Complement Component C5a

C5a Structure C5a Functions C5a Test C5a Deficiency Therapeutic Target

Complement component C5a, a cleavage product of the fifth component of the complement system (C5), is a potent anaphylatoxin and chemotactic agent that plays a key role in innate immune and inflammatory responses. Its dual role in pathogen clearance and immunomodulation makes it a key target for therapeutic intervention in inflammatory and autoimmune diseases.


Structure of C5a

C5a is a glycoprotein consisting of 74 amino acids and is approximately ~11 kDa. Its three-dimensional structural features include:

C5a docked to the C5aR. (Hagemann, Ian S., et al., 2006)Fig. 1 Model of C5a docked to the C5aR.1,3

Biosynthesis and Metabolism
  • C5 precursor synthesis
    • C5 is synthesized by hepatocytes and secreted into the bloodstream as a single-chain precursor.
    • During processing, C5 is enzymatically cleaved into a double-chain structure (α and β chains), linked by disulfide bonds and non-covalent interactions.
  • Production of C5a
    • Upon complement activation (via the classical/alternative/lectin pathway), C5 is cleaved by C5 convertase (C3bBb or C4b2a) to produce C5a and C5b fragments.
    • C5a is rapidly released as a potent anaphylatoxin, triggering a localized inflammatory response.
  • Metabolic regulation
    • C5a is rapidly metabolized by carboxypeptidase B to C5a-des-Arg (de-arginine C5a), which is significantly less active.
    • The metabolic process avoids excessive inflammation by regulating the bioavailability of C5a.
Cellular Sources and Regulation of Expression
  • Primary source: Hepatocytes are the main site of C5 synthesis, but immune cells such as macrophages can also express C5 and participate in local complement activation.
  • Regulatory mechanism: C5 synthesis is regulated by inflammatory factors (e.g., IL-6, TNF-α), and its expression is significantly up-regulated under pathological conditions.

Through precise structural design and dynamic metabolic regulation, C5a achieves a balance between high efficiency and safety in the immune response.


Functional Roles of C5a in Immunity

C5a, as a key active fragment of the complement system, is involved in immune regulation, inflammatory response and pathological processes through a variety of mechanisms. Its biological activities can be categorized into the following core aspects:

Table 1 Functional of C5a in immunity.

Functional Roles Mechanisms Specific Cases
Anaphylatoxin effects C5a induces histamine release by binding to mast cells or basophils, leading to vasodilation, smooth muscle contraction and increased tissue permeability.
  • For example, hyperactivation of C5a in neococcal pneumonia triggers a cytokine storm that leads to multi-organ damage.
Chemokine function C5a acts as a potent chemokine, attracting neutrophils, monocytes and T-cells to the lesion site through a concentration gradient.
  • In burn models, C5a upregulates CR1 and CR3 receptors on the surface of neutrophils, enhances their response to complement fragments, and promotes the formation of neutrophil extracellular traps (NETs), further amplifying the inflammatory response.
Inflammatory mediator regulation C5a promotes lymphoma cell motility and spread by activating the p38 MAPK signaling pathway. At the same time, C5a synergizes with C3a to stimulate immune cells to release pro-inflammatory factors such as IL-1, IL-6, and TNF-α, forming a positive feedback loop to exacerbate inflammation.
  • In the tumor microenvironment, C5a also induces matrix metalloproteinase (MMP-2, MMP-9) expression, destroys the extracellular matrix, and promotes tumor invasion.
Immune cell regulation T cell regulation: C5a directly attracts T cells to migrate to the site of inflammation and regulates adaptive immunity by enhancing T cell proliferation and inhibiting apoptotic signaling.
γδ T-cell activation: C5a binds to C5aR (CD88) on the surface of γδ T-cells and enhances their cytokine secretion capacity, influencing the subsequent immune response.
Suppression of anti-tumor immunity: a tumor microenvironment with high C5a concentrations reduces the number of CD4+ and CD8+ T cells and accelerates tumor progression.

Receptor-mediated Signaling

C5a's diverse biological activities are mediated through interactions with C5aR (CD88) and C5L2 (GPR77) receptors, expressed on immune and non-immune cells.

The bioactive components of C5a contribute to allergic reactions, inflammation amplification, immunomodulation and pathological processes, and its complex signaling regulatory network makes it one of the most influential effector molecules in the complement system.

C5a mediates various biological activities. (Horiuchi, Takahiko, and Hiroshi Tsukamoto, 2016)Fig. 2 C5a binds to C5a receptor and mediates various biological activities.2,3


Complement C5a Test

C5a functional assays are essential for assessing its role in immunomodulation, disease diagnosis and therapeutic monitoring. These assays assess C5a activity, receptor interactions, and downstream biological effects.

Table 2 Methods of C5a functional assays.

Methods Principle Applications
Hemolytic assays Measure C5a's role in complement-mediated lysis by combining serum with C5-deficient serum. Lysis of sheep red blood cells (SRBC) or liposomes indicates functional C5a activity.
  • Diagnosing C5 deficiencies
  • Monitoring complement-mediated diseases
ELISA-based methods Uses a matched antibody pair to detect C5a in serum, plasma, or cell culture medium. Quantify both natural and recombinant C5a via colorimetric or luminescent detection.
  • Detecting C5a levels in inflammatory or autoimmune conditions
  • Monitoring C5a des-Arg (metabolized form) due to cross-reactivity in ELISAs
Cell-based reporter gene assays Add test samples (serum, inhibitors) to assay-ready cells. Measure luminescence to quantify C5a-driven luciferase expression.
  • Drug discovery: Potency testing of C5a-targeted therapies
  • Mechanistic studies: Investigating C5a-C5aR1 interactions
Functional activity assays Use Fluo-4 AM dye in neutrophils to detect C5a-induced calcium mobilization.
  • Studying C5a's pro-inflammatory effects
  • Validating C5aR inhibitors using C5aR-deficient mice or antibodies


C5a and Disease Pathology

C5a plays complex pathophysiological roles through regulation, immunosuppression and tissue damage in a wide range of diseases. Excessive C5a production is associated with a variety of inflammatory and autoimmune diseases.

The pathophysiological role of C5a is at multiple levels including tumor microenvironment regulation, cytokine regulation, immunosuppression and chronic regulation. Its complex regulatory signaling network both reflects the driving force of disease progression and provides potential targets for precision therapy.


C5a as a Therapeutic Target

Overactivation of the complement component C5a, a key molecule in the complement system, is closely associated with inflammation, immunity, and infection. Current therapeutic strategies for C5a focus on direct neutralization of C5a activity or blockade of its receptor signaling inhibitors, and both approved and clinical-stage drugs show significant potential.

Monoclonal antibodies that directly inhibit C5a

Several drugs have been approved.The main mechanisms include:

  • Inhibition of C5a, inhibition of its binding to C5aR1/C5aR2, inhibition of regulatory cascade reactions.
  • Inhibition of C5, blocking the production of C5a/C5b and indirectly inhibiting C5a activity.
Inhibitors targeting C5a receptor

Several drugs have been approved.The main mechanisms include:

  • Block C5aR1 and inhibit C5a-directed neutrophil-mediated activation.
  • Neutralize C5a function by binding to its hotspot region.
Integrated targeting strategy

Modulate C5a-mediated neutrophil chemotaxis by inhibiting GM-CSF.

Table 3 Indication coverage and mechanism of action.

Disease Type Mechanism of Action
COVID-19 Neutralize C5a activity and inhibit neutrophil chemotaxis with cytokine storms
ANCA-associated vasculitis Block C5aR1 or directly neutralize C5a and reduce immune complex-mediated tissue damage
Infectious diseases Block C5 production and reduce the risk of systemic fungal infections
Tumor microenvironment regulation Potential inhibition of C5a promotes tumor invasion and immunosuppression

Research is expanding into:

Complement C5a is a multifunctional mediator. Its structural complexity and receptor interactions highlight its therapeutic potential. C5a-targeted therapies have led to breakthroughs in several disease areas by directly neutralizing its activity or blocking receptor signaling. In the future, there is a need to break through the complexity of the complement system, develop more efficient drugs with lower side effects, and explore combination therapies to expand the range of indications.

Creative Biolabs is a global leader in biotechnology solutions, specializing in antibody development, protein engineering and therapeutic discovery. For customized solutions for complement system research, please contact our team of experts.

If you want more information, please feel free to contact us.

References

  1. Hagemann, Ian S., et al. "Random mutagenesis of the complement factor 5a (C5a) receptor N terminus provides a structural constraint for C5a docking." Journal of Biological Chemistry 281.48 (2006): 36783-36792. https://doi.org/10.1074/jbc.M607686200
  2. Horiuchi, Takahiko, and Hiroshi Tsukamoto. "Complement-targeted therapy: development of C5-and C5a-targeted inhibition." Inflammation and regeneration 36 (2016): 1-5. https://doi.org/10.1186/s41232-016-0013-6
  3. under Open Access license CC BY 4.0, without modification.
For Research Use Only.
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