Serum antibodies to HIV-1 in recombinant vaccinia virus recipients boosted with purified recombinant gp160. NIAID AIDS Vaccine Clinical Trials Network

Fc receptors and the diversity of antibody responses to HIV infection and vaccination

The development of an effective vaccine against HIV is desperately needed. The successive failures of HIV vaccine efficacy trials in recent decades have shown the difficulty of inducing an appropriate protective immune response to fight HIV. Different correlates of antibody parameters associated with a decreased risk of HIV-1 acquisition have been identified. However, these parameters are difficult to reproduce and improve, possibly because they have an intricate and combined action. Here, we describe the numerous antibody (Ab) functions associated with HIV-1 protection and report the interrelated parameters regulating their complex functions. Indeed, besides neutralizing and Fc-mediated activity, additional factors such as Ab type, concentration and kinetics of induction, and Fc-receptor expression and binding capacity also influence the protective effect conferred by Abs. As these parameters were described to be associated with ethnicity, age and sex, these additional factors must be considered for the development of an effective immune response. Therefore, future vaccine designs need to consider these multifaceted Ab functions together with the demographic attributes of the patient populations.

Encoded novel forms of HSP70 or a cytolytic protein increase DNA vaccine potency

In humans, DNA vaccines have failed to demonstrate the equivalent levels of immunogenicity that were shown in smaller animals. Previous studies have encoded adjuvants, predominantly cytokines, within these vaccines in an attempt to increase antigen-specific immune responses. However, these strategies have lacked breadth of innate immune activation and have led to disappointing results in clinical trials. Damage associated molecular patterns (DAMPs) have been identified as pattern recognition receptor (PRR) agonists. DAMPs can bind to a wide range of PRRs on dendritic cells (DCs) and thus our studies have aimed to utilize this characteristic to act as an adjuvant in a DNA vaccine approach. Specifically, HSP70 has been identified as a DAMP, but has been limited by its lack of accessibility to PRRs in and on DCs. Here, we discuss the promising results achieved with the inclusion of membrane-bound or secreted HSP70 into a DNA vaccine encoding HIV gag as the model immunogen.

DNA vaccines encoding membrane-bound or secreted forms of heat shock protein 70 exhibit improved potency

Traditional vaccine strategies are inefficient against challenge with complex pathogens including HIV; therefore, novel vaccine technologies are required. DNA vaccines are attractive as they are relatively cheap and easy to manufacture, but a major limitation has been their lack of immunogenicity in humans, which may be overcome with the incorporation of an adjuvant. HSP70 is a recognised damage-associated molecular pattern, which is a potential adjuvant. We investigated the immunogenicity of a DNA vaccine encoding HIV gag and HSP70; the latter was genetically modified to produce cytoplasmic, secreted or membrane-bound HSP70, the expression of which was controlled by an independent promoter. The DNA was administered to C57BL/6 mice to evaluate gag-specific T-cell responses. Our results demonstrated the ability of membrane-bound and secreted HSP70 to significantly enhance gag-specific T-cell responses and increase the breadth of T-cell responses to include subdominant epitopes. Membrane-bound or secreted HSP70 also significantly improved the multifunctionality of HIV-specific T cells and T-cell proliferation, which is important for maintaining T-cell integrity. Most importantly, the inclusion of membrane-bound HSP70, secreted HSP70 or a combination significantly increased protection in mice challenged with EcoHIV, a chimeric virus that replicates in mouse leukocytes in vivo.

Soluble HIV-1 Env trimers in adjuvant elicit potent and diverse functional B cell responses in primates

Broadly neutralizing antibodies (bNAbs) against the HIV-1 envelope glycoproteins (Envs) have proven difficult to elicit by immunization. Therefore, to identify effective Env neutralization targets, efforts are underway to define the specificities of bNAbs in chronically infected individuals. For a prophylactic vaccine, it is equally important to define the immunogenic properties of the heavily glycosylated Env in healthy primates devoid of confounding HIV-induced pathogenic factors. We used rhesus macaques to investigate the magnitude and kinetics of B cell responses stimulated by Env trimers in adjuvant. Robust Env-specific memory B cell responses and high titers of circulating antibodies developed after trimer inoculation. Subsequent immunizations resulted in significant expansion of Env-specific IgG-producing plasma cell populations and circulating Abs that displayed increasing avidity and neutralization capacity. The neutralizing activity elicited with the regimen used was, in most aspects, superior to that elicited by a regimen based on monomeric Env immunization in humans. Despite the potency and breadth of the trimer-elicited response, protection against heterologous rectal simian-HIV (SHIV) challenge was modest, illustrating the challenge of eliciting sufficient titers of cross-reactive protective NAbs in mucosal sites. These data provide important information for the design and evaluation of vaccines aimed at stimulating protective HIV-1 immune responses in humans.

The role of antibodies in HIV vaccines

Licensed vaccines against viral diseases generate antibodies that neutralize the infecting virus and protect against infection or disease. Similarly, an effective vaccine against HIV-1 will likely need to induce antibodies that prevent initial infection of host cells or that limit early events of viral dissemination. Such antibodies must target the surface envelope glycoproteins of HIV-1, which are highly variable in sequence and structure. The first subunit vaccines to enter clinical trails were safe and immunogenic but unable to elicit antibodies that neutralized most circulating strains of HIV-1. However, potent virus neutralizing antibodies (NAbs) can develop during the course of HIV-1 infection, and this is the type of antibody response that researchers seek to generate with a vaccine. Thus, current vaccine design efforts have focused on a more detailed understanding of these broadly neutralizing antibodies and their epitopes to inform the design of improved vaccines.

Toward effective HIV vaccination: induction of binary epitope reactive antibodies with broad HIV neutralizing activity

We describe murine monoclonal antibodies (mAbs) raised by immunization with an electrophilic gp120 analog (E-gp120) expressing the rare ability to neutralize genetically heterologous human immunodeficiency virus (HIV) strains. Unlike gp120, E-gp120 formed covalent oligomers. The reactivity of gp120 and E-gp120 with mAbs to reference neutralizing epitopes was markedly different, indicating their divergent structures. Epitope mapping with synthetic peptides and electrophilic peptide analogs indicated binary recognition of two distinct gp120 regions by anti-E-gp120 mAbs, the 421-433 and 288-306 peptide regions. Univalent Fab and single chain Fv fragments expressed the ability to recognize both peptides. X-ray crystallography of an anti-E-gp120 Fab fragment revealed two neighboring cavities, the typical antigen-binding cavity formed by the complementarity determining regions (CDRs) and another cavity dominated by antibody heavy chain variable (V(H)) domain framework (FR) residues. Substitution of the FR cavity V(H) Lys-19 residue by an Ala residue resulted in attenuated binding of the 421-433 region peptide probe. The CDRs and V(H) FR replacement/silent mutation ratios exceeded the ratio for a random mutation process, suggesting adaptive development of both putative binding sites. All mAbs studied were derived from V(H)1 family genes, suggesting biased recruitment of the V gene germ line repertoire by E-gp120. The conserved 421-433 region of gp120 is essential for HIV binding to host CD4 receptors. This region is recognized weakly by the FR of antibodies produced without exposure to HIV, but it usually fails to induce adaptive synthesis of neutralizing antibodies. We present models accounting for improved CD4-binding site recognition and broad HIV neutralizing activity of the mAbs, long sought goals in HIV vaccine development.

Increased human immunodeficiency virus type 1 Env expression and antibody induction using an enhanced alphavirus vector

Viral vectors encoding heterologous vaccine antigens are potent inducers of cellular immune responses, but they are generally less efficient at stimulating humoral immunity. To improve the induction of antibody responses by Semliki Forest virus-based vaccines, a vector encoding a translation-enhancer element and a novel internal signal sequence for increased expression and secretion of soluble antigens was designed. Approximately tenfold more human immunodeficiency virus type 1 gp120 was secreted into culture supernatants of infected cells using the enhanced vector compared with the parental vector. This translated into a significant increase in gp120-specific antibodies in immunized mice, suggesting that antigen-expression levels from the parental vector are limiting for induction of antibody responses. These data encourage the use of the enhanced vector for elicitation of immune responses against heterologous antigens during vaccination.

Efficient protein boosting after plasmid DNA or recombinant adenovirus immunization with HIV-1 vaccine constructs

DNA plasmids and recombinant adenovirus serotype-5 (rAd5) vectors are being studied in human clinical trials as HIV-1 vaccine candidates. Each elicits robust T-cell responses and modest antibody levels. Since protein immunization alone elicits antibody but not CD8 T-cell responses, we studied protein boosting of DNA and rAd5 HIV-1 vaccine vectors. A single Env protein immunization provided a marked boost in antibody titer in guinea pigs primed with either DNA or rAd5 vaccines, and the resulting antibody binding and neutralization levels were similar to those attained after thee sequential protein immunizations. Since both T-cell immunity and neutralizing antibodies are thought to be required for protection against HIV-1, it may be possible to establish a balanced T-cell and antibody response with appropriate vectored vaccines and improve the neutralizing antibody titer with protein boosting.

Statistical evaluation of HIV vaccines in early clinical trials

The HIV pandemic is a pressing threat to global public health; HIV vaccine development is critical. Clinical evaluation of HIV vaccine candidates differs from the standard therapeutics trial framework primarily due to the fact that healthy individuals are studied. We present an early stage evaluation program developed for the HIV Vaccine Trials Network (HVTN) motivated by characteristics unique to the vaccine setting. The program consists of 3 prototypical stages (Phase I, Ib, II) that provide a unified yet flexible approach to the safety and immunogenicity evaluation of diverse vaccine regimens. The goal of these early trials is to narrow the number of candidate vaccines to the most promising candidates worthy of further study in efficacy trials.

Vaccines for viral and parasitic diseases produced with baculovirus vectors

The baculovirus-insect cell expression system is an approved system for the production of viral antigens with vaccine potential for humans and animals and has been used for production of subunit vaccines against parasitic diseases as well. Many candidate subunit vaccines have been expressed in this system and immunization commonly led to protective immunity against pathogen challenge. The first vaccines produced in insect cells for animal use are now on the market. This chapter deals with the tailoring of the baculovirus-insect cell expression system for vaccine production in terms of expression levels, integrity and immunogenicity of recombinant proteins, and baculovirus genome stability. Various expression strategies are discussed including chimeric, virus-like particles, baculovirus display of foreign antigens on budded virions or in occlusion bodies, and specialized baculovirus vectors with mammalian promoters that express the antigen in the immunized individual. A historical overview shows the wide variety of viral (glyco)proteins that have successfully been expressed in this system for vaccine purposes. The potential of this expression system for antiparasite vaccines is illustrated. The combination of subunit vaccines and marker tests, both based on antigens expressed in insect cells, provides a powerful tool to combat disease and to monitor infectious agents.

Maternal plasma viral load and neutralizing/enhancing antibodies in vertical transmission of HIV: a non-randomized prospective study

Background

We examined the association and interaction between maternal viral load and antibodies in vertical transmission of HIV in a non-randomized prospective study of 43 HIV-1 infected pregnant women who attended the San Juan City Hospital, Puerto Rico, and their 45 newborn infants. The women and infants received antiretroviral therapy.

Methods

A nested PCR assay of the HIV-1 envelope V3 region and infant PBMC culture were performed to determine HIV status of the infants. Maternal and infant plasma were tested for HIV neutralization or enhancement in monocyte-derived macrophages.

Results

Twelve (26.7%) infants were positive by the HIV V3 PCR assay and 3 of the 12 were also positive by culture. There was a trend of agreement between high maternal viral load and HIV transmission by multivariate analysis (OR = 2.5, CI = 0.92, p = 0.0681). Both maternal and infant plasma significantly (p = 0.001 for both) reduced HIV replication at 10^-1 dilution compared with HIV negative plasma. Infant plasma neutralized HIV (p = 0.001) at 10^-2 dilution but maternal plasma lost neutralizing effect at this dilution. At 10^-3 dilution both maternal and infant plasma increased virus replication above that obtained with HIV negative plasma but only the increase by maternal plasma was statistically significant (p = 0.005). There were good agreements in enhancing activity in plasma between mother-infant pairs, but there was no significant association between HIV enhancement by maternal plasma and vertical transmission.

Conclusion

Although not statistically significant, the trend of association between maternal viral load and maternal-infant transmission of HIV supports the finding that viral load is a predictor of maternal-infant transmission. Both maternal and infant plasma neutralized HIV at low dilution and enhanced virus replication at high dilution. The antiretroviral treatments that the women received and the small sample size may have contributed to the lack of association between HIV enhancement by maternal plasma and vertical transmission.

Neutralization and enhancement of HIV-1 infection by sera from HIV-1 infected individuals who progress to disease at different rates

We examined the neutralizing/enhancing activity in sera collected at an early and a later time point postinfection from 13 HIV-positive nonprogressors, 13 moderate progressors, and 13 rapid progressors to determine the relationship between neutralizing/enhancing activity and disease progression. Early sera from each group reduced virus replication at low dilutions (10(-1) to 10(-2)) when compared with negative sera. The reduction was statistically significant for moderate and rapid progressors at 10(-1) dilution (P = 0.02 and P = 0.02 respectively) but not for nonprogressors (P = 0.16). Late sera from nonprogressors and moderate progressors reduced virus replication at low dilution but late sera from rapid progressors lost neutralizing activity. These data suggest that an association exists between neutralizing activity in sera and nonprogression or slower progression to disease and that loss of neutralizing activity is associated with disease progression. At higher dilutions (10(-3) to 10(-6)), both early and late sera from each group increased virus replication over negative sera. The levels and frequency of enhancement were higher for sera from a subgroup of nonprogressors than a subgroup of rapid progressors who exhibited enhancement. This suggests that enhancement is not associated with disease progression. The neutralizing/enhancing activity observed in sera of these three groups of subjects suggest that enhancement levels may reflect the overall level of antibody response to HIV. The replication patterns observed for early and late sera from individuals in the different groups reflect changes in antibody activity that appear to be associated with protection or disease progression.

The diversity of T-cell co-stimulation in the induction of antitumor immunity

T-cell activation has now been shown to require at least two signals. The first signal is antigen-specific, is delivered through the T-cell receptor (TCR) via the peptide/major histocompatibility complex (MHC), and causes the T cell to enter the cell cycle. The second, or co-stimulatory, signal is required for cytokine production and proliferation, and is mediated through ligand interaction on the surface of the T cell. This chapter deals with: 1) comparative studies on the use of a dual-gene construct of a recombinant vaccinia (rV) vector containing a tumor-associated antigen (TAA) gene and a co-stimulatory molecule gene vs the use of admixtures of rV-TAA and rV containing the co-stimulatory molecule to induce anti-tumor immunity; 2) the use of an admixture of vaccinia viruses containing a TAA gene and the B7-1 co-stimulatory molecule gene to induce a therapeutic response in a lung metastasis tumor model; 3) the antitumor efficacy of whole-tumor-cell vaccines in which the B7-1 co-stimulatory molecule is expressed in a tumor-cell vaccine via a vaccinia vs a retroviral vector; 4) the use of recombinant poxviruses containing the genes for the co-stimulatory molecules ICAM-1 or LFA-3 to induce antitumor immunity; and 5) the use of poxvirus vectors containing a triad of co-stimulatory molecules (B7-1, ICAM-1 and LFA-3) that synergize to enhance both CD4+ and CD8+ T-cell responses to a new threshold.

Complement-mediated enhancement of HIV-1 infection in peripheral blood mononuclear cells

We investigated if complement-mediated enhancement of HIV infection occurs in peripheral blood mononuclear cells (PBMC). In 7 experiments, we evaluated the effect of human complement on HIVIIIB infection in vitro. We measured HIV antigen production on day 4 and found that pre-incubation of HIV with complement led to enhanced production of antigen with a median enhancement of 2.5-fold (range 1.1-6.8). This complement-mediated increase in antigen production was statistically significant (p < 0.02). Complement-mediated enhancement of HIV infection was also tested in CD4 cells enriched from PBMC, and CD4 cells persistently gave higher levels of infection enhancement than PBMC. Thus, CD4 cells appear to be sufficient for complement-mediated enhancement of HIV infection to occur. In addition, we tested if it was possible to detect complement-mediated enhancement of primary HIV isolates in PBMC. We tested 3 isolates and found only a minor effect on antigen production (median enhancement 1.2-fold, range 0.6-1.5). Furthermore, addition of HIV-specific antibodies in combination with complement resulted in enhanced antigen production in 2/3 sera tested. However, the combination of complement and antibodies resulted in only a minor increase in enhancement of HIV infection compared to that obtained with complement alone. Finally, we found evidence of complement-mediated enhancement of HIV infection in resting PBMC. In conclusion, we demonstrated that complement-mediated enhancement of HIV infection does occur in vitro in PBMC.

Restricted specificity of anti-V3 antibodies induced in humans by HIV candidate vaccines

We analyzed the fine specificity of anti-V3 antibodies elicited in three different species (human, guinea pig, and macaque) by various HIV candidate vaccines. Following immunization with recombinant canarypox virus expressing gp160MN or with recombinant gp160MN/LAI, this antibody response was shown to be directed against the NH2-terminal region of the V3 loop. Although this response was increased by a prime-boost regimen using immunization with canarypox expressing gp160 followed by an rgp160 boost, its specificity remained restricted mainly to the recognition of this region of the V3 loop. Pepscan analysis of sera confirmed the results obtained by ELISA and allowed the definition of an immunodominant common binding site for these sera located within the sequence NKRKRIHIGPGR. In contrast to these results, a boost with the V3 peptide was shown to broaden the antibody response and pepscan analysis showed that sera from individuals boosted with the V3 synthetic peptide recognize determinants all along the V3 loop. Similar fine specificity of anti-V3 antibodies was obtained in human, guinea pig, and macaque following immunization by a prime-boost regimen using canarypox recombinants expressing gp160 or gp120 and purified rgp160. In contrast, a V3 synthetic peptide boost stimulated the production of antibodies that recognize multiple epitopes within the V3 loop. Because the induction of antibodies that recognize multiple sites in the V3 loop could be of major importance to neutralize different HIV isolates, these results may have implications for the design and selection of HIV candidate vaccines.

The enhanced immune response to the HIV gp160/LAMP chimeric gene product targeted to the lysosome membrane protein trafficking pathway

The lysosome-associated membrane proteins (LAMP), found in the outer membrane of lysosomes and also in a multilaminar compartment that contains major histocompatibility complex class II (MHC II) proteins, are directed to their localization by a cytoplasmic carboxyl-terminal sequence. Our studies of the immune response to LAMP-targeted proteins has led to the application of a HIV-1 gp160/LAMP chimeric gene as a novel means to enhance the MHC II presentation of gp160. Immunofluorescence microscopy confirmed that the gp160/LAMP protein had a cellular localization corresponding to that of lysosomes. Pulse-chase analysis confirmed that the rates of synthesis of gp160/LAMP and wild type gp160 were comparable and that both proteins were processed to gp120 at similar rates. However, the gp160/LAMP was degraded more rapidly than the wild type gp160. MHC II-mediated T cell proliferation assays performed with cloned human cell lines showed that gp160/LAMP stimulated greater responses than did the wild type gp160. Moreover, mice vaccinated with recombinant vaccinia expressing gp160/LAMP had greater gp160-specific lymphoproliferation responses and higher titers of anti-V3 loop antibodies than mice vaccinated with recombinant vaccinia expressing wild type gp160.

Immunization trial of cats with a replication-defective adenovirus type 5 expressing the ENV gene of feline immunodeficiency virus

Our aim was to develop a recombinant replication-defective adenovirus suitable for the vaccination of cats against feline immunodeficiency virus. We first demonstrated that this vector was able to transfer a marker gene (E. coli beta-galactosidase) in feline cells in vitro. We then constructed an adenovirus type 5 expressing the Feline Immunodeficiency Virus (FIV) envelope (ENV) gene of the Wo isolate in the absence of the rev gene (Ad-ENV-Wo). Ad-ENV-Wo was then tested in four cats in a 3 injections scheme (at day 0, day 30 and day 210). Four other control cats received Ad-gp50, a similar recombinant adenovirus expressing gp50 (Ad-gp50) of pseudorabies virus (PRV). Viruses were formulated in two different kind of oil adjuvants (water/oil and water/oil/water), a protocol previously shown to enhance the immune response against the virus-induced protein. The control cats developed neutralizing antibodies against PRV, demonstrating the potency of recombinant human adenovirus 5 (Ad5) as a vector in cats. Antibody responses appeared after the first injection and were higher with the water/oil/water formulation than with the water/oil controls. However, none of the four cats vaccinated with Ad-ENV-Wo developed antibodies against two peptides of the envelope protein. Animals were challenged with 20 infectious doses 50% of the strain Wo. All of them developed antibodies against FIV within 4 to 5 weeks, and FIV virus could be isolated from all.

Studies of complement-activating antibodies in the SIV/macaque model of acute primary infection and vaccine protection

Questions regarding the potential impact of complement-activating antibodies on lentivirus pathogenesis and vaccine development were addressed in the SIV/macaque model by evaluating sera for activity related to complement-mediated, antibody-dependent enhancement (C'-ADE) of SIV infection in vitro. C'-ADE activity in sera obtained during acute primary infection in macaques inoculated with SIVmac251 appeared before neutralizing antibodies and coincided with the initial peak and decline of plasma antigenemia. The power of C'-ADE activity (i.e., virus production measured by p24 immunoassay) decreased as titers of neutralizing antibodies increased in these animals, suggesting a balance in the net effect between C'-ADE and neutralizing activities in vitro. Antibodies with C'-ADE activity were also induced in macaques immunized with live-attenuated SIVmac239/nef-deletion or primed with recombinant SIVmne gp120 vaccinia virus and boosted with SIVmne rgp160. The titer (i.e., last serum dilution to show enhancement), peak (i.e., serum dilution producing the greatest enhancement as measured by p24 production), and power (i.e., magnitude of p24 production at the peak titer) of C'-ADE activity in sera obtained from vaccinated macaques on the day of challenge were comparable to those of sera from infected macaques and showed no correlation with vaccine outcome, where some protected animals had C'-ADE profiles that resembled those of unprotected animals. The results of these studies suggest that antibodies having C'-ADE activity in vitro could contribute to virus replication or, alternatively, to virus clearance during the acute stage of SIV infection in macaques.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased adhesion as a mechanism of antibody-dependent and antibody-independent complement-mediated enhancement of human immunodeficiency virus infection

Enhancement of human immunodeficiency virus (HIV) infection by complement alone or in conjunction with antibodies was studied experimentally and theoretically. Experimental studies showed that while HIV-positive sera neutralize HIV infection, the addition of fresh complement abrogated neutralization and could even cause enhancement. Enhancement was blocked by anti-complement receptor 2 antibodies, and infection under enhancing conditions could be blocked by soluble CD4. Antibody-dependent complement-mediated enhancement (C'ADE) was dependent on the alternative complement activation pathway, as factor B-deficient serum could enhance only after the addition of factor B. The observed enhancement was also antibody dependent, since the addition of antibodies increased the level of enhancement. Under C'ADE conditions, infection reached a plateau within 5 min and was not caused by activation of cells by factors in the human serum. On the contrary, preincubation of cells with complement decreased the level of enhancement. A theoretical model of HIV infection in vitro which exhibited similar enhancement in an antibody- and complement concentration-dependent way was developed. Model studies indicated that the enhanced infection process could be explained by the fact that virions, because of complement deposition on the surface, bind more efficiently to cells. The model also indicated that the saturation of the enhanced infection process seen after a few minutes could be caused by saturation of the complement receptors. The effect of neutralizing antibodies can thus be overcome by the enhancing effect of complement that facilitates the contact between gp120 and CD4. These studies demonstrate that the main features of the complement-dependent enhancement phenomenon can be understood in terms of a simple mathematical model.

Vaccine-induced antibodies to native and recombinant human immunodeficiency virus type 1 envelope glycoproteins. NIAID AIDS Vaccine Clinical Trials Network

The ability of antibody induced by combination vaccination (human immunodeficiency virus type 1 (HIV-1LAV) gp160 live recombinant vaccinia virus priming followed by a booster injection with a baculovirus-expressed HIV-1LAV recombinant gp160 candidate vaccine) to bind to native and recombinant HIV-1 envelope glycoproteins was measured in 12 uninfected healthy adult volunteers. By using a flow cytometric indirect immunofluorescence assay (FIFA) to detect vaccine-induced antibody to native envelope glycoprotein expressed by target cells infected with HIV-1IIIB (infected-cell FIFA), sera from ten of 12 vaccinees before the rgp160 booster injection were positive, and all vaccinees were positive at higher levels after the rgp160 boost. Evidence for cross-reacting antibody to HIV-1MN, HIV-1RF and HIV-1CC expressed on infected cells was also present after the rgp160 booster injection. High titres of anti-rgp160 antibody were also measured in an enzyme-linked immunosorbent assay after the boost. None of the sera obtained immediately prior to the rgp160 booster injection, but all sera drawn after the boost, reacted with recombinant gp160 antigen bound to uninfected cell surfaces. The high anti-gp160 binding activity in these assays, the concomitant presence of positivity in infected-cell and rgp160-coated cell FIFA assays, and high anti-rgp160 antibody titre by ELISA in sera from recipients of this prime-boost vaccination regimen suggest that the live vector priming and rgp160 boosting strategy should be pursued in HIV-1 vaccine development.

V3-specific neutralizing antibodies in sera from HIV-1 gp160-immunized volunteers block virus fusion and act synergistically with human monoclonal antibody to the conformation-dependent CD4 binding site of gp120. NIH-NIAID AIDS Vaccine Clinical Trials Network

Sera from 11 volunteers immunized with a recombinant HIV-1 gp160-expressing vaccinia virus (HIVAC-1e; Oncogen/Bristol-Myers Squibb, Seattle, WA) and boosted with baculovirus-derived rgp160 (VaxSyn; MicroGeneSys, Inc., Meriden, CT) were evaluated for functional serum antibodies and their epitopes. Sera obtained prior to boosting had undetectable HIV-1-specific IgG and neutralizing activity, and did not block HIV-1 from binding or fusing to CD4+ MT-2 cells. 14 d after boosting, sera from each volunteer contained HIV-1-specific IgG titers of 1:40 to 1:1,280. Five of these sera also contained neutralizing antibodies, where most or all neutralizing activity was blocked by a synthetic peptide corresponding to amino acids 307-330 of the V3 loop of gp120, indicating that neutralizing antibodies were mostly V3 loop-specific. All sera obtained after boosting contained HIV-1 binding/fusion-inhibition antibodies, and a significant portion of their activity was blocked by the V3 loop peptide, a result consistent with the presence of antibodies against the region of the V3 loop that participates in fusion. Three sera with V3 loop-specific neutralizing and fusion-inhibition antibodies were studied further. In competitive antibody binding experiments, antibodies reactive with the conformation-dependent, CD4 binding site of gp120 were undetectable in each serum. When evaluated in combination with a monoclonal antibody to the CD4 binding site of gp120, two sera demonstrated synergism in neutralizing assays, and all three sera demonstrated synergism in binding/fusion-inhibition assays, further indicating that the functional antibodies were primarily V3 loop-specific. The synergism also suggests that a vaccine that elicits strong serum antibody responses to both regions of gp120 may improve the potential for inducing protective immunity.