Francisella tularensis, the causative agent for tularemia, can infect humans by a number of routes, including vector-borne transmission. However, it is inhalation of the bacterium, and the resulting pneumonic tularemia, that represents the most dangerous form of disease. This is due to the short incubation time (3-5 days), non-specific symptoms, and a high mortality rate (greater than 80%) in untreated individuals. Furthermore, F. tularensis has been weaponized by both the United States and the former Soviet Union making it a viable candidate for use as a biological weapon. Despite over 80 years of research on F. tularensis around the world, very little is understood about the dynamic interaction of this bacterium with the host, especially following aerosol infection. My laboratory has established that, similarly to murine cells, human dendritic cells and macrophages are acutely susceptible to infection with F. tularensis, but fail to produce pro-inflammatory cytokines or undergo maturation. Further, virulent F. tularensis actively interferes with the ability of human DC and macrophages to respond to secondary stimuli. Understanding the mechanism by which F. tularensis actively suppresses DC and macrophage function is a central directive of my laboratory. We are tackling this directive in two different ways. First, we are analyzing the role Francisella lipids play in mediating anti-inflammatory responses. Structures present on the surface of bacteria are the first components encountered by the host cell. Thus, it is possible that, in the context of F. tularensis infections, these structures contribute to the early, rapid suppression of human dendritic cells. Bacterial lipids represent one such structure. We recently published that lipids isolated from virulent, but not attenuated F. tularensis, potently suppress inflammatory responses in human dendritic cells and macrophages. Further, we identified the specific signal transduction proteins modulated by Francisella lipids. We are currently identifying the specific lipid(s) responsible for this suppression and the mechanism by which they interfere with human dendritic cell functions. Second, we are exploring the role of carbohydrates associated with the outer surface of F. tularensis in directing immunosuppressive programs in human cells. The major outer surface carbohydrate structure of F. tularensis is the O-Antigen (O-Ag) associated with LPS. Typically O-Ag is thought to simply cover up proteins present on the bacterial surface that could stimulate an inflammatory response. However, we have preliminary data which demonstrates that the O-Ag directly inhibits pro-inflammatory responses in human cells. Utilizing mutants with specific defects in O-Ag synthesis, we are currently identifying the specific receptors and host signaling pathways modulated by F. tularensis O-Ag to initiate an anti-inflammatory program.
