Why do we lack an effective vaccine against herpes simplex virus infections?

Hinge Region in DNA Packaging Terminase pUL15 of Herpes Simplex Virus: A Potential Allosteric Target for Antiviral Drugs

Approximately 80% of adults are infected with a member of the herpesviridae family. Herpesviruses establish life-long latent infections within neurons, which may reactivate into lytic infections due to stress or immune suppression. There are nine human herpesviruses (HHV) posing health concerns from benign conditions to life threatening encephalitis, including cancers associated with viral infections. The current treatment options for most HHV conditions mainly include several nucleoside and nucleotide analogs targeting viral DNA polymerase. Although these drugs help manage infections, their common mechanism of action may lead to the development of drug resistance, which is particularly devastating in immunocompromised patients. Therefore, new classes of drugs directed against novel targets in HHVs are necessary to alleviate this issue. We analyzed the conservation rates of all proteins in herpes simplex virus 1 (HHV-1), a representative of the HHV family and one of the most common viruses infecting the human population. Furthermore, we generated a full-length structure model of the most conserved HHV-1 protein, the DNA packaging terminase pUL15. A series of computational analyses were performed on the model to identify ATP and DNA binding sites and characterize the dynamics of the protein. Our study indicates that proteins involved in HHV-1 DNA packaging and cleavage are amongst the most conserved gene products of HHVs. Since the packaging protein pUL15 is the most conserved among all HHV-1 gene products, the virus will have a lower chance of developing resistance to small molecules targeting pUL15. A subsequent analysis of the structure of pUL15 revealed distinct ATP and DNA binding domains and the elastic network model identifies a functionally important hinge region between the two domains of pUL15. The atomic information on the active and allosteric sites in the ATP- and DNA-bound model of pUL15 presented in this study can inform the structure-based drug discovery of a new class of drugs to treat a wide range of HHVs.

The potential of currently unavailable herpes virus vaccines

INTRODUCTION

Despite overwhelming experimental work, there are no licensed vaccines against the most frequent Alphaherpesviruses, namely herpes simplex virus 1 and 2 (HSV1 and 2) nor against the Epstein-Barr virus (EBV), a member of the subfamily Gammaherpesvirus.

AREAS COVERED

Since the DNAs of both HSVs reside in the regional sensory ganglia in a latent state (i.e. as circularized episomal molecules), a corresponding vaccine might be useful for immunotherapy rather than for prevention of primary infection. Here we describe the design of a purified subunit vaccine as well as the preparation and efficacy of a recombinant fusion protein consisting of the gD ectodomain from our domestic attenuated HSV1 strain HSZP. The EBV vaccines considered so far, were destined for prevention of infectious mononucleosis (IM) or to prevent formation of EBV related tumors. To design the EBV peptide vaccine, at least 15 carefully selected immunogenic epitopes coming from 12 virus coded proteins were bound to synthetic micro-particle carriers along with a non-specific pathogen recognizing receptor (PRR) stimulating both the T as well as B lymphocytes.

EXPERT COMMENTARY

The efficacy of a novel EBV peptide in the rabbit model was based on criteria such as antibody formation (EA-D detected by ELISA, early and capsid proteins tested by immunoblot), presence of LMP1 antigen and of viral DNA in peripheral white blood cells. Out of 19 peptide combinations used for vaccination, at least 6 showed a satisfactory protective effect.

Mucosal herpes immunity and immunopathology to ocular and genital herpes simplex virus infections

Herpes simplex viruses type 1 and type 2 (HSV-1 and HSV-2) are amongst the most common human infectious viral pathogens capable of causing serious clinical diseases at every stage of life, from fatal disseminated disease in newborns to cold sores genital ulcerations and blinding eye disease. Primary mucocutaneous infection with HSV-1 & HSV-2 is followed by a lifelong viral latency in the sensory ganglia. In the majority of cases, herpes infections are clinically asymptomatic. However, in symptomatic individuals, the latent HSV can spontaneously and frequently reactivate, reinfecting the muco-cutaneous surfaces and causing painful recurrent diseases. The innate and adaptive mucosal immunities to herpes infections and disease remain to be fully characterized. The understanding of innate and adaptive immune mechanisms operating at muco-cutaneous surfaces is fundamental to the design of next-generation herpes vaccines. In this paper, the phenotypic and functional properties of innate and adaptive mucosal immune cells, their role in antiherpes immunity, and immunopathology are reviewed. The progress and limitations in developing a safe and efficient mucosal herpes vaccine are discussed.

Permissive and restricted virus infection of murine embryonic stem cells

Recent RNA interference (RNAi) studies have identified many host proteins that modulate virus infection, but small interfering RNA 'off-target' effects and the use of transformed cell lines limit their conclusiveness. As murine embryonic stem (mES) cells can be genetically modified and resources exist where many and eventually all known mouse genes are insertionally inactivated, it was reasoned that mES cells would provide a useful alternative to RNAi screens. Beyond allowing investigation of host-pathogen interactions in vitro, mES cells have the potential to differentiate into other primary cell types, as well as being used to generate knockout mice for in vivo studies. However, mES cells are poorly characterized for virus infection. To investigate whether ES cells can be used to explore host-virus interactions, this study characterized the responses of mES cells following infection by herpes simplex virus type 1 (HSV-1) and influenza A virus. HSV-1 replicated lytically in mES cells, although mES cells were less permissive than most other cell types tested. Influenza virus was able to enter mES cells and express some viral proteins, but the replication cycle was incomplete and no infectious virus was produced. Knockdown of the host protein AHCYL1 in mES cells reduced HSV-1 replication, showing the potential for using mES cells to study host-virus interactions. Transcriptional profiling, however, indicated the lack of an efficient innate immune response in these cells. mES cells may thus be useful to identify host proteins that play a role in virus replication, but they are not suitable to determine factors that are involved in innate host defence.

Improved DNA vaccination by skin-targeted delivery using dry-coated densely-packed microprojection arrays

HSV-2-gD2 DNA vaccine was precisely delivered to immunologically sensitive regions of the skin epithelia using dry-coated microprojection arrays. These arrays delivered a vaccine payload to the epidermis and the upper dermis of mouse skin. Immunomicroscopy results showed that, in 43 ± 5% of microprojection delivery sites, the DNA vaccine was delivered to contact with professional antigen presenting cells in the epidermal layer. Associated with this efficient delivery of the vaccine into the vicinity of the professional antigen presenting cells, we achieved superior antibody responses and statistically equal protection rate against an HSV-2 virus challenge, when compared with the mice immunized with intramuscular injection using needle and syringe, but with less than 1/10th of the delivered antigen.

Effector CD4+ T-cell involvement in clearance of infectious herpes simplex virus type 1 from sensory ganglia and spinal cords

In primary infection, CD8(+) T cells are important for clearance of infectious herpes simplex virus (HSV) from sensory ganglia. In this study, evidence of CD4(+) T-cell-mediated clearance of infectious HSV type 1 (HSV-1) from neural tissues was also detected. In immunocompetent mice, HSV-specific CD4(+) T cells were present in sensory ganglia and spinal cords coincident with HSV-1 clearance from these sites and remained detectable at least 8 months postinfection. Neural CD4(+) T cells isolated at the peak of neural infection secreted gamma interferon, tumor necrosis factor alpha, interleukin-2 (IL-2), or IL-4 after stimulation with HSV antigen. HSV-1 titers in neural tissues were greatly reduced over time in CD8(+) T-cell-deficient and CD8(+) T-cell-depleted mice, suggesting that CD4(+) T cells could mediate clearance of HSV-1 from neural tissue. To examine possible mechanisms by which CD4(+) T cells resolved neural infection, CD8(+) T cells were depleted from perforin-deficient or FasL-defective mice. Clearance of infectious virus from neural tissues was not significantly different in perforin-deficient or FasL-defective mice compared to wild-type mice. Further, in spinal cords and brains after vaginal HSV-1 challenge of chimeric mice expressing both perforin and Fas or neither perforin nor Fas, virus titers were significantly lower than in control mice. Thus, perforin and Fas were not required for clearance of infectious virus from neural tissues. These results suggest that HSV-specific CD4(+) T cells are one component of a long-term immune cell presence in neural tissues following genital HSV-1 infection and play a role in clearance of infectious HSV-1 at neural sites, possibly via a nonlytic mechanism.

Anti-HSV activity of digitoxin and its possible mechanisms

Herpes simplex virus type 1 (HSV-1) can establish latent infection in the nervous system and usually leads to life-threatening diseases in immunocompromised individuals upon reactivation. Treatment with conventional nucleoside analogue such as acyclovir is effective in most cases, but drug-resistance may arise due to prolonged treatment in immunocompromised individuals. In this study, we identified an in-use medication, digitoxin, which actively inhibited HSV-1 replication with a 50% effective concentration (EC(50)) of 0.05 microM. The 50% cytotoxicity concentration (CC(50)) of digitoxin is 10.66 microM and the derived selective index is 213. Several structural analogues of digitoxin such as digoxin, ouabain octahydrate and G-strophanthin also showed anti-HSV activity. The inhibitory effects of digitoxin are likely to be introduced at the early stage of HSV-1 replication and the virus release stage. The observation that digitoxin can inhibit acyclovir-resistant viruses further implicates that digitoxin represents a novel drug class with distinct antiviral mechanisms from traditional drugs.

Glycoprotein D adjuvant herpes simplex virus vaccine

Herpes simplex virus (HSV) Type-1 and -2 are common infections that can cause primary and recurrent herpes labialis and genitalis, as well as gingivostomatitis, keratoconjunctivitis, encephalitis, disseminated infections in immunocompromised persons and neonatal infections. Despite several decades of HSV vaccine development, no effective vaccine has been developed until recently. The following review of the genital HSV-2 glycoprotein D (gD2t, t is for truncated) subunit vaccine formulated with a new adjuvant (AS04) containing alum and 3-O deacylated monophosphoryl lipid A (MPL) provides a background in which to evaluate the vaccine as well as a brief review of other approaches to herpes vaccines. The gD2t-AS04 vaccine has been demonstrated to be safe in several large clinical trials. In two trials, the vaccine reduced genital herpes disease by 73 and 74%, but only in females with no previous HSV infection. A large ongoing trial in HSV seronegative females will provide additional data on protection from HSV disease and infection.

Developments in herpes simplex virus vaccines: old problems and new challenges

Vaccination has remained the best method for preventing virus spread. The herpes simplex virus (HSV) candidate vaccines tested till now were mostly purified subunit vaccines and/or recombinant envelope glycoproteins (such as gB and gD). In many experiments performed in mice, guinea pigs and rabbits, clear-cut protection against acute virus challenge was demonstrated along with the reduction of the extent of latency, when established in the immunized host. The immunotherapeutic effect of herpes vaccines seems less convincing. However, introduction of new adjuvants, which shift the cytokine production of helper T-cells toward stimulation of cytotoxic T-cells (TH1 type cytokine response), reveals a promising development. Mathematical analysis proved that overall prophylactic vaccination of seronegative women, even when eliciting 40-60 % antibody response only, would reduce the frequency of genital herpes within the vaccinated population. Even when partially effective, immunotherapeutic vaccination might represent a suitable alternative of chronic chemotherapy in recurrent labial and genital herpes.

Characterization of the IFN-gamma T-cell responses to immediate early antigens in humans with genital herpes

Background

The IFN-γ ELISPOT assay has been used to examine the T-cell repertoire for many disease states in humans but, as yet, not genital herpes. Using overlapping synthetic peptide libraries, an IFN-γ ELISPOT assay was established that could measure CD4 and CD8 T-cell responses to HSV-2 antigens in patients with genital herpes.

Results

In unexpanded T-cells isolated from peripheral blood, CD4 responses were readily measured against four immediate early antigens (ICP0, ICP4, ICP22 and ICP27), VP22 and gD. The CD4 responses were characterized by a low number of positive cells which produced large ELISPOTs. CD4 responses had a broad specificity and within individual patients several of the test antigens were recognized. In contrast, CD8 responses were found only in approximately 50% of patients and were typically specific to a single antigen. When disease status and immune responses were compared, an enhanced CD4 response to ICP4 in patients with a low recurrence rate was found. The ICP4 response was striking in three HSV-1 single positive genital herpes patients.

Conclusion

The survey of T-cell responses is an important step to understand the host cellular immune response in individuals with genital herpes. The assay described here has the capability of measuring CD4 and CD8 T-cell responses that may be used to correlate disease status with specific immune responses. In an evaluation of 18 subjects a trend of positive responses to an immediate early protein, ICP4, was found in individuals that had a low rate of disease recurrence.

Potent protective cellular immune responses generated by a DNA vaccine encoding HSV-2 ICP27 and the E. coli heat labile enterotoxin

A mouse model was employed to evaluate protective cellular immune responses induced by an immediate early antigen of HSV-2. Particle-mediated DNA vaccination of mice with a DNA plasmid-encoding ICP27 resulted in the induction of ICP27-specific IFN-gamma and TNF-alpha production in Balb/c mice, but little protection to intranasal challenge with wild type HSV-2. However, when the DNA vaccine was supplemented with as little as 50ng of a vector encoding the A and B subunits of the Escherichia coli heat labile enterotoxin (LT), animals were profoundly protected from morbidity and mortality. The ICP27+LT-mediated protection was correlated with a large increase in ICP27-specific IFN-gamma and TNF-alpha production but cytokine-specific monoclonal antibody treatment at the time of challenge showed that protection was mediated predominantly by IFN-gamma. Furthermore, depletion of T cell subsets prior to infectious challenge demonstrated that removal of either CD8+ or CD4+ T cells impaired protection with CD8+ T cells appearing to play a direct effector role. These data demonstrate that augmented cellular immune responses resulting from LT vector plus antigen vector administration to the skin are biologically significant, leading to enhanced protection against mucosal pathogenic challenge.

Herpes simplex virus in pregnancy: new concepts in prevention and management

Genital herpes simplex virus (HSV) infection is one of the most common viral sexually transmitted diseases in the United States. It is estimated that 45 million adolescents and adults are infected with genital HSV. Most genital herpes infections in the United States are caused by HSV type 2 (HSV-2), and 25% to 30% of women of reproductive age have HSV-2 antibodies. What is more striking is that genital herpes is frequently under-recognized, and that only 5% to 10% of these women have a history of genital herpes. Because such a small percentage of women are aware of being infected with HSV, the risk of maternal transmission of this virus to the fetus or newborn is a significant health issue.

Reduced immune responses after vaccination with a recombinant herpes simplex virus type 1 vector in the presence of antiviral immunity

Due to the continuous need for new vaccines, viral vaccine vectors have become increasingly attractive. In particular, herpes simplex virus type 1 (HSV-1)-based vectors offer many advantages, such as broad cellular tropism, large DNA-packaging capacity and the induction of pro-inflammatory responses. However, despite promising results obtained with HSV-1-derived vectors, the question of whether pre-existing virus-specific host immunity affects vaccine efficacy remains controversial. For this reason, the influence of pre-existing HSV-1-specific immunity on the immune response induced with a replication-defective, recombinant HSV-1 vaccine was investigated in vivo. It was shown that humoral as well as cellular immune responses against a model antigen encoded by the vaccine were strongly diminished in HSV-1-seropositive mice. This inhibition could be observed in mice infected with wild-type HSV-1 or with a replication-defective vector. Although these data clearly indicate that pre-existing antiviral host immunity impairs the efficacy of HSV-1-derived vaccine vectors, they also show that vaccination under these constraints might still be feasible.

Herpes Simplex Virus Type-1-Induced Activation of Myeloid Dendritic Cells: The Roles of Virus Cell Interaction and Paracrine Type I IFN Secretion

Adaptive cellular immunity is required to clear HSV-1 infection in the periphery. Myeloid dendritic cells (DCs) are the first professional Ag-presenting cell to encounter the virus after primary and secondary infection and thus the consequences of their infection are important in understanding the pathogenesis of the disease and the response to the virus. Following HSV-1 infection, both uninfected and infected human DCs acquire a more mature phenotype. In this study, we demonstrate that type I IFN secreted from myeloid DC mediates bystander activation of the uninfected DCs. Furthermore, we confirm that this IFN primes DCs for elevated IL-12 p40 and p70 secretion. However, secretion of IFN is not responsible for the acquisition of a mature phenotype by HSV-1-infected DC. Rather, virus binding to a receptor on the cell surface induces DC maturation directly, through activation of the NF-kappaB and p38 MAPK pathways. The binding of HSV glycoprotein D is critical to the acquisition of a mature phenotype and type I IFN secretion. The data therefore demonstrate that DCs can respond to HSV exposure directly through recognition of viral envelope structures. In the context of natural HSV infection, the coupling of viral entry to the activation of DC signaling pathways is likely to be counterbalanced by viral disruption of DC maturation. However, the parallel release of type I IFN may result in paracrine activation so that the DCs are nonetheless able to mount an adaptive immune response.

Herpes simplex virus type 2 vaccines: new ground for optimism?

The development of effective prophylactic and therapeutic vaccines against genital herpes has proven problematic. Difficulties are associated with the complexity of the virus life cycle (latency) and our relatively poor understanding of the mechanism of immune control of primary and recurrent disease. The types of effector cells and the mechanisms responsible for their activation and regulation are particularly important. Studies from my and other laboratories have shown that recurrent disease is prevented by virus-specific T helper 1 (Th1) cytokines (viz., gamma interferon) and activated innate immunity. Th2 cytokines (viz., interleukin-10 [IL-10]) and regulatory (suppressor) T cells downregulate this immune profile, thereby allowing unimpeded replication of reactivated virus and recurrent disease. Accordingly, an effective therapeutic vaccine must induce Th1 immunity and be defective in Th2 cytokine production, at least IL-10. These concepts are consistent with the findings of the most recent clinical trials, which indicate that (i) a herpes simplex virus type 2 (HSV-2) glycoprotein D (gD-2) vaccine formulated with a Th1-inducing adjuvant has prophylactic activity in HSV-2- and HSV-1-seronegative females, an activity attributed to the adjuvant function, and (ii) a growth-defective HSV-2 mutant (ICP10DeltaPK), which is deleted in the Th2-polarizing gene ICP10PK, induces Th1 immunity and has therapeutic activity in both genders. The ICP10DeltaPK vaccine prevents recurrent disease in 44% of treated subjects and reduces the frequency and severity of recurrences in the subjects that are not fully protected. Additional studies to evaluate these vaccines are warranted.

Glycoprotein-D-adjuvant vaccine to prevent genital herpes

BACKGROUND

An effective prophylactic vaccine would help control the spread of genital herpes.

METHODS

We conducted two double-blind, randomized trials of a herpes simplex virus type 2 (HSV-2) glycoprotein-D-subunit vaccine with alum and 3-O-deacylated-monophosphoryl lipid A in subjects whose regular sexual partners had a history of genital herpes. In Study 1, subjects were seronegative for herpes simplex virus type 1 (HSV-1) and HSV-2; in Study 2, subjects were of any HSV serologic status. At months 0, 1, and 6, subjects received either vaccine or a control injection and were evaluated for 19 months. The primary end point was the occurrence of genital herpes disease in all subjects in Study 1 and in HSV-2-seronegative female subjects in Study 2.

RESULTS

A total of 847 subjects who were seronegative for both HSV-1 and HSV-2 (268 of them women, in Study 1) and 1867 subjects who were seronegative for HSV-2 (710 of them women, in Study 2) underwent randomization and received injections. Vaccination was well tolerated and elicited humoral and cellular responses. Overall, the efficacy of the vaccine was 38 percent in Study 1 (95 percent confidence interval, -18 to 68 percent; 15 cases occurred in the vaccine group and 24 in the control group), and efficacy in female subjects was 42 percent in Study 2 (95 percent confidence interval, -31 to 74 percent; 9 cases occurred in the vaccine group and 16 in the control group). In both studies, further analysis showed that the vaccine was efficacious in women who were seronegative for both HSV-1 and HSV-2: efficacy in Study 1 was 73 percent (95 percent confidence interval, 19 to 91 percent; P=0.01), and efficacy in Study 2 was 74 percent (95 percent confidence interval, 9 to 93 percent; P=0.02). It was not efficacious in women who were seropositive for HSV-1 and seronegative for HSV-2 at base line or in men.

CONCLUSIONS

These studies suggest that the glycoprotein D vaccine has efficacy against genital herpes in women who are seronegative for both HSV-1 and HSV-2 at base line but not in those who are seropositive for HSV-1 and seronegative for HSV-2. It had no efficacy in men, regardless of their HSV serologic status.

Therapeutic vaccination with vhs(-) herpes simplex virus reduces the severity of recurrent herpetic stromal keratitis in mice

Virion host shutoff (vhs)-deficient herpes simplex virus (HSV) was tested as a therapeutic vaccine in a mouse model of UV light-induced recurrent herpetic stromal keratitis. Four weeks after primary corneal infection, mice were vaccinated intraperitoneally with vhs(-) vaccine or control. Four weeks after vaccination, the eyes of latently infected mice were UV-B irradiated to induce recurrent virus shedding and disease. Post-irradiation corneal opacity in latently infected, vhs(-)-vaccinated mice was significantly reduced compared to control-vaccinated mice (P=0.007 to 0.035). The incidence and duration of recurrent virus shedding were the same in both groups. Antibody titres were increased (P=0.05) and delayed type hypersensitive responses were unaffected by vhs(-) vaccination. Combined with studies using different vaccination timing and vhs(-) genotypes, these data suggest that deletion of vhs is a useful strategy in the development of a therapeutic HSV vaccine, and that temporal and genetic factors influence vaccination outcome.

Design and selection of vaccine adjuvants: animal models and human trials

The availability of hundreds of different adjuvants has prompted a need for identifying rational standards for selection of adjuvant formulations based on safety and sound immunological principles for human vaccines. Although many of the mechanisms of adjuvants have been elucidated, meaningful comparisons between different adjuvants derived from in vitro studies, or from studies using adjuvants in rodents or other animals, are often not predictive for safety, adjuvant effects, or vaccine efficacy in humans. A highly efficient and cost-effective method for comparison of adjuvants with a new antigen is to conduct multiple small-scale, phase 1, comparative studies in humans with a new antigen, using adjuvants previously found to be safe with other antigens in human trials. Studies in which highly immunogenic and safe adjuvant formulations have been evaluated in comparative adjuvant trials in humans using a single candidate vaccine antigen against malaria, HIV, and prostate cancer with multiple adjuvants are reviewed.

Mechanisms of pathogenesis in herpetic immunoinflammatory ocular lesions

This article reviews possible mechanisms by which ocular infections with herpes simplex virus result in a blinding immunoinflammatory lesion in the cornea. We conclude that this immunoinflammatory response involves multiple immune mechanisms including autoimmunity.

Therapeutic immunization with a virion host shutoff-defective, replication-incompetent herpes simplex virus type 1 strain limits recurrent herpetic ocular infection

Immunization of mice with herpes simplex virus type 1 (HSV-1) mutant viruses containing deletions in the gene for virion host shutoff (vhs) protein diminishes primary and recurrent corneal infection with wild-type HSV-1. vhs mutant viruses are severely attenuated in vivo but establish latent infections in sensory neurons. A safer HSV-1 mutant vaccine strain, Delta41Delta29, has combined vhs and replication (ICP8-) deficits and protects BALB/c mice against primary corneal infection equivalent to a vhs- strain (BGS41). Here, we tested the hypothesis that Delta41Delta29 can protect as well as BGS41 in a therapeutic setting. Because immune response induction varies with the mouse and virus strains studied, we first determined the effect of prophylactic Delta41Delta29 vaccination on primary ocular infection of NIH inbred mice with HSV-1 McKrae, a model system used to evaluate therapeutic vaccines. In a dose-dependent fashion, prophylactic Delta41Delta29 vaccination decreased postchallenge tear film virus titers and ocular disease incidence and severity while eliciting high levels of HSV-specific antibodies. Adoptive transfer studies demonstrated a dominant role for immune serum and a lesser role for immune cells in mediating prophylactic protection. Therapeutically, vaccination with Delta41Delta29 effectively reduced the incidence of UV-B-induced recurrent virus shedding in latently infected mice. Therapeutic Delta41Delta29 and BGS41 vaccination decreased corneal opacity and delayed-type hypersensitivity responses while elevating antibody titers, compared to controls. These data indicate that replication is not a prerequisite for generation of therapeutic immunity by live HSV mutant virus vaccines and raise the possibility that genetically tailored replication-defective viruses may make effective and safe therapeutic vaccines.

New treatments for genital herpes

Genital herpes is a sexually transmitted disease that has been widely investigated from both an epidemiological and a diagnostic viewpoint. Better knowledge of the disease has pointed to the important role of asymptomatic herpes simplex virus type 2 shedding as a major culprit for the spread of the virus in the world, the latest statistics showing that the incidence of genital herpes is constantly increasing. The reference compound for the treatment and prophylaxis of genital herpes is acyclovir. Recently, two drugs with better oral bioavailability, valaciclovir, the oral prodrug of acyclovir, and famciclovir, the oral prodrug of penciclovir, have been made available. Nevertheless, there is a need to develop additional approaches, to extend the therapeutic possibilities and to face the problem of emerging resistant viruses, mostly in immunocompromised patients. Therefore, efforts have been made in different directions including the exploration of new targets for antiviral chemotherapy, the use of immunomodulators and the development of specific vaccines.

Native-like, long synthetic peptides as components of sub-unit vaccines: practical and theoretical considerations for their use in humans

Vaccines have been used as a successful tool in medicine by way of controlling many major diseases. In spite of this, vaccines today represent only a handful of all infectious diseases. Therefore, there is a pressing demand for improvements of existing vaccines with particular reference to higher efficacy and undisputed safety profiles. To this effect, as an alternative to available vaccine technologies, there has been a drive to develop vaccine candidate polypeptides by chemical synthesis. In our laboratory, we have recently developed a technology to manufacture long synthetic peptides of up to 130 residues, which are correctly folded and biologically active. This paper discusses the advantages of the molecularly defined, long synthetic peptide approach in the context of vaccine design, development and use in human vaccination.