Eosinophil granule cationic proteins regulate the classical pathway of complement

Area of Science:

• Immunology
• Complement System Biology

Background:

• Eosinophil granule proteins, such as major basic protein (MBP), are key mediators of allergic inflammation and tissue damage.
• MBP and other eosinophil cationic proteins are known to regulate the alternative complement pathway by inhibiting C3b interaction with factor B.

Purpose of the Study:

• To investigate the capacity of eosinophil granule proteins (MBP, eosinophil peroxidase (EPX), eosinophil cationic protein (ECP), and eosinophil-derived neurotoxin (EDN)) to regulate the classical complement pathway.
• To determine the specific mechanisms and sites of action for these proteins on classical pathway components.

Main Methods:

• Assessing the inhibition of classical pathway C3 convertase (EAC1,4b,2a) formation and complement-mediated lysis by purified eosinophil granule proteins.
• Evaluating the impact of these proteins on the formation of the membrane attack complex (MAC) and terminal lysis.
• Analyzing the effects of polycations on kinetic parameters (Zmax, Tmax) of C3 convertase formation and their interaction with complement components C1, C4, and C2.

Main Results:

• EPX, ECP, and MBP inhibited the formation of cell-bound classical pathway C3 convertase (EAC1,4b,2a), with EDN showing no activity.
• EPX inhibited lysis mediated by EAC1,4b,2a,3b,5b, while MBP and ECP did not affect terminal lysis.
• Eosinophil granule polycations primarily regulated the early classical pathway by reducing Zmax, inhibiting EAC1,4b,2a formation proportional to C4, and directly interfering with C1 activity.

Conclusions:

• Eosinophil granule proteins, particularly EPX, ECP, and MBP, are potent regulators of the early classical complement pathway.
• These proteins interfere with C1 function, suggesting a novel mechanism for modulating complement activation in vivo.
• Eosinophil granule proteins may play a significant role in controlling immune responses and tissue homeostasis through complement regulation.