Persistent articular infection and host reactive response contribute to <i>Brucella</i> -induced spondyloarthritis in SKG mice

Brucellosis, one of the most prevalent zoonotic diseases worldwide, often results in osteoarticular complications including large joint and axial arthritis mimicking spondyloarthritis. To model this chronic manifestation, we infected autoimmunity-prone SKG mice containing a mutation in the T cell adaptor ZAP-70 with <i>Brucella</i> species. <i>B. melitensis</i> infection resulted in a fully penetrant, readily scoreable disease involving large joint wrist and foot arthritis, peri-ocular inflammation, and less frequent scaly paw rash. Infection with <i>B. abortus</i> resulted in similar manifestations, although with delayed arthritis onset, and <i>B. neotomae</i> revealed sex differences, with a more severe disease in females. Heat-killed <i>Brucella</i> did not induce arthritis, evincing a requirement for viable infection. Across species, splenic CFU correlated well with final clinical score at 12 weeks (ρ = 0.79 and <i>P</i> < 0.001). Moreover, viable <i>Brucella</i> was recovered from the paws at 12 weeks, consistent with persistent articular infection. Mice infected with a <i>Brucella</i>Δ<i>tcpB</i> mutant lacking a Type IV secretion system-dependent mediator displayed an intermediate phenotype without significant differences in splenic CFU. Thus, the degree of arthritis did not strictly correlate with the degree of systemic infection, but it suggested an additional reactive component. Together, these data suggest that <i>Brucella</i>-induced spondyloarthritis reflects both persistent colonization and excess host reactivity. Moreover, the sensitivity of the SKG model to different species and mutants will provide new opportunities for dissecting correlates of <i>Brucella</i> virulence and host immunity.IMPORTANCEBrucellosis, a bacterial infection acquired from herd animals, remains one of the most common zoonotic diseases worldwide. Chronic infection often results in spondyloarthritis-like complications. Investigation into pathogenesis has been limited by the lack of overt disease in standard laboratory mice. We addressed this issue using spondyloarthritis-susceptible SKG mice. Upon infection with <i>B. melitensis</i>, SKG mice develop robust, fully penetrant large joint arthritis. Arthritis development required viable bacteria, and live <i>Brucella</i> persisted in paw tissue out to 12 weeks. Disease onset, severity, and manifestations varied upon infection with different <i>Brucella</i> species and mutants, suggesting an additional immune reactive component. Together, these results suggest that this new model will be very useful to the scientific community for determining correlates of bacterial virulence leading to clinical disease.