Innate immune defense against pneumococcal pneumonia requires pulmonary complement component C3.

Complement is known to be involved in protection against systemic infection with Streptococcus pneumoniae. However, less is known about effects of complement within the lungs during pneumococcal pneumonia. By intranasally infecting transgenic mice unable to express complement C3, we investigated the role of complement in pulmonary defenses against S. pneumoniae. It was demonstrated that within the lungs, there is a requirement for C3 during the initial hours of infection. It was found that within 1 h of infection, bacterial loads decreased within lung airways of control mice as C3 protein increased. The lack of C3 resulted in the inability to control growth of wild-type or attenuated pneumococci within the lungs and bloodstream, resulting in an overwhelming inflammatory response and shorter survival times. Our results show that during the initial hours of infection with S. pneumoniae, C3 is protective within the lungs and subsequently plays an important role systemically.