Biosynthesis of the Third and Fifth Complement Components by Isolated Human Lung Cells

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Abstract
Increasing research efforts have been directed at determining the contribution of locally synthesized (cell-derived) complement in host defense and inflammation. In the studies presented here, we determined the ability of a continuous cell line of type II pneumocytes (A549) and a cell line of human lung fibroblasts (WI-38) to produce complement components in vitro. Complement biosynthesis by A549 pneumocytes and WI-38 fibroblasts was demonstrated by incorporation of [35S]methionine into immunoprecipitable complement proteins. Using this technique, A549 pneumocytes were demonstrated to synthesize Clr, Cls, C4, C3, C5, C6, C7, C8, C9, Factor B, Factor H, Factor I and C1s inactivator. In comparison, WI-38 fibroblasts were shown to synthesize Cls, C4, C3, C5, C6, C8, and C9. Because previous work has demonstrated the central role of C3, C5, and their activation products in regulating lung inflammation and tissue injury, we further investigated the production of C3 and C5 by both lung pneumocytes and fibroblasts using enzyme-linked immunospecific assays. A549 cells cultured in the presence of 15% fetal bovine serum (FBS) produced antigenic C3 (135 ng C3/ml/24 h) at a greater rate than did identical cells maintained in serum-free culture conditions (70 ng C3/ml/24 h). Similarly, antigenic C5 production by A549 pneumocytes was greatest in the presence of FBS when compared with cells maintained in serum-free culture conditions (245 ng C5/ml/24 h versus 155 ng C5/ml/24 h). Additionally, using a sensitive hemolytic titration assay, the C5 protein produced by these cells was found to be functional, and the rate of functional C5 production by A549 pneumocytes cultured in the presence of FBS was approximately twice the rate determined for cells cultured in serum-free medium (265 ng C5/ml/24 h versus 135 ng C5/ml/24 h). In contrast to the A549 pneumocytes, the WI-38 fibroblasts cultured in the absence of FBS produced antigenic C3 and C5 at 40% the rate determined for A549 pneumocytes (28 ng C3/ml/24 h and 60 ng C5/ml/24 h). The addition of 15% FBS to the WI-38 fibroblast cultures increased the rate of C3 and C5 production by approximately 30%. In summary, our findings demonstrate that lung cells such as human type II pneumocytes and fibroblasts synthesize and secrete a number of complement components including C3 and C5 in vitro, and these data suggest the potential for this to occur in vivo.