Alternative pathway amplification and infections.

Area of Science:

• Immunology
• Microbiology
• Complement System Biology

Background:

• The alternative pathway (AP) is the oldest component of the complement system, likely evolved for pathogen recognition.
• Pathogens employ sophisticated strategies to evade complement-mediated immunity, including protease production and inhibition of AP proteins.
• Factor H (FH) recruitment by microbes is a common evasion tactic, mimicking host cell interactions.

Purpose of the Study:

• To elucidate the mechanisms by which pathogens evade the complement alternative pathway.
• To explore the role of properdin in AP activation on microbial surfaces.
• To discuss the implications of antibody-dependent AP activation in immunity and therapeutics.

Main Methods:

• Analysis of AP protein function using purified components and AP-specific serum.
• Investigation of microbial evasion strategies, including FH mimicry.
• Examination of properdin's role in AP convertase stabilization.
• Discussion of antibody-dependent AP activation mechanisms.

Main Results:

• AP-mediated C3b amplification on bacteria shows a lag phase and is complement concentration-dependent.
• Pathogens utilize proteases and secreted proteins to disrupt AP function and evade complement.
• Microbial recruitment of FH is a convergent evolutionary strategy to inhibit the AP.
• Properdin primarily stabilizes AP convertases on microbes; its role as an initiator is limited.
• Therapeutic complement inhibition increases infection risk.

Conclusions:

• The complement alternative pathway is a critical defense against pathogens, but microbes have evolved effective evasion mechanisms.
• Understanding these interactions is vital for developing safe and effective immunotherapies.
• Antibody-dependent AP activation plays a significant role in vaccine-induced immunity, particularly when the classical pathway is impaired.