Complement-mediated host defense in the lung

Complement is a system of plasma proteins that aids in the elimination of p athogens from the body. We hypothesized that there is a functional compleme nt system present in the lung that aids in the removal of pathogens. Wester n blot analysis revealed complement proteins of the alternative and classic al pathways of complement in bronchoalveolar lavage fluids (BALF) from heal thy volunteers. Functional classical pathway activity was detected in human BALF, but there was no significant alternative pathway activity in lavage fluid, a finding that correlates with the low level of the alternative path way protein, factor B, in these samples. Although the classical pathway of complement was functional in lavage fluid, the level of the classical pathw ay activator C1q was very low. We tested the ability of the lung-specific s urfactant proteins, surfactant protein A (SP-A) and surfactant protein D (S P-D), to substitute for C1q in classical pathway activation, since they hav e structural homology to C1q. However, neither SP-A nor SP-D restored class ical pathway activity to C1q-depleted serum. These data suggest that the cl assical pathway of complement is functionally active in the lung where it m ay play a role in the recognition and clearance of bacteria.