Humoral and cell-mediated immune responses to recombinant vaccinia viruses in mice

Attenuation and genetic characteristics of a Moroccan strain of Camel pox virus

Camel pox (CML) is a widespread infectious viral disease of camels that causes huge economic losses to the camel industry. In this study, a local strain of Camel pox virus (CMLV) was attenuated by 175 serial passages in Vero cells and the residual pathogenicity and infectivity were tested in naïve camels at 120, 150 and 175 passage levels. Also, the safety and immunogenicity of the 175th passage were evaluated in camels using a dose of 10^4.0 Tissue Culture Dose 50% (TCID₅₀₎ and monitored for up to one-year post vaccination (pv) for neutralizing antibody. Seroconversion was noted at day 14 pv with neutralizing antibody titers ranging from 0.5 and 1.6 logs over the one-year of the study. Among 8 camels inoculated with the P175 strain, 4 were challenged at 12-month pv with 10^5.7 TCID₅₀/ml of the original virulent CMLV and complete protection was recorded in all animals. Whole genome sequencing detected six mutations in the original CMLV strain that were not present in the attenuated 175th passage of this strain. Overall, the findings of this study indicated that the 175th passage of the CMLV was attenuated, safe and afforded protection to camels against virulent CMLV, and is therefore, a promising vaccine candidate for the prevention of CML in camels.

Development and Evaluation of a Live Attenuated Egg-Based Camelpox Vaccine

Camelpox is an infectious viral disease of camels reported in all the camel-breeding areas of Africa, north of the equator, the Middle East and Asia. It causes huge economic loss to the camel industry. We developed a live camelpox virus vaccine candidate using an attenuated strain and evaluated its safety, immunogenicity and protective efficacy in camels. The attenuated virus strain was generated from the camelpox wild-type strain M-96 by 40 consecutive passages on the chorioallantoic membrane of 11-day-old embryonated chicken eggs, henceforth called KM-40 strain. Reversion to virulence of the KM-40 strain was evaluated in camels by three serial passages, confirmed its inability to revert to virulence and its overdose administration was also found safe. Studies of immunogenicity and protective efficacy of the candidate vaccine KM-40 strain in camels was carried out using the dose of 5 x 10^4.0 EID₅₀. Our data showed complete protection against the challenge infection using the virulent wild-type camelpox virus strain M-96 (dose of 10^5.0 EID₅₀) which was evaluated at 1, 3, 6 and 12 months post vaccination. In summary, our candidate live attenuated egg-based camelpox vaccine strain KM-40 was found safe, protective, and thus has the potential to use safely in field conditions.

Progresses in DNA-based heterologous prime-boost immunization strategies

Although recombinant DNA and recombinant viral vectors expressing HIV antigens have yielded positive outcomes in animal models, these vaccines have not been effectively translated to humans. Despite this, there is still a high level of optimism that poxviral-based vaccine strategies could offer the best hope for developing an effective vaccine against not only HIV-1 but also other chronic diseases where good-quality T and B cell immunity is needed for protection. In this chapter we discuss step by step (1) how recombinant poxviral vectors co-expressing HIV antigens and promising mucosal/systemic adjuvants (e.g., IL-13Rα2) are constructed, (2) how these vectors can be used in alternative heterologous prime-boost immunization strategies, (3) how systemic and mucosal samples are prepared for analysis, followed by (4) two immunological assays: multicolor intracellular cytokine staining and tetramer/homing maker analysis that are used to evaluate effective systemic and mucosal T cell immunity.

Chemokine binding protein vCCI attenuates vaccinia virus without affecting the cellular response elicited by immunization with a recombinant vaccinia vector carrying the HPV16 E7 gene

Viral CC chemokine inhibitor (vCCI) of the clone P13 vaccinia virus (VACV) strain PRAHA lacks eight amino acids in the signal peptide sequence. To study the influence of vCCI on virus biology, a virus with the vCCI gene coding for a prolonged signal sequence was prepared. We found that secreted vCCI attenuated the virus in vivo, and that it correlated with decreased levels of RANTES, eotaxin, TARC, and MDC in the blood in comparison with the parental virus. We determined the influence of vCCI on the CTL response against VACV E3((140-148)) (VGPSNSPTF) and HPV16 E7((49-57)) (RAHYNIVTF) H-2D(b)-restricted epitopes. The examination of the specific CTL response elicited by immunization with the recombinant VACV-expressing tumor-associated HPV16 E7 antigen by IFN-γ ELISPOT showed that the immunogenicity of the recombinant VACV-producing secretory vCCI was similar to that of the parent virus or deletion mutant in the C23L/B29R locus. Immunization with the secretory vCCI-producing recombinant virus has a lower therapeutic anti-tumor effect against TC-1 tumors. Viral CCI downregulated the E7-specific response induced by gene gun immunization with the DNA vaccines pBSC-SigE7 LAMP and pBSC-vCCI. We also observed that the immune response against vCCI elicited by the DNA vaccine did not affect the multiplication of VACV in vivo.

Reverse genetics technology for Rift Valley fever virus: current and future applications for the development of therapeutics and vaccines

The advent of reverse genetics technology has revolutionized the study of RNA viruses, making it possible to manipulate their genomes and evaluate the effects of these changes on their biology and pathogenesis. The fundamental insights gleaned from reverse genetics-based studies over the last several years provide a new momentum for the development of designed therapies for the control and prevention of these viral pathogens. This review summarizes the successes and stumbling blocks in the development of reverse genetics technologies for Rift Valley fever virus and their application to the further dissection of its pathogenesis and the design of new therapeutics and safe and effective vaccines.

Robust intrapulmonary CD8 T cell responses and protection with an attenuated N1L deleted vaccinia virus

Background

Vaccinia viruses have been used as a model for viral disease and as a protective live vaccine.

Methodology and Principal Findings

We investigated the immunogenicity of an attenuated strain of vaccinia virus engineered to inactivate the N1L gene (vGK5). Using the intranasal route, this recombinant virus was 2 logs less virulent compared to the wildtype VACV-WR. Infection by the intranasal, intraperitoneal, and tail scarification routes resulted in the robust induction of cytolytic virus-specific CD8 T cells in the spleens and the lungs. VACV-specific antibodies were also detected in the sera of mice infected 3–5 months prior with the attenuated vGK5 virus. Finally, mice immunized with vGK5 were significantly protected when challenged with a lethal dose of VACV-WR.

Conclusions

These results indicate that the attenuated vGK5 virus protects against subsequent infection and suggest that the N1L protein limits the strength of the early antiviral CD8 T cell response following respiratory infection.

Virulence in murine model shows the existence of two distinct populations of Brazilian Vaccinia virus strains

Brazilian Vaccinia virus had been isolated from sentinel mice, rodents and recently from humans, cows and calves during outbreaks on dairy farms in several rural areas in Brazil, leading to high economic and social impact. Some phylogenetic studies have demonstrated the existence of two different populations of Brazilian Vaccinia virus strains circulating in nature, but little is known about their biological characteristics. Therefore, our goal was to study the virulence pattern of seven Brazilian Vaccinia virus strains. Infected BALB/c mice were monitored for morbidity, mortality and viral replication in organs as trachea, lungs, heart, kidneys, liver, brain and spleen. Based on the virulence potential, the Brazilian Vaccinia virus strains were grouped into two groups. One group contained GP1V, VBH, SAV and BAV which caused disease and death in infected mice and the second one included ARAV, GP2V and PSTV which did not cause any clinical signals or death in infected BALB/c mice. The subdivision of Brazilian Vaccinia virus strains into two groups is in agreement with previous genetic studies. Those data reinforce the existence of different populations circulating in Brazil regarding the genetic and virulence characteristics.

Evasion of host immune responses by tumours and viruses

Viruses and tumours use various mechanisms to avoid immune surveillance. Oncogenic viruses have achieved a balance with the immune system through evolutionary time to ensure long-term persistence. Mutations that promote escape mechanisms favouring tumour growth to the detriment of host survival through reproductive age offer no selective advantage and will not generally be maintained in the viral genome that persists in nature. Conventional (non-oncogenic) and tumour viruses interact with various immune mediators and T cells in different ways. Oncogenic viruses cannot operate solely in the context of a lytic cycle, though this may be characteristic of the initial phase of infection that is limited by the acute immune response. Some oncogenic viruses interact with normal cellular growth control and signalling mechanisms. Synthesis of key viral proteins may be tightly controlled in replicating cells that are subject to T cell surveillance, such as basal epithelia, while productive infection occurs in non-proliferating progeny that are lost under normal physiological conditions, such as desquamating epithelia. Tumorigenesis may be an aberrant consequence of the molecular mechanisms needed to maintain this pattern of viral growth regulation in the context of the cell cycle. Vaccines designed to limit the acute phase of infection with cell-free oncogenic viruses should be as effective as those for conventional viruses.

Protective immune responses induced by different recombinant vaccine regimes to Rift Valley fever

The glycoprotein (GP) and nucleocapsid (NC) genes of Rift Valley fever virus (RVFV) were expressed in different expression systems and were evaluated for their ability to protect mice from virulent challenge using a prime-boost regime. Mice vaccinated with a lumpy skin disease virus-vectored recombinant vaccine (rLSDV-RVFV) expressing the two RVFV glycoproteins (G1 and G2) developed neutralising antibodies and were fully protected when challenged, as were those vaccinated with a crude extract of truncated G2 glycoprotein (tG2). By contrast mice vaccinated with a DNA vaccine expressing G1 and G2 did not sero-convert with only 20% of them surviving challenge. Mice vaccinated with the DNA vaccine and boosted with rLSDV-RVFV also failed to sero-convert but 40% survived challenge. Surprisingly, although none of the mice immunised with the purified NC protein sero-converted, 60% of them survived virulent challenge. The rLSDV-RVFV construct was then further evaluated in sheep for its dual protective abilities against RVFV and sheeppox virus (SPV). Vaccinated sheep sero-converted for both viruses and were protected against RVFV challenge, however, neither the immunised or negative control animals showed any significant reactions to the virulent SPV challenge.

Identification of a Bohle iridovirus thymidine kinase gene and demonstration of activity using vaccinia virus

In recent years interest in the family Iridoviridae has been renewed by the identification of a number of viruses, particularly from the genus Ranavirus, associated with disease in a range of poikilotherms. Ranaviruses have been isolated from amphibian, piscine and reptilian species. Here we describe an open reading frame (ORF) identified in the genome of Bohle iridovirus (BIV) which contains a nucleotide binding motif conserved within the thymidine kinase (TK) genes of iridoviruses from other genera (lymphocystis disease virus, LCDV, type species of the genus Lymphocystivirus; Chilo iridescent virus, CIV, type species of the genus Iridovirus). The ability of this putative gene to express a functional TK was confirmed by rescue of a TK negative mutant vaccinia virus in the presence of selective media, when expression was controlled by a vaccinia virus promoter. The sequence of the BIV TK was compared with the homologous sequences from epizootic haematopoietic necrosis virus (EHNV), a virus associated with disease in fish, from Wamena iridovirus (WIV) associated with systemic disease in green pythons, and from frog virus 3 (FV3) the ranavirus type species. Comparisons between these sequences and those available from other ranaviruses, other iridoviruses, other DNA viruses and cellular TKs are presented.

Induction of potent humoral and cell-mediated immune responses by attenuated vaccinia virus vectors with deleted serpin genes

Vaccinia virus (VV) has been effectively utilized as a live vaccine against smallpox as well as a vector for vaccine development and immunotherapy. Increasingly there is a need for a new generation of highly attenuated and efficacious VV vaccines, especially in light of the AIDS pandemic and the threat of global bioterrorism. We therefore developed recombinant VV (rVV) vaccines that are significantly attenuated and yet elicit potent humoral and cell-mediated immune responses. B13R (SPI-2) and B22R (SPI-1) are two VV immunomodulating genes with sequence homology to serine protease inhibitors (serpins) that possess antiapoptotic and anti-inflammatory properties. We constructed and characterized rVVs that have the B13R or B22R gene insertionally inactivated (vDeltaB13R and vDeltaB22R) and coexpress the vesicular stomatitis virus glycoprotein (v50DeltaB13R and v50DeltaB22R). Virulence studies with immunocompromised BALB/cBy nude mice indicated that B13R or B22R gene deletion decreases viral replication and significantly extends time of survival. Viral pathogenesis studies in immunocompetent CB6F(1) mice further demonstrated that B13R or B22R gene inactivation diminishes VV virulence, as measured by decreased levels of weight loss and limited viral spread. Finally, rVVs with B13R and B22R deleted elicited potent humoral, T-helper, and cytotoxic T-cell immune responses, revealing that the observed attenuation did not reduce immunogenicity. Therefore, inactivation of immunomodulating genes such as B13R or B22R represents a general method for enhancing the safety of rVV vaccines while maintaining a high level of immunogenicity. Such rVVs could serve as effective vectors for vaccine development and immunotherapy.

CEA-based vaccines

Carcinoembryonic antigen (CEA), the first tumor-associated antigen to be described, was cloned in 1987 and is expressed on nearly 50% of all human tumors. The identification of T-cell specific epitopes within the coding region of the CEA protein has led to the development of various vaccine strategies that target CEA and CEA-expressing tumors. These vaccines have shown evidence of therapeutic effectiveness in animal models and are being evaluated in early phase clinical trials. Although trials are not designed to elucidate clinical responses, they have provided important information about the ability of individual vaccines to induce CEA-specific immune responses through the use of newer in vitro monitoring assays. Continued clinical testing in patients with less advanced disease and the administration of vaccines in combination with other standard therapy will help define the role of CEA-based vaccines in the treatment and prevention of human cancer.

Immunotherapy for pancreatic cancer: current concepts

Despite advances in chemotherapy and surgical technique, patients with pancreatic cancer often succumb to local recurrence or metastatic spread. The need for new therapeutic strategies for this disease coupled with a better understanding of basic immunology have led to the development of novel anti-tumor vaccines. This review focuses on the historical development of tumor vaccines emphasizing the identification of potential pancreatic tumor antigens. The role of both B-cell and T-cell responses in tumor rejection will be reviewed. Methods for antigen presentation, including peptides, recombinant viral and bacterial vectors, dendritic cells, and whole cell approaches will be discussed. The use of immune adjuvants and improved methods of vaccine delivery will also be explored. The full potential for the immunotherapy of pancreatic cancer awaits the results of early phase clinical trials. The development of pancreatic cancer vaccines represents a useful paradigm for the translation of basic research into the clinical arena.

Construction and characterization of recombinant vaccinia viruses co-expressing a respiratory syncytial virus protein and a cytokine

Recombinant vaccinia viruses are well-characterized tools that can be used to define novel approaches to vaccine formulation and delivery. While vector co-expression of immune mediators has enormous potential for optimizing the composition of vaccine-induced immune responses, the impact on antigen expression and vector antigenicity must also be considered. Co-expression of IL-4 increased vaccinia virus vector titres, while IFN-gamma co-expression reduced vaccinia virus replication in BALB/c mice and in C57BL/6 mice infected with some recombinant viruses. Protection against respiratory syncytial virus (RSV) challenge was similar in mice immunized with vaccinia virus expressing RSV G glycoprotein and IFN-gamma, even though the replication efficiency of the vector was diminished. These data demonstrate the ability of vector-expressed cytokine to influence the virulence of the vector and to direct the development of selected immune responses. This suggests that the co-expression of cytokines and other immunomodulators has the potential to improve the safety of vaccine vectors while improving the immunogenicity of vaccine antigens.

Interleukin-12 (IL-12) enhancement of the cellular immune response against human immunodeficiency virus type 1 env antigen in a DNA prime/vaccinia virus boost vaccine regimen is time and dose dependent: suppressive effects of IL-12 boost are mediated by nitric oxide

We previously demonstrated that codelivery of interleukin-12 (IL-12) with the human immunodeficiency virus type 1 (HIV-1) Env antigen from a recombinant vaccinia virus (rVV) can enhance the specific anti-Env cell-mediated immune (CMI) response. In the present study, we have investigated the effects of IL-12 in mice when it is expressed in a DNA prime/VV boost vaccine regimen. The delivery of IL-12 and Env product during priming with a DNA vector, followed by a booster with VV expressing the Env gene (rVVenv), was found to trigger the optimal CMI response compared with other immunization schedules studied. Significantly, if IL-12 is also delivered as a booster from the viral vector, an impairment of the effects of IL-12 was observed involving nitric oxide (NO), since it was overcome by specific inhibitors of inducible NO synthase. NO caused transient immunosuppression rather than impairment of viral replication. Moreover, at certain viral doses, coadministration of the NO inhibitor during the booster resulted in IL-12-mediated enhancement of the specific CD8(+) T-cell response. In addition, the dose of the IL-12-encoding plasmid (pIL-12) and the route of administration of both vectors were relevant factors for optimal CMI responses. Maximal numbers of Env-specific CD8(+) gamma interferon-secreting cells were obtained when 50 microg of pIL-12 was administered intramuscularly at priming, followed by an intravenous rVVenv boost. Our results demonstrate, in a murine model, critical parameters affecting the success of vaccination schedules based on a combination of DNA and VV vectors in conjunction with immunomodulators.

Insertion sites for recombinant vaccinia virus construction: effects on expression of a foreign protein

The expression of antigens or other molecules from recombinant vaccinia viruses requires the insertion of coding sequence at specific sites in the viral genome. Here we investigate the influence of two different sites on the level of protein expressed during a viral infection. The level of immune response in mice to vaccinia virus-expressed murine interleukin 2 (IL-2) or IL-4 varied depending on whether the coding sequence was inserted into the vaccinia virus thymidine kinase (tk) gene or into the HindIII F fragment of the viral genome where herpes simplex virus (HSV) tk was used as a selectable marker. In each case the intensity of the response was greater when the relevant gene was expressed from the HindIII F insertion site. In order to quantify these differences a series of recombinant viruses expressing luciferase was constructed. Luciferase activity from coding sequence inserted into the HindIII F fragment was significantly higher than that from the tk gene insertion, provided HSV tk(+) constructs were compared. Insertion of a marker gene (HSV tk) into the HindIII F site with disruption of the F7L open reading frame led to a reduced level of luciferase expressed from the tk insert, despite more than 45 kb of intervening sequence. In mice, luciferase expression was higher from the HindIII F inserted gene than from the tk insert in both lungs and ovaries.

A model for vaccinia virus pathogenesis and immunity based on intradermal injection of mouse ear pinnae

Vaccinia virus (VV) proteins that interfere with the host response to infection are of interest because they provide insight into virus-host relationships and may affect the safety and immunogenicity of recombinant VV (rVV) vaccines. Such vaccines need assessment in animal models and with this aim a model of VV infection based on intradermal injection of BALB/c ear pinnae was developed and characterized. In this model, the outcome of infection is affected by the dose of virus inoculated but virus spread is minimal and the mice suffer no signs of systemic illness. Cellular and humoral immune responses to these infections were measured readily and were independent of virus dose over a 100-fold range. Thus the model seems suitable for the analysis of the safety and immunogenicity of VV mutants lacking specific immunomodulatory proteins or bearing foreign antigens.

Induction of immunity to antigens expressed by recombinant adeno-associated virus depends on the route of administration

Recombinant adeno-associated virus (rAAV) is a replication-defective parvovirus which is being explored as a vector for gene therapy because of its broad host range, excellent safety profile, and durable transgene expression in infected hosts. rAAV has also been reported by several groups to induce little or no immune response to its encoded transgene products. In this study we examined the immunogenicity of rAAV by studying the immune response of C57BL/6 mice to a single dose of rAAV-encoding ovalbumin (AAV-Ova) administered by a variety of routes. Mice injected with AAV-Ova intraperitoneally (ip), intravenously, or subcutaneously developed potent ovalbumin-specific cytotoxic T lymphocytes (CTL) as well as anti-ovalbumin antibodies and antibodies to AAV. In contrast, mice injected with AAV-Ova intramuscularly developed a humoral response to the virus and the transgene but minimal ovalbumin-specific CTLs. The induced CTL response after ip administration of AAV-Ova protected mice against a subsequent tumor challenge with an ovalbumin-transfected B16 melanoma cell line. Studies of the mechanism by which AAV-Ova induces CTL confirmed that the virus delivers the transgene product into the classical MHC class I pathway of antigen processing. Mice that previously had been exposed to rAAV vectors failed to develop ovalbumin-specific CTL following administration of AAV-Ova. Analysis of these mice revealed the presence of circulating anti-AAV antibodies that blocked rAAV transduction in vitro and inhibited CTL induction in vivo. These results suggest a possible role for rAAV in the immunotherapy of malignancies and viral infections, although induced antibody responses to AAV may limit its ability to be administered for repeated vaccinations.

A recombinant vaccinia-rabies virus in the immunocompromised host: oral innocuity, progressive parenteral infection, and therapeutics

With the emergence of raccoons (Procyon lotor) as the primary rabies reservoir in the United States of America, a recombinant vaccinia-rabies glycoprotein (V-RG) virus vaccine was developed that protected raccoons by the oral route from rabies infection. Despite extensive laboratory evaluation, vaccine safety concerns remained about free-choice distribution for wildlife rabies control. In this study, the oral innocuity of V-RG virus was demonstrated in immunodeficient mice but parenteral exposure resulted in systemic and progressive infection, albeit significantly abrogated in severity in comparison to vaccinia virus. Treatment with vaccinia immune globulin and hydroxyphosphonylmethoxy-propyl-cytosine resulted in significantly longer survival and minimized V-RG viral gross lesions.

Selective humoral immune response of Balb/C mice to Brucella abortus proteins expressed by vaccinia virus recombinants

Genes encoding Brucella abortus Cu/Zn superoxide dismutase (SOD) and a 54 kDa Escherichia coli HtrA homologue were cloned into shuttle plasmids pUV-1 and pSC11, and transfected into vaccinia virus to develop recombinants vUBSOD and vSB54. Control vaccinia virus recombinants vUV-1 and vSC11, carrying only the beta-gal reporter gene but no B. abortus DNA were also developed. Recombinants were analyzed in Western blotting using a polyclonal B. abortus immune serum. vUBSOD expressed a protein of apparent molecular weight of 28 kDa, composed of the 20 kDa B. abortus Cu/Zn-SOD and a protein approximately 8 kDa encoded by a portion of the vaccinia virus TK gene. vSB54 expressed a 54 kDa protein corresponding to the 54 kDa HtrA homologue. Recombinants vUSV-1 and vSC11 did not express B. abortus proteins. Groups of mice were inoculated intraperitoneally with 10(7) TCID50 of 1 of the 4 different recombinant vaccinia viruses and 5 weeks later their sera were analyzed for antibodies against vaccinia virus and B. abortus proteins. Each group of mice responded with antibodies to vaccinia virus. Sera of vSB54-inoculated mice recognized the 54 kDa HtrA homologue. vUBSOD did not induce a humoral immune response. These results represent the first report on the expression of B. abortus proteins by vaccinia virus recombinants and the first demonstrated immune response against a B. abortus protein expressed by such a recombinant.

Selective induction of immune responses by cytokines coexpressed in recombinant fowlpox virus

Avipoxviruses have recently been studied as potential vectors for the delivery of heterologous vaccine antigen. Because these viruses abortively infect mammalian cells yet still effectively present encoded foreign genes to the host immune system, they offer a safer but effective alternative to other live virus vectors. We have examined the effect of coexpressing the cytokine interleukin-6 or gamma interferon on immune responses to a recombinant fowlpox virus expressing influenza virus hemagglutinin. The encoded cytokine was expressed for prolonged periods in infected cell culture with little cytopathic effect due to the abortive nature of the infection. In mice, vector-expressed cytokine dramatically altered immune responses induced by the coexpressed hemagglutinin antigen. Expression of interleukin-6 augmented both primary systemic and mucosal antibody responses and primed for enhanced recall responses. In contrast, expression of gamma interferon markedly inhibited antibody responses without affecting the generation of cell-mediated immunity. The safety of these constructs was demonstrated in mice with severe combined immunodeficiency, and no side effects due to cytokine expression were observed. In summary, fowlpox virus vectors encoding cytokines represent a safe and effective vaccine strategy which may be used to selectively manipulate the immune response.

CD8+ T-cell memory to viruses

Recent experiments show that laboratory mice infected once with an influenza A virus or with the murine parainfluenza type 1 virus, called the Sendai virus, have enhanced numbers of cytotoxic T-lymphocyte precursors ( > 20x background) for life. Neither virus persists at the genome level, and the mice are maintained under conditions where there is no possibility of re-infection. These observations are highly relevant to any understanding of CD8+ cell memory and suggest that the clonal burst size associated with the primary challenge is a key determining factor.

Recombinant vaccinia immunization in the presence of passively administered antibody

Mice were injected with immune serum to vaccinia and/or influenza virus and then immunized by scarification with a recombinant vaccinia virus expressing the influenza haemagglutinin H1. The serum IgG antibody response to the foreign gene product, influenza H1, was suppressed by the passively administered anti-influenza antibody in a dose-dependent manner. Anti-vaccinia antibody alone had no effect on the anti-haemagglutinin antibody response to the recombinant vaccinia and did not suppress an anti-vaccinia antibody response. Secondary cytotoxic T-lymphocyte killing of influenza virus-infected target cells was relatively low in all animals that were immunized with the recombinant vaccinia, and showed some dose-dependent suppression by the passively administered antibody. The dose dependence of the inhibition suggests that while immunization with recombinant vaccinia viruses may not be effective at birth, they may be useful at several months of age.

Biological effects of recombinant vaccinia virus-expressed interleukin 4

Recombinant vaccinia viruses expressing murine interleukin 4 (IL-4), either alone or together with interleukin 2 (IL-2) or gamma interferon (gamma-IFN), were constructed. Unlike IL-2, IL-4 expressing viruses were not cleared from immunodeficient mice and the mice died. As they died more rapidly than immunodeficient mice inoculated with a control virus, it appeared that IL-4 contributed to their death and the IL-4 mediated toxicity was confirmed in normal immunocompetent mice. The toxicity was reversed by co-expression of either IL-2 or gamma-IFN, probably due to virus clearance and therefore lower levels of circulating IL-4. Vaccinia virus-expressed IL-4 did not increase antibody or natural killer cell levels and caused a slight decrease in cytotoxic T lymphocyte levels.

Vaccinia-rotavirus VP7 recombinants protect mice against rotavirus-induced diarrhoea

Recombinant vaccinia viruses expressing wild type intracellular VP7 (VP7wt) from rotavirus SA11 or VP7sc, a cell surface-anchored variant, boosted antibody titres in SA11-immune mice. Pups born to these mice were protected from diarrhoea following challenge with SA11. In rotavirus-naive mice, two immunizations with recombinant vaccinia virus expressing VP7sc stimulated protective immunity that could be transferred to pups, whereas viruses expressing VP7wt did not stimulate protective immunity. Recombinant vaccinia viruses expressing intracellular or cell surface-anchored VP6, the rotavirus group-reactive antigen from the inner capsid, did not stimulate protective immunity. These experiments demonstrate that a live viral vector expressing cell surface anchored VP7 may represent a strategy for the development of safe, effective vaccines against rotavirus-induced diarrhoea.

Induction of Plasmodium falciparum transmission-blocking antibodies by recombinant vaccinia virus

Many candidate antigens of malaria vaccines have limited immunological recognition. One exception is Pfs25, a cysteine-rich, 25-kilodalton sexual stage surface protein of Plasmodium falciparum. Pfs25 is a target of monoclonal antibodies that block transmission of malaria from vertebrate host to mosquito vector. The surface of mammalian cells infected with a recombinant vaccinia virus that expressed Pfs25 specifically bound transmission-blocking monoclonal antibodies. Furthermore, major histocompatibility complex-disparate congenic mouse strains immunized with recombinant Pfs25 elicited transmission-blocking antibodies, demonstrating that the capacity to develop transmission-blocking antibodies is not genetically restricted in mice. Live recombinant viruses may provide an inexpensive, easily administered alternative to subunit vaccines prepared from purified recombinant proteins to block transmission of malaria in developing countries.

Effects of vaccinia virus-expressed interleukin 2 on the immune system of sublethally irradiated mice

Vaccinia virus that expressed interleukin 2 (IL 2) was cleared from immunodeficient irradiated mice more efficiently than virus that did not express interleukin 2. These results extend the previously observed protection from nude mice to another model of immunodeficiency. No antibody or cytotoxic T lymphocyte response could be detected in sublethally irradiated mice that had been inoculated with IL 2-expressing vaccinia virus, but levels of splenic natural killer cell activity were elevated. Sublethally irradiated mice that had recovered from IL 2-plus hemagglutinin-expressing vaccinia virus were partially protected against both influenza virus and vaccinia virus. These results indicate that vaccinia virus-expressed IL 2 mediates clearance of primary viral infection via a mechanism that does not involve antibody or cytotoxic T lymphocytes. They also indicate that inclusion of lymphokine genes in live recombinant viral vaccine vectors may increase vaccine safety.

Poxvirus pathogenesis

Poxviruses are a highly successful family of pathogens, with variola virus, the causative agent of smallpox, being the most notable member. Poxviruses are unique among animal viruses in several respects. First, owing to the cytoplasmic site of virus replication, the virus encodes many enzymes required either for macromolecular precursor pool regulation or for biosynthetic processes. Second, these viruses have a very complex morphogenesis, which involves the de novo synthesis of virus-specific membranes and inclusion bodies. Third, and perhaps most surprising of all, the genomes of these viruses encode many proteins which interact with host processes at both the cellular and systemic levels. For example, a viral homolog of epidermal growth factor is active in vaccinia virus infections of cultured cells, rabbits, and mice. At least five virus proteins with homology to the serine protease inhibitor family have been identified and one, a 38-kDa protein encoded by cowpox virus, is thought to block a host pathway for generating a chemotactic substance. Finally, a protein which has homology with complement components interferes with the activation of the classical complement pathway. Poxviruses infect their hosts by all possible routes: through the skin by mechanical means (e.g., molluscum contagiosum infections of humans), via the respiratory tract (e.g., variola virus infections of humans), or by the oral route (e.g., ectromelia virus infection of the mouse). Poxvirus infections, in general, are acute, with no strong evidence for latent, persistent, or chronic infections. They can be localized or systemic. Ectromelia virus infection of the laboratory mouse can be systemic but inapparent with no mortality and little morbidity, or highly lethal with death in 10 days. On the other hand, molluscum contagiosum virus replicates only in the stratum spinosum of the human epidermis, with little or no involvement of the dermis, and does not spread systemically from the site of infection. The host response to infection is progressive and multifactorial. Early in the infection process, interferons, the alternative pathway of complement activation, inflammatory cells, and natural killer cells may contribute to slowing the spread of the infection. The cell-mediated response involving learned cytotoxic T lymphocytes and delayed-type hypersensitivity components appears to be the most important in recovery from infection. A significant role for specific antiviral antibody and antibody-dependent cell-mediated cytotoxicity has yet to be demonstrated in recovery from a primary infection, but these responses are thought to be important in preventing reinfection.

Expression of bovine leukaemia virus envelope gene by recombinant vaccinia viruses

Recombinant vaccinia viruses (VV) containing the envelope gene of bovine leukaemia virus (BLV) were constructed. Three virus constructs were designed: VV-BLV1 which contained the open reading frame for envelope glycoprotein gp51 alone, under control of VVP7.5 promoter; VV-BLV2 and VV-BLV3 contained the entire gene (gp51 + gp30) coding sequence downstream of VP7.5 and the fowlpox virus early/late promoter (PFE/L) respectively. All three VV recombinants expressed envelope glycoproteins as determined by the agar gel diffusion assay. By immunofluorescence techniques it was shown that while VV-BLV2 and VV-BLV3 expressed envelope glycoprotein on the surface of virus-infected cells, VV-BLV1 failed to do so. Rabbits inoculated with VV-BLV1 failed to show an anti envelope glycoprotein antibody response, however, significant levels of antibodies against envelope glycoprotein were detected in sera from rabbits inoculated with VV-BLV2 and VV-BLV3.

Vaccinia virus-mediated damage of murine ovaries and protection by virus-expressed interleukin-2

Vaccinia virus was shown to replicate in the ovaries of normal inbred mice and cause sufficient damage to ovarian follicles to decrease fertility of the mice. The mouse-adapted strain, VV-WR, had a greater affinity for growth in ovaries than the vaccine strains, VV-Elstree, VV-NYBH or VV-Copenhagen. Virus reached the ovaries after intravenous or intraperitoneal inoculation, but not after subcutaneous inoculation in the foot pad. Interleukin-2 (IL-2), when expressed by a recombinant vaccinia virus was able to decrease the titre of virus in the ovaries and prevent infertility. Both non-specific (natural killer cells) and antigen-specific mechanisms were active within the ovaries and may play a role in the IL-2-mediated clearance of vaccinia virus.

Protective efficacy of a recombinant vaccinia virus in vaccinia-immune mice

Recombinant viral vectors offer a potential means of vaccinating against diseases for which there are no current safe vaccines. One of the criteria on which a viral vaccine vector would be selected is that it either circulates in the human or livestock population without producing overt disease (e.g. adenovirus) or has a history as a safe vaccine (e.g. vaccinia virus). However, this selection criterion also means that the target population is likely to have circulating antibodies that are specific to the vaccine vector. Since a percentage of the world's population has been vaccinated during the World Health Organization's Smallpox Eradication Campaign, such antibody titres, which are likely to lower vaccine efficacy, have been raised as an objection to the use of recombinant vaccinia viruses as vaccines. We have tested the effect of vaccinia-specific immunity on the protective efficacy of a recombinant virus, VV-PR8-HA6 (1) which expresses the haemagglutinin of the influenza virus A/PR/8/34.