Soluble MAC is primarily released from MAC-resistant bacteria that potently convert complement component C5
- University Medical Center Utrecht, Netherlands
- Utrecht University, Netherlands
Abstract
The Membrane Attack Complex (MAC or C5b-9) is an important effector of the immune system to kill invading microbes. MAC formation is initiated 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 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 biomarker.
Data availability
All relevant data supporting the findings of this manuscript have been added in the main manuscript and supplemental information. Supporting source data (for figures 1 - 6a and supplements) have been uploaded to Dryad Digital Repository (doi:10.5061/dryad.g4f4qrfsd). The MS data (in figure 6, figure 6 supplement and figure 7) have been deposited to the ProteomeXchange partner MassIVE database and assigned the identifier MSV000088560 (doi:10.25345/C5QW00).
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Source data for: Soluble MAC is primarily released from MAC-resistant bacteria that potently convert complement component C5Dryad Digital Repository, doi:10.5061/dryad.g4f4qrfsd.
Article and author information
Author details
Suzan HM Rooijakkers
Funding
European Research Council ((639209-ComBact)
- Suzan HM Rooijakkers
Nederlandse Organisatie voor Wetenschappelijk Onderzoek (Aspasia)
- Suzan HM Rooijakkers
Utrecht Molecular Immunology HUB (eSTIMATE)
- Suzan HM Rooijakkers
Netherlands Proteomics Centre (184.034.019)
- Albert JR Heck
Independent Research Fund Denmark (9036-00007B)
- Marie V Lukassen
The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.
Copyright
© 2022, Doorduijn et al.
This article is distributed under the terms of the Creative Commons Attribution License permitting unrestricted use and redistribution provided that the original author and source are credited.
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Soluble MAC is primarily released from MAC-resistant bacteria that potently convert complement component C5
eLife 11:e77503.
https://doi.org/10.7554/eLife.77503
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