Inhibition of HIV-1 entry by antibodies: potential viral and cellular targets

Determination of Binding Affinity of Antibodies to HIV-1 Recombinant Envelope Glycoproteins, Pseudoviruses, Infectious Molecular Clones, and Cell-Expressed Trimeric gp160 Using Microscale Thermophoresis

Developing a preventative vaccine for HIV-1 has been a global priority. The elicitation of broadly neutralizing antibodies (bNAbs) against a broad range of HIV-1 envelopes (Envs) from various strains appears to be a critical requirement for an efficacious HIV-1 vaccine. To understand their ability to neutralize HIV-1, it is important to characterize the binding characteristics of bNAbs. Our work is the first to utilize microscale thermophoresis (MST), a rapid, economical, and flexible in-solution temperature gradient method to quantitatively determine the binding affinities of bNAbs and non-neutralizing monoclonal antibodies (mAbs) to HIV-1 recombinant envelope monomer and trimer proteins of different subtypes, pseudoviruses (PVs), infectious molecular clones (IMCs), and cells expressing the trimer. Our results demonstrate that the binding affinities were subtype-dependent. The bNAbs exhibited a higher affinity to IMCs compared to PVs and recombinant proteins. The bNAbs and mAbs bound with high affinity to native-like gp160 trimers expressed on the surface of CEM cells compared to soluble recombinant proteins. Interesting differences were seen with V2-specific mAbs. Although they recognize linear epitopes, one of the antibodies also bound to the Envs on PVs, IMCs, and a recombinant trimer protein, suggesting that the epitope was not occluded. The identification of epitopes on the envelope surface that can bind to high affinity mAbs could be useful for designing HIV-1 vaccines and for down-selecting vaccine candidates that can induce high affinity antibodies to the HIV-1 envelope in their native conformation.

Induction of Fc-Mediated Effector Functions Against a Stabilized Inner Domain of HIV-1 gp120 Designed to Selectively Harbor the A32 Epitope Region

Recent clinical trials and studies using nonhuman primates (NHPs) suggest that antibody-mediated protection against HIV-1 will require α-HIV envelope humoral immunity beyond direct neutralization to include Fc-receptor (FcR) mediated effector functions such as antibody-dependent cellular cytotoxicity (ADCC). There is also strong evidence indicating that the most potent ADCC response in humans is directed toward transitional non-neutralizing epitopes associated with the gp41-interactive face of gp120, particularly those within the first and second constant (C1–C2) region (A32-like epitopes). These epitopes were shown to be major targets of ADCC responses during natural infection and have been implicated in vaccine-induced protective immunity. Here we describe the immunogenicity of ID2, an immunogen consisting of the inner domain of the clade A/E 93TH057 HIV-1 gp120 expressed independently of the outer domain (OD) and stabilized in the CD4-bound conformation to harbor conformational A32 region epitopes within a minimal structural unit of HIV-1 Env. ID2 induced A32-specific antibody responses in BALB/c mice when injected alone or in the presence of the adjuvants Alum or GLA-SE. Low α-ID2 titers were detected in mice immunized with ID2 alone whereas robust responses were observed with ID2 plus adjuvant, with the greatest ID2 and A32-specific titers observed in the GLA-SE group. Only sera from groups immunized in the presence of GLA-SE were capable of mediating significant ADCC using NKr cells sensitized with recombinant BaL gp120 as targets and human PBMCs as effectors. A neutralization response to a tier 2 virus was not observed. Altogether, our studies demonstrate that ID2 is highly immunogenic and elicits A32-specific ADCC responses in an animal host. The ID2 immunogen has significant translational value as it can be used in challenge studies to evaluate the role of non-neutralizing antibodies directed at the A32 subregion in HIV-1 protection.

Current Peptide and Protein Candidates Challenging HIV Therapy beyond the Vaccine Era

Human immunodeficiency virus (HIV) is a causative agent of acquired immune deficiency syndrome (AIDS). Highly active antiretroviral therapy (HAART) can slow down the replication of HIV-1, leading to an improvement in the survival of HIV-1-infected patients. However, drug toxicities and poor drug administration has led to the emergence of a drug-resistant strain. HIV-1 immunotherapy has been continuously developed, but antibody therapy and HIV vaccines take time to improve its efficiency and have limitations. HIV-1-specific chimeric antigen receptor (CAR)-based immunotherapy founded on neutralizing antibodies is now being developed. In HIV-1 therapy, anti-HIV chimeric antigen receptors showed promising data in the suppression of HIV-1 replication; however, autologous transfusion is still a problem. This has led to the development of effective peptides and proteins for an alternative HIV-1 treatment. In this paper, we provide a comprehensive review of potent anti-HIV-1 peptides and proteins that reveal promising therapeutic activities. The inhibitory mechanisms of each therapeutic molecule in the different stages of the HIV-1 life cycle will be discussed herein.

"Fusion and binding inhibition" key target for HIV-1 treatment and pre-exposure prophylaxis: targets, drug delivery and nanotechnology approaches

More than 35 million people are living with HIV worldwide with approximately 2.3 million new infections per year. Cascade of events (cell entry, virus replication, assembly and release of newly formed virions) is involved in the HIV-1 transmission process. Every single step offers a potential therapeutic strategy to halt this progression and HIV fusion into the human host cell is one such stage. Controlling the initial event of HIV-1 transmission is the best way to control its dissemination especially when prophylaxis is concerned. Action is required either on the HIV’s or host’s cell surface which is logically more rational when compared with other intracellular acting moieties. Aim of this manuscript is to detail the significance and current strategies to halt this initial step, thus blocking the entry of HIV-1 for further infection. Both HIV-1 and the possible host cell’s receptors/co-receptors are under focus while specifying the targets available for inhibiting this fusion. Current and under investigation moieties are categorized based on their versatile mechanisms. Advanced drug delivery and nanotechnology approaches present a key tool to exploit the therapeutic potential in a boosted way. Current drug delivery and the impact of nanotechnology in potentiating this strategy are detailed.

Virome Analysis of Transfusion Recipients Reveals a Novel Human Virus That Shares Genomic Features with Hepaciviruses and Pegiviruses

To investigate the transmission of novel infectious agents by blood transfusion, we studied changes in the virome composition of blood transfusion recipients pre- and posttransfusion. Using this approach, we detected and genetically characterized a novel human virus, human hepegivirus 1 (HHpgV-1), that shares features with hepatitis C virus (HCV) and human pegivirus (HPgV; formerly called GB virus C or hepatitis G virus). HCV and HPgV belong to the genera Hepacivirus and Pegivirus of the family Flaviviridae. HHpgV-1 was found in serum samples from two blood transfusion recipients and two hemophilia patients who had received plasma-derived clotting factor concentrates. In the former, the virus was detected only in the posttransfusion samples, indicating blood-borne transmission. Both hemophiliacs were persistently viremic over periods of at least 201 and 1,981 days. The 5′ untranslated region (UTR) of HHpgV-1 contained a type IV internal ribosome entry site (IRES), structurally similar to although highly divergent in sequence from that of HCV and other hepaciviruses. However, phylogenetic analysis of nonstructural genes (NS3 and NS5B) showed that HHpgV-1 forms a branch within the pegivirus clade distinct from HPgV and homologs infecting other mammalian species. In common with some pegivirus variants infecting rodents and bats, the HHpgV-1 genome encodes a short, highly basic protein upstream of E1, potentially possessing a core-like function in packaging RNA during assembly. Identification of this new human virus, HHpgV-1, expands our knowledge of the range of genome configurations of these viruses and may lead to a reevaluation of the original criteria by which the genera Hepacivirus and Pegivirus are defined.

More than 30 million blood components are transfused annually in the United States alone. Surveillance for infectious agents in the blood supply is key to ensuring the safety of this critical resource for medicine and public health. Here, we report the identification of a new and highly diverse HCV/GB virus (GBV)-like virus from human serum samples. This new virus, human hepegivirus 1 (HHpgV-1), was found in serum samples from blood transfusion recipients, indicating its potential for transmission via transfusion products. We also found persistent long-term HHpgV-1 viremia in two hemophilia patients. HHpgV-1 is unique because it shares genetic similarity with both highly pathogenic HCV and the apparently nonpathogenic HPgV (GBV-C). Our results add to the list of human viruses and provide data to develop reagents to study virus transmission and disease association and for interrupting virus transmission and new human infections.

Role of human CD4 D1D2 domain in HIV-1 infection

Broadly neutralizing antibodies and appropriate immunogens are critical for preexposure prophylaxis and therapeutic HIV vaccines. In this study, we aimed to explore effective antibodies against the genetically diverse HIV-1 strains by investigating the roles of human CD4 D1D2 domain and nonvariable immugens. The human CD4 D1D2 domain and the chimeric protein of mouse D1 domain/human D2 domain were expressed in Sf9 insect cells and purified by gel-filtration chromatography. The human CD4 D1D2 domain potently inhibited the infection of 77.8% HIV-1 pseudoviruses, including the clades AE, B' and BC, with less than 20 μg/mL of IC(50). pcDNA3.1-mhD1D2m and pcDNA3.1-mhD2m plasmids were used for the production of mouse anti-human CD4 polyclonal antibodies. The neutralizing activities of the polyclonal antibodies were determined by using pseudotyped HIV-1 viruses. The antibodies induced by plasmids containing human CD4 D1D2 domain were able to potently inhibit all pseudotyped HIV-1 strains. The antibodies from mhD1D2m-immunized mice also showed strong binding capacity to CD4 expressed on the surface of TZM-bl cells. The potent and broad inhibitory activity of antibodies against the human CD4 D1D2 domain may be used to develop effective passive immunization agent to control the spread of HIV infection.

HIV-1 virus-like particles bearing pure env trimers expose neutralizing epitopes but occlude nonneutralizing epitopes

Hypothetically, since native HIV-1 Env trimers are exclusively recognized by neutralizing antibodies, they might induce the neutralizing antibodies in a vaccine setting. This idea has not been evaluated due to the difficulty of separating trimers from nonfunctional Env (uncleaved gp160 and gp41 stumps). The latter are immunodominant and induce nonneutralizing antibodies. We previously showed that nonfunctional Env can be selectively cleared from virus-like particle (VLP) surfaces by enzyme digests (E. T. Crooks, T. Tong(,) K. Osawa, and J. M. Binley, J.Virol. 85:5825, 2011). Here, we investigated the effects of these digests on the antigenicity of VLPs and their sensitivity to neutralization. Before digestion, WT VLPs (bearing wild-type Env) and UNC VLPs (bearing uncleaved gp160) were recognized by various Env-specific monoclonal antibodies (MAbs), irrespective of their neutralizing activity, a result which is consistent with the presence of nonfunctional Env. After digestion, only neutralizing MAbs recognized WT VLPs, consistent with selective removal of nonfunctional Env (i.e., "trimer VLPs"). Digests eliminated the binding of all MAbs to UNC VLPs, again consistent with removal of nonfunctional Env. An exception was MAb 2F5, which weakly bound to digested UNC VLPs and bald VLPs (bearing no Env), perhaps due to lipid cross-reactivity. Trimer VLPs were infectious, and their neutralization sensitivity was largely comparable to that of undigested WT VLPs. However, they were ∼100-fold more sensitive to the MAbs 4E10 and Z13e1, suggesting increased exposure of the gp41 base. Importantly, a scatterplot analysis revealed a strong correlation between MAb binding and neutralization of trimer VLPs. This suggests that trimer VLPs bear essentially pure native trimer that should allow its unfettered evaluation in a vaccine setting.

Nanotechnology Applications to HIV Vaccines and Microbicides

Human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS) remains one of the most serious threats to global health. Today there are no HIV vaccines which can prevent HIV infection. All of the candidates being studied are in the experimental stage. Preventive vaccine candidates are being tested in HIV-negative people to see if they can prevent infection. With of the development of a safe and effective vaccine still likely to be years away, topical microbicide formulations that are applied vaginally and rectally are receiving greater interest as an effective alternative to slow down the global spread of HIV. Current microbicide trials that aim to prevent the sexual transmission of HIV are using gels, creams, rings, films and there is also work underway to explore other types of 'delivery' systems. There have been numerous reports on safety and lack of toxicity of the application of nanotechnology for targeted delivery and slow, sustained release of drugs, proteins, peptides or nucleic acids by any route to maximize effectiveness and minimize adverse effects. The application of nanotechnology for targeting drugs and macromolecules to specific tissues or cells is one of the most important areas in nanomedicine research. Thus far nanoparticles provide a strong platform to combine protein and DNA based vaccines/microbicides and will facilitate the production, preclinical evaluation and clinical testing in the future.

Broad and potent immune responses to a low dose intradermal HIV-1 DNA boosted with HIV-1 recombinant MVA among healthy adults in Tanzania

BACKGROUND

We conducted a phase I/II randomized placebo-controlled trial with the aim of exploring whether priming with a low intradermal dose of a multiclade, multigene HIV-1 DNA vaccine could improve the immunogenicity of the same vaccine given intramuscularly prior to boosting with a heterologous HIV-1 MVA among healthy adults in Dar es Salaam, Tanzania.

METHODS

Sixty HIV-uninfected volunteers were randomized to receive DNA plasmid vaccine 1mg intradermally (id), n=20, or 3.8mg intramuscularly (im), n=20, or placebo, n=20, using a needle-free injection device. DNA plasmids encoding HIV-1 genes gp160 subtype A, B, C; rev B; p17/p24 gag A, B and Rtmut B were given at weeks 0, 4 and 12. Recombinant MVA (10(8)pfu) expressing HIV-1 Env, Gag, Pol of CRF01_AE or placebo was administered im at month 9 and 21.

RESULTS

The vaccines were well tolerated. Two weeks after the third HIV-DNA injection, 22/38 (58%) vaccinees had IFN-γ ELISpot responses to Gag. Two weeks after the first HIV-MVA boost all 35 (100%) vaccinees responded to Gag and 31 (89%) to Env. Two to four weeks after the second HIV-MVA boost, 28/29 (97%) vaccinees had IFN-γ ELISpot responses, 27 (93%) to Gag and 23 (79%) to Env. The id-primed recipients had significantly higher responses to Env than im recipients. Intracellular cytokine staining for Gag-specific IFN-γ/IL-2 production showed both CD8(+) and CD4(+) T cell responses. All vaccinees had HIV-specific lymphoproliferative responses. All vaccinees reacted in diagnostic HIV serological tests and 26/29 (90%) had antibodies against gp160 after the second HIV-MVA boost. Furthermore, while all of 29 vaccinee sera were negative for neutralizing antibodies against clade B, C and CRF01_AE pseudoviruses in the TZM-bl neutralization assay, in a PBMC assay, the response rate ranged from 31% to 83% positives, depending upon the clade B or CRF01_AE virus tested.

CONCLUSIONS

This vaccine approach is safe and highly immunogenic. Low dose, id HIV-DNA priming elicited higher and broader cell-mediated immune responses to Env after HIV-MVA boost compared to a higher HIV-DNA priming dose given im. Three HIV-DNA priming immunizations followed by two HIV-MVA boosts efficiently induced Env-antibody responses.

The hepatitis C virus glycan shield and evasion of the humoral immune response

Despite the induction of effective immune responses, 80% of hepatitis C virus (HCV)-infected individuals progress from acute to chronic hepatitis. In contrast to the cellular immune response, the role of the humoral immune response in HCV clearance is still subject to debate. Indeed, HCV escapes neutralizing antibodies in chronically infected patients and reinfection has been described in human and chimpanzee. Studies of antibody-mediated HCV neutralization have long been hampered by the lack of cell-culture-derived virus and the absence of a small animal model. However, the development of surrogate models and recent progress in HCV propagation in vitro now enable robust neutralization assays to be performed. These advances are beginning to shed some light on the mechanisms of HCV neutralization. This review summarizes the current state of knowledge of the viral targets of anti-HCV-neutralizing antibodies and the mechanisms that enable HCV to evade the humoral immune response. The recent description of the HCV glycan shield that reduces the immunogenicity of envelope proteins and masks conserved neutralizing epitopes at their surface constitutes the major focus of this review.

HIV microbicides: state-of-the-art and new perspectives on the development of entry inhibitors

Since the discovery of HIV at the beginning of the 1980s, numerous efforts have been devoted to the search of an efficient vaccine. There are at least 25 drugs available for HIV treatment, but no cure is available. The observation that therapy for HIV disease is life long and that these drugs are associated with a number of side effects underlines the need for approaches aimed at preventing rather than treating infection. Additionally, the economic burden of treatment for the HIV infection occupies an increasing share of healthcare expenditure, making current practices likely to become difficult to sustain in the long run. Unfortunately, no effective vaccine for this disease is foreseeable in the near future. Microbicides could be an alternate way to build preventative approaches to HIV infection. Strategies based on preventing the virus from reaching its target cells seem to have some room for development and application. In this review we explore the state-of-the-art of available microbicides, focusing on HIV entry inhibitors. In addition, we discuss new compounds that show anti-HIV activity, which could be effective candidates.

Enzyme digests eliminate nonfunctional Env from HIV-1 particle surfaces, leaving native Env trimers intact and viral infectivity unaffected

HIV-1 viruses and virus-like particles (VLPs) bear nonnative "junk" forms of envelope (Env) glycoprotein that may undermine the development of antibody responses against functional gp120/gp41 trimers, thereby blunting the ability of particles to elicit neutralizing antibodies. Here, we sought to better understand the nature of junk Env with a view to devising strategies for its removal. Initial studies revealed that native trimers were surprisingly stable in the face of harsh conditions, suggesting that junk Env is unlikely to arise by trimer dissociation or gp120 shedding. Furthermore, the limited gp120 shedding that occurs immediately after synthesis of primary HIV-1 isolate Envs is not caused by aberrant cleavage at the tandem gp120/gp41 cleavage sites, which were found to cleave in a codependent manner. A major VLP contaminant was found to consist of an early, monomeric form of gp160 that is glycosylated in the endoplasmic reticulum (gp160ER) and then bypasses protein maturation and traffics directly into particles. gp160ER was found to bind two copies of monoclonal antibody (MAb) 2G12, consistent with its exclusively high-mannose glycan profile. These findings prompted us to evaluate enzyme digests as a way to remove aberrant Env. Remarkably, sequential glycosidase-protease digests led to a complete or near-complete removal of junk Env from many viral strains, leaving trimers and viral infectivity largely intact. "Trimer VLPs" may be useful neutralizing antibody immunogens.

Replication competent molecular clones of HIV-1 expressing Renilla luciferase facilitate the analysis of antibody inhibition in PBMC

Effective vaccine development for human immunodeficiency virus type 1 (HIV-1) will require assays that ascertain the capacity of vaccine immunogens to elicit neutralizing antibodies (NAb) to diverse HIV-1 strains. To facilitate NAb assessment in peripheral blood mononuclear cell (PBMC)-based assays, we developed an assay-adaptable platform based on a Renilla luciferase (LucR) expressing HIV-1 proviral backbone. LucR was inserted into pNL4-3 DNA, preserving all viral open reading frames. The proviral genome was engineered to facilitate expression of diverse HIV-1 env sequences, allowing analysis in an isogenic background. The resulting Env-IMC-LucR viruses are infectious, and LucR is stably expressed over multiple replications in PBMC. HIV-1 neutralization, targeting TZM-bl cells, was highly correlative comparing virus (LucR) and cell (firefly luciferase) readouts. In PBMC, NAb activity can be analyzed either within a single or multiple cycles of replication. These results represent advancement toward a standardizable PBMC-based neutralization assay for assessing HIV-1 vaccine immunogen efficacy.

Innovative approaches to develop prophylactic and therapeutic vaccines against HIV/AIDS

The acquired immunodeficiency syndrome (AIDS) emerged in the human population in the summer of 1981. According to the latest United Nations estimates, worldwide over 33 million people are infected with human immunodeficiency virus (HIV) and the prevalence rates continue to rise globally. To control the alarming spread of HIV, an urgent need exists for developing a safe and effective vaccine that prevents individuals from becoming infected or progressing to disease. To be effective, an HIV/AIDS vaccine should induce broad and long-lasting humoral and cellular immune responses, at both mucosal and systemic level. However, the nature of protective immune responses remains largely elusive and this represents one of the major roadblocks preventing the development of an effective vaccine. Here we summarize our present understanding of the factors responsible for resistance to infection or control of progression to disease in human and monkey that may be relevant to vaccine development and briefly review recent approaches which are currently being tested in clinical trials. Finally, the rationale and the current status of novel strategies based on nonstructural HIV-1 proteins, such as Tat, Nef and Rev, used alone or in combination with modified structural HIV-1 Env proteins are discussed.

Current understanding of the mechanism of HPV infection

HPVs (human papillomaviruses) and other papillomaviruses have a unique mechanism of infection that has likely evolved to limit infection to the basal cells of stratified epithelium, the only tissue in which they replicate. Recent studies in a mouse cervicovaginal challenge model indicate that, surprisingly, the virus cannot initially bind to keratinocytes in vivo. Rather it must first bind via its L1 major capsid protein to heparan sulfate proteoglycans (HSPGs) on segments of the basement membrane (BM) exposed after epithelial trauma and undergo a conformational change that exposes the N-terminus of L2 minor capsid protein to furin cleavage. L2 proteolysis exposes a previously occluded surface of L1 that binds an as yet undetermined cell surface receptor on keratinocytes that have migrated over the BM to close the wound. Papillomaviruses are the only viruses that are known to initiate their infectious process at an extracellular site. In contrast to the in vivo situation, the virions can bind directly to many cultured cell lines through cell surface HSPGs and then undergo a similar conformational change and L2 cleavage. Transfer to the secondary receptor leads to internalization, uncoating in late endosomes, escape from the endosome by an L2-dependent mechanism, and eventual trafficking of an L2-genome complex to specific subnuclear domains designated ND10 bodies, where viral gene transcription is initiated. The infectious process is remarkably slow and asynchronous both in vivo and in cultured cells, taking 12-24h for initiation of transcription. The extended exposure of antibody neutralizing determinants while the virions reside on the BM and cell surfaces might, in part, account for the remarkable effectiveness of vaccines based on neutralizing antibodies to L1 virus-like particles or the domain of L2 exposed after furin cleavage.

Role of complex carbohydrates in human immunodeficiency virus type 1 infection and resistance to antibody neutralization

Complex N-glycans flank the receptor binding sites of the outer domain of HIV-1 gp120, ostensibly forming a protective "fence" against antibodies. Here, we investigated the effects of rebuilding this fence with smaller glycoforms by expressing HIV-1 pseudovirions from a primary isolate in a human cell line lacking N-acetylglucosamine transferase I (GnTI), the enzyme that initiates the conversion of oligomannose N-glycans into complex N-glycans. Thus, complex glycans, including those that surround the receptor binding sites, are replaced by fully trimmed oligomannose stumps. Conversely, the untrimmed oligomannoses of the silent domain of gp120 are likely to remain unchanged. For comparison, we produced a mutant virus lacking a complex N-glycan of the V3 loop (N301Q). Both variants exhibited increased sensitivities to V3 loop-specific monoclonal antibodies (MAbs) and soluble CD4. The N301Q virus was also sensitive to "nonneutralizing" MAbs targeting the primary and secondary receptor binding sites. Endoglycosidase H treatment resulted in the removal of outer domain glycans from the GnTI- but not the parent Env trimers, and this was associated with a rapid and complete loss in infectivity. Nevertheless, the glycan-depleted trimers could still bind to soluble receptor and coreceptor analogs, suggesting a block in post-receptor binding conformational changes necessary for fusion. Collectively, our data show that the antennae of complex N-glycans serve to protect the V3 loop and CD4 binding site, while N-glycan stems regulate native trimer conformation, such that their removal can lead to global changes in neutralization sensitivity and, in extreme cases, an inability to complete the conformational rearrangements necessary for infection.

The good and evil of complement activation in HIV-1 infection

The complement system, a key component of innate immunity, is a first-line defender against foreign pathogens such as HIV-1. The role of the complement system in HIV-1 pathogenesis appears to be multifaceted. Although the complement system plays critical roles in clearing and neutralizing HIV-1 virions, it also represents a critical factor for the spread and maintenance of the virus in the infected host. In addition, complement regulators such as human CD59 present in the envelope of HIV-1 prevent complement-mediated lysis of HIV-1. Some novel approaches are proposed to combat HIV-1 infection through the enhancement of antibody-dependent complement activity against HIV-1. In this paper, we will review these diverse roles of complement in HIV-1 infection.

Specificities of broadly neutralizing anti-HIV-1 sera

PURPOSE OF REVIEW

Effective vaccine-elicited immunity against HIV-1 infection will likely require broadly neutralizing antibodies to interrupt the fusion-promoting functions of the viral envelope glycoprotein spikes. Efforts in this area have, however, been fraught with challenges. The handful of existing broadly neutralizing monoclonal antibodies has provided information on some of the virus' sites of vulnerability, fueling a decade of structure-informed vaccine design. The fact that very few bnmAbs have been recovered to date illustrates the poor immunogenicity of these epitopes. Recognizing that progress may require more basic information, there has been a notable shift in the field toward identifying new chinks in HIV-1's armor. These efforts are based on the observation that some infected patients develop exceptionally broad serum neutralizing antibodies responses, a better understanding of which would be valuable for vaccine efforts aimed at eliciting similar specificities.

RECENT FINDINGS

New mapping methodologies are now providing an appreciation of the incidence of specificities similar to the existing known bnmAbs as well as some intriguing insights into novel specificities.

SUMMARY

The new information emerging from mapping efforts should help to sharpen efforts to isolate new bnmAbs and moreover, may provide crucial information for the rational design of novel vaccine candidates.

New virologic reagents for neutralizing antibody assays

PURPOSE OF REVIEW

This review summarizes current and novel virologic reagents employed for the development and application of in-vitro assays that assess neutralizing activity of antibodies against HIV-1. Characteristics of several virologic approaches are placed in context with various cellular targets and assay read-outs intended to determine potency and breadth of neutralization in patient cohorts and clinical vaccine trials.

RECENT FINDINGS

New molecular virologic reagents developed for in-vitro primary cell-based assays promise to facilitate rigorous and standardized assessment of anti-HIV-1-neutralizing antibody responses elicited by vaccine immunogens.

SUMMARY

Comprehensive assessment of anti-HIV-1 antibody potency and breadth is essential for evaluating vaccine immunogens, the advancement of vaccine candidates into clinical trials, and ultimately the development of effective vaccine strategies. Env-pseudovirion and recombinant reporter cell line neutralization assays are important tools for rapid and standardized measurement of neutralizing antibody activity. However, recent studies indicate that reporter cell lines fail to detect neutralization activity of certain antibodies observed when analyzed in peripheral blood mononuclear cells and may yield results on neutralizing antibody breadth that are discordant with peripheral blood mononuclear cell assays. Importantly, it remains unknown whether current in-vitro assays may be predictive of a protective neutralizing antibody response elicited by vaccine immunogens. This situation underscores the significance of standardizing existing, complementary methods as well as developing new assay concepts that assess neutralization in primary cells. Thus, this chapter focuses on new virologic reagents that promise to facilitate reaching this goal.

sCD4-17b bifunctional protein: extremely broad and potent neutralization of HIV-1 Env pseudotyped viruses from genetically diverse primary isolates

BACKGROUND

We previously described a potent recombinant HIV-1 neutralizing protein, sCD4-17b, composed of soluble CD4 attached via a flexible polypeptide linker to an SCFv of the 17b human monoclonal antibody directed against the highly conserved CD4-induced bridging sheet of gp120 involved in coreceptor binding. The sCD4 moiety of the bifunctional protein binds to gp120 on free virions, thereby enabling the 17b SCFv moiety to bind and block the gp120/coreceptor interaction required for entry. The previous studies using the MAGI-CCR5 assay system indicated that sCD4-17b (in concentrated cell culture medium, or partially purified) potently neutralized several genetically diverse HIIV-1 primary isolates; however, at the concentrations tested it was ineffective against several other strains despite the conservation of binding sites for both CD4 and 17b. To address this puzzle, we designed variants of sCD4-17b with different linker lengths, and tested the neutralizing activities of the immunoaffinity purified proteins over a broader concentration range against a large number of genetically diverse HIV-1 primary isolates, using the TZM-bl Env pseudotype assay system. We also examined the sCD4-17b sensitivities of isogenic viruses generated from different producer cell types.

RESULTS

We observed that immunoaffinity purified sCD4-17b effectively neutralized HIV-1 pseudotypes, including those from HIV-1 isolates previously found to be relatively insensitive in the MAGI-CCR5 assay. The potencies were equivalent for the original construct and a variant with a longer linker, as observed with both pseudotype particles and infectious virions; by contrast, a construct with a linker too short to enable simultaneous binding of the sCD4 and 17b SCFv moieties was much less effective. sCD4-17b displayed potent neutralizing activity against 100% of nearly 4 dozen HIV-1 primary isolates from diverse genetic subtypes (clades A, B, C, D, F, and circulating recombinant forms AE and AG). The neutralization breadth and potency were superior to what have been reported for the broadly neutralizing monoclonal antibodies IgG b12, 2G12, 2F5, and 4E10. The activity of sCD4-17b was found to be similar against isogenic virus particles from infectious molecular clones derived either directly from the transfected producer cell line or after a single passage through PBMCs; this contrasted with the monoclonal antibodies, which were less potent against the PMBC-passaged viruses.

CONCLUSIONS

The results highlight the extremely potent and broad neutralizing activity of sCD4-17b against genetically diverse HIV-1 primary isolates. The bifunctional protein has potential applications for antiviral approaches to combat HIV infection.

Correlates of antibody-mediated protection against HIV infection

PURPOSE OF REVIEW

The only unequivocal correlate of protection against primate immunodeficiency virus infection is the presence of neutralizing antibody at sufficient titre. This correlate has been determined experimentally using animal models, and the data are reproducible and robust. Recent advances have added further depth to this knowledge by moving us closer to understanding how antibodies neutralize HIV-1, and what effects they may have in vivo with regard to protection from infection and disease.

RECENT FINDINGS

This review will cover recent advances in our understanding of the structural basis of HIV-1 neutralization by antibody and how this understanding may relate to vaccine design, and incorporate this into the broader context of how antibodies may influence viral transmission, replication and disease.

SUMMARY

The sum of these findings provides a strong rationale for designing an HIV-1 vaccine on the principle of induction of neutralizing antibodies, although other effector functions of antibodies such as complement and antibody-mediated cellular immunity should also be borne in mind, as should CD4 and CD8 T cell responses.

Broad neutralization of human immunodeficiency virus type 1 mediated by plasma antibodies against the gp41 membrane proximal external region

We identified three cross-neutralizing plasma samples with high-titer anti-membrane proximal external region (MPER) peptide binding antibodies from among 156 chronically human immunodeficiency virus type 1-infected individuals. In order to establish if these antibodies were directly responsible for the observed neutralization breadth, we used MPER-coated magnetic beads to deplete plasmas of these specific antibodies. Depletion of anti-MPER antibodies from BB34, CAP206, and SAC21 resulted in 77%, 68%, and 46% decreases, respectively, in the number of viruses neutralized. Antibodies eluted from the beads showed neutralization profiles similar to those of the original plasmas, with potencies comparable to those of the known anti-MPER monoclonal antibodies (MAbs), 4E10, 2F5, and Z13e1. The anti-MPER neutralizing antibodies in BB34 were present in the immunoglobulin G3 subclass-enriched fraction. Alanine scanning of the MPER showed that the antibodies from these three plasmas had specificities distinct from those of the known MAbs, requiring one to three crucial residues at positions 670, 673, and 674. These data demonstrate the existence of MPER-specific cross-neutralizing antibodies in plasma, although the ability to elicit such potent antiviral antibodies during natural infection appears to be rare. Nevertheless, the identification of three novel antibody specificities within the MPER supports its further study as a promising target for vaccine design.

Improving the expression of recombinant soluble HIV Envelope glycoproteins using pseudo-stable transient transfection

The Envelope glycoprotein (Env) of the human immunodeficiency virus (HIV) is the target of neutralizing antibodies (NAbs). So far, HIV Env-derived immunogens have not been able to elicit broad neutralizing antibody responses against primary isolates. Identifying conditions that will permit the efficient production of different soluble HIV Env proteins will facilitate a high throughput comparative analysis of the immunogenicity of diverse Env constructs, potentially identifying Env forms that are more conducive to the elicitation of anti-HIV NAbs. Here we compared different cell types, transfection reagents, transfection conditions and different DNA expression vectors on soluble HIV Envelope expression levels. We identified optimal expression conditions and developed a protocol to streamline and maximize production of diverse HIV Env constructs. Using this optimized platform, milligram quantities of purified soluble HIV Env trimer can be routinely achieved in a rapid and cost-effective manner.

New cholesterol-specific antibodies remodel HIV-1 target cells' surface and inhibit their in vitro virus production

The importance of membrane rafts in HIV-1 infection is still in the focus of interest. Here, we report that new monoclonal anticholesterol IgG antibodies (ACHAs), recognizing clustered membrane cholesterol (e.g., in lipid rafts), rearrange the lateral molecular organization of HIV-1 receptors and coreceptors in the plasma membrane of HIV-1 permissive human T-cells and macrophages. This remodeling is accompanied with a substantial inhibition of their infection and HIV-1 production in vitro. ACHAs promote the association of CXCR4 with both CD4 and lipid rafts, consistent with the decreased lateral mobility of CXCR4, while Fab fragments of ACHAs do not show these effects. ACHAs do not directly mask the extracellular domains of either CD4 or CXCR4 nor do they affect CXCR4 internalization. No significant inhibition of HIV production is seen when the virus is preincubated with the antibodies prior to infection. Thus, we propose that the observed inhibition is mainly due to the membrane remodeling induced by cholesterol-specific antibodies on the target cells. This, in turn, may prevent the proper spatio-temporal juxtaposition of HIV-1 glycoproteins with CD4 and chemokine receptors, thus negatively interfering with virus attachment/entry.

Structural and immunogenicity studies of a cleaved, stabilized envelope trimer derived from subtype A HIV-1

SOSIP gp140 trimers represent a soluble, stabilized, proteolytically cleaved form of the HIV-1 envelope (Env) glycoproteins. SOSIP gp140 derived from a subtype A HIV-1 isolate, KNH1144, forms exceptionally stable trimers that resemble virion-associated Env in antigenicity and topology. Here, we used electron microscopy to demonstrate that KNH1144 SOSIP gp140 trimers bound three soluble CD4 molecules in a symmetrical orientation similar to that seen for native Env. We compared the immunogenicities of KNH1144 SOSIP gp140 trimers and gp120 monomers in rabbits and found that the trimers were superior at eliciting neutralizing antibodies (NAbs) to homologous virus as well as neutralization-sensitive subtype B and C viruses. The NAb specificities for SOSIP antisera mapped in part to the CD4 binding site on gp120. We also observed adjuvant-dependent induction of antibodies to the residual levels of host cell proteins (HCPs) contained in the purified Env preparations. When present, HCP antibodies enhanced pseudovirus infection. Our findings are relevant for the further development of Env-based vaccines for HIV-1.

Conserved peptides within the E2 region of Hepatitis C virus induce humoral and cellular responses in goats

The reason(s) why human antibodies raised against hepatitis C virus (HCV) E2 epitopes do not offer protection against multiple viral infections may be related to either genetic variations among viral strains particularly within the hypervariable region-1 (HVR-1), low titers of anti E2 antibodies or interference of non neutralizing antibodies with the function of neutralizing antibodies. This study was designed to assess the immunogenic properties of genetically conserved peptides derived from the C-terminal region of HVR-1 as potential therapeutic and/or prophylactic vaccines against HCV infection. Goats immunized with E2-conserved synthetic peptides termed p36 (a.a 430-446), p37(a.a 517-531) and p38 (a.a 412-419) generated high titers of anti-p36, anti-p37 and anti-P38 antibody responses of which only anti- p37 and anti- p38 were neutralizing to HCV particles in sera from patients infected predominantly with genotype 4a. On the other hand anti-p36 exhibited weak viral neutralization capacity on the same samples. Animals super-immunized with single epitopes generated 2 to 4.5 fold higher titers than similar antibodies produced in chronic HCV patients. Also the studied peptides elicited approximately 3 fold increase in cell proliferation of specific antibody-secreting peripheral blood mononuclear cells (PBMC) from immunized goats. These results indicate that, besides E1 derived peptide p35 (a.a 315-323) described previously by this laboratory, E2 conserved peptides p37 and p38 represent essential components of a candidate peptide vaccine against HCV infection.

Conserved peptides within the E2 region of Hepatitis C virus induce humoral and cellular responses in goats

The reason(s) why human antibodies raised against hepatitis C virus (HCV) E2 epitopes do not offer protection against multiple viral infections may be related to either genetic variations among viral strains particularly within the hypervariable region-1 (HVR-1), low titers of anti E2 antibodies or interference of non neutralizing antibodies with the function of neutralizing antibodies. This study was designed to assess the immunogenic properties of genetically conserved peptides derived from the C-terminal region of HVR-1 as potential therapeutic and/or prophylactic vaccines against HCV infection. Goats immunized with E2-conserved synthetic peptides termed p36 (a.a 430-446), p37(a.a 517-531) and p38 (a.a 412-419) generated high titers of anti-p36, anti-p37 and anti-P38 antibody responses of which only anti- p37 and anti- p38 were neutralizing to HCV particles in sera from patients infected predominantly with genotype 4a. On the other hand anti-p36 exhibited weak viral neutralization capacity on the same samples. Animals super-immunized with single epitopes generated 2 to 4.5 fold higher titers than similar antibodies produced in chronic HCV patients. Also the studied peptides elicited approximately 3 fold increase in cell proliferation of specific antibody-secreting peripheral blood mononuclear cells (PBMC) from immunized goats. These results indicate that, besides E1 derived peptide p35 (a.a 315-323) described previously by this laboratory, E2 conserved peptides p37 and p38 represent essential components of a candidate peptide vaccine against HCV infection.

HIV/AIDS vaccines: a need for new concepts?

HIV vaccine research is at a crossroads carefully contemplating on the next path. The unexpected results of the Merck vaccine trial, while providing a stunning blow to a field in dire need of a protective vaccine, has also raised several fundamental questions regarding the candidate immunogen itself, preexisting immunity to vaccine vectors, surrogate assays and animal models used for assessing preclinical protective responses, as well as relevant endpoints to be measured in a clinical trial. As a result, the research community is faced with the daunting task of identifying novel vaccine concepts and products to continue the search. This review highlights and addresses some of the scientific and practical concerns.

Broad immunogenicity of a multigene, multiclade HIV-1 DNA vaccine boosted with heterologous HIV-1 recombinant modified vaccinia virus Ankara

BACKGROUND

A human immunodeficiency virus (HIV) vaccine that limits disease and transmission is urgently needed. This clinical trial evaluated the safety and immunogenicity of an HIV vaccine that combines a plasmid-DNA priming vaccine and a modified vaccinia virus Ankara (MVA) boosting vaccine.

METHODS

Forty healthy volunteers were injected with DNA plasmids containing gp160 of HIV-1 subtypes A, B, and C; rev B; p17/p24 gag A and B, and RTmut B by use of a needle-free injection system. The vaccine was administered intradermally or intramuscularly, with or without recombinant granulocyte macrophage colony-stimulating factor, and boosted with a heterologous MVA containing env, gag, and pol of CRF01A_E. Immune responses were monitored with HIV-specific interferon (IFN)-gamma and interleukin (IL)-2 ELISpot and lymphoproliferative assays (LPAs).

RESULTS

Vaccine-related adverse events were mild and tolerable. After receipt of the DNA priming vaccine, 11 (30%) of 37 vaccinees had HIV-specific IFN-gamma responses. After receipt of the MVA boosting vaccine, ELISpot assays showed that 34 (92%) of 37 vaccinees had HIV-specific IFN-gamma responses, 32 (86%) to Gag and 24 (65%) to Env. IFN-gamma production was detected in both the CD8(+) T cell compartment (5 of 9 selected vaccinees) and the CD4(+) T cell compartment (9 of 9). ELISpot results showed that 25 (68%) of 37 vaccinees had a positive IL-2 response and 35 (92%) of 38 had a positive LPA response. Of 38 subjects, a total of 37 (97%) were responders. One milligram of HIV-1 DNA administered intradermally was as effective as 4 mg administered intramuscularly in priming for the MVA boosting vaccine.

CONCLUSION

This HIV-DNA priming-MVA boosting approach is safe and highly immunogenic.

TRIALS REGISTRATION

International Standard Randomised Controlled Trial number: ISRCTN32604572 .

Developing an HIV cytotoxic T-lymphocyte vaccine: issues of CD8 T-cell quantity, quality and location

Issues of quantity, quality and location impact the ability of CD8 T cells to mediate protection from infection. These issues are considered in light of human immunodeficiency virus (HIV)/simian immunodeficiency virus (SIV) vaccination. Methods are reviewed that result in 100- to 1000-fold higher frequencies of vaccine-specific memory CD8 T cells than that achieved by current HIV/SIV vaccine approaches. Data demonstrating that location within mucosal tissues has a direct impact on memory CD8 T-cell function are discussed. Arguments are made that establishing memory CD8 T cells within mucosal sites of transmission, a priori to natural infection, may be essential for conferring optimal and rapid protection. Lastly, it is proposed that heterologous prime-boost vaccination with recombinant live replicating vectors, which has the potential to induce tremendous numbers of cytolytic memory CD8 T cells within mucosal tissues, would provide a far more stringent test of the hypothesis that memory CD8 T cells could, in principal, form the basis for a preventative HIV vaccine.

Profiling the specificity of neutralizing antibodies in a large panel of plasmas from patients chronically infected with human immunodeficiency virus type 1 subtypes B and C

Identifying the viral epitopes targeted by broad neutralizing antibodies (NAbs) that sometimes develop in human immunodeficiency virus type 1 (HIV-1)-infected subjects should assist in the design of vaccines to elicit similar responses. Here, we investigated the activities of a panel of 24 broadly neutralizing plasmas from subtype B- and C-infected donors using a series of complementary mapping methods, focusing mostly on JR-FL as a prototype subtype B primary isolate. Adsorption with gp120 immobilized on beads revealed that an often large but variable fraction of plasma neutralization was directed to gp120 and that in some cases, neutralization was largely mediated by CD4 binding site (CD4bs) Abs. The results of a native polyacrylamide gel electrophoresis assay using JR-FL trimers further suggested that half of the subtype B and a smaller fraction of subtype C plasmas contained a significant proportion of NAbs directed to the CD4bs. Anti-gp41 neutralizing activity was detected in several plasmas of both subtypes, but in all but one case, constituted only a minor fraction of the overall neutralization activity. Assessment of the activities of the subtype B plasmas against chimeric HIV-2 viruses bearing various fragments of the membrane proximal external region (MPER) of HIV-1 gp41 revealed mixed patterns, implying that MPER neutralization was not dominated by any single specificity akin to known MPER-specific monoclonal Abs. V3 and 2G12-like NAbs appeared to make little or no contribution to JR-FL neutralization titers. Overall, we observed significant titers of anti-CD4bs NAbs in several plasmas, but approximately two-thirds of the neutralizing activity remained undefined, suggesting the existence of NAbs with specificities unlike any characterized to date.

Mechanisms of human papillomavirus type 16 neutralization by l2 cross-neutralizing and l1 type-specific antibodies

Pseudovirions of human papillomavirus type 16 (HPV16), the principal etiologic agent in 50% of cervical cancers, were used as a model system to investigate the cell surface interactions involved in the exposure of the broadly cross-neutralizing papillomavirus L2 epitopes. These neutralizing epitopes were exposed only after cell surface binding and a subsequent change in capsid conformation that permitted cleavage by the cellular protease furin at a specific highly conserved site in L2 that is immediately upstream of the cross-neutralizing epitopes. Unexpectedly, binding of L2 antibodies led to the release of the capsid/antibody complexes from the cell surface and their accumulation on the extracellular matrix. Study of the dynamics of exposure of the L2 epitopes further revealed that representatives of the apparently dominant class of L1-specific neutralizing antibodies induced by virus-like particle vaccination prevent infection, not by preventing cell surface binding but rather by preventing the conformation change involved in exposure of the L2 neutralizing epitope. These findings suggest a dynamic model of virion-cell surface interactions that has implications for both evolution of viral serotypes and the efficacy of current and future HPV vaccines.

The failed HIV Merck vaccine study: a step back or a launching point for future vaccine development?

The world of human immunodeficiency virus (HIV) vaccines has suffered a baffling setback. The first trial of a vaccine designed to elicit strong cellular immunity has shown no protection against infection. More alarmingly, the vaccine appeared to increase the rate of HIV infection in individuals with prior immunity against the adenovirus vector used in the vaccine. A new study in this issue suggests that a different vaccine approach—using a DNA prime/poxvirus boost strategy—induces polyfunctional immune responses to an HIV immunogen. The disappointing results of the recent vaccine trial suggest that a more thorough assessment of vaccine-induced immune responses is urgently needed, and that more emphasis should be placed on primate models before efficacy trials are undertaken.

Analysis of the human immunodeficiency virus type 1 gp41 membrane proximal external region arrayed on hepatitis B surface antigen particles

Vaccine immunogens derived from the envelope glycoproteins of the human immunodeficiency virus type 1 (HIV-1) that elicit broad neutralizing antibodies remain an elusive goal. The highly conserved 30 amino-acid membrane proximal external region (MPER) of HIV gp41 contains the hydrophobic epitopes for two rare HIV-1 broad cross-reactive neutralizing antibodies, 2F5 and 4E10. Both these antibodies possess relatively hydrophobic HCDR3 loops and demonstrate enhanced binding to their epitopes in the context of the native gp160 precursor envelope glycoprotein by the intimate juxtaposition of a lipid membrane. The hepatitis B surface antigen (HBsAg) S1 protein forms nanoparticles that can be utilized both as an immunogenic array of the MPER and to provide the lipid environment needed for enhanced 2F5 and 4E10 binding. We show that recombinant HBsAg particles with MPER (HBsAg-MPER) appended at the C-terminus of the S1 protein are recognized by 2F5 and 4E10 with high affinity compared to positioning the MPER at the N-terminus or the extracellular loop (ECL) of S1. Addition of C-terminal hydrophobic residues derived from the HIV-1 Env transmembrane region further enhances recognition of the MPER by both 2F5 and 4E10. Delipidation of the HBsAg-MPER particles decreases 2F5 and 4E10 binding and subsequent reconstitution with synthetic lipids restores optimal binding. Inoculation of the particles into small animals raised cross-reactive antibodies that recognize both the MPER and HIV-1 gp160 envelope glycoproteins expressed on the cell surface; however, no neutralizing activity could be detected. Prime:Boost immunization of the HBsAg-MPER particles in sequence with HIV envelope glycoprotein proteoliposomes (Env-PLs) did not raise neutralizing antibodies that could be mapped to the MPER region. However, the Env-PLs did raise anti-Env antibodies that had the ability to neutralize selected HIV-1 isolates. The first generation HBsAg-MPER particles represent a unique means to present HIV-1 envelope glycoprotein neutralizing determinants to the immune system.

Hepatitis C virus envelope glycoprotein immunization of rodents elicits cross-reactive neutralizing antibodies

Neutralizing antibody responses elicited during infection generally confer protection from infection. Hepatitis C virus (HCV) encodes two glycoproteins E1 and E2 that are essential for virus entry and are the major target for neutralizing antibodies. To assess whether both glycoproteins are required for the generation of a neutralizing antibody response, rodents were immunized with a series of glycoproteins comprising full length and truncated versions. Guinea pigs immunized with HCV-1 genotype 1a E1E2p7, E1E2 or E2 generated high titer anti-glycoprotein antibody responses that neutralized the infectivity of HCVpp and HCVcc expressing gps of the same genotype as the immunizing antigen. Less potent neutralization of viruses bearing the genotype 2 strain J6 gps was observed. In contrast, immunized mice demonstrated reduced anti-gp antibody responses, consistent with their minimal neutralizing activity. Immunization with E2 alone was sufficient to induce a high titer response that neutralized HCV pseudoparticles (HCVpp) bearing diverse glycoproteins and cell culture grown HCV (HCVcc). The neutralization titer was reduced 3-fold by the presence of lipoproteins in human sera. Cross-competition of the guinea pig anti-E1E2 immune sera with a panel of epitope mapped anti-E2 monoclonal antibodies for binding E2 identified a series of epitopes within the N-terminal domain that may be immunogenic in the immunized rodents. These data demonstrate that recombinant E2 and E1E2 can induce polyclonal antibody responses with cross-reactive neutralizing activity, supporting the future development of prophylactic and therapeutic vaccines.