Brucella induces an unfolded protein response via TcpB in macrophages

Abstract Brucella melitensis is a facultative intracellular bacterium that invades and replicates within macrophages and dendritic cells. With greater than 500,000 new infections per year, brucellosis is the most prevalent zoonosis worldwide. Currently, a safe and effective human vaccine does not exist and Brucella’s intracellular replicative niche renders the organism resistant to antibiotics. A better understanding of host-pathogen interactions supporting Brucella replication is necessary for vaccine and therapeutic development. Brucella fuses with the endoplasmic reticulum (ER) in order to replicate and may provoke a conserved ER stress response called the Unfolded Protein Response (UPR). B. melitensis infection in RAW 264.7 murine macrophages and bone marrow-derived macrophages upregulated the UPR target genes BiP, CHOP, and ERdj4, and induced XBP1 mRNA splicing. Upregulation of CHOP and ERdj4 was MyD88-independent whereas XBP1 splicing was MyD88-dependent. A Brucella TcpB mutant showed significantly reduced expression of BiP, CHOP, and ERdj4. Purified TcpB, a microtubule-modulating protein, induced UPR target genes. Also, purified TcpB caused ER restructuring similar to infection with B. melitensis. Finally, Brucella replication was significantly impaired by tauroursodeoxycholic acid, a pharmacologic UPR inhibitor. These results suggest that Brucella induces the UPR to enable its intracellular replication and thus provides a novel therapeutic target for brucellosis treatment.