Bovine herpesvirus type 1 (BoHV-1) anterograde neuronal transport from trigeminal ganglia to nose and eye requires glycoprotein E

A Quadruple Gene-Deleted Live BoHV-1 Subunit RVFV Vaccine Vector Reactivates from Latency and Replicates in the TG Neurons of Calves but Is Not Transported to and Shed from Nasal Mucosa

Bovine herpesvirus type 1 (BoHV-1) establishes lifelong latency in trigeminal ganglionic (TG) neurons following intranasal and ocular infection in cattle. Periodically, the latent virus reactivates in the TG due to stress and is transported anterogradely to nerve endings in the nasal epithelium, where the virus replicates and sheds. Consequently, BoHV-1 is transmitted to susceptible animals and maintained in the cattle population. Modified live BoHV-1 vaccine strains (BoHV-1 MLV) also have a similar latency reactivation. Therefore, they circulate and are maintained in cattle herds. Additionally, they can regain virulence and cause vaccine outbreaks because they mutate and recombine with other circulating field wild-type (wt) strains. Recently, we constructed a BoHV-1 quadruple mutant virus (BoHV-1qmv) that lacks immune evasive properties due to UL49.5 and glycoprotein G (gG) deletions. In addition, it also lacks the gE cytoplasmic tail (gE CT) and Us9 gene sequences designed to make it safe, increase its vaccine efficacy against BoHV-1, and restrict its anterograde neuronal transport noted above. Further, we engineered the BoHV-1qmv-vector to serve as a subunit vaccine against the Rift Valley fever virus (BoHV-1qmv Sub-RVFV) (doi: 10.3390/v15112183). In this study, we determined the latency reactivation and nasal virus shedding properties of BoHV-1qmv (vector) and BoHV-1qmv-vectored subunit RVFV (BoHV-1qmv sub-RVFV) vaccine virus in calves in comparison to the BoHV-1 wild-type (wt) following intranasal inoculation. The real-time PCR results showed that BoHV-1 wt- but not the BoHV-1qmv vector- and BoHV-1qmv Sub-RVFV-inoculated calves shed virus in the nose following dexamethasone-induced latency reactivation; however, like the BoHV-1 wt, both the BoHV-1qmv vector and BoHV-1qmv Sub-RVFV viruses established latency, were reactivated, and replicated in the TG neurons. These results are consistent with the anterograde neurotransport function of the gE CT and Us9 sequences, which are deleted in the BoHV-1qmv and BoHV-1qmv Sub-RVFV.

The Cell-Mediated Immune Response against Bovine alphaherpesvirus 1 (BoHV-1) Infection and Vaccination

Bovine Alphaherpesvirus 1 (BoHV-1) is one of the major respiratory pathogens in cattle worldwide. Infection often leads to a compromised host immune response that contributes to the development of the polymicrobial disease known as “bovine respiratory disease”. After an initial transient phase of immunosuppression, cattle recover from the disease. This is due to the development of both innate and adaptive immune responses. With respect to adaptive immunity, both humoral and cell-mediated immunity are required to control infection. Thus, several BoHV-1 vaccines are designed to trigger both branches of the adaptive immune system. In this review, we summarize the current knowledge on cell-mediated immune responses directed against BoHV-1 infection and vaccination.

Recombinant Bovine Herpesvirus Type I Expressing the Bovine Viral Diarrhea Virus E2 Protein Could Effectively Prevent Infection by Two Viruses

Bovine respiratory disease complex (BRDC) is a comprehensive disease in cattle caused by various viral and bacterial infections. Among them, bovine herpesvirus type I (BoHV−1) and bovine viral diarrhea virus (BVDV) play important roles and have caused huge financial losses for the cattle industry worldwide. At present, vaccines against BRDC include trivalent attenuated BoHV−1, BVDV−1, and BVDV−2 live vaccines, BoHV−1 live attenuated vaccines, and BoHV−1/BVDV bivalent live attenuated vaccines, which have limitations in terms of their safety and efficacy. To solve these problems, we optimized the codon of the BVDV−1 E2 gene, added the signal peptide sequence of the BoHV−1 gD gene, expressed double BVDV−1 E2 glycoproteins in tandem at the BoHV−1 gE gene site, and constructed a BoHV−1 genetics-engineered vectored vaccine with gE gene deletion, named BoHV−1 gE/E2−Linker−E2⁺ and BoHV−1 ΔgE. This study compared the protective effects in BoHV−1, BoHV−1 ΔgE, BoHV−1 gE/E2−Linker−E2⁺, and BVDV−1 inactivated antigen immunized guinea pigs and calves. The results showed that BoHV−1 gE/E2−Linker−E2⁺ could successfully induce guinea pigs and calves to produce specific neutralizing antibodies against BVDV−1. In addition, after BoHV−1 and BVDV−1 challenges, BoHV−1 gE/E2−Linker−E2⁺ can produce a specific neutralizing antibody response against BoHV−1 and BVDV−1 infections. Calves immunized with this type of virus can be distinguished as either vaccinated animals (gE^-) or naturally infected animals (gE⁺). In summary, our data suggest that BoHV−1 gE/E2−Linker−E2⁺ and BoHV−1 ΔgE have great potential to prevent BVDV−1 or BoHV−1 infection.

Concurrent Gene Insertion, Deletion, and Inversion during the Construction of a Novel Attenuated BoHV-1 Using CRISPR/Cas9 Genome Editing

Bovine herpesvirus type I (BoHV-1) is an important pathogen that causes respiratory disease in bovines. The disease is prevalent worldwide, causing huge economic losses to the cattle industry. Gene-deficient vaccines with immunological markers to distinguish them from wild-type infections have become a mainstream in vaccine research and development. In order to knock out the gE gene BoHV-1, we employed the CRISPR/Cas9 system. Interesting phenomena were observed at the single guide RNA (sgRNA) splicing site, including gene insertion, gene deletion, and the inversion of 5′ and 3′ ends of the sgRNA splicing site. In addition to the deletion of the gE gene, the US9 gene, and the non-coding regions of gE and US9, it was found that the US4 sequence, US6 sequence, and part of the US7 sequence were inserted into the EGFP sgRNA splicing site and the 3′ end of the EGFP sequence was deleted. Similar to the BoHV-1 parent, the BoHV-1 mutants induced high neutralizing antibodies titer levels in mice. In summary, we developed a series of recombinant gE-deletion BoHV-1 samples using the CRISPR/Cas9 gene editing system. The mutant viruses with EGFP⁺ or EGFP⁻ will lay the foundation for research on BoHV-1 and vaccine development in the future.

An Improved DNA Vaccine Against Bovine Herpesvirus-1 Using CD40L and a Chemical Adjuvant Induces Specific Cytotoxicity in Mice

Bovine herpesvirus-1 (BoHV-1) uses many mechanisms to elude the immune system; one of them is spreading intracellularly, even in the presence of specific antiviral antibodies. Cytotoxic T lymphocytes (CTLs) are necessary to eliminate the virus. The main preventive strategy is vaccination based on inactivated virus. These vaccines are poor inducers of cellular immune responses, and complicate serological diagnosis and determination of the real prevalence of infection. DNA vaccines are a good option because of the capacity of Differentiating Infected from Vaccinated Animals-(DIVA vaccine)-and may be the best way to induce cytotoxic responses. Although this type of vaccines leads to only weak "in vivo" expression and poor immune responses, incorporation of molecular and/or chemical adjuvants can improve the latter, both in magnitude and in direction. In this study, we have investigated the specific immune responses elicited in mice by DNA vaccines based on the BoHV-1 glycoprotein D (pCIgD) with and without two different adjuvants: a plasmid encoding for murine CD40L (pCD40L) or Montanide™ 1113101PR (101). Mice vaccinated with pCIgD⁺CD40L, pCIgD⁺101, and pCIgD⁺CD40L⁺101 developed significantly higher specific antibody titers against BoHV-1 than the pCIgD group (p < 0.01). The animals vaccinated with pCgD⁺pCD40L⁺101 raised significantly higher levels of IgG2a and IgG2b (p < 0.01 and p < 0.001, respectively) than mice vaccinated with pCIgD alone. On the contrary, when the activity of CTL against cells infected with BoHV-1 was measured, the vaccine pCgD⁺pCD40L⁺101 induced significantly higher levels of cytotoxicity activity (p < 0.001) than pCIgD alone. A significant increase in the CD4⁺ populations in the group receiving pCIgD⁺CD40L⁺101 in comparison with the pCIgD group was observed and, also, interferon gamma, interleukin (IL)-6, and IL-17A levels were higher. Considering the results obtained from this study for humoral and cellular responses in mice, the inclusion of pCD40L and 101 as adjuvants in a BoHV-1 DNA vaccine for cattle is highly recommendable.

Two Separate Tyrosine-Based YXXL/Φ Motifs within the Glycoprotein E Cytoplasmic Tail of Bovine Herpesvirus 1 Contribute in Virus Anterograde Neuronal Transport

Bovine herpesvirus 1 (BHV-1) causes respiratory infection and abortion in cattle. Following a primary infection, BHV-1 establishes lifelong latency in the trigeminal ganglia (TG). Periodic reactivation of the latent virus in TG neurons results in anterograde virus transport to nerve endings in the nasal mucosa and nasal virus shedding. The BHV-1 glycoprotein E cytoplasmic tail (gE-CT) is necessary for virus cell-to-cell spread in epithelial cells and neuronal anterograde transport. Recently, we identified two tyrosine residues, Y467 and Y563, within the tyrosine-based motifs ₄₆₇YTSL₄₇₀ and ₅₆₃YTVV₅₆₆, which, together, account for the gE CT-mediated efficient cell-to-cell spread of BHV-1 in epithelial cells. Here, we determined that in primary neuron cultures in vitro, the individual alanine exchange Y467A or Y563A mutants had significantly diminished anterograde axonal spread. Remarkably, the double-alanine-exchanged Y467A/Y563A mutant virus was not transported anterogradely. Following intranasal infection of rabbits, both wild-type (wt) and the Y467A/Y563A mutant viruses established latency in the TG. Upon dexamethasone-induced reactivation, both wt and the mutant viruses reactivated and replicated equally efficiently in the TG. However, upon reactivation, only the wt, not the mutant, was isolated from nasal swabs. Therefore, the gE-CT tyrosine residues Y467 and Y563 together are required for gE CT-mediated anterograde neuronal transport.

Bovine herpesvirus 1 (BHV-1) envelope protein gE subcellular trafficking is contributed by two separate YXXL/Φ motifs within the cytoplasmic tail which together promote efficient virus cell-to-cell spread

Bovine herpesvirus envelope glycoprotein E (gE) and, in particular, the gE cytoplasmic tail (CT) is a virulence determinant in cattle. Also, the gE CT contributes to virus cell-to-cell spread and anterograde neuronal transport. In this study, our goal was to map the gE CT sub-domains that contribute to virus cell-to-cell spread property. A panel of gE-CT specific mutant viruses was constructed and characterized, in vitro, with respect to their plaque phenotypes, gE recycling and gE basolateral membrane targeting. The results revealed that disruption of the tyrosine-based motifs, ₄₆₇YTSL₄₇₀ and ₅₆₃YTVV₅₆₆, individually produced smaller plaque phenotypes than the wild type. However, they were slightly larger than the gE CT-null virus plaques. The Y467A mutation affected the gE endocytosis, gE trans-Golgi network (TGN) recycling, and gE virion incorporation properties. However, the Y563A mutation affected only the gE basolateral cell-surface redistribution function. Notably, the simultaneous Y467A/Y563A mutations produced gE CT-null virus-like plaque phenotypes.

A DNA Vaccine Formulated with Chemical Adjuvant Provides Partial Protection against Bovine Herpes Virus Infection in Cattle

Bovine herpesvirus-1 (BoHV-1) is the causative agent of bovine infectious rhinotracheitis, an important disease worldwide. Although conventional BoHV-1 vaccines, including those based on the use of modified live virus and also inactivated vaccines, are currently used in many countries, they have several disadvantages. DNA vaccines have emerged as an attractive approach since they have the potential to induce both humoral and cellular immune response; nevertheless, it is largely known that potency of naked DNA vaccines is limited. We demonstrated previously, in the murine model, that the use of adjuvants in combination with a DNA vaccine against BoHV-1 is immunologically beneficial. In this study, we evaluate the immune response and protection against challenge elicited in bovines, by a DNA vaccine carrying the sequence of secreted version of glycoprotein D (gD) of BoHV-1 formulated with chemical adjuvants. Bovines were vaccinated with formulations containing the sequence of gD alone or in combination with adjuvants ESSAI 903110 or Montanide™ 1113101PR. After prime vaccination and two boosters, animals were challenged with infectious BoHV-1. Formulations containing adjuvants Montanide™ 1113101PR and ESSAI 903110 were both, capable of increasing humoral immune response against the virus and diminishing clinical symptoms. Nevertheless, only formulations containing adjuvant Montanide™ 1113101PR was capable of improving cellular immune response and diminishing viral excretion. To our knowledge, it is the first time that a BoHV-1 DNA vaccine is combined with adjuvants and tested in cattle. These results could be useful to design a vaccine for the control of bovine rhinotracheitis.

Identification of an epitope within the Bovine herpesvirus 1 glycoprotein E cytoplasmic tail and use of a monoclonal antibody directed against the epitope for the differentiation between vaccinated and infected animals

We constructed a recombinant bovine herpesvirus type 1 triple mutant virus (BoHV-1 tmv) that lacks UL49.5 residues 30-32 and 80-96, gE cytoplasmic tail (gE CT) residues 452-575 and the entire 435 bp long Us9 ORF. To develop a gE CT-specific blocking ELISA test that is necessary to distinguish the BoHV-1 tmv vaccinated calves from the wild-type (wt) virus-infected calves, a mouse monoclonal antibody (mAb) 2H8F3 was generated by using the Escherichia coli expressed gE CT residues 452-575. Further, by performing a PEPSCAN analysis of 12 mer overlapping peptides spanning the entire gE CT, the epitope sequence recognized by the mAb2H8F3 was mapped within the gE CT residues 499SDDDGPASN507. A blocking ELISA test was then developed for detecting antibodies in wild-type BoHV-1 infected calves against the gE CT epitope specified by 499SDDDGPASN507. The assay is based on the use of HRP conjugated mAb2H8F3 and the E. coli expressed gE CT protein as an indicator antibody and a coating antigen, respectively. In this assay, serum from entire gE-deleted and BoHV-1 tmv-infected calves scored negative, whereas serum from calves infected with BoHV-1 wt scored positive. Therefore, the gE CT-ELISA, based on the mAb2H8F3 and E. coli expressed gE CT protein, is suitable for differentiating the wt virus-infected and BoHV-1 tmv-vaccinated cattle.

A glycoprotein E gene-deleted bovine herpesvirus 1 as a candidate vaccine strain

A bovine herpesvirus 1 (BoHV-1) defective in glycoprotein E (gE) was constructed from a Brazilian genital BoHV-1 isolate, by replacing the full gE coding region with the green fluorescent protein (GFP) gene for selection. Upon co-transfection of MDBK cells with genomic viral DNA plus the GFP-bearing gE-deletion plasmid, three fluorescent recombinant clones were obtained out of approximately 5000 viral plaques. Deletion of the gE gene and the presence of the GFP marker in the genome of recombinant viruses were confirmed by PCR. Despite forming smaller plaques, the BoHV-1△gE recombinants replicated in MDBK cells with similar kinetics and to similar titers to that of the parental virus (SV56/90), demonstrating that the gE deletion had no deleterious effects on replication efficacy in vitro. Thirteen calves inoculated intramuscularly with BoHV-1△gE developed virus neutralizing antibodies at day 42 post-infection (titers from 2 to 16), demonstrating the ability of the recombinant to replicate and to induce a serological response in vivo. Furthermore, the serological response induced by recombinant BoHV-1△gE could be differentiated from that induced by wild-type BoHV-1 by the use of an anti-gE antibody ELISA kit. Taken together, these results indicated the potential application of recombinant BoHV-1 △gE in vaccine formulations to prevent the losses caused by BoHV-1 infections while allowing for differentiation of vaccinated from naturally infected animals.

A triple gene mutant of BoHV-1 administered intranasally is significantly more efficacious than a BoHV-1 glycoprotein E-deleted virus against a virulent BoHV-1 challenge

Bovine herpesvirus 1 (BoHV-1) causes respiratory infections and abortions in cattle, and is an important component of bovine respiratory disease complex, which causes a considerable economic loss worldwide. Several efforts have been made to produce safer and more effective vaccines. One of these vaccines is a glycoprotein E (gE)-deleted marker vaccine which is currently mandated for use in EU countries. In the present study, we have constructed a three-gene-mutated BoHV-1 vaccine virus (UL49.5 luminal domain residues 30-32 and cytoplasmic tail residues 80-96 deleted, gE cytoplasmic tail- and entire Us9-deleted) and compared its protective vaccine efficacy in calves after intranasal vaccination with that of a gE-deleted virus. Following vaccination, both the triple mutant and gE-deleted vaccine virus replicated well in the nasal epithelium of the calves. The vaccinated calves did not show any clinical signs. Four weeks post-vaccination, the animals were challenged intranasally with a virulent BoHV-1 wild-type virus. Based on clinical signs, both the gE-deleted and triple mutant group were protected equally against the virulent BoHV-1 challenge. However, based on the quantity and duration of nasal viral shedding, virus neutralizing antibody and cellular immune responses, the triple mutant virus vaccine induced a significantly better protective immune response than the gE-deleted virus vaccine. Notably, after the virulent BoHV-1 challenge, the triple mutant virus vaccinated group cleared the challenge virus three days earlier than the BoHV-1 gE-deleted virus vaccinated group.

Glycoproteins gE and gI are required for efficient KIF1A-dependent anterograde axonal transport of alphaherpesvirus particles in neurons

Alphaherpesviruses, including pseudorabies virus (PRV), spread directionally within the nervous systems of their mammalian hosts. Three viral membrane proteins are required for efficient anterograde-directed spread of infection in neurons, including Us9 and a heterodimer composed of the glycoproteins gE and gI. We previously demonstrated that the kinesin-3 motor KIF1A mediates anterograde-directed transport of viral particles in axons of cultured peripheral nervous system (PNS) neurons. The PRV Us9 protein copurifies with KIF1A, recruiting the motor to transport vesicles, but at least one unidentified additional viral protein is necessary for this interaction. Here we show that gE/gI are required for efficient anterograde transport of viral particles in axons by mediating the interaction between Us9 and KIF1A. In the absence of gE/gI, viral particles containing green fluorescent protein (GFP)-tagged Us9 are assembled in the cell body but are not sorted efficiently into axons. Importantly, we found that gE/gI are necessary for efficient copurification of KIF1A with Us9, especially at early times after infection. We also constructed a PRV recombinant that expresses a functional gE-GFP fusion protein and used affinity purification coupled with mass spectrometry to identify gE-interacting proteins. Several viral and host proteins were found to associate with gE-GFP. Importantly, both gI and Us9, but not KIF1A, copurified with gE-GFP. We propose that gE/gI are required for efficient KIF1A-mediated anterograde transport of viral particles because they indirectly facilitate or stabilize the interaction between Us9 and KIF1A.

A recombinant bovine herpesvirus 5 defective in thymidine kinase and glycoprotein E is attenuated and immunogenic for calves

Bovine herpesvirus 5 (BoHV-5) is an important pathogen of cattle in South America and efforts have been made to produce safer and more effective vaccines. In addition to afford protection, herpesvirus vaccines should allow serological differentiation of vaccinated from naturally, latently infected animals. We previously reported the construction and characterization in vitro of a double mutant BoHV-5 (BoHV-5gE/TK Δ) lacking the genes encoding thymidine kinase (tk) for attenuation, and glycoprotein E (gE) as the antigenic marker, as a vaccine candidate strain (Brum et al. 2010a). The present article reports an investigation on the attenuation and immunogenicity of this recombinant in calves. In a first experiment, 80 to 90-day-old seronegative calves (n=6) inoculated intranasally with the recombinant (titer of 10(7.5)TCID50) shed virus in low to moderate titers in nasal secretions for up to 6 days, yet did not develop any respiratory, systemic or neurological signs of infection. At day 30 post-infection (pi) all calves had BoHV-5 specific neutralizing (VN) antibodies in titers of 4 to 8 and were negative for anti-gE antibodies in a commercial ELISA test. Administration of dexamethasone (0.1mg/kg/day during 5 days) to four of these calves at day 42 pi did not result in virus shedding or increase in VN titers, indicating lack of viral reactivation. Secondly, a group of 8-month-old calves (n=9) vaccinated intramuscularly (IM) with the recombinant virus (10(7.5)TCID50/animal) did not shed virus in nasal secretions, remained healthy and developed VN titers from 2 to 8 at day 42 post-vaccination (pv), remaining negative for gE antibodies. Lastly, 21 calves (around 10 months old) maintained under field conditions were vaccinated IM with the recombinant virus (titer of 10(7.3)TCID50). All vaccinated animals developed VN titers from 2 to 16 at day 30 pv. A boost vaccination performed at day 240 pv resulted in a rapid and strong anamnestic antibody response, with VN titers reaching from 16 to 256 at day 14 post-booster. Again, serum samples remained negative for gE antibodies. Selected serum samples from vaccinated animals showed a broad VN activity against nine BoHV-5 and eight BoHV-1 field isolates. These results show that the recombinant virus is attenuated, immunogenic for calves and induces an antibody response differentiable from that induced by natural infection. Thus, the recombinant BoHV-5gE/TKΔ is an adequate candidate strain for a modified live vaccine.

A bovine herpesvirus type 1 mutant virus with truncated glycoprotein E cytoplasmic tail has defective anterograde neuronal transport in rabbit dorsal root ganglia primary neuronal cultures in a microfluidic chamber system

Bovine herpesvirus type 1 (BHV-1) is an important component of the bovine respiratory disease complex (BRDC) in cattle. Following primary intranasal and ocular infection of cattle, BHV-1 establishes lifelong latent infection in trigeminal ganglia (TG). Upon reactivation from latency, the virus is transported from neuronal cell bodies in the TG to projected nerve endings in nose and cornea of latently infected cattle where the virus shedding occurs. This property of BHV-1 plays a significant role in the pathogenesis of BRDC and maintenance of BHV-1 in the cattle population. Recently, we have reported that a glycoprotein E (gE) cytoplasmic tail-truncated BHV-1 (BHV-1 gEAm453) did not reactivate from latency and was not shed in the nasal and ocular secretions of calves and rabbits. Here we describe the methods to establish rabbit primary dorsal root ganglia (DRG) neuron cultures in a microfluidic chamber system and to characterize in vitro anterograde and retrograde axonal transport properties of BHV-1 gE-deleted and BHV-1 cytoplasmic tail-truncated gEAm453 mutant viruses relative to BHV-1 gEAm453-rescued/wild-type viruses. The results clearly demonstrated that whereas the BHV-1 gE-deleted, BHV-1 gEAm453, and BHV-1 gEAm453-rescued/wild-type viruses were transported equally efficiently in the retrograde direction, only the BHV-1 gEAm453-rescued/wild-type virus was transported anterogradely. Therefore, we have concluded that sequences within the BHV-1 gE cytoplasmic tail are essential for anterograde axonal transport and that primary rabbit DRG neuronal cultures in the microfluidic chambers are suitable for BHV-1 neuronal transport studies.

A bovine herpesvirus 5 recombinant defective in the thymidine kinase (TK) gene and a double mutant lacking TK and the glycoprotein E gene are fully attenuated for rabbits

Bovine herpesvirus 5 (BoHV-5), the agent of herpetic meningoencephalitis in cattle, is an important pathogen of cattle in South America and several efforts have been made to produce safer and more effective vaccines. In the present study, we investigated in rabbits the virulence of three recombinant viruses constructed from a neurovirulent Brazilian BoHV-5 strain (SV507/99). The recombinants are defective in glycoprotein E (BoHV-5gEDelta), thymidine kinase (BoHV-5TKDelta) and both proteins (BoHV-5gEDeltaTKDelta). Rabbits inoculated with the parental virus (N = 8) developed neurological disease and died or were euthanized in extremis between days 7 and 13 post-infection (pi). Infectivity was detected in several areas of their brains. Three of 8 rabbits inoculated with the recombinant BoHV-5gEDelta developed neurological signs between days 10 and 15 pi and were also euthanized. A more restricted virus distribution was detected in the brain of these animals. Rabbits inoculated with the recombinants BoHV-5TKDelta (N = 8) or BoHV-5gEDeltaTKDelta (N = 8) remained healthy throughout the experiment in spite of variable levels of virus replication in the nose. Dexamethasone (Dx) administration to rabbits inoculated with the three recombinants at day 42 pi did not result in viral reactivation, as demonstrated by absence of virus shedding and/or increase in virus neutralizing titers. Nevertheless, viral DNA was detected in the trigeminal ganglia or olfactory bulbs of all animals at day 28 post-Dx, demonstrating they were latently infected. These results show that recombinants BoHV-5TKDelta and BoHV-5gEDeltaTKDelta are attenuated for rabbits and constitute potential vaccine candidates upon the confirmation of this phenotype in cattle.