Innate and adaptive immunity to Francisella

Studies of immune responses to Francisella have been conducted for well over 50 years. Here, the basic parameters of innate and adaptive immune responses to Francisella are reviewed, with an emphasis on those that may contribute directly to protection against infection. Although older literature provides a wealth of information on human immune responses to infection and vaccination, most recent information has been derived largely from studies in animals and using animal cells, particularly mice. In experimental animals, activation of macrophages, a major and probably preferred host cell for Francisella, appears central to control of infection. Thus, in animal models and in vitro studies using mouse macrophages, cytokines such as IFN-gamma and TNF-alpha, derived first from both nonspecific cells such as natural killer cells and later from Francisella-specific T cells, collaborate to effect intracellular killing. In mice, these intracellular killing mechanisms include reactive nitrogen and oxygen species, but killing mechanisms remain to be identified in humans. Ultimately both CD4(+) and CD8(+) T cells develop into Francisella-specific memory cells and are important for control of primary Francisella infection or vaccination-induced protection. The effector mechanisms invoked by either CD4+ or CD8+ T cells, beyond production of IFN-gamma and TNF-alpha, are the subject of ongoing studies. Both specific antibodies and B cells may contribute to control of primary infection or vaccination-induced protection in some circumstances, particularly against lower virulence Francisella strains. Thus a number of known proinflammatory and Th-1 T cell related components of the immune system combat this virulent bacterium; no doubt others remain to be discovered.