A phase I/IIA clinical study with a chimeric mouse-human monoclonal antibody to the V3 loop of human immunodeficiency virus type 1 gp120

Multivariate analysis of FcR-mediated NK cell functions identifies unique clustering among humans and rhesus macaques

Rhesus macaques (RMs) are a common pre-clinical model used to test HIV vaccine efficacy and passive immunization strategies. Yet, it remains unclear to what extent the Fc-Fc receptor (FcR) interactions impacting antiviral activities of antibodies in RMs recapitulate those in humans. Here, we evaluated the FcR-related functionality of natural killer cells (NKs) from peripheral blood of uninfected humans and RMs to identify intra- and inter-species variation. NKs were screened for FcγRIIIa (human) and FcγRIII (RM) genotypes (FcγRIII(a)), receptor signaling, and antibody-dependent cellular cytotoxicity (ADCC), the latter mediated by a cocktail of monoclonal IgG1 antibodies with human or RM Fc. FcγRIII(a) genetic polymorphisms alone did not explain differences in NK effector functionality in either species cohort. Using the same parameters, hierarchical clustering separated each species into two clusters. Importantly, in principal components analyses, ADCC magnitude, NK contribution to ADCC, FcγRIII(a) cell-surface expression, and frequency of phosphorylated CD3ζ NK cells all contributed similarly to the first principal component within each species, demonstrating the importance of measuring multiple facets of NK cell function. Although ADCC potency was similar between species, we detected significant differences in frequencies of NK cells and pCD3ζ+ cells, level of cell-surface FcγRIII(a) expression, and NK-mediated ADCC (P<0.001), indicating that a combination of Fc-FcR parameters contribute to overall inter-species functional differences. These data strongly support the importance of multi-parameter analyses of Fc-FcR NK-mediated functions when evaluating efficacy of passive and active immunizations in pre- and clinical trials and identifying correlates of protection. The results also suggest that pre-screening animals for multiple FcR-mediated NK function would ensure even distribution of animals among treatment groups in future preclinical trials.

Anti-HIV-1 Antibodies: An Update

Even after more than 30 years since its discovery, there is no cure for HIV-1 infection. Combination antiretroviral therapy (cART) is currently the only HIV-1 infection management option in clinics. Despite its success in suppressing viral replication and converting HIV-1 from a lethal infection to a chronic and manageable disease, cART treatment is life long and long-term use can result in major drawbacks such as high cost, multiple side effects, and an increase in the development of multidrug-resistant escape mutants. Recently, antibody-based anti-HIV-1 treatment has emerged as a potential alternative therapeutic modality for HIV-1 treatment and cure strategies. These antibody-based anti-HIV-1 treatments comprising either receptor-targeting antibodies or broad neutralizing antibodies (bNAbs) are currently being developed and evaluated in clinical trials. These antibodies have demonstrated potent antiviral effects against multiple strains of HIV-1, and shown promise for prevention, maintenance, and prolonged remission of HIV-1 infection. This review gives an update on the current status of these antibody-based treatments for HIV-1, discusses their mechanism of action and the challenges in developing them, providing insight for their development as novel clinical therapies against HIV-1 infection.

Antibody-mediated prevention and treatment of HIV-1 infection

Novel broadly neutralizing antibodies targeting HIV-1 hold promise for their use in the prevention and treatment of HIV-1 infection. Pre-clinical results have encouraged the evaluation of these antibodies in healthy and HIV-1-infected humans. In first clinical trials, highly potent broadly neutralizing antibodies have demonstrated their safety and significant antiviral activity by reducing viremia and delaying the time to viral rebound in individuals interrupting antiretroviral therapy. While emerging antibody-resistant viral variants have indicated limitations of antibody monotherapy, strategies to enhance the efficacy of broadly neutralizing antibodies in humans are under investigation. These include the use of antibody combinations to prevent viral escape, antibody modifications to increase the half-life and the co-administration of latency-reversing agents to target the cellular reservoir of HIV-1. We provide an overview of the results of pre-clinical and clinical studies of broadly HIV-1 neutralizing antibodies, discuss their implications and highlight approaches for the ongoing advancement into humans.

Where are we with injectables against HIV infection and what are the remaining challenges?

INTRODUCTION

Drug adherence has been a recurring issue in the field of HIV treatment, and low treatment adherence is typically associated with emergence of drug resistance, treatment failure and increased risks of transmission. Injectable antiretroviral drugs offer a unique opportunity to counter this issue for the treatment of HIV-positive individuals. In addition, injectables offer a remarkable opportunity to reduce new HIV infections, if applied in the context of both treatment-as-prevention and pre-exposure prophylaxis. Areas covered: Researchers and drug companies are developing long-acting agents that possess long biological half-life and excellent pharmacokinetic profiles that can be administered intramuscularly, intravenously, or subcutaneously. These long-acting injectables are categorized as drugs that target different steps of HIV replication cycle or monoclonal antibodies that target HIV entry. Expert commentary: Injectables against HIV have the potential to revolutionize the fight against HIV by facilitating both treatment and prevention in a wide variety of clinical settings. Several challenges remain including the identification of potent two-drug combinations of drugs that can be formulated as injectables, and thorough drug-drug interaction studies with a broad variety of medications. Finally we believe that the healthcare benefits of injectables will require regulatory changes to allow self-injection before they reach their full potential.

Ibalizumab Targeting CD4 Receptors, An Emerging Molecule in HIV Therapy

The HIV infection is responsible for the most devastating global pandemic of the last century. More than 39 million people have died of HIV/AIDS since 1981. The development of the antiretroviral (ARV) treatment begins with the discovery of zidovudine a nucleoside reverse transcriptase inhibitor. This breakthrough was followed by other ARV drug classes and representatives. Presently, HIV treatment employs 27 ARV representatives belonging to five different classes. Despite the proven benefits of ARV treatment and its long-term control of the HIV infection, there is an increasing concern about the numerous adverse effects and resistance to current ARV drugs. Therefore, the new HIV treatment strategies focus on the development of new ARV agents with a high genetic barrier to resistance and low toxicity. Monoclonal antibodies (MAbs) belong to a new drug class with encouraging results in the treatment of cancer, autoimmune disorders and most recently against HIV infection. The advantages of using MAbs for HIV treatment are related to their antiviral effect, lack of toxicity, good resistance profile, additional synergy with other ARV drug classes and ability to restore CD4 T-cell responses. The current article is a short summary of ibalizumab, an anti-CD4 monoclonal antibody that interferes with HIV viral entry. Current studies on ibalizumab have underlined its antiviral potential, minimal adverse effects, and lack of crossed resistance with other ARV agents thus supporting its further therapeutic use in multidrug resistant HIV-infected patients.

Broadly Neutralizing Antibodies for HIV Eradication

Passive transfer of antibodies has long been considered a potential treatment modality for infectious diseases, including HIV. Early efforts to use antibodies to suppress HIV replication, however, were largely unsuccessful, as the antibodies that were studied neutralized only a relatively narrow spectrum of viral strains and were not very potent. Recent advances have led to the discovery of a large portfolio of human monoclonal antibodies that are broadly neutralizing across many HIV-1 subtypes and are also substantially more potent. These antibodies target multiple different epitopes on the HIV envelope, thus allowing for the development of antibody combinations. In this review, we discuss the application of broadly neutralizing antibodies (bNAbs) for HIV treatment and HIV eradication strategies. We highlight bNAbs that target key epitopes, such as the CD4 binding site and the V2/V3-glycan-dependent sites, and we discuss several bNAbs that are currently in the clinical development pipeline.

Clinical development of monoclonal antibody-based drugs in HIV and HCV diseases

Today there are many licensed antiviral drugs, but the emergence of drug resistant strains sometimes invalidates the effects of the current therapies used in the treatment of infectious diseases. Compared to conventional antiviral drugs, monoclonal antibodies (mAbs) used as pharmacological molecules have particular physical characteristics and modes of action, and, therefore, they should be considered as a distinct therapeutic class. Despite being historically validated, antibodies may represent a novel tool for combatting infectious diseases. The current high cost of mAbs' production, storage and administration (by injection only) and the consequent obstacles to development are outweighed by mAbs' clinical advantages. These are related to a low toxicity combined with high specificity and versatility, which allows a specific antibody to mediate various biological effects, ranging from the virus neutralization mechanisms to the modulation of immune responses.This review briefly summarizes the recent technological advances in the field of immunoglobulin research, and the current status of mAb-based drugs in clinical trials for HIV and HCV diseases. For each clinical trial the available data are reported and the emerging conceptual problems of the employed mAbs are highlighted.This overview helps to give a clear picture of the efficacy and challenges of the mAbs in the field of these two infectious diseases which have such a global impact.

Human monoclonal antibodies and engineered antibody domains as HIV-1 entry inhibitors

PURPOSE OF REVIEW

To summarize the in-vivo efficacy of neutralizing human monoclonal antibodies against HIV-1, to discuss the recent finding that an engineered human antibody VH domain, domain antibody (dAb), exhibits exceptionally potent and broadly cross-reactive neutralizing activity against HIV-1 primary isolates by targeting a hidden conserved epitope that is not accessible by larger antibodies and to suggest the possibility of developing a novel class of potent HIV-1 inhibitors based on human dAbs.

RECENT FINDINGS

HIV-1 has evolved a number of strategies to evade humoral immunity, including protecting highly conserved and important structures from the access of antibodies generated by the immune system. We have recently demonstrated that a human dAb (size approximately 15 kDa), m36, targets a highly protected structure on the HIV-1 envelope glycoprotein (Env), gp120, and exhibits exceptionally potent neutralizing activity against HIV-1 primary isolates, with potency on average higher than those of the broadly cross-reactive neutralizing human monoclonal antibody, scFv m9, and the inhibitory peptide, C34.

SUMMARY

The efficacy of the anti-HIV-1 therapy is significantly compromised by resistance to the currently used US Food and Drug Administration-approved antiretroviral drugs, which suggests an urgent need to develop novel classes of potent inhibitors. Several broadly cross-reactive neutralizing human monoclonal antibodies are highly effective against HIV-1 infection in vitro, but their administration to HIV-1-infected humans has only resulted in modest antiviral effects. Engineered human antibody fragments, dAbs, could be more potent because of their small size (about 10-fold smaller than that of an IgG), which allows targeting of highly conserved structures on the HIV-1 envelope glycoprotein that are not accessible by full-size antibodies and relatively efficient penetration into the densely packed lymphoid environment in which HIV-1 mostly replicates and spreads.

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.

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.

Long-term multiple-dose pharmacokinetics of human monoclonal antibodies (MAbs) against human immunodeficiency virus type 1 envelope gp120 (MAb 2G12) and gp41 (MAbs 4E10 and 2F5)

While certain antibodies directed against the human immunodeficiency virus (HIV) envelope have the potential to suppress virus replication in vitro, the impact of neutralizing antibodies in vivo remains unclear. In a recent proof-of-concept study, the broadly neutralizing monoclonal antibodies 2G12, 4E10, and 2F5 exhibited inhibitory activities in vivo, as exemplified by a delay of the viral rebound following the interruption of antiretroviral therapy. Unexpectedly, the antiviral effect seen was most prominently due to 2G12 activity. To further investigate whether differential HIV-inhibitory activity was due to different pharmacokinetic properties of the antibodies, we performed a formal pharmacokinetic analysis with 14 patients. Repeated infusions at high dose levels were well tolerated by the patients and did not elicit an endogenous immune response against the monoclonal antibodies. The pharmacokinetic parameters of all three antibodies correlated with each other. Mean estimates were 0.047, 0.035, and 0.044 liter/kg for the central volume of distribution of 2G12, 4E10, and 2F5, respectively, and 0.0018, 0.0058, and 0.0077 liter/kg . day for the systemic clearance of 2G12, 4E10, and 2F5, respectively. Monoclonal antibody 2G12 had a significantly longer elimination half-life (21.8 +/- 7.2 days [P < 0.0001]) than monoclonal antibodies 4E10 (5.5 +/- 2.2 days) and 2F5 (4.3 +/- 1.1 days). The comprehensive pharmacokinetic data from this long-term multiple-dose phase II study were coherent with those from previous short-term phase I studies, as assessed by compartmental and noncompartmental techniques. The anti-HIV type 1 antibodies studied showed distribution and elimination kinetics similar to those seen for other human-like antibodies. Further studies examining tissue concentrations to explain the differential in vivo activity of the anti-gp120 antibody compared with those of the two anti-gp41 antibodies are warranted.

Clinical experience with therapeutic AIDS vaccines

Recently, there has been a renewed interest in therapeutic vaccination as an adjunct or alternative to current treatment options for HIV. The first immunotherapeutic trial relevant to this topic was published in 1983. Since then, several dozen therapeutic vaccine trials have been carried out. The results have consistently shown that although in vitro-measured HIV-specific immune responses were evident as a result of vaccination, clinical improvement has been seldom observed. The instances of apparent clinical benefit however, were invariably associated with the usage of vaccines that acted in accord with the principles of allo- or autoimmunization. The majority of these vaccines were derived from the blood of HIV carriers or a cell culture and therefore inherently contained host-cell antigens unrelated to HIV. These observations raise the issue of whether this clinically successful approach has been unduly neglected. Most commercial vaccines on the market today are made the old-fashioned way, but very little support or attention has been given to the development of such vaccines for AIDS therapy. The current strategy, biased toward vaccines which have shown little evidence of clinical efficacy, is shortsighted and needs to be revised.

The F(ab′)2 fragment of an Aβ-specific monoclonal antibody reduces Aβ deposits in the brain

This work examines whether administering the F(ab' )2 fragment of an IgG1 monoclonal antibody (mAb) targeting the N-terminal 1-13 amino acids of the beta-amyloid peptide (Abeta mAb) reduces amyloid deposition in Alzheimer's disease (AD). The F(ab')2 fragment was injected intraperitoneally or intracranially into Tg2576 mice, a murine model of human AD. Both routes of administration significantly reduced Abeta plaque formation in the brain, as determined immunohistochemically and by monitoring levels of Abeta1-40 and Abeta1-42 peptide. Use of the F(ab')2 fragment significantly reduced phagocytic infiltration in the CNS when compared to intact mAb. Since IgG1 Abs do not fix complement, these findings suggest that effective in vivo clearance of amyloid deposits can be achieved without stimulation of FcR-reactive phagocytes or activation of the complement cascade.

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

To determine the protective potential of the humoral immune response against HIV-1 in vivo we evaluated the potency of three neutralizing antibodies (2G12, 2F5 and 4E10) in suppressing viral rebound in six acutely and eight chronically HIV-1-infected individuals undergoing interruption of antiretroviral treatment (ART). Only two of eight chronically infected individuals showed evidence of a delay in viral rebound during the passive immunization. Rebound in antibody-treated acutely infected individuals upon cessation of ART was substantially later than in a control group of 12 individuals with acute infection. Escape mutant analysis showed that the activity of 2G12 was crucial for the in vivo effect of the neutralizing antibody cocktail. By providing further direct evidence of the potency, breadth and titers of neutralizing antibodies that are required for in vivo activity, these data underline both the potential and the limits of humoral immunity in controlling HIV-1 infection.

Passive immunization with the anti-HIV-1 human monoclonal antibody (hMAb) 4E10 and the hMAb combination 4E10/2F5/2G12

OBJECTIVES

To study the safety, immunogenicity and pharmacokinetics of the human monoclonal antibody (hMAb) 4E10 alone and in combination with the hMAbs 2F5 and 2G12 in HIV-1-infected persons.

MATERIALS AND METHODS

Eight healthy volunteers with > or =350 CD4 cells/mm3 and < or =100 000 HIV-1 RNA copies/mL were enrolled, seven finished the study. A single 4E10 infusion was administered on day 0, followed by three doses of the hMAb combination 4E10/2F5/2G12 on days 7, 14 and 21 (total amount 8.5 g). Safety was assessed by physical examination, blood chemistry, complete blood cell count and recording of adverse events. 4E10, 2F5 and 2G12 plasma levels were determined before and at the end of each infusion and during the 7 week follow-up.

RESULTS

No drug-related adverse events were observed throughout the study. The median plasma concentrations immediately after the first infusion were 371, 253 and 139 microg/mL for 4E10, 2F5 and 2G12. Multiple infusions resulted in maximum plasma concentrations of 407, 294 and 210 microg/mL for 4E10, 2F5, and 2G12, respectively. The median elimination half-lives (t1/2beta) were 6.6, 3.2 and 14.1 days for 4E10, 2F5 and 2G12. A low level antibody response against 2G12 was found in two patients.

CONCLUSION

This Phase I trial showed that the hMAb 4E10 can be safely administered, both alone and in combination with 2F5 and 2G12 to HIV-1-infected patients.

Back to the future: antibody-based strategies for the treatment of infectious diseases

Before antibiotics, sera from immune animals and humans were used to treat a variety of infectious diseases, often with successful results. After the discovery of antimicrobial agents, serum therapy for bacterial infections was rapidly forsaken. In the last two decades, problems with treatment of newly emerged, re-emerged, or persistent infectious diseases necessitated researchers to develop new and/or improved antibody-based therapeutic approaches. This article reviews some information on the use of antibodies for the treatment of infectious diseases, with special reference to the most seminal discoveries and current advances as well as available treatment approaches in this field.

A phase I trial with two human monoclonal antibodies (hMAb 2F5, 2G12) against HIV-1

OBJECTIVE

To study the safety, immunogenicity and pharmacokinetics of two intravenously administered human monoclonal antibodies (hMAb 2F5, 2G12) against HIV-1 in humans.

DESIGN

Open label clinical phase I trial.

SETTING

Primary institutional care.

PATIENTS

Seven HIV-1-infected healthy volunteers with > or = 500 x 10(6)CD4 cells/l and < or = 10,000 HIV-1 RNA copies/ml, not treated with highly active antiretroviral therapy (HAART), entered and finished the study.

INTERVENTIONS

and main outcome measures: Eight separate infusions of the hMAb were administered over a 4-week period (total dose 14 g). The safety was assessed by physical examination, blood chemistry, complete blood cell count and recording adverse events. 2F5 and 2G12 plasma levels were determined prior to and at the end of each infusion and during the follow-up period of 22 weeks.

RESULTS

No clinical or laboratory abnormalities were observed throughout the study. The median distribution half-life (t(1/2 alpha)) of 2F5 and 2G12 was 1.02 (range, 0.77-1.47) days and 2.49 (range, 0.92-4.59) days, respectively. The elimination half-life (t(1/2 beta)) was calculated to be 7.94 (range, 3.46-8.31) days for 2F5 and 16.48 (range, 12.84-24.85) days for 2G12. The median plasma concentration immediately after the first infusion was 216 microg/ml (range, 158-409 microg/ml) for 2F5 and 238 microg/ml (range, 197-402 microg/ml) for 2G12. Multiple infusions resulted in maximum plasma concentrations of 374 microg/ml (range, 304-700 microg/ml) and 605 microg/ml (range, 479-897 microg/ml) for 2F5 and 2G12, respectively.

CONCLUSIONS

This study showed that the hMAb 2F5 and 2G12 are safe and well tolerated by HIV-1-infected subjects.

Alternatives to standard blood transfusion: availability and promise

Largely due to concerns over safety, a wide variety of alternatives to the conventional blood bank products of red cells, platelet concentrates, plasma and fractionated plasma products are under development. This review attempts to survey the alternative therapies that are being developed, whether they provide viable solutions and what impact they might have on transfusion practice.

Alternatives to human blood and blood resources

Biotechnology is increasingly providing alternatives to established transfusion products, with the promise of freedom from infectivity and unlimited supply. Such products must demonstrate a safety and efficacy at least as great as current products, and thereafter an improved cost-benefit ratio. Alternatives to established products may be categorised as: Alternatives made from blood donations. Substitutes (acting by distinct mechanisms). Recombinant analogs. Cellular therapies (including gene therapy). This overview covers the current status in each of these categories, and likely trends in the near future. Recombinant therapeutic proteins are already established in certain indications, and are likely to be extended to a wider range of applications as the methods for bulk production are improved. Alternative products from donated blood should find niche indications, especially for acute therapy, but cellular therapy is unlikely to be widely implemented on this timescale, except as an adjunct in life-threatening indications. Substitutes are perhaps the most interesting category and are expected to have a major impact, particularly through the use of "proteins" to stimulate endogenous production of circulating cell populations.

Phase I study of a human monoclonal antibody directed against the CD4-binding site of HIV type 1 glycoprotein 120

A phase I dose escalation study was conducted with the human monoclonal anti-gp120 antibody F105, to evaluate the safety, pharmacokinetics, and functional activity of F105 in HIV-1-infected individuals. F105 is an IgG1(kappa) antibody reactive with a discontinuous epitope that overlaps the CD4-binding site of gp120. F105 neutralizes laboratory strains of HIV-1 and some primary isolates, and synergizes with other antibodies in neutralizing an expanded spectrum of isolates. Four patients each with CD4 counts between 200 and 500/mm3 received a single dose of F105 at 100 or 500 mg/m2, intravenously. Sustained levels of F105 were obtained in plasma, and there was no evidence of an immune response to F105 as determined by a double-antigen immunoassay. No patient experienced any toxicity. Infused antibody retained full functional activity as detected by the ability of sera to block the binding of labeled F105 to HIV-1-infected cells. Of note, all patients had preexisting antibody to the gp120 CD4-binding site. The ability to culture virus by quantitative microculture remained unchanged by this single dose of antibody. Thus, it can be concluded that F105 is safe and nontoxic as a single injection at the doses tested. Furthermore, the antibody retains full gp120-binding activity. In these patients, with preexisting CD4-binding site antibody, there is no evidence of anti-HIV-1 activity following a single antibody infusion.

Involvement of the complement system in antibody-mediated post-exposure protection against human immunodeficiency virus type 1

We previously reported that passive transfer of a murine V3-specific monoclonal antibody (BAT123) to hu-PBL-SCID mice challenged with HIV-1LAI confers postexposure protection from infection. The role of the Fc fragment of this antibody as well as the involvement of the complement system in protection were evaluated in vivo. When we compared the postexposure protection offered by BAT123 and CGP 47439, a chimeric form of BAT123 in which the murine Fc domain has been replaced by a human IgG1 Fc domain, CGP 47439 failed to provide postexposure protection against HIV-1LAI despite having similar pharmacokinetics and in vitro neutralizing activity. Furthermore, when hu-PBL-SCID mice were treated with cobra venom factor, which inactivates serum complement activity, the postexposure protective ability of BAT123 was abrogated. These findings suggest that the complement system is involved in the passive protection against HIV-1 infection conferred by the murine monoclonal antibody BAT123 in hu-PBL-SCID mice.

Potent and synergistic neutralization of human immunodeficiency virus (HIV) type 1 primary isolates by hyperimmune anti-HIV immunoglobulin combined with monoclonal antibodies 2F5 and 2G12

Three antibody reagents that neutralize primary human immunodeficiency virus type 1 (HIV-1) isolates were tested for magnitude and breadth of neutralization when used alone or in double or triple combinations. Hyperimmune anti-HIV immunoglobulin (HIVIG) is derived from the plasma of HIV-1-infected donors, and monoclonal antibodies (MAbs) 2F5 and 2G12 bind to distinct regions of the HIV-1 envelope glycoprotein. The antibodies were initially tested against a panel of 15 clade B HIV-1 isolates, using a single concentration that is achievable in vivo (HIVIG, 2,500 microg/ml; MAbs, 25 microg/ml). Individual antibody reagents neutralized many of the viruses tested, but antibody potency varied substantially among the viruses. The virus neutralization produced by double combinations of HIVIG plus 2F5 or 2G12, the two MAbs together, or the triple combination of HIVIG, 2F5, and 2G12 was generally equal to or greater than that predicted by the effect of individual antibodies. Overall, the triple combination displayed the greatest magnitude and breadth of neutralization. Synergistic neutralization was evaluated by analyzing data from dose-response curves of each individual antibody reagent compared to the triple combination and was demonstrated against each of four viruses tested. Therefore, combinations of polyclonal and monoclonal anti-HIV antibodies can produce additive or synergistic neutralization of primary HIV-1 isolates. Passive immunotherapy for treatment or prophylaxis of HIV-1 should consider mixtures of potent neutralizing antibody reagents to expand the magnitude and breadth of virus neutralization.

Epitope mapping, V-region DNA sequence, and neutralizing Fab fragments of two monoclonal antibodies against the HIV-1 V3 loop

BACKGROUND

Experimental evidence suggests that neutralizing antibodies could constitute an important factor in the control of AIDS progression and that the V3 loop of gp120 constitutes the main target for such purposes. We have previously developed two neutralizing murine monoclonal antibodies (Mabs) against the V3 region of the HIV-1 MN strain.

OBJECTIVES

To characterize those Mabs in terms of fine specificity and DNA sequence of their V regions and to study if Fab fragments retain their neutralizing potential in vitro.

STUDY DESIGN

A set of 12-mer alanine substituted peptides were employed for epitope mapping using two ELISA procedures: (1) indirect, with each peptide bound to polystyrene plates, and (2) competitive, with co-incubation of peptides and Mabs in solution. The V regions of both Mabs were PCR amplified from cDNA and their nucleotide sequences were determined. Finally, Fab fragments of Mab 10F10 were generated and their neutralizing capacity against the MN isolated was assessed.

RESULTS

We first restricted the minimal length of the epitopes recognized by 2C4 and 10F10 to the 12-mer peptide KRIHIGPGRAFY. The core of the epitopes recognized by Mabs 2C4 and 10F10 were IHIGP-R and IHIG-R, respectively. While substitution of proline in position 7 completely abolished the binding of 2C4, it only reduced that of 10F10 by 50%. Finally, Fab fragments of Mab 10F10 were still able to neutralize the HIV-1 MN strain in vitro.

CONCLUSION

This subtle distinction in the fine mapping of the epitope recognized by Mabs 2C4 and 10F10 should correspond to three amino acid differences that we found in the heavy chain V-regions. The Fab fragments of Mab 10F10 retained the neutralizing capacity. This indicates that HIV neutralization by anti V3 Mabs is an Fc independent process.