Secretion, cleavage and binding of complement component C3 by the human monocytic cell line U937

Area of Science:

• Immunology
• Cell Biology
• Biochemistry

Background:

• The complement system plays a crucial role in innate and adaptive immunity.
• Complement component C3 is a central protein in complement activation.
• U937 cells are a human monocyte cell line often used in immunological studies.

Purpose of the Study:

• To investigate the secretion of complement component C3 by U937 cells.
• To identify and characterize cell-associated proteolytic activity specific for C3.
• To elucidate the mechanism of C3 fragment binding to U937 cell membranes.

Main Methods:

• SDS/polyacrylamide-gel electrophoresis to analyze C3 forms.
• Incubation of U937 cells with radiolabeled C3.
• Use of protease inhibitors to study C3 cleavage.
• Immunological characterization of cell-bound C3 fragments using monoclonal antibodies.

Main Results:

• U937 cells secrete C3, with stimulated secretion reaching 120-140 ng/10(6) cells/24 h.
• A cell-associated protease, distinct from classical or alternative pathway convertases, cleaves C3.
• C3 fragments bind covalently to U937 cell membranes via a thioester bond reaction, forming 43 kDa and 46 kDa fragments with C3dg-like characteristics.
• Proteolysis is specific for C3, as C4 is not cleaved or bound.

Conclusions:

• U937 cells actively participate in complement processing through C3 secretion and membrane-associated C3 modification.
• The identified proteolytic activity and binding mechanism represent a novel interaction between cells and complement component C3.
• These findings contribute to understanding the complex roles of complement in cellular immune responses.