RT Journal Article
SR Electronic
T1 Soluble MAC is primarily released from MAC-resistant bacteria that potently convert complement component C5
JF bioRxiv
FD Cold Spring Harbor Laboratory
SP 2021.12.15.472789
DO 10.1101/2021.12.15.472789
A1 Dennis J. Doorduijn
A1 Marie V. Lukassen
A1 Marije F.L. van ’t Wout
A1 Vojtech Franc
A1 Maartje Ruyken
A1 Bart W. Bardoel
A1 Albert J.R. Heck
A1 Suzan H. M. Rooijakkers
YR 2021
UL http://biorxiv.org/content/early/2021/12/15/2021.12.15.472789.abstract
AB The Membrane Attack Complex (MAC or C5b-9) is an important effector of the immune system to kill invading microbes. MAC is formed when complement enzymes on the bacterial surface convert complement component C5 into C5b. Although the MAC is a membrane-inserted complex, soluble forms of MAC (sMAC, or terminal complement complex (TCC)) are often detected in sera of patients suffering from infections. Consequently, sMAC has been proposed as a biomarker, but it remains unclear when and how it is formed during infections. Here, we studied mechanisms of MAC formation on bacteria and found that sMAC is primarily formed in human serum by bacteria resistant to MAC-dependent killing. Surprisingly, C5 was converted into C5b more potently by MAC-resistant compared to MAC-sensitive Escherichia coli strains. Both the increase in C5 conversion and sMAC generation were linked to the expression of lipopolysaccharide (LPS) O-Antigen in the bacterial outer membrane. In addition, we found that MAC precursors are released from the surface of MAC-resistant bacteria during MAC assembly. Release of MAC precursors from bacteria induced lysis of bystander human erythrocytes in the absence of other serum components. However, serum regulators vitronectin (Vn) and clusterin (Clu) can prevent this bystander lysis. Combining size exclusion chromatography with mass spectrometry profiling, we show that sMAC released from bacteria in serum is a heterogeneous mixture of complexes composed of C5b-8, up to 3 copies of C9 and multiple copies of Vn and Clu. Altogether, our data provide molecular insight into how sMAC is generated during bacterial infections. This fundamental knowledge could form the basis for exploring the use of sMAC as biomarker.Competing Interest StatementThe authors have declared no competing interest.