Absence of complement component 3 does not prevent classical pathway-mediated hemolysis.

Area of Science:

• Immunology
• Complement System Biology

Background:

• Complement component 3 (C3) is a central protein in the complement system.
• C3 plays a critical role in both classical and alternative complement pathways.
• C3 is being investigated as a therapeutic target for inflammatory and autoimmune diseases.

Purpose of the Study:

• To investigate the role of C3 in complement-mediated hemolysis via classical and alternative pathways.
• To evaluate the efficacy of targeting C3 in different complement activation scenarios.
• To determine the impact of C3 deficiency on in vitro and in vivo hemolysis.

Main Methods:

• Utilized normal and C3-depleted human sera for in vitro assays.
• Employed wild-type (WT) and C3-deficient rat sera and C3 knockout rat models for in vivo studies.
• Investigated hemolysis mechanisms using preassembled classical pathway C3 convertases.

Main Results:

• Loss of C3 did not impede classical pathway-mediated hemolysis.
• Alternative pathway-mediated hemolysis was almost abolished in the absence of C3.
• Classical pathway C3 convertase (C4b2a) directly activated complement component 5 (C5), leading to hemolysis.

Conclusions:

• Targeting C3 is a viable strategy to inhibit alternative pathway-mediated hemolysis and associated tissue damage.
• Therapeutic inhibition of C3 may have limited effectiveness against classical pathway-mediated pathological conditions.
• Understanding the distinct roles of C3 in complement pathways is crucial for developing targeted therapies.