Construção e caracterização de recombinante do herpesvírus bovino tipo 1 com deleção da glicoproteína E para uso em vacina

Abstract

Vaccines with antigenic markers also known as differential vaccines have been largely used for control and prevention of bovine herpesvirus 1 (BoHV-1) infection. With this purpose, a Brazilian BoHV-1 isolate (SV56/90) was submitted to deletion of the glycoprotein E (gE) gene for a potential use in vaccines. BoHV-1 gE gene deletion was performed by homologous recombination, being the gE gene replaced by 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 (and nominated as BoHV-1ΔgE). The recombinant viruses formed smaller plaques in MDBK cells yet with similar kinetics and grew to similar titers to those of the parental virus, showing that gE deletion had no deleterious effect on the replication efficiency in vitro. Thirteen calves inoculated intramuscularly (IM) with the recombinant BoHV-1ΔgE developed virus neutralizing (VN) antibodies at day 42pi (titers from 2 to 16), demonstrating his ability to replicate and to induce a serological response in vivo. Furthermore, the serological response induced by the recombinant virus could be differentiated from that induced by wild-type BoHV-1 by the use of an anti-gE antibody ELISA kit. Experiments to determine the safety, immunogenicity and protection were performed with the BoHV-1ΔgE candidate vaccine strain. In the safety test, five three months-old calves were inoculated with approximately 10-100 times the usual vaccine dose (108.5TCID50 per animal). The inoculated animals remained healthy and did not shed virus, confirmed by the absence of virus in nasal secretions and lack of seroconversion by sentinel calves kept in contact. In addition, the recombinant virus was not shed upon dexamethasone administration (at day 42pi) showing the inability of reactivation and/or shedding after attempts of reactivation of latent infection. In the immunogenicity test, calves (8 to 10 months-old) were vaccinated once IM (group I, n=8) or subcutaneously (group II, n=9) with live BoHV-1ΔgE or twice (30 days apart) with inactivated virus plus aluminum hydroxide (group IV, n=13) or MontanideTM Gel 1 (Seppic - group V, n=14). As controls, three animals (group III) were vaccinated once IM with the parental virus. All calves vaccinated with live virus developed VN titers of 2 to 8 (group I, GMT: 2; group II, GMT: 1.65; group III, GMT: 1.65) at day 42pv. Animals of groups IV and V developed VN titers of 2 to 16 (GMT: 2.45) and 2 to 128 (GMT: 3.9), respectively. All calves vaccinated with the BoHV-1ΔgE remained negative in the gE ELISA. In a vaccination-challenge experiment, six calves (three to four-months-old) were vaccinated with live virus (107.3TCID50/animal) and four calves were kept as controls. Forty-seven days after vaccination, the calves were challenged with a heterologous BoHV-1 strain (107.5TCID50/animal) by the intranasal route. Vaccinated animals developed only mild and transient nasal signs comparing with the control calves. Virus shedding by vaccinated animals was also significantly reduced compared to controls. These results demonstrate that the recombinant BoHV-1ΔgE is safe/attenuated, immunogenic for calves both in a live or inactivated, adjuvanted vaccine formulation. Moreover, it induces a humoral response that can be distinguished from that induced by the wild type virus. Thus, the recombinant BoHV-1ΔgE presents suitable properties to be used in vaccine formulations.