Polymerization of C9 enhances bacterial cell envelope damage and killing by membrane attack complex pores

Complement proteins can form Membrane Attack Complex (MAC) pores that directly kill Gram-negative bacteria. MAC pores assemble by stepwise binding of C5b, C6, C7, C8 and finally C9, which can polymerize into a transmembrane ring of up to 18 C9 monomers. It is still unclear if the assembly of a polymeric-C9 ring is necessary to sufficiently damage the bacterial cell envelope to kill bacteria, because a robust way to specifically prevent polymerization of C9 has been lacking. In this paper, polymerization of C9 was prevented without affecting the binding of C9 to C5b-8 by locking the first transmembrane helix domain of C9. We show that polymerization of C9 strongly enhanced bacterial cell envelope damage and killing by MAC pores for several Escherichia coli and Klebsiella strains. Moreover, we show that polymerization of C9 is impaired on complement-resistant E. coli strains that survive killing by MAC pores. Altogether, these insights are important to understand how MAC pores kill bacteria and how bacterial pathogens can resist MAC-dependent killing.