Comparison of the antibody repertoire generated in healthy volunteers following immunization with a monomeric recombinant gp120 construct derived from a CCR5/CXCR4-using human immunodeficiency virus type 1 isolate with sera from naturally infected individuals

Genetic diversity of pneumococcal surface protein A in invasive pneumococcal isolates from Korean children, 1991-2016

Pneumococcal surface protein A (PspA) is an important virulence factor of pneumococci and has been investigated as a primary component of a capsular serotype-independent pneumococcal vaccine. Thus, we sought to determine the genetic diversity of PspA to explore its potential as a vaccine candidate. Among the 190 invasive pneumococcal isolates collected from Korean children between 1991 and 2016, two (1.1%) isolates were found to have no pspA by multiple polymerase chain reactions. The full length pspA genes from 185 pneumococcal isolates were sequenced. The length of pspA varied, ranging from 1,719 to 2,301 base pairs with 55.7–100% nucleotide identity. Based on the sequences of the clade-defining regions, 68.7% and 49.7% were in PspA family 2 and clade 3/family 2, respectively. PspA clade types were correlated with genotypes using multilocus sequence typing and divided into several subclades based on diversity analysis of the N-terminal α-helical regions, which showed nucleotide sequence identities of 45.7–100% and amino acid sequence identities of 23.1–100%. Putative antigenicity plots were also diverse among individual clades and subclades. The differences in antigenicity patterns were concentrated within the N-terminal 120 amino acids. In conclusion, the N-terminal α-helical domain, which is known to be the major immunogenic portion of PspA, is genetically variable and should be further evaluated for antigenic differences and cross-reactivity between various PspA types from pneumococcal isolates.

Repeated Vaccination of Cows with HIV Env gp140 during Subsequent Pregnancies Elicits and Sustains an Enduring Strong Env-Binding and Neutralising Antibody Response

An important feature of a potential vaccine against HIV is the production of broadly neutralising antibodies (BrNAbs) capable of potentially blocking infectivity of a diverse array of HIV strains. BrNAbs naturally arise in some HIV infected individuals after several years of infection and their serum IgG can neutralise various HIV strains across different subtypes. We previously showed that vaccination of cows with HIV gp140 AD8 trimers resulted in a high titre of serum IgG against HIV envelope (Env) that had strong BrNAb activity. These polyclonal BrNAbs concentrated into the colostrum during the late stage of pregnancy and can be harvested in vast quantities immediately after calving. In this study, we investigated the effect of prolonged HIV gp140 vaccination on bovine colostrum IgG HIV Env-binding and BrNAb activity over subsequent pregnancies. Repeated immunisation led to a maintained high titre of HIV Env specific IgG in the colostrum batches, but this did not increase through repeated cycles. Colostrum IgG from all batches also strongly competed with sCD4 binding to gp140 Env trimer and with human-derived monoclonal VRC01 and b12 BrNAbs that bind the CD4 binding site (CD4bs). Furthermore, competition neutralisation assays using RSC3 Env gp120 protein core and a derivative CD4bs mutant, RSC3 Δ371I/P363N, showed that CD4bs neutralising antibodies contribute to the neutralising activity of all batches of purified bovine colostrum IgG. This result indicates that the high IgG titre/avidity of anti-CD4bs antibodies with BrNAb activity was achieved during the first year of vaccination and was sustained throughout the years of repeated vaccinations in the cow tested. Although IgG of subsequent colostrum batches may have a higher avidity towards the CD4bs, the overall breadth in neutralisation was not enhanced. This implies that the boosting vaccinations over 4 years elicited a polyclonal antibody response that maintained the proportion of both neutralising and non-neutralising CD4bs antibodies.

Recombinant HIV envelope proteins fail to engage germline versions of anti-CD4bs bNAbs

Vaccine candidates for HIV-1 so far have not been able to elicit broadly neutralizing antibodies (bNAbs) although they express the epitopes recognized by bNAbs to the HIV envelope glycoprotein (Env). To understand whether and how Env immunogens interact with the predicted germline versions of known bNAbs, we screened a large panel (N:56) of recombinant Envs (from clades A, B and C) for binding to the germline predecessors of the broadly neutralizing anti-CD4 binding site antibodies b12, NIH45-46 and 3BNC60. Although the mature antibodies reacted with diverse Envs, the corresponding germline antibodies did not display Env-reactivity. Experiments conducted with engineered chimeric antibodies combining the mature and germline heavy and light chains, respectively and vice-versa, revealed that both antibody chains are important for the known cross-reactivity of these antibodies. Our results also indicate that in order for b12 to display its broad cross-reactivity, multiple somatic mutations within its VH region are required. A consequence of the failure of the germline b12 to bind recombinant soluble Env is that Env-induced B-cell activation through the germline b12 BCR does not take place. Our study provides a new explanation for the difficulties in eliciting bNAbs with recombinant soluble Env immunogens. Our study also highlights the need for intense efforts to identify rare naturally occurring or engineered Envs that may engage the germline BCR versions of bNAbs.

Recombinant HIV Envelope glycoproteins (Env), the sole target of anti-HIV neutralizing antibodies, have, so far, not been able to elicit broadly neutralizing antibodies (bNAbs) although they express the corresponding epitopes. Such constructs elicit neutralizing antibodies of very narrow neutralizing breadth; antibodies whose epitopes are primarily located within variable domains of Env. Diverse approaches that have been evaluated over the past two decades to overcome this limitation were met with limited success. The exact reasons for the lack of elicitation of bNAbs during immunization with Env are not well understood. Here we show that recombinant Env proteins are inefficient in engaging the predicted germline BCRs of known bnAbs. Thus, our study provides new insights as to why recombinant Env immunogens have failed to elicit bNAbs. Our results indicate that, as a first step in eliciting bNAbs by immunization, Env immunogens should be designed that would engage the germline BCR versions of bNAbs.

The Thai Phase III HIV Type 1 Vaccine trial (RV144) regimen induces antibodies that target conserved regions within the V2 loop of gp120

The Thai Phase III clinical trial (RV144) showed modest efficacy in preventing HIV-1 acquisition. Plasma collected from HIV-1-uninfected trial participants completing all injections with ALVAC-HIV (vCP1521) prime and AIDSVAX B/E boost were tested for antibody responses against HIV-1 gp120 envelope (Env). Peptide microarray analysis from six HIV-1 subtypes and group M consensus showed that vaccination induced antibody responses to the second variable (V2) loop of gp120 of multiple subtypes. We further evaluated V2 responses by ELISA and surface plasmon resonance using cyclic (Cyc) and linear V2 loop peptides. Thirty-one of 32 vaccine recipients tested (97%) had antibody responses against Cyc V2 at 2 weeks postimmunization with a reciprocal geometric mean titer (GMT) of 1100 (range: 200-3200). The frequency of detecting plasma V2 antibodies declined to 19% at 28 weeks post-last injection (GMT: 110, range: 100-200). Antibody responses targeted the mid-region of the V2 loop that contains conserved epitopes and has the amino acid sequence KQKVHALFYKLDIVPI (HXB2 Numbering sequence 169-184). Valine at position 172 was critical for antibody binding. The frequency of V3 responses at 2 weeks postimmunization was modest (18/32, 56%) with a GMT of 185 (range: 100-800). In contrast, naturally infected HIV-1 individuals had a lower frequency of antibody responses to V2 (10/20, 50%; p=0.003) and a higher frequency of responses to V3 (19/20, 95%), with GMTs of 400 (range: 100-3200) and 3570 (range: 200-12,800), respectively. RV144 vaccination induced antibodies that targeted a region of the V2 loop that contains conserved epitopes. Early HIV-1 transmission events involve V2 loop interactions, raising the possibility that anti-V2 antibodies in RV144 may have contributed to viral inhibition.

Solution structure, conformational dynamics, and CD4-induced activation in full-length, glycosylated, monomeric HIV gp120

The gp120 subunit of the HIV Env glycoprotein is responsible for receptor interactions leading to viral entry and is a primary target for neutralizing antibodies. Most structural studies have focused on the heavily truncated, deglycosylated gp120 core, leaving fundamental aspects of the glycoprotein that are responsible for immune evasion and receptor-induced activation unresolved. Here we investigate full-length, glycosylated HIV gp120 in unliganded and CD4-bound forms by using small-angle X-ray scattering to visualize global structural reorganization and hydrogen/deuterium exchange to track changes in local conformational dynamics. The studies revealed unliganded full-length gp120 to be considerably more dynamic, particularly at the CD4 binding site, than suggested by previous studies of the subunit core alone. The large V1/V2 loops, previously unmapped, are positioned to mask the coreceptor binding site in an orientation that recapitulates that observed in the Env trimer. CD4 binding shifts V1/V2 to unmask the coreceptor binding site and triggers profound dynamic changes in gp120 spanning from the binding site to the gp41-interactive face of gp120. These findings provide further insights on the structural basis of Env antigenicity and immunogenicity and of allosteric effects upon receptor binding.

Binding interactions between soluble HIV envelope glycoproteins and quaternary-structure-specific monoclonal antibodies PG9 and PG16

PG9 and PG16 are antibodies isolated from a subject infected with HIV-1 and display broad anti-HIV neutralizing activities. They recognize overlapping epitopes, which are preferentially expressed on the membrane-anchored trimeric form of the HIV envelope glycoprotein (Env). PG9 and PG16 were reported not to bind to soluble mimetics of Env. The engineering of soluble Env proteins on which the PG9 and PG16 epitopes are optimally exposed will support efforts to elicit broad anti-HIV neutralizing antibodies by immunization. Here, we identified several soluble gp140 Env proteins that are recognized by PG9 and PG16, and we investigated the molecular details of those binding interactions. The IgG versions of PG9 and PG16 recognize the soluble trimeric gp140 form less efficiently than the corresponding monomeric gp140 form. In contrast, the Fab versions of PG9 and PG16 recognized the monomeric and trimeric gp140 forms with identical binding kinetics and with binding affinities similar to the high binding affinity of the anti-V3 antibody 447D to its epitope. Our data also indicate that, depending on the Env backbone, the interactions of PG9 and PG16 with gp140 may be facilitated by the presence of the gp41 ectodomain and are independent of the proper enzymatic cleavage of gp140 into gp120 and gp41. The identification of soluble Env proteins that express the PG9 and PG16 epitopes and the detailed characterization of the molecular interactions between these two antibodies and their ligands provide important and novel information that will assist in improving the engineering of future Env immunogens.

Clinical studies of experimental vaccines

PURPOSE OF REVIEW

The scope of this review is to provide the current status of HIV vaccine clinical development. A series of issues regarding the type of immune response stimulated by the candidate vaccines in the pipeline, the advances in the immune correlates of protection, the need for an effective decision-making process for selection of candidate vaccines into further clinical development and the rationale for clinical trials will also be discussed.

RECENT FINDINGS

Efforts in the development of HIV vaccines inducing broad neutralizing antibodies have failed so far. The current pipeline is predominantly composed of candidate vaccines designed to induce cellular immunity and particularly T-cell response. For these reasons, these candidate vaccines have been termed 'T-cell vaccines'. A large number of candidate vaccines or vaccine combinations have entered phase I-II clinical trials in 2005. Furthermore, an adenovirus vector-based vaccine has entered proof-of-concept efficacy trial and a canarypox vector in combination with a protein-based vaccine is currently being evaluated in phase III clinical trials. T-cell vaccines have been shown to be safe and the most recent generation of these vaccines also has substantial immunogenicity.

SUMMARY

Only clinical trials can provide the definitive answer to immune correlates of protection and vaccine efficacy.

Cross-subtype antibody and cellular immune responses induced by a polyvalent DNA prime-protein boost HIV-1 vaccine in healthy human volunteers

An optimally effective AIDS vaccine would likely require the induction of both neutralizing antibody and cell-mediated immune responses, which has proven difficult to obtain in previous clinical trials. Here we report on the induction of human immunodeficiency virus type-1 (HIV-1)-specific immune responses in healthy adult volunteers that received the multi-gene, polyvalent, DNA prime-protein boost HIV-1 vaccine formulation, DP6-001, in a Phase I clinical trial. Robust cross-subtype HIV-1 specific T cell responses were detected in IFN-gamma ELISPOT assays. Furthermore, we detected high titer serum antibody responses that recognized a wide range of primary HIV-1 Env antigens and also neutralized pseudotyped viruses that express the primary Env antigens from multiple HIV-1 subtypes. These findings demonstrate that the DNA prime-protein boost approach is an effective immunization method to elicit both humoral and cell-mediated immune responses in humans, and that a polyvalent Env formulation could generate broad immune responses against HIV-1 viruses with diverse genetic backgrounds.

Cross-subtype antibody and cellular immune responses induced by a polyvalent DNA prime-protein boost HIV-1 vaccine in healthy human volunteers

An optimally effective AIDS vaccine would likely require the induction of both neutralizing antibody and cell-mediated immune responses, which has proven difficult to obtain in previous clinical trials. Here we report on the induction of Human Immunodeficiency Virus Type-1 (HIV-1)-specific immune responses in healthy adult volunteers that received the multi-gene, polyvalent, DNA prime-protein boost HIV-1 vaccine formulation, DP6-001, in a Phase I clinical trial conducted in healthy adult volunteers of both genders. Robust cross-subtype HIV-1-specific T cell responses were detected in IFNgamma ELISPOT assays. Furthermore, we detected high titer serum antibody responses that recognized a wide range of primary HIV-1 Env antigens and also neutralized pseudotyped viruses that express the primary Env antigens from multiple HIV-1 subtypes. These findings demonstrate that the DNA prime-protein boost approach is an effective immunization method to elicit both humoral and cell-mediated immune responses in humans, and that a polyvalent Env formulation could generate broad immune responses against HIV-1 viruses with diverse genetic backgrounds.

Removal of a single N-linked glycan in human immunodeficiency virus type 1 gp120 results in an enhanced ability to induce neutralizing antibody responses

Glycans on human immunodeficiency virus (HIV) envelope protein play an important role in infection and evasion from host immune responses. To examine the role of specific glycans, we introduced single or multiple mutations into potential N-linked glycosylation sites in hypervariable regions (V1 to V3) of the env gene of HIV type 1 (HIV-1) 89.6. Three mutants tested showed enhanced sensitivity to soluble CD4. Mutant N7 (N197Q) in the carboxy-terminal stem of the V2 loop showed the most pronounced increase in sensitivity to broadly neutralizing antibodies (NtAbs), including those targeting the CD4-binding site (IgG1b12) and the V3 loop (447-52D). This mutant is also sensitive to CD4-induced NtAb 17b in the absence of CD4. Unlike the wild-type (WT) Env, mutant N7 mediates CD4-independent infection in U87-CXCR4 cells. To study the immunogenicity of mutant Env, we immunized pig-tailed macaques with recombinant vaccinia viruses, one expressing SIVmac239 Gag-Pol and the other expressing HIV-1 89.6 Env gp160 in WT or mutant forms. Animals were boosted 14 to 16 months later with simian immunodeficiency virus gag DNA and the cognate gp140 protein before intrarectal challenge with SHIV89.6P-MN. Day-of-challenge sera from animals immunized with mutant N7 Env had significantly higher and broader neutralizing activities than sera from WT Env-immunized animals. Neutralizing activity was observed against SHIV89.6, SHIV89.6P-MN, HIV-1 SF162, and a panel of subtype B primary isolates. Compared to control animals, immunized animals showed significant reduction of plasma viral load and increased survival after challenge, which correlated with prechallenge NtAb titers. These results indicate the potential advantages for glycan modification in vaccine design, although the role of specific glycans requires further examination.

Immunogenicity comparison between codon optimized HIV-1 CRF BC_07 gp140 and gp145 vaccines

To develop an effective vaccine against the most prevalent HIV strain "B'/C recombinant" in China, we compared the immunogenicity of B'/C-derived gp140 and gp145. The codon optimized gp140 and gp145 env gene derived from CN54, an ancestor-like B'/C recombinant strain, were synthesized and cloned into a plasmid as DNA vaccines, designated as pDRVISV140 and pDRVISV145, respectively. BALB/c mice were inoculated three times at week 0, 2, and 4 and sacrificed at week 7. Both T cell immunity and humoral immunity were determined. The mock vector pDRVISV1.0 carrying no HIV immunogen was included as control. Our data showed that B'/C recombinant-derived gp145 mounted stronger T cell and broader linear antibody but less binding antibody immune responses than gp140 did. Though both gp145 and gp140 raised neutralization antibodies against laboratory-adapted strain SF33, both failed to neutralize B' or B'/C clade primary strains. Overall, this is the first time the immunogenicity of B'/C recombinant-derived gp140 and gp145 was examined and compared; our data prefer B'/C-derived gp145 to gp140 as an HIV vaccine immunogen. The failure to induce neutralization antibodies against primary isolates indicates that it is insufficient to enhance the immunogenicity of conserved epitopes by simply employing gp145 or gp140; strategies to enhance antibody responses against conserved epitopes should be explored further.

Standardized assessment of NAb responses elicited in rhesus monkeys immunized with single- or multi-clade HIV-1 envelope immunogens

The genetic diversity of HIV-1 envelope glycoproteins (Env) remains a major obstacle to the development of an antibody-based AIDS vaccine. The present studies examine the breadth and magnitude of neutralizing antibody (NAb) responses in rhesus monkeys after immunization with DNA prime-recombinant adenovirus (rAd) boost vaccines encoding either single or multiple genetically distant Env immunogens, and subsequently challenged with a pathogenic simian-human immunodeficiency virus (SHIV-89.6P). Using a standardized multi-tier panel of reference Env pseudoviruses for NAb assessment, we show that monkeys immunized with a mixture of Env immunogens (clades A, B, and C) exhibited a greater breadth of NAb activity against neutralization-sensitive Tier 1 viruses following both vaccination and challenge compared to monkeys immunized with a single Env immunogen (clade B or C). However, all groups of Env-vaccinated monkeys demonstrated only limited neutralizing activity against Tier 2 pseudoviruses, which are more characteristic of the neutralization sensitivity of circulating HIV-1. Notably, the development of a post-challenge NAb response against SHIV-89.6P was similar in monkeys receiving either clade B, clade C, or clade A+B+C Env immunogens, suggesting cross-clade priming of NAb responses. In addition, vaccines encoding Env immunogens heterologous to SHIV-89.6P primed for a rapid anamnestic NAb response following infection compared to vaccines lacking an Env component. These results show that DNA/rAd immunization with multiple diverse Env immunogens is a viable approach for enhancing the breadth of NAb responses against HIV-1, and suggest that Env immunogens can prime for anamnestic NAb responses against a heterologous challenge virus.

Lessons From a Multisite International Trial in the Caribbean and South America of an HIV-1 Canarypox Vaccine (ALVAC-HIV vCP1452) With or Without Boosting With MN rgp120

BACKGROUND

The first multicenter, international National Institutes of Allergy and Infectious Diseases (NIAID)-sponsored HIV vaccine trial took place in Brazil, Haiti, Peru and Trinidad. This randomized, double-blind, placebo-controlled, phase 2 trial evaluated the safety and immunogenicity of a clade B-derived, live canarypox HIV vaccine, vCP1452. vCP1452 was administered alone or with a heterologous boost of MN rgp120 glycoprotein. The trial was pivotal in deciding whether these vaccines advanced to phase 3 efficacy trials.

METHODS

Forty seronegative volunteers per site were randomized to ALVAC alone, ALVAC plus MN rgp120, or placebo in a 0, 1, 3, and 6 month schedule. Immunogenicity was assayed by chromium-release cytotoxic T lymphocyte (CTL) responses; interferon-gamma (IFN-gamma) enzyme-linked immunosorbent spot assays (ELISpot); lymphocyte proliferation assays (LPA); neutralization; and enzyme-linked immunosorbent assays (ELISA).

RESULTS

Enrollment and follow-up were excellent. Both vaccines were well tolerated. Neutralizing antibody to the laboratory-adapted MN strain was detected. Cellular immune responses, as measured by CTL, ELISpot, and LPA, did not differ between vaccines and placebos.

CONCLUSIONS

The observation of disappointing immunogenicity in this and a parallel domestic study has informed future vaccine development. Equally important, challenges to doing an integrated trial across countries, cultures, languages, and differing at-risk populations were overcome. The identification of specific safety, ethical, logistic, and immunological issues in this trial established the foundation for current larger international studies.

Dissecting the neutralizing antibody specificities of broadly neutralizing sera from human immunodeficiency virus type 1-infected donors

Attempts to elicit broadly neutralizing antibody responses by human immunodeficiency virus type 1 (HIV-1) vaccine antigens have been met with limited success. To better understand the requirements for cross-neutralization of HIV-1, we have characterized the neutralizing antibody specificities present in the sera of three asymptomatic individuals exhibiting broad neutralization. Two individuals were infected with clade B viruses and the third with a clade A virus. The broadly neutralizing activity could be exclusively assigned to the protein A-reactive immunoglobulin G (IgG) fraction of all three donor sera. Neutralization inhibition assays performed with a panel of linear peptides corresponding to the third hypervariable (V3) loop of gp120 failed to inhibit serum neutralization of a panel of HIV-1 viruses. The sera also failed to neutralize chimeric simian immunodeficiency virus (SIV) and HIV-2 viruses displaying highly conserved gp41-neutralizing epitopes, suggesting that antibodies directed against these epitopes likely do not account for the broad neutralizing activity observed. Polyclonal IgG was fractionated on recombinant monomeric clade B gp120, and the neutralization capacities of the gp120-depleted samples were compared to that of the original polyclonal IgG. We found that the gp120-binding antibody population mediated neutralization of some isolates, but not all. Overall, the data suggest that broad neutralization results from more than one specificity in the sera but that the number of these specificities is likely small. The most likely epitope recognized by the monomeric gp120 binding neutralizing fraction is the CD4 binding site, although other epitopes, such as the glycan shield, cannot be excluded.

T cell line passage can select for pre-existing neutralization-sensitive variants from the quasispecies of primary human immunodeficiency virus type-1 isolates

Primary human immunodeficiency type 1 viruses (HIV-1) resist antibody neutralization but become sensitive after passage through T cell lines. We and others previously reported an increased neutralization sensitivity of HIV-1 after prolonged culture on primary peripheral blood mononuclear cells (PBMC). Hence we hypothesized that adaptation to growth in T cell lines is in fact selection of a pre-existing neutralization-sensitive HIV-1 variant from the quasispecies in the PBMC culture. Indeed, increased neutralization sensitivity was associated with largely identical synonymous and non-synonymous mutations between progeny of parallel H9 passages from the same split inoculum from 2 of 3 viruses. H9 T cell line adaptation of molecular cloned HIV-1 was less successful and associated with only a few de novo mutations that varied between parallel H9-adapted progeny from the same split inoculum. We conclude that T cell line adaptation of HIV-1 can indeed select for a pre-existing variant but that this most likely depends on the viral diversity in the inoculum.

Comparison of immunogenicity between codon optimized HIV-1 Thailand subtype B gp140 and gp145 vaccines

HIV-1 pandemic posed an unprecedented challenge to the global health and it is believed that an effective vaccine will be the final solution against HIV-1. HIV-1 envelope is the primary immunogen in developing neutralization antibody based HIV vaccine. To define the suitable Env derived immunogen, we systemically compared the immunogenicity of gp140 and gp145 in a DNA vaccination alone and a prime-boost modalities. Two DNA vaccines and two recombinant Tiantan vaccinia vaccines (rTTV) were constructed for vaccination of female Balb/c mice. Elispot assay was used to read out the T cell immunity and ELISA assay was used to quantify antibody immunity. PLL (poly-L-leucine)-ELISA assay was used in linear antibody epitope mapping. Mice primed with gp145 tended to elicit more Env-specific T cells responses than those primed with gp140, significant difference was observed in DNA immunization alone. The ultimate T cell responses in prime-boost regimen tend to be determined mainly by the priming efficacy. Linear antibody epitope mapping displayed that sera raised by gp145 priming were vigorously reactive to more peptides than that by gp140. Our data demonstrated HIV-1 Thailand B-derived gp145 may raise higher T-cell responses and broader linear peptide-specific antibody responses than gp140 does. However, it remains to be determined that how these observations are relevant to the neutralization of antibody activities.

Antigenic and immunogenic study of membrane-proximal external region-grafted gp120 antigens by a DNA prime-protein boost immunization strategy

The membrane-proximal external region (MPER) of human immunodeficiency virus type 1 (HIV-1) gp41 is a target of broadly neutralizing monoclonal antibodies (MAbs) 2F5, 4E10, and Z13. Here we engrafted the MPER into the V1/2 region of HIV-1 gp120 to investigate the ability of the engineered antigens to elicit virus-neutralizing antibodies (NAbs). To promote the correct folding and presentation of the helical 4E10 epitope, we flanked the epitope with helical domains and manipulated the helix by sequential deletion of residues preceding the epitope. Binding of the recombinant proteins to MAb 4E10 increased four- to fivefold with the deletion of one or two residues, but it returned to the wild-type level when three residues were deleted, suggesting rotation of the 4E10 epitope along the helix. Immunization of mice and rabbits by electroporation-mediated DNA priming and protein boosting with these constructs elicited high levels of gp120-specific antibodies. A consistent NAb response against the neutralization-resistant, homologous JR-FL virus was detected in rabbits but not in mice. Analysis of the neutralizing activity revealed that the NAbs do not target the MPER or the V1, V2, or V3 region. Through this study, we learned the following. (i) The 4E10 epitope can be manipulated using a "rotate-the-helix" strategy that alters the helix register. However, presentation of this epitope in the immunogenic V1/2 region did not render it immunogenic in mice or rabbits. (ii) DNA vaccination with monomeric gp120-based antigens can elicit a consistent NAb response against the homologous neutralization-resistant virus by targeting epitopes outside the V1, V2, and V3 regions.

Virus-like particles: designing an effective AIDS vaccine

Viruses that infect eukaryotic organisms have the unique characteristic of self-assembling into particles. The mammalian immune system is highly attuned to recognizing and attacking these viral particles following infection. The use of particle-based immunogens, often delivered as live-attenuated viruses, has been an effective vaccination strategy for a variety of viruses. The development of an effective vaccine against the human immunodeficiency virus (HIV) has proven to be a challenge, since HIV infects cells of the immune system causing severe immunodeficiency resulting in the syndrome known as AIDS. In addition, the ability of the virus to adapt to immune pressure and reside in an integrated form in host cells presents hurdles for vaccinologists to overcome. A particle-based vaccine strategy has promise for eliciting high titer, long-lived, immune responses to a diverse number of viral epitopes against different HIV antigens. Live-attenuated viruses are effective at generating both cellular and humoral immune responses. However, while these vaccines stimulate immunity, challenged animals rarely clear the viral infection and the degree of attenuation directly correlates with protection from disease. Further, a live-attenuated vaccine has the potential to revert to a pathogenic form. Alternatively, virus-like particles (VLPs) mimic the viral particle without causing an immunodeficiency disease. VLPs are self-assembling, non-replicating, non-pathogenic particles that are similar in size and conformation to intact virions. A variety of VLPs for lentiviruses are currently in preclinical and clinical trials. This review focuses on our current status of VLP-based AIDS vaccines, regarding issues of purification and immune design for animal and clinical trials.

Genetic and neutralization properties of subtype C human immunodeficiency virus type 1 molecular env clones from acute and early heterosexually acquired infections in Southern Africa

A standard panel of subtype C human immunodeficiency virus type 1 (HIV-1) Env-pseudotyped viruses was created by cloning, sequencing, and characterizing functional gp160 genes from 18 acute and early heterosexually acquired infections in South Africa and Zambia. In general, the gp120 region of these clones was shorter (most evident in V1 and V4) and less glycosylated compared to newly transmitted subtype B viruses, and it was underglycosylated but no different in length compared to chronic subtype C viruses. The gp120s also exhibited low amino acid sequence variability (12%) in V3 and high variability (39%) immediately downstream of V3, a feature shared with newly transmitted subtype B viruses and chronic viruses of both subtypes. When tested as Env-pseudotyped viruses in a luciferase reporter gene assay, all clones possessed an R5 phenotype and resembled primary isolates in their sensitivity to neutralization by HIV-1-positive plasmas. Results obtained with a multisubtype plasma panel suggested partial subtype preference in the neutralizing antibody response to infection. The clones were typical of subtype C in that all were resistant to 2G12 (associated with loss of N-glycosylation at position 295) and most were resistant to 2F5, but all were sensitive to 4E10 and many were sensitive to immunoglobulin G1b12. Finally, conserved neutralization epitopes in the CD4-induced coreceptor binding domain of gp120 were poorly accessible and were difficult to induce and stabilize with soluble CD4 on Env-pseudotyped viruses. These results illustrate key genetic and antigenic properties of subtype C HIV-1 that might impact the design and testing of candidate vaccines. A subset of these gp160 clones are suitable for use as reference reagents to facilitate standardized assessments of vaccine-elicited neutralizing antibody responses.

Aiming to induce broadly reactive neutralizing antibody responses with HIV-1 vaccine candidates

Neutralizing antibody induction is a key feature of many effective vaccines and is the only immune response that has proven to be capable of completely blocking AIDS virus infection in animal models. Unfortunately, the extensive genetic variability and complex immune-evasion strategies of HIV-1 have thwarted all attempts to date at eliciting an effective neutralizing antibody response with candidate HIV-1 vaccine immunogens. Recent advances in our understanding of how these evasion strategies operate, coupled with growing progress in unravelling the structure and immunobiology of the viral envelope glycoproteins, are contributing to novel immunogen designs to overcome the many barriers to inducing protective antibodies against HIV-1.

Antibody responses elicited in macaques immunized with human immunodeficiency virus type 1 (HIV-1) SF162-derived gp140 envelope immunogens: comparison with those elicited during homologous simian/human immunodeficiency virus SHIVSF162P4 and heterologous HIV-1 infection

The antibody responses elicited in rhesus macaques immunized with soluble human immunodeficiency virus (HIV) Env gp140 proteins derived from the R5-tropic HIV-1 SF162 virus were analyzed and compared to the broadly reactive neutralizing antibody responses elicited during chronic infection of a macaque with a simian/human immunodeficiency virus (SHIV) expressing the HIV-1 SF162 Env, SHIV(SF162P4), and humans infected with heterologous HIV-1 isolates. Four gp140 immunogens were evaluated: SF162gp140, DeltaV2gp140 (lacking the crown of the V2 loop), DeltaV3gp140 (lacking the crown of the V3 loop), and DeltaV2DeltaV3gp140 (lacking both the V2 and V3 loop crowns). SF162gp140 and DeltaV2gp140 have been previously evaluated by our group in a pilot study, but here, a more comprehensive analysis of their immunogenic properties was performed. All four gp140 immunogens elicited stronger anti-gp120 than anti-gp41 antibodies and potent homologous neutralizing antibodies (NAbs) that primarily targeted the first hypervariable region (V1 loop) of gp120, although SF162gp140 also elicited anti-V3 NAbs. Heterologous NAbs were elicited by SF162gp140 and DeltaV2gp140 but were weak in potency and narrow in specificity. No heterologous NAbs were elicited by DeltaV3gp140 or DeltaV2DeltaV3gp140. In contrast, the SHIV(SF162P4)-infected macaque and HIV-infected humans generated similar titers of anti-gp120 and anti-gp41 antibodies and NAbs of significant breadth against primary HIV-1 isolates, which did not target the V1 loop. The difference in V1 loop immunogenicity between soluble gp140 and virion-associated gp160 Env proteins derived from SF162 may be the basis for the observed difference in the breadth of neutralization in sera from the immunized and infected animals studied here.

Aiming to induce broadly reactive neutralizing antibody responses with HIV-1 vaccine candidates

Neutralizing antibody induction is a key feature of many effective vaccines and is the only immune response that has proven to be capable of completely blocking AIDS virus infection in animal models. Unfortunately, the extensive genetic variability and complex immune-evasion strategies of HIV-1 have thwarted all attempts to date at eliciting an effective neutralizing antibody response with candidate HIV-1 vaccine immunogens. Recent advances in our understanding of how these evasion strategies operate, coupled with growing progress in unravelling the structure and immunobiology of the viral envelope glycoproteins, are contributing to novel immunogen designs to overcome the many barriers to inducing protective antibodies against HIV-1.

Human immunodeficiency virus type 1 env clones from acute and early subtype B infections for standardized assessments of vaccine-elicited neutralizing antibodies

Induction of broadly cross-reactive neutralizing antibodies is a high priority for AIDS vaccine development but one that has proven difficult to be achieved. While most immunogens generate antibodies that neutralize a subset of T-cell-line-adapted strains of human immunodeficiency virus type 1 (HIV-1), none so far have generated a potent, broadly cross-reactive response against primary isolates of the virus. Even small increments in immunogen improvement leading to increases in neutralizing antibody titers and cross-neutralizing activity would accelerate vaccine development; however, a lack of uniformity in target strains used by different investigators to assess cross-neutralization has made the comparison of vaccine-induced antibody responses difficult. Thus, there is an urgent need to establish standard panels of HIV-1 reference strains for wide distribution. To facilitate this, full-length gp160 genes were cloned from acute and early subtype B infections and characterized for use as reference reagents to assess neutralizing antibodies against clade B HIV-1. Individual gp160 clones were screened for infectivity as Env-pseudotyped viruses in a luciferase reporter gene assay in JC53-BL (TZM-bl) cells. Functional env clones were sequenced and their neutralization phenotypes characterized by using soluble CD4, monoclonal antibodies, and serum samples from infected individuals and noninfected recipients of a recombinant gp120 vaccine. Env clones from 12 R5 primary HIV-1 isolates were selected that were not unusually sensitive or resistant to neutralization and comprised a wide spectrum of genetic, antigenic, and geographic diversity. These reference reagents will facilitate proficiency testing and other validation efforts aimed at improving assay performance across laboratories and can be used for standardized assessments of vaccine-elicited neutralizing antibodies.

Enhanced immunogenicity of gp120 protein when combined with recombinant DNA priming to generate antibodies that neutralize the JR-FL primary isolate of human immunodeficiency virus type 1

Strategies are needed for human immunodeficiency virus type 1 vaccine development that improves the neutralizing antibody response against primary isolates of the virus. Here we examined recombinant DNA priming followed by subunit protein boosting as a strategy to generate neutralizing antibodies. Both plasmid-based and recombinant protein envelope (Env) glycoprotein immunogens were derived from a primary viral isolate, JR-FL. Serum from rabbits immunized with either gp120 or gp140 DNA vaccines delivered by gene gun inoculation followed by recombinant gp120 protein boosting was capable of neutralizing JR-FL. Neither the DNA vaccines alone nor the gp120 protein alone generated a detectable neutralizing antibody response against this virus. Neutralizing antibody responses using gp120 DNA and gp140 DNA for priming were similar. The results suggest that Env DNA priming followed by gp120 protein boosting provides an advantage over either approach alone for generating a detectable neutralizing antibody response against primary isolates that are not easily neutralized.

Neutralization sensitivity of HIV-1 Env-pseudotyped virus clones is determined by co-operativity between mutations which modulate the CD4-binding site and those that affect gp120–gp41 stability

Adaptation of antibody neutralization-resistant human immunodeficiency virus type I (HIV-1) to growth in vitro generally results in the acquisition of a neutralization-sensitive phenotype, an alteration of viral growth kinetics, and an array of amino acid substitutions associated with these changes. Here we examine a panel of Env chimeras and mutants derived from these neutralization-resistant and -sensitive parental Envs. A range of neutralization and infectivity phenotypes was observed. These included a modulation of the CD4 binding site (CD4bs) towards recognition by neutralizing and non-neutralizing CD4bs-directed antibodies, resulting in a globally neutralization-sensitive Env; alterations which affected Env complex stability; and interactions which resulted in differential infectivity and CCR5/CXCR4 usage. This range of properties resulted from the complex interactions of no more than three amino acids found in key Env locations. These data add to a growing body of evidence that dramatic functional alterations of the native oligomeric Env protein complex can result from relatively minor amino acid substitutions.

Dichotomy in cross-clade reactivity and neutralization by HIV-1 sera: Implications for active and passive immunotherapy

The identification of broadly reactive and cross-clade neutralizing antibodies will facilitate the development of a more universally effective vaccine for human immunodeficiency virus (HIV). Antibodies in sera from individuals infected with Clade B HIV bind native primary viral isolates, and virus binding correlates with neutralization and stable clinical disease. In this study, we quantified cross-clade antibody reactivity and neutralization by Clades B and C sera. Primary viral isolates were captured by serum IgG bound to anti-human IgG and quantitated as p24 released by lysis of captured virus. Neutralization was determined using PHA-stimulated PBMC. Clade B antibodies reacted more frequently with Clade B R5 virus, but positive sera captured quantitatively more X4 virus than R5 and R5X4 virus. Clade B sera reacted less frequently and captured less Clade C virus than Clade B virus. Antibodies in Clade C sera captured Clades B and C isolates with equal frequency and quantity. There was no difference in neutralization of Clade B virus by either group of sera; however, Clade C sera neutralized Clade C virus, whereas Clade B sera were ineffective against Clade C virus. Thus, there are distinct differences in cross-clade reactivity of and neutralization by antibodies induced in response to Clade C infection compared to Clade B infection. Understanding antibody responses to native virions after Clade C infection and cross clade antibody behavior has implications for understanding pathogenesis and vaccine development.

Characterizing anti-HIV monoclonal antibodies and immune sera by defining the mechanism of neutralization

Understanding the nature of neutralization may provide information for crafting improvements in HIV vaccines. Using JR-FL as a prototype primary pseudovirus, we first investigated anti-HIV monoclonal antibodies (mAbs) in several neutralization formats designed to elucidate the timing of neutralization. MAb b12 was most effective before receptor binding, 2G12 neutralized effectively even after CD4 binding, and X5 and a V3 loop mAb (LE311) were inactive in a standard format but were induced by sCD4. Consistent with this latter finding, native PAGE indicated that X5 and V3 mAb binding to Envelope trimers was dependent on sCD4 binding. In contrast, 2F5 and 4E10 were active even post-CD4/CCR5 engagement. We next analyzed the neutralization mechanism of a panel of HIV+ donor plasmas of various potencies. All mediated high levels of post-CD4 neutralization that was not associated with activity in the standard format. None, however, neutralized effectively in the post-CD4/CCR5 format, suggesting that 2F5/4E10-like Abs were absent or at low concentrations. Finally, we analyzed a non-neutralizing plasma spiked with mAbs b12, 2G12 or 2F5, which resulted in increases in neutralization titers consistent with the activities of the mAbs. We conclude that these methods, together with other mapping approaches, may provide a better understanding of neutralization that could be useful in vaccine research.

Long-term memory B-cell responses in recipients of candidate human immunodeficiency virus type 1 vaccines

The efficacy and practical application of human immunodeficiency virus type 1 (HIV-1) vaccines may depend in part on the longevity of the immune responses generated, particularly those in the memory compartment. Candidate vaccines based on the HIV-1 envelope glycoproteins generate binding and neutralizing antibodies in humans but there have been no prior studies on the long-term persistence and recall of those responses. We evaluated six healthy, HIV non-infected adults who had received a combination of recombinant canarypox HIV-1 vaccines boosted by gp120 and who had achieved a high serum titer of neutralizing antibody to HIV-1 MN. These individuals were administered a gp160 boost 4-5 years after their last vaccination. Four volunteers had detectable binding and neutralizing antibodies at the time of boosting and all six volunteers exhibited a recall binding and neutralizing antibody response. The antibodies neutralized multiple T cell line-adapted (TCLA) strains of virus, including the vaccine strain, but not primary isolates. These results demonstrate that memory B-cell responses can last for many years following HIV-1 envelope glycoprotein immunization. In principle, similar long-term memory may be possible with improved immunogens that generate broadly cross-reactive neutralizing antibodies.

Progress on new vaccine strategies against chronic viral infections

Among the most cost-effective strategies for preventing viral infections, vaccines have proven effective primarily against viruses causing acute, self-limited infections. For these it has been sufficient for the vaccine to mimic the natural virus. However, viruses causing chronic infection do not elicit an immune response sufficient to clear the infection and, as a result, vaccines for these viruses must elicit more effective responses--quantitative and qualitative--than does the natural virus. Here we examine the immunologic and virologic basis for vaccines against three such viruses, HIV, hepatitis C virus, and human papillomavirus, and review progress in clinical trials to date. We also explore novel strategies for increasing the immunogenicity and efficacy of vaccines.

Progress on new vaccine strategies against chronic viral infections

Among the most cost-effective strategies for preventing viral infections, vaccines have proven effective primarily against viruses causing acute, self-limited infections. For these it has been sufficient for the vaccine to mimic the natural virus. However, viruses causing chronic infection do not elicit an immune response sufficient to clear the infection and, as a result, vaccines for these viruses must elicit more effective responses--quantitative and qualitative--than does the natural virus. Here we examine the immunologic and virologic basis for vaccines against three such viruses, HIV, hepatitis C virus, and human papillomavirus, and review progress in clinical trials to date. We also explore novel strategies for increasing the immunogenicity and efficacy of vaccines.

Native HIV type 1 virion surface structures: relationships between antibody binding and neutralization or lessons from the viral capture assay

Despite a vigorous antibody response following HIV-1 infection, antibodies which neutralize primary isolates tend to be of low titer or sporadic. Similarly, antibodies produced in response to HIV-1 vaccines in human and animals react with HIV but, only on occasion, do these antibodies neutralize primary isolates. The failure of the immune system to respond in an effective manner is related to the inherent structural properties of the HIV-1 envelope expressed on the native virion and the pathogenesis of HIV infection. Identification of effective antibody interactions with HIV, as judged by inhibition of virus, is crucial for the development of broadly effective HIV vaccines and immune therapeutics. It has been proposed that antibodies must bind and neutralize virus to be effective at controlling HIV infection. We propose that this hypothesis may limit the identification of effective antibodies that are desperately needed given the difficulty in preventing and treating HIV. We provide evidence that the viral capture assay (VCA) is an important adjunct to the study of antibody interactions with primary isolate virus. Further, we propose that antibodies that are ineffective in traditional neutralization assays may also be effective at limiting viral spread and preventing viral infection.

Prevention of disease induced by a partially heterologous AIDS virus in rhesus monkeys by using an adjuvanted multicomponent protein vaccine

Recombinant protein subunit AIDS vaccines have been based predominantly on the virus envelope protein. Such vaccines elicit neutralizing antibody responses that can provide type-specific sterilizing immunity, but in most cases do not confer protection against divergent viruses. In this report we demonstrate that a multiantigen subunit protein vaccine was able to prevent the development of disease induced in rhesus monkeys by a partially heterologous AIDS virus. The vaccine was composed of recombinant human immunodeficiency virus type 1 (HIV-1) gp120, NefTat fusion protein, and simian immunodeficiency virus (SIV) Nef formulated in the clinically tested adjuvant AS02A. Upon challenge of genetically unselected rhesus monkeys with the highly pathogenic and partially heterologous SIV/HIV strain SHIV(89.6p) the vaccine was able to reduce virus load and protect the animals from a decline in CD4-positive cells. Furthermore, vaccination prevented the development of AIDS for more than 2.5 years. The combination of the regulatory proteins Nef and Tat together with the structural protein gp120 was required for vaccine efficacy.

Vaccine development against HIV-1: current perspectives and future directions

The development of an efficacious vaccine against the human immunodeficiency virus (HIV) is of great urgency, because it is accepted that vaccination is the only means capable of controlling the AIDS pandemic. The foundation of HIV vaccine development is the analysis of immune responses during natural infection and the utilization of this knowledge for the development of protective immunization strategies. Initial vaccine development and experimentation are usually in animal models, including murine, feline, and nonhuman primates. Experimental vaccine candidates are closely studied for both efficacy and safety before proceeding to human clinical trials. There are a number of different therapeutic and prophylactic vaccine strategies currently being studied in human clinical trials. Vaccine strategies that are being tested, or have previously been tested, in humans include subunit, DNA plasmid, and viral vector, and combinations of these various strategies. Some of the results of these trials are promising, and additional research has focused on the development of appropriate chemical and genetic adjuvants as well as methods of vaccine delivery to improve the host immune response. This review summarizes the vaccine strategies that have been tested in both animal models and human clinical trials.

Crosslinked HIV-1 envelope-CD4 receptor complexes elicit broadly cross-reactive neutralizing antibodies in rhesus macaques

The identification of HIV envelope structures that generate broadly cross-reactive neutralizing antibodies is a major goal for HIV-vaccine development. In this study, we evaluated one such structure, expressed as either a gp120-CD4 or a gp140-CD4 complex, for its ability to elicit a neutralizing antibody response. In rhesus macaques, covalently crosslinked complexes of soluble human CD4 (shCD4) and HIV-1(IIIB) envelope glycoproteins (gp120 or gp140) generated antibodies that neutralized a wide range of primary HIV-1 isolates regardless of the coreceptor usage or genetic subtype. Ig with cross-reactive neutralizing activity was recovered by affinity chromatography with a chimeric single-chain polypeptide containing sequences for HIV(BaL) gp120 and a mimetic peptide that induces a CD4-triggered envelope structure. These results suggest that covalently crosslinked complexes of the HIV-1 surface envelope glycoprotein and CD4 elicit broadly neutralizing humoral responses that, in part, may be directed against a novel epitope(s) found on the HIV-1 envelope.

Induction of neutralizing antibodies and gag-specific cellular immune responses to an R5 primary isolate of human immunodeficiency virus type 1 in rhesus macaques

The ability to generate antibodies that cross-neutralize diverse primary isolates is an important goal for human immunodeficiency virus type 1 (HIV-1) vaccine development. Most of the candidate HIV-1 vaccines tested in humans and nonhuman primates have failed in this regard. Past efforts have focused almost entirely on the envelope glycoproteins of a small number of T-cell line-adapted strains of the virus as immunogens. Here we assessed the immunogenicity of noninfectious virus-like particles (VLP) consisting of Gag, Pro (protease), and Env from R5 primary isolate HIV-1(Bx08). Immunogens were delivered to rhesus macaques in the form of either purified VLP, recombinant DNA and canarypox (ALVAC) vectors engineered to express VLP, or a combination of these products. Seroconversion to Gag and Pro was detected in all of the immunized animals. Antibodies that could neutralize HIV-1(Bx08) were detected in animals that received (i) coinoculations with DNA(Bx08) and VLP(Bx08), (ii) DNA(Bx08) followed by ALVAC(Bx08) boosting, and (iii) VLP(Bx08) alone. The neutralizing antibodies were highly strain specific despite the fact that they did not appear to be directed to linear epitopes in the V3 loop. Virus-specific cellular immune responses also were generated, as judged by the presence of Gag-specific gamma interferon (IFN-gamma)-producing cells. These cellular immune responses required the inclusion of DNA(Bx08) in the immunization modality, since few or no IFN-gamma-producing cells were detected in animals that received either VLP(Bx08) or ALVAC(Bx08) alone. The results demonstrate the feasibility of generating neutralizing antibodies and cellular immune responses that target an R5 primary HIV-1 isolate by vaccination in primates.

Antibody-mediated neutralization of primary human immunodeficiency virus type 1 isolates: investigation of the mechanism of inhibition

Human immunodeficiency virus type 1 (HIV-1) neutralization occurs when specific antibodies, mainly those directed against the envelope glycoproteins, inhibit infection, most frequently by preventing the entry of the virus into target cells. However, the precise mechanisms of neutralization remain unclear. Previous studies, mostly with cell lines, have produced conflicting results involving either the inhibition of virus attachment or interference with postbinding events. In this study, we investigated the mechanisms of neutralization by immune sera and compared the inhibition of peripheral blood mononuclear cells (PBMC) infection by HIV-1 primary isolates (PI) with the inhibition of T-cell line infection by T-cell line-adapted (TCLA) strains. We followed the kinetics of neutralization to determine at which step of the viral cycle the antibodies act. We found that neutralization of the TCLA strain HIV-1MN/MT-4 required an interaction between antibodies and cell-free virions before the addition of MT-4 cells, whereas PI were neutralized even after adsorption onto PBMC. In addition, the dose-dependent inhibition of HIV-1MN binding to MT-4 cells was strongly correlated with serum-induced neutralization. In contrast, neutralizing sera did not reduce the adhesion of PI to PBMC. Postbinding inhibition was also detected for HIV-1MN produced by and infecting PBMC, demonstrating that the mechanism of neutralization depends on the target cell used in the assay. Finally, we considered whether the different mechanisms of neutralization may reflect the recognition of qualitatively different epitopes on the surface of PI and HIV-1MN or whether they reflect differences in virus attachment to PBMC and MT-4 cells.

Immunogenicity and protective efficacy of oligomeric human immunodeficiency virus type 1 gp140

The biologically active form of the human immunodeficiency virus type 1 (HIV-1) envelope (Env) glycoprotein is oligomeric. We previously described a soluble HIV-1 IIIB Env protein, gp140, with a stable oligomeric structure composed of uncleaved gp120 linked to the ectodomain of gp41 (P. L. Earl, C. C. Broder, D. Long, S. A. Lee, J. Peterson, S. Chakrabarti, R. W. Doms, and B. Moss, J. Virol. 68:3015-3026, 1994). Here we compared the antibody responses of rabbits to gp120 and gp140 that had been produced and purified in an identical manner. The gp140 antisera exhibited enhanced cross-reactivity with heterologous Env proteins as well as greater neutralization of HIV-1 compared to the gp120 antisera. To examine both immunogenicity and protective efficacy, we immunized rhesus macaques with oligomeric gp140. Strong neutralizing antibodies against a homologous virus and modest neutralization of heterologous laboratory-adapted isolates were elicited. No neutralization of primary isolates was observed. However, a substantial fraction of the neutralizing activity could not be blocked by a V3 loop peptide. After intravenous challenge with simian-HIV virus SHIV-HXB2, three of the four vaccinated macaques exhibited no evidence of virus replication.

Immunization with recombinant canarypox vectors expressing membrane-anchored glycoprotein 120 followed by glycoprotein 160 boosting fails to generate antibodies that neutralize R5 primary isolates of human immunodeficiency virus type 1

Antibodies generated by candidate HIV-1 vaccines in a phase I clinical trial were assessed for neutralizing activity with a panel of eight well-characterized, genetically diverse clade B primary isolates having an R5 phenotype. The vaccines consisted of one of three different recombinant canarypox vectors expressing membrane-anchored HIV-1(MN)gp120 (ALVAC vCP205, vCP1433, and vCP1452) followed by boosting with a soluble gp160 hybrid consisting of MNgp120 and the majority of gp41 from strain IIIB. Serum samples from a subset of volunteers in each arm of the trial, containing moderate to high titers of neutralizing antibodies to HIV-1 MN, were analyzed. Competition assays with peptides revealed that the majority of neutralizing activity was specific for the MN-V3 loop. Despite MN-specific neutralization titers that sometimes exceeded 1:500, no neutralization of primary isolates was detected and, in some cases, mild infection enhancement was observed. In addition, little or no neutralization of the HIV-1 IIIB heterologous T cell line-adapted strain of virus was detected. These results reinforce the notion that monovalent HIV-1 ENV is a poor immunogen for generating cross-reactive neutralizing antibodies.

Turning a corner on HIV neutralization?

HIV vaccine development has been hampered by the inability of conventional immunogens to elicit antibodies capable of neutralizing primary isolates of the virus. Recent studies using 'fusion-competent' immunogens that capture transitional intermediate structures of the functioning envelope protein suggest that this goal may now be achievable.

A phase I trial in HIV negative healthy volunteers evaluating the effect of potent adjuvants on immunogenicity of a recombinant gp120W61D derived from dual tropic R5X4 HIV-1ACH320

Thirty healthy HIV negative volunteers were randomised to receive 200 micrograms of rgp120W61D in either: 3D-MPL and QS21, with an oil and water emulsion (SBAS-2) (13); or 3D-MPL and QS21 (SBAS-1) (11); or alum (six). Immunizations were given at 0, 4 and 28 weeks and 23 (77%) participants completed the schedule. Adverse events were more frequent (P < 0.001) and more severe (P < 0.001) in the SBAS-2 group. Binding antibodies to the homologous rgp120W61D were detected after the first immunisation only in those receiving SBAS-1 and SBAS-2, were maximal after the third immunization in all three groups, and persisted to week 84 only in the novel adjuvant groups. These differences were significant (p = 0.02). Neutralising antibodies to TCLA-strains of HIV-1 were observed after the second immunization in all three groups, were maximal after the third immunization, but did not neutralise homologous or heterologous PBMC derived primary HIV-1 isolates. Proliferative T-cell responses to rgp120W61D were maximal after the second immunization and reached very high values in the SBAS-2 group. HIV-1 specific CD8+ MHC Class I restricted cytotoxic T-lymphocytes were not seen in a subset of participants tested at a single timepoint. SBAS-2 with rgp120W61D induced antibody titres as high as those seen in HIV infection, but the quality of the antibodies remained different in that there was no evidence of primary isolate neutralisation. Although cell-mediated immunity was enhanced by SBAS-2 in terms of lymphoproliferative responses, HIV-1 specific CD8+ cytotoxicity was not demonstrated.