env antigens by an added chaotrope (such as thiocyanate). Based on that assay, an avidity index was devised for assessing the affinity maturation of antibodies of unknown concentration in polyclonal sera. Since a high avidity index was linked to protection in animal models of HIV-1 infection, it has become a criterion for evaluating antibody responses to vaccine candidates. But what does the assay measure and what does an avidity index mean? Here, we have used a panel of monoclonal antibodies to well-defined epitopes on Env (gp120, gp41, and SOSIP.664 trimers) to explore how the chaotrope acts. We conclude that the chaotrope sensitivity of antibody binding to Env depends on several properties of the epitopes (continuity versus tertiary- and quaternary-structural dependence) and that the avidity index has no simple relationship to antibody affinity for functional Env spikes on virions. We show that the binding of broadly neutralizing antibodies against quaternary-structural epitopes is particularly sensitive to chaotrope treatment, whereas antibody binding to epitopes in variable loops and to nonneutralization epitopes in gp41 is generally resistant. As a result of such biases, the avidity index may at best be a mere surrogate for undefined antibody or other immune responses that correlate weakly with protection.

IMPORTANCE

An effective HIV-1 vaccine is an important goal. Such a vaccine will probably need to induce antibodies that neutralize typically transmitted variants of HIV-1, preventing them from infecting target cells. Vaccine candidates have so far failed to induce such antibody responses, although some do protect weakly against infection in animals and, possibly, humans. In the search for responses associated with protection, an avidity assay based on chemical disruption is often used to measure the strength of antibody binding. We have analyzed this assay mechanistically and found that the epitope specificity of an antibody has a greater influence on the outcome than does its affinity. As a result, the avidity assay is biased toward the detection of some antibody specificities while disfavoring others. We conclude that the assay may yield merely indirect correlations with weak protection, specifically when Env vaccination has failed to induce broad neutralizing responses.

Oligomer-specific conformations of the human immunodeficiency virus (HIV-1) gp41 envelope glycoprotein ectodomain recognized by human monoclonal antibodies

Trimerization of the human immunodeficiency virus (HIV-1) envelope glycoproteins is mediated by the ectodomain of the gp41 transmembrane glycoprotein. Here we investigate oligomer-specific conformations of gp41 by using monoclonal antibodies (MAbs) from HIV-1-infected humans. Human MAbs directed against the cluster I region of gp41 recognized trimeric, dimeric, and monomeric forms of soluble envelope glycoproteins; thus, the integrity of the cluster I epitopes is minimally affected by the oligomeric state. In contrast, human MAbs to the cluster II region were all oligomers specific. One cluster II MAb, 126-6, recognized exclusively the trimeric form of envelope glycoproteins, whereas the others recognized both trimeric and dimeric forms. Thus, a distinct trimer-specific conformation exists in the cluster II region of gp41. Analysis of soluble envelope glycoprotein mutants revealed that gp41 sequences immediately N-terminal to isoleucine 646 contribute to the formation of both the trimer and the trimer-specific conformational epitope.

Analysis of the neutralization breadth of the anti-V3 antibody F425-B4e8 and re-assessment of its epitope fine specificity by scanning mutagenesis

The identification of cross-neutralizing antibodies to HIV-1 is important for designing antigens aimed at eliciting similar antibodies upon immunization. The monoclonal antibody (mAb) F425-B4e8 had been suggested previously to bind an epitope at the base of V3 and shown to neutralize two primary HIV isolates. Here, we have assessed the neutralization breadth of mAb F425-B4e8 using a 40-member panel of primary HIV-1 and determined the epitope specificity of the mAb. The antibody was able to neutralize 8 clade B viruses (n=16), 1 clade C virus (n=11), and 2 clade D viruses (n=6), thus placing it among the more broadly neutralizing anti-V3 antibodies described so far. Contrary to an initial report, results from our scanning mutagenesis of the V3 region suggest that mAb F425-B4e8 interacts primarily with the crown/tip of V3, notably Ile(309), Arg(315), and Phe(317). Despite the somewhat limited neutralization breadth of mAb F425-B4e8, the results presented here, along with analyses from other cross-neutralizing anti-V3 mAbs, may facilitate the template-based design of antigens that target V3 and permit neutralization of HIV-1 strains in which the V3 region is accessible to antibodies.

Dissecting the neutralizing antibody specificities of broadly neutralizing sera from human immunodeficiency virus type 1-infected donors

Attempts to elicit broadly neutralizing antibody responses by human immunodeficiency virus type 1 (HIV-1) vaccine antigens have been met with limited success. To better understand the requirements for cross-neutralization of HIV-1, we have characterized the neutralizing antibody specificities present in the sera of three asymptomatic individuals exhibiting broad neutralization. Two individuals were infected with clade B viruses and the third with a clade A virus. The broadly neutralizing activity could be exclusively assigned to the protein A-reactive immunoglobulin G (IgG) fraction of all three donor sera. Neutralization inhibition assays performed with a panel of linear peptides corresponding to the third hypervariable (V3) loop of gp120 failed to inhibit serum neutralization of a panel of HIV-1 viruses. The sera also failed to neutralize chimeric simian immunodeficiency virus (SIV) and HIV-2 viruses displaying highly conserved gp41-neutralizing epitopes, suggesting that antibodies directed against these epitopes likely do not account for the broad neutralizing activity observed. Polyclonal IgG was fractionated on recombinant monomeric clade B gp120, and the neutralization capacities of the gp120-depleted samples were compared to that of the original polyclonal IgG. We found that the gp120-binding antibody population mediated neutralization of some isolates, but not all. Overall, the data suggest that broad neutralization results from more than one specificity in the sera but that the number of these specificities is likely small. The most likely epitope recognized by the monomeric gp120 binding neutralizing fraction is the CD4 binding site, although other epitopes, such as the glycan shield, cannot be excluded.

Expression and immunoreactivity of an epitope of HCV in a foreign epitope presenting system

AIM: To construct and highly express an epitope of hepatitis C virus (HCV) in a foreign epitope presenting vector based on an insect virus, and to study the antigenicity of the epitope.

METHODS: The HCV epitope sequence (amino acid residues 315 to 328: EGHRMAWDMMMNWS) of the E1 region was constructed at different positions of a foreign epitope presenting vector based on an insect virus, flock house virus (FHV) capsid protein encoding gene as a vector, and expressed in E. coli cells. Western blotting and ELISA were used to detect the immunoreactivity of these recombinant proteins.

RESULTS: The gene encoding of the concerned B-cell epitope of HCV E1 envelope protein was expressed on FHV capsid carrier protein at positions I1 (aa 106), I2 (aa 153) and I3 (aa 305), respectively, on the surface of FHV capsid protein. The recombinant proteins in this system could be highly expressed in more than 40% of total cell protein of E. coli BL21. All the expressed recombinant proteins were in inclusion body form, and showed obvious immunoreactivity by Western blotting. Further purified recombinant proteins were detected by indirect ELISA as coating antigen respectively. All recombinant proteins could still show immunoreactivity.

CONCLUSION: The epitope of HCV E1 envelope protein can be highly expressed in FHV carrier system as a chimeric protein with high immunoreactivity. This system has multiple entry sites conferring many possible conformations closer to the native one for a given sequence.

Inhibition of HIV infectivity by a natural human isolate of Lactobacillus jensenii engineered to express functional two-domain CD4

The predominant mode of HIV transmission worldwide is via heterosexual contact, with the cervico-vaginal mucosa being the main portal of entry in women. The cervico-vaginal mucosa is naturally colonized with commensal bacteria, primarily lactobacilli. To address the urgent need for female-controlled approaches to block the heterosexual transmission of HIV, we have engineered natural human vaginal isolates of Lactobacillus jensenii to secrete two-domain CD4 (2D CD4) proteins. The secreted 2D CD4 recognized a conformation-dependent anti-CD4 antibody and bound HIV type 1 (HIV-1) gp120, suggesting that the expressed proteins adopted a native conformation. Single-cycle infection assays using HIV-1HxB2 carrying a luciferase reporter gene demonstrated that Lactobacillus-derived 2D CD4 inhibited HIV-1 entry into target cells in a dose-dependent manner. Importantly, coincubation of the engineered bacteria with recombinant HIV-1HxB2 reporter virus led to a significant decrease in virus infectivity of HeLa cells expressing CD4-CXCR4-CCR5. Engineered lactobacilli also caused a modest, but statistically significant, decrease in infectivity of a primary isolate, HIV-1JR-FL. This represents an important first step toward the development of engineered commensal bacteria within the vaginal microflora to inhibit heterosexual transmission of HIV.

The ability of an oligomeric human immunodeficiency virus type 1 (HIV-1) envelope antigen to elicit neutralizing antibodies against primary HIV-1 isolates is improved following partial deletion of the second hypervariable region

Partial deletion of the second hypervariable region from the envelope of the primary-like SF162 virus increases the exposure of certain neutralization epitopes and renders the virus, SF162DeltaV2, highly susceptible to neutralization by clade B and non-clade B human immunodeficiency virus (HIV-positive) sera (L. Stamatatos and C. Cheng-Mayer, J. Virol. 78:7840-7845, 1998). This observation led us to propose that the modified, SF162DeltaV2-derived envelope may elicit higher titers of cross-reactive neutralizing antibodies than the unmodified SF162-derived envelope. To test this hypothesis, we immunized rabbits and rhesus macaques with the gp140 form of these two envelopes. In rabbits, both immunogens elicited similar titers of binding antibodies but the modified immunogen was more effective in eliciting neutralizing antibodies, not only against the SF162DeltaV2 and SF162 viruses but also against several heterologous primary HIV type 1 (HIV-1) isolates. In rhesus macaques both immunogens elicited potent binding antibodies, but again the modified immunogen was more effective in eliciting the generation of neutralizing antibodies against the SF162DeltaV2 and SF162 viruses. Antibodies capable of neutralizing several, but not all, heterologous primary HIV-1 isolates tested were elicited only in macaques immunized with the modified immunogen. The efficiency of neutralization of these heterologous isolates was lower than that recorded against the SF162 isolate. Our results strongly suggest that although soluble oligomeric envelope subunit vaccines may elicit neutralizing antibody responses against heterologous primary HIV-1 isolates, these responses will not be broad and potent unless specific modifications are introduced to increase the exposure of conserved neutralization epitopes.

Assessment of antibody assays for identifying and distinguishing recent from long-term HIV type 1 infection

We evaluated 16 antibody assays for their performance in discriminating recent from established HIV-1 infection. These approaches were based on antigen specificity, quantity, conformation dependence, and avidity/affinity of HIV-specific antibodies. A panel of 41 sera from subjects who had seroconverted in the previous 2-6 months (n = 20) and from subjects with established infection (>1 year, n = 21) were run in each assay. Compared with anti-Gag and anti-Pol responses, quantitative anti-Env antibody levels were initially lower and ultimately higher, resulting in the greatest spread and least overlap between incident and established infection. Quantitative measurement included end-point titer in Western blot, end-point titer or response at a given dilution in solid-phase enzyme immunoassays (EIAs) with recombinant proteins or synthetic peptides, and IgG capture assays that reflect the relative proportion of IgG that is anti-HIV antibody. Focusing on the anti-env response, we measured specific responses to the V3 region of gp120, to the CD4-binding site of gp120, to a peptide corresponding to the immunodominant region of gp41, and to conformation-dependent epitopes of gp120. We also measured antibody affinity for gp41 peptide and the relative avidity for gp120 or gp41 peptide by thermal or urea-elution assays. These assays also discriminated recent from established infection but were not necessarily superior to the quantitative anti-Env assays. Appropriate approaches, based on distinct principles or combination of principles, can be used to develop simple assays for identifying individuals recently infected with HIV-1.

Anti-F(ab')2 antibody in HIV type 1 infection: relationship to hypergammaglobulinemia and to antibody specific to the V3 loop region of glycoprotein 120

As HIV infection and autoimmune disease share certain similarities, it has been suggested that HIV may disrupt control of humoral immunity by the antiidiotype network, and that this may be evident as increased IgG antibody to F(ab')2. When anti-F(ab')2 was quantified by ELISA in sera of randomly chosen HIV-infected versus uninfected donors, some HIV-infected sera did contain increased anti-F(ab')2, resulting in a median amount twofold higher than in uninfected sera. Moreover, when data were grouped by blood CD4 lymphocyte count, anti-F(ab')2 in HIV+ groups appeared to rise as CD4 lymphocytes declined. However, increased anti-F(ab')2 mirrored the elevation in serum IgG closely, and normalization of anti-F(ab')2 to serum IgG concentration equalized the groups so that no relationship to CD4 lymphocytes remained. Hypergammaglobulinemia is therefore strongly implicated as a cause of variation in anti-F(ab')2. After dissociation of immune complexes, anti-F(ab')2 activity per microgram of monomeric IgG was slightly increased over normal only in the HIV-infected group with fewest CD4 lymphocytes, without statistical significance. In contrast, the proportion of IgG antibody to the V3-neutralizing determinant in HIV-1 decreased significantly as disease advanced. The same was true for 12 HIV+ individuals studied longitudinally for 500-1300 days. The data suggest that measuring serum anti-F(ab')2 is misleading when immune complexes are present: apparent increases as disease progresses are due to increased IgG and, possibly, to related technical artifacts. During HIV infection, the proportion of antiidiotypic IgG in fact remains unaltered or falls, making this an unlikely cause of suppressed humoral immunity to HIV-1.

An immunoassay with bovine serum albumin coupled peptides for the improved detection of anti V3 antibodies in HIV-1 positive human sera

The V3 loop of the gp120 envelope glycoprotein contains the principal neutralizing determinant of HIV-1. Many serological studies have been carried out to assess the reactivity of HIV-1 infected individuals against V3 loop synthetic peptides from different HIV-1 subtypes. V3 directed serology has also been used to demonstrate the association between ELISA reactivity and progression to AIDS in HIV patients, and to study the reactivity against the V3 region in sera from vaccinated animals and human volunteers. The advantage of the use of bovine serum albumin (BSA) conjugated V3 peptides over free V3 peptides for ELISA is described. 15 meric V3 peptides representing several HIV-1 isolates were synthesized, chemically coupled to BSA, and used to coat ELISA microplates. Conjugated peptides were compared with free peptides for the detection of anti V3 antibodies in the sera from rabbits immunized with V3 containing chimeric proteins and from HIV-1 infected individuals. No differences in reactivity against free or BSA-peptide were found for most rabbit sera, however human plasma recognized preferentially the BSA conjugated peptides. Although technically more complex, ELISA with BSA coupled V3 peptides is more sensitive and appropriate for serological studies of HIV-1 infected persons.

Immunological and virological analyses of persons infected by human immunodeficiency virus type 1 while participating in trials of recombinant gp120 subunit vaccines

We have studied 18 participants in phase I/II clinical trials of recombinant gp120 (rgp120) subunit vaccines (MN and SF-2) who became infected with human immunodeficiency virus type 1 (HIV-1) during the course of the trials. Of the 18 individuals, 2 had received a placebo vaccine, 9 had been immunized with MN rgp120, and seven had been immunized with SF-2 rgp120. Thirteen of the 18 infected vaccinees had received three or four immunizations prior to becoming infected. Of these, two were placebo recipients, six had received MN rgp120, and five had received SF-2 rgp120. Only 1 of the 11 rgp120 recipients who had multiple immunizations failed to develop a strong immunoglobulin G antibody response to the immunogen. However, the antibody response to rgp120 was transient, typically having a half-life of 40 to 60 days. No significant neutralizing activity against the infecting strain was detected in any of the infected individuals at any time prior to infection. Antibody titers in subjects infected despite vaccination and in noninfected subjects were not significantly different. Envelope-specific cytotoxic T-lymphocyte responses measured after infection were infrequent and weak in the nine vaccinees who were tested. HIV-1 was isolated successfully from all 18 individuals. Sixteen of these strains had a non-syncytium-inducing (NSI) phenotype, while two had a syncytium-inducing (SI) phenotype. NSI strains used the CCR5 coreceptor to enter CD4+ cells, while an SI strain from one of the vaccinees also used CXCR4. Viruses isolated from the blood of rgp120 vaccinees were indistinguishable from viruses isolated from control individuals in terms of their inherent sensitivity to neutralization by specific monoclonal antibodies and their replication rates in vitro. Furthermore, genetic sequencing of the env genes of strains infecting the vaccinees did not reveal any features that clearly distinguished these viruses from contemporary clade B viruses circulating in the United States. Thus, despite rigorous genetic analyses, using various breakdowns of the data sets, we could find no evidence that rgp120 vaccination exerted selection pressure on the infecting HIV-1 strains. The viral burdens in the infected rgp120 vaccine recipients were also determined, and they were found to be not significantly different from those in cohorts of placebo-vaccinated and nonvaccinated individuals. In summary, we conclude that vaccination with rgp120 has had,to date, no obvious beneficial or adverse effects on the individuals we have studied.

Most potential linear B cell epitopes of Env glycoproteins of feline immunodeficiency virus are immunogenically silent in infected cats

A battery of sixty-six 20- to 23-amino acid synthetic peptides, partially overlapping by 10-12 amino acids, spanning the entire sequence of the envelope (Env) glycoproteins of the Petaluma isolate of feline immunodeficiency virus (FIV-Pet), has been used to map Env linear B cell epitopes. By screening FIV-infected cat sera for anti-peptide reactivity, the existence of two immunodominat domains, namely the V3 region of the surface (SU) glycoprotein and the domain including the highly conserved sequence QNQFF of the transmembrane (TM) glycoprotein, was detected; antibody-binding sites were also mapped in the domain overlapping the cleavage site between SU and TM encompassing the V6 variable region. Moreover, at least two novel linear B epitopes, the former spanning residues 427M-H446 and the latter spanning residues 737N-N756 and likely representing a "type-specific" determinant, have been revealed. The battery of synthetic peptides was then used to immunize outbred Swiss mice in the attempt to reveal other potential sites of immunogenicity of the Env glycoproteins. Analysis of peptide-immunized mouse sera for anti-peptide reactivity revealed more numerous B cell epitopes, generally mapping in different peptides, as compared with those defined in the feline system. None of the mouse anti-peptide sera, however, proved neutralizing for FIV-Pet.

An investigation of the high-avidity antibody response to glycoprotein 120 of human immunodeficiency virus type 1

The avidity of antibodies for antigens can be measured by determining what remains bound after exposing the antibody-antigen complex to a chaotropic agent such as urea. This method has been gaining popularity for assessing the immune response to the human immunodeficiency virus type 1 (HIV-1) surface glycoprotein gp120 (or its counterpart from simian immunodeficiency virus), during natural infection or after subunit vaccination. High-avidity antibodies have been considered to be a possible correlate of protection. We have examined the avidity assay to determine what it, in fact, measures. First, we studied the development of the anti-gp120 response in seroconverting individuals. Urea elution reduced the polyclonal anti-gp120 titers by 3- to 10-fold. After allowing for the consequent reduction in assay sensitivity, there was no obvious change in the rate of development of the high-avidity and unfractionated antibody responses. Furthermore, in the one individual who developed a strong autologous, virus-neutralizing response, the appearance of neutralizing antibodies and high-avidity antibodies did not coincide. Antibodies to the V3 loop, when present, comprised a major fraction of the polyclonal response that survives urea elution. We next examined the effect of urea elution on the binding to gp120 of a panel of monoclonal antibodies (MAbs). Urea treatment preferentially eluted MAbs to discontinuous rather than continuous epitopes, independent of their affinities. Furthermore, these patterns of epitope stability were unaltered by the presence of polyclonal anti-gp120 antibodies. As most broadly neutralizing anti-gp120 antibodies recognize discontinuous epitopes, this skewing effect must be taken into account when interpreting studies using polyclonal sera.

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

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

Improved reactivity of hepatitis C virus core protein epitopes in a conformational antigen-presenting system

Recent studies have identified several epitopes in the N-terminal portion of the nucleocapsid protein which are predominantly recognized by sera of patients infected with hepatitis C virus (HCV). The characterization of the sequences recognized by theses antibodies and the evaluation of their reactivities have been performed mainly with synthetic peptides. However, synthetic peptides are notoriously unreliable as antigens when the immune response is directed against conformational epitopes. In order to improve the detection of antibody responses in HCV-infected patients, we have evaluated the reactivities of three immunodominant regions of the HCV core protein (residues 1 to 20, 21 to 40, and 32 to 46) displayed in a conformation-specific manner on the surface of the Flock House virus (FHV) capsid protein. The results obtained with these proteins in the analysis of 94 serum samples positive by anti-HCV enzyme-linked immunosorbent assay where then compared with those obtained with the corresponding synthetic peptides. The sequence most reactive both with the peptide and with the FHV protein was the region from residues 1 to 20, confirming the low conformational requirements for the display of these residues. On the other hand, the already reported conformational nature of residues 32 to 46 is in keeping with its observed high reactivity when displayed by the FHV recombinant protein and with the low reactivity displayed by its corresponding synthetic peptide. Finally, the high reactivity observed for the chimeric protein displaying the region from residues 21 to 40, as opposed to the results obtained with the synthetic peptide, also suggests that this sequence contains one or more conformational epitopes whose structures cannot be mimicked correctly with synthetic peptides.

V3 loop core region serotyping of HIV-1 infected patients using the FHV epitope presenting system

We have reported recently a new epitope presenting system based on the Flock House Virus (FHV) capsid protein. The HIV-1 V3 loop core sequence IGPGRAF was inserted in different sites of this carrier molecule. Immunoreactivity experiments and molecular modelling consistently showed that the most reactive recombinant protein displayed the IGPGRAF sequence in a conformation which is most similar to that of a V3 loop reference structure. The same insertion site was then used to display the V3 loop apex sequences of six different HIV-1 isolates. Sera from 32 HIV-1 infected patients were examined for their reactivity to our chimeric proteins and the results were compared with those obtained using synthetic V3 loop peptides. The data obtained were confirmed by nested PCR amplification and direct sequencing of the patient's V3 loops. The results showed that the V3 loop serotyping using the FHV hybrid proteins, was more specific than that obtained using synthetic peptides. This system will therefore be a useful tool for the correct evaluation of the immune response against different V3 loop core sequences.

Neutralization of HIV type 1 infectivity by serum antibodies from a subset of autoimmune patients with mixed connective tissue disease

Mixed connective tissue disease (MCTD) is a rheumatic disorder with clinical similarities to HIV-1 infection, and with characteristic autoimmune anti-RNP antibodies specific for the U1 snRNP splicing complex. Anti-RNP antibodies cross-react with the HIV-1 surface, owing to multiple homologies between the gp120/41 envelope complex and the 70K protein of U1 snRNP. A key epitope of 70K, its RNA-binding site, is homologous to a dominant B and T cell epitope in the third variable loop (V3) of gp120. In this study, we tested the ability of anti-RNP sera to inhibit HIV-1 infectivity in vitro. Of nine sera tested, five were 70-99% effective in neutralizing one or more HIV-1 strains. One serum was > 99% effective in neutralizing HIV-1MN, and 86 and 77% effective against the primary isolates HIV-1(CO) and HIV-1(JR-FL), respectively, an efficacy equal to that of a pool of broadly neutralizing antibodies from HIV-1-infected subjects (HIVIG). The mean neutralizing titer of anti-RNP sera against HIV-1(JR-FL) was 3.9-fold higher than that of HIVIG. Neutralizing potency was associated with high reactivity to gp120 by ELISA, and with the presence of serum rheumatoid factor, known to enhance antibody neutralization of other viruses. The current findings provide further evidence that individuals unexposed to HIV-1 may develop immunologic resistance by alternative mechanisms, possibly including molecular mimicry, or exposure to as yet unidentified retroviruses. Thus MCTD, which involves both B and T cell reactivity to self-epitopes homologous to HIV-1, may elucidate new strategies for generating protective immunity to this virus.

Conformational display of two neutralizing epitopes of HIV-1 gp41 on the Flock House virus capsid protein

We have developed an antigen presenting system based on the capsid protein of the Flock House virus (FHV) and used it to display, in different positions on its external surface, two neutralizing epitopes found at residues 735-752 of HIV-1 gp160. We have compared the immunoreactivity of these FHV chimeric proteins and of the corresponding synthetic peptide using a panel of neutralizing mouse monoclonal antibodies (mAbs) directed against two distinct sequences (IEEE and ERDRD) contained in this epitope of the gp41 region. We have observed that both the FHV chimeric protein and the synthetic peptide are clearly detected in ELISA procedures by the mAbs recognizing the sequence IEEE. The denaturation of these recombinant proteins had little effect on the recognition pattern of this group of monoclonals, suggesting minor conformational requirements for the display of this epitope. The FHV chimeric proteins were also recognized by the mAbs directed against the ERDRD epitope, whereas the corresponding synthetic peptide was not recognized. In this case, denaturation of these recombinant proteins completely abolished the reactivity of the second group of mAbs, arguing for the existence of strong conformational constraints. Additionally, we have investigated whether an isolated loop structure from the FHV protein was sufficient to provide the conformational requirements for the presentation of these epitopes. These experiments have shown that the stabilized loop structure, although improving the presentation of both epitopes, is not as efficient as the native loop in the intact FHV protein. The data obtained with these mAbs support the recently observed limitations in the use of synthetic peptides for the screening of the immune response against conformational epitopes. The establishment of appropriate tools able to present epitope sequences in a structure resembling the native conformation will be useful for accurate epidemiological studies and for the design of new epitope-specific vaccines.

Inter- and intraclade neutralization of human immunodeficiency virus type 1: genetic clades do not correspond to neutralization serotypes but partially correspond to gp120 antigenic serotypes

We have studied genetic variation among clades A through E of human immunodeficiency virus type 1 (HIV-1) at the levels of antibody binding to gp120 molecules and virus neutralization. We are unable to identify neutralization serotypes that correspond to the genetic clades. Instead, we observe that inter- and intraclade neutralization of primary isolates by HIV-1-positive sera is generally weak and sporadic; some sera show a reasonable degree of neutralization breadth and potency whereas others are relatively sensitive to neutralization, but no consistent pattern was found. However, a few sera were able to neutralize across clades with significant potency, an observation which may have implications for the feasibility of a broadly effective HIV-1 vaccine involving humoral immunity. Serological assays measuring anti-gp120 antibody binding also failed to identify serotypes that correspond precisely to the genetic clades, but some indications of clade-specific binding were observed, notably with sera from clades B and E. A representative protein for each clade (A through E) was selected on the basis of its specificity, defined as high seroreactivity with sera from individuals infected with virus of that clade and lower reactivity with sera from individuals infected with viruses from other clades. The seroreactivity patterns against these five proteins could be used to predict the genotype of the infecting virus with moderate success.

Genetically engineered HIV type 1 peptides as tools for the determination of anti-V3 loop antibody titers in sera from HIV type 1-infected patients

Recombinant peptides from Escherichia coli encoding the principal neutralizing domain (PND) and surrounding sequences of gp120 of human immunodeficiency virus type 1 (HIV-1) with a C-terminal polyhistidine tag were expressed and purified on Ni(2+)-nitrilotriacetate agarose. High yields of more than 99% pure protein were obtained. Their serological reactivity with anti-HIV-positive and -negative human sera was compared to chemically synthesized V3 loop peptides. Overall the genetic PND peptides of the HIV-1MN isolate showed higher and broader reactivity patterns (84%) with HIV-positive sera from German patients than the chemically synthesized peptides (74%). By their higher reactivity and easy way of production and purification, recombinant peptides seem to be highly preferable for the determination of antibody titers to the PND of HIV-infected patients.

Insertion of primary syncytium-inducing (SI) and non-SI envelope V3 loops in human immunodeficiency virus type 1 (HIV-1) LAI reduces neutralization sensitivity to autologous, but not heterologous, HIV-1 antibodies

The aim of the study was to investigate the influence of V3 loops from naturally occurring viruses on the neutralization sensitivity of a molecularly cloned virus. A selection of well-defined syncytium-inducing (SI) and non-SI V3 loops of a single human immunodeficiency virus type 1-infected individual (H594) and the V3 regions of two SI laboratory strains were inserted in an infectious molecular clone of human immunodeficiency type 1 LAI. Neutralization was performed with a heterologous serum pool and autologous patient serum, using the virus reduction neutralization assay and peripheral blood lymphocytes as target cells. High sensitivity of the chimeric viruses containing the laboratory strain V3 regions to neutralization by H594 sequential sera as well as the heterologous serum pool was found. A statistically significant correlation between the sensitivities of these viruses was seen. In contrast, insertion of the primary isolate NSI and SI envelope V3 loops significantly reduced the neutralization by autologous serum but not by the heterologous serum pool. No correlation was found between the neutralization of the viruses with laboratory strain-derived V3 regions and the viruses with primary isolate V3 domains. We conclude that heterologous antibodies are able to neutralize infectious molecular clones with V3 loops of both SI and NSI viruses, regardless of whether they originated from laboratory strains or primary isolates. However, serum of patient H594 discriminated between the two types of viruses and showed reduced neutralization of the viruses with the autologous NSI and SI primary isolate V3 loops. These results indicated that the neutralization sensitivity of the viruses depended on the capacity of the V3 region to influence the conformation of the virus envelope. These V3-dependent conformational changes partially explain the neutralization sensitivity of laboratory strains and the relative neutralization resistance of primary isolates.

Sera from HIV-1 infected individuals in all stages of disease preferentially recognize the V3 loop of the prototypic macrophage-tropic glycoprotein gp120 ADA

The outer membrane glycoprotein gp120 and the transmembrane glycoprotein gp41 are predominant targets of the humoral immune response to infection by human immunodeficiency virus type 1. The third hypervariable region (V3 loop) is the principal neutralizing domain and is the primary target of neutralizing antibodies directed against the envelope proteins of HIV-1. The V3 loop is also the major determinant for HIV-1 cell-specific tropism. To further characterize the humoral immune response directed against the gp120 envelope proteins, we expressed two prototypic gp120 envelope proteins (LAI/HXB2 and ADA) and chimeric gp120 envelope proteins in stable transfected Drosophila melanogaster Schneider 2 cells. Sera from four infected adults over the course of infection [McNearney et al. (1992) Proc. natn. Acad. Sci. U.S.A. 89, p. 10,242] were assayed for reactivity with the respective envelope proteins. Sera obtained at early stages preferentially recognized the gp120 envelope protein ADA, whereas in later stages of infection the sera showed diminished reactivity with both gp120 LAI/HXB2 and gp120 ADA. Chimeric envelope proteins revealed that the humoral response was directed primarily against the V3 loop of gp120 ADA. Furthermore, 22 sera from HIV-1 infected individuals in different stages of the disease were tested. Reactivity of sera with the gp120 envelope protein ADA was seven-fold higher than with the gp120 envelope protein LAI/HXB2. Our results suggest that the humoral immune response is preferentially elicited against the V3 loop of the prototypic macrophage-tropic gp120 envelope protein ADA.

Anti-V3 antibody reactivity correlates with clinical stage of HIV-1 infection and with serum neutralizing activity

During HIV-1 infection, the current tenet is that only anti-gag antibodies decline, while those directed at env remain stable. Among the latter antibodies, those directed to the V3 domain of gp120 are assumed to play a role in the immune surveillance against HIV-1. We investigated the correlation between anti-V3 antibody levels and the clinical stage of infection and the ability to neutralize syncytium formation. Using a V3-specific antigen-limited ELISA, we analysed the antibody levels of a panel of 93 HIV-1+ sera to V3 peptides derived from different HIV-1 strains and from the North American/European consensus sequence V3(Cs). Sera preferentially recognized V3 peptides from the representative V3(MN) strain and V3(Cs). Antibody reactivity to V3(MN) or V3(Cs) actually declined in relation with progression to AIDS, while antibodies against whole recombinant gp160 or gp41 immunodominant epitope remained stable. There was a strong correlation (P < 0.0001) between anti-V3 (Cs)/V3(MN) antibody levels and serum titres that neutralized HIV-1MN-mediated syncytia. Serology based on V3-specific antigen-limited ELISA indicates that anti-V3 antibody reactivity may decline during the course of HIV infection.

A human monoclonal antibody to a complex epitope in the V3 region of gp120 of human immunodeficiency virus type 1 has broad reactivity within and outside clade B

We have used virus neutralization and antibody-binding techniques to define the epitope for a human monoclonal antibody, designated 19b, within the V3 region of the gp120 surface glycoprotein of human immunodeficiency virus type 1. Unusually, the 19b epitope encompasses residues on both flanks of the V3 loop. However, 19b binding to gp120 is independent of sequences at the crown of the V3 loop, provided that they are compatible with the formation of a type II beta turn that is presumably necessary to juxtapose the antigenic residues on the V3 flanks. By comparing the V3 sequences of virus gp120s able and unable to bind 19b, we were able to define the canonical 19b epitope as -I----G--FY-T, where residues at the positions indicated by the gaps do not contribute directly to the 19b-binding site. A few conservative substitutions at the more critical residues are also compatible with 19b binding. Inspection of V3 sequences in the human immunodeficiency virus database indicated that the canonical 19b epitope is well conserved among isolates from the North American-European clade B and also among clade E isolates from Thailand and clade F isolates from Brazil. A minority of gp120s from clades A and C also possess the 19b epitope. Consistent with the theoretical predictions of its cross-clade reactivity, 19b was found to bind to gp120s from clades A, B, C, E, and F in immunoassays. However, 19b was not able to reduce the infectivity of primary viruses from clades A, E, and F that were predicted to possess the 19b epitope and only modestly reduced the infectivity of a clade C virus at low input virus concentrations. Cross-clade neutralization via V3-directed antibodies may, therefore, be difficult, even if the antibodies show broad reactivities in binding assays and the viruses theoretically possess the relevant binding site.

Exploration of antigenic variation in gp120 from clades A through F of human immunodeficiency virus type 1 by using monoclonal antibodies

The reactivities of a panel of 14 monoclonal antibodies (MAbs) with monomeric gp120 derived from 67 isolates of human immunodeficiency virus type 1 of clades A through F were assessed by using an antigen-capture enzyme-linked immunosorbent assay. The MAbs used were all raised against gp120 or gp120 peptides from clade B viruses and were directed at a range of epitopes relevant to human immunodeficiency virus type 1 neutralization: the V2 and V3 loops, discontinuous epitopes overlapping the CD4-binding site, and two other discontinuous epitopes. Four of the five V3 MAbs showed modest cross-reactivity within clade B but very limited reactivity with gp120s from other clades. These reactivity patterns are consistent with the known primary sequence requirements for the binding of these MAbs. One V3 human MAb (19b), however, was much more broadly reactive than the others, binding to 19 of 29 clade B and 10 of 12 clade E gp120s. The 19b epitope is confined to the flanks of the V3 loop, and these sequences are relatively conserved in clade B and E viruses. In contrast to the limited reactivity of V3 MAbs, CD4-binding site MAbs were much more broadly reactive across clades, two of these MAbs (205-46-9 and 21h) being virtually pan-reactive across clades A through F. Another human MAb (A-32) to a discontinuous epitope was also pan-reactive. The CD4-binding site is strongly conserved between clades; but when considering the epitopes near the CD4-binding site, clade D gp120 appears to be the most closely related to clade B and clade E appears to be the least related. A tentative rank order for these epitopes is B/D-A/C-E/F. V2 MAbs reacted sporadically within and between clades, and no clear pattern was observable. While results from binding assays do not predict neutralization serotypes, they suggest that there may be antigenic subtypes related, but not identical, to the genetic subtypes.

Correlation of perinatal transmission of human immunodeficiency virus type 1 with maternal viremia and lymphocyte phenotypes

OBJECTIVE

To determine whether maternal transmission of human immunodeficiency virus (HIV) is correlated with increased quantities of HIV, decreased frequencies of CD4+ T cells, or increased levels of CD8+ T cells in the transmitting mother.

METHODS

Peripheral blood obtained from HIV-infected women at different times during pregnancy was used to measure quantitative cell-associated HIV-1 and CD3+CD4+ and CD3+CD8+ proportions; the plasma was used to perform measurements of quantitative viremia by culture and subsequently to measure quantitative HIV-1 ribonucleic acid levels. These measurements were analyzed with respect to their association with HIV transmission to the baby, which occurred in one fourth of the cases. The children were also studied to determine whether HIV-1 was detected near birth or not until 1 to 8 weeks of life.

RESULTS

Increased clonal frequencies of HIV-1-infected peripheral blood mononuclear cells were found in mothers of infected children; fivefold fewer cells were required for a positive culture result (median cell numbers of 10(4.5) vs 10(5.2); p = 0.008). Higher frequencies of infected cells were seen in mothers of babies with evidence of infection at birth than in mothers of infected babies without evidence of infection at birth (p < 0.05). Plasma viremia was measured in 10% of cultures without regard to whether the mothers transmitted virus to their babies. Increased levels of ribonucleic acid as detected by the branched-chain DNA method were measurable more often (45% vs 17%) in the mothers of infected children than in mothers of uninfected children. Proportions of CD4+ and CD8+ T cells were indistinguishable in these two groups of women.

CONCLUSIONS

Increased viremia was present in mothers who transmitted HIV to their offspring. This variable could be used to select women at highest risk of transmitting HIV to their offspring for treatment to decrease the HIV burden five-fold.

Development of the anti-gp120 antibody response during seroconversion to human immunodeficiency virus type 1

We have studied the development of the antibody response to the surface glycoprotein gp120 of human immunodeficiency virus type 1 in three individuals who presented with primary human immunodeficiency virus type 1 infection syndrome. Serum anti-gp120 antibodies were first detected 4 to 23 days after presentation, after p24 antigen and infectious-virus titers in the peripheral blood had declined manyfold from their highest values. Whether anti-gp120 antibodies present at undetectable levels are involved in clearance of viremia remains unresolved. Among the earliest detectable anti-gp120 antibodies were those to conformationally sensitive epitopes; these antibodies were able to block the binding of gp120 monomers to soluble CD4 or to a human monoclonal antibody to a discontinuous epitope overlapping the CD4-binding site. Some of these antibodies were type specific to a degree, in that they were more effective at blocking ligand binding to autologous gp120 than to heterologous gp120. However, the appearance of these antibodies did not correlate with that of antibodies able to neutralize the autologous virus in vitro by a peripheral blood mononuclear cell-based assay. Antibodies to the V3 loop were detected at about the same time as, or slightly later than, those to the CD4-binding site. There was a weak correlation between the presence of antibodies to the V3 loop and autologous virus-neutralizing activity in two of three individuals studied. However, serum from the third individual contained V3 antibodies but lacked the ability to neutralize the autologous virus in vitro, even immediately after seroconversion. Thus, no simple, universal correlate of autologous virus-neutralizing activity in a peripheral blood mononuclear cell-based assay is apparent from in vitro assays that rely on detecting antibody interactions with monomeric gp120 or fragments thereof.

Neutralization of HIV-1

A Workshop on Neutralization of HIV-1: Technology and reagents for analysis of prophylactic vaccines clinical trials, sponsored by the Food and Drug Administration (FDA) and the Division of AIDS, National Institute of Allergy and Infectious Diseases (NIAID), National Institutes of Health (NIH), was held on April 19-20, 1993, in Bethesda, Maryland. This workshop brought together researchers who are involved in the development, testing, and evaluation of HIV-1 prophylactic vaccines. The major objectives were (1) to discuss critically the different neutralization and binding assays that are currently used in the evaluation of immune sera; (2) to identify assays that will measure the "most relevant" antibodies, which are likely to predict neutralization of primary isolates; and (3) to identify well-characterized reference reagents, which could be used to standardize neutralization assays used in laboratories around the world.

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.

Presentation of native epitopes in the V1/V2 and V3 regions of human immunodeficiency virus type 1 gp120 by fusion glycoproteins containing isolated gp120 domains

The immune response to viral glycoproteins is often directed against conformation- and/or glycosylation-dependent structures; synthetic peptides and bacterially expressed proteins are inadequate probes for the mapping of such epitopes. This report describes a retroviral vector system that presents such native epitopes on chimeric glycoproteins in which protein fragments of interest are fused to the C terminus of the N-terminal domain of the murine leukemia virus surface protein, gp70. The system was used to express two disulfide-bonded domains from gp120, the surface protein of human immunodeficiency virus type 1 (HIV-1), that include potent neutralization epitopes. The resulting fusion glycoproteins were synthesized at high levels and were efficiently transported and secreted. A fusion protein containing the HXB2 V1/V2 domain was recognized by an HIVIIIB-infected patient serum as well as by 17 of 36 HIV-1 seropositive hemophiliac, homosexual male and intravenous drug user patient sera. Many of these HIV+ human sera reacted with V1/V2 domains from several HIV-1 clones expressed in fusion glycoproteins, indicating the presence of cross-reactive antibodies against epitopes in the V1/V2 domain. Recognition of gp(1-263):V1/V2HXB2 by the HIVIIIB-infected human patient serum was largely blocked by synthetic peptides matching V1 but not V2 sequences, while recognition of this construct by a broadly cross-reactive hemophiliac patient serum was not blocked by individual V1 or V2 peptides or by mixtures of these peptides. A construct containing the V3 domain of the IIIB strain of HIV-1, gp(1-263):V3HXB2, was recognized by sera from a human and a chimpanzee that had been infected by HIVIIIB but not by sera from hemophiliac patients who had been infected with HIV-1 of MN-like V3 serotype. The reactive sera had significantly higher titers when assayed against gp(1-263):V3HXB2 than when assayed against matching V3 peptides. Immunoprecipitation of this fusion glycoprotein by the human serum was only partially blocked by V3 peptide, indicating that this infected individual produced antibodies against epitopes in V3 that were expressed on the fusion glycoprotein but not by synthetic peptides. These data demonstrated that the chimeric glycoproteins described here effectively present native epitopes present in the V1/V2 and V3 domains of gp120 and provide efficient methods for detection of antibodies directed against native epitopes in these regions and for characterization of such epitopes.

Probing the structure of the V2 domain of human immunodeficiency virus type 1 surface glycoprotein gp120 with a panel of eight monoclonal antibodies: human immune response to the V1 and V2 domains

We have analyzed a panel of eight murine monoclonal antibodies (MAbs) that depend on the V2 domain for binding to human immunodeficiency virus type 1 (HIV-1) gp120. Each MAb is sensitive to amino acid changes within V2, and some are affected by substitutions elsewhere. With one exception, the MAbs were not reactive with peptides from the V2 region, or only poorly so. Hence their ability to bind recombinant strain IIIB gp120 depended on the preservation of native structure. Three MAbs cross-reacted with strain RF gp120, but only one cross-reacted with MN gp120, and none bound SF-2 gp120. Four MAbs neutralized HIV-1 IIIB with various potencies, and the one able to bind MN gp120 neutralized that virus. Peptide serology indicated that antibodies cross-reactive with the HxB2 V1 and V2 regions are rarely present in HIV-1-positive sera, but the relatively conserved segment between the V1 and V2 loops was recognized by antibodies in a significant fraction of sera. Antibodies able to block the binding of V2 MAbs to IIIB or MN gp120 rarely exist in sera from HIV-1-infected humans; more common in these sera are antibodies that enhance the binding of V2 MAbs to gp120. This enhancement effect of HIV-1-positive sera can be mimicked by several human MAbs to different discontinuous gp120 epitopes. Soluble CD4 enhanced binding of one V2 MAb to oligomeric gp120 but not to monomeric gp120, perhaps by inducing conformational changes in the oligomer.

Immunochemical analysis of the gp120 surface glycoprotein of human immunodeficiency virus type 1: probing the structure of the C4 and V4 domains and the interaction of the C4 domain with the V3 loop

We have probed the structure of the C4 and V3 domains of human immunodeficiency virus type 1 gp120 by immunochemical techniques. Monoclonal antibodies (MAbs) recognizing an exposed gp120 sequence, (E/K)VGKAMYAPP, in C4 were differentially sensitive to denaturation of gp120, implying a conformational component to some of the epitopes. The MAbs recognizing conformation-sensitive C4 structures failed to bind to a gp120 mutant with an alteration in the sequence of the V3 loop, and their binding to gp120 was inhibited by both V3 and C4 MAbs. This implies an interaction between the V3 and C4 regions of gp120, which is supported by the observation that the binding of some MAbs to the V3 loop was often enhanced by amino acid changes in an around the C4 region.