Complement Regulator of Complement System: Others

The complement system is a pivotal arm of innate immunity, orchestrating host defense mechanisms through a cascade of proteolytic activations. However, uncontrolled complement activation can result in tissue damage, inflammation, and chronic disease. To mitigate this, a series of endogenous complement regulators have evolved, among which C1 Inhibitor (C1-INH), CD59 (Protectin), Vitronectin (VTN), and Clusterin (CLU) are of particular importance. These regulators operate at various checkpoints in the complement pathways—classical, lectin, and terminal—providing intricate control and ensuring immune homeostasis.

Creative Biolabs offers comprehensive research solutions to support investigation and therapeutic targeting of complement regulators.

Overview of Complement Regulators

Activation of the classical complement pathway (CP) and the lectin pathway (LP) depends to a large extent on foreign substances, but in some cases (such as tissue ischemia and reperfusion), both pathways can be activated and causes auto-injury. More relevant to complement-related human diseases, activation and expansion of C3b deposition through alternative pathways (AP) is non-discriminatory and, if not properly regulated, may rapidly damage host cells. Therefore, it is essential that the activity on the host cell is down-regulated while allowing efficient activation on the foreign target. Humans and other mammals have developed a variety of plasma and membrane-binding inhibitory proteins to regulate complement location and activity. These molecules regulate complement activation in different steps of the complement cascade, participate in the removal of complement-coated particles and / or protect host cells from damage.

Fig. 1 The important components and regulators of each complement pathway.^1,2

Complement Regulator of Complement System: C1 Inhibitor / C1INH / SERPING1

C1-INH is a serine protease inhibitor (serpin) that plays a fundamental role in regulating the classical and lectin pathways. By irreversibly binding and inactivating C1r, C1s, MASP-1, and MASP-2, it prevents the uncontrolled initiation of complement activation.

C1-INH deficiency causes paroxysmal angioedema. In addition to its role as a complement inhibitor, C1-INH is also a major inhibitor of factor XIIa and kallikrein in the system. C1-INH's insufficient inhibition of these enzymes can lead to inappropriate bradykinin production. This in turn mediates the vascular permeability characteristic of angioedema.

• HAE: Caused by mutations in the SERPING1 gene leading to C1-INH deficiency.
• Sepsis and Autoimmune Diseases: Dysregulated C1-INH activity has been implicated in systemic inflammatory responses.

Creative Biolabs provides:

• C1-INH activity assays for functional analysis.
• Recombinant C1-INH proteins for therapeutic development.
• Antibody generation services targeting C1-INH for diagnostic research.

Complement Regulator of Complement System: Protectin / CD59

Both CD59 and DAF are immobilized on the cell membrane by glycosylphosphatidylinositol molecules. CD59 is a key regulator of the terminal pathway and is a GPI-anchored cell surface protein. It inhibits the association of C9 with C5b-8 to prevent the formation of C5b-9. This action preserves host cell integrity and prevents bystander cell lysis.

• Expressed on nearly all human cells.
• Critical in preventing complement-mediated damage to red blood cells and endothelial cells.

Somatic mutations of the phosphatidylinositol-complementing class A gene in red blood cells can cause a rare clonal disease called PNH. Patients suffer from a deficiency in phosphatidylinositol-complex class A function, causing affected red blood cells lack all glycosylphosphatidylinositol-linked membrane proteins associated with autocomplement-mediated hemolytic anemia.

• PNH: Characterized by CD59 deficiency, resulting in chronic hemolysis.
• Cancer and autoimmunity: Overexpression or modulation of CD59 contributes to immune evasion or excessive inflammation.

Research services offered by Creative Biolabs include:

• CD59 detection kits for cell-surface analysis
• Functional CD59 assays to monitor MAC inhibition
• Custom antibody solutions targeting CD59

Complement Regulator of Complement System: Vitronectin / VTN

VTN, also known as protein S, is one of the MAC modulators. It is a multifunctional glycoprotein found in plasma and the extracellular matrix. It exerts complement regulatory effects by binding to the terminal complex components and inhibiting C5b-7 to C9 assembly, thus modulating MAC formation. It also includes other accessory features.

• Involved in cell adhesion, migration, and tissue remodeling.
• Interacts with integrins and plasminogen activator inhibitor-1 (PAI-1).

Elevated levels of VTN are associated with chronic inflammation, fibrosis, and tumor progression. A promising target in cancer immunotherapy for its role in immune escape.

Complement Regulator of Complement System: Clusterin / CLU

CLU (also known as apolipoprotein J) is a secreted chaperone protein that regulates complement activity by interacting with C5b-7 complexes, thereby inhibiting MAC insertion into the membrane. Its role overlaps with both VTN and CD59 in terminal pathway control.

Unlike most other chaperone proteins that help intracellular proteins, CLU is a Golgi protein chaperone that promotes the folding of secreted proteins in an ATP-independent manner. In addition to protecting its own cells in the complement pathway, CLU can promote cell survival in a variety of ways, including inhibiting BAX on the mitochondrial membrane, activating the phosphatidylinositol 3-kinase / protein kinase B pathway, and regulating extracellular signal-regulated kinase (ERK) 1/2 signaling and matrix metallopeptidase-9 expression promote angiogenesis and mediate the nuclear factor-κB (NF-κB) pathway.

Applications in Research and Drug Development

The understanding of complement regulators is reshaping therapeutic strategies in immunology and oncology. These proteins are not only biomarkers for disease activity, but also targets for biologics, small molecules, and gene therapies. Creative Biolabs provides integrated support from molecular characterization to drug screening, facilitating:

• Preclinical complement modulation assays
• Recombinant protein expression for structural studies
• High-throughput screening platforms for complement inhibitors

With over 20 years of experience in complement biology, protein engineering, and assay development, Creative Biolabs delivers:

• Custom assay design for complement function studies.
• Reagent supply including high-purity antibodies and protein.
• Integrated CRO services for drug discovery pipelines targeting complement pathways.

Unlock the power of immune modulation through our expert complement research services. Contact us today for a customized solution tailored to your project.

References

1. Dobó, József, et al. "The lectin pathway of the complement system—activation, regulation, disease connections and interplay with other (proteolytic) systems." International Journal of Molecular Sciences 25.3 (2024): 1566. https://doi.org/10.3390/ijms25031566
2. Distributed under Open Access license CC BY 4.0, without modification.

• RCA/CCP Family
• C1 Inhibitor/C1INH/SERPING1
• Protectin/CD59
• Vitronectin/VTN
• Clusterin/CLU