A human macrophage – hepatocyte co-culture model for comparative studies of infection and replication of Francisella tularensis LVS strain and subspecies holarctica and mediasiatica

Knut Rennert, Peter Otto, Harald Funke, Otmar Huber, Herbert Tomaso, Alexander S. Mosig

Francisella tularensis, a gram-negative bacterium replicates intracellularly within macrophages and efficiently evades the innate immune response. It is able to infect and replicate within Kupffer cells, specialized tissue macrophages of the liver, and to modulate the immune response upon infection to its own advantage. Studies on Francisella tularensis liver infection were mostly performed in animal models and difficult to extrapolate to the human situation, since human infections and clinical observations are rare. Using a human co-culture model of macrophages and hepatocytes we investigated the course of infection of three Francisella tularensis strains (subspecies holarctica - wildtype and live vaccine strain, and mediasiatica - wildtype) and analyzed the immune response triggered upon infection. We observed that hepatocytes support the intracellular replication of Franciscella species in macrophages accompanied by a specific immune response inducing TNFα, IL-1β, IL-6 and fractalkine (CX3CL1) secretion and the induction of apoptosis. We could demonstrate that this human macrophage / hepatocyte co-culture model reflects strain-specific virulence of Francisella tularensis. We developed a suitable tool for more detailed in vitro studies on the immune response upon liver cell infection by F. tularensis.