Expression of functional human C1 inhibitor in COS cells.

Full length human C1 inhibitor cDNA was cloned into a vector suitable for transient expression in COS-1 cells. Transfected COS cells secreted an immunoreactive protein of Mr approximately 110,000 that appeared to be functionally equivalent to the plasma-derived protein as established by the following criteria: 1) ability to form sodium dodecyl sulfate-stable complexes with C1s, factor XIIa, and kallikrein; 2) inhibition of C1s-mediated C4 consumption; and 3) susceptibility to inactivation by the nontarget proteinase elastase. Quantitation of secreted recombinant C1 inhibitor by radioimmunoassay indicated that 72 h after transfection the level was approximately 2.2 micrograms/ml. Treatment of transfected cells with tunicamycin resulted in secretion of a protein of Mr approximately 90,000 that was also capable of complex formation with C1s.