Soluble Membrane Attack Complex: Biochemistry and Immunobiology

Barnum, Scott R.; Bubeck, Doryen; Schein, Theresa N.

doi:10.3389/fimmu.2020.585108

Cited by 44 publications

(36 citation statements)

References 217 publications

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“…However, it remains unclear if a full polymeric-C9 ring of 18 C9 molecules is required. The aE11 antibody that is used for the detection of polymeric-C9 can also detect soluble MAC [32], which only contains up to 3 C9 molecules [39]. Although SDS-PAGE only revealed a single band for C9 polymers, it is still unknown if this band contains a mixture of C9 polymers of different sizes or if smaller C9 polymers are SDS-stable.…”

Section: Plos Pathogensmentioning

confidence: 99%

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

et al. 2021

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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. In this paper, polymerization of C9 was prevented without affecting binding of C9 to C5b-8, by locking the first transmembrane helix domain of C9. Using this system, we show that polymerization of C9 strongly enhanced damage to both the bacterial outer and inner membrane, resulting in more rapid killing of several Escherichia coli and Klebsiella strains in serum. By comparing binding of wildtype and ‘locked’ C9 by flow cytometry, we also show that polymerization of C9 is impaired when the amount of available C9 per C5b-8 is limited. This suggests that an excess of C9 is required to efficiently form polymeric-C9. Finally, we show that polymerization of C9 was impaired on complement-resistant E. coli strains that survive killing by MAC pores. This suggests that these bacteria can specifically block polymerization of C9. All tested complement-resistant E. coli expressed LPS O-antigen (O-Ag), compared to only one out of four complement-sensitive E. coli. By restoring O-Ag expression in an O-Ag negative strain, we show that the O-Ag impairs polymerization of C9 and results in complement-resistance. Altogether, these insights are important to understand how MAC pores kill bacteria and how bacterial pathogens can resist MAC-dependent killing.

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Section: Plos Pathogensmentioning

confidence: 99%

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

et al. 2021

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“…When inserted in a membrane, this complex can lead to breach of osmotic stability and lysis. Bound to S-protein (Vitronectin), the sC5b-9 complex is held in the fluid phase; it is this circulating form that is becoming a common measure of terminal pathway activation levels ( 75 , 76 ) to determine the level of activation or inhibition occurring in a patient ( 77 , 78 ). As such, a measure of terminal pathway activation sC5b-9/sTCC has gained favor as a potential way to assess the likelihood of a patient to respond to therapeutic complement inhibition ( 68 , 79 ) and then as a measure of the level of appropriate inhibition ( 80 ); however, this has yet to be firmly established ( 81 ).…”

Section: Complement Testing Addressed In the Standardization Effortsmentioning

confidence: 99%

Expanding Horizons in Complement Analysis and Quality Control

Frazer‐Abel¹,

Kirschfink²,

Prohászka³

2021

Front. Immunol.

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Complement not only plays a key role in host microbial defense but also modulates the adaptive immune response through modification of T- and B-cell reactivity. Moreover, a normally functioning complement system participates in hematopoiesis, reproduction, lipid metabolism, and tissue regeneration. Because of its powerful inflammatory potential, multiple regulatory proteins are needed to prevent potential tissue damage. In clinical practice, dysregulation and overactivation of the complement system are major causes of a variety of inflammatory and autoimmune diseases ranging from nephropathies, age-related macular degeneration (AMD), and systemic lupus erythematosus (SLE) to graft rejection, sepsis, and multi-organ failure. The clinical importance is reflected by the recent development of multiple drugs targeting complement with a broad spectrum of indications. The recognition of the role of complement in diverse diseases and the advent of complement therapeutics has increased the number of laboratories and suppliers entering the field. This has highlighted the need for reliable complement testing. The relatively rapid expansion in complement testing has presented challenges for a previously niche field. This is exemplified by the issue of cross-reactivity of complement-directed antibodies and by the challenges of the poor stability of many of the complement analytes. The complex nature of complement testing and increasing clinical demand has been met in the last decade by efforts to improve the standardization among laboratories. Initiated by the IUIS/ICS Committee for the Standardization and Quality Assessment in Complement Measurements 14 rounds of external quality assessment since 2010 resulted in improvements in the consistency of testing across participating institutions, while extending the global reach of the efforts to more than 200 laboratories in 30 countries. Worldwide trends of assay availability, usage, and analytical performance are summarized based on the past years’ experiences. Progress in complement analysis has been facilitated by the quality assessment and standardization efforts that now allow complement testing to provide a comprehensive insight into deficiencies and the activation state of the system. This in turn enables clinicians to better define disease severity, evolution, and response to therapy.

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“…C3a also stimulates the production of cachexia-associated inflammatory mediators such as TNF-α and IL-1 [ 23 , 24 ]. The assembly of the MAC is the ultimate consequence of complement activation and leads to the effective lysis of target cells by forming a pore in the membrane, disrupting cellular integrity and function [ 25 ]. When the MAC is formed on membranes, a soluble counterpart sMAC, also referred to as the Terminal Complement Complex (TCC), can be formed.…”

Section: Introductionmentioning

confidence: 99%

“…When the MAC is formed on membranes, a soluble counterpart sMAC, also referred to as the Terminal Complement Complex (TCC), can be formed. TCC levels are increased in various acute and chronic inflammatory diseases [ 25 ].…”

Section: Introductionmentioning

confidence: 99%

Activation of the Complement System in Patients with Cancer Cachexia

Deng

Vaes

Bijnen

et al. 2021

Cancers

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Systemic inflammation is thought to underlie many of the metabolic manifestations of cachexia in cancer patients. The complement system is an important component of innate immunity that has been shown to contribute to metabolic inflammation. We hypothesized that systemic inflammation in patients with cancer cachexia was associated with complement activation. Systemic C3a levels were higher in cachectic patients with inflammation (n = 23, C-reactive protein (CRP) ≥ 10 mg/L) as compared to patients without inflammation (n = 26, CRP < 10 mg/L) or without cachexia (n = 13) (medians 102.4 (IQR 89.4–158.0) vs. 81.4 (IQR 47.9–124.0) vs. 61.6 (IQR 46.8–86.8) ng/mL, respectively, p = 0.0186). Accordingly, terminal complement complex (TCC) concentrations gradually increased in these patient groups (medians 2298 (IQR 2022–3058) vs. 1939 (IQR 1725–2311) vs. 1805 (IQR 1552–2569) mAU/mL, respectively, p = 0.0511). C3a and TCC concentrations were strongly correlated (rs = 0.468, p = 0.0005). Although concentrations of C1q and mannose-binding lectin did not differ between groups, C1q levels were correlated with both C3a and TCC concentrations (rs = 0.394, p = 0.0042 and rs = 0.300, p = 0.0188, respectively). In conclusion, systemic inflammation in patients with cancer cachexia is associated with the activation of key effector complement factors. The correlations between C1q and C3a/TCC suggest that the classical complement pathway could play a role in complement activation in patients with pancreatic cancer.

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Soluble Membrane Attack Complex: Biochemistry and Immunobiology

Cited by 44 publications

References 217 publications

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

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

Expanding Horizons in Complement Analysis and Quality Control

Activation of the Complement System in Patients with Cancer Cachexia

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