Complement activation by myeloperoxidase products released from stimulated human polymorphonuclear leukocytes

Area of Science:

• Immunology
• Biochemistry
• Complement System

Background:

• Human myeloperoxidase (MPO) is an enzyme released by neutrophils.
• The complement system is a critical part of innate immunity.
• Activation of the terminal complement cascade (C5-C9) forms the membrane attack complex, leading to cell lysis.

Purpose of the Study:

• To investigate the role of MPO in activating the complement system.
• To identify the specific MPO-derived products responsible for complement activation.
• To elucidate the mechanism by which MPO-released factors activate complement component C5.

Main Methods:

• Purified human MPO was used to activate purified human C5 in vitro.
• Human polymorphonuclear leukocytes (PMNs) were stimulated with opsonized zymosan.
• Supernatants from stimulated PMNs were analyzed for C5-activating potency using reactive lysis assays.
• Experiments involved using taurine as a hypochlorite trap and inhibitors like azide and catalase.

Main Results:

• Purified MPO activated C5, leading to the formation of a hemolytic C5-9 complex, requiring chloride and hydrogen peroxide.
• Stimulated PMNs released MPO and hydrogen peroxide, which generated hypochlorite and chloramines.
• Taurine chloramine, formed by trapping hypochlorite, activated C5.
• Inhibitors of MPO and hydrogen peroxide blocked C5 activation and chloramine formation.
• The absence of taurine resulted in less oxidant and C5 activation, indicating MPO-generated hypochlorite is key.

Conclusions:

• MPO released from stimulated PMNs activates the terminal complement cascade.
• MPO-generated hypochlorite, subsequently forming chloramines (like taurine chloramine), is the primary C5-activating agent.
• This MPO-mediated complement activation represents a significant pathway contributing to inflammatory responses.