Osteoarticular brucellosis is the most common complication in Brucella-infected humans. The mechanism of bone destruction caused by Brucella remains partially unknown due to the lack of a suitable animal model. Of greater concern is the fact that development of vaccines to protect humans against brucellosis is hampered by safety concerns and no effective models exist to assess the safety of candidate vaccines in the context of osteoarticular disease. To address this issue, we explored the suitability of the NOD-scid IL2r?null (NSG) mouse and examined the potential use of this strain to evaluate the safety of live attenuated vaccine candidates. Mice were inoculated intraperitoneally with B. abortus S19 or the vaccine candidate B. abortus S19?vjbR. Hypothermia, weight loss, splenomegaly, and tail deformation were observed in mice inoculated with B. abortus S19 but not with S19?vjbR. Histologically, all S19 but not S19?vjbR inoculated mice exhibited severe dose-dependent osteomyelitis in the tail characterized by large numbers of neutrophils, macrophages, and osteoclasts with marked bone destruction. Interestingly, myriad bacteria were observed within osteoclasts of S19-infected mice. To further investigate the role of osteoclasts during Brucella infection, murine bone marrow-derived macrophages were derived into mature osteoclasts and infected with B. abortus 2308, S19, and attenuated mutants S19?vjbR and B. abortus2308?virB2. While B. abortus 2308 and S19 replicated inside mature osteoclasts, the attenuated mutants were progressively killed. Interestingly, B. abortus 2308 impaired the growth of osteoclasts without reducing resorptive activity while osteoclasts infected with B. abortus S19 and S19?vjbR were significantly larger and exhibited enhanced resorption. None of the Brucella strains induced apoptosis or stimulated nitric oxide or lactose dehydrogenase production in osteoclasts. Finally, infection of macrophages or osteoclast precursors with B. abortus 2308 resulted in generation of smaller osteoclasts with decreased resorptive activity. Overall, Brucella exhibits similar growth characteristics in mature osteoclasts compared to the primary target cell, the macrophage, but is able to impair the maturation and alter the resorptive capacity of these cells. These results suggest that osteoclasts play an important role in osteoarticular brucellosis and could serve as a useful in vitro model for both analyzing host-pathogen interactions and assessing vaccine safety.
