Bactericidal activity of the alternative complement pathway generated from 11 isolated plasma proteins.

Area of Science:

• Immunology
• Microbiology
• Biochemistry

Background:

• The complement system is crucial for innate immunity, offering defense against pathogens.
• The alternative pathway (AP) of complement is a key component in pathogen recognition and elimination.
• Understanding the AP's lytic mechanism is vital for developing novel antimicrobial strategies.

Purpose of the Study:

• To investigate the bactericidal activity of the AP using isolated proteins.
• To characterize the lytic process and compare it to complement-mediated lysis in serum.
• To determine the potential of the AP to eradicate gram-negative bacteria without immunoglobulin involvement.

Main Methods:

• Reconstitution of the AP using 11 purified pathway proteins.
• Exposure of E. coli K12 W1485 to the reconstituted AP with and without lysozyme.
• Assessment of bacterial viability and time-course/dose-dependency of lysis.
• Electron microscopy (TEM and SEM) for visualizing bacterial damage.

Main Results:

• The reconstituted AP, with lysozyme, induced bacterial killing and lysis.
• Lysis kinetics and dose-dependency mirrored those observed in C4-depleted serum.
• Bacteriolytic activity was concentration-dependent, diminishing at lower protein levels.
• Microscopy revealed complement membrane attack complexes and significant bacterial membrane damage.

Conclusions:

• The isolated alternative pathway proteins can effectively lyse gram-negative bacteria.
• Lysozyme is essential for the bacteriolytic activity of this reconstituted system.
• The AP demonstrates intrinsic bactericidal potential against E. coli, independent of antibodies.