Evidence that the human monocyte-like cell line U937 synthesizes functional C1̄-INA

Human C1̄ inhibitor (CI-INA) is a plasma protease inhibitor that inhibits several serine proteases including C1̄r, C1̄s, plasmin, Factor XIa, active Hageman Factor (XIIa), and kallikrein. In addition C1β-INA plays an important homeostatic role in the complement cascade by preventing spontaneous dissociation and autocatalytic activation of macromolecular Cl. While the bulk of plasma Cl-INA is most likely produced by hepatocytes, it has been shown recently that stimulated human monocytes can synthesize Cl-INA in vitro. In this report evidence is presented which demonstrates that the U937 cell line synthesizes C1̄-INA. This was shown in several ways including: 1) incorporation of tritiated amino acids into antigenic C1̄-INA, followed by immunoprecipitation, and detection by fluorography, 2) a sensitive ELISA assay which allowed quantitation of antigenic C1β-INA in cell lysates, 3) a hemolytic assay, in which ability of U937 derived C1̄-INA to prevent C1̄s dependent destruction of C2 was assessed. Data from the ELISA assays indicate that U937 cells contain between 2.1 to 12.8 ng C1̄-INA Cl-INA per 1 x 10⁶ cells. Furthermore, fluorescence activated cell sorter analysis revealed that approximately 16% of U937 cells carry C1β-INA as a surface bound antigen. Other proteins found to be synthesized by U937 cells include Clr, C8, and possibly alpha-2-macroglobulin. These studies suggest that the U937 cell line is a convenient and valuable model for the study of monocyte C1̄-INA synthesis and physiology.