Further development of a recombinant feline herpesvirus type 1 vector expressing feline calicivirus immunogenic antigen

The recombinant feline herpesvirus 1 expressing feline Calicivirus VP1 protein is safe and effective in cats

Feline herpesvirus type 1 (FHV) and feline calicivirus (FCV) are significant pathogens causing upper respiratory tract disease in cats. Existing inactivated or modified live vaccines against FCV and FHV face limitations in safety and efficacy. To overcome these challenges, a recombinant strain FHV ΔgI/gE-FCV VP1 was developed by deleting the gI/gE gene and concurrently expressing FCV VP1, using the FHV WX19 strain as the parental virus. Results indicated the presence of FCV VP1 in FHV ΔgI/gE-FCV VP1-infected CRFK cells, confirmed through protein blotting and immunofluorescence assays and virus-like particles (VLPs) of FCV were observed using transmission electron microscopy. For efficacy in cats, each animal received intranasal vaccination with 1 mL of FHV ΔgI/gE-FCV VP1 at 106 TCID50. Following completion of vaccination on day 28, animals were exposed to a potent FCV strain. Assessments included clinical signs, nasal shedding, virus neutralizing antibodies, cytokine expression and postmortem histological testing. All vaccinations with FHV ΔgI/gE-FCV VP1 were deemed safe, with significantly reduced clinical disease scores, pathological changes and viral nasal shedding following infection and robust immune responses were induced. These findings collectively suggest the effectiveness of FHV-based recombinant vaccines in preventing FCV infections.

Advancements, challenges, and future perspectives in developing feline herpesvirus 1 as a vaccine vector

As the most prevalent companion animal, cats are threatened by numerous infectious diseases and carry zoonotic pathogens such as Toxoplasma gondii and Bartonella henselae, which are the primary causes of human toxoplasmosis and cat-scratch disease. Vaccines play a crucial role in preventing and controlling the spread of diseases in both humans and animals. Currently, there are only three core vaccines available to prevent feline panleukopenia, feline herpesvirus, and feline calicivirus infections, with few vaccines available for other significant feline infectious and zoonotic diseases. Feline herpesvirus, a major component of the core vaccine, offers several advantages and a stable genetic manipulation platform, making it an ideal model for vaccine vector development to prevent and control feline infectious diseases. This paper reviews the technologies involved in the research and development of the feline herpesvirus vaccine vector, including homologous recombination, CRISPR/Cas9, and bacterial artificial chromosomes. It also examines the design and effectiveness of expressing antigens of other pathogens using the feline herpesvirus as a vaccine vector. Additionally, the paper analyzes existing technical bottlenecks and challenges, providing an outlook on its application prospects. The aim of this review is to provide a scientific basis for the research and development of feline herpesvirus as a vaccine vector and to offer new ideas for the prevention and control of significant feline infectious and zoonotic diseases.

A potential dual protection vaccine: Recombinant feline herpesvirus-1 expressing feline parvovirus VP2 antigen

Recently, herpesvirus viral vectors that stimulate strong humoral and cellular immunity have been demonstrated to be the most promising platforms for the development of multivalent vaccines, because they contain various nonessential genes and exhibit long-life latency characteristics. Previously, we showed that the feline herpesvirus-1 (FHV-1) mutant WH2020-ΔTK/gI/gE, which was safe for felines and provided efficacious protection against FHV-1 challenge, can be used as a vaccine vector. Moreover, previous studies have shown that the major neutralizing epitope VP2 protein of feline parvovirus (FPV) can elicit high levels of neutralizing antibodies. Therefore, to develop a bivalent vaccine against FPV and FHV-1, we first generated a novel recombinant virus by CRISPR/Cas9-mediated homologous recombination, WH2020-ΔTK/gI/gE-VP2, which expresses the VP2 protein of FPV. The growth characteristics of WH2020-ΔTK/gI/gE-VP2 were similar to those of WH2020-ΔTK/gI/gE, and WH2020-ΔTK/gI/gE-VP2 was stable for at least 30 generations in CRFK cells. As expected, we found that the felines immunized with WH2020-ΔTK/gI/gE-VP2 produced FPV-neutralizing antibody titers (27.5) above the positive cutoff (26) on day 14 after single inoculation. More importantly, recombinant WH2020-ΔTK/gI/gE-VP2 exhibited severely impaired pathogenicity in inoculated and cohabiting cats. The kittens immunized with WH2020-ΔTK/gI/gE and WH2020-ΔTK/gI/gE-VP2 produced similar levels of FHV-specific antibodies and IFN-β. Furthermore, felines immunized with WH2020-ΔTK/gI/gE-VP2 were protected against challenge with FPV and FHV-1. These data showed that WH2020-ΔTK/gI/gE-VP2 appears to be a potentially safe, effective, and economical bivalent vaccine against FPV and FHV-1 and that WH2020-ΔTK/gI/gE can be used as a viral vector to develop feline multivalent vaccines.

Use of feline herpesvirus as a vaccine vector offers alternative applications for feline health

Herpesviruses are attractive vaccine vector candidates due to their large double stranded DNA genome and latency characteristics. Within the scope of veterinary vaccines, herpesvirus-vectored vaccines have been well studied and commercially available vectored vaccines are used to help prevent diseases in different animal species. Felid alphaherpesvirus 1 (FHV-1) has been characterised as a vector candidate to protect against a range of feline pathogens. In this review we highlight the methods used to construct FHV-1 based vaccines and their outcomes, while also proposing alternative uses for FHV-1 as a viral vector.

Utilization of herpesviridae as recombinant viral vectors in vaccine development against animal pathogens

Throughout the past few decades, numerous viral species have been generated as vaccine vectors. Every viral vector has its own distinct characteristics. For example, the family herpesviridae encompasses several viruses that have medical and veterinary importance. Attenuated herpesviruses are developed as vectors to convey heterologous immunogens targeting several serious and crucial pathogens. Some of these vectors have already been licensed for use in the veterinary field. One of their prominent features is their capability to accommodate large amount of foreign DNA, and to stimulate both cell-mediated and humoral immune responses. A better understanding of vector-host interaction builds up a robust foundation for the future development of herpesviruses-based vectors. At the time, many molecular tools are applied to enable the generation of herpesvirus-based recombinant vaccine vectors such as BAC technology, homologous and two-step en passant mutagenesis, codon optimization, and the CRISPR/Cas9 system. This review article highlights the most important techniques applied in constructing recombinant herpesviruses vectors, advantages and disadvantages of each recombinant herpesvirus vector, and the most recent research regarding their use to control major animal diseases.

Intranasal immunization with inactivated feline calicivirus particles confers robust protection against homologous virus and suppression against heterologous virus in cats

The protective efficacy of intranasal (IN) administration of inactivated feline calicivirus (FCV) vaccine against homologous or heterologous FCV infection was investigated. Groups of cats immunized with the experimental inactivated, non-adjuvanted FCV vaccine via either the IN or subcutaneous (SC) route were exposed to homologous or highly heterologous FCV. Both the IN and SC immunization protocols established robust protection against homologous FCV infection. Although neither immunization regimen conferred protection against the heterologous strain, clinical scores and virus titres of oral swabs were lower in cats in the IN group compared to those in the SC group, accompanying a faster neutralizing antibody response against the heterologous virus in cats in the IN group. The IN group secreted more IgA specific to FCV proteins in oral washes (lavage fluids from the oral cavity) than the SC group. IN immunization with an inactivated whole FCV particle, which protects cats from homologous virus exposure and shortens the period of heterologous virus shedding, may serve as a better platform for anti-FCV vaccine.

Analysis of herpesvirus host specificity determinants using herpesvirus genomes as bacterial artificial chromosomes

Almost all mammalian alphaherpesviruses can grow in cells derived from several types of animals in vitro. However, FHV-1 can only infect feline cell lines. For this reason, FHV-1 should be a good model to investigate species barriers to herpesviruses in vivo. To apply bacterial mutagenesis of FHV-1, we cloned the FHV-1 genome as a BAC. Using lambda and flp recombinations, we introduced a monomeric red fluorescence protein into the C-terminus of glycoprotein D. Although GFP in the constructed recombinant FHV-1, a transfectant of the bacmid of FHV-1 that possessed the GFP, acted in non-feline cell lines, the virus could not enter non-feline cell lines, demonstrating that the host specificity of FHV-1 was restricted in an early step of infection. The host range of canine herpesvirus is limited to dogs in vitro and in vivo; it cannot enter non-canine cell lines as a result of infection but the GFP is active by transfection, revealing the same result that the restriction step is at an early stage of infection. These results suggest the possibility of breaking species barriers of FHV-1 and CHV by modifying the gene(s) that act at the early stage of infection.

Comparison of the ability of feline calicivirus (FCV) vaccines to neutralise a panel of current UK FCV isolates

Feline calicivirus (FCV) comprises a large number of strains which are related antigenically to varying degrees. The antigenic variability creates problems for choosing antigens to include in vaccines. Historically, these have been selected for use based on their cross-reactivity with a high proportion of field strains. However, it is important to determine the current level of cross-reactivity of vaccines and whether or not this may be decreasing owing to widespread vaccine use. In this in vitro study, we have compared the ability of antisera to two vaccine viruses (FCV strain F9 and FCV strain 255) to neutralise a panel of 40 recent UK field isolates. These 40 isolates were obtained by randomised, cross-sectional sampling of veterinary practices in different geographical regions of the UK so as to ensure they were representative of viruses circulating in the veterinary-visiting population of cats in the UK. Virus neutralisation assays showed that both vaccine strains are still broadly cross-reactive, with F9 antiserum neutralising 87.5% and 255 antiserum 75% of isolates tested with antiserum dilutions of 1 in 2 or greater. However, when antibody units were used, in order to take account of differences in homologous titres between antisera, fewer isolates were neutralised, with F9 antiserum showing a slightly higher proportion of isolates neutralised than 255. Multivariable analysis of the sample population of 1206 cats from which the 40 isolates were derived found that vaccinated cats were at a decreased risk of being positive for FCV, whereas cats from households with more than one cat, and cats with mouth ulcers were at increased risk. In addition as cats became older their risk of shedding FCV decreased.

Feline calicivirus

Feline calicivirus (FCV) is an important and highly prevalent pathogen of cats. It belongs to the family Caliciviridae which includes other significant pathogens of man and animals. As an RNA virus, high polymerase error rates convey upon FCV a high genome plasticity, and allow the virus to respond rapidly to environmental selection pressures. This makes the virus very adaptable and has important implications for clinical disease and its control. Being genetically diverse, FCV is associated with a range of clinical syndromes from inapparent infections to relatively mild oral and upper respiratory tract disease with or without acute lameness. More recently, highly virulent forms of the virus have emerged associated with a systemic infection that is frequently fatal. A proportion of FCV infected cats that recover from acute disease, remain persistently infected. In such cats, virus evolution is believed to help the virus to evade the host immune response. Such long-term carriers may only represent a minority of the feline population but are likely to be crucial to the epidemiology of the virus. Vaccination against FCV has been available for many years and has effectively reduced the incidence of clinical disease. However, the vaccines do not prevent infection and vaccinated cats can still become persistently infected. In addition, FCV strain variability means that not all strains are protected against equally. Much progress has been made in understanding the biology and pathogenesis of this important feline virus. Challenges for the future will necessarily focus on how to control the variability of this virus particularly in relation to emerging virulent strains and vaccination.

Feline herpesvirus

Feline herpesvirus (FHV-1; felid herpesvirus 1 (FeHV-1)) is an alphaherpesvirus of cats closely related to canine herpesvirus-1 and phocine herpesvirus-1. There is only one serotype of the virus and it is relatively homogenous genetically. FeHV-1 is an important cause of acute upper respiratory tract and ocular disease in cats. In addition, its role in more chronic ocular disease and skin lesions is increasingly being recognised. Epidemiologically, FeHV-1 behaves as a typical alphaherpesvirus whereby clinically recovered cats become latently infected carriers which undergo periodic episodes of virus reactivation, particularly after a stress. The primary site of latency is the trigeminal ganglion. Conventional inactivated and modified-live vaccines are available and protect reasonably well against disease but not infection, although viral shedding may be reduced. Genetically engineered vaccines have also been developed, both for FeHV-1 and as vector vaccines for other pathogens, but none is as yet marketed.

Mucosal delivery of vaccines in domestic animals

Mucosal vaccination is proving to be one of the greatest challenges in modern vaccine development. Although highly beneficial for achieving protective immunity, the induction of mucosal immunity, especially in the gastro-intestinal tract, still remains a difficult task. As a result, only very few mucosal vaccines are commercially available for domestic animals. Here, we critically review various strategies for mucosal delivery of vaccines in domestic animals. This includes live bacterial and viral vectors, particulate delivery-systems such as polymers, alginate, polyphosphazenes, immune stimulating complex and liposomes, and receptor mediated-targeting strategies to the mucosal tissues. The most commonly used routes of immunization, strategies for delivering the antigen to the mucosal surfaces, and future prospects in the development of mucosal vaccines are discussed.

Construction of an infectious clone of canine herpesvirus genome as a bacterial artificial chromosome

Canine herpesvirus (CHV) is an attractive candidate not only for use as a recombinant vaccine to protect dogs from a variety of canine pathogens but also as a viral vector for gene therapy in domestic animals. However, developments in this area have been impeded by the complicated techniques used for eukaryotic homologous recombination. To overcome these problems, we used bacterial artificial chromosomes (BACs) to generate infectious BACs. Our findings may be summarized as follows: (i) the CHV genome (pCHV/BAC), in which a BAC flanked by loxP sites was inserted into the thymidine kinase gene, was maintained in Escherichia coli; (ii) transfection of pCHV/BAC into A-72 cells resulted in the production of infectious virus; (iii) the BAC vector sequence was almost perfectly excisable from the genome of the reconstituted virus CHV/BAC by co-infection with CHV/BAC and a recombinant adenovirus that expressed the Cre recombinase; and (iv) a recombinant virus in which the glycoprotein C gene was deleted was generated by lambda recombination followed by Flp recombination, which resulted in a reduction in viral titer compared with that of the wild-type virus. The infectious clone pCHV/BAC is useful for the modification of the CHV genome using bacterial genetics, and CHV/BAC should have multiple applications in the rapid generation of genetically engineered CHV recombinants and the development of CHV vectors for vaccination and gene therapy in domestic animals.

Immunisation with a combination of two complementary feline calicivirus strains induces a broad cross-protection against heterologous challenges

Feline calicivirus (FCV) is characterised by a high degree of antigenic variation potentially compromising vaccine efficacy. Inclusion of several FCV strains or antigens in current vaccines could be a means to improve protection against antigenically distinct isolates. This study evaluated the synergy between two FCV strains (FCVG1 and FCV431) by comparing immunity induced by either strain with that provided by a combination of the two strains against an heterologous challenge with antigenically distant FCV strains (FCV393 and FCV220). Thirty-two SPF kittens were randomly allocated to four groups of eight cats in each group. Groups B, C and D cats were vaccinated once subcutaneously with strains FCVG1, FCV431, and FCVG1 + FCV431, respectively. Each kitten received a total dose of 10(3.4) CCID50 of FCV. Control group A was not immunised. On day 31, four cats from each group were challenged oronasally with FCV220 and four cats with FCV393. Following challenge, the cats were monitored for clinical signs, viral shedding and antibody responses. FCV220 and FCV393 induced severe clinical signs in control cats typical of FCV infection. Immunisation with both strains mixed together induced higher neutralizing antibody titres against FCV220 and FCV393 strains on average. Protection was observed in all groups, however combination of the two strains resulted in a better clinical protection and reduction of virus shedding after heterologous challenge. A moderate correlation was observed between neutralizing antibody titres at the time of challenge and protection against clinical signs. These results indicated that vaccines combining antigens from different FCV strains may induce a broader heterologous protection.

Experimental infection of recent field isolates of feline herpesvirus type 1

Two field isolates of feline herpesvirus type 1 (FHV-1) designated as 00-015 and 00-035, were obtained from cats diagnosed as feline viral rhinotracheitis (FVR) in Japan. To analyze the character of recent FHV-1, these two isolates and our laboratory strain C7301 were inoculated experimentally to specific-pathogen-free cats. Although all cats showed typical FVR symptoms, more severe clinical symptoms were observed on cats infected with the isolates 00-015 and 00-035 compared with those of C7301-infected cats. Severe ocular lesions including conjunctivitis were found in the cats infected with the isolates, indicating that the recent FHV-1 has a potential to induce severe FVR symptoms including ocular lesions.

Application of chimeric feline foamy virus-based retroviral vectors for the induction of antiviral immunity in cats

In order to define the potential and applicability of replication-competent foamy virus-based vaccine vectors, recombinant feline foamy virus (FFV) vectors encoding defined segments of the feline calicivirus (FCV) capsid protein E domain were constructed. In cell cultures, these FFV-FCV vectors efficiently transduced and expressed a hybrid fusion protein consisting of the essential FFV Bet protein and the attached FCV E domains. The stability of the vectors in vitro was inversely correlated to the size of the heterologous insert. The deletion of a part of the FFV U3 sequence in these FFV-FCV vectors did not interfere with replication and titer in cell cultures but increased the genetic stability of the hybrid vectors. Selected chimeric vectors were injected into immunocompetent cats and persisted in the transduced host concomitant with a strong and specific humoral immune response against vector components. In a substantial number of cats, antibodies directed against the FCV E domain were induced by the FFV-FCV vectors, but no FCV-neutralizing activities were detectable in vitro. When the vaccinated cats were challenged with a high-titer FCV dose, sterile immunity was not induced by any of the hybrid FFV-FCV vectors. However, the FFV-FCV vector with a truncated U3 region of the long terminal repeat promoter significantly reduced the duration of FCV shedding after challenge and suppressed the appearance of FCV-specific ulcers. Possible mechanisms contributing to the partial protection will be discussed.

Nucleotide sequence of glycoprotein genes B, C, D, G, H and I, the thymidine kinase and protein kinase genes and gene homologue UL24 of an Australian isolate of canine herpesvirus

We report the complete nucleotide (nt) sequence of nine genes of an Australian isolate of canine herpesvirus (CHV). Four of them are located in the unique short (US) region: glycoprotein (g) genes gG, gD and gI, and the protein kinase gene. Five are in the unique long (UL) region: the thymidine kinase gene, gB, gC, gH, and gene homologue UL24. Partial sequence was determined for four genes, two in the UL region (UL21 and virion protein) and two in the US region (US2 and gE). A repeat sequence of 382 nt with unknown function was identified in the 615 nt intergenic region between gH and UL21. A total of 16.93 kb was sequenced and compared with sequences from CHV isolates from the USA, France, Japan and Australia. Only minor nt and/or amino acid (aa) differences were observed.

Vaccination of cats with an attenuated recombinant myxoma virus expressing feline calicivirus capsid protein

Myxoma virus, a member of the Poxviridae family (genus Leporipoxvirus) is the agent responsible for myxomatosis in the European rabbit. Recombinant myxoma viruses expressing the capsid gene of an F9 strain of feline calicivirus (FCV) were constructed from an apathogenic, laboratory attenuated, isolate of myxoma virus. The FCV capsid genes were recombined into the myxoma growth factor (MGF) locus of the myxoma genome and expressed from synthetic poxvirus promoters. Myxoma virus is unable to replicate productively in feline cells in vitro, however, cells infected with recombinant viruses do express the heterologous antigens from both late and early/late synthetic promoters. Cats immunised with myxoma-FCV recombinant virus generated high levels of serum neutralising antibody and were protected from disease on subsequent challenge with virulent FCV. Furthermore, there was no evidence of transmission of myxoma-FCV recombinant virus from vaccinated to non-vaccinated cats. These results demonstrate the potential of myxoma virus as a safe vaccine vector for use in non-lepori species and in particular the cat.

Efficient expression of the envelope protein of feline immunodeficiency virus in a recombinant feline herpesvirus type 1 (FHV-1) using the gC promoter of FHV-1

We constructed two recombinant feline herpesvirus type 1 (FHV-1) expressing the envelope (Env) protein of feline immunodeficiency virus (FIV). One recombinant, designated dlTK-env, has the whole FIV env gene inserted at a thymidine kinase (TK) deletion site. The second recombinant, designated dlTK(gCp)-env, has a cassette containing a partial FIV env gene fused with the signal sequence of the gC protein of FHV-1 (under the control of the gC promoter) inserted at the same site. Growth kinetics of both the recombinants in Crandell feline kidney (CRFK) cells were similar to that of the parent strain of FHV-1. By indirect immunofluorescence assays and immunoblot analyses, we confirmed the expression of the FIV Env protein in CRFK cells infected with both recombinants. Enzyme-linked immunosorbent assays showed that the maximum Env expression level achieved by dlTK(gCp)-env was more than four times higher than that observed for dlTK-env. Flow cytometric analyses revealed that the Env protein produced by both recombinants was efficiently expressed on the cell surface. The dlTK(gCp)-env reported here may thus be a promising candidate for a live recombinant vaccine to protect against FIV infection.

Herpesviruses of carnivores

This review focuses on felid herpesvirus 1 (FHV-1), the most studied of the carnivore herpesviruses. Canid herpesvirus (CHV-1) and phocid (seal) herpesvirus 1 (PhHV-1) are also included where information is available. FHV-1 is a member of the Varicellovirus genus of the Alphaherpesvirinae, which appears to be closely related phylogenetically to both CHV-1 and PhHV-1. FHV-1 infects both domestic and some wild Felidae, such as cheetahs, and is predominantly a respiratory pathogen of cats. As in other herpesviruses, infection with FHV-1 is characterised by a latent carrier state, during which intermittent shedding of infectious virus may occur. Typical of an alphaherpesvirus, the primary site of FHV-1 latency is neurological tissue (trigeminal ganglion), though recent studies using the polymerase chain reaction have suggested that some latency may occur in non-neurological sites. Latently infected carriers are epidemiologically important as sources of infection for susceptible animals. Though conventional modified live and inactivated vaccines have been available for a number of years, they do not protect against infection nor the development of latency. Recently, work has focused on molecular characterisation of FHV-1, detecting genes such as glycoproteins or regulatory genes. Such work will enable better understanding of the interaction of FHV-1 with the natural host. Deletion mutants of some of these genes may also have potential as vaccine strains.