Immunization with Brucella VirB proteins elicits a protective Th1 immune response in mice and 1 a similar immune response in dogs

Abstract

VirB proteins from Brucella spp. constitute the Type IV secretion system, a key virulence factor mediating the intracellular survival of these bacteria. Here we assessed whether a Th1-type immune response against VirB proteins may protect mice from Brucella infection, and whether such response can be induced in the dog, a natural host for Brucella. Splenocytes from mice immunized with VirB7 or VirB9 responded to their respective antigens with a significant and specific production of IFN-γ, whereas IL-4 was not detected. Thirty days after intraperitoneal challenge with live B. abortus the spleen load of bacteria was almost one log lower in mice immunized with VirB proteins than in unvaccinated animals. As protection seemed to correlate with a Th1-type immune response against VirB proteins, we decided to assess whether such response could be elicited in the dog. PBMCs from dogs immunized with VirB proteins (three subcutaneous doses in QuilA adjuvant) produced significantly higher leves of IFN-γ than cells from control animals upon in vitro stimulation with VirB proteins. A skin test to assess specific delayed-type hypersensitivity was positive in 4 out of 5 dogs immunized with either VirB7 or VirB9. As both proteins are predicted to locate in the outer membrane of Brucella, the ability of anti-VirB antibodies to mediate complement-dependent bacteriolysis of B. canis was assessed in vitro. Sera from dogs immunized with either VirB7 or VirB9, but not from those receiving PBS, produced a significant bacteriolysis. These results suggest that VirB-specific responses potentially protective against B. canis infection can be elicited in dogs.