C1R

The complement system is an integral part of the innate immune system responsible for recognizing and eliminating pathogens, modulating inflammation, and facilitating the clearance of apoptotic cells. It functions through a cascade of reactions that activate a series of proteins, ultimately leading to the formation of the membrane attack complex (MAC) and the opsonization of pathogens. The system is divided into three major pathways: the classical pathway, the alternative pathway and the lectin pathway. The classical complement pathway, which plays a central role in immune defense, is activated when the C1 complex binds to antibody-antigen complexes. Central to this pathway is the C1 complex, which consists of three subunits: C1q, C1r, and C1s. Of these, C1r is a serine protease that plays a critical role in the initiation and regulation of the complement cascade. The study of C1r and its potential as a therapeutic target has attracted increasing interest due to its involvement in a number of pathological conditions, including autoimmune diseases, inflammatory disorders and certain infectious diseases.

NCBI Gene ID: 715

UniProtKB ID: P0036

Structure and Function of C1r

The C1r protein is an essential component of the C1 complex, which serves as the initiator of the classical complement pathway. Together with C1s, C1r is a serine protease that is activated by conformational changes in the C1q subunit upon binding to antigen-antibody complexes. Structurally, C1r exists as a homodimer composed of two identical subunits, each containing an N-terminal protease domain and a C-terminal collagen-like region. The protease domain is responsible for the enzymatic activity of C1r, while the collagen-like region helps mediate the interactions between C1q and C1r and stabilizes the C1 complex.

Figure 1. Complement component C1. C1 is comprised of three different subunits C1q, C1r, and C1s. C1q is produced from six identical subunits which form a collagenous stalk with globular heads at the carboxy-terminus. The globular heads can recognize and bind to either IgG/IgM or bind directly onto the surface of the target cell. Binding causes a conformational change which activates C1r to cleave C1s causing it to become enzymatically active. (Middleton et al., 2016)

In its inactive form, C1r exists as a zymogen that requires activation to become catalytically active. Upon binding of C1q to immune complexes, C1r undergoes a conformational change leading to its activation. Activated C1r then cleaves C1s, another serine protease in the C1 complex, initiating the complement cascade. This cleavage results in the activation of C4 and C2, leading to the formation of the C3 convertase enzyme. C3 convertase then cleaves C3 into C3a and C3b, triggering the downstream effects of the complement system, including opsonization, inflammation, and cell lysis.

Figure 2. In the classical pathway of the complement system, C1q binding to an antigen-antibody complex activates C1r, which in turn activates C1s. Activated C1s then cleaves C4 and C2, forming the C3 convertase (C4b2a) (Adapted from Batista et al., 2024)

Thus, C1r is critical for activation of the classical complement pathway and regulation of immune responses. Precise control of C1r activation is essential to maintain immune homeostasis, as excessive or inappropriate complement activation can lead to tissue damage and the development of pathological conditions.

C1r in Immune Responses

The complement system, including C1r, plays a critical role in the immune system's ability to respond to infection, clear apoptotic cells, and modulate inflammation. In particular, the classical pathway is integral to the body's defense against pathogens and regulation of immune tolerance.

• Immune Defense and Pathogen Clearance: C1r and the classical complement pathway help clear pathogens from the body. Efficient activation of C1r and its downstream effects are essential for controlling infections caused by bacteria, viruses and fungi.
• Clearance of Apoptotic Cells: Another critical function of C1r is the clearance of apoptotic cells. Apoptosis, or programmed cell death, is a normal physiological process that occurs during tissue remodeling, immune regulation, and development. However, failure to efficiently remove apoptotic cells can lead to chronic inflammation and the development of autoimmune diseases. C1q plays a critical role in the recognition and clearance of apoptotic cells through its interaction with C1r. Binding of C1q to specific molecular patterns on apoptotic cells promotes their recognition by phagocytes such as macrophages and dendritic cells. This process prevents the release of intracellular autoantigens that could otherwise trigger autoimmune responses.
• Regulation of Immune Tolerance: In addition to its role in pathogen clearance and apoptotic cell removal, the complement system, including C1r, also plays a critical role in maintaining immune tolerance. The complement system helps regulate the balance between immune activation and immune suppression, ensuring that the body does not mount inappropriate immune responses against self-antigens. C1r contributes to the maintenance of immune homeostasis through its involvement in the clearance of apoptotic cells and the regulation of immune responses.

C1r and Disease Pathogenesis

Dysregulation of the complement system, including C1r, has been implicated in a variety of pathological conditions, including autoimmune diseases, inflammatory disorders, and certain infectious diseases. The critical role of C1r in initiating the classical complement pathway makes it a key player in these diseases.

• Autoimmune Diseases: One of the best-established links between C1r and disease is in the context of autoimmune diseases, particularly systemic lupus erythematosus (SLE). In SLE, autoantibodies are produced against self-antigens, leading to the formation of immune complexes that are deposited in tissues, causing inflammation and tissue damage. These immune complexes activate the complement system, and defects in complement components such as C1q and C1r have been shown to increase susceptibility to SLE. C1r deficiency impairs the clearance of apoptotic cells and immune complexes, resulting in the accumulation of autoantigens and the development of autoimmune responses. Other autoimmune diseases such as rheumatoid arthritis (RA), vasculitis, and glomerulonephritis have also been associated with complement dysregulation. In these conditions, excessive activation of C1r and other complement components leads to chronic inflammation, tissue damage, and disease progression.
• Inflammatory Diseases: C1r also plays a role in inflammatory diseases where excessive complement activation can contribute to tissue damage. For example, in conditions such as sepsis, excessive complement activation can exacerbate the inflammatory response, leading to widespread tissue damage and organ failure. In these situations, targeting C1r could potentially reduce complement activation and mitigate the effects of excessive inflammation.
• Infectious Diseases: While complement activation is essential for pathogen clearance, certain pathogens have evolved mechanisms to evade the complement system. Some viruses, bacteria and fungi can inhibit complement activation by binding to complement regulators or by altering the expression of complement receptors. In these cases, modulation of C1r activity could enhance the immune response and improve pathogen clearance. For example, enhancing C1r activity could promote the recognition and destruction of pathogens by the complement system.

C1r as a Drug Target

Given its central role in immune responses and disease pathogenesis, C1r represents an attractive target for therapeutic intervention. Several strategies have been proposed to modulate C1r activity, either by inhibition or enhancement, depending on the clinical context.

Inhibition of C1r in Autoimmune and Inflammatory Diseases

In diseases characterized by excessive complement activation, such as SLE, rheumatoid arthritis and sepsis, inhibition of C1r may provide therapeutic benefit by preventing overactivation of the complement system. One potential strategy is to develop monoclonal antibodies or small molecules that specifically inhibit the activity of C1r. These therapies could block the activation of C1r, thereby preventing the downstream activation of C1s and the subsequent complement cascade.

Several complement inhibitors are already in clinical use, including eculizumab, a monoclonal antibody that inhibits C5, a downstream complement component. However, targeting earlier components of the complement system, such as C1r, may provide a more precise and effective approach to modulating complement activation.

Enhancement of C1r Activity in Infectious Diseases

Conversely, in certain infectious diseases, enhancing C1r activity could improve pathogen clearance by enhancing complement-mediated immune responses. This could be achieved through the development of small molecules or biologics that enhance activation of the classical complement pathway. By increasing C1r activity, these therapies could promote pathogen opsonization, enhance phagocytosis and facilitate MAC formation, leading to more efficient pathogen clearance.

Therapeutic Proteins Targeting C1r

• Conestat Alfa: Conestat alfa is a recombinant version of human C1-esterase inhibitor (C1-INH) specifically developed for the treatment of hereditary angioedema (HAE); a genetic disorder characterized by recurrent episodes of severe swelling due to excessive complement activation. In individuals with HAE, mutations in the C1-INH gene result in insufficient levels of the protein, leading to unregulated activation of C1r and C1s. This activation drives the classical complement cascade, triggering the release of inflammatory mediators such as bradykinin, which causes the characteristic tissue swelling.
• Native human C1-esterase inhibitor (hC1INH): Native human C1-esterase inhibitor (hC1INH) is the naturally occurring protein in the body that regulates the complement system, specifically by inhibiting C1r and C1s. It is derived from human plasma and functions as a potent regulator of the classical complement pathway. This protein is essential for maintaining complement homeostasis and preventing excessive immune activation.

Creative BioMart specializes in providing therapeutic proteins, including conestat alfa and hC1INH, targeting complement component 1r. Contact us with any questions or inquiries.

References

1. Batista AF, Khan KA, Papavergi MT, Lemere CA. The importance of complement-mediated immune signaling in alzheimer's disease pathogenesis. IJMS. 2024;25(2):817. doi:10.3390/ijms25020817
2. Middleton O, Wheadon H, Michie AM. Classical complement pathway. In: Encyclopedia of Immunobiology. Elsevier; 2016:318-324.