C5a

Return to Immune System Menu

C5a is a small protein fragment that plays a crucial role in the immune system, specifically as part of the complement system. The complement system is a component of the innate immune response, which is the body's first line of defense against pathogens. C5a is the most potent of these and also promotes granulocyte participation, leading to release of histamines and toxic oxygen metabolites into the tissues. Its a chemo-attractant for macrophages and neutrophils. C5a is known for its potent inflammatory properties and its ability to recruit immune cells to sites of infection or injury.

Structure and Formation of C5a

C5a is derived from the complement component C5, a protein that circulates in the blood in an inactive form. The activation of the complement system, through either the classical, lectin, or alternative pathways, leads to the cleavage of C5 by the enzyme C5 convertase. This cleavage results in the formation of two fragments: C5a and C5b. C5a is a 74-amino acid peptide that is highly conserved across different species, indicating its essential role in immune function.

Functions of C5a

C5a is a powerful anaphylatoxin, which means it can induce rapid inflammatory responses. Its primary functions include:

• Chemotaxis: C5a acts as a chemoattractant, guiding immune cells such as neutrophils, eosinophils, and macrophages to sites of infection or tissue damage. This recruitment is crucial for the clearance of pathogens and the initiation of the healing process.

• Activation of Immune Cells: C5a binds to specific receptors on the surface of immune cells, such as the C5a receptor (C5aR) and C5L2. This binding triggers a cascade of intracellular signaling events that activate these cells, enhancing their ability to phagocytize pathogens and release inflammatory mediators.

• Induction of Inflammation: By promoting the release of pro-inflammatory cytokines and chemokines, C5a amplifies the inflammatory response. This can lead to increased vascular permeability, allowing immune cells and proteins to access the affected tissue more easily.

• Modulation of Adaptive Immunity: C5a also influences the adaptive immune response by affecting the function of dendritic cells and T cells, which are critical for the development of long-term immunity.

Clinical Significance of C5a

While C5a is essential for effective immune responses, its dysregulation can contribute to various pathological conditions. Excessive or prolonged production of C5a can lead to chronic inflammation and tissue damage, which are hallmarks of several diseases.

C5a in Disease

1. Sepsis: In sepsis, an overwhelming systemic inflammatory response to infection, C5a levels are significantly elevated. This can lead to excessive inflammation, tissue damage, and organ failure. Targeting C5a or its receptors has been explored as a therapeutic strategy to mitigate the harmful effects of sepsis.

2. Autoimmune Diseases: Conditions such as rheumatoid arthritis and systemic lupus erythematosus are characterized by chronic inflammation and autoimmunity. C5a contributes to the pathogenesis of these diseases by promoting the recruitment and activation of immune cells that attack the body's own tissues.

3. Asthma and Allergic Reactions: C5a is involved in the pathophysiology of asthma and allergic reactions by enhancing the inflammatory response and contributing to airway hyperresponsiveness.

4. Ischemia-Reperfusion Injury: In conditions such as myocardial infarction and stroke, the restoration of blood flow to ischemic tissues can cause further damage due to inflammation. C5a is a key mediator of this process, and its inhibition has been shown to reduce tissue injury in experimental models.

Therapeutic Targeting of C5a

Given its central role in inflammation and disease, C5a and its receptors have become attractive targets for therapeutic intervention. Several strategies are being explored:

• C5a Receptor Antagonists: These compounds block the interaction between C5a and its receptors, thereby reducing the inflammatory response. Avacopan, a C5aR antagonist, has shown promise in treating conditions like ANCA-associated vasculitis.

• C5a Inhibitors: Direct inhibitors of C5a aim to neutralize its activity and prevent its pro-inflammatory effects. These inhibitors are being investigated for their potential to treat a range of inflammatory diseases.

• Monoclonal Antibodies: Antibodies that specifically target C5a or its receptors can effectively reduce inflammation and tissue damage. These biologics are being developed for use in autoimmune and inflammatory diseases.

New insights for C5a and C5a receptors in sepsis

Human C5a is composed of 74 amino acids, which is a glycosylated peptide. NMR spectroscopy demonstrated that C5a contains four helices, which are connected by loops. The helical structures are cross-linked by disulfide bonds, which make the molecule quite stable in the presence of oxidative stress (Ward, 2010). It has been well established that C5a production could be due to plasma complement activation pathways. In addition, studies indicated that C5a could also be generated through cleavage of C5 by phagocytic cell-derived serine proteases that have C5 convertase activity (Huber-Lang et al., 2002c). These cells include alveolar macrophages and neutrophils (Huber-Lang et al., 2002c). Interestingly, a recent study shows that M-ficolin, a pattern-recognition molecule which activates the complement system in a manner similar to MBL pathway, was released by phagocytes during bacterial sepsis, and its cord blood level was positively related to circulating phagocytes and early-onset sepsis in neonates (Schlapbach et al., 2012).

The roles of C5a in sepsis have been investigated in subhuman primate model of sepsis-induced by intravenous injection of Escherichia coli (E. coli) into monkeys. In this model, C5a neutralizing antibody reduced several septic parameters (Stevens et al., 1986; Hangen et al., 1987). As a result, all septic animals treated with anti-C5a antibody survived, and did not developed severe lung edema and decreased oxygenation (Stevens et al., 1986; Hangen et al., 1987). In contrast, 75% of animals treated with control IgG died with decreased oxygenation, increased extravascular lung water, and profound hypotension (Stevens et al., 1986; Hangen et al., 1987).

The molecular mechanisms underlying the harmful effects of excessive C5a on innate immune functions during sepsis are being defined. C5a inhibited phagocytic activity of normal blood neutrophil in a dose-dependent manner (Huber-Lang et al., 2002b). Furthermore, blood neutrophils from septic rats showed defect in phagocytosis (Huber-Lang et al., 2002b). In contrast, neutrophils from cecal ligation and puncture (CLP) rats treated with antibody to C5a preserved the phagocytic activity. C5a treatments also led to suppression of p47^phox phosphorylation, and its subsequent translocation to the cell membrane and assemble of NADPH oxidase, which resulted in inhibition of respiratory burst in neutrophils (Huber-Lang et al., 2002b). C5a-induced defects in phagocytosis and NADPH oxidase assembly caused defective bactericidal activity of neutrophils, leading to increased bacterial counts (Huber-Lang et al., 2002b). In CLP-induced sepsis model, 50% of rats receiving anti-C5a antibody treatment survived during a 10-day survival study, while the survival rate was only 9.5% in the septic group treated with normal IgG (Czermak et al., 1999). The improved survival was linked to reduced bacterial colony forming-units (CFU) in blood, spleen, and liver, and improved H₂O₂-generating ability of neutrophils by C5a blockade (Czermak et al., 1999).

Complement activation occurs during sepsis in human, leading to the generation of anaphylatoxins including C3a, C4a, and C5a (Nakae et al., 1996). Appearance of high levels of anaphylatoxins was correlated with MOF that is a key factor resulting in death, and lower anaphylatoxin levels could only be identified in surviving septic patients but not non-surviving persons (Bengtson and Heideman, 1986; Nakae et al., 1996). In addition, in vitro experiment demonstrated that neutrophils in patients surviving from sepsis-induced MOF had defect in chemotactic response to C5a, which might be related with inability of C5a to bind to neutrophils (Solomkin et al., 1981; Goya et al., 1994). In experimental sepsis, C5a blockade attenuated the parameters of MOF, and maintained normal chemotactic function of neutrophils (Huber-Lang et al., 2001a; Flierl et al., 2006). Importantly, C5a blockade given at 12 h after the initiation of sepsis has protective effects against detrimental influence of septic shock (Huber-Lang et al., 2001b). However, it remains to be determined whether, in human beings with sepsis, there may be a similar “time window” during which anti-C5a treatment can be an effective method to improve survival.

The Role of the Complement System and the Activation Fragment C5a in the Central Nervous System

The Complement System Component C5a Produces Thermal Hyperalgesia via Macrophage-to-Nociceptor Signaling That Requires NGF and TRPV1