Multimeric complement component C9 is necessary for killing of Escherichia coli J5 by terminal attack complex C5b-9

Area of Science:

• Immunology
• Molecular Biology
• Microbiology

Background:

• The complement system is crucial for innate immunity.
• The terminal complement complex (C5b-9) mediates bacterial killing.
• The role of C9 polymerization in C5b-9 function is not fully understood.

Purpose of the Study:

• To investigate the molecular requirements for complement-mediated killing of Escherichia coli.
• To determine the necessity of C9 polymerization for the formation and function of the C5b-9 complex.

Main Methods:

• Deposition of C5b-9 components (C7, C8, C9) on E. coli J5.
• Quantification of bound complement proteins using radiolabeled antibodies.
• Assessment of bacterial killing via dose-response curves.
• Electron microscopy and antibody binding assays to detect C9 polymerization.

Main Results:

• Bacterial killing correlated with C9 input and C9-to-C7 ratios.
• No killing occurred below a C9-to-C7 ratio of 3.3:1, irrespective of C5b-9 complex numbers.
• Multimeric C9 formation was confirmed at killing-permissive C9-to-C7 ratios.

Conclusions:

• C9 polymerization is biologically functional and necessary for optimal killing of E. coli J5 by C5b-9.
• The formation of multimeric C9 is a critical step in complement-mediated bacterial lysis.