High-level eucaryotic in vivo expression of biologically active measles virus hemagglutinin by using an adenovirus type 5 helper-free vector system

The potential of adenoviral vaccine vectors with altered antigen presentation capabilities

Introduction: Despite their appeal as vaccine vectors, adenoviral vectors are yet unable to induce protective immune responses against some weakly immunogenic antigens. Additionally, the maximum doses of adenovirus-based vaccines are limited by vector-induced toxicity, causing vector elimination and diminished immune responses against the target antigen. In order to increase immune responses to the transgene, while maintaining a moderate vector dose, new technologies for improved transgene presentation have been developed for adenoviral vaccine vectors.Areas covered: This review provides an overview of different genetic-fusion adjuvants that aim to improve antigen presentation in the context of adenoviral vector-based vaccines. The influence on both T cell and B cell responses are discussed, with a main focus on two technologies: MHC class II-associated invariant chain and virus-like-vaccines.Expert opinion: Different strategies have been tested to improve adenovirus-based vaccinations with varying degrees of success. The reviewed genetic adjuvants were designed to increase antigen processing and MHC presentation, or promote humoral immune responses with an improved conformational antigen display. While none of the introduced technologies is universally applicable, this review shall give an overview to identify potential improvements for future vaccination approaches.

Future Prospects for the Development of Cost-Effective Adenovirus Vaccines

Vaccination is one of the most efficient tools for disease prevention, and a continuously growing field of research. However, despite progress, we still need more efficient and cost-effective vaccines that would improve access to those in need. In this review, we will describe the status of virus-vectored vaccine technology with a focus on adenoviral-based vaccines. Adenovirus (Ad) vaccines have proven to be efficient in military vaccinations against Ad4 and Ad7 and as highly efficient vectored vaccines against rabies. The question of how other adenovirus-based vaccines can become as efficient as the rabies vaccine is the underlying theme in this review. Here, we will first give an overview of the basic properties of vectored vaccines, followed by an introduction to the characteristics of adenoviral vectors and previously tested modifications of the vector backbone and expression cassettes, with a focus on how they can contribute to increased vaccine cost-effectiveness. Finally, we will highlight a few successful examples of research that have attempted to improve the use of adenoviral-based vaccines by improving the transgene immunogenicity.

Advances and future challenges in recombinant adenoviral vectored H5N1 influenza vaccines

The emergence of a highly pathogenic avian influenza virus H5N1 has increased the potential for a new pandemic to occur. This event highlights the necessity for developing a new generation of influenza vaccines to counteract influenza disease. These vaccines must be manufactured for mass immunization of humans in a timely manner. Poultry should be included in this policy, since persistent infected flocks are the major source of avian influenza for human infections. Recombinant adenoviral vectored H5N1 vaccines are an attractive alternative to the currently licensed influenza vaccines. This class of vaccines induces a broadly protective immunity against antigenically distinct H5N1, can be manufactured rapidly, and may allow mass immunization of human and poultry. Recombinant adenoviral vectors derived from both human and non-human adenoviruses are currently being investigated and appear promising both in nonclinical and clinical studies. This review will highlight the current status of various adenoviral vectored H5N1 vaccines and will outline novel approaches for the future.

Sublingual immunization with recombinant adenovirus encoding SARS-CoV spike protein induces systemic and mucosal immunity without redirection of the virus to the brain

Background

Sublingual (s.l.) administration of soluble protein antigens, inactivated viruses, or virus-like particles has been shown to induce broad immune responses in mucosal and extra-mucosal tissues. Recombinant replication-defective adenovirus vectors (rADVs) infect mucosa surface and therefore can serve as a mucosal antigen delivery vehicle. In this study we examined whether s.l. immunization with rADV encoding spike protein (S) (rADV-S) of severe acute respiratory syndrome-associated coronavirus (SARS-CoV) induces protective immunity against SARS-CoV and could serve as a safe mucosal route for delivery of rADV.

Results

Here, we show that s.l. administration of rADV-S induced serum SARS-CoV neutralizing and airway IgA antibodies in mice. These antibody responses are comparable to those induced by intranasal (i.n.) administration. In addition, s.l. immunization induced antigen-specific CD8⁺ T cell responses in the lungs that are superior to those induced by intramuscular immunization. Importantly, unlike i.n. administration, s.l. immunization with rADV did not redirect the rADV vector to the olfactory bulb.

Conclusion

Our study indicates that s.l. immunization with rADV-S is safe and effective in induction of a broad spectrum of immune responses and presumably protection against infection with SARS-CoV.

The recombinant globular head domain of the measles virus hemagglutinin protein as a subunit vaccine against measles

Despite the availability of live attenuated measles virus (MV) vaccines, a large number of measles-associated deaths occur among infants in developing countries. The development of a measles subunit vaccine may circumvent the limitations associated with the current live attenuated vaccines and eventually contribute to global measles eradication. Therefore, the goal of this study was to test the feasibility of producing the recombinant globular head domain of the MV hemagglutinin (H) protein by stably transfected human cells and to examine the ability of this recombinant protein to elicit MV-specific immune responses. The recombinant protein was purified from the culture supernatant of stably transfected HEK293T cells secreting a tagged version of the protein. Two subcutaneous immunizations with the purified recombinant protein alone resulted in the production of MV-specific serum IgG and neutralizing antibodies in mice. Formulation of the protein with adjuvants (polyphosphazene or alum) further enhanced the humoral immune response and in addition resulted in the induction of cell-mediated immunity as measured by the production of MV H-specific interferon gamma (IFN-γ) and interleukin 5 (IL-5) by in vitro re-stimulated splenocytes. Furthermore, the inclusion of polyphosphazene into the vaccine formulation induced a mixed Th1/Th2-type immune response. In addition, the purified recombinant protein retained its immunogenicity even after storage at 37°C for 2 weeks.

Adenoviruses as vaccine vectors

Adenoviruses have transitioned from tools for gene replacement therapy to bona fide vaccine delivery vehicles. They are attractive vaccine vectors as they induce both innate and adaptive immune responses in mammalian hosts. Currently, adenovirus vectors are being tested as subunit vaccine systems for numerous infectious agents ranging from malaria to HIV-1. Additionally, they are being explored as vaccines against a multitude of tumor-associated antigens. In this review we describe the molecular biology of adenoviruses as well as ways the adenovirus vectors can be manipulated to enhance their efficacy as vaccine carriers. We describe methods of evaluating immune responses to transgene products expressed by adenoviral vectors and discuss data on adenoviral vaccines to a selected number of pathogens. Last, we comment on the limitations of using human adenoviral vectors and provide alternatives to circumvent these problems. This field is growing at an exciting and rapid pace, thus we have limited our scope to the use of adenoviral vectors as vaccines against viral pathogens.

Role for CCR5Delta32 protein in resistance to R5, R5X4, and X4 human immunodeficiency virus type 1 in primary CD4+ cells

CCR5Delta32 is a loss-of-function mutation that abolishes cell surface expression of the human immunodeficiency virus (HIV) coreceptor CCR5 and provides genetic resistance to HIV infection and disease progression. Since CXCR4 and other HIV coreceptors also exist, we hypothesized that CCR5Delta32-mediated resistance may be due not only to the loss of CCR5 function but also to a gain-of-function mechanism, specifically the active inhibition of alternative coreceptors by the mutant CCR5Delta32 protein. Here we demonstrate that efficient expression of the CCR5Delta32 protein in primary CD4(+) cells by use of a recombinant adenovirus (Ad5/Delta32) was able to down-regulate surface expression of both wild-type CCR5 and CXCR4 and to confer broad resistance to R5, R5X4, and X4 HIV type 1 (HIV-1). This may be important clinically, since we found that CD4(+) cells purified from peripheral blood mononuclear cells of individuals who were homozygous for CCR5Delta32, which expressed the mutant protein endogenously, consistently expressed lower levels of CXCR4 and showed less susceptibility to X4 HIV-1 isolates than cells from individuals lacking the mutation. Moreover, CD4(+) cells from individuals who were homozygous for CCR5Delta32 expressed the mutant protein in five of five HIV-exposed, uninfected donors tested but not in either of two HIV-infected donors tested. The mechanism of inhibition may involve direct scavenging, since we were able to observe a direct interaction of CCR5 and CXCR4 with CCR5Delta32, both by genetic criteria using the yeast two-hybrid system and by biochemical criteria using the coimmunoprecipitation of heterodimers. Thus, these results suggest that at least two distinct mechanisms may account for genetic resistance to HIV conferred by CCR5Delta32: the loss of wild-type CCR5 surface expression and the generation of CCR5Delta32 protein, which functions as a scavenger of both CCR5 and CXCR4.

Experimental vaccines against measles in a world of changing epidemiology

Vaccination with the current live attenuated measles vaccine is one of the most successful and cost-effective medical interventions. However, as a result of persisting maternal antibodies and immaturity of the infant immune system, this vaccine is poorly immunogenic in children <9 months old. Immunity against the live vaccine is less robust than natural immunity and protection less durable. There may also be some concern about (vaccine) virus spread during the final stage of an eventual measles eradication program. Opinions may differ with respect to the potential threat that some of these concerns may be to the World Health Organisation goal of measles elimination, but there is a consensus that the development of new measles vaccines cannot wait. Candidate vaccines are based on viral or bacterial vectors expressing recombinant viral proteins, naked DNA, immune stimulating complexes or synthetic peptides mimicking neutralising epitopes. While some of these candidate vaccines have proven their efficacy in monkey studies, aerosol formulated live attenuated measles vaccine are evaluated in clinical trials.

Immunosorbent assay based on recombinant hemagglutinin protein produced in a high-efficiency mammalian expression system for surveillance of measles immunity

Recombinant hemagglutinin (H) protein of the measles virus (MV) was produced in mammalian cells with a high-yield expression system based on the Semliki Forest virus replicon. Crude membrane preparations of H protein-transfected BHK-21 cells were used to coat microtiter plates to measure specific immunoglobulin G antibodies in 228 serologically defined serum samples mainly from measles late-convalescent adults. The titers by the enzyme-linked immunosorbent assay for the H protein (H-ELISA) closely correlated with neutralization test (NT) titers (R2 = 0.66), hemagglutination inhibition test (HI) titers (R2 = 0.64), with the titers from a certified commercial ELISA based on whole MV-infected cells (MV-ELISA; R2 = 0.45). The correlations described above were better than those of the commercial MV-ELISA titers with the NT (R2 = 0.52) or HI (R2 = 0.48) titers. By using the 2nd International Standard for anti-measles serum, the detection level of the assay corresponds to 215 mIU/ml for undiluted serum, which corresponds to the estimated threshold for protective immunity. The specificity, accuracy, and positive predictive value were, in general, better for the H-ELISA than for a commercial MV-ELISA, independent of whether HI, NT, or HI and NT were used as "gold standards." In contrast, the H-ELISA proved to be slightly less sensitive than the MV-ELISA (sensitivities, 98.6 versus 99.5%, respectively; P was not significant). The assays did not differ significantly in the number of serum samples with positive HI and NT results (n = 212) which measured false negative (H-ELISA, 2 of 212 [0.94%]; MV-ELISA, 1 of 212 [0.47%]), but the H-ELISA detected significantly more measles-susceptible individuals than the MV-ELISA (10 of 11 versus 3 of 11, respectively; P < 0.05) among the individuals whose sera had negative HI and NT results. Our data demonstrate that the H-protein preparation that we describe could be a cost-effective alternative to current whole-virus-based ELISAs for surveillance for immunity to measles and that such an assay could be more efficient in detecting susceptibility to measles. Furthermore, unlike whole MV-based antigens, H-protein would also be suitable for use in the development of a simple field test for the diagnosis of measles.

Recombinant viral vector vaccines for the veterinary use

Recently, genetically engineering using recombinant DNA techniques has been applied to design new viral vaccines in order to reduce some problems which present viral vaccines have. Up to now, many viruses have been investigated for development of recombinant attenuated vaccines or live viral vectors for delivery of foreign immunogenic antigens. In this review, we introduced three kind of viruses; herpesviruses, vaccinia viruses, and adenoviruses, which have best widely been studied as recombinant vaccines or delivery vaccines for the veterinary use.

Costimulatory protein B7-1 enhances the cytotoxic T cell response and antibody response to hepatitis B surface antigen

There is a need for more effective therapy for chronic virus infections. A principle natural mechanism for elimination of virus-infected host cells is activation of viral antigen-specific cytotoxic T lymphocytes (CTL). In an effort to develop methods of inducing virus-specific CTL responses that might be utilized in therapy of virus infections, we have investigated the effect of B7, a costimulatory factor for T-cell activation. In this study we show that delivery of genes encoding human B7-1 and a viral antigen in the same recombinant viral vector to cells of mice induces a greater viral antigen-specific CTL response than does similar delivery of the viral antigen gene alone. Two recombinant adenovirus vectors were constructed with the foreign genes inserted in the early region 3. One of them (Ad1312) directed expression of the surface antigen gene of hepatitis B virus (HBS); the other (Ad1310) directed coexpression of HBS and human B7-1 (CD80) by means of an internal ribosomal entry site placed between the two coding sequences. When inoculated into BALB/c mice, both vectors induced a viral surface antigen-specific CTL response. The response induced by Ad1310 was stronger than that by Adl312 as measured by a chromium release assay for CTL activity and limiting dilution analysis for CTL precursor frequency, indicating that the B7-1 gene co-delivered with the HBS gene had an enhancing effect on the CTL response against surface antigen. Ad1310 also induced a higher titer of antibody against surface antigen than did Ad1312. This result suggests that expression of a costimulatory protein and a viral antigen in the same cells in vivo induces stronger immune responses than expression of the antigen alone. This could be a novel strategy for development of both preventive and therapeutic vaccines against infectious agents.

Adenovirus vectors as recombinant viral vaccines

Adenoviruses can efficiently induce immunity in the lung following single enteric delivery. These viruses can also be engineered to express a number of heterologous proteins in vitro. In the past 10 years, recombinant adenoviruses expressing a variety of antigens have been constructed and tested. This article reviews the main properties of adenoviruses which render them attractive for vaccine development, as well as the results of the immunization studies performed to date. Some disadvantages of this technology and the desired characteristics of second generation adenoviral vectors are discussed.

Improved adenovirus vector provides herpes simplex virus ribonucleotide reductase R1 and R2 subunits very efficiently

We have constructed a new adenovirus (Ad) expression vector, pAdBM5, that allows for the production of unprecedented levels of recombinant protein in the human 293 cell line using the Ad expression system. The main feature of this vector is a combination of enhancer sequences that increases the activity of the ectopic major late promoter (MLP) in recombinant Ad. In 293 cells infected with helper-free Ad recombinants generated with the pAdBM5 transfer vector, both herpes simplex virus (HSV) ribonucleotide reductase R1 and R2 subunits represent the most abundant polypeptides, accounting for as much as 15-20% of total cellular proteins. Our data suggest that this level of expression is probably very close to the upper limit of the system. Furthermore, when compared to the widely utilized baculovirus (Bac)/Sf9 expression system, the improved Ad vector showed a better performance for the production and purification of active HSV-2 ribonucleotide reductase R1 and R2 subunits. The R2 subunit was about 5-fold more abundant in recombinant Ad-infected 293 cells than in Bac-infected Sf9 cells while the R1 subunit was produced at roughly similar levels with either system. However, the amount of active soluble R1 obtained from recombinant Ad-infected 293 cells was at least 5 times higher because most of the R1 produced in Sf9 cells was insoluble.

Functional and structural interactions between measles virus hemagglutinin and CD46

We analyzed the roles of the individual measles virus (MV) surface glycoproteins in mediating functional and structural interactions with human CD46, the primary MV receptor. On one cell population, recombinant vaccinia virus vectors were used to produce the MV hemagglutinin (H) and fusion (F) glycoproteins. As fusion partner cells, various cell types were examined, without or with human CD46 (endogenous or recombinant vaccinia virus encoded). Fusion between the two cell populations was monitored by a quantitative reporter gene activation assay and by syncytium formation. MV glycoproteins promoted fusion with primate cells but not with nonprimate cells; recombinant CD46 rendered nonprimate cells competent for MV glycoprotein-mediated fusion. Markedly different fusion specificity was observed for another morbillivirus, canine distemper virus (CDV): recombinant CDV glycoproteins promoted fusion with primate and nonprimate cells independently of CD46. Fusion by the recombinant MV and CDV glycoproteins required coexpression of H plus F in either homologous or heterologous combinations. To assess the role of H versus F in determining the CD46 dependence of MV fusion, we examined the fusion specificities of cells producing heterologous glycoprotein combinations. The specificity of HMV plus FCDV paralleled that observed for the homologous MV glycoproteins: fusion occurred with primate cells but not with nonprimate cells unless they produced recombinant CD46. By contrast, the specificity of HCDV plus FMV paralleled that for the homologous CDV glycoproteins: fusion occurred with either primate or nonprimate cells with no dependence on CD46. Thus, for both MV and CDV, fusion specificity was determined by H. In particular, the results demonstrate a functional interaction between HMV and CD46. Flow cytometry and antibody coprecipitation studies provided a structural correlate to this functional interaction: CD46 formed a molecular complex with HMV but not with FMV or with either CDV glycoprotein. These results highlight the critical role of the H glycoprotein in determining MV specificity for CD46-positive cells.

Human parainfluenza virus type 3: analysis of the cytoplasmic tail and transmembrane anchor of the hemagglutinin-neuraminidase protein in promoting cell fusion

The role of the cytoplasmic tail and transmembrane anchor of the human parainfluenza virus type 3 (HPIV3) hemagglutinin-neuraminidase (HN) glycoprotein in promoting cell fusion was investigated. A series of amino terminal deletion mutants (d10, d20, d27, d31, d40, d44, and d73) were compared for processing, cell surface expression, and maintenance of their biological attributes by recombinant expression of mutant genes using a plasmid vector (pcDL-SR alpha-296) in CV-1 and HeLa cells. To determine the fusion promoting activity (FPA) of the various mutant proteins, a simple assay was developed which quantified the fusion of two different HeLa cell types. One of the cell types, HeLa-tat, constitutively expressed the human immunodeficiency virus type I (HIV-1) tat protein from a Moloney murine leukemia virus long terminal repeat (LTR), while the second cell type, HeLa beta-gal, contained a reporter gene, beta-galactosidase, under the control of an HIV1-LTR. Fusion of mixed HeLa cell monolayers (50:50, HeLa-tat: HeLa beta-gal), following transfection with appropriate plasmids, resulted in transactivation of the reporter gene which was then measured by direct staining of cells or using cell lysates with appropriate substrates. Cell fusion was observed only when both the HPIV3 F and functional HN proteins were both co-transfected into cells. Of the seven deletion mutants examined, only d10, d20, d27 and d31 were expressed to significant levels on the cell surface and only these four mutant proteins maintained FPA. Compared with the wt HN at 48 h post transfection, d10 and d20 had enhanced FPA (119% and 158%, respectively), while d27 and d31 were diminished (74% and > 4%, respectively). Analysis of protein expression suggested that the reason for the increase in FPA of the mutant proteins was that the levels of protein expressed at the cell surface was twofold or threefold higher for d10 and d20, respectively, compared to the wt HN.

Characterization of a cleavage mutant of the measles virus fusion protein defective in syncytium formation

Membrane fusion caused by measles virus (MV) is a function of the fusion (F) protein. This process is essential for penetration into the host cell and subsequent initiation of the virus replicative cycle. The biological activity of the MV F protein is generated by endoproteolytic cleavage of a precursor protein (F0) into a large F1 subunit and a smaller F2 subunit held together by disulfide bonds. The cleavage site consists of a cluster of five basic amino acids (amino acids 108 to 112) within the predicted primary structure of the F protein. To investigate the role of the arginine residue at the carboxy terminus of the F2 subunit (arginine 112), site-directed mutagenesis was used to construct a cleavage mutant of the MV F protein in which this arginine residue was changed to a leucine residue. The mutated F gene, encoding four out of the five basic amino acids at the cleavage site, was inserted into the genome of vaccinia virus. The resulting recombinant virus was used to study expression of the mutant F protein in infected cells. Analysis of the Leu-112 mutant protein made in infected cells demonstrated that this single-amino-acid substitution resulted in a reduced rate of transport of the mutant protein to the cell surface, despite its efficient cleavage to yield F1 and F2 subunits. However, the electrophoretic mobilities of the Leu-112 polypeptides suggested that the protein was cleaved incorrectly. This aberrant cleavage appears to have abolished the ability of the F protein to cause syncytium formation. The data indicate that the arginine 112 residue is critical for the correct proteolytic cleavage that is required for the membrane fusion activity of the MV F protein.

CD46, a primate-specific receptor for measles virus

Measles virus normally infects only primate cells. The receptor for measles virus has recently been shown to be the complement regulator CD46, also known as membrane cofactor protein. Transfection of rodent cells with human CD46 renders them susceptible to the virus, suggesting that transgenic animals may prove useful for testing antiviral agents and vaccines.

Functional analysis of N-linked glycosylation mutants of the measles virus fusion protein synthesized by recombinant vaccinia virus vectors

The role of N-linked glycosylation in the biological activity of the measles virus (MV) fusion (F) protein was analyzed by expressing glycosylation mutants with recombinant vaccinia virus vectors. There are three potential N-linked glycosylation sites located on the F2 subunit polypeptide of MV F, at asparagine residues 29, 61, and 67. Each of the three potential glycosylation sites was mutated separately as well as in combination with the other sites. Expression of mutant proteins in mammalian cells showed that all three sites are used for the addition of N-linked oligosaccharides. Cell surface expression of mutant proteins was reduced by 50% relative to the wild-type level when glycosylation at either Asn-29 or Asn-61 was abolished. Despite the similar levels of cell surface expression, the Asn-29 and Asn-61 mutant proteins had different biological activities. While the Asn-61 mutant was capable of inducing syncytium formation, the Asn-29 mutant protein did not exhibit any significant cell fusion activity. Inactivation of the Asn-67 glycosylation site also reduced cell surface transport of mutant protein but had little effect on its ability to cause cell fusion. However, when the Asn-67 mutation was combined with mutations at either of the other two sites, cleavage-dependent activation, cell surface expression, and cell fusion activity were completely abolished. Our data show that the loss of N-linked oligosaccharides markedly impaired the proteolytic cleavage, stability, and biological activity of the MV F protein. The oligosaccharide side chains in MV F are thus essential for optimum conformation of the extracellular F2 subunit that is presumed to bind cellular membranes.

Paramyxovirus mediated cell fusion requires co-expression of both the fusion and hemagglutinin-neuraminidase glycoproteins

Syncytia formation in either CV-1 or HeLa T4+ cells required recombinant expression of both fusion (F) and hemagglutinin-neuraminidase (HN) glycoproteins from the human parainfluenza virus type 3 (HPIV3), human parainfluenza virus type 2 (HPIV2), and simian virus 5 (SV5). In this system, recombinant T7 transcription vectors (pT7-5 or pGEM) containing F or HN, were transfected individually or in combination into cells previously infected with a recombinant vaccinia virus expressing T7 RNA polymerase (vTF7-3). While both proteins were processed and expressed at the cell surface, syncytia formation occurred only when both glycoproteins were co-expressed. The function of HN in the fusion process could not be replaced using lectins or by co-expression of heterologous F and HN proteins. Further, cell fusion was not observed when experiments were performed using individually expressed F and HN proteins in adjacent cells. The data presented in this report support the notion that a specific interaction between both paramyxoviral glycoproteins is required for the formation of syncytia in tissue culture monolayers.

Establishment and characterisation of murine cells constitutively expressing the fusion, nucleoprotein and matrix proteins of measles virus

To advance our understanding of the immunobiology of measles virus (MV) infections, we have investigated the possibility of establishing cell lines constitutively expressing the individual MV antigens. In contrast to previously published studies, we show that it is possible to establish cell lines expressing high levels of fusion (F), nucleoprotein (NP) and matrix (M) MV proteins. Once cloned, the cell lines were stable with high levels of expression for more than six months. The size and cell distribution of the NP and F proteins were similar to those observed in MV- or vaccinia-MV recombinant-infected cells. In contrast, the distribution of the M protein, although being similar to that of MV-infected cells, differed from that of Vaccinia-M recombinant virus-infected cells. Preliminary results suggest that these cell lines will be useful tools for studying the contribution of individual MV antigens to the cell-mediated immune response to this virus.

Generation of mammalian cells expressing stably measles virus proteins via bicistronic RNA

The proteins of measles virus are believed to be cytotoxic, and have never been expressed stably from the cloned genes in cultured cells. We found that measles viral proteins can be expressed via a bicistronic RNA. The dominantly selectable DHFR* protein-coding region encoding a mutant dihydrofolate reductase was inserted into the 3'-untranslated regions of the measles viral genes encoding nucleoprotein (N), matrix (M) protein, and hemagglutinin (H). The tandemly arranged cistrons were placed under control by the inducible promoter of human metallothionein IIA gene, or the noninducible early promoter of simian virus 40. Upon transfecting into mammalian cells, these gene constructs synthesized bicistronic RNAs. The downstream DHFR* gene conferred resistance to methotrexate (MTX). Cells that survived MTX selection expressed stably the N, M, or H protein of measles virus. Expression of N protein was further inducible by cadmium chloride treatment. This system will be useful for studying the protein functions of measles virus, and could be applied to express other potentially toxic gene products.

Evaluation of the transfer and expression in mice of an enzyme-encoding gene using a human adenovirus vector

Mutant mice of the Spf-ash strain have an inherited defect in ornithine transcarbamylase (OTC) protein synthesis, and were used to ascertain the potential of recombinant adenoviruses for introducing and expressing the normal gene lacking in these mice. These OTC mutant mice are characterized by a reduction in the amount of OTC activity, resulting in hyperammonemia, pronounced orotic aciduria, growth retardation, and sparse fur until weaning. A recombinant adenovirus that harbors the rat OTC cDNA under the control of the viral major late promoter (MLP) was constructed and injected into such newborn mice. The effect of the virus was analyzed by monitoring the hepatic OTC enzyme during several months after the injection. An increase in OTC activity was detected and was accompanied by a diminution of orotic acid in the urine. The observation of MLP-OTC mRNA transcripts over 1 year following the injection attests to the relatively long-term presence of the transferred gene. In those mice showing the greatest OTC activity, a normalization of the fur was also observed. The experiments reported here document the feasibility of using adenovirus for the direct delivery in vivo of a gene to restore an impaired metabolism.

Intracellular processing, glycosylation, and cell-surface expression of the measles virus fusion protein (F) encoded by a recombinant adenovirus

The membrane fusion protein of measles virus (MVF) is a surface glycoprotein which is essential for initiation of viral infection. The F protein mediates penetration of the host cell through a process of membrane fusion between the viral envelope and the host cell plasma membrane. To study the structure-function relationship of the MVF protein, a recombinant adenovirus, Ad5MVF, was constructed which expressed the F protein in mammalian cells. The MVF gene was inserted into the Ad5 genome by homologous recombination, which resulted in replacement of most of the E1 region. This recombinant virus was stable and replicated efficiently in the 293 cell line which complemented the deleted E1 functions. Human 293 cells infected with Ad5MVF synthesized an authentic MVF protein precursor (F0) which appeared to be cleaved efficiently to the F1 and F2 polypeptides. This recombinant F protein was glycosylated, transported to the cell surface, and found to be capable of inducing syncytia formation and hemolysis of monkey erythrocytes. The hemagglutinin protein (HA), provided by a coinfecting adenovirus, was not able to increase the biological activity of the F protein. Treatment of MV or Ad5MVF-infected cells with tunicamycin, an inhibitor of N-linked glycosylation, abolished processing of the F protein. This observation suggests that glycosylation might play an important role in cleavage-dependent activation of the precursor F0 protein or in its transport to the subcellular region where proteolytic cleavage occurs.

Synthesis of the membrane fusion and hemagglutinin proteins of measles virus, using a novel baculovirus vector containing the beta-galactosidase gene

An improved baculovirus expression vector was developed to expedite screening and facilitate oligonucleotide-directed mutagenesis. This vector contained twin promoters derived from the P10 and polyhedrin genes of Autographica californica nuclear polyhedrosis virus. The P10 promoter directed the synthesis of beta-galactosidase, whereas the polyhedrin promoter controlled the synthesis of foreign gene products. These two genes recombined with wild-type virus genome to yield recombinants which were polyhedrin negative, produced the foreign gene product, and formed blue plaques when beta-galactosidase indicator was present in the agarose overlay. An origin of replication derived from M13 or f1 bacteriophage was also included in the plasmid to permit the synthesis of single-stranded DNA. This template DNA was used to introduce or delete sequences through the process of site-specific mutagenesis. The measles virus virion possesses a membrane envelope which contains two glycoproteins: the hemagglutinin (H) and membrane fusion (F) proteins. The H polypeptide has receptor-binding and hemagglutinating activity, whereas the F protein mediates virus penetration of the host cell, formation of syncytia, and hemolysis of erythrocytes. Genes for these two glycoproteins were inserted into the NheI cloning site of the modified expression vector described above. The vector and purified wild-type viral DNA were introduced into Sf9 insect cells by calcium phosphate precipitation. A mixture of wild-type and recombinant virus was generated and used to infect Sf9 cells, which were subsequently overlaid with agarose. After 3 days, 0.1 to 1% of the plaques became blue in the presence of beta-galactosidase indicator. At least 70% of these blue viral colonies contained the foreign gene of interest as determined by dot blot analysis. Recombinant virus was separated from contaminating wild-type virus through several rounds of plaque purification. Insect cells were then infected with the purified recombinants, and synthesis of H and F proteins were verified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by immunoblot detection and Coomassie blue staining. Glycosylation of the proteins appeared to be impaired somewhat, and the precursor to the F protein was not completely cleaved by the proteases present in insect host cells. On the other hand, both proteins appeared to be active in hemagglutination, hemolysis, and cell fusion assays. Levels of synthesis were in the order of 50 to 150 mg of protein per 10(8) cells.

Expression of bicistronic measles virus P/C mRNA by using hybrid adenoviruses: levels of C protein synthesized in vivo are unaffected by the presence or absence of the upstream P initiator codon

The measles virus (MV) P/C mRNA is functionally bicistronic. Translation is presumed to initiate at both the first and second 5'-proximal AUG codons, leading, respectively, to synthesis of the P and C polypeptides from different overlapping reading frames. To study the function and differential expression of these polypeptides, we have constructed hybrid human adenoviruses capable of expressing high levels of P and C together or of C alone. Cloned cDNA corresponding to the MV P/C gene was coupled to the adenovirus type 2 (Ad2) major late promoter, most of the Ad2 tripartite leader sequence, and the simian virus 40 3'-end processing signal and then used to replace most of the E1a-E1b region of the Ad5 genome in two hybrid adenoviruses: one (Ad5MV/PC13) which contained both 5'-proximal AUG codons of the P/C mRNA and another (Ad5MV/C3) which retained only the second. The sequence context for the P protein initiator AUG codon in Ad5MV/PC13 was made more favorable (GAGAUGG) than the relatively unfavorable context (CCGAUGG) seen in the native MV P/C mRNA. After infection of 293 cells (which provide complementary E1a-E1b functions), both viruses directed equal amounts of P/C-specific mRNA transcription. Ad5MV/PC13 directed the synthesis of both P and C proteins, while Ad5MV/C3 directed the synthesis of C protein alone. Ad5-expressed P protein was phosphorylated, while C was not. C protein had a similar diffuse cytoplasmic localization in both MV and Ad5-infected cells. Ad5MV/C3 and Ad5MV/PC13 directed equal amounts of C protein expression in 293 cells at a level approximately 15 times greater than that seen in MV-infected cells. Thus the level of C protein expression was unaffected by the presence or absence of an out-of-frame upstream AUG codon in a favorable sequence context. This observation cannot be explained by the scanning model for ribosomal initiation and suggests that ribosomes may be binding directly at an internal mRNA site at or near the initiator AUG codon for the C protein.