Solubilization of an activity regulating C3b function from Raji cell membranes

Area of Science:

• Immunology
• Complement System Biology

Background:

• The alternative pathway of complement is a crucial part of the innate immune system.
• Regulation of the alternative pathway prevents host cell damage.

Purpose of the Study:

• To investigate the inhibitory activity of a solubilized Raji cell membrane fraction on the alternative pathway of complement.
• To determine if this inhibitory activity is attributable to known complement regulatory proteins, factor H or factor I.

Main Methods:

• Solubilization of Raji cell membranes.
• Assay of alternative pathway-dependent hemolysis.
• Analysis of factor B decay.
• Immunoadsorption using anti-factor H and anti-factor I antibodies.
• Functional assays to assess C3 cleavage and synergy with factor I.

Main Results:

• The Raji cell membrane fraction inhibited alternative pathway-dependent hemolysis of sheep erythrocytes.
• The fraction accelerated the decay of factor B from erythrocytes but did not affect C2 decay.
• Inhibitory activity was not removed by immunoadsorption with anti-factor H or anti-factor I.
• The fraction did not cleave C3 or synergize with factor I, unlike factor H and I.

Conclusions:

• Raji cells contain a novel membrane-bound regulatory factor for the alternative pathway of complement.
• This factor is functionally and immunologically distinct from serum factors H and I.
• Further characterization of this Raji cell factor may reveal new insights into complement regulation.