Possible beneficial effects of neutralizing antibodies and antibody-dependent, cell-mediated cytotoxicity in human immunodeficiency virus infection

Natural killer cells during acute HIV-1 infection: clues for HIV-1 prevention and therapy

Despite progress in preexposure prophylaxis, the number of newly diagnosed cases with HIV-1 remains high, highlighting the urgent need for preventive and therapeutic strategies to reduce HIV-1 acquisition and limit disease progression. Early immunological events, occurring during acute infection, are key determinants of the outcome and course of disease. Understanding early immune responses occurring before viral set-point is established, is critical to identify potential targets for prophylactic and therapeutic approaches. Natural killer (NK) cells represent a key cellular component of innate immunity and contribute to the early host defence against HIV-1 infection, modulating the pathogenesis of acute HIV-1 infection (AHI). Emerging studies have identified tools for harnessing NK cell responses and expanding specialized NK subpopulations with adaptive/memory features, paving the way for development of novel HIV-1 therapeutics. This review highlights the knowns and unknowns regarding the role of NK cell subsets in the containment of acute HIV-1 infection, and summarizes recent advances in selectively augmenting NK cell functions through prophylactic and therapeutic interventions.

Antibody-dependent cellular cytotoxicity targeting CD4-inducible epitopes predicts mortality in HIV-infected infants

BACKGROUND

Antibody-dependent cellular cytotoxicity (ADCC) has been associated with improved infant outcome in mother-to-child transmission (MTCT) of HIV-1. Epitopes of these ADCC-mediating antibodies remain unidentified. CD4-inducible (CD4i) epitopes on gp120 are common ADCC targets in natural infection and vaccination. We tested whether CD4i epitope-specific ADCC mediated by maternal antibodies or passively-acquired antibodies in infants is associated with reduced MTCT and improved infant survival.

METHODS

We used variants of CD4i cluster A-specific antibodies, A32 and C11, and a cluster C-specific antibody, 17b, with mutations abolishing Fc-Fc receptor interactions as inhibitors in a competition rapid and fluorometric ADCC assay using gp120-coated CEM-nkr target cells with plasma from 51 non-transmitting and 21 transmitting breastfeeding mother-infant pairs.

FINDINGS

Cluster A-specific ADCC was common. Individually, neither A32-like nor C11-like ADCC was statistically significantly associated with risk of MTCT or infected infant survival. In combination, total maternal cluster A-specific ADCC was statistically significantly associated with decreased infected infant survival in a log-rank test (p = 0·017). There was a non-significant association for infant passively-acquired total cluster A-specific ADCC and decreased infected infant survival (p = 0·14). Surprisingly, plasma ADCC was enhanced in the presence of the defective Fc 17b competitor. Defective Fc 17b competitor-mediated maternal ADCC enhancement was statistically significantly associated with reduced infected infant survival (p = 0·011). A non-significant association was observed for passively-acquired infant ADCC enhancement and decreased survival (p = 0·19).

INTERPRETATIONS

These data suggest that ADCC targeting CD4i epitopes is not associated with protection against breast milk HIV transmission but is associated with decreased survival of infected infants. FUND: This study was funded by NIH grant R01AI076105 and NIH fellowship F30AI136636.

Mind the Gap: How Interspecies Variability in IgG and Its Receptors May Complicate Comparisons of Human and Non-human Primate Effector Function

The field of HIV research relies heavily on non-human primates, particularly the members of the macaque genus, as models for the evaluation of candidate vaccines and monoclonal antibodies. A growing body of research suggests that successful protection of humans will not solely rely on the neutralization activity of an antibody's antigen binding fragment. Rather, immunological effector functions prompted by the interaction of the immunoglobulin G constant region and its cognate Fc receptors help contribute to favorable outcomes. Inherent differences in the sequences, expression, and activities of human and non-human primate antibody receptors and immunoglobulins have the potential to produce disparate results in the observations made in studies conducted in differing species. Having a more complete understanding of these differences, however, should permit the more fluent translation of observations between model organisms and the clinic. Here we present a guide to such translations that encompasses not only what is presently known regarding the affinity of the receptor-ligand interactions but also the influence of expression patterns and allelic variation, with a focus on insights gained from use of this model in HIV vaccines and passive antibody therapy and treatment.

The Antibodiome-Mapping the Humoral Immune Response to HIV

PURPOSE OF REVIEW

The design of an HIV vaccine remains an elusive but top priority. Data from the non-human primate model and the first moderately protective HIV vaccine trial (RV144) point to a role for qualitative changes in humoral immune functions in protection from infection. Here, we review the current understanding of the antibody response throughout HIV infection, the known correlates of protection, and current strategies to manipulate antibodies to put an end to the epidemic.

RECENT FINDINGS

Recent studies point to innate immune-recruiting antibody function in preventing infection as well as controlling viremia following infection. These data have begun to inform next-generation design of HIV vaccines and antibody therapies by uncovering new viral targets and antibody architectures to improve potency and breadth. Emerging data illustrate a role for innate immune recruiting-antibodies in conferring protection against HIV infection as well as promoting viral control and clearance, offering an unprecedented opportunity to modulate and improve antibody function to fight HIV more effectively.

Use of broadly neutralizing antibodies for HIV-1 prevention

Antibodies have a long history in antiviral therapy, but until recently, they have not been actively pursued for HIV-1 due to modest potency and breadth of early human monoclonal antibodies (MAbs) and perceived insurmountable technical, financial, and logistical hurdles. Recent advances in the identification and characterization of MAbs with the ability to potently neutralize diverse HIV-1 isolates have reinvigorated discussion and testing of these products in humans, since new broadly neutralizing MAbs (bnMAbs) are more likely to be effective against worldwide strains of HIV-1. In animal models, there is abundant evidence that bnMAbs can block infection in a dose-dependent manner, and the more potent bnMAbs will allow clinical testing at infusion doses that are practically achievable. Moreover, recent advances in antibody engineering are providing further improvements in MAb potency, breadth, and half-life. This review summarizes the current state of the field of bnMAb protection in animal models as well as a review of variables that are critical for antiviral activity. Several bnMAbs are currently in clinical testing, and we offer perspectives on their use as pre-exposure prophylaxis (PrEP), potential benefits beyond sterilizing immunity, and a discussion of future approaches to engineer novel molecules.

Fc receptor-mediated phagocytosis in tissues as a potent mechanism for preventive and therapeutic HIV vaccine strategies

Although the development of a fully protective HIV vaccine is the ultimate goal of HIV research, to date only one HIV vaccine trial, the RV144, has successfully induced a weakly protective response. The 31% protection from infection achieved in the RV144 trial was linked to the induction of nonneutralizing antibodies, able to mediate antibody-dependent cell-mediated cytotoxicity (ADCC), suggestive of an important role of Fc-mediated functions in protection. Similarly, Fc-mediated antiviral activity was recently shown to play a critical role in actively suppressing the viral reservoir, but the Fc effector mechanisms within tissues that provide protection from or after infection are largely unknown. Here we aimed to define the landscape of effector cells and Fc receptors present within vulnerable tissues. We found negligible Fc receptor-expressing natural killer cells in the female reproductive and gastrointestinal mucosa. Conversely, Fc receptor-expressing macrophages were highly enriched in most tissues, but neutrophils mediated superior antibody-mediated phagocytosis. Modifications in Fc domain of VRC01 antibody increased phagocytic responses in both phagocytes. These data suggest that non-ADCC-mediated mechanisms, such as phagocytosis and neutrophil activation, are more likely to play a role in preventative vaccine or reservoir-eliminating therapeutic approaches.

Comparison of Antibody-Dependent Cell-Mediated Cytotoxicity and Virus Neutralization by HIV-1 Env-Specific Monoclonal Antibodies

Although antibodies to the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein have been studied extensively for their ability to block viral infectivity, little data are currently available on nonneutralizing functions of these antibodies, such as their ability to eliminate virus-infected cells by antibody-dependent cell-mediated cytotoxicity (ADCC). HIV-1 Env-specific antibodies of diverse specificities, including potent broadly neutralizing and nonneutralizing antibodies, were therefore tested for ADCC against cells infected with a lab-adapted HIV-1 isolate (HIV-1NL4-3), a primary HIV-1 isolate (HIV-1JR-FL), and a simian-human immunodeficiency virus (SHIV) adapted for pathogenic infection of rhesus macaques (SHIVAD8-EO). In accordance with the sensitivity of these viruses to neutralization, HIV-1NL4-3-infected cells were considerably more sensitive to ADCC, both in terms of the number of antibodies and magnitude of responses, than cells infected with HIV-1JR-FL or SHIVAD8-EO ADCC activity generally correlated with antibody binding to Env on the surfaces of virus-infected cells and with viral neutralization; however, neutralization was not always predictive of ADCC, as instances of ADCC in the absence of detectable neutralization, and vice versa, were observed. These results reveal incomplete overlap in the specificities of antibodies that mediate these antiviral activities and provide insights into the relationship between ADCC and neutralization important for the development of antibody-based vaccines and therapies for combating HIV-1 infection.

IMPORTANCE

This study provides fundamental insights into the relationship between antibody-dependent cell-mediated cytotoxicity (ADCC) and virus neutralization that may help to guide the development of antibody-based vaccines and immunotherapies for the prevention and treatment of HIV-1 infection.

Conformational Masking and Receptor-Dependent Unmasking of Highly Conserved Env Epitopes Recognized by Non-Neutralizing Antibodies That Mediate Potent ADCC against HIV-1

The mechanism of antibody-mediated protection is a major focus of HIV-1 vaccine development and a significant issue in the control of viremia. Virus neutralization, Fc-mediated effector function, or both, are major mechanisms of antibody-mediated protection against HIV-1, although other mechanisms, such as virus aggregation, are known. The interplay between virus neutralization and Fc-mediated effector function in protection against HIV-1 is complex and only partially understood. Passive immunization studies using potent broadly neutralizing antibodies (bnAbs) show that both neutralization and Fc-mediated effector function provides the widest dynamic range of protection; however, a vaccine to elicit these responses remains elusive. By contrast, active immunization studies in both humans and non-human primates using HIV-1 vaccine candidates suggest that weakly neutralizing or non-neutralizing antibodies can protect by Fc-mediated effector function, albeit with a much lower dynamic range seen for passive immunization with bnAbs. HIV-1 has evolved mechanisms to evade each type of antibody-mediated protection that must be countered by a successful AIDS vaccine. Overcoming the hurdles required to elicit bnAbs has become a major focus of HIV-1 vaccine development. Here, we discuss a less studied problem, the structural basis of protection (and its evasion) by antibodies that protect only by potent Fc-mediated effector function.

Development of IgG Mediated Antibody Dependent Cell-mediated Cytotoxicity (ADCC) in the Serum and Genital Mucosa of HIV Seroconverters

Background

We measured antibody-dependent cell mediated cytotoxicity (ADCC) activity in serum and genital fluids of heterosexually exposed women during HIV seroconversion.

Methods

Plasma and cervico-vaginal lavage (CVL) fluid from 11 seroconverters (SC) were analyzed biannually from one year pre- to 6 year post-seroconversion using a ⁵¹Cr-release assay to measure HIV-1 gp120 specific ADCC.

Results

No SC had significant HIV specific CVL ADCC activity before seroconversion or until 1.5 yr after seroconversion. One individual had a %Specific Release (SR) of 25.4 at 2 years, 26.7 at 3 years and 21.0 at 4 years after seroconversion in CVL. Another sample had 4.7% SR at 2 years, 5.3 at 3 years, 10.9 at 4 years, and 8.4 at 5 years after seroconversion in CVL. A third had no activity until 17% SR 5 years after seroconversion in CVL. A fourth showed activity of 36.5% SR at 6.5 years after seroconversion. Seven women had no ADCC activity in their CVL. Paired serum samples showed HIV specific ADCC activity prior to the appearance of CVL ADCC activity.

Conclusions

HIV specific ADCC activity in CVL rose 2 years after seroconversion; ADCC was present in the serum prior to this time. These data suggest that genital tract ADCC activity is not present until well after acute infection.

Opportunities to exploit non-neutralizing HIV-specific antibody activity

Antibodies act as a nexus between innate and adaptive immunity: they provide a means to engage a spectrum of innate immune effector cells in order to clear viral particles and infected cells and prime antigen presentation. This functional landscape is remarkably complex, and depends on antibody isotype, subclass, and glycosylation; the expression levels and patterns of a suite of Fc receptors with both complementary and opposing activities; and a host of innate immune cells capable of differential responses to opsonized particles and present at different sites. In vivo, even neutralizing antibodies rely on their ability to act as molecular beacons and recruit innate immune effector cells in order to provide protection, and results from both human and macaque studies have implicated these effector functions in vaccinemediated protection. Thus, while enhancing effector function is a tractable handle for potentiating antibody-mediated protection from HIV infection, success will depend critically on leveraging understanding of the means by which antibodies with specific functional profiles could be elicited, which effector functions could provide optimal protection, and perhaps most critically, how to efficiently recruit the innate effector cells present at sites of infection.

Role of Fc-mediated antibody function in protective immunity against HIV-1

The importance of Fc-mediated effector function in protective immunity to HIV-1 (hereafter referred to simply as HIV) is becoming increasingly apparent. A large of number of studies in natural infection cohorts, spanning the last 26 years, have associated Fc-mediated effector function, particularly antibody-dependent cellular cytotoxicity, with a favourable clinical course. These studies strongly suggest a role for Fc-mediated effector function in the post-infection control of viraemia. More recently, studies in both humans and non-human primates (NHPs) also implicate Fc-mediated effector function in blocking HIV acquisition. Accordingly, this review will discuss the results supporting a role of Fc-mediated effector function in both blocking acquisition and post-infection control of viraemia. Parallel studies in NHPs and humans will be compared for common themes. Context for this discussion will be provided by summarizing the temporal emergence of key host and virological events over the course of an untreated HIV infection framing where, when and how Fc-mediated effector function might be protective. A hypothesis that Fc-mediated effector function protects primarily in the early stages of both acquisition and post-infection control of viraemia will be developed.

Enhanced phagocytic activity of HIV-specific antibodies correlates with natural production of immunoglobulins with skewed affinity for FcγR2a and FcγR2b

While development of an HIV vaccine that can induce neutralizing antibodies remains a priority, decades of research have proven that this is a daunting task. However, accumulating evidence suggests that antibodies with the capacity to harness innate immunity may provide some protection. While significant research has focused on the cytolytic properties of antibodies in acquisition and control, less is known about the role of additional effector functions. In this study, we investigated antibody-dependent phagocytosis of HIV immune complexes, and we observed significant differences in the ability of antibodies from infected subjects to mediate this critical effector function. We observed both quantitative differences in the capacity of antibodies to drive phagocytosis and qualitative differences in their FcγR usage profile. We demonstrate that antibodies from controllers and untreated progressors exhibit increased phagocytic activity, altered Fc domain glycosylation, and skewed interactions with FcγR2a and FcγR2b in both bulk plasma and HIV-specific IgG. While increased phagocytic activity may directly influence immune activation via clearance of inflammatory immune complexes, it is also plausible that Fc receptor usage patterns may regulate the immune response by modulating downstream signals following phagocytosis--driving passive degradation of internalized virus, release of immune modulating cytokines and chemokines, or priming of a more effective adaptive immune response.

Compromised NK cell-mediated antibody-dependent cellular cytotoxicity in chronic SIV/SHIV infection

Increasing evidence indicates that antibody-dependent cellular cytotoxicity (ADCC) contributes to the control of HIV/SIV infection. However, little is known about the ADCC function of natural killer (NK) cells in non-human primate model. Here we demonstrated that ADCC function of NK cells was significantly compromised in chronic SIV/SHIV infection, correlating closely with the expression of FcγRIIIa receptor (CD16) on NK cells. CD32, another class of IgG Fc receptors, was identified on NK cells with higher expression in the infected macaques and the blockade of CD32 impacted the ability of NK cells to respond to antibody-coated target cells. The inhibition of matrix metalloproteases (MMPs), a group of enzymes normally involved in tissue/receptor remodeling, could restore NK cell-mediated ADCC with increased CD16 expression on macaque NK cells. These data offer a clearer understanding of NK cell-mediated ADCC in rhesus macaques, which will allow us to evaluate the ADCC repertoire arising from preclinical vaccination studies in non-human primates and inform us in the future design of effective HIV vaccination strategies.

Neutralizing and other antiviral antibodies in HIV-1 infection and vaccination

PURPOSE OF REVIEW

New findings continue to support the notion that broadly crossreactive neutralizing antibody induction is a worthwhile and achievable goal for HIV-1 vaccines. Immunogens are needed that can overcome the genetic variability and complex immune evasion tactics of the virus. Other antibodies might bridge innate and acquired immunity for possible beneficial vaccine effects. This review summarizes progress made over the past year that has enhanced our understanding of humoral immunity as it relates to HIV-1 vaccine development.

RECENT FINDINGS

Although a clear path to designing an effective neutralizing antibody-based HIV-1 vaccine remains elusive, there is new information on how antibodies neutralize HIV-1, the epitopes involved, and clues to the possible nature of protective immunogens that keep this goal alive. Moreover, there is a greater understanding of HIV-1 diversity and its possible limits under immune pressure. Other antibodies might possess antiviral activity by mechanisms involving Fc receptor engagement or complement activation that would be of value for HIV-1 vaccines.

SUMMARY

Recent developments strengthen the rationale for antibody-based HIV-1 vaccine immunogens and provide a stronger foundation for vaccine discovery.

B cell depletion in HIV-1 subtype A infected Ugandan adults: relationship to CD4 T cell count, viral load and humoral immune responses

To better understand the nature of B cell dysfunctions in subjects infected with HIV-1 subtype A, a rural cohort of 50 treatment-naïve Ugandan patients chronically infected with HIV-1 subtype A was studied, and the relationship between B cell depletion and HIV disease was assessed. B cell absolute counts were found to be significantly lower in HIV-1+ patients, when compared to community matched negative controls (p<0.0001). HIV-1-infected patients displayed variable functional and binding antibody titers that showed no correlation with viral load or CD4+ T cell count. However, B cell absolute counts were found to correlate inversely with neutralizing antibody (NAb) titers against subtype A (p = 0.05) and subtype CRF02_AG (p = 0.02) viruses. A positive correlation was observed between subtype A gp120 binding antibody titers and NAb breadth (p = 0.02) and mean titer against the 10 viruses (p = 0.0002). In addition, HIV-1 subtype A sera showed preferential neutralization of the 5 subtype A or CRF02_AG pseudoviruses, as compared with 5 pseudoviruses from subtypes B, C or D (p<0.001). These data demonstrate that in patients with chronic HIV-1 subtype A infection, significant B cell depletion can be observed, the degree of which does not appear to be associated with a decrease in functional antibodies. These findings also highlight the potential importance of subtype in the specificity of cross-clade neutralization in HIV-1 infection.

Activation of NK cells by ADCC responses during early HIV infection

Partial control of HIV occurs during acute infection, although the mechanisms responsible are poorly understood. We studied the ability of antibody-dependent cellular cytotoxicity (ADCC) antibodies in serum to activate natural killer (NK) cells in longitudinal samples from 8 subjects with well-defined early HIV infection who controlled viremia to low levels. NK cell activation by ADCC antibodies to gp140 Env proteins was detected in half of the subjects at the first time point studied, a mean of 111 d after the estimated time of infection. In contrast, ADCC-mediated NK cell activation in response to linear HIV peptides evolved more slowly, over the first 2 y of infection. Our studies suggest that HIV-specific ADCC responses to conformational epitopes occur early during acute HIV infection, and broaden to include linear epitopes over time. These findings have implications for the immune control of HIV.

Rational design of HIV vaccines and microbicides: report of the EUROPRISE network annual conference 2010

Novel, exciting intervention strategies to prevent infection with HIV have been tested in the past year, and the field is rapidly evolving. EUROPRISE is a network of excellence sponsored by the European Commission and concerned with a wide range of activities including integrated developmental research on HIV vaccines and microbicides from discovery to early clinical trials. A central and timely theme of the network is the development of the unique concept of co-usage of vaccines and microbicides. This review, prepared by the PhD students of the network captures much of the research ongoing between the partners. The network is in its 5^th year and involves over 50 institutions from 13 European countries together with 3 industrial partners; GSK, Novartis and Sanofi-Pasteur. EUROPRISE is involved in 31 separate world-wide trials of Vaccines and Microbicides including 6 in African countries (Tanzania, Mozambique, South Africa, Kenya, Malawi, Rwanda), and is directly supporting clinical trials including MABGEL, a gp140-hsp70 conjugate trial and HIVIS, vaccine trials in Europe and Africa.

A dimeric form of the HIV-1 antibody 2G12 elicits potent antibody-dependent cellular cytotoxicity

OBJECTIVE

Increasing data support a role for antibody-dependent cellular cytotoxicity (ADCC) in controlling HIV-1 infection. We recently isolated a naturally occurring dimeric form of the anti-HIV-1 antibody 2G12 and found it to be significantly more potent than 2G12 monomer in neutralizing primary virus strains. However, given the unusual structure of dimeric 2G12 with two Fc regions, it was not clear whether 2G12 dimer could bind to the CD16 Fc receptor on ADCC effector cells or trigger ADCC. Here we compared the in-vitro ADCC activities of 2G12 monomer and dimer and investigated the effects of including ADCC-enhancing mutations in both forms of 2G12.

METHODS

An in-vitro ADCC assay using target cells stably expressing gp160 was developed to evaluate the activities of 2G12 monomer and dimer with and without ADCC-enhancing mutations that increase the CD16-binding affinity of the 2G12 Fc region.

RESULTS

Both 2G12 monomer and 2G12 dimer elicited ADCC, although the dimer showed increased potency [lower half-maximal concentration (EC(50))] in triggering ADCC, thus confirming its ability to bind CD16 and trigger ADCC. The ADCC-enhancing mutations improved the ADCC activity of 2G12 monomer more than 2G12 dimer such that their EC(50) values were nearly equal. However, no increase in nonspecific ADCC activity was observed using 2G12 IgGs with these mutations.

CONCLUSION

Given the likelihood that ADCC plays a role in protecting against initial infection and/or controlling chronic infection, these data suggest 2G12 dimers and/or addition of ADCC-enhancing mutations could augment the prophylactic and/or therapeutic potential of 2G12.

Multiple vaccine-elicited nonneutralizing antienvelope antibody activities contribute to protective efficacy by reducing both acute and chronic viremia following simian/human immunodeficiency virus SHIV89.6P challenge in rhesus macaques

We have shown that following priming with replicating adenovirus type 5 host range mutant (Ad5hr)-human immunodeficiency virus (HIV)/simian immunodeficiency virus (SIV) recombinants, boosting with gp140 envelope protein enhances acute-phase protection against intravenous simian/human immunodeficiency virus (SHIV)(89.6P) challenge compared to results with priming and no boosting or boosting with an HIV polypeptide representing the CD4 binding site of gp120. We retrospectively analyzed antibodies in sera and rectal secretions from these same macaques, investigating the hypothesis that vaccine-elicited nonneutralizing antibodies contributed to the better protection. Compared to other immunized groups or controls, the gp140-boosted group exhibited significantly greater antibody activities mediating antibody-dependent cellular cytotoxicity (ADCC) and antibody-dependent cell-mediated viral inhibition (ADCVI) in sera and transcytosis inhibition in rectal secretions. ADCC and ADCVI activities were directly correlated with antibody avidity, suggesting the importance of antibody maturation for functionality. Both ADCVI and percent ADCC killing prechallenge were significantly correlated with reduced acute viremia. The latter, as well as postchallenge ADCVI and ADCC, was also significantly correlated with reduced chronic viremia. We have previously demonstrated induction by the prime/boost regimen of mucosal antibodies that inhibit transcytosis of SIV across an intact epithelial cell layer. Here, antibody in rectal secretions was significantly correlated with transcytosis inhibition. Importantly, the transcytosis specific activity (percent inhibition/total secretory IgA and IgG) was strongly correlated with reduced chronic viremia, suggesting that mucosal antibody may help control cell-to-cell viral spread during the course of infection. Overall, the replicating Ad5hr-HIV/SIV priming/gp140 protein boosting approach elicited strong systemic and mucosal antibodies with multiple functional activities associated with control of both acute and chronic viremia.

Complement, Fc receptors and antibodies: a Trojan horse in HIV infection?

PURPOSE OF REVIEW

Because complement is present in all fluids of the body, including serum, saliva and seminal fluid, and is found at mucosal surfaces and in the brain, all pathogens have to deal with complement proteins. Thus, immediately upon entering the host, independent on the route of infection, HIV activates the complement system. Although a first line of immune defense, complement cannot eliminate retroviral infections completely.

RECENT FINDINGS

Recent data indicate that complement, in concert with non-neutralizing antibodies, contributes to the control of HIV replication at early stages of infection. In parallel or at later stages, complement and non-neutralizing antibodies may counteract the immune response by enhancing HIV infection via complement and Fc-receptor-positive cells in 'cis' and 'trans'.

SUMMARY

This review highlights current knowledge in this field and emphasizes the contribution of complement and non-neutralizing antibodies in controlling versus and enhancing infection.

Role of complement and antibodies in controlling infection with pathogenic simian immunodeficiency virus (SIV) in macaques vaccinated with replication-deficient viral vectors

Background

We investigated the interplay between complement and antibodies upon priming with single-cycle replicating viral vectors (SCIV) encoding SIV antigens combined with Adeno5-SIV or SCIV pseudotyped with murine leukemia virus envelope boosting strategies. The vaccine was applied via spray-immunization to the tonsils of rhesus macaques and compared with systemic regimens.

Results

Independent of the application regimen or route, viral loads were significantly reduced after challenge with SIVmac239 (p < 0.03) compared to controls. Considerable amounts of neutralizing antibodies were induced in systemic immunized monkeys. Most of the sera harvested during peak viremia exhibited a trend with an inverse correlation between complement C3-deposition on viral particles and plasma viral load within the different vaccination groups. In contrast, the amount of the observed complement-mediated lysis did not correlate with the reduction of SIV titres.

Conclusion

The heterologous prime-boost strategy with replication-deficient viral vectors administered exclusively via the tonsils did not induce any neutralizing antibodies before challenge. However, after challenge, comparable SIV-specific humoral immune responses were observed in all vaccinated animals. Immunization with single cycle immunodeficiency viruses mounts humoral immune responses comparable to live-attenuated immunodeficiency virus vaccines.

Contribution of nonneutralizing vaccine-elicited antibody activities to improved protective efficacy in rhesus macaques immunized with Tat/Env compared with multigenic vaccines

Previously, chronic-phase protection against SHIV(89.6P) challenge was significantly greater in macaques primed with replicating adenovirus type 5 host range mutant (Ad5hr) recombinants encoding HIVtat and env and boosted with Tat and Env protein compared with macaques primed with multigenic adenovirus recombinants (HIVtat, HIVenv, SIVgag, SIVnef) and boosted with Tat, Env, and Nef proteins. The greater protection was correlated with Tat- and Env-binding Abs. Because the macaques lacked SHIV(89.6P)-neutralizing activity prechallenge, we investigated whether Ab-dependent cellular cytotoxicity (ADCC) and Ab-dependent cell-mediated viral inhibition (ADCVI) might exert a protective effect. We clearly show that Tat can serve as an ADCC target, although the Tat-specific activity elicited did not correlate with better protection. However, Env-specific ADCC activity was consistently higher in the Tat/Env group, with sustained cell killing postchallenge exhibited at higher levels (p < 0.00001) for a longer duration (p = 0.0002) compared with the multigenic group. ADCVI was similarly higher in the Tat/Env group and significantly correlated with reduced acute-phase viremia at wk 2 and 4 postchallenge (p = 0.046 and 0.011, respectively). Viral-specific IgG and IgA Abs in mucosal secretions were elicited but did not influence the outcome of the i.v. SHIV(89.6P) challenge. The higher ADCC and ADCVI activities seen in the Tat/Env group provide a plausible mechanism responsible for the greater chronic-phase protection. Because Tat is known to enhance cell-mediated immunity to coadministered Ags, further studies should explore its impact on Ab induction so that it may be optimally incorporated into HIV vaccine regimens.

Complement and antibodies: a dangerous liaison in HIV infection?

Due to ongoing recombination and mutations, HIV permanently escapes from neutralizing antibody (nAb) responses of the host. By the masking of epitopes or shedding of gp120, HIV-1 further impedes an efficient neutralization by Abs. Therefore, nAbs responses of the host are chasing behind a rapidly evolving virus and mainly non-neutralizing antibodies (non-nAbs) are present in the host. At the same time, complement deposition on immune-complexed HIV may counteract the immune response by enhancing the infection. On the other hand, complement-mediated lysis is a putative effector mechanism to control viral replication. Here we review the complex interplay between complement, neutralizing and non-neutralizing Abs during HIV infection and discuss the contribution of Abs and complement in blocking versus enhancing the course of infection.

Preclinical studies of human immunodeficiency virus/AIDS vaccines: inverse correlation between avidity of anti-Env antibodies and peak postchallenge viremia

A major challenge for human immunodeficiency virus (HIV)/AIDS vaccines is the elicitation of anti-Env antibodies (Ab) capable of neutralizing the diversity of isolates in the pandemic. Here, we show that high-avidity, but nonneutralizing, Abs can have an inverse correlation with peak postchallenge viremia for a heterologous challenge. Vaccine studies were conducted in rhesus macaques using DNA priming followed by modified vaccinia Ankara boosting with HIV type 1 (HIV-1) immunogens that express virus-like particles displaying CCR5-tropic clade B (strain ADA) or clade C (IN98012) Envs. Rhesus granulocyte-macrophage colony-stimulating factor was used as an adjuvant for enhancing the avidity of anti-Env Ab responses. Challenge was with simian/human immunodeficiency virus (SHIV)-162P3, a CCR5-tropic clade B chimera of SIV and HIV-1. Within the groups receiving the clade B vaccine, a strong inverse correlation was found between the avidity of anti-Env Abs and peak postchallenge viremia. This correlation required the use of native but not gp120 or gp140 forms of Env for avidity assays. The high-avidity Ab elicited by the ADA Env had excellent breadth for the Envs of incident clade B but not clade C isolates, whereas the high-avidity Ab elicited by the IN98012 Env had excellent breadth for incident clade C but not clade B isolates. High-avidity Ab elicited by a SHIV vaccine with a dual-tropic clade B Env (89.6) had limited breadth for incident isolates. Our results suggest that certain Envs can elicit nonneutralizing but high-avidity Ab with broad potential for blunting incident infections of the same clade.

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.

Evaluation of passively transferred, nonneutralizing antibody-dependent cellular cytotoxicity-mediating IgG in protection of neonatal rhesus macaques against oral SIVmac251 challenge

Previously, Ab-dependent cellular cytotoxicity (ADCC) was significantly correlated with reduced acute viremia upon intrarectal SIVmac251 challenge of immunized rhesus macaques. To directly assess ADCC protective efficacy, six neonatal macaques were infused s.c. with immune IgG (220 mg/kg) purified from the immunized animals and positive for ADCC and Ab-dependent cell-mediated viral inhibition (ADCVI) activities. Six neonates received control IgG. The neonates were challenged twice orally with 10(5) 50% inhibiting tissue culture-infective dose of SIVmac251 2 days post-IgG infusion. At challenge, plasma of neonates that received immune IgG did not neutralize SIVmac251 but had geometric mean ADCC titers of 48,130 and 232,850 against SIVmac251 -infected and gp120-coated targets, respectively. Peak ADCVI activity varied from 62 to 81%. ADCC activity declined with the 2-wk IgG half-life but was boosted at wk 4, together with de novo ADCC-mediating Abs in controls, by postchallenge viremia. ADCVI activity was similarly induced. No protection, assessed by viral burdens, CD4 counts, and time to euthanasia was observed. Possible factors contributing to the discrepancy between the previous correlation and lack of protection here include: the high oral challenge dose compared with the 400-fold lower intrarectal dose; the challenge route with regard to viral dissemination and distribution of infused IgG; insufficient NK effector activity and/or poor functionality in newborns; insufficient immune IgG; and the possibility that the previous correlation of ADCC with protection was augmented by cellular immune responses also present at challenge. Future studies should explore additional challenge routes in juvenile macaques using higher amounts of potent IgG preparations.

A simplified method for the rapid fluorometric assessment of antibody-dependent cell-mediated cytotoxicity

We demonstrate that the FATAL cytolysis assay can be adapted into a rapid and fluorometric antibody-dependent cellular cytotoxicity assay (RFADCC). The RFADCC relies on double-staining target cells with a membrane dye (PKH-26) and a viability dye (CFSE) prior to the addition of antibody and effector cells. We used the RFADCC to assess dose-dependent and envelope-specific anti-human immunodeficiency virus (HIV) ADCC responses mediated by monoclonal antibody-2G12 and human sera. Using the assay, we also detected early anti-simian immunodeficiency virus (SIV) ADCC responses in rhesus macaques infected with pathogenic SIV(mac251). Importantly, the RFADCC was further useful in monitoring anti-HIV and anti-SIV ADCC responses elicited by immunizing chimpanzees and rhesus macaques with replicating adenovirus-based AIDS vaccine candidates. In comparison to the standard chromium release assay, the RFADCC provides a higher cell killing readout and is advantageous in allowing use of viably frozen as well as fresh effector cells, thus facilitating assay standardization. The RFADCC is therefore a simple, reliable, and highly sensitive method that can be applied to assess the ADCC activity of monoclonal antibodies as well as ADCC responses elicited by HIV or SIV infection or by AIDS vaccine candidates.

Vaccine-elicited antibodies mediate antibody-dependent cellular cytotoxicity correlated with significantly reduced acute viremia in rhesus macaques challenged with SIVmac251

Effector cells armed with Abs can eliminate virus-infected target cells by Ab-dependent cellular cytotoxicity (ADCC), an immune mechanism that has been largely overlooked in HIV vaccine development. Here, we show that a prime/boost AIDS vaccine approach elicits potent ADCC activity correlating with protection against SIV in rhesus macaques (Macacca mulatta). Priming with replicating adenovirus type 5 host range mutant-SIV recombinants, followed by boosting with SIV gp120, elicited Abs with ADCC activity against SIV(mac251)-infected cells. In vitro ADCC activity correlated with in vivo reduced acute viremia after a mucosal challenge with pathogenic SIV. Our findings expose ADCC activity as an immune correlate that may be relevant in the rational design of an efficacious vaccine against HIV.

Neutralizing antibody responses to HIV: role in protective immunity and challenges for vaccine design

AIDS continues to be a major health problem throughout the world with a high degree of mortality and morbidity. Therefore, there is an urgent need for an effective anti-HIV vaccine. Although the correlates of protective immunity against infection by HIV remain unidentified, recent studies have demonstrated that both humoral and cellular responses are required for controlling viral replication. Vaccine efforts should therefore aim at developing broad and potent humoral as well as cellular responses. Anti-HIV T-cell responses can be generated both in animals and humans by several vaccine modalities. In contrast, broadly neutralizing antibody responses against HIV have not been elicited by any strategy tested in the clinic thus far. The presence of such responses has the potential to prevent the establishment of infection. If not, the presence of neutralizing antibodies may significantly reduce the number of cells that become infected, therefore reducing the inoculum, which may delay viral spread and allow for a better control of viral replication in the infected host. Finally, cytotoxic T-lymphocytes may facilitate the clearance of virally infected cells. One of the biggest challenges in HIV vaccine development is to design a HIV envelope immunogen that can induce protective neutralizing antibodies effective against the diverse HIV-1 strains that characterize the global pandemic. The focus of this article is to review the importance of antibodies and the strategies that are currently being used for inducing such antibodies.

Construction of artificial virus-like particles exposing HIV epitopes, and the study of their immunogenic properties

One of the major problems in the development of successful recombinant vaccines against human immunodeficiency virus (HIV) is that of correct identification of a safe and effective vaccine delivery system with which to induce protective immunity using soluble protein antigens. An original method for constructing artificial immunogens in the form of spherical particles with yeast dsRNA in the center and hybrid proteins exposing epitopes of an infectious agent on the surface is reported. The dsRNA and the proteins were linked with spermidine-polyglucin-glutathione conjugates. Particles exposing HIV-1 epitopes were constructed, and their immunogenicity tested.

Concordant modulation of neutralization resistance and high infectivity of the primary human immunodeficiency virus type 1 MN strain and definition of a potential gp41 binding site in gp120

Efforts to develop a vaccine against human immunodeficiency virus type 1 (HIV-1) are complicated by resistance of virus to neutralization. The neutralization resistance phenotype of HIV-1 has been linked to high infectivity. We studied the mechanisms determining this phenotype using clones of the T-cell-line-adapted (TCLA) MN strain (MN-TCLA) and the neutralization-resistant, primary MN strain (MN-P). Mutations in the amino- and carboxy-terminal halves of gp120 and the carboxy terminus of gp41 contributed to the neutralization resistance, high-infectivity phenotype but depended upon sequences in the leucine zipper (LZ) domain of gp41. Among 23 clones constructed to map the contributing mutations, there was a very strong correlation between infectivity and neutralization resistance (R(2) = 0.81; P < 0.0001). Mutations that distinguished the gp120s of MN-P and MN-TCLA clones were clustered in or near the CD4 and coreceptor binding sites and in regions distant from those binding sites. To test the hypothesis that some of these distant mutations may interact with gp41, we determined which of them contributed to high infectivity and whether those mutations modulated gp120-gp41 association in the context of MN-P LZ sequences. In one clone, six mutations in the amino terminus of gp120, at least four of which clustered closely on the inner domain, modulated infectivity. This clone had a gp120-gp41 association phenotype like MN-P: in comparison to MN-TCLA, spontaneous dissociation was low, and dissociation induced by soluble CD4 binding was high. These results identify a region of the gp120 inner domain that may be a binding site for gp41. Our studies clarify mechanisms of primary virus neutralization resistance.

Evidence for antibody-mediated enhancement of simian immunodeficiency virus (SIV) Gag antigen processing and cross presentation in SIV-infected rhesus macaques

By using the dominant simian immunodeficiency virus (SIV) Gag Mamu-A01 restricted major histocompatibility complex (MHC) class I epitope p11CM, we demonstrate antibody-mediated enhanced MHC class I cross presentation of SIV Gag. In vitro restimulation of peripheral blood mononuclear cells from SIV-infected rhesus macaques with recombinant full-length SIV Gag p55 plus p55 affinity-purified immunoglobulin G (p55 Gag/p55-IgG) led to the generation of markedly higher frequencies of p11CM specific precursor cytotoxic T lymphocytes (p-CTLs) compared with restimulation with (i) SIV Gag p55 alone or (ii) optimal concentrations of the p11CM peptide alone. These results, along with the finding that CD4 depletion abrogated the enhancement, suggest a prominent role for CD4(+) T cells. Testing for p-CTLs against other Mamu-A01-restricted SIV Gag epitopes suggested that this mechanism favored recognition of the dominant p11CM peptide, potentially further skewing of the CTL response. The p-CTL enhancing effect was also decreased or abrogated by pepsin digestion of the p55-specific IgG or by the addition of monoclonal antibodies to Fc receptor (FcR) II/III, suggesting that the effect was dependent on FcR-mediated uptake of the immune-complexed antigen. Finally, incubation of antigen-presenting cells with SIV Gag p55 immune complexes in the presence of lactacystin or of bafilomycin indicated that the mechanism of antibody-mediated enhancement of cross presentation required both the proteasomal and the endosomal pathways. These data demonstrate for the first time the cross presentation of antigens via immune complexes in lentiviral infection and indicate a heretofore-unrecognized role for antibodies in modulating the magnitude and potentially also the breadth of MHC class I-restricted antigen processing and presentation and CTL responses.

Phage-displayed mimotopes recognizing a biologically active anti-HIV-1 gp120 murine monoclonal antibody

Antibody-dependent cellular cytotoxicity (ADCC) is a host defense mechanism in which Fc receptor-bearing effector cells in combination with antigen-specific antibodies recognize and kill antigen-expressing target cells. The authors previously described a murine monoclonal antibody (MAb-ID6) that mediated ADCC activity against HIV-infected cells. It was demonstrated that the specificity of MAb-ID6 maps to the first 204 amino acids of gp120; however, the exact epitope was not identified. In the present work, by screening phage display libraries with MAb-ID6, the authors have mapped the corresponding epitope to amino acids 86-100 (HIV-1 gp120 sequence). This epitope lies within the C1 region of gp120 and is highly conserved among all subtypes and circulating recombinant forms of HIV-1. Thus, these phage mimotopes of C1 may serve as components of a vaccine for the induction of gp120-specific antibodies mimicking MAb-ID6.

Evidence for a correlation between antibody-dependent cellular cytotoxicity-mediating anti-HIV-1 antibodies and prognostic predictors of HIV infection

Using our gp120/41-expressing, NK cell activity-resistant CEM.NKR cell clones as targets in HIV-1-specific antibody-dependent cellular cytotoxicity (ADCC) assays, we demonstrate here that the serum titers of anti-HIV-1 ADCC antibodies bear a significant (P < 0.05) positive correlation with the peripheral blood CD4+ T cell counts and a negative one with the number of copies of HIV-1 RNA in the plasma of HIV-infected individuals. These findings underscore the importance of these antibodies as a protective immune parameter in these infections.

Primary virus envelope cross-reactivity of the broadening neutralizing antibody response during early chronic human immunodeficiency virus type 1 infection

To test the hypothesis that changing neutralizing antibody responses against human immunodeficiency virus type 1 (HIV-1) during chronic infection were a response to emergence of neutralization escape mutants, we cloned expressed and characterized envelope clones from patients in the Multicenter AIDS Cohort Study (MACS). Pseudotyped HIV-1 envelope clones obtained from differing time points were assessed for sensitivity to neutralization by using sera from different times from the same and different patients. Clones from early and late time points during chronic infection had similar neutralization sensitivity, and neutralizing antibody responses cross-reacted with early, late, and heterologous envelopes. The potential for broadly effective HIV-1 immunization is supported.

Antibody-dependent cell-mediated cytotoxicity can protect PBMC from infection by cell-associated HIV-1

We have developed a novel in vitro assay system to study the role of antibody-dependent cell-mediated cytotoxicity (ADCC) in protection against HIV-1 infection by cell-associated virus. HIV-1-infected NK-resistant cells are mixed with specific antibody and unstimulated PBMC and ADCC is allowed to occur over several hours. The PBMC are then activated and cultured to allow virus replication in newly infected T cells. To ensure that ADCC is the only mechanism by which protection could occur we have used haptenated (TNP) infected cells and anti-hapten antibody. Anti-hapten sera completely protected PBMC from infection by haptenated HIV-1-infected cells in ADCC protection assays. F(ab')2 fragments of anti-hapten IgG showed no protection, confirming that ADCC was responsible for protection by anti-hapten IgG. PCR analysis for HIV-1 DNA confirmed the elimination of infected cells. We believe this to be the first direct demonstration that ADCC alone can protect PBMC from infection by cell-associated HIV-1.

Mutations in both gp120 and gp41 are responsible for the broad neutralization resistance of variant human immunodeficiency virus type 1 MN to antibodies directed at V3 and non-V3 epitopes

The escape of human immunodeficiency virus type 1 from effects of neutralizing antibodies was studied by using neutralization-resistant (NR) variants generated by growing the neutralization-sensitive (NS) wild-type MN virus in the presence of human serum with neutralizing antibodies, more than 99% of which were directed at the V3 region of gp120. The variants obtained had broad neutralization resistance to human sera, without limitation with respect to the V3 specificity of the sera. The molecular basis for the resistance was evaluated with molecularly cloned viruses, as well as with pseudoviruses expressing envelope glycoproteins of the NS and NR phenotypes. Nucleotide sequence analyses comparing NS and NR clones revealed a number of polymorphisms, including six in the V1/V2 region, two in C4/V5 of gp120, three in the leucine zipper (LZ) domain of gp41, and two in the second external putative alpha-helix region of gp41. A series of chimeras from NS and NR env genes was constructed, and each was presented on pseudoviruses to locate the domain(s) which conferred the phenotypic changes. The neutralization phenotypes of the chimeric clones were found to be dependent on mutations in both the C4/V5 region of gp120 and the LZ region of gp41. Additionally, interaction between mutations in gp120 and gp41 was demonstrated in that a chimeric env gene consisting of a gp120 coding sequence from an NS clone and a gp41 sequence from an NR clone yielded a pseudovirus with minimal infectivity. The possible significance of predicted amino acid changes in these domains is discussed. The results indicate that polyvalent antibodies predominantly directed against V3 can induce NR through selection for mutations that alter interactions of other domains in the envelope complex.

gp120-directed antibody-dependent cellular cytotoxicity as a major determinant of the rate of decline in CD4 percentage in HIV-1 disease

OBJECTIVE

To determine the relationship between the rate of CD4 percentage decline and two factors postulated to be associated with CD4 cell destruction: circulating HIV-1 viral load and gp120-directed antibody-dependent cellular cytotoxicity (ADCC).

DESIGN

Four women and 16 men had serial determinations of CD4 percentage gp120-directed ADCC activity [using the cell-mediated cytotoxicity (CMC) assay] natural killer (NK) cell number, spontaneous NK lytic function, and plasma HIV-1 RNA.

METHODS

The rate of decline in CD4 percentage was modeled as a function of gp120-directed ADCC activity and circulating HIV-1 RNA using Pearson correlation and multiple regression analyses.

RESULTS

All individuals had at least four CMC assays performed and two HIV-1 RNA polymerase chain reaction measurements over a median follow-up of 27 months. Although the rate of CD4 percentage decline was associated with either CMC activity (r = -0.53, P = 0.02) or circulating HIV-1 RNA (r = -0.42, P = 0.07), it was strongly correlated with an interaction between CMC and HIV-1 RNA (r = -0.76, P < 0.0001). Mean CMC activity was associated with both mean percentage of circulating NK cells and mean spontaneous NK cell lysis.

CONCLUSIONS

The ability of cells from HIV-infected individuals to mediate gp120-directed ADCC, together with a sufficient circulating viral load, define conditions under which rapid CD4 cell destruction may occur. This relationship between viral load and an HIV-1-specific immune response lends important insights into the central causes of immunodeficiency in AIDS and suggests additional avenues for therapeutic intervention.

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

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

A novel antibody-dependent cellular cytotoxicity epitope in gp120 is identified by two monoclonal antibodies isolated from a long-term survivor of human immunodeficiency virus type 1 infection

Two monoclonal antibodies (MAbs), 42F and 43F, were isolated some 14 months apart from a single long-term survivor of human immunodeficiency virus type 1 (HIV-1) infection. These MAbs were found to be indistinguishable in terms of their isotypes, specificities, affinities, and biological activities. Both 42F and 43F directed substantial antibody-dependent cellular cytotoxicity (ADCC) against cells infected with four divergent lab-adapted strains of HIV-1, but no neutralizing activity against these strains was detectable. The ability of MAbs 42F and 43F, as well as that of MAbs against two other gp120 epitopes, to direct ADCC against uninfected CD4+ cells to which recombinant gp120SF2 had been adsorbed (i.e., "innocent bystanders") was demonstrated to be less efficient by at least an order of magnitude than their ability to direct ADCC against HIV-1-infected cells. Flow cytometry analyses showed that 42F and 43F also bind to native primary isolate Envs from clades B and E expressed on cell surfaces. By direct binding and competition assays, it was demonstrated that the 42F/43F epitope lies in a domain of gp120 outside the previously described CD4-binding site and V3 loop ADCC epitope clusters. Immunoblot analysis revealed that the 42F/43F epitope is not dependent on disulfide bonds or N-linked glycans in gp120. Epitope mapping of 42F and 43F by binding to linear peptides demonstrated specificity of these MAbs for a sequence of 10 amino acids in the C5 domain comprising residues 491 to 500 (Los Alamos National Laboratory numbering for the HXB2 strain). Thus, 42F and 43F define a new ADCC epitope in gp120. Because of the relative conservation of this epitope and the fact that it appears to have been significantly immunogenic in the individual from which these MAbs were derived, it may prove to be a useful component of HIV vaccines. Furthermore, these MAbs may be used as tools to probe the potential importance of ADCC as an antiviral activity in HIV-1 infection.

Development of T-cell lines expressing functional HIV-1 envelope glycoproteins for evaluation of immune responses in HIV-infected individuals

The human T-lymphoid cell line, CEM, was transfected with gp 160 cDNA of human immunodeficiency virus type 1 (HIV-1)pm213. Three clones expressing the envelope glycoproteins (env), designated CEM-213env1, -env4, and -env7, were isolated. These clones expressed high levels of surface gp41 and gp120, as demonstrated by flow cytometry with anti-HIV env monoclonal antibodies. Processing and function of env was shown by induction of syncytia with CD4-expressing HeLa cells and by immunoblot analysis. The env expression resulted in specific down-regulation of surface CD4 levels, supporting the role of HIV env in CD4 modulation. Furthermore, serum samples from nine of nine HIV-1-infected individuals bound specifically to the env-expressing transfectants, substantiating the presence of conserved antigenic determinants. These sera also mediated antibody-dependent cellular cytotoxicity (ADCC) of the env-expressing cell lines. The env-expressing cell lines provide a relevant, safe, and practical model for qualitative and quantitative analysis of humoral and cellular immune responses and their role in HIV-1 pathogenesis and therapy.

Human monoclonal antibody 2G12 defines a distinctive neutralization epitope on the gp120 glycoprotein of human immunodeficiency virus type 1

We have isolated and characterized human monoclonal antibody 2G12 to the gp120 surface glycoprotein of human immunodeficiency virus type 1 (HIV-1). This antibody potently and broadly neutralizes primary and T-cell line-adapted clade B strains of HIV-1 in a peripheral blood mononuclear cell-based assay and inhibits syncytium formation in the AA-2 cell line. Furthermore, 2G12 possesses neutralizing activity against strains from clade A but not from clade E. Complement- and antibody-dependent cellular cytotoxicity-activating functions of 2G12 were also defined. The gp120 epitope recognized by 2G12 was found to be distinctive; binding of 2G12 to LAI recombinant gp120 was abolished by amino acid substitutions removing N-linked carbohydrates in the C2, C3, V4, and C4 regions of gp120. This gp120 mutant recognition pattern has not previously been observed, indicating that the 2G12 epitope is unusual. consistent with this, antibodies able to block 2G12 binding to recombinant gp120 were not detected in significant quantities in 16 HIV-positive human serum samples.

Functional activities of 20 human immunodeficiency virus type 1 (HIV-1)-specific human monoclonal antibodies

Antibodies that are useful in the treatment of HIV infection should result in virus neutralization or lysis of infected cells but should not enhance infection. In this study, the potential clinical use of 20 HIV-1-specific human monoclonal antibodies (HuMAbs) was determined by measuring their enhancing (C-ADE) activities using HIVLAI as the target virus. Two HuMAbs mediated both C-ADE and ADCC, two exclusively neutralized, and five exclusively mediated ADCC. Ten HuMAbs demonstrated no activity in any of the three assays. Three antibodies that neutralized HIVLAI were tested against HIVSF2; all three also neutralized HIVSF2. Four of five HuMAbs mediating ADCC against HIVLAI that were also tested against HIVSF2 had ADCC activity against HIVSF2. These results demonstrate that many HuMAbs have unique functions, allowing the separation of potentially beneficial and harmful activities. Combinations of HuMAbs with ADCC and neutralizing functions may have therapeutic utility.

Broadly reactive HIV-2 and SIVmac specific antibody-dependent cellular cytotoxicity in immunized and infected cynomolgus monkeys

Antibody-dependent cellular cytotoxicity (ADCC) was analysed in groups of cynomolgus monkeys that had been immunized with either HIV-2 (strains SBL6669 or SBL-K135) or SIVmac. HIV-2- and SIVmac-infected monkeys were also studied for ADCC. Sequential serum samples were collected from the animals, which were followed for 1 to 3 years. Sera from the HIV-2-immunized monkeys had ADCC against both homologous and heterologous HIV-2 strains as well as cross-reactivity against SIVmac-infected target cells. This broadly reactive ADCC response could be detected within the first weeks after immunization. Homologous ADCC was also seen in seven of eight SIVmac-immunized monkeys which were all protected from later challenge with SIVmac or SVsm. ADCC titres sometimes decreased after a few months if the immunized monkeys were not boosted whereas most of the HIV-2-and SIVmac-infected monkeys developed a rapid and persistent ADCC response. The presence of ADCC, like the presence of neutralization in previous studies, did not predict whether the immunized monkeys would be protected upon challenge with live virus.

Induction of humoral and cell-mediated anti-human immunodeficiency virus (HIV) responses in HIV sero-negative volunteers by immunization with recombinant gp160

Development of an effective vaccine for prevention of infection with HIV would provide an important mechanism for controlling the AIDS epidemic. In the current study, the first clinical trial of a candidate HIV-1 vaccine initiated in the United States, the safety and immunogenicity of escalating doses (10-1,280 micrograms) of recombinant gp160 (rgp160), were evaluated in 138 HIV-negative volunteers. Maximal antibody responses, as evaluated by ELISA, were seen after immunization with three doses of 1,280 micrograms rgp160. Responses to some specific epitopes of HIV gp160, including the second conserved domain and the CD4 binding site, were seen more frequently than after natural infection. Neutralizing antibodies to the homologous HIV strain, but not heterologous strains, were induced by this regimen. Blastogenic responses to rgp160 were seen in most volunteers receiving at least two doses of > or = 20 micrograms. These envelope-specific T cell responses were also seen against heterologous strains of HIV. No major adverse reactions were seen after immunization. Thus, rgp160 is a safe and immunogenic candidate HIV vaccine; further studies are needed to determine if it will provide any clinical benefit in preventing HIV infection.

Immune response of chimpanzees after immunization with the inactivated whole immunodeficiency virus (HIV-1), three different adjuvants and challenge

Purified whole virus preparations of HIV-1 were produced from supernatants of infected cells and concentrated 5000-fold. After inactivation with formaldehyde, the concentrates were combined with one of three different adjuvants, and used to immunize three groups of three chimpanzees each. The chimpanzees were monitored for HIV-specific humoral and cellular immune responses by ELISA, immunoblot, virus neutralization, delayed-type hypersensitivity, lymphocyte proliferation and antibody-dependent cell-mediated cytotoxicity. Weak and inconsistent responses were observed in animals that received HIV-1 formulated with alum as adjuvant, whereas HIV-1 formulated with incomplete Freund's adjuvant or an experimental adjuvant (BWZL) induced good humoral and cellular immune responses to the virus. The three animals that received HIV-1 with the BWZL adjuvant generated overall the best immune responses; therefore, 2 weeks after the sixth immunization these animals were challenged with infectious HIV-1. Despite the presence of good humoral and cell-mediated immunity, all three immunized animals and a control animal became infected within 4 weeks, as evidenced by repeated isolation of HIV-1 from peripheral blood mononuclear cells and anamnestic antibody responses. The new experimental adjuvant has to be further investigated in other vaccine trials and different animal models.

Variable region gene utilization and mutation in a group of neutralizing murine anti-human immunodeficiency virus type 1 principal neutralizing determinant antibodies

The heavy (VH) and light (VL) chain variable region nucleotide sequences of four neutralizing anti-human immunodeficiency virus type 1 (HIV-1) murine monoclonal antibodies (mAbs) were determined. These mAbs bind to native gp120, recombinant gp120, and a linear HIV-1 principal neutralizing determinant (PND) peptide that spans amino acid 308-328. Three mAbs that bind to the same linear determinant, 110.3, 110.4, and 110.5, all use the same VL gene elements, a VK21 gene and JK2. These three mAbs also share the same VKJK junctional diversity and specific somatic mutations. They have identical VL immunoglobulin gene rearrangement patterns on Southern blot. Two of the antibodies, 110.4 and 110.5, also use the same VH gene elements, SB32-D-JH4, and have identical VD and DJ junctions and N sequences. Two different anti-HIV-1 PND murine mAbs reported by others, BAT123 and 0.5 beta, also use VK21-JK2, and BAT123 also uses the SB32 VH gene element. Although 110.3 uses the same VL region gene as 110.3 and 110.4, it uses a different VH gene that appears to be a member of the 7183 VH family. 110.6, an mAb that recognizes a discrete, overlapping PND compared to 110.3, 110.4, and 110.5, uses entirely different VH and VL gene elements and has unique immunoglobulin VH and VL rearrangement patterns. Our data, taken together with reports of the BAT123 and 0.5 beta mAb sequences, suggest that the murine antibody response to HIV-1 PND may be restricted to a small subset of VH and VL gene elements.