Delay of HIV-1 rebound after cessation of antiretroviral therapy through passive transfer of human neutralizing antibodies

The membrane-proximal external region of the human immunodeficiency virus type 1 envelope: dominant site of antibody neutralization and target for vaccine design

Enormous efforts have been made to produce a protective vaccine against human immunodeficiency virus type 1; there has been little success. However, the identification of broadly neutralizing antibodies against epitopes on the highly conserved membrane-proximal external region (MPER) of the gp41 envelope protein has delineated this region as an attractive vaccine target. Furthermore, emerging structural information on the MPER has provided vaccine designers with new insights for building relevant immunogens. This review describes the current state of the field regarding (i) the structure and function of the gp41 MPER; (ii) the structure and binding mechanisms of the broadly neutralizing antibodies 2F5, 4E10, and Z13; and (iii) the development of an MPER-targeting vaccine. In addition, emerging approaches to vaccine design are presented.

Generation of glyco-engineered Nicotiana benthamiana for the production of monoclonal antibodies with a homogeneous human-like N-glycan structure

A common argument against using plants as a production system for therapeutic proteins is their inability to perform authentic human N-glycosylation (i.e. the presence of beta1,2-xylosylation and core alpha1,3-fucosylation). In this study, RNA interference (RNAi) technology was used to obtain a targeted down-regulation of the endogenous beta1,2-xylosyltransferase (XylT) and alpha1,3-fucosyltransferase (FucT) genes in Nicotiana benthamiana, a tobacco-related plant species widely used for recombinant protein expression. Three glyco-engineered lines with significantly reduced xylosylated and/or core alpha1,3-fucosylated glycan structures were generated. The human anti HIV monoclonal antibody 2G12 was transiently expressed in these glycosylation mutants as well as in wild-type plants. Four glycoforms of 2G12 differing in the presence/absence of xylose and core alpha1,3-fucose residues in their N-glycans were produced. Notably, 2G12 produced in XylT/FucT-RNAi plants was found to contain an almost homogeneous N-glycan species without detectable xylose and alpha1,3-fucose residues. Plant-derived glycoforms were indistinguishable from Chinese hamster ovary (CHO)-derived 2G12 with respect to electrophoretic properties, and exhibited functional properties (i.e. antigen binding and HIV neutralization activity) at least equivalent to those of the CHO counterpart. The generated RNAi lines were stable, viable and did not show any obvious phenotype, thus providing a robust tool for the production of therapeutically relevant glycoproteins in plants with a humanized N-glycan structure.

Characteristics of the env genes of HIV type 1 quasispecies in long-term nonprogressors with broadly neutralizing antibodies

Primary isolates of different subtypes of HIV-1 can be neutralized in vitro by the broadly neutralizing antibodies (NAbs) found in the sera of a small number of HIV-1-infected patients. This broad response is most frequent in long-term nonprogressors (LTNPs). We investigated whether the presence of NAbs in the sera of some LTNPs was associated with particular properties of the envelope glycoproteins of the variants found in these patients. Toward that aim, 147 env gene fragments (encoding almost the entire gp120) amplified from the proviral DNA of 5 LTNPs who developed broadly NAbs (NAb+) and of 4 LTNPs who did not develop such broadly NAbs (NAb-) were cloned, sequenced, and compared. We found that the development of broadly NAbs was associated with high viral loads, greater diversity in the gp120 of the viruses infecting these patients, and longer V1 sequences and additional N-gly sites in V1. In addition, a higher proportion of defective clones was found among the env genes of NAb- patients (25% to 93%)-particularly those with lower viral loads and low levels of env diversity-than among those of NAb+ patients (7% to 19%).

A glycoconjugate antigen based on the recognition motif of a broadly neutralizing human immunodeficiency virus antibody, 2G12, is immunogenic but elicits antibodies unable to bind to the self glycans of gp120

The glycan shield of human immunodeficiency virus type 1 (HIV-1) gp120 contributes to viral evasion from humoral immune responses. However, the shield is recognized by the HIV-1 broadly neutralizing antibody (Ab), 2G12, at a relatively conserved cluster of oligomannose glycans. The discovery of 2G12 raises the possibility that a carbohydrate immunogen may be developed that could elicit 2G12-like neutralizing Abs and contribute to an AIDS vaccine. We have previously dissected the fine specificity of 2G12 and reported that the synthetic tetramannoside (Man(4)) that corresponds to the D1 arm of Man(9)GlcNAc(2) inhibits 2G12 binding to gp120 as efficiently as Man(9)GlcNAc(2) itself, indicating the potential use of Man(4) as a building block for creating immunogens. Here, we describe the development of neoglycoconjugates displaying variable copy numbers of Man(4) on bovine serum albumin (BSA) molecules by conjugation to Lys residues. The increased valency enhances the apparent affinity of 2G12 for Man(4) up to a limit which is achieved at approximately 10 copies per BSA molecule, beyond which no further enhancement is observed. Immunization of rabbits with BSA-(Man(4))(14) elicits significant serum Ab titers to Man(4). However, these Abs are unable to bind gp120. Further analysis reveals that the elicited Abs bind a variety of unbranched and, to a lesser extent, branched Man(9) derivatives but not natural N-linked oligomannose containing the chitobiose core. These results suggest that Abs can be readily elicited against the D1 arm; however, potential differences in the presentation of Man(4) on neoglycoconjugates, compared to glycoproteins, poses challenges for eliciting anti-mannose Abs capable of cross-reacting with gp120 and HIV-1.

Antiviral NK cell responses in HIV infection: II. viral strategies for evasion and lessons for immunotherapy and vaccination

As is the case in other viral infections, humans respond to HIV infection by activating their NK cells. However, the virus uses several strategies to neutralize and evade the host's NK cell responses. Consequently, it is not surprising that NK cell functions become compromised in HIV-infected individuals in early stages of the infection. The compromised NK cell functions also adversely affect several aspects of the host's antiviral adaptive immune responses. Researchers have made significant progress in understanding how HIV counters NK cell responses of the host. This knowledge has opened new avenues for immunotherapy and vaccination against this infection. In the first part of this review article, we gave an overview of our current knowledge of NK cell biology and discussed how the genes encoding NK cell receptors and their ligands determine innate genetic resistance/susceptibilty of humans against HIV infections and AIDS. In this second part, we discuss NK cell responses, viral strategies to counter these responses, and finally, their implications for anti-HIV immunotherapy and vaccination.

HIV-neutralizing immunoglobulin A and HIV-specific proliferation are independently associated with reduced HIV acquisition in Kenyan sex workers

OBJECTIVES

HIV-neutralizing immunoglobulin A (IgA) and HIV-specific cellular immunity have been described in highly exposed, persistently seronegative (HEPS) individuals, but well controlled studies have not been performed. We performed a prospective, nested case-control study to examine the association of genital IgA and systemic cellular immune responses with subsequent HIV acquisition in high-risk Kenyan female sex workers (FSWs).

DESIGN AND METHODS

A randomized trial of monthly antibiotic prophylaxis to prevent sexually transmitted disease/HIV infection was performed from 1998 to 2002 in HIV-uninfected Kenyan FSWs. After the completion of trial, FSWs who had acquired HIV (cases) were matched 1: 4 with persistently uninfected controls based on study arm, duration of HIV-seronegative follow-up, and time of cohort enrolment. Blinded investigators assayed the ability at enrolment of genital IgA to neutralize primary HIV isolates as well as systemic HIV-specific cellular IFNgamma-modified enzyme-linked immunospot and proliferative responses.

RESULTS

The study cohort comprised 113 FSWs: 24 cases who acquired HIV and 89 matched controls. Genital HIV-neutralizing IgA was associated with reduced HIV acquisition (P = 0.003), as was HIV-specific proliferation (P = 0.002), and these associations were additive. HIV-specific IFNgamma production did not differ between case and control groups. In multivariable analysis, HIV-neutralizing IgA and HIV-specific proliferation each remained independently associated with lack of HIV acquisition. Genital herpes (HSV2) was associated with increased HIV risk and with reduced detection of HIV-neutralizing IgA.

CONCLUSION

Genital HIV-neutralizing IgA and systemic HIV-specific proliferative responses, assayed by blinded investigators, were prospectively associated with HIV nonacquisition. The induction of these immune responses may be an important goal for HIV vaccines.

[Virological and immunological bases for HIV-1 vaccine design]

A logical approach for prophylactic HIV-1 vaccine development begins by recognition that the regimen needs to contain viruses which are not cleared by primary host immune responses and develop persistent infection. Hence the required strategy is different from the one against self-remitting acute infections which aims at eliciting robust host immune responses in advance by infection mimicry. Host adaptive immune responses do play a central role in primary resolution from acute HIV-1 and simian immunodeficiency virus (SIV) infection, but as observed in the non-remitting disease course, their function is not fully exerted, leading to failure in viral containment. Either overcoming the limitations of antiviral immunity in natural infection or augmenting the effectors potentially capable of controlling virus replication would be essential for development of an effective HIV-1 vaccine. This approach is hand-in-hand with understanding of the reversibility of various steps leading to establishment of persistent HIV-1 infection. This article reviews the interplay between HIV-1/SIV and the infected host, mainly focusing on macaque models of SIV infection and characterization of the two major wings of adaptive immunity, cytotoxic T lymphocytes (CTLs) and neutralizing antibodies. Discussed in parallel are the up-to-date topics of HIV-1 vaccine development including our recent progress.

Assessment of antibody responses against gp41 in HIV-1-infected patients using soluble gp41 fusion proteins and peptides derived from M group consensus envelope

Human immunodeficiency virus type 1 (HIV-1) transmembrane glycoprotein gp41 is targeted by broadly-reactive neutralizing antibodies 2F5 and 4E10, making it an attractive target for vaccine development. To better assess immunogenic properties of gp41, we generated five soluble glutathione S-transferase fusion proteins encompassing C-terminal 30, 64, 100, 142, or 172 (full-length) amino acids of gp41 ectodomain from M group consensus envelope sequence. Antibody responses in HIV-1-infected patients were evaluated using these proteins and overlapping peptides. We found (i) antibody responses against different regions of gp41 varied tremendously among individual patients, (ii) patients with stronger antibody responses against membrane-proximal external region exhibit broader and more potent neutralizing activity, and (iii) several patients mounted antibodies against epitopes that are near, or overlap with, those targeted by 2F5 or 4E10. These soluble gp41 fusion proteins could be an important source of antigens for future vaccine development efforts.

4E10-resistant variants in a human immunodeficiency virus type 1 subtype C-infected individual with an anti-membrane-proximal external region-neutralizing antibody response

The broadly neutralizing monoclonal antibody (MAb) 4E10 recognizes a linear epitope in the C terminus of the membrane-proximal external region (MPER) of gp41. This epitope is particularly attractive for vaccine design because it is highly conserved among human immunodeficiency virus type 1 (HIV-1) strains and neutralization escape in vivo has not been observed. Multiple env genes were cloned from an HIV-1 subtype C virus isolated from a 7-year-old perinatally infected child who had anti-MPER neutralizing antibodies. One clone (TM20.13) was resistant to 4E10 neutralization as a result of an F673L substitution in the MPER. Frequency analysis showed that F673L was present in 33% of the viral variants and in all cases was linked to the presence of an intact 2F5 epitope. Two other envelope clones were sensitive to 4E10 neutralization, but TM20.5 was 10-fold less sensitive than TM20.6. Substitutions at positions 674 and 677 within the MPER rendered TM20.5 more sensitive to 4E10 but had no effect on TM20.6. Using chimeric and mutant constructs of these two variants, we further demonstrated that the lentivirus lytic peptide-2 domain in the cytoplasmic tail affected the accessibility of the 4E10 epitope, as well as virus infectivity. Collectively, these genetic changes in the face of a neutralizing antibody response to the MPER strongly suggested immune escape from antibody responses targeting this region.

A plant-derived human monoclonal antibody induces an anti-carbohydrate immune response in rabbits

A common argument against using plants as a production system for therapeutic proteins is their inability to perform authentic N-glycosylation. A major concern is the presence of beta 1,2-xylose and core alpha 1,3-fucose residues on complex N-glycans as these nonmammalian N-glycan residues may provoke unwanted side effects in humans. In this study we have investigated the potential antigenicity of plant-type N-glycans attached to a human monoclonal antibody (2G12). Using glyco-engineered plant lines as expression hosts, four 2G12 glycoforms differing in the presence/absence of beta 1,2-xylose and core alpha 1,3-fucose were generated. Systemic immunization of rabbits with a xylose and fucose carrying 2G12 glycoform resulted in a humoral immune response to both N-glycan epitopes. Furthermore, IgE immunoblotting with sera derived from allergic patients revealed binding to plant-produced 2G12 carrying core alpha 1,3 fucosylated N-glycan structures. Our results provide evidence for the adverse potential of nonmammalian N-glycan modifications present on monoclonal antibodies produced in plants. This emphasizes the need for the use of glyco-engineered plants lacking any potentially antigenic N-glycan structures for the production of plant-derived recombinant proteins intended for parenteral human application.

Full-length characterization of A1/D intersubtype recombinant genomes from a therapy-induced HIV type 1 controller during acute infection and his noncontrolling partner

To increase the understanding of mechanisms of HIV control we have genetically and immunologically characterized a full-length HIV-1 isolated from an acute infection in a rare case of undetectable viremia. The subject, a 43-year-old Danish white male (DK1), was diagnosed with acute HIV-1 infection after 1 year in Uganda. Following transient antiretroviral therapy DK1 maintained undetectable viral load for more than 10 years. His Ugandan wife (UG1) developed high viral load. HIV-1 sequences from both individuals were compared by bootscanning for recombination break points. Diversity plots and phylogenic trees were constructed and diversity and evolutionary distances were calculated. Intracellular IFN-gamma in CD8(+)CD3(+) T-lymphocyte reactions was investigated by intracellular flow cytometry (IC-FACS). Virus isolates from both patients were A1D intersubtype recombinants showing 98% sequence homology in shared regions. Four of seven crossover points were identical; however, the env gene from UG1 was subtype D, but A1 in DK1. Both viruses encoded proteins of the expected length and replicated equally well in vitro. DK1 and UG1 shared the HLA-A02 tissue type. HLA-A02-restricted CD8(+) T cell IFN-gamma IC-FACS response in DK1 was detected against only one (Pol(476)) of 23 conserved epitopes. Neutralizing antibodies were induced only to the homologous isolate. These results indicate an A1D intersubtype recombination or transmission of a minor variant. Transient early antiretroviral therapy may have induced full HIV-1 control in this individual mediated by a narrow specific cytotoxic T lymphocyte and neutralizing antibody response and/or other factors yet to be characterized.

The growth and potential of human antiviral monoclonal antibody therapeutics

Monoclonal antibodies (mAbs) have long provided powerful research tools for virologists to understand the mechanisms of virus entry into host cells and of antiviral immunity. Even so, commercial development of human (or humanized) mAbs for the prophylaxis, preemptive and acute treatment of viral infections has been slow. This is surprising, as new antibody discovery tools have increased the speed and precision with which potent neutralizing human antiviral mAbs can be identified. As longstanding barriers to antiviral mAb development, such as antigenic variability of circulating viral strains and the ability of viruses to undergo neutralization escape, are being overcome, deeper insight into the mechanisms of mAb action and engineering of effector functions are also improving the efficacy of antiviral mAbs. These successes, in both industrial and academic laboratories, coupled with ongoing changes in the biomedical and regulatory environments, herald an era when the commercial development of human antiviral mAb therapies will likely surge.

A bispecific antibody composed of a nonneutralizing antibody to the gp41 immunodominant region and an anti-CD89 antibody directs broad human immunodeficiency virus destruction by neutrophils

In addition to the direct neutralization of virus, there is a broader potential for antibody-mediated inhibition of human immunodeficiency virus (HIV) by targeting HIV to effector cells. We demonstrate here that a bispecific antibody incorporating a broadly reactive anti-gp41 antibody, F240, and an anti-IgA receptor (CD89) antibody is effective at directing neutrophils to destroy HIV. Not only are neutrophils the predominant type of white blood cells and very efficient at mediating cell cytotoxicity, they are relatively resistant to infection with HIV. Therefore, they represent a significant weapon against infection if they can be directed and armed to destroy HIV and infected cells.

In vivo efficacy of human immunodeficiency virus neutralizing antibodies: estimates for protective titers

The definition of plasma neutralizing antibody titers capable of controlling human immunodeficiency virus (HIV) infection in vivo is considered a critical step in vaccine development. Here we provide estimates for effective neutralization titers by assessing samples from a recent passive immunization trial with the neutralizing monoclonal antibodies (MAbs) 2G12, 2F5, and 4E10 using an analytic strategy that dissects the contributions of these MAbs to the total neutralization activity in patient plasma. Assessment of neutralization activities for six responding patients with partial or complete control of viremia during the MAb treatment and for the eight nonresponding patients revealed a significant difference between these groups: Among responders, MAb-mediated activity exceeded the autologous neutralization response by 1 to 2 log units (median difference, 43.3-fold), while in the nonresponder group, the autologous activity prevailed (median difference, 0.63-fold). In order to reach a 50% proportion of the responders in our study cohort, MAb neutralizing titers higher than 1:200 were required based on this analysis. The disease stage appears to have a significant impact on the quantities needed, since titers above 1:1,000 were needed to reach the same effect in chronic infection. Although our analysis is based on very small sample numbers and thus cannot be conclusive, our data provide a first estimate on how in vitro-measured neutralizing antibody activity can relate to in vivo efficacy in controlling HIV infection and may therefore provide valuable information for vaccine development. Interestingly, lower neutralizing antibody levels showed an effect in acute compared to chronic infection, suggesting that in early disease stages, therapeutic vaccination may show promise. Equally, this raises hopes that a preventive vaccine could become effective at comparatively lower neutralizing antibody titers.

Recombinant antibody 2G12 produced in maize endosperm efficiently neutralizes HIV-1 and contains predominantly single-GlcNAc N-glycans

Antibody 2G12 is one of a small number of human immunoglobulin G (IgG) monoclonal antibodies exhibiting potent and broad human immunodeficiency virus-1 (HIV-1)-neutralizing activity in vitro, and the ability to prevent HIV-1 infection in animal models. It could be used to treat or prevent HIV-1 infection in humans, although to be effective it would need to be produced on a very large scale. We have therefore expressed this antibody in maize, which could facilitate inexpensive, large-scale production. The antibody was expressed in the endosperm, together with the fluorescent marker protein Discosoma red fluorescent protein (DsRed), which helps to identify antibody-expressing lines and trace transgenic offspring when bred into elite maize germplasm. To achieve accumulation in storage organelles derived from the endomembrane system, a KDEL signal was added to both antibody chains. Immunofluorescence and electron microscopy confirmed the accumulation of the antibody in zein bodies that bud from the endoplasmic reticulum. In agreement with this localization, N-glycans attached to the heavy chain were mostly devoid of Golgi-specific modifications, such as fucose and xylose. Surprisingly, most of the glycans were trimmed extensively, indicating that a significant endoglycanase activity was present in maize endosperm. The specific antigen-binding function of the purified antibody was verified by surface plasmon resonance analysis, and in vitro cell assays demonstrated that the HIV-neutralizing properties of the maize-produced antibody were equivalent to or better than those of its Chinese hamster ovary cell-derived counterpart.

Potent human immunodeficiency virus-neutralizing and complement lysis activities of antibodies are not obligatorily linked

To evaluate the contribution of complement-mediated lysis to the in vivo activities of neutralizing antibodies, we analyzed the influence of complement activation on treatment success in a recent passive immunization trial with the neutralizing monoclonal antibodies 2G12, 2F5, and 4E10. Administration of monoclonal antibodies led to an immediate, high activation of the complement system even in the absence of viremia in the 14 participating human immunodeficiency virus-infected individuals. Lysis activity measured in patient plasma increased during passive immunization; however, the increases were modest and only partially attributable to the administration of antibodies. We found that unlike neutralization activity, lysis activity was not associated with treatment success in this trial. Compared to complement lysis mounted by the polyclonal antibody response in vivo, monoclonal antibodies were weak inducers of this activity, suggesting that polyclonal responses are more effective in reaching the required threshold of complement activation. Importantly, strong neutralization activity of the monoclonal antibodies did not predict complement lysis activity against patient and reference viruses, suggesting that these activities are not linked. In summary, our data support the notion that the in vivo activities of 2G12, 2F5, and 4E10 are likely due to direct neutralization or Fc receptor-mediated mechanisms such as phagocytosis and antibody-dependent cellular cytotoxicity.

A vaccine strategy against AIDS: an HIV gp41 peptide immunization prevents NKp44L expression and CD4+ T cell depletion in SHIV-infected macaques

We previously showed that a gp41 peptide (3S) induces expression of a natural killer (NK) ligand (NKp44L) on CD4+ T cells during HIV-1 infection and that those cells are highly sensitive to NK lysis. In HIV-infected patients, anti-3S antibodies are associated with the maintenance of CD4+ T cell counts close to their baseline values, and CD4+ T cells decrease with the antibody titer. This study sought to determine whether anti-3S immunization could prevent NKp44L expression on these CD4+ T cells in vivo and inhibits the subsequent decline in CD4+ T cell counts by immunizing macaques with 3S and then infecting them with simian HIV(162P3). The results show that anti-3S antibodies inhibited NKp44L expression and NK activity and cytotoxicity. They also decreased the apoptosis rate of CD4+ T cells in peripheral blood and lymph nodes. These data raise questions about the pathogenesis of HIV and present opportunities for both preventive and therapeutic HIV vaccine strategies.

Stimulation of the primary anti-HIV antibody response by IFN-alpha in patients with acute HIV-1 infection

Type I IFNs are needed for the production of antiviral antibodies in mice; whether they also stimulate primary antibody responses in vivo during human viral infections is unknown. This was assessed in patients acutely infected with HIV-1 and treated with IFN-alpha2b. Patients with acute HIV-1 infection were randomized to receive antiretroviral therapy alone (Group A, n=60) or combined for 14 weeks with pegylated-IFN-alpha2b (Group B, n=30). Emergence of anti-HIV antibodies was monitored during 32 weeks by Western blot (WB) analyses of serum samples. IFN-alpha2b treatment stimulated the production of anti-HIV antibodies. On Week 32, 19 weeks after the last IFN-alpha2b administration, there were 8.5 (6.5-10.0) HIV WB bands (median, interquartile range) in Group B and 7.0 (5.0-10.0) bands in Group A (P=0.054), and band intensities were stronger in Group B (P<0.05 for p18, p24, p34, p40, and p55 HIV antigens). IFN-alpha2b treatment also increased circulating concentrations of the B cell-activating factor of the TNF family (P<0.001) and ex vivo production of IL-12 (P<0.05), reflecting its effect on innate immune cells. Withdrawal of antiretroviral treatment on Week 36 resulted in a lower rebound of HIV replication in Group B than in Group A (P<0.05). Therefore, type I IFNs stimulate the emerging anti-HIV immune response in patients with acute HIV-1 infection, resulting in an improved control of HIV replication. Type I IFNs are thus critical in the development of efficient antiviral immune responses in humans, including the production of antiviral antibodies.

B cell immunopathology during HIV-1 infection: lessons to learn for HIV-1 vaccine design

Induction of broad HIV-1 neutralizing antibodies should be a major goal of an effective HIV-1 vaccine. However, B cells are severely damaged during HIV-1 infection with loss of memory B cells and decline of serological memory. The molecular events leading to B cell damage must be further characterized with the aim of selecting vaccine components allowing preservation of B cell functions. This review focuses on B cell damage and antibody responses in HIV-1-infected patients during vaccination studies with viral and bacterial antigens. In addition novel data indicate that B cell activation may be at the basis of impaired immune responses.

Monoclonal antibodies for prophylaxis and therapy of infectious diseases

Monoclonal antibodies (mAb) are attractive biologic drugs due to their exquisite specificity and well understood mechanisms of action, which results in a higher predictability and lower attrition rate compared with other drugs. Therefore, it may seem surprising that only a single mAb is presently marketed for an infectious disease indication. However, the antibiotic resistance crisis, emerging viral diseases and bioterroristic threats have recently spurred the development of anti-infective mAbs, of which more than a dozen are being tested in clinical trials. Conceptually, and validated in many preclinical models, mAbs will be most effective when used prophylactically against acute viral infections and bacterial toxins. The acute bacterial and chronic viral infections, which are medically and economically far more important, are much more difficult to control by antibodies, as the recent clinical failure of some polyclonal antibody products has shown. In these situations, the synergistic action of two or more mAbs together with a small molecule drug will most likely be required for therapeutic efficacy. This review aims to highlight the scientific and economic opportunities and obstacles that are encountered in the quest to add mAbs to the armament of anti-infective drugs.

Antibodies for HIV treatment and prevention: window of opportunity?

Monoclonal antibodies are routinely used as therapeutics in a number of disease settings and have thus also been explored as potential treatment for human immunodeficiency virus (HIV)-1 infection. Antibodies targeting viral antigens, and those directed to the cellular receptors, have been considered for use in prevention and therapy. For virus-targeted antibodies, attention has focused primarily on their neutralizing activity, but such antibodies also have the potential to exert antiviral effects via effector functions, such as antibody-dependent cellular cytotoxicity (ADCC), opsonization, or complement activation. Anti-cell antibodies act through occlusion or down-modulation of the viral receptors with notable impact in vivo, as recent trials have shown. This review summarizes the diverse specificities and modes of action of therapeutic antibodies against HIV-1 infection. Successes, challenges, and future opportunities of harnessing antibodies for therapy of HIV-1 infection are discussed.

Endogenous cytotoxic T-cell response contributes to the long-term antiretroviral protection induced by a short period of antibody-based immunotherapy of neonatally infected mice

Neutralizing monoclonal antibodies (MAbs) are increasingly being considered for blunting human viral infections. However, whether they can also exert indirect effects on endogenous antiviral immune responses has been essentially overlooked. We have recently shown that a short (several-day) period of immunotherapy with the neutralizing 667 MAb of mouse neonates shortly after infection with the lethal FrCas(E) retrovirus not only has an immediate effect on the viral load but also permits an endogenous antiviral immunity to emerge. Even though passive immunotherapy was administered during the particular period of immunocompetence acquisition, the endogenous response eventually arising was protective and persisted long (>1 year) after the MAb has disappeared. As very high levels of anti-FrCas(E) antibodies, predominantly of the immunoglobulin G2a (IgG2a) isotype and showing strong neutralization activity, were found in the sera of MAb-treated mice, it was necessary to address whether this humoral immunity was sufficient on its own to confer full protection against FrCas(E) or whether a cytotoxic T-lymphocyte (CTL) response was also necessary. Using a variety of in vivo assays in young and adult animals previously infected by FrCas(E) and treated by 667, we show here that transient 667 immunotherapy is associated with the emergence of a CTL response against virus-infected cells. This cytotoxic activity is indispensable for long-term antiviral protective immunity, as high neutralizing antibody titers, even enhanced in in vivo CD8(+) cell depletion experiments, cannot prevent the FrCas(E)-induced death of infected/treated mice. Our work may have important therapeutic consequences, as it indicates that a short period of MAb-based immunotherapy conducted at a stage where the immune system is still developing can be associated with the mounting of a functional Th1-type immune response characterized by both CTL and IgG2a-type humoral contributions, the cooperation of which is known to be essential for the containment of chronic infections by a variety of viruses.

Development of an HIV-1 reference panel of subtype B envelope clones isolated from the plasma of recently infected individuals

We have developed an HIV-1 reference panel of 20 subtype B envelope clones isolated from the plasma of recently infected individuals. It is widely accepted that a prophylactic vaccine against HIV-1 requires the development of novel immunogens that are capable of eliciting broadly protective neutralizing antibody responses. Historically, patient serum has been screened for such antibodies by assaying against laboratory strains, but these viruses typically have increased neutralization sensitivity compared with primary isolates. To create a more standardized and relevant assay system for vaccine evaluation, we have developed a panel of primary envelopes derived from the plasma of individuals with documented acute/early subtype B HIV-1 infection occurring between 2000 and 2004. The HIV-1 envelopes from this panel vary in mode of transmission, coreceptor tropism, fusogenicity, and overall sensitivity to neutralization. These envelope sequences represent a broad spectrum of subtype B genetic diversity with an average pairwise genetic distance of 12% and a range from 10% to 19%. This well-characterized HIV-1 envelope panel should be a valuable resource for optimizing and standardizing vaccine assessment and improving vaccine design.

Subtype-specific conformational differences within the V3 region of subtype B and subtype C human immunodeficiency virus type 1 Env proteins

The V3 region of the human immunodeficiency virus type 1 gp120 Env protein is a key domain in Env due to its role in interacting with the coreceptors CCR5 and CXCR4. We examined potential subtype-specific V3 region differences by comparing patterns of amino acid variability and probing for subtype-specific structures using 11 anti-V3 monoclonal antibodies (V3 MAbs). Differences between the subtypes in patterns of variability were most evident in the stem and turn regions of V3 (positions 9 to 24), with the two subtypes being very similar in the base region. The characteristics of the binding of V3 MAbs to Env proteins of the subtype B virus JR-FL and the subtype C virus BR025 suggested three patterns, as each group of MAbs recognized a specific conformation- or sequence-based epitope. Viruses pseudotyped with Env from JR-FL and BR025 were resistant to neutralization by the V3 MAbs, although the replacement of the Env V3 region of the SF162 virus with the JR-FL V3 created a pseudotyped virus that was hypersensitive to neutralization. A single mutation in V3 (H13R) made this chimeric Env selectively resistant to one group of V3 MAbs, consistent with the mAb binding properties. We hypothesize that there are intrinsic differences in V3 conformation between subtype B and subtype C that are localized to the stem and turn regions and that these differences have two important biological consequences: first, subtype B and subtype C V3 regions can have subtype-specific epitopes that will inherently limit antibody cross-reactivity, and second, V3 conformational differences may potentiate the frequent evolution of R5- into X4-tropic variants of subtype B but limit subtype C virus from using the same mechanism to evolve X4-tropic variants as efficiently.

Difficulties in eliciting broadly neutralizing anti-HIV antibodies are not explained by cardiolipin autoreactivity

OBJECTIVE

In a recent report [Haynes et al. Science 2005; 308:1906-1908], difficulties in eliciting broadly neutralizing antibodies to HIV were linked to the binding of prototypic broadly neutralizing monoclonal antibodies to autoantigens and in particular, to the binding of two antigp41 antibodies, 2F5 and 4E10, to the autoantigen cardiolipin. We used a number of assays to understand whether 2F5 and 4E10 are autoreactive, polyreactive, or have a generalized affinity for lipids that may facilitate recognition of their membrane proximal epitopes.

METHODS

2F5 and 4E10 were evaluated for autoreactivity using diagnostic assays developed to detect serum antibodies associated with antiphospholipid syndrome (APS). As an indication of polyreactivity, we measured the binding of 2F5 and 4E10 to liposomal bilayers of differing composition using surface plasmon resonance (SPR) spectroscopy and to protein microarrays using biochip technology.

RESULTS

2F5 showed completely negative results in the APS and SPR studies, indicating that it is neither autoreactive nor absolutely requires phospholipid binding for epitope recognition. In contrast, 4E10 bound to more than one lipid and showed weak activity in the APS studies. The activity displayed by 4E10 more closely resembles that of antiphospholipid antibodies elicited during many infections than that of autoimmune APS antibodies, at variance with the notion that difficulites in eliciting 4E10-like antibodies can be attributed to tolerance mechanisms. The microarray studies further indicated that broadly neutralizing anti-HIV mAb are not exceptionally polyreactive.

CONCLUSION

These results suggest that autoantigen mimicry cannot be reliably invoked as a general mechanism for HIV immune evasion.

Reassessment of autoreactivity of the broadly neutralizing HIV antibodies 4E10 and 2F5 and retrospective analysis of clinical safety data

BACKGROUND

The broadly neutralizing recombinant human HIV-1 antibodies 4E10, 2F5 and Igh1b12 are reported to have autoreactive potential, which is significant for HIV-1 vaccine development and passive immunotherapy using these antibodies.

OBJECTIVE

To investigate the clinical relevance of these findings in subjects receiving passive immunotherapy with these antibodies.

METHODS

Four types of investigations were performed: (1) Investigation of clotting parameters in an ongoing clinical study with 4E10, 2F5 and 2G12. (2) Mixing experiments of pooled plasma with the same antibodies. (3) Retrospective analysis of serum from patients who received passive immunotherapy with 4E10, 2F5 and 2G12 either alone or in combination. (4) Assessment of clinical safety data obtained after 418 infusions with these antibodies.

RESULTS

Standard clinical assays confirmed that 4E10 showed low-level cross-reactivity with cardiolipin, while previously reported cardiolipin cross-reactivity for 2F5 could not be confirmed. High serum titers of 4E10 induced mild prolongation of the activated partial thromboplastin time, which resolved with the wash out of 4E10. Neither 2F5 nor 2G12 affected coagulation. Repeated high-dose infusions of the monoclonal antibody combination were well tolerated with no incidence for thrombotic complications after 418 infusions in 39 subjects.

CONCLUSIONS

Monoclonal antibody 4E10 but not 2F5 or 2G12 showed autoreactive binding specificities. Infusion of 4E10 resulted in transient low anticardiolipin titers. Although an increased thromboembolic risk cannot definitely be excluded, this risk appears to be low and likely depend on underlying disorders.

Antibody-based inhibitors of HIV infection

The demand for new treatment options against HIV is becoming increasingly desperate as the side effects and the expansion and spread of drug-resistant virus within the infected population limit the clinical benefits provided by available anti-HIV drugs. Preparations of polyclonal antibodies have a long history of proven clinical utility against some viruses; however, they have enjoyed very limited success against HIV. Recent clinical trials and in vitro experiments suggest that monoclonal antibodies against HIV may have promise clinically. These antibodies and antibody-based reagents target either the viral envelope glycoprotein, the receptor (CD4) or coreceptor (CCR5) molecules, or transition-state structures that appear during viral entry. The challenge is whether an antibody-based therapy can be identified (with or without their small molecule brethren) that presents long-term clinical efficacy, low toxicity and minimal risk of clinical failure from viral resistance.

N-linked glycan modifications in gp120 of human immunodeficiency virus type 1 subtype C render partial sensitivity to 2G12 antibody neutralization

The monoclonal antibody (MAb) 2G12 recognizes a cluster of high-mannose oligosaccharides on the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein gp120 and is one of a select group of MAbs with broad neutralizing activity. However, subtype C viruses are generally resistant to 2G12 neutralization. This has been attributed to the absence of a glycosylation site at position 295 in most subtype C gp120s, which instead is typically occupied by a Val residue. Here we show that N-linked glycans in addition to the one at position 295 are important in the formation of the 2G12 epitope in subtype C gp120. Introduction of the glycosylation site at position 295 into three subtype C molecular clones, Du151.2, COT9.6, and COT6.15, did increase 2G12 binding to all three mutagenized gp120s, but at various levels. The COT9-V295N mutant showed the strongest 2G12 binding and was the only mutant to become sensitive to 2G12 neutralization, although very high antibody concentrations were required. Introduction of a glycosylation site at position 448 into mutant COT6-V295N, which occurs naturally in COT9, resulted in a virus that was partially sensitive to 2G12. Interestingly, a glycosylation site at position 442, which is common among subtype C viruses, also contributed to the 2G12 epitope. The addition of this glycan increased virus neutralization sensitivity to 2G12, whereas its deletion conferred resistance. Collectively, our results indicate that the 2G12 binding site cannot readily be reconstituted on the envelopes of subtype C viruses, suggesting structural differences from other HIV subtypes in which the 2G12 epitope is naturally expressed.

Utility of human immunodeficiency virus type 1 envelope as a T-cell immunogen

Human immunodeficiency virus (HIV)-specific CD8 T lymphocytes are important for the control of viremia, but the relative utility of responses to the various HIV proteins is controversial. Immune responses that force escape mutations that exact a significant fitness cost from the mutating virus would help slow progression to AIDS. The HIV envelope (Env) protein is subject to both humoral and cellular immune responses, suggesting that multiple rounds of mutation are needed to facilitate viral escape. The Gag protein, however, has recently been shown to elicit a more effective CD8 T-cell immune response in humans. We studied 30 pigtail macaques for their CD8 T-lymphocyte responses to HIV-1 Env and simian immunodeficiency virus (SIV) Gag following prime/boost vaccination and intrarectal challenge with simian-human immunodeficiency virus SHIV(mn229). Eight CD8 Env-specific T-cell epitopes were identified and mapped in 10 animals. Animals that generated Env-specific CD8 T-cell responses had equivalent viral loads and only a modest advantage in retention of peripheral CD4 T lymphocytes compared to those animals without responses to Env. This contrasts with animals that generated CD8 T-cell responses to SIV Gag in the same trial, demonstrating superior control of viral load and a larger advantage in retention of peripheral CD4 T cells than Gag nonresponders. Mutational escape was common in Env but, in contrast to mutations in Gag, did not result in the rapid emergence of dominant escape motifs, suggesting modest selective pressure from Env-specific T cells. These results suggest that Env may have limited utility as a CD8 T-cell immunogen.

Adjunctive passive immunotherapy in human immunodeficiency virus type 1-infected individuals treated with antiviral therapy during acute and early infection

Three neutralizing monoclonal antibodies (MAbs), 2G12, 2F5, and 4E10, with activity in vitro and in vivo were administered in an open-label, nonrandomized, proof-of-concept study to attempt to prevent viral rebound after interruption of antiretroviral therapy (ART). Ten human immunodeficiency virus type 1-infected individuals identified and treated with ART during acute and early infection were enrolled. The first six patients were administered 1.0 g of each of the three MAbs per infusion. The remaining four patients received 2G12 at 1.0 g/infusion and 2.0 g/infusion of 2F5 and 4E10. The MAbs were well tolerated. Grade I post-partial thromboplastin time prolongations were noted. Viral rebound was observed in 8/10 subjects (28 to 73 days post-ART interruption), and 2/10 subjects remained aviremic over the course of the study. In seven of eight subjects with viral rebound, clear resistance to 2G12 emerged, whereas reductions in the susceptibilities of plasma-derived recombinant viruses to 2F5 and 4E10 were neither sustained nor consistently measured. Viral rebound was associated with a preferential depletion of CD4(+) T cells within the gastrointestinal tract. Though safe, the use of MAbs generally delayed, but did not prevent, virologic rebound. Consideration should be given to further pilot studies with alternative combinations of MAbs and perhaps additional novel treatment modalities.

In vivo and in vitro escape from neutralizing antibodies 2G12, 2F5, and 4E10

Recently, passive immunization of human immunodeficiency virus (HIV)-infected individuals with monoclonal antibodies (MAbs) 2G12, 2F5, and 4E10 provided evidence of the in vivo activity of 2G12 but raised concerns about the function of the two membrane-proximal external region (MPER)-specific MAbs (A. Trkola, H. Kuster, P. Rusert, B. Joos, M. Fischer, C. Leemann, A. Manrique, M. Huber, M. Rehr, A. Oxenius, R. Weber, G. Stiegler, B. Vcelar, H. Katinger, L. Aceto, and H. F. Gunthard, Nat. Med. 11:615-622, 2005). In the light of MPER-targeting vaccines under development, we performed an in-depth analysis of the emergence of mutations conferring resistance to these three MAbs to further elucidate their activity. Clonal analysis of the MPER of plasma virus samples derived during antibody treatment confirmed that no changes in this region had occurred in vivo. Sequence analysis of the 2G12 epitope relevant N-glycosylation sites of viruses derived from 13 patients during the trial supported the phenotypic evaluation, demonstrating that mutations in these sites are associated with resistance. In vitro selection experiments with isolates of four of these individuals corroborated the in vivo finding that virus strains rapidly escape 2G12 pressure. Notably, in vitro resistance mutations differed, in most cases, from those found in vivo. Importantly, in vitro selection with 2F5 and 4E10 demonstrated that resistance to these MAbs can be difficult to achieve and can lead to selection of variants with impaired infectivity. This remarkable vulnerability of the virus to interference within the MPER calls for a further evaluation of the safety and efficacy of MPER-targeting therapeutic and vaccination strategies.

Inhibition of replication of primary HIV-1 isolates in huPBL-NOD/Scid mice by antibodies from HIV-1 infected patients

Although a limited number of HIV-infected patients have broadly neutralizing antibodies, it has not been examined whether these antibodies can protect against infection with primary virus in vivo. Here we screened the plasma of 23 HIV-1-infected patients for broadly neutralizing antibodies. Purified antibodies from subjects with broad and more narrow responses were administered to huPBL-NOD/Scid mice that were subsequently challenged with primary viruses of clade A, B and CRF01_AE. Although we observed a lack of correlation between the data from the in vitro neutralization assay and the results from the passive immunization experiments, we report for the first time that antibodies from HIV-infected persons can inhibit replication of primary virus isolates in an animal model.

Modulation of HIV and SIV neutralization sensitivity by DC-SIGN and mannose-binding lectin

The C-type lectin DC-SIGN binds to oligosaccharides on the human and simian immunodeficiency virus (HIV, SIV) envelope glycoproteins and promotes infection of susceptible cells. Here, we show that DC-SIGN recognizes glycans involved in SIV sensitivity to neutralizing antibodies and that binding to DC-SIGN confers neutralization resistance to an otherwise sensitive SIV variant. Moreover, we provide evidence that mannose-binding lectin (MBL) can interfere with HIV-1 neutralization by the carbohydrate-specific antibody 2G12.

Genetic characteristics of HIV-1 subtype C envelopes inducing cross-neutralizing antibodies

This study aimed to characterize genetic features of HIV-1 subtype C envelope glycoproteins capable of eliciting cross-reactive neutralizing antibodies during natural infections. The gp160 sequences were determined for 36 HIV-1 subtype C isolates (donor viruses) from infected individuals residing in Malawi, Zimbabwe, Zambia and South Africa, whose sera displayed a range of cross-neutralizing activities against a panel of 5 subtype C and 5 subtype B viruses (panel viruses). Hierarchical clustering analysis of neutralization data of the panel viruses predicted phylogenetic relationships between subtype B and C panel viruses, suggesting some subtype-specific neutralization determinants. A similar comparison of subtype C donor viruses showed no significant correlation; however of three donor sequence pairs resolvable by phylogenetic analysis, two were also associated within the neutralization clustering dendrogram, suggesting that closely related viruses may elicit antibodies targeting common neutralization determinants. Significantly, viruses that had shorter V1-V4 loops induced antibodies that showed more neutralization breadth against the subtype C panel viruses (p=0.0135). This study indicates that that some structural features of envelope, such as shorter variable loops, may facilitate the elicitation of cross-reactive neutralizing antibodies in natural infections. Collectively these data provide some insights into design features of an envelope immunogen aimed at inducing neutralizing antibodies.

Inhibition of HIV-1 entry by antibodies: potential viral and cellular targets

Vaccine-induced antibodies that interfere with viral entry are the protective correlate of most existing prophylactic vaccines. However, for highly variable viruses such as HIV-1, the ability to elicit broadly neutralizing antibody responses through vaccination has proven to be extremely difficult. The major targets for HIV-1 neutralizing antibodies are the viral envelope glycoprotein trimers on the surface of the virus that mediate receptor binding and entry. HIV-1 has evolved many mechanisms on the surface of envelope glycoproteins to evade antibody-mediated neutralization, including the masking of conserved regions by glycan, quaternary protein interactions and the presence of immunodominant variable elements. The primary challenge in the development of an HIV-1 vaccine that elicits broadly neutralizing antibodies therefore lies in the design of suitable envelope glycoprotein immunogens that circumvent these barriers. Here, we describe neutralizing determinants on the viral envelope glycoproteins that are defined by their function in receptor binding or by rare neutralizing antibodies isolated from HIV-infected individuals. We also describe the nonvariable cellular receptors involved in the HIV-1 entry process, or other cellular proteins, and ongoing studies to determine if antibodies against these proteins have efficacy as therapeutic reagents or, in some cases, as vaccine targets to interfere with HIV-1 entry.

Mucosal immune responses in the genital tract of HIV-1-exposed uninfected women

The development of HIV-1 vaccines and microbicides remains hindered by our limited understanding of correlates of immune protection to infection. Evidence indicating that resistance to HIV-1 infection is indeed possible comes from HIV-1-exposed yet uninfected individuals, including cohorts of commercial sex workers and discordant couples. Despite their uninfected status some of these individuals have mucosal and systemic HIV-1-specific humoral and cellular immune responses in addition to their innate immune response. The combined contribution of innate and adaptive immunity as well as genetic factors is most likely of great importance for this protection against infection. Here we review data on the antibody responses and secreted immune molecules of the innate immune system in the female genital tract with emphasis on individuals who seem to resist HIV-1-infection despite repeated exposure to the virus.

Humoral immunity to HIV-1: neutralization and beyond

Humoral immunity is considered a key component of effective vaccines against HIV-1. Hence, an enormous effort has been put into investigating the neutralizing antibody response to HIV-1 over the past 20 years which generated key information on epitope specificity, potency, breadth and in vivo activity of the neutralizing antibodies. Less clear is still the role of antibody-mediated effector functions (antibody-dependent cellular cytotoxicity, phagocytosis, complement system) and uncertainty prevails whether Fc-mediated mechanisms are largely beneficial or detrimental for the host. The current knowledge on the manifold functions of the humoral immune response in HIV infection, their underlying mechanisms and potential in vaccine-induced immunity will be discussed in this review.

Immunogenicity of recombinant human immunodeficiency virus type 1-like particles expressing gp41 derivatives in a pre-fusion state

The conserved membrane proximal external region (MPER) of the ectodomain of human immunodeficiency virus type 1 (HIV-1) gp41 is the target of two broadly neutralizing antibodies, 2F5 and 4E10. However, no neutralizing antibodies have been elicited against immunogens bearing these epitopes. Given that structural and biochemical studies suggest that the lipid membrane of the virion is involved in their proper configuration, HIV-1 gp41 derivatives in a pre-fusion state were expressed on the surface of immature virus like particles (VLP) derived from Sf9 cells. Guinea pigs were immunized with three doses of VLPs or Sf9 cells presenting gp41 derivatives with or without E. coli heat-labile enterotoxin (LT) as an adjuvant. While immune sera contained high titer anti-VLP antibodies, the specific anti-gp41 antibody responses were low with no neutralizing antibodies detected. An explanation for this absence may be the low level of gp41 expression relative to the many other proteins derived from host cells which are incorporated onto the VLP surface. In addition, the anti-gp41 immune response was preferentially directed to the C-helical domain, away from the MPER. Future vaccine design needs to contend with the complexity of epitope display as well as immunodominance.

Post-infection immunodeficiency virus control by neutralizing antibodies

Background

Unlike most acute viral infections controlled with the appearance of virus-specific neutralizing antibodies (NAbs), primary HIV infections are not met with such potent and early antibody responses. This brings into question if or how the presence of potent antibodies can contribute to primary HIV control, but protective efficacies of antiviral antibodies in primary HIV infections have remained elusive; and, it has been speculated that even NAb induction could have only a limited suppressive effect on primary HIV replication once infection is established. Here, in an attempt to answer this question, we examined the effect of passive NAb immunization post-infection on primary viral replication in a macaque AIDS model.

Methods and Findings

The inoculums for passive immunization with simian immunodeficiency virus mac239 (SIVmac239)-specific neutralizing activity were prepared by purifying polyclonal immunoglobulin G from pooled plasma of six SIVmac239-infected rhesus macaques with NAb induction in the chronic phase. Passive immunization of rhesus macaques with the NAbs at day 7 after SIVmac239 challenge resulted in significant reduction of set-point plasma viral loads and preservation of central memory CD4 T lymphocyte counts, despite the limited detection period of the administered NAb responses. Peripheral lymph node dendritic cell (DC)-associated viral RNA loads showed a remarkable peak with the NAb administration, and DCs stimulated in vitro with NAb-preincubated SIV activated virus-specific CD4 T lymphocytes in an Fc-dependent manner, implying antibody-mediated virion uptake by DCs and enhanced T cell priming.

Conclusions

Our results present evidence indicating that potent antibody induction post-infection can result in primary immunodeficiency virus control and suggest direct and indirect contribution of its absence to initial control failure in HIV infections. Although difficulty in achieving requisite neutralizing titers for sterile HIV protection by prophylactic vaccination has been suggested, this study points out a possibility of non-sterile HIV control by prophylactic vaccine-induced, sub-sterile titers of NAbs post-infection, providing a rationale of vaccine-based NAb induction for primary HIV control.

The role of antibody polyspecificity and lipid reactivity in binding of broadly neutralizing anti-HIV-1 envelope human monoclonal antibodies 2F5 and 4E10 to glycoprotein 41 membrane proximal envelope epitopes

Two neutralizing human mAbs, 2F5 and 4E10, that react with the HIV-1 envelope gp41 membrane proximal region are also polyspecific autoantibodies that bind to anionic phospholipids. To determine the autoantibody nature of these Abs, we have compared their reactivities with human anti-cardiolipin mAbs derived from a primary antiphospholipid syndrome patient. To define the role of lipid polyreactivity in binding of 2F5 and 4E10 mAbs to HIV-1 envelope membrane proximal epitopes, we determined the kinetics of binding of mAbs 2F5 and 4E10 to their nominal gp41 epitopes vs liposome-gp41 peptide conjugates. Both anti-HIV-1 mAbs 2F5 and 4E10 bound to cardiolipin with K(d) values similar to those of autoimmune anti-cardiolipin Abs, IS4 and IS6. Binding kinetics studies revealed that mAb 2F5 and 4E10 binding to their respective gp41 peptide-lipid conjugates could best be defined by a two-step (encounter-docking) conformational change model. In contrast, binding of 2F5 and 4E10 mAbs to linear peptide epitopes followed a simple Langmuir model. A mouse mAb, 13H11, that cross-blocks mAb 2F5 binding to the gp41 epitope did not cross-react with lipids nor did it neutralize HIV-1 viruses. Taken together, these data demonstrate the similarity of 2F5 and 4E10 mAbs to known anti-cardiolipin Abs and support the model that mAb 2F5 and 4E10 binding to HIV-1 involves both viral lipid membrane and gp41 membrane proximal epitopes.

Susceptibility of recently transmitted subtype B human immunodeficiency virus type 1 variants to broadly neutralizing antibodies

The ability of the broadly neutralizing human immunodeficiency virus type 1 (HIV-1) specific human monoclonal antibodies (MAbs) b12, 2G12, 2F5, and 4E10 to neutralize recently transmitted viruses has not yet been explored in detail. We investigated the neutralization sensitivity of subtype B HIV-1 variants obtained from four primary HIV infection cases and six transmission couples (four homosexual and two parenteral) to these MAbs. Sexually transmitted HIV-1 variants isolated within the first 2 months after seroconversion were generally sensitive to 2F5, moderately resistant to 4E10 and b12, and initially resistant but later more sensitive to 2G12 neutralization. In the four homosexual transmission couples, MAb neutralization sensitivity of HIV in recipients did not correlate with the MAb neutralization sensitivity of HIV from their source partners, whereas the neutralization sensitivity of donor and recipient viruses involved in parenteral transmission was more similar. For a fraction (11%) of the HIV-1 variants analyzed here, neutralization by 2G12 could not be predicted by the presence of N-linked glycosylation sites previously described to be involved in 2G12 binding. Resistance to 2F5 and 4E10 neutralization did also not correlate with mutations in the respective core epitopes. Overall, we observed that the neutralization resistance of recently transmitted subtype B HIV-1 variants was relatively high. Although 8 of 10 patients had viruses that were sensitive to neutralization by at least one of the four broadly neutralizing antibodies studied, 4 of 10 patients harbored at least one virus variant that seemed resistant to all four antibodies. Our results suggest that vaccine antigens that only elicit antibodies equivalent to b12, 2G12, 2F5, and 4E10 may not be sufficient to protect against all contemporary HIV-1 variants and that additional cross-neutralizing specificities need to be sought.

Resistance of HIV-1 to the broadly HIV-1-neutralizing, anti-carbohydrate antibody 2G12

The 2G12 mAb inhibits the infection of HIV-1 laboratory-adapted viruses at 50% inhibitory concentrations (IC(50)) ranging from 0.02 to 0.2 microg/ml when evaluated in different cell-types. However, isolates from various HIV-1 subtypes (such as clade C, D, A/E, F and group O) were not inhibited by 2G12 mAb (IC(50) >20 microg/ml). 2G12 mAb pressure in HIV-1 IIIB- and NL4.3-infected T cell cultures selected for resistant viruses containing only few (1 to 3 N-glycosylation) deletions in gp120. The 2G12-resistant viruses keep their full sensitivity to various mannose-specific lectins and other known HIV entry inhibitors. Moreover, we observed that the NL4.3-2G12-resistant virus, with the N295K mutation in gp120, became significantly more sensitive to several mannose-specific lectins. This is, to our knowledge, the first report showing that a resistant virus generated in vitro against a neutralizing mAb and containing a mutation in gp120, has increased sensitivity to another class of HIV entry inhibitors.

Mimotopes selected with antibodies from HIV-1-neutralizing long-term non-progressor plasma

A promising approach to identify HIV-1 vaccine candidates is to dissect the natural immune response against the virus in persons controlling the infection over decades without any antiviral therapy. Here we focus on a group of such persons, eight long-term non-progressors (LTNP), in which we proved the presence of broadly neutralizing antibodies against HIV-1 in the plasma as very likely cause for their LTNP status. The aim of this study was to identify the epitopes for these neutralizing antibodies, as these should represent immunogens potentially able to elicit neutralizing antibodies upon vaccination. We screened random peptide phage libraries with plasma antibodies from eight LTNP. After several rounds of positive and negative selection, about 700 HIV-specific mimotopes were sequenced. The mimotope sequences were analyzed for homology to HIV-1 Env, in particular for their capacity to represent conformational epitopes on the surface of the gp120 structure using our software 3DEX. Related phage groups were analyzed for crossreactivity with the LTNP plasma by ELISA as well as for their capacity to induce HIV-1-neutralizing antibodies in mice. Based on this study interesting mimotopes can now be selected for further immunization studies.

Macaques infected with a CCR5-tropic simian/human immunodeficiency virus (SHIV) develop broadly reactive anti-HIV neutralizing antibodies

The development of anti-human immunodeficiency virus (anti-HIV) neutralizing antibodies and the evolution of the viral envelope glycoprotein were monitored in rhesus macaques infected with a CCR5-tropic simian/human immunodeficiency virus (SHIV), SHIVSF162P4. Homologous neutralizing antibodies developed within the first month of infection in the majority of animals, and their titers were independent of the extent and duration of viral replication during chronic infection. The appearance of homologous neutralizing antibody responses was preceded by the appearance of amino acid changes in specific variable and conserved regions of gp120. Amino acid changes first appeared in the V1, V2, C2, and V3 regions and subsequently in the C3, V4, and V5 regions. Heterologous neutralizing antibody responses developed over time only in animals with sustained plasma viremia. Within 2 years postinfection the breadth of these responses was as broad as that observed in certain patients infected with HIV type 1 (HIV-1) for over a decade. Despite the development of broad anti-HIV-1 neutralizing antibody responses, viral replication persisted in these animals due to viral escape. Our studies indicate that cross-reactive neutralizing antibodies are elicited in a subset of SHIVSF162P4 infected macaques and that their development requires continuous viral replication for extended periods of time. More importantly, their late appearance does not prevent progression to disease. The availability of an animal model where cross-reactive anti-HIV neutralizing antibodies are developed may facilitate the identification of virologic and immunologic factors conducive to the development of such antibodies.

Rapid induction of virus-neutralizing antibodies and viral clearance in a single-source outbreak of hepatitis C

In contrast to a detailed understanding of antiviral cellular immune responses, the impact of neutralizing antibodies for the resolution of acute hepatitis C is poorly defined. The analysis of neutralizing responses has been hampered by the fact that patient cohorts as well as hepatitis C virus (HCV) strains are usually heterogeneous, and that clinical data from acute-phase and long-term follow-up after infection are not readily available. Using an infectious retroviral HCV pseudoparticle model system, we studied a cohort of women accidentally exposed to the same HCV strain of known sequence. In this single-source outbreak of hepatitis C, viral clearance was associated with a rapid induction of neutralizing antibodies in the early phase of infection. Neutralizing antibodies decreased or disappeared after recovery from HCV infection. In contrast, chronic HCV infection was characterized by absent or low-titer neutralizing antibodies in the early phase of infection and the persistence of infection despite the induction of cross-neutralizing antibodies in the late phase of infection. These data suggest that rapid induction of neutralizing antibodies during the early phase of infection may contribute to control of HCV infection. This finding may have important implications for understanding the pathogenesis of HCV infection and for the development of novel preventive and therapeutic antiviral strategies.