Feline leukemia virus envelope protein expression encoded by a recombinant vaccinia virus: apparent lack of immunogenicity in vaccinated animals

Production and Immunogenicity of FeLV Gag-Based VLPs Exposing a Stabilized FeLV Envelope Glycoprotein

The envelope glycoprotein (Env) of retroviruses, such as the Feline leukemia virus (FeLV), is the main target of neutralizing humoral response, and therefore, a promising vaccine candidate, despite its reported poor immunogenicity. The incorporation of mutations that stabilize analogous proteins from other viruses in their prefusion conformation (e.g., HIV Env, SARS-CoV-2 S, or RSV F glycoproteins) has improved their capability to induce neutralizing protective immune responses. Therefore, we have stabilized the FeLV Env protein following a strategy based on the incorporation of a disulfide bond and an Ile/Pro mutation (SOSIP) previously used to generate soluble HIV Env trimers. We have characterized this SOSIP-FeLV Env in its soluble form and as a transmembrane protein present at high density on the surface of FeLV Gag-based VLPs. Furthermore, we have tested its immunogenicity in DNA-immunization assays in C57BL/6 mice. Low anti-FeLV Env responses were detected in SOSIP-FeLV soluble protein-immunized animals; however, unexpectedly no responses were detected in the animals immunized with SOSIP-FeLV Gag-based VLPs. In contrast, high humoral response against FeLV Gag was observed in the animals immunized with control Gag VLPs lacking SOSIP-FeLV Env, while this response was significantly impaired when the VLPs incorporated SOSIP-FeLV Env. Our data suggest that FeLV Env can be stabilized as a soluble protein and can be expressed in high-density VLPs. However, when formulated as a DNA vaccine, SOSIP-FeLV Env remains poorly immunogenic, a limitation that must be overcome to develop an effective FeLV vaccine.

Review of companion animal viral diseases and immunoprophylaxis

In this article we review important established, newly emergent and potential viral diseases of cats, dogs and rabbits. Topics covered include virus epidemiology, disease pathogenesis, existing and prospective immunoprophylaxis against the viruses. For some feline viruses, notably the immunodeficiency virus, leukaemia virus and peritonitis virus, available vaccines are poorly efficacious but there are good prospects for this. A further challenge for the industry is likely to be due to viruses jumping species and the emergence of more virulent variants of established viruses resulting from mutations as has been the case for the canine parvovirus, coronaviruses and feline calicivirus.

Feline leukemia virus DNA vaccine efficacy is enhanced by coadministration with interleukin-12 (IL-12) and IL-18 expression vectors

The expectation that cell-mediated immunity is important in the control of feline leukemia virus (FeLV) infection led us to test a DNA vaccine administered alone or with cytokines that favored the development of a Th1 immune response. The vaccine consisted of two plasmids, one expressing the gag/pol genes and the other expressing the env gene of FeLV-A/Glasgow-1. The genetic adjuvants were plasmids encoding the feline cytokines interleukin-12 (IL-12), IL-18, or gamma interferon (IFN-gamma). Kittens were immunized by three intramuscular inoculations of the FeLV DNA vaccine alone or in combination with plasmids expressing IFN-gamma, IL-12, or both IL-12 and IL-18. Control kittens were inoculated with empty plasmid. Following immunization, anti-FeLV antibodies were not detected in any kitten. Three weeks after the final immunization, the kittens were challenged by the intraperitoneal inoculation of FeLV-A/Glasgow-1 and were then monitored for a further 15 weeks for the presence of virus in plasma and, at the end of the trial, for latent virus in bone marrow. The vaccine consisting of FeLV DNA with the IL-12 and IL-18 genes conferred significant immunity, protecting completely against transient and persistent viremia, and in five of six kittens protecting against latent infection. None of the other vaccines provided significant protection.

Viral vectors for veterinary vaccines

Whatever strategy is adopted for the development of viral vectors for delivery of veterinary vaccines there are several key points to consider: (1) Will the vectored vaccine give a delivery advantage compared to what's already available? (2) Will the vectored vaccine give a manufacturing advantage compared to what's already available? (3) Will the vectored vaccine provide improved safety compared to what's already available? (5) Will the vectored vaccine increase the duration of immunity compared to what's already available? (6) Will the vectored vaccine be more convenient to store compared to what's already available? (7) Is the vectored vaccine compatible with other vaccines? If there is no other alternative available then the answer to these questions is easy. However, if there are alternative vaccines available then the answers to these questions become very important because the answers will determine whether a vectored vaccine is merely a good laboratory idea or a successful vaccine.

Efficacy of an inactivated feline leukemia virus vaccine

An inactivated whole-virus FeLV vaccine, developed from a molecularly cloned FeLV isolate (FeLV-61E-A), was assessed for its ability to protect cats against homologous and heterologous virulent viral challenge. The fractions of cats that resisted the induction of persistent viremia after FeLV challenge were as follows: FeLV-61E-A vaccine, 95%; adjuvant controls, 26%; and established commercial control FeLV vaccine, 35%. The prechallenge mean neutralizing antibody titers for each group were as follows: FeLV-61E-A vaccine, 1:43; adjuvant controls, < 1:8; and commercial control FeLV vaccine, 1:12. The prototype FeLV-61E-A vaccine was developed commercially for immunization of pet cats by substitution of a proprietary adjuvant and development of stable, high antigen production cell lines. This vaccine (Fel-O-Vax) has been studied extensively, alone and in multivalent combination with other feline virus vaccines, in seven efficacy trials involving a total of 150 immunized cats. These studies yielded an FeLV-resistant fraction of 87% in vaccinated cats as compared with 8% in adjuvant controls. The duration of immunity induced by an FeLV-61E-A commercial vaccine (Fel-O-Vax-LvK IV) was also assessed. One year after vaccination, 100% of challenged vaccinated cats and none of challenged controls resisted induction of persistent viremia. The results of these studies demonstrate that an inactivated FeLV vaccine prepared from a molecularly cloned subgroup A FeLV produces a high level of protective immunity against heterologous and homologous FeLV infection. This vaccine-induced immunity is durable for at least 1 year without intervening booster immunization or exposure to virus.

An HTLV-I/II vaccine: from animal models to clinical trials?

A human T-lymphotropic virus type I/II (HTLV-I/II) vaccine is necessary in view of two etiologically related, life-threatening diseases, namely, adult T-cell leukemia/lymphoma and tropical spastic paraparesis/HTLV-I-associated myelopathy. When the risk of developing autoimmune diseases such as uveitis, polymyositis, and arthritis is included, one can estimate the life-long risk of infected individuals to develop an HTLV associated pathology as approximately 10%. The populations at risk are, in a large majority, from developing countries but the epidemic of HTLV-II infection in intravenous drug users (IVDU) represents a possible reservoir for dissemination in the general population. The number of HTLV-I-infected individuals (15 to 25 million), together with the severity of associated disease, justifies the development of a vaccine. Different vaccine preparations have been developed, using mostly recombinant pox and adenoviruses, but DNA plasmid technology will soon become a feasible approach. Various animal models exist for experimental viral infections, involving rats, rabbits, or monkeys, but up to now, neither hematological nor neurological disorders have been induced by HTLV infection in such animal models. For long-term protection from HTLV-I-associated diseases, vaccination should induce both neutralizing antibodies and specific cell-mediated immunity. This will require the incorporation of both env and gag coding sequences in the vaccine preparations. Preventive clinical trials may involve different cohorts of seronegative young girls from endemic areas prior to sexual activity and IVDU in the industrialized world. In parallel, one should consider therapeutic vaccine trials in HTLV-I-positive mothers and IVDU to protect them against disease development. The observed rate of seroconversion in these different cohorts makes such trials feasible.

Interferon induction in peripheral blood mononuclear leukocytes of man and farm animals by poxvirus vector candidates and some poxvirus constructs

Prototypes of three poxvirus genera--orthopoxvirus (OPV), parapoxvirus (PPV), avipoxvirus (APV)--and Newcastle disease virus (NDV) as a control, as well as three recombinant OPV strains and one recombinant APV strain, were incubated in vitro with peripheral blood mononuclear leukocytes (PBML) of man, sheep and swine. Antiviral activity was determined in PBML culture supernatants at different time intervals after virus cell interaction using a cytopathic effect inhibition bioassay. Additionally, supernatants derived from human PBML were screened for interferons (IFN) alpha and gamma as well as for tumor necrosis factor by enzyme-linked immunosorbent assay. IFN titers reached a maximum 24 h after PBML stimulation at a multiplicity of infection (MOI) greater than 1. IFN alpha/beta was found to be responsible for the antiviral effect. Using a MOI > or = 1 the highly attenuated strain MVA was the only representant of vaccinia virus (VV) that induced significant amounts of IFN also as a lacZ recombinant. Replicable virus from five well-known VV strains as well as the Chinese VV strain Tien Tan (VVTT) as a recombinant vaccine failed to induce leukocyte IFN. Inactivated VV strain Elstree and the recombinant TT strain induced high titers of leukocyte IFN. Supernatants derived from human, porcine and ovine PBML stimulated with replicable PPV, native VV MVA and MVA lacZ recombinant or native APV and APV lacZ recombinant virus regularly contained IFN alpha. In contrast to NDV, neither specific antisera nor monoclonal antibodies were able to block the INF induction by VV and PPV.

Delayed hypersensitivity, immune deviation, antigen processing and T-cell subset selection in syphilis pathogenesis and vaccine design

T-cell mediated delayed-type hypersensitivity (DTH) is the predominant immune mechanism for clearing tissues of infecting organisms in the primary lesion of syphilis. Here, Stewart Sell and Pei-Ling Hsu propose a strategy for vaccination against syphilis in which selective induction of DTH may be accomplished by using BCG vectored vaccines containing selected DNA sequences for T. pallidum antigens.

Protection of cats against feline leukemia virus by vaccination with a canarypox virus recombinant, ALVAC-FL

Two ALVAC (canarypox virus)-based recombinant viruses expressing the feline leukemia virus (FeLV) subgroup A env and gag genes were assessed for their protective efficacy in cats. Both recombinant viruses contained the entire gag gene. ALVAC-FL also expressed the entire envelope glycoprotein, while ALVAC-FL(dl IS) expressed an env-specific gene product deleted of the putative immunosuppressive region. Although only 50% of the cats vaccinated with ALVAC-FL(dl IS) were protected against persistent viremia after oronasal exposure to a homologous FeLV isolate, all cats administered ALVAC-FL resisted the challenge exposure. Significantly, protection was afforded in the absence of detectable FeLV-neutralizing antibodies. These results represent the first effective vaccination of cats against FeLV with a poxvirus-based recombinant vector and have implications that are relevant not only to FeLV vaccine development but also to developing vaccines against other retroviruses, including human immunodeficiency virus.

Feline leukemia virus. Immunization and prevention

Our understanding of the pathogenesis of FeLV infection is little changed from what was described by Hardy and his colleagues in the mid-1970s. The prevention of FeLV infection consists, first, of avoiding the agent and, second, of providing optimum immunologic resistance. In multi-cat environments, the former is achieved through test-and-removal methods perennially reviewed in the literature and by minimizing exposure to outdoor cats. The latter is possible by attempting to maintain a low-stress, pathogen-free household and by the use of appropriate, effective immunization programs. Simple immunologic concepts used for the development of vaccines against feline distemper and rabies have evolved to enable generation of products that can now protect against retroviruses. The use of more complex biologic methods, such as recombinant technology and the manipulation of antigen presentation, bears encouragement, so that perhaps one day the most destructive of feline infectious diseases may be checked.

Applications of genetic engineering technology in feline medicine

Genetic engineering technology is a rapidly developing field that has almost unlimited potential for the production of safer and more effective vaccines, therapeutic proteins, and more specific and sensitive diagnostic reagents. Although applications in veterinary medicine of genetically engineered products are presently limited by availability of species-specific reagents, the use of recombinant DNA products is increasing. Because of the recent discovery of FIV and the relevance of FIV as an animal model for the study of human immunodeficiency virus, feline genetic research is gaining in importance. Research using FIV as a human AIDS model ideally will yield many new species-specific feline recombinant DNA products that have important applications in feline medicine and research.

Recombinant feline herpesviruses expressing feline leukemia virus envelope and gag proteins

We constructed recombinant feline herpesviruses (FHVs) expressing the envelope (env) and gag genes of feline leukemia virus (FeLV). Expression cassettes, utilizing the human cytomegalovirus immediate-early promoter, were inserted within the thymidine kinase gene of FHV. The FeLV env glycoprotein expressed by recombinant FHV was processed and transported to the cell surface much as in FeLV infection, with the exception that proteolytic processing to yield the mature gp70 and p15E proteins was less efficient in the context of herpesvirus infection. Glycosylation of the env protein was not affected; modification continued in the absence of efficient proteolytic processing to generate terminally glycosylated gp85 and gp70 proteins. A recombinant FHV containing the FeLV gag and protease genes expressed both gag and gag-protease precursor proteins. Functional protease was produced which mediated the proteolytic maturation of the FeLV gag proteins as in authentic FeLV infection. Use of these recombinant FHVs as live-virus vaccines may provide insight as to the role of specific retroviral proteins in protective immunity. The current use of conventional attenuated FHV vaccines speaks to the wider potential of recombinant FHVs for vaccination in cats.

A recombinant vaccinia virus expressing hepatitis B virus middle surface protein. Restricted expression of HBV antigens in human diploid cells

Several vaccinia virus recombinants inducing the synthesis of the middle surface (M) protein of hepatitis B virus (HBV) were constructed. One of them, denoted v137, was examined in some detail. The virus replicated nearly to the same extent in various cell lines, viz. human embryo diploid fibroblast LEP and MRC-5 cells, rabbit embryo fibroblast REF cells, TK- rat RAT-2 cells, and green monkey CV-1 cells. However, the production of M protein was found considerably lower in the human LEP and MRC-5 than in the other cells examined. In addition, the kinetics of M formation were different in these two cell systems, LEP cells lagging significantly behind CV-1 cells. The low-level production of M protein in LEP cells was not increased by repeated v137 passages in LEP cells, nor by a passage in a laboratory worker accidentally infected with the v137 virus, nor by shortening the leader sequence preceding the translation initiation codon. The greater part of the M antigen was found to be cell associated, more so in the cells of human than monkey origin. From the major HBV S antigen (HBsAg) isolated from the plasma of chronically infected subjects, the antigen released by cell destruction differed by binding to polymerized human albumin. This property was utilized in ELISA to detect anti-preS2 antibody. Rabbits inoculated intradermally with the v137 virus developed antibodies reactive in this assay as well as with a synthetic peptide corresponding in the amino acids 14-34 of the NH2 terminus of the HBsAg preS2 region.

Feline retroviruses: a brief review

Representatives of all three retrovirus subfamilies are recognized in the cat: feline leukaemia virus (an Oncovirus), feline syncytium-forming virus (a Spumavirus) and feline immunodeficiency virus (a Lentivirus). Each of these is briefly reviewed.

Vaccinia virus vectors: new strategies for producing recombinant vaccines

The development and continued refinement of techniques for the efficient insertion and expression of heterologous DNA sequences from within the genomic context of infectious vaccinia virus recombinants are among the most promising current approaches towards effective immunoprophylaxis against a variety of protozoan, viral, and bacterial human pathogens. Because of its medical relevance, this area is the subject of intense research interest and has evolved rapidly during the past several years. This review (i) provides an updated overview of the technology that exists for assembling recombinant vaccinia virus strains, (ii) discusses the advantages and disadvantages of these approaches, (iii) outlines the areas of outgoing research directed towards overcoming the limitations of current techniques, and (iv) provides some insight (i.e., speculation) about probable future refinements in the use of vaccinia virus as a vector.

Identification of the thymidine kinase gene of feline herpesvirus: use of degenerate oligonucleotides in the polymerase chain reaction to isolate herpesvirus gene homologs

Feline herpesvirus 1 (FHV) is the causative agent of viral rhinotracheitis in cats. Current vaccination programs employing attenuated live and killed FHV vaccines have been effective in reducing the incidence of this disease. As an initial step in the development of recombinant FHVs for use in the vaccination of cats, we have identified the thymidine kinase (TK) gene of this feline-specific alphaherpesvirus. Comparisons of the amino acid sequences of other herpesvirus TK proteins have shown that these proteins are highly divergent, sharing only short regions of imperfect amino acid identity. We have used the polymerase chain reaction method of DNA amplification to increase the specificity associated with the use of short, highly degenerate oligonucleotide probes derived from regions of imperfect amino acid conservation. These methods were used to isolate the TK gene of FHV and should prove to be useful in the identification of new members of other viral and cellular gene families. A recombinant FHV bearing a deletion in the identified TK gene was constructed and shown to possess the expected TK- phenotype. The FHV TK gene is located at a position of approximately 40% in the long unique component of the FHV genome. The location of the TK gene and the location and orientation of flanking FHV genes, homologs of herpes simplex virus type 1 UL24 and UL22, are conserved among alphaherpesviruses.

Vaccines against tumor antigens

Effective vaccines against tumor antigens have not yet been produced. However, immunomodulators hold much promise in cancer therapy. Such treatments will probably involve using combinations of various immunomodulators together with activated killer cells. Development of vaccines against tumor-causing viruses seems to be a rational approach to preventing the onset of virus-induced cancers. It seems that efficient vaccines have already been developed for hepatitis B virus; such vaccines have the potential to decrease the incidence of its associated hepatoma. However, successful vaccines against RNA-containing tumor viruses have yet to be developed, although they hold much promise.

Present and future applications of vaccinia virus as a vector

The use of live recombinant vaccinia virus strains, which are capable of expressing immunoreactive epitopes from pathogenic agents, holds great promise as a means of immunoprophylaxis against a variety of human and animal diseases. This review will outline the basic methodology employed by this approach, summarize some recent technological advancements which should facilitate the construction of genetically-engineered recombinant viruses, and consider potential future applications of this avenue of research.

Prospects for molecular vaccines in veterinary parasitology

Despite the profound developments in recombinant DNA technology there is only one marketed recombinant vaccine (for human viral hepatitis B). The development of others proceeds with great difficulty. Molecular vaccines against veterinary parasites are at the utmost pole of complexity in the spectrum of potential vaccines since these parasites are complex eukaryotic organisms, often dwelling at mucosal surfaces where anamnestic responses are problematic, where the immunogenicity of the parasite components is poorly understood and where the effector mechanisms of immunity are unresolved. Cloning a "protective" gene is only the first step, and perhaps the easiest, in a long process which will be necessary to develop vaccines against parasites. Additional steps will involve comprehensive analyses of the immunological responses to ensure that vaccine antigens contain the correct epitopes to induce appropriate immune effector mechanisms for parasite elimination and immunological memory and that these responses are not genetically restricted. The great expectations for recombinant vaccinia-based vaccines must be modified substantially in the light of recent evidence indicating immunological and other constraints on this approach. The use of anti-idiotype vaccines is an underexplored opportunity for practical parasite vaccines since they have several potentially important advantages. The need to include T cell antigenic peptides in peptide vaccines to extend the range of genetic responsiveness and to induce anamnestic responses is now clear. New algorithms for the prediction of such sites exist and these can be tested experimentally with synthetic peptides. There are no major technical obstacles to the development of vaccines for parasites which cannot be overcome. However substantial long term basic research is needed over a range of disciplines to achieve this worthwhile objective.