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JAIDS Journal of Acquired Immune Deficiency Syndromes:
doi: 10.1097/01.qai.0000230318.40170.60
Basic Science

An Adenovirus-Based HIV Subtype B Prime/Boost Vaccine Regimen Elicits Antibodies Mediating Broad Antibody-Dependent Cellular Cytotoxicity Against Non-Subtype B HIV Strains

Gómez-Román, V. Raúl PhD*†; Florese, Ruth H PhD*; Peng, Bo PhD*; Montefiori, David C PhD‡; Kalyanaraman, Vaniambadi S PhD§; Venzon, David PhD‖; Srivastava, Indresh PhD†; Barnett, Susan W PhD†; Robert-Guroff, Marjorie PhD*

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Received for publication March 25, 2006; accepted May 22, 2006.

From the *Vaccine Branch, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD; †Novartis Vaccines, Emeryville, CA; ‡Department of Surgery, Duke University Medical Center, Durham, NC; §Advanced BioScience Laboratories, Inc., Kensington, MD; and ‖Biostatistics and Data Management Section, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD.

Supported by National Institute of Allergy and Infectious Diseases, National Institute of Health (NIH) grant P01 AI48225-03 to Chiron Corporation and, in part, by the Intramural Research Program of the NIH, National Cancer Institute, Center for Cancer Research.

Reprints: Marjorie Robert-Guroff, PhD, National Institutes of Health, National Cancer Institute, 41 Medlars Drive, Building 41, Room D804, Bethesda, MD 20892-5065 (e-mail: guroffm@mail.nih.gov).

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Abstract

Summary: Although HIV subtype B predominates in North America and Western Europe, most HIV infections worldwide are non-subtype B. Globally effective AIDS vaccines need to elicit broad immunity against multiple HIV strains. In this study, 10 chimpanzees were intranasally primed sequentially with adenovirus type 5 (Ad5)- and Ad7-HIVMNenv/rev recombinants and boosted twice intramuscularly with heterologous oligomeric HIVSF162 gp140ΔV2 protein in MF59 adjuvant. Sera were evaluated for binding, neutralizing, and antibody-dependent cellular cytotoxicity (ADCC) against HIV clades A, B, C, and CRF01_AE. The vaccine regimen elicited high-titered HIV subtype A, B, C and CRF01_AE gp120-binding antibodies. Sera from 7 of 10 vaccinated chimpanzees cross-neutralized the heterologous South African subtype C primary HIVTV-1 isolate. Significant cross-clade neutralization against other subtype A, C and E isolates was not observed. Sera from all animals mediated ADCC of cells coated with gp120 from HIV subtypes A and B. Nine of 10 animals also exhibited ADCC activity against HIV subtype C and CRF01_AE gp120-coated targets. This subtype B Ad-HIV recombinant prime/envelope protein boost regimen is a promising approach for eliciting broad ADCC activity against diverse HIV clades. Incorporating additional non-subtype B envelope genes and protein boosts in a multivalent strategy may be required to elicit broader neutralizing antibodies against non-subtype B HIV strains.

According to the United Nations Program on AIDS, at least 20 million people have died of AIDS since the beginning of the epidemic; another 38 million are now living with HIV.1 Most HIV infections worldwide involve strains of HIV genetically and antigenically different from the subtype B strains that circulate in the United States and Western Europe.2 HIV-1 subtype C, for example, is the most prevalent HIV subtype, accounting for more than 40% of all HIV infections. Subtype C is mainly distributed in sub-Saharan Africa, India, and parts of China. Seven countries in southern Africa, where subtype C HIV strains are predominant, report prevalence rates greater than 17%; Botswana and Swaziland have prevalence rates greater than 35%.1 Subtype A HIV and circulating recombinant forms (CRFs) are the second most prevalent strains, accounting for 27% of HIV infections worldwide.2 Subtype A CRFs are mainly distributed in West Africa and Asia. Subtype B HIV strains, the third most prevalent subtype in the epidemic, have been estimated to account for 12.3% of all HIV infections.2 Thus, safe and effective prophylactic vaccines are critically needed to control the spread of subtype B HIV and the more prevalent non-subtype B HIV strains circulating worldwide.

Passive immunization with combinations of human monoclonal antibodies derived from people infected with clade B HIV have effectively protected rhesus macaques against intravenous and mucosal challenges with chimeric simian/human immunodeficiency viruses (SHIVs) containing HIV clade B envelopes.3-6 These monoclonal antibodies also neutralize a broad spectrum of non-clade B primary HIV isolates in vitro,7-10 suggesting the potential for broad cross-clade protective efficacy, although passive transfer studies followed by challenge with non-clade B SHIV isolates have not yet been published. The induction of similarly potent cross-clade neutralizing antibodies by active immunization has not been achieved.

We have been pursuing a replicating adenovirus (Ad)-HIV prime/protein subunit boost AIDS vaccine approach. The advantages of using Ad-based vectors for HIV vaccine development have been described in detail.11,12 Replication-defective Ad-HIV recombinants have protected rhesus monkeys from an intravenous SHIV challenge,13 whereas the replication-competent Ad-HIV/gp120 prime-boost vaccine approach has protected chimpanzees from low- and high-dose intravenous challenges with HIV,14 including a primary heterologous HIV isolate.15 The replication-competent Ad-simian immunodeficiency virus (SIV) priming/envelope subunit boosting vaccine approach has also conferred increasing degrees of protection of rhesus macaques from vaginal and rectal challenges with pathogenic SIVmac251 as Ad-SIV multigenic recombinants were incorporated into the vaccine regimen.16-18 Comparisons in chimpanzees of replication-defective and replication-competent Ad-HIVMNenv/rev priming, in combination with oligomeric HIVSF162 gp140ΔV2 boosting, have shown that the replicating recombinants are more effective at eliciting cell-mediated immunity and priming humoral immune responses, including envelope-binding antibodies, antibody-dependent cellular cytotoxicity (ADCC), and neutralizing antibodies against some primary subtype B CCR5-tropic HIV strains.19 The difference in antibody priming was largely overcome after a second intramuscular boost with the gp140ΔV2 subunit.

Neutralization of a subset of HIV subtype B primary isolates observed in the chimpanzee study prompted us to explore further the breadth of the induced humoral immunity against other HIV clades. Here, we report that the HIV subtype B vaccine regimen composed of priming with Ad-HIVMNenv/rev and boosting with HIVSF162 gp140ΔV2 in MF59 adjuvant elicits cross-clade antibody-binding activity, heterologous neutralizing antibodies against the South African subtype C TV1 isolate, and weak neutralizing antibody responses against other non-subtype B HIV strains. The vaccine-induced antibodies are shown to mediate cross-clade ADCC activity at high titers, however. Our results therefore suggest that although this subtype B Ad-HIVMNenv/rev priming/HIVSF162 gp140ΔV2 boosting regimen is a promising vaccine strategy capable of eliciting broad ADCC against different HIV strains circulating worldwide, incorporating additional non-subtype B envelope genes and/or protein boosts into a multivalent vaccine approach may be required to elicit neutralizing antibodies against a broader spectrum of non-subtype B HIV strains.

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METHODS

Animals and Sera

Sera (or plasma) samples from 10 chimpanzees were obtained from a previous study in which cellular and humoral immunity elicited by replication-competent and replication-defective Ad-HIV envelope vaccines were compared. The immunization regimen was described in detail.19 Briefly, the chimpanzees were primed twice intranasally with the indicated doses of Ad5-HIVMNenv/rev and then Ad7-HIVMNenv/rev and were subsequently boosted with 2 intramuscular immunizations of oligomeric SF162 gp140ΔV2 in MF59 adjuvant. Sera and plasma were filter sterilized and heat inactivated before use.

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Cross-Clade-Binding Antibodies

Binding antibody titers were assessed by enzyme-linked immunosorbent assay (ELISA). Test antigens included HIV gp120 proteins produced in CHO cells. The recombinant gp120 proteins were cloned from the following primary isolates: HIV92UG037.8 (clade A), HIVBal (clade B), HIVCzm962M651 (clade C), and HIV93TH976 (clade E)20 as well as HIVTV-1 (clade C).21 Binding titers are defined as the reciprocal serum dilution at which the optical density of the test serum was twice that of the negative control serum diluted at a ratio of 1:50.

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Cross-Clade Neutralizing Antibody Assays

Neutralizing antibody titers against the South African subtype C HIVTV-1 strain were assessed using an M7-luciferase based assay22 and 5000 50% tissue culture infectious dose (TCID50) of the virus isolate. Neutralization titers are the highest serum dilution at which the relative luminescence units (RLUs) were reduced 50% compared with virus control wells after subtraction of background RLU.

Sera at 1:15 dilutions were evaluated for additional neutralizing antibodies against a panel of 7 non-clade B primary HIV isolates. The isolates included 2 subtype A (92RW020 and 92UG03723), 3 subtype C (Du12324, S021, and S08025), and 2 CRF01_AE (CM24326 and CM24427) HIV strains isolated in Rwanda, Uganda, South Africa, Malawi, and Thailand. The CCR5-tropic sexually transmitted viruses were obtained from chronically infected individuals, except for Du123, which was obtained from an early seroconverter. Results are reported as percent neutralization, calculated as percent reduction in RLU relative to a 1:15 dilution of corresponding preimmune serum. Values ≥50% are considered positive.

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Cross-Clade Antibody-Dependent Cellular Cytotoxicity Assays

A rapid fluorometric ADCC assay (RFADCC) was previously described.28 Briefly, human peripheral blood mononuclear cells (PBMCs) were used as effector cells and CEM-natural killer-resistant (NKr) cells (AIDS Research and Reference Reagent Program, National Institute of Allergy and Infectious Diseases [NIAID], National Institutes of Health [NIH]) were used as targets. The target cells were double-labeled with PKH-26 and CFSE [5(6)-carboxyfluorescein diacetate, succinimidyl ester; Molecular Probes, Eugene, OR], and coated with 15 μg of native monomeric gp120 for 1 hour at room temperature, followed by washing. ADCC was allowed to occur for 4 hours at 37°C using an effector/target ratio of 50:1 and serum samples diluted as indicated. Cells were subsequently washed and fixed in paraformaldehyde. Analysis by flow cytometry was carried out within 18 hours. The ADCC titer was defined as the highest 10-fold serum dilution yielding an ADCC activity higher than the mean background ADCC activity of each prebleed serum plus 3 SDs.

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Statistical Analyses

Comparisons between antibody titers elicited by priming with replication-competent and replication-defective Ad recombinants were analyzed using the Wilcoxon rank-sum test. Correlations between antibody-binding titers induced against different HIV clades were analyzed by the Spearman rank-correlation method and corrected for multiple tests by the Sidak method.

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RESULTS

Anti-Envelope-Binding Antibodies

We have previously shown that the Ad-HIVMNenv/rev/HIVSF162 gp140ΔV2 immunization regimen elicited high binding antibody titers in chimpanzee sera against subtype B HIVIIIB and HIVSF162 envelopes.19 The strong recognition of CCR5- and CXCR4-tropic HIV envelopes suggested that the antibodies might possess relatively broad reactivity. We further characterized them for binding to HIV envelopes of different clades. Figure 1 shows that the immunization regimen induced cross-clade gp120-binding antibodies. Replicating and nonreplicating Ad-HIV recombinants primed clade A, B, C, and CRF01_AE gp120-binding antibodies, suggesting that the combination prime/boost vaccine approach generates humoral immune responses directed against highly conserved epitopes. Geometric mean antibody titers were uniformly higher after priming with the replication-competent Ad recombinants compared with the replication-defective recombinants at 2 weeks after the second gp140ΔV2 boost (Table 1A) but did not reach statistical significance. Binding titers to clade A envelope after the second envelope boost were significantly correlated with the clade B-binding titers (P = 0.0001) and the clade C-binding titers (P = 0.0005) (Figs. 2A, B). (By a conservative application of the Sidak statistical method used to account for the multiple titer comparisons, only P values less than 0.001 are significant at the <0.05 level). The significant correlations suggest that further investigation of the epitopes recognized might reveal highly immunogenic conserved envelope epitopes across the 3 HIV clades for future targeting in vaccine design.

Figure 1
Figure 1
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Figure 2
Figure 2
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Table 1
Table 1
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Neutralizing Antibodies

We previously demonstrated that the Ad-HIVenv/rev/gp140ΔV2 vaccine approach elicited neutralizing antibodies against a subset of 8 subtype B HIV primary isolates.19 To evaluate neutralizing antibody breadth further, we first examined neutralization of HIVTV-1, a South African clade C primary isolate. HIVTV-1 anti-envelope-binding antibodies were well recognized in ELISAs by all the chimpanzee sera (Fig. 3A). Seven of 10 chimpanzees also exhibited neutralizing antibody activity against HIVTV-1, with titers ranging from 150 to 540 (see Fig. 3B). Peak neutralizing antibody activity was most frequently observed after the first gp140ΔV2 booster immunization and, in most cases, was no longer detectable after the second envelope boost (see Fig. 3B), although gp120 ELISA titers were boosted. This result suggests that the second protein immunization boosted humoral immune responses against nonneutralizing epitopes and, in general, may not be beneficial for induction of cross-clade neutralizing antibody responses. The lack of correlation between the binding and neutralizing antibody titers further suggests that the neutralizing activity is directed against envelope epitopes not present on the gp120 monomer.

Figure 3
Figure 3
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In contrast to neutralization of clade C HIVTV-1 by 70% of the chimpanzee sera, no positive cross-neutralizing activity (defined as ≥50% neutralization) was observed against the panel of 7 non-clade B primary HIV isolates described in the methods section (data not shown). A rare exception was the serum from chimpanzee A163, which neutralized a clade A isolate from Rwanda (92RW020) at 57%. Serum from this animal was previously shown to exhibit the broadest neutralizing activity against a panel of 8 clade B primary isolates.19

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Antibody-Dependent Cellular Cytotoxicity-Mediating Antibodies

We previously reported that priming rhesus macaques with replicating Ad-SIV followed by SIVgp120 boosting in monophosphoryl lipid A-stable emulsion (MPL-SE) adjuvant elicited envelope-specific-binding antibodies significantly correlated with a reduction in acute-phase viremia after intrarectal challenge with SIVmac251.18 These binding antibodies, which lacked the ability to neutralize the challenge virus, mediated ADCC activity assessed in vitro against SIV-infected target cells. The ADCC activity was also significantly correlated with an in vivo reduction in acute viremia after the SIVmac251 challenge.29 In the chimpanzees studied here, the Ad-HIVenv/rev/gp140ΔV2 vaccine approach also elicited antibodies with ADCC activity against CEM-NKr cells coated with gp120 from the CXCR4-tropic subtype B isolate HIVIIIB19 and the CCR5-tropic HIVBal.28 In view of the broad cross-reactive-binding antibody seen here against HIV envelopes of different clades, we assayed the chimpanzee sera for cross-clade ADCC activity. As shown in Figure 4, the chimpanzee sera mediated ADCC activity against CEM-NKr cells coated with monomeric gp120 derived from primary isolates belonging to HIV clades A, B, C, and CRF01-AE. The Ad-HIV recombinants themselves elicited cross-clade ADCC activity, with somewhat higher titers observed after replicating Ad-HIV recombinant priming compared with priming with nonreplicating Ad recombinants (see Table 1B). After boosting with gp140ΔV2, except for ADCC against targets coated with clade B gp120, the higher titers were seen in the nonreplicating Ad-HIV recombinant group. None of these differences were statistically significant, however. Of interest, antibody titers mediating ADCC activity were higher than corresponding binding antibody titers. We speculate that this result arises because of a conformational envelope epitope recognized by the ADCC-mediating antibodies and uncovered in the envelope protein after binding of the gp120 to CD4 on the CEM-NKr target cells.

Figure 4
Figure 4
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DISCUSSION

To our knowledge, this is the first demonstration of a subtype B prime/boost HIV vaccine approach that elicits neutralizing antibodies to a heterologous clade C isolate (HIVTV-1) and cross-clade-binding and ADCC-mediating antibodies in primates. Only one prime/boost vaccine approach, the AIDS Vaccine Evaluation Group protocol 029 (AVEG 029), has evaluated cross-clade-binding and neutralizing antibody responses in humans.30 This protocol involved priming with canarypox virus expressing Env, Gag, and Pol of HIVMN, followed by boosting with monomeric HIVSF2 gp120 in MF59 adjuvant. Sera from most of the human vaccinees exhibited strong cross-reactivity with V3 peptides derived from HIV clades B, C, and F and extensive cross-clade-binding reactivity against recombinant HIV envelope proteins from clades B, D, and E. The vaccine regimen elicited antibodies with low neutralizing activity (24%-45% neutralization at a 1:20 dilution) against a panel of clade C and clade F isolates, however. In contrast, 70% of the vaccinated chimpanzees described here neutralized the heterologous clade C HIVTV-1 isolate with 50% end point titers as high as 540. Significant breadth against other non-clade B isolates was not observed, suggesting that the clade C HIVTV-1 envelope may share critical neutralizing epitopes with clade B viruses. Neutralization of HIVTV-1 with soluble CD4 (sCD4), monoclonal antibodies, and HIV-positive clade B and C sera shows that it is typical of other HIV primary isolates and not unusually neutralization sensitive. Unlike most clade C viruses, however, it is sensitive to monoclonal 2G12 (D. C. Montefiori, PhD, unpublished data, 2006).

Consistent with previously described increases in breadth and immunogenicity of envelope immunogens on deletion of N-linked glycosylation sites on the V2 loop,31,32 the gp140SF162ΔV2 protein boost clearly contributed to induction of cross-clade humoral responses (see Figs. 1, 3, 4; Table 1). A similar V2 loop deletion of HIVSF162 renders the resulting mutant virus more susceptible to neutralization with sera obtained from patients infected with HIV clades A, B, C, D, E, and F, suggesting that the ΔV2 deletion exposes cross-clade neutralizing epitopes relevant for vaccine design.33

The extent to which the mismatched nature of the prime/boost immunogens contributed to cross-clade antibody development is not known. This question should be examined in appropriately designed studies comparing homologous and heterologous prime/boost immunization strategies. The CXCR4-tropic HIVMN envelope in the Ad recombinants and the CCR5-tropic envelope of the gp140SF162ΔV2 boost induced antibody breadth. It is not unreasonable to hypothesize that such a heterologous vaccine regimen would induce enhanced immunity to epitopes conserved between the mismatched components. In addition, the AVEG 029 human protocol and the chimpanzee study referenced here used protein subunits formulated in MF59 adjuvant. Whether protein immunogens formulated with MF59 better elicit cross-clade humoral immunity compared with other adjuvants is currently under investigation.

In addition to cross-clade neutralizing antibodies, the clade B vaccine regimen elicited cross-clade antibodies mediating envelope-specific ADCC (see Fig. 4). ADCC is relevant to an HIV/AIDS vaccine for several reasons. First, ADCC as a non-major histocompatibility complex (MHC)-restricted immune response is broadly applicable to the human population. Second, ADCC combines adaptive and innate immune defense mechanisms, because killing involves antigen-specific antibodies and nonantigen-specific Fcγ receptor-bearing effectors, such as NK cells, neutrophils, and γδ T cells. Third, ADCC may contribute to protective efficacy, because nonneutralizing antibodies with ADCC activity have been shown to confer protection against cells infected with other enveloped viruses.34 Fourth, in line with this observation, we have shown that ADCC activity correlates with reduced acute-phase viremia in the rhesus macaque SIV model.29 Although canarypox-based prime/boost HIV vaccine approaches have elicited homologous ADCC activity against envelope proteins included in the vaccine components,35,36 this is the first report to demonstrate that antibodies with heterologous ADCC activity against other clades can also be induced by vaccination. It remains to be established whether this cross-clade ADCC activity is relevant in the context of primary HIV-infected cells or an HIV challenge. As in the case of cross-neutralizing antibodies, reliable and standard methods must be implemented to compare the potency, breadth, and relevance of cross-clade ADCC activity elicited by various HIV vaccine candidates adequately.

Whether induction of cross-clade ADCC-mediating antibodies is unique to the immunization regimen used here must be determined by future comparative studies. Not all antibodies able to bind HIV envelope antigens mediate ADCC. ADCC is dependent not only on antibody binding to a specific antigen but on the ability of the antibody to bind Fcγ receptors on the surface of effector cells. In this regard, monoclonal antibody IgG1 b12 mutants with changes in the heavy-chain constant region that diminished binding to complement and Fcγ receptors retained equivalent binding and neutralizing activity against HIV yet differed markedly in their ability to mediate ADCC and complement-dependent cytotoxicity.37 Glycosylation of IgG also affects binding to Fcγ receptors and, as a result, ADCC activity. Depletion of fucose from IgG1 antibodies has enhanced Fcγ receptor binding and ADCC activity.38 Of interest, multiple ovalbumin injections of mice led to increased fucosylation of the Fc domains of anti-ovalbumin antibodies,39 suggesting that repetitive protein immunizations might elicit antibodies with diminished ADCC activity. Overall, a spectrum of antibody characteristics, in addition to binding specificity, has an impact on ADCC activity. With our present knowledge of antibody induction, one cannot predict whether particular immunization strategies are going to elicit potent broad ADCC activity.

Corollaries may be drawn from our ADCC results. Our ADCC assay uses gp120 bound to CD4 molecules on the surface of more than 97% of CEM-NKr target cells.28 Because gp120 is bound via its CD4-binding site, and because cross-clade killing occurs when this binding site is occupied by the CD4 molecule on the target cell, we speculate that the vaccine-induced cross-clade ADCC antibodies must be recognizing epitopes different from the gp120-CD4-binding site. These epitopes may include linear and conformational V3 epitopes and CD4-inducible epitopes exposed on gp120 binding to cell-surface CD4. Further, because ADCC activity is boosted by gp140ΔV2, which lacks an N-linked glycosylation site and 30 amino acids of the V2 loop, we assume that the observed cross-clade ADCC activity is not directed against these 30 amino acids. The fine specificity of these cross-clade ADCC-mediating antibodies remains to be established. Whether boosting with intact gp140, including the V2 loop, would provide additional B-cell epitopes for enhanced induction of antibodies mediating ADCC could be addressed in a future study. Here, the gp140ΔV2 protein, previously shown to elicit broader neutralizing antibodies than intact gp140, was chosen, because our primary goal was to elicit neutralizing antibodies. The induction of antibodies mediating broad ADCC was an added benefit of the vaccine strategy.

Overall, the Ad-HIV/gp140 vaccine regimen did not elicit cross-clade neutralization as consistently as it induced cross-clade binding and ADCC antibodies. Although broader neutralization should continue to be a major goal of vaccination to block HIV infection, nonneutralizing antibodies may contribute to the clearance of virus-infected cells by 1 or more mechanisms, including direct or indirect ADCC,29,40 the related antibody-dependent cell-mediated viral inhibition,41 or antibody-mediated complement-dependent mechanisms.42 Several recent studies have shown that vaccine regimens incorporating envelope immunogens confer better protection than those lacking the envelope component, despite the inability to elicit neutralizing antibodies.43-45 The extent to which this enhanced protective efficacy might be attributable to nonneutralizing antibodies rather than envelope-specific cellular immune responses remains to be determined.

The possibility that nonneutralizing binding antibodies may be detrimental to protection against HIV infection or disease progression should also be considered, however. Antibody-dependent enhancement of infection can occur via complement and Fc receptors.46 Inhibition or enhancement of infection by antibody is dependent on several factors, including antibody titer and affinity for complement and Fc receptors. Recently, Fcγ IIa and IIIa receptor genetic polymorphisms, known to increase avidity of the receptors for immune complexes, were associated with increased risk of HIV infection in individuals vaccinated with recombinant gp120 in the VAX004 trial.47 Among vaccinees with the polymorphism, infected individuals had higher vaccine-induced antibody titers than uninfected individuals, suggesting possible in vivo antibody-dependent enhancement of HIV infection. Thus, delineation of factors that lead to induction of protective as opposed to enhancing antibodies should be an additional goal of vaccine studies.

In summary, vaccination of chimpanzees with a subtype B Ad-HIV/gp140 vaccine approach elicited broad ADCC but limited neutralization against HIV strains representing the most prevalent HIV clades worldwide. We did not evaluate binding and ADCC activity against all the non-clade B isolates assayed for neutralization. Thus, it is possible that the vaccine regimen did not elicit binding antibodies against all the non-clade B isolates tested. Sera from all 10 chimpanzees bound gp120 proteins from clade A (HIV92UG037.8), clade B (HIVBal), and clade C (HIVTV-1) isolates (see Figs. 3, 4), yet not all sera neutralized each isolate. HIV92UG037.8 was not neutralized by any of the 10 sera (D. C. Montefiori, PhD, unpublished data, 2006), Bal was neutralized by 4 of the 10 sera,19 and HIVTV-1 was neutralized by 7 of the 10 sera (see Fig. 3). Although we cannot rule out the possibility that the other non-clade B isolates tested were simply not recognized by the chimpanzee sera, what is clear is that the binding and ADCC responses elicited were broader than the neutralizing antibody response.

We cannot directly assess whether the broad ADCC responses elicited affect AIDS vaccine efficacy, because chimpanzees cannot be challenged with pathogenic HIV. Alternative approaches to evaluate broad protective efficacy would include immunization of rhesus macaques and challenge with pathogenic non-subtype B SHIV isolates or passive antibody transfer studies. Our data extend previous observations on the breadth and potency of humoral immune responses elicited by the clade B Ad-HIV/gp140 vaccine regimen. Incorporating additional non-subtype B envelope genes and protein boosts in a multivalent vaccine combination should elicit broader neutralizing antibodies against non-subtype B HIV strains.

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ACKNOWLEDGMENTS

The authors thank Nelle Cronen for assistance in preparation of the figures. The CEM-NKr reagent was obtained from Dr. Peter Cresswell through the AIDS Research and Reference Reagent Program, Division of AIDS, NIAID, NIH.

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Keywords:

AIDS vaccine; adenovirus; antibody-dependent cellular cytotoxicity; HIV clades; chimpanzee

© 2006 Lippincott Williams & Wilkins, Inc.

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