Complement factor H-binding protein of Raji cells and tonsil B lymphocytes

Area of Science:

• Immunology
• Complement System
• Cell Biology

Background:

• Factor H plays a crucial role in regulating the complement system.
• Previous studies suggested the existence of Factor H receptors on certain cell populations.
• A Factor H-binding protein was previously purified from Raji B-lymphoblastoid cell supernatants.

Purpose of the Study:

• To re-investigate the existence of cell-surface Factor H-binding proteins on B cells.
• To characterize the identified Factor H-binding proteins.
• To differentiate the binding protein from Factor H itself.

Main Methods:

• Surface iodination of Raji and tonsil B cells.
• Factor H-Sepharose affinity chromatography of cell extracts.
• SDS-PAGE analysis under reducing and non-reducing conditions.
• Biosynthetic radiolabelling studies.
• Characterization using monoclonal antibodies and Zn2+-chelate resin.

Main Results:

• A single radioactive species binding to Factor H-Sepharose was eluted from surface-labeled B cells, with an apparent molecular weight of 170,000.
• Biosynthetic studies confirmed this species is synthesized by Raji cells.
• A soluble form of the binding protein (approx. 105,000 Mr) was detected in cell supernatants.
• The cell-surface protein is distinct from Factor H, as shown by antibody binding, Zn2+-chelate resin interaction, and lack of binding to Factor H on U937 cells.

Conclusions:

• A cell-surface Factor H-binding protein, synthesized by Raji cells, exists on B cells.
• This protein is distinct from Factor H and may be related to previously purified soluble forms.
• The findings contribute to understanding the interactions between Factor H and B lymphocytes.