A solid-phase radioimmunoassay for C1q-binding immune complexes. I. Delta IgG as indicator molecule

Area of Science:

• Immunology
• Biochemistry
• Assay Development

Background:

• Circulating immune complexes (IC) are implicated in various autoimmune diseases.
• Accurate detection of IC and related markers like heat-aggregated IgG (delta IgG) is crucial for diagnosis and monitoring.
• Existing methods may have limitations in sensitivity or practicality.

Purpose of the Study:

• To develop and validate a novel solid-phase radioimmunoassay for detecting circulating C1q-binding immune complexes (IC) and heat-aggregated IgG (delta IgG).
• To establish the sensitivity, stability, and utility of the developed assay for clinical applications.

Main Methods:

• Adsorption of purified human C1q protein onto polystyrene tubes to create a solid-phase.
• Incubation with samples containing IC or delta IgG, followed by detection using radiolabeled tracers.
• Optimization of binding conditions, including ionic strength (mu) and temperature.
• Assessment of assay stability under storage conditions.
• Evaluation of detection limits using competitive binding assays with [125I]delta IgG.

Main Results:

• The assay demonstrated effective binding of delta IgG and IC to solid-phase C1q, enhanced at low ionic strength.
• Heating C1q significantly reduced binding capacity, indicating the importance of native C1q structure.
• The assay showed good stability, retaining binding capacity after several weeks of refrigerated storage.
• Detection limits were established at 2-5 ng for delta IgG and <50 ng for IC.
• Preliminary testing on human sera confirmed the assay's potential for detecting IC in patient samples.

Conclusions:

• A robust and sensitive solid-phase radioimmunoassay for C1q-binding IC and delta IgG has been successfully developed.
• The assay exhibits favorable stability and sensitivity, making it suitable for clinical diagnostics.
• This assay offers a valuable tool for the detection of immune complexes in patient sera, aiding in the diagnosis of related diseases.