Source data for: Soluble MAC is primarily released from MAC-resistant bacteria that potently convert complement component C5

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
different Gram-negative and Gram-positive 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. In addition, we found that MAC precursors are released from the
surface of MAC-resistant bacteria during MAC assembly. Although the
release of MAC precursors from bacteria induced lysis of bystander human
erythrocytes, serum regulators vitronectin (Vn) and clusterin (Clu) can
prevent this. 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 a biomarker.