Pathogenic Rickettsia are obligate intracellular bacteria and the etiologic agents of many life-threatening
infectious diseases. Due to the serious nature of these
infections, it is imperative to both identify the responsive immune sensory pathways and understand the associated immune mechanisms that restrict Rickettsia proliferation. Previous studies have demonstrated that the mammalian
complement system is both activated during
Rickettsia infection and contributes to the immune response to
infection. To further define this component of the mammalian anti-Rickettsia immune response, we sought to identify the mechanism(s) of complement activation during
Rickettsia infection. We have employed a series of in vitro and in vivo models of
infection to investigate the role of the classical complement activation pathway during
Rickettsia infection. Depletion or elimination of
complement activity demonstrates that both C1q and pre-existing
IgM contribute to complement activation; thus implicating the classical
complement system in Rickettsia-mediated complement activation. Elimination of the classical complement pathway from mice increases susceptibility to R. australis
infection with both increased bacterial loads in multiple tissues and decreased immune activation markers. This study highlights the role of the classical complement pathway in immunity against Rickettsia and implicates resident Rickettsia-responsive
IgM in the response to
infection.