Bacteria of the genus Brucella are the etiologic agents of brucellosis, a worldwide zoonosis that is highly transmissible to humans. Due to its high infectivity through inhalation, brucellae are included in the CDC Category B list of Select Agents. Virulence of this pathogen mostly depends upon its ability to survive and replicate within macrophages of the infected host. Following phagocytosis, intracellular brucellae reside in a membrane-bound vacuole, the Brucella-containing vacuole (BCV), that progressively mature over several hours into a replicative organelle derived from the host endoplasmic reticulum (ER) where the bacteria proliferate. The VirB type IV secretion system, a major determinant of Brucella virulence that is induced intracellularly, is required for the conversion of the BCV into a replicative organelle, likely through the translocation of effector molecules into the macrophage that modulate host functions. Yet, very few effectors have been identified, impairing advances in the understanding of Brucella molecular pathogenesis. We have recently shown that BCVs mature along the endocytic pathway and fuse in a limited manner with terminal lysosomes. These events provide intravacuolar cues necessary to the expression of the VirB Type IV secretion system, and consequently, biogenesis of the replicative organelle (Starr et al., 2008 Traffic 9(5): 678). In 2009-2010, we have further examined late events in the Brucella intracellular cycle in the context of bacterial egress following intracellular proliferation. We have found that Brucella proliferation in the ER is followed by bacterial translocation into endosomal organelles with ultrastructural features of autophagy. Surprisingly, this process was Atg5- and LC3-independent in both primary macrophages and HeLa cells, but required the autophagy proteins Beclin 1 and Ulk1. Moreover, formation of these endosomal vacuoles was important for completion of the Brucella intracellular cycle and reinfection events. These results demonstrate a novel membrane trafficking process between the ER and endosomes that involves unconventional autophagy and that is associated with bacterial egress and reinfection.
