The complement system participates in the local immune system of the lung. In this study, we investigated the secretion of complement components of the alternative pathway (C3 and factor B) in the alveolar type II epithelial cell line A549. The levels of C3 and factor B protein in the culture medium were determined by enzyme-linked immunosorbent assay (ELISA). The C3 and factor B mRNA expression was assessed by reverse transcription polymerase chain reaction (RT-PCR). The addition of interleukin (IL)-1beta or tumor necrosis factor (TNF)-alpha induced a dose- and time-dependent increase in C3 and factor B secretion. The addition of IL-6 or interferon (IFN)-gamma also induced a weak but significant increase in C3 and factor B secretion. These responses at the protein level were also observed at the mRNA level. Furthermore, the combination of IL-1beta plus TNF-alpha induced a marked increase in C3 and factor B secretion. Similarly, IL-6 and IFN-gamma potently enhanced IL-1beta- or TNF-alpha-induced C3 and factor B secretion, respectively. In this study, we demonstrated C3 and factor B secretion in A549 cells, and showed that the proinflammatory cytokines, IL-1beta, IL-6, TNF-alpha and IFN-gamma, acted as potent inducers of C3 and factor B secretion. It is likely that alveolar type II epithelial cells are the local sites of complement biosynthesis, and that various cytokines act as regulators of this local immune protection system.

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