Receptor-ligand interactions between serum amyloid P component and model soluble immune complexes

Area of Science:

• Immunology
• Biochemistry
• Molecular Biology

Background:

• Soluble immune complexes, such as aggregated IgG (AAg), play a role in various immune responses.
• Serum amyloid P (SAP) is a plasma protein found in various tissues and bodily fluids.
• Understanding the interactions of SAP with immune complexes is crucial for elucidating immune clearance mechanisms.

Purpose of the Study:

• To investigate the binding characteristics of isolated human serum amyloid P (SAP) to aggregated human IgG (AAg).
• To determine the specificity, affinity, and reversibility of the SAP:AAg interaction.
• To compare SAP binding to AAg with its binding to monomeric IgG and explore potential in vivo implications.

Main Methods:

• Protein binding assays were performed using SAP adsorbed to microtiter plate wells.
• Aggregated IgG (AAg) preparations of varying molecular weights were used as ligands.
• Binding isotherms, dissociation constants (Kd), and competitive binding assays (IC50) were analyzed.

Main Results:

• SAP demonstrated specific, saturable, high-affinity, and reversible binding to AAg.
• Dissociation constants (Kd) for SAP:AAg binding ranged from 0.60 to 1.9 nM.
• SAP showed significantly higher affinity for AAg compared to monomeric IgG (IC50 of 22-51 nM vs 10 μM).
• Binding was independent of calcium ions but not supported by cuprous ion.

Conclusions:

• SAP exhibits receptor:ligand-like binding to aggregated IgG (AAg).
• This interaction is specific and of high affinity, distinguishing it from binding to monomeric IgG.
• The findings suggest a potential role for SAP in the in vivo clearance of immune complexes.