Analysis of neutralizing and enhancing antibodies to human immunodeficiency virus type 1 primary isolates in plasma of individuals infected with env Genetic subtype B and E viruses in Thailand

Human Transbodies to Reverse Transcriptase Connection Subdomain of HIV-1 Gag-Pol Polyprotein Reduce Infectiousness of the Virus Progeny

HIV-1 progeny are released from infected cells as immature particles that are unable to infect new cells. Gag-Pol polyprotein dimerization via the reverse transcriptase connection domain (RTCDs) is pivotal for proper activation of the virus protease (PR protein) in an early event of the progeny virus maturation process. Thus, the RTCD is a potential therapeutic target for a broadly effective anti-HIV agent through impediment of virus maturation. In this study, human single-chain antibodies (HuscFvs) that bound to HIV-1 RTCD were generated using phage display technology. Computerized simulation guided the selection of the transformed Escherichia coli-derived HuscFvs that bound to the RTCD dimer interface. The selected HuscFvs were linked molecularly to human-derived-cell-penetrating peptide (CPP) to make them cell-penetrable (i.e., become transbodies). The CPP-HuscFvs/transbodies produced by a selected transformed E. coli clone were tested for anti-HIV-1 activity. CPP-HuscFvs of transformed E. coli clone 11 (CPP-HuscFv11) that presumptively bound at the RTCD dimer interface effectively reduced reverse transcriptase activity in the newly released virus progeny. Infectiousness of the progeny viruses obtained from CPP-HuscFv11-treated cells were reduced by a similar magnitude to those obtained from protease/reverse transcriptase inhibitor-treated cells, indicating anti-HIV-1 activity of the transbodies. The CPP-HuscFv11/transbodies to HIV-1 RTCD could be an alternative, anti-retroviral agent for long-term HIV-1 treatment.

Mechanism for complement-mediated, antibody-dependent enhancement of human immunodeficiency virus type 1 infection in MT2 cells is enhanced entry through CD4, CD21, and CXCR4 chemokine receptors

Some antibodies neutralize Human Immunodeficiency Virus (HIV). However, antibody to HIV and complement can enhance HIV replication if cells express both complement receptors and CD4, a phenomenon described as complement-mediated, antibody-dependent enhancement (C'ADE). Although increased binding of opsonized virions has been reported, the mechanism by which C'ADE enhances HIV replication remains unproven. In this study, real-time polymerase chain reaction to detect HIV cDNA indicates that complement and anti-HIV antibodies enhance HIV entry 8- to 30- fold with similar increases in integrated provirus. Thus, complement increases HIV replication through a mechanism of enhanced entry. To further refine the mechanism of C'ADE, chemokine receptor antagonists were employed. JM2987, a CXCR4 chemokine receptor antagonist, blocked HIV infection and C'ADE; thus CD4, complement receptors, and CXCR4 chemokine receptors are required for enhanced entry of HIV into MT2 cells. Finally, anti-HIV immunoglobulin enhanced replication of not only group M clade B HIV but also group M clade D and group O isolates. These data demonstrate that antibodies mediating C'ADE of HIV infection are broadly reactive.

N-linked glycosylation in C2 region of HIV-1 envelope reduces sensitivity to neutralizing antibodies

N-linked glycosylation at specific sites on human immunodeficiency virus (HIV)--1 gp120 envelope glycoprotein is believed to act as a glycan shield to protect the viral neutralizing epitopes. Various glycosylation sites have been shown to affect the sensitivity to antibody-mediated neutralization. These include sites on V1V2, C2, base of V3, V5 and C5. Among these, the sites around the base of V3 loop have been most consistently found to associate with neutralization sensitivity in subtype B viruses. In contrast, we found that N-linked glycosylation sites at the junction of V2--C2 and in the middle of C2 were responsible for the neutralization resistance in CRF01_A/E, whereas sites at the base of V3 loop and in V1 and V5 did not affect the neutralization phenotype.

Molecular makeup of HIV-1 envelope protein

A broad range of structural, functional, and immunological similarities between HIV-1 gp120 and human proteins, especially those participating in immune responses, highlight gp120 as a pleiotropic protein that can in different ways affect many important functions of the human immune system. Here we described some of these properties of HIV-1 gp120 that represent the main obstacle in the development of effective and safe AIDS vaccine.

Antibody-mediated enhancement of human immunodeficiency virus type 1 infectivity is determined by the structure of gp120 and depends on modulation of the gp120-CCR5 interaction

In this study, we characterized the viral determinants of coreceptor usage in relation to susceptibility to antibody-mediated neutralization or enhancement of infectivity by using chimeras of three highly related human immunodeficiency virus type 1 (HIV-1) isolates of different phenotypes. We found that the V3 region was the main determinant of antibody-mediated enhancement and coreceptor specificity but that the overall structure of gp120 was also important for these properties. Constructs susceptible to antibody-mediated enhancement preferentially use CCR5 as a coreceptor, in contrast to constructs that were neutralized or not affected. Using monoclonal antibodies directed against CD4 or CCR5, we were able to show that antibody-mediated enhancement was CD4 dependent. Altogether, our results suggest that the modulation of the interaction of gp120 with CCR5 is the mechanism underlying antibody-mediated enhancement of HIV-1 infectivity.

AIDS epidemic at the beginning of the third millennium: time for a new AIDS vaccine strategy

Current expansion of AIDS pandemic significantly accelerates AIDS vaccine research resulting in development and clinical testing of several AIDS vaccine candidates. At the same time, available experimental and clinical data demonstrate that current AIDS vaccine strategy is unsuccessful resulting in development of inefficient and harmful vaccines. This overview briefly summarizes reported results which point out the requirement for moratorium on the current clinical trials of HIV-1 gp120/160 vaccines and urgent need for development of a new, efficient and safe AIDS vaccine strategy.

HIV type 1 subtype E-infected patients with broadened, dual (B/E) V3 loop serology have increased cross-neutralizing antibodies

The two prevalent subtypes of HIV-1 circulating in Thailand are subtypes E and B. While the most prevalent subtype continues to be E using molecular typing assays, immunologically, a subset of subtype E-infected patients (3.4% in 1997) have binding antibodies to both the E and B V3 loops in a peptide ELISA. To assess the potential function of this dual (B/E) V3 reactivity, plasmas from patients with genetically defined HIV-1 subtype E infection and either E or B/E V3 serotypes were compared for magnitude and breadth of neutralization of seven primary and laboratory-adapted subtype B and E viruses. Dually reactive (B/E) plasmas showed significantly increased cross-neutralizing activity against subtype B viruses (p < 0.001), and increased neutralization of the panel of viruses overall (p < 0.02), as compared to monoreactive E serotype plasmas. While the total envelope binding antibody titers to both subtype B and E envelopes did not differ significantly between the E and B/E plasmas, 67% of B/E plasmas neutralized >50% of the viruses in the panel, and only 14% of E plasmas showed this broadened neutralizing activity. These data suggest that dual (B/E) V3 loop reactivity may be a marker of broader immune recognition of HIV envelope epitopes in subtype E-infected patients. V3 loop antibody, perhaps in conjunction with antibodies to additional epitopes, may play a role in neutralization of virus isolates from Thailand.

Primary isolate neutralization by HIV type 1-infected patient sera in the era of highly active antiretroviral therapy

Sera from highly selected HIV-1-positive patients are known to have the ability to neutralize a diverse array of primary isolates of HIV-1. The human osteosarcoma cell line that expresses CD4 and chemokine receptors (GHOST cells) was adapted to study HIV-1 neutralization in 37 HIV-1-infected individuals who were selected because of slow disease progression or nonprogression. Many of these individuals were receiving combination drug therapy. Molecularly cloned HIV-1 JR-FL and NL4-3 viruses were used as prototypes to define assay conditions. Sera were then tested at a 1:40 dilution against six additional primary isolates, three of which utilized CCR5 and three of which used both CCR5 and CXCR4. The assay was highly reproducible and independent of viral input titer, with a readout at 48 hr equivalent to that at later time points. As previously reported, neutralization sensitivity was entirely independent of coreceptor usage. Only a few sera from slow progressors were able to neutralize a broad array of primary isolates at a 1:40 dilution, and the best clinical predictor of broadly neutralizing antibody for primary isolates was the present use of antiretroviral agents. In further studies it was found that purified antibody accounted for the majority of the measured neutralization. However, experiments with exogenous addition of antiviral agents showed that the use of nucleosides also greatly contributed to the measured neutralization in some patients. Measurement of neutralization of HIV-1 primary isolates by sera from patients receiving antiretroviral therapy must be carried out with some caution.

Characterization of sera from subjects infected with HIV-1 subtypes B and E in Thailand by antibody binding and neutralization

The range and specificity of the humoral immune response to HIV-1 subtypes B and E was investigated in Thai samples. Sera from HIV-1-positive subjects, consisting of subtypes B (n = 24) and E (n = 138), were characterized in relation to the neutralization of primary isolates and T-cell line-adapted (TCLA) strains and binding to monomeric gp120, the CD4/gp120 binding site (BS), and V3 peptides. A subtype-specific pattern of antibody binding was observed with the exception of the CD4/gp 120MN BS. Neutralization of TCLA strains (n = 4) was strongly type-specific (p = .002); however, neutralization of primary isolates (n = 8) was weak and group specific. Thus, the subtype specificity of B and E sera in the neutralization of TCLA strains, but not primary isolates, supports the dominance of the V3 region in TCLA virus neutralization but does not support the distinction of subtypes B and E as discrete neutralization serotypes in Thailand.

Neutralization profiles of primary human immunodeficiency virus type 1 isolates in the context of coreceptor usage

Most strains of human immunodeficiency virus type 1 (HIV-1) which have only been carried in vitro in peripheral blood mononuclear cells (primary isolates) can be neutralized by antibodies, but their sensitivity to neutralization varies considerably. To study the parameters that contribute to the differential neutralization sensitivity of primary HIV-1 isolates, we developed a neutralization assay with a panel of genetically engineered cell lines (GHOST cells) that express CD4, one of eight chemokine receptors which function as HIV-1 coreceptors, and a Tat-dependent green fluorescent protein reporter cassette which permits the evaluation and quantitation of HIV-1 infection by flow cytometry. All 21 primary isolates from several clades could grow in the various GHOST cell lines, and their use of one or more coreceptors could easily be defined by flow cytometric analysis. Ten of these primary isolates, three that were CXCR4 (X4)-tropic, three that were CCR5 (R5)-tropic, and four that were dual- or polytropic were chosen for study of their sensitivity to neutralization by human monoclonal and polyclonal antibodies. Viruses from the X4-tropic category of viruses were first tested since they have generally been considered to be particularly neutralization sensitive. It was found that the X4-tropic virus group contained both neutralization-sensitive and neutralization-resistant viruses. Similar results were obtained with R5-tropic viruses and with dual- or polytropic viruses. Within each category of viruses, neutralization sensitivity and resistance could be observed. Therefore, sensitivity to neutralization appears to be the consequence of factors that influence the antibody-virus interaction and its sequelae rather than coreceptor usage. Neutralization of various viruses by the V3-specific monoclonal antibody, 447-52D, was shown to be dependent not only on the presence of the relevant epitope but also on its presentation. An epitope within the envelope of a particular virus is not sufficient to render a virus sensitive to neutralization by an antibody that recognizes that epitope. Moreover, conformation-dependent factors may overcome the need for absolute fidelity in the match between an antibody and its core epitope, permitting sufficient affinity between the viral envelope protein and the antibody to neutralize the virus. The studies indicate that the neutralization sensitivity of HIV-1 primary isolates is a consequence of the complex interaction between virus, antibody, and target cell.