Protection Provided by an Encapsulated Live Attenuated ΔabcBAStrain of Brucella ovis against Experimental Challenge in a Murine Model

Abstract

dThis study aimed to evaluate the Brucella ovis ⌬abcBA strain as a vaccine candidate in the murine model. BALB/c mice were subcutaneously or intraperitoneally immunized with a single dose or three doses of the B. ovis ⌬abcBA strain and then were challenged with wild-type B. ovis. Single or multiple immunizations provided only mild protection, with significantly smaller numbers of wild-type B. ovis CFU in the livers of immunized mice but not in the spleens. Encapsulation of B. ovis ⌬abcBA significantly improved protection against experimental challenges in both BALB/c and C57BL/6 mice. Furthermore, immunization with encapsulated B. ovis ⌬abcBA markedly prevented lesions in the spleens and livers of experimentally challenged mice. These results demonstrated that the encapsulated B. ovis ⌬abcBA strain confers protection to mice; therefore, this strain has potential as a vaccine candidate for rams. Brucella ovis is a facultative, intracellular, Gram-negative coccobacillus (1) that causes economic losses due to its potential to induce reproductive failure in sheep (2, 3). The most important lesions caused by B. ovis in rams are epididymitis and seminal vesiculitis (4), and B. ovis occasionally causes abortion in ewes (5, 6).In spite of attempts to develop safe, effective vaccines (7-10), there is no available vaccine specifically against B. ovis. The ideal vaccine against B. ovis must prevent infection and clinical signs or at least reduce the risk of infection and transmission (11). Rev-1, a live attenuated Brucella melitensis strain, is the most commonly used vaccine against B. melitensis, and it also protects against B. ovis (12). However, the Rev-1 strain retains pathogenic potential, so it is capable of infecting humans. It may cause abortions in ruminants, it may interfere with serological tests, and it is resistant to streptomycin (13,14).Mice have been extensively used as a model for brucellosis, which is very important due to the limitations of experimentation with larger animals (15-18). The murine model has allowed studies for the development of safe, effective brucellosis vaccines (15,19,20).Inactivation of a predicted abcEDCBA-encoded ATP-binding cassette (ABC) transporter of B. ovis (21), which is located in B. ovis pathogenicity island 1 (BOPI-1), results in strong attenuation of this pathogen in the murine model (22). Although the substrate of this ABC transporter is not known, it is required for expression of B. ovis virulence proteins, including Omp31, Sod, and the virB-encoded type IV secretion system (23), which is required for B. ovis intracellular survival and in vivo persistence (17). Furthermore, a B. ovis ⌬abcBA strain is capable of triggering humoral and cellular immune responses like the wild-type strain in rams; importantly, however, the mutant strain is not excreted in semen and urine (24). Therefore, this mutant strain has great potential as an attenuated vaccine strain. However, considering that the ⌬abcBA mutant strain is attenuated at very early stages during infection, resulting in p...

Meta-Analysis and Advancement of Brucellosis Vaccinology

“…ovis attenuated mutant vaccine candidate strain that lacks an ABC transporter [36], which influences the virB -encoded Type IV secretion system [146] thus interfering with intracellular trafficking [147]. This vaccine strain provided only moderate protection in the mouse model, yielding a protection index of approximately 1.0 [120], whereas it surprisingly provided a very strong protection against experimental challenge in rams, preventing shedding of the wild type strain in the semen and urine, accumulation of inflammatory cells in the semen, and gross or microscopic lesions induced by wild type B . ovis , resulting in sterile immunity under experimental conditions [148].…”

“…A limiting factor for these vaccines is the fact that some of these mutants have poor persistence in the host, which may not allow enough time for exposure of the vaccine strain to the immune system, thus preventing appropriate levels of protection [152–154]. However, delivery systems that promote a slow delivery of the vaccine strain may overcome this limitation [120,148]. The mutagenesis in these cases usually targets genes that are required for virulence or survival in the host [93,153,155,156].…”

Pathogenic potential of Brucella ovis field isolates with different genotypic profile and protection provided by the vaccine strain B. ovis ΔabcBA against B. ovis field isolates in mice

“…Bacterial strains. As detailed in Table 1, this study included seven field Brucella ovis strains isolated from semen of naturally infected rams, which have been previously genotypically characterized by MLVA-16 (Dorneles et al 2014), the reference wild-type strain B. ovis ATCC 25840, and the candidate vaccine strain B. ovis ΔabcBA (Silva et al 2011b(Silva et al , 2015a(Silva et al , 2015b. Bacteria were grown on the tryptic soy agar (TSA) plates supplemented with 1% hemoglobin, for 3 days at 37°C with 5% CO 2 .…”

“…Vaccine experiments. Encapsulation of the B. ovis ΔabcBA vaccine strain was performed as previously described (Silva et al 2015a). Briefly, 2 x 10 10 CFU of B. ovis ΔabcBA were resuspended in 2mL of 1% alginate solution (Sigma-Aldrich) and dripped in 10mL of polymerization solution (0.5 mM CaCl 2 ), using a 0.23mm x 4mm needle, followed by homogenization for 15 minutes.…”

“…In spite of its attenuation, B. ovis ΔabcBA triggers humoral and cellular immune responses in rams that are indistinguishable from those triggered by the wild-type parental strain (Silva et al 2013). Therefore, B. ovis ΔabcBA has been tested as an experimental candidate vaccine strain and provided protection in a mouse model of infection (Silva et al 2015a). Furthermore, when tested in the natural host, this vaccine strain prevented any clinical sign of disease, macro-and microscopic lesions, and induced sterile immunity in experimentally challenged rams (Silva et al 2015b).…”