Introduction The complement system is part of innate immunity and is comprised of
an intricate network of proteins that are vital for host defense and host homeostasis.
A distinct mechanism by which complement defends against invading pathogens is through
the membrane attack complex (MAC), a lytic structure that forms on target surfaces.
The MAC is made up of several complement components, and one indispensable component
of the MAC is C7. The role of C7 in MAC assembly is well documented, however, inherent
characteristics of C7 are yet to be investigated.Methods To shed light on the molecular
characteristics of C7, we examined the properties of serum-purified C7 acquired using
polyclonal and novel monoclonal antibodies. The properties of serum-purified C7 were
investigated through a series of proteolytic analyses, encompassing Western blot and
mass spectrometry. The nature of C7 protein-protein interactions were further examined
by a novel enzyme-linked immunosorbent assay (ELISA), as well as size-exclusion chromatography.Results
Protein analyses showcased an association between C7 and clusterin, an inhibitory
complement regulator. The distinct association between C7 and clusterin was also demonstrated
in serum-purified clusterin. Further assessment revealed that a complex between C7
and clusterin (C7-CLU) was detected. The C7-CLU complex was also identified in healthy
serum and plasma donors, highlighting the presence of the complex in circulation.Discussion
Clusterin is known to dissociate the MAC structure by binding to polymerized C9, nevertheless,
here we show clusterin binding to the native form of a terminal complement protein
in vivo. The presented data reveal that C7 exhibits characteristics beyond that of
MAC assembly, instigating further investigation of the effector role that the C7-CLU
complex plays in the complement cascade.
