Safety and immunogenicity of recombinant human immunodeficiency virus-like particles in rodents and rhesus macaques

Preventive, Diagnostic and Therapeutic Applications of Baculovirus Expression Vector System

Different strategies are being worked out for engineering the original baculovirus expression vector (BEV) system to produce cost-effective clinical biologics at commercial scale. To date, thousands of highly variable molecules in the form of heterologous proteins, virus-like particles, surface display proteins/antigen carriers, heterologous viral vectors and gene delivery vehicles have been produced using this system. These products are being used in vaccine production, tissue engineering, stem cell transduction, viral vector production, gene therapy, cancer treatment and development of biosensors. Recombinant proteins that are expressed and post-translationally modified using this system are also suitable for functional, crystallographic studies, microarray and drug discovery-based applications. Till now, four BEV-based commercial products (Cervarix^®, Provenge^®, Glybera^® and Flublok^®) have been approved for humans, and myriad of others are in different stages of preclinical or clinical trials. Five products (Porcilis^® Pesti, BAYOVAC CSF E2^®, Circumvent^® PCV, Ingelvac CircoFLEX^® and Porcilis^® PCV) got approval for veterinary use, and many more are in the pipeline. In the present chapter, we have emphasized on both approved and other baculovirus-based products produced in insect cells or larvae that are important from clinical perspective and are being developed as preventive, diagnostic or therapeutic agents. Further, the potential of recombinant adeno-associated virus (rAAV) as gene delivery vector has been described. This system, due to its relatively extended gene expression, lack of pathogenicity and the ability to transduce a wide variety of cells, gained extensive popularity just after the approval of first AAV-based gene therapy drug alipogene tiparvovec (Glybera^®). Numerous products based on AAV which are presently in different clinical trials have also been highlighted.

Viruses and Virus-Like Particles in Biotechnology: Fundamentals and Applications

Although viruses are simple biological systems, they are capable of evolving highly efficient techniques for infecting cells, expressing their genomes, and generating new copies of themselves. It is possible to genetically manipulate most of the different classes of known viruses in order to produce recombinant viruses that express foreign proteins. Recombinant viruses have been used in gene therapy to deliver selected genes into higher organisms, in vaccinology and immunotherapy, and as important research tools to study the structure and function of these proteins.

Virus-like particles (VLPs) are multiprotein structures that mimic the organization and conformation of authentic native viruses but lack the viral genome. They have been applied not only as prophylactic and therapeutic vaccines but also as vehicles in drug and gene delivery and, more recently, as tools in nanobiotechnology.

In this chapter, basic and advanced features of viruses and VLPs are presented and their major applications are discussed. The different production platforms based on animal cell technology are explained, and their main challenges and future perspectives are explored. The implications of large-scale production of viruses and VLPs are discussed in the context of process control, monitoring, and optimization. The main upstream and downstream technical challenges are identified and discussed accordingly.

Virus-like particles in vaccine development

Virus-like particles (VLPs) are multiprotein structures that mimic the organization and conformation of authentic native viruses but lack the viral genome, potentially yielding safer and cheaper vaccine candidates. A handful of prophylactic VLP-based vaccines is currently commercialized worldwide: GlaxoSmithKline's Engerix (hepatitis B virus) and Cervarix (human papillomavirus), and Merck and Co., Inc.'s Recombivax HB (hepatitis B virus) and Gardasil (human papillomavirus) are some examples. Other VLP-based vaccine candidates are in clinical trials or undergoing preclinical evaluation, such as, influenza virus, parvovirus, Norwalk and various chimeric VLPs. Many others are still restricted to small-scale fundamental research, despite their success in preclinical tests. This article focuses on the essential role of VLP technology in new-generation vaccines against prevalent and emergent diseases. The implications of large-scale VLP production are discussed in the context of process control, monitorization and optimization. The main up- and down-stream technical challenges are identified and discussed accordingly. Successful VLP-based vaccine blockbusters are briefly presented concomitantly with the latest results from clinical trials and the recent developments in chimeric VLP-based technology for either therapeutic or prophylactic vaccination.

Viruses and Virus-Like Particles in Biotechnology

Although viruses are simple biological systems, they are capable of evolving highly efficient techniques for infecting cells, expressing their genomes, and generating new copies of themselves. It is possible to genetically manipulate most of the different classes of known viruses in order to produce recombinant viruses that express foreign proteins. Recombinant viruses have been used in gene therapy to deliver selected genes into higher organisms, in vaccinology and immunotherapy, and as important research tools to study the structure and function of these proteins.

Virus-like particles (VLPs) are multiprotein structures that mimic the organization and conformation of authentic native viruses but lack the viral genome. They have been applied not only as prophylactic and therapeutic vaccines but also as vehicles in drug and gene delivery and, more recently, as tools in nanobiotechnology.

In this article, basic and advanced features of viruses and VLPs are presented and their major applications are discussed. The different production platforms based on animal cell technology are explained, and their main challenges and future perspectives are explored. The implications of large-scale production of viruses and VLPs are discussed in the context of process control, monitorization, and optimization. The main upstream and downstream technical challenges are identified and discussed accordingly.

Virus-like particles as a vaccine delivery system: myths and facts

Vaccines against viral disease have traditionally relied on attenuated virus strains or inactivation of infectious virus. Subunit vaccines based on viral proteins expressed in heterologous systems have been effective for some pathogens, but have often suffered from poor immunogenicity due to incorrect protein folding or modification. In this chapter we focus on a specific class of viral subunit vaccine that mimics the overall structure of virus particles and thus preserves the native antigenic conformation of the immunogenic proteins. These virus-like particles (VLPs) have been produced for a wide range of taxonomically and structurally distinct viruses, and have unique advantages in terms of safety and immunogenicity over previous approaches. With new VLP vaccines for papillomavirus beginning to reach the market place we argue that this technology has now ‘come-of-age’ and must be considered a viable vaccine strategy.

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.

Virus-like particle vaccines and adjuvants: the HPV paradigm

Complex antigen structures currently represent the most-studied approach for prophylactic as well as therapeutic vaccines. Different types of complex vaccines, including virus-like particles and virosomes, have been developed depending on the nature of the viral pathogen they are trying to replicate (enveloped vs naked) or the modality to express antigenic epitopes (i.e., the binding of envelope protein on liposomic structures). The complex structure of these vaccines provides them with some adjuvanted properties, not uniformly present for all virus-like particle types. The further inclusion of specific adjuvants in vaccine preparations can modify the presentation modality of such particles to the immune system with a specific Th1 versus Th2 polarization efficacy. A paradigm of the relevance of these new adjuvants are the immunological results obtained with the inclusion of monophosphoryl lipid A adjuvant in the formulation of L1-based human papillomavirus-naked virus-like particles to reduce a Th1 cellular immunity impairment, peculiar for alum-derived adjuvants, along with the induction of highly enhanced humoral and memory B-cellular immunity.

Protection against Mucosal SHIV Challenge by Peptide and Helper-Dependent Adenovirus Vaccines

Groups of rhesus macaques that had previously been immunized with HIV-1 envelope (env) peptides and first generation adenovirus serotype 5 (FG-Ad5) vaccines expressing the same peptides were immunized intramuscularly three times with helper-dependent adenovirus (HD-Ad) vaccines expressing only the HIV-1 envelope from JRFL. No gag, pol, or other SHIV genes were used for vaccination. One group of the FG-Ad5-immune animals was immunized three times with HD-Ad5 expressing env. One group was immunized by serotype-switching with HD-Ad6, HD-Ad1, and HD-Ad2 expressing env. Previous work demonstrated that serum antibody levels against env were significantly higher in the serotype-switched group than in the HD-Ad5 group. In this study, neutralizing antibody and T cell responses were compared between the groups before and after rectal challenge with CCR5-tropic SHIV-SF162P3. When serum samples were assayed for neutralizing antibodies, only weak activity was observed. T cell responses against env epitopes were higher in the serotype-switched group. When these animals were challenged rectally with SHIV-SF162P3, both the Ad5 and serotype-switch groups significantly reduced peak viral loads 2 to 10-fold 2 weeks after infection. Peak viral loads were significantly lower for the serotype-switched group as compared to the HD-Ad5-immunized group. Viral loads declined over 18 weeks after infection with some animals viremia reducing nearly 4 logs from the peak. These data demonstrate significant mucosal vaccine effects after immunization with only env antigens. These data also demonstrate HD-Ad vectors are a robust platform for vaccination.

A prime-boost immunisation regimen using recombinant BCG and Pr55(gag) virus-like particle vaccines based on HIV type 1 subtype C successfully elicits Gag-specific responses in baboons

Mycobacterium bovis BCG is considered an attractive live bacterial vaccine vector. In this study, we investigated the immune response of baboons to a primary vaccination with recombinant BCG (rBCG) constructs expressing the gag gene from a South African HIV-1 subtype C isolate, and a boost with HIV-1 subtype C Pr55(gag) virus-like particles (Gag VLPs). Using an interferon enzyme-linked immunospot assay, we show that although these rBCG induced only a weak or an undetectable HIV-1 Gag-specific response on their own, they efficiently primed for a Gag VLP boost, which strengthened and broadened the immune responses. These responses were predominantly CD8+ T cell-mediated and recognised similar epitopes as those targeted by humans with early HIV-1 subtype C infection. In addition, a Gag-specific humoral response was elicited. These data support the development of HIV-1 vaccines based on rBCG and Pr55(gag) VLPs.

Live attenuated measles vaccine expressing HIV-1 Gag virus like particles covered with gp160DeltaV1V2 is strongly immunogenic

Although a live attenuated HIV vaccine is not currently considered for safety reasons, a strategy inducing both T cells and neutralizing antibodies to native assembled HIV-1 particles expressed by a replicating virus might mimic the advantageous characteristics of live attenuated vaccine. To this aim, we generated a live attenuated recombinant measles vaccine expressing HIV-1 Gag virus-like particles (VLPs) covered with gp160DeltaV1V2 Env protein. The measles-HIV virus replicated efficiently in cell culture and induced the intense budding of HIV particles covered with Env. In mice sensitive to MV infection, this recombinant vaccine stimulated high levels of cellular and humoral immunity to both MV and HIV with neutralizing activity. The measles-HIV virus infected human professional antigen-presenting cells, such as dendritic cells and B cells, and induced efficient presentation of HIV-1 epitopes and subsequent activation of human HIV-1 Gag-specific T cell clones. This candidate vaccine will be next tested in non-human primates. As a pediatric vaccine, it might protect children and adolescents simultaneously from measles and HIV.

Human immunodeficiency virus type-1 (HIV-1) Pr55gag virus-like particles are potent activators of human monocytes

Human immunodeficiency virus type-1 (HIV-1) Pr55(Gag) virus-like particles (VLP) represent an interesting HIV vaccine component since they stimulate strong humoral and cellular immune responses. We demonstrated that VLP expressed by recombinant baculoviruses activate human PBMC to release pro-inflammatory (lL-6, TNF-alpha), anti-inflammatory (IL-10) and Th1-polarizing (IFN-gamma) cytokines as well as GM-CSF and MIP-1alpha in a dose-and time-dependent manner. Herein, residual baculoviruses within the VLP preparations showed no or minor effects. Monocytes could be identified as a main target for VLP to induce cytokine production. Furthermore, VLP-induced monocyte activation was shown by upregulation of molecules involved in antigen presentation (MHC II, CD80, CD86) and cell adhesion (CD54). Exposure of VLP to serum inactivates its capacity to stimulate cytokine production. In summary, these investigations establish VLP as strong activators of PBMC and monocytes therein, potently enhancing their functionality and potency to promote an efficient immune response. This capacity makes VLP an interesting component of combination vaccines.

HIV-1 subtype C Pr55gag virus-like particle vaccine efficiently boosts baboons primed with a matched DNA vaccine

A DNA vaccine expressing human immunodeficiency virus type 1 (HIV-1) southern African subtype C Gag (pTHGag) and a recombinant baculovirus Pr55gag virus-like particle prepared using a subtype C Pr55gag protein (Gag VLP) was tested in a prime-boost inoculation regimen in Chacma baboons. The response of five baboons to Gag peptides in a gamma interferon (IFN-gamma) enzyme-linked immunospot (ELISPOT) assay after three pTHGag immunizations ranged from 100 to 515 spot-forming units (s.f.u.) per 10(6) peripheral blood mononuclear cells (PBMCs), whilst the response of two baboons to the Gag VLP vaccine ranged from 415 to 465 s.f.u. per 10(6) PBMCs. An increase in the Gag-specific response to a range of 775-3583 s.f.u. per 10(6) PBMCs was achieved by boosting with Gag VLPs the five baboons that were primed with pTHGag. No improvement in Gag responses was achieved in this prime-boost inoculation regimen by increasing the number of pTHGag inoculations to six. IFN-gamma responses were mapped to several peptides, some of which have been reported to be targeted by PBMCs from HIV-1 subtype C-infected individuals. Gag VLPs, given as a single-modality regimen, induced a predominantly CD8+ T-cell IFN-gamma response and interleukin-2 was a major cytokine within a mix of predominantly Th1 cytokines produced by a DNA-VLP prime-boost modality. The prime-boost inoculation regimen induced high serum p24 antibody titres in all baboons, which were several fold above that induced by the individual vaccines. Overall, this study demonstrated that these DNA prime/VLP boost vaccine regimens are highly immunogenic in baboons, inducing high-magnitude and broad multifunctional responses, providing support for the development of these products for clinical trials.

Chimaeric HIV-1 subtype C Gag molecules with large in-frame C-terminal polypeptide fusions form virus-like particles

HIV-1 Pr55 Gag virus-like particles (VLPs) are strong immunogens with potential as candidate HIV vaccines. VLP immunogenicity can be broadened by making chimaeric Gag molecules: however, VLPs incorporating polypeptides longer than 200 aa fused in frame with Gag have not yet been reported. We constructed a range of gag-derived genes encoding in-frame C-terminal fusions of myristoylation-competent native Pr55Gag and p6-truncated Gag (Pr50Gag) to test the effects of polypeptide length and sequence on VLP formation and morphology, in an insect cell expression system. Fused sequences included a modified reverse transcriptase-Tat-Nef fusion polypeptide (RTTN, 778 aa), and truncated versions of RTTN ranging from 113 aa to 450 aa. Baculovirus-expressed chimaeric proteins were examined by western blot and electron microscopy. All chimaeras formed VLPs which could be purified by sucrose gradient centrifugation. VLP diameter increased with protein MW, from approximately 100 nm for Pr55Gag to approximately 250 nm for GagRTTN. The presence or absence of the Gag p6 region did not obviously affect VLP formation or appearance. GagRT chimaeric particles were successfully used in mice to boost T-cell responses to Gag and RT that were elicited by a DNA vaccine encoding a GagRTTN polypeptide, indicating the potential of such chimaeras to be used as candidate HIV vaccines.

A comparative immunogenicity study of HIV-1 virus-like particles bearing various forms of envelope proteins, particles bearing no envelope and soluble monomeric gp120

To assess the potential of native Envelope glycoprotein (Env) trimers as neutralizing antibody vaccines, we immunized guinea pigs with three types of VLPs and soluble gp120. Particles included "SOS-VLPs" (bearing disulfide-shackled functional trimers), "UNC-VLPs" (bearing uncleaved nonfunctional Env) and "naked VLPs" (bearing no Env). The SOS-VLPs were found to have a density of about 27 native trimers per particle, approximately twice that of live inactivated HIV-1 preparations. As immunogens, UNC- and SOS-VLP rapidly elicited anti-gp120 antibodies focused on the V3 loop and the gp120 coreceptor binding site. Reactivity to the gp41 immunodominant domain was absent in SOS-VLP sera, presumably because gp120-gp41 association is stabilized, effectively covering this epitope. Gp120-immune sera reacted with the receptor binding sites of gp120 and were less focused on the V3 loop. Some Env-VLP sera neutralized primary isolates at modest titers. The measurement of neutralization was found to be affected by the cell lines used. Depending on the assay particulars, non-Env specific antibodies in VLP sera could enhance infection, or nonspecifically neutralize. However, a neutralization assay using TZM-BL cells was essentially clear of these effects. We also describe a native trimer binding assay to confirm neutralization activity in a manner that completely eliminates nonspecific effects. Overall, our data suggests that Env-VLP sera were primarily focused on nonfunctional forms of Env on VLP surfaces, possibly gp120/gp41 monomers and not the trimers. Therefore, to make progress toward a more effective VLP-based vaccine, we will need to find ways to refocus the attention of B cells on native trimers.

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.

Virus-like particles as immunogens

Subunit vaccines based on recombinant proteins can suffer from poor immunogenicity owing to incorrect folding of the target protein or poor presentation to the immune system. Virus-like particles (VLPs) represent a specific class of subunit vaccine that mimic the structure of authentic virus particles. They are recognized readily by the immune system and present viral antigens in a more authentic conformation than other subunit vaccines. VLPs have therefore shown dramatic effectiveness as candidate vaccines. Here, we review the current status of VLPs as vaccines, and discuss the characteristics and problems associated with producing VLPs for different viruses.

[Status and current strategies of HIV vaccine development]

Despite intensive research efforts a vaccine against HIV has not yet been developed twenty years even after the onset of the HIV-epidemic. The problems in the development of an HIV-vaccine as well as former and current strategies to overcome these problems are presented here. The current status of human studies of different candidate vaccines is outlined.

Systemic, mucosal, and heterotypic immune induction in mice inoculated with Venezuelan equine encephalitis replicons expressing Norwalk virus-like particles

Norwalk-like viruses (NLVs) are a diverse group of single-stranded, nonenveloped, positive-polarity RNA viruses and are the leading cause of epidemic acute gastroenteritis in the United States. In this study, the major capsid gene of Norwalk virus, the prototype NLV, has been cloned and expressed in mammalian cells using a Venezuelan equine encephalitis (VEE) replicon expression system. Upon infection of baby hamster kidney (BHK) cells with VEE replicon particles (VRPs), the Norwalk virus capsid proteins self-assemble to generate high titers of Norwalk virus-like particles (VLPs) that are morphologically and antigenically analogous to wild-type Norwalk virus. Mice inoculated subcutaneously with VRPs expressing the Norwalk virus capsid protein (VRP-NV1) developed systemic and mucosal immune responses to Norwalk VLPs, as well as heterotypic antibody responses to the major capsid protein from another genogroup I NLV strain (NCFL) isolated from a recent outbreak. A second Norwalk virus capsid clone (NV2) containing three amino acid codon mutations from the NV1 clone was also expressed using VEE replicons (VRP-NV2), but upon infection of BHK cells failed to confer VLP self-assembly. Mice inoculated with VRP-NV2 elicited reduced systemic and mucosal immune responses to Norwalk VLPs, demonstrating the importance and potential utility of endogenous VLP presentation for maximum immune induction. Inoculation with either VRP-NV1 or VRP-NV2 resulted in serum antibody responses far superior to the induction in mice dosed orally with VLPs that were prepared using the VEE-NV1 replicon construct, a regimen similar to current models for NLV vaccination. Expression of NLV VLPs in mammalian cells offers a powerful approach for the design of novel NLV vaccines, either alone or in combination with current vaccination models.

Characterization of a virtually full-length human immunodeficiency virus type 1 genome of a prevalent intersubtype (C/B') recombinant strain in China

A molecular epidemiology study was conducted among more than 100 human immunodeficiency virus type 1 (HIV-1) subtype C seropositive intravenous drug users (IDUs) from China. Genotyping based on the envelope C2V3 coding region revealed the highest homology of the most prevalent virus strains circulating throughout China to subtype C sequences of Indian origin. Based on these results, a virtually full-length genome representing the most prevalent class of clade C strains circulating throughout China was directly amplified from peripheral blood mononuclear cells of a selected HIV-infected IDU and subcloned. Sequence analysis identified a mosaic structure, suggesting extensive intersubtype recombination events between genomes of the prevalent clade C and (B')-subtype Thai virus strains of that geographic region. Recombinant Identification Program analysis and phylogenetic bootstrapping suggested that there were 10 breakpoints (i) in the gag-pol coding region, (ii) in vpr and at the 3' end of the vpu gene, and (iii) in the nef open reading frame. (B')-sequences therefore include (i) several insertions in the gag-pol coding region; (ii) 3'-vpr, the complete vpu gene, and the first exons of tat and rev; and (iii) the 5' half of the nef gene. Breakpoints located in the vpr/vpu coding region as well as in the nef gene of 97cn54 were found at almost identical positions of all subtype C strains isolated from IDUs living in different areas of China, suggesting a common ancestor for the C/B' recombinant strains. More than 50% of well-defined subtype B-derived cytotoxic T-lymphocyte epitopes within Gag and Pol and 10% of the known epitopes in Env were found to exactly match sequences within in this clade C/B' chimeric reference strain. These results may substantially facilitate a biological comparison of clade C-derived reference strains as well as the generation of useful reagents supporting vaccine-related efforts in China.

New chimaeric hepatitis B virus core particles carrying hantavirus (serotype Puumala) epitopes: immunogenicity and protection against virus challenge

Virus-like particles generated by the heterologous expression of virus structural proteins are able to potentiate the immunogenicity of foreign epitopes presented on their surface. In recent years epitopes of various origin have been inserted into the core antigen of hepatitis B virus (HBV) allowing the formation of chimaeric HBV core particles. Chimaeric core particles carrying the 45 N-terminal amino acids of the Puumala hantavirus nucleocapsid protein induced protective immunity in bank voles, the natural host of this hantavirus. Particles applied in the absence of adjuvant are still immunogenic and partially protective in bank voles. Although a C-terminally truncated core antigen of HBV (HBcAg delta) tolerates the insertion of extended foreign sequences, for the construction of multivalent vaccines the limited insertion capacity is still a critical factor. Recently, we have described a new system for generating HBV 'mosaic particles' in an Escherichia coli suppressor strain based on a readthrough mechanism on a stop linker located in front of the insert. Those mosaic particles are built up by both HBcAg delta and the HBcAg delta/Puumala nucleocapsid readthrough protein. The particles formed presented the 114 amino acid (aa) long hantavirus sequence, at least in part, on their surface and induced antibodies against the hantavirus sequence in bank voles. Variants of the stop linker still allowed the formation of mosaic particles demonstrating that stop codon suppression alone is sufficient for the packaging of longer foreign sequences in mosaic particles. Another approach to increase the insertion capacity is based on the simultaneous insertion of different Puumala nucleocapsid protein sequences (aa 1-45 and aa 75-119) into two different positions (aa 78 and behind aa 144) of a single HBcAg molecule. The data presented are of high relevance for the generation of multivalent vaccines requiring a high insertion capacity for foreign sequences.

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.

Core particles of hepatitis B virus as carrier for foreign epitopes

To be effective as vaccines, most monomeric proteins and peptides either require chemical coupling to high molecular weight carriers or application together with adjuvants. More recently, recombinant DNA techniques have been used to insert foreign epitopes into proteins with inherent multimerization capacity, such as particle-forming viral capsid or envelope proteins. The core protein of hepatitis B virus (HBcAg), because of its unique structural and immunological properties, has gained widespread interest as a potential antigen carrier. Foreign sequences of up to approximately 40 amino acid residues at the N terminus, 50 or 100 amino acids in the central immunodominant c/e 1 epitope region of HBcAg, and up to 100 or even more residues at the C terminus, did not interfere with particle formation. The humoral immunogenicity of inserted epitopes is determined by the immunogenicity of the peptide itself and its surface exposure, and is influenced by the route of application. The probably flexible and surface-exposed c/e1 region emerged as the most promising insertion site. When applied together with adjuvants approved for human and veterinary use, or even without adjuvants, such chimeric particles induced B and T cell immune responses against the inserted epitopes. In some cases neutralizing antibodies, cytotoxic T cells and protection against challenge with the intact pathogen were demonstrated. Major factors for the potentiated immune response against the foreign epitopes are the multimeric structure of chimeric HBcAg that results in a high epitope density per particle, and the provision of T cell help by the carrier moiety. Beyond its use as subunit vaccine, chimeric HBcAg produced in attenuated Salmonella strains may be applicable as live vaccine.

Scope for using plant viruses to present epitopes from animal pathogens

Epitope presentation to the immune system for vaccination purposes can be achieved either via an inactivated or attenuated form of a pathogen or via its isolated antigenic sequences. When free, these peptides can adopt a variety of conformations, most of which will not exist in their native environment. Conjugation to carrier proteins restricts mobility of the peptides and increases their immunogenicity. A high local concentration of epitopes boosts the immune response further and can be generated by the use of self-aggregating carriers, such as the capsid proteins of viruses. In this regard plant viruses have in recent years started to make an impact as safer alternatives to the use of bacterial and attenuated animal viruses: the latter both require propagation in costly cell-culture systems where they can undergo reversion towards a virulent form and/or become contaminated by other pathogens. Plant virus-based vectors can be multiplied cheaply and to high yields (exceeding 1 mg/g plant tissue) in host plants. Both helical (tobacco mosaic virus, potato virus X, alfalfa mosaic virus) and icosahedral (cowpea mosaic virus, tomato bushy stunt virus) particles have been used to express a number of animal B-cell epitopes, whose immunogenic properties have been explored to varying degrees. Copyright 1998 John Wiley & Sons, Ltd.

Chimeric HIV-1 virus-like particles containing gp120 epitopes as a result of a ribosomal frameshift elicit Gag- and SU-specific murine cytotoxic T-lymphocyte activities

Insect cell expression of the HIV-1 Gag precursor protein by recombinant baculoviruses results in the assembly and budding of noninfectious virus-like particles (VLPs). The VLPs resemble immature virus in ultrastructural morphology and can be purified by conventional retroviral techniques. The virus-like appearance of the particles suggested that they could be used to package additional peptides. The retroviral frameshift mechanism was used to translate the pol gene products by expressing additional genetic information as chimeric Gag-Pol fusion proteins. Sequences encoding the carboxyl 65% of the HIV-1 surface glycoprotein (gp120, SU) were inserted into the Gag-Pol reading frame immediately downstream of the Gag stop codon. The assembly and budding of large quantities of Gag and chimeric Gag-SU VLPs were observed by standard transmission electron microscopy. The presence of gp120 epitopes in the Gag-SU VLPs was confirmed by immunoelectron microscopy and Western blot analysis using monoclonal anti-gp120 antibodies. Mice inoculated with the Gag-SU pseudovirions developed cytotoxic lymphocyte responses to both HIV-1 Gag and Env epitopes yet humoral immune responses only to Gag epitopes. The chimeric Gag-SU particles may have applications as vaccines or immunotherapeutic treatments for HIV-1 infection. In addition, the frameshift mechanism can be applied to the packaging of other viral or cellular proteins.