Varicella-zoster virus expressing HSV-2 glycoproteins B and D induces protection against HSV-2 challenge

The Quest for Immunity: Exploring Human Herpesviruses as Vaccine Vectors

Herpesviruses are large DNA viruses that have long been used as powerful gene therapy tools. In recent years, the ability of herpesviruses to stimulate both innate and adaptive immune responses has led to their transition to various applications as vaccine vectors. This vaccinology branch is growing at an unprecedented and accelerated rate. To date, human herpesvirus-based vectors have been used in vaccines to combat a variety of infectious agents, including the Ebola virus, foot and mouth disease virus, and human immunodeficiency viruses. Additionally, these vectors are being tested as potential vaccines for cancer-associated antigens. Thanks to advances in recombinant DNA technology, immunology, and genomics, numerous steps in vaccine development have been greatly improved. A better understanding of herpesvirus biology and the interactions between these viruses and the host cells will undoubtedly foster the use of herpesvirus-based vaccine vectors in clinical settings. To overcome the existing drawbacks of these vectors, ongoing research is needed to further advance our knowledge of herpesvirus biology and to develop safer and more effective vaccine vectors. Advanced molecular virology and cell biology techniques must be used to better understand the mechanisms by which herpesviruses manipulate host cells and how viral gene expression is regulated during infection. In this review, we cover the underlying molecular structure of herpesviruses and the strategies used to engineer their genomes to optimize capacity and efficacy as vaccine vectors. Also, we assess the available data on the successful application of herpesvirus-based vaccines for combating diseases such as viral infections and the potential drawbacks and alternative approaches to surmount them.

Recombinant Simian Varicella Virus-Simian Immunodeficiency Virus Vaccine Induces T and B Cell Functions and Provides Partial Protection against Repeated Mucosal SIV Challenges in Rhesus Macaques

HIV vaccine mediated efficacy, using an expanded live attenuated recombinant varicella virus-vectored SIV rSVV-SIVgag/env vaccine prime with adjuvanted SIV-Env and SIV-Gag protein boosts, was evaluated in a female rhesus macaques (RM) model against repeated intravaginal SIV challenges. Vaccination induced anti-SIV IgG responses and neutralizing antibodies were found in all vaccinated RMs. Three of the eight vaccinated RM remained uninfected (vaccinated and protected, VP) after 13 repeated challenges with the pathogenic SIVmac251-CX-1. The remaining five vaccinated and infected (VI) macaques had significantly reduced plasma viral loads compared with the infected controls (IC). A significant increase in systemic central memory CD4+ T cells and mucosal CD8+ effector memory T-cell responses was detected in vaccinated RMs compared to controls. Variability in lymph node SIV-Gag and Env specific CD4+ and CD8+ T cell cytokine responses were detected in the VI RMs while all three VP RMs had more durable cytokine responses following vaccination and prior to challenge. VI RMs demonstrated predominately SIV-specific monofunctional cytokine responses while the VP RMs generated polyfunctional cytokine responses. This study demonstrates that varicella virus-vectored SIV vaccination with protein boosts induces a 37.5% efficacy rate against pathogenic SIV challenge by generating mucosal memory, virus specific neutralizing antibodies, binding antibodies, and polyfunctional T-cell responses.

A recombinant varicella vaccine harboring a respiratory syncytial virus gene induces humoral immunity

The varicella-zoster virus (VZV) Oka vaccine strain (vOka) is highly efficient and causes few adverse events; therefore, it is used worldwide. We previously constructed recombinant vOka (rvOka) harboring the mumps virus gene. Immunizing guinea pigs with rvOka induced the production of neutralizing antibodies against the mumps virus and VZV. Here, we constructed recombinant vOka viruses containing either the respiratory syncytial virus (RSV) subgroup A fusion glycoprotein (RSV A-F) gene or RSV subgroup B fusion glycoprotein (RSV B-F) gene (rvOka-RSV A-F or rvOka-RSV B-F). Indirect immunofluorescence and Western blot analyses confirmed the expression of each recombinant RSV protein in virus-infected cells. Immunizing guinea pigs with rvOka-RSV A-F or rvOka-RSV B-F led to the induction of antibodies against RSV proteins. These results suggest that the current varicella vaccine genome can be used to generate custom-made vaccine vectors to develop the next generation of live vaccines.

Novel polyvalent live vaccine against varicella-zoster and mumps virus infections

The varicella-zoster virus (VZV) Oka vaccine strain (vOka) is a highly immunogenic and safe live vaccine that has long been used worldwide. Because its genome is large, making it suitable for inserting foreign genes, vOka is considered a candidate vector for novel polyvalent vaccines. Previously, a recombinant vOka, rvOka-HN, that expresses mumps virus (MuV) hemagglutinin-neuraminidase (HN) was generated by the present team. rvOka-HN induces production of neutralizing antibodies against MuV in guinea pigs. MuV also expresses fusion (F) protein, which is important for inducing neutralizing antibodies, in its viral envelope. To induce a more robust immune response against MuV than that obtained with rvOka-HN, here an rvOka expressing both HN and F (rvOka-HN-F) was generated. However, co-expression of HN and F caused the infected cells to form syncytia, which reduced virus titers. To reduce the amount of cell fusion, an rvOka expressing HN and a mutant F, F(S195Y) were generated. Almost no syncytia formed among the rvOka-HN-F(S195Y)-infected cells and the growth of rvOka-HN-F(S195Y) was similar to that of the original vOka clone. Moreover, replacement of serine 195 with tyrosine had no effect on the immunogenicity of F in mice and guinea pigs. Although obvious augmentation of neutralizing antibody production was not observed after adding F protein to vOka-HN, the anti-F antibodies did have neutralizing activity. These data suggest that F protein contributes to induction of immune protection against MuV. Therefore this recombinant virus is a promising candidate vaccine for polyvalent protection against both VZV and MuV.

A recombinant Newcastle disease virus (NDV) expressing infectious laryngotracheitis virus (ILTV) surface glycoprotein D protects against highly virulent ILTV and NDV challenges in chickens

Infectious laryngotracheitis (ILT) is a highly contagious acute respiratory disease of chickens caused by infectious laryngotracheitis virus (ILTV). Currently, modified live ILTV vaccines are used to control ILT infections. However, the live ILTV vaccines can revert to virulence after bird-to-bird passage and are capable of establishing latent infections, suggesting the need to develop safer vaccines against ILT. We have evaluated the role of three major ILTV surface glycoproteins, namely, gB, gC, and gD in protection and immunity against ILTV infection in chickens. Using reverse genetics approach, three recombinant Newcastle disease viruses (rNDVs) designated rNDV gB, rNDV gC, and rNDV gD were generated, each expressing gB, gC, and gD, respectively, of ILTV. Chickens received two immunizations with rNDVs alone (gB, gC, and gD) or in combination (gB+gC, gB+gD, gC+gD, and gB+gC+gD). Immunization with rNDV gD induced detectable levels of neutralizing antibodies with the magnitude of response greater than the rest of the experimental groups including those vaccinated with commercially available vaccines. The birds immunized with rNDV gD showed complete protection against virulent ILTV challenge. The birds immunized with rNDV gC alone or multivalent vaccines consisting of combination of rNDVs displayed partial protection with minimal disease and reduced replication of challenge virus in trachea. Immunization with rNDV gB neither reduced the severity of the disease nor the replication of challenge virus in trachea. The superior protective efficacy of rNDV gD vaccine compared to rNDV gB or rNDV gC vaccine was attributed to the higher levels of envelope incorporation and infected cell surface expression of gD than gB or gC. Our results suggest that rNDV expressing gD is a safe and effective bivalent vaccine against NDV and ILTV.

Vaccination with a HSV-2 UL24 mutant induces a protective immune response in murine and guinea pig vaginal infection models

The rational design and development of genetically attenuated HSV-2 mutant viruses represent an attractive approach for developing both prophylactic and therapeutic vaccines for genital herpes. Previously, HSV-2 UL24 was shown to be a virulence determinant in both murine and guinea pig vaginal infection models. An UL24-βgluc insertion mutant produced syncytial plaques and replicated to nearly wild type levels in tissue culture, but induced little or no pathological effects in recipient mice or guinea pigs following vaginal infection. Here we report that immunization of mice or guinea pigs with high or low doses of UL24-βgluc elicited a highly protective immune response. UL24-βgluc immunization via the vaginal or intramuscular routes was demonstrated to protect mice from a lethal vaginal challenge with wild type HSV-2. Moreover, antigen re-stimulated splenic lymphocytes harvested from immunized mice exhibited both HSV-2 specific CTL activity and IFN-γ expression. Humoral anti-HSV-2 responses in serum were Th1-polarized (IgG2a>IgG1) and contained high-titer anti-HSV-2 neutralizing activity. Guinea pigs vaccinated subcutaneously with UL24-βgluc or the more virulent parental strain (186) were challenged with a heterologous HSV-2 strain (MS). Acute disease scores were nearly indistinguishable in guinea pigs immunized with either virus. Recurrent disease scores were reduced in UL24-βgluc immunized animals but not to the same extent as those immunized with strain 186. In addition, challenge virus was not detected in 75% of guinea pigs subcutaneously immunized with UL24-βgluc. In conclusion, disruption of the UL24 gene is a prime target for the development of a genetically attenuated live HSV-2 vaccine.

Recombinant varicella-zoster virus vaccines as platforms for expression of foreign antigens

Varicella-zoster virus (VZV) vaccines induce immunity against childhood chickenpox and against shingles in older adults. The safety, efficacy, and widespread use of VZV vaccines suggest that they may also be effective as recombinant vaccines against other infectious diseases that affect the young and the elderly. The generation of recombinant VZV vaccines and their evaluation in animal models are reviewed. The potential advantages and limitations of recombinant VZV vaccines are addressed.

The varicella-zoster virus genome

The varicella-zoster virus (VZV) genome contains at least 70 genes, and all but six have homologs in herpes simplex virus (HSV). Cosmids and BACs corresponding to the VZV parental Oka and vaccine Oka viruses have been used to "knockout" 34 VZV genes. Seven VZV genes (ORF4, 5, 9, 21, 29, 62, and 68) have been shown to be required for growth in vitro. Recombinant viruses expressing several markers (e.g., beta-galactosidase, green fluorescence protein, luciferase) and several foreign viral genes (from herpes simplex, Epstein-Barr virus, hepatitis B, mumps, HIV, and simian immunodeficiency virus) have been constructed. Further studies of the VZV genome, using recombinant viruses, may facilitate the development of safer and more effective VZV vaccines. Furthermore, VZV might be useful as a vaccine vector to immunize against both VZV and other viruses.

Recombinant simian varicella viruses expressing respiratory syncytial virus antigens are immunogenic

Recombinant simian varicella viruses (rSVVs) were engineered to express respiratory syncytial virus (RSV) antigens. The RSV surface glycoprotein G and second matrix protein M2 (22k) genes were cloned into the SVV genome, and recombinant viruses were characterized in vitro and in vivo. rSVVs were also engineered to express the membrane-anchored or secreted forms of the RSV-G protein as well as an RSV G lacking its chemokine mimicry motif (CX3C), which may have different effects on priming the host immune response. The RSV genes were efficiently expressed in rSVV/RSV-infected Vero cells as RSV-G and -M2 transcripts were detected by RT-PCR, and RSV antigens were detected by immunofluorescence and immunoblot assays. The rSVVs replicated efficiently in Vero cell culture. Rhesus macaques immunized with rSVV/RSV-G and rSVV/RSV-M2 vaccines produced antibody responses to SVV and RSV antigens. The results demonstrate that recombinant varicella viruses are suitable vectors for the expression of RSV antigens and may represent a novel vaccine strategy for immunization against both pathogens.

Recombinant simian varicella viruses induce immune responses to simian immunodeficiency virus (SIV) antigens in immunized vervet monkeys

The varicella-zoster virus (VZV) Oka vaccine offers potential as a recombinant vaccine against other pathogens. In this study, recombinant simian varicella viruses (rSVV) expressing simian immunodeficiency virus (SIV) envelope (env, gp130) and gag antigens were constructed. Expression of the SIV env and gag transcripts and antigens in rSVV-infected Vero cells was confirmed. The rSVV-SIVenv and rSVV-SIVgag viruses replicated as efficiently as wild-type SVV in cell culture. The immunogenicity of rSVV-SIVenv and rSVV-SIVgag was investigated in immunized vervet monkeys. Humoral immune responses to the SIV gp130 and gag antigens were detected as early as 4 weeks after the initial immunization with higher antibody titers following a booster immunization. Cellular immune responses against the SIV gp130 antigen were detected by ELISPOT assay. The rSVV established latent infection in neural ganglia. A subsequent study will evaluate the ability of rSVV vaccines expressing SIV antigens to protect nonhuman primates against simian AIDS.

Recombinant vesicular stomatitis virus vectors expressing herpes simplex virus type 2 gD elicit robust CD4+ Th1 immune responses and are protective in mouse and guinea pig models of vaginal challenge

Recombinant vesicular stomatitis virus (rVSV) vectors offer an attractive approach for the induction of robust cellular and humoral immune responses directed against human pathogen target antigens. We evaluated rVSV vectors expressing full-length glycoprotein D (gD) from herpes simplex virus type 2 (HSV-2) in mice and guinea pigs for immunogenicity and protective efficacy against genital challenge with wild-type HSV-2. Robust Th1-polarized anti-gD immune responses were demonstrated in the murine model as measured by induction of gD-specific cytotoxic T lymphocytes and increased gamma interferon expression. The isotype makeup of the serum anti-gD immunoglobulin G (IgG) response was consistent with the presence of a Th1-CD4+ anti-gD response, characterized by a high IgG2a/IgG1 IgG subclass ratio. Functional anti-HSV-2 neutralizing serum antibody responses were readily demonstrated in both guinea pigs and mice that had been immunized with rVSV-gD vaccines. Furthermore, guinea pigs and mice were prophylactically protected from genital challenge with high doses of wild-type HSV-2. In addition, guinea pigs were highly protected against the establishment of latent infection as evidenced by low or absent HSV-2 genome copies in dorsal root ganglia after virus challenge. In summary, rVSV-gD vectors were successfully used to elicit potent anti-gD Th1-like cellular and humoral immune responses that were protective against HSV-2 disease in guinea pigs and mice.

A cosmid-based system for inserting mutations and foreign genes into the simian varicella virus genome

Simian varicella is a natural varicella-like disease of nonhuman primates. The etiologic agent, simian varicella virus (SVV), is genetically related to varicella-zoster virus (VZV) and SVV infection of nonhuman primates is a useful model to investigate VZV pathogenesis and latency. In this study, we report development of a cosmid-based genetic system to generate SVV mutant viruses. SVV subgenomic DNA fragments (32-38kb) that span the viral genome were cloned into cosmid vectors. Co-transfection of Vero cells with four overlapping cosmid clones representing the entire SVV genome resulted in recombination and generation of infectious virus. SVV mutants were produced by manipulation of one cosmid and substitution into the genetic system. This genetic approach was used to insert a site-specific mutation within the SVV open reading frame 14 which encodes the nonessential glycoprotein C gene. In a subsequent experiment, the green fluorescent protein (GFP) gene was inserted into the SVV genome within ORF 14. These SVV mutants replicate as efficiently as wild-type SVV in cell culture. This cosmid-based genetic system will be useful to investigate the effect of viral mutations on SVV pathogenesis and latency and also to develop and evaluate recombinant varicella vaccines that express foreign antigens.

The varicella-zoster virus open reading frame 63 latency-associated protein is critical for establishment of latency

Varicella-zoster virus (VZV) expresses at least six viral transcripts during latency. One of these transcripts, derived from open reading frame 63 (ORF63), is one of the most abundant viral RNAs expressed during latency. The VZV ORF63 protein has been detected in human and experimentally infected rodent ganglia by several laboratories. We have deleted >90% of both copies of the ORF63 gene from the VZV genome. Animals inoculated with the ORF63 mutant virus had lower mean copy numbers of latent VZV genomes in the dorsal root ganglia 5 to 6 weeks after infection than animals inoculated with parental or rescued virus, and the frequency of latently infected animals was significantly lower in animals infected with the ORF63 mutant virus than in animals inoculated with parental or rescued virus. In contrast, the frequency of animals latently infected with viral mutants in other genes that are equally or more impaired for replication in vitro, compared with the ORF63 mutant, is similar to that of animals latently infected with parental VZV. Examination of dorsal root ganglia 3 days after infection showed high levels of VZV DNA in animals infected with either ORF63 mutant or parental virus; however, by days 6 and 10 after infection, the level of viral DNA in animals infected with the ORF63 mutant was significantly lower than that in animals infected with parental virus. Thus, ORF63 is not required for VZV to enter ganglia but is the first VZV gene shown to be critical for establishment of latency. Since the present vaccine can reactivate and cause shingles, a VZV vaccine based on the ORF63 mutant virus might be safer.