VirB type IV secretory system does not contribute to Brucella suis' avoidance of human dendritic cell maturation

Dendritic cells (DCs), which are critical components of adaptive immunity, are highly susceptible to infection with the intracellular bacteria Brucella. Infection with living Brucella prevents infected human DCs from engaging in maturation processes, thus impairing their capacity to present antigens to naive T cells and to secrete IL-12. Recently, we have established that several attenuated mutants of Brucella (rough, omp25, bvrR) are unable to control DCs maturation and thus effectively stimulate naive T cells, which could be the origin of the protective immunity elicited by these mutants in vivo. In this study, we investigate the interactions of a VirB-defective Brucella mutant with human DCs to determine whether its attenuation could be attributed to the induction of an adaptive immune response. We show here that in contrast to previously studied strains and similar to wild-type strains, this virB mutant was unable to trigger significant DC maturation. Together with recently published data describing infection with virB mutants in vivo, these results suggest that Brucella T4SS VirB is not involved in the control of DC maturation and does not interfere with the establishment of a T-helper type 1 adaptive immune response.