Increased incorporation of chimeric human immunodeficiency virus type 1 gp120 proteins into Pr55gag virus-like particles by an Epstein-Barr virus gp220/350-derived transmembrane domain

Insect cell production of chimeric virus-like particles based on human immunodeficiency virus GAG proteins and yellow fever virus envelope protein

The yellow fever virus (YFV) is a single stranded RNA virus belonging to the genus Orthoflavivirus that is capable of zoonotic transmissions that infect nonhuman and human primates. It is endemic in Brazil with recurrent epidemics of the disease, and it is transmitted through mosquitoes. The detection and immunization against YFV and other flaviviruses are fundamental for the management of the impacts of the disease in human environments. In an ongoing effort to develop new approaches for diagnostics and immunizations, we expressed VLPs displaying the yellow fever virus envelope protein (YFE) using recombinant baculovirus in insect cells. By co-expressing HIV-1 Pr55Gag protein (GAG) together with YFE we were able to generate chimeric VLPs containing a GAG core together with an envelope containing the YFE protein. The YFE and the chimeric GAG-YFE VLPs have potential as vaccine candidates and as reagents for serological assays in the detection of these viruses in human sera.

Nanoadjuvants: Promising Bioinspired and Biomimetic Approaches in Vaccine Innovation

Adjuvants are the important part of vaccine manufacturing as they elicit the vaccination effect and enhance the durability of the immune response through controlled release. In light of this, nanoadjuvants have shown unique broad spectrum advantages. As nanoparticles (NPs) based vaccines are fast-acting and better in terms of safety and usability parameters as compared to traditional vaccines, they have attracted the attention of researchers. A vaccine nanocarrier is another interesting and promising area for the development of next-generation vaccines for prophylaxis. This review looks at the various nanoadjuvants and their structure-function relationships. It compiles the state-of-art literature on numerous nanoadjuvants to help domain researchers orient their understanding and extend their endeavors in vaccines research and development.

Design Concepts of Virus-Like Particle-Based HIV-1 Vaccines

Prophylactic vaccines remain the best approach for controlling the human immunodeficiency virus-1 (HIV-1) transmission. Despite the limited efficacy of the RV144 trial in Thailand, there is still no vaccine candidate that has been proven successful. Consequently, great efforts have been made to improve HIV-1 antigens design and discover delivery platforms for optimal immune elicitation. Owing to immunogenic, structural, and functional diversity, virus-like particles (VLPs) could act as efficient vaccine carriers to display HIV-1 immunogens and provide a variety of HIV-1 vaccine development strategies as well as prime-boost regimes. Here, we describe VLP-based HIV-1 vaccine candidates that have been enrolled in HIV-1 clinical trials and summarize current advances and challenges according to preclinical results obtained from five distinct strategies. This mini-review provides multiple perspectives to help in developing new generations of VLP-based HIV-1 vaccine candidates with better capacity to elicit specific anti-HIV immune responses.

Critical design criteria for engineering a nanoparticulate HIV-1 vaccine

Inducing a long-lasting as well as broad and potent immune response by generating broadly neutralizing antibodies is a major goal and at the same time the main challenge of preventive HIV-1 vaccine design. Immunization with soluble, stabilized and native-like envelope (Env) glycoprotein so far only led to low neutralization breadth and displayed low immunogenicity. A promising approach to generate a potent immune response is the presentation of Env on the surface of nanoparticles. In this review, we will focus on two key processes essential for the induction of immune response that can be addressed by specific features of nanoparticulate carriers: first, the trafficking to and within distinct compartments of the lymph node, and second, the use of multivalent Env display allowing for high avidity interactions. To optimize these pivotal steps critical design criteria should be considered for the presentation of Env on nanoparticles. These include an optimal particle size below 100 nm, distances between two adjacent Env antigens of approximately 10-15 nm, an appropriate orientation of Env, and finally, the stability of both the Env attachment and the nanoparticle platform. Hence, an interdisciplinary approach that combines a suitable delivery system and a straightforward presentation of the Env antigen may have the potential to drive the immune response towards increased breadth and potency.

Nanoparticle Vaccines Against Infectious Diseases

Due to emergence of new variants of pathogenic micro-organisms the treatment and immunization of infectious diseases have become a great challenge in the past few years. In the context of vaccine development remarkable efforts have been made to develop new vaccines and also to improve the efficacy of existing vaccines against specific diseases. To date, some vaccines are developed from protein subunits or killed pathogens, whilst several vaccines are based on live-attenuated organisms, which carry the risk of regaining their pathogenicity under certain immunocompromised conditions. To avoid this, the development of risk-free effective vaccines in conjunction with adequate delivery systems are considered as an imperative need to obtain desired humoral and cell-mediated immunity against infectious diseases. In the last several years, the use of nanoparticle-based vaccines has received a great attention to improve vaccine efficacy, immunization strategies, and targeted delivery to achieve desired immune responses at the cellular level. To improve vaccine efficacy, these nanocarriers should protect the antigens from premature proteolytic degradation, facilitate antigen uptake and processing by antigen presenting cells, control release, and should be safe for human use. Nanocarriers composed of lipids, proteins, metals or polymers have already been used to attain some of these attributes. In this context, several physico-chemical properties of nanoparticles play an important role in the determination of vaccine efficacy. This review article focuses on the applications of nanocarrier-based vaccine formulations and the strategies used for the functionalization of nanoparticles to accomplish efficient delivery of vaccines in order to induce desired host immunity against infectious diseases.

Particle-based delivery of the HIV envelope protein

PURPOSE OF REVIEW

A major focus in HIV vaccine research is the development of suitable antigens that elicit broadly neutralizing antibody responses targeting HIV's envelope protein (Env). Delivery of Env in a repetitive manner on particle-based carriers allows higher avidity interactions and is therefore expected to efficiently engage B cells, thus leading to affinity maturation that results in superior antibody responses characterized by improved breadth, potency, and durability. This review summarizes current work that is evaluating diverse types of such particulate carriers for Env delivery.

RECENT FINDINGS

Various types of particle scaffolds are being investigated, encompassing group-specific antigen-derived virus-like particles, bacteria-derived proteins that self-assemble into symmetrical nanoparticles, as well as liposomes assembled from membrane components and recombinantly produced Env isoforms. Env-derived antigens from peptides over selected isolates to improved, stabilized next-generation designer Envs have been attached to such carriers. Immunological evaluation in animal models showed that these structures often elicit superior humoral immune responses.

SUMMARY

The findings reviewed here emphasize the potential of particle-based delivery modalities to elicit better antibody responses. Together with advances in Env antigen design, these approaches may synergistically act together on the way to obtain vaccine candidates that potentially induce protective immune responses against HIV.

Dense Array of Spikes on HIV-1 Virion Particles

HIV-1 is rare among viruses for having a low number of envelope glycoprotein (Env) spikes per virion, i.e., ∼7 to 14. This exceptional feature has been associated with avoidance of humoral immunity, i.e., B cell activation and antibody neutralization. Virus-like particles (VLPs) with increased density of Env are being pursued for vaccine development; however, these typically require protein engineering that alters Env structure. Here, we used instead a strategy that targets the producer cell. We employed fluorescence-activated cell sorting (FACS) to sort for cells that are recognized by trimer cross-reactive broadly neutralizing antibody (bnAb) and not by nonneutralizing antibodies. Following multiple iterations of FACS, cells and progeny virions were shown to display higher levels of antigenically correct Env in a manner that correlated between cells and cognate virions (P = 0.027). High-Env VLPs, or hVLPs, were shown to be monodisperse and to display more than a 10-fold increase in spikes per particle by electron microscopy (average, 127 spikes; range, 90 to 214 spikes). Sequencing revealed a partial truncation in the C-terminal tail of Env that had emerged in the sort; however, iterative rounds of "cell factory" selection were required for the high-Env phenotype. hVLPs showed greater infectivity than standard pseudovirions but largely similar neutralization sensitivity. Importantly, hVLPs also showed superior activation of Env-specific B cells. Hence, high-Env HIV-1 virions, obtained through selection of producer cells, represent an adaptable platform for vaccine design and should aid in the study of native Env.IMPORTANCE The paucity of spikes on HIV is a unique feature that has been associated with evasion of the immune system, while increasing spike density has been a goal of vaccine design. Increasing the density of Env by modifying it in various ways has met with limited success. Here, we focused instead on the producer cell. Cells that stably express HIV spikes were screened on the basis of high binding by bnAbs and low binding by nonneutralizing antibodies. Levels of spikes on cells correlated well with those on progeny virions. Importantly, high-Env virus-like particles (hVLPs) were produced with a manifest array of well-defined spikes, and these were shown to be superior in activating desirable B cells. Our study describes HIV particles that are densely coated with functional spikes, which should facilitate the study of HIV spikes and their development as immunogens.

Intracellular targeting with engineered proteins

If the isolation, production, and clinical use of insulin marked the inception of the age of biologics as therapeutics, the convergence of molecular biology and combinatorial engineering techniques marked its coming of age. The first wave of recombinant protein-based drugs in the 1980s demonstrated emphatically that proteins could be engineered, formulated, and employed for clinical advantage. Yet despite the successes of protein-based drugs such as antibodies, enzymes, and cytokines, the druggable target space for biologics is currently restricted to targets outside the cell. Insofar as estimates place the number of proteins either secreted or with extracellular domains in the range of 8000 to 9000, this represents only one-third of the proteome and circumscribes the pathways that can be targeted for therapeutic intervention. Clearly, a major objective for this field to reach maturity is to access, interrogate, and modulate the majority of proteins found inside the cell. However, owing to the large size, complex architecture, and general cellular impermeability of existing protein-based drugs, this poses a daunting challenge. In recent years, though, advances on the two related fronts of protein engineering and drug delivery are beginning to bring this goal within reach. First, prompted by the restrictions that limit the applicability of antibodies, intense efforts have been applied to identifying and engineering smaller alternative protein scaffolds for the modulation of intracellular targets. In parallel, innovative solutions for delivering proteins to the intracellular space while maintaining their stability and functional activity have begun to yield successes. This review provides an overview of bioactive intrabodies and alternative protein scaffolds amenable to engineering for intracellular targeting and also outlines advances in protein engineering and formulation for delivery of functional proteins to the interior of the cell to achieve therapeutic action.

High-Density Array of Well-Ordered HIV-1 Spikes on Synthetic Liposomal Nanoparticles Efficiently Activate B Cells

A major step toward an HIV-1 vaccine is an immunogen capable of inducing neutralizing antibodies. Envelope glycoprotein (Env) mimetics, such as the NFL and SOSIP designs, generate native-like, well-ordered trimers and elicit tier 2 homologous neutralization (SOSIPs). We reasoned that the display of well-ordered trimers by high-density, particulate array would increase B cell activation compared to soluble trimers. Here, we present the design of liposomal nanoparticles displaying well-ordered Env spike trimers on their surface. Biophysical analysis, cryo- and negative stain electron microscopy, as well as binding analysis with a panel of broadly neutralizing antibodies confirm a high-density, well-ordered trimer particulate array. The Env-trimer-conjugated liposomes were superior to soluble trimers in activating B cells ex vivo and germinal center B cells in vivo. In addition, the trimer-conjugated liposomes elicited modest tier 2 homologous neutralizing antibodies. The trimer-conjugated liposomes represent a promising initial lead toward the development of more effective HIV vaccine immunogens.

Formulation and stabilization of recombinant protein based virus-like particle vaccines

Vaccine formulation development has traditionally focused on improving antigen storage stability and compatibility with conventional adjuvants. More recently, it has also provided an opportunity to modify the interaction and presentation of an antigen/adjuvant to the immune system to better stimulate the desired immune responses for maximal efficacy. In the last decade, there has been a paradigm shift in vaccine antigen and formulation design involving an improved physical understanding of antigens and a better understanding of the immune system. In addition, the discovery of novel adjuvants and delivery systems promises to further improve the design of new, more effective vaccines. Here we describe some of the fundamental aspects of formulation design applicable to virus-like-particle based vaccine antigens (VLPs). Case studies are presented for commercially approved VLP vaccines as well as some investigational VLP vaccine candidates. An emphasis is placed on the biophysical analysis of vaccines to facilitate formulation and stabilization of these particulate antigens.

Biological and biochemical characterization of HIV-1 Gag/dgp41 virus-like particles expressed in Nicotiana benthamiana

The transmembrane HIV-1 envelope protein gp41 has been shown to play critical roles in the viral mucosal transmission and infection of CD4⁺ cells. Gag is a structural protein configuring the enveloped viral particles and has been suggested to constitute a target of the cellular immunity that may control viral load. We hypothesized that HIV enveloped virus-like particles (VLPs) consisting of Gag and a deconstructed form of gp41 comprising the membrane proximal external, transmembrane and cytoplasmic domains (dgp41) could be expressed in plants. To this end, plant-optimized HIV-1 genes were constructed and expressed in Nicotiana benthamiana by stable transformation, or transiently using a Tobamovirus-based expression system or a combination of both. Our results of biophysical, biochemical and electron microscopy characterization demonstrates that plant cells could support not only the formation of enveloped HIV-1 Gag VLPs, but also the accumulation of VLPs that incorporated dgp41. These findings provide further impetus for the journey towards a broadly efficacious and inexpensive subunit vaccine against HIV-1.

Developments in virus-like particle-based vaccines for HIV

Virus-like particles (VLPs) hold great promise for the development of effective and affordable vaccines. VLPs, indeed, are suitable for presentation and efficient delivery to antigen-presenting cells of linear as well as conformational antigens. This will ultimately result in a crosspresentation with both MHC class I and II molecules to prime CD4(+) T-helper and CD8(+) cytotoxic T cells. This review describes an update on the development and use of VLPs as vaccine approaches for HIV.

From virus-like particles to engineered exosomes for a new generation of vaccines

Over the last two decades, virus-like particles (VLPs) have been the focus of countless investigations on innovative vaccines. The number of monotypic, multipartite and chimeric VLP-based vaccines proposed have increased even further in the last few years as part of the continuous effort to improve the safety, efficacy and cost–effectiveness of immunogens. As compared with monomer- or subunit-based vaccines, VLPs show several advantages in terms of potency of the elicited immune responses. Chimeric VLPs are quite flexible tools to accommodate foreign peptides, cell proteins and nonself-assembling viral products. However, their use often meets with still unresolved hurdles such as induction of undesired immune responses, neutralization by pre-existing immunity and complex methods of production. Among strategies aimed at developing new nanoparticle-based vaccines, exosomes hold much promise. They are nanovesicles constitutively released by eukaryotic cells that originate from intraluminal vesicles accumulating in multivesicular bodies. Exosomes have immunogenic properties, the strength of which correlates with the amounts of associated antigens. Engineering antigens of interest to target them in exosomes represents the last frontier in terms of nanoparticle-based vaccines.

Developments in virus-like particle-based vaccines for infectious diseases and cancer

Virus-like particles hold great promise for the development of effective and affordable vaccines. Indeed, virus-like particles are suitable for presentation and efficient delivery of linear as well as conformational antigens to antigen-presenting cells. This will ultimately result in optimal B-cell activation and cross-presentation with both MHC class I and II molecules to prime CD4(+) T-helper as well as CD8(+) cytotoxic T cells. This article provides an update on the development and use of virus-like particles as vaccine approaches for infectious diseases and cancer.

Co-expression vs. co-infection using baculovirus expression vectors in insect cell culture: Benefits and drawbacks

The baculovirus expression vector system (BEVS) is a versatile and powerful platform for protein expression in insect cells. With the ability to approach similar post-translational modifications as in mammalian cells, the BEVS offers a number of advantages including high levels of expression as well as an inherent safety during manufacture and of the final product. Many BEVS products include proteins and protein complexes that require expression from more than one gene. This review examines the expression strategies that have been used to this end and focuses on the distinguishing features between those that make use of single polycistronic baculovirus (co-expression) and those that use multiple monocistronic baculoviruses (co-infection). Three major areas in which researchers have been able to take advantage of co-expression/co-infection are addressed, including compound structure-function studies, insect cell functionality augmentation, and VLP production. The core of the review discusses the parameters of interest for co-infection and co-expression with time of infection (TOI) and multiplicity of infection (MOI) highlighted for the former and the choice of promoter for the latter. In addition, an overview of modeling approaches is presented, with a suggested trajectory for future exploration. The review concludes with an examination of the gaps that still remain in co-expression/co-infection knowledge and practice.

Design and immunogenicity assessment of HIV-1 virus-like particles as a candidate vaccine

The rapid growth of the global HIV/AIDS epidemic makes it a high priority to develop an effective vaccine. Since a live attenuated or inactivated HIV vaccine is not likely to be approved for clinical application due to safety concerns, HIV virus like particles (VLPs) offer an attractive alternative because they are considered safer since they lack viral genome. We got a stable eukaryotic cell line by G418 resistance selection, engineered to express the HIV-1 structure protein Gag and Env efficiently and stably. We confirmed the presence of Gag and Env proteins in the cell culture supernatant and that they could self-assemble into VLPs. These VLPs were found to be able to elicit specific humoral and cellular immune response after immunization without any adjuvant.

Generation of HIV-1 Virus-Like Particles expressing different HIV-1 glycoproteins

Elicitation of a potent and broadly neutralizing antibody response is the main goal of an effective preventive HIV-1 vaccine. It has been shown by us and others that the expression of Env glycoproteins on the surface of particulate structures, such as Virus-Like Particles (VLPs), could be a more efficient strategy to deliver conformational epitopes to the immune system. To this aim, VLPs expressing native HIV Env gp140 or gp41 glycoproteins have been produced in insect cells using a baculovirus expression system and characterized for appropriate protein expression. VLP-bound HIV gp140 glycoprotein showed the appropriate expression and trimeric conformation. Immunogenicity studies have been performed in BALB/C mice by intra-peritoneal administration and sera from immunized mice have been tested in ELISA assays, for their reactivity with HIV specific antigens, as well as in ex vivo neutralization assay. Sera from immunized animals showed a high reactivity with individual HIV proteins expressed in VLPs. Results of TZM-bl based neutralization assay show that combined sera from animals independently immunized with gp140- or full-length-gp41-expressing VLPs have an additive/synergistic effect in the neutralization activity of HIV pseudoviruses. In conclusion, novel VLPs expressing different HIV Env glycoproteins with native trimeric conformation have been generated, showing the induction of effective antibody response with neutralization activity in TZM-bl neutralization assay. These results confirm the effectiveness of VLPs as presentation and delivery system for conformational proteins and show the improved neutralization activity upon the combination of anti-sera elicited by different HIV envelope antigens, suggesting the possibility of broadening the spectrum of viral epitopes targeted by immune response.

HIV-Gag VLPs presenting trimeric HIV-1 gp140 spikes constitutively expressed in stable double transfected insect cell line

We have previously described the establishment and characterization of a stably transfected insect cell line for the constitutive and efficient expression of Pr55 HIV Gag proteins, which auto-assemble into enveloped Virus-Like Particles (VLPs) released into the cell culture supernatant. Such HIV-Gag VLPs have been shown to elicit a specific systemic humoral response in vivo, proving the appropriate antigenic presentation of the HIV Gag protein to the immune system. Here we describe the establishment of a stable double transfected insect cell line for the constitutive and reproducible production of Pr55Gag-VLPs expressing on their surface trimeric forms of HIV-1 envelope glycoproteins. The persistence of HIV coding genes has been verified in clonal resistant insect cells, the protein expression and conformation has been verified by Western blot analysis. The resulting HIV-VLPs have been visualized by standard transmission electron microscopy and their immunogenicity has been evaluated in vivo. This represents, to our knowledge, the first example of stable double transfected insect cell line for the constitutive production of enveloped HIV-Gag VLPs presenting trimeric HIV-gp140 on their surface.

Conformational HIV-1 envelope on particulate structures: a tool for chemokine coreceptor binding studies

The human immunodeficiency virus type 1 (HIV-1) external envelope glycoprotein gp120 presents conserved binding sites for binding to the primary virus receptor CD4 as well as the major HIV chemokine coreceptors, CCR5 and CXCR4.

Concerted efforts are underway to understand the specific interactions between gp120 and coreceptors as well as their contribution to the subsequent membrane fusion process.

The present review summarizes the current knowledge on this biological aspect, which represents one of the key and essential points of the HIV-host cell interplay and HIV life cycle. The relevance of conformational HIV-1 Envelope proteins presented on Virus-like Particles for appropriate assessment of this molecular interaction, is also discussed.

Retroviral display in gene therapy, protein engineering, and vaccine development

The display and analysis of proteins expressed on biological surfaces has become an attractive tool for the study of molecular interactions in enzymology, protein engineering, and high-throughput screening. Among the growing number of established display systems, retroviruses offer a unique and fully mammalian platform for the expression of correctly folded and post-translationally modified proteins in the context of cell plasma membrane-derived particles. This is of special interest for therapeutic applications such as gene therapy and vaccine development and also offers advantages for the engineering of mammalian proteins toward customized binding affinities and catalytic activities. This review critically summarizes the basic concepts and applications of retroviral display and analyses its benefits in comparison to other display techniques.

Current progress in the development of a prophylactic vaccine for HIV-1

Since its discovery and characterization in the early 1980s as a virus that attacks the immune system, there has been some success for the treatment of human immunodeficiency virus-1 (HIV-1) infection. However, due to the overwhelming public health impact of this virus, a vaccine is needed urgently. Despite the tireless efforts of scientist and clinicians, there is still no safe and effective vaccine that provides sterilizing immunity. A vaccine that provides sterilizing immunity against HIV infection remains elusive in part due to the following reasons: 1) degree of diversity of the virus, 2) ability of the virus to evade the hosts' immunity, and 3) lack of appropriate animal models in which to test vaccine candidates. There have been several attempts to stimulate the immune system to provide protection against HIV-infection. Here, we will discuss attempts that have been made to induce sterilizing immunity, including traditional vaccination attempts, induction of broadly neutralizing antibody production, DNA vaccines, and use of viral vectors. Some of these attempts show promise pending continued research efforts.

The membrane-spanning domain of gp41 plays a critical role in intracellular trafficking of the HIV envelope protein

Background

The sequences of membrane-spanning domains (MSDs) on the gp41 subunit are highly conserved among many isolates of HIV-1. The GXXXG motif, a potential helix-helix interaction motif, and an arginine residue (rare in hydrophobic MSDs) are especially well conserved. These two conserved elements are expected to locate on the opposite sides of the MSD, if the MSD takes a α-helical secondary structure. A scanning alanine-insertion mutagenesis was performed to elucidate the structure-function relationship of gp41 MSD.

Results

A circular dichroism analysis of a synthetic gp41 MSD peptide determined that the secondary structure of the gp41 MSD was α-helical. We then performed a scanning alanine-insertion mutagenesis of the entire gp41 MSD, progressively shifting the relative positions of MSD segments around the helix axis. Altering the position of Gly694, the last residue of the GXXXG motif, relative to Arg696 (the number indicates the position of the amino acid residues in HXB2 Env) around the axis resulted in defective fusion. These mutants showed impaired processing of the gp160 precursor into gp120 and gp41. Furthermore, these Env mutants manifested inefficient intracellular transport in the endoplasmic reticulum and Golgi regions. Indeed, a transplantation of the gp41 MSD portion into the transmembrane domain of another membrane protein, Tac, altered its intracellular distribution. Our data suggest that the intact MSD α-helix is critical in the intracellular trafficking of HIV-1 Env.

Conclusions

The relative position between the highly conserved GXXXG motif and an arginine residue around the gp41 MSD α-helix is critical for intracellular trafficking of HIV-1 Env. The gp41 MSD region not only modulates membrane fusion but also controls biosynthesis of HIV-1 Env.

Immunization with a Mixture of HIV Env DNA and VLP Vaccines Augments Induction of CD8 T Cell Responses

The immune response induced by immunization with HIV Env DNA and virus-like particle (VLP) vaccines was investigated. Immunization with the HIV Env DNA vaccine induced a strong CD8 T cell response but relatively weak antibody response against the HIV Env whereas immunization with VLPs induced higher levels of antibody responses but little CD8 T cell response. Interestingly, immunization with a mixture the HIV Env DNA and VLP vaccines induced enhanced CD8 T cell and antibody responses. Further, it was observed that the mixing of DNA and VLP vaccines during immunization is necessary for augmenting induction of CD8 T cell responses and such augmentation of CD8 T cell responses was also observed by mixing the HIV Env DNA vaccine with control VLPs. These results show that immunization with a mixture of DNA and VLP vaccines combines advantages of both vaccine platforms for eliciting high levels of both antibody and CD8 T cell responses.

Lentivirus-based virus-like particles as a new protein delivery tool

Virus Like Particles (VLPs) are self-assembling, nonreplicating, nonpathogenic, genomeless particles similar in size and conformation to intact infectious virions. The possibility of engineering VLPs to incorporate heterologous polypeptides/proteins renders VLPs attractive candidates for vaccine strategies, as well as for protein delivery for basic science. Among the wide number of VLP types, our expertise focused on both retro- and lentivirus based VLPs as protein delivery tools. In particular, here we describe a system relying on the finding that some HIV-1 Nef mutants are incorporated at high levels into both Human Immunodeficiency virus (HIV)-1 and Moloney Leukemia Virus (MLV)-based VLPs. Most importantly, these Nef mutants can efficiently act as anchoring proteins upon fusion with heterologous proteins up to 630 amino acids in length. This chapter describes the preparation of prototypic HIV-1 based VLPs incorporating Nef mutant-GFP fusion molecules. Besides having potential utility in the field of basic virology, these VLPs represent a useful reference model for recovering alternative retro- or lentiviral based VLPs for the cell delivery of polypeptides/proteins of interest.

Virus-like particle vaccine activates conventional B2 cells and promotes B cell differentiation to IgG2a producing plasma cells

We have previously shown that immunization with SIV-, SHIV-, or HA (influenza hemagglutinin)-virus-like particles (VLPs) elicits a strong humoral immune response in mice. However, little is known about the action VLPs exert on immune effector cells, including B cells. In this study, we found that all three types of VLPs could directly bind and activate B cells in vitro. VLPs stimulated the proliferation of B220(+)IgM(+)CD43(-)CD5(-) B2 cells and their differentiation to plasma cells that preferentially produce IgG2a antibodies. Up-regulation of Blimp-1, XBP-1, IRF4, and AID genes, which are responsible for class-switch recombination and somatic hypermutation, was observed in VLP-activated B2 cells. Stimulation of naïve splenocytes with VLPs led to a high expression of IL-12, RANTES and MIP, the cytokine milieu that favors B cell differentiation into IgG2a secreting cells. VLP immunization of C57BL/6 mice corroborated our in vitro data showing enlarged germinal centers and expanded conventional B2 cells, but no enlarged marginal zone B1 cells, in the spleen. Enhanced antigen-specific plasma cell formation, antibody production, and IgG2a class switching were found in VLP-immunized groups. The current study details the interaction between VLPs and B cells which result in preferential IgG2a antibody production following VLP immunization.

Influenza vaccines based on virus-like particles

The simultaneous expression of structural proteins of virus can produce virus-like particles (VLPs) by a self-assembly process in a viral life cycle even in the absence of genomic material. Taking an advantage of structural and morphological similarities of VLPs to native virions, VLPs have been suggested as a promising platform for new viral vaccines. In the light of a pandemic threat, influenza VLPs have been recently developed as a new generation of non-egg based cell culture-derived vaccine candidates against influenza infection. Animals vaccinated with VLPs containing hemagglutinin (HA) or HA and neuraminidase (NA) were protected from morbidity and mortality resulting from lethal influenza infections. Influenza VLPs serve as an excellent model system of an enveloped virus for understanding the properties of VLPs in inducing protective immunity. In this review, we briefly describe the characteristics of influenza VLPs assembled with a lipid bilayer containing glycoproteins, and summarize the current progress on influenza VLPs as an alternative vaccine candidate against seasonal as well as pandemic influenza viruses. In addition, the protective immune correlates induced by vaccination with influenza VLPs are discussed.

Influenza virus-like particles as pandemic vaccines

There is an urgent need to develop novel approaches for vaccination against emerging pathogenic avian influenza viruses as a priority for pandemic preparedness. Influenza virus-like particles (VLPs) have been suggested and developed as a new generation of non-egg-based cell culture-derived vaccine candidates against influenza infection. Influenza VLPs are formed by a self-assembly process incorporating structural proteins into budding particles composed of the hemagglutinin (HA), neuraminidase (NA) and M1 proteins, and may include additional influenza proteins such as M2. Animals vaccinated with VLPs were protected from morbidity and mortality resulting from lethal influenza infections. The protective mechanism of influenza VLP vaccines was similar to that of the currently licensed influenza vaccines inducing neutralizing antibodies and hemagglutination inhibition activities. Current studies demonstrate that influenza VLP approaches can be a promising alternative approach to developing a vaccine for pandemic influenza viruses. The first human clinical trial of a recombinant pandemic-like H5N1 influenza VLP vaccine was initiated in July 2007 (Bright et al., unpublished).

Incorporation of high levels of chimeric human immunodeficiency virus envelope glycoproteins into virus-like particles

The human immunodeficiency virus (HIV) envelope (Env) protein is incorporated into HIV virions or virus-like particles (VLPs) at very low levels compared to the glycoproteins of most other enveloped viruses. To test factors that influence HIV Env particle incorporation, we generated a series of chimeric gene constructs in which the coding sequences for the signal peptide (SP), transmembrane (TM), and cytoplasmic tail (CT) domains of HIV-1 Env were replaced with those of other viral or cellular proteins individually or in combination. All constructs tested were derived from HIV type 1 (HIV-1) Con-S DeltaCFI gp145, which itself was found to be incorporated into VLPs much more efficiently than full-length Con-S Env. Substitution of the SP from the honeybee protein mellitin resulted in threefold-higher chimeric HIV-1 Env expression levels on insect cell surfaces and an increase of Env incorporation into VLPs. Substitution of the HIV TM-CT with sequences derived from the mouse mammary tumor virus (MMTV) envelope glycoprotein, influenza virus hemagglutinin, or baculovirus (BV) gp64, but not from Lassa fever virus glycoprotein, was found to enhance Env incorporation into VLPs. The highest level of Env incorporation into VLPs was observed in chimeric constructs containing the MMTV and BV gp64 TM-CT domains in which the Gag/Env molar ratios were estimated to be 4:1 and 5:1, respectively, compared to a 56:1 ratio for full-length Con-S gp160. Electron microscopy revealed that VLPs with chimeric HIV Env were similar to HIV-1 virions in morphology and size and contained a prominent layer of Env spikes on their surfaces. HIV Env specific monoclonal antibody binding results showed that chimeric Env-containing VLPs retained conserved epitopes and underwent conformational changes upon CD4 binding.

Immunogenicity of recombinant human immunodeficiency virus type 1-like particles expressing gp41 derivatives in a pre-fusion state

The conserved membrane proximal external region (MPER) of the ectodomain of human immunodeficiency virus type 1 (HIV-1) gp41 is the target of two broadly neutralizing antibodies, 2F5 and 4E10. However, no neutralizing antibodies have been elicited against immunogens bearing these epitopes. Given that structural and biochemical studies suggest that the lipid membrane of the virion is involved in their proper configuration, HIV-1 gp41 derivatives in a pre-fusion state were expressed on the surface of immature virus like particles (VLP) derived from Sf9 cells. Guinea pigs were immunized with three doses of VLPs or Sf9 cells presenting gp41 derivatives with or without E. coli heat-labile enterotoxin (LT) as an adjuvant. While immune sera contained high titer anti-VLP antibodies, the specific anti-gp41 antibody responses were low with no neutralizing antibodies detected. An explanation for this absence may be the low level of gp41 expression relative to the many other proteins derived from host cells which are incorporated onto the VLP surface. In addition, the anti-gp41 immune response was preferentially directed to the C-helical domain, away from the MPER. Future vaccine design needs to contend with the complexity of epitope display as well as immunodominance.

Prime-boost immunization with DNA, recombinant fowlpox virus and VLP(SHIV) elicit both neutralizing antibodies and IFNgamma-producing T cells against the HIV-envelope protein in mice that control env-bearing tumour cells

Different primings with DNA and fowlpox virus (FP) recombinants or FP alone were used in a pre-clinical trial to evaluate and compare immunogenicity and efficacy against HIV/SHIV. Three immunization regimens were tested in three groups of mice in which the SIV gag/pol and HIV-1 env transgenes were separately expressed by DNA and FP vectors, followed by VLP(SHIV) boosting. All of the protocols were effective in eliciting homologous neutralizing antibodies, although the mice immunized with DNA followed by FP recombinants or DNA+FP recombinants showed both high titres of neutralizing antibodies and high frequencies of env-specific IFNgamma-producing T lymphocytes. Vaccine efficacy, as demonstrated by growth control of env-expressing tumours, was obtained in both of these two groups of mice. These results establish a preliminary profile for the combined use of these recombinant vectors in protocols to be tested in the SHIV-macaque model of HIV-1 infection.

Incorporation of glycosylphosphatidylinositol-anchored granulocyte- macrophage colony-stimulating factor or CD40 ligand enhances immunogenicity of chimeric simian immunodeficiency virus-like particles

The rapid worldwide spread of human immunodeficiency virus (HIV) mandates the development of successful vaccination strategies. Since live attenuated HIV is not accepted as a vaccine due to safety concerns, virus-like particles (VLPs) offer an attractive safe alternative because they lack the viral genome yet they are perceived by the immune system as a virus particle. We hypothesized that adding immunostimulatory signals to VLPs would enhance their efficacy. To accomplish this we generated chimeric simian immunodeficiency virus (SIV) VLPs containing either glycosylphosphatidylinositol (GPI)-anchored granulocyte-macrophage colony-stimulating factor (GM-CSF) or CD40 ligand (CD40L) and investigated their biological activity and ability to enhance immune responses in vivo. Immunization of mice with chimeric SIV VLPs containing GM-CSF induced SIV Env-specific antibodies as well as neutralizing activity at significantly higher levels than those induced by standard SIV VLPs, SIV VLPs containing CD40L, or standard VLPs mixed with soluble GM-CSF. In addition, mice immunized with chimeric SIV VLPs containing either GM-CSF or CD40L showed significantly increased CD4(+)- and CD8(+)-T-cell responses to SIV Env, compared to standard SIV VLPs. Taken together, these results demonstrate that the incorporation of immunostimulatory molecules enhances humoral and cellular immune responses. We propose that anchoring immunostimulatory molecules into SIV VLPs can be a promising approach to augmenting the efficacy of VLP antigens.

Elicitation of immunity to HIV type 1 Gag is determined by Gag structure

The gag gene of the human immunodeficiency virus type 1 (HIV-1) encodes for viral proteins that self-assemble into viral particles. The primary Gag gene products (capsid, matrix, and nucleocapsid) elicit humoral and cellular immune responses during natural infection, and these proteins are included in many preclinical and clinical HIV/AIDS vaccines. However, the structure (particulate or soluble) of these proteins may influence the immunity elicited during vaccination. In this study, mice were inoculated with four different HIV-1 Gag vaccines to compare the elicitation of immune responses by the same Gag immunogen presented to the immune system in different forms. The immunity elicited by particles produced in vivo by DNA plasmid (pGag) was compared to these same proteins retained intracellularly (pGag(DMyr)). In addition, the elicitation of anti- Gag immunity by Gag(p55) virus-like particles (VLPs) or soluble, nonparticulate Gag(p55) proteins was compared. Enhanced cellular responses, but almost no anti-Gag antibodies, were elicited with intracellularly retained Gag proteins. In contrast, DNA vaccines expressing VLPs elicited both anti-Gag antibodies and cellular responses. Mice vaccinated with purified Gag(p55) VLPs elicited robust humoral and cellular immune responses, which were significantly higher than the immunity elicited by soluble, nonparticulate Gag(p55) protein. Overall, purified particles of Gag effectively elicited the broadest and highest titers of anti-Gag immunity. The structural form of Gag influences the elicited immune responses and should be considered in the design of HIV/AIDS vaccines.

Molecular and biological characterization of simian-human immunodeficiency virus-like particles produced by recombinant fowlpox viruses

Virus-like particles (VLPs) mimicking the simian-human immunodeficiency virus SHIV89.6P (VLPSHIV) were produced by co-infection of Vero cells with fowlpox SIVgag/pol (FPgag/polSIV) and fowlpox HIV-1env89.6P (FPenv89.6P) recombinant viruses. As a necessary prerequisite for a more efficient vaccine approach, ultrastructural, functional and molecular characterizations of VLP(SHIV) were performed in the SHIV-macaque model to verify the similarity of these particles to SHIV89.6P. Here we show that VLPSHIV can infect T cells by fusion without replication, as demonstrated by the absence of new viral progeny in VLPSHIV-infected C8166 cells. Biochemical characterization showed identical protein profiles of VLPSHIV and SHIV89.6P, and ultrastructural analysis of Vero cells releasing VLPSHIV also confirmed the morphological similarity of these pseudovirions to SHIV89.6P particles. Viral mRNAs were also found packaged inside the core of VLPSHIV by RT-PCR and reverse transcriptase assays.

Induction of systemic and mucosal cross-clade neutralizing antibodies in BALB/c mice immunized with human immunodeficiency virus type 1 clade A virus-like particles administered by different routes of inoculation

We have recently developed a candidate human immunodeficiency virus type 1 (HIV-1) vaccine model, based on virus-like particles (VLPs) expressing gp120 from a Ugandan HIV-1 isolate of clade A (HIV-VLP(A)s), which shows the induction of neutralizing antibodies as well as cytotoxic T lymphocytes (CTL) in BALB/c mice by intraperitoneal (i.p.) administration. In the present study, immunization experiments based on a multiple-dose regimen have been performed with BALB/c mice to compare different routes of administration. i.p. and intranasal (i.n.), but not oral, administration induce systemic as well as mucosal (vaginal and intestinal) immunoglobulin G (IgG) and IgA responses. These immune sera exhibit >50% ex vivo neutralizing activity against both autologous and heterologous primary isolates. Furthermore, the administration of HIV-VLP(A)s by the i.n. immunization route induces a specific CTL activity, although at lower efficiency than the i.p. route. The HIV-VLP(A)s represent an efficient strategy to stimulate both arms of immunity; furthermore, the induction of specific humoral immunity at mucosal sites, which nowadays represent the main port of entry for HIV-1 infection, is of great interest. All these properties, and the possible cross-clade in vivo protection, could make these HIV-VLP(A)s a good candidate for a mono- and multicomponent worldwide preventive vaccine approach not restricted to high-priority regions, such as sub-Saharan countries.

Circumventing tolerance to the prion protein (PrP): vaccination with PrP-displaying retrovirus particles induces humoral immune responses against the native form of cellular PrP

Passive immunization with antibodies directed against the cellular form of the prion protein (PrPC) can protect against prion disease. However, active immunization with recombinant prion protein has so far failed to induce antibodies directed against native PrPC expressed on the cell surface. To develop an antiprion vaccine, a retroviral display system presenting either the full-length mouse PrP (PrP209) or the C-terminal 111 amino acids (PrP111) fused to the transmembrane domain of the platelet-derived growth factor receptor was established. Western blot analysis and immunogold electron microscopy of the retroviral display particles revealed successful incorporation of the fusion proteins into the particle membrane. Interestingly, retroviral particles displaying PrP111 (PrPD111 retroparticles) showed higher incorporation efficiencies than those displaying PrP209. Already 7 days after intravenous injection of PrPD111 retroparticles, PrPC-deficient mice (Prnp(o/o)) showed high immunoglobulin M (IgM) and IgG titers specifically binding the native PrPC molecule as expressed on the surface of T cells isolated from PrPC-overexpressing transgenic mice. More importantly, heterozygous Prnp(+/o) mice and also wild-type mice showed PrPC-specific IgM and IgG antibodies upon vaccination with PrPD111 retroparticles, albeit at considerably lower levels. Bacterially expressed recombinant PrP, in contrast, was unable to evoke IgG antibodies recognizing native PrPC in wild-type mice. Thus, our data show that PrP or parts thereof can be functionally displayed on retroviral particles and that immunization with PrP retroparticles may serve as a novel promising strategy for vaccination against transmissible spongiform encephalitis.

Modified HIV envelope proteins with enhanced binding to neutralizing monoclonal antibodies

The target for neutralizing antibodies against human immunodeficiency virus (HIV) is the trimeric Env protein on the native virion. Conserved neutralizing epitopes of receptor binding sites are located in the recessed core of the Env protein, partially masked by glycosylations and variable loops. In this study, we have investigated the effects of modifications of the HIV Env protein by glycosylation site mutations, deletions of variable loops, or combinations of both types of mutations on their protein functions and reactivities with neutralizing antibodies. Modified Env proteins were expressed in insect or mammalian cells, and their reactivity with epitope-specific broadly neutralizing monoclonal antibodies (Mabs) was determined by flow cytometry. A unique mutant designated 3G with mutations in three glycosylation motifs within the V3/C3 domains surrounding the CD4 binding site showed higher levels of binding to most broadly neutralizing Mabs (b12 and 2F5) in both insect and mammalian expression systems. Mutants with a deletion of both V1 and V2 loop domains or with a unique combination of both types of mutations also bound to most neutralizing Mabs at higher levels compared to the wild-type control. Most mutants maintained the ability to bind CD4 and to induce syncytium formation at similar or higher levels as compared to that of the wild-type Env protein, except for a mutant with a combination of variable loop deletions and deglycosylation mutations. Our study suggests that modified HIV Env proteins with reduced glycosylation in domains surrounding the CD4 binding site or variable loop-deleted mutants expose important neutralizing epitopes at higher levels than wild type and may provide novel vaccine immunogens.

Recombinant HIV-1 Pr55gag virus-like particles: potent stimulators of innate and acquired immune responses

Several previous reports have clearly demonstrated the strong effectiveness of human immunodeficiency virus (HIV) Gag polyprotein-based virus-like particles (VLP) to stimulate humoral and cellular immune responses in complete absence of additional adjuvants. Yet, the mechanisms underlying the strong immunogenicity of these particulate antigens are still not very clear. However, current reports strongly indicate that these VLP act as "danger signals" to trigger the innate immune system and possess potent adjuvant activity to enhance the immunogenicity of per se only weakly immunogenic peptides and proteins. Here, we review the current understanding of how various particle-associated substances and other impurities may contribute to the observed immune-activating properties of these complex immunogens.

Virus-like particles as HIV-1 vaccines

Traditional successful antiviral vaccines have relied mostly on live-attenuated viruses. Live-attenuated HIV vaccine candidates are not ideal as they pose risks of reversion, recombination or mutations. Other current HIV vaccine candidates have difficulties generating broadly effective neutralising antibodies and cytotoxic T cell immune responses to primary HIV isolates. Virus-like-particles (VLPs) have been demonstrated to be safe to administer to animals and human patients as well as being potent and efficient stimulators of cellular and humoral immune responses. Therefore, VLPs are being considered as possible HIV vaccines. Chimeric HIV-1 VLPs constructed with either HIV or SIV capsid protein plus HIV immune epitopes and immuno-stimulatory molecules have further improved on early VLP designs, leading to enhanced immune stimulation. The administration of VLP vaccines via mucosal surfaces has also emerged as a promising strategy with which to elicit mucosal and systemic humoral and cellular immune responses. Additionally, new information on antigen processing and the presentation of particulate antigens by dendritic cells (DCs) has created new strategies for improved VLP vaccine candidates. This paper reviews the field of HIV-1 VLP vaccine development, focusing on recent studies that will likely uncover promising prospects for new HIV vaccines.

Mucosal immunization with virus-like particles of simian immunodeficiency virus conjugated with cholera toxin subunit B

To enhance the efficiency of antigen uptake at mucosal surfaces, CTB was conjugated to simian immunodeficiency virus (SIV) virus-like particles (VLPs). We characterized the immune responses to the Env and Gag proteins after intranasal administration. Intranasal immunization with a mixture of VLPs and CTB as an adjuvant elicited higher levels of SIV gp160-specific immunoglobulin G (IgG) in sera and IgA in mucosae, including saliva, vaginal-wash samples, lung, and intestine, as well as a higher level of neutralization activities than immunization with VLPs alone. Conjugation of CTB to VLPs also enhanced the SIV VLP-specific antibodies in sera and in mucosae to similar levels. Interestingly, CTB-conjugated VLPs showed higher levels of cytokine (gamma interferon)-producing splenocytes and cytotoxic-T-lymphocyte activities of immune cells than VLPs plus CTB, as well as an increased level of both IgG1 and IgG2a serum antibodies, which indicates enhancement of both Th1- and Th2-type cellular immune responses. These results demonstrate that CTB can be an effective mucosal adjuvant in the context of VLPs to induce enhanced humoral, as well as cellular, immune responses.

Enhancement of mucosal immunization with virus-like particles of simian immunodeficiency virus

Cholera toxin (CT) is the most potent known mucosal adjuvant, but its toxicity precludes its use in humans. Here, in an attempt to develop safe and effective mucosal adjuvants, we compared immune responses to simian immunodeficiency virus (SIV) virus-like particles (VLPs) after intranasal coimmunization with RANTES, CpG oligodeoxynucleotides (ODN), or CT. Antibody analysis demonstrated that RANTES and CpG ODN had capacities for mucosal adjuvanticity, i.e., for enhancing serum and vaginal antibodies specific to SIV Env, similar to those for CT. RANTES and CpG ODN skewed serum antibodies predominantly to the immunoglobulin G2a isotype. Most importantly, RANTES and CpG ODN were more effective than CT in increasing neutralizing titers of both serum and vaginal antibodies. After intranasal coadministration with VLPs, RANTES or CpG ODN also induced increased levels of gamma interferon (IFN-gamma)-producing lymphocyte and cytotoxic T-lymphocyte activities in both spleen and lymph nodes but did not increase the levels of interleukin-4-producing lymphocytes. The results suggest that RANTES and CpG ODN enhance immune responses in a T-helper-cell-type-1 (Th1)-oriented manner and that they can be used as effective mucosal adjuvants for enhancing both humoral and cellular immune responses in the context of VLPs, which are particulate antigens.

Induction of neutralizing antibodies and cytotoxic T lymphocytes in Balb/c mice immunized with virus-like particles presenting a gp120 molecule from a HIV-1 isolate of clade A

We have recently developed a candidate HIV-1 vaccine based on virus-like particles (VLPs) expressing a gp120 from an Ugandan HIV-1 isolate of the clade A (HIV-VLP(A)s). In vivo immunogenicity experiments were performed in Balb/c mice, with an immunization schedule based on a multiple-dose regimen of HIV-VLP(A)s without adjuvants, showing a significant induction of both humoral and cellular immunity. The Env-specific cellular response was investigated in vitro, scoring for both the proliferative response of T helper cells and the cytolytic activity of cytotoxic T lymphocytes (CTLs). Furthermore, immune sera showed >50% neutralization activity against both the autologous field isolate and the heterologous T cell adapted B-clade HIV-1(IIIB) viral strain. This is one of the first examples of HIV-1 vaccines based on antigens derived from the A clade, which represents >25% of all isolates identified world wide. In particular, the A clade is predominant in sub-Saharan countries, where 70% of the global HIV-1 infections occur, and where vaccination is the only rational strategy for an affordable prevention against HIV-1 infection.

Production and characterization of simian--human immunodeficiency virus-like particles

We have produced and characterized, in a baculovirus expression system, simian-human immunodeficiency virus-like particles (SHIV VLPs) containing SIV Gag and HIV envelope (Env) proteins. Recombinant SIV gag (SIVmac239) and full-length or cytoplasmic domain-truncated HIV env from either HIV BH10 or HIV 89.6 virus were coexpressed in insect cells and Env incorporation into released SHIV VLPs was characterized. The expression level of the Env protein was found to be about 20-50% higher in both strains producing the truncated Env. Cell surface expression of the truncated Env proteins was found to be about eightfold higher than that of the full-length Env proteins. Furthermore, the truncated Env proteins exhibited higher levels of cleavage into gp120 and gp41 compared with the full-length Env. The SHIV VLPs produced by the coexpression of SIV gag and truncated HIV env contained both precursor (gp160) and gp120, while predominantly gp160 was found in the VLPs containing full-length Env. Coinfection of a recombinant virus expressing the protease furin also resulted in more efficient cleavage of gp160 to gp120. Both full-length and truncated Env were found to induce CD4+ cell fusion. Analysis of VLPs by immunoelectron microscopy demonstrated the incorporation of both full-length and truncated Env on the surface of VLPs. Truncated Env also was incorporated at higher levels on the surfaces of VLPs than full-length Env. The assembly of VLPs containing biologically active Env proteins may be useful in vaccine development and in functional studies of the HIV envelope protein.

Accelerated clearance of SHIV in rhesus monkeys by virus-like particle vaccines is dependent on induction of neutralizing antibodies

Recombinant, insect cell derived SIV Pr56(gag) virus-like particles (VLPs) have been modified either by inserting HIV-1 Gp160 derived peptides into the Pr56(gag) precursor or by integrating the complete HIV-1 gp120 in the particle membrane. To investigate the protective efficacy of these particulate antigens, rhesus macaques were immunized with VLPs both adjuvant-free or adsorbed to alum. In addition, recombinant Semliki Forest viruses (SFV) expressing proteins corresponding to the VLP constructs were established and administered as live vaccines in combination with particulate antigens. Vaccination induced specific humoral responses irrespective of the immunization regimen. However, in contrast to Pr56(gag)-specific antibodies, Env-specific antibody titers could not be increased by booster immunizations in this study. Cell-mediated immune responses were detected following vaccination with VLP-preparations as well as recombinant SFVs. A tendency towards stimulating both enhanced cell mediated as well as humoral immune responses was observed following priming with recombinant SFVs. Upon challenge with SHIV-4 all vaccinated monkeys became infected. However, animals, that were vaccinated with VLPs presenting the complete gp120, managed to clear virus faster than nonimmunized controls. The observed virus elimination significantly correlated with an anamnestic antibody response and an accelerated appearance of neutralizing antibodies postchallenge.

Correlates of protection, antigen delivery and molecular epidemiology: basics for designing an HIV vaccine

Major obstacles in the development of HIV vaccines are the high variability of the virus and its complex interaction with the immune system. Recent studies demonstrated, that CTLs recognizing highly conserved epitopes in the group-specific antigen are capable of controlling HIV-replication in long-term nonprogressors. Necessary consequences for novel vaccine concepts are the presentation of a large repertoire of antigenic sites as well as the stimulation of different effectors of the immune system. Accordingly, different types of recombinant HIV-1 virus-like particles (VLPs) have been constructed stimulating the induction of neutralizing antibodies and HIV-specific CD8-positive CTL responses in preclinical studies. With respect to future vaccine trials, HIV vaccine formulations may need to be tailored to the local strains circulating within a geographical region. The expert group of the joint United Nations Programme on AIDS recently identified Yunnan, a southwestern province of China, as a region, in which the HIV epidemic is starting to gain speed, resembling to the situation in Thailand 10 years ago. A molecular clone of a representative virus strain is now available for the development of innovative antigen delivery systems aiming to be evaluated in future clinical vaccine trials throughout this area.

Construction and characterization of recombinant VLPs and Semliki-Forest virus live vectors for comparative evaluation in the SHIV monkey model

For testing of recombinant virus-like particles (VLPs) in the SHIV monkey model, SIVmac239 Pr56gag precursor-based pseudovirions were modified by HIV-1 gp160 derived peptides. First, well-characterized epitopes from the HIV-1 envelope glycoprotein were inserted into the Pr56gag precursor by replacing defined regions that were shown to be dispensable for virus particle formation. Expression of these chimeric proteins in a baculovirus expression system resulted in efficient assembly and release of non-infectious, hybrid VLPs. In a second approach the HIV-1IIIB external glycoprotein gp120 was covalently linked to an Epstein-Barr virus derived transmembrane domain. Coexpression of the hybrid envelope derivative with the Pr56gag precursor yielded recombinant SIV derived Pr56gag particles with the HIV-1 gp120 firmly anchored on the VLP surface. Immunization of rhesus monkeys with either naked VLPs or VLPs adsorbed to alum induced substantial serum antibody titers and promoted both T helper cell and cytotoxic T lymphocyte responses. Furthermore, priming macaques with the corresponding set of recombinant Semliki-Forest viruses tended to enhance the immunological outcome. Challenge of the immunized monkeys with chimeric SHIV resulted in a clearly accelerated reduction of the plasma viremia as compared to control animals.

Immunostimulatory CpG motifs trigger a T helper-1 immune response to human immunodeficiency virus type-1 (HIV-1) gp 160 envelope proteins

Bacterial DNA sequences containing unmethylated CpG motifs have recently been proposed to exhibit immunostimulatory effects on B-, T- and NK cells, leading to the induction of humoral and cell-mediated immune responses. In the present study we investigated the immunomodulatory effects of a CpG-containing oligodeoxynucleotide (CpG ODN) to the HIV-1 gp 160 envelope (Env) protein in the BALB/c mouse model. Priming and boosting of mice with gp 160 adsorbed to aluminium hydroxide (Alum) induced a typical T helper-2 (Th2)-dominated immune response with high titers of gp 160-specific immunoglobulin (Ig)G1 isotypes but a weak IgG2a response. Specifically re-stimulated splenocytes from these mice predominantly secreted interleukin (IL)-5 but only minute amounts of interferon-gamma (IFN-gamma) upon specific re-stimulation. In contrast, a boost immunisation of gp 160/Alum primed mice with a gp 160/Alum/CpG combination resulted in a seven times higher production of IgG2a antibodies, without affecting the titers of IgG1 isotypes. Furthermore, approximately 10-fold increased levels of IFN-gamma, but significantly reduced amounts of IL-5, were secreted from gp 160-restimulated splenic cells. A further greater than 30-fold increase in the levels of specific IgG2a responses and a substantially elevated secretion of IFN-gamma were observed when the mice received gp160/Alum/CpG combinations for priming and boost injections. Thus, CpG ODNs are useful as an adjuvant to induce a typical Th0/Th1 response to HIV gp 160 proteins. However, despite the induction of a more Th1-like immune response, gp 16O/Alum/CpG combinations were not sufficient to prime an Env-specific cytotoxic T-cell (CTL) response.

Cytotoxic T cells and neutralizing antibodies induced in rhesus monkeys by virus-like particle HIV vaccines in the absence of protection from SHIV infection

HIV Pr55gag has in the absence of other viral components the capacity to self assemble in budding noninfectious virus-like particles (VLP). The immunological spectrum of the HIV-1IIIB gag-derived VLP was expanded either by stable anchoring of chimeric modified gp 120 on the surface of the VLP (type 1) or by replacing sequences of the Pr55gag precursor by the V3 loop and a linear portion of the CD4 binding domain (type 2). This noninfectious antigen delivery system was evaluated for immunogenicity and efficacy in rhesus macaques without adjuvants. Intramuscular immunization with both types of VLP induced high titers of gag-specific antibodies ranging from 1/8000 to 1/510,000 for type 1 VLP and from 1/4000 to 1/16,000 for type 2 VLP. Only animals immunized with type 1 VLP developed substantial endpoint titers of env-specific antibodies (1/2000-1/32,000) with a neutralizing capacity at serum dilutions of 1/32-1/128. Gag- and env-specific cytotoxic T lymphocyte (CTL) activity was induced by both types of VLP at similar levels. Four weeks after the last immunization animals were challenged intravenously with 20 MID50 of the cell free homologous envelope simian/HIV-1IIIB chimeric challenge stock Despite HIV-1-specific neutralizing and CTL responses, all vaccinated animals became infected.